An indispensable part of the structure determination of crystalline two-dimensional (2D) materials and epitaxial thin films is the correct indexing of the acquired diffraction patterns. In our previous work, we described an effective algorithm to determine the 3D unit-cell parameters of complex systems comprising different orientations and polymorphs. In this work, we adapt the indexing method to 2D lattices in reciprocal space. Analyzing low-energy electron diffraction and Fourier-transformed scanning tunneling microscopy measurements, the method is exemplarily applied to thin films of conjugated molecules like 3,4:9,10-perylenetetracarboxylic dianhydride (PTCDA), 6,13-pentacenequinone (P2O), and vanadyl phthalocyanine (VOPc) grown by physical vapor deposition on Ag(111). In all cases unit cells (rhomboids) along with their sixfold rotationally or mirror symmetric counterparts are determined. The already known commensurate epitaxial relationship is reproduced for PTCDA on Ag(111), demonstrating the validity of our method. In the case of P2O/Ag(111) a point-on-line epitaxial condition is found. Our algorithm can be equally well applied to all kinds of 2D patterns in reciprocal space where a crystallographic indexing is required, e.g., electron diffraction data [such as transmission electron diffraction, selected area electron diffraction (SAED)] and fast Fourier transforms (FFTs) of scanning probe images. To demonstrate this aspect, we evaluate FFTs of scanning tunneling microscopy data for stacked VOPc/PTCDA heteroepitaxial layers on Ag(111) as well as SAED data of an epitaxial ${\mathrm{TiO}}_2/{\mathrm{LaAlO}}_3\left(100\right)$ heterostructure in cross section.

We consider closed quantum many-body systems subject to stochastic resetting. This means that their unitary time evolution is interrupted by resets at randomly selected times. When a reset takes place, the system is reinitialized to a state chosen from a set of reset states conditionally on the outcome of a measurement taken immediately before resetting. We construct analytically the resulting nonequilibrium stationary state, thereby establishing an explicit connection between quantum quenches in closed systems and the emergent open system dynamics induced by stochastic resetting. We discuss as an application the paradigmatic transverse-field quantum Ising chain. We show that signatures of its ground-state quantum phase transition are visible in the steady state of the reset dynamics as a sharp crossover. Our findings show that a controlled stochastic resetting dynamics allows one to design nonequilibrium stationary states of quantum many-body systems, where uncontrolled dissipation and heating can be prevented. These states can thus be created on demand and exploited, e.g., as a resource for quantum enhanced sensing on quantum simulator platforms.

A wide spectrum of disordered many-body systems that are inherent structures (i.e., local minima of potential energy landscape) was recently discovered to possess a novel hidden long-range order known as hyperuniformity, yet the mechanisms associated with the emergence of disordered hyperuniformity in such systems are not well understood. Here, we address this important fundamental problem by linking together the concepts of topological defects, inherent structures, and hyperuniformity. We consider representative examples of disordered inherent structures that are topological variants of crystals obtained by continuously introducing into the crystalline state randomly distributed topological defects such as dislocations and disclinations. Using large-scale numerical simulations and analysis based on a continuum theory, we show that the topological defects and the associated transformation pathways linking the disordered inherent structures to the crystalline state preserve hyperuniformity of the latter, as long as the displacements induced by the defects are sufficiently localized (i.e., the volume integrals of the displacements and squared displacements caused by individual defect are finite) and the displacement-displacement correlation matrix of the system is diagonalized and isotropic. Our results provide insights to the discovery, design, and generation of novel disordered hyperuniform materials.

An effective Hamiltonian is developed and used to investigate the different equilibrium phases that $\mathrm{Ba}\mathrm{Ce}{\mathrm{O}}_3$ can possess, as a function of temperature. Such atomistic technique predicts that monodomain $\mathrm{Ba}\mathrm{Ce}{\mathrm{O}}_3$ adopts a phase transition sequence that differs from the one commonly experimentally reported in this specific important perovskite, even if the end members of this sequence (namely, the high-temperature cubic $Pm\overline{3}m$ state and the orthorhombic $Pbnm$ ground state) are identical between our simulations and measurements. In contrast with this experimental phase transition sequence, the predicted one is, in fact, in line with a rule denoted here as “the gradual tilting rule” that guides the progressive change of oxygen octahedral tiltings about the three Cartesian axes from $Pm\overline{3}m$ to $Pbnm$. The fact that this rule is obeyed in many perovskites, along with some of our analyses and previous works, leads us to strongly suggest that intermediate phases experimentally reported in $\mathrm{Ba}\mathrm{Ce}{\mathrm{O}}_3$ pertain to multidomains, twinning, and/or antiphase boundaries. Such suggestion should also apply to other perovskites for which the structural phase transition sequence does not follow “the gradual tilting rule.”

