Braiding errors in interacting Majorana quantum wires Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 Michael Sekania, Stephan Plugge, Martin Greiter, Ronny Thomale, and Peter Schmitteckert
Avenues of Majorana bound states (MBSs) have become one of the primary directions towards a possible realization of topological quantum computation. For a Y junction of Kitaev quantum wires, we numerically investigate the braiding of MBSs while considering the full quasiparticle background. The two central sources of braiding errors are found to be the fidelity loss due to the incomplete adiabaticity of the braiding operation as well as the finite hybridization of the MBSs. The explicit extraction of the braiding phase from the full many-particle states allows us to analyze the breakdown of the independent-particle picture of Majorana braiding. Furthermore, we find nearest-neighbor interactions to significantly affect the braiding performance for better or worse, depending on the sign and magnitude of the coupling.
Spin-reorientation transitions in the Cairo pentagonal magnetBi4Fe5O13F Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 Alexander A. Tsirlin, Ioannis Rousochatzakis, Dmitry Filimonov, Dmitry Batuk, Matthias Frontzek, and Artem M. Abakumov
We show that interlayer spins play a dual role in the Cairo pentagonal magnet Bi4Fe5O13F, on one hand mediating the three-dimensional magnetic order, and on the other driving spin-reorientation transitions both within and between the planes. The corresponding sequence of magnetic orders unraveled by neutron diffraction and Mössbauer spectroscopy features two orthogonal magnetic structures described by opposite local vector chiralities, and an intermediate, partly disordered phase with nearly collinear spins. A similar collinear phase has been predicted theoretically to be stabilized by quantum fluctuations, but Bi4Fe5O13F is very far from the relevant parameter regime. While the observed in-plane reorientation cannot be explained by any standard frustration mechanism, our ab initio band-structure calculations reveal strong single-ion anisotropy of the interlayer Fe3+ spins that turns out to be instrumental in controlling the local vector chirality and the associated interlayer order.
Terahertz-frequency magnetoelectric effect in Ni-dopedCaBaCo4O7 Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 Shukai Yu, C. Dhanasekhar, Venimadhav Adyam, Skylar Deckoff-Jones, Michael K. L. Man, Julien Madéo, E Laine Wong, Takaaki Harada, M. Bala Murali Krishna, Keshav M. Dani, and Diyar Talbayev
We present a study of the terahertz-frequency magnetoelectric effect in ferrimagnetic pyroelectric CaBaCo4O7 and its Ni-doped variants. The terahertz absorption spectrum of these materials consists of spin excitations and low-frequency infrared-active phonons. We studied the magnetic-field-induced changes in the terahertz refractive index and absorption in magnetic fields up to 17 T. We find that the magnetic field modulates the strength of infrared-active optical phonons near 1.2 and 1.6 THz. We use the Lorentz model of the dielectric function to analyze the measured magnetic-field dependence of the refractive index and absorption. We propose that most of the magnetoelectric effect is contributed by the optical phonons near 1.6 THz and higher frequency resonances. Our experimental results can be used to construct and validate more detailed theoretical descriptions of magnetoelectricity in CaBaCo4−xNixO7.
Negative permeability in magnetostatics and its experimental demonstration Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 Rosa Mach-Batlle, Albert Parra, Jordi Prat-Camps, Sergi Laut, Carles Navau, and Alvaro Sanchez
The control of magnetic fields, essential for our science and technology, is currently achieved by magnetic materials with positive permeability, including ferromagnetic, paramagnetic, and diamagnetic types. Here we introduce materials with negative static permeability as a new paradigm for manipulating magnetic fields. As a first step, we extend the solutions of Maxwell magnetostatic equations to include negative-permeability values. The understanding of these new solutions allow us to devise a negative-permeability material as a suitably tailored set of currents arranged in space, overcoming the fact that passive materials with negative permeability do no exist in magnetostatics. We confirm the theory by experimentally creating a spherical shell that emulates a negative-permeability material in a uniform magnetic field. Our results open new possibilities for creating and manipulating magnetic fields, which can be useful for practical applications.
Magnetic structure and spin-wave excitations in the multiferroic magnetic metal-organic framework(CD3)2ND2[Mn(DCO2)3] Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 H. C. Walker, H. D. Duncan, M. D. Le, D. A. Keen, D. J. Voneshen, and A. E. Phillips
We report the magnetic diffraction pattern and spin-wave excitations in (CD3)2ND2[Mn(DCO2)3] measured using elastic and inelastic neutron scattering. The magnetic structure is shown to be a G-type antiferromagnet with moments pointing along the b axis. By comparison with simulations based on linear spin-wave theory, we have developed a model for the magnetic interactions in this multiferroic metal-organic framework material. The interactions form a three-dimensional network with antiferromagnetic nearest-neighbor interactions along three directions of J1=−0.103(8) meV, J2=−0.032(8) meV, and J3=−0.035(8) meV.
Pair breaking of multigap superconductivity under parallel magnetic fields in the electric-field-induced surface metallic state Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 Masahiro Nabeta, Kenta K. Tanaka, Seiichiro Onari, and Masanori Ichioka
The roles of paramagnetic and diamagnetic pair-breaking effects in superconductivity in the electric-field-induced surface metallic state are studied using the Bogoliubov–de Gennes equation when magnetic fields are applied parallel to the surface. The multigap states of the subbands are related to the depth dependence and the magnetic field dependence of the superconductivity. In the Fermi-energy density of states and the spin density, subband contributions successively appear from higher-level subbands with increasing magnetic fields. The characteristic magnetic field dependence may be a key feature to identify the multigap structure of the surface superconductivity.
Effect of proton irradiation on the normal-state low-energy excitations ofBa(Fe1−xRhx)2As2superconductors Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 M. Moroni, L. Gozzelino, G. Ghigo, M. A. Tanatar, R. Prozorov, P. C. Canfield, and P. Carretta
We present a As75 nuclear magnetic resonance (NMR) and resistivity study of the effect of 5.5 MeV proton irradiation on the optimal electron doped (x=0.068) and overdoped (x=0.107) Ba(Fe1−xRhx)2As2 iron based superconductors. While the proton induced defects only mildly suppress the critical temperature and increase residual resistivity in both compositions, sizable broadening of the NMR spectra was observed in all the irradiated samples at low temperature. The effect is significantly stronger in the optimally doped sample where the Curie Weiss temperature dependence of the line width suggests the onset of ferromagnetic correlations coexisting with superconductivity at the nanoscale. 1/T2 measurements revealed that the energy barrier characterizing the low energy spin fluctuations of these compounds is enhanced upon proton irradiation, suggesting that the defects are likely slowing down the fluctuations between (0,π) and (π,0) nematic ground states.
Real-space observation of nanoscale magnetic phase separation in dysprosium by aberration-corrected Lorentz microscopy Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 Takuro Nagai, Koji Kimoto, Koji Inoke, and Masaki Takeguchi
Transport properties across the many-body localization transition in quasiperiodic and random systems Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 F. Setiawan, Dong-Ling Deng, and J. H. Pixley
We theoretically study transport properties in one-dimensional interacting quasiperiodic systems at infinite temperature. We compare and contrast the dynamical transport properties across the many-body localization (MBL) transition in quasiperiodic and random models. Using exact diagonalization we compute the optical conductivity σ(ω) and the return probability R(τ) and study their average low-frequency and long-time power-law behavior, respectively. We show that the low-energy transport dynamics is markedly distinct in both the thermal and MBL phases in quasiperiodic and random models and find that the diffusive and MBL regimes of the quasiperiodic model are more robust than those in the random system. Using the distribution of the dc conductivity, we quantify the contribution of sample-to-sample and state-to-state fluctuations of σ(ω) across the MBL transition. We find that the activated dynamical scaling ansatz works poorly in the quasiperiodic model but holds in the random model with an estimated activation exponent ψ≈0.9. We argue that near the MBL transition in quasiperiodic systems, critical eigenstates give rise to a subdiffusive crossover regime on finite-size systems.
