We numerically investigate the growth of the entanglement entropy $S_{\mathrm{ent}}\left(t\right)$ in time $t$, after a global quench from a product state, in quantum chains with various kinds of disorder. The main focus is, in particular, on fermionic chains with bond disorder. In the noninteracting case at criticality we numerically test recent predictions by the real-space renormalization group for the entanglement growth in time, the maximal entanglement as a function of block size, and the decay of a density-wave order parameter. We show that multiprecision calculations are required to reach the scaling regime and perform such calculations for specific cases. For interacting models with binary bond disorder we present data based on infinite-size density matrix renormalization group calculations and exact diagonalizations. We obtain numerical evidence of a many-body localized phase in bond-disordered systems where $S_{\mathrm{ent}}\left(t\right)\sim lnt$ seems to hold. Our results for bond disorder are contrasted with the well-studied case of potential disorder.

We consider a nonequilibrium system of interacting emitters described by the XXZ model, whose excitonic transitions are spatially and spectrally coupled to a single mode cavity. We demonstrate that the output radiation field is sensitive to an interplay between the hopping ($J$) and the interactions ($U$) of the excitons. Moderate values of the short-ranged interaction are shown to induce laser with maximal output at the Heisenberg point ($U=J$). In the laser regime, charge-charge correlations emerge and they are shown to strongly depend on the interaction-hopping ratio. In particular, the system shows charge-density correlations below the Heisenberg point and ferromagnetic correlations beyond the Heisenberg point. This contrast to the equilibrium behavior of the XXZ chain occurs since the laser explores highly excited states of the emitters.

We study electromagnetic properties of a double-layer graphene system in which electrons from one layer are coupled with holes from the other layer. The gauge invariant linear response functions are obtained. The frequency dependences of the transmission, reflection, and absorption coefficients are computed. We predict a peak in the reflection and absorption at the frequency equal to the gap in the quasiparticle spectrum. It is shown that the electron-hole pairing results in an essential modification of the spectrum of surface TM plasmons. We find that the optical TM mode splits into a low frequency undamped branch and a high frequency damped branch. At zero temperature the lower branch disappears. It is established that the pairing does not influence the acoustic TM mode. It is also shown that the pairing opens the frequency window in the subgap range for the surface TE wave.

Electric fields are known to favor long-range polar order through the aligning of electric dipoles in relation to Coulomb's force. Therefore, it would be surprising to observe a disordered polar state induced from an ordered state by electric fields. Here we show such an unusual phenomenon in a polycrystalline oxide where electric fields induce a ferroelectric-to-relaxor phase transition. The nonergodic relaxor phase with disordered dipoles appears as an intermediate state under electric fields during polarization reversal of the ferroelectric phase. Using the phenomenological theory, the underlying mechanism for this unexpected behavior can be attributed to the slow kinetics of the ferroelectric-to-relaxor phase transition, as well as its competition against domain switching during electric reversal. The demonstrated material could also serve as a model system to study the transient stages in first-order phase transitions; the slow kinetics does not require the use of sophisticated ultrafast tools.

Local distortions of the ideal periodic structure in crystals around impurity atoms play an important role in various physical properties of materials. The aim of this study was to investigate the static distortions around cadmium impurity atoms in a ${\text{SnCd}}_{0.0026}$ single crystal using atomic resolution neutron holography technique. The cadmium nucleus was used as an inside detector to measure the holographic interference pattern from which the three-dimensional (3D) atomic arrangement of tin nuclei around the cadmium impurities was reconstructed. Detailed analysis of the reconstructed image revealed the 3D static displacements of Sn atoms around the impurity. It was found that the crystal structure contracts around the cadmium impurity atom and the displacements tend to transform the crystal to the $\alpha$ phase. The local contraction of the lattice was used to explain the slower phase transformation to $\alpha$-Sn phase when Cd impurity atoms are present. The study shows the ability of neutron holography to measure 3D displacements around impurities which can be used, e.g., to understand the mechanisms that block the phase transformations in the presence of impurities.

