We show that the macroscopic magnetic and electronic properties of strongly correlated electron systems can be manipulated by coupling them to a cavity mode. As a paradigmatic example we consider the Fermi-Hubbard model and find that the electron-cavity coupling enhances the magnetic interaction between the electron spins in the ground-state manifold. At half filling this effect can be observed by a change in the magnetic susceptibility. At less than half filling, the cavity introduces a next-nearest-neighbor hopping and mediates a long-range electron-electron interaction between distant sites. We study the ground-state properties with tensor network methods and find that the cavity coupling can induce a phase characterized by a momentum-space pairing effect for electrons.

We propose a modified interaction strength interpolation method that is more robust in the quasi-two-dimensional density regime and more accurate than the original method [M. Seidl et al., Phys. Rev. Lett. 84, 5070 (2000)] for the correlation energy and adiabatic integrand of small atoms. This method also allows us to construct correlation energy densities as simple local interpolations along the adiabatic connection. As an example, we develop a generalized gradient approximation correlation functional that performs in line with, and often better than, popular semilocal correlation functionals.

Learning from data has led to a paradigm shift in computational materials science. In particular, it has been shown that neural networks can learn the potential energy surface and interatomic forces through examples, thus bypassing the computationally expensive density functional theory calculations. Combining many-body techniques with a deep-learning approach, we demonstrate that a fully connected neural network is able to learn the complex collective behavior of electrons in strongly correlated systems. Specifically, we consider the Anderson-Hubbard (AH) model, which is a canonical system for studying the interplay between electron correlation and strong localization. The ground states of the AH model on a square lattice are obtained using the real-space Gutzwiller method. The obtained solutions are used to train a multitask multilayer neural network, which subsequently can accurately predict quantities such as the local probability of double occupation and the quasiparticle weight, given the disorder potential in the neighborhood as the input.

It is shown that closed magnetic barriers defined in two-dimensional electron gases generate a collimated emerging electron beam at the edge of the Hall bar. The collimation takes place through two mechanisms: electron selection for $\vec{E}\times \vec{B}$ drift at the edge and bulk collimation of the emerging electrons due to the magnetic field gradient. The collimation effect is verified by experiments and simulations: a second magnetic barrier used as a probe will reflect the collimated beam differently when positioned at various distances from the first one, resulting in oscillations in the resistance.

Planar microcavities filled with active materials containing excitonic resonances host radiative exciton-polariton (polariton) modes with in-plane wave vectors within the light cone. They also host at least one mode that is guided in the cavity plane by total internal reflection and that is not radiatively coupled to the vacuum modes except through defects or sample edges. We show that polariton lasing mediated by polariton stimulated scattering can occur concomitantly in both types of modes in a microcavity. By adjusting the detuning between the exciton and the radiative photon mode one can favor polariton lasing either in the radiative or in the guided modes. Our results suggest that the competition between these two types of polariton lasing modes may have played a role in many previous observations of polariton lasing and polariton Bose-Einstein condensation.

When slow ions incident on a surface are neutralized, the excess potential energy is passed on to an electron inside the surface, leading to emission of secondary electrons. The microscopic description of this process, as well as the calculation of the secondary electron yield, is a challenging problem due to its complexity as well as its sensitivity to surface properties. One of the first quantitative descriptions was articulated in the 1950s by Hagstrum, who based his calculation on a parametrization of the density of states of the material. In this paper, we present a model for calculating the secondary electron yield, derived from Hagstrum's initial approach. We use first-principles density functional theory calculations to acquire the necessary input and introduce the concept of electron cascades to Hagstrum's model in order to improve the calculated spectra, as well as remove its reliance on fitting parameters. We apply our model to ${\mathrm{He}}^{+}$ and ${\mathrm{Ne}}^{+}$ ions incident on $\mathrm{Ge}\left(111\right)$ and $\mathrm{Si}\left(111\right)$ and obtain yield spectra that match closely to the experimental results of Hagstrum.

We present first-principles calculations of structural, electronic, magnetic, and optical properties of zigzag-oriented silicene nanoribbons, which, being endowed with spin-polarized edge states, are promising candidates as building blocks of future spintronic devices. The minimal width for a structurally stable planar structure having zigzag edges corresponds to a 4-chain ribbon, whose ground state presents antiferromagnetically coupled spin-polarized edges, and a lattice parameter along the nanoribbon axis contracted ($\sim 5\%$) with respect to the bulk value. Starting from the dependence of structural and electronic properties on the ribbon width, we present theoretical predictions for the optical spectra of narrow nanoribbons, in which excitonic effects are relevant due to the confinement in a quasi-one-dimensional structure. Especially for light polarized parallel to the ribbon axis, we find significant differences in the position of optical absorption peaks of ribbons with ferro- or antiferromagnetically coupled edges, showing that optical spectra can be used as a fingerprint of the magnetic coupling of electronic edge states.