Bounds on complex polarizabilities and a new perspective on scattering by a lossy inclusion Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 Graeme W. Milton
Here, we obtain explicit formulas for bounds on the complex electrical polarizability at a given frequency of an inclusion with known volume that follow directly from the quasistatic bounds of Bergman and Milton on the effective complex dielectric constant of a two-phase medium. We also describe how analogous bounds on the orientationally averaged bulk and shear polarizabilities at a given frequency can be obtained from bounds on the effective complex bulk and shear moduli of a two-phase medium obtained by Milton, Gibiansky, and Berryman, using the quasistatic variational principles of Cherkaev and Gibiansky. We also show how the polarizability problem and the acoustic scattering problem can both be reformulated in an abstract setting as “Y problems.” In the acoustic scattering context, to avoid explicit introduction of the Sommerfeld radiation condition, we introduce auxiliary fields at infinity and an appropriate “constitutive law” there, which forces the Sommerfeld radiation condition to hold. As a consequence, we obtain minimization variational principles for acoustic scattering that can be used to obtain bounds on the complex backwards scattering amplitude. Some explicit elementary bounds are given.
Coherent generation of symmetry-forbidden phonons by light-induced electron-phonon interactions in magnetite Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 S. Borroni, E. Baldini, V. M. Katukuri, A. Mann, K. Parlinski, D. Legut, C. Arrell, F. van Mourik, J. Teyssier, A. Kozlowski, P. Piekarz, O. V. Yazyev, A. M. Oleś, J. Lorenzana, and F. Carbone
Symmetry breaking across phase transitions often causes changes in selection rules and emergence of optical modes which can be detected via spectroscopic techniques or generated coherently in pump-probe experiments. In second-order or weakly first-order transitions, fluctuations of the ordering field are present above the ordering temperature, giving rise to intriguing precursor phenomena, such as critical opalescence. Here, we demonstrate that in magnetite (Fe3O4) light excitation couples to the critical fluctuations of the charge order and coherently generates structural modes of the ordered phase above the critical temperature of the Verwey transition. Our findings are obtained by detecting coherent oscillations of the optical constants through ultrafast broadband spectroscopy and analyzing their dependence on temperature. To unveil the coupling between the structural modes and the electronic excitations, at the origin of the Verwey transition, we combine our results from pump-probe experiments with spontaneous Raman scattering data and theoretical calculations of both the phonon dispersion curves and the optical constants. Our methodology represents an effective tool to study the real-time dynamics of critical fluctuations across phase transitions.
Photomagnonic nanocavities for strong light–spin-wave interaction Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 P. A. Pantazopoulos, N. Stefanou, E. Almpanis, and N. Papanikolaou
The interaction of visible and near-infrared light with spin waves in appropriately designed dual nanocavities, for both photons and magnons, is investigated by means of rigorous calculations, correct to arbitrary order in the magneto-optical coupling parameter. It is shown that the concurrent localization of the interacting photon and magnon fields in the same region of space for a long period of time enhances their mutual interaction, provided that specific selection rules are fulfilled. Our results provide evidence for the occurrence of strong effects, beyond the linear response approximation, which lead to enhanced modulation of light by spin waves through multimagnon absorption and emission processes by a photon.
Off-axis spin orientation in goethite nanoparticles Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 Erik Brok, Kim Lefmann, Gøran Jan Nilsen, Mathias Kure, and Cathrine Frandsen
Neutron diffraction is a powerful technique for determining the magnetic structure of antiferromagnetic materials. However, for some of these, determining the detailed magnetic structure remains a challenge. In goethite (α-FeOOH) the antiferromagnetic unit cell coincides with the chemical unit cell and, consequently, nuclear and magnetic diffraction peaks occur at the same positions. Analysis of diffraction data from goethite is further complicated by finite-size peak broadening, resulting from goethite commonly occurring in nanocrystalline form. For these reasons, determining the magnetic structure of goethite has been challenging, and few detailed studies have been published. Even today, not all aspects of the magnetic structure are well established. Here, we investigate the magnetic structure of three samples of goethite nanoparticles with polarized neutron powder diffraction (xyz-polarization analysis). Two samples consist of acicular goethite particles that are approximately 40 nm long and with different thicknesses, and one sample consists of pseudo-spherical particles with a diameter of approximately 5 nm. The larger particles consist of several crystallites whereas the 5-nm particles are mostly single crystalline. The polarization analysis enables us to separate magnetic scattering from nuclear and spin-incoherent scattering, resulting in data that can readily be analyzed. For the two samples with the larger particle size, we find nuclear correlation lengths in the  direction that are approximately 3 nm longer than the magnetic correlation lengths, indicating a magnetically disordered layer perpendicular to the antiferromagnetic modulation direction. We find no evidence of a magnetically disordered surface layer in the 5-nm particles. We find the magnetic structure to be antiferromagnetic but, in contrast to most previous studies, we find the spin orientation in all three samples to make an angle of 28-30∘ with respect to the crystallographic b axis.
Transient terahertz photoconductivity of insulating cuprates Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 J. C. Petersen, A. Farahani, D. G. Sahota, Ruixing Liang, and J. S. Dodge
We establish a detailed phenomenology of photocarrier transport in the copper oxide plane by studying the transient terahertz photoconductivity of Sr2CuO2Cl2 and YBa2Cu3O6. The peak photoconductivity saturates with fluence, decays on multiple picosecond timescales, and evolves into a state characterized by activated transport. The time dependence shows little change with fluence, indicating that the decay is governed by first-order recombination kinetics. We find that most photocarriers make a negligible contribution to the dc photoconductivity, and we estimate the intrinsic photocarrier mobility to be 0.6–0.7 cm2/Vs at early times, comparable to the mobility in chemically doped materials.
Examining real-time time-dependent density functional theory nonequilibrium simulations for the calculation of electronic stopping power Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 Dillon C. Yost, Yi Yao, and Yosuke Kanai
In ion irradiation processes, electronic stopping power describes the energy transfer rate from the irradiating ion to the target material's electrons. Due to the scarcity and significant uncertainties in experimental electronic stopping power data for materials beyond simple solids, there has been growing interest in the use of first-principles theory for calculating electronic stopping power. In recent years, advances in high-performance computing have opened the door to fully first-principles nonequilibrium simulations based on real-time time-dependent density functional theory (RT-TDDFT). While it has been demonstrated that the RT-TDDFT approach is capable of predicting electronic stopping power for a wide range of condensed matter systems, there has yet to be an exhaustive examination of the physical and numerical approximations involved and their effects on the calculated stopping power. We discuss the results of such a study for crystalline silicon with protons as irradiating ions. We examine the influences of key approximations in RT-TDDFT nonequilibrium simulations on the calculated electronic stopping power, including approximations related to basis sets, finite size effects, exchange-correlation approximation, pseudopotentials, and more. Finally, we propose a simple and efficient correction scheme to account for the contribution from core-electron excitations to the stopping power, as it was found to be significant for large proton velocities.
