In the past decade, topological materials have been continuously attracting the interest of the condensed-matter physics community because of their unique band structures and transport properties. Recently, ZrTe${}_5$ is becoming a promising platform to study topological phase transitions, as it could possibly be a 3D Dirac semimetal, a 3D weak topological insulator (TI), or a 3D strong TI, which are distinguished by whether there is a finite band gap and whether there is a topological surface state (TSS). This paper performs a systematic high-momentum-resolution photoemission study on ZrTe${}_5$ using 6 eV photon energy. The conduction and valence bands near $\mathrm{\Gamma }$ are measured with a gap between 18 and 29 meV. The gap size is smaller than former studies. This work also examines the spectral difference at different photon energies, which is attributed to final-state effects and the 3D nature of the band structure. By doing so, the authors confirm the gap is not due to some specific out-of-plane momentum, and conclude that there is no TSS. The final-state interpretation also reconciles the discrepancies of previous studies regarding the existence of the TSS. Hence, the results are consistent with ZrTe${}_5$ being a 3D weak topological insulator.

Here, the authors investigate the possibility of building up nonequilibrium spin accumulation in a ferromagnetic thin film composed of a metallic alloy of nickel and iron, commonly known as permalloy. Analogously to thermoelectric generators, a temperature gradient in a ferromagnet is known to generate a distribution of accumulated electron spins over macroscopic distances, which may be used as an efficient source of spin-polarized electrons in future spintronic devices. To enable the detection and quantitative determination of this imbalance of electron spins, the authors have developed an entirely optical technique, based on the magneto-optical Kerr effect, using a Sagnac interferometer microscope with a spatial resolution of one micrometer and an absolute polarization angle resolution of ten nanoradians. This new technique is not only robust against electrical artifacts, but allows the user to measure previously inaccessible experimental geometries where the magnetization is not restricted to lying in the plane of the film.

Here, the authors argue that the recent experimental discovery of Majorana zero modes in topological superconductors may be the key to realizing an elusive state of matter, the odd-frequency superconductor, in which Cooper pairing is entirely dynamical in nature. Via an exactly soluble model, it is shown that when Majorana zero modes couple to a spin-polarized metallic system, equal-spin $s$-wave odd-frequency pairing is generically induced in the spin-polarized metal. The key properties of odd-frequency superconductivity, such as a vanishing static superconducting order parameter and a paramagnetic Meissner effect, are demonstrated in this system by explicit calculations.

Due to its highly tunable unconventional superconductivity, FeSe is one of the most intriguing materials among the iron-based superconductors. In addition, it exhibits a tetragonal to orthorhombic “nematic” phase transition in the absence of static magnetic order. This article presents a set of tight-binding parameters, optimized directly with respect to experimental angle-resolved photoemission spectroscopy (ARPES) data obtained at 100 K. It thus provides a quantitatively accurate model of the electronic structure of the tetragonal phase of FeSe and, hence, a coherent foundation for the understanding of the unique physical properties of this system. The authors go on to predict a feature that has so far remained unnoticed in simpler DFT-based tight-binding-model analyses, namely, a significant temperature dependence of the chemical potential in FeSe. This is confirmed by performing ARPES on FeSe single crystals, whereby a 25-meV rigid chemical potential shift is detected across the entire Brillouin zone over the temperature range between 100 K and 300 K. The finding has important implications for any future theoretical models of nematicity and superconductivity in FeSe and related materials.

Antiferromagnets are rapidly gaining importance as crucial ingredients in many applications. They are abundant in nature and they are robust against externally applied fields. Rather usefully, their spin resonances often lie within the THz regime, which makes them ideal candidates for optical studies. In this paper, the authors present a thorough experimental and theoretical exploration of the optical spin excitation in antiferromagnetic NiO. Based on a phenomenological theory, they derive expressions for the optically induced magnetization via the inverse Faraday effect and the inverse Cotton-Mouton effect. Light polarization is conserved by pumping and probing along the optical axis of the material, facilitating the comparison between theory and experiment. Those agree amazingly well, making possible the identification of the driving mechanism behind the ultrafast magnon excitations. Moreover, the authors succeed in obtaining information about the otherwise elusive spin-domain distribution in NiO and in showing that the energy transfer into the magnon mode is about three orders of magnitude more efficient via the inverse Cotton-Mouton effect than via the inverse Faraday effect.

