Characterization of topological phases of dimerized Kitaev chain via edge correlation functions Phys. Rev. B (IF 3.836) Pub Date : 2017-11-20 Yucheng Wang, Jian-Jian Miao, Hui-Ke Jin, and Shu Chen
We study analytically topological properties of a noninteracting modified dimerized Kitaev chain and an exactly solvable interacting dimerized Kitaev chain under open boundary conditions by analyzing two introduced edge correlation functions. The interacting dimerized Kitaev chain at the symmetry point Δ = t and the chemical potential μ = 0 can be exactly solved by applying two Jordan-Wigner transformations and a spin rotation, which permits us to calculate the edge correlation functions analytically. We demonstrate that the two edge correlation functions can be used to characterize the trivial, Su-Schrieffer-Heeger-like topological and topological superconductor phases of both the noninteracting and interacting systems and give their phase diagrams.
Single layers and multilayers of GaN and AlN in square-octagon structure: Stability, electronic properties, and functionalization Phys. Rev. B (IF 3.836) Pub Date : 2017-11-20 E. Gürbüz, S. Cahangirov, E. Durgun, and S. Ciraci
Further to planar single-layer hexagonal structures, GaN and AlN can also form free-standing, single-layer structures constructed from squares and octagons. We performed an extensive analysis of dynamical and thermal stability of these structures in terms of ab initio finite-temperature molecular dynamics and phonon calculations together with the analysis of Raman and infrared active modes. These single-layer square-octagon structures of GaN and AlN display directional mechanical properties and have wide, indirect fundamental band gaps, which are smaller than their hexagonal counterparts. These density functional theory band gaps, however, increase and become wider upon correction. Under uniaxial and biaxial tensile strain, the fundamental band gaps decrease and can be closed. The electronic and magnetic properties of these single-layer structures can be modified by adsorption of various adatoms, or by creating neutral cation-anion vacancies. The single-layer structures attain magnetic moment by selected adatoms and neutral vacancies. In particular, localized gap states are strongly dependent on the type of vacancy. The energetics, binding, and resulting electronic structure of bilayer, trilayer, and three-dimensional (3D) layered structures constructed by stacking the single layers are affected by vertical chemical bonds between adjacent layers. In addition to van der Waals interaction, these weak vertical bonds induce buckling in planar geometry and enhance their binding, leading to the formation of stable 3D layered structures. In this respect, these multilayers are intermediate between van der Waals solids and wurtzite crystals, offering a wide range of tunability.
Open-boundary reflection of quantum well states at Pb(111) Phys. Rev. B (IF 3.836) Pub Date : 2017-11-20 M. Müller, N. Néel, S. Crampin, and J. Kröger
Using a scanning tunneling microscope, confined electron states are studied that exist above subsurface nanometer-sized voids at Pb(111), where potential barriers at the parallel vacuum-Pb(111) and Pb(111)-void interfaces establish a principal series of quantum well states that are further confined laterally by strong reflection at the open boundaries at the edges of the void. The influence of the size, depth, and shape of the voids on the effectiveness of the lateral confinement is discussed. Standing wave patterns observed in differential conductance maps unravel the dispersion of the relevant underlying Pb electron states.
Spin-charge conversion in disordered two-dimensional electron gases lacking inversion symmetry Phys. Rev. B (IF 3.836) Pub Date : 2017-11-20 Chunli Huang, Mirco Milletarì, and Miguel A. Cazalilla
We study the spin-charge conversion mechanisms in a two-dimensional gas of electrons moving in a smooth disorder potential by accounting for both Rashba-type and Mott's skew scattering contributions. We find that the quantum interference effects between spin-flip and skew scattering give rise to anisotropic spin precession scattering (ASP), a direct spin-charge conversion mechanism that was discovered in an earlier study of graphene decorated with adatoms [Huang et al., Phys. Rev. B 94, 085414 (2016)]. Our findings suggest that, together with other spin-charge conversion mechanisms such as the inverse galvanic effect, ASP is a fairly universal phenomenon that should be present in disordered two-dimensional systems lacking inversion symmetry.
Spin-polarized ballistic conduction through correlated Au-NiMnSb-Au heterostructures Phys. Rev. B (IF 3.836) Pub Date : 2017-11-20 C. Morari, W. H. Appelt, A. Östlin, A. Prinz-Zwick, U. Schwingenschlögl, U. Eckern, and L. Chioncel
We examine the ballistic conduction through Au-NiMnSb-Au heterostructures consisting of up to four units of the half-metallic NiMnSb in the scattering region, using density functional theory (DFT) methods. For a single NiMnSb unit the transmission function displays a spin polarization of around 50 % in a window of 1 eV centered around the Fermi level. By increasing the number of layers, an almost complete spin polarization of the transmission is obtained in this energy range. Supplementing the DFT calculations with local electronic interactions, of Hubbard-type on the Mn sites, leads to a hybridization between the interface and many-body states. The significant reduction of the spin polarization seen in the density of states is not apparent in the spin polarization of the conduction electron transmission, which suggests that the hybridized interface and many-body induced states are localized.
Optical conductivity of a two-dimensional metal near a quantum critical point: The status of the extended Drude formula Phys. Rev. B (IF 3.836) Pub Date : 2017-11-20 Andrey V. Chubukov and Dmitrii L. Maslov
Spin-orbit coupling, optical transitions, and spin pumping in monolayer and few-layer InSe Phys. Rev. B (IF 3.836) Pub Date : 2017-11-20 S. J. Magorrian, V. Zólyomi, and V. I. Fal'ko
We show that spin-orbit coupling (SOC) in InSe enables the optical transition across the principal band gap to couple with in-plane polarized light. This transition, enabled by p x , y ↔ p z hybridization due to intra-atomic SOC in both In and Se, can be viewed as a transition between two dominantly s - and p z -orbital based bands, accompanied by an electron spin-flip. Having parametrized k · p theory using first-principles density functional theory we estimate the absorption for σ ± circularly polarized photons in the monolayer as ∼ 1.5 %, which saturates to ∼ 0.3 % in thicker films (3–5 layers). Circularly polarized light can be used to selectively excite electrons into spin-polarized states in the conduction band, which permits optical pumping of the spin polarization of In nuclei through the hyperfine interaction.
Chemically exfoliatedMoS2layers: Spectroscopic evidence for the semiconducting nature of the dominant trigonal metastable phase Phys. Rev. B (IF 3.836) Pub Date : 2017-11-20 Banabir Pal, Anjali Singh, Sharada G., Pratibha Mahale, Abhinav Kumar, S. Thirupathaiah, H. Sezen, M. Amati, Luca Gregoratti, Umesh V. Waghmare, and D. D. Sarma
A metastable trigonal phase, existing only as small patches on a chemically exfoliated few-layered, thermodynamically stable 1 H phase of Mo S 2 , is believed to critically influence the properties of Mo S 2 -based devices. The electronic structure of this metastable phase is little understood in the absence of a direct experimental investigation of its electronic properties, complicated further by conflicting claims from theoretical investigations. We address this issue by investigating the electronic structure of this minority phase in chemically exfoliated Mo S 2 few-layered systems by enhancing its contributions with the use of highly spatially resolved ( ≤ 120 nm resolution) photoemission spectroscopy and Raman spectroscopy in conjunction with state-of-the-art electronic structure calculations. Based on these results, we establish that the ground state of this phase, arrived at by the chemical exfoliation of Mo S 2 using the usual Li intercalation technique, is a small gap ( ∼ 90 ± 40 meV ) semiconductor in contrast to most claims in the literature; we also identify the specific trigonal structure it has among many suggested ones.
