A superconductor/ferromagnet/superconductor Josephson junction with anomalous phase shift $({\phi }_0\text{−}\mathrm{S}/\mathrm{F}/\mathrm{S} \mathrm{JJ})$ is a system in which the anomalous ground state shift ${\phi }_0$ provides direct magnetoelectric coupling between a magnetic moment and a phase of the superconducting condensate. If a chain of such ${\phi }_0\text{−}\mathrm{S}/\mathrm{F}/\mathrm{S}$ JJs are coupled via superconducting leads, the condensate phase, being a macroscopic quantity, mediates a long-range interaction between the magnetic moments ${\mathit{M}}_i$ of the weak links. We study the static and dynamic magnetic properties of such a system. It is shown that it manifests properties of an $n$-level system, in which the energies of the levels are determined by only projections of the total magnetic moment $\sum {\mathit{M}}_i$ onto the easy magnetic axis. It is similar to a magnetic atom in a Zeeman field, but the role of the field is played by the magnetoelectric coupling. However, unlike an atom in a magnetic field, the relative order of energies of different states is controlled by electrical means. It is also demonstrated that $\sum {\mathit{M}}_i$ can be fully controlled by a supercurrent and the response of the magnetic system to local external perturbations is highly nonlocal.

Two-dimensional (2D) ferromagnets with controllable magnetic anisotropy hold great promise for quantum information processing and sensing. However, realizing large magnetic anisotropy and strong exchange interactions in stable 2D magnetic materials remains a major challenge. Here, we present the identification of strong in-plane magnetic anisotropy in CrSBr-type chromium chalcohalide monolayers. Importantly, the different easy axes of magnetization can be further controlled by applying mechanical strain. The underlying physics are related to the change in the orbital magnetic moment dependent on the lattice. Furthermore, our investigation demonstrates that the optical properties of the ${\mathrm{Cr}}_2{\mathrm{SSeBr}}_2$ monolayer are sensitive to the magnetization direction. This study is interesting for achieving new 2D ferromagnets and advancing nanoelectronic devices.

The physics of skyrmions, and in particular the issue of how to isolate and manipulate them individually, is a subject of major importance nowadays in the community of magnetism. In this article we present an in-depth extension of a study on this issue that was recently proposed by some of the authors [H. D. Rosales et al., Phys. Rev. Lett. 130, 106703 (2023)]. More precisely, we analyze the competition between skyrmions and a chiral spin liquid in a model on the kagome lattice. We first present an analytical overview of the low-energy states using the Luttinger-Tisza approximation. We then study the effect of thermal fluctuations thanks to large-scale Monte Carlo simulations and explore the entire parameter space with a magnetic field $B$, in-plane $D^{xy}$, and out-of-plane $D{}^z$ Dzyaloshinskii-Moriya interactions. While skyrmions and the chiral spin liquid live in different regions of the parameter space, we show how to bring them together, stabilizing a skyrmion fluid in between; a region where the density of well-defined skyrmions can be tuned before obtaining an ordered phase. We investigate in particular the melting of the skyrmion solid. Our analysis also brings to light a long-range ordered phase with ${\mathrm{Z}}_3$ symmetry. At last, we initiate the study of this rich magnetic background on conduction electrons that are coupled to the local spins. We study how the different chiral magnetic textures stabilized in this model (skyrmion solid, liquid, and gas and chiral spin liquid) induce a topological quantum Hall effect. We observe in the ordered skyrmion phase the appearance of Landau levels which persist even in the skyrmion-liquid regime and gradually disappear as the skyrmion density decreases to form a gas.

Heavy-fermion superconductors exhibit unusual properties such as low transition temperatures, large specific heat jumps, and normal states with universal two-fluid behaviors. Standard theoretical models such as the Anderson or Kondo lattices often require uncontrolled approximations to solve. Here, we propose an exactly solvable Kondo lattice model with a local-in-momentum Kondo interaction, which can be derived from an Anderson lattice with a Hatsugai-Kohmoto interaction between $f$ electrons. As the Kondo coupling increases, the normal state evolves from a Fermi liquid to a non-Fermi liquid two-fluid state. With a pairing interaction between electrons, the model exhibits a crossover from a weak-coupling superconductor to a strong-coupling one showing many important features of heavy-fermion superconductivity, including the formation of Kondo-induced composite-fermion Cooper pairs, the analog of heavy-electron Cooper pairs in real materials.

