We connect a family of gauge theories (Maxwell theories with a magnetoelectric coupling $\theta =2\pi k,k\in \mathbb{Z}$) to the family of 3D topological lattice models introduced by Walker and Wang. In particular, we show that the lattice Hamiltonians capture a certain strong-coupling limit of these gauge theories, in which the system enters a gapped (confined) phase. We discuss the relationship between the topological order exhibited by certain of these lattice Hamiltonians and the characteristic electromagnetic response of the symmetry-protected bosonic topological insulator.

Nonequilibrium dynamical mean-field theory (DMFT) solves correlated lattice models by obtaining their local correlation functions from an effective model consisting of a single impurity in a self-consistently determined bath. The recently developed mapping of this impurity problem from the Keldysh time contour onto a time-dependent single-impurity Anderson model (SIAM) [C. Gramsch et al., Phys. Rev. B 88, 235106 (2013)] allows one to use wave-function-based methods in the context of nonequilibrium DMFT. Within this mapping, long times in the DMFT simulation become accessible by an increasing number of bath orbitals, which requires efficient representations of the time-dependent SIAM wave function. These can be achieved by the multiconfiguration time-dependent Hartree (MCTDH) method and its multilayer extensions. We find that MCTDH outperforms exact diagonalization for large baths in which the latter approach is still within reach and allows for the calculation of SIAMs beyond the system size accessible by exact diagonalization. Moreover, we illustrate the computation of the self-consistent two-time impurity Green's function within the MCTDH second quantization representation.

The transport and thermoelectric properties of the interface between ${\mathrm{SrTiO}}_3$ and a 26-monolayer-thick ${\mathrm{LaAlO}}_3$ layer grown at high oxygen pressure have been investigated at temperatures from 4.2 to 100 K and in magnetic fields up to 18 T. For $T>4.2\mathrm{K}$, two different electronlike charge carriers originating from two electron channels which contribute to transport are observed. We probe the contributions of a degenerate and a nondegenerate band to the thermoelectric power and develop a consistent model to describe the temperature dependence of the thermoelectric tensor. Anomalies in the data point to an additional magnetic field dependent scattering.

A recently developed bounds-analysis approach has been used to interpret density-functional-theory (DFT) results for the As and Ga antisites in GaAs. The bounds analysis and subsequent processing of DFT results for the As antisite yielded levels—defined as the Fermi levels at which the defect charge state changes—in very good agreement with measurements, including the −1/0 level which is within 0.1 eV of the conduction-band edge. Good agreement was also obtained for the activation energies to transform the $\mathrm{A}{\mathrm{s}}_{\mathrm{Ga}}$ from its metastable state to its stable state. For the Ga antisite, the bounds analysis revealed that the −1 and 0 charge states are hole states weakly bound to a localized −2 charge state. The calculated levels are in good agreement with measurements.

The magnetic proximity effect between the topological insulator (TI) and ferromagnetic insulator (FMI) is considered to have great potential in spintronics. However, a complete determination of interfacial magnetic structure has been highly challenging. We theoretically investigate the interlayer exchange coupling of two FMIs separated by a TI thin film, and show that the particular electronic states of the TI contributing to the proximity effect can be directly identified through the coupling behavior between two FMIs, together with a tunability of the coupling constant. Such an FMI/TI/FMI structure not only serves as a platform to clarify the magnetic structure of the FMI/TI interface, but also provides insights in designing the magnetic storage devices with ultrafast response.

A strong electronic reconstruction resulting in a quenching of the Fe magnetic moments has recently been predicted to be at the origin of the giant magnetocaloric effect displayed by ${\mathrm{Fe}}_2\mathrm{P}$-based materials. To verify this scenario, x-ray magnetic circular dichroism experiments have been carried out at the L edges of Mn and Fe for two typical compositions of the ${(\mathrm{Mn},\mathrm{Fe})}_2(\mathrm{P},\mathrm{Si},\mathrm{B})$ system. The dichroic absorption spectra of Mn and Fe have been measured in the vicinity of the first-order ferromagnetic transition. The experimental spectra are compared with first-principles calculations and charge-transfer multiplet simulations in order to derive the magnetic moments. Even though signatures of a metamagnetic behavior are observed either as a function of the temperature or the magnetic field, the similarity of the Mn and Fe moment evolution suggests that the quenching of the Fe moment is weaker than previously predicted.