Anisotropic pseudopotential characterization of quantum Hall systems under a tilted magnetic field Phys. Rev. B (IF 3.836) Pub Date : 2017-11-20 Bo Yang, Ching Hua Lee, Chi Zhang, and Zi-Xiang Hu
We analytically derived the effective two-body interaction for a finite thickness quantum Hall system with a harmonic perpendicular confinement and an in-plane magnetic field. The anisotropic effective interaction in the lowest Landau level (LLL) and first Landau level (1LL) are expanded in the basis of the generalized pseudopotentials (PPs), and we analyze how the coefficients of some prominent isotropic and anisotropic PPs depend on the thickness of the sample and the strength of the in-plane magnetic field. We also investigate the stability of the topological quantum Hall states, especially the Laughlin state and its emergent guiding center metric, which we can now compute analytically. An interesting reorientation of the anisotropy direction of the Laughlin state in the 1LL is revealed, and we also discuss various possible experimental ramifications for this quantum Hall system with broken rotational symmetry.
Quantum field theory of X-cube fracton topological order and robust degeneracy from geometry Phys. Rev. B (IF 3.836) Pub Date : 2017-11-20 Kevin Slagle and Yong Baek Kim
We propose a quantum field theory description of the X-cube model of fracton topological order. The field theory is not (and cannot be) a topological quantum field theory (TQFT) since, unlike the X-cube model, TQFTs are invariant (i.e., symmetric) under continuous space-time transformations. However, the theory is instead invariant under a certain subgroup of the conformal group. We describe how braiding statistics and ground-state degeneracy are reproduced by the field theory, and how the the X-cube Hamiltonian and field theory can be minimally coupled to matter fields. We also show that even on a manifold with trivial topology, spatial curvature can induce a ground-state degeneracy that is stable to arbitrary local perturbations! Our formalism may allow for the description of other fracton field theories, where the only necessary input is an equation of motion for a charge density.
Principal component analysis for fermionic critical points Phys. Rev. B (IF 3.836) Pub Date : 2017-11-20 Natanael C. Costa, Wenjian Hu, Z. J. Bai, Richard T. Scalettar, and Rajiv R. P. Singh
We use determinant quantum Monte Carlo (DQMC), in combination with the principal component analysis (PCA) approach to unsupervised learning, to extract information about phase transitions in several of the most fundamental Hamiltonians describing strongly correlated materials. We first explore the zero-temperature antiferromagnet to singlet transition in the periodic Anderson model, the Mott insulating transition in the Hubbard model on a honeycomb lattice, and the magnetic transition in the 1/6-filled Lieb lattice. We then discuss the prospects for learning finite temperature superconducting transitions in the attractive Hubbard model, for which there is no sign problem. Finally, we investigate finite temperature charge density wave (CDW) transitions in the Holstein model, where the electrons are coupled to phonon degrees of freedom, and carry out a finite size scaling analysis to determine T c . We examine the different behaviors associated with Hubbard-Stratonovich auxiliary field configurations on both the entire space-time lattice and on a single imaginary time slice, or other quantities, such as equal-time Green's and pair-pair correlation functions.
High-pressure versus isoelectronic doping effect on the honeycomb iridateNa2IrO3 Phys. Rev. B (IF 3.836) Pub Date : 2017-11-20 V. Hermann, J. Ebad-Allah, F. Freund, I. M. Pietsch, A. Jesche, A. A. Tsirlin, J. Deisenhofer, M. Hanfland, P. Gegenwart, and C. A. Kuntscher
We study the effect of isoelectronic doping and external pressure in tuning the ground state of the honeycomb iridate Na 2 IrO 3 by combining optical spectroscopy with synchrotron x-ray diffraction measurements on single crystals. The obtained optical conductivity of Na 2 IrO 3 is discussed in terms of a Mott-insulating picture versus the formation of quasimolecular orbitals and in terms of Kitaev interactions. With increasing Li content x , ( Na 1 − x Li x ) 2 IrO 3 moves deeper into the Mott-insulating regime, and there are indications that up to a doping level of 24% the compound comes closer to the Kitaev limit. The optical conductivity spectrum of single-crystalline α − Li 2 IrO 3 does not follow the trends observed for the series up to x = 0.24 . There are strong indications that α − Li 2 IrO 3 is not as close to the Kitaev limit as Na 2 IrO 3 and lies closer to the quasimolecular orbital picture instead. Except for the pressure-induced hardening of the phonon modes, the optical properties of Na 2 IrO 3 seem to be robust against external pressure. Possible explanations of the unexpected evolution of the optical conductivity with isolectronic doping and the drastic change between x = 0.24 and x = 1 are given by comparing the pressure-induced changes of lattice parameters and the optical conductivity with the corresponding changes induced by doping.
Amplitude mode oscillations in pump-probe photoemission spectra from ad-wave superconductor Phys. Rev. B (IF 3.836) Pub Date : 2017-11-20 B. Nosarzewski, B. Moritz, J. K. Freericks, A. F. Kemper, and T. P. Devereaux
Recent developments in the techniques of ultrafast pump-probe photoemission have made possible the search for collective modes in strongly correlated systems out of equilibrium. Including inelastic scattering processes and a retarded interaction, we simulate time- and angle-resolved photoemission spectroscopy (trARPES) to study the amplitude mode of a d -wave superconductor, a collective mode excited through the nonlinear light-matter coupling to the pump pulse. We find that the amplitude mode oscillations of the d -wave order parameter occur in phase at a single frequency that is twice the quasi-steady-state maximum gap size after pumping. We comment on the necessary conditions for detecting the amplitude mode in trARPES experiments.
Magnetic solitons and magnetic phase diagram of the hexagonal chiral crystalCrNb3S6in oblique magnetic fields Phys. Rev. B (IF 3.836) Pub Date : 2017-11-20 Jun-ichiro Yonemura, Yusuke Shimamoto, Takanori Kida, Daichi Yoshizawa, Yusuke Kousaka, Sadafumi Nishihara, Francisco Jose Trindade Goncalves, Jun Akimitsu, Katsuya Inoue, Masayuki Hagiwara, and Yoshihiko Togawa
We investigate the magnetic torque and magnetoresistance (MR) responses in oblique magnetic fields in micrometer-sized specimens of the hexagonal chiral magnetic crystal CrNb 3 S 6 . The results exhibit hysteresis over a wide range of applied field angles, while reversible behavior appears only when the magnetic field is closely aligned to the helical axis of the crystal. Stepwise changes of the magnetic torque and MR detected in the hysteresis region indicate the existence of chiral solitons in the oblique magnetic fields. A magnetic phase diagram is derived from the experimental results, and the stability of the chiral magnetic phases, such as the chiral soliton lattice and chiral conical phase, and the nature of the phase transition between them are discussed.
Magnetic-field-induced decrease of the spin Peltier effect inPt/Y3Fe5O12system at room temperature Phys. Rev. B (IF 3.836) Pub Date : 2017-11-20 Ryuichi Itoh, Ryo Iguchi, Shunsuke Daimon, Koichi Oyanagi, Ken-ichi Uchida, and Eiji Saitoh
We report the observation of magnetic-field-induced decrease of the spin Peltier effect (SPE) in a junction of a paramagnetic metal Pt and a ferrimagnetic insulator Y 3 Fe 5 O 12 (YIG) at room temperature. For driving the SPE, spin currents are generated via the spin Hall effect from applied charge currents in the Pt layer, and injected into the adjacent thick YIG film. The resultant temperature modulation is detected by a commonly used thermocouple attached to the Pt/YIG junction. The output of the thermocouple shows sign reversal when the magnetization is reversed and linearly increases with the applied current, demonstrating the detection of the SPE signal. We found that the SPE signal decreases with the magnetic field. The observed decreasing rate was found to be comparable to that of the spin Seebeck effect (SSE), suggesting the dominant and similar contribution of the low-energy magnons in the SPE as in the SSE.