Semiconductor quantum dots in close proximity to superconductors may provoke localized bound states within the superconducting energy gap known as the Yu-Shiba-Rusinov state, which is a promising candidate for constructing Majorana zero modes and topological qubits. Side-coupled double quantum dot systems are ideal platforms revealing the secondary proximity effect. Numerical renormalization group calculations show that if the central quantum dot can be treated as a noninteracting resonant level, it acts as a superconducting medium due to the ordinary proximity effect. The bound state in the side dot behaves as the case of a single impurity connected to two superconducting leads. The side dot undergoes a quantum phase transition between a spin-singlet state and a doublet state as the Coulomb repulsion, the interdot coupling strength, or the energy level sweeps. Phase diagrams indicate that the phase boundaries could be well illustrated by $\mathrm{\Delta }\approx c{T}_{K2}$ in all cases, where $\mathrm{\Delta }$ is the superconducting gap, $T_{K2}$ is the side Kondo temperature and $c$ is of the order 1.0. These findings offer valuable insights into the secondary proximity effect, which is a promising approach for realizing superconducting couplings between quantum dots and reducing the random-disorder potential via quantum interferences.

Motivated by the emergence of higher-order van Hove singularities (VHS) with power-law divergent density of states (DOS) $({\rho }_c\left(\omega \right)={\rho }_0/{\left|\omega \right|}^r, 0<r<1)$ in materials, we investigate a multichannel Kondo model involving conduction electrons near the higher-order van Hove filling, termed power-law diverging multichannel Kondo model. This model considers $M$ channel and $N$ spin degrees of freedom. Employing a renormalization group analysis and dynamical large-$N$ approach, our results reveal the emergence of a pseudogap behavior in the spectral properties of the local impurity at the overscreened fixed point. We precisely determine the conditions under which the pseudogap behavior occurs, either by tuning the exponent $r$ or the ratio $\kappa =M/N$ to values such as $r^{*}$ or ${\kappa }^{*}$. This pseudogap phase of spinon exhibits distinct physical properties that could have an impact on the properties of real systems. The results of this study provide novel insights into the pseudogap behaviors in strongly correlated systems and offer a playground to study the interplay between higher-order van Hove singularities and multichannel Kondo physics.

Kohn's theorem places strong constraints on the cyclotron response of Fermi liquids. Recent observations of a doping dependence in the cyclotron mass of ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ [Legros et al., Phys. Rev. B 106, 195110 (2022)] are therefore surprising because the cyclotron mass can only be renormalized by large momentum umklapp interactions, which are not expected to vary significantly with doping. We show that a version of Kohn's theorem continues to apply to disorder-free non-Fermi-liquids with a critical boson near zero momentum. However, marginal Fermi liquids arising from a spatially random Yukawa coupling between the electrons and bosons do give rise to significant corrections to the cyclotron mass that we compute. This is the same theory that yields linear-in-temperature resistivity and other properties of strange metals at zero fields [Patel et al., Science 381, 790 (2023)].

Pair spin-orbit interaction can emerge in strongly interacting systems characterized by a large spin-orbit coupling. Here we study the role of this interaction in stabilizing ordered and unconventional superconducting phases. We find that, if the system avoids superconductivity, the order realized is a combination of charge density and spin-vorticity waves. The latter is reminiscent of a loop-current state, albeit in the spin channel, rather than in the charge channel. If the system becomes superconducting, the order parameter assumes the form of a paired density wave, i.e., pairing occurs at finite momentum. Intriguingly, one of the possible pairings acquires a form analogous to Amperean superconductivity. However, the order parameter here is always a blend of paired density wave and Amperean pairing, rather than being purely one or the other.

In this work, we study unconventional anisotropic topologically ordered phases in $3d$ that manifest type-II fractonic physics along submanifolds. While they behave as usual topological order along a preferred spatial direction, their physics along perpendicular planes is dictated by the presence of fractal subsystem symmetries, completely restricting the mobility of anyonic excitations and their bound states. We consider an explicit lattice model realization of such phases and proceed to study their properties under periodic boundary conditions and, later, in the presence of boundaries. We find that for specific lattice sizes, the system possesses line and fractal membrane symmetries that are mutually anomalous, resulting in a nontrivially gapped ground state space. This amounts to the spontaneous breaking of the fractal symmetries, implying a subextensive ground state degeneracy. For the remaining system sizes the fractal symmetries are explicitly broken by the periodic boundary conditions, which is intrinsically related to the uniqueness of the ground state. Despite that, the system is still topologically ordered since locally created quasiparticles have nontrivial mutual statistics and, in the presence of boundaries, it still presents anomalous edge modes. The intricate symmetry interplay dictated by the lattice size is a wild manifestation of ultraviolet/infrared (UV/IR) mixing.

