The superbosonization formula aims at rigorously calculating fermionic integrals via employing supersymmetry. We derive such a supermatrix representation of superfield integrals and specify integration contours for the supermatrices. The derivation is essentially based on the supersymmetric generalization of the Itzykson-Zuber integral in the presence of anomalies in the Berezinian and shows how an integral over supervectors is eventually reduced to an integral over commuting variables. The approach is tested by calculating both one and two point correlation functions in a class of random matrix models. It is argued that the approach is capable of producing nonperturbative results in various systems with disorder, including physics of many-body localization, and other situations hosting localization phenomena.

We present an investigation of the multiferroic lacunar spinel compound ${\mathrm{GeV}}_4{\mathrm{S}}_8$ using time-domain terahertz spectroscopy. We find three absorptions which either appear or shift at the antiferromagnetic transition temperature, $T_N=17\mathrm{K}$, as $S=1$ magnetic moments develop on vanadium tetrahedra. Two of these absorptions are coupled to the magnetic state and one only appears below the Néel temperature, and is interpreted as a magnon. We also observe isosbestic points in the dielectric constant in both the temperature and frequency domains. Further, we perform an analysis on the isosbestic features to reveal an interesting collapse into a single curve as a function of both frequency and temperature, behavior which exists throughout the phase transitions. This analysis suggests the importance of spectral changes in the terahertz range which are linear in frequency and temperature.

The domain wall depinning field represents the minimum magnetic field needed to move a domain wall, typically pinned by samples' disorder or patterned constrictions. Conventionally, such a field is considered independent on the Gilbert damping since it is assumed to be the field at which the Zeeman energy equals the pinning energy barrier (both damping independent). Here we analyze numerically the domain wall depinning field as a function of the Gilbert damping in a system with perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya interaction. Contrary to expectations, we find that the depinning field depends on the Gilbert damping and that it strongly decreases for small damping parameters. We explain this dependence with a simple one-dimensional model and we show that the reduction of the depinning field is related to the finite size of the pinning barriers and to the domain wall internal dynamics, connected to the Dzyaloshinskii-Moriya interaction and the shape anisotropy.

Iron-based superconductors can exhibit different magnetic ground states and are in a critical magnetic region where frustrated magnetic interactions strongly compete with each other. Here we investigate the longitudinal modes of spin fluctuations in a unified effective magnetic model for iron-based superconductors. We focus on the collinear antiferromagnetic phase and calculate the behavior of the longitudinal mode when different phase boundaries are approached. The results can help to determine the nature of the magnetic fluctuations in iron-based superconductors.

The emergent integrability of the many-body localized phase is naturally understood in terms of localized quasiparticles. As a result, the occupations of the one-particle density matrix in eigenstates show a Fermi-liquid-like discontinuity. Here, we show that in the steady state reached at long times after a global quench from a perfect density-wave state, this occupation discontinuity is absent, reminiscent of a Fermi liquid at a finite temperature, while the full occupation function remains strongly nonthermal. We discuss how one can understand this as a consequence of the local structure of the density-wave state and the resulting partial occupation of quasiparticles. This partial occupation can be controlled by tuning the initial state and can be described by an effective temperature.

We present a rigorous procedure for evaluating the photoelastic coefficients of a layered medium in which the periodicity is smaller than the wavelengths of all optical and acoustic fields. Analytical expressions are given for the coefficients of a composite material comprising thin layers of optically isotropic materials. These photoelastic coefficients include artificial contributions that are unique to structured media and arise from the optical and mechanical contrast between the constituents. Using numerical examples, we demonstrate that the acousto-optic properties of layered structures can be enhanced beyond those of the constituent materials. Furthermore, we show that the acousto-optic response can be tuned as desired.

Type-I clathrate compounds with off-center guest ions can be used to realize the concept of phonon-glass electron-crystal because they exhibit lattice thermal conductivities ${\kappa }_{\mathrm{L}}$ that are almost identical to those observed in network-forming glasses. This is in contrast with type-I clathrates with on-center guest ions, which show ${\kappa }_{\mathrm{L}}$ of conventional crystalline structures. Glasslike ${\kappa }_{\mathrm{L}}$ stems from the peculiar THz frequency dynamics in off-center type-I clathrates, in which there appear three kinds of modes classified as extended, weakly localized (WL) and strongly localized (SL) modes, as demonstrated by Liu et al. [Phys. Rev. B 93, 214305 (2016)]. Our calculated results based on the hopping mechanism of SL modes via anharmonic interactions show fairly good agreement with the observed $T$-linear rise of ${\kappa }_{\mathrm{L}}$ above the plateau at a few tens Kelvin. We emphasize that both the magnitude and the temperature dependence are in accord with the experimental data of off-center type-I clathrates.

