Er-doped Si is a promising candidate for quantum information applications due to its telecom wavelength optical transition and its compatibility with Si nanofabrication technologies. Recent spectroscopic studies based on photoluminescence excitation have shown multiple well-defined lattice sites that Er occupies in Si. Here we report a measurement of the Zeeman and hyperfine tensors of a single ${}^{167}\mathrm{Er}^{3+}$ ion in Si. All the obtained tensors are highly anisotropic with the largest value principal axes aligning in nearly the same direction, and the trace of the lowest crystal field level $\mathbf{g}$-tensor is $17.78\pm 0.40$. The results indicate that this specific Er site is likely to be a distorted cubic site that exhibits monoclinic (${\mathrm{C}}_1$) symmetry. Finally, zero first-order-Zeeman fields are identified for this site and could be used to reduce decoherence of hyperfine spin states in future experiments.

Plasmonic coupling is one of the most important effects in compact plasmonic systems, and it has been studied intensively. In contrast, magnetoplasmonic coupling (MPC) is rarely mentioned, even in graphene nanostructures supporting the strong magneto-optic effect. Here, we theoretically investigate MPC in graphene nanodisk dimers in the presence of either parallel (case I) or antiparallel (case II) magnetic fields. We find the hybridized modes always appear for two states with same chirality, while their excitations depend on incident polarization. Moreover, two antisymmetric modes are dark in case I, but all four modes are bright in case II. To provide better insight, an extended coupled-dipole model is presented, in which the fundamental circularly polarized magnetoplasmons are decomposed into two linear and orthogonal dipoles, with a $\pi /2$ phase difference, and then the coupling is described by two linear dipoles along the two orthogonal directions separately. The parameters for the magneto-optic effect and coupling strengths are independent and can be easily extracted from their individual simulations. The eigenvalues and wave functions obtained from the model can describe well the resonance frequency and excitation strength of each hybridized mode. We finally discuss MPC in touching graphene nanodisks, where the charge transfer plasmon is immune to the magneto-optic effect, and in case II, a circular resonance state will be replaced by a linear one with the incident electric field along the touching direction. In this paper, we provide a general framework for investigating mode coupling of two circular states and pave the way to magneto-optic and plasmonic applications.

We present an investigation on the structural and magnetic properties of the interfaces of ${\mathrm{Fe}}_3{\mathrm{O}}_4/\mathrm{MgO}\left(001\right)$ and ${\mathrm{Fe}}_3{\mathrm{O}}_4/\mathrm{NiO}/\mathrm{MgO}\left(001\right)$ by extracting cation-selective magneto-optical depth profiles by means of x-ray resonant magnetic reflectivity in combination with charge-transfer multiplet simulations of x-ray magnetic circular dichroism data. For ${\mathrm{Fe}}_3{\mathrm{O}}_4/\mathrm{MgO}\left(001\right)$, the magneto-optical depth profiles at the ${\mathrm{Fe}}_{\mathrm{oct}}^{2+}$ and the ${\mathrm{Fe}}_{\mathrm{oct}}^{3+}$ resonant energies follow exactly the structural profile, while the magneto-optical depth profile at the ${\mathrm{Fe}}_{\mathrm{tet}}^{3+}$ resonance is offset by $3.2\pm 1.3$ Å from the interface, consistent with a $B$-site interface termination of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ with fully intact magnetic order. In contrast, for ${\mathrm{Fe}}_3{\mathrm{O}}_4/\mathrm{NiO}\left(001\right)$, the magneto-optical depth profiles at the ${\mathrm{Fe}}_{\mathrm{oct}}^{2+}$ and the ${\mathrm{Ni}}^{2+}$ resonances agree with the structural profile, but the interface positions of the magneto-optical depth profiles at the ${\mathrm{Fe}}_{\mathrm{oct}}^{3+}$ and the ${\mathrm{Fe}}_{\mathrm{tet}}^{3+}$ resonances are spatially shifted by $3.3\pm 1.4$ and $2.7\pm 0.9$ Å, respectively, not consistent with a magnetically ordered stoichiometric interface. This may be related to an intermixed $(\mathrm{Ni},\mathrm{Fe})\mathrm{O}$ layer at the interface. The dichroic depth profile at the Ni $L_3$ edge might hint at uncompensated magnetic moments throughout the NiO film.