Contrary to the original expectation, ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ is not a Kitaev's quantum spin liquid (QSL) but shows a zigzag-type antiferromagnetic order in experiments. Here, we propose experimental clues and criteria to measure how a material in hand is close to the Kitaev's QSL state. For this purpose, we systematically study thermal and spin excitations of a generalized Kitaev-Heisenberg model studied by Chaloupka et al., Phys. Rev. Lett. 110, 097204 (2013) and an effective ab initio Hamiltonian for ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ proposed by Yamaji et al., Phys. Rev. Lett. 113, 107201 (2014), by employing a numerical diagonalization method. We reveal that closeness to the Kitaev's QSL is characterized by the following properties, besides trivial criteria such as reduction of magnetic ordered moments and Néel temperatures. (1) Two peaks in the temperature dependence of specific heat at $T_{\ell }$ and $T_h$ caused by the fractionalization of spin to two types of Majorana fermions. (2) In between the double peak, a prominent plateau or shoulder pinned at $\frac{R}{2}ln2$ in the temperature dependence of entropy, where $R$ is the gas constant. (3) Failure of the linear spin wave approximation at the low-lying excitations of dynamical structure factors. (4) Small ratio $T_{\ell }/T_h$ close to or less than 0.03. According to the proposed criteria, ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ is categorized to a compound close to the Kitaev's QSL, and is proven to be a promising candidate for the realization of the QSL if the relevant material parameters can further be tuned by making thin film of ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ on various substrates or applying axial pressure perpendicular to the honeycomb networks of iridium ions. Applications of these characterization to ${\left({\mathrm{Na}}_{1-x}{\mathrm{Li}}_x\right)}_2{\mathrm{IrO}}_3$ and other related materials are also discussed.

Determining the symmetry of the superconducting order parameter is the most important but also the most complicated step in elucidating the mechanism of superconductivity. Here we present an experimental approach to investigate the order parameter symmetry of unconventional multiband superconductors, which is based on a disorder-induced change from sign-reversed $\left(s_{\pm }\right)$ to sign-preserved $\left(s_{++}\right)$ symmetry. Therefore, we investigated a $\mathrm{Ba}({\mathrm{Fe}}_{0.9}{\mathrm{Co}}_{0.1}){}_2{\mathrm{As}}_2$ thin film by THz spectroscopy and stepwise proton irradiation. In our experiments, the low-energy superconducting gap first vanishes but recovers at higher irradiation doses. At the same time, the decrease of the superfluid density with disorder comes to a halt. Thus, we confirm with the method presented here that the superconducting order parameter in the pristine sample possesses $s_{\pm }$ symmetry.

A metallic state enabled by the metal-insulator transition (MIT) in single crystal $\mathrm{V}{\mathrm{O}}_2\left(\mathrm{A}\right)$ nanorods is demonstrated, which provides important physical foundation in experimental understanding of MIT in $\mathrm{V}{\mathrm{O}}_2$. The observed tetragonal metallic state at ∼28 GPa should be interpreted as a distinct metastable state, while increasing pressure to ∼32 GPa, it transforms into a metallic amorphous state completely. The metallization is due to V $3d$ orbital electrons delocalization, and the amorphization is attributed to the unique variation of V-O-V bond angle. A metallic amorphous $\mathrm{V}{\mathrm{O}}_2$ state is found under pressure, which is beneficial to explore the phase diagram of $\mathrm{V}{\mathrm{O}}_2$. Furthermore, this work proves the occurrence of both the metallization and amorphization in octahedrally coordinated materials.