Effect of different in-chain impurities on the magnetic properties of the spin chain compoundSrCuO2probed by NMR Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 Yannic Utz, Franziska Hammerath, Roberto Kraus, Tobias Ritschel, Jochen Geck, Liviu Hozoi, Jeroen van den Brink, Ashwin Mohan, Christian Hess, Koushik Karmakar, Surjeet Singh, Dalila Bounoua, Romuald Saint-Martin, Loreynne Pinsard-Gaudart, Alexandre Revcolevschi, Bernd Büchner, and Hans-Joachim Grafe
The S=1/2 Heisenberg spin chain compound SrCuO2 doped with different amounts of nickel (Ni), palladium (Pd), zinc (Zn), and cobalt (Co) has been studied by means of Cu nuclear magnetic resonance (NMR). Replacing only a few of the S=1/2 Cu ions with Ni, Pd, Zn, or Co has a major impact on the magnetic properties of the spin chain system. In the case of Ni, Pd, and Zn an unusual line broadening in the low temperature NMR spectra reveals the existence of an impurity-induced local alternating magnetization (LAM), while strongly decaying spin-lattice relaxation rates T1−1 towards low temperatures indicate the opening of spin gaps. A distribution of gap magnitudes is implied by a stretched spin-lattice relaxation and a variation of T1−1 within the broad resonance lines. These observations depend strongly on the impurity concentration and therefore can be understood using the model of finite segments of the spin 1/2 antiferromagnetic Heisenberg chain, i.e., pure chain segmentation due to S=0 impurities. This is surprising for Ni as it was previously assumed to be a magnetic impurity with S=1 which is screened by the neighboring copper spins. In order to confirm the S=0 state of the Ni, we performed x-ray absorption spectroscopy (XAS) and compared the measurements to simulated XAS spectra based on multiplet ligand-field theory. Furthermore, Zn doping leads to much smaller effects on both the NMR spectra and the spin-lattice relaxation rates, indicating that Zn avoids occupying Cu sites. For magnetic Co impurities, T1−1 does not obey the gaplike decrease, and the low-temperature spectra get very broad. This could be related to an increase of the Néel temperature and is most likely an effect of the impurity spin S≠0.
Universal boundary entropies in conformal field theory: A quantum Monte Carlo study Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 Wei Tang, Lei Chen, Wei Li, X. C. Xie, Hong-Hao Tu, and Lei Wang
Scanning tunneling microscopy and spectroscopy of finite-size twisted bilayer graphene Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 Wen-Xiao Wang, Hua Jiang, Yu Zhang, Si-Yu Li, Haiwen Liu, Xinqi Li, Xiaosong Wu, and Lin He
Finite-size twisted bilayer graphene (TBG, where here the TBG is of nanoscale size) is quite unstable and will change its structure to a Bernal (or AB-stacking) bilayer with a much lower energy. Therefore, the lack of finite-size TBG makes its electronic properties difficult to access in experiments. In this paper, a special confined TBG is obtained in the overlaid area of two continuous misoriented graphene sheets. The width of the confined region of the TBG changes gradually from about 22 to 0 nm. By using scanning tunneling microscopy, we study carefully the structure and the electronic properties of finite-size TBG. Our results indicate that the low-energy electronic properties, including twist-induced Van Hove singularities (VHSs) and spatial modulation of the local density of states, are strongly affected by the translational symmetry breaking of the finite-size TBG. However, the electronic properties above the energy of the VHSs are almost not influenced by quantum confinement even when the width of the TBG is reduced to only a single moiré spot.
Field-enhanced direct tunneling in ultrathin atomic-layer-deposition-grownAu−Al2O3-Cr metal-insulator-metal structures Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 L. Fry-Bouriaux, M. C. Rosamond, D. A. Williams, A. G. Davies, and C. Wälti
Metal-insulator-metal structures based on ultrathin high-k dielectric films are underpinning a rapidly increasing number of devices and applications. Here, we report detailed electrical characterizations of asymmetric metal-insulator-metal devices featuring atomic layer deposited 2-nm-thick Al2O3 films. We find a high consistency in the current density as a function of applied electric field between devices with very different surface areas and significant asymmetries in the IV characteristics. We show by TEM that the thickness of the dielectric film and the quality of the metal-insulator interfaces are highly uniform and of high quality, respectively. In addition, we develop a model which accounts for the field enhancement due to the small sharp features on the electrode surface and show that this can very accurately describe the observed asymmetry in the current-voltage characteristic, which cannot be explained by the difference in work function alone.
Faraday rotation spectrum of magneto-optical nanoparticle aggregates Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 Mahdiyeh Sadrara and MirFaez Miri
The interaction of light with a cluster of gyrotropic spherical particles is studied in view of a miniaturized Faraday rotator. The electromagnetic fields are expanded in terms of the vector multipole fields and the expansion of the scattered field is related to that of the incident field. An incident linearly polarized light with polarization azimuth ψ becomes elliptically polarized upon scattering from the cluster. The polarization azimuth rotation and ellipticity angle variation are almost sinusoidal functions of 2ψ. With planar disordered clusters of bismuth-substituted yttrium iron garnet nanoparticles of radius 50 nm, polarization rotations about ±4∘ are achievable.
Loss of adiabaticity with increasing tunneling gap in nonintegrable multistate Landau-Zener models Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 Rajesh K. Malla and M. E. Raikh
We consider the simplest nonintegrable model of the multistate Landau-Zener transition. In this model, two pairs of levels in two tunnel-coupled quantum dots are swept past each other by the gate voltage. Although this 2×2 model is nonintegrable, it can be solved analytically in the limit when the interlevel energy distance is much smaller than their tunnel splitting. The result is contrasted to the similar 2×1 model, in which one of the dots contains only one level. The latter model does not allow interference of the virtual transition amplitudes, and it is exactly solvable. In the 2×1 model, the probability for a particle, residing at time t→−∞ in one dot, to remain in the same dot at t→∞, falls off exponentially with tunnel coupling. By contrast, in the 2×2 model, this probability grows rapidly with tunnel coupling. The physical origin of this growth is the formation of the tunneling-induced collective states in the system of two dots. This can be viewed as a manifestation of the Dicke effect.
Low-energy electron diffraction from ferroelectric surfaces: Dead layers and surface dipoles in cleanPb(Zr,Ti)O3(001) Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 Cristian M. Teodorescu, Lucian Pintilie, Nicoleta G. Apostol, Ruxandra M. Costescu, George A. Lungu, Luminiţa Hrib, Lucian Trupină, Liviu C. Tănase, Ioana C. Bucur, and Amelia E. Bocîrnea
The positions of the low energy electron diffraction (LEED) spots from ferroelectric single crystal films depend on its polarization state, due to electric fields generated outside of the sample. One may derive the surface potential energy, yielding the depth where the mobile charge carriers compensating the depolarization field are located (δ). On ferroelectric Pb(Zr,Ti)O3(001) samples, surface potential energies are between 6.7 and 10.6 eV, and δ values are unusually low, in the range of 1.8±0.4Å. When δ is introduced in the values of the band bending inside the ferroelectric, a considerably lower value of the dielectric constant and/or of the polarization near the surface than their bulk values is obtained, evidencing either that the intrinsic ‘dielectric constant’ of the material has this lower value or the existence of a ‘dead layer’ at the free surface of clean ferroelectric films. The inwards polarization of these films is explained in the framework of the present considerations by the formation of an electron sheet on the surface. Possible explanations are suggested for discrepancies between the values found for surface potential energies from LEED experiments and those derived from the transition between mirror electron microscopy and low energy electron microscopy.
Adiabatic and nonadiabatic spin torques induced by a spin-triplet supercurrent Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 Rina Takashima, Satoshi Fujimoto, and Takehito Yokoyama
We study spin-transfer torques induced by a spin-triplet supercurrent in a magnet with the superconducting proximity effect. By a perturbative approach, we show that spin-triplet correlations realize new types of torques, which are analogous to the adiabatic and nonadiabatic (β) torques, without extrinsic spin-flip scattering. Remarkable advantages compared to conventional spin-transfer torques are highlighted in domain-wall manipulation. Oscillatory motions of a domain wall do not occur for a small Gilbert damping, and the threshold current density to drive its motion becomes zero in the absence of extrinsic pinning potentials due to the nonadiabatic torque controlled by the triplet correlations.
