Resistive switching, i.e., the remanent (reversible) change of a device's resistance, is a widely investigated phenomenon as it holds the prospect for realizing high density memory devices. Resistive switching has also been observed in organic semiconductors; however, a clear understanding of the underlying physics could not yet be obtained. Possible options are for example interface effects at the electrodes or the formation and destruction of filaments. Here we present resistive switching in an organic spin valve based on tunneling anisotropic magnetoresistance. Similar to experiments in conventional spin-valve devices with two ferromagnetic electrodes we observe a modulation of the magnetoresistance by the electrical switching. However, as the magnetoresistance effect's origin is unambiguously clear, which is not always the case for effects in conventional structures, it can be exploited to prove that a tunnel barrier exists at the interface between the ferromagnetic oxide electrode and the organic semiconductor. Furthermore our experiments reveal that this barrier is reversibly modified during the switching, which causes both the change in magnetoresistance and total device resistance. Quantitative analysis indicates that the barrier is situated in the oxide layer. A phenomenological model provides a full description of the microscopic processes involved in the resistive switching.

At Coulomb blockade valleys inelastic cotunneling processes generate particle-hole excitations in quantum dots (QDs), and lead to energy dissipation. We have analyzed the probability distribution function (PDF) of energy dissipated in a QD due to such processes during a given time interval. We obtained analytically the cumulant generating function, and extracted the average, variance, and Fano factor. The latter diverges as $T^3/{\left(eV\right)}^2$ at bias $eV$ smaller than the temperature $T$, and reaches the value $3eV/5$ in the opposite limit. The PDF is further studied numerically. As expected, the Crooks fluctuation relation is not fulfilled by the PDF. Our results can be verified experimentally utilizing transport measurements of charge.

The properties of two quantum spin chain materials, ${\mathrm{K}}_2{\mathrm{CuSO}}_4{\mathrm{Cl}}_2$ and ${\mathrm{K}}_2{\mathrm{CuSO}}_4{\mathrm{Br}}_2$, are studied by a variety of experimental techniques, including bulk measurements, neutron spectroscopy, and electron spin resonance. The hierarchy of relevant terms in the magnetic Hamiltonian is established. It is shown that these two compounds feature substantial Dzyaloshinskii-Moriya interactions that are uniform within each chain, but antiparallel in adjacent chains. The result is a peculiar type of frustration of interchain interactions, which leads to an unusual field-temperature phase diagram.

We revisit the problem that relevant parts of band structures for a given cell choice can reflect exact or approximate higher symmetries of subsystems in the cell and can therefore be significantly simplified by an unfolding procedure that recovers the higher symmetry. We show that band-structure unfolding can be understood as projection onto induced irreducible representations of a group obtained by extending the original group of translations with a number of additional symmetry operations. The resulting framework allows us to define a generalized unfolding procedure that includes the point group operations and can be applied to any quantity in the reciprocal space. The unfolding of the Brillouin zone follows naturally from the properties of the induced irreducible representations. In this context, we also introduce a procedure to derive tight-binding models of reduced dimensionality by making use of point group symmetries. Further, we show that careful consideration of unfolding has important consequences on the interpretation of angle-resolved photoemission experiments. Finally, we apply the unfolding procedure to various representative examples of Fe-based superconductor compounds and show that the one-iron picture arises as an irreducible representation of the glide-reflection group, and we comment on the consequences for the interpretation of one-iron versus two-iron Brillouin zone representations.

The valley degeneracy of electron states in graphene stimulates intensive research of valley-related optical and transport phenomena. While many proposals on how to manipulate valley states have been put forward, experimental access to the valley polarization in graphene is still a challenge. Here, we develop a theory of the second optical harmonic generation in graphene and show that this effect can be used to measure the degree and sign of the valley polarization. We show that, at the normal incidence of radiation, the second harmonic generation stems from an imbalance of carrier populations in the valleys. The effect has a specific polarization dependence reflecting the trigonal symmetry of the electron valley and is resonantly enhanced if the energy of the incident photons is close to the Fermi energy.