The relation of $5f$-electron localization and valence-band x-ray and ultraviolet photoelectron spectra (XPS, UPS) is analyzed on the example of uranium binary hydrides ${\mathrm{UH}}_3$ and ${\mathrm{UH}}_2$. Confronted with results of density functional theory (DFT) and $\mathrm{DFT}+U$ calculations, it is recognized that electron-electron correlations play an important role. The spectra can be well accounted for by dynamical mean-field theory (DMFT) calculations providing insight into the final state remaining after the photoexcitation event. It is concluded that details of the spectra reflect the dominant final-state $5f^2$ multiplets with the main line ${}^{3}H_4$ adjacent to the Fermi level and the first excited line ${}^{3}F_2$ responsible for a shoulder at 0.5 eV binding energy. Calculations with a varied strength of hybridization between $5f$ and non-$f$ states allow us to visualize how the individual multiplet lines shift in energy. The one-to-one correspondence between features seen in the DMFT calculations and free-ion multiplets remains noticeable even with the realistic strength of the hybridization included, but the positions of some of the lines can vary substantially. The width of the distribution characterizing fluctuations of the $5f$ filling among the individual integral $5f^N$ states, approximated by a Gaussian distribution with a certain width, is suggested as a quantification of the $5f$ delocalization. In this respect, the hydrides are found to be more delocalized than ${\mathrm{UGa}}_2$ or $\delta$-Pu.

We present here a Hofstadter's butterfly spectrum for the magic-angle twisted bilayer graphene obtained using an ab initio-based multimillion-atom tight-binding model. We incorporate a hexagonal boron nitride substrate and out-of-plane atomic relaxation. The effects of a magnetic field are introduced via the Peierls modification of the long-range tight-binding matrix elements and the Zeeman spin splitting effects. A nanoribbon geometry is studied, and the quantum size effects for the sample widths up to 1 $\mu \mathrm{m}$ are analyzed both for a large energy window and for the flat band around the Fermi level. For sufficiently wide ribbons, where the role of the finite geometry is minimized, we obtain and plot the Hofstadter spectrum and identify the in-gap Chern numbers by counting the total number of chiral edge states crossing these gaps. Subsequently, we examine the Wannier diagrams to identify the insulating states at charge neutrality. We establish the presence of three types of electronic states: moiré, mixed, and conventional. These states describe both the bulk Landau levels and the edge states crossing gaps in the spectrum. The evolution of the bulk moiré flat band wave functions in the magnetic field is investigated, predicting a decay of the electronic density from the moiré centers as the magnetic flux increases. Furthermore, the spatial properties of the three types of edge states are studied, illustrating the evolution of their localization as a function of the nanoribbon momentum.

The optical phonon spectra in an HgTe quantum well surrounded by CdHgTe barriers are calculated, taking into account the contribution of free electrons to the dielectric permittivity of the quantum well. It is shown that free electrons not only change the phonon spectrum, but they can also change the number of branches of the surface optical phonons. The frequencies of the electron-optical phonon collisions, the wave-vector relaxation rates, and the energy relaxation rates are calculated for different temperatures and electron concentrations. The dependencies of the momentum and energy scattering frequencies on the electron kinetic energy are found.

We investigate the effect of oblique magnetic field configurations on a singly charged self-assembled quantum dot system as a means to tune the spin composition of the ground electron spin eigenstates. Using magneto-optical spectroscopy and Stokes polarimetry techniques, we evaluate the anisotropic $g$ factors and characterize the polarization properties of the charged quantum dot system under oblique fields. We compare the results to a simple model that captures the resulting level structure and polarization selection rules for arbitrary magnetic field orientations. Under oblique magnetic fields, the system's ground spin eigenstates are composed of unequal superpositions of the electron spins. This provides an additional degree of freedom to tailor the composition of the ground spin states in charged quantum dots and based on this we demonstrate spin pumping and initialization of the tailored ground states, confirming that the double-Λ level structure of the charged quantum dot persists in oblique magnetic fields. These combined results show that the uneven weightings of the tailored spin states can yield systems with interesting behaviors, with a potential towards developing spin-selective readout schemes to further enhance the capabilities of spin qubits.

