In Weyl semimetals, the chiral charge is not conserved in the presence of external nonorthogonal magnetic and electric fields; this chiral anomaly manifests itself in a negative longitudinal magnetoresistance. In this paper, we report on detailed calculations of transport properties of type-I Weyl semimetals with broken time-reversal and broken inversion symmetries, respectively, within a semiclassical Boltzmann approach. The use of Fermi surface harmonics provides a comprehensive and closed solution of the Boltzmann equation including the influence of the Lorentz force as well as $\mathbf{k}$-dependent scattering-out and scattering-in terms. Respecting a modified phase space volume, we identify additional contributions to the charge conductivity, which scale linearly with the magnetic field and can change the sign of the magnetoresistance in systems with broken inversion symmetry. Considering the scattering properties, the energy dependence of the chiral anomaly-related contribution to the charge conductivity is more pronounced than usually discussed. On top of this, we show for the Weyl semimetal TaAs that a misalignment of an applied magnetic field with the crystal axes can destroy the negative longitudinal magnetoresistance.

Excitonic insulator (EI) was proposed in the 1960s as a distinct insulating state originating from pure electronic interaction, but its material realization has been elusive with extremely few material candidates and with only limited evidence such as anomalies in transport properties, band dispersions, or optical transitions. We investigate the real-space electronic states of the low-temperature phase in ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ with atomic resolution to clearly identify the quasiparticle energy gap together with the strong electron-hole band renormalization using scanning tunneling microscopy and spectroscopy (STS). These results are in good agreement with the EI transition scenario in ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$. Our spatially resolved STS data and theoretical calculations reveal further the orbital inversion at band edges, which indicates the exciton condensation close to the Bardeen-Cooper-Schrieffer regime.

Dirac materials have unique transport properties, partly due to the presence of surface states. A new type of two-dimensional Dirac material, protected by nonsymmorphic symmetries, was recently proposed by S. M. Young and C. L. Kane [Phys. Rev. Lett. 115, 126803 (2015)]. By breaking of time-reversal or inversion symmetry one can split the Dirac cones into Weyl nodes. The latter are characterized by local Chern numbers, which makes them two-dimensional analogs of Weyl semimetals. We find that the formation of the Weyl nodes is accompanied by an emergence of one-dimensional surface states, similar to Fermi arcs in Weyl semimetals and edge states in two-dimensional graphene. We explore these states for a quasi-one-dimensional nonsymmorphic ribbon. The type and strength of applied deformation control the location and Weyl nodes and their composition. This determines the properties of emerging edge states. The sensitivity of these edge states to the external deformations makes nonsymmorphic materials potentially useful as a new type of electromechanical sensor.

The metal-insulator transition (MIT) is an exceptional test bed for studying strong electron correlations in two dimensions in the presence of disorder. In the present study, it is found that in contrast to previous experiments on lower mobility samples, in ultrahigh mobility SiGe/Si/SiGe quantum wells the critical electron density $n_{\text{c}}$ of the MIT becomes smaller than the density $n_{\text{m}}$, where the effective mass at the Fermi level tends to diverge. Near the topological phase transition expected at $n_{\text{m}}$, the metallic temperature dependence of the resistance should be strengthened, which is consistent with the experimental observation of more than an order of magnitude resistance drop with decreasing temperature below $\sim 1$ K.

The localized-to-itinerant transition of $f$ electrons lies at the heart of heavy-fermion physics, but has only been directly observed in single-layer Ce-based materials. Here, we report a comprehensive study on the electronic structure and nature of the Ce $4f$ electrons in the heavy-fermion superconductor ${\mathrm{Ce}}_2{\mathrm{PdIn}}_8$, a typical $n=2 {\mathrm{Ce}}_n{\mathrm{M}}_m{\mathrm{In}}_{3n+2m}$ compound, using high-resolution and $4d-4f$ resonant photoemission spectroscopies. The electronic structure of this material has been studied over a wide temperature range, and hybridization between $f$ and conduction electrons can be clearly observed to form a Kondo resonance near the Fermi level at low temperatures. The characteristic temperature of the localized-to-itinerant transition is around 120 K, which is much higher than its coherence temperature $T_{\text{coh}}\sim 30\mathrm{K}$.

