Room-temperature optically detected magnetic resonance experiments on spin-$\frac{3}{2}$ silicon vacancies in $4H$-SiC are reported. The $m_s=+\frac{1}{2}\leftrightarrow -\frac{1}{2}$ transition is accessed using a two-microwave-frequency excitation protocol. The ratio of the Rabi frequencies of the $+\frac{3}{2}\leftrightarrow +\frac{1}{2}$ and $+\frac{1}{2}\leftrightarrow -\frac{1}{2}$ transitions is measured to be $(0.901\pm 0.013)$. The deviation from $\sqrt{3}/2$ is attributed to small difference in $g$ factor for different magnetic dipole transitions. Whereas a spin-$\frac{1}{2}$ system is characterized by a single-spin lifetime $T_1$, we experimentally demonstrate that the spin-$\frac{3}{2}$ system has three distinct relaxation modes that can be preferentially excited and detected. The measured relaxation times are $(0.41\pm 0.02)T_{\mathrm{slow}}=T_d=(3.3\pm 0.5)T_{\mathrm{fast}}$. This differs from the values of $T_p/3=T_d=2T_f$ expected for pure dipole ($T_p$), quadrupole ($T_d$), and octupole ($T_f$) relaxation modes, respectively, and implies admixing of the slow dipole and fast octupole relaxation modes.

Magnetic noise from randomly fluctuating nuclear spin ensembles is the dominating source of decoherence for many multi-quantum-dot multielectron spin qubits. Here, we investigate in detail the effect of a DC electric current on the coupled electron-nuclear spin dynamics in double and triple quantum dots tuned to the regime of a Pauli spin blockade. We consider both systems with and without significant spin-orbit coupling and find that in all cases the flow of electrons can induce a process of dynamical nuclear spin polarization that effectively suppresses the nuclear polarization gradients over neighboring dots. Since exactly these gradients are the components of the nuclear fields that act harmfully in the qubit subspace, we believe that this presents a straightforward way to extend coherence times in multielectron spin qubits by at least one order of magnitude.

We study the band structure of electrons hopping on a honeycomb lattice with $p/q$ ($p,q$ are coprime integers) flux quanta through each elementary hexagon. In the nearest-neighbor hopping model the two bands that eventually form the $n=0$ Landau level have $2q$ zero-energy Dirac touchings. In this work we study the conditions needed for these Dirac points and their stability to various perturbations. We prove that these touchings and their locations are guaranteed by a combination of an antiunitary particle-hole symmetry and the lattice symmetries of the honeycomb structure. We also study the stability of the Dirac touchings to one-body perturbations that explicitly lower the symmetry.

Supercooled silicate liquids exhibit several orders of magnitude increase in viscosity at the glass-transition temperature ($T_{\mathrm{g}}$) towards the glassy state. Such a drastic dynamical slowdown leads to an abrupt change in the slope of temperature-dependent thermodynamic properties because the measurements reflect the equilibrium-to-nonequilibrium change from liquid to glass. However, an underlying structural change associated with such a transition remains elusive. For instance, understanding the structural origin of the variation in the coefficient of thermal expansion (CTE) of silicate glasses upon vitrification is critical for glass-manufacturing processes and applications. Here, based on temperature-dependent neutron diffraction, we demonstrate that the temperature dependences of both short- and medium-range order structural parameters show a pronounced change of slope at $T_{\mathrm{g}}$ for a range of silicate glasses of industrial importance. Interestingly, the short- and medium-range order structural parameters are found to be mutually correlated, both below and above $T_{\mathrm{g}}$. Based on these results, we find that the slope change of the area of the first sharp diffraction peak at $T_{\mathrm{g}}$ is correlated with the extent of the CTE jump at $T_{\mathrm{g}}$, which offers a structural origin for the discontinuity in the CTE of glasses at $T_{\mathrm{g}}$. This study can therefore shine light on solving critical industrial problems, such as glass relaxation.