It has been shown that a Weyl point in a superconducting nanostructure may give rise to a Weyl disk where two quantum states are almost degenerate in a two-dimensional manifold in the parametric space. This opens up the possibility of a holonomic quantum manipulation: A transformation of the wave function upon an adiabatic change of the parameters within the degenerate manifold. In this paper, we investigate in detail the opportunities for holonomic manipulation in Weyl disks. We compute the connection at the manifold in quasiclassical approximation to show it is Abelian and can be used for a phase gate. To provide a closed example of quantum manipulation that includes a state preparation and readout, we augment the holonomic gate with a change of parameters that brings the system out of the degenerate subspace. For numerical illustrations, we use a finite value of quasiclassical parameter and exact quantum dynamics. We investigate the fidelity of an example gate for different execution times. We evaluate the decoherence rate and show it can be made small to ensure a wide frequency range where an adiabatic manipulation remains coherent.

A recently proposed theoretical framework for the description of electronic quantum friction for constant-velocity nuclear projectiles traversing periodic crystals is here implemented using a local basis representation. The theory requires a change of reference frame to the projectile's, and a basis set transformation for the target basis functions to a “gliding basis” is presented, which is time-periodic but does not displace in space with respect to the projectile, allowing a local-basis Floquet impurity-scattering formalism to be used. It is illustrated for a one-dimensional single-band tight-binding model, as the simplest paradigmatic example, displaying the qualitative behavior of the formalism. The time-dependent nonorthogonality of the gliding basis requires care in the proper (simplest) definition of a local projectile perturbation. The Fermi level is tilted with a slope given by the projectile velocity, which complicates integration over occupied states. It is solved by a recurrent application of the Lippmann-Schwinger equation, in analogy with the previous nonequilibrium treatment of electron ballistic transport. Aiming towards a first-principles mean-field-like implementation, the final result is the time-periodic particle density in the region around the projectile, describing the stroboscopically stationary perturbation cloud around the projectile, out of which other quantities can be obtained, such as the electronic stopping power.

Using angle-resolved photoemission spectroscopy, we investigated the evolution of the electronic structure of ${\mathrm{Fe}}_{1-x}{\mathrm{Cu}}_x\mathrm{Se}$ from $x=0$ to 0.10. We found that the substitution of Fe by Cu introduces extra electron carriers. The hole bands near the $\mathrm{\Gamma }$ point were observed to shift downward with increasing doping $x$ and completely sank down below the Fermi level ($E_F$) for $x\ge 0.05$. Meanwhile, the electron pockets near the M point became larger but lost the spectral weight near $E_F$. Concomitantly, the effective mass of the electron bands increased with doping. Our results show how a metal-insulator transition behavior occurs upon Cu doping in view of the electronic structure and provide a platform to further investigation on the origin of emergent magnetic fluctuation in ${\mathrm{Fe}}_{1-x}{\mathrm{Cu}}_x\mathrm{Se}$.

The most fundamental symmetry in condensed matter physics is the crystallographic symmetry, such that solids could exhibit unconventional excitations beyond Dirac and Weyl fermions. Among the unconventional fermions, the sextuple excitation has been predicted in many nonmagnetic materials. However, the study of magnetic sextuple excitations has fallen behind because of the complicated magnetic structures of materials. Here we perform a systematic search for ferromagnetic (FM) sextuple points (SPs) in the absence of spin-orbit coupling. Based on 230 space groups (SGs), we find there are 5 SGs that could host SPs at high-symmetry points. Regarding the stability of the sextuple excitation, we reveal that the minimum little point group at where the SP locates is ${\mathrm{T}}_d$. To support this result, we construct the effective model based on the $k·p$ method, from which the linear dispersion can be read off. Via first-principles calculations, we also identify 67 FM material candidates, which realize the SPs with completely/incompletely spin polarized. To support our theory, we pick one of the FM materials, ${\mathrm{Rb}}_4{\mathrm{O}}_6$, as an example in which there is a fully spin-polarized SP. Finally, our work paves the way to study the spin-polarized SPs and provides a platform to realize the spintronic associated with sextuple excitations.