Coupling of structure to magnetic and superconducting orders in quasi-one-dimensionalK2Cr3As3 Phys. Rev. B (IF 3.836) Pub Date : 2017-11-20 K. M. Taddei, Q. Zheng, A. S. Sefat, and C. de la Cruz
Quasi-one-dimensional A 2 Cr 3 As 3 (with A = K , Cs, Rb) is an intriguing new family of superconductors which exhibit many similar features to the cuprate and iron-based unconventional superconductor families. Yet, in contrast to these systems, no charge or magnetic ordering has been observed which could provide the electronic correlations presumed necessary for an unconventional superconducting pairing mechanism—an absence which defies predictions of first-principles models. We report the results of neutron scattering experiments on polycrystalline K 2 Cr 3 As 3 ( T c ∼ 7 K ) which probed the low-temperature dynamics near T c . Neutron diffraction data evidence a subtle response of the nuclear lattice to the onset of superconductivity while inelastic scattering reveals a highly dispersive column of intensity at the commensurate wave vector q = 00 1 2 which loses intensity beneath T c —indicative of short-range magnetic fluctuations. Using linear spin-wave theory, we model the observed scattering and suggest a possible structure to the short-range magnetic order. These observations suggest that K 2 Cr 3 As 3 is in close proximity to a magnetic instability and that the incipient magnetic order both couples strongly to the lattice and competes with superconductivity, in direct analogy with the iron-based superconductors.
Doping-induced quantum crossover inEr2Ti2−xSnxO7 Phys. Rev. B (IF 3.836) Pub Date : 2017-11-20 M. Shirai, R. S. Freitas, J. Lago, S. T. Bramwell, C. Ritter, and I. Živković
We present the results of the investigation of magnetic properties of the Er 2 Ti 2 − x Sn x O 7 series. For small doping values, the ordering temperature decreases linearly with x , while the moment configuration remains the same as in the x = 0 parent compound. Around x = 1.7 doping level, we observe a change in the behavior, where the ordering temperature starts to increase and new magnetic Bragg peaks appear. For the first time, we present evidence of a long-range order (LRO) in Er 2 Sn 2 O 7 ( x = 2.0 ) below T N = 130 mK. It is revealed that the moment configuration corresponds to a Palmer-Chalker type with a value of the magnetic moment significantly renormalized compared to x = 0 . We discuss our results in the framework of a possible quantum phase transition occurring close to x = 1.7 .
Determining the vortex tilt relative to a superconductor surface Phys. Rev. B (IF 3.836) Pub Date : 2017-11-20 V. G. Kogan and J. R. Kirtley
It is of interest to determine the exit angle of a vortex from a superconductor surface, since this affects the intervortex interactions and their consequences. Two ways to determine this angle are to image the vortex magnetic fields above the surface, or the vortex core shape at the surface. In this work we evaluate the field h ( x , y , z ) above a flat superconducting surface x , y and the currents J ( x , y ) at that surface for a straight vortex tilted relative to the normal to the surface, for both the isotropic and anisotropic cases. In principle, these results can be used to determine the vortex exit tilt angle from analyses of magnetic field imaging or density of states data.
Unconventional superconductivity and an ambient-pressure magnetic quantum critical point in single-crystalLaNiC2 Phys. Rev. B (IF 3.836) Pub Date : 2017-11-20 J. F. Landaeta, D. Subero, P. Machado, F. Honda, and I. Bonalde
Superconductivity in noncentrosymmetric LaNiC 2 is expected to be induced by electron-phonon interactions due to its lack of magnetic instabilities. The non-Bardeen-Cooper-Schrieffer (BCS) behaviors found in this material call into question the long-standing idea that relates unconventional superconductivity with magnetic interactions. Here we report magnetic penetration-depth measurements in a high-purity single crystal of LaNiC 2 at pressures up to 2.5 GPa and temperatures down to 0.04 K. At ambient pressure and below 0.5 T c the penetration depth goes as T 4 for the in-plane and T 2 for the out-of-plane component, firmly implying the existence of point nodes in the energy gap and the unconventional character of this superconductor. The present study also provides evidence of magnetism in LaNiC 2 by unraveling a pressure-induced antiferromagnetic phase inside the superconducting state at temperatures below 0.5 K, with a quantum critical point around ambient pressure. The results presented here maintain a solid base for the notion that unconventional superconductivity only arises near magnetic order or fluctuations.
Thickness and angular dependence of the magnetocurrent of hot electrons in a magnetic tunnel transistor with crossed anisotropies Phys. Rev. B (IF 3.836) Pub Date : 2017-11-20 C. Vautrin, D. Lacour, G. Sala, Y. Lu, F. Montaigne, and M. Hehn
We have studied the thickness and angular dependence of the magnetocurrent of hot electrons in a magnetic tunnel transistor (MTT) with crossed magnetic anisotropies. In a first step, we show that the magnetocurrent increases with ferromagnetic layer thickness as for MTTs with collinear magnetic configurations. The maximum magnetocurrent value is obtained to be 85%, which is close to the theoretical maximum value of 100% for MTTs with crossed magnetic configurations. In a second step, we demonstrate that we are able to reproduce both current vs field direction and current vs field intensity measurements in a framework taking into account a reduced number of magnetic parameters and a simple cosine dependence of the hot-electron current on the angle between magnetizations.
Magnetization, specific heat, and thermal conductivity of hexagonalErMnO3single crystals Phys. Rev. B (IF 3.836) Pub Date : 2017-11-20 J. D. Song, C. Fan, Z. Y. Zhao, F. B. Zhang, J. Y. Zhao, X. G. Liu, X. Zhao, Y. J. Liu, J. F. Wang, and X. F. Sun
We report a study of magnetism and magnetic transitions of hexagonal ErMnO 3 single crystals by magnetization, specific heat, and heat transport measurements. Magnetization data show that the c -axis magnetic field induces three magnetic transitions at 0.8, 12, and 28 T. The specific heat shows a peak at 2.2 K, which is due to a magnetic transition of Er 3 + moments. For low- T thermal conductivity ( κ ) , a clear diplike feature appears in the κ ( H ) isotherm at 1–1.25 T for H ∥ a b , while in the case of H ∥ c , a steplike increase is observed at 0.5–0.8 T. The transition fields in κ ( H ) are in good agreement with those obtained from magnetization, and the anomaly of κ can be understood by a spin-phonon scattering scenario. The natures of magnetic structures and corresponding field-induced transitions at low temperatures are discussed.
Spin excitations and quantum criticality in the quasi-one-dimensional Ising-like ferromagnetCoCl2·2D2Oin a transverse field Phys. Rev. B (IF 3.836) Pub Date : 2017-11-20 J. Larsen, T. K. Schäffer, U. B. Hansen, S. L. Holm, S. R. Ahl, R. Toft-Petersen, J. Taylor, G. Ehlers, J. Jensen, H. M. Rønnow, K. Lefmann, and N. B. Christensen
We present experimental evidence for a quantum phase transition in the easy-axis S = 3 / 2 anisotropic quasi-one-dimensional ferromagnet CoCl 2 · 2 D 2 O in a transverse field. Elastic neutron scattering shows that the magnetic order parameter vanishes at a transverse critical field μ 0 H c = 16.05 (4) T, while inelastic neutron scattering shows that the gap in the magnetic excitation spectrum vanishes at the same field value, and reopens for H > H c . The field dependence of the order parameter and the gap are well described by critical exponents β = 0.45 ± 0.09 and z ν close to 1 / 2 , implying that the quantum phase transition in CoCl 2 · 2 D 2 O differs significantly from the textbook version of a S = 1 / 2 Ising chain in a transverse field. We attribute the difference to weak but finite three-dimensionality of the magnetic interactions.
