Subsystem symmetries are intermediate between global and gauge symmetries. One can treat these symmetries either like global symmetries that act on subregions of a system, or gauge symmetries that act on the regions transverse to the regions acted upon by the symmetry. We show that this latter interpretation can lead to an understanding of global, topology-dependent features in systems with subsystem symmetries. We demonstrate this with an exactly solvable lattice model constructed from a two-dimensional system of bosons coupled to a vector field with a one-dimensional subsystem symmetry. The model is shown to host a robust ground-state degeneracy that depends on the spatial topology of the underlying manifold, and localized zero-energy modes on corners of the system. A continuum field theory description of these phenomena is derived in terms of an anisotropic, modified version of the Abelian $K$-matrix Chern-Simons field theory. We show that this continuum description can lead to geometric-type effects such as corner states and edge states whose character depends on the orientation of the edge.

It is known that ${\left(\mathrm{ZnO}\right)}_{1-x}{\left(\mathrm{GaN}\right)}_x$ alloys demonstrate remarkable energy band bowing, making the material absorb in the visible range, in spite of the binary components being classical wide band gap semiconductors. However, the origin of this bowing is not settled; two major mechanisms are under debate: Influence of the orbital repulsion and/or formation of a defect band. In the present work, we applied a combination of the absorption and emission measurements on the samples exhibiting an outstanding nanoscale level of ${\left(\mathrm{ZnO}\right)}_{1-x}{\left(\mathrm{GaN}\right)}_x$ homogeneity as monitored by the high resolution electron microscopy equipped with the energy dispersive x-ray analysis and the electron energy loss spectroscopy; moreover the experimental data were set in the context of the computational analysis of the alloys employing density functional theory and quasiparticle $GW$ approximation. A prominent discrepancy in the band gap values as deduced from the absorption and emission experiments was observed systematically for the alloys with different compositions and interpreted as evidence for the absorption gap shrinking due to the defect band formation. Computational data support the argument, revealing only minor variations in the bulk of the conduction and valence band structures of the alloys, except for a characteristic “tail” in the vicinity of the valence band maximum. As such, we conclude that the energy gap bowing in ${\left(\mathrm{ZnO}\right)}_{1-x}{\left(\mathrm{GaN}\right)}_x$ alloys is due to the defect band formation, presumably at the top of the valence band maximum.

The (001) surface of heavily boron (B) doped silicon is investigated by employing the scanning tunneling microscopy measurements and the density functional theory calculations. Two different defect structures are found in the surface layers, both of which evolve the characteristic spectral features near the valence band maximum. One of the two incorporates a B atom in the second layer with the Si dimers intact and is scattered randomly in the layer. The other one incorporates a B atom in the fourth layer with a dimer vacancy produced directly above it and tends to get together with nearby dimer vacancies to grow the extended or line defects along the perpendicular direction to the dimer rows. Such defect formation is energetically favored to enhance the B populations in the second and fourth layers by $\sim 120$ and $\sim 80$ times, respectively, when compared to that in the bulk layer.

We investigate the electronic physics of layered Ni-based trichalcogenide ${\mathrm{NiPX}}_3$ (X = S, Se), a member of transition metal trichalcogenides (TMTs) with the chemical formula ${\mathrm{ABX}}_3$. These Ni-based TMTs distinguish themselves from other TMTs as their low energy electronic physics can be effectively described by the two $e_g d$ orbitals. The major band kinematics is characterized by the unusual long-range effective hopping between two third nearest-neighbor (TNN) Ni sites in the two-dimensional Ni honeycomb lattice so that the Ni lattice can be equivalently viewed as four weakly coupled honeycomb sublattices. Within each sublattice, the electronic physics is described by a strongly correlated two-orbital graphene-type model that results in an antiferromagnetic (AFM) ground state near half-filling. We show that the low energy physics in a paramagnetic state is determined by the eight Dirac cones which locate at $K, K^{\prime }, \frac{K}{2}$, and $\frac{K^{\prime }}{2}$ points in the first Brillouin zone with a strong AFM fluctuation between two $K \left(K^{\prime }\right)$ and $\frac{K^{\prime }}{2} \left(\frac{K}{2}\right)$ Dirac cones and carrier doping can sufficiently suppress the long-range AFM order and allow other competing orders, such as superconductivity, to emerge. The material can be an ideal system to study many exotic phenomena emerged from strong electron-electron correlation, including a potential $d\pm id$ superconducting state at high temperature.

