The rare-earth-based ternary intermetallic compounds $R_{2}T{X}_3$ ($R$ = rare earth, $T$ = transition metal, $X$ = Si, Ge, Ga, In) have attracted considerable interest due to a wide range of interesting low-temperature properties. Here we investigate the magnetic state of ${\mathrm{Nd}}_2{\mathrm{PdSi}}_3$ using neutron diffraction, muon spin relaxation ($\mu \mathrm{SR}$), and inelastic neutron scattering (INS). This compound appears anomalous among the $R_2{\mathrm{PdSi}}_3$ series since it was proposed to order ferromagnetically, whereas others in this series are antiferromagnets. Although some members of the $R_{2}T{X}_3$ series have been reported to form ordered superstructures, our data are well described by ${\mathrm{Nd}}_2{\mathrm{PdSi}}_3$ adopting the ${\mathrm{AlB}}_2$-type structure with a single Nd site, and we do not find evidence of superlattice peaks in neutron diffraction. Our results confirm the onset of long-range magnetic order below $T_0=17$ K, where the whole sample enters the ordered state. Neutron diffraction measurements establish the presence of a ferromagnetic component in this compound, as well as an antiferromagnetic one which has a propagation vector ${\mathbf{k}}_{\mathbf{2}}=(1/2,1/2,1/4-\delta )$ with a temperature-dependent $\delta \approx 0.02–0.04$, and moments orientated exclusively along the $c$ axis. $\mu \mathrm{SR}$ measurements suggest that these components coexist on a microscopic level, and therefore the magnetic structure of ${\mathrm{Nd}}_2{\mathrm{PdSi}}_3$ is predominantly ferromagnetic, with a sinusoidally modulated antiferromagnetic contribution which reaches a maximum amplitude at 11 K and becomes smaller upon further decreasing the temperature. INS results show the presence of crystalline electric field (CEF) excitations above $T_0$, and from our analysis we propose a CEF level scheme.

We introduce a machine-learning interatomic potential for tungsten using the Gaussian approximation potential framework. We specifically focus on properties relevant for simulations of radiation-induced collision cascades and the damage they produce, including a realistic repulsive potential for the short-range many-body cascade dynamics and a good description of the liquid phase. Furthermore, the potential accurately reproduces surface properties and the energetics of vacancy and self-interstitial clusters, which have been longstanding deficiencies of existing potentials. The potential enables molecular dynamics simulations of radiation damage in tungsten with unprecedented accuracy.

We show that the recently proposed cooling-by-doping mechanism allows one to efficiently prepare interesting nonequilibrium states of the Hubbard model. Using nonequilibrium dynamical mean field theory and a particle-hole symmetric setup with dipolar excitations to full and empty bands we produce cold photodoped Mott insulating states with a sharp Drude peak in the optical conductivity, a superconducting state in the repulsive Hubbard model with an inverted population, and $\eta$-paired states in systems with a large density of doublons and holons. The reshuffling of entropy into full and empty bands not only provides an efficient cooling mechanism, it also allows one to overcome thermalization bottlenecks and slow dynamics that have been observed in systems cooled by the coupling to boson baths.

In this paper we continue to explore “hybrid” quantum circuit models in one-dimension with both unitary and measurement gates, focusing on the entanglement properties of wave function trajectories at long times, in the steady state. We simulate a large class of Clifford circuits, including models with or without randomness in the unitary gates, and with or without randomness in the locations of measurement gates, using stabilizer techniques to access the long-time dynamics of systems up to 512 qubits. In all models we find a volume-law entangled phase for low measurement rates, which exhibits a subdominant logarithmic behavior in the entanglement entropy, $S_A=\alpha ln\left|A\right|+s\left|A\right|$, with subsystem size $\left|A\right|$. With increasing measurement rate the volume-law phase is unstable to a disentangled area-law phase, passing through a single entanglement transition at a critical rate of measurement. At criticality we find a purely logarithmic entanglement entropy, $S_A=\alpha \left(p_c\right)ln\left|A\right|$, a power-law decay and conformal symmetry of the mutual information, with exponential decay off criticality. Various spin-spin correlation functions also show slow decay at criticality. Critical exponents are consistent across all models, indicative of a single universality class. These results suggest the existence of an effective underlying statistical mechanical model for the entanglement transition. Beyond Clifford circuit models, numerical simulations of up to 20 qubits give consistent results.