Disorder-induced topological phase transitions on Lieb lattices Phys. Rev. B (IF 3.836) Pub Date : 2017-11-17 Rui Chen, Dong-Hui Xu, and Bin Zhou
Motivated by the very recent experimental realization of electronic Lieb lattices and research interest on topological states of matter, we study the topological phase transitions driven by Anderson-type disorder on spin-orbit coupled Lieb lattices in the presence of spin-independent and -dependent staggered potentials. By combining the recursive Green's-function and self-consistent Born approximation methods, we found that both time-reversal-invariant and time-reversal-symmetry-broken spin-orbit coupled Lieb lattice systems can host the disorder-induced gapful topological phases, including the quantum spin Hall insulator (QSHI) and quantum anomalous Hall insulator (QAHI) phases. For the time-reversal-invariant case, the disorder induces a topological phase transition directly from a normal insulator (NI) to the QSHI, while for the time-reversal-symmetry-broken case, the disorder can induce either a QAHI-QSHI phase transition or a NI-QAHI-QSHI phase transition, depending on the initial state of the system. Remarkably, the time-reversal-symmetry-broken QSHI phase can be induced by Anderson-type disorder on the spin-orbit coupled Lieb lattices without time-reversal symmetry.
Half-metal phases in a quantum wire with modulated spin-orbit interaction Phys. Rev. B (IF 3.836) Pub Date : 2017-11-17 D. C. Cabra, G. L. Rossini, A. Ferraz, G. I. Japaridze, and H. Johannesson
We propose a spin filter device based on the interplay of a modulated spin-orbit interaction and a uniform external magnetic field acting on a quantum wire. Half-metal phases, where electrons with only a selected spin polarization exhibit ballistic conductance, can be tuned by varying the magnetic field. These half-metal phases are proven to be robust against electron-electron repulsive interactions. Our results arise from a combination of explicit band diagonalization, bosonization techniques, and extensive density matrix renormalization group computations.
Voltage-dependent cluster expansion for electrified solid-liquid interfaces: Application to the electrochemical deposition of transition metals Phys. Rev. B (IF 3.836) Pub Date : 2017-11-17 Stephen E. Weitzner and Ismaila Dabo
The detailed atomistic modeling of electrochemically deposited metal monolayers is challenging due to the complex structure of the metal-solution interface and the critical effects of surface electrification during electrode polarization. Accurate models of interfacial electrochemical equilibria are further challenged by the need to include entropic effects to obtain accurate surface chemical potentials. We present an embedded quantum-continuum model of the interfacial environment that addresses each of these challenges and study the underpotential deposition of silver on the gold (100) surface. We leverage these results to parametrize a cluster expansion of the electrified interface and show through grand canonical Monte Carlo calculations the crucial need to account for variations in the interfacial dipole when modeling electrodeposited metals under finite-temperature electrochemical conditions.
Critical divergence of the symmetric (A1g) nonlinear elastoresistance near the nematic transition in an iron-based superconductor Phys. Rev. B (IF 3.836) Pub Date : 2017-11-17 J. C. Palmstrom, A. T. Hristov, S. A. Kivelson, J.-H. Chu, and I. R. Fisher
We report the observation of a nonlinear elastoresistivity response for the prototypical underdoped iron pnictide Ba ( Fe 0.975 Co 0.025 ) 2 As 2 . Our measurements reveal a large quadratic term in the isotropic ( A 1 g ) electronic response that was produced by a purely shear ( B 2 g ) strain. The divergence of this quantity upon cooling towards the structural phase transition reflects the temperature dependence of the nematic susceptibility. This observation shows that nematic fluctuations play a significant role in determining even the isotropic properties of this family of compounds.
Calibration of the fine-structure constant of graphene by time-dependent density-functional theory Phys. Rev. B (IF 3.836) Pub Date : 2017-11-17 A. Sindona, M. Pisarra, C. Vacacela Gomez, P. Riccardi, G. Falcone, and S. Bellucci
One of the amazing properties of graphene is the ultrarelativistic behavior of its loosely bound electrons, mimicking massless fermions that move with a constant velocity, inversely proportional to a fine-structure constant α g of the order of unity. The effective interaction between these quasiparticles is, however, better controlled by the coupling parameter α g * = α g / ε , which accounts for the dynamic screening due to the complex permittivity ε of the many-valence electron system. This concept was introduced in a couple of previous studies [Reed et al., Science 330, 805 (2010) and Gan et al., Phys. Rev. B 93, 195150 (2016)], where inelastic x-ray scattering measurements on crystal graphite were converted into an experimentally derived form of α g * for graphene, over an energy-momentum region on the eV Å − 1 scale. Here, an accurate theoretical framework is provided for α g * , using time-dependent density-functional theory in the random-phase approximation, with a cutoff in the interaction between excited electrons in graphene, which translates to an effective interlayer interaction in graphite. The predictions of the approach are in excellent agreement with the above-mentioned measurements, suggesting a calibration method to substantially improve the experimental derivation of α g * , which tends to a static limiting value of ∼ 0.14 . Thus, the ab initio calibration procedure outlined demonstrates the accuracy of perturbation expansion treatments for the two-dimensional gas of massless Dirac fermions in graphene, in parallel with quantum electrodynamics.
Thermoelectric power factor of nanocomposite materials from two-dimensional quantum transport simulations Phys. Rev. B (IF 3.836) Pub Date : 2017-11-17 Samuel Foster, Mischa Thesberg, and Neophytos Neophytou
Nanocomposites are promising candidates for the next generation of thermoelectric materials since they exhibit extremely low thermal conductivities as a result of phonon scattering on the boundaries of the various material phases. The nanoinclusions, however, should not degrade the thermoelectric power factor, and ideally should increase it, so that benefits to the ZT figure of merit can be achieved. In this work we employ the nonequilibrium Green's function quantum transport method to calculate the electronic and thermoelectric coefficients of materials embedded with nanoinclusions. For computational effectiveness we consider two-dimensional nanoribbon geometries, however, the method includes the details of geometry, electron-phonon interactions, quantization, tunneling, and the ballistic to diffusive nature of transport, all combined in a unified approach. This makes it a convenient and accurate way to understand electronic and thermoelectric transport in nanomaterials, beyond semiclassical approximations, and beyond approximations that deal with the complexities of the geometry. We show that the presence of nanoinclusions within a matrix material offers opportunities for only weak energy filtering, significantly lower in comparison to superlattices, and thus only moderate power factor improvements. However, we describe how such nanocomposites can be optimized to limit degradation in the thermoelectric power factor and elaborate on the conditions that achieve the aforementioned mild improvements. Importantly, we show that under certain conditions, the power factor is independent of the density of nanoinclusions, meaning that materials with large nanoinclusion densities which provide very low thermal conductivities can also retain large power factors and result in large ZT figures of merit.
Fast pulse sequences for dynamically corrected gates in singlet-triplet qubits Phys. Rev. B (IF 3.836) Pub Date : 2017-11-17 Robert E. Throckmorton, Chengxian Zhang, Xu-Chen Yang, Xin Wang, Edwin Barnes, and S. Das Sarma
We present a set of experimentally feasible pulse sequences that implement any single-qubit gate on a singlet-triplet spin qubit and demonstrate that these new sequences are up to three times faster than existing sequences in the literature. We show that these sequences can be extended to incorporate built-in dynamical error correction, yielding gates that are robust to both charge and magnetic field noise and up to twice as fast as previous dynamically corrected gate schemes. We present a thorough comparison of the performance of our new sequences with that of several existing ones using randomized benchmarking, considering both quasistatic and 1 / f α noise models. We provide our results both as a function of evolution time and as a function of the number of gates, which respectively yield both an effective coherence time and an estimate of the number of gates that can be performed within this coherence time. We determine which set of pulse sequences gives the best results for a wide range of noise strengths and power spectra. Overall, we find that the traditional, slower sequences perform best when there is no field noise or when the noise contains significant high-frequency components; otherwise, our new, fast sequences exhibit the best performance.