We explore quantum and classical correlations along with coherence in the ground states of spin-1 Heisenberg chains, namely the one-dimensional XXZ model and the one-dimensional bilinear biquadratic model, with the techniques of density matrix renormalization group theory. Exploiting the tools of quantum information theory, that is, by studying quantum discord, quantum mutual information, and three recently introduced coherence measures in the reduced density matrix of two nearest neighbor spins in the bulk, we investigate the quantum phase transitions and special symmetry points in these models. We point out the relative strengths and weaknesses of correlation and coherence measures as figures of merit to witness the quantum phase transitions and symmetry points in the considered spin-1 Heisenberg chains. In particular, we demonstrate that, as none of the studied measures can detect the infinite-order Kosterlitz-Thouless transition in the XXZ model, they appear to be able to signal the existence of the same type of transition in the biliear biquadratic model. However, we argue that what is actually detected by the measures here is the SU(3) symmetry point of the model rather than the infinite-order quantum phase transition. Moreover, we show in the XXZ model that examining even single site coherence can be sufficient to spotlight the second-order phase transition and the SU(2) symmetry point.

Recent experiments have demonstrated that the metamaterial approach is capable of a drastic increase of the critical temperature $T_{\mathrm{c}}$ of epsilon near zero (ENZ) metamaterial superconductors. For example, tripling of the critical temperature has been observed in $\mathrm{Al}-\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ ENZ core-shell metamaterials. Here, we perform theoretical modeling of $T_{\mathrm{c}}$ increase in metamaterial superconductors based on the Maxwell-Garnett approximation of their dielectric response function. Good agreement is demonstrated between theoretical modeling and experimental results in both aluminum- and tin-based metamaterials. Taking advantage of the demonstrated success of this model, the critical temperature of hypothetic niobium-, $\mathrm{Mg}{\mathrm{B}}_{2^{-}}$, and ${\mathrm{H}}_2\mathrm{S}$-based metamaterial superconductors is evaluated. The $\mathrm{Mg}{\mathrm{B}}_2$-based metamaterial superconductors are projected to reach the liquid nitrogen temperature range. In the case of a ${\mathrm{H}}_2\mathrm{S}$-based metamaterial $T_{\mathrm{c}}$ appears to reach ∼250 K.

We theoretically study the quantum anomalies in the superconducting Weyl metals based on the topological field theory. It is demonstrated that the Fermi arc and the surface Andreev bound state, characteristic of the superconducting Weyl metals, are the manifestations of two underlying phenomena, namely, the chiral anomaly and the paritylike anomaly, respectively. The first anomaly is inherited from the Berry curvature around the original Weyl points, while the second is the result of the superconductivity. We show that all the fascinating topological behavior of the superconducting Weyl metals, either the intranode Fulde-Ferrell-Larkin-Ovchinnikov or the internode Bardeen-Cooper-Schrieffer pairing state, can be satisfactorily described and predicted by our topological field theory.

We present measurements of the superconducting and charge-density-wave (CDW) critical temperatures $(T_c$ and $T_{\text{CDW}})$ as a function of pressure in the transition metal dichalchogenides $2H\text{−}{\text{TaSe}}_2$ and $2H\text{−}{\mathrm{TaS}}_2$. Resistance and susceptibility measurements show that $T_c$ increases from temperatures below 1 K up to $8.5$ K at $9.5$ GPa in $2H\text{−}{\mathrm{TaS}}_2$ and $8.2$ K at 23 GPa in $2H\text{−}{\mathrm{TaSe}}_2$. We observe a kink in the pressure dependence of $T_{\text{CDW}}$ at about 4 GPa that we attribute to the lock-in transition from incommensurate CDW to commensurate CDW. Above this pressure, the commensurate $T_{\text{CDW}}$ slowly decreases, coexisting with superconductivity within our full pressure range.

The topological property of ${\mathrm{SrRu}}_2{\mathrm{O}}_6$ and isostructural ${\mathrm{CaOs}}_2{\mathrm{O}}_6$ under various strain conditions is investigated using density functional theory. Based on an analysis of parity eigenvalues, we anticipate that a three-dimensional strong topological insulating state should be realized when band inversion is induced at the A point in the hexagonal Brillouin zone. For ${\mathrm{SrRu}}_2{\mathrm{O}}_6$, such a transition requires rather unrealistic tuning, where only the $c$ axis is reduced while other structural parameters are unchanged. However, given the larger spin-orbit coupling and smaller lattice constants in ${\mathrm{CaOs}}_2{\mathrm{O}}_6$, the desired topological transition does occur under uniform compressive strain. Our study paves a way to realize a topological insulating state in a complex oxide, which has not been experimentally demonstrated so far.

