We report an efficient method to observe single photon emissions in monolayer $\mathrm{WS}{\mathrm{e}}_2$ by applying hydrostatic pressure. The photoluminescence peaks of typical two-dimensional excitons show a nearly identical pressure-induced blueshift, whereas the energy of pressure-induced discrete emission lines (quantum emitters) demonstrates a pressure insensitive behavior. The decay time of these discrete line emissions is approximately 10 ns, which is at least one order longer than the lifetime of the broad localized ($L$) excitons. These characteristics lead to a conclusion that the excitons bound to deep-level defects can be responsible for the observed single photon emissions.

We model quantum Hall skyrmions in graphene monolayer at quarter filling by a theory of ${\mathrm{CP}}^3$ fields and study the energy minimizing skyrmions in the presence of valley pseudospin anisotropy and Zeeman coupling. We present a diagram of all types of skyrmions in a wide range of the anisotropy parameters. For each type of skyrmion, we visualize it on three Bloch spheres, and present the profiles of its texture on the graphene honeycomb lattice, thus providing references for the scanning-tunneling microscopy and spectroscopy imaging of spin-pseudospin textures in graphene monolayer in the quantum Hall regime. Besides the spin and pseudospin skyrmions for the corresponding degrees of freedom of an electron in the $N=0$ Landau level, we discuss two unusual types—the “entanglement skyrmion”, the texture of which lies in the space of the entanglement of spin and pseudospin, as well as the “deflated pseudospin skyrmion” with partial entanglement. For all skyrmion types, we study the dependence of the energy and the size of a skyrmion on the anisotropy parameters and perpendicular magnetic field. We also propose three ways to modify the anisotropy energy, namely, the sample tilting, the substrate anisotropy, and the valley pseudospin analog of Zeeman coupling.

We measure subnanometer displacements of thin samples vibrated by piezotransducer. Samples contain ${}^{57}\mathrm{Fe}$ nuclei, which are exposed to 14.4-keV $\gamma$ radiation. Vibration produces sidebands from a single absorption line of the sample. The sideband intensities depend on the vibration amplitude and its distribution along the sample. We developed a model of this distribution, which adequately describes the spectra of powder and stainless steel (SS) absorbers. We propose to filter $\gamma$ radiation through a small round hole in the lead mask, placed before the absorber. In this case only a small spot of the vibrated absorber is observed. We found for SS foil that nuclei, exposed to $\gamma$ radiation in this small spot, vibrate with almost the same amplitudes whose difference does not exceed a few picometers within the irradiated area.

Magnetic skyrmions are topological spin configurations in materials with chiral Dzyaloshinskii-Moriya interaction (DMI), that are potentially useful for storing or processing information. To date, DMI has been found in few bulk materials, but can also be induced in atomically thin magnetic films in contact with surfaces with large spin-orbit interactions. Recent experiments have reported that isolated magnetic skyrmions can be stabilized even near room temperature in few-atom-thick magnetic layers sandwiched between materials that provide asymmetric spin-orbit coupling. Here we present the minimum-energy path analysis of three distinct mechanisms for the skyrmion collapse, based on ab initio input and the performed atomic-spin simulations. We focus on the stability of a skyrmion in three atomic layers of Co, either epitaxial on the Pt(111) surface or within a hybrid multilayer where DMI nontrivially varies per monolayer due to competition between different symmetry breaking from two sides of the Co film. In laterally finite systems, their constrained geometry causes poor thermal stability of the skyrmion toward collapse at the boundary, which we show to be resolved by designing the high-DMI structure within an extended film with lower or no DMI.

