Generalized multifractality characterizes system size dependence of pure scaling local observables at Anderson transitions in all 10 symmetry classes of disordered systems. Recently, the concept of generalized multifractality has been extended to boundaries of critical disordered noninteracting systems. Here we study the generalized boundary multifractality in the presence of electron-electron interaction, focusing on the spin quantum Hall symmetry class (class C). Employing the two-loop renormalization group analysis within the Finkel'stein nonlinear sigma model, we compute the anomalous dimensions of the pure scaling operators located at the boundary of the system. We find that generalized boundary multifractal exponents are twice larger than their bulk counterparts. Exact symmetry relations between generalized boundary multifractal exponents in the case of noninteracting systems are explicitly broken by the interaction.

The power-law random banded matrix (PLRBM) is a paradigmatic ensemble to study the Anderson localization transition (AT). In $d$ dimensions, the PLRBMs are random matrices with algebraic decaying off-diagonal elements $H_{\mathbf{n}\mathbf{m}}\sim 1/{|\mathbf{n}-\mathbf{m}|}^{\alpha }$, having AT at $\alpha =d$. In this work, we investigate the fate of the PLRBM to non-Hermiticity (nH). We consider the case where the random on-site diagonal potential takes complex values, mimicking an open system, subject to random gain-loss terms. We understand the model analytically by generalizing the Anderson-Levitov resonance counting technique to the nH case. We identify two competing mechanisms due to nH: favoring localization and delocalization. The competition between the two gives rise to AT at $d/2\le \alpha \le d$. The value of the critical $\alpha$ depends on the strength of the on-site potential, like in Hermitian disordered short-range models in $d>2$. Within the localized phase, the wave functions are algebraically localized with an exponent $\alpha$ even for $\alpha <d$. This result provides an example of non-Hermiticity-induced localization and finds immediate application in phase transitions driven by weak measurements.

We identify the triple-$\mathbf{q}$ (3Q) state as magnetic ground state in Pd/Mn and Rh/Mn bilayers on Re(0001) using spin-polarized scanning tunneling microscopy and density-functional theory. An atomistic model reveals that in general the 3Q state with tetrahedral magnetic order and zero net spin moment is coupled to a hexagonal atomic lattice in a highly symmetric orientation via the anisotropic symmetric exchange interaction, whereas other spin-orbit coupling terms cancel due to symmetry. Our experiments are in agreement with the predicted orientation of the 3Q state. A distortion from the ideal tetrahedral angles would lead to other orientations of the 3Q state with a reduced topological orbital magnetization compared to the ideal 3Q state.

Motivated by the report of superconductivity in bilayer ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ at high pressure, we examine the interacting electrons in this system. First-principles many-body theory is utilized to study the normal-state electronic properties. Below 100 K, a multiorbital non-Fermi-liquid state resulting from a loss of Ni-ligand coherence within a flat-band-dominated low-energy landscape is uncovered. The incoherent low-temperature Fermi surface displays strong mixing between $\mathrm{Ni}\text{−}{d}_{z^2}$ and $\mathrm{Ni}\text{−}{d}_{x^2-y^2}$ orbital character. In a model Hamiltonian picture, spin fluctuations originating mostly from the $\mathrm{Ni}\text{−}{d}_{z^2}$ orbital give rise to strong tendencies towards a superconducting instability with a $B_{1g}$ or $B_{2g}$ order parameter. The dramatic enhancement of $T_{\mathrm{c}}$ in pressurized ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ is due to stronger $\mathrm{Ni}\text{−}{d}_{z^2}$ correlations compared to those in the infinite-layer nickelates.

Recently, above-room-temperature superconductivity was reported in the Cu-doped lead apatite ${\mathrm{Pb}}_{10-x}{\mathrm{Cu}}_x{\left({\mathrm{PO}}_4\right)}_6\mathrm{O}$, dubbed LK-99. By relaxing the structure with Cu substitution, we derive a four-band low-energy model with two 3/4 filled bands of predominantly $\mathrm{Cu}\text{−}{d}_{xz,yz}$ character and two filled $\mathrm{O}\text{−}{p}_{x,y}$ bands. This model is further downfolded to a two-band $\mathrm{Cu}\text{−}{d}_{xz/yz}$ model. Using ab initio derived interaction parameters, we perform dynamical mean-field theory calculations to determine the correlated electronic structure in the normal state. These calculations yield a Mott insulator at $x=1$ and a strongly correlated non-Fermi-liquid metal upon doping. The very large interaction versus bandwidth ratio $U/W\approx 30-50$ and the local moment paramagnetic behavior in the relevant filling regime are hard to reconcile with diamagnetism and high-temperature superconductivity. Our calculations suggest that this behavior comes from a component with a different stoichiometry.

