We report the planar and anomalous Nernst effects in epitaxial thin films of spin polarized ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_3$. The thermal counterpart of the anomalous Hall effect in this material (i.e., the anomalous Nernst effect) shows an extreme sensitivity to any parasitic thermal gradient, resulting in large asymmetric voltages under small temperature differences. This should be considered when interpreting the magnitude of the electrical response in nanostructures and devices that operate under high current densities. Finally, none of the observed magnetothermoelectric signals is related to the spin Seebeck effect in this material.

Polarized neutron scattering experiments on the slightly overdoped superconductor ${\mathrm{Ba}}_{0.5}{\mathrm{K}}_{0.5}{\mathrm{Fe}}_2{\mathrm{As}}_2$ reveal broad magnetic resonance scattering peaking at approximately 15 meV. In spite of doping far beyond the suppression of magnetic order, this compound exhibits dispersive and anisotropic magnetic excitations. At energies below the resonance maximum, magnetic correlations polarized parallel to the layers but perpendicular to the propagation vector are reduced by a factor of 2 compared to those in the two orthogonal directions; in contrast, correlations at the peak maximum are isotropic.

Some polymer properties, such as conductivity, are very sensitive to short- and intermediate-range orientational and positional ordering of anisotropic molecular functional groups, and yet means to characterize orientational order in disordered systems are very limited. We demonstrate that resonant scattering at the carbon $K$ edge is uniquely sensitive to short-range orientation correlations in polymers through depolarized scattering at high momentum transfers, using atactic polystyrene as a well-characterized test system. Depolarized scattering is found to coexist with unpolarized fluorescence and to exhibit pronounced anisotropy. We also quantify the spatially averaged optical anisotropy from low-angle reflectivity measurements, finding anisotropy consistent with prior visible, x-ray absorption, and theoretical studies. The average anisotropy is much smaller than that in the depolarized scattering and the two have different character. Both measurements exhibit clear spectral signatures from the phenyl rings and the polyethylenelike backbone. Discussion focuses on analysis considerations and prospects for using this depolarized scattering for studies of disorder in soft condensed matter.

We present an exact expansion of the master equation for an open quantum system. The resulting equation is time local and enables us to calculate clearly defined higher-order corrections to the Born-Markov approximation. In particular, we show that non-Markovian terms are of the same order of magnitude as higher-order terms in the system-bath coupling. As a result, we emphasize that analyzing non-Markovian behavior of a system implies going beyond the Born approximation. Additionally, we address with this approach the initial-state problem occurring in non-Markovian master equations.

Using real-space dynamical mean-field theory with hybridization-expansion quantum Monte Carlo as a solver, we study the optical conductivity of two-dimensional topological Kondo insulating states. We consider model parameters which allow us to consider mixed-valence and local-moment regimes. The real-space resolution inherent to our approach reveals a renormalization of the hybridization gap as one approaches the edge. Low-energy transport is dominated by the helical edge state and the corresponding Drude weight scales as the coherence scale of the heavy fermion state. The concomitant renormalization of the edge state velocity leads to a constant edge local density of states. We discuss the implication of our results for the three-dimensional case.

We investigate the interplay of band structure topology and localization properties of Wannier functions. To this end, we extend a recently proposed compressed sensing based paradigm for the search for maximally localized Wannier functions [Ozolins et al., Proc. Natl. Acad. Sci. USA 110, 18368 (2013)]. We develop a practical toolbox that enables the search for maximally localized Wannier functions which exactly obey the underlying physical symmetries of a translationally invariant quantum lattice system under investigation. Most saliently, this allows us to systematically identify the most localized representative of a topological equivalence class of band structures, i.e., the most localized set of Wannier functions that is adiabatically connected to a generic initial representative. We also elaborate on the compressed sensing scheme and find a particularly simple and efficient implementation in which each step of the iteration is an $O(NlogN)$ algorithm in the number of lattice sites $N$. We present benchmark results on one-dimensional topological superconductors demonstrating the power of these tools. Furthermore, we employ our method to address the open question of whether compact Wannier functions can exist for symmetry-protected topological states such as topological insulators in two dimensions. The existence of such functions would imply exact flat-band models with finite range hopping. Here, we find numerical evidence for the absence of such functions. We briefly discuss applications in dissipative-state preparation and in devising variational sets of states for tensor network methods.

