The power of universal pictures Nat. Phys. (IF 22.806) Pub Date : 2017-09-05
The sky map presented by the Dark Energy Survey showcases the power of images to reach scientists and the wider public alike.
Physics students unite Nat. Phys. (IF 22.806) Pub Date : 2017-09-05
The International Conference of Physics Students continues its remarkable tradition.
Time to fix science prizes Nat. Phys. (IF 22.806) Pub Date : 2017-08-28 Shivaji Sondhi, Steven Kivelson
Science prizes should better reflect how modern science is carried out, argue Shivaji Sondhi and Steven Kivelson.
Exciton-polaritons: In full flow Nat. Phys. (IF 22.806) Pub Date : 2017-06-05 Thilo Stöferle
Flow without friction is a strange phenomenon usually seen in quantum fluids that are cooled to temperatures near absolute zero, but features of superfluidity have now been seen with polaritons at ambient conditions.
Heavy ion collisions: A clash of photons Nat. Phys. (IF 22.806) Pub Date : 2017-08-14 Spencer R. Klein
The ATLAS Collaboration observed photons elastically scattering from other photons — an effect predicted by quantum electrodynamics over 80 years ago.
Rotational superradiant scattering in a vortex flow Nat. Phys. (IF 22.806) Pub Date : 2017-06-12 Theo Torres, Sam Patrick, Antonin Coutant, Maurício Richartz, Edmund W. Tedford, Silke Weinfurtner
The amplification of waves reflected from a rotating obstacle, or superradiance, has been predicted in hydrodynamics and black-hole physics. An experiment with rotating vortex flows confirms this phenomenon.
Spin conversion on the nanoscale Nat. Phys. (IF 22.806) Pub Date : 2017-07-10 YoshiChika Otani, Masashi Shiraishi, Akira Oiwa, Eiji Saitoh, Shuichi Murakami
Spins can act as mediators to interconvert electricity, light, sound, vibration and heat. This Progress article gives an overview of the recent advances associated with nanoscale spin conversion.
Room-temperature superfluidity in a polariton condensate Nat. Phys. (IF 22.806) Pub Date : 2017-06-05 Giovanni Lerario, Antonio Fieramosca, Fábio Barachati, Dario Ballarini, Konstantinos S. Daskalakis, Lorenzo Dominici, Milena De Giorgi, Stefan A. Maier, Giuseppe Gigli, Stéphane Kéna-Cohen, Daniele Sanvitto
Superfluidity is a phenomenon usually restricted to cryogenic temperatures, but organic microcavities provide the conditions for a superfluid flow of polaritons at room temperature.
Direct optical detection of Weyl fermion chirality in a topological semimetal Nat. Phys. (IF 22.806) Pub Date : 2017-05-29 Qiong Ma, Su-Yang Xu, Ching-Kit Chan, Cheng-Long Zhang, Guoqing Chang, Yuxuan Lin, Weiwei Xie, Tomás Palacios, Hsin Lin, Shuang Jia, Patrick A. Lee, Pablo Jarillo-Herrero, Nuh Gedik
Measuring the photocurrent response to circularly polarized mid-infrared light provides direct access to the chirality of Weyl fermions in Weyl semimetals — the property responsible for a range of exotic phenomena.
Numerical test of the Edwards conjecture shows that all packings are equally probable at jamming Nat. Phys. (IF 22.806) Pub Date : 2017-06-26 Stefano Martiniani, K. Julian Schrenk, Kabir Ramola, Bulbul Chakraborty, Daan Frenkel
A decades-old proposal that all distinct packings are equally probable in granular media has gone unproven due to the sheer number of packings involved. Numerical simulation now demonstrates that it holds — precisely at the jamming threshold.
Evidence for light-by-light scattering in heavy-ion collisions with the ATLAS detector at the LHC Nat. Phys. (IF 22.806) Pub Date : 2017-08-14 ATLAS Collaboration
Light-by-light scattering (γγ γγ) is a quantum-mechanical process that is forbidden in the classical theory of electrodynamics. This reaction is accessible at the Large Hadron Collider thanks to the large electromagnetic field strengths generated by ultra-relativistic colliding lead ions. Using 480 μb−1 of lead–lead collision data recorded at a centre-of-mass energy per nucleon pair of 5.02 TeV by the ATLAS detector, here we report evidence for light-by-light scattering. A total of 13 candidate events were observed with an expected background of 2.6 ± 0.7 events. After background subtraction and analysis corrections, the fiducial cross-section of the process Pb + Pb (γγ) Pb(∗) + Pb(∗)γγ, for photon transverse energy ET > 3 GeV, photon absolute pseudorapidity |η| < 2.4, diphoton invariant mass greater than 6 GeV, diphoton transverse momentum lower than 2 GeV and diphoton acoplanarity below 0.01, is measured to be 70 ± 24 (stat.) ± 17 (syst.) nb, which is in agreement with the standard model predictions.