Within a microscopic theory, we study the quantum Brownian motion of a skyrmion in a magnetic insulator coupled to a bath of magnonlike quantum excitations. The intrinsic skyrmion-bath coupling gives rise to an effective mass for the skyrmion, which remains finite down to zero temperature due to the quantum nature of the magnon bath. We show that the quantum version of the fluctuation-dissipation theorem acquires a nontrivial temperature dependence. As a consequence, the skyrmion mean-square displacement is finite at zero temperature and has a fast thermal activation that scales quadratically with temperature, contrary to the linear increase predicted by the classical phenomenological theory. The effects of an external oscillating drive which couples directly to the magnon bath are investigated. We generalize the standard quantum theory of dissipation and we show that the external drive generates a time-periodic term linear in the skyrmion velocity, and a time-periodic magnus force correction, which are both absent in the static limit. The skyrmion response function inherits the time periodicity of the driving field and is thus enhanced and lowered over a driving cycle. Finally, we provide a generalized version of the nonequilibrium fluctuation-dissipation theorem valid for weakly driven baths.

The statistics and form of avalanches in a driven system reveal the nature of the underlying energy landscape and dynamics. In conventional metallic ferromagnets, eddy-current back action can dominate the dynamics. Here, we study Barkhausen noise in Li(Ho,Y)${\mathrm{F}}_4$, an insulating Ising ferromagnet that cannot sustain eddy currents. For large avalanches at temperatures approaching the Curie point, we find a symmetric response free of drag effects. In the low-temperature limit, drag effects contribute to the dynamics, which we link to enhanced pinning from local random fields that are enabled by the microscopic dipole-coupled Hamiltonian (the Ising model in transverse field).

Frustration in classical spin models can lead to degenerate ground states without long-range order. In reciprocal space, these degeneracies appear as manifolds of wave vectors, their dimensionality increasing with the degree of frustration and the robustness of the disordered spin-liquid state. Here, we present a recipe to explicitly construct Heisenberg models on Bravais lattices with codimension-one manifolds, i.e., lines in two dimensions and surfaces with different Euler characteristics in three dimensions. Furthermore, we discuss the role of thermal and quantum fluctuations in stabilizing ordered states.

Exploiting the possibility of temporal variation of the winding number, we have prepared a Su-Schrieffer-Heeger (SSH) chain in its stroboscopic topological state, starting from the trivial one, by application of a periodic perturbation. The periodic driving we employ here is adiabatically switched on to break the particle-hole symmetry and generate a chiral mass term in the effective Floquet Hamiltonian; consequently, the Floquet Hamiltonian also gets deformed without crossing the gapless quantum critical point. The particle-hole symmetry is subsequently restored in the Floquet Hamiltonian by adiabatically switching off a part of the periodic potential. Thereafter, the Floquet Hamiltonian develops a symmetry-protected nontrivial topological winding number. Furthermore, we also observe stroboscopic topologically protected localized edge states in a long open chain and show that a bulk-boundary correspondence survives a unitary nonequilibrium situation in one-dimensional BDI Hamiltonians. Moreover, considering an extended SSH chain with higher neighbor hoppings, we dynamically prepare the system in a stroboscopic out-of-equilibrium topological insulator state starting from a metallic regime. At the same time, we establish the dynamical preparation of higher winding phases in an extended SSH chain with stroboscopic bulk-boundary correspondence in the nonequilibrium state of the system.

In this paper, we present a detailed study of the oscillating magnetic relaxation in the synthetic antiferromagnet (SAF) with two ferromagnetic Co layers of different thicknesses separated by an Ir spacer. The four stable magnetic states of the SAF are determined by the mutual alignment of magnetic moments in the layers and are controlled by both the magnetic interlayer exchange interaction and the Zeeman energy. The specific variations in the thicknesses of the layers and/or temperature allow the existence of a “triple point,” which corresponds to a coincidence of the critical switching fields for two or three interstate transitions. In this case, two or even three different types of magnetization reversals occur simultaneously and competitively. A nonmonotonic dependence of the domain-wall speed $v_{\mathrm{DW}}$ on magnetic field $H$ and an oscillating time dependence of magnetic moment $M$ in a constant magnetic field were observed in a Pt/Co/Ir/Co/Pt synthetic antiferromagnet with perpendicular anisotropy due to interplay between the magnetic nuclei produced by Dzyaloshinskii-Moriya interaction. The proximity of two or three (triple-point) critical fields of SAF switching is the necessary condition for both a nonmonotonic magnetic relaxation and the oscillating time variations of the magnetic moment. The dynamical model describing the interaction and subsequent evolution of the magnetic nuclei demonstrates that this nontrivial magnetic relaxation obeys a simple Schrödinger equation.