We have studied spin excitations in a single-domain crystal of antiferromagnetic ${\mathrm{LiCoPO}}_4$ by terahertz absorption spectroscopy. By analyzing the selection rules and comparing the strengths of the absorption peaks in the different antiferromagnetic domains, we found electromagnons and magnetoelectric (ME) spin resonances in addition to conventional magnetic dipole active spin-wave excitations. Using the sum rule for the ME susceptibility, we determined the contribution of the spin excitations to all the different off-diagonal elements of the static ME susceptibility tensor in zero and finite magnetic fields. We conclude that the ME spin resonances are responsible for the static ME response of the bulk when the magnetic field is along the $x$ axis, and the symmetric part of the ME tensor with zero diagonal elements dominates over the antisymmetric components.

Compressing FeSi induces a progressive semiconductor to metal transition, onset at $P$≥15 GPa at temperatures below $T_{\max }$ determined by the degree of disorder in the sample. At high pressure preceding charge-gap closure, a broad maximum manifests in the ρ($T$) data at $T_{\max }$ and is a feature which persists into the metallic state. The extremum in ρ($T$) occurs at $T_{\max }\sim 40\mathrm{K}$ at ∼11 GPa and shifts monotonically to ∼240 K as pressure is increased to ∼32 GPa, in the most detailed example of three series of measurements involving pressurized FeSi with different degrees of disorder. The transition to a metallic phase is an electronic change only, in that the B20-type crystal structure is retained up to 30 GPa, with no evidence of a discontinuity in the volume-pressure equation of state data. Samples from the same ingot subjected to different quasihydrostatic conditions reveal different values of the critical pressure of the electronic transition, its width, and pressure dependences of $T_{\max }$. This attests to sensitivity of the electronic transition to the degree of disorder in the investigated sample. The metallic state has neither Fermi-liquid nor non-Fermi-liquid behavior. Such an unusual pressure-induced correlated metallic state in FeSi is attributed to extended states within the $3d-3p$ hybridization gap originating from disorder and compression tuning of the mobility edge relative to the Fermi level. The metallic state has also been investigated in external magnetic fields up to 8 T at low temperatures $(2\mathrm{K}\le T\le 20\mathrm{K})$ at 15 and 19 GPa. This reveals a positive magnetoresistance, as observed in doped $\mathrm{F}{\mathrm{e}}_{1-x}\mathrm{C}{\mathrm{o}}_x\mathrm{Si}$ samples at ambient pressure, suggesting that in the majority and minority spin bands there is a field-induced modification of the respective magnitudes of charge-carrier populations which have different mobilities.

We introduce a class of phase transitions separating quantum states with different entanglement features. An example of such an “entanglement phase transition” is provided by the many-body localization transition in disordered quantum systems, as it separates highly entangled thermal states at weak disorder from many-body localized states with low entanglement at strong disorder. In the spirit of random matrix theory, we describe a simple model for such transitions where a physical quantum many-body system lives at the “holographic” boundary of a bulk random tensor network. Using a replica trick approach, we map the calculation of the entanglement properties of the boundary system onto the free energy cost of fluctuating domain walls in a classical statistical mechanics model. This allows us to interpret transitions between volume-law and area-law scaling of entanglement as ordering transitions in this statistical mechanics model. Our approach allows us to get an analytic handle on the field theory of these entanglement transitions.

Using intrinsic multiple-Andreev-reflection-effect spectroscopy, we studied ballistic superconductor-normal metal-superconductor (SnS) contacts in layered oxypnictide superconductors ${\mathrm{NdFeAsO}}_{0.6}{\mathrm{H}}_{0.36}$ with critical temperatures $T_c=45-48$ K. We directly determined the magnitude of two bulk superconducting order parameters, the large gap ${\mathrm{\Delta }}_L\approx 10.4$ meV, a possible small gap ${\mathrm{\Delta }}_S\approx 1.8$ meV, and their temperature dependence. Additionally, a resonant coupling with a characteristic bosonic mode was observed—the boson energy at 4.2 K, ${\varepsilon }_0=10.5-11.0$ meV being less than the indirect gap (${\mathrm{\Delta }}_L<{\varepsilon }_0<{\mathrm{\Delta }}_L+{\mathrm{\Delta }}_S$).