Spectroscopic evidence of a new energy scale for superconductivity in H3S Nat. Phys. (IF 22.806) Pub Date : 2017-06-19 F. Capitani, B. Langerome, J.-B. Brubach, P. Roy, A. Drozdov, M. I. Eremets, E. J. Nicol, J. P. Carbotte, T. Timusk
The discovery of a superconducting phase in sulfur hydride under high pressure with a critical temperature above 200 K has provided fresh impetus to the search for superconductors at ever higher temperatures. Although this system displays all of the hallmarks of superconductivity, the mechanism through which it arises remains to be determined. Here we provide a first optical spectroscopy study of this superconductor. Experimental results for the optical reflectivity of H3S, under hydrostatic pressure of 150 GPa, for several temperatures and over the range 60 to 600 meV of photon energies, are compared with theoretical calculations based on Eliashberg theory. Two significant features stand out: some remarkably strong infrared-active phonons at around 160 meV, and a band with a depressed reflectance in the superconducting state in the region from 450 meV to 600 meV. In this energy range H3S becomes more reflecting with increasing temperature, a change that is traced to superconductivity originating from the electron–phonon interaction. The shape, magnitude and energy dependence of this band at 150 K agrees with our calculations. This provides strong evidence of a conventional mechanism. However, the unusually strong optical phonon suggests a contribution of electronic degrees of freedom.
Large orbital polarization in a metallic square-planar nickelate Nat. Phys. (IF 22.806) Pub Date : 2017-06-12 Junjie Zhang, A. S. Botana, J. W. Freeland, D. Phelan, Hong Zheng, V. Pardo, M. R. Norman, J. F. Mitchell
High-temperature cuprate superconductivity remains a defining problem in condensed-matter physics. Among myriad approaches to addressing this problem has been the study of alternative transition metal oxides with similar structures and 3d electron count that are suggested as proxies for cuprate physics. None of these analogues has been superconducting, and few are even metallic. Here, we report that the low-valent, quasi-two-dimensional trilayer compound Pr4Ni3O8 avoids a charge-stripe-ordered phase previously reported for La4Ni3O8, leading to a metallic ground state. X-ray absorption spectroscopy shows that metallic Pr4Ni3O8 exhibits a low-spin configuration with significant orbital polarization and pronounced dx2−y2 character in the unoccupied states above the Fermi energy, a hallmark of the cuprate superconductors. Density functional theory calculations corroborate this finding, and reveal that the dx2−y2 orbital dominates the near-Ef occupied states as well. Belonging to a regime of 3d electron count found for hole-doped cuprates, Pr4Ni3O8 thus represents one of the closest analogues to cuprates yet reported and a singularly promising candidate for high-Tc superconductivity if electron doping could be achieved.
Direct measurement of polariton–polariton interaction strength Nat. Phys. (IF 22.806) Pub Date : 2017-06-05 Yongbao Sun, Yoseob Yoon, Mark Steger, Gangqiang Liu, Loren N. Pfeiffer, Ken West, David W. Snoke, Keith A. Nelson
Exciton–polaritons in a microcavity are composite two-dimensional bosonic quasiparticles, arising from the strong coupling between confined light modes in a resonant planar optical cavity and excitonic transitions. Quantum phenomena such as Bose–Einstein condensation, superfluidity, quantized vortices, and macroscopic quantum states have been realized at temperatures from tens of kelvin up to room temperatures. Crucially, many of these effects of exciton–polaritons depend on the polariton–polariton interaction strength. Despite the importance of this parameter, it has been difficult to make an accurate experimental measurement, mostly because of the difficulty in determining the absolute densities of polaritons and bare excitons. Here we report a direct measurement of the polariton–polariton interaction strength in a very high-Q microcavity structure. By allowing polaritons to propagate over 20 μm to the centre of a laser-generated annular trap, we are able to separate the polariton–polariton interactions from polariton–exciton interactions. The interaction strength is deduced from the energy renormalization of the polariton dispersion as the polariton density is increased, using the polariton condensation as a benchmark for the density. We find that the interaction strength is about two orders of magnitude larger than previous theoretical estimates, putting polaritons in the strongly interacting regime.
