Weyl semimetals are examples of a new class of topological states of matter, which are gapless in the bulk with protected surface states. Their low-energy sector is characterized by massless chiral fermions, which are robust against translationally invariant perturbations. A variant of these systems has two nondegenerate bands touching along lines rather than points. A proposal to realize such a phase involves alternating layers of topological insulators and magnetic insulators, where the magnetization lies perpendicular to the symmetry axis of the heterostructure. The shape, size, and the dispersion in the vicinity of the nodal lines varies with the strength of the magnetization, offering a new knob to control the properties of the system. In this paper, we map out the evolution of the nodal lines and the dependence of the conductivity on magnetization and identify signatures of the low-energy sector in quantum oscillation measurements.

We develop and exactly solve a model for electrons driven by pulsed or continuous ac fields. The theory includes both the photoexcitation process as well as the subsequent acceleration of the electrons. In the case of an ac response, we examine both the nonequilibrium density of states and the current. In the case of pulsed light for high harmonic generation, we find the radiated light assumes a nearly universal behavior, with only limited dependence on the parameters of the system, except for the amplitude of the driving field, which determines the range of high harmonics generated and a tendency toward a narrowing of the peaks in a charge density wave versus a metal. This type of high harmonic generation can potentially be used for the creation of solid-state-based ultrafast light sources.

We study the unoccupied region of the electronic structure of the fivefold symmetric surface of an icosahedral ($i$) Al-Pd-Mn quasicrystal. A feature that exhibits parabolic dispersion with an effective mass of ($1.15\pm 0.1$)${\mathrm{m}}_e$ and tracks the change in the work function is assigned to an image potential resonance because our density functional calculation shows an absence of band gap in the respective energy region. We show that Sn grows pseudomorphically on $i\text{−}\mathrm{Al}\text{−}\mathrm{Pd}\text{−}\mathrm{Mn}$ as predicted by density functional theory calculations, and the energy of the image potential resonance tracks the change in the work function with Sn coverage. The image potential resonance appears much weaker in the spectrum from the related crystalline Al-Pd-Mn surface, demonstrating that its strength is related to the compatibility of the quasiperiodic wave functions in $i\text{−}\mathrm{Al}\text{−}\mathrm{Pd}\text{−}\mathrm{Mn}$ with the free-electron-like image potential states. Our investigation of the energy region immediately above $E_F$ provides unambiguous evidence for the presence of a pseudogap, in agreement with our density functional theory calculations.

Nanostructuring of graphene is in part motivated by the requirement to open a gap in the electronic band structure. In particular, a periodically perforated graphene sheet in the form of an antidot lattice may have such a gap. Such systems have been investigated with a view towards application in transistor or waveguiding devices. The desired properties have been predicted for atomically precise systems, but fabrication methods will introduce significant levels of disorder in the shape, position and edge configurations of individual antidots. We calculate the electronic transport properties of a wide range of finite graphene antidot devices to determine the effect of such disorders on their performance. Modest geometric disorder is seen to have a detrimental effect on devices containing small, tightly packed antidots, which have optimal performance in pristine lattices. Larger antidots display a range of effects which strongly depend on their edge geometry. Antidot systems with armchair edges are seen to have a far more robust transport gap than those composed from zigzag or mixed edge antidots. The role of disorder in waveguide geometries is slightly different and can enhance performance by extending the energy range over which waveguiding behavior is observed.

We consider gapped ${\mathbb{Z}}_2$ spin liquids, where spinon quasiparticles may carry fractional quantum numbers of space-group symmetry. In particular, spinons can carry fractional crystal momentum. We show that such quantum number fractionalization has dramatic, spectroscopically accessible consequences, namely, enhanced periodicity of the two-spinon density of states in the Brillouin zone, which can be detected via inelastic neutron scattering. This effect is a sharp signature of certain topologically ordered spin liquids and other symmetry-enriched topological phases. Considering square lattice space-group and time-reversal symmetry, we show that exactly four distinct types of spectral periodicity are possible.

