We formulate a first-principle scheme for structural optimization at finite temperature ($T$) based on the self-consistent phonon (SCP) theory, which accurately takes into account the effect of strong phonon anharmonicity. The $T$ dependence of the shape of the unit cell and internal atomic configuration is determined by minimizing the variational free energy in the SCP theory. At each optimization step, the interatomic force constants in the new structure are calculated without running additional electronic structure calculations, which makes the method dramatically efficient. We demonstrate that the thermal expansion of silicon and the three-step structural phase transitions in ${\mathrm{BaTiO}}_3$ and its pressure-temperature ($p\text{−}T$) phase diagram are successfully reproduced. The present formalism will open the way to the nonempirical prediction of physical properties at finite $T$ of materials having a complex structural phase diagram.

The rare-earth monopnictide compounds have recently received considerable attention in condensed matter physics because of their correlation between magnetism, crystallographic structure, and transport. Here, we report experimental observations of high-magnetic-field-induced multiple metamagnetic transitions and magnetic structures in DyBi single crystals. The external field up to 32 T is applied in three distinct crystallographic directions as $H//\left[001\right]$, $H//\left[011\right]$, and $H//\left[111\right]$, which reveals anisotropic magnetism as well as multiple metamagnetic transitions. Two field-induced magnetic phase transitions at 3.7 T and 4.9 T along $H//\left[001\right]$, two at 3.8 T and 24 T along $H//\left[011\right]$, and one at 3.8 T along $H//\left[111\right]$ are identified. In order to unveil the nature of magnetic interaction in DyBi, the critical behavior analysis for $H//$[001], a direction with typical magnetism and transitions, is performed. The yielded critical exponents $\beta =0.234\left(8\right)$, $\gamma =0.904\left(3\right)$, and $\delta =5.03\left(2\right)$ agree well with the theoretical prediction of a tricritical mean-field model, indicating a field-induced tricritical phenomenon in this system. Comprehensive magnetic phase diagrams for $H//\left[001\right]$, $H//\left[011\right]$, and $H//\left[111\right]$ are constructed based on detailed magnetization measurement and scaling, which reveal multiple phases such as NiO-type antiferromagnetic (AFM), HoP-type AFM, forced ferromagnetic (FFM), and paramagnetic (PM) phases. Two tricritical points (TCPs) are determined at the intersections of the AFM, FFM, and PM phases, with TCP = (8.6 K, 85 kOe) for $H//$[001] and (16.8 K, 243 kOe) for $H//$[011]. The recognition of multiple phases suggests delicate and complex competition and balance between variable couplings in this system.

We theoretically investigate the propagation of spin waves confined in a Néel-type ferrimagnetic domain-wall channel in the presence of the interfacial Dzyaloshinskii-Moriya interaction (DMI). We observe a nonreciprocal spin-wave propagation induced by the antisymmetric chiral DMI, which can be manipulated by changing the net angular momentum or the polarity of domain walls. Considering that the band gap of spin waves excited in domain walls is significantly reduced, the three-magnon processes are applied to nonlinearly detect this bounded mode in the narrow channel, which enables us to quantify the frequency and the wave vector by analyzing the spectrum of the transmitted spin wave outside the domain wall and to determine the DMI strength. These findings are useful for fabricating energy-efficient magnetic devices and measuring the interfacial DMI in ultra narrow ferrimagnetic stripes.

Typically, atomic scale disorder is essential to show glassy behavior and amorphous materials can be the optimum candidates in this regard. Generating local disorder in an ordered material by substitution of another metal is a challenging task. In this paper, we successfully substituted Ni in ${\mathrm{EuGa}}_4$ (${\mathrm{EuNi}}_{0.37}{\mathrm{Ga}}_{3.63}$) to induce spin glass (SG) behavior. The compound was characterized by single-crystal x-ray diffraction, dc magnetization, frequency-dependent ac susceptibility, isothermal magnetization at various temperatures, spin relaxation, specific heat, and electric resistivity. Two distinct anomalies were observed in dc susceptibility $\sim 16.5$ and 5.2 K corresponding to long-range antiferromagnetic (AFM) ordering and SG transition, respectively. Their characteristic peaks in specific heat capacity further support the AFM and SG behavior. The irreversibility temperature is fitted with the Almeida-Thouless equation, confirming the Ising SG. The SG behavior is further confirmed by shift of maxima to a high temperature in ac susceptibility with increasing frequency. Nonzero spin relaxation after 7200 s, fitted with stretched exponential model, provides the value of $\beta =0.41$. This material can be identified as a possible reentrant SG as AFM and SG behavior coexist in the low-temperature range. Additionally, a spin-flip transition is observed in the AFM region ($5.2\mathrm{K}<T<16.5\mathrm{K}$). Our first-principles calculations show Ni substitution induces the distortion in electronic band structure as compared with parent ${\mathrm{EuGa}}_4$. A minimal difference between the different magnetic configurations for Ni-substituted ${\mathrm{EuGa}}_4$ suggests the frustration in the lattice, pointing toward SG behavior.