We analyze the strong hexagonal warping of the Dirac cone of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ by angle-resolved photoemission. Along $\overline{\Gamma }\overline{\mathrm{M}}$, the dispersion deviates from a linear behavior meaning that the Dirac cone is warped outwards and not inwards. We show that this introduces an anisotropy in the lifetime broadening of the topological surface state which is larger along $\overline{\Gamma }\overline{\mathrm{K}}$. The result is not consistent with an explanation by nesting properties. Based on the theoretically predicted modifications of the ground-state spin texture of a strongly warped Dirac cone, we propose spin-dependent scattering processes as explanation for the anisotropic scattering rates. These results could help paving the way for optimizing future spintronic devices using topological insulators and controlling surface-scattering processes via external gate voltages.

The recently synthesized iridate $\beta \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$ has been proposed as a candidate to display novel magnetic behavior stabilized by frustration effects from bond-dependent, anisotropic interactions (Kitaev model) on a three-dimensional “hyperhoneycomb” lattice. Here we report a combined study using neutron powder diffraction and magnetic resonant x-ray diffraction to solve the complete magnetic structure. We find a complex, incommensurate magnetic order with non-coplanar and counter-rotating Ir moments, which surprisingly shares many of its features with the related structural polytype “stripyhoneycomb” $\gamma \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$, where dominant Kitaev interactions have been invoked to explain the stability of the observed magnetic structure. The similarities of behavior between those two structural polytypes, which have different global lattice topologies but the same local connectivity, are strongly suggestive that the same magnetic interactions and the same underlying mechanism govern the stability of the magnetic order in both materials, indicating that both $\beta$- and $\gamma \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$ are strong candidates to realize dominant Kitaev interactions in a solid state material.

We use video-rate magneto-optical Kerr effect microscopy to study zero-field magnetic fluctuations in a perpendicularly magnetized cobalt film at room temperature, prepared near a spin reorientation transition. Globally, the film has zero net moment but local areas continually undergo thermal magnetic fluctuations between saturated states. Position resolved hysteresis loops show the fluctuations occur at sample locations close to the spin reorientation transition. Furthermore, nearby fluctuations interact magnetostatically, acting to maintain the overall zero net magnetization. The measured scaling exponent of the fluctuation areas $\tau$ is consistent with $4/3$.

We study how coexisting states of superconductivity and magnetism interact with each other by local measurements of the magnetic fields, with micron scale spatial resolution, above a single crystal of the ferromagnetic superconductor UCoGe using scanning superconducting quantum interference device microscopy. Our measurements show that the spontaneous ferromagnetic transition at $T_{\mathrm{C}}=2.5$ K is characterized by Ising-like magnetization along the easy axis ($c$ direction), with domain sizes of the order of $\mathrm{10}\mu \mathrm{m}$, magnetization amplitudes of 45 G, and are consistent with estimates of domain-wall widths of several angstroms. The measured magnetization amplitudes are in agreement with bulk magnetization measurements, implying that domain reconstruction at the sample surface is negligible. In the superconducting state, which coexists with ferromagnetism below $T_{\mathrm{SC}}=0.67$ K, both diamagnetic screening and Meissner expulsion of flux, but no shrinkage of the ferromagnetic domains, are detected. Although we could not resolve individual vortices, our measurements provide evidence for the existence of the spontaneous vortex state in UCoGe.

We suggest a construction of a large class of topological states using an array of quantum wires. First, we show how to construct a Chern insulator using an array of alternating wires that contain electrons and holes, correlated with an alternating magnetic field. This is supported by semiclassical arguments and a full quantum-mechanical treatment of an analogous tight-binding model. We then show how electron-electron interactions can stabilize fractional Chern insulators (Abelian and non-Abelian). In particular, we construct a non-Abelian ${\mathbb{Z}}_3$ parafermion state. Our construction is generalized to wires with alternating spin-orbit couplings, which give rise to integer and fractional (Abelian and non-Abelian) topological insulators. The states we construct are effectively two dimensional, and are therefore less sensitive to disorder than one-dimensional systems. The possibility of experimental realization of our construction is addressed.