Proximity effect model of ultranarrow NbN strips Phys. Rev. B (IF 3.836) Pub Date : 2017-11-17 I. Charaev, T. Silbernagel, B. Bachowsky, A. Kuzmin, S. Doerner, K. Ilin, A. Semenov, D. Roditchev, D. Yu. Vodolazov, and M. Siegel
We show that narrow superconducting strips in superconducting (S) and normal (N) states are universally described by the model presenting them as lateral NSN proximity systems in which the superconducting central band is sandwiched between damaged edge bands with suppressed superconductivity. The width of the superconducting band was experimentally determined from the value of magnetic field at which the band transits from the Meissner state to the static vortex state. Systematic experimental study of 4.9-nm-thick NbN strips with widths in the interval from 50 nm to 20 μm, which are all smaller than the Pearl's length, demonstrates gradual evolution of the temperature dependence of the critical current with the change of the strip width.
Resonant inelastic x-ray scattering probes the electron-phonon coupling in the spin liquidκ-(BEDT-TTF)2Cu2(CN)3 Phys. Rev. B (IF 3.836) Pub Date : 2017-11-17 V. Ilakovac, S. Carniato, P. Foury-Leylekian, S. Tomić, J.-P. Pouget, P. Lazić, Y. Joly, K. Miyagawa, K. Kanoda, and A. Nicolaou
Resonant inelastic x-ray scattering at the N K edge reveals clearly resolved harmonics of the anion plane vibrations in the κ -(BEDT- TTF ) 2 Cu 2 ( CN ) 3 spin-liquid insulator. Tuning the incoming light energy at the K edge of two distinct N sites permits us to excite different sets of phonon modes. The cyanide (CN) stretching mode is selected at the edge of the ordered N sites which are more strongly connected to the bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) molecules, while positionally disordered N sites show multimode excitation. Combining measurements with calculations on an anion plane cluster permits us to estimate the site-dependent electron-phonon coupling of the modes related to nitrogen excitation.
Towards accurate models for amorphous GeTe: Crucial effect of dispersive van der Waals corrections on the structural properties involved in the phase-change mechanism Phys. Rev. B (IF 3.836) Pub Date : 2017-11-17 M. Micoulaut, A. Piarristeguy, H. Flores-Ruiz, and A. Pradel
The effect of van der Waals dispersion correction in combination with density functional theory is investigated on a canonical amorphous phase-change material. Density functional theory (DFT), using the generalized gradient approximation, usually fails to reproduce the structure of amorphous tellurides, which manifests by an overestimation of the interatomic bond distances, and particularly the Ge-Te one involved in local geometries (tetrahedral or defect octahedral). Here, we take into account dispersion forces in a semiempirical way and apply such DFT simulations to amorphous GeTe. We obtain a substantial improvement of the simulated structure factor and pair-correlation function, which now reproduce the experimental counterparts with an unprecedented accuracy, including on a recent partial contribution from anomalous x-ray scattering and from x-ray absorption. A detailed analysis of the corresponding structures indicates that the dispersion correction reduces the Ge-Te bond length, increases the fraction of tetrahedral germanium, and reduces the presence of heteropolar so-called fourfold ABAB rings. Given that these structural features have been stressed to be central for the understanding of the phase-change mechanism, the present results challenge our current understanding of the crystal to amorphous transformation at play.
Superconductivity in engineered two-dimensional electron gases Phys. Rev. B (IF 3.836) Pub Date : 2017-11-17 Andrey V. Chubukov and Steven A. Kivelson
We consider Kohn-Luttinger mechanism for superconductivity in a two-dimensional electron gas confined to a narrow well between two grounded metallic planes with two occupied subbands with Fermi momenta k F L > k F S . On the basis of a perturbative analysis, we conclude that non- s -wave superconductivity emerges even when the bands are parabolic. We analyze the conditions that maximize T c as a function of the distance to the metallic planes, the ratio k F L / k F S , and r s , which measures the strength of Coulomb correlations. The largest attraction is in p -wave and d -wave channels, of which p wave is typically the strongest. For r s = O ( 1 ) we estimate that the dimensionless coupling λ ≈ 10 − 1 , but it likely continues increasing for larger r s (where we lose theoretical control).
One dimensionalization in the spin-1 Heisenberg model on the anisotropic triangular lattice Phys. Rev. B (IF 3.836) Pub Date : 2017-11-17 M. G. Gonzalez, E. A. Ghioldi, C. J. Gazza, L. O. Manuel, and A. E. Trumper
We investigate the effect of dimensional crossover in the ground state of the antiferromagnetic spin-1 Heisenberg model on the anisotropic triangular lattice that interpolates between the regime of weakly coupled Haldane chains ( J ′ ≪ J ) and the isotropic triangular lattice ( J ′ = J ) . We use the density-matrix renormalization group (DMRG) and Schwinger boson theory performed at the Gaussian correction level above the saddle-point solution. Our DMRG results show an abrupt transition between decoupled spin chains and the spirally ordered regime at ( J ′ / J ) c ∼ 0.42 , signaled by the sudden closing of the spin gap. Coming from the magnetically ordered side, the computation of the spin stiffness within Schwinger boson theory predicts the instability of the spiral magnetic order toward a magnetically disordered phase with one-dimensional features at ( J ′ / J ) c ∼ 0.43 . The agreement of these complementary methods, along with the strong difference found between the intra- and the interchain DMRG short spin-spin correlations for sufficiently large values of the interchain coupling, suggests that the interplay between the quantum fluctuations and the dimensional crossover effects gives rise to the one-dimensionalization phenomenon in this frustrated spin-1 Hamiltonian.
Green's function formalism for spin transport in metal-insulator-metal heterostructures Phys. Rev. B (IF 3.836) Pub Date : 2017-11-17 Jiansen Zheng, Scott Bender, Jogundas Armaitis, Roberto E. Troncoso, and Rembert A. Duine
We develop a Green's function formalism for spin transport through heterostructures that contain metallic leads and insulating ferromagnets. While this formalism in principle allows for the inclusion of various magnonic interactions, we focus on Gilbert damping. As an application, we consider ballistic spin transport by exchange magnons in a metal-insulator-metal heterostructure with and without disorder. For the former case, we show that the interplay between disorder and Gilbert damping leads to spin current fluctuations. For the case without disorder, we obtain the dependence of the transmitted spin current on the thickness of the ferromagnet. Moreover, we show that the results of the Green's function formalism agree in the clean and continuum limit with those obtained from the linearized stochastic Landau-Lifshitz-Gilbert equation. The developed Green's function formalism is a natural starting point for numerical studies of magnon transport in heterostructures that contain normal metals and magnetic insulators.
Evolution of magnetic phases inSmCrO3: A neutron diffraction and magnetometric study Phys. Rev. B (IF 3.836) Pub Date : 2017-11-17 Malvika Tripathi, R. J. Choudhary, D. M. Phase, T. Chatterji, and H. E. Fischer
The classical belief about the mechanism of spin reorientation phase transition (SRPT) and ground-state magnetic structure in SmCrO 3 has become intriguing because of inconsistent bulk magnetization observations. The presence of highly neutron-absorbing Sm atom has so far evaded the determination of microscopic magnetic structure. In the present report, we have utilized very high-energy “hot neutrons” to overcome the Sm absorption and to determine the thermal evolution of magnetic configurations. Unambiguously, three distinct phases are observed: the uncompensated canted antiferromagnetic structure Γ 4 ( G x , A y , F z ; F z R ) occurring below the Néel temperature ( T N = 191 K), the collinear antiferromagnetic structure Γ 1 ( A x , G y , C z ; C z R ) occurring below 10 K, and a nonequilibrium configuration with cooccurring Γ 1 and Γ 4 phases in the neighborhood of the SRPT (10 K ≤ T ≤ 40 K). In differing to the earlier predictions, we divulge the SRPT to be a discontinuous transition where chromium spins switch from the a − b crystallographic plane to the b − c crystallographic plane in a discrete manner with no allowed intermediate configuration. The canting angle of chromium ions in the a − b plane is unusually not a thermal constant, rather it is empirically discerned to follow exponential behavior. The competition between magnetocrystalline anisotropy and free energy derived by isotropic and antisymmetric exchange interactions between different pairs of magnetic ions is observed to govern the mechanism of SRPT.