The elementary excitation in the antiferromagnetic spin-1 model known as the Affleck-Kennedy-Lieb-Tasaki (AKLT) Hamiltonian has been described alternatively as magnons or kinklike solitons (triplons). The latter, which we call the triplon throughout this paper, has been proven equivalent descriptions of the same magnon excitation and not an independent branch of excited states. On the other hand, no careful examination of multimagnon and multitriplon equivalence was made in the past. In this paper we prove that two-magnon and two-triplon states are also identical descriptions of the same excited states and furthermore that their energies break down as the sum of one-triplon energies exactly for the AKLT Hamiltonian. The statement holds despite the fact that the model is nonintegrable. Such magnon/triplon dichotomy is conjectured to hold for arbitrary $n$-magnon and $n$-triplon states. The one- and two-triplon states form orthogonal sets that can be used to span the low-energy Hilbert space. We construct an effective version of the AKLT Hamiltonian within such a subspace and work out the correction to the one-triplon energy gap that finds excellent agreement with the known exact value.

The interplay of photon spin and orbital angular momentum (OAM) in the optical fiber (one-dimensional waveguide) has recently risen to the forefront of quantum nanophotonics. Here, we introduce the fermionic dual of the optical fiber, the Dirac wire, which exhibits unique electronic spin and OAM properties arising from confined solutions of the Dirac equation. The Dirac wires analyzed here represent cylindrical generalizations of the Jackiw-Rebbi domain wall and the minimal topological insulator, which are of significant interest in spintronics. We show the unique longitudinal spin arising from electrons confined to propagation in a wire, an effect which is fundamentally prohibited in planar geometries. Our work sheds light on the universal spatial dynamics of electron spin in confined geometries and the duality between electronic and photonic spin.

A semiconductor quantum dot (QD) can generate highly indistinguishable single photons at a high rate. For application in quantum communication and integration in hybrid systems, control of the QD optical properties is essential. Understanding the connection between the optical properties of a QD and the growth process is therefore important. Here, we show for GaAs QDs, grown by infilling droplet-etched nanoholes, that the emission wavelength, the neutral-to-charged exciton splitting, and the diamagnetic shift are strongly correlated with the capture-zone area, an important concept from nucleation theory. We show that the capture-zone model applies to the growth of this system even in the limit of a low QD density in which atoms diffuse over $\mu \mathrm{m}$ distances. The strong correlations between the various QD parameters facilitate preselection of QDs for applications with specific requirements on the QD properties; they also suggest that a spectrally narrowed QD distribution will result if QD growth on a regular lattice can be achieved.

In this paper we study an approach for nanoscale spatially inhomogeneous excitation of quasistatic magnetic fields by the plasmon-induced inverse Faraday effect (IFE) in graphene-covered semiconductors and we present analytical and numerical results for the induced magnetic field distribution. The effective magnetic field is predicted to reach about 1 T and the direction of the magnetic field can be switched by surface plasmon polaritons propagating into the opposite direction. By electrically controlling the chemical potential of the graphene sheet the spatial inhomogeneity of the magnetization near field can be broadly tuned. The response of the induced magnetization to the plasmon propagation is manifested by a nonlinear phase shift which is measurable in the far field. By using the Lorentz reciprocity theorem we analytically calculated the nonlinear susceptibility and the nonlinear absorption coefficient in dependence on the chemical potential, the frequency, and the other material parameters. The plasmon-induced IFE and the nonlinearity can be very strong by decreasing the chemical potential which is flexibly controllable by using the graphene's gate voltage. The studied system could pave the way for an alternative approach for nanometer spatial all-optical magnetization control.

We investigate the consequences of applying electric fields perpendicularly to thin films of topological semimetals. In particular, we consider Weyl and Dirac semimetals in a configuration such that their surface Fermi arcs lie on opposite edges of the films. We develop an analytical approach based on perturbation theory and a single-surface approximation and we compare our analytical results with numerical calculations. The effect of the electric field on the dispersion is twofold: It shifts the dispersion relation and renormalizes the Fermi velocity, which would, in turn, have direct effects on quantum transport measurements. Additionally, it modifies the spatial decay properties of surface states which will impact the connection of the Fermi arcs in opposite sides of a narrow thin film.