Three mechanisms are known to contribute to second-order nonlinear current: Extrinsic nonlinear Drude, Berry curvature dipole, and intrinsic Berry connection polarizability. Here, we predict an intrinsic contribution to the current related to the quantum metric, a quantum geometric property of the electronic wave function. This contribution manifests in systems that simultaneously break the time-reversal and the inversion symmetry. Interestingly, this contribution is dissipative and contributes to both longitudinal and transverse response. The quantum metric-induced NL current dominates transport in parity-time reversal symmetric systems near the band edges, something we show explicitly for topological antiferromagnets.

Nitrogen-vacancy (NV) centers in diamond are a leading competitor for quantum sensing. The resolution of DC magnetic field sensing is limited by dephasing of the NV electron spin, with the main dephasing source being the thermal and quantum noise from the ${}^{13}\mathrm{C}$ nuclear spin bath. In this work, we propose an effective method to extend the dephasing time of a single NV electron spin by suppressing the nuclear spin noise. On the one hand, we improve the technique of bath driving to accommodate nuclear spin bath decoupling, and extend the dephasing time from 2.6 to 10.2 $\mu \mathrm{s}$ at a magnetic field of 455 G. On the other hand, our method enables decoupling the quantum noise by inserting $\pi$ rotations around the $Z$ axis of the nuclear spin bath into the free evolution. By suppressing both the thermal and quantum noise, the dephasing time is extended by more than four times at 100 G. Our results provide a new route to explore strategies of suppressing the thermal noise and quantum noise in quantum information processing, and can be applied to other solid-state defects.

We present a unified understanding of the experimentally observed minimal dc conductivity in Dirac materials. First, based on the linear response theory, we reveal that the momentum-dependent self-energy function induces a tunable minimal conductivity in Dirac electrons, the magnitude of which is directly related to the coefficient of the momentum-dependent part in the self-energy function. Taking the long-ranged Gaussian and Coulomb potentials as examples, the momentum-dependent self-energy function is perturbatively derived using the Born approximation and supplemented by the self-consistent Born approximation and renormalization group analysis. Moreover, we further validate our theory via numerical simulations using the large-scale Lanczos algorithm. The explicit momentum dependence of the self-energy on the intensity, concentration, and range of potential are critically addressed. Therefore, our theory provides a reasonable interpretation of the sample-dependent minimal conductivity observed in experiments.

We theoretically study the linear and nonlinear dynamics of a system of mutually coupled magnetic vortices. We consider the excitation of vortex dynamics by both an external harmonic magnetic field and a spin-polarized current. The results based on the Thiele equations for the motion of coupled vortices are compared with the numerical integrations. Theoretical analysis shows that the appearance of the exceptional point corresponding to the coincidence of the normal modes of the system allows one to observe such effects as the death of amplitudes and parametric resonance. We propose a method for observing the effect of amplitude death in coupled magnetic vortices. In the investigated system, the emergence of exceptional points is controlled by the spin-polarized current and the strength of the coupling between vortices. Thus, possibilities are uncovered for the development of tunable spintronic devices.

We have studied the nature of optically induced short-range magnetic fluctuations occurring at 10-nm length scale during ultrafast demagnetization in ferromagnetic Co/Pt multilayers. The time resolved probing of magnetization dynamics was performed with femtosecond soft x-ray pulses at the European x-ray free-electron laser. A transient high-$q$ magnetic scattering accompanying and directly correlated to the destruction of the maze domain network has been observed at picosecond time scale in the wave vector region of $0.2–0.8\mathrm{n}{\mathrm{m}}^{-1}$. This high-q scattering has a purely magnetic nature and is ascribed to the optically induced short-range magnetic fluctuations developing in the disturbed but not fully destroyed magnetic domain network. Finally, we have simulated the optically induced response of the domain system using a two-temperature atomistic spin model and have concluded that the reason behind the high-q scattering is the laser-driven heating inducing thermal fluctuations of the domain magnetic structure.