Spin-orbital texture in topological insulators due to the spin locking with the electron momentum play an important role in spintronic phenomena that arise from the interplay between charge and spin degrees of freedom. We have explored interfaces between a ferromagnetic system $\left({\mathrm{CrI}}_3\right)$ and a topological insulator $\left({\mathrm{Bi}}_2{\mathrm{Se}}_3\right)$ that allow the manipulation of spin-orbital texture. Within an ab initio approach we have extracted the spin-orbital-texture dependence of experimentally achievable interface designs. The presence of the ferromagnetic system introduces anisotropic transport of the electronic spin and charge. From a parametrized Hamiltonian model we capture the anisotropic backscattering behavior, showing its extension to other ferromagnetic/topological insulator interfaces. We verified that the van der Waals TI/MI interface is an excellent platform for controlling the spin degree of freedom arising from topological states, providing a rich family of unconventional spin-texture configurations.

Motivated by the recent experimental breakthroughs in observing fractional quantum anomalous Hall (FQAH) states in moiré materials, we propose and study various unconventional phase transitions between quantum Hall phases and Fermi liquids or charge ordered phases upon tuning the bandwidth. At a fixed rational lattice filling $\nu$, we describe a quantum Ginzburg-Landau theory to describe the intertwinement between the FQAH and charge density wave (CDW) orders. We use this theory to describe phase transitions between the FQAH and a CDW insulator. The critical theory for a direct second-order transition resembles that of the familiar deconfined quantum critical point (DQCP) but with an additional Chern-Simons term. At filling $\nu =-1/2$, we study the possibility of a continuous transition between the composite Fermi liquid (CFL) and the Fermi liquid (FL) building on and refining previous work by Barkeshli and McGreevy [Phys. Rev. B 86, 075136 (2012)]. Crucially we show that filling constraints, ignored in that work, ensure that translation symmetry alone is enough to enable a second-order CFL-FL transition. We argue that there must be critical CDW fluctuations though neither phase has long-range CDW order. We present experimental signatures the most striking of which is a universal jump of both longitudinal and Hall resistivities at the critical point. With disorder, we argue that the CDW order gets pinned and the CFL-FL evolution happens through an intermediate electrically insulating phase with mobile neutral fermions. A clean analog of this insulating phase with long-range CDW order and a neutral Fermi surface can potentially also exist. We discuss the properties of this phase and the nature of its phase transitions. We also present a critical theory for the CFL to FL transition at filling $\nu =-3/4$. Our work opens up a new avenue to realize deconfined criticality and fractionalized phases beyond familiar Landau level physics in the moiré Chern band system.

Multiferroic tunnel junctions (MFTJs) have attracted considerable attention due to their multifunctional properties, which are valuable for nonvolatile memory devices. The recent advancements in van der Waals (vdW) multiferroic materials, combining ferromagnetic and ferroelectric properties, provide an excellent platform for exploring MFTJs at the atomic scale. In this study, we employ a combination of nonequilibrium Green's function and density functional theory to theoretically investigate the spin-dependent transport properties of vdW MFTJs, which consist of metal electrodes and sliding multiferroic layered ${\mathrm{VSi}}_2{\mathrm{N}}_4$ barrier layers. Our findings demonstrate that asymmetric $\mathrm{Ag}\left(111\right)/\text{bilayer}\text{−}{\mathrm{VSi}}_2{\mathrm{N}}_4/\mathrm{Au}\left(111\right)$ MFTJs can exhibit multiple nonvolatile resistance states by manipulating the ferroelectric polarization and magnetization alignment of the bilayer ${\mathrm{VSi}}_2{\mathrm{N}}_4$, achieving maximum tunneling magnetoresistance (TMR) and tunneling electroresistance (TER) ratios of up to $1.01\times {10}^5\%$ and 37.3%, respectively. More intriguingly, the TER ratio can be further increased to 448.3% by employing left and right symmetric Au(111) electrodes and trilayer ${\mathrm{VSi}}_2{\mathrm{N}}_4$ barrier layers. Additionally, we reveal that layered ${\mathrm{VSi}}_2{\mathrm{N}}_4$ possesses intrinsic multiferroicity with the coexistence of the out of plane ferroelectricity and interlayer A-type antiferromagnetism. Through an analysis of electronic structure and Berry curvature, we elucidate the coupling between ferroelectricity and antiferromagnetism via a ferrovalley, enabling electrically controlled magnetism in the bilayer ${\mathrm{VSi}}_2{\mathrm{N}}_4$ by interlayer sliding. Our results demonstrate that giant TMR, large TER, and multiferroic coupling can coexist in layered ${\mathrm{VSi}}_2{\mathrm{N}}_4$, with potential applications in other vdW layered multiferroics. The controllable interlayer sliding of vdW MFTJs offers promising opportunities for the design of next-generation logic and memory devices.

