Showing papers in "Nature Physics in 2016"
TL;DR: In this article, the acoustic analogue of a topological insulator is shown: a metamaterial exhibiting one-way sound transport along its edge, a graphene-like array of stainless-steel rods.
Abstract: The acoustic analogue of a topological insulator is shown: a metamaterial exhibiting one-way sound transport along its edge. The system — a graphene-like array of stainless-steel rods — is a promising new platform for exploring topological phenomena.
901 citations
TL;DR: In this article, the superconducting properties of NbSe2 as it approaches the monolayer limit are investigated by means of magnetotransport measurements, uncovering evidence of spin-momentum locking.
Abstract: The superconducting properties of NbSe2 as it approaches the monolayer limit are investigated by means of magnetotransport measurements, uncovering evidence of spin–momentum locking.
888 citations
TL;DR: In this paper, an Ising Hamiltonian with long-range interactions and programmable random disorder is used to generate many-body localization (MBL) states in a small system with programmable disorder.
Abstract: Interacting quantum systems are expected to thermalize, but in some situations in the presence of disorder they can exist in localized states instead. This many-body localization is studied experimentally in a small system with programmable disorder. When a system thermalizes it loses all memory of its initial conditions. Even within a closed quantum system, subsystems usually thermalize using the rest of the system as a heat bath. Exceptions to quantum thermalization have been observed, but typically require inherent symmetries1,2 or noninteracting particles in the presence of static disorder3,4,5,6. However, for strong interactions and high excitation energy there are cases, known as many-body localization (MBL), where disordered quantum systems can fail to thermalize7,8,9,10. We experimentally generate MBL states by applying an Ising Hamiltonian with long-range interactions and programmable random disorder to ten spins initialized far from equilibrium. Using experimental and numerical methods we observe the essential signatures of MBL: initial-state memory retention, Poissonian distributed energy level spacings, and evidence of long-time entanglement growth. Our platform can be scaled to more spins, where a detailed modelling of MBL becomes impossible.
835 citations
TL;DR: A magnetotransport study of zirconium pentatelluride, ZrTe5, has been carried out in this paper, which reveals evidence for a chiral magnetic effect, a striking macroscopic manifestation of the quantum and relativistic nature of Weyl semimetals.
Abstract: A magnetotransport study of zirconium pentatelluride now reveals evidence for a chiral magnetic effect, a striking macroscopic manifestation of the quantum and relativistic nature of Weyl semimetals The chiral magnetic effect is the generation of an electric current induced by chirality imbalance in the presence of a magnetic field It is a macroscopic manifestation of the quantum anomaly1,2 in relativistic field theory of chiral fermions (massless spin 1/2 particles with a definite projection of spin on momentum)—a remarkable phenomenon arising from a collective motion of particles and antiparticles in the Dirac sea The recent discovery3,4,5,6 of Dirac semimetals with chiral quasiparticles opens a fascinating possibility to study this phenomenon in condensed matter experiments Here we report on the measurement of magnetotransport in zirconium pentatelluride, ZrTe5, that provides strong evidence for the chiral magnetic effect Our angle-resolved photoemission spectroscopy experiments show that this material’s electronic structure is consistent with a three-dimensional Dirac semimetal We observe a large negative magnetoresistance when the magnetic field is parallel with the current The measured quadratic field dependence of the magnetoconductance is a clear indication of the chiral magnetic effect The observed phenomenon stems from the effective transmutation of a Dirac semimetal into a Weyl semimetal induced by parallel electric and magnetic fields that represent a topologically non-trivial gauge field background We expect that the chiral magnetic effect may emerge in a wide class of materials that are near the transition between the trivial and topological insulators
806 citations
TL;DR: In this paper, it was shown that MoTe2 is a type-II Weyl semimetal, hosting Weyl fermions that have no counterpart in high-energy physics.
Abstract: Observations of topological surface states provide strong evidence that MoTe2 is a type-II Weyl semimetal, hosting Weyl fermions that have no counterpart in high-energy physics.
