When a semiconductor is subjected to a strong electric field, carriers are generated via quantum tunneling; this is termed as dielectric breakdown. Thus, using a terahertz pulse to drive the dielectric breakdown in Mott insulators, which exhibit variations in their electronic structures under carrier doping, a filling-controlled transition can be induced in the subpicosecond time scale. However, to generate carriers via quantum tunneling in a material with a band gap in the visible or near-infrared regions, an electric field pulse significantly exceeding $1 \mathrm{MV} {\mathrm{cm}}^{-1}$ is necessary. In this paper, using an organic molecular compound, bis(ethylenedithio)tetrathiafulvalene-difluorotetracyanoquinodimethane, which is a typical one-dimensional (1D) Mott insulator with a Mott gap of 0.7 eV, we aimed at realizing carrier generation and metallization via a strong electric field component of a terahertz pulse enhanced with an organic nonlinear optical crystal up to $2.8 \mathrm{MV} {\mathrm{cm}}^{-1}$. Even after the terahertz electric field decays, the reflectivity change caused by the terahertz pulse remains; this is different from the case involving the use of weaker electric fields. More importantly, this response indicates a threshold behavior against the electric field amplitude, which is characteristic of carrier generation via the quantum tunneling process. Furthermore, transient reflectivity spectra across the mid-infrared region could be reproduced well by numerical simulations using the Drude model, in which inhomogeneous carrier distributions are considered. The observed Drude response of the doublons and holons was ascribed to the spin-charge separation characteristic of 1D strongly correlated electron systems. We also demonstrate that the energy efficiency of such carrier generation by the terahertz pulse excitation is at least five times greater than that when using photoexcitation beyond the Mott gap. This indicates that excitation with the strong terahertz pulse is more effective for carrier doping in solids; thus, the proposed method is expected to be widely applicable for the electronic-state control of various correlated electron materials in which chemical carrier doping is currently difficult.

We develop a formalism for constructing particle-number-conserving Gaussian fermionic projected entangled pair states [U(1)-GfPEPSs] and show that these states can describe ground states of band insulators and gapless fermions with band touching points. When using them as variational Ansätze for two Dirac fermion systems (the $\pi$-flux model on the square lattice and the $[0,\pi ]$-flux model on the kagome lattice), we find that the U(1)-GfPEPSs, even with a relatively small bond dimension, can accurately approximate the Dirac Fermi sea ground states. By applying Gutzwiller projectors on top of these U(1)-GfPEPSs, we obtain a PEPS representation of U(1)-Dirac spin liquid states for spin-1/2 systems. With state-of-the-art tensor network numerics, the critical exponent in the spin-spin correlation function of the Gutzwiller-projected $\pi$-flux state is estimated to be $\eta \approx 1.7$.

The light-yield enhancement and decay-time shortening by Ca codoped with ${\mathrm{Lu}}_2\mathrm{Si}{\mathrm{O}}_5$:Ce have been observed in experiment. However, the underlying mechanisms remain enigmatic. First-principles calculations are performed to gain insight into the effect of Ca codoping. We show that first, the formation of ${\mathrm{Ca}}_{\mathrm{Lu}}\text{−}{V}_{\mathrm{O}}$ complex defects is favored, which causes the dissociation of ${\mathrm{Ce}}_{\mathrm{Lu}}\text{−}{V}_{\mathrm{O}}$; therefore, the nonradiative decay of Ce via $V_{\mathrm{O}}$ is suppressed, leading to light-yield enhancement. Secondly, the downward shift of the Fermi level by the codoping effect causes the observation of ${\mathrm{Ce}}^{4+}$ ions, and the electron trap depth of $V_{\mathrm{O}}$ decreases by the combination with Ca; thus, the trapping/detrapping rates increase, leading to decay-time shortening. Hence, our study has demonstrated an effective way to gain insight into the mechanisms for tuning properties of optical materials by codoping. The principles of the obtained mechanisms provide guidance for designing and optimizing a broad range of functional materials.