The longitudinal spin Seebeck effect (SSE) has been investigated using Pt/ferrite bilayers employing two Y-hexagonal ferrites ${\mathrm{Ba}}_2{\mathrm{Zn}}_2{\mathrm{Fe}}_{12}{\mathrm{O}}_{22}$ (Zn2Y) and ${\mathrm{Ba}}_2{\mathrm{Co}}_2{\mathrm{Fe}}_{12}{\mathrm{O}}_{22}$ (Co2Y) deposited by a spin-coating method on ${\mathrm{SrTiO}}_3\left(111\right)$ substrates. The prepared hexagonal ferrites are highly oriented with $c$ axes perpendicular to the substrate plane. The room-temperature magnetic moments of both ferrimagnetic ferrites amount to similar values and, most importantly, both have easy magnetization normal to the $c$ axis. Despite their similar magnetic response the notable SSE signal is only observed for Zn2Y whereas the SSE signal of Co2Y is below the experimental noise level. A plausible explanation for this surprising discrepancy is magnetic disorder induced by cobalt cations, the random distribution of which in the Co2Y ferrite structure might critically limit the spin-wave propagation. This results in suppression of the SSE signal in Co2Y, while the Zn2Y with nonmagnetic substituent exhibits significant SSE signal. The temperature dependence of SSE for Zn2Y was measured over the $30\text{–}300\text{−}\mathrm{K}$ range and quantitatively analyzed considering the heat flow through the Pt/Zn2Y bilayer and thermal gradient across the Zn2Y thin layer as the most relevant parameters. Using this approach the normalized SSE smoothly increases with lowering temperature, which correlates to increasing magnon propagation length and magnetization with decreasing temperature.

Spectral characteristics of THz generation in a parallel array of inductively coupled Josephson junctions with intermediate-to-large damping in the presence of thermal noise have been studied numerically. The influence of the number of junctions and coupling between them on the spectral linewidth has been investigated. We show that known theoretical formulas for radiation linewidth of a single Josephson junction, divided by the number of junctions in the chain, gives good agreement with numerical results for overdamped chains, while for chains with intermediate damping a factor of 1/2 has to be introduced into the formula in order to describe the linewidth on the I-V curve steps corresponding to lag-synchronization (soliton) regimes.

A major challenge in creating a quantum computer is to find a quantum system that can be used to implement the qubits. For this purpose, deep centers are prominent candidates, and ab initio calculations are one of the most important tools to theoretically study their properties. However, these calculations are highly involved, due to the large supercell needed, and the computational cost can be even larger when one goes beyond the Kohn-Sham scheme to correct the band gap problem and achieve good accuracy. In this work, we present a method that overcomes these problems and provides the optical transition energies as a difference of Kohn-Sham eigenvalues; even more, provides a complete and accurate band structure of the defects in a semiconductor. Despite the original motivations, the presented methodology is a general procedure, which can be used to systematically study the optical transitions between localized levels within the band gap of any system. The method is an extension of the low-cost and parameter-free DFT-1/2 approximate quasiparticle correction, and allows it to be applied in the study of complex defects. As a benchmark, we apply the method to the ${\mathrm{NV}}^{-}$ center in diamond. The agreement with experiments is remarkable, with an accuracy of 0.1 eV. The band structure agrees with the expected qualitative features of this system, and thus provides a good intuitive physical picture by itself.

We study the many-body localization of spin chain systems with quasiperiodic fields. We identify the lower bound for the critical disorder necessary to drive the transition between the thermal and many-body localized phase to be $W_c>1.85$, based on finite-size scaling of entanglement entropy and fluctuations of the bipartite magnetization. We also examine the time evolution of the entanglement entropy of an initial product state where we find power-law and logarithmic growth for the thermal and many-body localized phases, respectively, with a transition point $W_c\sim 2.5$. For larger disorder strength, both imbalance and spin-glass order are preserved at long times, while spin-glass order shows dependence on system size. Quasiperiodic fields have been applied in different experimental systems, and our study finds that such fields are very efficient at driving the many-body localized phase transition.

We study the electronic orders in a honeycomb-Kondo lattice. For the ground state, we use variational quantum Monte Carlo to find the transition from antiferromagnetic insulator to Kondo insulator is continuous, in contrast to the discontinuous transition in mean-field theory. Moreover, the hybridization parameter between the conduction electron and the Kondo spin is nonzero even within the antiferromagnetic phase. At finite temperatures, we resort to dynamical mean-field theory, which not only captures local quantum fluctuations but also accesses the thermodynamic limit directly. There are three phases, namely, antiferromagnetic insulator, Kondo insulator, and paramagnetic phase. The transition from antiferromagnetic phase to paramagnetic phase is likely discontinuous, while that from antiferromagnetic phase to Kondo insulator phase remains to be continuous at finite temperatures. There is a crossover from the paramagnetic phase, where spin excitations are gapless, to the Kondo insulator phase, where spin excitations are gapped. Our results indicate a significant effect of fluctuations beyond mean-field theory in the honeycomb-Kondo lattice. Since the transition from the antiferromagnetic phase to the Kondo insulating phase occurs at a sizable Kondo coupling, where the Kondo lattice model is inequivalent to the Anderson lattice model, our results are complementary to that for a honeycomb-Anderson lattice.

