Dynamic friction-force microscopy with fractional-resonance excitation is used to analyze defect structures on graphite. The sample is modulated laterally at one third of the resonance of the cantilever in contact with the surface. Nonlinear tip-sample interactions then lead to cantilever oscillations at resonance, the amplitude of which is recorded as the dynamic friction signal during scanning. We show that this method is extremely sensitive to subtle surface defects, allowing direct imaging of grain boundaries. In order to understand the image contrast mechanism, we compare the dynamic friction signal to simulations for different surface defects. Further, the complete resonance behavior of the cantilever is analyzed using a fast band excitation scheme. We find that the surface elasticity plays an important role for the image contrast formation.

Strong interactions can give rise to new fermionic symmetry-protected topological phases which have no analogs in free fermion systems. As an example, we have systematically studied a spinless fermion model with $U\left(1\right)$ charge conservation and time-reversal symmetry on a three-leg ladder using density-matrix renormalization group. In the noninteracting limit, there are no topological phases. Turning on interactions, we found two gapped phases. One is trivial and is adiabatically connected to a band insulator, while another one is a nontrivial symmetry-protected topological phase resulting from strong interactions.

We show that the modulation of the hopping amplitudes in the honeycomb lattice of the monolayer graphene uniquely defines a metric which corresponds to the shape of the Dirac cone near the Dirac points. The spin connection derived from this effective metric field agrees exactly with the microscopic tight-binding Hamiltonian. The paradoxical mismatch between the quantum field theory (QFT) approach and the microscopic tight-binding model in the literature is resolved. The effective metric as seen by the sublattice pseudospin is different from the real-space metric as defined by the two-dimensional manifold of the graphene monolayer. All relevant terms of the effective gauge field from the microscopic model are calculated exactly for a unimodular effective metric.

We report first-principles $GW$ results on the dispersion of the bulk band-gap edges in the three-dimensional topological insulator ${\mathrm{Sb}}_2{\mathrm{Te}}_3$. We find that, independently of the reference density-functional-theory band structure and the crystal-lattice parameters used, the one-shot $GW$ corrections enlarge the fundamental band gap, bringing its value in close agreement with experiment. We conclude that the $GW$ corrections cause the displacement of the valence-band maximum (VBM) to the $\mathrm{\Gamma }$ point, ensuring that the surface-state Dirac point lies above the VBM. We extend our study to the analysis of the electron-energy-loss spectrum (EELS) of bulk ${\mathrm{Sb}}_2{\mathrm{Te}}_3$. In particular, we perform energy-filtered transmission electron microscopy and reflection EELS measurements. We show that the random-phase approximation with the $GW$ quasiparticle energies and taking into account virtual excitations from the semicore states leads to good agreement with our experimental data.

We propose a simple theoretical construction of certain short-range entangled phases of interacting fermions, by putting the bound states of three fermions (which we refer to as clustons) into topological bands. We give examples in two and three dimensions, and show that they are distinct from any free fermion state. We further argue that these states can be viewed as combinations of certain free fermion topological states and bosonic symmetry-protected topological (SPT) states. This provides a conceptually simple understanding of various SPT phases, and the possibility of realizing them in cold atom systems. New parton constructions of these SPT phases in purely bosonic systems are proposed. We also discuss a related anomaly in two dimensional Dirac theories, which is the gravitational analog of the parity anomaly.

We investigate the Hubbard model on the anisotropic triangular lattice as a suggested effective description of the Mott phase in various triangular organic compounds. Employing the variational cluster approximation and the ladder dual-fermion approach as complementary methods to adequately treat the zero-temperature and the finite-temperature domains, we obtain a consistent picture of the phase diagram as a function of anisotropy and interaction strength. The metal-insulator transition substantially depends on the anisotropy, and so does the nature of magnetism and the emergence of a nonmagnetic insulating phase. We further find that geometric anisotropy significantly influences the thermodynamics of the system. For increased frustration induced by anisotropy, the entropy of the system increases with interaction strength, opening the possibility of adiabatically cooling a frustrated system by an enhancement of electronic correlations.

