We have studied the nature of the three-dimensional multiband electronic structure in the two-dimensional triangular lattice Ir${}_{1-x}$Pt${}_x$Te${}_2$ ($x=0.05$) superconductor using angle-resolved photoemission spectroscopy (ARPES), x-ray photoemission spectroscopy (XPS), and a band structure calculation. ARPES results clearly show a cylindrical (almost two-dimensional) Fermi surface around the zone center. Near the zone boundary, the cylindrical Fermi surface is truncated into several pieces in a complicated manner with strong three dimensionality. The XPS result and the band structure calculation indicate that the strong Te 5$p$-Te 5$p$ hybridization between the IrTe${}_2$ triangular lattice layers is responsible for the three dimensionality of the Fermi surfaces and the intervening of the Fermi surfaces observed by ARPES.

At the monolayer limit both ${\mathrm{MoS}}_2$ and the graphitic phase of ZnO have a direct band gap. Biaxial tensile strain has been found to induce a transition into an indirect band-gap semiconductor with the strain percentage required for the transition equal to 0.83% for ${\mathrm{MoS}}_2$ and 8% for ZnO, respectively. A low strain percentage is desirable for possible device applications. We identify a simple design principle which could be used to identify materials requiring a small strain to induce such a transition. A scaling of the hopping interaction strengths according to Harrison's law within a tight-binding model for ${\mathrm{MoS}}_2$ is able to capture the effect.

We demonstrate that entangled electron-hole pairs can be produced and detected in a quantum spin Hall insulator with a constriction that allows for a weak interedge tunneling. A violation of a Bell inequality, which can be constructed in terms of low-frequency nonlocal current-current correlations, serves as a detection of the entanglement. We show that the maximum violation of a Bell inequality can be naturally achieved in this setup, without a need to fine tune tunneling parameters. This may provide a viable route to producing orbital entanglement in the absence of any correlations and pairing, which is enabled by the helical edge structure of a quantum spin Hall insulator.

We have synthesized ferromagnetic InMnP, a member of the III-Mn-V ferromagnetic semiconductor family, by Mn ion implantation and pulsed laser annealing. Clear ferromagnetic hysteresis loops and a perpendicular magnetic anisotropy are observed up to a Curie temperature of 42 K. Large values of negative magnetoresistance and magnetic circular dichroism as well as an anomalous Hall effect are further evidence of a ferromagnetic order in InMnP. An effort is made to understand the transport mechanism in InMnP using the theoretical models. We find that the valence band of InP does not merge with the impurity band of the heavily doped ferromagnetic InMnP. Our results suggest that impurity band conduction is a characteristic of Mn-doped III-V semiconductors which have deep Mn-acceptor levels.

The geometry of two-dimensional crystalline membranes dictates their mechanical, electronic, and chemical properties. The local geometry of a surface is determined from the two invariants of the metric and the curvature tensors. Here we discuss those invariants directly from atomic positions in terms of angles, areas, and vertex and normal vectors from carbon atoms on the graphene lattice, for arbitrary elastic regimes and atomic conformations, and without recourse to an effective continuum model. The geometrical analysis of graphene membranes under mechanical load is complemented with a study of the local density of states (LDOS), discrete induced gauge potentials, velocity renormalization, and nontrivial electronic effects originating from the scalar deformation potential. The asymmetric LDOS is related to sublattice-specific deformation potential differences, giving rise to the pseudomagnetic field. The results here enable the study of geometrical, mechanical, and electronic properties for arbitrarily shaped graphene membranes in experimentally relevant regimes without recourse to differential geometry and continuum elasticity.

A comparison study of theoretical approaches to the description of the Casimir interaction in layered systems including graphene is performed. It is shown that at zero temperature, the approach using the polarization tensor leads to the same results as the approach using the longitudinal density-density correlation function of graphene. An explicit expression for the zero-temperature transverse density-density correlation function of graphene is provided. We further show that the computational results for the Casimir free energy of graphene-graphene and graphene-Au plate interactions at room temperature, obtained using the temperature-dependent polarization tensor, deviate significantly from those using the longitudinal density-density correlation function defined at zero temperature. We derive both the longitudinal and transverse density-density correlation functions of graphene at nonzero temperature. The Casimir free energy in layered structures including graphene, computed using the temperature-dependent correlation functions, is exactly equal to that found using the polarization tensor.

