We present several results relating to the contraction of generic tensor networks and discuss their application to the simulation of quantum many-body systems using variational approaches based upon tensor network states. Given a closed tensor network $\mathcal{T}$, we prove that if the environment of a single tensor from the network can be evaluated with computational cost $\kappa$, then the environment of any other tensor from $\mathcal{T}$ can be evaluated with identical cost $\kappa$. Moreover, we describe how the set of all single tensor environments from $\mathcal{T}$ can be simultaneously evaluated with fixed cost $3\kappa$. The usefulness of these results, which are applicable to a variety of tensor network methods, is demonstrated for the optimization of a multiscale entanglement renormalization Ansatz for the ground state of a one-dimensional quantum system, where they are shown to substantially reduce the computation time.

Kondo insulators combine strong electronic correlations with spin-orbit coupling and thereby provide a potential realization of correlated topological insulators. We present model calculations that allow us to study the onset of bulk coherence and concomitant topological edge states from the mixed valence to local moment regimes. Our real-space dynamical mean-field results include the detailed temperature dependence of the single-particle spectral function on slab geometries as well as the temperature dependence of the topological invariant. The relevance of our calculations for candidate materials like SmB6 is discussed.

We study the effects of intraband relaxation processes on optical manipulation protocols for $\mathit{sp}$ biexcitons hosted by CdTe disk-shaped quantum dots. The many-body states are calculated within the configuration interaction method starting from single-particle states given by the $k·p$ theory. The time-dependent occupations of relevant many-body states are extracted from the von Neumann-Lindblad equation for the density operator. We mainly investigate the generation of $\mathit{sp}$ biexcitons with two pulses of different polarizations ${\sigma }_{+}$ and ${\sigma }_{-}$. The fast hole relaxation processes prevent a high-fidelity controlled operation on $\mathit{sp}$ biexcitons and lead to the occupation of some transient states which can be optically probed. More importantly, the many-body structure of the transient states consists of two holes on the $s$ shell and antiparallel $\mathit{sp}$ triplet states for electrons. Our simulations show that these triplet states are more stable against decoherence as they can only be damaged through slow electron relaxation. The configuration mixing due to correlation effects is also discussed.

Bulk electrical dissipation caused by charge-density-wave (CDW) depinning and sliding is a classic subject. We present a local, nanoscale mechanism describing the occurrence of mechanical dissipation peaks in the dynamics of an atomic force microscope tip oscillating above the surface of a CDW material. Local surface $2\pi$ slips of the CDW phase are predicted to take place, giving rise to mechanical hysteresis and large dissipation at discrete tip surface distances. The results of our static and dynamic numerical simulations are believed to be relevant to recent experiments on $\mathrm{Nb}{\mathrm{Se}}_2$; other candidate systems in which similar effects should be observable are also discussed.

Motivated by recent interest in perovskite surfaces and heterostructures, we present a theoretical analysis of the optical conductivity of a two-dimensional electron gas (2DEG) formed in the $t_{2g}$ bands of an oxide with perovskite structure based on the Kubo formula and a realistic electronic structure model. We find that, when the electric field is polarized in the plane of the 2DEG, the optical conductivity is dominated by nearly independent Drude contributions from all occupied two-dimensional subbands, whereas for perpendicular-to-plane polarization it has strong intersubband features. For perpendicular-to-plane polarization, Coulomb interactions couple different intersubband modes, transferring spectral weight to higher energy absorption features and inducing a strong excitation that is collective in nature. Our analysis suggests that perpendicular-to-plane optical conductivity studies may help advance understanding of the roles of lattice distortions and electron-electron interactions in complex oxide 2DEG quantum confinement physics.

We study the crossover from classical to quantum phase transitions at zero temperature within the framework of ${\varphi }^4$ theory. The classical transition at zero temperature can be described by the Landau theory, turning into a quantum Ising transition with the addition of quantum fluctuations. We perform a calculation of the transition line in the regime where the quantum fluctuations are weak. The calculation is based on a renormalization group analysis of the crossover between classical and quantum transitions, and is well controlled even for space-time dimensionality $D$ below 4. In particular, for $D=2$ we obtain an analytic expression for the transition line which is valid for a wide range of parameters, as confirmed by numerical calculations based on the density matrix renormalization group. This behavior could be tested by measuring the phase diagram of the linear-zigzag instability in systems of trapped ions or repulsively interacting dipoles.