We explore the Fock-space structure of eigenstates across the many-body localization (MBL) transition in a disordered, interacting quantum spin-$\frac{1}{2}$ chain. Eigenstate expectation values of spatially local observables, which distinguish an MBL phase from an ergodic one, can be represented in terms of eigenstate amplitudes on the Fock space. Motivated by this, we introduce and study spatial correlations on the Fock space. From these, a correlation length emerges, which is found to vary discontinuously across the MBL transition, and is intimately connected to the discontinuous jump in the multifractal exponents characterizing the Fock-space wave functions. Exploiting the direct connection between the local observables and Fock-space correlations, we show that the discontinuity in the length scale also implies discontinuous behavior of the local observables across the transition. A scaling theory based on these Fock-space correlations is constructed, which is closely connected to that for the inverse participation ratio. It yields a volume scale in the ergodic phase and a length scale in the MBL phase, whose critical properties suggest a Kosterlitz-Thouless–type scenario for the MBL transition, as is predicted by recent phenomenological theories. Finally, we also show how correlation functions on the Fock space reveal the inhomogeneities in eigenstate amplitudes on the Fock space in the MBL phase.

We study the transmission and reflection of a paraxial acoustic vortex beam (AVB) at a metasurface with a prescribed phase gradient. By using a plane-wave decomposition method, we obtain a closed-form expression for the transverse shift of the beam's gravity center. The transverse shift is proportional to the topological charge of the AVB and derives from momentum conservation and the reflection/transmission coefficient modulation. It corresponds to the acoustic Fedorov-Imbert linear shift and represents the orbital Hall effect for spinless longitudinal sound. We develop a generalized momentum conservation relation to understand the phenomena and find that the metasurface induces both linear momentum and angular momentum, which have different dependences on the incident angle. In particular, this mechanism can enable the Fedorov-Imbert linear shift even for normal incidence and total reflection. The results provide deeper insights into the physics underlying the interaction of AVBs with metasurfaces and may help the development of orbital angular momentum based acoustic devices.

Magnetic isotropic and Dzyaloshinskii-Moriya interactions in yttrium iron garnet have been obtained by ab initio fully relativistic calculations. The calculated coupling constants are in agreement with available experimental data. Using linear spin-wave theory, we are able to reproduce the experimental magnon spectrum including the spin-wave gap and stiffness. The way to calculate the exchange coupling constants using the Korringa-Kohn-Rostoker formalism for large magnetic systems such as complex oxides is discussed in detail.

The phenomenon of spin-transfer torque (STT) in an antiferromagnet (AFM) is reexamined in terms of the incoming and outgoing spin currents. In contrast to the conventional approach, our model treats the STT as a result of the electron spin angular momentum transfer to the entire monodomain magnetic structure rather than to the individual sublattice magnetizations. This treatment enables the analysis to account for not only the destructive role of electron spin relaxation but also the potentially incomplete loss of electron spin phases that can either enhance the STT or suppress it depending on the structure parameters. Application of the developed model to the dynamical equation of the AFM order parameter illustrates the qualitative and quantitative differences with the conventional approach. Unlike the latter, our results predict a strong dependence of the STT on the orientation of the electron spin polarization relative to the magnetic anisotropy axes of the AFM. A similar characteristic also reveals a complex interplay in the Néel vector dynamics that can lead to a sublinear response of the oscillation frequency to the strength of the spin current. The impact of the fieldlike torque that can arise likewise from the spin injection is examined as well for a comprehensive account. The numerical calculations elucidate further the conditions that can elicit efficient manipulation of the magnetic states in an AFM under a spin-polarized current.

A generic two-dimensional disordered topological superconductor in symmetry class D exhibits rich phenomenology and multiple phases: diffusive thermal metal (DTM), Anderson insulator (AI), and thermal quantum Hall (TQH) phase (a topological superconductor). We numerically investigate the phase diagram of a lattice model of such class-D superconductor, specifically focusing on transitions between the phases and the associated universal critical behaviors. We confirm the existence of a tricritical point and its repulsive nature at the point on the phase diagram where the three phases meet. We characterize the critical behaviors at various critical points and the tricritical point using numerical evaluation of the localization length, the conductance (or conductivity), and the density of states. We conclude that the two metal-insulator transitions (DTM-TQH and DTM-AI) belong to the same universality class, whereas the tricritical point represents a distinct universality class.