Based on accurate band structures of AlN, GaN, and InN, we report physical quantities related to high-field electron transport, including effective masses, energies of inflection points, and satellite valleys in the conduction band. The band structures are obtained from density functional theory with a hybrid functional, as well as many-body perturbation theory based on the $G_0{W}_0$ approach. We also calculate the electron-energy relaxation time due to the electron-longitudinal-optical-phonon interaction within the Fröhlich model. Our results provide insights into the physical origin of negative differential resistance and the high-frequency characteristics of group-III nitrides and their alloys under high-field operation.

Quantum Monte Carlo methods provide in principle a highly accurate treatment of the many-body problem of calculating the ground and excited states of condensed systems. In practice, however, uncontrolled errors, such as those arising from the fixed-node and pseudopotential approximations can be problematic. We show that the accuracy of some quantum Monte Carlo calculations is limited by the properties of currently available pseudopotentials. The use of pseudopotentials involves several approximations, and we will focus on one that is relatively simple to correct during the pseudopotential design phase. It is necessary to include angular-momentum channels in the pseudopotential for excited angular-momentum states and to choose the local channel appropriately to obtain accurate results. Variational and diffusion Monte Carlo calculations for Zn, O, and Si atoms and ions demonstrate these issues. Adding higher-angular-momentum channels into the pseudopotential description reduces such errors without a significant increase in computational cost.

The Hubbard model is a prototype for strongly correlated many-particle systems, including electrons in condensed matter and molecules, as well as for fermions or bosons in optical lattices. While the equilibrium properties of these systems have been studied in detail, the nonequilibrium dynamics following a strong nonperturbative excitation only recently came into the focus of experiments and theory. It is of particular interest how the dynamics depend on the coupling strength and on the particle number and whether there exist universal features in the time evolution. Here, we present results for the dynamics of finite Hubbard clusters based on a self-consistent nonequilibrium Green functions (NEGF) approach invoking the generalized Kadanoff-Baym ansatz (GKBA). We discuss the conserving properties of the GKBA with Hartree-Fock propagators in detail and present a generalized form of the energy conservation criterion of Baym and Kadanoff for NEGF. Furthermore, we demonstrate that the HF-GKBA cures some artifacts of prior two-time NEGF simulations. Besides, this approach substantially speeds up the numerical calculations and thus presents the capability to study comparatively large systems and to extend the analysis to long times allowing for an accurate computation of the excitation spectrum via time propagation. Our data obtained within the second Born approximation compare favorably with exact diagonalization results (available for up to 13 particles) and are expected to have predictive capability for substantially larger systems in the weak-coupling limit.

Finite lattice models are a prototype for interacting quantum systems and capture essential properties of condensed matter systems. With the dramatic progress in ultracold atoms in optical lattices, finite fermionic Hubbard systems have become directly accessible in experiments, including their ultrafast dynamics far from equilibrium. Here, we present a theoretical approach that is able to treat these dynamics in any dimension and fully includes inhomogeneity effects. The method consists in stochastic sampling of mean-field trajectories and is—for not too large two-body interaction strength—found to be much more accurate than time-dependent mean-field at the same order of numerical costs. Furthermore, it can well compete with recent nonequilibrium Green function approaches using second-order Born approximation, which are of substantially larger complexity. The performance of the stochastic mean-field approach is demonstrated for Hubbard clusters with up to 512 particles in one, two, and three dimensions.

In the singlet-triplet qubit architecture, the two-qubit interactions required in universal quantum computing can be implemented by capacitative coupling, by exploiting the charge distribution differences of the singlet and triplet states. The efficiency of this scheme is limited by decoherence, that can be mitigated by stronger coupling between the qubits. In this paper, we study the capacitative coupling of singlet-triplet qubits in different geometries of the two-qubit system. The effects of the qubit-qubit distance and the relative orientation of the qubits on the capacitative coupling strength are discussed using an accurate microscopic model and exact diagonalization of it. We find that trapezoidal quantum dot formations allow strong coupling with low charge distribution differences between the singlet and triplet states. The analysis of geometry on the capacitative coupling is also extended to the many-qubit case and the creation of cluster states.