Tunneling of an electron from one composite fermion liquid into another in a bilayer system offers a unique spectroscopic probe into the short-distance high-energy physics of this highly nontrivial strongly correlated state. The authors identify the interlayer exciton responsible for the maximum current, and find excellent quantitative agreement with the experimentally measured energy as well as its dependence on an in-plane magnetic field. They also predict that the spin-polarization transitions of the fractional quantum Hall states as a function of the Zeeman energy will be marked by discontinuous jumps in the energy of this exciton.

The topological phase of a one-dimensional triplet superconductor support fractionalized edge states described by Majorana fermions. Generically, these edge states are not completely decoupled. Their wave functions decay exponentially, and the tails induce a coupling that decays exponentially with the length of the system. This is not the whole story, however. In an appropriate parameter range (if the pairing is smaller than the hopping in the simplest model, the Kitaev model), the Majorana wave functions oscillate on top of decaying, and these oscillations induce exact zero modes, at which the Majorana edge states are rigorously decoupled. In the present paper, motivated by the observation of ground-state level crossings in chains of cobalt adatoms, which, up to a Jordan-Wigner transformation, are described by the same model, the authors explicitly calculate the Majorana edge-state wave functions, and they fully characterize the number and positions of the level crossings induced by the wave-function oscillations.

Chiral magnetism plays an increasingly important role in condensed matter investigations driven by the discovery of exotic spin textures, such as the topologically protected chiral skyrmions that can form a lattice under magnetic fields. The nature and universality of the phase diagram in cubic chiral magnets has been the subject of numerous experimental and theoretical investigations, including the transition to the helimagnetic state. In MnSi, the archetypal system in this family, this transition is of first order and involves a precursor phase, where strong chiral fluctuating correlations build up. This work presents an experimental investigation of the structural and dynamical aspects of the phase transition in Fe${}_{1-x}$Co${}_x$Si, a system that belongs to the same family as MnSi but with the additional possibility to tune important physical interactions and parameters through variation of the Fe and Co concentration. In this system, the combination of small-angle neutron scattering and neutron spin echo spectroscopy uncovers that the scenario of the transition is qualitatively very different from that in MnSi. This goes beyond what can be expected from a comparison of the relevant length scales and thus challenges the validity of a universal approach to the helimagnetic transition in cubic chiral magnets.

The strong spin-spin exchange interaction in some low-dimensional magnetic materials can give rise to a high group velocity and thermal conductivity contribution from magnons, which are energy quanta of collective spin excitations. Examples are the incommensurate layered compounds (Sr,Ca,La)${}_{14}$Cu${}_{24}$O${}_{41}$, with strong antiferromagnetic interaction along the ladders. The effects of grain boundaries and defects on quasi-one-dimensional magnon transport in these compounds are not well understood. Here, the authors report the microstructures and anisotropic thermal transport properties of textured Sr${}_{14}$Cu${}_{24}$O${}_{41}$. TEM clearly reveals nanolayered grains and the presence of dislocations and planar defects. The thermal conductivity contribution and mean free paths of magnons in the textured samples are evaluated with the use of a kinetic model for one-dimensional magnon transport, and found to be suppressed significantly compared to single crystals at low temperatures. The experimental results can be explained by a one-dimensional magnon-defect scattering model, provided that the magnon–grain boundary scattering mean free path in the anisotropic magnetic structure is smaller than the average length of these nanolayers along the $c$ axis. The finding suggests low transmission coefficients for energy-carrying magnons across grain boundaries.