Nondiagonal (bond) disorder in graphene broadens Landau levels (LLs) in the same way as random potential. The exception is the zeroth LL, $n=0$, which is robust to the bond disorder, since it does not mix different $n=0$ states within a given valley. The mechanism of broadening of the $n=0$ LL is the intervalley scattering. Several numerical simulations of graphene with bond disorder had established that $n=0$ LL is not only anomalously narrow but also that its shape is very peculiar with three maxima, one at zero energy, $E=0$, and two others at finite energies $\pm E$. We study theoretically the structure of the states in $n=0$ LL in the presence of bond disorder. Adopting the assumption that the bond disorder is strongly anisotropic, namely, that one type of bonds is perturbed much stronger than other two, allowed us to get an analytic expression for the density of states which agrees with numerical simulations remarkably well. On the qualitative level, our key finding is that delocalization of the $E=0$ state has a dramatic back effect on the density of states near $E=0$. The origin of this unusual behavior is the strong correlation of eigenstates in different valleys.

We show theoretically that conversion between spin and charge by spin-orbit interaction in metals occurs even in a nonlocal setup where magnetization and spin-orbit interaction are spatially separated if electron diffusion is taken into account. Calculation is carried out for the Rashba spin-orbit interaction treating the coupling with a ferromagnet perturbatively. The results indicate the validity of the concept of effective spin gauge field (spin motive force) in the nonlocal configuration. The inverse Rashba-Edelstein effect observed for a trilayer of a ferromagnet, a normal metal and a heavy metal can be explained in terms of the nonlocal effective spin gauge field.

The physical properties of the single-crystalline ${\mathrm{U}}_2{\mathrm{Ir}}_3{\mathrm{Si}}_5$, a new ternary uranium compound with ${\mathrm{U}}_2{\mathrm{Co}}_3{\mathrm{Si}}_5$-type orthorhombic structure, are investigated by means of magnetic susceptibility $\chi \left(T\right)$, specific heat $C\left(T\right)$, electrical resistivity $\rho \left(T\right)$, and high-field magnetization $M(H,T)$ measurements. ${\mathrm{U}}_2{\mathrm{Ir}}_3{\mathrm{Si}}_5$ undergoes an antiferromagnetic transition at $T_N=36$ K followed by a first-order phase transition at $T_0=25.5$ K. The sharp peak in $C\left(T\right)$ at $T_0$ and the obvious hysteresis behavior in $\chi \left(T\right),\rho \left(T\right)$, and $M(H,T)$ around $T_0$ provide strong evidence for the first-order phase transition. The $\rho \left(T\right)$ measurements along the $a$ and $b$ axes reveal the negative temperature coefficients of resistance over a wide temperature range, which can be understood based on the semiconductorlike narrow band gap model or the Kondo effect. The $M\left(H\right)$ curve measured at 4.2 K along the $b$ axis shows three-step metamagnetic transitions within a narrow field region around 200 kOe and a large hysteresis near the first transition field, while the $M_{H\parallel a}\left(H\right)$ and $M_{H\parallel c}\left(H\right)$ curves show no transitions up to 560 kOe suggesting the strong magnetic anisotropy. A possible mechanism of the first-order phase transition at $T_0$ is the occurrence of a magnetic quadrupolar order, resulting from the quasi-one-dimensional uranium zigzag chain.

We show that the diagrammatic approach to quantum spin systems developed in a seminal work by Vaks, Larkin, and Pikin [Sov. Phys. JETP 26, 188 (1968)] can be embedded in the framework of the functional renormalization group. The crucial insight is that the generating functional of the time-ordered connected spin correlation functions of an arbitrary quantum spin system satisfies an exact renormalization group flow equation which resembles the corresponding flow equation of interacting bosons. The $SU\left(2\right)$ spin algebra is implemented via a nontrivial initial condition for the renormalization group flow. Our method is rather general and offers a different nonperturbative approach to quantum spin systems.

