${\mathrm{Y}}_2{\mathrm{O}}_3$ codoped with $\mathrm{B}{\mathrm{i}}^{3+}$ and $\mathrm{Y}{\mathrm{b}}^{3+}$ is considered as an efficient downconversion material combining strong broadband absorption of $\mathrm{B}{\mathrm{i}}^{3+}$ with photon splitting by cooperative energy transfer from $\mathrm{B}{\mathrm{i}}^{3+}$ to two $\mathrm{Y}{\mathrm{b}}^{3+}$ neighbors. However, evidence for photon splitting is lacking. Here we investigate the $\mathrm{B}{\mathrm{i}}^{3+}\text{−}\mathrm{to}\text{−}\mathrm{Y}{\mathrm{b}}^{3+}$ energy-transfer mechanism. For cooperative energy transfer the $\mathrm{Y}{\mathrm{b}}^{3+}$-concentration-dependent luminescence decay will show clear characteristics of cooperative dipole-dipole transfer. Analysis of $\mathrm{Y}{\mathrm{b}}^{3+}$-concentration and temperature-dependent decay curves however demonstrates that the energy-transfer mechanism is not cooperative but single step, probably through a $\mathrm{B}{\mathrm{i}}^{4+}\text{−}\mathrm{Y}{\mathrm{b}}^{2+}$ charge-transfer state. The temperature dependence of the $\mathrm{B}{\mathrm{i}}^{3+}\text{−}\mathrm{to}\text{−}\mathrm{Y}{\mathrm{b}}^{3+}$ energy-transfer efficiency is unusual as it decreases with temperature, unlike commonly observed thermally activated energy transfer. This is a signature of energy transfer via exchange interaction. The present results provide evidence for the absence of photon splitting in ${\mathrm{Y}}_2{\mathrm{O}}_3:\mathrm{B}{\mathrm{i}}^{3+},\mathrm{Y}{\mathrm{b}}^{3+}$ and form a convincing demonstration of exchange interaction mediated energy transfer.

Electron-phonon $\left(e\text{−}p\right)$ interaction and transport are important for laser-matter interactions, hot-electron relaxation, and metal-nonmetal interfacial thermal transport. A widely used approach is the two-temperature model (TTM), where $e\text{−}p$ coupling is treated with a gray approach with a lumped coupling factor $G_{ep}$ and the assumption that all phonons are in local thermal equilibrium. However, in many applications, different phonon branches can be driven into strong nonequilibrium due to selective $e\text{−}p$ coupling, and a TTM analysis can lead to misleading or wrong results. Here, we extend the original TTM into a general multitemperature model (MTM), by using phonon branch-resolved $e\text{−}p$ coupling factors and assigning a separate temperature for each phonon branch. The steady-state thermal transport and transient hot electron relaxation processes in constant and pulse laser-irradiated single-layer graphene (SLG) are investigated using our MTM respectively. Results show that different phonon branches are in strong nonequilibrium, with the largest temperature rise being more than six times larger than the smallest one. A comparison with TTM reveals that under steady state, MTM predicts 50% and 80% higher temperature rises for electrons and phonons respectively, due to the “hot phonon bottleneck” effect. Further analysis shows that MTM will increase the predicted thermal conductivity of SLG by 67% and its hot electron relaxation time by 60 times. We expect that our MTM will prove advantageous over TTM and gain use among experimentalists and engineers to predict or explain a wide ranges of processes involving laser-matter interactions.

As an alternative angular momentum carrier, magnons or spin waves can be utilized to encode information and breed magnon-based circuits with ultralow power consumption and non-Boolean data processing capability. In order to construct such a circuit, it is indispensable to design some electronic components with both long magnon decay and coherence length and effective control over magnon transport. Here we show that an all-insulating magnon junction composed of a magnetic insulator $\left({\mathrm{MI}}_1\right)$/antiferromagnetic insulator (AFI)/magnetic insulator $\left({\mathrm{MI}}_2\right)$ sandwich $({\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$/NiO/${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12})$ can completely turn a thermogradient-induced magnon current on or off as the two ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ layers are aligned parallel or antiparallel. The magnon decay length in NiO is about $3.5–4.5$ nm between 100 and 200 K for thermally activated magnons. The insulating magnon valve (magnon junction), as a basic building block, possibly sheds light on the naissance of efficient magnon-based circuits, including non-Boolean logic, memory, diodes, transistors, magnon waveguides, and switches with sizable on-off ratios.

