Electronic transport in the $\alpha \text{−}{\mathcal{T}}_3$ model of pseudospin-1 fermions with a finite gap is studied within the semiclassical Boltzmann approximation. We show that coupling of the orbital magnetic moment to the external magnetic field, which is otherwise absent in the massless model, breaks valley symmetry, results in finite and measurable corrections to the longitudinal and Hall conductivity, and yields a geometrical contribution to the Hall conductivity due to the Berry curvature. We also show that, for a generic two-dimensional system, remarkably, magnetoresistance induced by the orbital magnetic moment can be either positive or negative; the sign depends on the amount of disorder and is different for both conventional and geometric contributions to the magnetoresistance.

We develop a theory for the coaxial configuration of a Néel-type skyrmion and a Pearl vortex in thin superconductor-chiral ferromagnetic heterostructures. Using an exact numerical solution of the Euler-Lagrange equation and micromagnetic simulations, we demonstrate that the inhomogeneous magnetic field of the Pearl vortex significantly modifies the skyrmion profile with respect to the same profile in the absence of the vortex. We discover drastic enlargement of the radius of the skyrmion and inversion of the chirality of the skyrmion. To unravel the physics behind these effects, we invent a two-parameter ansatz for the magnetization profile of the skyrmion in the presence of the vortex. Chirality inversion and radius blowup are controlled not only by the material parameters of the heterostructure but also by the thickness of the superconductor. Our findings can have implications for Majorana modes localized at skyrmion-vortex pairs.

The nature of the gap observed at the zone border in the spin excitation spectrum of ${\mathrm{CrI}}_3$ quasi-two-dimensional single crystals is still controversial. We perform first-principles calculations based on time-dependent density functional perturbation theory, which indicate that the observed gap results from a combination of spin-orbit and interlayer interaction effects. The former give rise to the anisotropic spin-spin interactions that are responsible for its very existence, while the latter determine both its displacement from the $K$ point of the Brillouin zone, due to the in-plane lattice distortions induced by them, and an enhancement of its magnitude, in agreement with experiments and previous theoretical work based on a lattice model.

The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state is an exotic superconducting phase formed by Cooper pairs with finite center-of-mass momentum $\mathit{q}$. On theoretical grounds, the superconducting order parameter in the FFLO state is spatially modulated along the $\mathit{q}$ vector, and the emergence of an associated anisotropy is expected at the phase transition from the Abrikosov state to the FFLO state. Here, we report the results of high-resolution magnetostriction measurements for a single crystal of ${\mathrm{CeCoIn}}_5$ around $B\parallel c$. We find two anomalies in the magnetostriction along the $c$ axis, parallel to the magnetic-field orientation. In sharp contrast, this $B_{\mathrm{K}}$ anomaly disappears in the magnetostriction along the $a$-axis direction, perpendicular to the magnetic-field orientation. To explain this uniaxial expansion, we suggest a possibility that the FFLO transition occurs slightly below the upper critical field, and the FFLO modulation vector parallel to the applied magnetic field gives rise to the anisotropic response.

Recent advances in numerical methods significantly pushed forward the understanding of electrons coupled to quantized lattice vibrations. At this stage, it becomes increasingly important to also account for the effects of physically inevitable environments. Here, we combine state-of-the-art tensor-network and quantum trajectories methods in order to study the impact of dissipation on realistic condensed matter models including highly excited phononic modes. In particular, we study the transport properties of the Hubbard-Holstein Hamiltonian that models a large class of materials characterized by strong electron-phonon coupling, in contact with a dissipative environment. We combine the non-Markovian hierarchy of pure states method and the Markovian quantum jumps method with the newly introduced projected purified density-matrix renormalization group, creating powerful tensor-network methods for dissipative quantum many-body systems. Investigating their numerical properties, we find a significant speedup up to a factor $\approx 30$ compared to conventional tensor-network techniques. We apply these methods to study dissipative quenches, aiming for an in-depth understanding of the formation, stability, and quasiparticle properties of bipolarons. Surprisingly, our results show that in the metallic phase dissipation localizes the bipolarons, which is reminiscent of an indirect quantum Zeno effect. However, the bipolaronic binding energy remains mainly unaffected, even in the presence of strong dissipation, exhibiting remarkable bipolaron stability. These findings shed light on the problem of designing real materials exhibiting phonon-mediated high-$T_{\mathrm{C}}$ superconductivity.

A solution to energy-efficient magnetization switching in a nanoparticle with biaxial anisotropy is presented. Optimal control paths minimizing the energy cost of magnetization reversal are calculated numerically as functions of the switching time and materials properties, and used to derive energy-efficient switching pulses of external magnetic field. Hard-axis anisotropy reduces the minimum energy cost of magnetization switching due to the internal torque in the desired switching direction. Analytical estimates quantifying this effect are obtained based on the perturbation theory. The optimal switching time providing a tradeoff between fast switching and energy efficiency is obtained. The energy cost of switching and the energy barrier between the stable states can be controlled independently in a biaxial nanomagnet. This provides a solution to the dilemma between energy-efficient writability and good thermal stability of magnetic memory elements.