The tetragonal heavy-fermion compound ${\mathrm{CeAuSb}}_2$ (space group $P4/nmm$) exhibits incommensurate spin-density wave (SDW) order below $T_N\approx 6.5\text{K}$ with the propagation vector ${\mathbf{q}}_A=({\delta }_A,{\delta }_A,1/2)$. The application of uniaxial stress along the [010] direction induces a sudden change in the resistivity ratio ${\rho }_a/{\rho }_b$ at a compressive strain of $\epsilon \approx -0.5$%. Here we use neutron scattering to show that the uniaxial stress induces a first-order transition to a SDW state with a different propagation vector $(0,{\delta }_B,1/2)$ with ${\delta }_B=0.25$. The magnetic structure of the new (B) phase consists of Ce layers with ordered moments alternating with layers with zero moment stacked along the $c$ axis. The ordered layers have an up-up-down-down configuration along the $b$ axis. This is an unusual situation in which the loss of spatial inversion in a metallic system is driven by the magnetic order. We argue that the change in SDW wave vector leads to Fermi-surface reconstruction and a concomitant change in the transport properties.

We predict sound anomalies at the doping ${\delta }_p$ where the pseudogap ends in the normal state of hole-doped cuprates. Our prediction is based on the two-dimensional compressible Hubbard model using cluster dynamical mean-field theory. We find sharp anomalies (dips) in the velocity of sound as a function of doping and interaction. These dips are a signature of supercritical phenomena, stemming from an electronic transition without symmetry breaking below the superconducting dome. If experimentally verified, these signatures may help to solve the fundamental question of the nature of the pseudogap, pinpointing its origin as due to Mott physics and resulting short-range correlations.

The observation of flat energy bands in transition metal dichalcogenide bilayers such as twisted ${\mathrm{WSe}}_2$ makes those materials interesting prospects for reproducing the behavior observed in graphene-based systems. We use an effective Hubbard model providing a description of twisted ${\mathrm{WSe}}_2$ to explore the presence of superconductivity, which was previously reported in experiments. Using both the variational cluster approximation and cluster dynamical mean-field theory, we predict the existence of chiral superconductivity of type $d\pm id$ that can be tuned by the twist angle and by the application of a perpendicular displacement field in both electron- and hole-doped systems.

Two-dimensional layered materials with different anions on both faces of each layer, called Janus monolayers, have attracted significant interest due to their unique structural asymmetry perpendicular to the layer, which gives rise to an electric dipole moment that further diversifies the versatile properties of the transition-metal dichalcogenide materials class. However, the synthesis of a material such as MoSSe is prone to the introduction of various point defects, which could significantly modify the electric and optical properties as well as vibrational spectra. Using density functional theory, we provide an in-depth insight into the thermal stability of numerous point-vacancy and antisite defects in the Janus MoSSe monolayer. The structural changes are discussed in terms of the local strain induced by the modified atomic bonding around the defect sites. The electronic structure and linear optical response of Janus MoSSe monolayer with various point defects are studied, and possible fingerprints of electronic transitions due to defects are rationalized. First-principles calculations of phonons are carried out to spot the fingerprint of each point defect in the vibrational spectrum of the Janus MoSSe monolayer. Our systemic study will provide a broad picture of the roles the point defects could play in modifying and tuning the electronic and optical properties of two-dimensional Janus materials and thus help customizing them for certain applications.

We study the possibility of spontaneous symmetry breaking in systems with both charge and dipole symmetries. For $d$-dimensional systems at a positive temperature, we show that charge symmetry cannot be spontaneously broken for $d\le 4$, while dipole symmetry cannot be spontaneously broken for $d\le 2$. For $T=0$, we show that charge symmetry cannot be spontaneously broken for $d\le 2$ if the compressibility is finite. We also show that continuum systems with a dipole symmetry have infinite inertial mass density.

