We discuss the optical conductivity of several noninteracting two-dimensional semiconducting systems focusing on gapped Dirac and Schrödinger fermions as well as on a system mixing these two types. Close to the band gap, we can define a universal optical conductivity quantum of ${\sigma }_0=\frac{1}{16}\frac{e^2}{\hslash }$ for the pure systems. The effective optical conductivity then depends on the degeneracy factors $g_s$ (spin) and $g_v$ (valley) and on the curvature around the band gap $\nu$, i.e., it generally reads $\sigma =g_s{g}_v\nu {\sigma }_0$. For a system composed of both types of carriers, the optical conductivity becomes nonuniversal.

We present a first principles method for calculating the inelastic electron tunneling spectroscopy (IETS) on gated graphene. We reproduce experiments on pristine graphene and point out the importance of including several phonon modes to correctly estimate the local doping from IETS. We demonstrate how the IETS of typical imperfections in graphene can yield characteristic fingerprints revealing, e.g., adsorbate species or local buckling. Our results show how care is needed when interpreting scanning tunneling microscopy images of defects due to suppression of the elastic tunneling on graphene.

Photoinduced melting of the spin-Peierls phase was investigated in Na-tetracyanoquinodimethane by pump-probe reflection spectroscopy with a time resolution of 49 fs. Photoirradiation generates coherent molecular oscillation, which indicates that the spin-Peierls dimerization decreased. Increasing the excitation photon density to $>0.02$ photon/molecule doubles the oscillation frequency attributable to recovery of the original symmetry, that is, complete melting of the spin-Peierls phase. We observed a complete photoinduced transition with symmetry recovery revealed by frequency doubling of coherent oscillations.

In ${\mathrm{FeSc}}_2{\mathrm{S}}_4$ spin-orbital exchange competes with strong spin-orbit coupling, suppressing long-range spin and orbital order and, hence, this material represents one of the rare examples of a spin-orbital liquid ground state. Moreover, it is close to a quantum-critical point separating the ordered and disordered regimes. Using terahertz and far-infrared spectroscopy we study low-lying excitations in ${\mathrm{FeSc}}_2{\mathrm{S}}_4$ and provide clear evidence for a spin-orbiton, an excitation of strongly entangled spins and orbitals. It becomes particularly well pronounced upon cooling, when advancing deep into the quantum-critical regime. Moreover, indications of an underlying structureless excitation continuum are found, a possible signature of quantum criticality.

We report on the coherent control and transport of indirect-exciton (IX) spins in GaAs double quantum well (DQW) nanostructures. The spin dynamics was investigated by optically generating spins using a focused, circularly polarized light spot and by probing their spatial distribution using spatially and polarization resolved photoluminescence spectroscopy. Optically injected IX spins precess while moving over distances exceeding $20\mu \mathrm{m}$ from the excitation spot with a precession length that depends on the spin transport direction as well as on the bias applied across the DQW structure. This behavior is attributed to the spin precession in the effective magnetic field induced by the spin-orbit interaction. From the dependence of the spin dynamics on the transport direction, bias, and external magnetic fields we directly determined the Dresselhaus and Rashba electron spin splitting coefficients for the DQW structure. The precession dynamics is essentially independent of the IX density, thus indicating that the long spin lifetimes are not associated with IX collective effects. The long IX lifetimes, together with the negligible contribution of holes to the spin dynamics, are rather attributed to spatial separation of the electron and hole wave functions by the electric field, which reduces the electron-hole exchange interaction. If extended to the single-exciton regime, the present results on coherent spin precession over long transport distances as well as the control of the spin vector using electric and magnetic fields open the way for the application of IX spins in quantum information processing.