Collective charge excitations of the two-dimensional electrideCa2N Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 Pierluigi Cudazzo and Matteo Gatti
Ca2N is a layered material that has been recently identified as a two-dimensional (2D) electride, an unusual ionic compound in which electrons serve as anions. The electronic properties of 2D electrides attract considerable interest as the anionic electrons, which form a 2D layer sandwiched between atomic planes, are highly mobile as they are not attached to any ion. Here, on the basis of first-principles time-dependent density-functional theory calculations, we investigate the collective excitations of the electrons—i.e., the plasmons—in Ca2N as a function of wave vector q. Our calculations reveal an intrinsic negative in-plane dispersion of the anionic plasmon, in striking contrast with the homogeneous electron gas. Moreover, for wave vectors q normal to the planes, we find a long-lived plasmon that continues to exist well beyond the first Brillouin zone. This is a mark of the electronic inhomogeneities in the charge response that Ca2N shares with other layered materials like transition-metal dichalcogenides and MgB2. Finally, we compare the plasmon properties of Ca2N in its bulk and monolayer forms, which shows the effect of the different electronic structures and dimensionalities.
First-principles study of the luminescence ofEu2+-doped phosphors Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 Yongchao Jia, Anna Miglio, Samuel Poncé, Masayoshi Mikami, and Xavier Gonze
The luminescence of fifteen representative Eu2+-doped phosphors used for white-LED and scintillation applications is studied through a constrained density functional theory. Transition energies and Stokes shift are deduced from differences of total energies between the ground and excited states of the systems, in the absorption and emission geometries. The general applicability of such methodology is first assessed: for this representative set, the calculated absolute error with respect to experiment on absorption and emission energies is within 0.3 eV. This set of compounds covers a wide range of transition energies that extents from 1.7 to 3.5 eV. The information gained from the relaxed geometries and total energies is further used to evaluate the thermal barrier for the 4f−5d crossover, the full width at half maximum of the emission spectrum and the temperature shift of the emission peak, using a one-dimensional configuration-coordinate model. The former results indicate that the 4f−5d crossover cannot be the dominant mechanism for the thermal quenching behavior of Eu2+-doped phosphors and the latter results are compared to available experimental data and yield a 30% mean absolute relative error. Finally, a semiempirical model used previously for Ce3+-doped hosts is adapted to Eu2+-doped hosts and gives the absorption and emission energies within 0.9 eV of experiment, underperforming compared to the first-principles calculation.
Manipulating light at subwavelength scale by exploiting defect-guided spoof plasmon modes Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 A. Ourir, A. Maurel, S. Félix, J.-F. Mercier, and M. Fink
We study the defect-guided modes supported by a set of metallic rods structured at the subwavelength scale. Following the idea of photonic crystal waveguide, we show that spoof plasmon surface waves can be manipulated at subwavelength scale. We demonstrate that these waves can propagate without leakage along a row of rods having a different length than the surrounding medium and we provide the corresponding dispersion relation. The principle of this subwavelength colored guide is validated experimentally. This allows us to propose the design of a wavelength demultiplexer whose efficiency is illustrated in the microwave regime.
Plasmonic modes of polygonal rods calculated using a quantum hydrodynamics method Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 Kun Ding and C. T. Chan
Plasmonic resonances of nanoparticles have drawn lots of attention due to their interesting and useful properties such as strong field enhancements. The self-consistent hydrodynamics model has the advantage that it can incorporate the quantum effect of the electron gas into classical electrodynamics in a consistent way. We use the method to study the plasmonic response of polygonal rods under the influence of an external electromagnetic wave, and we pay particular attention to the size and shape of the particle and the effect of charging. We find that the particles support edge modes, face modes, and hybrid modes. The charges induced by the external field in the edge (face) modes mainly localize at the edges (faces), while the induced charges in the hybrid modes are distributed nearly evenly in both the edges and faces. The edge modes are less sensitive to particle size than the face modes but are sensitive to the corner angles of the edges. When the number of sides of regular polygons increases, the edge and face modes gradually change into the classical dipole plasmonic mode of a cylinder. The hybrid modes are found to be the precursor of the Bennett mode, which cannot be found in classical electrodynamics.
Structural and electronic properties ofα-(BEDT-TTF)2I3,β-(BEDT-TTF)2I3, andκ-(BEDT-TTF)2X3(X= I, F, Br, Cl) organic charge transfer salts Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 Benjamin Commeau, R. Matthias Geilhufe, Gayanath W. Fernando, and Alexander V. Balatsky
(BEDT−TFF)2I3 charge transfer salts are reported to show superconductivity and pressure-induced quasi-two-dimensional Dirac cones at the Fermi level. By performing state of the art ab initio calculations in the framework of density functional theory, we investigate the structural and electronic properties of the three structural phases α, β, and κ. We furthermore report about the irreducible representations of the corresponding electronic band structures, symmetry of their crystal structure, and the origin of band crossings. Additionally, we discuss the chemically induced strain in κ−(BEDT−TTF)2I3 achieved by replacing the iodine layer with other halogens: fluorine, bromine, and chlorine. In the case of κ−(BEDT−TTF)2F3, we identify topologically protected crossings within the band structure. These crossings are forced to occur due to the nonsymmorphic nature of the crystal. The calculated electronic structures presented here are added to the organic materials database (OMDB).
Finite-size effects in a nanowire strongly coupled to a thin superconducting shell Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 Christopher Reeg, Daniel Loss, and Jelena Klinovaja
We study the proximity effect in a one-dimensional nanowire strongly coupled to a finite superconductor with a characteristic size which is much shorter than its coherence length. Such geometries have become increasingly relevant in recent years in the experimental search for Majorana fermions with the development of thin epitaxial Al shells which form a very strong contact with either InAs or InSb nanowires. So far, however, no theoretical treatment of the proximity effect in these systems has accounted for the finite size of the superconducting film. We show that the finite-size effects become very detrimental when the level spacing of the superconductor greatly exceeds its energy gap. Without any fine tuning of the size of the superconductor (on the scale of the Fermi wavelength), the tunneling energy scale must be larger than the level spacing in order to reach the “hard gap” regime which is seen ubiquitously in the experiments. However, in this regime, the large tunneling energy scale induces a large shift in the effective chemical potential of the nanowire and pushes the topological phase transition to magnetic field strengths which exceed the critical field of Al.
Finite-size nanowire at a surface: Unconventional power laws of the van der Waals interaction Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 K. A. Makhnovets and A. K. Kolezhuk
We study the van der Waals interaction of a metallic or narrow-gap semiconducting nanowire with a surface, in the regime of intermediate wire-surface distances (vF/c)L≪d≪L or L≪d≪(c/vF)L, where L is the nanowire length, d is the distance to the surface, and vF is the characteristic velocity of nanowire electrons (for a metallic wire, it is the Fermi velocity). Our approach, based on the Luttinger liquid framework, allows one to analyze the dependence of the interaction on the interplay between the nanowire length, wire-surface distance, and characteristic length scales related to the spectral gap and temperature. We show that this interplay leads to nontrivial modifications of the power law that governs van der Waals forces, in particular to a nonmonotonic dependence of the power-law exponent on the wire-surface separation.
Dispersion of the nonlinear susceptibility in gold nanoantennas Phys. Rev. B (IF 3.836) Pub Date : 2017-09-19 V. Knittel, M. P. Fischer, M. Vennekel, T. Rybka, A. Leitenstorfer, and D. Brida
Femtosecond optical pulses tunable in the near infrared are exploited to drive third harmonic generation (THG) and incoherent multiphoton photoluminescence (MPPL) in gold plasmonic nanoantennas. By comparing the yield of the two processes concurrently occurring on the same nanostructure, we extract the coherent third-order response of the antenna. Its contribution is enhanced at shorter excitation wavelengths allowing the observation of dispersion in the nonlinear susceptibility of gold.
