A study of spherical maghemite nanoparticles on a two-dimensional triangular array was carried out using a stochastic Landau-Lifshitz-Gilbert approach. The simulation method was first validated with a triangular array of simple dipoles, where results show the expected phase transition to a ferromagnetic state at a finite temperature. The ground state exhibited a continuous degeneracy that was lifted by an order-from-disorder mechanism at infinitesimal temperatures with the appearance of a sixfold planar anisotropy. The nanoparticle array consisted of 7.5 nm diameter maghemite spheres with bulklike superexchange interactions between Fe ions in the core, and weaker exchange between surface Fe ions and a radial anisotropy. The triangular nanoparticle array ordered at the same reduced temperature as the simple dipole array, but exhibited different behavior at low temperatures due to the surface anisotropy. We find that the vacancies on the octahedral sites in the nanoparticles combine with the surface anisotropy to produce an effective random temperature-dependent anisotropy for each particle. This leads to a reduction in the net magnetization of the nanoparticle array at zero temperature compared to the simple dipole array.

Recent experimental studies of magnetic domain expansion under easy-axis drive fields in materials with a perpendicular magnetic anisotropy have shown that the domain wall velocity is asymmetric as a function of an external in-plane magnetic field. This is understood as a consequence of the inversion asymmetry of the system, yielding a finite chiral Dzyaloshinskii-Moriya interaction. Numerous attempts have been made to explain these observations using creep theory, but, in doing so, these have not included all contributions to the domain wall energy or have introduced additional free parameters. In this article we present a theory for creep motion of chiral domain walls in the creep regime that includes the most important contributions to the domain-wall energy and does not introduce new free parameters beyond the usual parameters that are included in the micromagnetic energy. Furthermore, we present experimental measurements of domain wall velocities as a function of in-plane field that are well described by our model, and from which material properties such as the strength of the Dzyaloshinskii-Moriya interaction and the demagnetization field are extracted.

The coupling between antiferrodistortion (AFD) and ferroelectric (FE) polarization, universal for all tilted perovskite multiferroics, is known to strongly correlate with domain wall functionalities in the materials. The intrinsic mechanisms of domain wall phenomena, especially AFD-FE coupling-induced phenomena at the domain walls, have continued to intrigue the scientific and technological communities because of the need to develop future nanoscale electronic devices. Over the past years, theoretical studies have often shown controversial results, owing to the fact that they are neither sufficiently nor directly corroborated with experimental evidence. In this work, the AFD-FE coupling at uncharged 180° and 71° domain walls in $\mathrm{BiFe}{\mathrm{O}}_3$ films is investigated by means of aberration-corrected scanning transmission electron microscopy with high-resolution scanning transmission electron microscopy and rationalized by phenomenological Landau-Ginzburg-Devonshire (LGD) theory. We reveal a peculiar morphology at the AFD-FE walls, including kinks, meandering, and trianglelike regions with opposite oxygen displacements and curvature near the interface. The LGD theory confirms that the tilt gradient energy induces these unusual morphologies and the features would change delicately with different kinds of domain walls. Moreover, the 180° AFD-FE walls are proved to be conductive with an unexpected reduction of the Fe-O-Fe bond angle, which is distinct from theoretical predictions. By exploring AFD-FE coupling at the domain walls, and its induced functionalities, we provide exciting evidence into the links between structural distortions and its electronic properties, which provide great benefit for fundamental understanding of domain wall functionalities as well as functional manipulations for novel nanodevices.

We develop an embedded-atom method (EAM) model to perform classical molecular-dynamics computer simulations of a model of Zr-Ni-Al ternary melts, based on the existing binary ones. The EAM potential is validated against a broad range of experimental data for the liquid melt, including both static-structure factors and dynamical data on the mass-transport coefficients. We use our simulation model to address the structural and dynamical changes induced by a systematic replacement of Zr by Al in ${\mathrm{Zr}}_{75-x}{\mathrm{Ni}}_{25}{\mathrm{Al}}_x\left(x=0–30\right)$ ternary alloys. We find strong chemical-ordering effects exhibited as the locally preferred structure when the Al-concentration $c_{\text{Al}}$ is increased. Along with the chemical effects, effective-power-law relations are found between the self-diffusion coefficients in the melts, with an exponent that monotonically decreases with increasing Al concentration. The associated Stokes-Einstein relation between diffusivity and viscosity breaks down at higher temperature upon Al addition. We also address the influence of Al admixture on the vibrational spectrum of the melt. With increasing $c_{\text{Al}}$, sound waves move faster, and an optical vibrational mode is found.

