Many-body-localized (MBL) phases can be topologically distinct, but distinguishing these phases using order parameters can be challenging. Here we show how topologically distinct local integrals of motion, variationally parametrized by quantum circuits, can be used to numerically demonstrate the topological inequivalence of MBL phases. We illustrate our approach on a fermionic chain where both topologically distinct MBL phases and benchmark comparisons to order parameters are possible. We also use our approach, augmented by the excited state density-matrix renormalization group algorithm, to extract high-energy topological doublets. We describe applying our methods to higher dimensions to identify MBL topological order and topological multiplets hidden by the dense many-body spectrum.

Magnetic Weyl semimetals (MWSMs) have attracted significant attention due to their intriguing physical properties and potential applications in spin-electronic devices. Here we report the characterization of NdAlSi including transport, magnetization, and heat capacity on single crystals, as well as band structure calculation. It is a newly proposed MWSM candidate which breaks both time-reversal and spatial inversion symmetries. A temperature–magnetic field phase diagram is experimentally established. Remarkably, on the angular magnetoresistance, a twofold symmetric sharp peak instead of a smooth variation is observed across the phase boundary between the ferrimagnetic and antiferromagnetic phases. We argue that the tunability of both the electronic structure and magnetic properties in NdAlSi is crucial for realizing such a behavior. Our results indicate that $4f$-electron-based MWSM could provide a unique platform to explore new and intriguing quantum phenomena arising from the interaction between magnetism and electron topology.

The developments in the field of quantum optics raise expectations that laser-matter coupling is a promising building block for magnonics. Here, we propose a method for the generation of direct and alternating spin currents of magnons across the junction interface irradiated by linearly polarized laser. In a junction of ferromagnetic insulators with a large electronic gap, the spin angular momentum is exchanged during the tunneling process of magnons across the junction interface. The advanced technology in the field of plasmonics and metamaterials realizes that spins irradiated by the laser field interact only with the magnetic component of the laser through the Zeeman coupling. Using an analytic perturbation theory, we provide a general formula for magnon transport induced by the inversion symmetry breaking across the junction interface. Then, we show that those spin currents are enhanced by the ferromagnetic resonance, and the period of the ac spin current is one-half that of the laser magnetic field. Finally, we estimate the magnitude of the spin current, and find that it will be within experimental reach.

The Ginzburg-Landau theory is used to model the order parameter of a finite-size mesoscopic ring to investigate the effects of the onset of screening currents on the transport of incoming ones. The magnetic flux breaks the symmetry of currents between input and output stubs by means of an induced spatial ordering upon diamagnetic and paramagnetic supercurrents circulating in the ring. The distribution of those screening currents drives the interference of incoming/outgoing supercurrents resulting into a sinusoidal variation of resistance as a function of the magnetic flux even if the density of quasiparticles is not modified by the external magnetic field.

The two-electron states and exchange couplings are investigated for a phosphorous donor pair in silicon using an atomistic full configuration interaction method for donor separations spanning from 0.4 to 15 nm. Three distinct donor separation regimes appear from our large basis calculations, from which the validity of simplified methods such as Heitler-London and Hartree-Fock type approaches can be assessed. For bulk donors, the exchange coupling saturates below 5 nm due to excited bonding orbital contributions to the wave functions. Ionic contributions to the two-electron state decrease between 5 and 14 nm, and a fully correlated Heitler-London-like state is reached from 14 nm onwards. Oscillations in exchange couplings can be strongly suppressed by placing the donors in the same $z$ plane and at a small depth, $D$, from the surface. This is a consequence of the $z$-valley terms becoming dominant within the dopant's wave function and of small changes with $x$ and $y$ separations no longer having much effect. We find the depth to be an important parameter in determining the exchange coupling for subsurface dopants, not only through valley repopulation ($D<10$ nm) but also through additional interface effects for ultrashallow depths ($D<2.5$ nm). Our full configuration interaction method provides insights in the exchange interaction for various regimes of donor separation and depths, from the Heitler-London limit at large distances to the 0.4–5 nm range relevant for scanning tunneling microscope based quantum state imaging and spectroscopy experiments. The precise control of electron-electron quantum correlations in such engineered atoms in the solid state is useful to design quantum logic gates and quantum simulators.