Supercells are often used in ab initio calculations to model compound alloys, surfaces, and defects. One of the main challenges of supercell electronic structure calculations is to recover the Bloch character of electronic eigenstates perturbed by disorder. Here we apply the spectral weight approach to unfolding the electronic structure of group III-V and II-VI semiconductor solid solutions. The illustrative examples include formation of donorlike states in dilute Ga(PN) and associated enhancement of its optical activity, direct observation of the valence band anticrossing in dilute GaAs:Bi, and a topological band crossover in ternary (HgCd)Te alloy accompanied by emergence of high-mobility Kane fermions. The analysis facilitates interpretation of optical and transport characteristics of alloys that are otherwise ambiguous in traditional first-principles supercell calculations.

We describe a broadly applicable theory of spin relaxation in materials with incoherent charge transport; examples include amorphous inorganic semiconductors, organic semiconductors, quantum dot arrays, and systems displaying trap-controlled transport or transport within an impurity band. The theory can incorporate many different relaxation mechanisms, so long as electron-electron correlations can be neglected. We focus primarily on spin relaxation caused by spin-orbit effects, which manifest through inhomogeneities in the $g$ factor and non-spin-conserving carrier hops, scattering, trapping, or detrapping. Analytic and numerical results from the theory are compared in various regimes with Monte Carlo simulations. Our results should assist in evaluating the suitability of various disordered materials for spintronic devices.

We investigate the relationship between charge concentration, exciton concentration, and photocurrent generation in fullerene-containing heterojunction diodes. Impedance measurements on ${\mathrm{C}}_{60}$ diodes reveal a charge buildup at the ${\mathrm{C}}_{60}$/bathocuproine (BCP) interface that can be swept out under reverse bias. In solar cell structures, a similar charge buildup is observed in dark conditions, and increases as a function of incident light intensity. Photoluminescence measurements reveal that the ${\mathrm{C}}_{60}$ exciton concentration is voltage dependent, explained via the process of exciton-polaron annihilation. This process has a negative impact on the generated photocurrent of the solar cells and thereby decreases the fill factor. A combination of electroabsorption, photoluminescence, and impedance measurements reveal a decrease in charge buildup and the associated exciton-polaron annihilation through the use of a BCP/3,4,9,10-perylenetetracarboxylic bis-benzimidazole/Ag cathode.

We propose an experiment that would establish the entanglement of Majorana zero modes in semiconductor nanowires by testing the Bell and Clauser-Horne-Shimony-Holt inequalities. Our proposal is viable with realistic system parameters, simple “keyboard” gating, and projective measurement. Theoretical models and simulation results indicate entanglement can be demonstrated with moderately accurate gate operations. In addition to providing further evidence for the existence of the Majorana bound states, our proposal could be used as an experimental stepping stone to more complicated braiding experiments.

The electrical resistance $R$ of metallic nanocontacts subjected to controlled cyclic electromigration in ultrahigh vacuum has been investigated in situ as a function of applied voltage $V$. For sufficiently small contacts, i.e., large resistance, a decrease of $R\left(V\right)$ while increasing $V$ is observed. This effect is tentatively attributed to the presence of contacts separated by thin vacuum barriers in parallel to ohmic nanocontacts. Simple model calculations indicate that both thermal activation or tunneling can lead to this unusual behavior. We describe our data by a tunneling model whose key parameter, i.e., the tunneling distance, changes because of thermal expansion due to Joule heating and/or electrostatic strain arising from the applied voltage. Oxygen exposure during electromigration prevents the formation of negative $R\left(V\right)$ slopes, and at the same time enhances the probability of uncontrolled melting, while other gases show little effects. In addition, indication for field emission has been observed in some samples.

Motivated by recent observations of charge order in the pseudogap regime of hole-doped cuprates, we show that crisscrossed stripe order can be stabilized by coherent, momentum-dependent interlayer tunneling, which is known to be present in several cuprate materials. We further describe how subtle variations in the couplings between layers can lead to a variety of stripe ordering arrangements, and discuss the implications of our results for recent experiments in underdoped cuprates.

The ultrathin film limit has been shown to be a rich playground for unusual low-dimensional physics. Taking the example of ${\mathrm{SrRuO}}_3$, which is ferromagnetic and metallic at the bulk limit, one finds that it becomes antiferromagnetic and insulating at the three-monolayers limit when grown on ${\mathrm{SrTiO}}_3$. The origin of the insulating state is traced to strongly orbital-dependent exchange splittings. A modest compressive strain of 1% of the ${\mathrm{SrTiO}}_3$ substrate is then found to drive the system into a highly confined two-dimensional 100% spin polarized metallic state. This metal-insulator transition driven by a modest strain could be useful in two-state device applications.