The bulk electric polarization works as a nonlocal order parameter that characterizes topological quantum matters. Motivated by a recent paper [H. Watanabe et al., Phys. Rev. B 103, 134430 (2021)], we discuss magnetic analogs of the bulk polarization in one-dimensional quantum spin systems, that is, quantized magnetizations on the edges of one-dimensional quantum spin systems. The edge magnetization shares the topological origin with the fractional edge state of the topological odd-spin Haldane phases. Despite this topological origin, the edge magnetization can also appear in topologically trivial quantum phases. We develop straightforward field theoretical arguments that explain the characteristic properties of the edge magnetization. The field theory shows that a U(1) spin-rotation symmetry and a site-centered or bond-centered inversion symmetry protect the quantization of the edge magnetization. We proceed to discussions that quantum phases on nonzero magnetization plateaus can also have the quantized edge magnetization that deviates from the magnetization density in bulk. We demonstrate that the quantized edge magnetization distinguishes two quantum phases on a magnetization plateau separated by a quantum critical point. The edge magnetization exhibits an abrupt stepwise change from zero to $1/2$ at the quantum critical point because the quantum phase transition occurs in the presence of the symmetries protecting the quantization of the edge magnetization. We also show that the quantized edge magnetization can result from the spontaneous ferrimagnetic order.

Competing magnetic interactions in low-dimensional quantum magnets can lead to the exotic ground state with fractionalized excitations. Herein, we present our results on an $S=\frac{5}{2}$ quasi-one-dimensional spin system ${\mathrm{Bi}}_3{\mathrm{FeMo}}_2{\mathrm{O}}_{12}$. The structure of ${\mathrm{Bi}}_3{\mathrm{FeMo}}_2{\mathrm{O}}_{12}$ consists of very well separated, infinite zigzag $S=\frac{5}{2}$ spin chains. The observation of a broad maximum around 10 K in the magnetic susceptibility $\chi \left(T\right)$ suggests the presence of short-range spin correlations. $\chi \left(T\right)$ data do not fit the $S=\frac{5}{2}$ uniform spin chain model due to the presence of second-nearest-neighbor coupling ($J_2$) along with the first-nearest-neighbor coupling ($J_1$) of the zigzag chain. The electronic structure calculations infer that the value of $J_1$ is comparable with $J_2$ ($J_2/J_1\approx 1.1$) with a negligible interchain interaction ($J^{\prime }/J\approx 0.01$) implying that ${\mathrm{Bi}}_3{\mathrm{FeMo}}_2{\mathrm{O}}_{12}$ is a highly frustrated triangular chain system. The absence of magnetic long-range ordering down to 0.2 K is seen in the heat-capacity data, despite a relatively large antiferromagnetic Curie-Weiss temperature ${\theta }_{CW}\approx –40$ K. The magnetic heat capacity follows nearly a linear behavior at low temperatures indicating that the $S=\frac{5}{2}$ anisotropic triangular chain exhibits the gapless excitations.

We analyze fermions after an interaction quantum quench in one spatial dimension and study the growth of the steady state entanglement entropy density under either a spatial mode or particle bipartition. For integrable lattice models we find excellent agreement between the increase of spatial and particle entanglement entropy, and for chaotic models an examination of two further neighbor interaction strengths suggests similar correspondence. This result highlights the applicability of a statistical ensemble to compute expectation values of local observables after a quantum quench

Robust boundary states have been the focus of much recent research, both as topologically protected states and as non-Hermitian skin states. In this paper, we show that many-body effects can also induce analogs of these robust states in place of actual physical boundaries. Particle statistics or suitably engineered interactions, i.e., in ultracold atomic lattices can restrict the accessible many-body Hilbert space, and introduce effective boundaries in a spatially periodic higher-dimensional configuration space. We demonstrate the emergence of topological chiral modes in a two-fermion hopping model without open boundaries, with fermion pairs confined and asymmetrically propagated by suitably chosen fluxes. Heterogeneous nonreciprocal hoppings across different particle species can also result in robust particle clumping in a translation invariant setting, reminiscent of skin mode accumulation at an open boundary. But unlike fixed open boundaries, effective boundaries correspond to the locations of impenetrable particles and are mobile, giving rise to fundamentally different many-body versus single-body spectra and corresponding dynamics. Since nonreciprocal accumulation is agnostic to the dimensionality of restricted Hilbert spaces, our many-body skin states generalize directly in the thermodynamic limit. The many-body topological states, however, are nontrivially dimension-dependent, and their detailed exploration will stimulate further studies in higher dimensional topological invariants.