The Kondo semiconductor ${\text{CeOs}}_2$${\text{Al}}_{10}$ exhibits an antiferromagnetic (AFM) order at $T_{\mathrm{N}}=28.5$ K, whose temperature is unexpectedly high for the small ordered moment of ${\mu }_{\mathrm{AF}}=0.3$${\mu }_{\mathrm{B}}/$Ce. We have studied the effects of electron and hole doping on the hybridization gap and AFM order by measuring the magnetization $M$, magnetic susceptibility $\chi$, electrical resistivity $\rho$, and specific heat $C$ on single crystals of Ce(${\text{Os}}_{1-x}$${\text{Ir}}_x$)${}_2$${\text{Al}}_{10}$($x\le 0.15$) and Ce(${\text{Os}}_{1-y}$${\text{Re}}_y$)${}_2$${\text{Al}}_{10}$($y\le 0.1$). The results of $M\left(B\right)$ indicate that the direction of ${\mu }_{\mathrm{AF}}$ changes from the $c$ axis for $x=0$ to the $a$ axis for $x=0.03$. With increasing $x$ up to 0.15, $T_{\mathrm{N}}$ gradually decreases although the $4f$ electron state becomes localized and the magnitude of ${\mu }_{\mathrm{AF}}$ is increased to 1${\mu }_{\mathrm{B}}/$Ce. With increasing $y$, on the contrary, the $4f$ electron state is more delocalized and the AFM order disappears at a small doping level $y=0.05$. In both electron- and hole-doped systems, the suppression of $T_{\mathrm{N}}$ is well correlated with the increase of the Sommerfeld coefficient $\gamma$ in $C\left(T\right)$. Furthermore, the simultaneous suppression of $T_{\mathrm{N}}$ and the semiconducting gap in $\rho \left(T\right)$ at $T>T_{\mathrm{N}}$ indicates that the presence of the hybridization gap is necessary for the unusual AFM order in ${\text{CeOs}}_2$${\text{Al}}_{10}$.

The gyrotropic eigenmodes in magnetic vortex crystals are studied by means of broadband ferromagnetic-resonance measurements. An artificial crystal structure is realized with two staggered ellipses as building block. Making use of the anisotropic field dependence of the vortex resonance frequency in a ferromagnetic ellipse, distinct resonance modes can be observed that are easily tunable via a small magnetic bias field. The gap between the modes can be continuously adjusted. In the presence of dipolar interaction, a bending and shifting of the modes is observed. A rigid particle model is adopted to analytically describe the experimental results. Calculations for the crystal basis reveal an anticrossing of the resonance modes depending on the vortex-core polarization configuration.

Supercurrents carry charge but not spin in the vast majority of superconductors. This is because the charge carriers are Cooper pairs which are formed of electrons with antiparallel spins (singlet pairs). It is now established that if singlet pairs propagate through a superconductor interface with an inhomogeneous ferromagnet, triplet pair correlations form so the supercurrents can acquire a net spin component. Although the spins at sputter-deposited Fe/Cr interfaces can be frustrated due to surface roughness and interdiffusion, an antiferromagnetic spin density wave (SDW) state can still form in Cr close to the interface. Here, we show evidence for triplet pair correlations in Josephson junctions with Cr/Fe and Cr/Fe/Cr barriers. Although the exact mechanism of pair conversion is unknown, we propose a simple model in which a SDW state in Cr and frustrated spins at the Cr/Fe interfaces serve as spin-mixers for generating triplet supercurrents and so provide a potential means to generating large superconducting spin current densities.

