To observe, by microscopic probe, how low Zn doping in $\mathrm{F}{\mathrm{e}}_{3-x}\mathrm{Z}{\mathrm{n}}_x{\mathrm{O}}_4$ ($x$ is below 1%) changes the Verwey transition, we have performed Mössbauer spectroscopy measurements on three single crystalline samples with various Zn doping. In spectra analysis we used the recently published model of Mössbauer data treatment formulated as a result of ab initio calculations for a low-temperature monoclinic structure (of Cc symmetry) of magnetite. It was suggested there that the hyperfine parameters for all 24 Fe distinct positions in the lattice can be grouped into four major components with very similar hyperfine parameters within each set. Using these parameters as starting values, very good fits were obtained for magnetite with low doping level, while for higher doping, $x=0.03$, where the Verwey transition changes its character, one component is significantly different. In particular, low hyperfine field $B_{\mathrm{eff}}=36\mathrm{T}$, considered as a characteristic feature of the Cc phase spectrum, is absent here. Also, in this case, the high-temperature spectra are different from those for lower doped magnetite showing more pronounced continuous alteration with temperature. This might be due to crystal structure of lower than Fd-3m symmetry, a fact suggested by our x-ray synchrotron studies. All this triggered a discussion about an experimental fingerprint for the difference between these two classes of magnetite, frequently referred to as magnetite of first- and second-order Verwey transition, and about the electronic structure of both kinds of systems.

We calculate the ground-state energy, pair correlation function, static structure factor, and momentum density of the one-dimensional electron fluid at high density using variational quantum Monte Carlo simulation. For an infinitely thin cylindrical wire the predicted correlation energy is found to fit nicely with a quadratic function of coupling parameter $r_s$. The extracted exponent $\alpha$ of the momentum density for $k\sim k_F$ is used to determine the Tomonaga-Luttinger parameter $K_{\rho }$ as a function of $r_s$ in the high-density regime. We find that the simulated static structure factor and pair correlation function for infinitely thin wires agree with our recent high-density theory [K. Morawetz et al., Phys. Rev. B 97, 155147 (2018)].

We investigate the induced orbital magnetization density in a Rashba electron gas with magnetic impurities. Relying on classical electrodynamics, we obtain this quantity through the bound currents composed of paramagnetic and diamagneticlike contributions which emerge from the spin-orbit interaction. Similar to Friedel charge ripples, the bound currents and the orbital magnetization density oscillate as a function of distance away from the impurity with characteristic wavelengths defined by the Fermi energy and the strength of the Rashba spin-orbit interaction. The net induced orbital magnetization was found to be of the order of magnitude of its spin counterpart. In addition to the exploration of the impact of the electronic filling of the impurity states, we investigate and analyze the orbital magnetization induced by an equilateral frustrated trimer in various noncollinear magnetic states. On the one hand, we confirm that nonvanishing three-spin chiralities generate a finite orbital magnetization density. On the other hand, higher-order contributions lead to multiple-spin chiralities affecting nontrivially and significantly the overall magnitude and sign of the orbital magnetization. This study substantiates the notion that the orbital degrees of freedom are an essential aspect of nanoscale magnetism, calling for further theoretical and experimental attention.

Methods that are devised to achieve reversal of quantum dynamics in time have been named “quatum time mirrors.” Such a time mirror can be considered as a generalization of Hahn's spin echo to systems with continuous degrees of freedom. We extend the quantum time mirror protocol originally proposed for Dirac dispersions to arbitrary two-band systems and establish the general requirements for its efficient implementation. We further discuss its sensitivity to various nonhomogeneous perturbations including disorder potentials and the effect of external static magnetic and electric fields. Our general statements are verified for a number of exemplary Hamiltonians, whose phase-coherent dynamics are studied both analytically and numerically. The Hamiltonians considered can be used to describe the low-energy properties of systems as diverse as cold atom-optics setups, direct band gap semiconductors, or (mono- or bilayer) graphene. We discuss the consequences of many-body effects at a qualitative level, and consider the protocol feasibility in state-of-the-art experimental setups.

