Recently, it has been suggested that operational properties connected to quantum computation can be alternative indicators of quantum phase transitions. In this work we systematically study these operational properties in one-dimensional systems that present phase transitions of different orders. For this purpose, we evaluate the local convertibility between bipartite ground states. Our results suggest that the operational properties, related to nonanalyticities of the entanglement spectrum, are good detectors of explicit symmetries of the model, but not necessarily of phase transitions. We also show that thermodynamically equivalent phases, such as Luttinger liquids, may display different convertibility properties depending on the underlying microscopic model.

We have measured the current noise in a device with Corbino geometry to investigate the dynamics of electrons in the breakdown regime of the integer quantum Hall effect (QHE). In the breakdown regime, the Fano factor of the current noise exceeds ${10}^3$, which indicates the presence of electron bunching. As super-Poissonian current noise is observed only in the breakdown regime, the bunching effect is related to the QHE breakdown. These observations support a QHE breakdown mechanism that involves an electron avalanche.

Artificial graphene is a recently realized, man-made nanosystem that exhibits graphene-like physics in a tunable setup. The system can be created by, e.g., positioning molecules in a triangular lattice on a metal surface. Here, we model finite flakes of artificial graphene on a real-space grid and calculate their single-electron properties as a function of the flake size and the strength of an external magnetic field. Our calculations reveal the gradual formation of Dirac cones as well as a self-similar Hofstadter butterfly as the flake size is increased. Moreover, the density of states qualitatively agrees with the experimental data with and without the magnetic field.

We study Raman scattering in the superconducting state of alkali-intercalated iron selenide materials $A_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ ($A=\text{K},\text{Rb},\text{Cs}$) in which the Fermi surface has only electron pockets. Theory predicts that both $s$-wave and $d$-wave pairing channels are attractive in this material, and the gap can have either $s$-wave or $d$-wave symmetry, depending on the system parameters. ARPES data favor $s$-wave superconductivity. We present the theory of Raman scattering in $A_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ assuming that the ground state has s-wave symmetry but $d$ wave is a close second. We argue that Raman profile in $d$-wave $B_{2g}$ channel displays two collective modes. One is a particle-hole exciton, another is a Bardasis-Schrieffer-type mode associated with superconducting fluctuations in $d$-wave channel. At a finite damping, the two modes merge into one broad peak. We present Raman data for $A_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ and compare them with theoretical Raman profile.

Plasmon enhanced electroluminescence of a single as well as a couple of molecules is discussed theoretically. The considered system is formed by a metal nanoparticle (MNP) placed in the proximity of a junction with molecules sandwiched between a left and a right lead. In continuing recent work on a junction with a single molecule [Phys. Rev. B 88, 155426 (2013)] the case of strong charge injection and resulting further enhanced electroluminescence is investigated. Photoemission and plasmon excitation saturate if the molecule-lead coupling enters a range of about 50 meV. This saturation appears long before the molecule-lead coupling approaches the limit at which the considered sequential type of charge transfer becomes invalid. Turning to junctions with up to five molecules stronger plasmon excitation is achieved and a noticeable population of the second and third excited dipole plasmon level appears. This goes along with a nonlinear increase of the photoemission intensity and a decrease of the emission linewidth displaying the onset of cooperative molecule-MNP coupling.

Resonant Raman spectra of single-layer ${\mathrm{WS}}_2$ flakes are presented. A second-order Raman peak (2LA) appears under resonant excitation with a separation from the ${\mathrm{E}}_{2g}^1$ mode of only ${4\mathrm{cm}}^{-1}$. Depending on the intensity ratio and the respective linewidths of these two peaks, any analysis that neglects the presence of the 2LA mode can lead to an inaccurate estimation of the position of the ${\mathrm{E}}_{2g}^1$ mode, leading to a potentially incorrect assignment for the number of layers. Our results show that the intensity of the 2LA mode strongly depends on the angle between the linear polarization of the excitation and detection, a parameter that is neglected in many Raman studies.