X-ray powder diffraction (XRD), magnetic susceptibility $\chi$, isothermal magnetization $M$, heat capacity $C_{\mathrm{p}}$, and in-plane electrical resistivity $\rho$ measurements as a function of temperature $T$ and magnetic field $H$ are presented for ${\mathrm{CaCo}}_{1.86}{\mathrm{As}}_2$ single crystals. The electronic structure is probed by angle-resolved photoemission spectroscopy (ARPES) measurements of ${\mathrm{CaCo}}_{1.86}{\mathrm{As}}_2$ and by full-potential linearized augmented-plane-wave calculations for the supercell ${\mathrm{Ca}}_8{\mathrm{Co}}_{15}{\mathrm{As}}_{16}$ (${\mathrm{CaCo}}_{1.88}{\mathrm{As}}_2$). Our XRD crystal structure refinement is consistent with the previous combined refinement of x-ray and neutron powder diffraction data showing a collapsed-tetragonal ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type structure with 7(1)% vacancies on the Co sites corresponding to the composition ${\mathrm{CaCo}}_{1.86}{\mathrm{As}}_2$ [D. G. Quirinale et al., Phys. Rev. B 88, 174420 (2013)]. The anisotropic $\chi \left(T\right)$ data are consistent with the magnetic neutron diffraction data of Quirianale et al. that demonstrate the presence of A-type collinear antiferromagnetic order below the Néel temperature $T_{\mathrm{N}}=52$(1) K with the easy axis being the tetragonal $c$ axis. However, no clear evidence from the $\rho \left(T\right)$ and $C_{\mathrm{p}}\left(T\right)$ data for a magnetic transition at $T_{\mathrm{N}}$ is observed. A metallic ground state is demonstrated from the band calculations and the $\rho \left(T\right)$, $C_{\mathrm{p}}\left(T\right)$, and ARPES data, and spin-polarized calculations indicate a competition between the A-type AFM and FM ground states. The $C_{\mathrm{p}}\left(T\right)$ data exhibit a large Sommerfield electronic coefficient reflecting a large density of states at the Fermi energy $\mathcal{D}\left(E_{\mathrm{F}}\right)$ that is enhanced compared with the band structure calculation where the bare $\mathcal{D}\left(E_{\mathrm{F}}\right)$ arises from Co $3d$ bands. At 1.8 K, the $M\left(H\right)$ data for $H\parallel c$ exhibit a well-defined first-order spin-flop transition at an applied field of 3.5 T. The small ordered moment of $\approx$$0.3{\mu }_{\mathrm{B}}$/Co obtained from the $M\left(H\right)$ data at low $T$, the large exchange enhancement of $\chi$ and the lack of a self-consistent interpretation of the $\chi \left(T\right)$ and $M(H,T)$ data in terms of a local moment Heisenberg model together indicate that the magnetism of ${\mathrm{CaCo}}_{1.86}{\mathrm{As}}_2$ is itinerant.

We report the angle-resolved photoemission spectroscopy observation of the lifting of symmetry-protected band degeneracy, and consequently the breakdown of local tetragonal symmetry in the superconducting state of Li(${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$)As. Supported by theoretical simulations, we analyze the doping and temperature dependences of this band splitting and demonstrate an intimate connection between ferro-orbital correlations and superconductivity.

Employing a sapphire whispering gallery mode resonator, we demonstrate features of the spin-photon interaction in cavities with broken time-reflection symmetry. The broken symmetry leads to a lifting of the degeneracy between left-handed and right-handed polarized cavity photons, which results in an observable gyrotropic effect. In the high-$Q$ cavity limit, such a situation requires a modification of the Tavis-Cummings Hamiltonian to take into account conservation of spin angular momentum and the corresponding selection rules. As a result, the system is represented by a system of two linearly coupled bosonic modes, with each one coupled to its own subensemble of two-level systems with different energy splittings. In the experimental example, these subensembles originate from ${\text{Fe}}^{3+}$ impurity ions effectively seen as a two-level system at the interaction frequency. The temperature dependence of the population of each subensemble (in terms of effective susceptibility of the medium) is determined experimentally in accordance with the theoretical predictions revealing various paramagnetic impurity types in the solid. The regimes of backscatterer and spin ensemble domination are discussed and compared.

We report on infrared (IR) optical experiments on ${\mathrm{Bi}}_2$${\mathrm{Te}}_3$ and Mn-doped ${\mathrm{Bi}}_2$${\mathrm{Te}}_3$ epitaxial thin films. In the latter film, dilute Mn doping (4.5%) of the topologically nontrivial semiconductor host results in a time-reversal-symmetry-breaking ferromagnetic order below $T_C=15$ K. Our spectroscopic study shows that both materials share the ${\mathrm{Bi}}_2$${\mathrm{Te}}_3$ crystal structure, as well as classification as bulk degenerate semiconductors. Hence the Fermi energy is located in the ${\mathrm{Bi}}_2$${\mathrm{Te}}_3$ conduction band in both materials, and furthermore, there is no need to invoke topological surface states to describe the conductivity spectra. We also demonstrate that the Drude oscillator strength gives a simple metric with which to distinguish the possibility of topological surface state origins of the low frequency conductance, and we conclude that in both the pristine and Mn-doped ${\mathrm{Bi}}_2$${\mathrm{Te}}_3$ samples the electromagnetic response is indeed dominated by the bulk material properties, rather than those of the surface. An encouraging aspect for taking advantage of the interplay between nontrivial topology and magnetism, however, is that the temperature dependence of the Mn-doped ${\mathrm{Bi}}_2$${\mathrm{Te}}_3$ film suggests bulk charge carriers do not play a significant role in mediating ferromagnetism. Thus, a truly insulating bulk may still be suitable for the formation of a ferromagnetic ground state in this dilute magnetic topological semiconductor.