We investigate the phase diagram of hard-core bosons in two-leg ladders in the presence of soft-shoulder potentials. We show how the competition between local and nonlocal terms gives rise to a phase diagram with liquid phases with dominant cluster, spin-, and density-wave quasi-long-range ordering. These phases are separated by Berezinskii-Kosterlitz-Thouless, Gaussian, and supersymmetric (SUSY) quantum critical transitions. For the latter, we provide a phenomenological description of coupled SUSY conformal field theories, whose predictions are confirmed by matrix product state simulations. Our results are motivated by, and directly relevant to, recent experiments with Rydberg-dressed atoms in optical lattices, where ladder dynamics has already been demonstrated, and emphasize the capabilities of these setups to investigate exotic quantum phenomena such as cluster liquids and coupled SUSY conformal field theories.

In order to investigate the possibility of construction of the higher-order topological superconductor on the triangular lattice, the different types of the coupling of chiral topological superconductor edge states are considered. It is demonstrated that the widespread criterion of the topological corner mode's appearance consisting in edge-state effective mass changing at the corner has to be modified in the case of multiple Dirac points in the edge-state spectrum. It is shown that the effective mass must be of the same sign at all Dirac points on one edge adjacent to the corner and be opposite at all the Dirac points on another one. For a two-dimensional system constructed from two chiral topological superconductors on the triangular lattice with inverted bands, two types of interactions leading to the higher-order topological superconducting phase are revealed.

Quantum Hall phases are gapped in the bulk but support chiral edge modes, both charged and neutral. Here, we consider a circuit where the path from the source of electric current to the drain necessarily passes through a segment consisting solely of neutral modes. We find that upon biasing the source, a dc electric current is detected at the drain, provided there is backscattering between counterpropagating modes under the contacts placed in certain locations. Thus, neutral modes carry information that can be used to nonlocally reconstruct a dc charge current. Our protocol can be used to detect any neutral mode that counterpropagates with respect to all charge modes. Our protocol applies not only to the edge modes of a quantum Hall system, but also to systems that have neutral modes of nonquantum Hall origin. We conclude with a possible experimental realization of this phenomenon.

Dynamical screening is a key property of charged many-particle systems. Its theoretical description is based on the $GW$ approximation that is extensively applied for ground-state and equilibrium situations but also for systems driven out of equilibrium. The main limitation of the $GW$ approximation is the neglect of strong electronic correlation effects that are important in many materials as well as in dense plasmas. Here we derive the dynamically screened ladder (DSL) approximation that self-consistently includes, in addition to the $GW$ diagrams, also particle-particle and particle-hole $T$-matrix diagrams. The derivation is based on reduced-density-operator theory and the result is equivalent to the recently presented G1-G2 scheme [N. Schlünzen et al., Phys. Rev. Lett. 124, 076601 (2020); J.-P. Joost et al., Phys. Rev. B 101, 245101 (2020)]. We perform extensive time-dependent DSL simulations for finite Hubbard clusters and present tests against exact results that confirm excellent accuracy as well as total-energy conservation of the approximation. At strong coupling and for long simulation durations, instabilities are observed. These problems are solved by enforcing contraction consistency and applying a purification approach.

We study the ergodic and nonergodic behaviors of a clean Jaynes-Cummings-Hubbard chain for different parameters based on the average level spacing ratios and the generalized fractal dimensions of eigenstates by using exact diagonalization. It can be found that a transition from ergodic to nonergodic regimes happens when increasing the atom-photon detuning, and the nonergodic phases should exist in the thermodynamic limit. We also find that the nonergodic phase violates the eigenstate thermalization hypothesis and displays many-body-localization-like behavior. Finally, we study the many-body multifractality of the ground state and find that the derivative of the generalized fractal dimensions can determine the critical point of the superfluid-Mott-insulation phase transition in a small range of parameters under different boundary conditions and there is no ergodicity for the ground state.

We consider a model Hamiltonian with two $\mathrm{SU}\left(4\right)$ fermions per site on a square lattice, showing a competition between bilinear and biquadratic interactions. This model has generated interest due to possible realizations in ultra-cold-atom experiments and existence of spin-liquid ground states. Using a basis transformation, we show that part of the phase diagram is amenable to quantum Monte Carlo simulations without a sign problem. We find evidence for spin nematic and valence-bond crystalline phases, which are separated by a weak first-order phase transition. A $\mathrm{U}(1$) symmetry is found to emerge in the valence-bond crystal histograms, suggesting proximity to a deconfined quantum critical point. Our results are obtained with the help of a loop algorithm which allows large-scale simulations of bilinear-biquadratic $\mathrm{SO}(N$) models on arbitrary lattices in a certain parameter regime.