We construct a low-energy effective action for a two-dimensional nonrelativistic topological (i.e., gapped) phase of matter in a continuum, which completely describes all of its bulk electrical, thermal, and stress-related properties in the limit of low frequencies, long distances, and zero temperature, without assuming either Lorentz or Galilean invariance. This is done by generalizing Luttinger's approach to thermoelectric phenomena, via the introduction of a background vielbein (i.e., gravitational) field and spin connection a la Cartan, in addition to the electromagnetic vector potential, in the action for the microscopic degrees of freedom (the matter fields). Crucially, the geometry of spacetime is allowed to have timelike and spacelike torsion. These background fields make all natural invariances—under U(1) gauge transformations, translations in both space and time, and spatial rotations—appear locally, and corresponding conserved currents and the stress tensor can be obtained, which obey natural continuity equations. On integrating out the matter fields, we derive the most general form of a local bulk induced action to first order in derivatives of the background fields, from which thermodynamic and transport properties can be obtained. We show that the gapped bulk cannot contribute to low-temperature thermoelectric transport other than the ordinary Hall conductivity; the other thermoelectric effects (if they occur) are thus purely edge effects. The coupling to “reduced” spacelike torsion is found to be absent in minimally coupled models, and, using a generalized Belinfante stress tensor, the stress response to time-dependent vielbeins (i.e., strains) is the Hall viscosity, which is robust against perturbations and related to the spin current, as in earlier work.

We investigate the local electron temperature distribution in graphene nanoribbon and graphene junctions subject to an applied thermal gradient. Using a realistic model of a scanning thermal microscope, we predict quantum temperature oscillations whose wavelength is related to that of Friedel oscillations. Experimentally this wavelength can be tuned over several orders of magnitude by gating or doping, bringing quantum temperature oscillations within reach of the spatial resolution of existing measurement techniques.

We present a numerically efficient technique to evaluate the Green's function for extended two-dimensional systems without relying on periodic boundary conditions. Different regions of interest, or “patches,” are connected using self-energy terms which encode the information of the extended parts of the system. The calculation scheme uses a combination of analytic expressions for the Green's function of infinite pristine systems and an adaptive recursive Green's function technique for the patches. The method allows for an efficient calculation of both local electronic and transport properties, as well as the inclusion of multiple probes in arbitrary geometries embedded in extended samples. We apply the patched Green's function method to evaluate the local densities of states and transmission properties of graphene systems with two kinds of deviations from the pristine structure: bubbles and perforations with characteristic dimensions of the order of 10–25 nm, i.e., including hundreds of thousands of atoms. The strain field induced by a bubble is treated beyond an effective Dirac model, and we demonstrate the existence of both Friedel-type oscillations arising from the edges of the bubble, as well as pseudo-Landau levels related to the pseudomagnetic field induced by the nonuniform strain. Second, we compute the transport properties of a large perforation with atomic positions extracted from a transmission electron microscope image and show that current vortices may form near the zigzag segments of the perforation.

Many-body localization in a disordered system of interacting spins coupled by the long-range interaction $1/R^{\alpha }$ is investigated combining analytical theory considering resonant interactions and a finite-size scaling of exact numerical solutions with number of spins $N$. The numerical results for a one-dimensional system are consistent with the general expectations of analytical theory for a $d$-dimensional system including the absence of localization in the infinite system at $\alpha <2d$ and a universal scaling of a critical energy disordering $W_c\propto N^{\frac{2d-\alpha }{d}}$.

