Frustrated magnets, especially the ones with spin-disordered ground state, have raised intensive attention as excellent magnetocaloric materials for achieving sub-Kelvin magnetic cooling. Here, we investigate the ultralow-temperature magnetic behaviors and magnetocaloric effect (MCE) down to 60 mK on a frustrated Shastry-Sutherland lattice magnet $\mathrm{Y}{\mathrm{b}}_2\mathrm{B}{\mathrm{e}}_2\mathrm{Ge}{\mathrm{O}}_7$ crystallized into the tetragonal structure with space group $P\overline{4}{2}_{1}m$. The magnetic susceptibility, isothermal magnetization $M$($B$), and specific heat results reveal the Ising-like magnetic anisotropy with easy magnetization along the [001] axis. No sign of long-range magnetic order or spin freezing is detected down to 50 mK, where magnetic susceptibility and specific heat data can be understood by a spin dimer system with small spin gap ∼0.78 K. Benefiting from the compatibility of high magnetic ion density ($N_{\mathrm{mag}}$) ∼15.6 $\mathrm{n}{\mathrm{m}}^{-3}$ and large Landé factor $g_{\mathrm{c}}\sim 4.88$ ($B$ // [001]) with the disordered ground state, $\mathrm{Y}{\mathrm{b}}_2\mathrm{B}{\mathrm{e}}_2\mathrm{Ge}{\mathrm{O}}_7$ exhibits an excellent magnetocaloric response in sub-Kelvin temperature regimes. From the analysis of magnetic entropy $S$($T, B$) landscape, it can be cooled down to the minimum temperatures of $T_{\min }\sim 95$ mK and $T_{\min }\sim 240$ mK from $T$ = 2 K after the adiabatic demagnetization from initial fields of $B_i=4$ T and $B_i=2$ T, respectively. The attainable $T_{\min }$ and $N_{\mathrm{mag}}$ of $\mathrm{Y}{\mathrm{b}}_2\mathrm{B}{\mathrm{e}}_2\mathrm{Ge}{\mathrm{O}}_7$ outperform most of Yb-based quantum magnets and the recently reported spin supersolid candidate $\mathrm{N}{\mathrm{a}}_2\mathrm{BaCo}{\left(\mathrm{P}{\mathrm{O}}_4\right)}_2$ in the similar working temperature regimes. Therefore, $\mathrm{Y}{\mathrm{b}}_2\mathrm{B}{\mathrm{e}}_2\mathrm{Ge}{\mathrm{O}}_7$ provides a platform for exploring the exotic magnetic states as well as potential application in magnetic refrigeration at sub-Kelvin temperatures.

We study the microscopic mechanism of nucleation of the helical superconducting state maintained by a spin-splitting field and weak Rashba spin-orbit coupling in a 2D superconductor/ferromagnet system and present an exact solution for the Gorkov equations, which provides a full description of the thermodynamic properties of the system under consideration. This permits us to obtain the Ginzburg-Landau expansion and use it to analyze the possibility of controlling magnetization using the transport supercurrent in both dc and quasistatic ac regimes. We show that these properties are related to the manifestation of the diode effect in superconducting systems with spin-orbit interaction.

The Josephson junction under consideration is composed of two s-wave superconducting contacts deposited on the top of a two-dimensional antiferromagnet (AF). Triplet Cooper correlations in AF are provided by thin ferromagnetic spacers between AF and superconducting contacts. The problem is considered in the regime of a weak tunneling of electrons between these contacts and AF. The transport of electron Cooper's pairs under the stationary phase bias in the disordered AF is treated within the formalism of equilibrium Green's functions and the Born approximation for electrons which are scattered by nonmagnetic impurities. The system of diffusion equations is derived for three triplet components of Cooper-correlated electron pairs. These spin projections are coupled to each other due to interplay of the spin-orbit interaction and AF order. Moreover, the diffusion equations are sublattice-dependent. It is shown that such a staggered behavior will lead to the nonreciprocal Josephson current, if the tunneling of electrons between contacts and AF is different for different sublattices.

Nonlinear response signatures are increasingly recognized as useful probes of condensed matter systems, in particular for the characterization of topologically nontrivial states. The circular photogalvanic effect (CPGE) is particularly useful in the study of topological semimetals, as the CPGE tensor quantizes for well-isolated topological degeneracies in strictly linearly dispersing band structures. Here, we study multiplicative Weyl semimetal band structures, and find that the multiplicative structure robustly protects the quantization of the CPGE even in the case of nonlinear dispersion. Computing phase diagrams as a function of Weyl node tilting, we find a variety of quantized values for the CPGE tensor, revealing that the CPGE is also a useful tool in detecting and characterizing the parent topology of multiplicative topological states.