First-principles-based Landau-Devonshire potential forBiFeO3 Phys. Rev. B (IF 3.836) Pub Date : 2017-11-17 P. Marton, A. Klíč, M. Paściak, and J. Hlinka
The work describes a first-principles-based computational strategy for studying structural phase transitions, and in particular, for determination of the so-called Landau-Devonshire potential—the classical zero-temperature limit of the Gibbs energy, expanded in terms of order parameters. It exploits the configuration space attached to the eigenvectors of the modes frozen in the ground state, rather than the space spanned by the unstable modes of the high-symmetry phase, as done usually. This allows us to carefully probe the part of the energy surface in the vicinity of the ground state, which is most relevant for the properties of the ordered phase. We apply this procedure to BiFeO 3 and perform ab initio calculations in order to determine potential energy contributions associated with strain, polarization, and oxygen octahedra tilt degrees of freedom, compatible with its two-formula unit cell periodic boundary conditions.
Photomagnetoelectric and magnetophotovoltaic effects in multiferroicBiFeO3 Phys. Rev. B (IF 3.836) Pub Date : 2017-11-16 Bruno Mettout and Pierre Tolédano
A polarized light beam modifies the equilibrium tensors associated with any measurable property of a crystal. The light-induced linear magnetoelectric susceptibility and electric current density of bismuth ferrite (BFO) are investigated theoretically using their expansion in Wigner spherical functions. Under illumination, the interplay between magnetoelectric and photovoltaic properties yields the emergence in the paramagnetic and multiferroic phases of BFO of linear photomagnetoelectric tensor components changing sign in domains with opposite electric polarization or magnetization, and to magnetophotocurrents changing sense in opposite ferroelectric and magnetic domains.
Hall effect in cuprates with an incommensurate collinear spin-density wave Phys. Rev. B (IF 3.836) Pub Date : 2017-11-16 M. Charlebois, S. Verret, A. Foley, O. Simard, D. Sénéchal, and A.-M. S. Tremblay
The presence of incommensurate spiral spin-density waves (SDW) has been proposed to explain the p (hole doping) to 1 + p jump measured in the Hall number n H at a doping p * . Here we explore incommensurate collinear SDW as another possible explanation of this phenomenon, distinct from the incommensurate spiral SDW proposal. We examine the effect of different SDW strengths and wave vectors, and we find that the n H ∼ p behavior is hardly reproduced at low doping. Furthermore, the calculated n H and Fermi surfaces give characteristic features that should be observed; thus, the lack of these features in experiment suggests that the incommensurate collinear SDW is unlikely to be a good candidate to explain the n H ∼ p observed in the pseudogap regime.
Quantized charge transport in chiral Majorana edge modes Phys. Rev. B (IF 3.836) Pub Date : 2017-11-16 Stephan Rachel, Eric Mascot, Sagen Cocklin, Matthias Vojta, and Dirk K. Morr
Non-necessity of band inversion process in two-dimensional topological insulators for bulk gapless states and topological phase transitions Phys. Rev. B (IF 3.836) Pub Date : 2017-11-16 Wenjie Xi (奚文杰) and Wei Ku (顧威)
In commonly employed models for two-dimensional (2D) topological insulators, bulk gapless states are well known to form at the band inversion points where the degeneracy of the states is protected by symmetries. It is thus sometimes quite tempting to consider this feature, the occurrence of gapless states, a result of the band inversion process under protection of the symmetries. Similarly, the band inversion process might even be perceived as necessary to induce 2D topological phase transitions. To clarify these misleading perspectives, we propose a simple model with a flexible Chern number to demonstrate that the bulk gapless states emerge at the phase boundary of topological phase transitions, despite the absence of a band inversion process. Furthermore, the bulk gapless states do not need to occur at the special k points protected by symmetries. Given the significance of these fundamental conceptual issues and their widespread influence, our clarification should generate strong general interests and significant impacts. Furthermore, the simplicity and flexibility of our general model with an arbitrary Chern number should prove useful in a wide range of future studies of topological states of matter.
Erratum: Reliability of Raman measurements of thermal conductivity of single-layer graphene due to selective electron-phonon coupling: A first-principles study [Phys. Rev. B93, 125432 (2016)] Phys. Rev. B (IF 3.836) Pub Date : 2017-11-16 Ajit K. Vallabhaneni, Dhruv Singh, Hua Bao, Jayathi Murthy, and Xiulin Ruan
Geometrical phase shift in Friedel oscillations Phys. Rev. B (IF 3.836) Pub Date : 2017-11-16 C. Dutreix and P. Delplace
This work addresses the problem of elastic scattering on a localized impurity in a one-dimensional crystal with sublattice degrees of freedom. The impurity yields long-range interferences in the local density of states known as Friedel oscillations. Here we show that the internal degrees of freedom of Bloch waves are responsible for a geometrical phase shift in Friedel oscillations. The Fourier transform of the energy-resolved interference pattern reveals a topological property of this phase shift, which is intrinsically related to a wave-function topological property (Zak phase) in the absence of impurity. As a result, Friedel oscillations in the local density of states can be regarded as a probe of wave topological properties in a broad class of classical and quantum systems, such as acoustic and photonic crystals, ultracold atomic gases in optical lattices, and electronic compounds.
Current-induced spin polarization in InGaAs and GaAs epilayers with varying doping densities Phys. Rev. B (IF 3.836) Pub Date : 2017-11-16 M. Luengo-Kovac, S. Huang, D. Del Gaudio, J. Occena, R. S. Goldman, R. Raimondi, and V. Sih
The current-induced spin polarization and momentum-dependent spin-orbit field were measured in In x Ga 1 − x As epilayers with varying indium concentrations and silicon doping densities. Samples with higher indium concentrations and carrier concentrations and lower mobilities were found to have larger electrical spin generation efficiencies. Furthermore, current-induced spin polarization was detected in GaAs epilayers despite the absence of measurable spin-orbit fields, indicating that the extrinsic contributions to the spin-polarization mechanism must be considered. Theoretical calculations based on a model that includes extrinsic contributions to the spin dephasing and the spin Hall effect, in addition to the intrinsic Rashba and Dresselhaus spin-orbit coupling, are found to reproduce the experimental finding that the crystal direction with the smaller net spin-orbit field has larger electrical spin generation efficiency and are used to predict how sample parameters affect the magnitude of the current-induced spin polarization.
Role of the transition state in muon implantation Phys. Rev. B (IF 3.836) Pub Date : 2017-11-16 R. C. Vilão, R. B. L. Vieira, H. V. Alberto, J. M. Gil, and A. Weidinger
In muon-spin-rotation experiments, positive muons are implanted in the material and come to rest in the unrelaxed host lattice. The formation of the final configuration requires a lattice relaxation which does not occur instantly. The present paper is concerned with the transition from the initial stopping state to the final muon configuration. We identify the often observed fast relaxing signal in muon experiments (e.g., in several oxides studied recently) with the transition state in this conversion process. This state is paramagnetic with a small hyperfine interaction (in the order of MHz) which fluctuates and averages to almost zero. Because of its apparent diamagnetic frequency behavior, the fast signal was in the past assigned to Mu + or Mu − . We present evidence that this state is actually paramagnetic. The model presented in this paper is of importance for the interpretation of past and future μ SR measurements.