The composite fermion (CF) theory gives both a phenomenological description for many fractional quantum Hall (FQH) states, as well as a microscopic construction for large scale numerical calculation of these topological phases. The fundamental postulate of mapping FQH states of electrons to integer quantum Hall (IQH) states of CFs, however, was not formally established. The Landau level (LL) projection needed for the microscopic calculations is in some sense uncontrolled and unpredictable. We rigorously derive the unitary relationship between electrons and the CFs, showing the latter naturally emerge from special interactions within a single LL, without resorting to any projection by hand. In this framework, all FQH states topologically equivalent to those described by the conventional CF theory (e.g., the Jain series) have exact model Hamiltonians that can be explicitly derived, and we can easily generalize to FQH states from interacting CFs. Our derivations reveal fundamental connections between the CF theory and the pseudopotential/Jack polynomial constructions, and argue that all Abelian CF states are physically equivalent to the IQH states, while a plethora of non-Abelian CF states can be systematically constructed and classified. We also discuss about implications to experiments and effective field theory descriptions based on the descriptions with CFs as elementary particles.

We calculate the resonant inelastic x-ray scattering (RIXS) spectra of a $4d$-electron Mott insulator ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$ that exhibits spin-orbit entangled magnetic ordering. Considering the Ru $L_3$-edge RIXS, we identify four different types of excitations: magnetic dipolar transitions, quadrupolar transitions changing total angular momentum by $\mathrm{\Delta }J=2$, spin-state transitions with $\mathrm{\Delta }S=1$, and $t_{2g}\to e_g$ orbital transitions across the crystal-field and Hund's coupling derived levels. We provide analytical expressions for the magnetic transitions and the $\mathrm{\Delta }J=2$ transitions in the tetragonal compression limit, which are generally applicable to other compounds with $t_{2g}^4$ configurations in a square lattice. Intriguingly, we find that upon tetragonal compression the dipole-forbidden $\mathrm{\Delta }J=2$ transitions acquire large spectral weights with pronounced polarization dependencies, which are consistent with the experimental observations. Our numerical simulations show that this is due to a constructive interference between dipolar and quadrupolar scattering channels.

We study the rich properties of a topolectrical (TE) circuit array consisting of lossless basic electrical components, such as capacitors and inductors, which can be designed to exhibit higher-order topological phases (HOTP). The HOTP of the circuit exhibits the characteristics of higher-order topology, i.e., unconventional bulk-boundary correspondence with strongly localized corner modes, and higher winding numbers. More interestingly, a type-II corner mode emerges in the presence of long-range interaction, which is realized in the TE circuit by the introduction of next-nearest neighbor (NNN) coupling capacitances. Unlike the usual (i.e., “type-I”) corner modes that are localized at a particular sublattice node due to the chiral symmetry, the type-II corner modes possess a spatial extent with an exponential decay length. We analytically derive this decay length as a function of the circuit parameters. The NNN coupling is also associated with the tilt parameter in the admittance spectrum of the circuit. The admittance spectrum is reminiscent of that of Dirac fermions. Changing the tilt parameter can lead to a transition from the type-I to the overtilted type-II Dirac dispersion. This overtilting results in a hybridization of the bulk and corner modes in which the distinct corner modes disappear. Furthermore, the type-I and type-II corner modes can be distinguished by their impedance readout. By virtue of their flexibility, the TE circuits provide an ideal platform to demonstrate unusual features of HOTPs arising from long-range interactions, and to engineer different types of robust topological corner modes.

Relaxation inherently relates to the nonequilibrium nature of glassy solids and has profound influences on their properties. Exploring diverse relaxation modes of glasses is central to understanding their complicated dynamic behaviors and the nature of glass transition. Here, we report a hidden and universal relaxation mode (termed ${\alpha }^{\prime }$ relaxation)—which emerges between α relaxation and β relaxation in metallic glasses with relatively simple atomic structure—by systematically investigating their dynamic modulus spectrum. We find that the excitation of the ${\alpha }^{\prime }$ relaxation mode is controlled by the energy state and the local potential energy topology around the state. A notable ${\alpha }^{\prime }$ relaxation can only appear at an intermediate range of energy states, and it can be manipulated by annealing or rejuvenating. The activation energy and the nonmonotonic behavior of ${\alpha }^{\prime }$ relaxation reveal that it intrinsically connects α relaxation and β relaxation. Furthermore, we also show that the excitation of the ${\alpha }^{\prime }$ relaxation mode, rather than the level of the energy state, can notably improve the homogeneous tensile deformability of metallic glasses.

Multiferroics have found renewed interest in topological magnetism and for logic-in-memory applications. Among them, ${\mathrm{SrMnO}}_3$, possessing strong magnetoelectric coupling, is gaining attention for the design of coexisting magnetic and polar orders upon straining. Here we demonstrate antiferromagnetic exchange interactions in strained SMO thin films extracted from a new feature in the phase response of spin Hall magnetoresistance, which has not been explored in earlier works, such as in magnetic insulators. We explain our findings with a model that incorporates magnetic anisotropy along the [110] direction, corroborates with density functional theory studies, and is consistent with the direction of ferroelectric polarization in ${\mathrm{SrMnO}}_3$. The fundamental insights obtained from our studies establishes the potential of this material in magnetoelectrically coupled devices for different logic and memory applications.

