Correlated band theory implemented as a combination of density functional theory with exact diagonalization [$\mathrm{DFT}+U$(ED)] of the Anderson impurity term with Coulomb repulsion $U$ in the open 14-orbital $5f$ shell is applied to ${\mathrm{UTe}}_2$. The small gap for $U=0$, evidence of the half-filled $j=\frac{5}{2}$ subshell of $5f^3$ uranium, is converted for $U=3\mathrm{eV}$ to a flat-band semimetal with small heavy-carrier Fermi surfaces that will make properties sensitive to pressure, magnetic field, and off-stoichiometry, as observed experimentally. Two means of identification from the Green's function give a mass enhancement of the order of 12 for already heavy (flat) bands, consistent with the common heavy-fermion characterization of ${\mathrm{UTe}}_2$. The predicted Kondo temperature around 100 K matches the experimental values from resistivity. The electric field gradients for the two Te sites are calculated by $\mathrm{DFT}+U$(ED) to differ by a factor of 7, indicating a strong site distinction, while the anisotropy factor $\eta =0.18$ is similar for all three sites. The calculated uranium moment ${〈M^2〉}^{1/2}$ of $3.5{\mu }_B$ is roughly consistent with the published experimental Curie-Weiss values of 2.8 and $3.3{\mu }_B$ (which are field-direction dependent), and the calculated separate spin and orbital moments are remarkably similar to Hund's rule values for an $f^3$ ion. The $U=3\mathrm{eV}$ spectral density is compared with angle-integrated and angle-resolved photoemission spectra, with agreement that there is strong $5f$ character at, and for several hundred meV below, the Fermi energy. Our results support the picture that the underlying ground state of ${\mathrm{UTe}}_2$ is that of a half-filled $j=\frac{5}{2}$ subshell with two half-filled $m_j=\pm \frac{1}{2}$ orbitals forming a narrow gap by hybridization and then driven to a conducting state by configuration mixing (spin-charge fluctuations). ${\mathrm{UTe}}_2$ displays similarities to ${\mathrm{UPt}}_3$ with its $5f$-dominated Fermi surfaces rather than a strongly localized Kondo lattice system.

Photoinduced Mott transition has been revealed as a superior method to control the optical and electronic properties in excited semiconductors on ultrashort timescales but the corresponding ultrafast carrier dynamics and the underlying many-body interactions, together with their responses to external stimuli besides optical excitation, are still poorly understood. Herein, we experimentally demonstrate that the photoinduced Mott transition in few-layer ${\mathrm{MoS}}_2$ at room temperature can be achieved and subtly tuned under mild optical excitation via hydrostatic pressure. Utilizing ultrafast pump-probe spectroscopy, significant reduction of the Mott density by more than two orders of magnitude and a gradual acceleration of the optical switching to the metallic electron-hole plasma state under elevated pressures up to $\sim 3$ GPa are experimentally revealed, which can be attributed to the reduction of the exciton binding energy and the acceleration of the exciton dissociation under hydrostatic pressure.

The plasmon excitations in small metal-atom clusters have been extensively investigated in recent years, and most of the efforts are focused on freestanding clusters. However, freestanding clusters are inherently unstable and have to be stabilized by interacting with a two-dimensional or three-dimensional substrate; hence, the effects of substrates should be taken into account in the calculations of electronic excitations. With the use of the random-phase approximation, we extensively investigate the polarization effects of substrates on the plasmon resonances of small Na chains on Si surfaces. Our results show that due to the polarization effects of Si substrates, the redshifts of plasmon peaks appear in both longitudinal and transverse absorption spectra, and the strengths of the main plasmon resonant peaks have some variation; in particular, the strength gets a very large enhancement when the external field is applied in the direction perpendicular to the surface of Si. In addition, we study the formation mechanism of the end mode and the central mode of transverse-plasmon excitations and show that the two modes are caused by electron-electron interactions, which reflects the collective excitations induced by the interactions.

In condensed matter systems it is necessary to distinguish between the momentum of the constituents of the system and the pseudomomentum of quasiparticles. The same distinction is also valid for angular momentum and pseudoangular momentum. Based on Noether's theorem, we demonstrate that the recently discussed orbital angular momenta of phonons and magnons are pseudoangular momenta. This conceptual difference is important for a proper understanding of the transfer of angular momentum in condensed matter systems, especially in spintronics applications.