By quantitative low-energy electron diffraction (LEED) we investigate the extensively studied commensurate charge density wave (CDW) phase of trigonal tantalum disulphide $(1T-{\mathrm{TaS}}_2)$, which develops at low temperatures with a $(\sqrt{13}\times \sqrt{13})R13.9^{\circ }$ periodicity. A full-dynamical analysis of the energy dependence of diffraction spot intensities reveals the entire crystallographic surface structure, i.e., the detailed atomic positions within the outermost two trilayers consisting of 78 atoms as well as the CDW stacking. The analysis is based on an unusually large data set consisting of spectra for 128 inequivalent beams taken in the energy range 20–250 eV and an excellent fit quality expressed by a best-fit Pendry $R$ factor of $R=0.110$. The LEED intensity analysis reveals that the well-accepted model of star-of-David-shaped clusters of Ta atoms for the bulk structure also holds for the outermost two ${\mathrm{TaS}}_2$ trilayers. Specifically, in both layers the clusters of Ta atoms contract laterally by up to 0.25 Å and also slightly rotate within the superstructure cell, causing respective distortions as well as heavy bucklings (up to 0.23 Å) in the adjacent sulfur layers. Most importantly, our analysis finds that the CDWs of the first and second trilayers are vertically aligned, while there is a lateral shift of two units of the basic hexagonal lattice (6.71 Å) between the second and third trilayers. The results may contribute to a better understanding of the intricate electronic structure of the reference compound $1T-{\mathrm{TaS}}_2$ and guide the way to the analysis of complex structures in similar quantum materials.

Chains of superconducting circuit devices provide a natural platform for studies of synthetic bosonic quantum matter. Motivated by the recent experimental progress in realizing disordered and interacting chains of superconducting transmon devices, we study the bosonic many-body localization phase transition using the methods of exact diagonalization as well as matrix product state dynamics. We estimate the location of transition separating the ergodic and the many-body localized phases as a function of the disorder strength and the many-body on-site interaction strength. The main difference between the bosonic model realized by superconducting circuits and similar fermionic model is that the effect of the on-site interaction is stronger due to the possibility of multiple excitations occupying the same site. The phase transition is found to be robust upon including longer-range hopping and interaction terms present in the experiments. Furthermore, we calculate experimentally relevant local observables and show that their temporal fluctuations can be used to distinguish between the dynamics of Anderson insulator, many-body localization, and delocalized phases. While we consider unitary dynamics, neglecting the effects of dissipation, decoherence, and measurement back action, the timescales on which the dynamics is unitary are sufficient for observation of characteristic dynamics in the many-body localized phase. Moreover, the experimentally available disorder strength and interactions allow for tuning the many-body localization phase transition, thus making the arrays of superconducting circuit devices a promising platform for exploring localization physics and phase transition.

Superconducting quantum circuits are potential candidates to realize a large-scale quantum computer. The envisioned large density of integrated components, however, requires a proper thermal management and control of dissipation. To this end, it is advantageous to utilize tunable dissipation channels and to exploit the optimized heat flow at exceptional points (EPs). Here, we experimentally realize an EP in a superconducting microwave circuit consisting of two resonators. The EP is a singularity point of the effective Hamiltonian, and corresponds to critical damping with the most efficient heat transfer between the resonators without back and forth oscillation of energy. We observe a crossover from underdamped to overdamped coupling across the EP by utilizing photon-assisted tunneling as an in situ tunable dissipative element in one of the resonators. These methods can be used to obtain fast dissipation, for example, for initializing qubits to their ground states. In addition, these results pave the way for thorough investigation of parity-time symmetry and the spontaneous symmetry breaking at the EP in superconducting quantum circuits operating at the level of single energy quanta.