We consider a quantum well embedded in a zero-dimensional microcavity with a subwavelength grated mirror, where the $x$-linearly polarized exciton mode is strongly coupled to the cavity photon, while $y$-polarized excitons remain in the weak-coupling regime. Under incoherent optical or electric pumping, we demonstrate polariton bistability associated with parametric scattering processes. Such bistability is useful for constructing polaritonic devices with optical or electrical incoherent pumping.

We present a detailed experimental and theoretical characterization of the adsorption of the perylene derivative 4,9-diaminoperylene-quinone-3,10-diimine (DPDI) on Cu(111) and compare it to its threefold dehydrogenated derivative 3deh-DPDI, which forms in a surface reaction upon annealing. While DPDI itself does not give rise to long-range ordered structures due to lack of appropriate functional groups, 3deh-DPDI acts as an exoligand in a Cu-coordinated honeycomb network on Cu(111). The main focus of this work lies on the analysis of intermolecular and molecule-substrate interactions by combining results from scanning tunneling microscopy, x-ray photoelectron spectroscopy, x-ray standing wave measurements, and density functional theory. We show, in particular, that the interactions between metal atoms and organic ligands effectively weaken the molecule-surface interactions for 3deh-DPDI leading to an increase in molecule-substrate distances compared to the DPDI precursor. Our experimental findings also shed light on the applicability of current theories, namely van der Waals corrections to density functional theory.

The ab initio calculations of phonon dispersions and nonthermal forces along structural deformation paths were used to study nonthermal solid-to-solid phase transitions in photoexcited tungsten. We assumed that electronic excitation can be described by an electronic temperature and demonstrated that nonthermal, i.e., caused purely by electronic excitation, bcc-to-fcc and bcc-to-hcp phase transitions can occur for electronic temperatures between 1.7 and 4.3 eV. These transitions result from soft modes along the $\Sigma$ line of the Brillouin zone. Structural path calculations at different electronic temperatures indicate that both transitions are likely to take place in nonequilibrium conditions. We further predict that transient fcc and hcp phases of tungsten could be observed for several ps.

We develop an approach that combines the power of nonlinear dynamics with the evolution equations for the mobile and immobile dislocation densities and force to explain force fluctuations in nanoindentation experiments. The model includes nucleation, multiplication, and propagation thresholds for mobile dislocations, and other well known dislocation transformation mechanisms. The model predicts all the generic features of nanoindentation such as the Hertzian elastic branch followed by several force drops of decreasing magnitudes, and residual plasticity after unloading. The stress corresponding to the elastic force maximum is close to the yield stress of an ideal solid. The predicted values for all the quantities are close to those reported by experiments. Our model allows us to address the indentation-size effect including the ambiguity in defining the hardness in the force drop dominated regime. At large indentation depths, the hardness remains nearly constant with a marginal decreasing trend.

We investigate two-dimensional frustrated Heisenberg magnets using nonperturbative renormalization group techniques. These magnets allow for pointlike topological defects which are believed to unbind and drive either a crossover or a phase transition which separates a low-temperature, spin-wave-dominated regime from a high-temperature regime where defects are abundant. Our approach can account for the crossover qualitatively and both the temperature dependence of the correlation length as well as a broad but well-defined peak in the specific heat are reproduced. We find no signatures of a finite-temperature transition and an accompanying diverging length scale. Our analysis is consistent with a rapid crossover driven by topological defects.

Electronic properties of ${\mathrm{Dy}}_3{\mathrm{Ru}}_4{\mathrm{Al}}_{12}$ (hexagonal crystal structure, Dy atoms form distorted kagome nets) are studied on a single crystal by means of magnetization, neutron diffraction, specific heat, and resistivity measurements. The onset of a long-range magnetic order of Dy moments occurs at 7 K through a first-order phase transition. The compound has a noncollinear antiferromagnetic structure with a propagation vector (1/2 0 1/2). The configuration of the Dy moments is consistent with the monoclinic Shubnikov group $C_{c}2/c$. The γ coefficient in the temperature linear term of the specific heat is strongly enhanced to 500 mJ ${\mathrm{mol}}^{-1}$ ${\mathrm{K}}^{-2}$ taking into account the localized nature of Dy magnetism. An additional contribution originates from spin fluctuations induced in the $4d$ subsystem of Ru by the exchange field acting from the Dy $4f$ moments. In an applied magnetic field ${\mathrm{Dy}}_3{\mathrm{Ru}}_4{\mathrm{Al}}_{12}$ displays magnetization jumps along all crystallographic directions. All the metamagnetic transitions are accompanied by large positive magnetoresistance. The maximum effect (125%–140%) is attained for current along the [100] axis and field along the [120] or [001] axes. The large positive effect is explained by changes in the conduction electron spectra through the jumps as the conduction electrons interact with localized magnetic moments.