Using small-angle neutron scattering, we investigated the behavior of a metastable vortex lattice state in ${\mathrm{MgB}}_2$ as it is driven towards equilibrium by an ac magnetic field. This shows an activated behavior, where the ac field amplitude and cycle count are equivalent to, respectively, an effective “temperature” and “time.” The activation barrier increases as the metastable state is suppressed, corresponding to an aging of the vortex lattice. Furthermore, we find a crossover from a partial to a complete suppression of metastable domains depending on the ac field amplitude, which may empirically be described by a single free parameter. This represents an unconventional kind of collective vortex behavior, not governed by pinning, most likely due to the nucleation and growth of equilibrium vortex lattice domains.

A high-throughput screening based on first-principles calculations was performed to search for new ternary inorganic electrides. From the available materials database, we identified three new thermodynamically stable materials (${\mathrm{Li}}_{12}{\mathrm{Mg}}_3{\mathrm{Si}}_4, {\mathrm{NaBa}}_2\mathrm{O}$, and ${\mathrm{Ca}}_5{\mathrm{Ga}}_2{\mathrm{N}}_4$) as potential electrides made by main group elements, in addition to the well known mayenite based electride (C12A7:$e^{-}$). Different from those conventional inorganic electrides in which the excess electrons play only the role of anions, the three new materials, resembling the electrides found in simple metals under high pressure, possess mixed ionic and metallic bonding. The interplay between two competing mechanisms, together with the different crystal packing motifs, gives rise to a variety of geometries in anionic electrons and rich physical phenomena such as ferromagnetism, superconductivity, and metal-insulator transition. Our finding here bridges the gap between electrides found at ambient and high-pressure conditions.

Unlike monolayer Fe-Chalcogenide (Fe-$Ch$)$/\mathrm{SrTi}{\mathrm{O}}_3$ (STO), which possesses the potential for high-temperature superconductivity (HTS), a regular Fe-$Ch$ thin film grown on a non-STO substrate by the pulsed laser deposition method shows totally different superconducting behavior and a different mechanism. Although regular Fe-$Ch$ thick films grown on $\mathrm{Ca}{\mathrm{F}}_2$ generally show the highest superconducting transition temperature ($T_{\mathrm{c}}$) compared with any other substrates, the disappearance of superconductivity always takes place when the thickness of the Fe-$Ch$ film is reduced to a critical value ($\sim 20\mathrm{nm}$ for Fe-Se and $\sim 30\mathrm{nm}$ for Fe-Se-Te) with the reason still under debate. Here, we report an enhanced $T_{\mathrm{c}}\approx 17.6\mathrm{K}$ in a 7-nm-FeTe/7-nm-FeSe bilayer heterostructure grown on $\mathrm{Ca}{\mathrm{F}}_2$ substrate. Generally, the Fe-$Ch$ film on $\mathrm{Ca}{\mathrm{F}}_2$ is supposed to be one order of magnitude greater in thickness to achieve similar performance. Hall measurements manifest the dominant nature of hole-type carriers in the films in this work, which is similar to the case of a pressurized bulk FeSe single crystal, while in sharp contrast to heavily electron-doped HTS Fe-$Ch$ systems. According to the electron energy loss spectroscopy results, we observed direct evidence of nanoscale phase separation in the form of a fluctuation of the Fe-$L_3/L_2$ ratio near the FeTe/FeSe interface. In detail, a several-unit-cell-thick Fe(Se,Te) diffusion layer shows a higher Fe-$L_3/L_2$ ratio than either an FeTe or an FeSe layer, indicating low Fe $3d$ electron occupancy, which is, to some extent, consistent with the hole-dominant scenario obtained from the Hall results. It also implies a possible relationship between the state of Fe $3d$ electron occupancy and the enhanced $T_{\mathrm{c}}$ in this work. Our work clarifies the importance of the FeTe/FeSe interface in reviving the superconductivity in Fe-$Ch$ ultrathin films, contributing to a more unified understanding of unconventional Fe-$Ch$ superconductivity.

In materials with nearly commensurate band filling the electron liquid may spontaneously separate into components with distinct properties, yielding complex intra- and interunit cell ordering patterns and a reduced dimensionality. Polarization-dependent angle-resolved photoemission data with submicron spatial resolution demonstrate such an electronic self-organization in ${\mathrm{NbSe}}_3$, a compound considered to be a paradigm of charge order. The new data indicate the emergence of a novel order, and reveal the one-dimensional (1D) physics hidden in a material which naively could be considered the most three dimensional of all columnar chalcogenides. The 1D physics is evidenced by a new selection rule—in two polarizations we observe two strikingly different dispersions each closely resembling apparently contradicting results of previous studies of this material.