We consider a thin ferromagnetic layer to which an external field or a current are applied along an in-plane easy axis. The perpendicular hard-axis anisotropy constant is large so that the out-of-plane magnetization component is much smaller than the in-plane components. A perturbation approach is used to obtain the profile and velocity of the moving domain wall. The dynamics of the in-plane components of the magnetization is governed by a reaction diffusion equation which determines the speed of the profile. We find a simple analytic expression for the out-of-plane magnetization showing a symmetric distortion due to the motion in addition to the asymmetric component due to the Dzyaloshinskii-Moriya interaction. The results obtained complement previous studies in which either the Dzyalozhinskii vector or the out-of-plane hard-axis anisotropy were assumed small. In the regime studied the Walker breakdown is not observed but the reaction diffusion dynamics predicts a slowing down of the domain wall for a sufficiently large magnetic field. The transition point depends on the applied field, saturation magnetization, and easy-axis anisotropy.

Using a classical Heisenberg model with anisotropic nearest-neighbor superexchange and uniaxial anisotropy, I investigate the magnetic order of the transition metals on the $B$ sites in $A\mathrm{M}{\mathrm{n}}_{1\text{−}x}\mathrm{F}{\mathrm{e}}_x{\mathrm{O}}_3(A=\mathrm{La}$, Nd; $x=\left[0,1\right])$ with Monte Carlo methods. Magnetic parameters are extracted from preexisting experimental data when available and otherwise estimated with calculations. Superexchange energies are compared to density functional theory and anisotropy energies to ligand field theory, and the potential for doping-induced strain affecting these parameters is considered. I find coexistence regions of different types of magnetic order ($A$-type and $G$-type antiferromagnetism), regions with distinct magnetic transitions for the two order parameters, noncollinear ground states for intermediate values of $x$ that decrease the amplitude of the ordered moment in a way consistent with experiment for low dopings, and a reproduction of the experimentally determined, nonmonotonic interpolation of the ordering temperatures. The detailed shape of the phase boundaries in $(x,T$) space is found to be diagnostic of the magnetic interactions. These results also suggest a ferrimagnetic ground state for the $A_2\mathrm{MnFe}{\mathrm{O}}_6$ double perovskite with $T_{\mathrm{N}}\approx 270\mathrm{K}$ for $A=\mathrm{La}$ and Nd.

We study the effects of disorder on a Kitaev chain with longer-range hopping and pairing terms which is capable of forming local zero energy excitations and, hence, serves as a minimal model for localization-protected edge qubits. The clean phase diagram hosts regions with 0, 1, and 2 Majorana zero-modes (MZMs) per edge. Using a semianalytic approach corroborated by numerical calculations of the entanglement degeneracy, we show how phase boundaries evolve under the influence of disorder. While in general the 2 MZM region is stable with respect to moderate disorder, stronger values drive transition towards the topologically trivial phase. We uncover regions where the addition of disorder induces local zero-modes absent for the corresponding clean system. Interestingly, we discover that disorder destroys any direct transition between phases with zero and two MZMs by creating a tricritical point at the 2-0 MZM boundary of the clean system. Finally, motivated by recent experiments, we calculate the characteristic signatures of the disorder phase diagram as measured in dynamical local and nonlocal “qubit” correlation functions. Our work provides a minimal starting point to investigate the coherence properties of local qubits in the presence of disorder.

We have explored the origin of charge density wave (CDW) instabilities in ${\mathrm{LuPd}}_2\mathrm{In}$ and ${\mathrm{LuPt}}_2\mathrm{In}$ cubic-Heusler compounds, employing band structure and phonon dispersion calculations based on the ab initio density functional theory. We have found that, despite quite enhanced electronic susceptibilities along $\mathbf{q}=M-R$ for both compounds, the phonon softening instability occurs only for ${\mathrm{LuPt}}_2\mathrm{In}$ at $\mathbf{q}=M$, as is consistent with experimental result. We have demonstrated that the key factor inducing the CDW in ${\mathrm{LuPt}}_2\mathrm{In}$ is neither the Fermi-surface nesting nor the spin-orbit coupling strength but the electron-phonon interaction with large deformation potential at $\mathbf{q}=M$. In the CDW ground state, the Pt cubes are rotated within the $2\times 2\times 1$ Heusler structure, retaining the mirror symmetry through all Lu-In planes, which results in the suppression of density of states of Pt $d$ band near the Fermi level.