Soft magnetic dots in the form of thin rings have unique topological properties. They can be in a vortex state with no vortex core. Here, we study the magnon modes of such systems both analytically and numerically. In an external magnetic field, magnetic rings are characterized by easy-cone magnetization and shows a giant splitting of doublets for modes with the opposite value of the azimuthal mode quantum number. The effect of the splitting can be refereed as a magnon analog of the topology-induced Aharonov-Bohm effect. For this we develop an analytical theory to describe the nonmonotonic dependence of the mode frequencies on the azimuthal mode number, influenced by the balance between the local exchange and nonlocal dipole interactions.

Identifying and measuring the “localization length” in many-body systems in the vicinity of a many-body localization transition is difficult. Following Hatano and Nelson, a recent paper [S. Heußen, C. D. White, and G. Refael, Phys. Rev. B 103, 064201 (2021)] introduced an “imaginary vector potential” to a disordered ring of interacting fermions, in order to define a many-body localization length (corresponding, in the noninteracting case, to the end-to-end Green's function of the Hermitian system). We extend these results, by connecting this localization length to the length scale appearing in the avalanche model of delocalization. We use this connection to derive the distribution of the localization length at the MBL transition, finding good agreement with our numerical observations. Our results demonstrate how a localization length defined as such probes the localization of the underlying ring, without the need to explicitly construct the l-bits.

Although topological spin textures are not commonly found in centrosymmetric magnetic systems, noncoplanar monodomain magnetic states such as flower and curling states do emerge due to the contribution of surfaces and edges in nanoparticles. In this work, we studied the topological nature of these intriguing noncoplanar spin textures and their manifestation in electric transport phenomena due to the Berry phase accumulation. Specifically, we calculated the topological charges $Q$ associated with these spin textures and the corresponding topological Hall effect. We assessed these spin textures across various particle sizes and along magnetic hysteresis loops and mapped the spin structures in confined geometries using magnetic force microscopy. We show that $Q$, as a fractional number, increases with particle size and saturates as the system transits from the flower state to the curling state. Along magnetic hysteresis loops, smaller particles that show flower states in zero field, exhibit a peak in $Q$ near the coercive field, a signature of the topological Hall effect demonstrated in other systems. In contrast, larger particles that show curling states during the magnetization reversal, exhibit transitions between the homogeneous state, flower state, and curling state, which generates jumps in $Q$ and the topological Hall effects. These results reveal the rich topological nature of centrosymmetric magnetic nanoparticles, offer control using magnetic field and probe using electric transport, suggesting promising potential applications.

We consider a Josephson bijunction consisting of three superconducting reservoirs connected through two quantum dots. In equilibrium, the interdot coupling is sizable only for distances smaller than the superconducting coherence length. Application of commensurate dc voltages results in a time-periodic Hamiltonian and induces an interdot coupling at large distances. The basic mechanism of this long-range coupling is shown to be due to local multiple Andreev reflections on each dot, followed by quasiparticle propagation at energies larger than the superconducting gap. At large interdot distances, we derive an effective non-Hermitian Hamiltonian describing two resonances coupled through a continuum.

The topological properties of the Su-Schrieffer-Heeger (SSH) model in the presence of nearest-neighbor interaction are investigated by means of a topological marker, generalized from a noninteracting one by utilizing the single-particle Green's function of the many-body ground state. We find that despite the marker not being perfectly quantized in the presence of interactions, it always remains finite in the topologically nontrivial phase while converging to zero in the trivial phase when approaching the thermodynamic limit, and hence correctly judges the topological phases in the presence of interactions. The marker also correctly captures the interaction-driven, second-order phase transitions between a topological phase and a Landau-ordered phase, which is a charge-density wave order in our model with a local-order parameter, as confirmed by the calculation of entanglement entropy and the many-body Zak phase. Our paper thus points to the possibility of generalizing topological markers to interacting systems through Green's function, which may be feasible for topological insulators in any dimension and symmetry class.