Similarity transformation of the Hubbard Hamiltonian using a Gutzwiller correlator leads to a non-Hermitian effective Hamiltonian, which can be expressed exactly in momentum-space representation and contains three-body interactions. We apply this methodology to study the two-dimensional Hubbard model with repulsive interactions near half filling in the intermediate interaction strength regime $\left(U/t=4\right)$. We show that at optimal or near optimal strength of the Gutzwiller correlator, the similarity-transformed Hamiltonian has extremely compact right eigenvectors, which can be sampled to high accuracy using the full configuration interaction quantum Monte Carlo (FCIQMC) method and its initiator approximation. Near-optimal correlators can be obtained using a simple projective equation, thus obviating the need for a numerical optimization of the correlator. The FCIQMC method, as a projective technique, is well suited for such non-Hermitian problems, and its stochastic nature can handle the three-body interactions exactly without undue increase in computational cost. The highly compact nature of the right eigenvectors means that the initiator approximation in FCIQMC is not severe and that large lattices can be simulated, well beyond the reach of the method applied to the original Hubbard Hamiltonian. Results are provided in lattice sizes up to 50 sites and compared to auxiliary-field QMC. New benchmark results are provided in the off-half-filling regime, with no severe sign problem being encountered. In addition, we show that methodology can be used to calculate excited states of the Hubbard model and lay the groundwork for the calculation of observables other than the energy.

The vibrational thermal conductances ($G$) across GaN-AlN interfaces are computed using a nonequilibrium Green's function formalism in the harmonic limit with bulk and interfacial interatomic force constants (IFCs) fully from density functional theory. Several numerical methods and supercell configurations are employed to examine the sensitivity of $G$ to variances of IFCs. In particular, the effects of supercell size, the enforcement of symmetry constraints, and truncation of IFCs near the interface, and atomic relaxation on phonon transmission and conductance are explored. Our fully first-principles calculations are compared with common approximations and measured $G$ values inferred from thermal conductivity measurements for GaN-AlN superlattices. Our calculated value, $G\sim 300\mathrm{MW}{\mathrm{m}}^{-2}{\mathrm{K}}^{-1}$, is nearly half that from measurements. This discrepancy is critically analyzed in terms of the physical assumptions of the calculations and the derivation of the experimental values. This work provides guidelines to determine “physically correct” sets of interfacial IFCs from first principles for thermal conductance calculations using minimal computational resources. It also contributes toward developing predictive calculations and a more complete picture of thermal conduction across interfaces, a step toward first-principles multiscale thermal transport.

Transport spectroscopy of two-spin states in a double quantum dot can be performed by an ac electric field which tunes the energy detuning. However, a problem arises when the transition rate between the states is small and, consequently, the ac-induced current is suppressed. Here we show that if the ac field tunes the interdot tunnel barrier, then for large detuning the transition rate increases drastically, resulting in high current. Multiphoton resonances are enhanced by orders of magnitude. Our study demonstrates an efficient way for fast two-spin transitions.

We study the luminescence of hexagonal boron nitride $\left(h\text{−}\mathrm{BN}\right)$ by means of nonequilibrium Green's functions plus finite-difference electron-phonon coupling. We derive a formula for light emission in solids in the limit of a weak excitation that includes perturbatively the contribution of electron-phonon coupling at the first order. This formula is applied to study luminescence in bulk $h\text{−}\mathrm{BN}$. This material has attracted interest due to its strong luminescence in the ultraviolet region of the electromagnetic spectrum [K. Watanabe et al., Nat. Mater. 3, 404 (2004)]. The origin of this intense luminescence signal has been widely discussed, but only recently a clear signature of phonon-mediated light emission started emerging from the experiments [G. Cassabois et al., Nat. Photonics 10, 262 (2016)]. By means of our theoretical framework, we provide a clear and full explanation of light emission in $h\text{−}\mathrm{BN}$.

Parafermions are emergent quasiparticles which generalize Majorana fermions and possess intriguing anyonic properties. The theoretical investigation of effective models hosting them is gaining considerable importance in view of present-day condensed-matter realizations where they have been predicted to appear. Here we study the simplest number-conserving model of particlelike Fock parafermions, namely a one-dimensional tight-binding model. By means of numerical simulations based on exact diagonalization and on the density-matrix renormalization group, we prove that this quadratic model is nonintegrable and displays bound states in the spectrum due to its peculiar anyonic properties. Moreover, we discuss its many-body physics, characterizing anyonic correlation functions and discussing the underlying Luttinger-liquid theory at low energies. In the case when Fock parafermions behave as fractionalized fermions, we are able to unveil interesting similarities with two counterpropagating edge modes of two neighboring Laughlin states at filling $1/3$.