Modulation of the superconducting critical temperature due to quantum confinement at theLaAlO3/SrTiO3interface Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 D. Valentinis, S. Gariglio, A. Fête, J.-M. Triscone, C. Berthod, and D. van der Marel
Superconductivity develops in bulk doped SrTiO 3 and at the LaAlO 3 / SrTiO 3 interface with a dome-shaped density dependence of the critical temperature T c , despite different dimensionalities and geometries. We propose that the T c dome of LaAlO 3 / SrTiO 3 is a shape resonance due to quantum confinement of superconducting bulk SrTiO 3 . We substantiate this interpretation by comparing the exact solutions of a three-dimensional and quasi-two-dimensional two-band BCS gap equation. This comparison highlights the role of heavy bands for T c in both geometries. For bulk SrTiO 3 , we extract the density dependence of the pairing interaction from the fit to experimental data. We apply quantum confinement in a square potential well of finite depth and calculate T c in the confined configuration. We compare the calculated T c to transport experiments and provide an explanation as to why the optimal T c 's are so close to each other in two-dimensional interfaces and the three-dimensional bulk material.
Low-energy spin dynamics and critical hole concentrations inLa2−xSrxCuO4(0.07≤x≤0.2)revealed byLa139andCu63nuclear magnetic resonance Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 S.-H. Baek, A. Erb, and B. Büchner
We report a comprehensive La 139 and Cu 63 nuclear magnetic resonance study on La 2 − x Sr x CuO 4 ( 0.07 ≤ x ≤ 0.2 ) single crystals. The La 139 spin-lattice relaxation rate T 1 − 1 139 is drastically influenced by Sr doping x at low temperatures. A detailed field dependence of T 1 − 1 139 at x = 1 / 8 suggests that charge ordering induces the critical slowing down of spin fluctuations toward glassy spin order and competes with superconductivity. On the other hand, the Cu 63 relaxation rate T 1 − 1 63 is well described by a Curie-Weiss law at high temperatures, yielding the Curie-Weiss temperature Θ as a function of doping. Θ changes sharply through a critical hole concentration x c ∼ 0.09 . x c appears to correspond to the delocalization limit of doped holes, above which the bulk nature of superconductivity is established.
Intraband and interband conductivity in systems of strongly interacting bosons Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 B. Grygiel, K. Patucha, and T. A. Zaleski
Motivated by recent experimental progress on measuring various correlation functions in systems of ultracold atoms in optical lattices, we study properties of the Bose-Hubbard model in external synthetic magnetic field to describe transport phenomena in a multiband strongly interacting bosonic systems. We calculate the conductivity both in the Mott insulator and superfluid phases and investigate its two main contributions: intra- and interband. It appears that the interband processes dominate the transport properties by at least an order of magnitude. Also, at finite temperatures, additional transport channels appear due to coupling of the thermally excited particles or holes to the external field.
Electronic properties, low-energy Hamiltonian, and superconducting instabilities inCaKFe4As4 Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 Felix Lochner, Felix Ahn, Tilmann Hickel, and Ilya Eremin
We analyze the electronic properties of the recently discovered stoichiometric superconductor CaKFe 4 As 4 by combining an ab initio approach and a projection of the band structure to a low-energy tight-binding Hamiltonian, based on the maximally localized Wannier orbitals of the 3 d Fe states. We identify the key symmetries as well as differences and similarities in the electronic structure between CaKFe 4 As 4 and the parent systems CaFe 2 As 2 and KFe 2 As 2 . In particular, we find CaKFe 4 As 4 to have a significantly more quasi-two-dimensional electronic structure than the latter systems. Finally, we study the superconducting instabilities in CaKFe 4 As 4 by employing the leading angular harmonics approximation and find two potential A 1 g -symmetry representations of the superconducting gap to be the dominant instabilities in this system.
Quasiparticle excitations and evidence for superconducting double transitions in monocrystallineU0.97Th0.03Be13 Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 Yusei Shimizu, Shunichiro Kittaka, Shota Nakamura, Toshiro Sakakibara, Dai Aoki, Yoshiya Homma, Ai Nakamura, and Kazushige Machida
Novel mechanism for order patterning in alloys driven by irradiation Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 C. R. Lear, P. Bellon, and R. S. Averback
Kinetic Monte Carlo simulations have been performed to investigate the evolution of ordered domains in model alloys under irradiation. The alloys investigated were equiatomic binary alloys on a simple square lattice with first and second nearest-neighbor interactions, chosen so that a 2 × 2 ordered structure is the equilibrium phase below a critical order-disorder transition temperature T c . The ratio of second to first nearest-neighbor interactions R was varied from 0 to 0.45 to explore the effect of the thermodynamic frustrations induced by the proximity of the 2 × 1 phase boundary, which occurs at R = 0.5 for T = 0 . The atomic mixing produced by nuclear collisions was modeled by forcing the ballistic exchange of pairs of atoms at a controlled rate Γ b . This disordering process competed with thermodynamic reordering, resulting in nonequilibrium steady states. Two trivial steady states were found, a disordered state at high Γ b and low T , and a long-range ordered state at low Γ b and low T . In the R = 0.45 alloy, however, a third steady state was identified at intermediate Γ b and T values, where multiple long-range ordered domains coexisted dynamically. It is shown that this state of patterning of order resulted from the coupling of the thermodynamic frustrations present in that alloy with the disorder introduced by irradiation. The practical relevance of this novel mechanism for patterning of order under irradiation is discussed in the context of recent observations of domain coexistence in irradiated Cu 3 Au .
Nonequilibriumab initiomolecular dynamics determination of Ti monovacancy migration rates inB1TiN Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 D. Gambino, D. G. Sangiovanni, B. Alling, and I. A. Abrikosov
We use the color diffusion (CD) algorithm in nonequilibrium (accelerated) ab initio molecular dynamics simulations to determine Ti monovacancy jump frequencies in NaCl-structure titanium nitride (TiN), at temperatures ranging from 2200 to 3000 K. Our results show that the CD method extended beyond the linear-fitting rate-versus-force regime [Sangiovanni et al., Phys. Rev. B 93, 094305 (2016)] can efficiently determine metal vacancy migration rates in TiN, despite the low mobilities of lattice defects in this type of ceramic compound. We propose a computational method based on gamma-distribution statistics, which provides unambiguous definition of nonequilibrium and equilibrium (extrapolated) vacancy jump rates with corresponding statistical uncertainties. The acceleration-factor achieved in our implementation of nonequilibrium molecular dynamics increases dramatically for decreasing temperatures from 500 for T close to the melting point T m , up to 33 000 for T ≈ 0.7 T m .
Magnetic properties of epitaxial CoO/Fe(001) bilayers: The onset of exchange bias as a function of sublayer thickness and temperature Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 Jacek Gurgul, Ewa Młyńczak, Anna Kozioł-Rachwał, Krzysztof Matlak, Kinga Freindl, Ewa Madej, Nika Spiridis, Tomasz Ślęzak, and Józef Korecki
We study the magnetic properties of epitaxial double-wedge CoO/Fe bilayers grown on MgO(001). We present a comprehensive set of data derived from the hysteresis loops measured as a function of temperature in a wide range of CoO and Fe thicknesses using Kerr microscopy imaging. We focus on relatively high temperatures and CoO layers with small thicknesses to address the onset of the exchange bias. We identify a characteristic CoO thickness of 30–40 Å, above which the films can be considered bulklike. However, a considerable exchange bias is still observed even for CoO films as thin as 12 Å if the temperature is sufficiently low. From the results of the x-ray photoelectron spectroscopy measurements, we deduce the presence of an interfacial iron oxide layer of mixed stoichiometry and a uniform composition of the CoO layer along the CoO wedge.