We study the spin injection efficiency into single and bilayer graphene on the ferrimagnetic insulator yttrium-iron-garnet (YIG) through an exfoliated tunnel barrier of bilayer hexagonal boron nitride (hBN). The contacts of two samples yield a resistance-area product between 5 and 30 $\mathrm{k}\mathrm{\Omega }\mu {\mathrm{m}}^2$. Depending on an applied dc bias current, the magnitude of the nonlocal spin signal can be increased or suppressed below the noise level. The differential spin injection efficiency reaches values from $-60\%$ to $+25\%$. The results are confirmed with both spin valve and spin precession measurements. The proximity induced exchange field is found in single layer graphene on YIG to be ($85\pm 30$) mT and in bilayer graphene on YIG close to the detection limit. Our results show that the exceptional spin injection properties of bilayer hBN tunnel barriers reported by Gurram et al. [Nat. Commun. 8, 248 (2017)] are not limited to fully encapsulated graphene systems but are also valid in graphene/YIG devices. This further emphasizes the versatility of bilayer hBN as an efficient and reliable tunnel barrier for graphene spintronics.

The unconventional exchange-bias (EB) effect in single crystals of $R\mathrm{Fe}{\mathrm{O}}_3\left(R=\mathrm{Nd},\mathrm{Sm}\right)$ compensated ferrimagnets (fMs), composed of two antiferromagnetically (AFM) coupled $R$ and Fe sublattices with opposite ferromagnetic (FM) moments, exhibiting the FM moment reversal via a fast spin switching is reported. In ${\mathrm{NdFeO}}_3$, the EB anisotropy field emerges and diverges upon approaching the compensation temperature $T_{\mathrm{comp}}^{\mathrm{Nd}}=9.2\mathrm{K}$, and changes sign with crossing $T_{\mathrm{comp}}^{\mathrm{Nd}}$, in similarity to the behavior observed recently in ${\mathrm{ErFeO}}_3$. In contrast, ${\mathrm{SmFeO}}_3$ exhibits a substantial EB, not only at its $T_{\mathrm{comp}}^{\mathrm{Sm}}=4.8\mathrm{K}$, but also at higher temperatures up to 100 K, and the EB changes its sign with increasing cooling field. It is shown that in all known $R{\mathrm{FeO}}_3$ ($R=\mathrm{Nd},\mathrm{Sm},\mathrm{Er}$) compensated fMs the field-induced FM moment reversal is similarly exchange biased near $T_{\mathrm{comp}}$.

We have performed ${}^{119}\mathrm{Sn}$-NMR measurements on the antiperovskite oxide superconductor ${\mathrm{Sr}}_{3-x}\mathrm{SnO}$ to investigate how its normal state changes with the Sr deficiency. A two-peak structure was observed in the NMR spectra of all the measured samples. This suggests that the phase separation tends to occur between the nearly stoichiometric and heavily Sr-deficient ${\mathrm{Sr}}_{3-x}\mathrm{SnO}$ phases. The measurement of the nuclear spin-lattice relaxation rate $1/T_1$ indicates that the Sr-deficient phase shows a conventional metallic behavior due to the heavy hole doping. In contrast, the nearly stoichiometric phase exhibits unusual temperature dependence of $1/T_1$, attributable to the presence of a Dirac-electron band.

As out-of-equilibrium materials, glasses continually tend to relax toward the metastable supercooled liquid state. Glass relaxation can result in a nonreversible glass transition upon a cooling/reheating cycle. Here, based on molecular dynamics simulations, we present a methodology combining thermal cycles and inherent configuration analysis to investigate the features of relaxation and glass transition reversibility. By considering three archetypical silicate glasses, viz., silica, sodium silicate, and calcium aluminosilicate, we show that, for all the glasses considered herein, the enthalpy relaxation can be well described by mode-coupling theory. Further, we demonstrate the existence of a decoupling between enthalpy and volume relaxation. Finally, we show that enthalpy relaxation results in a nonreversible glass transition—the degree of nonreversibility being strongly system-specific.