We review different matrix-product-state (MPS) approaches to study the spreading of operators in generic nonintegrable quantum systems. As a common ground to all methods, we quantify this spreading by means of the Frobenius norm of the commutator of a spreading operator with a local operator, which is usually referred to as the out-of-time-order correlation (OTOC) function. We compare two approaches based on matrix-product states in the Schrödinger picture: the time-dependent block decimation (TEBD) and the time-dependent variational principle (TDVP), as well as TEBD based on matrix-product operators directly in the Heisenberg picture. The results of all methods are compared to numerically exact results using Krylov space exact time evolution. We find that for the Schrödinger picture, the TDVP algorithm performs better than the TEBD algorithm.

Moreover, the tails of the OTOC are accurately obtained both by TDVP MPS and TEBD MPO. They are in very good agreement with exact results at short times, and appear to be converged in bond dimensions even at longer times. However, the growth and saturation regimes are not well captured by either of the methods.

We investigated the electronic superstructure of a graphite surface in the vicinity of monoatomic armchair step edges with scanning tunneling microscopy and spectroscopy. Only the ($\sqrt{3}\times \sqrt{3}$)$R{30}^{\circ }$ superstructure is visualized near atomically clean armchair edges, while the honeycomb superstructure is absent. The spectroscopic mapping near the clean armchair edge clearly reveals the ($\sqrt{3}\times \sqrt{3}$)$R{30}^{\circ }$ superstructure on both sides of the step edge. We have also visualized a mixture of ($\sqrt{3}\times \sqrt{3}$)$R{30}^{\circ }$ and honeycomb superstructures near structurally defective armchair edges. Our results suggest that the honeycomb superstructure pattern results from a superposition of two sets of ($\sqrt{3}\times \sqrt{3}$)$R{30}^{\circ }$ superstructures with different phases. Our observation solves the mystery of the coexistence of two types of superstructures reported by prior studies.

Motivated by recent quantum gas microscope experiments for fermions in optical lattices, we present proof-of-principle calculations showing that it is possible to obtain the complete information about the quantum state on a small subsystem from equilibrium determinantal quantum Monte Carlo simulations. Both diagonal (in the occupation number basis) and off-diagonal elements of the reduced density matrix are calculated for a square plaquette, which is embedded in a much larger system of the two-dimensional Hubbard model, both at half filling and in the doped case. The diagonalization of the reduced density matrix is done by exploiting the point group symmetry and particle number conservation, which allows one to attach symmetry labels to its eigenvalues. Knowledge of the probabilities of plaquette occupation number configurations is useful for meticulous benchmarking of quantum gas microscope experiments. As the quantum state on the plaquette is exact and self-consistently embedded in an exact, correlated bath, the present approach connects to various cluster approximation techniques.

An electrodynamic response of graphene to a strong electromagnetic radiation is considered. A hot electron model (HEM) is introduced and a corresponding system of nonlinear equations is formulated. Solutions of this system are found and discussed in detail for intrinsic and doped graphene: the hot electron temperature, nonequilibrium electron and hole densities, absorption coefficient, and other physical quantities are calculated as functions of the incident wave frequency $\omega$ and intensity $I$, of the equilibrium chemical potential ${\mu }_0$, and temperature $T_0$, scattering parameters, as well as of the ratio ${\tau }_{\epsilon }/{\tau }_{\mathrm{rec}}$ of the intraband energy relaxation time ${\tau }_{\epsilon }$ to the recombination time ${\tau }_{\mathrm{rec}}$. The influence of the radiation intensity on the absorption coefficient $A$ at low ($\hslash \omega \lesssim 2|{\mu }_0|, dA/dI>0$) and high ($\hslash \omega \gtrsim 2|{\mu }_0|, dA/dI<0$) frequencies is studied. The results are shown to be in good agreement with recent experimental data.

Topologically nontrivial phases are linked to the appearance of localized modes in the boundaries of an open insulator. On the other hand, the existence of geometric frustration gives rise to degenerate localized bulk states. The interplay of these two phenomena may, in principle, result in an enhanced protection/localization of edge states. In this paper, we study a two-dimensional Lieb-based topological insulator with staggered hopping parameters and diagonal open boundary conditions. This system belongs to the $C_{2v}$ class and sustains one-dimensional (1D) boundary modes except at the topological transition point, where the $C_{4v}$ symmetry allows for the existence of localized (0D) corner states. Our analysis reveals that, while a large set of boundary states have a common well-defined topological phase transition, other edge states reflect a topological nontrivial phase for any finite value of the hopping parameters, are completely localized (compact) due to destructive interference, and evolve into corner states when reaching the higher symmetry point. We consider the robustness of these compact edge states with respect to time-dependent perturbations and indicate ways that these states could be prepared and measured in experiments with ultracold atoms.