Microwave spectroscopy of spinful Andreev bound states in ballistic semiconductor Josephson junctions Nat. Phys. (IF 22.806) Pub Date : 2017-06-05 David J. van Woerkom, Alex Proutski, Bernard van Heck, Daniël Bouman, Jukka I. Väyrynen, Leonid I. Glazman, Peter Krogstrup, Jesper Nygård, Leo P. Kouwenhoven, Attila Geresdi
The superconducting proximity effect in semiconductor nanowires has recently enabled the study of new superconducting architectures, such as gate-tunable superconducting qubits and multiterminal Josephson junctions. As opposed to their metallic counterparts, the electron density in semiconductor nanosystems is tunable by external electrostatic gates, providing a highly scalable and in situ variation of the device properties. In addition, semiconductors with large g-factor and spin–orbit coupling have been shown to give rise to exotic phenomena in superconductivity, such as φ0 Josephson junctions and the emergence of Majorana bound states. Here, we report microwave spectroscopy measurements that directly reveal the presence of Andreev bound states (ABS) in ballistic semiconductor channels. We show that the measured ABS spectra are the result of transport channels with gate-tunable, high transmission probabilities up to 0.9, which is required for gate-tunable Andreev qubits and beneficial for braiding schemes of Majorana states. For the first time, we detect excitations of a spin-split pair of ABS and observe symmetry-broken ABS, a direct consequence of the spin–orbit coupling in the semiconductor.
Controlled release of multiphoton quantum states from a microwave cavity memory Nat. Phys. (IF 22.806) Pub Date : 2017-06-05 Wolfgang Pfaff, Christopher J. Axline, Luke D. Burkhart, Uri Vool, Philip Reinhold, Luigi Frunzio, Liang Jiang, Michel H. Devoret, Robert J. Schoelkopf
Signal transmission loss in a quantum network can be overcome by encoding quantum states in complex multiphoton fields. But transmitting quantum information encoded in this way requires that locally stored states can be converted to propagating fields. Here we experimentally show the controlled conversion of multiphoton quantum states, such as Schrödinger cat states, from a microwave cavity quantum memory into propagating modes. By parametric conversion using the nonlinearity of a single Josephson junction, we can release the cavity state in ~500 ns, about three orders of magnitude faster than its intrinsic lifetime. This mechanism—which we dub Schrödinger’s catapult—faithfully converts arbitrary cavity fields to travelling signals with an estimated efficiency of >90%, enabling the on-demand generation of complex itinerant quantum states. Importantly, the release process can be precisely controlled on fast timescales, allowing us to generate entanglement between the cavity and the travelling mode by partial conversion.
Gate-tunable black phosphorus spin valve with nanosecond spin lifetimes Nat. Phys. (IF 22.806) Pub Date : 2017-05-29 Ahmet Avsar, Jun Y. Tan, Marcin Kurpas, Martin Gmitra, Kenji Watanabe, Takashi Taniguchi, Jaroslav Fabian, Barbaros Özyilmaz
Two-dimensional materials offer new opportunities for both fundamental science and technological applications, by exploiting the electron’s spin. Although graphene is very promising for spin communication due to its extraordinary electron mobility, the lack of a bandgap restricts its prospects for semiconducting spin devices such as spin diodes and bipolar spin transistors. The recent emergence of two-dimensional semiconductors could help overcome this basic challenge. In this letter we report an important step towards making two-dimensional semiconductor spin devices. We have fabricated a spin valve based on ultrathin (~5 nm) semiconducting black phosphorus (bP), and established fundamental spin properties of this spin channel material, which supports all electrical spin injection, transport, precession and detection up to room temperature. In the non-local spin valve geometry we measure Hanle spin precession and observe spin relaxation times as high as 4 ns, with spin relaxation lengths exceeding 6 μm. Our experimental results are in a very good agreement with first-principles calculations and demonstrate that the Elliott–Yafet spin relaxation mechanism is dominant. We also show that spin transport in ultrathin bP depends strongly on the charge carrier concentration, and can be manipulated by the electric field effect.