Elastic waves are complex mixtures of transverse and longitudinal oscillations even in isotropic and homogeneous media, in contrast to the quantum, electromagnetic, or acoustic waves which could share the same formalism of Hamiltonian and application techniques. Here, we reformulate the elastic wave equation into a set of polarization-dependent decoupled Hamiltonians, to enable the quantum analogous techniques for higher functionalities. As an application example, we adopt the supersymmetric transformation from particle physics and apply it to elastic Hamiltonians, for the demonstration of spatial- and polarization-selective separation of guided elastic waves. Enabling the application of quantum-analogous techniques under the established elastic Hamiltonian formulation, our approach provides a pathway for controlling elastic waves, not limited to the control of an individual guided mode for arbitrary elastic waves, demonstrated here with supersymmetric technique.

We have investigated lattice dynamics of antiferromagnetic and ferromagnetic phases of cubic $\left(B2\right)$ FeRh at various temperatures from first principles using the temperature-dependent effective potential method. We have shown that already at low temperature the cubic structure of the antiferromagnetic phase becomes dynamically stable, which eliminates the contradiction between experimental observations and previous theoretical results showing its dynamical instability at temperature $T$ = 0 K. In addition, we have observed a significant difference in the temperature dependence of lattice vibrations of the ferromagnetic and antiferromagnetic phases. The phonon spectrum of the FM phase softens much stronger than that of the AFM phase, which provides additional contribution to the increase of vibrational entropy of the FM phase at high temperatures. The calculated difference between the vibrational entropies of the FM and AFM phases at a metamagnetic transition temperature (350 K) is 16 J/kg/K. This value is comparable with the experimental value of the total entropy change. We therefore conclude that the lattice dynamics plays a decisive role in the metamagnetic phase transition in FeRh and its remarkable magnetocaloric properties.

As scanning tunneling microscopy is pushed towards fast local dynamics, a quantitative understanding of tunnel junctions under the influence of a fast ac driving signal is required, especially at the ultralow temperatures relevant to spin dynamics and correlated electron states. We subject a superconductor-insulator-superconductor junction to a microwave signal from an antenna mounted in situ and examine the dc response of the contact to this driving signal. Quasiparticle tunneling and the Josephson effect can be interpreted in the framework of Tien-Gordon theory. The situation is more complex when it comes to higher-order effects such as multiple Andreev reflections. Microwave-assisted tunneling unravels these complex processes, providing deeper insights into tunneling than are available in a pure dc measurement.

The discovery of room-temperature skyrmions in some magnetic materials has boosted the investigation of their dynamics in view of future applications. We study the dynamics of skyrmions in the presence of defects or borders using a deterministic (finding and solving a deterministic Fokker-Planck differential equation) while probabilistic (the solution is the probability density for the presence of a skyrmion) approach. The probability that a skyrmion becomes trapped in a pinning center or the probability of the survival of a skyrmion along a racetrack are obtained as a function of temperature. The present work can be relevant in the design of skyrmionic devices where the probability of finding skyrmions at a given position and time is crucial for their feasibility.

We use inelastic neutron scattering to study the effect of a magnetic field on the neutron spin resonance $(E_r=3.6$ meV) of superconducting FeSe $(T_c=9$ K). While a field aligned along the in-plane direction broadens and suppresses the resonance, a $c$-axis aligned field does so much more efficiently, consistent with the anisotropic field-induced suppression of the superfluid density from the heat capacity measurements. These results suggest that the resonance in FeSe is associated with the superconducting electrons arising from orbital selective quasiparticle excitations between the hole and electron Fermi surfaces.

The relationship among structural transition, superconductivity, and doping in the Weyl semimetal ${\mathrm{Mo}}_{1-x}{\mathrm{W}}_x{\mathrm{Te}}_2$ has been established through a systematic study of the doping and pressure effects. Doping-dependent resistivity measurements at ambient pressure revealed that the structural transition temperature increases linearly with increasing W content in ${\mathrm{Mo}}_{1-x}{\mathrm{W}}_x{\mathrm{Te}}_2$. The observed structural transition temperature $\left(T_s\right)$ of ${\mathrm{MoTe}}_2$ at ambient pressure is 249 K and that of ${\mathrm{WTe}}_2$ is 613 K. Temperature-dependent synchrotron x-ray diffraction measurements further confirmed the structural transition in ${\mathrm{WTe}}_2$ at ambient pressure. Pressure was found to continuously suppress the $T_s$ in ${\mathrm{Mo}}_{0.90}{\mathrm{W}}_{0.10}{\mathrm{Te}}_2,{\mathrm{Mo}}_{0.60}{\mathrm{W}}_{0.40}{\mathrm{Te}}_2$, and ${\mathrm{Mo}}_{0.25}{\mathrm{W}}_{0.75}{\mathrm{Te}}_2$, and superconductivity emerges in ${\mathrm{Mo}}_{0.90}{\mathrm{W}}_{0.10}{\mathrm{Te}}_2$ and ${\mathrm{Mo}}_{0.60}{\mathrm{W}}_{0.40}{\mathrm{Te}}_2$ above 1.25 K when $T_s$ is suppressed to a lower temperature.

