We construct minimal electronic models for a newly discovered superconductor LaO${}_{1-x}$F${}_x$BiS${}_2$ ($T_c=10.6$ K) possessing BiS${}_2$ layers based on a first-principles band calculation. First, we obtain a model consisting of two Bi $6p$ and two S $3p$ orbitals, which give nearly electron-hole symmetric bands. Further focusing on the bands that intersect the Fermi level, we obtain a model with two $p$ orbitals. The two bands (per BiS${}_2$ layer) have a quasi-one-dimensional character with a double minimum dispersion, which gives good nesting of the Fermi surface. At around $x\sim 0.5$ the topology of the Fermi surface changes, so that the density of states at the Fermi level becomes large. Possible pairing states are discussed.

Gate-modulated and laser-dependent Raman spectroscopy have been widely used to study $q=0$ zone center phonon modes, their self-energy, and their coupling to electrons in graphene systems. In this work we use gate-modulated Raman of $q\ne 0$ phonons as a technique to understand the nature of five second-order Raman combination modes observed in the frequency range of 1700–2300 cm${}^{-1}$ of single-layer graphene (SLG). Anomalous phonon self-energy renormalization phenomena are observed in all five combination modes within this intermediate frequency region, which can clearly be distinguished from one another. By combining the anomalous phonon renormalization effect with the double resonance Raman theory, which includes both phonon dispersion relations and angular dependence of the electron-phonon scattering matrix elements, and by comparing it to the experimentally obtained phonon dispersion, measured by using different laser excitation energies, we can assign each Raman peak to the proper phonon combination mode. This approach should also shed light on the understanding of more complex structures such as few-layer graphene (FLG) and its stacking orders as well as other two-dimensional (2D)-like materials.

We have successfully synthesized a layered iron oxychalcogenide BaFe${}_2$Se${}_2$O single crystal. This compound is built up of Ba and Fe-Se(O) layers alternatively stacked along the $c$ axis. The Fe-Se(O) layers contain double chains of edge-shared Fe-Se(O) tetrahedra that propagate along the $b$ axis and are bridged by oxygen along the $a$ axis. Physical property measurements indicate that BaFe${}_2$Se${}_2$O is a semiconductor without the Curie-Weiss behavior up to 350 K. There is a possible long-range antiferromagnetic transition at 240 K, corresponding to the peak in specific-heat measurement, and two transitions at 115 K and 43 K where magnetic susceptibility drops abruptly. The magnetic entropy up to 300 K is much smaller than the expected value for Fe${}^{2+}$ in tetrahedral crystal fields and the Mössbauer spectrum indicates that long-range magnetic order is unlikely at 294 K. Our results suggest that BaFe${}_2$Se${}_2$O is a magnetic insulator at the borderline between a long-range antiferromagnetic spin ordering and possible spin dimerization.

The breaking of time-reversal symmetry by ferromagnetism is predicted to yield profound changes to the electronic surface states of a topological insulator. Here, we report on a concerted set of structural, magnetic, electrical, and spectroscopic measurements of Mn-Bi${}_2$Se${}_3$ thin films wherein photoemission and x-ray magnetic circular dichroism studies have recently shown surface ferromagnetism in the temperature range $15\text{K}\le T\le 100$ K, accompanied by a suppressed density of surface states at the Dirac point. Secondary-ion mass spectroscopy and scanning tunneling microscopy reveal an inhomogeneous distribution of Mn atoms, with a tendency to segregate towards the sample surface. Magnetometry and anisotropic magnetoresistance measurements are insensitive to the high-temperature ferromagnetism seen in surface studies, revealing instead a low-temperature ferromagnetic phase at $T\lesssim 5$ K. The absence of both a magneto-optical Kerr effect and an anomalous Hall effect suggests that this low-temperature ferromagnetism is unlikely to be a homogeneous bulk phase but likely originates in nanoscale near-surface regions of the bulk where magnetic atoms segregate during sample growth. Although the samples are not ideal, with both bulk and surface contributions to electron transport, we measure a magnetoconductance whose behavior is qualitatively consistent with predictions that the opening of a gap in the Dirac spectrum drives quantum corrections to the conductance in topological insulators from the symplectic to the orthogonal class.