Recent experiments on Kitaev spin liquid candidate materials reported nonmonotonic behavior of thermal conductivity as a function of magnetic field, which lead to conflicting interpretations of its origin. Motivated by this development, we study the magnetic field dependence of thermal conductivity of a generalized Kitaev model, which allows the phase transitions between different flux sectors as a function of the magnetic field. The thermal conductivity due to Majorana fermions shows dip-bump structures as the magnetic field increases, which is caused by either the transitions between different flux sectors of Kitaev spin liquids or the topological transitions that change the Majorana Chern number within the same flux sector. It is shown that the change of Chern number is closely related to the four-Majorana-fermion interaction induced by the magnetic field. The nonmonotonic behavior in thermal conductivity emerges at finite temperature, and it becomes weaker when temperature decreases toward zero. Our model provides a generic mechanism for the Kitaev spin liquids to develop nonmonotonic magnetic-field dependence of thermal conductivity while the comparison to realistic materials remains an open question for future investigation.

Non-Hermitian systems exhibit nontrivial band topology and a strong sensitivity of the energy spectrum on the boundary conditions. Remarkably, a macroscopic number of bulk states get squeezed toward the lattice edges under open boundary conditions, an effect dubbed the non-Hermitian skin effect (NHSE). A well-established dynamical signature of the NHSE in real space is the directional bulk flow (or persistent current) for arbitrary initial excitation of the system, which is observed at long times. Here we unravel a different dynamical signature of the NHSE in real space that manifests itself in the early-time dynamics of the system, namely, self-acceleration of the wave function. Self-acceleration is demonstrated to occur rather generally in single–band lattice models probed by single-site excitation, where the acceleration turns out to be proportional to the area enclosed by the energy spectrum of the Bloch Hamiltonian under periodic boundary conditions. The observation of wave packet self-acceleration at early times is a clear signature of the NHSE and should be experimentally accessible using synthetic non-Hermitian matter, for example, in discrete-time photonic quantum walks.

The thermoelectric (TE) properties of the three pyrite-type IIB-${\mathrm{VIA}}_2$ dichalcogenides (${\mathrm{ZnS}}_2, {\mathrm{CdS}}_2$, and ${\mathrm{CdSe}}_2$) are systematically investigated and compared with those of the prototype ${\mathrm{ZnSe}}_2$ to optimize their TE properties. Using the phonon Boltzmann transport equation, we find that they all have ultralow lattice thermal conductivities. By analyzing their vibrational properties, this is attributed to soft phonon modes derived from the loosely bound rattlinglike metal atoms and to strong anharmonicity caused by the vibrations of all atoms perpendicular to the strongly bound nonmetallic dimers. Additionally, by correlating those properties along the series, we elucidate several chemical trends. We find that the heavier atom masses, larger atomic displacement parameters, and longer bond lengths between metal and nonmetal atoms can be beneficial to the looser rattling behaviors of the metal atoms and therefore lead to the softer phonon modes and that the stronger nonmetallic dimer bonds can boost the anharmonicities, both leading to the lower thermal conductivities. Furthermore, we find that all three compounds have complex energy isosurfaces at valence and conduction band edges that simultaneously allow for large density-of-states effective masses and small conductivity effective masses for both $p$- and $n$-type carriers. Consequently, the calculated TE figures of merit ($ZT$) can reach large values for both $p$- and $n$-type doping. In this paper, we illustrate the effects of the rattlinglike metal atoms and the localized nonmetallic dimers on the thermal transport properties and the importance of different carrier effective masses to electrical transport properties in these pyrite-type dichalcogenides, which can be used to predict and optimize the TE properties of other TE compounds in the future.