Recently, it was suggested that metastable spin supercurrents (spin superfluidity) are possible in the magnon condensate observed in yttrium iron garnet (YIG) magnetic films under strong magnon pumping. In YIG, magnetic anisotropy is rather weak, and this material can be treated as an isotropic ferromagnet. Its order-parameter space is a sphere of radius equal to the spontaneous magnetization $M$. A current state maps on the equatorial circumference on the sphere, and topology of the sphere allows us to continuously transform the equatorial circumference to the point. This rules out metastable current states. Only easy-plane anisotropy reducing the order-parameter space from the sphere to the equatorial circumference makes current states topologically stable. However, by pumping magnons to the isotropic ferromagnet in a magnetic field, it is possible to support a strongly nonequilibrium state with fixed average $M_z$. This confines the precessing spin to an “easy plane” of dynamical rather than topological origin and makes metastable spin currents possible. But final judgment requires evaluation of the Landau superfluidity criterion as done in the present work. The conclusion is that spin superfluidity in YIG films is possible in principle, although the recently published claim of its observation is not justified.

The tenfold way classification of noninteracting topological insulators represents a great success towards understanding topological states of matter. The question the authors try to address is: can all possible topological insulators be represented by the prototypes in the tenfold way classification (perhaps enriched with some extra symmetries), or are there exceptions fundamentally different from those prototypes? The Hopf insulator was considered as a special type of TI that is not obviously represented by any prototype in the tenfold way classification, but its stability and classification were never clarified. In this work, the authors identify a generalized particle-hole symmetry that gives the Hopf insulator and its higher-dimensional analog a rigorous definition and classification. Moreover, they provide a very heuristic understanding of the minimal models for the 3$d$ and 4$d$ Hopf insulators, based on which possible experimental realization of a Hopf insulator can be found.

Recently, there has been a surge of interest in effects arising from the interplay between the electron-electron interaction and spin-orbit coupling (SOC). One such effect is a novel type of collective spin excitations (chiral spin waves), which are oscillations of the magnetization that exist even without an external magnetic field. However, a typical experimental setup in a semiconductor heterostructure includes a magnetic field as well as both Rashba and Dresselhaus types of SOC. In this case, the spectrum of chiral spin waves becomes fairly complicated, with some branches running into or splitting off the continuum of spin-flip excitations. The authors show that this complicated physics can be understood by exact mapping of the quantum kinetic equation for a two-dimensional Fermi liquid onto an effective one-dimensional tight-binding model. This mapping is a useful tool that helps us to understand the nature of the collective modes in a Fermi liquid of arbitrary type.

Experiments with ultracold fermionic atoms in optical lattices have attracted much interest in condensed matter physics as they serve as a model for electrons in solids. While experiments now allow the observation of nonequilibrium transport processes with single-site resolution, the theoretical description, especially for strongly correlated fermions, remains very challenging. Standard methods, such as the density matrix renormalization group (DMRG) and nonequilibrium Green functions (NEGF), have been primarily applied to one-dimensional systems and weak to moderate coupling, respectively. Here, the authors perform a detailed comparison of DMRG and NEGF and demonstrate that both can be benchmarked against each other. They observe complementary applicability ranges suggesting that a combination of both will allow to substantially expand the range and duration of first-principles quantum dynamics simulations for fermionic lattice systems.

This paper reports on the observation of the magnetic quantum ratchet effect in (Cd,Mn)Te- and CdTe-based quantum well structures with an asymmetric lateral dual grating gate superlattice subjected to an external magnetic field ($B$), applied normal to the quantum well plane. An electric current excited by terahertz laser radiation shows 1/$B$-periodic oscillations with amplitude much larger than the photocurrent at zero magnetic field. It is shown that the photocurrent generation is due to the combined action of a spatially periodic lateral potential and the spatially modulated radiation due to the near-field effects of light diffraction. The magnitude and direction of the photocurrent are determined by the degree of the lateral asymmetry controlled by the variation of voltages applied to the individual gates. The observed magneto-oscillations with enhanced photocurrent amplitude result from Landau quantization. For (Cd,Mn)Te-based structures at low temperatures, a beating-like pattern of the oscillations is observed, caused by the interplay of Zeeman and Landau splitting. A theoretical analysis, considering the magnetic quantum ratchet effect in the framework of a semiclassical approach, describes quite well the experimental results.