We investigate magnetotransport across an interface between two Weyl semimetals (finite in both directions) whose Weyl nodes project onto two different surfaces which are twisted with respect to each other before being coupled. This gives rise to a novel contribution to the conductance through the junction purely through Fermi arc states, even in the absence of a magnetic field perpendicular to the junction. When the perpendicular magnetic field is included, we find that for a mesoscopic or smaller samples, the transverse Fermi arc states have a significant contribution to the conductance for experimentally relevant fields and need to be taken into account along with the conductance through the bulk chiral Landau levels.

Motivated by the discovery of superconductivity in infinite-layer nickelates ${\mathrm{RNiO}}_2$ (R = rare-earth), and the subsequent enhancement of their $T_c$ with pressure, we investigate the evolution of the electronic structure and magnetic properties of this family of materials via first-principles calculations employing hydrostatic and chemical pressure as tuning knobs. Overall, our analysis shows that pressure tends to increase the $\mathrm{R}\text{−}5d$ self-doping effect, as well as the $\mathrm{Ni}\text{−}{d}_{x^2-y^2}$ bandwidth, the $e_g$ energy splitting, the charge transfer energy, and the superexchange $\left(J\right)$. Using the energy scale of $J$ as a predictor of superconducting tendencies, we anticipate that pressure can indeed be a feasible means to further increase the $T_c$ in this family of materials.

Realizing Majorana bound states (MBSs) in short, well-controllable chains of coupled quantum dots sidesteps the problem of disorder but requires finetuning and does not give the true topological protection inherent to long chains. Here, we introduce a quality measure that is also applicable in the presence of strong electron-electron interactions and that quantifies the closeness to topological protection of finetuned MBSs in short quantum dot chains. We call this measure local distinguishability (LD) because it puts a bound to the degree an arbitrary local measurement can distinguish between two states. We study the LD for quantum dot chains of different length. The three-dot chain is studied in detail, and we find that it may not always be an improvement over the two-dot case, a fact that can be understood within an effective model derived from perturbation theory. For longer chains, the LD vanishes exponentially, signaling a transition to a topological phase with two ground states that cannot be distinguished by any local measurement.

We introduce a versatile machine-learning scheme for predicting dipole moments of molecular liquids to study dielectric properties, implemented in https://github.com/ToAmano/MLWC. We attribute the center of mass of Wannier functions, called Wannier centers, to each chemical bond and create neural network models that predict the Wannier centers for each chemical bond. Application to liquid methanol and ethanol shows that our neural network models successfully predict the dipole moment of various liquid configurations in close agreement with DFT calculations. We show that the dipole moment and dielectric constant in the liquids are greatly enhanced by the polarization of Wannier centers due to local intermolecular interactions. The calculated dielectric spectra quantitatively agree with experiments over terahertz (THz) to infrared regions. Furthermore, we investigate the physical origin of THz absorption spectra of methanol, confirming the importance of translational and librational motions. Our method is applicable to other molecular liquids and can be widely used to study their dielectric properties.

We introduce a class of intrinsic symmetry-protected topological mixed state (mSPT) in open quantum systems that feature modulated symmetries, such as dipole and subsystem symmetries. Intriguingly, these mSPT phases cannot be realized as the ground states of a gapped Hamiltonian under thermal equilibrium. The microscopic form of the density matrix characterizing these intrinsic mixed-state SPT ensembles is constructed using solvable coupled-wire models that incorporate quenched disorder or quantum channels. A detailed comparison of the hierarchical structure of boundary anomalies in both pure and mixed states is presented, utilizing flux insertion and Laughlin's charge pumping arguments. Finally, we explore the salient features of boundary anomalies in the mixed-state ensemble, which can be detected through the Rényi-$N$ correlation function of charged observables.