We study in this work the ground-state entanglement properties of two models of noninteracting fermions moving in one dimension, that exhibit metal-insulator transitions. We find that entanglement entropy grows either logarithmically or in a power-law fashion in the metallic phase, thus violating the (one-dimensional version of) entanglement area law. No such violation is found in the insulating phase. We further find that characteristics of single fermion states at the Fermi energy (which can not be obtained from the many-fermion Slater determinant) is captured by the lowest energy single fermion mode of the entanglement Hamiltonian; this is particularly true at the metal-insulator transition point. Our results suggest entanglement is a powerful way to detect metal-insulator transitions, without knowledge of the Hamiltonian of the system.

Bi possesses intriguing properties due to its large spin-orbit coupling, e.g., as a constituent of topological insulators. While its electronic structure and the dynamics of electron-phonon coupling have been studied in the past, photoinduced charge carriers have not been observed in the early phases of their respective relaxation pathways. Using two-photon photoemission (2PPE) we follow the deexcitation pathway of electrons along the unoccupied band structure and into a bulk hole pocket. Two decay channels are found, one of which involves an Auger process. In the hole pocket, the electrons undergo an energetic stabilization and recombine with the corresponding holes with an inverse rate of 2.5 ps. Our results contribute to the understanding of the charge carrier relaxation processes immediately upon photoexcitation, particularly along the $\Gamma T$ line where the electron dynamics have not been probed with time-resolved 2PPE so far.

We investigated the onset of the many-body coherence in the $f$-orbital single crystalline alloys ${\text{Ce}}_{1-x}$${\text{Yb}}_x$${\text{CoIn}}_5$ ($0\le x\le 0.775$) through thermodynamic and magnetotransport measurements. Our study shows the evolution of the many-body electronic state as the Kondo lattice of Ce moments is transformed into an array of Ce impurities. Specifically, we observe a crossover from the predominantly localized Ce moment regime to the predominantly itinerant Yb $f$-electronic states regime for about 60$\%$ of Yb doping. Our analysis of the residual resistivity data unveils the presence of correlations between Yb ions, while from our analysis of specific heat data we conclude that for $0.65\le x\le 0.775$, Yb $f$ electrons strongly interact with the conduction electrons while the Ce moments remain completely decoupled. The sublinear temperature dependence of resistivity across the whole range of Yb concentrations suggest the presence of a unconventional scattering mechanism for the conduction electrons.

In our previous study [Phys. Rev. B 86, 201104 (2012).] we introduced the so-called quasi-non-uniform gradient-level exchange-correlation approximation (QNA) and demonstrated its strength in producing highly accurate equilibrium volumes for metals and their alloys within density-functional theory. In this paper we extend the scheme to include the accuracy of the bulk modulus as an additional figure of merit and show that this scheme is flexible enough to allow the computation of accurate equilibrium volumes and bulk moduli at the same time. The power and feasibility of this scheme is demonstrated on NiAl and FeV binary alloys.

The optical diffraction limit has stood for a long time in the way of achieving higher optical resolution in far-field imaging, photolithography, and optical data storage. We present here a simple and original concept for broadband far-field imaging in the visible and ultraviolet range that beats this limit. A finite-sized ultrathin metallic slab is used to encode subwavelength details of the broadband field radiated by an object. This field excites a set of surface plasmon modes on the finite-sized slab that radiates in the far field. A numerical time reversal imaging process is applied to reconstruct the image of the object with a resolution smaller than λ/6 for a gold slab and λ/8 for a silver slab. With these structures, the highest spatial frequencies are no longer limited by the pitch of the array of the subwavelength resonators as in classical metalenses. We show that the resolution depends mainly on the intrinsic properties of the metal but can be slightly controlled by the geometry design of the slab. Thanks to advances in the control of light in space and time, this concept would provide a promising alternative for high-resolution imaging techniques in the visible and ultraviolet range.

We report on the emergence of laser-induced chiral edge states in graphene ribbons. Insights on the nature of these Floquet states is provided by an analytical solution which is complemented with numerical simulations of the transport properties. Guided by these results we show that graphene can be used for realizing nonequilibrium topological states with striking tunability: while the laser intensity can be used to control their velocity and decay length, changing the laser polarization switches their propagation direction.