We study dynamic response of a Fermi liquid in the spin, charge, and nematic channels beyond the random phase approximation for the dynamically screened Coulomb potential. In all the channels, one-loop order corrections to the irreducible susceptibility result in a nonzero spectral weight of the corresponding fluctuations above the particle-hole continuum boundary. It is shown that the imaginary part of the spin susceptibility, $\mathrm{Im}{\chi }_s(\mathbf{q},\omega )$, falls off as $q^2/\omega$ for frequencies above the continuum boundary ($\omega \gg v_{\mathrm{F}}q$) and below the model-dependent cutoff frequency, whereas the imaginary part of the charge susceptibility, $\mathrm{Im}{\chi }_c(\mathbf{q},\omega )$, falls off as ${(q/k_F)}^2{q}^2/\omega$ for frequencies above the plasma frequency. An extra factor of ${(q/k_F)}^2$ in $\mathrm{Im}{\chi }_c(\mathbf{q},\omega )$ as compared to $\mathrm{Im}{\chi }_s(\mathbf{q},\omega )$ is a direct consequence of Galilean invariance. The imaginary part of the nematic susceptibility increases linearly with $\omega$ up to a peak at the ultraviolet energy scale—the plasma frequency and/or Fermi energy—and then decreases with $\omega$. We also obtain explicit forms of the spin susceptibility from the kinetic equation in the collisionless limit and for the Landau function that contains up to the first three harmonics.

Composite fermions (CFs) of the fractional quantum Hall effect (FQHE) are described as spherical products of electron and vortex spinors, built from underlying $L=1/2$ ladder operators aligned so that the spinor angular momenta, $L_e$ and $L_v$, are maximal. We identify the CF's quantum numbers as the angular momentum $L$ in $\left(L_e{L}_v\right)L$, its magnetic projection $m_L$, the electron number $N [L_v=(N-1)/2]$, and magnetic $\nu$ spin, $m_{\nu }=L_e-L_v$. Translationally invariant FQHE states are formed by fully filling $p$ subshells with their respective CFs, in order of ascending $L$ for fixed $L_e$ and $L_v$, beginning with the lowest allowed value, $L=|m_{\nu }|$. CF subshells are contained entirely within the first Landau level (FLL). Alternatively, we provide an equivalent hierarchical wave function in which the underlying objects are vortices with $L_v=\frac{p}{2}$, correlated pairwise via ${\vec{L}}_{v_i}·{\vec{L}}_{v_j}$. We show that CFs can be written as a valence operator carrying the angular momentum quantum numbers $L,m_{\nu },m_L$ acting on a scalar half-filled intrinsic state. This scalar state serves as the vacuum for the valence electron (${\mathit{b}}^{†},\tilde{\mathit{b}}$) and vortex (${\mathit{v}}^{†},\tilde{\mathit{v}}$) ladder operators that create FQHE states. With respect to this vacuum, FQHE states can be grouped into $\nu$-spin multiplets mirror symmetric around $m_{\nu }=0$, in which $N$ is held constant. $m_{\nu }\ne 0$ states have a net electron particle or hole number. Particle-hole conjugation with respect to this vacuum is identified as the mirror symmetry relating FQHE states of the same $N$ but distinct fillings $\nu =p/(2p+1)$ and $\overline{\nu }=p/(2p-1)$, e.g., $2/5\leftrightarrow 2/3$. Alternatively, mirror symmetric $\nu$-spin multiplets can be constructed in which the magnetic field strength is held fixed: the valence states are electron particle-vortex hole excitations relative to the half-filled vacuum ($m_{\nu }>0$) and their mirror conjugates ($m_{\nu }<0$). Multiplet members are linked by the $\nu$-spin raising/lowering operators, ${\widehat{S}}_{\pm }^{\nu }$. Particle-hole (PH) symmetry—relating the $N$-particle FQHE state $\mathrm{\Psi }$ of filling $\nu =p/(2p+1)$ to the $\overline{N}$-particle state $\overline{\mathrm{\Psi }}$ of filling $\overline{\nu }=(p+1)/(2p+1)$, e.g., $2/5\leftrightarrow 3/5$—is shown to be equivalent to electron-vortex exchange, ${\mathit{b}}^{†}\leftrightarrow {\mathit{v}}^{†}$ and $\tilde{\mathit{b}}\leftrightarrow \tilde{\mathit{v}}$. The $N$-particle states $\mathrm{\Psi }$ and ${\widehat{S}}_{-}\overline{\mathrm{\Psi }}$ are connected by this mirror symmetry. In this construction, $\overline{N}-N$ CFs of the state $\overline{\mathrm{\Psi }}$ occupy an extra zero-mode subshell that is annihilated by ${\widehat{S}}_{-}^{\nu }$. We link this structure, familiar from supersymmetric quantum mechanics, to the CF Pauli Hamiltonian, which we show is isospectral, quadratic in the $\nu$-spin raising and lowering operators ${\widehat{S}}_{\pm }^{\nu }$, and fourfold degenerate in $\mathrm{\Psi },{\widehat{S}}_{-}^{\nu }\mathrm{\Psi },\overline{\mathrm{\Psi }}$, and ${\widehat{S}}_{-}^{\nu }\overline{\mathrm{\Psi }}$. On linearization, it takes a Dirac form similar to that found in the integer quantum Hall effect (IQHE).