Cu${}_x$Bi${}_2$Se${}_3$ was recently found to be likely the first example of a time-reversal-invariant topological superconductor accompanied by helical Majorana fermions on the surface. Here we present that progressive Cu intercalation into this system introduces significant disorder and leads to an anomalous suppression of the superfluid density which was obtained from the measurements of the lower critical field. At the same time, the transition temperature $T_{\mathrm{c}}$ is only moderately suppressed, which agrees with a recent prediction for the impurity effect in this class of topological superconductors bearing strong spin-orbit coupling. Those unusual disorder effects give support to the possible odd-parity pairing state in Cu${}_x$Bi${}_2$Se${}_3$.

We report on spin-resolved measurements of single cobalt phthalocyanine molecules which are coupled to a ferromagnetic cobalt support. The same molecule is probed while the magnetization direction of the tip is rotated from parallel to antiparallel by the application of an external magnetic biasing field. Highly spin-polarized states near the Fermi energy demonstrate the realization of a single molecule spin filter. Intramolecular variations of the molecular spin polarization reveal a spin-polarized resonance for the molecular cobalt ion as well as a spin polarization carried by the organic ligand. The polarization measured for the molecular ion is of the same sign as the polarization of the cobalt support and has an opposite sign compared to the polarization measured for the molecular ligand. We argue that both effects arise due to a delicate balance in the hybridization between substrate states and molecular orbitals: surface and interface states, molecular ligand $\pi$ orbitals, and molecular cobalt-ion $d$ orbitals. Moreover, the degree of hybridization will influence the amount of charge transfer from the substrate into the unoccupied molecular orbitals, thereby affecting the molecular magnetic moment.

Superconductivity in iron selenides has experienced a rapid growth, but not without major inconsistencies in the reported properties. For alkali-intercalated iron selenides, even the structure of the superconducting phase is a subject of debate, in part because the onset of superconductivity is affected much more delicately by stoichiometry and preparation than in cuprate or pnictide superconductors. If high-quality, pure, superconducting intercalated iron selenides are ever to be made, the intertwined physics and chemistry must be explained by systematic studies of how these materials form and by and identifying the many coexisting phases. To that end, we prepared pure K${}_2$Fe${}_4$Se${}_5$ powder and superconductors in the K${}_x$Fe${}_{2-y}$Se${}_2$ system, and examined differences in their structures by high-resolution synchrotron and single-crystal x-ray diffraction. We found four distinct phases: semiconducting K${}_2$Fe${}_4$Se${}_5$, a metallic superconducting phase K${}_x$Fe${}_2$Se${}_2$ with $x$ ranging from 0.38 to 0.58, the phase KFe${}_{1.6}$Se${}_2$ with full K occupancy and no Fe vacancy ordering, and a oxidized phase K${}_{0.51\left(5\right)}$Fe${}_{0.70\left(2\right)}$Se that forms the PbClF structure upon exposure to moisture. We find that the vacancy-ordered phase K${}_2$Fe${}_4$Se${}_5$ does not become superconducting by doping, but the distinct iron-rich minority phase K${}_x$Fe${}_2$Se${}_2$ precipitates from single crystals upon cooling from above the vacancy ordering temperature. This coexistence of separate metallic and semiconducting phases explains a broad maximum in resistivity around 100 K. Further studies to understand the solubility of excess Fe in the K${}_x$Fe${}_{2-y}$Se${}_2$ structure will shed light on the maximum fraction of superconducting K${}_x$Fe${}_2$Se${}_2$ that can be obtained by solid state synthesis.

The spatially, spectrally, and temporally resolved emission from a nonsymmetric polariton condensate prepared in a staircase shape is investigated. The results show a strong dependence of the spatial emission profile on the excitation density due to polariton redistribution. At large excitation densities the onset of conventional photon lasing has a crucial effect on the system. It is possible to vary the position from which the peak emission originates, and an emission profile showing switching of the predominant emission position from one end of the pump spot to the other is demonstrated. The findings are compared to a numerical simulation.

We study the photoluminescence spectra excited at an edge side of $n$-InP slabs and observed from the broadside. In a moderately doped sample the intensity drops off as a power-law function of the distance from the excitation—up to several millimeters—with no change in the spectral shape. The hole distribution is described by a stationary Lévy-flight process over more than two orders of magnitude in both the distance and hole concentration. For heavily doped samples, the power law is truncated by free-carrier absorption. Our experiments are nearly perfectly described by the Biberman-Holstein transport equation with parameters found from independent optical experiments.