We report the observation of strongly temperature ($T$)-dependent spectral lines in electronic Raman-scattering spectra of graphite in a high magnetic field up to 45 T applied along the $c$ axis. The magnetic field quantizes the in-plane motion, while the out-of-plane motion remains free, effectively reducing the system dimension from 3 to 1. Optically created electron-hole pairs interact with, or shake up, the one-dimensional Fermi sea in the lowest Landau subbands. Based on the Tomonaga-Luttinger liquid theory, we show that interaction effects modify the spectral line shape from ${(\omega -\Delta )}^{-1/2}$ to ${(\omega -\Delta )}^{2\alpha -1/2}$ at $T$ = 0. At finite $T$, we predict a thermal broadening factor that increases linearly with $T$. Our model reproduces the observed $T$-dependent line shape, determining the electron-electron interaction parameter $\alpha$ to be $\sim$0.05 at 40 T.

Based on numerical renormalization group calculations, we demonstrate that experimentally realized double quantum dots constitute a minimal thermoelectric generator. In the Kondo regime, one quantum dot acts as an $n$-type and the other one as a $p$-type thermoelectric device. Properly connected, a capacitively coupled double quantum dot provides a miniature power supply utilizing the thermal energy of the environment.

We investigate theoretically the use of nonideal ferromagnetic contacts as a means to detect quantum entanglement of electron spins in transport experiments. We use a designated entanglement witness and find a minimal spin polarization of $\eta >1/\sqrt{3}\approx 58\%$ required to demonstrate spin entanglement. This is significantly less stringent than the ubiquitous tests of Bell's inequality with $\eta >1/\sqrt[4]{2}\approx 84\%$. In addition, we discuss the impact of decoherence and noise on entanglement detection and apply the presented framework to a simple quantum cryptography protocol. Our results are directly applicable to a large variety of experiments.

Using nonequilibrium renormalized perturbation theory to second order in the renormalized Coulomb repulsion, we calculate the lesser ${\Sigma }^{<}$ and and greater ${\Sigma }^{>}$ self-energies of the impurity Anderson model, which describes the current through a quantum dot, in the general asymmetric case. While in general a numerical integration is required to evaluate the perturbative result, we derive an analytical approximation for small frequency $\omega$, bias voltage $V$, and temperature $T$, which is exact to total second order in these quantities. The approximation is valid when the corresponding energies $\hslash \omega$, $eV$, and $k_{B}T$ are small compared to $k_B{T}_K$, where $T_K$ is the Kondo temperature. The result of the numerical integration is compared with the analytical one and with Ng approximation, in which ${\Sigma }^{<}$ and ${\Sigma }^{>}$ are assumed proportional to the retarded self-energy ${\Sigma }^r$ times an average Fermi function. While it fails at $T=0$ for $\hslash \left|\omega \right|\lesssim eV$, we find that the Ng approximation is excellent for $k_{B}T>eV/2$ and improves for asymmetric coupling to the leads. Even at $T=0$, the effect of the Ng approximation on the total occupation at the dot is very small. The dependence on $\omega$ and $V$ are discussed in comparison with a Ward identity that is fulfilled by the three approaches. We also calculate the heat currents between the dot and any of the leads at finite bias voltage. One of the heat currents changes sign with the applied bias voltage at finite temperature.

We present a photoluminescence study of freestanding and Si/${\text{SiO}}_2$ supported single- and few-layer ${\text{MoS}}_2$. The single-layer exciton peak (A) is only observed in freestanding ${\text{MoS}}_2$. The photoluminescence of supported single-layer ${\text{MoS}}_2$ instead originates from the A${}^{-}$ (trion) peak as the ${\text{MoS}}_2$ is $n$-type doped from the substrate. In bilayer ${\text{MoS}}_2$, the van der Waals interaction with the substrate decreases the indirect band gap energy by up to $\approx 80$ meV. Furthermore, the photoluminescence spectra of suspended ${\text{MoS}}_2$ can be influenced by interference effects.