Magnesium silicides are favorable thermoelectric materials considering resource abundance and cost. Chromium (Cr) doping in magnesium silicides has not yet been explored. Using first-principles calculations, we have studied the stability of ${\mathrm{Mg}}_2\mathrm{Si}$ with chromium (1.85, 3.7, 5.55, and 6.25% Cr) and tin (12.5 and 50% Sn). Three ${\mathrm{Mg}}_2\mathrm{Si}$ compounds doped with Sn, (Sn + Bi), and (Sn + Bi + Cr) are used to explain doping effects on thermoelectric performance. Notably, Cr behaves nonmagnetically for $\le 2\%\mathrm{Cr}$, after which ferromagnetic ordering is favored ($\le 12.96\%$ Cr), despite its elemental antiferromagnetic state. With alloying of Sn (70.4%), ${\mathrm{Mg}}_2\mathrm{Si}$ remains an indirect-band-gap semiconductor, but adding small amounts of Bi (3.7%) increases the carrier concentration such that electrons occupy conduction bands, making it a degenerate semiconductor. ${\mathrm{Mg}}_2{\mathrm{Si}}_{0.296}{\mathrm{Sn}}_{0.666}{\mathrm{Bi}}_{0.037}$ is found to give the highest thermoelectric figure of merit (ZT) and power factor (PF) at 700 K, i.e., 1.75 and 7.04 $\text{mW}{\text{m}}^{-1}{\text{K}}^{-2}$, respectively. Adding small %Cr decreases ZT and PF to 0.78 and 4.33 $\text{mW}{\text{m}}^{-1}{\text{K}}^{-2}$, respectively. Such a degradation in thermoelectric (TE) performance is attributed to two factors: (i) uniform doping acting as an electron acceptor, decreasing conduction, and (ii) the loss of low-lying conduction band degeneracy with doping, decreasing the Seebeck coefficients. A study of configurations of Cr doping suggests that Cr has a tendency to form clusters inside the lattice, which play a crucial role in tuning the magnetic and TE performance of doped ${\mathrm{Mg}}_2\mathrm{Si}$ compounds.

Electromagnetic cloaking for large objects in free space using a lossless impenetrable tensor impedance surface is proposed. The key enabling feature is enforcement of the pointwise lossless condition via introduction and optimization of surface waves that carry power along the curved bounding surface of the cloaked region. In a pointwise lossless and reciprocal manner, the cloaking surface converts the incident plane wave into surface waves on the lit side, carries the power along the surface into the shadow side, and continuously releases a propagating wave to reconstruct the incident field behind the object. Overcoming the difficulties associated with currently available active and passive metasurface cloaks, the proposed cloaks may be realized entirely using passive constituents and they are effective for electrically large free-standing objects that cast shadows. The surface cloaks may be realized as subwavelength-thin passive tensor gradient metasurfaces.