A scheme is proposed to construct integer and fractional topological quantum states of fermions in two spatial dimensions. We devise models for such states by coupling wires of nonchiral Luttinger liquids of electrons that are arranged in a periodic array. Which interwire couplings are allowed is dictated by symmetry and the compatibility criterion that they can simultaneously acquire a finite expectation value, opening a spectral gap between the ground state(s) and all excited states in the bulk. First, with these criteria at hand, we reproduce the tenfold classification table of integer topological insulators, where their stability against interactions becomes immediately transparent in the Luttinger liquid description. Second, we construct an example of a strongly interacting fermionic topological phase of matter with short-range entanglement that lies outside of the tenfold classification. Third, we expand the table to long-range entangled topological phases with intrinsic topological order and fractional excitations.

Molecular transport of a two-component system of liquid and vapor in a porous medium can be anomalously increased owing to fast exchange between the two phases [Phys. Rev. Lett. 63, 43 (1989)]. We have investigated this phenomenon measuring the self-diffusion coefficient of methanol adsorbed in a 98% porosity aerogel using nuclear magnetic resonance field gradient techniques. We found enhancement of several orders of magnitude from which we determined the ballistic mean-free path in the vapor phase. We have grown globally uniform anisotropic aerogels and applied the diffusion measurements to characterize the anisotropy. Our results are important for understanding the novel properties of superfluid ${}^3\mathrm{He}$ confined within an aerogel framework and for application to other physical systems.

The quest for broadening the materials base of topological insulators (TIs) beyond the handful of presently known examples has recently led to exploratory calculations of the topological $Z_2$ metric from the band structure of various candidate compounds in assumed crystal structures. However, structural transformations such as volume compression, lattice straining, or atom swaps that are used to instigate in a trivial insulator the band inversion underlying TI-ness, might also destabilize the system to the point that it either distorts into a more stable structure or does not form at all. Whether the more stable form of the candidate material is a TI, it is to be determined. Yet, often TI discovery calculations do not assess whether the postulated structural forms predicted to be TIs are also the stable forms of these compounds. Here, we show that in the broad family of III-Bi-${\mathrm{O}}_3$ oxides $(\text{III}=\text{Al}$, Ga, In, Sc, Y, and La), the cubic $A^{\text{III}}{\mathrm{BiO}}_3$ perovskite structure, which has been recently predicted to be TI for ${\mathrm{YBiO}}_3$, is unstable, whereas the stable $A^{\text{III}}{\mathrm{BiO}}_3$ structural forms are trivial band insulators.

We present an improved first-principles description of melting under pressure based on thermodynamic integration comparing density functional theory (DFT) and quantum Monte Carlo (QMC) treatments. The method is applied to address the longstanding discrepancy between DFT calculations and diamond anvil cell (DAC) experiments on the melting curve of xenon, a noble gas solid where van der Waals binding is challenging for traditional DFT methods. The calculations show agreement with data below 20 GPa and that the high-pressure melt curve is well described by a Lindemann behavior up to at least 80 GPa, in contrast to DAC data.

We present a study of thermal conductivity in superconducting ${\mathrm{SrTi}}_{1-x}{\mathrm{Nb}}_x{\mathrm{O}}_3$, sufficiently doped to be near its maximum critical temperature. The bulk critical temperature, determined by the jump in specific heat, occurs at a significantly lower temperature than the resistive $T_c$. Thermal conductivity, dominated by the electron contribution, deviates from its normal-state magnitude at bulk $T_c$, following a Bardeen-Rickayzen-Tewordt behavior, which is expected for thermal transport by Bogoliubov excitations. The absence of a T-linear term at very low temperatures rules out the presence of nodal quasiparticles. On the other hand, the field dependence of thermal conductivity points to the existence of at least two distinct superconducting gaps. We conclude that optimally doped strontium titanate is a multigap nodeless superconductor.