Monte Carlo simulations are useful tools for modeling quantum systems, but in some cases they suffer from a sign problem, leading to an exponential slow down in their convergence to a value. While solving the sign problem is generically NP hard, many techniques exist for mitigating the sign problem in specific cases; in particular, the technique of deforming the Monte Carlo simulation's plane of integration onto Lefschetz thimbles (complex hypersurfaces of stationary phase) has seen significant success in the context of quantum field theories. We extend this methodology to spin systems by utilizing spin coherent state path integrals to reexpress the spin system's partition function in terms of continuous variables. Using some toy systems, we demonstrate its effectiveness at lessening the sign problem in this setting, despite the fact that the initial mapping to spin coherent states introduces its own sign problem. The standard formulation of the spin coherent path integral is known to make use of uncontrolled approximations; despite this, for large spins they are typically considered to yield accurate results, so it is somewhat surprising that our results show significant systematic errors. Therefore, possibly of independent interest, our use of Lefschetz thimbles to overcome the intrinsic sign problem in spin coherent state path integral Monte Carlo enables a novel numerical demonstration of a breakdown in the spin coherent path integral.

We predict that the amplitude of the Josephson current through an antiferromagnetic weak link changes by several orders of magnitude upon rotation of the Néel order parameter characterizing the staggered magnetic order. This occurs due to the presence of spin-orbit coupling arising from structural inversion asymmetry which makes the band gap in the antiferromagnet highly sensitive to the staggered order parameter direction. We also demonstrate that when phase-biasing the junction, magnetization dynamics is induced which switches the direction of the Néel vector in the antiferromagnet. These results reveal an interesting versatility of antiferromagnetic Josephson junctions as they offer both a large tunability of the supercurrent magnitude via the staggered magnetization and phase-coherent control over the Néel order parameter.

We theoretically investigate how the Berry curvature, which arises in multiband structures when the electrons can be described by an effective single-band Hamiltonian, affects the superconducting properties of two-dimensional electronic systems. Generically, the Berry curvature is coupled to electric fields beyond those created by the periodic crystal potential. A potential source of such electric fields, which vary slowly on the lattice scale, is the mutual interaction between the electrons. We show that the Berry curvature provides additional terms in the Hamiltonian describing interacting electrons within a single band. When these terms are taken into account in the framework of the usual BCS weak-coupling treatment of a generic attractive interaction that allows for the formation of Cooper pairs, the coupling constant is modified. In pure singlet and triplet superconductors, we find that the Berry curvature generally lowers the coupling constant and thus the superconducting gap and the critical temperature as a function of doping. From an experimental point of view, a measured deviation from the expected BCS critical temperature upon doping, e.g., in doped two-dimensional transition-metal dichalcogenides, may unveil the strength of the Berry curvature.

We study the effects of a random magnetic field on a one-dimensional (1D) spin-1 chain with correlated nearest-neighbor XY interaction. We show that this spin model can be exactly mapped onto the 1D disordered tight-binding model of ${\mathbb{Z}}_3$ Fock parafermions (FPFs), exotic anyonic quasiparticles that generalize usual spinless fermions. Thus, we have a peculiar case of a disordered Hamiltonian that despite being bilinear in the creation and annihilation operators, exhibits a many-body localization (MBL) transition owing to the nontrivial statistics of FPFs. This is in sharp contrast to conventional bosonic and fermionic quadratic disordered Hamiltonians that show single-particle (Anderson) localization. We perform finite-size exact diagonalization calculations of level-spacing statistics, fractal dimensions, and entanglement entropy, and provide convincing evidence for the MBL transition at finite disorder strength.

Recently, we have predicted, solely on the basis of symmetry arguments, the existence of strong magnetochiral anisotropy in the displacement current in chiral dielectrics and reported its experimental observation in chiral ferroelectrics near their ferroelectric-paraelectric phase transitions [Nat. Commun. 13, 3564 (2022).]. Here, we present a microscopic model that describes both direct and inverse dielectric magnetochiral anisotropy (dMChA) in the chiral ferroelectric triglycine sulfate (TGS), based on the specifics of the charge movement in a time-varying electric field. We predict a strong inverse dMChA in TGS that should be within reach of experimental observation and we find good agreement between our model and the experimental observation of the direct dMChA in TGS.