Atomically thin ferromagnetic (FM) films were recently prepared by mechanical exfoliation of bulk FM semiconductor ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$. They provide a platform to explore novel two-dimensional (2D) magnetic phenomena, and they offer exciting prospects for new technologies. By performing systematic ab initio density functional calculations, here we study two relativity-induced properties of these 2D materials (monolayer, bilayer, and trilayer as well as bulk), namely magnetic anisotropy energy (MAE) and magneto-optical (MO) effects. Competing contributions of both magnetocrystalline anisotropy energy (C-MAE) and magnetic dipolar anisotropy energy (D-MAE) to the MAE are computed. The calculated MAEs of these materials are large, being on the order of $\sim 0.1$ meV/Cr. Interestingly, we find that out-of-plane magnetic anisotropy is preferred in all the systems except the monolayer, where in-plane magnetization is favored because here the D-MAE is larger than the C-MAE. Crucially, this explains why long-range FM order was observed in all the few-layer ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ except the monolayer because the out-of-plane magnetic anisotropy would open a spin-wave gap and thus suppress magnetic fluctuations so that long-range FM order could be stabilized at finite temperature. In the visible frequency range, large Kerr rotations up to $\sim 2.2^{\circ }$ in these materials are predicted, and they are comparable to that observed in famous MO materials such as PtMnSb and ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$. Moreover, they are $\sim 100$ times larger than that of $3d$ transition metal monolayers deposited on Au surfaces. Faraday rotation angles in these 2D materials are also large, being up to $\sim {120}^{\circ }/\mu \mathrm{m}$, and they are thus comparable to the best-known MO semiconductor ${\mathrm{Bi}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$. These findings thus suggest that with large MAE and MO effects, atomically thin ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ films would have potential applications in novel magnetic, MO, and spintronic nanodevices.

We explore various aspects of magnetoconductance oscillations in semiconductor nanowires, developing quantum transport models based on the nonequilibrium Green's function formalism. In the clean case, Aharonov-Bohm (AB, $h/e$) oscillations are found to be dominant, contingent upon the surface confinement of electrons in the nanowire. We also numerically study disordered nanowires of finite length, extending the existing literature. By varying the nanowire length and disorder strength, we identify the transition where Al'tshuler-Aronov-Spivak (AAS, $h/2e$) oscillations start dominating, noting the effects of considering an open system. Moreover, we demonstrate how the relative magnitudes of the scattering length and the device dimensions govern the relative dominance of these harmonics with energy, revealing that the AAS oscillations emerge and start dominating from the center of the band, much higher in energy than the conduction band edge. We also show the ways of suppressing the oscillatory components (AB and AAS) to observe the nonoscillatory weak localization corrections, noting the interplay of scattering, incoherence/dephasing, the geometry of electronic distribution, and orientation of magnetic field. This is followed by a study of surface roughness which shows contrasting effects depending on its strength and type, ranging from magnetic depopulation to strong AAS oscillations. Subsequently, we show that dephasing causes a progressive degradation of the higher harmonics, explaining the reemergence of the AB component even in long and disordered nanowires. Lastly, we show that our model qualitatively reproduces the experimental magnetoconductance spectrum of Holloway et al. [Phys. Rev. B 91, 045422 (2015)] reasonably well while demonstrating the necessity of spatial correlations in the disorder potential and dephasing.

Recent experiments with film-coupled nanoparticles suggest that the impact of spatial dispersion is enhanced in plasmonic structures where high wave-vector guided modes are excited. More advanced descriptions of the optical response of metals than Drude's are thus probably necessary in plasmonics. We show that even in classical prism coupler experiments, the plasmonic enhancement of spatial dispersion can be leveraged to make such experiments two orders of magnitude more sensitive. The realistic multilayered structures involved rely on layers that are thick enough to rule our any other phenomenon as the spill-out. Optical evanescent excitation of plasmonic wave guides using prism couplers thus constitutes an ideal platform to study spatial dispersion.