Developing materials with low thermal conductivity requires an understanding of phonon transport, and it is of significance for practical applications such as thermoelectrics. Traditionally, weak chemical bonding, heavier atomic masses, and complex structure are the most common features in high-performance thermoelectric materials with low thermal conductivity. In this work, we applied the ab initio phonon Boltzmann transport method to investigate the abnormally low lattice thermal conductivity observed in the Zintl-phase compound, SrAgSb, among BaCuSb, SrCuSb, and the isomorphism heavier BaAgSb. The optical-acoustic branches coupling is considered the main reason for the diminishing thermal conductivity of SrAgSb, whereas one low-energy optical branch with low and negative Grüneisen parameters. The hybridization between optical and acoustic branches plays an essential role as the scattering center in reciprocal space, like defects in the crystal lattice. Our analysis illustrates that the soft and anharmonic lowest-lying optical mode contributes to the lower thermal conductivity of SrAgSb and suggests more potential chances for achieving high thermoelectric performance with intrinsic low ${\kappa }_{\mathrm{lat}}$ among abundant Zintl-phase compounds.

We report $\beta$-detected nuclear magnetic resonance of ultradilute ${}^8\mathrm{Li}^{+}$ implanted in highly oriented pyrolytic graphite (HOPG). The absence of motional narrowing and diffusional spin-lattice relaxation implies ${\mathrm{Li}}^{+}$ is not appreciably mobile up to 400 K, in sharp contrast to the highly lithiated stage compounds. However, the relaxation is remarkably fast and persists down to cryogenic temperatures. Ruling out extrinsic paramagnetic impurities and intrinsic ferromagnetism, we conclude the relaxation is due to paramagnetic centers correlated with implantation. While the resulting effects are not consistent with a Kondo impurity, they also differ from free paramagnetic centers, and we suggest that a resonant scattering approach may account for much of the observed phenomenology.

An effective action for the bulk dynamics of the quantum Hall effect in arbitrary, even spatial dimensions was obtained some time ago in terms of a Chern-Simons term associated with the Dolbeault index theorem. Here we explore further properties of this action, showing how electronic band structures can be incorporated, obtaining Hall currents and conductivity (for arbitrary dimensions) in terms of integrals of Chern classes for the bands. We also derive the expression for Hall viscosity from the effective action. Explicit formulas for the Hall viscosity are given for $2+1$ and $4+1$ dimensions.

Understanding competing instabilities in systems with correlated fermions remains one of the holy grails of modern condensed matter physics. Among the fermionic lattice models used to this effect, the extended Hubbard model occupies a prime place due to the potential relevance of its repulsive and attractive versions for both electronic materials and artificial systems. Using the recently introduced multichannel fluctuating field approach, we address the interplay of fluctuations in the charge density wave, $s$-wave superconducting, and phase separation channels in the attractive extended Hubbard model. Despite the fact that this model has been intensively studied for decades, our approach allows us to identify a phase that has not been analyzed before and which is characterized by the coexistence of collective $s$-wave superconducting and phase separation fluctuations. Our findings resonate with previous observations of interplaying phase separation and superconducting phases in electronic systems, most importantly in high-temperature superconductors.

The recently synthesized ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ monolayer [Science 369, 670 (2020)] exhibit outstanding environmental stability, moderate band gap, and excellent mechanical properties, which opens up a new avenue for the explorations of two-dimensional (2D) ${\mathrm{MA}}_2{\mathrm{Z}}_4$ materials. Inspired by this finding, we perform comprehensive structural predictions of ${\mathrm{MSi}}_2{\mathrm{N}}_4$ monolayers with lanthanide and actinide metals at the M site. Using the CALYPSO structural search method and first-principles calculations, we identify seven ${\mathrm{MSi}}_2{\mathrm{N}}_4$ monolayers with robust ambient stabilities, four of which are metals (M = Tm, Lu, Pa, Np) and three are semiconductors (M = Ce, Th, U). Of particular interest is the ${\mathrm{ThSi}}_2{\mathrm{N}}_4$ monolayer, which is an indirect-gap semiconductor with ultra-high electron mobility of 14384 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$. Our results enrich the 2D ${\mathrm{MA}}_2{\mathrm{Z}}_4$ family and offer insights into the design and synthesis of novel multifunctional materials.