We study one-dimensional Kondo lattices (KLs) which consist of itinerant electrons interacting with Kondo impurities (KIs)—localized quantum magnetic moments. We focus on KLs with isotropic exchange interaction between electrons and KIs and with a high KI density. The latter determines the principal role of the indirect interaction between KIs for the low-energy physics. Namely, the Kondo physics becomes suppressed and all properties are governed by spin ordering. We present a comprehensive analytical theory of such KLs at an arbitrary doping and predict a variety of regimes with different electronic phases. They range from commensurate insulators (at filling factors 1/2, 1/4, and 3/4) to metals with strongly interacting conduction electrons (close to these three special cases) to an exotic phase of a helical metal. The helical metals can provide a unique platform for realization of an emergent protection of ballistic transport in quantum wires. We compare our theory with previously obtained numerical results and discuss possible experiments where the theory could be tested.

Magic-angle twisted bilayer graphene (MA-tBLG) has appeared as a tunable testing ground to investigate the conspiracy of electronic interactions, band structure, and lattice degrees of freedom to yield exotic quantum many-body ground states in a two-dimensional (2D) Dirac material framework. While the impact of external parameters such as doping or magnetic field can be conveniently modified and analyzed, the all-surface nature of the quasi-2D electron gas combined with its intricate internal properties pose a challenging task to characterize the quintessential nature of the different insulating and superconducting states found in experiments. We analyze the interplay of internal screening and dielectric environment on the intrinsic electronic interaction profile of MA-tBLG. We find that interlayer coupling generically enhances the internal screening. The influence of the dielectric environment on the effective interaction strength depends decisively on the electronic state of MA-tBLG. Thus, we propose the experimental tailoring of the dielectric environment, e.g., by varying the capping layer composition and thickness, as a promising pursuit to provide further evidence for resolving the hidden nature of the quantum many-body states in MA-tBLG.

We propose a concept of three-dimensional topological magnon systems which are the magnonic analog of topological insulators in three dimensions. We define a set of ${\mathbb{Z}}_2$ topological invariants that characterizes different topological phases and determines the presence or absence of surface Dirac cones. The validity of the classification scheme based on these ${\mathbb{Z}}_2$ invariants is demonstrated by considering three concrete examples. One of them is a bosonic counterpart of the Fu-Kane-Mele model on a diamond lattice, which is found to exhibit three distinct phases analogous to strong topological, weak topological, and trivial insulator phases of the original fermionic model. We also discuss a possible realization of the thermal Hall effect of surface magnons in the presence of a magnetic field in proximity to a normal ferromagnet. The topological characterization in this paper can also be applied to other systems, such as spin liquids or paramagnets, described by Schwinger bosons.

The polarization and piezoelectric response of the $\mathrm{BiTe}X$ $(X=\mathrm{Cl},\mathrm{Br},and\mathrm{I})$ layered tellurohalides are computed from first principles. The results confirm a mixed ionic-covalent character of the bonding, and demonstrate that the internal structure within each triple layer is only weakly affected by the external stress, while the changes in the charge distribution with stress produce a substantial negative piezoelectric response. This suggests a mechanism for negative piezoelectric response that should remain robust even in the ultrathin film form in this class of materials.

As realized by Kapitza long ago, a rigid pendulum can be stabilized upside down by periodically driving its suspension point with tuned amplitude and frequency. While this dynamical stabilization is feasible in a variety of systems with few degrees of freedom, it is natural to search for generalizations to multiparticle systems. In particular, a fundamental question is whether, by periodically driving a single parameter in a many-body system, one can stabilize an otherwise unstable phase of matter against all possible fluctuations of its microscopic degrees of freedom. In this paper, we show that such stabilization occurs in experimentally realizable quantum many-body systems: A periodic modulation of a transverse magnetic field can make ferromagnetic spin systems with long-range interactions stably trapped around unstable paramagnetic configurations as well as in other unconventional dynamical phases with no equilibrium counterparts. We demonstrate that these quantum Kapitza phases have a long lifetime and can be observed in current experiments with trapped ions.