We predict here the fine structure of an electrically tunable negatively charged exciton (trion) composed of two electrons and a hole confined in a gated bilayer graphene quantum dot (QD). We start with an atomistic approach, allowing us to compute confined electron and confined hole QD states for a structure containing over one million atoms. Using atomistic wave functions we compute Coulomb matrix elements and self-energies. In the next step, by solving the Bethe-Salpeter-like equation for trions, we describe a negatively charged exciton, built as a strongly interacting interlayer complex of two electrons in the conduction band, and one hole in the valence band. Unlike in conventional semiconducting QDs, we show that the trion contains a fine structure composed of ten states arising from the valley and spin degrees of freedom. Finally, we obtain absorption into and emission from the trion states. We predict the existence of bright low-energy states and propose to extract the fine structure of the trion using the temperature dependence of emission spectra.

A chiral spin liquid (CSL) phase has been recently reported in the Hubbard model on a triangular lattice. It emerges in an intermediate coupling regime at half-filling, which is sandwiched between a ${120}^{\circ }$ antiferromagnetic (AFM) phase and a metallic phase as a function of on-site repulsion $U$. In this work, we examine the mechanism of the CSL and complex phase diagram via analytic analysis and numerical density matrix renormalization group (DMRG) method. First, we identify an exact Berry-phase-like sign structure in the partition function of the model at arbitrary $U$, which is originated from the Fermi sign structure at $U=0$.The spin and charge degrees of freedom are generally entangled via a singular phase string in such a many-body sign structure. In the large-$U$ limit, the suppression of the charge fluctuation at half-filling can render the phase string ineffective, resulting in the AFM order. However, if one precisely switches off such a phase string in the DMRG calculation, the ${120}^{\circ }$ AFM is shown to survive all the way down to a much weaker $U$ without the emergence of the CSL and metallic phases. It indicates that the phase-string sign structure plays the key role to mediate the mutual interaction between the charge and spin fluctuations, which results in the CSL and metallic phases at finite $U$. General implications for the Mott physics in the Hubbard model will also be discussed.

The variety of two-dimensional (2D) discommensurate charge density wave (DC-CDW) phases has been explained by a phenomenological CDW free-energy theory, where the wave vector of different DC-CDW phases corresponds to different local minima of a multivalley free-energy landscapes. Here, we discover that experimental 2D DC-CDW structures in transition-metal dichalcogenides can be understood elegantly by a conformal transformation with complex numbers. Specifically, we represent a 2D CDW wave vector $\mathbf{Q}$ by a complex number $Q$ and find that by minimizing the CDW free energy with respect to $Q$, different wave vectors $Q$ and $Q^{\prime }$ are connected by a conformal transformation. Consequently, $\sqrt{n}\times \sqrt{n}(n=7,9,13)$ honeycomb and (quasi-)stripe DC-CDWs can be understood using simple conformal groups, which link DC-CDW wave vectors (multivalley landscape) to the incommensurate and commensurate wave vectors.

Scattering theory has been suggested as a convenient method to identify topological phases of matter, in particular of disordered systems for which the Bloch band-theory approach is inapplicable. Here we examine this idea, employing as a benchmark a one-dimensional triangle chain whose versatility yields a system that “flows” in parameter space among several members of the topology classification scheme. Our results show that the reflection amplitudes (from both ends of long chains) indicate the appearance of edge states in all (topological and nontopological) cases. For the topological cases, the transmission has a peak at the topological phase transition, located at the Fermi energy. A peak still exists as one moves into the nontopological regions, where another transmission peak may occur at nonzero energy, at which an edge state appears in the isolated chain. For finite chains, the transmission peak depends strongly on their coupling with the leads, and not on the phase transition of the isolated chain. In any case, the appearance of a transmission peak is insufficient to conclude that the system undergoes a topological phase transition.