Hall effect, photoluminescence, magneto-photoluminescence, and electron paramagnetic resonance measurements performed on the undoped $p$-type ZnTe and ${\mathrm{Zn}}_{1-x}{\mathrm{Mn}}_x\mathrm{Te}$ crystals have revealed a strong coupling between ${\mathrm{Mn}}^{2+}$ ions and holes trapped at singly ionized zinc vacancy defects ($V_{Zn}^{-}$), resulting in the formation of the ${\mathrm{Mn}}^{2+}\text{−}{V}_{Zn}^{-}$ charge transfer complexes. These complexes cause the transformation of the 0.047 eV $V_{Zn}^{-/0}$ shallow acceptor level, present in ZnTe, into a 0.138 eV $V_{Zn}^{--/-}$ deep acceptor level and induce a new 1.44 eV PL band associated with ${\mathrm{Mn}}^{3+}$ ions in ${\mathrm{Zn}}_{1-x}{\mathrm{Mn}}_x\mathrm{Te}$. The coexistence of the mixed valence ${\mathrm{Mn}}^{2+}$ and ${\mathrm{Mn}}^{3+}$ states leads to the local ferromagnetic exchange interaction between ${\mathrm{Mn}}^{2+}$ and ${\mathrm{Mn}}^{3+}$ ions. The competition between the ferromagnetic exchange interaction and the intrinsic ${\mathrm{Mn}}^{2+}\text{−}{\mathrm{Mn}}^{2+}$ antiferromagnetic superexchange coupling gives rise to the paramagnetic–spin glass phase transition at $T_f=3.2$ K in the ${\mathrm{Zn}}_{1-x}{\mathrm{Mn}}_x\mathrm{Te}$ samples with $x=0.06$. We suggest a charge transfer mechanism, which transforms the ${\mathrm{Mn}}^{2+}\text{−}{V}_{Zn}^{-}$ complexes into ${\mathrm{Mn}}^{3+}\text{−}{V}_{Zn}^{--}$ ones. The effect of ${\mathrm{Mn}}^{2+}$ on the $p$-type doping of the II-VI compounds is discussed.

One of the key pathways toward scalability of spin-based quantum computing systems lies in achieving long-range interactions between electrons and increasing their interconnectivity. Coherent spin transport is one of the most promising strategies to achieve this architectural advantage. Experimental results have previously demonstrated high-fidelity transportation of spin qubits between two quantum dots in silicon and identified possible sources of error. In this theoretical study, we investigate these errors and analyze the impact of tunnel coupling, magnetic field, and spin-orbit effects on the spin transfer process. The interplay between these effects gives rise to double dot configurations that include regimes of enhanced decoherence that should be avoided for quantum information processing. These conclusions permit us to extrapolate previous experimental conclusions and rationalize the future design of large-scale quantum processors.

We show, by means of dynamical mean-field theory calculations, that the experimental fingerprints of the nematic order in iron-based superconductors are intrinsically connected with the electronic correlations in the Hund's correlated metallic state and they cannot be accessed via a renormalized quasiparticle picture. In particular, our results show that (i) in a metal in which correlations are dominated by the Hund's coupling the nematic ordering does not produce a rigid energy shift in the photoemission spectra, but a much richer spectral weight redistribution which mirrors the experimental results, and (ii) the nematic ordering is characterized by an orbital-selective coherence induced by the Hund's physics in agreement with the experimental picture.

Organic polaritons, strongly hybridized light-matter excitations, have arisen as a platform to device optical interfaces at room temperature. Despite their inherent complexity, polaritons are commonly regarded as coherent excitations described by Landau's quasiparticle theory. Here, we theoretically study and experimentally demonstrate the role of incoherent matter excitations in terms of generalized Hopfield coefficients, which unveil the fading of the upper polariton at its entrance to a continuum of molecular excitations. This marks the breakdown of the simplistic quasiparticle picture and the formation of a more complex quantum state beyond Landau's formalism.

We report on experimental study of the effect of disorder on electronic parameters and inelastic scattering mechanisms in ultrathin superconducting NbN films, which are commonly used in single-photon detectors. An increase in disorder in the studied 2.5-nm-thick NbN films characterized by Ioffe-Regel parameter from 6.3 to 1.6 is accompanied by a decrease in the critical temperature $T_c$ from 11.5 to 3.4 K. By measuring magnetoconductance in the range from $T_c$ to $\sim 3T_c$, we extract the inelastic scattering rates of electrons, including electron-phonon (e-ph) scattering rate ${\tau }_{\text{e-ph}}^{-1}$. We observe that ${\tau }_{\text{e-ph}}^{-1}$ and their temperature dependencies are insensitive to disorder that is not described by the existing models of the e-ph scattering in disordered metals. As temperature decreases, the temperature dependence of ${\tau }_{\text{e-ph}}^{-1}$ varies from $T^3$ to $T^2$, which can be result of a decrease in the dimensionality of phonons involved in the e-ph scattering process. Our results call for further theoretical and experimental studies of the e-ph scattering in ultrathin disordered films.

