The family of “Kitaev materials” provides an ideal platform to study quantum spin liquids and their neighboring magnetic orders. Motivated by the possibility of a quantum spin liquid ground state in pressurized hyperhoneycomb iridate $\beta \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$, we systematically classify and study symmetric quantum spin liquids on the hyperhoneycomb lattice, using the Abrikosov-fermion representation. Among the 176 symmetric $U\left(1\right)$ spin liquids (and 160 $Z_2$ spin liquids), we identify eight “root” $U\left(1\right)$ spin liquids in proximity to the ground state of the solvable Kitave model on the hyperhonecyomb lattice. These eight states are promising candidates for possible $U\left(1\right)$ spin liquid ground states in pressurized $\beta \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$. We further discuss physical properties of these eight $U\left(1\right)$ spin liquid candidates, and show that they all support nodal-line-shaped spinon Fermi surfaces.

A kinetic equation of nonlocal and noninstantaneous character unifies the achievements of transport in dense quantum gases with the Landau theory of quasiclassical transport in Fermi systems. Large cancellations in the off-shell motion appear, which are usually hidden in non-Markovian behaviors. The remaining corrections are expressed in terms of shifts in space and time that characterize the nonlocality of the scattering process. In this way, it is possible to recast quantum transport into a quasiclassical picture. In addition to the quasiparticle, the balance equations for density, momentum, energy, and entropy also include correlated two-particle contributions beyond the Landau theory. The medium effects on binary collisions are shown to mediate the latent heat, i.e., an energy conversion between correlation and thermal energy. For Maxwellian particles with time-dependent $s$-wave scattering, the correlated parts of the observables are calculated and a sign change of the latent heat is reported at a universal ratio of scattering length to the thermal de Broglie wavelength. This is interpreted as a change from correlational heating to cooling.

We consider a family of spin-$1/2$ models with few-body, SU(2)-invariant Hamiltonians and analytical ground states related to the one-dimensional (1D) Haldane-Shastry wave function. The spins are placed on the surface of a cylinder, and the standard 1D Haldane-Shastry model is obtained by placing the spins with equal spacing in a circle around the cylinder. Here, we show that another interesting family of models with two-body exchange interactions is obtained if we instead place the spins along one or two lines parallel to the cylinder axis, giving rise to chain and ladder models, respectively. We can change the scale along the cylinder axis without changing the radius of the cylinder. This gives us a parameter that controls the ratio between the circumference of the cylinder and all other length scales in the system. We use Monte Carlo simulations and analytical investigations to study how this ratio affects the properties of the models. If the ratio is large, we find that the two legs of the ladder decouple into two chains that are in a critical phase with Haldane-Shastry-like properties. If the ratio is small, the wave function reduces to a product of singlets. In between, we find that the behavior of the correlations and the Renyi entropy depends on the distance considered. For small distances the behavior is critical, and for long distances the correlations decay exponentially and the entropy shows an area law behavior. The distance up to which there is critical behavior gets larger as the ratio increases.

We have measured the dynamic admittance of an interacting coherent capacitor in the quantum Hall regime. Our experiments demonstrate that, in the fully coherent regime, the charge relaxation resistance is universal and independent of the transmission even in the presence of strong charge interactions. Conversely, we observe strong suppression of the electrochemical capacitance, which is related to the density of states of the charge excitations due to strong interactions. Our experiments form the building blocks for the realization of electron quantum optics experiments with strong charge interactions, and they should prove useful for quantum bits in interacting ballistic conductors.

In this Rapid Communication, we construct a large-$N$ exactly solvable model to study the interplay between interaction and topology, by connecting the Sachdev-Ye-Kitaev (SYK) model with constant hopping. The hopping forms a band structure that can exhibit both topologically trivial and nontrivial phases. Starting from a topologically trivial insulator with zero Hall conductance, we show that the interaction can drive a phase transition to a topologically nontrivial insulator with quantized nonzero Hall conductance, and a single gapless Dirac fermion emerges when the interaction is fine tuned to the critical point. The finite temperature effect is also considered, and we show that the topological phase with a stronger interaction is less stable against temperature. Our model provides a concrete example to illustrate the interacting topological phases and phase transitions, and can shed light on similar problems in physical systems.

We explore the possibility of a Berezinskii-Kosterlitz-Thouless-like critical phase for the charge degrees of freedom in the intermediate-temperature regime between the charge-ordered and disordered phases in two-dimensional systems with competing short-range Coulomb repulsion. As the simplest example, we investigate the extended Hubbard model with on-site and nearest-neighbor Coulomb interactions on a triangular lattice at half filling in the atomic limit by using a classical Monte Carlo method, and find a critical phase, characterized by algebraic decay of the charge correlation function, belonging to the universality class of the two-dimensional XY model with a ${\mathbb{Z}}_6$ anisotropy. Based on the results, we discuss possible conditions for the critical phase in materials.

