The helimagnets ${\mathrm{Cr}}_{1/3}M{\mathrm{S}}_2(M$ = Nb or Ta) have attracted renewed attention due to the discovery of a chiral soliton lattice (CSL) stabilized in an applied magnetic field, but reports of unusual low-temperature transport and magnetic properties in this system lack a unifying explanation. Here, we present electronic structure calculations that demonstrate the materials are half metals. There is also a gaplike feature (width in range 40–100 meV) in the density of states of one spin channel. This electronic structure explains the low-temperature electronic and magnetic properties of ${\mathrm{Cr}}_{1/3}M{\mathrm{S}}_2(M$ = Nb or Ta), with the gaplike feature particularly important for explaining the magnetic behavior. Our magnetometry measurements confirm the existence of this gap. Dynamic spin fluctuations driven by excitations across this gap are seen over a wide range of frequencies (0.1 Hz to MHz) with ac susceptibility and muon-spin relaxation $({\mu }^{+}\text{SR}$) measurements. We show further how effects due to the CSL in ${\mathrm{Cr}}_{1/3}{\mathrm{NbS}}_2$, as detected with ${\mu }^{+}\text{SR}$, dominate over the gap driven magnetism when the CSL is stabilized as the majority phase.

Thin films of $\beta$-tungsten host superconductivity in the presence of strong spin-orbit coupling. This nonequilibrium crystalline phase of tungsten has attracted considerable attention in recent years due to its giant spin Hall effect and the potential promise of exotic superconductivity. However, more than 60 years after its discovery, superconductivity in this material is still not well understood. Using time-domain terahertz spectroscopy, we measure the frequency response of the complex optical conductivity of $\beta$-tungsten thin film with a $T_c$ of 3.7 K in its superconducting state. At temperatures down to 1.6 K, we find that both the superconducting gap and the superfluid spectral weight are much smaller than that expected for a weakly coupled superconductor given the $T_c$. The conclusion of a small gap holds up even when accounting for possible inhomogeneities in the system, which could come from other crystalline forms of tungsten (that are not superconducting at these temperatures) or surface states on $\beta$ tungsten grains. Using detailed x-ray diffraction measurements, we preclude the possibility of significant amounts of other tungsten allotropes, suggesting the topological surface states of $\beta$-tungsten could play the role of inhomogeneity in these films. Our observations pose a challenge and opportunity for a theory of strongly anisotropic normal metals with strong spin-orbit coupling to be described.

The existence of localization and mobility edges in one-dimensional lattices is commonly thought to depend on disorder (or quasidisorder). We investigate localization properties of a disorder-free lattice subject to an equally spaced electric field. We analytically show that, even though the model has no quenched disorder, this system manifests an exact mobility edge and the localization regime extends to weak fields, in contrast to gigantic field for the localization of a usual Stark lattice. For strong fields, the Wannier-Stark ladder is recovered, and the number of localized eigenstates is inversely proportional to the spacing. Moreover, we study the time dependence of an initially localized excitation and dynamically probe the existence of mobility edge.

We study the formation of local magnetic moments in the strongly correlated Hubbard model within dynamical mean-field theory and associate peculiarities of the temperature dependence of local charge ${\chi }_c$ and spin ${\chi }_s$ susceptibilities with different stages of local moment formation. The local maximum of the temperature dependence of the charge susceptibility ${\chi }_c$ is associated with the beginning of local magnetic moment formation, while the minimum of the susceptibility ${\chi }_c$ and double occupation, as well as the low-temperature boundary of the plateau of the effective local magnetic moment ${\mu }_{\mathrm{eff}}^2=T{\chi }_s$ temperature dependence are connected with the full formation of local moments. We also obtain the interaction dependence of the Kondo temperature $T_K$, which is compared to the fingerprint criterion of Chalupa et al. [Phys. Rev. Lett. 126, 056403 (2021)]. Near the Mott transition the two criteria coincide, while further away from the Mott transition the fingerprint criterion somewhat overestimates the Kondo temperature. The relation of the observed features to the behavior of eigenvectors/eigenvalues of fermionic frequency-resolved charge susceptibility and divergences of irreducible vertices is discussed.

A mechanism of electron-mediated pumping of heat in the absence of net charge transfer is proposed. It may be realized in charge-neutral electron systems, such as graphene, coupled to an external electric potential. The flow of heat in this pumping cycle is not accompanied by a buildup of voltage along the system, which offers advantages over traditional thermoelectric cooling setups. The efficiency of heat pumping and the magnitude of heat flux are studied in the hydrodynamic regime for weak disorder. It is shown that the cycle efficiency may approach the Carnot limit. In a pristine system, even for an infinitesimal pumping potential, the heat flux remains finite. In particular, for a potential in the form of a traveling wave moving with velocity $c$, the pumping is perfect; the entire heat content of the electron liquid is advected with velocity $c$. For a general pumping cycle the heat flux is determined by the cycle geometry and disorder strength.