We study the interaction of a (classical) light field with the magnetic degrees of freedom in the two-dimensional antiferromagnet ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. The reduced space group symmetry of the crystal allows for several channels for spin-operator bilinears to couple to the electric field. Integrating out high-energy degrees of freedom in a Keldysh framework, we derive induced effective fields, which enter the equations of motion of the low-energy mode of in-plane rotations, which couple to the out-of-plane magnetization. Considering a pump-probe protocol, these induced fields excite magnetization oscillations, which can subsequently be probed, e.g., using Kerr rotation. We discuss how the induced fields depend on polarization and frequency of the driving light, and our study applies to both resonant and nonresonant regimes. Crucially, the induced fields depend on the two-magnon density of states, thus allowing for further insight into properties of the magnetic excitation spectrum. Furthermore, these effects rely upon (weak) magnon interactions, and so are beyond a Floquet magnon description.

Computational materials screening studies require fast calculation of the properties of thousands of materials. The calculations are often performed with density functional theory (DFT), but the necessary computer time sets limitations for the investigated material space. Therefore, the development of machine-learning models for prediction of DFT-calculated properties is currently of interest. A particular challenge for new materials is that the atomic positions are generally not known. We present a machine-learning model for the prediction of DFT-calculated formation energies based on Voronoi quotient graphs and local symmetry classification without the need for detailed information about atomic positions. The model is implemented as a message passing neural network and tested on the Open Quantum Materials Database (OQMD) and the Materials Project Database. The test mean absolute error is 22 meV on the OQMD and 43 meV on Materials Project Database. The possibilities for prediction in a realistic computational screening setting are investigated on a data set of 5976 $AB{\mathrm{Se}}_3$ selenides with very limited overlap with the OQMD training set. Pretraining on OQMD and subsequent training on 100 selenides result in a mean absolute error below 0.1 eV for the formation energy of the selenides.

We theoretically investigate magnon-mediated superconductivity in a heterostructure consisting of a normal metal and a two-sublattice antiferromagnetic insulator. The attractive electron-electron pairing interaction is caused by an interfacial exchange coupling with magnons residing in the antiferromagnet, resulting in $p$-wave, spin-triplet superconductivity in the normal metal. Our main finding is that one may significantly enhance the superconducting critical temperature by coupling the normal metal to only one of the two antiferromagnetic sublattices employing, for example, an uncompensated interface. Employing realistic material parameters, the critical temperature increases from vanishingly small values to significantly larger values than 1 K as the interfacial coupling becomes strongly sublattice asymmetric. We provide a general physical picture of this enhancement mechanism based on the notion of squeezed bosonic eigenmodes.

Topological antiferromagnetic (AFM) spintronics is an emerging field of research, which involves the topological electronic states coupled to the AFM order parameter known as the Néel vector. The control of these states is envisioned through manipulation of the Néel vector by spin-orbit torques driven by electric currents. Here we propose a different approach favorable for low-power AFM spintronics, where the control of the topological states in a two-dimensional material, such as graphene, is performed via the proximity effect by the voltage induced switching of the Néel vector in an adjacent magnetoelectric AFM insulator, such as chromia. Mediated by the symmetry protected boundary magnetization and the induced Rashba-type spin-orbit coupling at the interface between graphene and chromia, the emergent topological phases in graphene can be controlled by the Néel vector. Using density functional theory and tight-binding Hamiltonian approaches, we model a $\mathrm{graphene}/{\mathrm{Cr}}_2{\mathrm{O}}_3$ (0001) interface and demonstrate nontrivial band gap openings in the graphene Dirac bands asymmetric between the $K$ and $K^{\prime }$ valleys. This gives rise to an unconventional quantum anomalous Hall effect (QAHE) with a quantized value of $2e^2/h$ and an additional steplike feature at a value close to $e^2/2h$, and the emergence of the spin-polarized valley Hall effect (VHE). Furthermore, depending on the Néel vector orientation, we predict the appearance and transformation of different topological phases in graphene across the 180° AFM domain wall, involving the QAHE, the valley-polarized QAHE, and the quantum VHE, and the emergence of the chiral edge states along the domain wall. These topological properties are controlled by voltage through magnetoelectric switching of the AFM insulator with no need for spin-orbit torques.