Identifying and understanding interacting systems that can host non-Abelian topological phases with fractionalized quasiparticles have attracted intense attention in the past 20 years. Theoretically, it is possible to realize a rich variety of such states by coupling together two Abelian fractional quantum Hall (FQH) states through gapping out part of the low-energy degrees of freedom. So far, there are some indications, but no robust examples have been established in bilayer systems for realizing the non-Abelian state in the past. Here, we present a phase diagram of a double-layer bosonic FQH system based on exact diagonalization and density-matrix renormalization group (DMRG) calculations, which demonstrates a potential regime with the emergence of a non-Abelian bosonic Moore-Read state. Starting from the Abelian phase with fourfold topological degeneracy at weak coupling, with an increase of interlayer tunneling, we find an intermediate regime with a threefold ground-state degeneracy and a finite drag Hall conductance. We find different topological sectors consistent with the Moore-Read state by inserting different fluxes in an adiabatic DMRG study. We also extract the modular $\mathcal{S}$ matrix, which supports the emergence of a non-Abelian Ising anyon quasiparticle in this system.

The structural and electrical properties of metastable defects in various types of hydrogenated amorphous silicon have been studied using a powerful combination of continuous wave electron-paramagnetic resonance spectroscopy, electron spin echo (ESE) decay measurements, and Doppler broadening positron annihilation spectroscopy. The observed dependence of the paramagnetic defect density on the Doppler $S$ parameter indicates that porous, nanosized void-rich materials exhibit higher spin densities, while dense, divacancy-dominated materials show smaller spin densities. However, after light soaking more similar spin densities are observed, indicating a long-term defect creation process in the Staebler-Wronski effect that does not depend on the a-Si:H nanostructure. From ESE decays it appears that there are fast and slowly relaxing defect types, which are linked to various defect configurations in small and large open volume deficiencies. A nanoscopic model for the creation of light-induced defects in the a-Si:H nanostructure is proposed.

Optoelectronic properties of site-controlled AlGaAs/GaAs quasi-one-dimensional structures are investigated. Controllable linear variation of the band gap along the vertical axis of the quantum wire, symmetrical with respect to its center, results in a set of closely separated (by a few meV) confined electron and hole states giving rise to multiple excitonic lines well resolved in the photoluminescence spectra. Exciton recombination and relaxation dynamics in this multilevel system is studied by time-resolved and photon correlation spectroscopy and described with a master-equation model. Increasing carrier occupation in the higher-energy levels results in appearance of new multiexciton lines in the emission from the lowest-energy states. Typical features of quantum dot multiexciton dynamics such as antibunching photon statistics and cascaded biexciton-exciton emission are observed for the ground state. Pronounced bunching photon statistics and strong temporal correlations between photons emitted from the different excited states indicate sequential photon emission from multiple discrete energy levels. These structures thus represent a class of quasi-one-dimensional systems with tailored energy levels and photon emission statistics.

Graphene formation on top of SiC(0001) by decoupling the carbon buffer layer through lithium intercalation is investigated. Low-energy electron diffraction and core-level photoemission spectroscopy results show that graphene formation already occurs at room temperature, and that the interface morphology is improved after thermal annealing. Angle-resolved photoemission spectroscopy (ARPES) shows that the resulting graphene layer is strongly $n$-type doped, and in spite of the decoupling by lithium intercalation, a persistent interaction with the substrate imposes a superperiodicity on the graphene band structure that modulates the $\pi$ band intensity and gives rise to quasi-$\left(2\times 2\right) \pi$ replica bands. Through a comparison of the ARPES-derived band structure with density-functional-theory calculations, we assign the observed bands to SiC-derived states and interface-related ones; this assignment permits us to establish that the intercalated lithium occupies the T4 site on the topmost SiC layer.

We derive the effective one-dimensional Schrödinger-Pauli equation for electrons constrained to move on a space curve. The electrons are confined using a double thin-wall quantization procedure with adiabatic separation of fast and slow quantum degrees of freedom. This procedure is capable of yielding a correct Hermitian one-dimensional Schrödinger-Pauli operator. We find that the torsion of the space curve generates an additional quantum geometric potential, adding to the well-known curvature-induced one. Finally, we derive an analytic form of the one-dimensional Hamiltonian for spin-orbit coupled electrons in a nanoscale helical wire.