We model the coherent deposition of a mismatched material on the top facet of a nanowire and investigate the possible formation of a cylindrical island that is narrower than the nanowire stem. We calculate the elastic relaxation and the total energy of the system and determine the optimal shape of the deposit as a function of misfit, nanowire radius, and deposit thickness. If the values of any two of these parameters are set, then there is a critical value of the third one above which the formation of a genuine island that is narrower than the stem is favored compared to that of a disk of equal volume covering entirely the top facet. These critical values are easily accessible in current nanowire systems, and we predict that islanding can lead to a large reduction of the total energy. We discuss the similarities and differences between the present effect and the standard Volmer-Weber and Stranski-Krastanow growth modes. For semiconductor nanowires, islanding is likely to occur primarily in the case of catalyst-free growth. We argue that it might already have been observed in heterostructures of nanowires of group-III nitrides.

We describe the superconducting proximity effect taking place in a contact between a noncentrosymmetric superconductor and a diffusive normal/ferromagnetic metal within the quasiclassical theory of superconductivity. By solving numerically the Usadel equation with boundary conditions valid for arbitrary interface transparency, we show that the analysis of the proximity-modified local density of states in the normal side can be used to obtain information about the exotic superconductivity of noncentrosymmetric materials. We point out the signatures of triplet pairing, the coexistence of triplet and singlet pairing, and particular orbital symmetries of the pair potential. Exploiting the directional dependence of the spin polarization pair breaking effect on the triplet correlations, we show how the order relation between triplet and singlet gaps can be discriminated and that an estimation of the specific gap ratio is possible in some cases.

We study one-dimensional, interacting, gapped fermionic systems described by variants of the Peierls-Hubbard model, and we characterize their phases via a topological invariant constructed out of their Green's functions. We demonstrate that the existence of topologically protected, zero-energy states at the boundaries of these systems can be tied to the value of the topological invariant, just like when working with the conventional, noninteracting topological insulators. We use a combination of analytical methods and the numerical density matrix renormalization group method to calculate the values of the topological invariant throughout the phase diagrams of these systems, thus deducing when topologically protected boundary states are present. We are also able to study topological states in spin systems because, deep in the Mott insulating regime, these fermionic systems reduce to spin chains. In this way, we associate the zero-energy states at the end of an antiferromagnetic spin-1 Heisenberg chain with a topological invariant equal to 2.

A nontrivial Hall effect has been observed as an anomaly of the Hall conductivity in the magnetization process in a typical itinerant helimagnet MnP. Since the Dzyaloshinsky-Moriya interaction slightly modulates the spin structure, the anomaly is identified only in the specific magnetic (fanlike) phase that can have finite scalar spin chirality. The decrease in the magnitude of the anomaly as observed by doping Co is well accounted for in terms of the topological Hall effect arising from the spin-chirality-induced Berry phase in real space. A fictitious magnetic flux in real space is estimated to be $\sim 1$ T, which is consistent with the small modulation of the spin structure.

We investigate transport through normal-superconductor nanowire junctions in the presence of spin-orbit coupling and magnetic field. As the Zeeman field crosses the critical bulk value $B_c$ of the topological transition, a Majorana bound state (MBS) is formed, giving rise to a sharp zero-bias anomaly (ZBA) in the tunneling differential conductance. We identify novel features beyond this picture in wires with inhomogeneous depletion, such as the appearance of two MBSs inside a long depleted region for $B<B_c$. The resulting ZBA is in most cases weakly split and may coexist with Andreev bound states near zero energy. The ZBA may appear without evidence of a topological gap closing. This latter aspect is more evident in the multiband case and stems from a smooth pinch-off barrier. Most of these features are in qualitative agreement with recent experiments [V. Mourik et al., Science 336, 1003 (2012)]. We also discuss the rich phenomenology of the problem in other regimes which remain experimentally unexplored.

We measured the Dresselhaus spin-orbit interaction coefficient ${\beta }_1$ for (001)-grown GaAs/Al${}_{0.3}$Ga${}_{0.7}$As quantum wells for six different well widths $w$ between 6 and 30 nm. The varying size quantization of the electron wave vector $z$-component $\langle k_z^2\rangle \sim {(\pi /w)}^2$ influences ${\beta }_1=-\gamma \langle k_z^2\rangle$ linearly. The value of the bulk Dresselhaus coefficient $\gamma =(-11\pm 2)$ eVÅ${}^3$ was determined. We discuss the absolute sign of the Land$\acute{\mathrm{e}}$ $g$ factors and the effective momentum scattering times.