We have investigated lithium fluoride (LiF) in the monolayer range deposited by thermal sublimation onto the (100) surface of a silver single crystal in ultrahigh vacuum. Our study combines scanning tunneling microscopy, low-energy electron diffraction, and polarized x-ray absorption spectroscopy. The results reveal that LiF grows epitaxially on Ag(100), with a morphology strongly depending on the growth temperature: at room temperature, dendritic patterns are formed with branches along the [011] and $\left[0\overline{1}1\right]$ directions, exhibiting the same lattice orientation as the substrate. Conversely, at a substrate temperature of 500 K LiF forms square islands exhibiting a moiré pattern, with the islands’ borders oriented along the [001] and [010] directions. While the lattice constant of the surface deposited samples is close to the one of bulk fcc LiF, the appearance of a strong x-ray linear dichroism for dendritic LiF points to a tetragonal distortion arising from the LiF-substrate interaction. The less prominent dichroism and the presence of the moiré pattern on the square islands both suggest the relaxation of the LiF grown on the hot Ag substrate.

Giant optical circular birefringence (GCB) induced by a single quantum-dot-confined spin in an optical microcavity finds wide applications in quantum and optical technologies, such as quantum gates, quantum transistors, quantum repeaters, quantum routers, etc. If the system is probed with a classical light, such as the laser light where the photons are uncorrelated with each other, then the single-photon GCB is detected. In this work we develop a general approach to investigate the many-body dynamics of $n$ photons bound to one quantum emitter and apply this method to calculate the $n$-photon GCB probed with quantum light in Fock states. With suppressed atomic saturation, quantum light loads photons into dressed states more efficiently than classical light such that the whole Jaynes-Cummings energy ladder and the quantum nonlinearity related to the multiphoton transitions can be observed. The $n$-photon GCB lying at the cavity mode resonance allows the spin-cavity quantum gates and quantum transistors to extend from single-photon to $n$-photon operations and generate entangled photonic Fock states, i.e., the NOON states which are useful for quantum metrology and lithography.

We theoretically propose a family of structurally stable monolayer halide perovskite ${\mathrm{A}}_3{\mathrm{B}}_2{\mathrm{C}}_9$ (A = Rb, Cs; B = Pd, Pt; C = Cl, Br) with easy magnetization planes. The family materials are all half metals with large spin gaps beyond 1 eV accompanying a single spin Dirac point located at K point. When the spin-orbit coupling is switched on, we further show that ${\mathrm{Rb}}_3{\mathrm{Pt}}_2{\mathrm{Cl}}_9, {\mathrm{Cs}}_3{\mathrm{Pd}}_2{\mathrm{Cl}}_9$, and ${\mathrm{Cs}}_3{\mathrm{Pt}}_2{\mathrm{Cl}}_9$ monolayers can open up large band gaps from 63 to 103 meV to harbor quantum anomalous Hall effect with Chern numbers of $\mathcal{C}=\pm 1$ whenever the mirror symmetry is broken by the in-plane magnetization, and the corresponding Berezinskii-Kosterlitz-Thouless transition temperatures are over 248 K. Our findings provide a potentially realizable platform to explore quantum anomalous Hall effect and spintronic applications at high temperatures.

We study spin and valley transport through a magnetic ${\mathrm{WSe}}_2$ superlattice in the presence of off-resonance circularly polarized light and gate voltage. It is demonstrated by using appropriate values of off-resonance light on the ferromagnetic region that the junction becomes gapless with a linear dispersion spectrum for each spin-valley flavor. Therefore, for normal incidence, the junction is totally transparent ($T=1$), a manifestation of spin-valley dependent Klein tunneling. By increasing the number of barriers, we find optically and electrically tunable spin-valley dependent transmission gaps and transmission resonances, which are also confirmed using the Bloch theorem. Consequently, a perfect spin- and valley-polarized current can be obtained, which is tunable and switchable using an exchange field, off-resonance light, and gate voltage. Our findings show the potential applications of the proposed superlattices for the spin-valley polarization and the spin-valley filtering of ${\mathrm{WSe}}_2$-based devices.

The recent reported robust surface states in a photonic metamaterial are usually limited to the gapless type. In this work, we propose a photonic topological metamaterial that can support surface states with gaps. The gaps appear when the electromagnetic duality symmetry of the material system is broken. Despite the existence of gaps, the surface waves can still bypass sharp corners robustly. We further demonstrate the filtering effect which can be viewed as a fingerprint of the existence of the gapped surface states. By changing the electromagnetic parameters of the covering materials, the filtering effect with controllable bandwidth can be realized.