Spin-polarized exciton quantum beating in hybrid organic–inorganic perovskites Nat. Phys. (IF 22.806) Pub Date : 2017-05-29 Patrick Odenthal, William Talmadge, Nathan Gundlach, Ruizhi Wang, Chuang Zhang, Dali Sun, Zhi-Gang Yu, Z. Valy Vardeny, Yan S. Li
Hybrid organic–inorganic perovskites have emerged as a new class of semiconductors that exhibit excellent performance as active layers in photovoltaic solar cells. These compounds are also highly promising materials for the field of spintronics due to their large and tunable spin–orbit coupling, spin-dependent optical selection rules, and their predicted electrically tunable Rashba spin splitting. Here we demonstrate the optical orientation of excitons and optical detection of spin-polarized exciton quantum beating in polycrystalline films of the hybrid perovskite CH3NH3PbClxI3−x. Time-resolved Faraday rotation measurement in zero magnetic field reveals unexpectedly long spin lifetimes exceeding 1 ns at 4 K, despite the large spin–orbit couplings of the heavy lead and iodine atoms. The quantum beating of exciton states in transverse magnetic fields shows two distinct frequencies, corresponding to two g-factors of 2.63 and −0.33, which we assign to electrons and holes, respectively. These results provide a basic picture of the exciton states in hybrid perovskites, and suggest they hold potential for spintronic applications.
Separating the configurational and vibrational entropy contributions in metallic glasses Nat. Phys. (IF 22.806) Pub Date : 2017-05-29 Hillary L. Smith, Chen W. Li, Andrew Hoff, Glenn R. Garrett, Dennis S. Kim, Fred C. Yang, Matthew S. Lucas, Tabitha Swan-Wood, J. Y. Y. Lin, M. B. Stone, D. L. Abernathy, Marios D. Demetriou, B. Fultz
Glassy materials exist in nature and play a critical role in technology, but key differences between the glass, liquid and crystalline phases are not well understood. Over several decades there has been controversy about the specific heat absorbed as a glass transforms to a liquid—does this originate from vibrational entropy or configurational entropy? Here we report direct in situ measurements of the vibrational spectra of strong and fragile metallic glasses in the glass, liquid and crystalline phases. For both types of material, the measured vibrational entropies of the glass and liquid show a tiny excess over the crystal, representing less than 5% of the total excess entropy measured with step calorimetry. These results reveal that the excess entropy of metallic glasses is almost entirely configurational in origin, consistent with the early theories of Gibbs and co-workers describing the glass transition as a purely configurational transition.
Membrane fluctuations mediate lateral interaction between cadherin bonds Nat. Phys. (IF 22.806) Pub Date : 2017-06-12 Susanne F. Fenz, Timo Bihr, Daniel Schmidt, Rudolf Merkel, Udo Seifert, Kheya Sengupta, Ana-Sunčana Smith
The integrity of living tissues is maintained by adhesion domains of trans-bonds formed between cadherin proteins residing on opposing membranes of neighbouring cells. These domains are stabilized by lateral cis-interactions between the cadherins on the same cell. However, the origin of cis-interactions remains perplexing since they are detected only in the context of trans-bonds. By combining experimental, analytical and computational approaches, we identify bending fluctuations of membranes as a source of long-range cis-interactions, and a regulator of trans-interactions. Specifically, nanometric membrane bending and fluctuations introduce cooperative effects that modulate the affinity and binding/unbinding rates for trans-dimerization, dramatically affecting the nucleation and growth of adhesion domains. Importantly, this regulation relies on physical principles and not on details of protein–protein interactions. These omnipresent fluctuations can thus act as a generic control mechanism in all types of cell adhesion, suggesting a hitherto unknown physiological role for recently identified active fluctuations of cellular membranes.
Dynamic scaling in natural swarms Nat. Phys. (IF 22.806) Pub Date : 2017-06-19 Andrea Cavagna, Daniele Conti, Chiara Creato, Lorenzo Del Castello, Irene Giardina, Tomas S. Grigera, Stefania Melillo, Leonardo Parisi, Massimiliano Viale
Collective behaviour in biological systems presents theoretical challenges beyond the borders of classical statistical physics. The lack of concepts such as scaling and renormalization is particularly problematic, as it forces us to negotiate details whose relevance is often hard to assess. In an attempt to improve this situation, we present here experimental evidence of the emergence of dynamic scaling laws in natural swarms of midges. We find that spatio-temporal correlation functions in different swarms can be rescaled by using a single characteristic time, which grows with the correlation length with a dynamical critical exponent z ≈ 1, a value not found in any other standard statistical model. To check whether out-of-equilibrium effects may be responsible for this anomalous exponent, we run simulations of the simplest model of self-propelled particles and find z ≈ 2, suggesting that natural swarms belong to a novel dynamic universality class. This conclusion is strengthened by experimental evidence of the presence of non-dissipative modes in the relaxation, indicating that previously overlooked inertial effects are needed to describe swarm dynamics. The absence of a purely dissipative regime suggests that natural swarms undergo a near-critical censorship of hydrodynamics.
