We construct an effective field theory for the $2d$ spinless $p$-wave paired superconductor that faithfully describes the topological properties of the bulk state, and also provides a model for the subgap states at vortex cores and edges. In particular, it captures the topologically protected zero modes and has the correct ground-state degeneracy on the torus. We also show that our effective field theory becomes a topological field theory in a well defined scaling limit and that the vortices have the expected non-Abelian braiding statistics.

The hydrostatic pressure effect on multiferroic properties of rare-earth manganites $R\mathrm{Mn}{\mathrm{O}}_3$ ($R=\mathrm{Dy},\mathrm{Tb}$, and $\mathrm{Gd}$) with the $Pbnm$ orthorhombic structure was investigated by using a diamond anvil cell. Under high pressure, all of these manganites were found to exhibit ferroelectricity in which spontaneous polarization $(\approx {10}^0\mu \mathrm{C}/\mathrm{c}{\mathrm{m}}^2)$ develops along the $a$ axis. The application of a magnetic field $B$ along the $a$ axis to the high-pressure ferroelectric phase causes a drastic change in the electric polarization $P$. Especially in $\mathrm{GdMn}{\mathrm{O}}_3$, the $B$-induced change in $P$ reached $\Delta P\left(B\right)=1.3\mu \mathrm{C}/\mathrm{c}{\mathrm{m}}^2$. A possible origin of the observed $R$-dependent pressure effect is discussed.

Ultrafast carrier dynamics have great significance for our understanding of the transport properties of the surface state in topological insulator (TI) materials. We report midinfrared pump-probe measurements on the intrinsic TI material $\mathrm{B}{\mathrm{i}}_{1.5}\mathrm{S}{\mathrm{b}}_{0.5}\mathrm{T}{\mathrm{e}}_{1.7}\mathrm{S}{\mathrm{e}}_{1.3}$ and show that the change in photoinduced reflectivity can be decomposed into a fast negative part and a slow positive part. Calculations of the dielectric function made at various carrier temperatures and densities reveal that the fast negative component corresponds to the disappearance of the phase-space filling effect due to hot carriers around the probe energy and the decay component corresponds to the recombination of carriers near the band edge. The ratio of the fast negative component to the slow positive component is larger in the excitations conducted at the higher carrier densities, which suggests that the carrier temperature increases through Auger recombination. A qualitative analysis using rate equations reinforces this assumption, so we conclude that Auger recombination is the main cause of the population relaxation at carrier densities higher than ${10}^{18}\mathrm{c}{\mathrm{m}}^{-3}$ and that we determined the Auger coefficient for $\mathrm{B}{\mathrm{i}}_{1.5}\mathrm{S}{\mathrm{b}}_{0.5}\mathrm{T}{\mathrm{e}}_{1.7}\mathrm{S}{\mathrm{e}}_{1.3}$ as $C=0.4\times {10}^{-26}\mathrm{c}{\mathrm{m}}^6/\mathrm{s}$.

The strain field of an ${\mathrm{AlO}}_x$ current aperture, fabricated by selective oxidation of an AlAs/GaAs layer buried in a circular GaAs mesa, is studied. Components of the strain tensor for a thin cross-section lamella cut out of such a structure are evaluated from dark-field electron holography, proving the validity of simulations based on linear elasticity. Simulation of the entire structure is utilized to prepare mesa surfaces with tailored strain fields for controlling the nucleation site of InGaAs quantum dots. The experimental proof of strain simulations allows estimating the magnitude of piezoelectricity, yielding for the studied mesa structures a piezoelectric potential up to 50 mV.

We present a large-scale exact diagonalization study of the one-dimensional $\text{spin}-1/2$ Heisenberg model in a random magnetic field. In order to access properties at varying energy densities across the entire spectrum for system sizes up to $L=22$ spins, we use a spectral transformation which can be applied in a massively parallel fashion. Our results allow for an energy-resolved interpretation of the many-body localization transition including the existence of an extensive many-body mobility edge. The ergodic phase is well characterized by Gaussian orthogonal ensemble statistics, volume-law entanglement, and a full delocalization in the Hilbert space. Conversely, the localized regime displays Poisson statistics, area-law entanglement, and nonergodicity in the Hilbert space where a true localization never occurs. We perform finite-size scaling to extract the critical edge and exponent of the localization length divergence.