711 citations
TL;DR: Using optical lattices to trap ultracold atoms provides a powerful platform for probing topological phases, analogues to those found in condensed matter as discussed by the authors. But as these systems are highly tunable, they could be used to engineer even more exotic phases.
Abstract: Using optical lattices to trap ultracold atoms provides a powerful platform for probing topological phases, analogues to those found in condensed matter. But as these systems are highly tunable, they could be used to engineer even more exotic phases.
590 citations
TL;DR: In this article, the idea of a topological charge pump with topologically protected transport has been realized with ultracold bosonic atoms, where the quantized motion of charge due to the slow cyclic variation of a periodic potential has been quantized.
Abstract: Thouless introduced the idea of a topological charge pump: the quantized motion of charge due to the slow cyclic variation of a periodic potential. This topologically protected transport has now been realized with ultracold bosonic atoms.
556 citations
TL;DR: Progress is surveyed towards attaining a deeper understanding of spreading processes on multilayer networks, and some of the physical phenomena related to spreading processes that emerge from multilayered structure are highlighted.
Abstract: Despite the success of traditional network analysis, standard networks provide a limited representation of complex systems, which often include different types of relationships (or ‘multiplexity’) between their components. Such structural complexity has a significant effect on both dynamics and function. Throwing away or aggregating available structural information can generate misleading results and be a major obstacle towards attempts to understand complex systems. The recent multilayer approach for modelling networked systems explicitly allows the incorporation of multiplexity and other features of realistic systems. It allows one to couple different structural relationships by encoding them in a convenient mathematical object. It also allows one to couple different dynamical processes on top of such interconnected structures. The resulting framework plays a crucial role in helping to achieve a thorough, accurate understanding of complex systems. The study of multilayer networks has also revealed new physical phenomena that remain hidden when using ordinary graphs, the traditional network representation. Here we survey progress towards attaining a deeper understanding of spreading processes on multilayer networks, and we highlight some of the physical phenomena related to spreading processes that emerge from multilayer structure. Reshaping network theory to describe the multilayered structures of the real world has formed a focus in complex networks research in recent years. Progress in our understanding of dynamical processes is but one of the fruits of this labour.
541 citations
TL;DR: In this paper, the Thouless pump was demonstrated in a dynamically controlled optical superlattice with ultracold fermionic atoms and the topological invariance of the pumping process was analyzed.
Abstract: Charge transport in a cyclically time-modulated periodic potential, also known as a topological Thouless pump, has been realized in an ultracold gas of fermionic atoms. An electron gas in a one-dimensional periodic potential can be transported even in the absence of a voltage bias if the potential is slowly and periodically modulated in time. Remarkably, the transferred charge per cycle is sensitive only to the topology of the path in parameter space. Although this so-called Thouless charge pump was first proposed more than thirty years ago1, it has not yet been realized. Here we report the demonstration of topological Thouless pumping using ultracold fermionic atoms in a dynamically controlled optical superlattice. We observe a shift of the atomic cloud as a result of pumping, and extract the topological invariance of the pumping process from this shift. We demonstrate the topological nature of the Thouless pump by varying the topology of the pumping path and verify that the topological pump indeed works in the quantum regime by varying the speed and temperature.
535 citations
Ikerbasque1, University of California, Berkeley2, Lawrence Berkeley National Laboratory3, Nanjing University4, SLAC National Accelerator Laboratory5, Tsinghua University6, California State University, Long Beach7, Monash University, Clayton campus8, Vienna University of Technology9, Geballe Laboratory for Advanced Materials10
TL;DR: In this paper, it was shown that superconductivity and charge density wave ordering can remain intact in just a single layer of niobium diselenide, even when the material is thinned.
Abstract: What happens to correlated electronic phases—superconductivity and charge density wave ordering—as a material is thinned? Experiments show that both can remain intact in just a single layer of niobium diselenide.