We have employed muon spin relaxation and rotation $\left(\mu \mathrm{SR}\right)$ to investigate the superconducting properties of the noncentrosymmetric superconductor ${\mathrm{CaIrSi}}_3$. Measurements of single-crystal specimens confirm the development of a robust superconducting state below $T_{\mathrm{c}}=3.55\pm 0.1\mathrm{K}$ with a ground-state magnetic penetration depth of ${\lambda }_L=288\pm 10\mathrm{nm}$ and a coherence length of $\xi =28.8\pm 0.1\mathrm{nm}$. The temperature evolution of the superfluid density indicates a nodeless superconducting gap structure dominated by an isotropic spin-singlet component in the dirty limit with a carrier density of $n=(4.6\pm 0.2)\times {10}^{22}{\mathrm{cm}}^{-3}$ as determined by Hall resistance measurements. We find no evidence of spontaneous time-reversal symmetry breaking in the superconducting state within an accuracy of 0.05 G. These observations suggest that the influence of any spin-triplet pairing component or multiple gap structure associated with noncentrosymmetric physics is very weak or entirely absent in ${\mathrm{CaIrSi}}_3$.

We report temperature-dependent neutron scattering measurements of the local dynamic structure factor in the quantum spin ladder ${\left({\mathrm{C}}_7{\mathrm{H}}_{10}\mathrm{N}\right)}_2{\mathrm{CuBr}}_4$ in a magnetic field $H=9$ T, in its gapless quantum-critical phase. We show that the measured quantity has a scaling form consistent with expectations for a Tomonaga-Luttinger liquid with attraction. The measured Luttinger parameter $K\approx 1.25$ and scaling function are in excellent agreement with density matrix renormalization group numerical calculations for the underlying spin Hamiltonian.

The phase stability of fcc and bcc magnetic binary Fe-Cr, Fe-Ni, and Cr-Ni alloys, and ternary Fe-Cr-Ni alloys is investigated using a combination of density functional theory (DFT), cluster expansion (CE), and magnetic cluster expansion (MCE) approaches. Energies, magnetic moments, and volumes of more than 500 alloy structures have been evaluated using DFT, and the predicted most stable configurations are compared with experimental observations. Deviations from the Vegard law in fcc Fe-Cr-Ni alloys, resulting from the nonlinear variation of atomic magnetic moments as functions of alloy composition, are observed. The accuracy of the CE model is assessed against the DFT data, where for ternary Fe-Cr-Ni alloys the cross-validation error is found to be less than 12 meV/atom. A set of cluster interaction parameters is defined for each alloy, where it is used for predicting new ordered alloy structures. The fcc ${\mathrm{Fe}}_2\mathrm{CrNi}$ phase with ${\mathrm{Cu}}_2\mathrm{NiZn}$-like crystal structure is predicted to be the global ground state of ternary Fe-Cr-Ni alloys, with the lowest chemical ordering temperature of 650 K. DFT-based Monte Carlo (MC) simulations are applied to the investigation of order-disorder transitions in Fe-Cr-Ni alloys. The enthalpies of formation of ternary alloys predicted by MC simulations at 1600 K, combined with magnetic correction derived from MCE, are in excellent agreement with experimental values measured at 1565 K. The relative stability of fcc and bcc phases is assessed by comparing the free energies of alloy formation. The evaluation of the free energies involved the application of a dedicated algorithm for computing the configurational entropies of the alloys. Chemical order is analyzed, as a function of temperature and composition, in terms of the Warren-Cowley short-range order (SRO) parameters and effective chemical pairwise interactions. In addition to compositions close to binary intermetallic phases ${\mathrm{CrNi}}_2$, FeNi, ${\mathrm{FeNi}}_3$, and ${\mathrm{FeNi}}_8$, pronounced chemical order is found in fcc alloys near the center of the ternary alloy composition triangle. The calculated SRO parameters compare favorably with experimental data on binary and ternary alloys. Finite temperature magnetic properties of fcc Fe-Cr-Ni alloys are investigated using an MCE Hamiltonian parameterized using a DFT database of energies and magnetic moments computed for a large number of alloy configurations. MCE simulations show that the ordered ternary ${\mathrm{Fe}}_2\mathrm{CrNi}$ alloy phase remains magnetic up to 850–900 K due to the strong antiferromagnetic coupling between (Fe,Ni) and Cr atoms in the ternary Fe-Cr-Ni matrix.