Two-dimensional periodically driven topological insulators have been shown to exhibit numerous topological phases, including ones which have no static analog, such as anomalous Floquet topological phases. We study a two-dimensional model of spinless fermions on a honeycomb lattice with periodic driving. We show that this model exhibits a rich mixture of weak and strong topological phases, which we identify by computing their scattering matrix invariants. Furthermore, we do an in-depth analysis of these topological phases in the presence of spatial disorder and show the relative robustness of these phases against imperfections. Making use of this robustness against spatial disorder, we propose a filter which allows the passage of only edge states, and which can be realized using existing experimental techniques.

We develop a hierarchical equations of motion (HEOM) approach to compute real-time single-particle correlation functions and thermodynamic properties of the Holstein model at finite temperature. We exploit the conservation of the total momentum of the system to formulate the momentum-space HEOM whose dynamical variables explicitly keep track of momentum exchanges between the electron and phonons. Our symmetry-adapted HEOM enable us to overcome the numerical instabilities inherent to the commonly used real-space HEOM. The HEOM method is then used to study the spectral function and thermodynamic quantities of chains containing up to ten sites. The HEOM results compare favorably to existing literature. To provide an independent assessment of the HEOM approach and to gain insight into the importance of finite-size effects, we devise a quantum Monte Carlo (QMC) procedure to evaluate finite-temperature single-particle correlation functions in imaginary time and apply it to chains containing up to twenty sites. QMC results reveal that finite-size effects are quite weak, so that the results on 5 to 10-site chains, depending on the parameter regime, are representative of larger systems. A detailed comparison between the HEOM and QMC data place our HEOM method among reliable methods to compute real-time finite-temperature correlation functions in parameter regimes ranging from low- to high-temperature, and weak- to strong-coupling regime.

By performing resonant inelastic x-ray scattering (RIXS) measurements at the Ir ${\mathit{L}}_3$ edge, we have investigated the low-energy elementary excitations in a series of double-perovskite iridate single crystals, ${\mathrm{La}}_2\mathit{M}{\mathrm{IrO}}_6 (\mathit{M}=\mathrm{Co},\mathrm{Ni},\text{and}\mathrm{Zn})$. Almost dispersionless magnetic excitations at $\sim 42\left(6\right)$ meV and $\sim 35$(5) meV have been observed in crystals containing magnetic $3\mathit{d}$ ions, ${\mathrm{La}}_2{\mathrm{CoIrO}}_6$ and ${\mathrm{La}}_2{\mathrm{NiIrO}}_6$, respectively. In contrast, this low-energy magnetic excitation is absent in ${\mathrm{La}}_2{\mathrm{ZnIrO}}_6$ in which the $3\mathit{d}$ ions are nonmagnetic, suggesting the importance of $3\mathit{d}-5\mathit{d}$ hybridization in the magnetic properties of these double-perovskite iridates. The magnetic excitation is suppressed completely above the magnetic ordering temperature, suggesting the inadequacy of using a simple spin Hamiltonian to describe magnetism of these materials.

The $J_1\text{−}{J}_2$ quantum spin sawtooth chain is a paradigmatic one-dimensional frustrated quantum spin system exhibiting unconventional ground-state and finite-temperature properties. In particular, it exhibits a flat energy band of one-magnon excitations accompanied by an enhanced magnetocaloric effect for two singular ratios of the basal interactions $J_1$ and the zigzag interactions $J_2$. In our paper, we demonstrate that one can drive the spin system into a flat-band scenario by applying an appropriate electric field, thus overcoming the restriction of fine-tuned exchange couplings $J_1$ and $J_2$ and allowing one to tune more materials towards flat-band physics, that is, to show a macroscopic magnetization jump when crossing the magnetic saturation field, a residual entropy at zero temperature, as well as an enhanced magnetocaloric effect. While the magnetic field acts on the spin system via the ordinary Zeeman term, the coupling of an applied electric field with the spins is given by the sophisticated Katsura-Nagaosa-Balatsky (KNB) mechanism, where the electric field effectively acts as a Dzyaloshinskii-Moriya spin-spin interaction. The resulting features are corresponding reciprocal effects: We find a magnetization jump driven by the electric field as well as a jump of the electric polarization driven by the magnetic field; i.e., the system exhibits an extraordinarily strong magnetoelectric effect. Moreover, in analogy to the enhanced magnetocaloric effect, the system shows an enhanced electrocaloric effect.