A fractional quantum Hall effect (FQHE) has been predicted in a topological flat band (FB) by a single-particle band structure combined with phenomenological theory or solution of a many-body lattice Hamiltonian with fuzzy parameters. A long-standing roadblock toward the realization of a FB-FQHE is lacking the many-body solution of specific materials under realistic conditions. We demonstrate a combined study of single-particle Floquet band theory with exact diagonalization (ED) of a many-body Hamiltonian. We show that a time-periodic circularly polarized laser inverts the sign of second-nearest-neighbor hopping in a kagome lattice and enhances spin-orbit coupling in one spin channel to produce a Floquet FB with a high flatness ratio of bandwidth over band gap, as exemplified in monolayer ${\mathrm{Pt}}_3{\mathrm{C}}_{36}{\mathrm{S}}_{12}{\mathrm{H}}_{12}$. The ED of the resultant Floquet-kagome lattice Hamiltonian gives a one-third-filling ground state with a laser-dependent excitation gap of a FQH state, up to an estimated temperature above 70 K. Our findings pave the way for exploring the alluding high-temperature FB-FQHE.

Collective phenomena in solids can be sensitive to the dimensionality of the system; a case of special interest is ${\mathrm{VSe}}_2$, which shows a $\left(\sqrt{7}\times \sqrt{3}\right)$ charge density wave (CDW) in the single layer with threefold symmetry in the normal phase spontaneously broken, in contrast to the $\left(4\times 4\right)$ in-plane CDW in the bulk. Angle-resolved photoemission spectroscopy (ARPES) from ${\mathrm{VSe}}_2$ ranging from a single layer to the bulk reveals the evolution of the electronic structure including the Fermi surface contours and the CDW gap. At a thickness of two layers, the ARPES maps are already nearly bulklike, but the transition temperature $T_{\mathrm{C}}$ for the $\left(4\times 4\right)$ CDW is much higher than the bulk value of 110 K. These results can be understood as due to dimensional crossover of phonon instability driven by a competition of nesting vectors. In this letter, we provide key insights into the CDW mechanisms and offer a perspective in the search and control of emergent phases in quantum materials.

We present numerical tests of the fluctuation-dissipation theorem (FDT) in the dumbbell model of spin ice with parameters suitable for dysprosium titanate. The tests are made for local spin variables, magnetic monopole density, and energy. We are able to achieve local equilibrium in which the FDT is satisfied down to $T=0.4\mathrm{K}$ below which the system completely freezes. Nonequilibrium dynamics, together with violation of the FDT, are nonetheless observed following a thermal quench into the noncontractable monopole pair regime. Despite FDT violation, an approximate linear response regime allows for the identification of effective nonequilibrium temperatures which are different for each variable. The spin variable appears hotter than the heat reservoir and the monopole concentration responds with a lower effective temperature, while the energy has a negative effective temperature. Results are discussed in the context of the monopole picture of spin ice and compared to the structure of FDT violations in other glassy materials. Prospectives for future experiments are reviewed.

Time dependence of the size and chirality of a ferromagnetic skyrmion in a Heisenberg model with the magnetic field on a square lattice has been studied analytically and numerically. The lattice and the magnetic field generate strong time dependence of the skyrmion chirality. Due to nonlinearity, the lattice alone also generates strong intrinsic damping that leads to the skyrmion collapse via the emission of spin waves. In the absence of the magnetic field, the collapse is slow for a large skyrmion, but it becomes exponentially fast in the presence of the Landau-Lifshitz damping when the field is turned on. Magnons emitted by a collapsing skyrmion must have a discrete spectrum due to the quantization of the skyrmion magnetic moment.

The superconducting order parameter of the noncentrosymmetric superconductor LaRhSn is investigated by means of low-temperature measurements of the specific heat, muon-spin relaxation/rotation $\left(\mu \mathrm{SR}\right)$, and the tunnel-diode oscillator (TDO)-based method. The specific heat and magnetic penetration depth $\left[\lambda \right(T\left)\right]$ show an exponentially activated temperature dependence, demonstrating fully gapped superconductivity in LaRhSn. The temperature dependences of ${\lambda }^{-2}\left(T\right)$ deduced from the TDO-based method and $\mu \mathrm{SR}$ show nearly identical behavior, which can be well described by a single-gap $s$-wave model, with a zero-temperature gap value of $\mathrm{\Delta }\left(0\right)=1.77\left(4\right)k_B{T}_c$. The zero-field $\mu \mathrm{SR}$ spectra do not show detectable changes upon cooling below $T_c$, and therefore there is no evidence for time-reversal-symmetry breaking in the superconducting state.