533 citations
TL;DR: A review of the current state of the art in inertial confinement fusion research can be found in this paper, where the authors describe the underlying physical principles of fusion energy production from controlled nuclear fusion reactions.
Abstract: The quest for controlled fusion energy has been ongoing for over a half century. The demonstration of ignition and energy gain from thermonuclear fuels in the laboratory has been a major goal of fusion research for decades. Thermonuclear ignition is widely considered a milestone in the development of fusion energy, as well as a major scientific achievement with important applications in national security and basic sciences. The US is arguably the world leader in the inertial confinement approach to fusion and has invested in large facilities to pursue it, with the objective of establishing the science related to the safety and reliability of the stockpile of nuclear weapons. Although significant progress has been made in recent years, major challenges still remain in the quest for thermonuclear ignition via laser fusion. Here, we review the current state of the art in inertial confinement fusion research and describe the underlying physical principles. The quest for energy production from controlled nuclear fusion reactions has been ongoing for many decades. Here, the inertial confinement fusion approach, based on heating and compressing a fuel pellet with intense lasers, is reviewed.
TL;DR: In this paper, the electric field-induced superconductivity in molybdenum disulphide (MoS2) was investigated by means of magneto-transport measurements, uncovering evidence of spin-momentum locking.
Abstract: The electric-field-induced superconducting properties of MoS2 are investigated by means of magneto-transport measurements, uncovering evidence of spin–momentum locking. Symmetry-breaking has been known to play a key role in non-centrosymmetric superconductors with strong spin–orbit interactions (SOIs; refs 1,2,3,4,5,6). The studies, however, have been so far mainly focused on a particular type of SOI, known as the Rashba SOI (ref. 7), whereby the electron spin is locked to its momentum at a right-angle, thereby leading to an in-plane helical spin texture. Here we discuss electric-field-induced superconductivity in molybdenum disulphide (MoS2), which exhibits a fundamentally different type of intrinsic SOI, manifested by an out-of-plane Zeeman-type spin polarization of energy valleys8,9,10. We find an upper critical field of approximately 52 T at 1.5 K, which indicates an enhancement of the Pauli limit by a factor of four as compared to that in centrosymmetric conventional superconductors. Using realistic tight-binding calculations, we reveal that this unusual behaviour is due to an inter-valley pairing that is symmetrically protected by Zeeman-type spin–valley locking against external magnetic fields. Our study sheds light on the interplay of inversion asymmetry with SOIs in confined geometries, and its role in superconductivity.
TL;DR: A 3D-printed fetal brain undergoes constrained expansion to reproduce the shape of the human cerebral cortex, mimicking cortical growth and revealing the mechanical origin of the brain’s folded geometry.
Abstract: A 3D-printed fetal brain undergoes constrained expansion to reproduce the shape of the human cerebral cortex. The soft gels of the model swell in solvent, mimicking cortical growth and revealing the mechanical origin of the brain’s folded geometry.
TL;DR: In this paper, the entanglement between the pairs of particles inside and outside a black hole has been studied, with tantalizing insights into the field of black hole thermodynamics.
Abstract: Hawking radiation is observed emanating from an analogue black hole, with measurements of the entanglement between the pairs of particles inside and outside the hole offering tantalizing insights into the field of black hole thermodynamics.
TL;DR: In this article, a new approach based on machine learning was proposed to reveal a correlation between softness and glassy dynamics, which is strongly correlated with local structure and is strongly associated with dynamics.