We show that vertex corrections to the quasiharmonic Green-Kubo theory of heat transport in insulators naturally lead to a generalization of the expression for the conductivity that could be derived from the linearized Boltzmann equation, when the effects of the full scattering matrix are accounted for. Our results, which are obtained from the Mori-Zwanzig memory-function formalism, provide a fully ab initio derivation of the linearized Boltzmann transport equation and establish a connection between two recently proposed unified approaches to heat transport in insulating crystals and glasses.

Centrosymmetric skyrmions attract much attention from the research community because of their small sizes and high concentrations. These features can be useful for applications. Such skyrmions are originated due to the Ruderman-Kittel-Kasuya-Yosida and Kondo interactions rather than the Dzyaloshinski-Moria interaction. We study a topological Hall effect in such systems using the Boltzmann equation for a nonequilibrium distribution function. For the relaxation, we choose the electron-acoustic phonon and electron-skyrmion interactions. We find that the topological Hall resistivity exhibits nonlinear behavior depending on chemical potential. Because of noncubic lattice symmetry, we investigate the dependence of the resistivity tensor components along the $x$ direction (parallel to an applied electric field), ${\rho }_{xx}$; ${\rho }_{zz}$ (the component along the $z$ axis); and ${\rho }_{xy}$ (a topological Hall component) with respect to the effective mass ratio, $m_z/m_x$. We assume that the skyrmions are spaced on the $\mathit{xy}$ plane and stretched out along the $z$ direction. The temperature dependence of the resistivity tensor reveals the monotonic growth for all components. There is some concern in the interpretation of experiments. Sometimes it can be very difficult to measure the topological Hall resistivity. Indeed, we find that ${\rho }_{xy}$ is one to two orders of magnitude less than ${\rho }_{xx}$ and ${\rho }_{zz}$. Additionally, there is another important factor, which complicates the problem. The $z$ axis and an applied electric field are not exactly perpendicular because of experimental conditions. Thus, the perpendicular to the electric field resistivity contains a linear combination of ${\rho }_{xy}$ and $|{\rho }_{xx}-{\rho }_{zz}|$. To determine ${\rho }_{xy}$ from an experiment, we propose an experimental setup to measure the topological Hall effect and provide the equations that allow us to determine ${\rho }_{xy}$.

A one band Hubbard model with intermediate coupling is shown to describe the two most important unusual features of a normal state: linear resistivity strange metal and the pseudogap. Both the spectroscopic and transport properties of the cuprates are considered on the same footing by employing a relatively simple post-Gaussian approximation valid for the intermediate couplings $U/t=1.5–4$ in relevant temperatures $T>100\mathrm{K}.$ In the doping range $p=0.1–0.3$, the value of $U$ is smaller than that in the parent material. For a smaller doping, especially in the Mott insulator phase, the coupling is large compared to the effective tight binding scale and a different method is required. This scenario provides an alternative to the paradigm that the coupling should be strong, say $U/t>6$, in order to describe the strange metal. We argue that, to obtain phenomenologically acceptable underdoped normal state characteristics like $T^{*}$, pseudogap values, and spectral weight distribution, a large value of $U$ is detrimental. Surprisingly the resistivity in the above temperature range is linear, $\rho ={\rho }_0+\alpha \frac{m^{*}}{e^{2}n\hslash }T$, with the “Planckian” coefficient $\alpha$ of order 1.

The current burst in research activities on disordered semiconductors calls for the development of appropriate theoretical tools that reveal the features of electron states in random potentials while avoiding the time-consuming numerical solution of the Schrödinger equation. Among various approaches suggested so far, the low-pass filter approach of Halperin and Lax (HL) and the so-called localization landscape technique (LLT) have received most recognition in the community. We prove that the HL approach becomes equivalent to the LLT for the specific case of a Lorentzian filter when applied to the Schrödinger equation with a constant mass. Advantageously, the low-pass filter approach allows further optimization beyond the Lorentzian shape. We propose the global HL filter as optimal filter with only a single length scale, namely, the size of the localized wave packets. As an application, we design an optimized potential landscape for a (semi)classical calculation of the number of strongly localized states that faithfully reproduce the exact solution for a random white-noise potential in one dimension.