We investigate the structural, static, and dynamic magnetic properties of exchange spring (ES) nanohetero-structured, stress-free, optically smooth films of amorphous/nanocrystalline Co-rich cobalt phosphorous (CoP) prepared by electrodeposition technique at room temperature. Static magnetic measurement reveals different hysteresis loop structures of the thin films — evolution from the low coercivity non-ES loop to the staircase-ES loop, giving rise to multiple coercivities for relatively higher thickness films. The first-order reversal curve (FORC) distributions demonstrate the different reversal mechanisms present in the samples and confirm the non-ES and ES natures of the films. The field-dependent Brillouin light scattering (BLS) spectra of the ES thin film unveil two well-resolved spin wave peaks associated with the bulk modes ($B$) and the so-called Damon-Eshbach (DE) surface spin wave modes ($S$) while that for non-ES low coercivity sample show a doublet of modes below a certain value of the applied field. Observation of the $S$ mode only on one side of the measured spectra depends on the direction of the external magnetic field due to the nonreciprocal character of the $S$ wave in the micrometer thickness of the investigated films. The external magnetic field-dependent BLS spectra yield an almost linear dependence of the mode frequencies versus the magnetic field intensity. Studied BLS measurements demonstrate the evolution of the ES structure in $5.7\text{−}\mu \text{m}$-thick nanohetero-structured CoP film compared to the $1.4\text{−}\mu \text{m}$-thick non-ES sample. Additionally, the increase in the interfacial exchange energy value ($J_I^{ES}-10.12\text{erg}/{\text{cm}}^2$) in the $5.7\mu \text{m}$ film compared to that ($J_I^{\text{non-ES}}-3.68\text{erg}/{\text{cm}}^2$) of $1.4\text{−}\mu \text{m}$ film (calculated from corresponding asymmetric peak fittings to the measured BLS spectra) reduces the interfacial exchange energy ratio ($J_I^{\text{non-ES}}/J_I^{\text{ES}}$) below unity, confirming the enhanced strength of the exchange coupling in the developed ES film.

The magnetic textures present in Weyl semimetals are intricately linked to their topological structure and exhibit dynamic interactions with it. In this study, we investigate the structural phase transitions in domain walls induced by electric fields, mediated by the spin transfer torque, and explore their impact on charge transport. Specific electric field orientations are found to result in the complete disappearance of a N'eel domain wall, while a Bloch domain wall undergoes a reconfiguration into a head-to-head structure as the magnetization on either side of it is rotated. These findings highlight the remarkable dynamics of domain walls and their influence on the local environment. Interestingly, the dynamics of domain walls give rise to transient, localized, nonlinear anomalous Hall effects and nonlinear anomalous drift currents, which can directly probe the magnetization dynamics and the precise location of the domain wall. We also discuss the experimental observations conducted using cutting-edge samples.

Using machine learning (ML) to recognize different phases of matter and to infer the entire phase diagram has proven to be an effective tool given a large dataset. In our previous proposals, we have successfully explored phase transitions for topological phases of matter at low dimensions either in a supervised or an unsupervised learning protocol with the assistance of quantum-information-related quantities. In this work, we adopt our previous ML procedures to study quantum phase transitions of magnetism systems such as the $XY$ and $XXZ$ spin chains by using spin-spin correlation functions as the input data. We find that our proposed approach not only maps out the phase diagrams with accurate phase boundaries, but also indicates some features that have not been observed in the field of machine learning before. In particular, we define so-called relevant correlation functions to some corresponding phases that can always distinguish between those and their neighbors. Based on the unsupervised learning protocol we proposed [Phys. Rev. B 104, 165108 (2021)], the reduced latent representations of the inputs combined with the clustering algorithm show the connectedness or disconnectedness between neighboring clusters (phases) just corresponding to the continuous or disrupt quantum phase transition, respectively. This property reminds us of the behavior of order parameters. Moreover, in the silhouette analysis we show that the ferromagnetic states in the $XXZ$ model with various anisotropy parameters correspond to almost the same silhouette value, while the critical or antiferromagnetic states behave quite differently. The analysis further indicates that the minima of silhouette values are close to the phase-transition points, showing strong positive correlation. These results again justify the usefulness of our proposed ML procedures, and they move us a step further toward understanding the relation between ML and quantum phase transitions from correlation function aspects.