Based on the exact solution of one-dimensional Fermi gas systems with $SU$(N) symmetry in a hard wall, we demonstrate that we are able to sort the ordering of the lowest-energy eigenvalues of states with all allowed permutation symmetries, which can be solely marked by certain quantum numbers in the Bethe ansatz equations. Our results give examples beyond the scope of the generalized Lieb-Mattis theorem, which can only compare the ordering of energy levels of states belonging to different symmetry classes if they are comparable according to the pouring principle. In the strongly interacting regime, we show that the ordering of energy levels can be determined by an effective spin-exchange model, and we extend our results to a nonuniform system trapped in a harmonic potential.

We report the direct observation of two mid-gap core $d$ states of differing symmetry for a single Fe atom embedded in GaAs. These states are distinguished by the strength of their hybridization with the surrounding host electronic structure. The midgap state of Fe that does not hybridize via $\sigma$ bonding is strongly localized to the Fe atom, whereas the other, which does, is extended and comparable in size to other acceptor states. Tight-binding calculations of these midgap states agree with the spatial structure of the measured wave functions and illustrate that such measurements can determine the degree of hybridization via $\pi$ bonding of impurity $d$ states. These single-dopant midgap states with strong $d$ character, which are intrinsically spin-orbit-entangled, provide an opportunity for probing and manipulating local magnetism and may be of use for high-speed electrical control of single spins.

The propagator of a dynamically screened Coulomb interaction $W$ in a sandwichlike structure consisting of two graphene layers separated by a slab of ${\mathrm{Al}}_2{\mathrm{O}}_3$ (or vacuum) is derived from single-layer graphene response functions and by using a local dielectric function for the bulk ${\mathrm{Al}}_2{\mathrm{O}}_3$. The response function of graphene is obtained using two approaches within the random phase approximation (RPA): an ab initio method that includes all electronic bands in graphene and a computationally less demanding method based on the massless Dirac fermion (MDF) approximation for the low-energy excitations of electrons in the $\pi$ bands. The propagator $W$ is used to derive an expression for the effective dielectric function of our sandwich structure, which is relevant for the reflection electron energy loss spectroscopy of its surface. Focusing on the range of frequencies from THz to mid-infrared, special attention is paid to finding an accurate optical limit in the ab initio method, where the response function is expressed in terms of a frequency-dependent conductivity of graphene. It was shown that the optical limit suffices for describing hybridization between the Dirac plasmons in graphene layers and the Fuchs-Kliewer phonons in both surfaces of the ${\mathrm{Al}}_2{\mathrm{O}}_3$ slab, and that the spectra obtained from both the ab initio method and the MDF approximation in the optical limit agree perfectly well for wave numbers up to about 0.1 ${\mathrm{nm}}^{-1}$. Going beyond the optical limit, the agreement between the full ab initio method and the MDF approximation was found to extend to wave numbers up to about 0.3 ${\mathrm{nm}}^{-1}$ for doped graphene layers with the Fermi energy of 0.2 eV.

The effects of induced optical polarization by Dirac electrons in graphene on the hybridization of radiative and evanescent fields are found. Such effects result in a localized polarization field which significantly modifies an incident surface-plasmon-polariton (SPP) field. This yields a high sensitivity to local dielectric environments and provides an investigative tool for molecules or proteins selectively bound with carbons. A scattering matrix is utilized with varied frequencies in the vicinity of the surface-plasmon (SP) resonance for the increase, decrease, and even a full suppression of the polarization field, which enables accurate effective-medium theories to be constructed for Maxwell-equation finite-difference time-domain methods. Moreover, double peaks in the absorption spectra for hybrid SP and graphene-plasmon modes are significant only with a large conductor plasma frequency, but are overshadowed by a round SPP peak at a small plasma frequency as the graphene is placed close to the conductor surface. These resonant absorptions facilitate polariton-only excitations, leading to polariton condensation for a threshold-free laser.

Motivated by the current interest in the understanding of the Mott insulators away from half-filling, observed in many perovskite oxides, we study the Mott metal-insulator transition in the doped Hubbard-Holstein model using the Hartree-Fock mean field theory. The Hubbard-Holstein model is the simplest model containing both the Coulomb and the electron-lattice interactions, which are important ingredients in the physics of the perovskite oxides. In contrast to the half-filled Hubbard model, which always results in a single phase (either metallic or insulating), our results show that away from half-filling, a mixed phase of metallic and insulating regions occurs. As the dopant concentration is increased, the metallic part progressively grows in volume, until it exceeds the percolation threshold, leading to percolative conduction. This happens above a critical dopant concentration ${\delta }_c$, which, depending on the strength of the electron-lattice interaction, can be a significant fraction of unity. This means that the material could be insulating even for a substantial amount of doping, in contrast to the expectation that doped holes would destroy the insulating behavior of the half-filled Hubbard model. While effects of fluctuation beyond the mean field remain an open question, our results provide a starting point for the understanding of the density-driven metal-insulator transition observed in many complex oxides.