We study the transport and equilibration properties of a classical Heisenberg chain, whose couplings are random variables drawn from a one-parameter family of power-law distributions. The absence of a scale in the couplings makes the system deviate substantially from the usual paradigm of diffusive spin hydrodynamics and exhibit a regime of subdiffusive transport with an exponent changing continuously with the parameter of the distribution. We propose a solvable phenomenological model that correctly yields the subdiffusive exponent, thereby linking local fluctuations in the coupling strengths to the long-time, large-distance behavior. It also yields the finite-time corrections to the asymptotic scaling, which can be important in fitting the numerical data. We show how such exponents undergo transitions as the distribution of the coupling gets wider, marking the passage from diffusion to a regime of slow diffusion, and finally to subdiffusion.

We investigate multiple-$Q$ instability on a square lattice at particular ordering wave vectors. We find that a superposition of quadruple-$Q$ spin density waves, which are connected by fourfold rotational and mirror symmetries, gives rise to a checkerboard bubble lattice with a collinear spin texture as a result of the geometry among the constituent ordering wave vectors in the Brillouin zone. By performing the simulated annealing for a fundamental spin model, we show that such a checkerboard bubble lattice is stabilized under an infinitesimally small easy-axis two-spin anisotropic interaction and biquadratic interaction at zero field, while it is degenerate with an anisotropic double-$Q$ state in the absence of the biquadratic interaction. The obtained multiple-$Q$ structures have no intensities at high-harmonic wave vectors in contrast to other multiple-$Q$ states, such as a magnetic skyrmion lattice. We also show that the checkerboard bubble lattice accompanies the charge density wave and exhibits a nearly flat-band dispersion in the electronic structure. Our results provide another route to realize exotic multiple-$Q$ spin textures by focusing on the geometry and symmetry in terms of the wave vectors in momentum space.

We present a group theoretical and ab initio analysis of lattice point defects in fluorographene, with a focus on neutral and negative ${\text{V}}_{\text{CF}}$ vacancies. By using a combination of density functional theory calculations and group theory analysis, we investigate the many-body configurations of the defects and calculate the vertical absorption and zero-phonon line energies of the excited states and their dependence with strain. The description of the defects is extended by computing their formation energy, as well as further relevant parameters as the Jahn-Teller energy for neutral ${\text{V}}_{\text{CF}}$ and the zero field splitting for negative ${\text{V}}_{\text{CF}}$ vacancies. Based on our results, we discuss possible quantum applications of these color centers when coupled to mechanical oscillation modes of the hosting two-dimensional material. The symmetry and active orbitals of the defects exhibit a parallelism with those of the extensively studied nitrogen vacancy (NV) centers in diamond. In this context, the studied defects emerge as interesting candidates for the development of two-dimensional quantum devices based on fluorographene.

In quadratic fermionic models, we determine a quantum correction to the work statistics after both a sudden quench and a time-dependent driving. Such a correction lies in the noncommutativity of the initial quantum state and the time-dependent Hamiltonian, and is revealed via the Kirkwood-Dirac quasiprobability (KDQ) approach to two-times correlators. Thanks to the latter, one can assess the onset of nonclassical signatures in the KDQ distribution of work, in the form of negative and complex values that no classical theory can reveal. By applying these concepts on the one-dimensional transverse-field Ising model, we relate nonclassical behaviors of the KDQ statistics of work in correspondence of the critical points of the model. Finally, we also prove the enhancement of the extracted work in nonclassical regimes where the noncommutativity takes a role.

The orbital angular momenta (OAM) of electrons play an increasingly important role in ultrafast electron and magnetization dynamics. In this theoretical study, we investigate the electron dynamics induced by femtosecond laser pulses in a normal metal, a ferromagnet, and a ferromagnet/normal metal heterostructure. We analyze the spatiotemporal distributions of the laser-induced OAM and their respective currents. Our findings demonstrate that a circularly polarized laser pulse can induce a sizable and long-lasting OAM component in a normal metal. Furthermore, an interface between a ferromagnet and a normal metal facilitates the demagnetization of the magnet by the OAM contribution to the total magnetization. Finally, to transfer OAM from a ferromagnet into a normal metal, it is advantageous to use a laser setup that induces the desired OAM component in the ferromagnet, but not in the normal metal.