Among various superconductors, the series of the layered organic superconductors, $\lambda$-(BETS)${}_2$Fe${}_{1-x}$Ga${}_x$Cl${}_4$, where BETS=bis(ethylenedithio)tetraselenafulvalene, is known to show very unique phase diagrams because of the exchange interaction between the conduction electrons in the BETS layer and the localized 3$d$ spins of the Fe ions in the insulating layers. The large internal field ($H_{\text{int}}$) by the 3$d$ spins is theoretically predicted to induce a peculiar superconducting phase diagram, where a paramagnetic phase with unconventional vortices appears. By systematic measurements of the magnetic torque and resistance, the authors determine the magnetic phase diagram for $x$=0.37 for an in-plane field, which has a maximum $T_c$ at 14 T (=$H_{\text{int}}$). They also find anomalously large energy dissipation due to Josephson vortex dynamics for $H$=$H_{\text{int}}$, in which microscopic distributions of the paramagnetic and super-currents play an essential role. These results will help develop a new field in vortex matter physics.

Graphene, a two-dimensional monatomic layer of crystal carbon, has recently emerged as a potential material for next-generation optoelectronic devices owing to unique properties arising from its massless Dirac fermions. However, the intrinsic carrier dynamics of graphene has remained a mystery even for the simple case of carrier cooling after the photoexcitation. This is because the observed temporal variations of the nonequilibrium carriers in graphene have been thoroughly described in terms of a defect-induced extrinsic effect known as “supercollision” (SC). The SC process is based on defect-mediated electron-acoustic phonon scattering and theoretically has been predicted to reduce with the increase of mobility of a material. Here, the authors have prepared extremely high mobility graphene and traced the dynamics of photoexcited carriers in the Dirac bands directly by time- and angle-resolved photoemission spectroscopy. They successfully observed suppression of SC and extracted the intrinsic dynamical properties of graphene, such as anharmonic decay of the optical phonons and the bottleneck relaxation at the Dirac point. Breaking the limit of SC, their research also has technological significance in developing graphene-based optoelectronic devices.

Many-body localization plays an increasing role in condensed matter theory, both because it challenges the fundaments of statistical physics, and because it allows us to engineer several new, exotic, stable phases of matter. In this paper, the authors address the issue of the stability of a many-body localized material in contact with an ergodic grain, i.e., an imperfect bath made of a few interacting degrees of freedom. Thanks to detailed microscopic analysis and numerics, they conclude that such an ergodic grain eventually destabilizes the localized phase in the following cases: if the spatial dimension is higher than one, or if the spatial dimension is one but the localization length of the localized material is larger than a fixed threshold value. In realistic materials, these ergodic grains are always present as Griffiths regions where the disorder is anomalously small, and hence, the authors conclude that the localized phase in such materials is unstable, strictly speaking. Transport and thermalization are however exponentially suppressed in the distance between ergodic grains.

Essentially all high temperature superconductors display a “strange metal” regime above their critical temperatures, where electrical and thermal transport cannot be explained within the traditional Fermi liquid paradigm of quasiparticle excitations. This paper focuses on the first, and newly discovered, solvable theory of a disordered metal without quasiparticle excitations, obtained by extending the Sachdev-Ye-Kitaev (SYK) models to finite spatial dimensions. The complete thermoelectric conductivity matrix is computed, and a surprising exact relation is found between the Seebeck coefficient and the derivative of the thermodynamic entropy with respect to the density. These computations are then compared with holographic theories which map strange metals onto black holes in theories of quantum gravity with one extra spatial dimension.

Researchers predict new two-dimensional materials whose structures differ from their three-dimensional counterparts.

Time-periodic driving enables new nonequilibrium quantum phases of matter with topologically protected and quantum coherent motion that would be forbidden in thermal equilibrium. This work uncovers two new classes of driven topological phases in driven two-dimensional spin systems: i) Floquet symmetry protected topological phases, which are driven and interacting analogs of topological insulators whose edge states are protected against disorder and localization; and ii) Floquet enriched topological orders, in which time-dependent driving “breaks” the spins into anyonic particles with fractional statistics and oscillating topological charge. The physics of these phases is explored by constructing toy models that can be explicitly solved despite the presence of strong interactions.