We study the nonequilibrium dynamics of an electronic model with competing spin-density-wave and unconventional superconductivity in the context of iron pnictides. Focusing on the collisionless regime, we find that magnetic and superconducting order parameters may coexist dynamically after a sudden quench, even though the equilibrium thermodynamic state supports only one order parameter. We consider various initial conditions concomitant with the phase diagram and in a certain regime identify different oscillatory amplitude modes with incommensurate frequencies for magnetic and superconducting responses. At the technical level we solve the equations of motion for the electronic Green's functions and self-consistency conditions by reducing the problem to a closed set of Bloch equations in a pseudospin representation. For certain quench scenarios the nonadiabatic dynamics of the pairing amplitude is completely integrable and in principle can be found exactly.

Previous research has found that the pairing symmetry in the iron-based superconductor $\mathrm{B}{\mathrm{a}}_{1-x}{\mathrm{K}}_x\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ changes from nodeless $s$ wave near optimally doped, $x\approx 0.4-0.55$ and $T_c>30\mathrm{K}$, to nodal (either $d$ wave or $s$ wave) at the pure end point, $x=1$ and $T_c<4\mathrm{K}$. Intense theoretical interest has been focused on this possibility of changing pairing symmetry, where in the transition region both order parameters would be present and time-reversal symmetry would be broken. Here we report specific heat measurements in zero and applied magnetic fields down to 0.4 K of three individual single crystals, free of low-temperature magnetic anomalies, of heavily overdoped $\mathrm{B}{\mathrm{a}}_{1-x}{\mathrm{K}}_x\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2,x=0.91$, 0.88, and 0.81. The values for ${T_c}^{\mathrm{mid}}$ are 5.6, 7.2, and 13 K and for $H_{c2}\approx 4.5$, 6, and 20 T, respectively. The data can be analyzed in a two-gap scenario, ${\mathrm{\Delta }}_2/{\mathrm{\Delta }}_1\approx 4$, with the magnetic field dependence of $\gamma (=C/T\mathrm{as}T\to 0)$ showing an anisotropic “$\mathsf{S}$-shaped” behavior vs $H$, with the suppression of the lower gap by 1 T and $\gamma \approx H^{1/2}$ overall. Although such a nonlinear γ vs $H$ is consistent with deep minima or nodes in the gap structure, it is not clear evidence for one, or both, of the gaps being nodal in these overdoped samples. Thus, following the established theoretical analysis of the specific heat of $d$-wave cuprate superconductors containing line nodes, we present the specific heat normalized by $H^{1/2}$ plotted vs $T/H^{1/2}$ of these heavily overdoped $\mathrm{B}{\mathrm{a}}_{1-x}{\mathrm{K}}_x\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ samples which—thanks to the absence of magnetic impurities in our sample—convincingly shows the expected scaling for line node behavior for the larger gap for all three compositions. There is, however, no clear observation of the nodal behavior $C\propto \alpha T^2$ in zero field at low temperatures, with $\alpha \le 2\mathrm{mJ}/\mathrm{mol}{\mathrm{K}}^3$ being consistent with the data. This, together with the scaling, leaves open the possibility of extreme anisotropy in a nodeless larger gap, ${\mathrm{\Delta }}_2$, such that the scaling works for fields above 0.25–0.5 T (0.2–0.4 K in temperature units), where this is an estimate for the size of the deep minima in the ${\mathrm{\Delta }}_2\sim 20-25\mathrm{K}$ gap. Therefore, the location of the change from nodeless to nodal gaps between optimally doped and heavily overdoped $\mathrm{B}{\mathrm{a}}_{1-x}{\mathrm{K}}_x\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ based on the present work may be closer to the $\mathrm{KF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ end point than $x=0.91$.