The Anderson metal-insulator transition (MIT) is central to our understanding of the quantum mechanical nature of disordered materials. Despite extensive efforts by theory and experiment, there is still no agreement on the value of the critical exponent $\nu$ describing the universality of the transition—the so-called “exponent puzzle.” In this Rapid Communication, going beyond the standard Anderson model, we employ ab initio methods to study the MIT in a realistic model of a doped semiconductor. We use linear-scaling density functional theory to simulate prototypes of sulfur-doped silicon (Si:S). From these we build larger tight-binding models close to the critical concentration of the MIT. When the dopant concentration is increased, an impurity band forms and eventually delocalizes. We characterize the MIT via multifractal finite-size scaling, obtaining the phase diagram and estimates of $\nu$. Our results suggest an explanation of the long-standing exponent puzzle, which we link to the hybridization of conduction and impurity bands.

We consider a bilayer system of two-dimensional Bose-Einstein-condensed dipolar dark excitons (upper layer) and bright ones (bottom layer). We demonstrate that the interlayer interaction leads to a mixing between excitations from different layers. This mixing leads to the appearance of a second spectral branch in the spectrum of bright condensate. The excitation spectrum of the condensate of dark dipolar excitons then becomes optically accessible during luminescence spectra measurements of the bright condensate, which allows one to probe its kinetic properties. This approach is relevant for experimental setups, where detection via conventional techniques remains challenging; in particular, the suggested method is useful for studying dark dipolar excitons in transition metal dichalcogenide monolayers.

The intrinsic peaks due to the topological transition of the Fermi surface shape, which appear in the resistivity as a function of carrier density and perpendicular electric field, were studied in AB-stacked six-layer graphene as a system with three bilayerlike bands. By the Landau level structure measured in low-temperature magnetotransport experiments, and by band structure calculations, it is shown that the ridge structures correspond one to one with the characteristic positions in the dispersion relations. Some ridges originated from electronic states near the bottoms of the bilayerlike bands that appear at the carrier density of the zero-mode Landau levels. Some ridges originated from the mini–Dirac cones formed in the perpendicular electric field because of hybridization of bilayerlike bands.

Berry curvature in Weyl semimetals leads to intriguing magnetoconductivity and magnetothermal transport properties. Here, we explore the impact of the tilting of the Weyl nodes on the magnetoconductivity of type-I and type-II Weyl semimetals using the Berry-curvature-connected Boltzmann transport formalism. We find that in addition to the quadratic magnetic field ($B$) corrections induced by the tilt, there are also anisotropic and $B$-linear corrections in several elements of the conductivity matrix. For the case of magnetic field applied perpendicular to the tilt direction, we show the existence of previously unexplored $B$-linear transverse conductivity components. For the other case of magnetic field applied parallel to the tilt axis, the $B$-linear corrections appear in the longitudinal conductivity, giving rise to anisotropic magnetoresistance measurements. Our systematic analysis of the full magnetoconductivity matrix predicts several specific experimental signatures related to the tilting of the Weyl nodes in both type-I and type-II Weyl semimetals.

A higher-order topological insulator is a new concept of topological states of matter, which is characterized by the emergent boundary states whose dimensionality is lower by more than two compared with that of the bulk, and draws a considerable interest. Yet, its robustness against disorders is still unclear. In this work, we investigate a phase diagram of higher-order topological insulator phases in a breathing kagome model in the presence of disorders by using a state-of-the-art machine learning technique. We find that the corner states survive against the finite strength of disorder potential as long as the energy gap is not closed, indicating the stability of the higher-order topological phases against the disorders.