Abstract: The relation between structure and dynamics in glasses is not fully understood. A new approach based on machine learning now reveals a correlation between softness—a structural property—and glassy dynamics. In contrast with crystallization, there is no noticeable structural change at the glass transition. Characteristic features of glassy dynamics that appear below an onset temperature, T0 (refs 1,2,3), are qualitatively captured by mean field theory4,5,6, which assumes uniform local structure. Studies of more realistic systems have found only weak correlations between structure and dynamics7,8,9,10,11. This raises the question: is structure important to glassy dynamics in three dimensions? We answer this question affirmatively, using machine learning to identify a new field, ‘softness’ which characterizes local structure and is strongly correlated with dynamics. We find that the onset of glassy dynamics at T0 corresponds to the onset of correlations between softness (that is, structure) and dynamics. Moreover, we construct a simple model of relaxation that agrees well with our simulation results, showing that a theory of the evolution of softness in time would constitute a theory of glassy dynamics.
TL;DR: This work reports an experimental realization of a Carnot engine with a single optically trapped Brownian particle as the working substance and analyses the fluctuations of the finite-time efficiency, showing that the Carnot bound can be surpassed for a small number of non-equilibrium cycles.
Abstract: The Carnot cycle imposes a fundamental upper limit to the efficiency of a macroscopic motor operating between two thermal baths. However, this bound needs to be reinterpreted at microscopic scales, where molecular bio-motors and some artificial micro-engines operate. As described by stochastic thermodynamics, energy transfers in microscopic systems are random and thermal fluctuations induce transient decreases of entropy, allowing for possible violations of the Carnot limit. Here we report an experimental realization of a Carnot engine with a single optically trapped Brownian particle as the working substance. We present an exhaustive study of the energetics of the engine and analyse the fluctuations of the finite-time efficiency, showing that the Carnot bound can be surpassed for a small number of non-equilibrium cycles. As its macroscopic counterpart, the energetics of our Carnot device exhibits basic properties that one would expect to observe in any microscopic energy transducer operating with baths at different temperatures. Our results characterize the sources of irreversibility in the engine and the statistical properties of the efficiency-an insight that could inspire new strategies in the design of efficient nano-motors.
TL;DR: In this article, the authors reported the experimental realization of 2D spin-orbit coupling in ultracold 40K Fermi gases using three lasers, each of which dresses one atomic hyperfine spin state.
Abstract: Spin–orbit coupling in two dimensions is essential for observing topological phases in ultracold atoms. Such a coupling was produced in a gas of potassium atoms and a robust Dirac point was observed in the energy dispersions of the dressed atoms. Spin–orbit coupling (SOC) is central to many physical phenomena, including fine structures of atomic spectra and topological phases in ultracold atoms. Whereas, in general, SOC is fixed in a system, laser–atom interaction provides a means to create and control synthetic SOC in ultracold atoms1. Despite significant experimental progress in this area2,3,4,5,6,7,8, two-dimensional (2D) synthetic SOC, which is crucial for exploring two- and three-dimensional topological phases, is lacking. Here, we report the experimental realization of 2D SOC in ultracold 40K Fermi gases using three lasers, each of which dresses one atomic hyperfine spin state. Through spin-injection radiofrequency (rf) spectroscopy4, we probe the spin-resolved energy dispersions of the dressed atoms, and observe a highly controllable Dirac point created by the 2D SOC. These results constitute a step towards the realization of new topological states of matter.
TL;DR: The crystal structure of the superconducting phase of hydrogen sulfide (and deuterium sulfide) in the normal andsuperconducting states obtained by means of synchrotron X-ray diffraction measurements, combined with electrical resistance measurements at both room and low temperatures are reported.
Abstract: A superconducting critical temperature above 200 K has recently been discovered in H2S (or D2S) under high hydrostatic pressure1, 2. These measurements were interpreted in terms of a decomposition of these materials into elemental sulfur and a hydrogen-rich hydride that is responsible for the superconductivity, although direct experimental evidence for this mechanism has so far been lacking. Here we report the crystal structure of the superconducting phase of hydrogen sulfide (and deuterium sulfide) in the normal and superconducting states obtained by means of synchrotron X-ray diffraction measurements, combined with electrical resistance measurements at both room and low temperatures. We find that the superconducting phase is mostly in good agreement with theoretically predicted body-centered cubic (bcc) structure for H3S (Ref.3). The presence of elemental sulfur is also manifest in the X-ray diffraction patterns, thus proving the decomposition mechanism of H2S to H3S + S under pressure4-6.