Reversible magnetic exchange springs can be formed in the magnetically soft $\mathrm{YF}{\mathrm{e}}_2$ layers of epitaxial $\mathrm{DyF}{\mathrm{e}}_2/\mathrm{YF}{\mathrm{e}}_2$ multilayer films. Here we show that the insertion of just two monolayers of $\mathrm{DyF}{\mathrm{e}}_2$, placed directly in the middle of the $\mathrm{YF}{\mathrm{e}}_2$ layers, brings about substantial changes. Results are presented for a Dy-doped (110)-oriented ${[{\mathrm{DyFe}}_2\left(60\AA \right)/{\mathrm{YFe}}_2\left(120\AA \right)/{\mathrm{DyFe}}_2\left(8\AA \right)/{\mathrm{YFe}}_2\left(120\AA \right)]}_{15}$ multilayer film, measured at 100 K in fields of up to ±10 T. Using bulk magnetometry, micromagnetic modeling, and Dy-specific x-ray magnetic circular dichroism, it is shown that Dy doping substantially increases the number of spin states available to the system. Altogether 12 distinct spring states are identified which bring additional complexity to the magnetic reversal process. In particular, the exchange springs are no longer reversible, exhibiting magnetic exchange-spring collapse. Full and partial magnetic loops are presented for fields applied along the in-plane easy [001] axis and the in-plane hard $\left[\overline{1}10\right]$ axis. In particular, it is demonstrated that exchange-spring collapse is sharpest when the field is applied along a hard in-plane $\left[\overline{1}10\right]$ axis.

For coupled-dimer Heisenberg magnets, a paradigm of magnetic quantum phase transitions, we develop a systematic expansion in $1/d$, the inverse number of space dimensions. The expansion employs a formulation of the bond-operator technique and is based on the observation that a suitably chosen product-state wave function yields exact zero-temperature expectation values of local observables in the $d\to \infty$ limit, with corrections vanishing as $1/d$. We demonstrate the approach for a model of dimers on a hypercubic lattice, which generalizes the square-lattice bilayer Heisenberg model to arbitrary $d$. In this paper, we use the $1/d$ expansion to calculate static and dynamic observables at zero temperature in the paramagnetic singlet phase, up to the quantum phase transition, and compare the results with numerical data available for $d=2$. Contact is also made with previously proposed refinements of bond-operator theory as well as with a perturbative expansion in the interdimer coupling. In a companion paper, the present $1/d$ expansion will be extended to the ordered phase, where it is shown to consistently describe the entire phase diagram including the quantum critical point.

We extend to magnetically ordered phases a recently developed expansion in $1/d$ for coupled-dimer Heisenberg magnets, where $d$ is the number of space dimensions. This extension utilizes generalized bond operators describing spin excitations on top of a reference state involving triplet condensates. We explicitly consider a model of dimers on a hypercubic lattice which displays, in addition to the paramagnetic singlet phase, a collinear antiferromagnetic phase for which we calculate static and dynamic observables at zero temperature. In particular, we show that the $1/d$ expansion smoothly connects the paramagnetic and antiferromagnetic phases and produces sensible results at and near the quantum phase transition point. Among others, we determine the dispersion and spectral-weight distribution of the amplitude (i.e., Higgs) mode of the ordered phase. In the limit of vanishing intradimer coupling, we connect our approach to spin-wave theory.

We use the coupled cluster method in order to calculate the ground-state phase diagram of the XXZ spin-$s$ kagome antiferromagnet with easy-plane anisotropy, i.e., the anisotropy parameter $\Delta$ varies between $\Delta =1$ (isotropic Heisenberg model) and $\Delta =0(XY$ model). We find that for the extreme quantum case $s=1/2$ the ground state is magnetically disordered in the entire region $0\le \Delta \le 1$. For $s=1$ the ground state is disordered for $0.818<\Delta \le 1$, it exhibits $\sqrt{3}\times \sqrt{3}$ magnetic long-range order for $0.281<\Delta <0.818$, and $\mathbf{q}=\mathbf{0}$ magnetic long-range order for $0\le \Delta <0.281$. We confirm the recent result of Chernyshev and Zhitomirsky [Phys. Rev. Lett. 113, 237202 (2014)] that the selection of the ground state by quantum fluctuations is different for small $\Delta$ ($XY$ limit) and for $\Delta$ close to 1 (Heisenberg limit), i.e., $\mathbf{q}=\mathbf{0}$ magnetic order is favored over $\sqrt{3}\times \sqrt{3}$ for $0\le \Delta <{\Delta }_c$ and vice versa for ${\Delta }_c<\Delta \le 1$. We calculate the critical anisotropy parameter ${\Delta }_c$ as a function of the spin quantum number $s$.