We determine the quantum phase diagram of the antiferromagnetic spin-1/2 XXZ model on the triangular lattice as a function of magnetic field and anisotropic coupling $J_z$. Using the density matrix renormalization group algorithm in two dimensions, we establish the locations of the phase boundaries between a plateau phase with 1/3 Néel order and two distinct coplanar phases. The two coplanar phases are characterized by a simultaneous breaking of both translational and U(1) symmetries, which is reminiscent of supersolidity. A translationally invariant umbrella phase is entered via a first-order phase transition at relatively small values of $J_z$ compared to the corresponding case of ferromagnetic hopping and the classical model. The phase transition lines meet at two tricritical points on the tip of the lobe of the plateau state, so that the two coplanar states are completely disconnected. Interestingly, the phase transition between the plateau state and the upper coplanar state changes from second order to first order for large values of $J_z\gtrsim 2.5J$.

We employ ultrabroadband terahertz-midinfrared probe pulses to characterize the optical response of photoinduced charge-carrier plasmas in high-resistivity silicon in a reflection geometry, over a wide range of excitation densities $({10}^{15}–{10}^{19}{\mathrm{cm}}^{-3})$ at room temperature. In contrast to conventional terahertz spectroscopy studies, this enables one to directly cover the frequency range encompassing the resultant plasma frequencies. The intensity reflection spectra of the thermalized plasma, measured using sum-frequency (up-conversion) detection of the probe pulses, can be modeled well by a standard Drude model with a density-dependent momentum scattering time of $\sim 200\mathrm{fs}$ at low densities, reaching $\sim 20\mathrm{fs}$ for densities of $\sim {10}^{19}{\mathrm{cm}}^{-3}$, where the increase of the scattering rate saturates. This behavior can be reproduced well with theoretical results based on the generalized Drude approach for the electron-hole scattering rate, where the saturation occurs due to phase-space restrictions as the plasma becomes degenerate. We also study the initial subpicosecond temporal development of the Drude response and discuss the observed rise in the scattering time in terms of initial charge-carrier relaxation, as well as the optical response of the photoexcited sample as predicted by finite-difference time-domain simulations.

The efficiencies and coefficients of performance of three-terminal devices, comprising two electronic terminals and a thermal one (e.g., a boson bath), are discussed. In particular, two procedures are analyzed. (a) One of the electronic terminals is cooled by investing thermal power (from the thermal bath) and electric power (from voltage applied across the electronic junction); (b) the invested thermal power from the boson bath is exploited to cool one electronic terminal and to produce electric power. Rather surprisingly, the coefficient of performance of (b) can be enhanced as compared to that of (a).

Graphene supports strongly confined transverse-magnetic sheet plasmons whose spectral characteristics depend on the energetic distribution of Dirac particles. The question arises whether plasmons can become amplified when graphene is pumped into a state of inversion. In establishing a theory for the dynamic nonequilibrium polarizability, we are able to determine the exact complex-frequency plasmon dispersion of photoinverted graphene and study the impact of doping, collision loss, and temperature on the plasmon gain spectrum. We calculate the spontaneous emission spectra and carrier recombination rates self-consistently and compare the results with approximations based on Fermi's golden rule. Our results show that amplification of plasmons is possible under realistic conditions but inevitably competes with ultrafast spontaneous emission, which, for intrinsic graphene, is a factor of 5 faster than previously estimated. This work casts new light on the nature of nonequilibrium plasmons and may aid the experimental realization of active plasmonic devices based on graphene.

Transport measurements are presented on thin-film superconducting spin-valve systems, where the controlled noncollinear arrangement of two ferromagnetic Co layers can be used to influence the superconducting state of Nb. We observe a very clear oscillation of the superconducting transition temperature with the relative orientation of the two ferromagnetic layers. Our measurements allow us to distinguish between the competing influences of domain averaging, stray dipolar fields, and the formation of superconducting spin triplets. Domain averaging is shown to lead to a weak enhancement of transition temperature for the antiparallel configuration of exchange fields, while much larger changes are observed for other configurations, which can be attributed to drainage currents due to spin triplet formation.

We performed a magneto-optical study of flux distributions in yttrium barium copper oxide (YBCO) crystal under various applied crossed field orientations to elucidate the complex nature of magnetic flux cutting in superconductors. Our work reveals unusual vortex patterns induced by the interplay between flux cutting and vortex pinning. We observe strong flux penetration anisotropy of the normal flux $B_{\perp }$ in the presence of an in-plane field ${\mathbf{H}}_{\left|\right|}$ and associate the modified flux dynamics with the staircase structure of tilted vortices in YBCO and the flux-cutting process. We demonstrate that flux cutting can effectively delay vortex entry in the direction transverse to ${\mathbf{H}}_{\left|\right|}$. Finally, we elucidate details of the vortex cutting and reconnection process using time-dependent Ginzburg-Landau simulations.