Recent discoveries of high-$T_c$ superconducting hydrides at high pressure have opened up new possibilities for improving the superconducting transition temperature $\left(T_c\right)$ using hydrogenation. Here, a unique thermodynamic approach is developed based on the concept of rapid thermal annealing and is adopted to study the hydrogenation effect on the superconducting properties of Nb/Pd bilayer films. Below ${300}^{\circ }\mathrm{C}$ annealing temperatures, the $T_c$ is enhanced from 8.77 to 9.06 K and is correlated with the compression of the Nb unit cell. A weak lattice expansion occurs at higher annealing temperatures, and the $T_c$ is gradually suppressed. Furthermore, the rapid thermal hydrogenation affects the $T_c$ differently depending on the substrate on which the Nb/Pd bilayers are grown. For the $c$-cut ${\mathrm{Al}}_2{\mathrm{O}}_3$ substrate, the $T_c$ reduction starts at ${250}^{\circ }\mathrm{C}$, while for the $r$-cut ${\mathrm{Al}}_2{\mathrm{O}}_3$, this occurs at ${350}^{\circ }\mathrm{C}$. We associate these features with the elastic behavior of Nb film upon hydrogenation. A proposed model shows that the increase of $T_c$ could be caused by the compressive stress related to the rapid nucleation of hydrides or the removal of impurities. Our discoveries provide insights into how superconductivity can be manipulated by rapid thermal hydrogenation.

A flat band is an electronic state with quenched kinetic energy and an extremely high density of states. In such a state, a small electron-electron correlation can drive multiple intriguing quantum phenomena. By using angle-resolved photoemission spectroscopy, we report the existence of a dispersionless flat band with a sharp quasiparticle peak below $E_{\mathrm{F}}$ in the charge density wave state of $4H_b\text{−}{\mathrm{TaS}}_{1.3}{\mathrm{Se}}_{0.7}$. This band extends over the $\sqrt{13}\times \sqrt{13}$ reconstructed Brillouin zone. In combination with the density functional theory calculation, we identify that the flat band is due to the formation of the Star of David at the top-most $T$ layer. In addition, the scanning tunneling spectroscopy observes a sharp peak above $E_{\mathrm{F}}$. We attribute it to the electron-correlation-driven splitting, forming a small gap of $\sim 45$ meV. Our observation of the flat band and splitting in $4H_b\text{−}{\mathrm{TaS}}_{1.3}{\mathrm{Se}}_{0.7}$ provides a platform to manipulate the electron-correlation-related quantum phenomena at a low energy scale.