By applying auxiliary-field quantum Monte Carlo, we calculate the equation of state (EOS) and B1-B2 phase transition of magnesium oxide (MgO) up to 1 TPa. The results agree with available experimental data at low pressures and are used to benchmark the performance of various exchange-correlation functionals in density functional theory calculations. We determine PBEsol is an optimal choice for the exchange-correlation functional and perform extensive phonon and quantum molecular-dynamics calculations to obtain the thermal EOS. Our results provide a preliminary reference for the EOS and B1-B2 phase boundary of MgO from 0 to 10 500 K.

Cadmium arsenide (${\mathrm{Cd}}_3{\mathrm{As}}_2$) is a prototypical Dirac semimetal that manifests topological properties in a three-dimensional (3D) bulk material. In defect-free ${\mathrm{Cd}}_3{\mathrm{As}}_2$, the Fermi level $E_{\mathrm{F}}$ lies at a minimum in the density of states at the Dirac point, but experimentally it forms with excess electron carriers and an elevated $E_{\mathrm{F}}$, thereby masking the topological features. To computationally study the self-doping of ${\mathrm{Cd}}_3{\mathrm{As}}_2$, we combine density functional theory (DFT) calculations for defect formation energies with quasiparticle self-consistent GW (QSGW) electronic structure calculations. We demonstrate an innate dependence of the point defect formation energies on carrier concentrations and use the QSGW calculated density of states to extrapolate formation energies to arbitrary electron concentrations. This approach allows the quantitative modeling of thermodynamic defect equilibria in topological semimetals and is used to predict how control of growth conditions might be utilized to achieve doping-neutral ${\mathrm{Cd}}_3{\mathrm{As}}_2$.

We obtain approximate solutions defining the mobility edge separating localized and extended states for several classes of generic one-dimensional quasiperiodic models. We validate our analytical ansatz with exact numerical calculations. Rather amazingly, we provide a single simple ansatz for the generic mobility edge, which is satisfied by quasiperiodic models involving many different types of nonsinusoidal incommensurate potentials as well as many different types of long-range hopping models. Our ansatz agrees precisely with the well-known limiting cases of the sinusoidal Aubry-André model (which has no mobility edge) and the generalized Aubry-André models (which have analytical mobility edges). Our work provides a practical tool for estimating the location of mobility edges in quasiperiodic systems.

We investigate the infinite-temperature dynamics of the complex Sachdev-Ye-Kitaev model $\left({\mathrm{SYK}}_4\right)$ complemented with a single-particle hopping term $\left({\mathrm{SYK}}_2\right)$, leading to the chaos-to-integrable crossover of the many-body eigenstates. Due to the presence of the all-to-all connected ${\mathrm{SYK}}_2$ term, a nonequilibrium prethermal state emerges for a finite time window $t_{\mathrm{th}}\propto 2^{a/{\lambda }^{2/5}}$ that scales with the relative interaction strength $\lambda$, between the SYK terms before eventually exhibiting thermalization for all $\lambda$. The scaling of the plateau with $\lambda$ is consistent with the many-body Fock-space structure of the time-evolved wave function. In the integrable limit, the wave function in the Fock space has a stretched exponential dependence on distance. On the contrary, in the ${\mathrm{SYK}}_4$ limit, it is distributed equally over the Fock-space points characterizing the ergodic phase at long times.

Transition-metal dichalcogenides (TMDs) are layered compounds that support many electronic phases, including various charge density waves, superconducting, and Mott insulating states. Their intercalation with magnetic ions introduces magnetic sublayers, which strongly influence the coupling between host layers, and feature various magnetic states adjustable by external means. ${\mathrm{Co}}_{1/3}\mathrm{Nb}{\mathrm{S}}_2$ hosts a particularly sensitive magnetic subsystem with the lowest magnetic ordering temperature in the family of magnetically intercalated TMDs and is the only one where the complete suppression of magnetic order under pressure has been recently suggested. By combining the results of several experimental methods, electronic ab initio calculations, and modeling, we develop insights into the mechanisms of electric transport, magnetic ordering, and their interaction in this compound. The elastic neutron scattering is used to directly follow the evolution of the antiferromagnetic order parameter with pressure and temperature. Our results unambiguously disclose the complete suppression of the observed magnetic order around $1.7\mathrm{GPa}$. We delve into possible mechanisms of magnetic order suppression under pressure, highlighting the role of magnetic frustrations indicated by magnetic susceptibility measurements and ab initio calculations. Electronic conduction anisotropy is measured in a wide temperature and pressure range. Here, we show that the transport in directions along and perpendicular to layers respond differently to the appearance of magnetic ordering or the application of the hydrostatic pressure. We propose a spin-valve mechanism where the intercalated Co ions act as spin-selective electrical transport bridges between host layers. The mechanism applies to various magnetic states and can be extended to other magnetically intercalated TMDs.