The position of the critical point determines the top of the liquid-vapor coexistence dome, and it is a physical parameter of fundamental importance in the study of high-energy shocks, including those associated with large planetary impacts. For most major planetary materials, such as oxides and silicates, the estimated position of the critical point is below $1\mathrm{g}/{\mathrm{cm}}^3$ at temperatures above 5000 K. Here we compute the position of the critical point of one of the most ubiquitous materials: MgO. For this we perform first-principles molecular dynamics simulations. We find the critical density to be in the 0.45–$0.6\mathrm{g}/{\mathrm{cm}}^3$ range and the critical temperature in the 6500–7000 K range. We investigate in detail the behavior of MgO in the subcritical and supercritical regimes, and we provide insight into the structure and chemical speciation. We see a change in Mg-O speciation toward lower degrees of coordination as the temperature is increased from 4000 to 10 000 K. This change in speciation is less pronounced at higher densities. We observe the liquid-gas separation in nucleating nanobubbles at densities below the liquid spinodal. The majority of the chemical species forming the incipient gas phase consists of isolated Mg and O atoms and some MgO and ${\mathrm{O}}_2$ molecules. We find that the ionization state of the atoms in the liquid phase is close to the nominal charge, but it almost vanishes close to the liquid-gas boundary and in the gas phase, which is consequently largely atomic.

Compressional $\left(V_P\right)$ and shear $\left(V_S\right)$ wave velocities of polycrystalline $\mathrm{Ce}{\mathrm{Al}}_2$ have been measured up to 12.5 GPa at room temperature by ultrasonic interferometry in a multi-anvil apparatus. Both $V_P$ and $V_S$ show a weak pressure dependence up to 7.5 GPa, and then a remarkable discontinuity is observed which supports the earlier report about a volume collapse in this material. An anomaly has also been found in the pressure derivative of resistance $dR/dP$ at around $P=7.5$ GPa. Based on the experimental data, the high-pressure behaviors of the bulk and shear moduli are investigated. Complementary to the measured data, the single-crystal elastic constants are computed by first-principles calculations. Both the pressure-dependent $C_{11}$ and $C_{44}$ exhibit a change of slope at about $P=7.5$ GPa. At low pressure, the observed flat curve for the shear modulus may be due to $C_{44}$ which shows negligible pressure dependence. At higher pressure, $C_{44}$ softens more rapidly and the mechanical instability occurs at $P>15$ GPa. The calculated electronic densities of states show that sharp peaks near the Fermi level are mainly originated from the $4f$ and $d$ states, which may be responsible for the Ce-Ce covalent bond. High pressure leads to a broadening of $4f$ band and an increase of the $d/f$ hybridization.

Periodically driven systems with internal and spatial symmetries can exhibit a variety of anomalous boundary behaviors at both the zero and $\pi$ quasienergies despite the trivial bulk Floquet bands. These phenomena are called anomalous Floquet topology (AFT) as they are unconnected from their static counterpart, emerging from the winding of the time-evolution unitary rather than the bulk Floquet bands at the end of the driving period. In this paper, we systematically derive the first and inversion-symmetric second-order AFT bulk-boundary correspondence for Altland-Zirnbauer (AZ) classes BDI, D, DIII, and AII. For each AZ class, we start a dimensional hierarchy with a parent dimension having $\mathbb{Z}$ classification, then use it as an interpolating map to classify the lower-dimensional descendants. From the Atiyah-Hirzebruch spectral sequence, we identify the subspace that contains topological information and faithfully derive the AFT bulk-boundary correspondence for both the parent and descendants. Our theory provides analytic tools for out-of-equilibrium topological phenomena.

The increasing demand for computer data storage with a higher recording density can be addressed by using smaller magnetic objects, such as bubble domains. Small bubbles predominantly require a strong saturation magnetization combined with a large magnetocrystalline anisotropy to resist self-demagnetization. These conditions are well satisfied for highly anisotropic materials. Here, we study the domain structure of thin ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ lamellae. Magnetic bubbles with a minimum diameter of 74 nm were observed at room temperature, approaching even the range of magnetic skyrmions. The stripe domain width and the bubble size are both thickness dependent. Furthermore, a kind of bubble was observed below the spin-reorientation transition temperature that combine bubbles with opposite helicity. In this paper, we reveal ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ to be a good candidate for a high-density magnetic bubble-based memory.

We show that in superlattices with alternating superconducting ($\mathrm{S}$) and ferromagnetic ($\mathrm{F}$) layers the spontaneous magnetic field induced in the superconducting layers due to the electromagnetic proximity effect becomes dramatically enhanced compared to the previously studied $\mathrm{S}/\mathrm{F}$ bilayers. The effect reveals itself for the in-plane orientation of the magnetic moments, both for ferromagnetic and antiferromagnetic ordering of the moments in the $\mathrm{F}$ layers. In the finite-size samples the magnetic field decays from the sample surface towards the bulk of the structure and the decay length strongly depends on the relative orientation of the sample surface, the layers planes and magnetic moments in the $\mathrm{F}$ layers. The obtained results provide additional insights into experimental data on the neutron scattering in Nb/Gd superlattices.