Charge puddles in the bulk and on the surface of the topological insulatorBiSbTeSe2studied by scanning tunneling microscopy and optical spectroscopy Phys. Rev. B (IF 3.836) Pub Date : 2017-11-16 T. Knispel, W. Jolie, N. Borgwardt, J. Lux, Zhiwei Wang, Yoichi Ando, A. Rosch, T. Michely, and M. Grüninger
Bulk boundary correspondence and the existence of Majorana bound states on the edges of 2D topological superconductors Phys. Rev. B (IF 3.836) Pub Date : 2017-11-16 Nicholas Sedlmayr, Vardan Kaladzhyan, Clément Dutreix, and Cristina Bena
The bulk-boundary correspondence establishes a connection between the bulk topological index of an insulator or superconductor, and the number of topologically protected edge bands or states. For topological superconductors in two dimensions, the first Chern number is related to the number of protected bands within the bulk energy gap, and is therefore assumed to give the number of Majorana band states in the system. Here we show that this is not necessarily the case. As an example, we consider a hexagonal-lattice topological superconductor based on a model of graphene with Rashba spin-orbit coupling, proximity-induced s -wave superconductivity, and a Zeeman magnetic field. We explore the full Chern number phase diagram of this model, extending what is already known about its parity. We then demonstrate that, despite the high Chern numbers that can be seen in some phases, these do not strictly always contain Majorana bound states.
Surface impedance and optimum surface resistance of a superconductor with an imperfect surface Phys. Rev. B (IF 3.836) Pub Date : 2017-11-16 Alex Gurevich and Takayuki Kubo
We calculate a low-frequency surface impedance of a dirty, s -wave superconductor with an imperfect surface incorporating either a thin layer with a reduced pairing constant or a thin, proximity-coupled normal layer. Such structures model realistic surfaces of superconducting materials which can contain oxide layers, absorbed impurities, or nonstoichiometric composition. We solved the Usadel equations self-consistently and obtained spatial distributions of the order parameter and the quasiparticle density of states which then were used to calculate a low-frequency surface resistance R s ( T ) and the magnetic penetration depth λ ( T ) as functions of temperature in the limit of local London electrodynamics. It is shown that the imperfect surface in a single-band s -wave superconductor results in a nonexponential temperature dependence of Z ( T ) at T ≪ T c which can mimic the behavior of multiband or d -wave superconductors. The imperfect surface and the broadening of the gap peaks in the quasiparticle density of states N ( ε ) in the bulk give rise to a weakly temperature-dependent residual surface resistance. We show that the surface resistance can be optimized and even reduced below its value for an ideal surface by engineering N ( ε ) at the surface using pair-breaking mechanisms, particularly by incorporating a small density of magnetic impurities or by tuning the thickness and conductivity of the normal layer and its contact resistance. The results of this work address the limit of R s in superconductors at T ≪ T c , and the ways of engineering the optimal density of states by surface nanostructuring and impurities to reduce losses in superconducting microresonators, thin-film strip lines, and radio-frequency cavities for particle accelerators.
Boundary-driven Heisenberg chain in the long-range interacting regime: Robustness against far-from-equilibrium effects Phys. Rev. B (IF 3.836) Pub Date : 2017-11-16 Leon Droenner and Alexander Carmele
We investigate the Heisenberg X X Z chain with long-range interactions in the Z dimension. By applying two magnetic boundary reservoirs, we drive the system out of equilibrium and induce a nonzero steady-state current. The long-range coupled chain shows nearly ballistic transport and linear response for all potential differences of the external reservoirs. In contrast, the common isotropic nearest-neighbor coupling shows negative differential conductivity and a transition from diffusive to subdiffusive transport for a far-from-equilibrium driving. Adding disorder, the change in the transport for nearest-neighbor coupling is therefore highly dependent on the driving. We find for the disordered long-range coupled X X Z chain, any change in the transport behavior is independent of the potential difference and the coupling strengths of the external reservoirs.
High-resolution resonant inelastic extreme ultraviolet scattering from orbital and spin excitations in a Heisenberg antiferromagnet Phys. Rev. B (IF 3.836) Pub Date : 2017-11-16 Antonio Caretta, Martina Dell'Angela, Yi-De Chuang, Alexandra M. Kalashnikova, Roman V. Pisarev, Davide Bossini, Florian Hieke, Wilfried Wurth, Barbara Casarin, Roberta Ciprian, Fulvio Parmigiani, Surge Wexler, L. Andrew Wray, and Marco Malvestuto
We report a high-resolution resonant inelastic extreme ultraviolet (EUV) scattering study of the quantum Heisenberg antiferromagnet KCoF 3 . By tuning the EUV photon energy to the cobalt M 23 edge, a complete set of low-energy 3 d spin-orbital excitations is revealed. These low-lying electronic excitations are modeled using an extended multiplet-based mean-field calculation to identify the roles of lattice and magnetic degrees of freedom in modifying the resonant inelastic x-ray scattering (RIXS) spectral line shape. We have demonstrated that the temperature dependence of RIXS features upon the antiferromagnetic ordering transition enables us to probe the energetics of short-range spin correlations in this material.
Raman scattering study of the tetragonal magnetic phase inSr1−xNaxFe2As2: Structural symmetry and electronic gap Phys. Rev. B (IF 3.836) Pub Date : 2017-11-16 Li Yue, Xiao Ren, Tingting Han, Jianqing Guo, Zhicheng Wu, Yan Zhang, and Yuan Li
We use inelastic light scattering to study Sr 1 − x Na x Fe 2 As 2 ( x ≈ 0.34 ), which exhibits a robust tetragonal magnetic phase that restores the fourfold rotation symmetry inside the orthorhombic magnetic phase. With cooling, we observe splitting and recombination of an E g phonon peak upon entering the orthorhombic and tetragonal magnetic phases, respectively, consistent with the reentrant phase behavior. Our electronic Raman data reveal a pronounced feature that is clearly associated with the tetragonal magnetic phase, suggesting the opening of a different electronic gap. The energy of this gap is substantially smaller than the gap observed in the orthorhombic magnetic phase, and the two gaps compete only weakly for spectral weight. These observations are consistent with the notion that the reentrant phase behavior is driven by spin-orbit interactions, which seem to dictate the manifestation of magnetism in Fe-based superconductors. No phonon back folding can be detected above the noise level, which implies that any lattice translation symmetry breaking in the tetragonal magnetic phase must be very weak.
Nearly isotropic superconductivity in the layered Weyl semimetalWTe2at 98.5 kbar Phys. Rev. B (IF 3.836) Pub Date : 2017-11-16 Yuk Tai Chan, P. L. Alireza, K. Y. Yip, Q. Niu, K. T. Lai, and Swee K. Goh
The layered transition metal dichalcogenide WTe 2 has recently attracted significant attention due to the discovery of an extremely large magnetoresistance, a predicted type-II Weyl semimetallic state, and a pressure-induced superconducting state. By a careful measurement of the superconducting upper critical fields as a function of the magnetic field angle at a pressure as high as 98.5 kbar, we provide the first detailed examination of the dimensionality of the superconducting condensate in WTe 2 . Despite the layered crystal structure, the upper critical field exhibits a negligible field anisotropy. The angular dependence of the upper critical field can be satisfactorily described by the anisotropic mass model from 2.2 K ( T / T c ∼ 0.67 ) to 0.03 K ( T / T c ∼ 0.01 ), with a practically identical anisotropy factor γ ∼ 1.7 . The temperature dependence of the upper critical field, determined for both H ⊥ a b and H ∥ a b , can be understood by a conventional orbital depairing mechanism. A comparison of the upper critical fields along the two orthogonal field directions results in the same value of γ ∼ 1.7 , leading to a temperature-independent anisotropy factor from near T c to < 0.01 T c . Our findings thus identify WTe 2 as a nearly isotropic superconductor, with an anisotropy factor among one of the lowest known in superconducting transition metal dichalcogenides.