TL;DR: In this article, vorticity is reported as a signature feature of electron viscosity in graphene, which leads to negative nonlocal resistance, which can be observed experimentally.
Abstract: In analogy to fluids, electric currents can exhibit viscosity — albeit with effects difficult to observe experimentally. Now, vorticity is reported as a signature feature of electron viscosity in graphene, which leads to negative nonlocal resistance.
TL;DR: In this article, coherent Rydberg dressing was used to implement a two-dimensional synthetic spin lattice, where ground state atoms were coupled to Rydgberg states via off-resonant laser coupling.
Abstract: The control of long-range interactions is an essential ingredient for the study of exotic phases of matter using atoms in optical lattices. Such control is demonstrated using Rydberg dressing: the coupling of ground state atoms to Rydberg states. Ultracold atoms in optical lattices are ideal to study fundamentally new quantum many-body systems1,2 including frustrated or topological magnetic phases3,4 and supersolids5,6. However, the necessary control of strong long-range interactions between distant ground state atoms has remained a long-standing goal. Optical dressing of ground state atoms via off-resonant laser coupling to Rydberg states is one way to tailor such interactions5,6,7,8. Here we report the realization of coherent Rydberg dressing to implement a two-dimensional synthetic spin lattice. Our single-atom-resolved interferometric measurements of the many-body dynamics enable the microscopic probing of the interactions and reveal their highly tunable range and anisotropy. Our work marks the first step towards the use of laser-controlled Rydberg interactions for the study of exotic quantum magnets3,4,9 in optical lattices.
TL;DR: In this paper, the spin-dependent long-range interaction known as Rydberg dressing is exploited to entangle a pair of ultracold neutral atoms, which has practical applications in quantum technologies.
Abstract: Tunable interactions in quantum many-body systems have practical applications in quantum technologies. The effective spin-dependent long-range interaction known as Rydberg dressing is now exploited to entangle a pair of ultracold neutral atoms.
TL;DR: In this paper, the spin-momentum locking of Dirac surface states offers intriguing possibilities for converting between charge and spin currents, and experiments show that fine tuning of the Fermi level is critical for maximizing the efficiency of such conversions.
Abstract: The spin–momentum locking of Dirac surface states offers intriguing possibilities for converting between charge and spin currents. Experiments show that fine tuning of the Fermi level is critical for maximizing the efficiency of such conversions.
TL;DR: Fusion materials research started in the early 1970s following the observation of the degradation of irradiated materials used in the first commercial fission reactors as mentioned in this paper, and has been the subject of decades of worldwide research efforts underpinning the present maturity of the fusion materials research program.
Abstract: Fusion materials research started in the early 1970s following the observation of the degradation of irradiated materials used in the first commercial fission reactors. The technological challenges of fusion energy are intimately linked with the availability of suitable materials capable of reliably withstanding the extremely severe operational conditions of fusion reactors. Although fission and fusion materials exhibit common features, fusion materials research is broader. The harder mono-energetic spectrum associated with the deuterium–tritium fusion neutrons (14.1 MeV compared to <2 MeV on average for fission neutrons) releases significant amounts of hydrogen and helium as transmutation products that might lead to a (at present undetermined) degradation of structural materials after a few years of operation. Overcoming the historical lack of a fusion-relevant neutron source for materials testing is an essential pending step in fusion roadmaps. Structural materials development, together with research on functional materials capable of sustaining unprecedented power densities during plasma operation in a fusion reactor, have been the subject of decades of worldwide research efforts underpinning the present maturity of the fusion materials research programme. For achieving proper safety and efficiency of future fusion power plants, low-activation materials able to withstand the extreme fusion conditions are needed. Here, the irradiation physics at play and fusion materials research is reviewed.