To catch a chameleon Nat. Phys. (IF 22.806) Pub Date : 2017-09-05 Tobias Jenke
High-precision laboratory experiments with neutrons and atoms are converging to a verdict on 'chameleon fields' as a possible explanation of dark energy, explains Tobias Jenke.
The thing about data Nat. Phys. (IF 22.806) Pub Date : 2017-08-02
The rise of big data represents an opportunity for physicists. To take full advantage, however, they need a subtle but important shift in mindset.
The physics of data Nat. Phys. (IF 22.806) Pub Date : 2017-07-03 Jeff Byers
Physicists are accustomed to dealing with large datasets, yet they are fortunate in that the quality of their experimental data is very good. The onset of big data has led to an explosion of datasets with a far more complex structure — a development that requires new tools and a different mindset.
Gravitational-wave detection: Entanglement at work Nat. Phys. (IF 22.806) Pub Date : 2017-05-15 Raffaele Flaminio
The Einstein–Podolsky–Rosen type of quantum entanglement can be used to improve the sensitivity of laser interferometer gravitational-wave detectors beyond the quantum limit.
Quantum simulation: Probing information scrambling Nat. Phys. (IF 22.806) Pub Date : 2017-05-22 Monika Schleier-Smith
Quantum information encoded in one of many interacting particles quickly becomes scrambled. A set of tools for tracking this process is on its way.
Van der waals heterostructures: Exciting double bilayers Nat. Phys. (IF 22.806) Pub Date : 2017-05-22 Koji Muraki
An excitonic Bose–Einstein condensate has so far been realized only in particular semiconductor heterostructure setups. Now, experiments show that such condensates can form in double graphene bilayers separated by hexagonal boron nitride.
Cell mechanics: The benefits of getting high Nat. Phys. (IF 22.806) Pub Date : 2017-05-01 Klaus Kroy
Standard rheology tells us how a cell responds to deformation. But ramping up the frequency reveals more about its internal dynamics and morphology, mapping a route to improved drug treatments — and possible insight into the malignancy of cancers.
Testing universality of Efimov physics across broad and narrow Feshbach resonances Nat. Phys. (IF 22.806) Pub Date : 2017-05-15 Jacob Johansen, B. J. DeSalvo, Krutik Patel, Cheng Chin
The emergence of Efimov states in ultracold atomic systems is expected to have a universal behaviour, but a new experimental study defies this expectation, reporting a clear deviation around a narrow Feshbach resonance.
Topological triplon modes and bound states in a Shastry–Sutherland magnet Nat. Phys. (IF 22.806) Pub Date : 2017-05-08 P. A. McClarty, F. Krüger, T. Guidi, S. F. Parker, K. Refson, A. W. Parker, D. Prabhakaran, R. Coldea
A detailed experimental investigation on the spin excitations in SrCu2(BO3)2 under an external magnetic confirms the existence of topological triplon modes in this experimental realization of the Shastry–Sutherland model.
Multidimensional entropy landscape of quantum criticality Nat. Phys. (IF 22.806) Pub Date : 2017-05-08 K. Grube, S. Zaum, O. Stockert, Q. Si, H. v. Löhneysen
Thermal-expansion measurements of CeCu6−xAux reveal the thermodynamic landscape of this material’s entropy, offering insights into the behaviour of quantum critical fluctuations as the system approaches its quantum critical point.
Quantum Hall drag of exciton condensate in graphene Nat. Phys. (IF 22.806) Pub Date : 2017-05-22 Xiaomeng Liu, Kenji Watanabe, Takashi Taniguchi, Bertrand I. Halperin, Philip Kim
An electronic double layer, subjected to a high magnetic field, can form an exciton condensate: a Bose–Einstein condensate of Coulomb-bound electron–hole pairs. Now, exciton condensation is reported for a graphene/boron-nitride/graphene structure.
Excitonic superfluid phase in double bilayer graphene Nat. Phys. (IF 22.806) Pub Date : 2017-05-22 J. I. A. Li, T. Taniguchi, K. Watanabe, J. Hone, C. R. Dean
Strongly interacting bosons have been predicted to display a transition into a superfluid ground state, similar to Bose–Einstein condensation. This effect is now observed in a double bilayer graphene structure, with excitons as the bosonic particles.
Tunnelling spectroscopy of Andreev states in graphene Nat. Phys. (IF 22.806) Pub Date : 2017-05-01 Landry Bretheau, Joel I-Jan Wang, Riccardo Pisoni, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero
Van der Waals heterostructures provide a tunable platform for probing the Andreev bound states responsible for proximity-induced superconductivity, helping to establish a connection between Andreev physics at finite energy and the Josephson effect.