We show that the Kohn-Sham positive-definite kinetic energy (KE) density significantly differs from the von Weizsäcker (VW) one at the nuclear cusp as well as in the asymptotic region. At the nuclear cusp, the VW functional is shown to be linear, and the contribution of $p$-type orbitals to the KE density is theoretically derived and numerically demonstrated in the limit of infinite nuclear charge as well in the semiclassical limit of neutral large atoms. In the latter case, it reaches 12% of the KE density. In the asymptotic region we find new exact constraints for meta-generalized gradient approximation (meta-GGA) exchange functionals: with an exchange enhancement factor proportional to $\sqrt{\alpha }$, where $\alpha$ is the common meta-GGA ingredient, both the exchange energy density and the potential are proportional to the exact ones. In addition, this describes exactly the large-gradient limit of quasi-two-dimensional systems.

Using the tight-binding model, we classify the eigenstates of symmetric graphene quantum dots based on their discrete rotational symmetry. In the presence of an external magnetic field, we identify a Zeeman-like behavior arising from the electronic orbital wave functions, with a constant $g$ value for a variety of quantum dot shapes, sizes, and edge structures. The discrete rotational symmetry of the dots also leads to anomalous optical selection rules, thereby suggesting a way to optically induce a magnetic moment in these nanostructures.

While the role of sampling of the electron momentum $k$ in supercell calculations of the elastic electron transmission is well understood, its influence in the case of inelastic electron tunneling (IET) has not yet been systematically explored. Here we compare ab initio IET spectra of molecular monolayers in the commonly used $\Gamma$-point approximation to rigorously $k$-converged results. We study four idealized molecular junctions with either alkanedithiolates or benzenedithiolates, and explore variations due to varying molecular tilt angle, density, as well as chemical identity of the monolayer. We show that the $\Gamma$-point approximation is reasonable for a range of systems, but that a rigorous convergence is needed for accurate signal amplitudes. We also describe an approximative scheme which reduces the computational cost of the $k$-averaged calculation in our implementation.

We present a class of Lie algebraic similarity transformations generated by exponentials of two-body on-site Hermitian operators whose Hausdorff series can be summed exactly without truncation. The correlators are defined over the entire lattice and include the Gutzwiller factor $n_{i\uparrow }{n}_{i\downarrow }$, and two-site products of density $(n_{i\uparrow }+n_{i\downarrow })$ and spin $(n_{i\uparrow }-n_{i\downarrow })$ operators. The resulting non-Hermitian many-body Hamiltonian can be solved in a biorthogonal mean-field approach with polynomial computational cost. The proposed similarity transformation generates locally weighted orbital transformations of the reference determinant. Although the energy of the model is unbound, projective equations in the spirit of coupled cluster theory lead to well-defined solutions. The theory is tested on the one- and two-dimensional repulsive Hubbard model where it yields accurate results for small and medium sized interaction strengths.

Image potential states (IPSs) have been observed for various adsorbed carbon structures, such as graphene or carbon nanotubes. Graphene nanoribbons (GNRs) are intriguing nanostructures with a significant band gap which promise applications in nanotechnology. In the present paper we employ two-photon photoemission (2PPE) to investigate the unoccupied electronic structure and particularly the IPS of chevron-shaped GNR which are synthesized in a thermally activated on-surface synthesis on Au(111). Angle- and time-resolved 2PPE are utilized to gain further insights into the properties of the IPS. Compared to the pristine surface, reduced effective masses between 0.6 and 0.8 electron masses are observed and the lifetimes of the IPS are below the experimental detection limit, which is in the femtosecond regime. Independent of the concentration of N dopant atoms introduced in the GNR we observe a constant binding energy with respect to the vacuum level of the system.