The space- and temperature-dependent electron distribution $n(\mathbf{r},T)$ is essential for the theoretical description of the optoelectronic properties of disordered semiconductors. We present two powerful techniques to access $n(\mathbf{r},T)$ without solving the Schrödinger equation. First, we derive the density for nondegenerate electrons by applying the Hamiltonian recursively to random wave functions (RWF). Second, we obtain a temperature-dependent effective potential from the application of a universal low-pass filter (ULF) to the random potential acting on the charge carriers in disordered media. Thereby, the full quantum-mechanical problem is reduced to the quasiclassical description of $n(\mathbf{r},T)$ in an effective potential. We numerically verify both approaches by comparison with the exact quantum-mechanical solution. Both approaches prove superior to the widely used localization landscape theory (LLT) when we compare our approximate results for the charge carrier density and mobility at elevated temperatures obtained by RWF, ULF, and LLT with those from the exact solution of the Schrödinger equation.

We study increasing subsequences (ISs) for an ensemble of sequences given by a permutation of numbers $\{1,2,...,n\}$. We consider a Boltzmann ensemble at temperature $T$. Thus each IS appears with the corresponding Boltzmann probability where the energy is the negative length $-l$ of the IS. For $T\to 0$, only ground states, i.e., the longest IS (LIS), contribute, also called Ulam's problem. We introduce an algorithm which allows us to directly sample ISs in perfect equilibrium in polynomial time, for any given sequence and any temperature. Thus, we can study very large sizes. We obtain averages for the first and second moments of the number of ISs as functions of $n$ and confirm analytical predictions. Furthermore, we analyze for low temperature $T$ the sampled ISs by computing the distribution of overlaps and performing hierarchical cluster analyses. In the thermodynamic limit $n\to \infty$ the distribution of overlaps stays broad and the configuration landscape remains complex. Thus, Ulam's problem exhibits replica symmetry breaking (RSB). This means it constitutes a model with complex equilibrium behavior which can be studied numerically exactly in a highly efficient way. This is in contrast to other models, where RSB becomes exponentially irrelevant for the equilibrium behavior in the thermodynamic limit, such as in a random exclusive-or satisfiability (XORSAT) problem, or models where RSB remains relevant, such as spin glasses or NP-hard optimization problems, but where no fast exact algorithms are known.

Magnetic skyrmions are topological spin textures and promising candidates for novel spintronic applications. Recent studies on the current-driven dynamics of ferromagnetic (FM) skyrmions revealed that they exhibit an undesirable transverse motion, the skyrmion Hall effect. For antiferromagnetic (AFM) skyrmions, a vanishing skyrmion Hall effect was predicted, along with faster dynamics. However, their zero net magnetization obstructs efficient detection. Ferrimagnetic (FI) materials promise to combine both advantages: fast, AFM-like dynamics and easy read-out via stray fields. Here, we investigate the current-driven and Brownian dynamics of skyrmions in a FI with a compensation point. We perform atomistic spin dynamics simulations based on a model Hamiltonian and the stochastic Landau-Lifshitz-Gilbert equation supplemented with spin-orbit torques, accompanied by analytical calculations based on a collective coordinate approach. Our results unveil a nonmonotonic temperature dependence of the velocities and the diffusion coefficient with a strong enhancement at the angular momentum compensation temperature, due to scaling from FM- to AFM-like dynamics. These findings open up a new pathway for the efficient manipulation of skyrmion dynamics via temperature.

We probe the spin configuration of manganese's $d$ states, namely ${}^4\mathrm{T}_1$ and ${}^6\mathrm{A}_1$, in doped semiconductor quantum dots (QDs) through spin-polarized scanning tunneling spectroscopy (SP-STS). We show that the two levels have opposing spin states. Experimentally, we probed the density of states (DOS) of the undoped and manganese-doped ZnS QDs, which have a wide band gap so that the ${}^4\mathrm{T}_1$ and ${}^6\mathrm{A}_1$ energies lie within the gap. The states were viewed with a nonmagnetic and a spin-polarized tip of a scanning tunneling microscope (STM). While a nonmagnetic tip could inject electrons into the ${}^4\mathrm{T}_1$ and withdraw electrons from the ${}^6\mathrm{A}_1$ state as well, the DOS derived with a spin-polarized tip evidenced that the tip could “see” only one of the two energy levels, whose spin state remained parallel to that of the tip. The intensity of DOS representing the ${}^4\mathrm{T}_1$ and ${}^6\mathrm{A}_1$ of manganese thereby depended on the mutual alignment of the magnetization vectors representing the tip and the $d$ states. We have inferred that the spin configuration of ${}^4\mathrm{T}_1$ and ${}^6\mathrm{A}_1$ states is mutually antiparallel in nature; such results have been the rationale of the photoluminescence (PL) emission in such manganese-doped QDs to possess an exceptionally long lifetime.