Hotspot-mediated non-dissipative and ultrafast plasmon passage Nat. Phys. (IF 22.806) Pub Date : 2017-05-15 Eva-Maria Roller, Lucas V. Besteiro, Claudia Pupp, Larousse Khosravi Khorashad, Alexander O. Govorov, Tim Liedl
Strong plasmonic hotspots can facilitate ultrafast energy transfer between metallic nanoparticles with almost no energy loss.
Attosecond chronoscopy of electron scattering in dielectric nanoparticles Nat. Phys. (IF 22.806) Pub Date : 2017-05-22 L. Seiffert, Q. Liu, S. Zherebtsov, A. Trabattoni, P. Rupp, M. C. Castrovilli, M. Galli, F. Süßmann, K. Wintersperger, J. Stierle, G. Sansone, L. Poletto, F. Frassetto, I. Halfpap, V. Mondes, C. Graf, E. Rühl, F. Krausz, M. Nisoli, T. Fennel, F. Calegari, M. F. Kling
Attosecond streaking is used to study the dynamics of electron scattering in dielectric nanoparticles in real time. Revealing the mechanisms involved is the first step towards understanding electron scattering in more complex dielectrics.
High-frequency microrheology reveals cytoskeleton dynamics in living cells Nat. Phys. (IF 22.806) Pub Date : 2017-05-01 Annafrancesca Rigato, Atsushi Miyagi, Simon Scheuring, Felix Rico
Microrheology of cells suggests that the dynamics of single filaments in the cytoskeleton dominate at high frequencies. This response can be used to detect differences between cell types and states — including benign and malignant cancer cells.
Proposal for gravitational-wave detection beyond the standard quantum limit through EPR entanglement Nat. Phys. (IF 22.806) Pub Date : 2017-05-15 Yiqiu Ma, Haixing Miao, Belinda Heyun Pang, Matthew Evans, Chunnong Zhao, Jan Harms, Roman Schnabel, Yanbei Chen
In continuously monitored systems the standard quantum limit is given by the trade-off between shot noise and back-action noise. In gravitational-wave detectors, such as Advanced LIGO, both contributions can be simultaneously squeezed in a broad frequency band by injecting a spectrum of squeezed vacuum states with a frequency-dependent squeeze angle. This approach requires setting up an additional long baseline, low-loss filter cavity in a vacuum system at the detector’s site. Here, we show that the need for such a filter cavity can be eliminated, by exploiting Einstein–Podolsky–Rosen (EPR)-entangled signals and idler beams. By harnessing their mutual quantum correlations and the difference in the way each beam propagates in the interferometer, we can engineer the input signal beam to have the appropriate frequency-dependent conditional squeezing once the out-going idler beam is detected. Our proposal is appropriate for all future gravitational-wave detectors for achieving sensitivities beyond the standard quantum limit.
Measuring out-of-time-order correlations and multiple quantum spectra in a trapped-ion quantum magnet Nat. Phys. (IF 22.806) Pub Date : 2017-05-22 Martin Gärttner, Justin G. Bohnet, Arghavan Safavi-Naini, Michael L. Wall, John J. Bollinger, Ana Maria Rey
Controllable arrays of ions and ultracold atoms can simulate complex many-body phenomena and may provide insights into unsolved problems in modern science. To this end, experimentally feasible protocols for quantifying the buildup of quantum correlations and coherence are needed, as performing full state tomography does not scale favourably with the number of particles. Here we develop and experimentally demonstrate such a protocol, which uses time reversal of the many-body dynamics to measure out-of-time-order correlation functions (OTOCs) in a long-range Ising spin quantum simulator with more than 100 ions in a Penning trap. By measuring a family of OTOCs as a function of a tunable parameter we obtain fine-grained information about the state of the system encoded in the multiple quantum coherence spectrum, extract the quantum state purity, and demonstrate the buildup of up to 8-body correlations. Future applications of this protocol could enable studies of many-body localization, quantum phase transitions, and tests of the holographic duality between quantum and gravitational systems.
A dissipative quantum reservoir for microwave light using a mechanical oscillator Nat. Phys. (IF 22.806) Pub Date : 2017-05-15 L. D. Tóth, N. R. Bernier, A. Nunnenkamp, A. K. Feofanov, T. J. Kippenberg
Engineered dissipation can be used for quantum state preparation. This is achieved with a suitably engineered coupling to a dissipative cold reservoir usually formed by an electromagnetic mode. In the field of cavity electro- and optomechanics, the electromagnetic cavity naturally serves as a cold reservoir for the mechanical mode. Here, we realize the opposite scenario and engineer a mechanical oscillator cooled close to its ground state into a cold dissipative reservoir for microwave photons in a superconducting circuit. By tuning the coupling to this dissipative mechanical reservoir, we demonstrate dynamical backaction control of the microwave field, leading to stimulated emission and maser action. Moreover, the reservoir can function as a useful quantum resource, allowing the implementation of a near-quantum-limited phase-preserving microwave amplifier. Such engineered mechanical dissipation extends the toolbox of quantum manipulation techniques of the microwave field and constitutes a new ingredient for optomechanical protocols.