Inelastic x-ray scattering and thermal diffuse scattering experiments on calcite, dolomite, and magnesite show an anomalous dispersion of the transverse acoustic $\mathrm{\Gamma }\text{−}F$ branch in calcite and dolomite. We show that the high-pressure transition from calcite to calcite-II is associated with a freezing of the transverse acoustic phonon at the $F$ point of the Brillouin zone. We also observe a softening of the transverse acoustic, $F$-point phonon in dolomite, and extrapolation of our experimental data is consistent with the presence of a phase transition to dolomite-II at 17 GPa. No anomaly of the $\mathrm{\Gamma }\text{−}F$ branch was observed in magnesite. We have performed density functional theory based calculations to obtain the displacement pattern of the soft phonon, which allows us to conclude that both phase transitions from calcite to calcite-II and from dolomite to dolomite-II are driven by the soft phonon at the $F$ point of the Brillouin zone.

We introduce a clean cluster spin chain coupled to fully interacting spinless fermions, forming an unconstrained ${\mathbb{Z}}_2$ lattice gauge theory (LGT), which possesses dynamical proximity effect controlled by the entanglement structure of the initial state. We expand the machinery of interaction-driven localization to the realm of LGTs such that for any starting product state, the matter fields exhibit emergent statistical bubble localization, which is driven solely by the cluster interaction, having no topologically trivial noninteracting counterpart, and thus is of a pure dynamical many-body effect. In this vein, our proposed setting provides possibly the minimal model dropping all the conventional assumptions regarding the existence of many-body localization. Through projective measurement of local constituting species, we also identify the coexistence of the disentangled nonergodic matter and thermalized gauge degrees of freedom, which stands completely beyond the standard established phenomenology of quantum disentangled liquids. As a byproduct of self-localization of the proximate fermions, the spin subsystem hosts the long-lived topological edge zero modes, which are dynamically decoupled from the thermalized background ${\mathbb{Z}}_2$ charges of the bulk, and hence remains cold at arbitrary high-energy density. This provides a convenient platform for strong protection of the quantum bits of information, which are embedded at the edges of completely ergodic subsystem; the phenomenon that in the absence of such proximity-induced self-localization could, at best, come about with a prethermal manner in translational invariant systems. Finally, by breaking local ${\mathbb{Z}}_2$ symmetry of the model, we argue that such admixture of particles no longer remains disentangled and the ergodic gauge degrees of freedom act as a “small bath” coupled to the localized components.

In the cooling concept by adiabatic melting, solid ${}^4\mathrm{He}$ is converted to liquid and mixed with ${}^3\mathrm{He}$ to produce cooling power directly in the liquid phase. This method overcomes the thermal boundary resistance that conventionally limits the lowest available temperatures in the helium fluids and hence makes it possible to reach for the temperatures significantly below $100\mu \mathrm{K}$. In this paper we focus on the thermodynamics of the melting process, and examine the factors affecting the lowest temperatures achievable. We show that the amount of ${}^3\mathrm{He}-{}^4\mathrm{He}$ mixture in the initial state, before the melting, can substantially lift the final temperature, as its normal Fermi fluid entropy will remain relatively large compared to the entropy of superfluid ${}^3\mathrm{He}$. We present the collection of formulas and parameters to work out the thermodynamics of the process at very low temperatures, study the heat capacity and entropy of the system with different liquid ${}^3\mathrm{He}$, mixture, and solid ${}^4\mathrm{He}$ contents, and use them to estimate the lowest temperatures achievable by the melting process, as well as compare our calculations to the experimental saturated ${}^3\mathrm{He}-{}^4\mathrm{He}$ mixture crystallization pressure data. Realistic expectations in the execution of the actual experiment are considered. Further, we study the cooling power of the process, and find the coefficient connecting the melting rate of solid ${}^4\mathrm{He}$ to the dilution rate of ${}^3\mathrm{He}$.