We report on zone center terahertz excitations of non-Jahn Teller ${\mathrm{LaFeO}}_3, {\mathrm{PrFeO}}_3, {\mathrm{ErFeO}}_3$, and ${\mathrm{LuFeO}}_3$ distorted perovskites under external magnetic fields up 7 T. Our measurements on low-temperature/low-energy absorptions of ${\mathrm{LaFeO}}_3$ show quasiantiferromagnetic (q-AFM) and quasiferromagnetic (q-FM) magnons at ${\omega }_{q\mathrm{AFM}}\sim 31.4\mathrm{c}{\mathrm{m}}^{-1}$ and ${\omega }_{q\mathrm{FM}}\sim 26.7\mathrm{c}{\mathrm{m}}^{-1}$ in the ${\mathrm{\Gamma }}_4\left(G_x,A_y,F_z\right)$ representation with near degeneracy linearly lifted by the field. ${\mathrm{LuFeO}}_3$ is characterized by zero-field magnetic resonances at ${\omega }_{q\mathrm{AFM}}\sim 26.3\mathrm{c}{\mathrm{m}}^{-1}$ and ${\omega }_{q\mathrm{FM}}\sim 22.4\mathrm{c}{\mathrm{m}}^{-1}$ in addition to ${\mathrm{Fe}}^{3+}$ Zeeman-split crystal field (CF) $6{\mathrm{A}}_1$ ground transitions at $\sim 10.4\mathrm{c}{\mathrm{m}}^{-1}$ triggered by subtle structural deviations induced by the $\mathrm{Lu}4f^{1}4$ smaller ionic radius at the A site. This local quasinoncentrosymmetric departure is also found in ${\mathrm{ErFeO}}_3$ (Kramers $4f^{11} {\mathrm{Er}}^{3+}\left({}^4\mathrm{I}_{15/2}\right)$; ${\mathrm{\Gamma }}_2\left(F_x,C_y,G_z\right)<T_{\mathrm{SR}}\sim 93\mathrm{K})$ but with the $\sim 4\mathrm{c}{\mathrm{m}}^{-1} {\mathrm{Fe}}^{3+}$ Zeeman branching strongly biased toward higher energies due to $3d\text{−}4f$ exchange. Magnons at ${\omega }_{q\mathrm{AFM}}\sim 31.5\mathrm{c}{\mathrm{m}}^{-1}$ and ${\omega }_{q\mathrm{FM}}\sim 21.5\mathrm{c}{\mathrm{m}}^{-1}$ in ${\mathrm{ErFeO}}_3$ do not undergo field-induced band splits but a 13-fold increase in the q-AFM (${\omega }_{q\mathrm{AMF}}$)/q-FM (${\omega }_{q\mathrm{AFM}}$) intensity ratio. There is a remarkable field-dependent CF matching population balance between ${\mathrm{Fe}}^{3+}$ higher and ${\mathrm{Er}}^{3+}$ lower Zeeman branches. The ${\mathrm{Er}}^{3+}\left({}^4\mathrm{I}_{15/2}\right)$ multiplet, at the 49.5, 110.5, and $167.3\mathrm{c}{\mathrm{m}}^{-1}$, coincides with external lattice mode frequencies, suggesting strong lattice-driven spin-phonon interactions. Far-infrared absorption ratios under mild external fields reveal magnetic dependence only for those zone-center phonons involving moving magnetic ions. Overall, our results support the viability of magnetic state manipulation by phonons. Quasiantiferroresonances and quasiferroresonances in ${\mathrm{PrFeO}}_3$ turn much broader as non-Kramers ${\mathrm{Pr}}^3$ introduces ligand changes at the A site, leading into near degeneracy the q-AFM mode and the lowest ${\mathrm{Pr}}^{3+}$ CF transition. They merge into a single broad mostly unresolved feature at 7 T. We conclude that low-energy excitations in $R{\mathrm{FeO}}_3$ ($R=\mathrm{rare}\mathrm{earth}$) strongly depend on the lanthanide ionic size, thus indivisibly tied to the mechanism associated with the origin of canted FM. In addition, minute lattice displacements also underlie considering noncentrosymmetric the most distorted $R{\mathrm{FeO}}_3$ ($R=\mathrm{rare}\mathrm{earth}$). In these perovskites, the changes triggered in the lattice by the smaller rare earth and the nonlinear intrinsic oxygen ion polarizability, known to drive lattice instabilities, provide grounds for interplay of ionic and electronic interactions yielding ferroelectric spontaneous polarization.