The electronic structure of atomically precise armchair graphene nanoribbons of width $N=7$ (7-AGNRs) are investigated by scanning tunneling spectroscopy (STS) on Au(111). We record the standing waves in the local density of states of finite ribbons as a function of sample bias and extract the dispersion relation of frontier electronic states by Fourier transformation. The wave-vector-dependent contributions from these states agree with density functional theory calculations, thus enabling the unambiguous assignment of the states to the valence band, the conduction band, and the next empty band with effective masses of $0.41\pm 0.08m_e,0.40\pm 0.18m_e$, and $0.20\pm 0.03m_e$, respectively. By comparing the extracted dispersion relation for the conduction band to corresponding height-dependent tunneling spectra, we find that the conduction band edge can be resolved only at small tip-sample separations and has not been observed before. As a result, we report a band gap of $2.37\pm 0.06$ eV for 7-AGNRs adsorbed on Au(111).

We study the thermopower of a three-terminal setup composed of a quantum dot attached to three electrodes, one of which is a topological superconductor. In the model, superconductivity is explicitly taken into account. We compare the results for $s$-wave (trivial) and $p$-wave (topological) superconductors and observe that for small temperatures the thermopower has different sign in the two cases. This behavior is strongly dependent on temperature and we estimate an energy scale that controls the sign in the $p$-wave case, which results proportional to the square root of the gap and the coupling to superconductor. The analytical results obtained with a simple 1D model are confirmed by a more realistic tight-binding model.

A generalization of the Aubry-André-Harper (AAH) model is developed, containing a tunable phase shift between on-site and off-diagonal modulations. A localization transition can be induced by varying just this phase, keeping all other model parameters constant. The complete localization phase diagram is obtained. Unlike the original AAH model, the generalized model can exhibit a transition between topologically trivial band structures and topologically nontrivial band structures containing protected boundary states. These boundary states can be pumped across the system by adiabatic variations in the phase shift parameter. The model can also be used to demonstrate the phenomenon of adiabatic pumping breakdown due to localization.

The local structure of two skutterudite families—$\mathrm{Ce}{M}_4{\mathrm{As}}_{12}$ ($M=\mathrm{Fe}$, Ru, Os) and $Ln{\mathrm{Cu}}_3{\mathrm{Ru}}_4{\mathrm{O}}_{12}$ ($Ln=\mathrm{La}$, Pr, and Nd)—have been studied using the extended x-ray absorption fine structure (EXAFS) technique with a focus on the lattice vibrations about the rare-earth “rattler atoms” and the extent to which these vibrations can be considered local modes, with the rattler vibrating inside a nearly rigid cage. X-ray absorption data at all the metal edges were collected over a temperature range of 4 to 300 K and analyzed using standard procedures. The pair distances from EXAFS results agree quite well with the average structure obtained from diffraction. The cage structure is formed by the $M$ and As atoms in $\mathrm{Ce}{M}_4{\mathrm{As}}_{12}$ and by Cu, O, and Ru atoms in $Ln{\mathrm{Cu}}_3{\mathrm{Ru}}_4{\mathrm{O}}_{12}$. Although some of the bonds within the cage are quite stiff (correlated Debye temperatures, ${\theta }_{\mathrm{cD}}$, are $\sim 500$ K for $\mathrm{Ce}{M}_4{\mathrm{As}}_{12}$ and above 800 K for $Ln{\mathrm{Cu}}_3{\mathrm{Ru}}_4{\mathrm{O}}_{12}$), we show that the structure is not completely rigid. For the rattler atom the nearest-neighbor pairs have a relatively low Einstein temperature, ${\theta }_{\mathrm{E}}:\sim 100–120$ K for Ce-As and $\sim 130$ K for $Ln\text{−}\mathrm{O}$. Surprisingly, the behaviors of the second-neighbor pairs are quite different: for $\mathrm{Ce}{M}_4{\mathrm{As}}_{12}$ the second-neighbor pairs $\left(\mathrm{Ce}\text{−}M\right)$ have a weaker bond while for $Ln{\mathrm{Cu}}_3{\mathrm{Ru}}_4{\mathrm{O}}_{12}$ the $Ln\text{−}\mathrm{Ru}$ second-neighbor pair has a stiffer effective spring constant than the first-neighbor pair. In addition, we show that the ${\mathrm{As}}_4$ or ${\mathrm{CuO}}_4$ rings are relatively rigid units and that their vibrations are anisotropic within these cubic structures, with stiff restoring forces perpendicular to the rings and much weaker restoring forces in directions parallel to the rings. Consequently vibrations of the rings may also act as “rattlers” and help suppress thermal conductivity. In general neither the rigid-cage approximation nor the simple reduced-mass approximation are sufficient for describing rattler behavior.