Spin excitation anisotropy in the optimally isovalent-doped superconductorBaFe2(As0.7P0.3)2 Phys. Rev. B (IF 3.836) Pub Date : 2017-11-16 Ding Hu, Wenliang Zhang, Yuan Wei, Bertrand Roessli, Markos Skoulatos, Louis Pierre Regnault, Genfu Chen, Yu Song, Huiqian Luo, Shiliang Li, and Pengcheng Dai
We use neutron polarization analysis to study spin excitation anisotropy in the optimally isovalent-doped superconductor BaFe 2 ( As 0.7 P 0.3 ) 2 ( T c = 30 K). Different from optimally hole- and electron-doped BaFe 2 As 2 , where there is a clear spin excitation anisotropy in the paramagnetic tetragonal state well above T c , we find no spin excitation anisotropy for energies above 2 meV in the normal state of BaFe 2 ( As 0.7 P 0.3 ) 2 . Upon entering the superconducting state, significant spin excitation anisotropy develops at the antiferromagnetic (AF) zone center Q AF = ( 1 , 0 , L = odd ) , while the magnetic spectrum is isotropic at the zone boundary Q = ( 1 , 0 , L = even ) . By comparing the temperature, wave vector, and polarization dependence of the spin excitation anisotropy in BaFe 2 ( As 0.7 P 0.3 ) 2 and hole-doped Ba 0.67 K 0.33 Fe 2 As 2 ( T c = 38 K), we conclude that such anisotropy arises from spin-orbit coupling and is associated with the nearby AF order and superconductivity.
Single-parameter scaling and maximum entropy inside disordered one-dimensional systems: Theory and experiment Phys. Rev. B (IF 3.836) Pub Date : 2017-11-16 Xiaojun Cheng, Xujun Ma, Miztli Yépez, Azriel Z. Genack, and Pier A. Mello
The single-parameter scaling hypothesis relating the average and variance of the logarithm of the conductance is a pillar of the theory of electronic transport. We use a maximum-entropy ansatz to explore the logarithm of the particle, or energy density ln W ( x ) at a depth x into a random one-dimensional system. Single-parameter scaling would be the special case in which x = L (the system length). We find the result, confirmed in microwave measurements and computer simulations, that the average of ln W ( x ) is independent of L and equal to − x / ℓ , with ℓ the mean free path. At the beginning of the sample, var [ ln W ( x ) ] rises linearly with x and is also independent of L , with a sublinear increase and then a drop near the sample output. At x = L we find a correction to the value of var [ ln T ] predicted by single-parameter scaling.
Publisher's Note: Domain-width model for perpendicularly magnetized systems with Dzyaloshinskii-Moriya interaction [Phys. Rev. B96, 144408 (2017)] Phys. Rev. B (IF 3.836) Pub Date : 2017-11-16 T. N. G. Meier, M. Kronseder, and C. H. Back
Band structure and Fermi surfaces of the reentrant ferromagnetic superconductorEu(Fe0.86Ir0.14)2As2 Phys. Rev. B (IF 3.836) Pub Date : 2017-11-16 S. Xing, J. Mansart, V. Brouet, M. Sicot, Y. Fagot-Revurat, B. Kierren, P. Le Fèvre, F. Bertran, J. E. Rault, U. B. Paramanik, Z. Hossain, A. Chainani, and D. Malterre
The electronic structure of the reentrant superconductor Eu ( Fe 0.86 Ir 0.14 ) 2 As 2 ( T c = 22 K) with coexisting ferromagnetic order ( T M = 18 K) is investigated using angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy. We study the in-plane and out-of-plane band dispersions and Fermi surface (FS) of Eu ( Fe 0.86 Ir 0.14 ) 2 As 2 . The near- E F Fe- 3 d -derived band dispersions near the Γ and X high-symmetry points show changes due to Ir substitution, but the FS topology is preserved. From momentum-dependent measurements of the superconducting gap measured at T = 5 K, we estimate an essentially isotropic s -wave gap ( Δ ∼ 5.25 ± 0.25 meV), indicative of strong-coupling superconductivity with 2 Δ / k B T c ≃ 5.8 . The gap gets closed at temperatures T ≥ 10 K, and this is attributed to the resistive phase which sets in at T M = 18 K due to the Eu 2 + -derived magnetic order. The modification of the FS with Ir substitution clearly indicates an effective hole doping with respect to the parent compound.
Emergent exotic superconductivity in artificially engineered tricolor Kondo superlattices Phys. Rev. B (IF 3.836) Pub Date : 2017-11-16 M. Naritsuka, T. Ishii, S. Miyake, Y. Tokiwa, R. Toda, M. Shimozawa, T. Terashima, T. Shibauchi, Y. Matsuda, and Y. Kasahara
In the quest for exotic superconducting pairing states, the Rashba effect, which lifts the electron-spin degeneracy as a consequence of strong spin-orbit interaction (SOI) under broken inversion symmetry, has attracted considerable interest. Here, to introduce the Rashba effect into two-dimensional (2D) strongly correlated electron systems, we fabricate noncentrosymmetric (tricolor) superlattices composed of three kinds of f -electron compounds with atomic thickness; d -wave heavy fermion superconductor CeCoIn 5 sandwiched by two different nonmagnetic metals, YbCoIn 5 and YbRhIn 5 . We find that the Rashba SOI-induced global inversion symmetry breaking in these tricolor Kondo superlattices leads to profound changes in the superconducting properties of CeCoIn 5 , which are revealed by unusual temperature and angular dependencies of upper critical fields that are in marked contrast with the bulk CeCoIn 5 single crystals. We demonstrate that the Rashba effect incorporated into 2D CeCoIn 5 block layers is largely tunable by changing the layer thickness. Moreover, the temperature dependence of in-plane upper critical field exhibits an anomalous upturn at low temperatures, which is attributed to a possible emergence of a helical or stripe superconducting phase. Our results demonstrate that the tricolor Kondo superlattices provide a new playground for exploring exotic superconducting states in the strongly correlated 2D electron systems with the Rashba effect.
Intrinsic ac anomalous Hall effect of nonsymmorphic chiral superconductors with an application toUPt3 Phys. Rev. B (IF 3.836) Pub Date : 2017-11-16 Zhiqiang Wang, John Berlinsky, Gertrud Zwicknagl, and Catherine Kallin
Spin-wave propagation and spin-polarized electron transport in single-crystal iron films Phys. Rev. B (IF 3.836) Pub Date : 2017-11-16 O. Gladii, D. Halley, Y. Henry, and M. Bailleul
The techniques of propagating spin-wave spectroscopy and current-induced spin-wave Doppler shift are applied to a 20-nm-thick Fe/MgO(001) film. The magnetic parameters extracted from the position of the spin-wave resonance peaks are very close to those tabulated for bulk iron. From the zero-current propagating wave forms, a group velocity of 4 km/s and an attenuation length of about 6 μ m are extracted for 1.6- μ m -wavelength spin wave at 18 GHz. From the measured current-induced spin-wave Doppler shift, we extract a surprisingly high degree of spin polarization of the current of 83 % , which constitutes the main finding of this work. This set of results makes single-crystalline iron a promising candidate for building devices utilizing high-frequency spin waves and spin-polarized currents.
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