A normal metallic film sandwiched between two insulators may have strong spin-orbit coupling near the metal-insulator interfaces, even if spin-orbit coupling is negligible in the bulk of the film. In this paper, we study two technologically important and deeply interconnected effects that arise from interfacial spin-orbit coupling in metallic films. The first is the spin Hall effect, whereby a charge current in the plane of the film is partially converted into an orthogonal spin current in the same plane. The second is the Edelstein effect, in which a charge current produces an in-plane, transverse spin polarization. At variance with strictly two-dimensional Rashba systems, we find that the spin Hall conductivity has a finite value even if spin-orbit interaction with impurities is neglected and “vertex corrections” are properly taken into account. Even more remarkably, such a finite value becomes “universal” in a certain configuration. This is a direct consequence of the spatial dependence of spin-orbit coupling on the third dimension, perpendicular to the plane of the film. The nonvanishing spin Hall conductivity has a profound influence on the Edelstein effect, which we show to consist of two terms, the first with the standard form valid in a strictly two-dimensional Rashba system, and a second arising from the presence of the third dimension. Whereas the standard term is proportional to the momentum relaxation time, the new one scales with the spin relaxation time. Our results, although derived in a specific model, should be valid rather generally, whenever a spatially dependent Rashba spin-orbit coupling is present and the electron motion is not strictly two dimensional.

Ab initio effective Hamiltonian simulations reveal a strain-induced control of domain morphology in epitaxial ${\mathrm{PbTiO}}_3$ ultrathin films being under open-circuit electrical boundary conditions. More precisely, rather different out-of-plane domain structures are found to be the ground state, depending on the value of the misfit strain. Examples include domain walls lying in different crystallographic planes or even being wandering. Analysis of the computations allows us to reveal the precise interactions responsible for such strain-driven domain reorganization.

We investigate the nature of the SDW (spin density wave) transition in the underdoped regime of an iron-based high-$T_c$ superconductor ${\mathrm{Ba(Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$ by ${}^{75}\mathrm{As}$ NMR, with primary focus on a composition with $x=0.02$ ($T_{\mathrm{SDW}}=99$ K). We demonstrate that critical slowing down toward the three-dimensional SDW transition sets in at the tetragonal to orthorhombic structural phase transition $T_s=105$ K, suggesting strong interplay between structural distortion and spin correlations. In the critical regime between $T_s$ and $T_{\mathrm{SDW}}$, the dynamical structure factor of electron spins $S(\mathbf{q},{\omega }_n)$ measured with the longitudinal NMR relaxation rate $1/T_1$ exhibits a divergent behavior obeying a power law $1/T_1\propto {\Sigma }_{\mathbf{q}}S(\mathbf{q},{\omega }_n)\sim {(T/T_{\mathrm{SDW}}-1)}^{-\delta }$ with the critical exponent $\delta \sim 0.33$.

We report a magnetoresistance study of the superconducting ferromagnet UCoGe. The data, taken on single-crystalline samples, show a pronounced structure at $B^{*}=8.5$ T for a field applied along the ordered moment $m_0$. Angle-dependent measurements reveal that this field-induced phenomenon has an uniaxial anisotropy. Magnetoresistance measurements under pressure show a rapid increase of $B^{*}$ to 12.8 T at 1.0 GPa. We discuss $B^{*}$ in terms of a field-induced polarization change. Upper critical field measurements corroborate the unusual S-shaped $B_{c2}\left(T\right)$ curve for a field along the $b$ axis of the orthorhombic unit cell.

Angle-resolved photoemission spectroscopy of ${\text{Ca}}_{10}\left({\text{Ir}}_4{\text{As}}_8\right)({\text{Fe}}_{2-x}{\text{Ir}}_x{\text{As}}_2{)}_5$ shows that the Fe $3d$ electrons in the FeAs layer form the holelike Fermi pocket at the zone center and the electronlike Fermi pockets at the zone corners as commonly seen in various Fe-based superconductors. The FeAs layer is heavily electron doped and has relatively good two dimensionality. On the other hand, the Ir $5d$ electrons are metallic and glassy probably due to atomic disorder related to the Ir $5d$ orbital instability. ${\text{Ca}}_{10}\left({\text{Ir}}_4{\text{As}}_8\right)({\text{Fe}}_{2-x}{\text{Ir}}_x{\text{As}}_2{)}_5$ exhibits a unique electronic state where the Bloch electrons in the FeAs layer coexist with the glassy electrons in the ${\text{Ir}}_4{\text{As}}_8$ layer.