TL;DR: The first experimental demonstration of optical anti-PT symmetry in a warm atomic-vapour cell was reported in this paper, where fast coherence transport via flying atoms leads to a dissipative coupling between two long-lived atomic spin waves, allowing for the observation of the essential features of anti-parity-time symmetry with unprecedented precision on the phase-transition threshold.
Abstract: The recently developed notion of parity–time (PT) symmetry in optical systems has spawned intriguing prospects. So far, most experimental implementations have been reported in solid-state systems. Here, we report the first experimental demonstration of optical anti-PT symmetry—the counterpart of conventional PT symmetry—in a warm atomic-vapour cell. Rapid coherence transport via flying atoms leads to a dissipative coupling between two long-lived atomic spin waves, allowing for the observation of the essential features of anti-PT symmetry with unprecedented precision on the phase-transition threshold, as well as refractionless light propagation. Moreover, we show that a linear or nonlinear interaction between the two spatially separated beams can be achieved. Our results advance non-Hermitian physics by bridging to the field of atomic, molecular and optical physics, where new phenomena and applications in quantum and nonlinear optics aided by (anti-)PT symmetry could be anticipated. Parity–time symmetry in optics is studied in a warm atomic vapour, where its counterpart, anti-parity–time symmetry, as well as refractionless propagation, can also be observed.
TL;DR: In this paper, the authors explore topological physics, bringing bosonic topological states that equip us with the ability to make perfect photonic devices using imperfect interfaces, which is a key role in the discovery of geometric phase.
Abstract: Optics played a key role in the discovery of geometric phase. It now joins the journey of exploring topological physics, bringing bosonic topological states that equip us with the ability to make perfect photonic devices using imperfect interfaces.
TL;DR: The conformal bootstrap was proposed in the 1970s as a strategy for calculating the properties of second-order phase transitions in higher dimensions as discussed by the authors, but little progress was made on higher dimensions until a recent renaissance beginning in 2008.
Abstract: The conformal bootstrap was proposed in the 1970s as a strategy for calculating the properties of second-order phase transitions. After spectacular success elucidating two-dimensional systems, little progress was made on systems in higher dimensions until a recent renaissance beginning in 2008. We report on some of the main results and ideas from this renaissance, focusing on new determinations of critical exponents and correlation functions in the three-dimensional Ising and O(N) models.
TL;DR: In this article, a unique feature of the band structure has been exploited to achieve similar levels of magnetic damping in a metallic alloy, which is important for a range of applications but is typically insulating.
Abstract: Materials with low magnetic damping are important for a range of applications but are typically insulating, which limits their use. Thanks to a unique feature of the band structure, similar levels of damping can now be achieved in a metallic alloy.
TL;DR: A series of 77Se nuclear magnetic resonance measurements on the electron-doped topological insulator Cu0.3Bi2Se3 reveal a spontaneous breaking of the rotational spin symmetry below its superconducting transition temperature as discussed by the authors.
Abstract: A series of 77Se nuclear magnetic resonance measurements on the electron-doped topological insulator Cu0.3Bi2Se3 reveal a spontaneous breaking of the rotational spin symmetry below its superconducting transition temperature.
TL;DR: A series of transport experiments on lanthanum antimonide reveal a plateau in its resistivity and an extremely large magnetoresistance that are consistent with topologically protected electronic states as mentioned in this paper.
Abstract: A series of transport experiments on lanthanum antimonide reveal a plateau in its resistivity and an extremely large magnetoresistance that are consistent with topologically protected electronic states.
TL;DR: In this article, the detection of a single photon heralds the projection of two remote spins onto a maximally entangled state, which is demonstrated for quantum-dot hole spins, featuring a fast generation rate that could enable quantum technology applications.
Abstract: The detection of a single photon heralds the projection of two remote spins onto a maximally entangled state. This has been demonstrated for quantum-dot hole spins, featuring a fast generation rate that could enable quantum technology applications.