The Kitaev model offers a platform for quantum spin liquids (QSLs) with fractional excitations, itinerant Majorana fermions and localized fluxes. Since these fractional excitations could be utilized for quantum computing, how to create, observe, and control them through the spin degree of freedom is a central issue. Here, we study dynamical spin transport in a wide range of frequencies for the Kitaev-Heisenberg model, by applying an AC magnetic field to an edge of the system. We find that, in the Kitaev QSL phase, spin polarizations at the other edge are resonantly induced in a specific spin component, even though the static spin correlations are vanishingly small. This interedge spin resonance appears around the input frequency over the broad frequency range. Comparing with the dynamical spin correlations, we clarify that the resonance is governed by the itinerant Majorana fermions with a broad continuum excitation spectrum, which can propagate over long distances, although it vanishes for the pure Kitaev model because of accidental degeneracy and requires weak Heisenberg interactions. We also find that the spin polarizations in the other spin components are weakly induced at an almost constant frequency close to the excitation gap of the localized fluxes, irrespective of the input frequency. These results demonstrate that the dynamical spin transport is a powerful probe of the fractional excitations in the Kitaev QSL. Possible experimental realization of the interedge spin resonance is discussed.

We theoretically investigate the binding energy and electron-hole (e-h) overlap of excitonic states confined at the interface between two-dimensional materials with type-II band alignment, i.e., with lowest conduction and highest valence band edges placed in different materials, arranged in a side-by-side planar heterostructure. We propose a variational procedure within the effective mass approximation to calculate the exciton ground state and apply our model to a monolayer ${\mathrm{MoS}}_2/{\mathrm{WS}}_2$ heterostructure. The role of nonabrupt interfaces between the materials is accounted for in our model by assuming a ${\mathrm{W}}_x{\mathrm{Mo}}_{1-x}{\mathrm{S}}_2$ alloy around the interfacial region. Our results demonstrate that (i) interface-bound excitons are energetically favorable only for small interface thickness and/or for systems under high dielectric screening by the materials surrounding the monolayer, and that (ii) the interface exciton binding energy and its e-h overlap are controllable by the interface width and dielectric environment.

As conceptual milestones of nontrivial phenomenon, ${\mathbb{Z}}_2$ topological insulators (TIs) and higher-order TIs (HOTIs) have greatly reshaped the landscape of fundamental physics and materials. However, despite the exciting progress, a tunable topological phase transition between ${\mathbb{Z}}_2$ TIs and HOTIs remains elusive. Here, using a tight-binding model and first-principles calculations, we propose that ferroelectric switching can be a straightforward and efficient way for engineering the ${\mathbb{Z}}_2$ TIs and HOTIs phases with strikingly different bulk-boundary correspondence. Remarkably, based on the Wannier charge centers, edge states, and corner states analysis, we identify the ferroelectric heterobilayer composed of ${\mathrm{MgAl}}_2{\mathrm{Se}}_4$ and ${\mathrm{In}}_2{\mathrm{S}}_3$ as a material candidate of the predicted topological phase transition. Obviously, the ferroelectric switching opens up a technological avenue to bridge the first- and higher-order topologies with high possibility of innovative applications in topotronic and ferroelectric devices.

Dielectric nanoparticles not showing distinct Mie resonances have been discarded as useful elements of resonant nanophotonics. However, the incorporation of the excitonic transition into such nanoparticles may allow us to generate sharp resonances, in particular, Fano resonances, thereby enhancing the utility of such nanoparticles. In this work, on the basis of the Mie theory and its electrostatics approximation, we analyze analytically and numerically optical responses and resonant behaviors of internal fields in spherical excitonic nanoparticles. The excitonic sphere is characterized by a dielectric constant consisting of a background dielectric constant and a Lorentzian response of the exciton excitation. From equations of the electrostatics approximation, by appropriately accounting for the background scattering, it is shown analytically that the absorption efficiency is expressed as a Lorentzian function, while the efficiencies of the scattering and extinction are expressed in the form of the generalized Fano function (external Fano resonance). From the same procedures, it is also shown that the spectra of the enhancement factor of internal fields are described by the same generalized Fano function (internal Fano resonance). Equations appearing in the derivation clearly indicate that both the external and internal Fano resonances are caused by the interference between a nonresonant component and a resonant component, corresponding to a broad background and sharp excitonic transition, respectively. Assuming a model excitonic sphere that mimics a polymer sphere doped with J-aggregates of excitonic molecules, spectra of scattering, absorption and extinction efficiencies, as well as that of the enhancement factor of internal fields, are calculated including a size range beyond the small particle limit, for which the exact Mie theory is used. The generalized Fano functions are shown to reproduce very well the calculated spectra even beyond the small particle limit, provided that the sphere radius is less than an upper bound. The results of the present paper provide a firm basis for discussing the formation of the Fano line shapes in optical responses and spectra of internal fields of excitonic nanospheres.