Quasistatic remanence in Dzyaloshinskii-Moriya interaction driven weak ferromagnets and piezomagnets Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 Namrata Pattanayak, Arpan Bhattacharyya, A. K. Nigam, Sang-Wook Cheong, and Ashna Bajpai
We explore remanent magnetization ( μ ) as a function of time and temperature, in a variety of rhombohedral antiferromagnets (AFMs) which are also weak ferromagnets (WFMs) and piezomagnets (PzMs). These measurements, across samples with length scales ranging from nano to bulk, firmly establish the presence of a remanence that is quasistatic in nature and exhibits a counterintuitive magnetic field dependence. These observations unravel an ultraslow magnetization relaxation phenomenon related to this quasistatic remanence. This feature is also observed in a defect-free single crystal of α − Fe 2 O 3 , which is a canonical WFM and PzM. Notably, α − Fe 2 O 3 is not a typical geometrically frustrated AFM, and in single crystal form it is also devoid of any size or interface effects, which are the usual suspects for a slow magnetization relaxation phenomenon. The underlying pinning mechanism appears exclusive to those AFMs which either are symmetry allowed WFMs, driven by Dzyaloshinskii-Moriya interaction, or can generate this trait by tuning of size and interface. The qualitative features of the quasistatic remanence indicate that such WFMs are potential piezomagnets, in which magnetization can be tuned by stress alone.
Intrinsic and spatially nonuniform ferromagnetism in Co-doped ZnO films Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 L. T. Tseng, A. Suter, Y. R. Wang, F. X. Xiang, P. Bian, X. Ding, A. Tseng, H. L. Hu, H. M. Fan, R. K. Zheng, X. L. Wang, Z. Salman, T. Prokscha, K. Suzuki, R. Liu, S. Li, E. Morenzoni, and J. B. Yi
Co doped ZnO films have been deposited by a laser-molecular beam epitaxy system. X-ray diffraction and UV spectra analysis show that Co effectively substitutes the Zn site. Transmission electron microscopy (TEM) and secondary ion mass spectroscopy analysis indicate that there are no clusters. Co dopants are uniformly distributed in ZnO film. Ferromagnetic ordering is observed in all samples deposited under an oxygen partial pressure, PO 2 = 10 − 3 , 10 − 5 , and 10 − 7 torr, respectively. However, the magnetization of PO 2 = 10 − 3 and 10 − 5 is very small at room temperature. At low temperature, the ferromagnetic ordering is enhanced. Muon spin relaxation ( μ SR ) measurements confirm the ferromagnetism in all samples, and the results are consistent with magnetization measurements. From μ SR and TEM analysis, the film deposited under PO 2 = 10 − 7 torr shows intrinsic ferromagnetism. However, the volume fraction of the ferromagnetism phase is approximately 70%, suggesting that the ferromagnetism is not carrier mediated. Resistivity versus temperature measurements indicate Efros variable range hopping dominates the conductivity. From the above results, we can confirm that a bound magnetic polaron is the origin of the ferromagnetism.
Multifaceted impact of a surface step on superconductivity in atomically thin films Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 L.-F. Zhang, L. Flammia, L. Covaci, A. Perali, and M. V. Milošević
Recent experiments show that an atomic step on the surface of atomically thin metallic films can strongly affect electronic transport. Here we reveal multiple and versatile effects that such a surface step can have on superconductivity in ultrathin films. By solving the Bogoliubov-de Gennes equations self-consistently in this regime, where quantum confinement dominates the emergent physics, we show that the electronic structure is profoundly modified on the two sides of the step, as is the spatial distribution of the superconducting order parameter and its dependence on temperature and electronic gating. Furthermore, the surface step changes nontrivially the transport properties both in the proximity-induced superconducting pair correlations and the Josephson effect, depending on the step height. These results offer a new route to tailor superconducting circuits and design atomically thin heterojunctions made of one same material.
Defect-induced large spin-orbit splitting in monolayerPtSe2 Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 Moh. Adhib Ulil Absor, Iman Santoso, Harsojo, Kamsul Abraha, Fumiyuki Ishii, and Mineo Saito
The effect of spin-orbit coupling on the electronic properties of monolayer (ML) PtSe 2 is dictated by the presence of the crystal inversion symmetry to exhibit a spin-polarized band without the characteristic of spin splitting. Through fully relativistic density-functional theory calculations, we show that large spin-orbit splitting can be induced by introducing point defects. We calculate the stability of native point defects such as a Se vacancy ( V Se ), a Se interstitial ( Se i ), a Pt vacancy ( V Pt ), and a Pt interstitial ( Pt i ) and find that both the V Se and Se i have the lowest formation energy. We also find that, in contrast to the Se i case exhibiting spin degeneracy in the defect states, the large spin-orbit splitting up to 152 meV is observed in the defect states of the V Se . Our analyses of orbital contributions to the defect states show that the large spin splitting is originated from the strong hybridization between Pt- d x 2 + y 2 + d x y and Se- p x + p y orbitals. Our study clarifies that the defects play an important role in the spin-splitting properties of the PtSe 2 ML, which is important for designing future spintronic devices.
Stable unitary integrators for the numerical implementation of continuous unitary transformations Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 Samuel Savitz and Gil Refael
The technique of continuous unitary transformations has recently been used to provide physical insight into a diverse array of quantum mechanical systems. However, the question of how to best numerically implement the flow equations has received little attention. The most immediately apparent approach, using standard Runge-Kutta numerical integration algorithms, suffers from both severe inefficiency due to stiffness and the loss of unitarity. After reviewing the formalism of continuous unitary transformations and Wegner's original choice for the infinitesimal generator of the flow, we present a number of approaches to resolving these issues including a choice of generator which induces what we call the “uniform tangent decay flow” and three numerical integrators specifically designed to perform continuous unitary transformations efficiently while preserving the unitarity of flow. We conclude by applying one of the flow algorithms to a simple calculation that visually demonstrates the many-body localization transition.
Interplay between short-range correlated disorder and Coulomb interaction in nodal-line semimetals Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 Yuxuan Wang and Rahul M. Nandkishore
In nodal-line semimetals, Coulomb interactions and short-range correlated disorder are both marginal perturbations to the clean noninteracting Hamiltonian. We analyze their interplay using a weak-coupling renormalization group approach. In the clean case, the Coulomb interaction has been found to be marginally irrelevant, leading to Fermi liquid behavior. We extend the analysis to incorporate the effects of disorder. The nodal line structure gives rise to kinematical constraints similar to that for a two-dimensional Fermi surface, which plays a crucial role in the one-loop renormalization of the disorder couplings. For a twofold degenerate nodal loop (Weyl loop), we show that disorder flows to strong coupling along a unique fixed trajectory in the space of symmetry inequivalent disorder couplings. Along this fixed trajectory, all symmetry inequivalent disorder strengths become equal. For a fourfold degenerate nodal loop (Dirac loop), disorder also flows to strong coupling, however, the strengths of symmetry inequivalent disorder couplings remain different. We show that feedback from disorder reverses the sign of the beta function for the Coulomb interaction, causing the Coulomb interaction to flow to strong coupling as well. However, the Coulomb interaction flows to strong coupling asymptotically more slowly than disorder. Extrapolating our results to strong coupling, we conjecture that at low energies nodal line semimetals should be described by a noninteracting nonlinear sigma model. We discuss the relation of our results with possible many-body localization at zero temperatures in such materials.