Hexagonal warping (HW) in three-dimensional topological insulators is, by now, well known. We show that non-Hermitian (NH) loss/gain can generate an exceptional HW effect in double Weyl semimetals (DWSMs). This unique feature of DWSMs has distinctive effects on Fermi surface topology. Importantly, in the presence of such a $k^3$ spin orbit coupling mimicking term, the symmetry associated with the DWSMs is changed, leading to four exceptional points, among which two are degenerate. Introducing a driving field removes this degeneracy. The combined action of the NH warping and driving parameters leads to notable effects, including merging and tuning of exceptional points. We analyze the topological nature of the generated exceptional contours by evaluating several topological invariants, such as winding number, vorticity, and NH Berry curvature. We hope that our theoretical results will initiate possible experiments exploring NH HW effects.

In photoinduced phase transitions, the relationship between the crystal orientation and the direction of the polarization or wave vector of light seems important, but surprisingly, such directional properties in photoinduced phase transitions have not been well studied experimentally. ${\mathrm{Ti}}_3{\mathrm{O}}_5$ exhibits a structural phase transition involving the anisotropic migration of Ti and O ions, and its nanocrystals exhibit a persistent photoinduced phase transition. We found that a photoinduced phase transition occurs even in a single crystal of this compound, but it significantly depends on the orientation of the plane on which the light is irradiated. Such a selection rule for photoinduced phase transitions is dominated by the anisotropy of the strain associated with the phase transition.

The Bose-Hubbard model subjected to an effective magnetic field hosts a plethora of phases with different topological orders when tuning the chemical potential. Using the density matrix renormalization group method, we identify several gapped phases near the first Mott lobe at strong interactions. They are connected by a particle-hole symmetry to a variety of quantum Hall states stabilized at low fillings. We characterize phases of both particle and hole type and identify signatures compatible with Laughlin, Moore-Read, and bosonic integer quantum Hall states by calculating the quantized Hall conductance and by extracting the topological entanglement entropy. Furthermore, we analyze the entanglement spectrum of Laughlin states of bosonic particles and holes for a range of interaction strengths, as well as the entanglement spectrum of a Moore-Read state. These results further corroborate the existence of topological states at high fillings, close to the first Mott lobe, as hole analogs of the respective low-filling states.

Longitudinal magnetoresistance (LMR) refers to the change in resistance due to a magnetic field when the current and the magnetic field are parallel to each other. For this to be nonzero in weak magnetic fields, kinetic theory stipulates that the electronic dispersion must satisfy certain conditions: it should either be sufficiently anisotropic or have topological features. The former results in a positive LMR, while the latter results in a negative LMR. Here, I propose a different mechanism that leads to LMR in any dispersion without a need to satisfy the above requirements. The mechanism is quantum in origin but is applicable in the said regime. It arises due to the change in the density of states with the magnetic field and is not kinetic in origin. Remarkably, LMR is found to be negative even if the dispersion is nontopological, provided it is nonparabolic. An analytical expression is derived for this contribution to LMR. It is found to depend on the orbital magnetic susceptibility. The analytical findings are confirmed by numerical calculations.

We develop a systematically general theory of one-dimensional (1D) non-Hermitian systems, elaborating on the energy bands, the band degeneracy, and the defectiveness of eigenstates under open boundary conditions. We analyze the band degeneracy and defectiveness of two typical 1D non-Hermitian models. We obtain the unusual presence and absence of the exceptional points in the generalized non-Hermitian Su-Schrieffer-Heeger model under open boundary conditions. Beyond the general theory, we discover that infernal points exist in 1D non-Hermitian systems, where the energy spectra under open boundary conditions converge on some discrete energy values. We analyze two relevant 1D non-Hermitian models with the existence of infernal points. Moreover, we generalize the infernal points to the infernal knots in four-dimensional systems. The general theory and the infernal points of non-Hermitian systems developed in this paper are also valid in Hermitian systems.

Although phase diagrams can be leveraged to investigate high transition temperature (high-$T_c$) superconductivity, the issue has not been discussed thoroughly. In this study, we elucidate the phase diagram of the overdoped side of high-$T_c$ cuprates via systematic anisotropic transport measurements for Pb-doped Bi-2212 single crystals. We demonstrate that the characteristic temperatures of the “weak” pseudogap opening and electronic coherence cross each other at a critical doping level, while those of the “strong” pseudogap merges into that of superconducting fluctuations above the critical doping level. Our results indicate the importance of Mottness in high-$T_c$ superconductivity.