Transitions between the two states of an ac Stark-split doublet are forbidden in centrosymmetric systems, and thus almost impossible to observe in experiments performed with atomic clouds. However, electrons trapped in nanoscopic heterostructures can behave as artificial atoms, with the advantage that the wave-function symmetry can be broken by using asymmetric confining potentials. Here we develop the many-body theory describing the intradoublet emission of a resonantly pumped intersubband transition in a doped asymmetric quantum well, showing that in such a system the intradoublet emission can be orders of magnitude higher than in previously studied systems. This emission channel, which lies in the terahertz range and whose frequency depends upon the pump power, opens the way to the realization of monolithic and tunable terahertz emitters.

We present a machine learning approach to solve electronic quantum transport equations of one-dimensional nanostructures. The transmission coefficients of disordered systems were computed to provide training and test data sets to the machine. The system's representation encodes energetic as well as geometrical information to characterize similarities between disordered configurations, while the Euclidean norm is used as a measure of similarity. Errors for out-of-sample predictions systematically decrease with training set size, enabling the accurate and fast prediction of new transmission coefficients. The remarkable performance of our model to capture the complexity of interference phenomena lends further support to its viability in dealing with transport problems of undulatory nature.

We calculate the conductance of a quantum wire with two occupied subbands in presence of a scatterer taking into account the interaction between electrons. We extend the renormalization-group equation for the scattering matrix of the obstacle to the case of intersubband interactions, find its fixed points, and investigate their stability. Depending on the interaction parameters, the conductance may be equal to 0, $e^2/h$, or $2e^2/h$ per spin projection. In some parameter ranges, two stable fixed points may coexist, so the ultimate conductance depends on the properties of the bare scatterer. For spinful electrons, the conductance of the wire may nonmonotonically depend on the Fermi level and temperature.

The electronic properties of graphene under any arbitrary uniaxial strain field are obtained by an exact mapping of the corresponding tight-binding Hamiltonian into an effective one-dimensional modulated chain. For a periodic modulation, the system displays a rich behavior, including quasicrystals and modulated crystals with a complex spectrum, including gaps and peaks at the Fermi energy and localization transitions. All these features are explained by the incommensurate or commensurate nature of the potential, which leads to a dense filling by diffraction spots of the reciprocal space in the former case. The essential features of strain are made specially clear by analyzing a special momenta that uncouples the model into dimers.

We present a new algorithm to analytically continue the self-energy of quantum many-body systems from Matsubara frequencies to the real axis. The method allows straightforward, unambiguous computation of electronic spectra for lattice models of strongly correlated systems from self-energy data that has been collected with state-of-the-art continuous time solvers within dynamical mean-field simulations. Using well-known analytical properties of the self-energy, the analytic continuation is cast into a constrained minimization problem that can be formulated as a quadratic programmable optimization with linear constraints. The algorithm is validated against exactly solvable finite-size problems, showing that all features of the spectral function near the Fermi level are very well reproduced and coarse features are reproduced for all energies. The method is applied to two well-known lattice problems, the two-dimensional Hubbard model at half filling, where the momentum dependence of the gap formation is studied, as well as a multiband model of NiO, for which the spectral function can be directly compared to experiment. Agreement with published results is very good.

The mobility of electron small polarons in hematite, $\alpha$-${\text{Fe}}_2$${\text{O}}_3$, is calculated by density functional theory within the generalized gradient approximation including Hubbard $U$ corrections. Our work goes beyond previous computational investigations of this system by computing both the prefactor and activation energies for adiabatic polaron transport. The results obtained using a Hubbard $U$ value of 4.3 eV yield a calculated value of the room-temperature basal plane mobility of 0.009 S*${\text{cm}}^2$/s, which compares to within an order of magnitude with experimental measurements. Further, the values of the electronic-coupling parameter in the Marcus theory for small-polaron transport are estimated from DFT$+U$ calculations of the defect energy levels in the stable and saddle-point configurations. Our results predict an adiabatic polaron transfer, in good agreement with previous wave function based calculations.

The spin Seebeck effect is studied across a charge insulating magnetic junction, in which thermal-spin conjugate transport is assisted by the exchange interactions between the localized spin in the center and electrons in metallic leads. We show that, in contrast with bulk spin Seebeck effect, the figure of merit of such nanoscale thermal-spin conversion can be infinite, leading to the ideal Carnot efficiency in the linear response regime. We also find that in the nonlinear spin Seebeck transport regime the device possesses the asymmetric and negative differential spin Seebeck effects. In the last, the situations with leaking electron tunneling are also discussed. This nanoscale thermal spin rectifier, by tuning the junction parameters, can act as a spin Seebeck diode, spin Seebeck transistor, and spin Seebeck switch, which could have substantial implications for flexible thermal and information control in molecular spin caloritronics.