The large anomalous Hall effect (AHE) has recently been found in noncollinear antiferromagnet $\mathrm{M}{\mathrm{n}}_3\mathrm{Sn}$. However, the complex magnetic structure and its correlation with the AHE remain unclear. Here we investigate the magnetic structure of $\mathrm{M}{\mathrm{n}}_3\mathrm{Sn}$ completely by means of both the temperature and magnetic field dependence of neutron power diffraction. This study establishes the possible incommensurate magnetic structure models and extracts the temperature evolution of the magnetic moment of the Mn atom over the whole temperature range for $\mathrm{M}{\mathrm{n}}_3\mathrm{Sn}$. The large AHE of polycrystalline bulk $\mathrm{M}{\mathrm{n}}_3\mathrm{Sn}$ is measured in detail. By comparing the temperature dependence of the spontaneous component of the AHE and the magnetic moment, direct experimental evidence reveals that the triangular antiferromagnetic ordered moment of the Mn atom controls the change in the AHE of $\mathrm{M}{\mathrm{n}}_3\mathrm{Sn}$. This study provides the possibility to control the AHE of functional materials via changing the magnetic structure.

 ${\mathrm{LaCoO}}_3$ provides a poignant example of a transition metal oxide where the cobalt cations display multiple spin states and spin transitions and which continues to garner substantial attention. In this work, we describe first principles studies, based on $\mathrm{DFT}+U$ theory, of superlattices containing ${\mathrm{LaCoO}}_3$, specifically ${\left({\mathrm{LaCoO}}_3\right)}_n+{\left({\mathrm{LaTiO}}_3\right)}_n$ for $n=1,2$. The superlattices show strong electron transfer from Ti to Co resulting in ${\mathrm{Co}}^{2+}$, significant structural distortions and a robust orbital polarization of the ${\mathrm{Co}}^{2+}$. We predict high-spin ${\mathrm{Co}}^{2+}$ and a checkerboard (G-type) antiferromagnetic ground state. We provide a detailed analysis of the magnetic interactions and phases in the superlattices. We predict that ferromagnetic order on the ${\mathrm{Co}}^{2+}$ can be stabilized by hole doping (e.g., replacing La by Sr), which is rather unusual for ${\mathrm{Co}}^{2+}$ cations.

The formation of bound states between mobile impurity particles and fermionic atoms has been demonstrated in spin-polarized Fermi gases with attractive interspecies interaction. We investigate bound states of mobile impurities immersed in a two-dimensional system with a symmetry-protected quadratic band touching. In addition to the standard $s$-wave interaction, we consider an anisotropic dipolar exchange interaction that locally breaks point group symmetries. Using a weak-coupling renormalization group approach and a ladder approximation for the impurity-fermion propagator, we establish that the number of bound states can be controlled by varying the anisotropy of the exchange interaction. Our results show that the degeneracy and momentum dependence of the binding energies reflect some distinctive properties of the quadratic band touching.

We consider spontaneous symmetry-breaking transitions of strongly interacting two-dimensional Dirac fermions minimally coupled to mass and emergent gauge field order parameters. Using a renormalization group analysis, we show that the presence of gauge fields leads to fermion-induced quantum (multi)criticality where the putative emergent Lorentz invariance is violated. We illustrate this with the example of translational symmetry breaking due to charge-density wave order on the honeycomb lattice. Finally, we identify that topological phase transitions are well described by this effective field theory.