We study the time evolution of a one-dimensional interacting fermion system described by the Luttinger model starting from a nonequilibrium state defined by a smooth temperature profile $T\left(x\right)$. As a specific example we consider the case when $T\left(x\right)$ is equal to $T_L$ $\left(T_R\right)$ far to the left (right). Using a series expansion in $\epsilon =2(T_R-T_L)/(T_L+T_R)$, we compute the energy density, the heat current density, and the fermion two-point correlation function for all times $t\ge 0$. For local (delta-function) interactions, the first two are computed to all orders, giving simple exact expressions involving the Schwarzian derivative of the integral of $T\left(x\right)$. For nonlocal interactions, breaking scale invariance, we compute the nonequilibrium steady state (NESS) to all orders and the evolution to first order in $\epsilon$. The heat current in the NESS is universal even when conformal invariance is broken by the interactions, and its dependence on $T_{L,R}$ agrees with numerical results for the $XXZ$ spin chain. Moreover, our analytical formulas predict peaks at short times in the transition region between different temperatures and show dispersion effects that, even if nonuniversal, are qualitatively similar to ones observed in numerical simulations for related models, such as spin chains and interacting lattice fermions.

Topological insulators and superconductors are characterized by their gapless boundary modes. In this paper, we develop a recursive approach to the boundary Green function, which encodes this nontrivial boundary physics. Our approach describes the various topologically trivial and nontrivial phases as fixed points of a recursion and provides direct access to the phase diagram, the localization properties of the edge modes, as well as topological indices. We illustrate our approach in the context of various familiar models such as the Su-Schrieffer-Heeger model, the Kitaev chain, and a model for a Chern insulator. We also show that the method provides an intuitive approach to understand recently introduced topological phases which exhibit gapless corner states.

Exciton transport in nanomaterials is sensitive to fluctuations in the confinement potential that are intrinsic to heterogeneous solid-state systems. Redistribution of exciton population manifests as spectral diffusion in which the exciton energy shifts. It is generally assumed that increase or decrease in the exciton energy are equally probable. We show that this assumption is not necessarily valid using two-dimensional coherent spectroscopy on a disordered GaAs quantum well. High-energy excitons relax into lower-energy localized states over a time scale of tens of picoseconds at low sample temperatures $\left(\sim 5\mathrm{K}\right)$. A transition to uniform spectral diffusion of excitons is observed as the temperature is increased to $\sim 20\mathrm{K}$. Numerical simulations reveal the contribution of exciton-phonon interactions to spectral diffusion of excitons. These results provide a perspective on the process of dynamic localization and the effect of the correlation length of disorder on spectral diffusion of excitons.

We address the electronic structure of the surface states of topological-insulator thin films with embedded local nonmagnetic and magnetic impurities. Using the $T$-matrix expansion of the real-space Green's function, we derive the local density of electron states and corresponding spin-resolved densities. We show that the effects of the impurities can be tuned by applying an electric field between the surface layers. The emerging magnetic states are expected to play an important role both in the ferromagnetic mechanism of magnetic topological insulators and in its transport properties. In the case of magnetic impurities, we have categorized the possible cases for different spin directions of the impurities as well as the spin direction in which the spin-resolved density of electron states is calculated and have related them to the spin susceptibility of the system.

An intense laser field in the high-frequency regime drives carriers in graphene nanoribbons (GNRs) out of equilibrium and creates topologically protected edge states. Using Floquet theory on driven GNRs, we calculate the time evolution of local excitations of these edge states and show that they exhibit a robust dynamics also in the presence of very localized lattice defects (atomic vacancies), which is characteristic of topologically nontrivial behavior. We show how it is possible to control them by a modulated electrostatic potential: They can be fully transmitted on the same edge, reflected on the opposite one, or can be split between the two edges, in analogy with Hall edge states, making them promising candidates for flying-qubit architectures.