We show how temperature-induced disorder can be combined in a direct way with first-principles scattering theory to study diffusive transport in real materials. Excellent (good) agreement with experiment is found for the resistivity of Cu, Pd, Pt (and Fe) when lattice (and spin) disorder are calculated from first principles. For Fe, the agreement with experiment is limited by how well the magnetization (of itinerant ferromagnets) can be calculated as a function of temperature. By introducing a simple Debye-like model of spin disorder parameterized to reproduce the experimental magnetization, the temperature dependence of the average resistivity, the anisotropic magnetoresistance, and the spin polarization of a ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ alloy are calculated and found to be in good agreement with existing data. Extension of the method to complex, inhomogeneous materials as well as to the calculation of other finite-temperature physical properties within the adiabatic approximation is straightforward.

Using an optimized fabrication and magnetic thermal annealing process, we have obtained a high quality β-Ta/CoFeB/MgO layered structure with strong spin-orbit coupling. We have studied electron transport, magnetotransport, and magnetic properties of this system over a wide temperature range between 5 and 300 K. We present the results of resistivity, magnetization, giant spin Hall effect (GSHE), perpendicular magnetic anisotropy, and magnetic switching phase diagram. β-Ta exhibits a large spin Hall angle of 0.14, and shows evidence of spin Hall angle's scaling with resistivity quadratically. The optimized β-Ta/CoFeB/MgO system displays the lowest switching current density among similar systems. Our comprehensive study will benefit applications of GSHE in spintronics.

In this paper we propose a protocol to achieve coherent population transfer between two states in a three-level atom by using two ac fields. It is based on the physics of stimulated Raman adiabatic passage (STIRAP), but it is implemented with the constraint of a reduced control, namely, one of the fields cannot be switched off. A combination of frequency chirps is used with resonant fields, allowing us to achieve approximate destructive interference, despite the fact that an exact dark state does not exist. This chirped STIRAP protocol is tailored for applications to artificial atoms, where architectures with several elementary units can be strongly coupled but where the possibility of switching on and off such couplings is often very limited. Demonstration of this protocol would be a benchmark for the implementation of a class of multilevel advanced control procedures for quantum computation and microwave quantum photonics in artificial atoms.

The temperature dependence of in-plane optical conductivity has been investigated for ${\mathrm{Ca}}_{0.86}{\mathrm{Pr}}_{0.14}{\mathrm{Fe}}_2{\mathrm{As}}_2$, which shows a structural transition from tetragonal (T) to collapsed tetragonal (cT) phase at $T_{cT}\sim 73$ K. Upon entering the cT phase, drastic change characterized by the formation of a midinfrared peak near $3200 {\text{cm}}^{-1}$ (0.4 eV) in the optical conductivity is observed. Analysis of the spectral weight reveals reduced electron correlation after the cT phase transition. Based on the calculated band structure and simulated optical conductivity, we attribute the new feature around 0.4 eV to the formation of an interlayer As-As bond. The As-As bond strongly affects the Fe-As hybridizations and, in turn, drastically changes the ${\mathrm{Ca}}_{0.86}{\mathrm{Pr}}_{0.14}{\mathrm{Fe}}_2{\mathrm{As}}_2$ into a nonmagnetic Fermi liquid system without bulk superconductivity in the cT phase.

Weyl semimetals are a class of topological materials that exhibit a bulk Hall effect due to time-reversal symmetry breaking. We show that for the idealized semi-infinite case, the Casimir force between two identical Weyl semimetals is repulsive at short range and attractive at long range. Considering plates of finite thickness, we can reduce the size of the long-range attraction even making it repulsive for all distances when thin enough. In the thin-film limit, we study the appearance of an attractive Casimir force at shorter distances due to the longitudinal conductivity. Magnetic field, thickness, and chemical potential provide tunable nobs for this effect, controlling the Casimir force: whether it is attractive or repulsive, the magnitude of the effect, and the positions and existence of a trap and antitrap.

We investigate the energy structure of a quantum dot-in-a-well system by tracing the population of optically created excitons in pump-probe experiments and numerical simulations. The combination of a zero-dimensional and a two-dimensional substructure in this system gives rise to crossed excitons, with the electron confined in the quantum dot and the hole in the quantum well. An analysis of a large set of experiments at low carrier density and variable extraction potential shows that excitons created with no excess energy remain Coulomb-coupled and equilibrate and diffuse as correlated pairs. Individual quantum dots reach their quasi thermal equilibrium after few picoseconds, spatial equilibration of the whole system is reached after 300 ps.