Möbius Kondo insulators Nat. Phys. (IF 22.806) Pub Date : 2017-04-10 Po-Yao Chang, Onur Erten, Piers Coleman
Heavy fermion materials have recently attracted attention for their potential to combine topological protection with strongly correlated electron physics. To date, the ideas of topological protection have been restricted to the heavy fermion or ‘Kondo’ insulators with the simplest point-group symmetries. Here we argue that the presence of nonsymmorphic crystal symmetries in many heavy fermion materials opens up a new family of topologically protected heavy electron systems. Re-examination of archival resistivity measurements in the nonsymmorphic heavy fermion insulators Ce3Bi4Pt3 and CeNiSn reveals the presence of a low-temperature conductivity plateau, making them candidate members of the new class of material. We illustrate our ideas with a specific model for CeNiSn, showing how glide symmetries generate surface states with a novel Möbius braiding that can be detected by ARPES or non-local conductivity measurements. One of the interesting effects of strong correlation is the development of partially localization or ‘Kondo breakdown’ on the surfaces, which transforms Möbius surface states into quasi-one-dimensional conductors, with the potential for novel electronic phase transitions.
Quasiparticle interference and strong electron–mode coupling in the quasi-one-dimensional bands of Sr2RuO4 Nat. Phys. (IF 22.806) Pub Date : 2017-05-08 Zhenyu Wang, Daniel Walkup, Philip Derry, Thomas Scaffidi, Melinda Rak, Sean Vig, Anshul Kogar, Ilija Zeljkovic, Ali Husain, Luiz H. Santos, Yuxuan Wang, Andrea Damascelli, Yoshiteru Maeno, Peter Abbamonte, Eduardo Fradkin, Vidya Madhavan
The single-layered ruthenate Sr2RuO4 is presented as a potential spin-triplet superconductor with an order parameter that may break time-reversal invariance and host half-quantized vortices with Majorana zero modes. Although the actual nature of the superconducting state is still a matter of controversy, it is believed to condense from a metallic state that is well described by a conventional Fermi liquid. In this work we use a combination of Fourier transform scanning tunnelling spectroscopy (FT-STS) and momentum-resolved electron energy loss spectroscopy (M-EELS) to probe interaction effects in the normal state of Sr2RuO4. Our high-resolution FT-STS data show signatures of the β-band with a distinctly quasi-one-dimensional (1D) character. The band dispersion reveals surprisingly strong interaction effects that dramatically renormalize the Fermi velocity, suggesting that the normal state of Sr2RuO4 is that of a ‘correlated metal’ where correlations are strengthened by the quasi-1D nature of the bands. In addition, kinks at energies of approximately 10 meV, 38 meV and 70 meV are observed. By comparing STM and M-EELS data we show that the two higher energy features arise from coupling with collective modes. The strong correlation effects and the kinks in the quasi-1D bands could provide important information for understanding the superconducting state.
Mottness at finite doping and charge instabilities in cuprates Nat. Phys. (IF 22.806) Pub Date : 2017-05-08 S. Peli, S. Dal Conte, R. Comin, N. Nembrini, A. Ronchi, P. Abrami, F. Banfi, G. Ferrini, D. Brida, S. Lupi, M. Fabrizio, A. Damascelli, M. Capone, G. Cerullo, C. Giannetti
The influence of Mott physics on the doping–temperature phase diagram of copper oxides represents a major issue that is the subject of intense theoretical and experimental efforts. Here, we investigate the ultrafast electron dynamics in prototypical single-layer Bi-based cuprates at the energy scale of the O-2p Cu-3d charge-transfer (CT) process. We demonstrate a clear evolution of the CT excitations from incoherent and localized, as in a Mott insulator, to coherent and delocalized, as in a conventional metal. This reorganization of the high-energy degrees of freedom occurs at the critical doping pcr ≈ 0.16 irrespective of the temperature, and it can be well described by dynamical mean-field theory calculations. We argue that the onset of low-temperature charge instabilities is the low-energy manifestation of the underlying Mottness that characterizes the p < pcr region of the phase diagram. This discovery sets a new framework for theories of charge order and low-temperature phases in underdoped copper oxides.