Here we present bulk property measurements and electronic structure calculations for PuFeAsO, an actinide analog of the iron-based rare-earth superconductors $R$FeAsO. Magnetic susceptibility and heat capacity data suggest the occurrence of an antiferromagnetic transition at $T_N=50$ K. No further anomalies have been observed down to 2 K, the minimum temperature that we have been able to achieve. Structural measurements indicate that PuFeAsO, with its more localized $5f$ electrons, bears a stronger resemblance to the $R$FeAsO compounds with larger $R$ ions than NpFeAsO does.

The sign and profile of the thermal Hall conductivity ${\kappa }_{xy}$ gives important insights into the gap structure of multiband superconductors. With this perspective, we have investigated ${\kappa }_{xy}$ and the thermal conductivity ${\kappa }_{xx}$ in ${\text{Ba}}_{1-x}{\text{K}}_x{\text{Fe}}_2{\text{As}}_2$ which display large peak anomalies in the superconducting state. The anomalies imply that a large holelike quasiparticle (QP) population exists below the critical temperature $T_c$. We show that the QP mean free path inferred from ${\kappa }_{xx}$ reproduces the observed anomaly in ${\kappa }_{xy}$, providing a consistent estimate of a large QP population. Further, we demonstrate that the peak structure and holelike signal are consistent with a theoretical scenario where, despite potentially large gap variations on the electron pockets, the minimal homogeneous gap of the superconducting phase resides at a hole pocket. Implications for probing the gap structure in the broader class of pnictide superconductors are discussed.

High intensity ultrafast acoustics is a powerful tool to modulate excitons in semiconductors on picosecond time scales. Here we give a particular example for the potential of this technique by demonstrating ultrafast destruction of exciton states with their subsequent, similarly fast recurrence, making the method's impact volatile, in stark contrast to other techniques. The origin of the destruction is strain modulation of single particle states on time scales so short that their contribution to exciton formation is hindered. This is demonstrated for an exciton confined in a quantum well, which is placed in a high quality microresonator. For this system the strong light-matter interaction leads to the formation of polaritons. The itinerant exciton destruction enforces a transfer to the weak coupling regime with fast subsequent recovery of strong coupling. Thereby a tool for controlling light-matter coupling in condensed matter systems has been realized, which may be of interest for emerging fields such as exciton polaritonics and quantum optics.

We report ${}^{75}$As nuclear magnetic resonance studies on an underdoped single-crystal Ba${}_{0.77}$K${}_{0.23}$Fe${}_2$As${}_2$ with $T_{\mathrm{c}}=16.5$ K. Below $T_{\mathrm{N}}=46$ K, the NMR peaks for $H\parallel c$ split and those for $H\parallel a$ shift to higher frequencies, which indicates that an internal magnetic field along the $c$ axis develops below $T_{\mathrm{N}}$. The spin-lattice relaxation rate $1/T_1$ measured at the shifted peak with $H\parallel a$, which experiences the internal field, shows a distinct decrease below $T_{\mathrm{c}}({\mu }_{0}H=12\mathrm{T})=16$ K, following a $T^3$ relation at low temperatures. Our results show unambiguously that antiferromagnetic order and superconductivity coexist microscopically. The unusual superconducting state with the coexisting magnetism is highlighted.

We present a correlative study of structural and optical properties of natural defects in planar semiconductor microcavities grown by molecular beam epitaxy, which are showing a localized polariton spectrum as reported in Zajac et al. [Phys. Rev. B 85, 165309 (2012)]. The three-dimensional spatial structure of the defects was studied using combined focused ion beam (FIB) and scanning electron microscopy (SEM). We find that the defects originate from a local increase of a GaAs layer thickness. Modulation heights of up to 140 nm for oval defects and 90 nm for round defects are found, while the lateral extension is about 2 $\mu$m for oval and 4 $\mu$m for round defects. The GaAs thickness increase is attributed to a Ga droplet deposited during growth due to Ga cell spitting. Following the droplet deposition, the thickness modulation expands laterally while reducing its height, yielding oval to round mounds of the interfaces and the surface. With increasing growth temperature, the ellipticity of the mounds is decreasing and their size is increasing. This suggests that the expansion is related to the surface mobility of Ga, which, with increasing temperature, is increasing and reducing its anisotropy between the $\left[110\right]$ and $\left[1\overline{1}0\right]$ crystallographic directions. Comprehensive data consisting of surface profiles of defects measured using differential interference contrast microscopy, volume information obtained using FIB/SEM, and characterization of the resulting confined polariton spectrum are presented.