In this paper, we address the optomechanical effects in surface-enhanced resonant Raman scattering (SERRS) from a single molecule in a nanoparticle on mirror (NPoM) nanocavity by developing a quantum master-equation theory, which combines macroscopic quantum electrodynamics and electron-vibration interaction within the framework of open quantum system theory. We supplement the theory with electromagnetic simulations and time-dependent density functional theory calculations in order to study the SERRS of a methylene blue molecule in a realistic NPoM nanocavity. The simulations allow us not only to identify the conditions to achieve conventional optomechanical effects, such as vibrational pumping, nonlinear scaling of Stokes and anti-Stokes scattering, but also to discovery distinct behaviors, such as the saturation of exciton population, the emergence of Mollow triplet side bands, and higher-order Raman scattering. All in all, our study might guide further investigations of optomechanical effects in resonant Raman scattering.

We report single-crystal growth and demonstrate Ising-type ferromagnetism with a Curie temperature of 25 K in ${\mathrm{UNi}}_4{\mathrm{P}}_2$ by means of magnetization, electrical transport, and specific heat measurements. Uniaxial anisotropy in magnetism and $cf$ hybridization are well reflected not only in the crystallographic $ac$ plane but also within the basal plane of the tetragonal unit cell owing to the quasi-one-dimensional crystal and electronic structure of ${\mathrm{UNi}}_4{\mathrm{P}}_2$. The strong anisotropy between the $c$ axis and the tetragonal basal plane is retained up to 57 T, while the intraplane anisotropy in the basal plane gradually vanishes at higher fields. Our results show that anisotropic electron correlations may stabilize the elusive Ising-type ferromagnetic order in one dimension at nonzero temperatures, presumably with its own characteristic universality class.

Magnesium aluminate scandium oxide ($\mathrm{ScAlMg}{\mathrm{O}}_4$) is a promising lattice-matched substrate material for GaN- and ZnO-based optoelectronic devices. Yet, despite its clear advantages over substrates commonly used in heteroepitaxial growth, several fundamental properties of $\mathrm{ScAlMg}{\mathrm{O}}_4$ remain unsettled. Here, we provide a comprehensive picture of its optical, electronic, and structural properties by studying $\mathrm{ScAlMg}{\mathrm{O}}_4$ single crystals grown by the Czochralski method. We use variable-angle spectroscopic ellipsometry to determine complex in-plane and out-of-plane refractive indices in the range from 193 to 1690 nm. An oscillator-based model provides a phenomenological description of the ellipsometric spectra with excellent agreement over the entire range of wavelengths. For convenience, we supply the reader also with Cauchy formulas describing the real part of the anisotropic refractive index for wavelengths above 400 nm. Ab initio many-body perturbation theory modeling provides information about the electronic structure of $\mathrm{ScAlMg}{\mathrm{O}}_4$, and successfully validated experimentally obtained refractive-index values. Simulations also show exciton binding energy as large as a few hundred of meV, indicating $\mathrm{ScAlMg}{\mathrm{O}}_4$ as a promising material for implementation in low-threshold, deep-UV lasing devices operating at room temperature. X-ray diffraction measurements confirm lattice constants of $\mathrm{ScAlMg}{\mathrm{O}}_4$ previously reported, but in addition, reveal that dominant crystallographic planes (001) are mutually inclined by about $0.{009}^{\circ }$. In view of our work, $\mathrm{ScAlMg}{\mathrm{O}}_4$ is a highly transparent, low refractive index, birefringent material similar to a sapphire, but with a much more favorable lattice constant and simpler processing.

Non-Hermitian systems reveal rich physics beyond the Hermitian regime, and have aroused great interest. One remarkable physical phenomenon is the non-Hermitian skin effect. Recently, topological switching for the non-Hermitian skin effect has been theoretically proposed in cold-atom systems. However, experimental realization of such a phenomenon remains a great challenge. Here, we theoretically propose and experimentally demonstrate a topological switch for the non-Hermitian skin effect in electrical circuit networks. By controlling the operational amplifier and other electric components, the nonreciprocal transport of probability for electrical signals is observed when the switch is turned on. Furthermore, the robustness of such a topological switch is demonstrated both theoretically and experimentally when perturbations are added. Our study provides an avenue for controlling electrical signals in circuit networks, with potential applications in the field of integrated circuit design.