We present a refined and improved study of the influence of screening on the effective fine structure constant of graphene, ${\alpha }^{*}$, as measured in graphite using inelastic x-ray scattering. This followup to our previous study [J. P. Reed et al., Science 330, 805 (2010)] was carried out with two times better energy resolution, five times better momentum resolution, and an improved experimental setup with lower background. We compare our results to random-phase approximation (RPA) calculations and evaluate the relative importance of interlayer hopping, excitonic corrections, and screening from high energy excitations involving the $\sigma$ bands. We find that the static, limiting value of ${\alpha }^{*}$ falls in the range 0.25–0.35, which is higher than our previous result of 0.14, but still below the value expected from RPA. We show the reduced value is not a consequence of interlayer hopping effects, which were ignored in our previous analysis, but of a combination of excitonic effects in the $\pi \to {\pi }^{*}$ particle-hole continuum, and background screening from the $\sigma$-bonded electrons. We find that $\sigma$-band screening is extremely strong at distances of less than a few nanometers, and should be highly effective at screening out short-distance, Hubbard-like interactions in graphene as well as other carbon allotropes.

We show that, with a system of electrically gated wide quantum wells embedded inside a simple dielectric waveguide structure, it is possible to excite, control, and observe waveguided exciton polaritons that carry an electric dipole moment. We demonstrate that the energy of the propagating dipolariton can be easily tuned using local electrical gates, that their excitation and extraction can be easily done using simple evaporated metal gratings, and that the dipolar interactions between polaritons and between polaritons and excitons can also be controlled by the applied electric fields. This system of gated flying dipolaritons thus exhibits the ability to locally control both the single polariton properties as well as the interactions between polaritons, which should open up opportunities for constructing complex polaritonic circuits and for studying strongly interacting, correlated polariton gases.

We engineer topological insulating phases in a fermion-fermion mixture on the honeycomb lattice, without resorting to artificial gauge fields or spin-orbit couplings and considering only local interactions. Essentially, upon integrating out the fast component (characterized by a larger hopping amplitude) in a finite region of dopings, we obtain an effective interaction between the slow fermions at half-filling, which acquires a Haldane mass with opposite parity in the two valleys of the Dirac cones, thus triggering a quantum anomalous Hall effect. We carefully analyze the competition between the induced Semenoff-type mass (producing charge density wave orders in real space) versus the Haldane mass (quantum anomalous Hall phase), as a function of the chemical potential of the fast fermions. If the second species involves spin-1/2 particles, this interaction may induce a quantum spin Hall phase. Such fermion-fermion mixtures can be realized in optical lattices or in graphene heterostructures.

Using in situ surface electron microscopy, we show that the surface chemical potential of GaAs (001), and hence the surface phase, can be systematically controlled by varying temperature with liquid Ga droplets present as Ga reservoirs. With decreasing temperature, the surface approaches equilibrium with liquid Ga. This provides access to a regime where we find phases ultrarich in Ga, extending the range of surface phases available in this technologically important system. The same behavior is expected to occur for similar binary or multicomponent semiconductors such as InGaAs.

We present the optimized design of an antireflection coating to efficiently couple an incident plane wave into a metamaterial with a complex field profile. We show that such an antireflection coating must enable spatial engineering of the field profiles at the coating/metamaterial interface to achieve high transmission, and therefore it is required to be inhomogeneous. As a demonstration, we investigate theoretically a waveguide-based negative-index metamaterial, which under normal incidence cannot be excited due to the antisymmetric propagating eigenmode. Through careful engineering of the field profile, lateral position, and thickness of the coating layer, we enhance the transmission under normal incidence from $0\%$ to $100\%$. This principle may generally be applied to overcome low coupling efficiency between incident plane waves and complex mode profiles in metamaterials.