Recent experiments, in which terahertz (THz) light has been used to induce nonequilibrium superconducting states, have raised a number of intriguing fundamental questions. Theoretically, these experiments are most often described within the Floquet formalism, which suffers a number of well-known limitations (e.g., Floquet heating). Alternative approaches rely on heavy numerical methods. In this paper, we develop an analytical theory of nonequilibrium superconductivity that combines path integrals on the Kostantinov-Perel' time contour with adiabatic perturbation theory [G. Rigolin, G. Ortiz, and V. H. Ponce, Phys. Rev. A 78, 052508 (2008)]. We consider a general system of electrons and Raman phonons coupled by the Fröhlich interaction, in the presence of a time-dependent external field which acts on the phonon subsystem. The latter is supposed to model the THz light-induced excitation of nonlinear interactions between infrared and Raman phonons. Assuming that the external field has a slow dependence on time, we derive equations for the dynamical superconducting gap, calculating the leading adiabatic term and the first nonadiabatic correction. Our nonequilibrium formulas can be solved numerically with a minimal increase of computational complexity with respect to that needed to calculate the superconducting gap at equilibrium.

We propose an operational framework to study the nonequilibrium thermodynamics of a quantum system $S$ that is coupled to a detector $D$ whose state is continuously monitored, allowing us to single out individual quantum trajectories of $S$. We focus on detailed fluctuation theorems and characterize the entropy production of the system. We establish fundamental differences with respect to the thermodynamics of unmonitored, unitarily evolved systems. We consider the paradigmatic example of circuit QED, where superconducting qubits can be coupled to a continuously monitored resonator and show numerical simulations using state-of-the-art experimental parameters.

We present a theoretical study of diffusive superconducting systems with extrinsic spin-orbit coupling and arbitrarily strong impurity potential. We derive from a microscopic Hamiltonian a diffusion equation for the quasi-classical Green function, and demonstrate that all mechanisms related to the spin-orbit coupling are expressed in terms of three kinetic coefficients: the spin Hall angle, the spin current swapping coefficient, and the spin relaxation rate due to Elliott-Yafet mechanism. The derived diffusion equation contains a hitherto unknown term describing a spin-orbit torque that appears exclusively in the superconducting state. As an example, we provide a qualitative description of a magnetic vortex in a superconductor with triplet correlations, and show that the novel term describes a spin torque proportional to the vector product between the spectral angular momentum of the condensate and the triplet vector. Our equation opens up the possibility to explore spintronic effects in superconductors with no counterparts in the normal metallic state.

Hybrid functionals, which mix a fraction of Hartree-Fock exchange with local or semilocal exchange, have become increasingly popular in quantum chemistry and computational materials science. Here, we assess the accuracy of the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional to describe many-electron interactions and charge localization in semiconductors. We perform diffusion quantum Monte Carlo calculations to obtain the accurate ground-state spin densities of the negatively charged (${\mathrm{SiV})}^{-}$ and the neutral (${\mathrm{SiV})}^0$ silicon-vacancy center in diamond and of the cubic silicon carbide (3C-SiC) with an extra electron. We compare our diffusion quantum Monte Carlo results with those obtained with the HSE functional and find a good agreement between the two methods for (${\mathrm{SiV})}^{-}$ and (${\mathrm{SiV})}^0$, whereas the correct description of 3C-SiC with an extra electron crucially depends on the amount of Hartree-Fock exchange included in the functional. Also, we examine the case of the neutral Cd vacancy in CdTe, for which we assess the performance of HSE versus the many-body GW approximation for the description of the position of the defect states in the band gap.

We describe an efficient approximation for the electron-electron interaction in the determination of the low-energy effective interaction in multiband lattice systems. By using ideas for channel decomposition, form-factor expansion, and the truncated-unity technique we describe the interaction as arising from the nonlocal and orbital-dependent coupling of particle-hole and particle-particle bilinears formed by fields residing in the same one or two orbitals. This allows us to employ the constrained functional renormalization group (cfRG) with a suitable momentum and frequency discretization. The approach gives insights into the nonlocal screening of spin and charge interactions when bands away from the Fermi level are integrated out. Specifically, we compute the effective low-energy interactions in the low-energy target band of a three-band model with on-site and nonlocal bare interactions. We show that the cfRG adds important features to the effective target-band interaction that cannot be found using the constrained random phase approximation (cRPA).

We investigate the bias dependence of the hybrid charge transfer state emission at planar heterojunctions between the metal oxide acceptor ZnO and three donor molecules. The electroluminescence peak energy linearly increases with the applied bias, saturating at high fields. Variation of the organic layer thickness and deliberate change of the ZnO conductivity through controlled photodoping allow us to confirm that this bias-induced spectral shift relates to the internal electric field in the organic layer rather than the filling of states at the hybrid interface. We show that existing continuum models overestimate the hole delocalization and propose a simple electrostatic model in which the linear and quadratic Stark effects are explained by the electrostatic interaction of a strongly polarizable molecular cation with its mirror image.