We present a comprehensive statistical study of free decay of the quantized vortex tangle in superfluid ${}^4\mathrm{He}$ at low and ultralow temperatures $0\le T\le 1.1$ K. Using high-resolution vortex filament simulations with full Biot-Savart vortex dynamics, we show that for ultralow temperatures $T\lesssim 0.5$ K, when the mutual friction parameters $\alpha \simeq {\alpha }^{\prime }<{10}^{-5}$, the vortex reconnections excite Kelvin waves with wavelengths $\lambda$ of the order of the intervortex distance $\ell$. These excitations cascade down to the resolution scale $\Delta \xi$ which in our simulations is of the order $\Delta \xi \sim \ell /100$. At this scale, the Kelvin waves are numerically damped by a line-smoothing procedure, that is supposed to mimic the dissipation of Kelvin waves by phonon and roton emission at the scale of the vortex core. We show that the Kelvin wave cascade is statistically important: the shortest available Kelvin waves at the end of the cascade determine the mean vortex-line curvature $S$, giving $S\gtrsim 30/\ell$, and play a major role in the tangle decay at ultralow temperatures below $0.6$ K. The found dependence of $\ell S$ on the resolution scale $\Delta \xi$ agrees with the L'vov-Nazarenko energy spectrum of weakly interacting Kelvin waves $E_{\mathrm{LN}}\propto k^{-5/3}$ rather than the spectrum $E_{\mathrm{LN}}\propto k^{-1}$, suggested by Vinen for turbulence of Kelvin waves with large amplitudes. We also show that already at $T=0.8$ K, when $\alpha$ and ${\alpha }^{\prime }$ are still very low, $\alpha \simeq {\alpha }^{\prime }<{10}^{-3}$, the Kelvin wave cascade is fully damped, vortex lines are very smooth, $S\simeq 2/\ell$, and the tangle decay is predominantly caused by the mutual friction.

Complex ac susceptibility, $\chi ={\chi }^{\prime }-i{\chi }^{\prime \prime }$, measurements of the clathrate compound ${\mathrm{Pr}}_3{\mathrm{Pd}}_{20}{\mathrm{Ge}}_6$ were performed in static fields up to 10 mT for $H\parallel \left[001\right]$ and at temperatures down to $500 \mu \mathrm{K}$. Praseodymium (Pr) nuclear magnetic moments at the $8c$ site, where quadrupole moments of $4f$ electrons order at ${\mathit{\text{T}}}_{\text{Q1}}=250$ mK, were found to order antiferromagnetically at 9 mK, as shown by a peak in ${\chi }^{\prime }$ and a substantial increase in thermal relaxation time. The large enhancement factor $\left(1+K_{8c}\right)$ obtained by calculation of the hyperfine-enhanced nuclear susceptibility of Pr at the $8c$ site accounts for the high transition temperature of Pr nuclear magnetic moments and the large ${\chi }^{\prime }$ below 30 mK. From analysis of the crystalline electric field and the mean-field approximation, we conclude that a $\chi$ peak at 77 mK can be ascribed to an antiferromagnetic ordering of magnetic moments of $4f$ electrons at the $4a$ site. We found that nuclear and $\mathit{\text{f}}$-electron moments order separately on two sublattices in this compound. The temperature and magnetic field dependence of ${\chi }^{\prime }$ and ${\chi }^{\prime \prime }$ between 30 and 60 mK are discussed in terms of dissipation phenomena.