We report measurements of electrical resistivity under pressure to $5.8$ GPa, magnetization to $6.7$ GPa, and ac susceptibility to $7.1$ GPa in $\mathrm{K}{\mathrm{Fe}}_2{\mathrm{As}}_2$. The previously reported change of slope in the pressure dependence of the superconducting transition temperature $T_c\left(p\right)$ at a pressure $p^{*}\sim 1.8$ GPa is confirmed, and $T_c\left(p\right)$ is found to be nearly constant above $p^{*}$ up to $7.1$ GPa. The $T\text{−}p$ phase diagram is very sensitive to the pressure conditions as a consequence of the anisotropic uniaxial pressure dependence of $T_c$. Across $p^{*}$, a change in the behavior of the upper critical field is revealed through a scaling analysis of the slope of $H_{c_2}$ with the effective mass as determined from the $A$ coefficient of the $T^2$ term of the temperature-dependent resistivity. We show that this scaling provides a quantitative test for the changes of the superconducting gap structure and suggests the development of a $k_z$ modulation of the superconducting gap above $p^{*}$ as a most likely explanation.

We investigate a model which couples diffusional melting and nanoscale structural forces via a combined nano-mesoscale description. Specifically, we obtain analytic and numerical solutions for melting processes at grain boundaries influenced by structural disjoining forces in the experimentally relevant regime of small deviations from the melting temperature. Though spatially limited to the close vicinity of the tip of the propagating melt finger, the influence of the disjoining forces is remarkable and leads to a strong modification of the penetration velocity. The problem is represented in terms of a sharp interface model to capture the wide range of relevant length scales, predicting the growth velocity and the length scale describing the pattern, depending on temperature, grain boundary energy, strength, and length scale of the exponential decay of the disjoining potential. Close to equilibrium the short-range effects near the triple junctions can be expressed through a contact angle renormalization in a mesoscale formulation. For higher driving forces strong deviations are found, leading to a significantly higher melting velocity than predicted from a purely mesoscopic description.

We report on the different roles of two orbital-active ${\mathrm{Fe}}^{2+}$ at the $A$ site and ${\mathrm{V}}^{3+}$ at the $B$ site in the magnetic excitations and on the anomalous spin-wave broadening in $\mathrm{Fe}{\mathrm{V}}_2{\mathrm{O}}_4$. $\mathrm{Fe}{\mathrm{V}}_2{\mathrm{O}}_4$ exhibits three structural transitions and successive paramagnetic (PM)–collinear ferrimagnetic (CFI)–noncollinear ferrimagnetic (NCFI)/ferroelectric transitions. The high-temperature tetragonal/PM–orthorhombic/CFI transition is accompanied by the appearance of a large energy gap in the magnetic excitations due to strong spin-orbit-coupling-induced anisotropy at the ${\mathrm{Fe}}^{2+}$ site. While there is no measurable increase in the energy gap from the orbital ordering of ${\mathrm{V}}^{3+}$ at the orthorhombic/CFI–tetragonal/NCFI transition, anomalous spin-wave broadening is observed in the orthorhombic/CFI state due to ${\mathrm{V}}^{3+}$ spin fluctuations at the $B$ site. The spin-wave broadening is also observed at the zone boundary without softening in the NCFI/ferroelectric phase, which is discussed in terms of magnon-phonon coupling. Our study also indicates that the ${\mathrm{Fe}}^{2+}$ spins without the frustration at the $A$ site may not play an important role in inducing ferroelectricity in the tetragonal/NCFI phase of $\mathrm{Fe}{\mathrm{V}}_2{\mathrm{O}}_4$.

In-plane resistivity, magnetoresistance, and Hall-effect measurements have been conducted on quenched ${\text{K}}_x{\text{Fe}}_{2-y}{\text{Se}}_2$ single crystals in order to analyze the normal-state transport properties. It is found that Kohler's rule is well obeyed below about 80 K, but clearly violated above 80 K. Measurements of the Hall coefficient reveal a strong but nonmonotonic temperature dependence with a maximum at about 80 K, in contrast to any other FeAs-based superconductors. With the two-band model analysis on the Hall coefficient, we conclude that a gap may open below 65 K. The data above 65 K are interpreted as a temperature-induced crossover from a metallic state at a low temperature to an orbital-selective Mott phase at a high temperature. This is consistent with the recent data of angle-resolved photoemission spectroscopy. These results call for a refined theoretical understanding, especially when the hole pockets are absent or become trivial in ${\text{K}}_x{\text{Fe}}_{2-y}{\text{Se}}_2$ superconductors.