Motivated by recent demands of the nanoplasmonic community, quasi-one-dimensional nanoribbons have attracted significant attention as promising materials in logic applications. By using the linear response theory and the Green's function formulation, we demonstrate how external local perturbations affect both intra- and interband plasmons in semiconductor armchair nanoribbons (aNRs). To do so practically, we focus on the silicon carbide aNRs subjected to a test photon beam. Particularly, the interplay between intra- and interband charge-density excitations is compared by taking into account the impact of ribbon width, the electronic dopant, the Zeeman magnetic field, temperature, and the incident photon wave vector on the dielectric function qualitatively. Furthermore, the combined effect of both weak and strong perturbations is studied. We show that both the intraband and interband plasmon excitations have a high susceptibility to perturbations, leading to the different optical features of the system. Moreover, we found various perturbation-dependent threshold frequencies in which the undamped plasmon modes and plasmon resonances take place. Finally, the invalidity of the linear response theory at strong perturbations is discussed concisely. Generically, the propagation and confinement of plasmon modes in the presence of weak and strong perturbations are reported. Our findings are useful for experimentalists to tune the optical properties of low-dimensional materials by perturbations.

The energy spectrum of massless Dirac fermions in graphene under two-dimensional periodic magnetic modulation with square-lattice symmetry is calculated. We show that the translation symmetry of the problem is similar to that of the Hofstadter or Thouless–Kohmoto–Nightingale–den Nijs problem, and in the weak-field limit the tight-binding energy eigenvalue equation is indeed given by the Harper-Hofstadter Hamiltonian. We show that due to its magnetic translational symmetry the Hall conductivity can be identified as a topological invariant and hence quantized. We thus extend the idea of topologically quantized Hall conductance to a two-dimensional electron system under periodic magnetic modulation. Finally, we indicate possible experimental systems where this may be verified.

We use classical electrodynamics, time-dependent density functional theory, and random-phase approximation to study the gap plasmons propagating in the nm-wide gap between metal surfaces. Particular emphasis is given to the quantum effects emerging when the junction is functionalized with a nanostructure supporting unoccupied gap localized electronic states. With the example of a quantum well (QW) introduced in the junction we show that the optically assisted electron transport across the junction via the gateway QW localized electronic states might strongly affect the lifetime and the propagation length of the gap plasmon. The coupling to the single-particle electron-hole excitations from occupied electronic states at metal surfaces into the QW-localized electronic states provides an efficient decay channel of the gap plasmon mode. Different from the through-gap electron tunneling discussed in the plasmonics literature, the electron transport involving the gateway electronic state is characterized by the threshold behavior with plasmon frequency. As a consequence, the dynamics of the gap plasmon can be controlled by varying the binding energy of the QW-localized electronic state. In more general terms, our results demonstrate strong sensitivity of the gap plasmons to the optically assisted electron transport properties of the junction which opens further perspectives in design of nanosensors and integrated active optical devices.

In the framework of a simple physical model, we demonstrate the existence of a system of discrete short-lifetime quantum levels for electrons in the potential well of the self-consistent field of charged fullerenes and onionlike structures. For electrons, in the case of positively charged fullerenes and onionlike structures, combining analytic and numeric considerations, we find that the energy of the volume-localized levels ranges from 1 to 100 eV.

Being motivated by recent achievements in the rapidly developing fields of optical bound states in the continuum (BICs) and excitons in monolayers of transition metal dichalcogenides, we analyze strong coupling between BICs in ${\mathrm{Ta}}_2{\mathrm{O}}_5$ periodic photonic structures and excitons in ${\mathrm{WSe}}_2$ monolayers. We demonstrate that the giant radiative lifetime of a BIC allows us to engineer the exciton-polariton lifetime, enhancing it by two orders of magnitude compared to a bare exciton. We show that the maximal lifetime of a hybrid light-matter state can be achieved at any point of $\mathbf{k}$ space by shaping the geometry of the photonic structure. Our findings open a different route for the realization of moving exciton-polariton condensates with a nonresonant pump and without Bragg mirrors, which is of paramount importance for polaritonic devices.

Charge order has now been observed in several cuprate high-temperature superconductors. We report a resonant inelastic x-ray scattering experiment on the electron-doped cuprate ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x\mathrm{Cu}{\mathrm{O}}_4$ that demonstrates the existence of dynamic correlations at the charge-order wave vector. Upon cooling we observe a softening in the electronic response, which has been predicted to occur for a $d$-wave charge order in electron-doped cuprates. At low temperatures, the energy range of these excitations coincides with that of the dispersive magnetic modes known as paramagnons. Furthermore, measurements where the polarization of the scattered photon is resolved indicate that the dynamic response at the charge-order wave vector primarily involves spin-flip excitations. Overall, our findings indicate a coupling between dynamic magnetic and charge-order correlations in the cuprates.