We address atomic collapse and its magnetic modulation in gapped graphene. Based on the exact solution of the Dirac equation, we obtain the complete supercritical Coulomb impurity spectra and accurate wave functions. The unique degeneracy, electron probability densities, pseudospin of supercritical states, and mass and core-size effects are investigated in the supercritical region. When a magnetic field is applied, atomic collapse can be effectively restrained. Both supercritical and subcritical impurity states exhibit obviously different behaviors in two valleys in the magnetic field. The Landau levels also exhibit different changes in supercritical and subcritical impurity potentials. This study is helpful in understanding relativistic quantum phenomena in graphene and related systems.

Bismuth Selenide $({\mathrm{Bi}}_2{\mathrm{Se}}_3)$ is a topological insulator exhibiting helical spin polarization and strong spin-orbit coupling. The spin-orbit coupling links the charge current to spin current via the spin Hall effect (SHE). We demonstrate a ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ spin detector by injecting the pure spin current from a magnetic permalloy layer to a ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ thin film and detect the inverse SHE in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. The spin Hall angle of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is found to be 0.0093 ± 0.0013 and the spin diffusion length in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ to be 6.2 ± 0.15 nm at room temperature. Our results suggest that topological insulators with strong spin-orbit coupling can be used in functional spintronic devices.

We find unconventional Mott insulators in a quasi-2D version of the Shastry-Sutherland model in a magnetic field. In our realization on a four-leg tube geometry, these are stabilized by correlated hopping of localized magnetic excitations. Using perturbative continuous unitary transformations (pCUTs, plus classical approximation or exact diagonalization) and the density matrix renormalisation group method (DMRG), we identify prominent magnetization plateaus at magnetizations $M=1/8,3/16,1/4$, and $1/2$. While the plateau at $M=1/4$ can be understood in a semiclassical fashion in terms of diagonal stripes, the plateau at $M=1/8$ displays highly entangled wheels in the transverse direction of the tube. Finally, the $M=3/16$ plateau is most likely to be viewed as a classical 1/8 structure on which additional triplets are fully delocalized around the tube. The classical approximation of the effective model fails to describe all these plateau structures which benefit from correlated hopping. We relate our findings to the full 2D system, which is the underlying model for the frustrated quantum magnet $\mathrm{SrCu}{\left({\mathrm{BO}}_3\right)}_2$.

A spin-orbit-coupled quantum wire, with one end proximate to an $s\text{-wave}$ superconductor, can become a topological superconductor, with a Majorana mode localized at each end of the superconducting region. It was recently shown that coupling one end of such a topological superconductor to two normal channels of interacting electrons leads to a novel type of frustration and a quantum critical point when both channels couple with equal strength. We propose an experimental method to access this critical point in a single quantum wire and show its resilience to disorder.

We consider Coulomb blockade effects for tunneling through a piece of wire with large resistance $R\gg 1$. This system can not be treated as a zero-dimensional one, as the dynamics of internal inhomogeneous degrees of freedom is crucial. At moderately high temperatures the linear conductance $G$ of the system is suppressed due to the one-dimensional Coulomb zero bias anomaly effect. At low $T$, besides the standard activational factor, there is an additional $T$-independent (though also exponentially strong) suppression of $G$. It arises due to the tunneling evolution of the charge in the wire to the equivipotential distribution. In the intermediate range of $T$ the $G\left(T\right)$ dependence is a power law, as in the phenomenological environmental theory. The effective “environmental resistance” entering the power exponent is found explicitly. It depends on the length of the wire and on the positions of the contacts.

We report the observation of the fractional quantum Hall effect in the lowest Landau level of a two-dimensional electron system (2DES), residing in the diluted magnetic semiconductor ${\mathrm{Cd}}_{1-x}{\mathrm{Mn}}_x\mathrm{Te}$. The presence of magnetic impurities results in a giant Zeeman splitting leading to an unusual ordering of composite fermion Landau levels. In experiment, this results in an unconventional opening and closing of fractional gaps around the filling factor $\nu =3/2$ as a function of an in-plane magnetic field, i.e., of the Zeeman energy. By including the s-d exchange energy into the composite Landau level spectrum the opening and closing of the gap at filling factor $5/3$ can be modeled quantitatively. The widely tunable spin-splitting in a diluted magnetic 2DES provides a means to manipulate fractional states.