Two-dimensional (2D) materials are intrinsically anisotropic, and an accurate description of their out-of-plane response to an electromagnetic field is more and more important as new materials with diverse properties are proposed. Their electromagnetic properties are often modeled using a single sheet with a surface susceptibility or conductivity or by means of a thin film of finite thickness with an effective bulk permittivity. The discordances between these two approaches lead to two irreconcilable interpretations of the optical characterizations and uncertain predictions of electromagnetic responses. Here, we fully account for the particular anisotropy of 2D materials and reconcile both approaches. We propose a unified description for the electromagnetic properties that applies to 2D heterostructures for all polarizations and at all angles of incidence. In particular, we determine the class of materials for which both models can be used indifferently and when particular care should be taken to select the thickness and the tensorial response of the effective thin film. We illustrate our conclusions on extensively studied experimental quantities such as transmittance and ellipsometric data of graphene and metal dichalcogenides. We discuss similarities and discrepancies reported in the literature when single-sheet or thin-film models are used.

A direct and local deep learning (DL) model for atomic forces is presented. We demonstrate the model performance in bulk aluminum, sodium, and silicon and show that its errors are comparable to those found in state-of-the-art machine learning and DL models. We then analyze the model's performance as a function of the number of neighbors included and show that one can ascertain physical attributes of the system from the analysis of the deep learning model's behavior. Finally, we test the size scaling performance of the model and the transferability between different temperatures and show that our model performs well in both scaling to larger systems and high- to low-temperature predictability.

A phase-field model is implemented to investigate the microstructure formation and evolution of ferromagnetic alloys consisting of ferromagnetic nanoprecipitates of low-symmetry phase coherently embedded into a ferromagnetic cubic matrix. It is shown that the nanodispersions produced at the early stage of decomposition are structurally in single-domain state because their nanosize makes the formation of multidomain state energetically unfavorable. An application of magnetic fields could rearrange the crystal lattices within the nanosized precipitates through a gradual switching of the crystal orientations in the case of strong elastic anisotropy and magnetocrystalline coupling. The resultant macroscopic strain responses (magnetostrictions) could be largely enhanced and low hysteretic. We also formulate conditions for finding such materials with a combination of excellent mechanical and functional properties but free from the rare-earth elements.

The impact of geometry on many-body localization (MBL) is studied on simple, exemplary systems amenable to exact diagonalization treatment. The crossover between ergodic and MBL phase for uniform as well as quasirandom disorder is analyzed using energy-level statistics. It is observed that the transition to many-body localized phase is correlated with the number of nearest coupled neighbors. The crossover from extended to localized systems is approximately described by the so called plasma model.

SnSe has drawn considerable attention on a global scale due to its intrinsic low thermal conductivity and large figure of merit along the $b$ axis. In Na-doped SnSe, further enhancement of the thermoelectric performance has been reported. Using angle-resolved photoemission spectroscopy and inelastic neutron scattering, we have studied how electronic structures and lattice dynamics evolve with temperature in Na-doped SnSe. Our data show that the effective mass of the Se $p_z$ orbital along the $\mathrm{\Gamma }\text{−}Z$ direction has a very weak temperature dependence, while the chemical potential shifts significantly along with the increase in the gap size evidenced by infrared absorption measurements. Inelastic neutron scattering reveals one acoustic TA and two low-lying optical (TO1 and TO2) phonon modes. Their temperature-dependent behaviors indicate that the TO1 and TA modes contribute more to the reduction of the lattice thermal conductivity with temperature increases. The estimated value of the lattice thermal conductivity based on the lattice dynamics is significantly larger than that determined by transport measurements, suggesting that extrinsic factors, such as the imperfection of the lattice, could drastically suppress the lattice thermal conductivity. Our data suggest that temperature-dependent properties of both electronic structures and phonon dynamics need to be taken into account for the investigation of the underlying physics of hole-doped SnSe.