Following the discovery of superconductivity in quasi-one-dimensional ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ containing ${\left[{\left({\text{Cr}}_3{\mathrm{As}}_3\right)}^{2-}\right]}_{\infty }$ chains (J. K. Bao et al., arXiv:1412.0067), we succeeded in synthesizing an analogous compound, ${\mathrm{Rb}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$, which also crystallizes in a hexagonal lattice. The replacement of K by Rb results in an expansion of the $a$ axis by 3%, indicating a weaker interchain coupling in ${\mathrm{Rb}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$. Bulk superconductivity emerges at 4.8 K, above which the normal-state resistivity shows a linear temperature dependence up to 35 K. The estimated upper critical field at zero temperature exceeds the Pauli paramagnetic limit by a factor of 2. Furthermore, the electronic specific-heat coefficient extrapolated to zero temperature in the mixed state increases with $\sqrt{H}$, suggesting the existence of nodes in the superconducting energy gap. Hence ${\mathrm{Rb}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ manifests itself as another example of an unconventional superconductor in the ${\mathrm{Cr}}_3{\mathrm{As}}_3$-chain based system.

We present a detailed study of single crystalline ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ and analyze its thermodynamic and transport properties, anisotropic $H_{c2}\left(T\right)$, and initial pressure dependence of $T_c$. In zero field, the temperature-dependent resistivity is metallic. Deviation from a linear temperature dependence is evident below 100 K and a $T^3$ dependence is roughly followed from just above $T_c$ ($\sim 10\mathrm{K}$) to $\sim 40\mathrm{K}$. Anisotropic $H_{c2}\left(T\right)$ data were measured up to 140 kOe with field applied along and perpendicular to the rodlike crystals. For the applied field perpendicular to the rod, $H_{c2}\left(T\right)$ is linear with a slope $\sim -70$ kOe/K. For field applied along the rod, the slope is about $-120$ kOe/K below 70 kOe. Above 70 kOe, the magnitude of the slope decreases to $\sim -70$ kOe/K. The electronic specific heat coefficient $\gamma$, just above $T_c$, is 73 mJ/mol ${\mathrm{K}}^2$; the Debye temperature ${\Theta }_D$ is 220 K. The specific heat jump at the superconducting transition $\Delta C\sim 2.2\gamma T_c$. Finally, for hydrostatic pressures up to $\sim 7$ kbar, $T_c$ decreases under pressure linearly at a rate of $-0.034\mathrm{K}/\mathrm{kbar}$.

We address the kinetic competition between charge striped order and superconductivity in $\mathrm{L}{\mathrm{a}}_{1.675}\mathrm{E}{\mathrm{u}}_{0.2}\mathrm{S}{\mathrm{r}}_{0.125}\mathrm{Cu}{\mathrm{O}}_4$. Ultrafast optical excitation is tuned to a midinfrared vibrational resonance that destroys charge order and promptly establishes transient coherent interlayer coupling in this material. This effect is evidenced by the appearance of a longitudinal plasma mode reminiscent of a Josephson plasma resonance. We find that coherent interlayer coupling can be generated up to the charge-order transition $T_{\mathrm{CO}}\approx 80\mathrm{K}$, far above the equilibrium superconducting transition temperature of any single layer cuprate. Two key observations are extracted from the relaxation kinetics of the interlayer coupling. First, the plasma mode relaxes through a collapse of its coherence length and not its density. Second, two distinct kinetic regimes are observed for this relaxation, above and below spin-order transition $T_{\mathrm{SO}}\approx 25\mathrm{K}$. In particular, the temperature-independent relaxation rate observed below $T_{\mathrm{SO}}$ is anomalous and suggests coexistence of superconductivity and stripes rather than competition. Both observations support arguments that a low temperature coherent stripe (or pair density wave) phase suppresses $c$-axis tunneling by disruptive interference rather than by depleting the condensate.

We theoretically investigate lasing action in plasmonic crystals incorporating optically pumped four-level gain media. By using detailed simulations based on a time-domain generalization of the finite-element method, we show that the excitation of dark plasmonic resonances (via the gain medium) enables accessing the optimal lasing characteristics of the considered class of systems. Moreover, our study reveals that, in general, arrays of nanowires feature lower lasing thresholds and larger slope efficiencies than those corresponding to periodic arrays of subwavelength apertures. These findings are of relevance for further engineering of active devices based on plasmonic crystals.