Scrambling of information in a quantum many-body system, quantified by the out-of-time-ordered correlator (OTOC), is a key manifestation of quantum chaos. A regime of exponential growth in the OTOC, characterized by a Lyapunov exponent, has so far mostly been observed in systems with a high-dimensional local Hilbert space and in weakly-coupled systems. Here, we propose a general criterion for the existence of a well-defined regime of exponential growth of the OTOC in spatially extended systems with local interactions. In such systems, we show that a parametrically long period of exponential growth requires the butterfly velocity to be much larger than the Lyapunov exponent times a microscopic length scale, such as the lattice spacing. As an explicit example, we study a random unitary circuit with tunable interactions. In this model, we show that in the weakly interacting limit, the above criterion is satisfied, and there is a prolonged window of exponential growth. Our results are based on numerical simulations of both Clifford and universal random circuits supported by an analytical treatment.

In a magnetic multilayer, magnetizations can be manipulated by spin transfer torque. Both spin transfer torque and its reciprocal effect, spin pumping, are governed by spin mixing conductance. The magnitude of spin mixing conductance at the interface of nearly magnetic metal has been theoretically shown to be enhanced by electron-electron interaction. However, experiments show both increasing and decreasing values of spin mixing conductance for metals with larger electron-electron interaction. Here we take into account the effect of electron-electron interaction on the screening of the Coulomb interaction at the magnetic interface to correctly describe the experiment.

The quantum dimer model is a low-energy effective model for many magnetic systems (materials) that are candidates for quantum spin liquids. It has strict local constraints which is described by gauge field theory. However, since constraints hinder the application of numerical algorithms, the phase diagrams of quantum dimer models are still controversial, even on a square lattice. The core controversy is whether the mixed state exists due to the restriction. In this paper, we give strong evidence to solve this dispute. With our sweeping cluster quantum Monte Carlo method, we studied the phase diagram of a large parameter region by introducing the definition of the pair correlation function and other supporting evidence to distinguish the mixed phase from the columnar phase with high precision. In particular, we find that the ground state belongs to the mixed phase for a vast parameter region.

The magnetic equiatomic alloys of the nominal compositions ${\mathrm{MnNiGe}}_{0.9}{\mathrm{Al}}_{0.1}$ and ${\mathrm{MnNiGe}}_{0.928}{\mathrm{Ga}}_{0.072}$ have been investigated through the neutron powder diffraction (NPD) technique. Both these alloys undergo a martensitic phase transition from the high-temperature hexagonal (${\mathrm{Ni}}_2\mathrm{In}$ type, with space group $P{6}_3/mmc$) austenite phase to the low-temperature orthorhombic (TiNiSi type, with space group $Pnma$) martensite phase. Our NPD analysis confirmed an incommensurate antiferromagnetic ordering of the low-temperature martensitic phase with helical modulation. The incommensurate propagation vector $\mathbf{k}$ is found to be (0.2065(1),0,0) and (0.2088(4),0,0) for Al and Ga doped alloys, respectively, at 1.5 K and shows a monotonic increase with increasing sample temperature. On the other hand, a commensurate antiferromagnetic ordering was observed for the high-temperature austenite phase. The incomplete martensitic transition allowed both these incommensurate and commensurate antiferromagnetic structures to coexist down to the lowest temperature of measurement (1.5 K).

We study superconductor/ferromagnet/superconductor junctions with CuNi, PtNi, or Ni interlayers. Remarkably, we observe that supercurrents through Ni can be significantly larger than through diluted alloys. The phenomenon is attributed to the dirtiness of disordered alloys leading to a short coherence length despite a small exchange energy. To the contrary, pure Ni is clean resulting in a coherence length as long as in a normal metal. Analysis of temperature dependencies of critical currents reveals a crossover from short (dirty) to long (clean) range proximity effects in ${\mathrm{Pt}}_{1-x}{\mathrm{Ni}}_x$ with increasing Ni concentration. Our results point out that structural properties of a ferromagnet play a crucial role for the proximity effect and indicate that conventional strong-but-clean ferromagnets can be advantageously used in superconducting spintronic devices.

The quality of a quantum operation determines the performance of quantum information processing, such as the sensitivity of quantum sensing. Different from the fidelity of quantum operation in quantum computation, we present an effective function to evaluate the performance and diagnose the imperfection of operations in multipulse based quantum sensing. The evaluation function directly links the realistic sensitivity with intrinsic sensitivity in a simple way. Moreover, guided by this evaluation function, we optimize a composite-pulse sequence for high sensitivity nitrogen-vacancy- center based magnetometry against spectrum inhomogeneities and control errors to improve the signal-to-noise ratio of nanoscale nuclear magnetic resonance by one order of magnitude. It marks an important step towards quantitative quantum sensing with imperfect quantum control in practical applications.