We have investigated the orbital states of the orbital-glassy (short-range orbital ordered) spinel vanadate ${\mathrm{Co}}_{1.21}{\mathrm{V}}_{1.79}{\mathrm{O}}_4$ using x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and subsequent configuration-interaction cluster-model calculation. From the sign of the XMCD spectra, it was found that the spin magnetic moment of the Co ion is aligned parallel to the applied magnetic field and that of the V ion antiparallel to it, consistent with neutron scattering studies. It was revealed that the excess Co ions at the octahedral site take the trivalent low-spin state and induce a random potential to the V sublattice. The orbital magnetic moment of the V ion is small, suggesting that the ordered orbitals mainly consist of real-number orbitals.

The two-dimensional excitons of transition metal dichalcogenide (TMDC) monolayers make these materials extremely promising for optical and optoelectronic applications. When the excitons interact with the electromagnetic field, they will give rise to exciton-polaritons, i.e., modes that propagate in the material plane while being confined in the out-of-plane direction. In this work, we derive the characteristic equations that determine both radiative and polaritonic modes in TMDC monolayers and we analyze the dispersion and decay rate of the modes. The condition for the existence of exciton-polaritons can be described in terms of a strong-coupling regime for the interaction between the exciton and the three-dimensional continuum of free-space electromagnetic modes. We show that the threshold for the strong-coupling regime critically depends on the interplay between nonradiative losses and the dielectric function imbalance at the two sides of the monolayer. Our results illustrate that a fine control of the dielectric function of the embedding media is essential for realizing exciton-polaritons in the strong-coupling regime.

Massless Dirac fermions (MDFs) emerge as quasiparticles in various novel materials such as graphene and topological insulators, and they exhibit several intriguing properties, of which Veselago focusing is an outstanding example with a lot of possible applications. However, up to now Veselago focusing merely occurred in p-n junction devices based on the isotropic MDF, which lacks the tunability needed for realistic applications. Here, motivated by the emergence of novel Dirac materials, we investigate the propagation behaviors of anisotropic MDFs in such a p-n junction structure. By projecting the Hamiltonian of the anisotropic MDF to that of the isotropic MDF and deriving an exact analytical expression for the propagator, precise Veselago focusing is demonstrated without the need for mirror symmetry of the electron source and its focusing image. We show a tunable focusing position that can be used in a device to probe masked atom-scale defects. This study provides an innovative concept to realize Veselago focusing relevant for potential applications, and it paves the way for the design of novel electron optics devices by exploiting the anisotropic MDF.

We report a systematic investigation of dc magnetization and ac susceptibility, as well as anisotropic magnetocaloric effect in bulk ${\mathrm{CrI}}_3$ single crystals. A second-stage magnetic transition was observed just below the Curie temperature $T_c$, indicating a two-step magnetic ordering. The low temperature thermal demagnetization could be well fitted by the spin-wave model rather than the single-particle model, confirming its localized magnetism. The maximum magnetic entropy change $-\mathrm{\Delta }{S}_M^{\text{max}}\sim 5.65$ J ${\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ and the corresponding adiabatic temperature change $\mathrm{\Delta }{T}_{\text{ad}}\sim 2.34$ K are achieved from heat capacity analysis with the magnetic field up to 9 T. Anisotropy of $\mathrm{\Delta }{S}_M(T,H)$ was further investigated by isothermal magnetization, showing that the difference of $-\mathrm{\Delta }{S}_M^{\text{max}}$ between the $ab$ plane and the $c$ axis reaches a maximum value $\sim 1.56$ J ${\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ with the field change of 5 T. With the scaling analysis of $\mathrm{\Delta }{S}_M$, the rescaled $\mathrm{\Delta }{S}_M(T,H)$ curves collapse onto a universal curve, indicating a second-order type of the magnetic transition. Furthermore, the $-\mathrm{\Delta }{S}_M^{\text{max}}$ follows the power law of $H^n$ with $n=0.64\left(1\right)$, and the relative cooling power depends on $H^m$ with $m=1.12\left(1\right)$.