Modern XAFS (x-ray absorption fine structure) data analysis is based on accurate multiple-scattering (MS) calculations of the x-ray absorption cross section, usually carried out solving the nonrelativistic Schrödinger equation for complex effective optical muffin-tin potentials describing the scattering of the atoms. The introduction of relativistic effects in extended XAFS (EXAFS) multiple-scattering calculations has been described in several papers and shown to be important for heavy atoms. However, few examples of applications and detailed studies of relativistic effects were given so far. In this work, we have performed a systematic investigation of relativistic corrections in systems of increasing atomic number, using a reliable simulation scheme recently developed and based on the incorporation of a pseudo-Schrödinger equation effectively replacing the Dirac relativistic form. Calculations have been put to a test in 12 different pure-element condensed-state systems, with the atomic number ranging from $Z=10$ for crystalline Ne to $Z=90$ for crystalline Th. The importance of accounting for relativistic effects has been highlighted for elements with $Z\gtrsim 60$, as ones for which relativistic corrections for amplitudes of calculated XAFS MS signals exceed $10\%$. The size of relativistic effects for calculated higher-order XAFS signal (with respect to the dominant single-scattering first-neighbor signal) has been shown, taking as example the $L_3$-edge spectra of crystalline Au and Pb. The size of relativistic effects for the $K$ and $L_3$ edges has been also evaluated, showing a slight increase of relativistic corrections for the $L_3$ edge. The improvement in the accuracy of XAFS simulations has been demonstrated comparing the results obtained for structural refinements of the $L_3$ edge of crystalline Au at 300 K.

We develop a Floquet protocol for long-range entanglement generation in the one-dimensional quantum Potts model, which generalizes the transverse-filed Ising model by allowing each spin to have $n>2$ states. We focus on the case of $n=3$, so that the model describes a chain of qutrits. The suggested protocol creates qutrit Bell-like pairs with nonlocal long-range entanglement that spans over the entire chain. We then conjecture that the proposed Floquet protocol is integrable and explicitly construct a few first nontrivial conserved quantities that commute with the stroboscopic evolution operator. Our analysis of the Floquet integrability relies on the deep connection between the quantum Potts model and a much broader class of models described by the Temperley-Lieb algebra. We work at the purely algebraic level and our results on Floquet integrability are valid for any representation of the Temperley-Lieb algebra. We expect that our findings can be probed with present experimental facilities using Rydberg programmable quantum simulators and can find various applications in quantum technologies.

To elucidate the origin of the reenhanced high-$T_c$ phase in the heavily electron-doped Fe pnictides, systematic ${}^{75}\mathrm{As}$ NMR studies are performed on heavily electron-doped $\mathrm{LaFe}Pn{\mathrm{O}}_{0.75}{\mathrm{H}}_{0.25}$ by controlling the pnictogen height $\left(h_{Pn}\right)$ from the Fe plane through the substitution at $Pn(=\text{As})$ site with Sb or P. The measurements of nuclear spin relaxation rate $(1/T_1)$ and Knight shift $\left(K\right)$ reveal that the moderate spin fluctuations at high temperatures are suppressed toward low temperatures. Such characteristic spin fluctuations with gaplike feature at low energies are more enlarged in higher $T_c$ compounds with higher $h_{Pn}$, while those are totally suppressed in nonsuperconducting compounds with lower $h_{Pn}$. This implies that the contribution of the finite-energy part in the spin-fluctuation spectrum is crucial for enhancing $T_c$ in the heavily electron-doped regime. This is in contrast to many cases of typical Fe-based compounds with hole and electron Fermi surfaces of similar sizes, where the spin fluctuations at low energies develop significantly at low temperatures. The features in the heavily electron-doped states are discussed in relation with the characteristics of the faint hole Fermi surface derived from the $d_{xy}$ orbital that rises when $h_{Pn}$ is high, together with the enhanced electron correlation effects.