Many questions are still open regarding the physical mechanisms behind the magnetic switching in Gd-Fe-Co alloys by single optical pulses. Phenomenological models suggest a femtosecond scale exchange relaxation between sublattice magnetization as the driving mechanism for switching. The recent observation of thermally induced switching in Gd-Fe-Co by using both several picosecond optical laser pulse as well as electric current pulses has questioned this previous understanding. This has raised the question of whether or not the same switching mechanics are acting at the femtosecond and picosecond scales. In this work, we aim at filling this gap in the understanding of the switching mechanisms behind thermal single-pulse switching. To that end, we have studied experimentally thermal single-pulse switching in Gd-Fe-Co alloys, for a wide range of system parameters, such as composition, laser power, and pulse duration. We provide a quantitative description of the switching dynamics using atomistic spin dynamics methods with excellent agreement between the model and our experiments across a wide range of parameters and timescales, ranging from femtoseconds to picoseconds. Furthermore, we find distinct element-specific damping parameters as a key ingredient for switching with long picosecond pulses and argue that switching with pulse durations as long as 15 ps is possible due to a low damping constant of Gd. Our findings can be easily extended to speed up dynamics in other contexts where ferrimagnetic Gd-Fe-Co alloys have been already demonstrated to show fast and energy-efficient processes, e.g., domain-wall motion in a track and spin-orbit torque switching in spintronics devices.

We theoretically study the effects of spin-orbit interactions on the symmetry of a Cooper pair in a spin-singlet $d$-wave superconductor and on the chiral property of the surface Andreev bound states at zero energy. The pairing symmetry is analyzed by using the anomalous Green's function, which is obtained by solving the Gor'kov equation analytically. The chiral property of surface bound states is discussed by using an index that represents a number of zero-energy states in the presence of potential disorder at a surface. A spin-orbit interaction induces a spatially uniform spin-triplet $p$-wave pairing correlation in a superconductor and an odd-frequency spin-triplet $s$-wave pairing correlation at a surface. The spin-orbit interaction splits the Fermi surface into two depending on the spin configuration. As a result of splitting, the index can be a nontrivial nonzero value. On the basis of the close relationship among the odd-frequency $s$-wave pairing correlation at a surface, the nonzero index of surface bound states, and the anomalous proximity effect, we provide a design of a superconductor that causes the strong anomalous proximity effect.

Ni $LVV$ Auger spectra, in coincidence with the corresponding $2p_{1/2}$, $2p_{3/2}$, and $6\text{eV}$ satellite photoelectrons, have been used to examine electron correlation and itinerance effects in Ni. In coincidence with the $2p_{3/2}$ core level, the Auger spectral shape is represented by localized $3d^8$ and itinerant valence final states with an additional $3d^7$ Auger shake-up contribution. The spectra in coincidence with the $6\mathrm{eV}$ satellite probe the decay of localized $2p^{5}3d^9$ double hole states, leading to $3d^7$ final states. It is found that a fraction of the double hole states delocalize before the Auger decay. A similar delocalization is observed for the double hole states produced by the $L_2{L}_3{M}_{45}$ Coster-Kronig process, and the delocalization rates have been determined.

${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ is an archetypal spin-orbit-coupled Mott insulator with an antiferromagnetic state below 240 K. Here we report results of our study on single crystals of ${\mathrm{Sr}}_2{\mathrm{Ir}}_{1-x}\mathbf{F}{\mathbf{e}}_x{\mathrm{O}}_4$ ($0\le x<0.32$) and ${\mathrm{Sr}}_2{\mathrm{Ir}}_{1-x}\mathbf{C}{\mathbf{o}}_x{\mathrm{O}}_4$ ($0\le x<0.22$). Fe doping retains the antiferromagnetic state but simultaneously precipitates an emergent metallic state whereas Co doping causes a rapid collapse of both the antiferromagnetic and Mott states, giving rise to a confined metallic state featuring a pronounced linearity of the basal-plane resistivity up to 700 K. The results indicate tetravalent ${\mathrm{Fe}}^{4+}\left(3d^4\right)$ ions in the intermediate spin state with $S=1$ and ${\mathrm{Co}}^{4+}\left(3d^5\right)$ ions in the high spin state with $S=5/2$ substituting for ${\mathrm{Ir}}^{4+}\left(5d^5\right)$ ions in ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$, respectively. The effective magnetic moment closely tracks the Néel temperature as doping increases, suggesting that the spin state of the dopant predominately determines the magnetic properties in doped ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$. Furthermore, all relevant properties including charge-carrier density (e.g., ${10}^{28}/{\mathrm{m}}^3$), Sommerfeld coefficient (e.g., $19\mathrm{mJ}/\mathrm{mole}{\mathrm{K}}^2$), and Wilson ratio (e.g., 2.6) consistently demonstrate an emergent metallic state that is both robust and highly correlated in the two systems, arising from the percolation of bound states and the weakening of structural distortions. This study strongly indicates that the antiferromagnetic and Mott states are correlated in an unconventional manner in ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$.