We study the problem of phase transitions from three-dimensional topological to normal insulators without inversion symmetry. In contrast with the conclusions of some previous work, we show that a Weyl semimetal always exists as an intermediate phase regardless of any constriant from lattice symmetries, although the interval of the critical region is sensitive to the choice of path in the parameter space and can be very narrow. We demonstrate this behavior by carrying out first-principles calculations on the noncentrosymmetric topological insulators ${\mathrm{LaBiTe}}_3$ and ${\mathrm{LuBiTe}}_3$ and the trivial insulator BiTeI. We find that a robust Weyl-semimetal phase exists in the solid solutions ${\mathrm{LaBi}}_{1-x}{\mathrm{Sb}}_x{\mathrm{Te}}_3$ and ${\mathrm{LuBi}}_{1-x}{\mathrm{Sb}}_x{\mathrm{Te}}_3$ for $x\approx 38.5\%\text{–}41.9\%$ and $x\approx 40.5\%\text{–}45.1\%$, respectively. A low-energy effective model is also constructed to describe the critical behavior in these two materials. In BiTeI, a Weyl semimetal also appears with applied pressure, but only within a very small pressure range, which may explain why it has not been experimentally observed.

We show how the superconducting phase difference in a Josephson junction may be used to split the Kramers degeneracy of its energy levels and to remove all the properties associated with time-reversal symmetry. The superconducting phase difference is known to be ineffective in two-terminal short Josephson junctions, where irrespective of the junction structure the induced Kramers degeneracy splitting is suppressed and the ground state fermion parity must stay even, so that a protected zero-energy Andreev level crossing may never appear. Our main result is that these limitations can be completely avoided by using multiterminal Josephson junctions. There the Kramers degeneracy breaking becomes comparable to the superconducting gap, and applying phase differences may cause the change of the ground state fermion parity from even to odd. We prove that the necessary condition for the appearance of a fermion parity switch is the presence of a “discrete vortex” in the junction: the situation when the phases of the superconducting leads wind by $2\pi$. Our approach offers strategies for creation of Majorana bound states as well as spin manipulation. Our proposal can be implemented using any low density, high spin-orbit material such as InAs quantum wells, and can be detected using standard tools.

We report room-temperature lasing in two-dimensional diffractive lattices of silver and gold plasmon particle arrays embedded in a dye-doped polymer that acts both as waveguide and gain medium. As compared to conventional dielectric distributed feedback (DFB) lasers, a central question is how the underlying band structure from which lasing emerges is modified by both the much stronger scattering and the disadvantageous loss of metal. We use spectrally resolved back-focal plane imaging to measure the wavelength and angle dependence of emission below and above threshold, thereby mapping the band structure. We find that, for silver particles, the band structure is strongly modified compared to dielectric reference DFB lasers since the strong scattering gives large stop gaps. In contrast, gold particles scatter weakly and absorb strongly, so that thresholds are higher, but the band structure is not strongly modified. The experimental findings are supported by finite element and Fourier modal method calculations of the single-particle scattering strength and lattice extinction.

We consider a multiband metal with deep primary bands and a shallow secondary one. In the normal state the system undergoes a Lifshitz transition when the bottom of the shallow band crosses the Fermi level. In the superconducting state Cooper pairing in the shallow band is induced by the deep ones. As a result, the density of electrons in the shallow band remains finite even when the bottom of the band is above the Fermi level. We study the density of states in the system and find qualitatively different behaviors on the two sides of the Lifshitz transition. On one side of the transition the density of states diverges at an energy equal to the induced gap, whereas on the other side it vanishes. We argue that this physical picture describes the recently measured gap structure in shallow bands of iron pnictides and selenides.

Since the seminal work of Marzari and Vanderbilt, maximally localized Wannier functions have become widely used as a real-space representation of the electronic structure of periodic materials. In this paper we introduce selectively localized Wannier functions which extend the method of Marzari and Vanderbilt in two important ways. First, our method allows us to focus on localizing a subset of orbitals of interest. Second, our method allows us to fix centers of these orbitals, and ensure the preservation of the point-group symmetry. These characteristics are important when Wannier functions are used in methodologies that go beyond density functional theory by treating a local subspace of the Hamiltonian more effectively. Application of our method to GaAs, ${\mathrm{SrMnO}}_3$, and Co demonstrates that selectively localized Wannier functions can offer improvements over the maximally localized Wannier function technique.