Electrical currents in a magnetic-insulator/heavy-metal heterostructure can induce two simultaneous effects, namely, spin Hall magnetoresistance (SMR) on the heavy-metal side and spin-orbit torques (SOTs) on the magnetic-insulator side. Within the framework of a pure spin current model based on the bulk spin Hall effect (SHE), the ratio of the spin Hall-induced anomalous Hall effect (SH-AHE) to SMR should be equal to the ratio of the fieldlike torque (FLT) to the dampinglike torque (DLT). We perform a quantitative study of SMR, SH-AHE, and SOTs in a series of thulium iron garnet/platinum or $\mathrm{T}{\mathrm{m}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}/\mathrm{Pt}$ heterostructures with different $\mathrm{T}{\mathrm{m}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}$ thicknesses, where $\mathrm{T}{\mathrm{m}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}$ is a ferrimagnetic insulator with perpendicular magnetic anisotropy. We find the ratio between the measured effective fields of FLT and DLT is at least two times larger than the ratio of the SH-AHE to SMR. In addition, the bulk SHE model grossly underestimates the spin-torque efficiency of FLT. Our results reveal deficiencies of the bulk SHE model and also address the importance of interfacial effects such as the Rashba and magnetic proximity effects in magnetic-insulator/heavy-metal heterostructures.

In electronic systems with long-range Coulomb interaction, the nonlocal Fock-exchange term has a band-widening effect. While this effect is included in combined many-body perturbation theory and dynamical mean field theory (DMFT) schemes, it is not taken into account in standard extended DMFT (EDMFT) calculations. Here, we include this instantaneous term in both approaches and investigate its effect on the phase diagram and dynamically screened interaction. We show that the largest deviations between previously presented EDMFT and $GW$+EDMFT results originate from the nonlocal Fock term, and that the quantitative differences are especially large in the strong-coupling limit. Furthermore, we show that the charge-ordering phase diagram obtained in $GW$+EDMFT methods for moderate interaction values is very similar to the one predicted by dual-boson methods that include the fermion-boson or four-point vertex.

We report on a combined soft x-ray absorption and magnetic circular dichroism (XMCD) study at the Co-$L_{3,2}$ edge on the hybrid $3d/5d$ solid state oxide ${\mathrm{Sr}}_2{\mathrm{Co}}_{0.5}{\mathrm{Ir}}_{0.5}{\mathrm{O}}_4$ with ${\mathrm{K}}_2{\mathrm{NiF}}_4$ structure. Our data indicate unambiguously a pure high spin state $(S=2)$ for the ${\mathrm{Co}}^{3+}$ ($3d^6$) ions with a significant unquenched orbital moment $L_z/2S_z=0.25$ despite the sizable elongation of the ${\mathrm{CoO}}_6$ octahedra. Using quantitative model calculations based on parameters consistent with our spectra, we have investigated the stability of this high spin state with respect to the competing low spin and intermediate spin states.

We present a conceptually simple method for treating electron-phonon scattering and phonon limited mobilities. By combining Green's function based transport calculations and molecular dynamics, we obtain a temperature dependent transmission from which we evaluate the mobility. We validate our approach by comparing to mobilities and conductivities obtained by the Boltzmann transport equation for different bulk and one-dimensional systems. For bulk silicon and gold we compare against experimental values. We discuss limitations and advantages of each of the computational approaches.

We study driven-dissipative Bose-Einstein condensates in a two-mode Josephson system, such as a double-well potential, with asymmetrical pumping. We investigate nonlinear effects on the condensate populations and mode transitions. The generalized Gross-Pitaevskii equations are modified in order to treat pumping of only a single mode. We characterize the steady-state solutions in such a system as well as criteria for potential trapping of a condensate mode. There are many possible steady states, with different density and/or phase profiles. Transitions between different condensate modes can be induced by varying the parameters of the junction or the initial conditions, or by applying external fields.

We show that in $AB$ stacked bilayer graphene low-energy excitations around the semimetallic points are described by massless, four-dimensional Dirac fermions. There is an effective reconstruction of the four-dimensional spacetime, including in particular the dimension perpendicular to the sheet, that arises dynamically from the physical graphene sheet and the interactions experienced by the carriers. The effective spacetime is the Eisenhart-Duval lift of the dynamics experienced by Galilei invariant Lévy-Leblond spin-$\frac{1}{2}$ particles near the Dirac points. We find that changing the intrinsic curvature of the bilayer sheet induces a change in the energy level of the electronic bands, switching from a conducting regime for negative curvature to an insulating one when curvature is positive. In particular, curving graphene bilayers allow opening or closing the energy gap between conduction and valence bands, a key effect for electronic devices. Thus, using curvature as a tunable parameter opens the way for the beginning of curvatronics in bilayer graphene.