Many-body electron-electron interaction effects are theoretically considered in monolayer graphene from a continuum effective field-theoretic perspective by going beyond the standard leading-order single-loop perturbative renormalization group (RG) analysis. Given that the effective (bare) coupling constant (i.e., the fine structure constant) in graphene is of order unity, which is neither small to justify a perturbative expansion nor large enough for strong-coupling theories to be applicable, the problem is a difficult one, with some similarity to (2+1)-dimensional strong-coupling quantum electrodynamics (QED). In this work, we take a systematic and comprehensive analytical approach in theoretically studying graphene many-body effects, primarily at the Dirac point (i.e., in undoped, intrinsic graphene), by going up to three loops in the diagrammatic expansion to both ascertain the validity of a perturbative expansion in the coupling constant and to develop a RG theory that can be used to estimate the actual quantitative renormalization effect to higher-order accuracy. Electron-electron interactions are expected to play an important role in intrinsic graphene due to the absence of screening at the Dirac (charge neutrality) point, potentially leading to strong deviations from the Fermi-liquid description around the charge neutrality point where the graphene Fermi velocity should manifest an ultraviolet logarithmic divergence because of the linear band dispersion. While no direct signatures for non-Fermi-liquid behavior at the Dirac point have yet been observed experimentally, there is ample evidence for the interaction-induced renormalization of the graphene velocity as the Dirac point is approached by lowering the carrier density. We provide a critical comparison between theory and experiment, using both higher-order diagrammatic and random phase approximation (i.e., infinite-order bubble diagrams) calculations, emphasizing future directions for a deeper understanding of the graphene effective field theory. We find that while the one-loop RG analysis gives reasonable quantitative agreement with the experimental data, both for graphene in vacuum and graphene on substrates, particularly when dynamical screening effects and finite carrier density effects are incorporated in the theory through the random phase approximation, the two-loop analysis reveals an interacting strong-coupling critical point in graphene suspended in vacuum signifying either a quantum phase transition or a breakdown of the weak-coupling renormalization group approach. By adapting a version of Dyson's argument for the breakdown of the QED perturbative expansion to the case of graphene, we show that in contrast to QED where the asymptotic perturbative series in the coupling constant converges to at least 137 orders (and possibly to much higher order) before diverging in higher orders, the graphene perturbative series in the coupling constant may manifest asymptotic divergence already in the first or second order in the coupling constant, favoring the conclusion that perturbation theory may be inadequate, particularly for graphene suspended in vacuum. We propose future experiments and theoretical directions to make further progress on this important and difficult problem. The question of convergence of the asymptotic perturbative expansion for graphene many-body effects is discussed critically in the context of the available experimental results and our theoretical calculations.

We theoretically investigate the surface plasmon polariton (SPP) at the interface between a three-dimensional strong topological insulator (TI) and a layered superconductor/magnetic insulator structure, within the random phase approximation. The tunability of the SPP through electronic doping can be enhanced when the magnetic permeability of the layered structure becomes higher. When the interface is gapped by superconductivity or perpendicular magnetism, the SPP dispersion is further distorted, accompanied by a shift of group velocity and penetration depth. Such a shift of the SPP reaches a maximum when the magnitude of the Fermi level approaches the gap value, and may lead to observable effects. The tunable SPP at the interface between layered-superconductor and magnetic materials in proximity to the TI surface may provide new insight in the detection of Majorana fermions.

The effect of magnetic field on the static and dynamic spin correlations in the noncentrosymmetric heavy-fermion superconductor $\mathrm{Ce}{\mathrm{Pt}}_3\mathrm{Si}$ was investigated by neutron scattering. The application of a magnetic field $\mathit{B}$ increases the antiferromagnetic (AFM) peak intensity. This increase depends strongly on the field direction: for $\mathit{B}\parallel$[0 0 1] the intensity increases by a factor of 4.6 at a field of 6.6 T, which corresponds to more than a doubling of the AFM moment, while the moment increases by only 10% for $\mathit{B}\parallel$[1 0 0] at 5 T. This is in strong contrast to the inelastic response near the antiferromagnetic ordering vector, where no marked field variations are observed for $\mathit{B}\parallel$[0 0 1] up to 3.8 T. The results reveal that the AFM state in $\mathrm{Ce}{\mathrm{Pt}}_3\mathrm{Si}$, which coexists with superconductivity, is distinctly different from other unconventional superconductors.

We present the design, characterization, and experimental verification of broadband and tunable metamaterials based on reconfigurable fluid channels. We demonstrate through simulation that a finite-height cylinder of water in a background medium behaves as a nonresonant metamaterial. By changing liquid type or adjusting liquid level, we tune the effective material parameters of the metamaterial through altering the composition and amount of water used. We verify the efficacy of the metamaterial through experiment, demonstrating a design procedure for a metamaterial with broadband and tunable properties.