We introduce designs for high-$Q$ photonic cavities in slab architectures in hyperuniform disordered solids displaying isotropic band gaps. Despite their disordered character, hyperuniform disordered structures have the ability to tightly confine the transverse electric-polarized radiation in slab configurations that are readily fabricable. The architectures are based on carefully designed local modifications of otherwise unperturbed hyperuniform dielectric structures. We identify a wide range of confined cavity modes, which can be classified according to their approximate symmetry (monopole, dipole, quadrupole, etc.) of the confined electromagnetic wave pattern. We demonstrate that quality factors $Q>{10}^9$ can be achieved for purely two-dimensional structures, and that for three-dimensional finite-height photonic slabs, quality factors $Q>20000$ can be maintained.

We report angle-resolved photoemission spectroscopic measurements of the evolution of the thickness-dependent electronic band structure of the atomically heavy two-dimensional layered dichalcogenide, tungsten diselenide $\left(\mathrm{WS}{\mathrm{e}}_2\right)$. Our data, taken on mechanically exfoliated $\mathrm{WS}{\mathrm{e}}_2$ single crystals, provide direct evidence for shifting of the valence-band maximum from $\overline{\Gamma }$ (multilayer $\mathrm{WS}{\mathrm{e}}_2$) to $\overline{K}$ (single-layer $\mathrm{WS}{\mathrm{e}}_2$). Further, our measurements also set a lower bound on the energy of the direct band gap and provide direct measurement of the hole effective mass.

We report nonlinear terahertz $\left(\mathrm{THz}\right)$ field transmission through photoexcited monolayer epitaxial graphene via differential transmission measurements enabled by optical-pump/intense-terahertz-probe (OPITP) spectroscopy. After photoexcitation of graphene, a transmission enhancement, defined by a positive differential transmission of the intense terahertz probe pulse, is observed. This is due to suppression of the graphene photoconductivity arising from an increased carrier scattering rate due to the increase in the carrier density and the extra energy from the photoexcited hot carriers. Thus, the transient enhancement in transmission increases as the optical pump fluence increased. Most interestingly, we observe that the transmission enhancement after photoexcitation decreases as the THz field strength is increased, which we attribute to the combined effects of the intense THz electric field and the optical pump fluence on the carrier scattering rate. We model the carrier dynamics in the graphene using the length gauge interaction Hamiltonian with the inclusion of short-range scattering by neutral impurities and the interaction of the carriers with optical phonons. Comparing the experimental and simulated transmission results, we extract the nonequilibrium effective lattice temperature of graphene as a function of the optical pump fluence and THz field strength.

While the simplest quantum Hall plateaus, such as the $\nu =1/3$ state in GaAs, can be conveniently analyzed by assuming only a single active Landau level participates, for many phases the spin, valley, bilayer, subband, or higher-Landau-level indices play an important role. These “multicomponent” problems are difficult to study using exact diagonalization because each component increases the difficulty exponentially. An important example is the plateau at $\nu =5/2$, where scattering into higher Landau levels chooses between the competing non-Abelian Pfaffian and anti-Pfaffian states. We address the methodological issues required to apply the infinite density matrix renormalization group to quantum Hall systems with multiple components and long-range Coulomb interactions, greatly extending accessible system sizes. As an initial application we study the problem of Landau-level mixing in the $\nu =5/2$ state. Within the approach to Landau-level mixing used here, we find that at the Coulomb point the anti-Pfaffian state is preferred over the Pfaffian state over a range of Landau-level mixing up to the experimentally relevant values.

We have studied the magnetic and superconducting properties of $\mathrm{Ba}{\left({\mathrm{Fe}}_{0.95}{\mathrm{Co}}_{0.05}\right)}_2{\mathrm{As}}_2$ as a function of temperature and external magnetic field using neutron scattering and muon spin rotation. Below the superconducting transition temperature the magnetic and superconducting order parameters coexist and compete. A magnetic field can significantly enhance the magnetic scattering in the superconducting state, roughly doubling the Bragg intensity at 13.5 T. We perform a microscopic modeling of the data by use of a five-band Hamiltonian relevant to iron pnictides. In the superconducting state, vortices can slow down and freeze spin fluctuations locally. When such regions couple they result in a long-range ordered antiferromagnetic phase producing the enhanced magnetic elastic scattering in agreement with experiments.