We study the incomplete oxidation of graphene or reduction of graphene oxide for hydroxyl and epoxy oxidant groups. While in wet oxidation hydroxyl groups are favorable, in a drier environment an oxygen atom can bridge two neighboring carbon atoms. We model composition variations and structural disorder within a statistical theory, the generalized quasichemical approximation, combined with density functional theory calculations of the local atomic geometries. A generalization of the statistical approach is developed to account for the antiparallel orientation of hydroxyl groups and a fourfold coordination of C atoms. The theoretical framework enables a thermodynamic treatment of graphene oxide as a function of oxygen content, allowing us to derive temperature-composition phase diagrams and investigate possible clustering and segregation. The resulting geometries, local and average electronic structures, and optical absorption spectra are discussed and compared with available experimental data.

In this work we discuss the existence of time-translation symmetry breaking in a kicked infinite-range-interacting clean spin system described by the Lipkin-Meshkov-Glick model. This Floquet time crystal is robust under perturbations of the kicking protocol, its existence being intimately linked to the underlying ${\mathbb{Z}}_2$ symmetry breaking of the time-independent model. We show that the model being infinite range and having an extensive amount of symmetry-breaking eigenstates is essential for having the time-crystal behavior. In particular, we discuss the properties of the Floquet spectrum, and show the existence of doublets of Floquet states which are, respectively, even and odd superposition of symmetry-broken states and have quasienergies differing of half the driving frequencies, a key essence of Floquet time crystals. Remarkably, the stability of the time-crystal phase can be directly analyzed in the limit of infinite size, discussing the properties of the corresponding classical phase space. Through a detailed analysis of the robustness of the time crystal to various perturbations we are able to map the corresponding phase diagram. We finally discuss the possibility of an experimental implementation by means of trapped ions.

Magnetic moments in an ultrathin Pt film on a ferrimagnetic insulator ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ (YIG) have been investigated at high magnetic fields and low temperatures by means of x-ray magnetic circular dichroism (XMCD). We observed an XMCD signal due to the magnetic moments in a Pt film at the Pt $L_3$ and $L_2$ edges. By means of the element-specific magnetometry, we found that the XMCD signal at the Pt $L_3$ edge gradually increases with increasing the magnetic field even when the field is much greater than the saturation field of YIG. Importantly, the observed XMCD intensity was found to be much greater than the intensity expected from the Pauli paramagnetism of Pt when the Pt film is attached to YIG. These results imply the emergence of induced paramagnetic moments in Pt on YIG and explain the characteristics of the unconventional Hall effect in Pt/YIG systems.

Temperature-dependent magnetic experiments such as pump-probe measurements generated by a pulsed laser have become a crucial technique for switching the magnetization in the picosecond time scale. Apart from having practical implications on the magnetic storage technology, the research field of ultrafast magnetization poses also fundamental physical questions. To correctly describe the time evolution of the atomic magnetic moments under the influence of a temperature-dependent laser pulse, it remains crucial to know if the magnetic material under investigation has magnetic excitation spectrum that is more or less dependent on the magnetic configuration, e.g., as reflected by the temperature dependence of the exchange interactions. In this paper, we demonstrate from first-principles theory that the magnetic excitation spectra in Co in fcc, bcc, and hcp structures are nearly identical in a wide range of noncollinear magnetic configurations. This is a curious result of a balance between the size of the magnetic moments and the strength of the Heisenberg exchange interactions, that in themselves vary with configuration, but put together in an effective spin Hamiltonian results in a configuration-independent effective model. We have used such a Hamiltonian, together with ab initio calculated damping parameters, to investigate the magnon dispersion relationship as well as ultrafast magnetization dynamics of Co and Co-rich CoMn alloys.