Recently, an exotic quantum Hall ferromagnet with spin-filtered helical edge modes was observed in monolayer graphene on a high-dielectric constant substrate at moderate magnetic fields, withstanding temperatures of up to 110 Kelvin [L. Veyrat et al., Science 367, 781 (2020)]. However, the characteristic quantized longitudinal resistance mediated by these edge modes departs from quantization with decreasing temperature. In this work, we investigate the transport properties of helical edge modes in a graphene nanoribbon under a perpendicular magnetic field using the Landauer-Büttiker transport formalism. We find that the departure of quantization of longitudinal conductance is due to the helical-edge gap opened by the Rashba spin-orbital coupling. The quantization can be restored by weak nonmagnetic Anderson disorder at low temperature, increasing the localization length, or by raising temperature at weak disorder, through thermal broadening. The resulted conductance is very close to the quantized value $2e^2/h$, which is qualitatively consistent with the experimental results. Furthermore, we suggest that the helical quantum Hall phase in graphene could be a promising platform for creating Majorana zero modes by introducing superconductivity.

Niobium halides, ${\mathrm{Nb}}_3{X}_8$ $(X=\mathrm{Cl},\mathrm{Br},\mathrm{I})$, which are predicted two-dimensional magnets, have recently gotten attention due to their breathing kagome geometry. Here we have studied the electronic structure of ${\mathrm{Nb}}_3{\mathrm{Br}}_8$ by using angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations. ARPES results depict the presence of multiple flat and weakly dispersing bands. These bands are well explained by the theoretical calculations, which show they have $\mathrm{Nb}d$ character indicating their origination from the $\mathrm{Nb}$ atoms forming the breathing kagome plane. This van der Waals material can be easily thinned down via mechanical exfoliation to the ultrathin limit and such ultrathin samples are stable as depicted from the time-dependent Raman spectroscopy measurements at room temperature. These results demonstrate that ${\mathrm{Nb}}_3{\mathrm{Br}}_8$ is an excellent material not only for studying breathing kagome induced flat band physics and its connection with magnetism but also for heterostructure fabrication and for practical application.

We investigate the ground state properties of quantum skyrmions in a ferromagnet using variational Monte Carlo with the neural network quantum state as variational ansatz. We study the ground states of a two-dimensional quantum Heisenberg model in the presence of the Dzyaloshinskii-Moriya interaction (DMI) and show that the ground state accommodates a quantum skyrmion for a large range of parameters, especially at large DMI. The spins in these quantum skyrmions are weakly entangled, and the entanglement increases with decreasing DMI. We also find that the central spin is completely disentangled from the rest of the lattice, establishing a non-destructive way of detecting this type of skyrmion by local magnetization measurements. While neural networks are well suited to detect weakly entangled skyrmions with large DMI, they struggle to describe skyrmions in the small DMI regime due to nearly degenerate ground states and strong entanglement. In this paper, we propose a method to identify this regime and a technique to alleviate the problem. Finally, we analyze the workings of the neural network and explore its limits by pruning. Our work shows that neural network quantum states can be efficiently used to describe the quantum magnetism of large systems exceeding the size manageable in exact diagonalization by far.