We use the coupled cluster method (CCM) to study the zero-temperature ground-state (GS) properties of a spin-$\frac{1}{2}$ $J_1-J_2$ Heisenberg antiferromagnet on a triangular lattice with competing nearest-neighbor and next-nearest-neighbor exchange couplings $J_1>0$ and $J_2\equiv \kappa J_1>0$, respectively, in the window $0\le \kappa <1$. The classical version of the model has a single GS phase transition at ${\kappa }^{\mathrm{cl}}=\frac{1}{8}$ in this window from a phase with 3-sublattice antiferromagnetic (AFM) ${120}^{\circ }$ Néel order for $\kappa <{\kappa }^{\mathrm{cl}}$ to an infinitely degenerate family of 4-sublattice AFM Néel phases for $\kappa >{\kappa }^{\mathrm{cl}}$. This classical accidental degeneracy is lifted by quantum fluctuations, which favor a 2-sublattice AFM striped phase. For the quantum model we work directly in the thermodynamic limit of an infinite number of spins, with no consequent need for any finite-size scaling analysis of our results. We perform high-order CCM calculations within a well-controlled hierarchy of approximations, which we show how to extrapolate to the exact limit. In this way we find results for the case $\kappa =0$ of the spin-$\frac{1}{2}$ model for the GS energy per spin, $E/N=-0.5521\left(2\right)J_1$, and the GS magnetic order parameter, $M=0.198\left(5\right)$ (in units where the classical value is $M^{\mathrm{cl}}=\frac{1}{2}$), which are among the best available. For the spin-$\frac{1}{2}$ $J_1-J_2$ model we find that the classical transition at $\kappa ={\kappa }^{\mathrm{cl}}$ is split into two quantum phase transitions at ${\kappa }_1^c=0.060\left(10\right)$ and ${\kappa }_2^c=0.165\left(5\right)$. The two quasiclassical AFM states (viz., the ${120}^{\circ }$ Néel state and the striped state) are found to be the stable GS phases in the regime $\kappa <{\kappa }_1^c$ and $\kappa >{\kappa }_2^c$, respectively, while in the intermediate regimes ${\kappa }_1^c<\kappa <{\kappa }_2^c$ the stable GS phase has no evident long-range magnetic order.

A first-principles-based effective Hamiltonian is used to reveal the effects of the growth direction on properties of films made of the prototype of ferroelectric materials (namely, ${\mathrm{BaTiO}}_3$) and grown on one of the most common and available substrates (that is, ${\mathrm{SrTiO}}_3$). Continuously varying the growth direction from the pseudocubic [001] to [110] direction, via the high-symmetry [111] direction, results in (i) an original temperature-versus-growth angle phase diagram possessing different structural states, (ii) a tuning of the Curie temperature $T_c$ by more than 450 K, (iii) a minimal value of $T_c$ being near room temperature, and (iv) optimization of important physical responses at room temperature, such as dielectric and piezoelectric coefficients, for some specific growth directions. The origins of these features are also revealed, including the finding of a law correlating $T_c$ with the growth direction.