We observe a broadband chaotic signal of terahertz frequency emitted from a superconducting junction. The generated radiation has a wide spectrum reaching 0.7 THz and power sufficient to drive on-chip circuit elements. Our experimental finding is fully confirmed by numerical modeling based on microscopic theory and reveals the unrealized potential of superconducting systems in chaos-based terahertz communication, fast generation of true random numbers, and noninvasive terahertz spectroscopy.

We investigate the dynamics of exchange-coupled ferromagnetic films in the nonlinear limit and find a number of interesting features. The introduction of asymmetries in the magnetic driving field can produce a power-dependent localization of the magnetization oscillations, in one film or the other, by way of the optic mode frequency shifting in the large field limit. A nonlinear mixing of the acoustic and optic modes in the antiferromagnetically coupled system leads to quasiperiodic and multiperiodic composite modes. Numerical ferromagnetic resonance experiments will show additional absorption peaks (in the 20–50 GHz range) which have a rapid increase in strength with input power. The results are calculated through linearization techniques and numerical solutions of the Landau-Lifshitz-Gilbert equation. The nonlinear (and linear) behavior is illustrated through Poincaré section bifurcation diagrams.

Understanding interfacial spin transport is key to developing magnetoelectonic devices, however, the exact nature of the parameters involved is unclear. Here, we report a detailed ferromagnetic resonance-based spin-transport analysis on a variety of structures of both ferromagnetic (Co, CoFeB) and heavy metal layers (Pt, Ru) in order to fully quantify the interfacial spin-transport parameters. Enhanced spin-mixing conductance is observed for more closely matched ferromagnet and heavy metal crystal structures, and, significantly, the inclusion of a thickness-dependent spin-diffusion length gives a bulk value of $9.4\pm 0.7$ nm for Pt, resolving reported discrepancies.

We explore the effects of electron doping in lanthanum ferrite, ${\mathrm{LaFeO}}_3$ by doping Mo at the Fe sites. Based on magnetic, transport, scanning tunneling spectroscopy, and x-ray photoelectron spectroscopy measurements, we find that the large gap, charge-transfer, antiferromagnetic (AFM) insulator ${\mathrm{LaFeO}}_3$ becomes a small gap AFM band insulator at low Mo doping. With increasing doping concentration, Mo states, which appear around the Fermi level, is broadened and become gapless at a critical doping of 20%. Using a combination of calculations based on density functional theory plus Hubbard $U$ (DFT+U) and x-ray absorption spectroscopy measurements, we find that the system shows charge disproportionation (CD) in Fe ions at 25% Mo doping, where two distinct Fe sites, having ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ nominal charge states appear. A local breathing-type lattice distortion induces the charge disproportionation at the Fe site without destroying the antiferromagnetic order. Our combined experimental and theoretical investigations establish that the Fe states form a CD antiferromagnet at 25% Mo doping, which remains insulating, while the appearance of Mo states around the Fermi level is showing an indication towards the insulator-metal transition.

We propose a path for constructing diverse interacting spin systems from topological nanowires in Cooper-pair boxes. The wires are grouped into a three-wire building block called a “hexon,” consisting of six Majorana zero modes. In the presence of a strong charging energy, the hexon becomes a Cooper-pair box equivalent to two spin-$1/2$ degrees of freedom. By considering arrays of hexons and controlling the distances between the various wires, one can tune the Hamiltonian governing the low-energy spins, thus providing a route for controllably constructing interacting spin systems in one and two dimensions. We explicitly present realizations of the one-dimensional spin-$1/2$ XXZ chain, as well as the transverse field Ising model. We propose an experiment capable of revealing the nature of critical points in such effective spin systems by applying a local gate voltage and measuring the induced charge at a distance. To demonstrate the applicability of this approach to two dimensions, we provide a scheme for realizing the topologically ordered Yao-Kivelson spin-liquid model, which has a collective Majorana edge mode, similar to the $B$ phase of Kitaev's honeycomb model.