The use of artificial neural networks to represent quantum wave functions has recently attracted interest as a way to solve complex many-body problems. The potential of these variational parametrizations has been supported by analytical and numerical evidence in controlled benchmarks. While approaching the end of the early research phase in this field, it becomes increasingly important to show how neural-network states perform for models and physical problems that constitute a clear open challenge for other many-body computational methods. In this paper, we start addressing this aspect, concentrating on a presently unsolved model describing two-dimensional frustrated magnets. Using a fully convolutional neural network model as a variational ansätz, we study the frustrated spin-1/$2J_1\text{−}{J}_2$ Heisenberg model on the square lattice. We demonstrate that the resulting predictions for both ground-state energies and properties are competitive with, and often improve upon, existing state-of-the-art methods. In a relatively small region in the parameter space, corresponding to the maximally frustrated regime, our ansätz exhibits comparatively good but not the best performance. The gap between the complexity of the models adopted here and those routinely adopted in deep-learning applications is, however, still substantial, such that further improvements in future generations of neural-network quantum states are likely to be expected.

We investigate long-range coherent and dissipative coupling between two spatially separated magnets while both are coupled to a microwave cavity in the strong coupling limit. A careful examination of the system shows that the indirect interaction between two magnon modes is dependent on their individual mechanisms of direct coupling to the cavity. If both magnon modes share the same form of coupling to the cavity (either coherent or dissipative), then the indirect coupling between them will produce level repulsion. Conversely, if the magnon modes have different forms of coupling to the cavity (one coherent and one dissipative), then their indirect coupling will produce level attraction. We further demonstrate the cavity-mediated nature of the indirect interaction through investigating the dependence of the indirect coupling strength on the frequency detuning between the magnon and cavity modes. Our work theoretically and experimentally explores indirect cavity mediated interactions in systems exhibiting both coherent and dissipative coupling, which opens an avenue for controlling and utilizing light-matter interactions.

At large scales, magnetostatics of superconductors is described by a massive vector field theory: the London model. The magnetic field cannot penetrate into the bulk unless quantum vortices are formed. These are topological excitations characterized by an invariant: the phase winding. The London model dictates that loops of such vortices are not stable because the kinetic energy of superflow and the magnetic energy are smaller, the smaller vortex loops are. We demonstrate that in two-component superconductors, under certain conditions, such as the proximity to pair-density-wave instabilities, the hydromagnetostatics of the superconducting state and topological excitation changes dramatically: the excitations acquire the form of stable vortex loops and knots characterized by the different topological invariant: the Hopf index and hence termed hopfions. This demonstrates that magnetic properties in a superconducting state can be dramatically different from those of a London's massive vector field theory.

In contrast to ${\mathrm{MgB}}_2$ superconducting at $39\mathrm{K}$, the isostructural ${\mathrm{AlB}}_2$ has no superconductivity due to the absence of metallization driven by the covalent state of boron in ${\mathrm{MgB}}_2$. In this paper, we investigate from first principles the superconducting behavior in ultrathin limit of ${\mathrm{AlB}}_2$, and reveal a two-gap superconducting nature with transition temperatures $T_c\mathrm{s}$ in the range of $17-26.5\mathrm{K}$. Our results indicate that the superconductivity in ${\mathrm{AlB}}_2$ films originates mainly from the surface-boron-plane-driven covalent state which crosses the Fermi level because of the quantum confinement. Therefore, the hyperstoichiometric ${\mathrm{AlB}}_{2+x}$ films with the surface boron layer existing on each side of the systems also promise admirable superconductivity. In particular, a high $T_c\sim 47\mathrm{K}$ with an exciting three-gap nature is captured in the thinnest ${\mathrm{AlB}}_{2+x}$ films that possess a stoichiometry of ${\mathrm{AlB}}_4$ and a sandwich configuration of ${\mathrm{B}}_2\text{−}\mathrm{Al}\text{−}{\mathrm{B}}_2$. In addition, the effect of external strain on superconducting behavior in the thinnest ${\mathrm{AlB}}_2$ film and sandwich ${\mathrm{AlB}}_4$ film is also involved, which shows that a small compressive strain can further boost the $T_c\mathrm{s}$ in both films and a three-gap nature can be also observed in the thinnest ${\mathrm{AlB}}_2$ film under a biaxial tensile strain of $2.0\%-2.5\%$. These findings pave the way not only for study of superconductivity in ${\mathrm{AlB}}_2$-based materials but also the search for multigap superconductors.