We report x-ray absorption and photoemission spectroscopy of the electronic structure in the normal state of metallic ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$. The data reveal evidence for large fluctuating spin moments on the Fe sites, as indicated by exchange multiplets appearing in the Fe $3s$ core-level photoemission spectra, even though the compound does not show magnetic order. The magnitude of the multiplet splitting is comparable to that observed in the normal state of the Fe-pnictide superconductors. This shows a connection between ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$ and the Fe-based superconductors even though it contains neither pnictogens nor chalcogens. The implication is that the chemical range of compounds showing at least one of the characteristic magnetic signatures of the Fe-based superconductors is broader than previously thought.

Topologically ordered phases of matter, in particular so-called symmetry-enriched topological phases, can exhibit quantum number fractionalization in the presence of global symmetry. In ${\mathbb{Z}}_2$ topologically ordered states in two dimensions, fundamental translations $T_x$ and $T_y$ acting on anyons can either commute or anticommute. This property, crystal momentum fractionalization, can be seen in a periodicity of the excited-state spectrum in the Brillouin zone. We present a numerical method to detect the presence of this form of symmetry enrichment given a projected entangled pair state; we study the minima of the spectrum of correlation lengths of the transfer matrix for a cylinder. As a benchmark, we demonstrate our method using a modified toric code model with perturbation. An enhanced periodicity in momentum clearly reveals the nontrivial anticommutation relation $\{T_x,T_y\}=0$ for the corresponding quasiparticles in the system.

We performed scanning tunneling microscopy at room temperature on the conducting surface of $\alpha -(\mathrm{BEDT-TTF}{)}_2{\mathrm{I}}_3$, which undergoes charge ordering below $T_{\mathrm{CO}}=135\mathrm{K}$. An $\alpha -\text{type}$ BEDT-TTF molecular arrangement was clearly observed. We observed a charge stripe structure with some inhomogeneity at room temperature. The horizontal charge stripe developed in most of the observed area. However, the coherence was restricted by the presence of charge uniform regions. The dimerization was found only between $A$ and $A^{\prime }$. The charge disproportionation state developed on the donor arrangement, which differs from that in the charge ordered state below $T_{\mathrm{CO}}$. A certain amount of stripe structure inhomogeneity is expected to be caused by the absence of the dimer of $B$ and $C$ BEDT-TTF molecules. We also found a local diagonal stripe that formed the boundary between two types of horizontal stripes.

Electronic states confined to zero-angle grain boundaries in single-layer graphene are analyzed using topological band theoretic arguments. We identify a hidden chiral symmetry which supports symmetry-protected zero modes in projected bulk gaps. These branches occupy a finite fraction of the interface-projected Brillouin zone and terminate at bulk gap closures, manifesting topological transitions in the occupied manifolds of the bulk systems that are joined at an interface. These features are studied by numerical calculations on a tight-binding lattice and by analysis of the geometric phases of the bulk ground states.

Using density functional theory (DFT) we demonstrate that the adsorption of large cations such as potassium or cesium facilitates the epitaxial growth of polar ${\mathrm{LaAlO}}_3\left(\mathrm{LAO}\right)$ on ${\mathrm{SrTiO}}_3\left(\mathrm{STO}\right)$. The low ionization potential of K favors efficient electron transfer to the STO conduction band and results in a two-dimensional electron gas which exactly compensates for the diverging potential with increasing layer thickness. For large cations like K or Cs, DFT total energy considerations show that they remain adsorbed on the LAO surface and do not enter substitutionally into LAO. These results suggest a scheme for growing clean LAO/STO interface systems, and polar systems in general, by performing the growth process in the presence of large, low ionization potential alkali-metal ions.