Magnetic skyrmions in multilayers are particularly appealing as next generation memory devices due to their topological compact size, the robustness against external perturbations, the capability of electrical driving and detection, and the compatibility with the existing spintronic technologies. To date, Néel-type skyrmions at room temperature (RT) have been studied mostly in multilayers with easy-axis magnetic anisotropy. Here, we systematically broadened the evolution of magnetic skyrmions with sub-50-nm size in a series of Pt/Co/Ta multilayers where the magnetic anisotropy is tuned continuously from easy axis to easy plane by increasing the ferromagnetic Co layer thickness. The existence of nontrivial skyrmions is identified via the combination of in situ Lorentz transmission electron microscopy (L-TEM) and Hall transport measurements. A high density of magnetic skyrmions over a wide temperature range is observed in the multilayers with easy-plane anisotropy, which will stimulate further exploration for new materials and accelerate the development of skyrmion-based spintronic devices.

We report first-principles study of shot noise and tunneling magnetoresistance in Fe/MgO/Fe- /MgO/Fe double-barrier magnetic tunnel junctions (MTJs). We mainly investigate the effects of disordered interfacial oxygen vacancies and barrier asymmetry on the spin-dependent tunneling. It is found that interchannel scattering induced by interfacial oxygen vacancies can substantially enhance the tunneling conductance of the antiparallel magnetic configurations, and results in the dramatic decrease of tunneling magnetoresistance. Moreover, we find the interfacial disorder scattering favors the sub-Poissonian tunneling process. As a result, Fano factors of symmetric MTJs maintain at around 0.5, or are suppressed, while Fano factors of asymmetric MTJs can all be significantly suppressed, illustrating the important correlations in tunneling induced by interfacial disorders. Interchannel scattering induced by interfacial oxygen vacancies can effectively couple the electron to high-transmission channels, enhancing the transmission and reducing the shot noise. In comparison with interfacial disorder, middle-layer disordered Fe vacancies present limited modulation on the Fano factor. Increasing the asymmetry of barriers can quickly decrease high-transmission channels, and make the tunneling process Poissonian in double-barrier MTJs.

Tetradymite-type topological insulator Sn-doped $\mathrm{B}{\mathrm{i}}_{1.1}\mathrm{S}{\mathrm{b}}_{0.9}\mathrm{T}{\mathrm{e}}_2\mathrm{S}$ (Sn-BSTS), with a surface state Dirac point energy well isolated from the bulk valence and conduction bands, is an ideal platform for studying the topological transport phenomena. Here, we present high-pressure transport studies on single-crystal Sn-BSTS, combined with Raman scattering and synchrotron x-ray diffraction measurements. Over the studied pressure range of 0.7–37.2 GPa, three critical pressure points can be observed: (i) At $\sim 9$ GPa, a pressure-induced topological insulator-to-metal transition is revealed due to closure of the bulk band gap, which is accompanied by changes in slope of the Raman frequencies and a minimum in $c$/$a$ within the pristine rhombohedral structure ($R\text{−}3m$); (ii) at $\sim 13$ GPa, superconductivity is observed to emerge, along with the $R\text{−}3m$ to a $C2$/$c$ (monoclinic) structural transition; (iii) at $\sim 24$ GPa, the superconducting transition onset temperature $T_{\mathrm{C}}$ reaches a maximum of $\sim 12\mathrm{K}$, accompanied by a second structural transition from the $C2$/$c$ to a body-centered cubic $Im\text{−}3m$ phase.

We propose a computational methodology capable of harvesting isothermal heat and entropy change in molecular dynamics simulations. The methodology is applied to study high-frequency dynamics of the electrocaloric effect (ECE) in ferroelectric ${\mathrm{PbTiO}}_3$. ECE is associated with a reversible change in temperature under adiabatic application of electric field or with a reversible change in entropy under isothermal application of the electric field. Accurate assessment of electrocaloric performance requires the knowledge of three quantities: isothermal heat, isothermal entropy change, and adiabatic temperature change. Our methodology allows computations of all these quantities directly, that is, without restoring to the reversible thermodynamical models. Consequently, it captures both reversible and irreversible effects, which is critical for ECE simulations. The approach is well suited to address the dynamics of the ECE, which so far remains underexplored. We report the following basic features of the intrinsic dynamics of ECE: (i) the ECE is independent of the electric field frequency, rate of application, or field profile; (ii) the effect persists up to the frequencies associated with the onset of dielectric losses and deteriorates from there due to the creation of irreversible entropy; and (iii) in the vicinity of the phase transition and in the paraelectric phase the onset of irreversible dynamics occurs at lower frequency as compared to the ferroelectric phase. The latter is attributed to lower intrinsic soft-mode frequencies and and larger losses in the paraelectric phase.