In the reported experiment, a picosecond strain pulse induces a sharp transition between the steady states in a bistable cavity-polariton system. The strain pulse of 10-ps duration, generated in the GaAs substrate and injected into a high-$Q$ GaAs/AlAs microcavity, modulates the exciton resonance energies of the embedded quantum wells and correspondingly of the polariton resonances. When the microcavity is pumped by a laser with the photon energy slightly above the lower-polariton resonance, the strain-induced energy shift triggers the irreversible switching of the bistable polariton system from the lower to the upper state. This transition is accompanied by an instant increase of the optical emission from the microcavity by more than an order of magnitude.

We report ${}^{75}\mathrm{As}$-NMR results for CrAs under pressure, which shows superconductivity adjoining a helimagnetically ordered state. We successfully evaluated the Knight shift from the spectrum, which is strongly affected by the quadrupole interaction. The Knight shift shows the remarkable feature that the uniform spin susceptibility increases toward low temperatures in the paramagnetic state. This is in sharp contrast to CrAs at ambient pressure, and also to cuprates and Fe pnictides, where antiferromagnetic correlations are dominant. Superconductivity emerges in CrAs under unique magnetic correlations, which probably originate in the three-dimensional zigzag structure of its nonsymmorphic symmetry.

A quantum-chemical study of the positive charge-state of the nitrogen-vacancy center in diamond is presented. Charge control of this promising qubit candidate is a focus of diamond quantum technology research, as currently charge stability relating to surfaces and nearby defects causes some difficulties for quantum applications. To demonstrate full charge control over the nitrogen vacancy, all three charge states should be identified and the processes that lead to charge state changes understood. However, experimental markers for the positive state remain elusive compared to the readily detectable zero-phonon lines of the neutral and negative. In this work we present predicted hyperfine and zero-field splitting tensors as clear signatures of the normally spinless ${\mathrm{NV}}^{+}({}^1\mathrm{A}_1$ ground state) by probing a long-lived spin-triplet excited-state $\sim 0.7$ eV above the ${\mathrm{NV}}^{+}$ ground state. We find a relatively narrow excitation energy range of approximately 0.7–1.1 eV between excitation into an ${}^3\mathrm{E}$ state and conversion into the neutral charge state. To provide insight into the thermal stability of the positive charge state, we have calculated binding and diffusion energies for both charged and uncharged systems. We predict, given that the activation energies are only weakly charge state dependent, all three charge states would diffuse in the 1600–$1900{}^{\circ }\text{C}$ range, but the positive state has a significantly lower binding energy, suggesting that it will dissociate at temperatures around $1000{}^{\circ }\mathrm{C}$ rather than migrate.

We have studied the evolution of magnetic and orbital excitations as a function of hole doping in single-crystal samples of ${\mathrm{Sr}}_2{\mathrm{Ir}}_{1-x}{\mathrm{Rh}}_x{\mathrm{O}}_4\left(0.07\le x\le 0.42\right)$ using high-resolution Ir $L_3$-edge resonant inelastic x-ray scattering. Within the antiferromagnetically ordered region of the phase diagram $\left(x\le 0.17\right)$ we observe highly dispersive magnon and spin-orbit exciton modes. Interestingly, both the magnon gap energy and the magnon bandwidth appear to increase as a function of doping, resulting in a hardening of the magnon mode with increasing hole doping. As a result, the observed spin dynamics of hole-doped iridates more closely resemble those of the electron-doped, rather than hole-doped, cuprates. Within the paramagnetic region of the phase diagram $\left(0.17\le x\le 0.42\right)$ the low-lying magnon mode disappears, and we find no evidence of spin fluctuations in this regime. In addition, we observe that the orbital excitations become essentially dispersionless in the paramagnetic phase, indicating that magnetic order plays a crucial role in the propagation of the spin-orbit exciton.

Superconductivity of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ single crystals was investigated in small magnetic fields. In magnetic measurements the superconducting transition for $\mathbf{H}\parallel c$ appears 0.4 K higher than for $\mathbf{H}\perp c$. In this temperature range superconductivity is two dimensional and the total thickness of superconducting layers is about 0.83 of the sample thickness, which is a consequence of the occurrence of the quasi-insulating plane in the unit cell of the crystal structure. Resistivity in the ab plane and along the $c$ axis was measured simultaneously. In these measurements two-dimensional superconductivity was observed in a temperature range of 0.6–0.8 K with the clear signs of the Berezinskii-Kosterlitz-Touless (BKT) transition which occurs approximately 0.15 K below $T_c$, the mean-field transition temperature.