This study is devoted to the profound implications of tilted Dirac cones on the quantum transport properties of two-dimensional (2D) Dirac materials. These materials, characterized by their linear conic energy dispersions in the vicinity of Dirac points, exhibit unique electronic behaviors, including the emulation of massless Dirac fermions and the manifestation of relativistic phenomena such as Klein tunneling. Expanding beyond the well-studied case of graphene, the manuscript focuses on materials with tilted Dirac cones, where the anisotropic and tilted nature of the cones introduces additional richness to their electronic properties that arises from an emergent underlying spacetime geometry. The investigation begins by considering a heterojunction of 2D Dirac materials, where electrons undergo quantum tunneling between regions with upright and tilted Dirac cones. The role of tilt in characterizing the transmission of electrons across these interfaces is thoroughly examined, shedding light on the influence of the tilt parameter on the transmission probability and the fate of the pseudospin of the Dirac electrons, particularly upon a sudden change in the tilting. We also investigate the probability of reflection and transmission from an intermediate slab with arbitrary subcritical tilt, focusing on the behavior of electron transmission across regions with varying Dirac cone tilts. The study demonstrates that for certain thicknesses of the middle slab, the transmission probability is equal to unity, and both reflection and transmission exhibit periodic behavior with respect to the slab thickness. This is reminiscent of Klein tunneling across scalar potential barriers in PNP junctions, no gate voltage is applied. Such a tilt-induced potential can be considered as the quantum transport manifestation of the “gravitomagnetic” effect of underlying spacetime structure.

Magnetic order, when coupled with band topology, gives rise to diverse magnetic topological phases. In this study, we focus on space group 173, particularly emphasizing the out-of-plane ferromagnetic order, to analyze how magnetic order and spin-orbit coupling (SOC) influence band symmetry changes. Our first-principles calculations and symmetry analysis predict that ferromagnetic ${\mathrm{La}}_3{\mathrm{AlCrS}}_7$ is an ideal candidate for the triple Weyl semimetal. In the absence of SOC, the spin-majority bands are conductive while the spin-minority bands are insulating, resulting in a fully polarized Weyl semimetal. Furthermore, within the framework of spin group theory, the triple hourglass Weyl point is stabilized by both $C_6$ screw rotation symmetry and a redefined time-reversal symmetry. Remarkably, this triple Weyl point remains robust against SOC, and the topological sextuple-helicoid surface states are clearly visible regardless of the presence of SOC. This study not only provides a promising platform for the experimental observation of triple Weyl fermions, but also opens avenues for exploring unprecedented phenomena in triple Weyl semimetals.

A smooth edge potential could lead to edge reconstruction of a $\nu =1$ quantum Hall state, resulting in an additional $\nu =1/3$ QH side strip, which, in turn, may yield Majorana/parafermion zero modes at the domain wall of the superconductor and ferromagnet on the edge. We demonstrate that the constraints imposed by the $\nu =1$ bulk permit the existence of multiple types of electron operators, leading to distinct and immutable species of Majorana modes. Different velocities of the reconstructed edge modes provide a clear signature of these species of Majoranas in the $4\pi$ fractional Josephson current.

The pressure and temperature conditions at the phase boundary of ${\mathrm{H}}_2\mathrm{O}$ are important for modeling the internal structure of icy bodies, such as Uranus and Neptune. However, disagreements have arisen regarding this from the viewpoints of experiments using static and dynamic compression techniques. Here, we investigated the phase relations of hot dense ${\mathrm{H}}_2\mathrm{O}$ using millisecond time-resolved synchrotron x-ray diffraction (XRD) measurements and a laser-heated diamond anvil cell technique at up to approximately 120 GPa and 6000 K. Homogeneous laser heating of a ${\mathrm{H}}_2\mathrm{O}$ sample and accurate pyrometry temperature measurements were achieved by implementing a sample assembly comprising two metal-coated $\mathrm{Si}{\mathrm{O}}_2$ glass disks sandwiching the ${\mathrm{H}}_2\mathrm{O}$ sample. The XRD measurements showed that the transformation from bcc ice to the fcc ice phase (presumably superionic state) was not completed in 1 s, implying the strong kinetics of ${\mathrm{H}}_2\mathrm{O}$ from the bcc to the fcc structure. The study results provide tighter constraints on the equilibrium phase diagram of ${\mathrm{H}}_2\mathrm{O}$ and a better understanding of the internal structure and dynamics of icy bodies.

Charged domain walls (CDWs) in ferroelectrics display electrical conductivity several orders of magnitude higher than that of the bulk material, and they can be created/erased by an external electric field, which makes them promising for integration into nanoelectronic devices. However, a comprehensive understanding of the mechanisms governing the kinetics of CDWs and their stabilization is still lacking. Here, we conduct both experimental and theoretical investigations into the phenomenon of CDW stability and its correlation with their enhanced conductivity. We demonstrate that the conductivity along the domain walls facilitates the redistribution of mobile charge, resulting in a reduction of the depolarization field at the CDWs with respective stabilization of the domains.