Renormalized Landau quasiparticle dispersion revealed by photoluminescence spectra from a two-dimensional Fermi liquid at the MgZnO/ZnO heterointerface Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 V. V. Solovyev and I. V. Kukushkin
We analyze the low-temperature photoluminescence spectra from two-dimensional electron systems (2DESs) confined at Mg x Zn 1 − x O /ZnO heterojunctions as the electron density is decreased from 2.3 × 10 12 to 3.5 × 10 11 cm − 2 . The value of the quasiparticle optical density-of-states mass is directly extracted from the width of the 2DES photoluminescence band and is shown to renormalize and double from the value close to that of bulk ZnO material, 0.3 m 0 ( m 0 is the bare electron mass), to 0.6 m 0 due to electron-electron interactions as the interaction parameter r s increases from 2.4 to 6.5. The experimentally probed quasiparticle energies far exceed the limits of Landau's Fermi-liquid theory.
Electronic band structure ofReS2by high-resolution angle-resolved photoemission spectroscopy Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 James L. Webb, Lewis S. Hart, Daniel Wolverson, Chaoyu Chen, Jose Avila, and Maria C. Asensio
The rhenium-based transition metal dichalcogenides (TMDs) are atypical of the TMD family due to their highly anisotropic crystalline structure and are recognized as promising materials for two-dimensional heterostructure devices. The nature of the band gap (direct or indirect) for bulk, few-, and single-layer forms of ReS 2 is of particular interest, due to its comparatively weak interplanar interaction. However, the degree of interlayer interaction and the question of whether a transition from indirect to direct gap is observed on reducing thickness (as in other TMDs) are controversial. We present a direct determination of the valence band structure of bulk ReS 2 using high-resolution angle-resolved photoemission spectroscopy. We find a clear in-plane anisotropy due to the presence of chains of Re atoms, with a strongly directional effective mass which is larger in the direction orthogonal to the Re chains ( 2.2 m e ) than along them ( 1.6 m e ). An appreciable interplane interaction results in an experimentally measured difference of ≈ 100 − 200 meV between the valence band maxima at the Z point (0,0, 1 2 ) and the Γ point (0,0,0) of the three-dimensional Brillouin zone. This leads to a direct gap at Z and a close-lying but larger gap at Γ , implying that bulk ReS 2 is marginally indirect. This may account for recent conflicting transport and photoluminescence measurements and the resulting uncertainty about the nature of the band gap in this material.
Generation of Schrödinger cat type states in a planar semiconductor heterostructure Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 J. Pawłowski, M. Górski, G. Skowron, and S. Bednarek
We propose a nanodevice based on a typical planar semiconductor heterostructure with lateral confinement potential created by voltages applied to local gates. We show how to obtain near parabolical confinement along the nanodevice, and how to use coherent states of the harmonic oscillator for spatial separation of electron densities corresponding to opposite spin directions. In such a way, an entangled state of Schrödinger's cat type is created. We have performed simulations of a realistic nanodevice model by numerically solving the time-dependent Schrödinger equation together with simultaneous tracking of the controllable confinement potential via solution of the Poisson's equation at every time step.
Magnetic properties of bilayer graphene quantum dots in the presence of uniaxial strain Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 J. S. Nascimento, D. R. da Costa, M. Zarenia, Andrey Chaves, and J. M. Pereira, Jr.
Using the tight-binding approach coupled with mean-field Hubbard model, we theoretically study the effect of mechanical deformations on the magnetic properties of bilayer graphene (BLG) quantum dots (QDs). Results are obtained for AA- and AB(Bernal)-stacked BLG QDs, considering different geometries (hexagonal, triangular and square shapes) and edge types (armchair and zigzag edges). In the absence of strain, our results show that (i) the magnetization is affected by taking different dot sizes only for hexagonal BLG QDs with zigzag edges, exhibiting different critical Hubbard interactions, and (ii) the magnetization does not depend on the interlayer hopping energies, except for the geometries with zigzag edges and AA stacking. In the presence of in-plane and uniaxial strain, for all geometries we obtain two different magnetization regimes depending on the applied strain amplitude. The appearance of such different regimes is due to the breaking of layer and sublattice symmetries in BLG QDs.
Mechanism of nucleation and incipient growth of Re clusters in irradiated W-Re alloys from kinetic Monte Carlo simulations Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 Chen-Hsi Huang, Leili Gharaee, Yue Zhao, Paul Erhart, and Jaime Marian
High-temperature, high-dose, neutron irradiation of W results in the formation of Re-rich clusters at concentrations one order of magnitude lower than the thermodynamic solubility limit. These clusters may eventually transform into brittle W-Re intermetallic phases, which can lead to high levels of hardening and thermal conductivity losses. Standard theories of radiation-enhanced diffusion and precipitation cannot explain the formation of these precipitates and so understanding the mechanism by which nonequilibrium clusters form under irradiation is crucial to predict material degradation and devise mitigation strategies. Here we carry out a thermodynamic study of W-Re alloys and conduct kinetic Monte Carlo simulations of Re cluster formation in irradiated W-2Re alloys. We use a generalized Hamiltonian for crystals containing point defects parametrized entirely with electronic structure calculations. Our model incorporates recently gained mechanistic information of mixed-interstitial solute transport, which is seen to control cluster nucleation and growth by forming quasispherical nuclei after an average incubation time of 13.5( ± 8.5 ) s at 1800 K. These nuclei are seen to grow by attracting more mixed interstitials bringing solute atoms, which in turn attracts vacancies leading to recombination and solute agglomeration. Owing to the arrival of both Re and W atoms from the mixed dumbbells, the clusters are not fully dense in Re, which amounts to no more than 50% of the atomic concentration of the cluster near the center. Our simulations are in qualitative agreement with recent atom probe examinations of ion-irradiated W-2Re systems at 773 K.
Prethermal time crystals in a one-dimensional periodically driven Floquet system Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 Tian-Sheng Zeng and D. N. Sheng
Motivated by experimental observations of time-symmetry breaking behavior in a periodically driven (Floquet) system, we study a one-dimensional spin model to explore the stability of such Floquet discrete time crystals (DTCs) under the interplay between interaction and the microwave driving. For intermediate interactions and high drivings, from the time evolution of both stroboscopic spin polarization and mutual information between two ends, we show that Floquet DTCs can exist in a prethermal time regime without the tuning of strong disorder. For much weak interactions the system is a symmetry-unbroken phase, while for strong interactions it gives its way to a thermal phase. Through analyzing the entanglement dynamics, we show that large driving fields protect the prethermal DTCs from many-body localization and thermalization. Our results suggest that by increasing the spin interaction, one can drive the experimental system into optimal regime for observing a robust prethermal DTC phase.
Atomic theory of viscoelastic response and memory effects in metallic glasses Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 Bingyu Cui, Jie Yang, Jichao Qiao, Minqiang Jiang, Lanhong Dai, Yun-Jiang Wang, and Alessio Zaccone
An atomic-scale theory of the viscoelastic response of metallic glasses is derived from first principles, using a Zwanzig-Caldeira-Leggett system-bath Hamiltonian as a starting point within the framework of nonaffine linear response to mechanical deformation. This approach provides a generalized Langevin equation (GLE) as the average equation of motion for an atom or ion in the material, from which non-Markovian nonaffine viscoelastic moduli are extracted. These can be evaluated using the vibrational density of states (DOS) as input, where the boson peak plays a prominent role in the mechanics. To compare with experimental data for binary ZrCu alloys, a numerical DOS was obtained from simulations of this system, which also take electronic degrees of freedom into account via the embedded-atom method for the interatomic potential. It is shown that the viscoelastic α -relaxation, including the α -wing asymmetry in the loss modulus, can be very well described by the theory if the memory kernel (the non-Markovian friction) in the GLE is taken to be a stretched-exponential decaying function of time. This finding directly implies strong memory effects in the atomic-scale dynamics and suggests that the α -relaxation time is related to the characteristic time scale over which atoms retain memory of their previous collision history. This memory time grows dramatically below the glass transition.