We study nonequilibrium thermodynamic properties of a driven one-dimensional quantum fluid by combining nonlinear Luttinger liquid theory with the quantum kinetic equation. In particular, we derive an entropy production consistent with the laws of thermodynamics for a system subject to an arbitrary perturbation varying slowly in space and time. Working in a basis of weakly interacting fermionic quasiparticles, we show that the leading contribution to the entropy production results from three-particle collisions, and we derive its scaling law at low temperatures.

The temperature and pressure dependence of structural phase transitions determine the structure-functionality relationships in many technologically important materials. Harmonic Hamiltonians have proven successful in predicting the vibrational properties of many materials. However, they are inadequate for modeling structural phase transitions in crystals with potential energy surfaces that are either strongly anharmonic or nonconvex with respect to collective atomic displacements or homogeneous strains. In this paper we develop a framework to express highly anharmonic first-principles potential energy surfaces as polynomials of collective cluster deformations. We further adapt the approach to a nonlinear extension of the cluster expansion formalism through the use of an artificial neural net model. The machine learning models are trained on a large database of first-principles calculations and are shown to reproduce the potential energy surface with low error.

${\mathrm{BaZrO}}_3$ is a perovskite that remains in the simple cubic phase at all temperatures, hence with no first-order Raman-active phonon mode allowed by symmetry. Yet, it exhibits an intense Raman spectrum with sharp and well-defined features. Here, we report the evolution of the Raman spectrum of ${\mathrm{BaZrO}}_3$ single crystals in a broad temperature range (4–1200 K) and discuss its origin with the support of detailed first-principle calculations of the lattice dynamics. Phonon calculations are performed not only for the cubic phase of ${\mathrm{BaZrO}}_3$, but also for the low-symmetry phases with octahedra tilts that have been suspected to exist at the nanoscale. We show that the Raman spectrum shows no direct evidence for these nanodomains, but can instead be explained by classical second-order Raman scattering. We provide an assignment of the dominant features to phonon mode combinations. In particular, we show that the high frequency range of the spectrum is dominated by overtones and shows an image of the phonon density of states corresponding to the stretching modes of the oxygen octahedra.

We analyze quasiperiodically driven quantum systems that can be mapped exactly to periodically driven ones and find Floquet time spirals in analogy with spatially incommensurate spiral magnetic states. Generalizing the mechanism to many-body systems, we discover that a form of discrete-time translation symmetry breaking can also occur in quasiperiodically driven systems. We construct a discrete-time quasicrystal stabilized by many-body localization. Crucially, it persists also under perturbations that break the equivalence with periodic systems. As such, it provides evidence of a stable quasiperiodically driven many-body quantum system which does not heat up to the featureless infinite-temperature state.

Spin currents are generated within the bulk of magnetic materials due to heat flow, an effect called intrinsic spin Seebeck. This bulk bosonic spin current consists of a diffusing thermal magnon cloud, parametrized by the magnon chemical potential (${\mu }_{\mathrm{m}}$), with a diffusion length of several microns in yttrium iron garnet (YIG). Transient optothermal measurements of the spin-Seebeck effect (SSE) as a function of temperature reveal the time evolution of ${\mu }_{\mathrm{m}}$ due to intrinsic SSE in YIG. The interface SSE develops at times $<2$ ns while the intrinsic SSE signal continues to evolve at times $>500\mu \mathrm{s}$, dominating the temperature dependence of SSE in bulk YIG. Time-dependent SSE data are fit to a multitemperature model of coupled spin/heat transport using the finite-element method (FEM), where the magnon spin lifetime ($\tau$) and magnon-phonon thermalization time (${\tau }_{\mathrm{mp}}$) are used as fit parameters. From 300 to 4 K, ${\tau }_{\mathrm{mp}}$ varies from 1 to 10 ns, whereas $\tau$ varies from 2 to $60\mu \mathrm{s}$ with the spin lifetime peaking at 90 K. At low temperature, a reduction in $\tau$ is observed consistent with impurity relaxation reported in ferromagnetic resonance measurements. These results demonstrate that the thermal magnon cloud in YIG contains extremely low-frequency magnons ($\sim 10$ GHz), providing spectral insight to the microscopic scattering processes involved in magnon spin/heat diffusion.