The electronic structure of phosphorene atomic chains (PACs) embedded in various dielectric environments is studied theoretically by using a configuration interaction approach beyond the conventional double-excitation scheme. While the nominal single-particle gap of the PACs is shown to roughly obey an expected scaling law of $L^{-2}$, where $L$ measures the length of PACs, the quasiparticle shift is found to gradually converge to a value that is insensitive to the dielectric environment as $L$ increases. In the meantime, exciton binding energies are revealed to obey a simple scaling law of $0.96/\left(\kappa L\right)-0.02$, with $\kappa$ being the effective dielectric constant. As $L$ goes to infinity, the quasiparticle and excitonic effect are both found to be greatly suppressed as if the long-range electron-electron interactions are quenched in long PACs.

Herein, we establish a hybrid coupling model (HCM) containing both near- and far-field couplings to describe the electromagnetic response of the coupled-grating system composed of two parallelly aligned subwavelength dielectric gratings. The HCM shows that the near-field coupling strength only contributes to the frequency splitting of two resonant modes, while the far-field one contributes to the frequency splitting and the linewidths of two resonant modes simultaneously. By changing the distance between two dielectric gratings, both the near- and far-field coupling strengths can be flexibly tuned, giving rise to rich electromagnetic responses. In addition, the formation of Fabry-Perot bound states in the continuum in coupled-grating systems can be clearly explained by the HCM. In this paper, we not only provide an all-dielectric platform for simultaneously manipulating near- and far-field couplings but also offer a viable approach to achieve reflectance/transmittance spectra with diverse shapes.

Many-body localization (MBL) is an example of a dynamical phase of matter that avoids thermalization. While the MBL phase is robust to weak local perturbations, the fate of an MBL system coupled to a thermalizing quantum system that represents a “heat bath” is an open question that is actively investigated theoretically and experimentally. In this work, we consider the stability of an Anderson insulator with a finite density of particles interacting with a single mobile impurity—a small quantum bath. We give perturbative arguments that support the stability of localization in the strong interaction regime. Large-scale tensor network simulations of dynamics are employed to corroborate the presence of the localized phase and give quantitative predictions in the thermodynamic limit. We develop a phenomenological description of the dynamics in the strong interaction regime, and we demonstrate that the impurity effectively turns the Anderson insulator into an MBL phase, giving rise to nontrivial entanglement dynamics well captured by our phenomenology.

The magnetization of a nonmagnetic semiconductor by femtosecond light pulses is crucial to achieve an all-optical control of the spin dynamics in materials and to develop faster memory devices. However, the conditions for its detection are largely unknown. In this paper, we identify the criteria for the observation of ultrafast magnetization and critically discuss the difficulties hindering its experimental detection. We show that ultrafast magnetization of a nonmagnetic semiconductor can be observed in compounds with very localized conduction band states and more delocalized valence bands, such as in the case of a p-d charge transfer gap. By using constrained and time-dependent density functional theory simulations, we demonstrate that a transient ferrimagnetic state can be induced in diamagnetic semiconductor ${\mathrm{V}}_2{\mathrm{O}}_5$ via ultrafast pulses at realistic fluences. The ferrimagnetic state has opposite magnetic moments on vanadium (conduction) and oxygen (valence) states. Our methodology outruns the case of ${\mathrm{V}}_2{\mathrm{O}}_5$ as it identifies the key requirements for a computational screening of ultrafast magnetism in nonmagnetic semiconductors.

The mechanism underlying charge transport in strongly correlated quantum systems, such as doped antiferromagnetic Mott insulators, remains poorly understood. Here, we study the expansion dynamics of an initially localized hole inside a two-dimensional (2D) Ising antiferromagnet at variable temperature. Using a combination of classical Monte Carlo and truncated-basis methods, we reveal two dynamically distinct regimes: a spin-charge confined region below a critical temperature $T^{*}$, characterized by slow spreading, and a spin-charge deconfined region above $T^{*}$, characterized by an unbounded diffusive expansion. The deconfinement temperature $T^{*}\approx 0.65J_z$ we find is around the Néel temperature $T_{\mathrm{N}}=0.567J_z$ of the Ising background in 2D, but we expect $T^{*}<T_{\mathrm{N}}$ in higher dimensions. In both regimes we find that the mobile hole does not thermalize with the Ising spin background on the considered time scales, indicating weak effective coupling of spin and charge degrees of freedom. Our results can be qualitatively understood by an effective parton model and can be tested experimentally in state-of-the-art quantum gas microscopes.