Mixed electrochemical–ferroelectric states in nanoscale ferroelectrics Nat. Phys. (IF 22.806) Pub Date : 2017-05-01 Sang Mo Yang, Anna N. Morozovska, Rajeev Kumar, Eugene A. Eliseev, Ye Cao, Lucie Mazet, Nina Balke, Stephen Jesse, Rama K. Vasudevan, Catherine Dubourdieu, Sergei V. Kalinin
Ferroelectricity on the nanoscale has been the subject of much fascination in condensed-matter physics for over half a century. In recent years, multiple reports claiming ferroelectricity in ultrathin ferroelectric films based on the formation of remnant polarization states, local electromechanical hysteresis loops, and pressure-induced switching were made. However, similar phenomena were reported for traditionally non-ferroelectric materials, creating a significant level of uncertainty in the field. Here we show that in nanoscale systems the ferroelectric state is fundamentally inseparable from the electrochemical state of the surface, leading to the emergence of a mixed electrochemical–ferroelectric state. We explore the nature, thermodynamics, and thickness evolution of such states, and demonstrate the experimental pathway to establish its presence. This analysis reconciles multiple prior studies, provides guidelines for studies of ferroelectric materials on the nanoscale, and establishes the design paradigm for new generations of ferroelectric-based devices.
The invention of dimension Nat. Phys. (IF 22.806) Pub Date : 2017-08-02 Steven T. Bramwell
Assigning dimensions to physical quantities is not just for practicality. Steven T. Bramwell reflects on the deeper physical connotations of it all.
Tools of the trade — and how to use them Nat. Phys. (IF 22.806) Pub Date : 2017-07-04
The role of physicists in finance is changing, as quantitative trading opens an exciting alternative to traditional financial modelling, and data science lures would-be 'quants' away. But the void is being steadily filled by a new type of analyst.
A matter of responding to stress Nat. Phys. (IF 22.806) Pub Date : 2017-07-04 Mark Buchanan
The eternal question Nat. Phys. (IF 22.806) Pub Date : 2017-07-04 Abigail Klopper
Exhibition: Destination unknown Nat. Phys. (IF 22.806) Pub Date : 2017-07-04 Adam Cox
Biomimetics: A sticky job for suckers Nat. Phys. (IF 22.806) Pub Date : 2017-07-04 Abigail Klopper
Applied physics: Martian dust blower Nat. Phys. (IF 22.806) Pub Date : 2017-07-04 Bart Verberck
Statistical physics: Dialectic magnetism Nat. Phys. (IF 22.806) Pub Date : 2017-07-04 Federico Levi
Anyons: Not just another statistic Nat. Phys. (IF 22.806) Pub Date : 2017-07-04 Luke Fleet
Meteoritics: Silica puzzle Nat. Phys. (IF 22.806) Pub Date : 2017-07-04 Iulia Georgescu
Superconductivity: Ferroelectricity woos pairing Nat. Phys. (IF 22.806) Pub Date : 2017-04-17 Marc Gabay, Jean-Marc Triscone
Ferroelectricity and superconductivity do not have much in common. Now, a superconducting and a ferroelectric-like state have been found to coexist in a doped perovskite oxide.
Superconductivity: When Andreev meets Hall Nat. Phys. (IF 22.806) Pub Date : 2017-07-04 Gleb Finkelstein, François Amet
A device with superconducting contacts connected to graphene in the quantum Hall regime hints at a novel Andreev scattering mechanism.
High-harmonic generation: The bright side of downsizing Nat. Phys. (IF 22.806) Pub Date : 2017-05-08 Alexandra Landsman
The shorter the antenna, the higher the frequency — so what happens when nanoantennas hit optical frequencies? One answer may lead to high-harmonic generation without the need for high-powered lasers.
Quantum simulation: Solid-state platforms Nat. Phys. (IF 22.806) Pub Date : 2017-04-24 Dario Bercioux, Sander Otte
Solid-state systems capable of simulating the theoretical predictions of condensed matter are in short supply. Demonstrations of electronic Lieb lattices using two different platforms suggest this may be about to change.
Magnetic skyrmions: On a wedge Nat. Phys. (IF 22.806) Pub Date : 2017-07-04 Bart Verberck
Topological materials: Monolayers have the edge Nat. Phys. (IF 22.806) Pub Date : 2017-06-26 Aravind Devarakonda, Joseph G. Checkelsky
Two studies show evidence that single layers of a transition metal dichalcogenide are two-dimensional topological insulators.
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