We introduce a family of $\mathrm{SO}(n$)-symmetric spin chains which generalize the transverse-field Ising chain for $n=1$. These spin chains are defined with gamma matrices and can be exactly solved by mapping to $n$ species of itinerant Majorana fermions coupled to a static ${\mathbb{Z}}_2$ gauge field. Their phase diagrams include a critical point described by the $\mathrm{Spin}{\left(n\right)}_1$ conformal field theory as well as two distinct gapped phases. We show that one of the gapped phases is a trivial phase and the other realizes a symmetry-protected topological phase when $n\ge 2$. These two gapped phases are proved to be related to each other by a Kramers-Wannier duality. Furthermore, other elegant structures in the transverse-field Ising chain, such as the infinite-dimensional Onsager algebra, also carry over to our models.

A detailed investigation of ${\mathrm{Ba}}_2{\mathrm{MnGe}}_2{\mathrm{O}}_7$ was performed in its low-temperature magnetoelectric state combining neutron diffraction with magnetization measurements on single crystals. In the paramagnetic state at 10 K, polarized neutron diffraction was applied to reveal the components of the susceptibility tensor. The crystal and magnetic structures below the antiferromagnetic transition temperature of $T_{\text{N}}\approx 4\mathrm{K}$ were determined using unpolarized neutron diffraction. These data imply no structural phase transition from 10 K down to 2.5 K and are well described within the tetragonal space group $P\overline{4}{2}_{1}m$. We found that in zero magnetic field the magnetic space group is either $C_{c}mc{2}_1$ or $P_c{2}_1{2}_1{2}_1$ with antiferromagnetic order along the [110] or [100] direction, respectively, while neighboring spins along the [001] axis are ordered antiferromagnetically. A noncollinear spin arrangement due to small canting within the $ab$ plane is allowed by symmetry and observed experimentally. The ordered moment is found to be 3.24(3) ${\mu }_{\text{B}}/{\mathrm{Mn}}^{2+}$ at 2.5 K and the temperature-field dependent magnetic phase diagram is mapped out by macroscopic magnetization. Distinct differences between the magnetic structure of ${\mathrm{Ba}}_2{\mathrm{MnGe}}_2{\mathrm{O}}_7$ as compared to those of ${\mathrm{Ba}}_2{\mathrm{CoGe}}_2{\mathrm{O}}_7$ and ${\mathrm{Ca}}_2{\mathrm{CoSi}}_2{\mathrm{O}}_7$ are discussed.

In this paper, we study the combined electrical and magnetic control of the spin-lattice configuration and the magnon-ferron hybridization, i.e., the collective excitation of the magnetic and electric orders, in a two-dimensional multiferroic model, where the magnetic and ferroelectric orders are strongly coupled via the Dzyaloshinskii-Moriya interaction. Based on a systematic analysis on the evolution of the spin and lattice configuration under varying electric and magnetic fields, we find that both electric and magnetic susceptibilities show tunable enhancements around the transitions of the magnetic and electric orders. The calculation of the excitation spectrum for the coupled spin-lattice dynamics shows that not only the magnon and phonon spectra, but also the hybridization behaviors between them can be efficiently controlled electrically.

Achievement of various topological spin textures, such as magnetic skyrmion and chiral domain walls, in heavy-metal/ferromagnet (FM) multilayer films with interfacial Dzyaloshinskii-Moriya interaction have attracted enormous attention owing to their topological nature, emergent electromagnetic properties, and potential applications in spintronics. The topological Hall effect (THE), an indicator to distinguish spin textures with nontrivial topology from the trivial collinear magnetic configurations, is usually characterized by the dome-shaped peak in anomalous Hall resistivity (AHR) loops. However, several other magnetoresistance effects could also cause this aberrant hump in AHR curves. Here, we systematically study the AHR loops with these aberrant humps under the coercivity ${\mu }_0{H}_{\mathrm{c}}$ as a function of the orientation of the magnetic field to the film plane in several Pt/FM/Pt films. Combining the simulation model of AHR and planar Hall magnetoresistance (PHMR), we demonstrate that this THE-like hump originates from the PHMR of the in-plane magnetization component due to magnetization is not saturated below saturation field in the out-of-plane AHR loop measurements. Our results indicate that the origin of these dome-shaped anomalous in transport measurements is manifold and should be examined carefully.