We theoretically study transport signatures associated with a spontaneous two-valley to one-valley quantum phase transition in a two-dimensional electron gas (2DEG) tuned by decreasing the 2D carrier density, as claimed in a recent experiment [M. S. Hossain et al., Phys. Rev. Lett. 127, 116601 (2021)]. The key issue we focus on is whether the experimentally measured 2D resistivity as a function of carrier density is consistent (or not) with an underlying spontaneous valley-polarization transition as assumed uncritically in the experimental report. Our theoretical analysis is particularly germane since the experiment does not directly measure the change in the Fermi surface resulting from the valley polarization transition, but infers such a transition indirectly through transport measurements. We validate the experimental claim, showing that indeed the observed sudden change in the 2D resistivity is quantitatively consistent with a sudden change in the valley polarization from 2 to 1 at the critical density.

Integrable quantum systems of finite size are generically robust against weak enough integrability-breaking perturbations, but become quantum chaotic and thermalizing if the integrability breaking is strong enough. We argue that the onset of quantum chaos can be described as a Fock-space delocalization process, with the eigenstates of the integrable system being taken as the Fock states. The integrability-breaking perturbation introduces hopping in this Fock space, and chaos sets in when this hopping delocalizes the many-body eigenstates in this space. Depending on the range of the dominant Fock-space hopping, delocalization can occur either through a crossover or via a transition that becomes sharp in the appropriate large-system dynamic limit. In either case, the perturbation strength at the onset of chaos scales to zero in the usual thermodynamic limit, with a size dependence that we estimate analytically and compute numerically for a few specific models. We also identify two intermediate finite-size-dependent regimes: There is generally an intermediate nonchaotic regime in which integrability is broken strongly enough to produce some systemwide many-body resonances but not enough to thermalize the system. In spatially extended systems (but not in quantum dots) there is also a crossover or transition between chaotic regimes where the ratio of the system size to the mean free path of the quasiparticles of the integrable system is small versus large compared to unity.

It has been a puzzle for a century about how “hard” (coercive) a ferromagnet can be. Seven decades ago, W. Brown gave his famous theorem to correlate coercivity of a ferromagnet to its magnetocrystalline anisotropy field. However, the experimental coercivity values are far below the calculated level given by the theorem, which is called Brown's Coercivity Paradox. The paradox has been considered to be related to the complex microstructures of the magnets in experiments because coercivity is an extrinsic property that is sensitive to any imperfections in the specimens. To date, coercivity cannot be predicted and calculated by quantitative modeling. In this investigation, we carried out a case study on the high magnetic coercivity of Co nanowires exceeding the magnetocrystalline anisotropy field as predicted by Brown's theorem. It is found that the aspect ratio and diameter of the nanocrystals have a strong effect on the coercivity. When the nanocrystals have an increased aspect ratio, the coercivity is significantly higher than the magnetocrystalline anisotropy field of a hcp Co crystal. Micromagnetic simulations give a coercivity aspect-ratio dependence that is well consistent with the experimental results. It is also revealed that a coercivity limit exists based on the geometrical structures of the nanocrystals that govern the demagnetizing process. The quantitative correlation obtained between the structure and coercivity enables material design of advanced permanent magnets in the future.

Detection of a bosonic metallic state that breaks the $Z_2$ time-reversal symmetry has been recently reported in ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ with a doping level $x\approx 0.8$. This is a metallic state of fermionic quadruplets that breaks time-reversal symmetry. As such, it has no condensed Cooper pairs but has a long-range order between fermionic quartets. In the present paper, we investigate the emergence of this phase in a two-component London model via Monte Carlo simulations as a function of various intercomponent couplings.