We study a system of fermions in one spatial dimension with linearly confining interactions and short-range disorder. We focus on the zero-temperature properties of this system, which we characterize using bosonization and the Gaussian variational method. We compute the static compressibility and ac conductivity, and thereby demonstrate that the system is incompressible, but exhibits gapless optical conductivity. This corresponds to a “Mott glass” state, distinct from an Anderson and a fully gapped Mott insulator, arising due to the interplay of disorder and charge confinement. We argue that this Mott glass phenomenology should persist to nonzero temperatures.

We report on the impact of the Dzyaloshinskii-Moriya interaction on the coherent backscattering of spin waves in a disordered magnetic material. This interaction breaks the inversion symmetry of the spin-wave dispersion relation, such that ${\omega }_{\mathbf{k}}={\omega }_{2{\mathbf{K}}^{\mathrm{I}}-\mathbf{k}}\ne {\omega }_{-\mathbf{k}}$, where ${\mathbf{K}}^{\mathrm{I}}$ is related to the Dzyaloshinskii-Moriya vectors. The nonequivalence of $\mathbf{k}$ and $-\mathbf{k}$ also means that time-reversal symmetry is broken. As a result of numerical investigations we find that the backscattering peak of a wave packet with initial wave vector ${\mathbf{k}}_0$ shifts from $-{\mathbf{k}}_0$ to $2{\mathbf{K}}^{\mathrm{I}}-{\mathbf{k}}_0$, such that the backscattering wave vector and the initial wave vector are in general no longer antiparallel. The shifted coherence condition is explained by a diagrammatic approach and opens up an avenue to measure sign and magnitude of the Dzyaloshinskii-Moriya interaction in weakly disordered chiral magnets. Surprisingly, although time-reversal symmetry is broken, our system shows coherent backscattering as a manifestation of weak localization, which is due to the fact that reciprocity is still preserved.

We report the ${}^{207}\mathrm{Pb}$ nuclear magnetic resonance (NMR) measurements on polycrystalline sample of ${\mathrm{PbTaSe}}_2$ with noncentrosymmetric crystal structure and topological electronic band. The nuclear spin-lattice relaxation rate $1/T_1$ shows a suppressed coherence peak below the superconducting transition temperature $T_{\mathrm{c}}=4.05\mathrm{K}$ and decreases as an exponential function of temperature. The penetration depth derived from the NMR spectrum is almost temperature independent below $T=0.7$ $T_{\mathrm{c}}$. The Knight shift $K$ decreases below $T_{\mathrm{c}}$. These results suggest spin-singlet superconductivity with a fully opened gap $2\mathrm{\Delta }=3.5k_{\mathrm{B}}{T}_{\mathrm{c}}$ in ${\mathrm{PbTaSe}}_2$.

Recently, HfSiS was found to be a new type of Dirac semimetal with a line of Dirac nodes in the band structure. Meanwhile, Rashba-split surface states are also pronounced in this compound. Here we report a systematic study of HfSiS by scanning tunneling microscopy/spectroscopy at low temperature and high magnetic field. The Rashba-split surface states are characterized by measuring Landau quantization and standing waves, which reveal a quasilinear dispersive band structure. First-principles calculations based on density-functional theory are conducted and compared with the experimental results. Based on these investigations, the properties of the Rashba-split surface states and their interplay with defects and collective modes are discussed.

Geometrical and topological properties of quantum ground state manifolds permit to characterize phases of matter and identify phase transitions. Here, we study the effect of a quantum dissipative environment on the geometrical properties of the ground state manifold of a single spin 1/2 in an external effective magnetic field. We show that the quantum phase transition at zero temperature in this model is associated with a universal metric singularity related to the divergence of the spin susceptibility. The absence of transition at finite temperature corresponds to a smooth variation of the associated metric with temperature, without singular points.

$\alpha$-Sn is well known as a typical Luttinger semimetal with a quadratic band touching at the $\mathrm{\Gamma }$ point. Based on the effective $k·p$ analysis as well as first-principles calculations, we demonstrate that multiple topological phases with a rich diagram, including topological insulator, Dirac semimetal, and Weyl semimetal phases, can be induced and engineered in $\alpha$-Sn by external strains, magnetic fields, and circularly polarized light (CPL). Intriguingly, not only the conventional type-I Weyl nodes but also type-II Weyl nodes and double-Weyl nodes can be generated directly from the quadratic semimetal by applying a magnetic field or CPL. Our results apply equally well to other Luttinger semimetals with similar crystal and electronic structures, and thus open an avenue for realizing and engineering multiple topological phases on a versatile platform.