We present an extension of constrained-path auxiliary-field quantum Monte Carlo (CP-AFQMC) for the treatment of correlated electronic systems coupled to phonons. The algorithm follows the standard CP-AFQMC approach for description of the electronic degrees of freedom while phonons are described in first quantization and propagated via a diffusion Monte Carlo approach. Our method is tested on the one- and two-dimensional Holstein and Hubbard-Holstein models. With a simple semiclassical trial wave function, our approach is remarkably accurate for $\omega /\left(2dt\lambda \right)<1$ for all parameters in the Holstein model considered in this study where $d$ is the dimensionality, $\omega$ is the phonon frequency, $t$ is the electronic hopping strength, and $\lambda$ is the dimensionless electron-phonon coupling strength. In addition, we empirically show that the autocorrelation timescales as $1/\omega$ for $\omega /t\lesssim 1$, which is an improvement over the $1/{\omega }^2$ scaling of the conventional determinant quantum Monte Carlo algorithm. In the Hubbard-Holstein model, the accuracy of our algorithm is found to be consistent with that of standard CP-AFQMC for the Hubbard model when the Hubbard $U$ term dominates the physics of the model, and is nearly exact when the ground state is dominated by the electron-phonon coupling scale $\lambda$. The ap- proach developed in this work should be valuable for understanding the complex physics arising from the interplay between electrons and phonons in both model lattice problems and ab initio systems.

Recently, the concept of ferrovalley materials has been proposed. These systems, in which ferromagnetism or ferroelectricity triggers spontaneous valley polarization, can be applied to nonvolatile valleytronic devices. In addition, a fabricated Janus transition metal dichalcogenide has been attracting immense attention due to its intrinsic valley properties and piezoelectricity. Inspired by these works, we predict a ferrovalley material, Janus VClBr monolayer, by performing first-principles calculations. The Janus VClBr monolayer possesses a large valley polarization due to the spin-orbit coupling coexisting with the intrinsic exchange interaction of vanadium $3d$ electrons, which can be illustrated by a $\mathbf{k}·\mathbf{p}$ model. Combining the valley band structures with group theory analysis, we also find the chirality-dependent optical band gap that could be used to detect the valley state of the system. An anomalous valley Hall effect is confirmed by calculating the Berry curvatures in such a material system. Meanwhile, mirror symmetry breaking along the perpendicular direction not only results in an emergent internal electric dipole and a giant in-plane piezoelectricity, but also generates an out-of-plane second-harmonic-generation coefficient. Our findings of these distinctive properties, particularly the coupling of piezoelectricity and valley, could pave the way to design the Janus ferrovalley material as multifunctional valleytronics and optoelectronics devices.

Recently, layered copper chalcogenides ${\mathrm{Cu}}_{2}X$ family $(X=\mathrm{S}, \mathrm{Se}, \mathrm{Te})$ has attracted tremendous research interests due to their high thermoelectric performance, which is partly due to the superionic behavior of mobile Cu ions, making these compounds “phonon liquids.” Here, we systematically investigate the electronic structure and its temperature evolution of the less studied single crystal ${\mathrm{Cu}}_{2-x}\mathrm{Te}$ by the combination of angle resolved photoemission spectroscopy (ARPES) and scanning tunneling microscope/spectroscopy (STM/STS) experiments. While the band structure of the ${\mathrm{Cu}}_{2-x}\mathrm{Te}$ shows agreement with the calculations, we clearly observe a $2\times 2$ surface reconstruction from both our low temperature ARPES and STM/STS experiments which survives up to room temperature. Interestingly, our low temperature STM experiments further reveal multiple types of reconstruction patterns, which suggests the origin of the surface reconstruction being the distributed deficiency of liquidlike Cu ions. Our findings reveal the electronic structure and impurity level of ${\mathrm{Cu}}_2\mathrm{Te}$, which provides knowledge about its thermoelectric properties from the electronic degree of freedom.