Broadband sound pressure enhancement in passive metafluids Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 Bogdan-Ioan Popa
Acoustic sensors operating in lossy environments, such as water, require significant sensitivity to overcome the sound attenuation in the environment and thus see farther. We show here that a surprisingly large class of passive fluids has the ability to enhance the sound pressure propagating inside them without employing active actuation. Specifically, the general requirements for this remarkable property are fluid impedance higher than the impedance of the environment and negligible insertion loss as sound propagates from the environment into the high impedance fluid. We demonstrate the pressure enhancing effect by designing a broadband isotropic metafluid that increases the pressure of sound waves impinging from water. We validate the design in numerical simulations showing that significant sound pressure level increases are achievable in realistic metafluid structures in large bandwidths covering several octaves. Our approach opens up unexplored avenues towards improving acoustic transducer sensitivity, which is critical in applications, such as medical ultrasound imaging, sonar, and acoustic communications.
Kinks and antikinks of buckled graphene: A testing ground for theφ4field model Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 R. D. Yamaletdinov, V. A. Slipko, and Y. V. Pershin
Kinks and antikinks of the classical φ 4 field model are topological solutions connecting its two distinct ground states. Here we establish an analogy between the excitations of a long graphene nanoribbon buckled in the transverse direction and φ 4 model results. Using molecular dynamics simulations, we investigated the dynamics of a buckled graphene nanoribbon with a single kink and with a kink-antikink pair. Several features of the φ 4 model have been observed including the kink-antikink capture at low energies, kink-antikink reflection at high energies, and a bounce resonance. Our results pave the way towards the experimental observation of a rich variety of φ 4 model predictions based on graphene.
Dynamic simulation of structural phase transitions in magnetic iron Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 Pui-Wai Ma, S. L. Dudarev, and Jan S. Wróbel
The occurrence of bcc-fcc ( α − γ ) and fcc-bcc ( γ − δ ) phase transitions in magnetic iron stems from the interplay between magnetic excitations and lattice vibrations. However, this fact has never been confirmed by a direct dynamic simulation, treating noncollinear magnetic fluctuations and dynamics of atoms, and their coupling at a finite temperature. Starting from a large set of data generated by ab initio simulations, we derive noncollinear magnetic many-body potentials for bcc and fcc iron, describing fluctuations of atomic coordinates in the vicinity of near perfect lattice positions. We then use spin-lattice dynamic simulations to evaluate the difference between the free energies of bcc and fcc phases, assessing their relative stability within a unified dynamic picture. We find two intersections between the bcc and fcc free energy curves, which correspond to the α − γ bcc-fcc and γ − δ fcc-bcc phase transitions. The maximum bcc-fcc free energy difference over the temperature interval between the two phase transitions is 2 meV per atom, in agreement with other experimental and theoretical estimates.
Ferromagnetic transition in a one-dimensional spin-orbit-coupled metal and its mapping to a critical point in smectic liquid crystals Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 Vladyslav Kozii, Jonathan Ruhman, Liang Fu, and Leo Radzihovsky
We study the quantum phase transition between a paramagnetic and ferromagnetic metal in the presence of Rashba spin-orbit coupling in one dimension. Using bosonization, we analyze the transition by means of renormalization group, controlled by an ɛ expansion around the upper critical dimension of two. We show that the presence of Rashba spin-orbit coupling allows for a new nonlinear term in the bosonized action, which generically leads to a fluctuation driven first-order transition. We further demonstrate that the Euclidean action of this system maps onto a classical smectic-A–C phase transition in a magnetic field in two dimensions. We show that the smectic transition is second order and is controlled by a new critical point.
Superconducting gaps in FeSe studied by soft point-contact Andreev reflection spectroscopy Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 Yu. G. Naidyuk, O. E. Kvitnitskaya, N. V. Gamayunova, D. L. Bashlakov, L. V. Tyutrina, G. Fuchs, R. Hühne, D. A. Chareev, and A. N. Vasiliev
FeSe single crystals have been studied by soft point-contact Andreev reflection spectroscopy. Superconducting gap features in the differential resistance d V / d I ( V ) of point contacts such as a characteristic Andreev reflection double-minimum structure have been measured versus temperature and magnetic field. Analyzing d V / d I within the extended two-gap Blonder-Tinkham-Klapwijk model allows one to extract both the temperature and magnetic field dependence of the superconducting gaps. The temperature dependence of both gaps is close to the standard BCS behavior. Remarkably, the magnitude of the double-minimum structure gradually vanishes in magnetic field, while the minima position only slightly shifts with field, indicating a weak decrease of the superconducting gaps. Analyzing the d V / d I ( V ) spectra for 25 point contacts results in the averaged gap values 〈 Δ L 〉 = 1.8 ± 0.4 meV and 〈 Δ S 〉 = 1.0 ± 0.2 meV and reduced values 2 〈 Δ L 〉 / k B T c = 4.2 ± 0.9 and 2 〈 Δ S 〉 / k B T c = 2.3 ± 0.5 for the large ( L ) and small ( S ) gap, respectively. Additionally, the small gap contribution was found to be within tens of percent, decreasing with both temperature and magnetic field. No signatures in the d V / d I spectra were observed, testifying to a gapless superconductivity or the presence of even smaller gaps.
Atomic defects and dopants in ternary Z-phase transition-metal nitridesCrMNwithM=V, Nb, Ta investigated with density functional theory Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 Daniel F. Urban and Christian Elsässer
A density functional theory study of atomic defects and dopants in ternary Z-phase transition-metal nitrides Cr M N with M = V , Nb, or Ta is presented. Various defect formation energies of native point defects and of substitutional atoms of other metal elements which are abundant in the steel as well are evaluated. The dependence thereof on the thermodynamic environment, i.e., the chemical conditions of a growing Z-phase precipitate, is studied, and different growth scenarios are compared. The results obtained may help to relate results of experimental atomic-scale analysis by atom probe tomography or transmission electron microscopy to the theoretical modeling of the formation process of the Z phase from binary transition-metal nitrides.
Universality of electronic friction: Equivalence of von Oppen's nonequilibrium Green's function approach and the Head-Gordon–Tully model at equilibrium Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 Wenjie Dou and Joseph E. Subotnik
For a molecule moving near a single metal surface at equilibrium, following von Oppen and coworkers [N. Bode, S. V. Kusminskiy, R. Egger, and F. von Oppen, Beilstein J. Nanotechnol. 3, 144 (2012)] and using a nonequilibrium Green's-function (NEGF) approach, we derive a very general form of electronic friction that includes non-Condon effects. We then demonstrate that the resulting NEGF friction tensor agrees exactly with the Head-Gordon–Tully model, provided that finite temperature effects are incorporated correctly. The present results are in agreement with our recent claim that there is only one universal electronic friction tensor arising from the Born-Oppenheimer approximation [W. Dou, G. Miao, and J. E. Subotnik, Phys. Rev. Lett. 119, 046001 (2017)].
Electron-phonon scattering rates in complex polar crystals Phys. Rev. B (IF 3.836) Pub Date : 2017-09-18 M. P. Prange, L. W. Campbell, and S. Kerisit
The thermalization of fast electrons by phonons is studied in CsI, NaI, SrI 2 , and YAlO 3 . This numerical study uses an improvement to a recently developed method based on a density functional perturbation description of the phonon modes that provides a way to go beyond widely used phonon models based on binary crystals. The method is compared to standard ab initio approaches to the electron-phonon interaction. Improvements to this method are described, and scattering rates are presented and discussed. The relative activity of the numerous phonon modes in materials with complicated structures is discussed, and a simple criterion for finding the modes that scatter strongly is presented.
Some contents have been Reproduced by permission of The Royal Society of Chemistry.
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