Changing some of its parameters over time is a paradigmatic way of driving an otherwise isolated many-body quantum system out of equilibrium, and a vital ingredient for building quantum computers and simulators. Here, we further develop a recently proposed nonlinear response theory which is based on typicality and random-matrix methods, and which is applicable to a wide variety of such parametrically perturbed systems in and out of equilibrium: We derive analytical approximations of the characteristic response function for the two limiting cases of fast driving and of strong and short-ranged-in-energy driving. Furthermore, we work out implications and predictions for common applications, including finite-time quenches and time-dependent forcing that breaks conservation laws of the underlying undriven system. Finally, we verify all predictions by numerical examples and discuss the theory's scope and limitations.

Using first-principles calculations, we explore the magnetoelectric properties of the room-temperature multiferroic crystal ${\mathrm{BiCoO}}_3$. We use both applied magnetic field and finite-difference techniques to show that ${\mathrm{BiCoO}}_3$ is antimagnetoelectric at the linear level. The calculation of the dynamical effective charges reveals that the total magnetoelectric response is zero due to the compensating nonzero magnetoelectric response of each magnetic sublattice. This calculation also highlights that the orbital contribution to the response is remarkably larger than the spin one and that each sublattice has a rather large total magnetoelectric response of 85 ps/m. Furthermore, we provide an intuitive recipe to visualize the dynamical magnetic effective charge, allowing to examine its multipolar nature, which we confirm by means of ab initio calculations. Given the large value of the local response, we investigate the ferromagnetic phase as well, which gives a giant magnetoelectric response of about 1000 ps/m and coming mainly from the spin contribution this time. Finally, we discuss the possible reasons for such a large magnetoelectric response in ${\mathrm{BiCoO}}_3$ and propose possible strategies to unveil this potentially large response.

${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$ is the archetype of a ferromagnetic one-dimensional Ising compound having antiferromagnetically coupled chains distributed on a triangular lattice. In the present paper, we investigate a first-order transition, not detected before, which takes place along the transformation from the spin-density-wave to the ferrimagnetic state. This $T^{*}\left(H\right)$ line has been derived from magnetization and specific heat measurements recorded vs $T$ and vs $H$ and carried out along the anisotropy axis on a set of single crystals. The entropy jump $\mathrm{\Delta }S$ at the transition has been derived from both the specific heat and magnetization data. A direct link between $\mathrm{\Delta }S$ and the period of the underlying spin modulations in the spin-density-wave state has been established. All the results about $T^{*}\left(H\right)$ are compared to the recent theoretical predictions of Kamiya [Phys. Rev. B 107, 134409 (2023)] and the degree of consistency was evaluated from the derived values of the magnetic couplings. Comparisons to similar transitions in related materials are also discussed.

We report that a quasi-one-dimensional ternary compound ${\mathrm{Ca}}_3{\mathrm{Pt}}_4{\mathrm{Bi}}_8$ exhibits bulk superconductivity at 0.6 K probed by dc magnetic susceptibility and specific-heat experiments, coexisting with a surface superconductivity at 1.17 K probed by electrical resistivity measurements. The two superconducting phases exhibit significantly different upper critical fields. In electrical resistivity measurements, the resistivity exhibits strong anisotropy and the obtained upper critical field is strongly anisotropic as well, implying two-dimensional (2D) superconductivity. Meanwhile, the bulk upper critical field measured by the dc magnetic susceptibility and specific heat is much smaller than that obtained by electrical resistivity measurements. Moreover, the angular dependence of the upper critical field ${\mu }_0{H}_{c2}$ based on the resistivity measurements is consistent with the 2D Tinkham model. Combined with the study of transmission electron microscopy (TEM), 2D superconductivity should occur on the surface of the sample where a thin shell surrounding the bulk phase forms possibly due to a kind of spontaneous chemical change. The first-principles calculations unveil a complicated Fermi surface with ten bands crossing the Fermi level. Some steep bands among these reflect the quasi-one-dimensional nature. This quasi-one-dimensional ternary compound exhibits interesting surface superconductivity in addition to the bulk superconductivity which is worthy of further study.

Two-dimensional mirror symmetry enriched topological (SET) orders can be studied using the folding approach: it can be folded along the mirror axis and turned into a bilayer system on which the mirror symmetry acts as a ${\mathbb{Z}}_2$ layer-exchange symmetry. How mirror symmetry enriches the topological order is then encoded at the mirror axis, which is a gapped boundary of the folded bilayer system. Based on anyon-condensation theory, we classify possible ${\mathbb{Z}}_2$-symmetric gapped boundaries of the folded system. In particular, we derive an $H^2$ obstruction function, which corresponds to an $H^3$ obstruction for topological orders enriched by the time-reversal symmetry instead of mirror symmetry. We demonstrate that states with a nontrivial $H^2$ obstruction function can be constructed on the surface of a three-dimensional mirror SET order.