Many frustrated spin models on three-dimensional (3D) lattices are currently being investigated, both experimentally and theoretically, and develop new types of long-range orders in their respective phase diagrams. They present finite-temperature phase transitions, most likely in the Heisenberg 3D universality class. However, the combination between the 3D character and frustration makes them hard to study. We present here several methods derived from high-temperature series expansions (HTSEs), which give exact coefficients directly in the thermodynamic limit up to a certain order; for several 3D lattices, supplementary orders than in previous literature are reported for the HTSEs. We introduce an interpolation method able to describe thermodynamic quantities at $T>T_c$, which we use here to reconstruct the magnetic susceptibility and the specific heat and to extract universal and nonuniversal quantities (for example, critical exponents, temperature, energy, entropy, and other parameters related to the phase transition). While the susceptibility associated with the order parameter is not usually known for more exotic long-range orders, the specific heat is indicative of a phase transition for any kind of symmetry breaking. We present examples of applications on ferromagnetic and antiferromagnetic models on various 3D lattices and benchmark our results whenever possible.

${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ is predicted to be a candidate for high-temperature superconductivity upon carrier doping, whereas extensive research has proved it challenging to obtain a metallic phase in this compound, especially in thin films. Here, we explore the impact of stoichiometry on the crystallinity, as well as carrier doping and epitaxial strain on the transport properties of ${\mathrm{Sr}}_{2-x}{\mathrm{Nd}}_x\mathrm{Ir}{\mathrm{O}}_4$ thin films. Via fine tuning the stoichiometry, the crystallinity of ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ films can be greatly enhanced, as indicated by the minimized systematic deviations of the x-ray diffraction peak positions. As the cation doping level increases, the resistivity of ${\mathrm{Sr}}_{2-x}{\mathrm{Nd}}_x\mathrm{Ir}{\mathrm{O}}_4$ films decreases, but it remains semiconducting even at a high level of $x=0.4$ where the resistivity has dropped by three orders of magnitude. By further applying compressive epitaxial strain, the ${\mathrm{Sr}}_{1.8}{\mathrm{Nd}}_{0.2}\mathrm{Ir}{\mathrm{O}}_4$ films exhibit a metalliclike behavior with an upturn at low temperature. Our finding reveals a promising combination of electron doping and compressive strain to narrow the band gap and tune the transport properties in iridate films for potential superconductivity.

Non-Hermitian physics in open quantum many-body systems provides novel opportunities for the discovery of exotic quantum phenomena unexpected in Hermitian systems. A previous study of the non-Hermitian Kondo problem in ultracold atoms reports reversion of renormalization group flows which violates the $g$ theorem and produces an unusual quantum phase transition. In this work, we study the effect of electron-electron interactions by considering the non-Hermitian Kondo problem in a Luttinger liquid. By performing a perturbative renormalization group analysis to two-loop order, we find that the interplay between non-Hermitian Kondo couplings and electron-electron interactions can produce a pair of complex fixed points. Complex fixed points have often been discussed in an attempt to understand the extremely long correlation length of Hermitian systems with weakly first-order transitions. Here, we show that complex fixed points arise naturally and can be physically realized in open quantum systems. We discuss consequences of the complex fixed points and future directions.

For two-dimensional (2D) polarized monolayers or type-II heterostructure semiconductors, the semiconductor-to-metal transition (SMT) process usually occurs under doping engineering or external electric field. In this work, we point out that the evolution of band offset, surface charge redistribution, and inner-layer charge transfer are synergetic in polarized monolayer semiconductors. Taking quintuple-layer (QL) ${\mathrm{Al}}_2{\mathrm{S}}_3$ as an example, the SMT process occurs by substituting the surface S atom with the O atom and applying external electric field by two methods. Our results reveal that the SMT derives from the increased surface charge on two-surface atoms. The SMT mechanism in polarized QL-${\mathrm{Al}}_2{\mathrm{S}}_3$ is also applicable to other 2D polarized monolayers and heterostructure semiconductors.

Topological mosaic pattern (TMP) can be formed in two-dimensional (2D) moiré superlattices, a set of periodic and spatially separated domains with distinct topologies that give rise to periodic edge states on the domain walls. In this study, we demonstrate that these periodic edge states play a crucial role in determining global topological properties. By developing a continuum model for periodic edge states with $C_{6z}$ and $C_{3z}$ rotational symmetry, we predict that a global topological phase transition at the charge neutrality point (CNP) can be driven by the size of domain walls and moiré period. The Wannier representation analysis reveals that these periodic edge states are fundamentally chiral $p_x\pm i{p}_y$ orbitals. The interplay between on-site chiral orbital rotation and neighboring hopping among chiral orbitals leads to band inversion and a topological phase transition. Our work establishes a general model for tuning local and global topological phases, paving the way for future research on strongly correlated topological flat minibands within topological mosaic patterns.