The linear absorption spectra in monolayers of transition metal dichalcogenides show pronounced signatures of the exceptionally strong exciton-phonon interaction in these materials. To account for both exciton and phonon physics in such optical signals, we compare different theoretical methods to calculate the absorption spectra using the example of ${\mathrm{MoSe}}_2$. In this paper, we derive the equations of motion for the polarization either using a correlation expansion up to fourth Born approximation or a time convolutionless master equation. We show that the Born approximation might become problematic when not treated in high enough order, especially at high temperatures. In contrast, the time convolutionless formulation gives surprisingly good results despite its simplicity when compared to higher-order correlation expansion and therefore provides a powerful tool to calculate the line shape of linear absorption spectra in the very popular monolayer materials.

Molecule-electrode interfaces in molecular electronic junctions are prone to chemical reactions, structural changes, and localized heating effects caused by electric current. These can be exploited for device functionality or may be degrading processes that limit performance and device lifetime. We develop a nonequilibrium Green's function based transport theory in which the central region atoms and, more importantly, atoms on molecule-electrode interfaces are allowed to move. The separation of timescales of slow nuclear motion and fast electronic dynamics enables the algebraic solution of the Kadanoff-Baym equations in the Wigner space. As a result, analytical expressions for dynamical corrections to the adiabatically computed Green's functions are produced. These dynamical corrections depend not only on the instantaneous molecular geometry but also on the nuclear velocities. To make the theoretical approach fully self-consistent, the same time-separation approach is used to develop expressions for the adiabatic, dissipative, and stochastic components of current-induced forces in terms of adiabatic Green's functions. Using these current-induced forces, the equation of motion for the nuclear degrees of freedom is cast in the form of a Langevin equation. The theory is applied to model molecular electronic junctions. We observe that the interplay between the value of the spring constant for the molecule-electrode chemical bond and electronic coupling strength to the corresponding electrode is critical for the appearance of structural instabilities and, consequently, telegraphic switching in the electric current. The range of model parameters is identified to observe structurally stable molecular junctions as well as various different kinds of current-induced telegraphic switching. The interfacial structural instabilities are also quantified based on current noise calculations.

Topological corner modes (TCMs) can exist in two-dimensional square lattices with either quantized dipole moments or a quantized bulk quadrupole moment. In both cases, four TCMs, each localized at one corner, can be found degenerate at zero energy. Here, we analytically show that in finite-sized systems, adjacent TCMs can couple, with hopping strength and sign determined by the edge configuration, which is directly associated with the magnetic flux of the unit cell. Consequently, we found that the response functions of the coupled corner modes of different origins, i.e., quantized edge dipoles and a quantized bulk quadrupole, possess distinctive line shapes. The response functions can therefore be used as a hallmark to determine the corner modes’ nature. We verify our findings in simulations using phononic crystals.

First-principles calculations of charged defects have become a cornerstone of research in semiconductors and insulators by providing insights into their fundamental physical properties. But, current standard approach using the so-called “jellium model” has encountered both conceptual ambiguity and computational difficulty, especially for low-dimensional semiconducting materials. In this paper, we propose a more physical and straightforward “transfer to real state” model to calculate the formation energies of charged defects in both three-dimensional (3D) bulk and low-dimensional semiconductors. Within this universal model, the ionized electrons or holes are placed on the realistic host band-edge states instead of the virtual jellium state; therefore, rendering it not only naturally keeps the supercell charge neutral, but also has clear physical meaning, thus can be easily extended to study other physical properties such as exciton binding energy, free carrier bound to the defect in low-dimensional systems. This realistic model reproduces the same accuracy as the traditional jellium model for most of the 3D semiconducting materials, and remarkably, for the low-dimensional structures, it can cure the divergence caused by the artificial long-range electrostatic energy encountered in the jellium model, and hence gives meaningful formation energies of defects in charged-state and transition-energy levels of the corresponding defects. Our realistic method, therefore, will have significant impact for the study of defect physics in all low-dimensional systems.

Nuclear quantum effect and many-body interaction importantly interplay in the hydrogen on the Pt(111) system under the high coverage conditions of electrochemical interest, as revealed by our ab initio path integral and ring polymer molecular dynamics simulations done at room temperature. At the full monolayer coverage, hydrogen atoms are close-packed either at the atop sites or the fcc sites owing to their strong repulsion and the nearly degenerate nature of the adsorption sites. While at the 2/3 monolayer, they are delocalized over the fcc and hcp sites via the bridge sites because of the hopping. The quantum many-body effect is thus crucially important in determining the coverage dependence and provides a clue for reconciling the long-standing controversy on this system.