We report an experimental study on the tunneling of holes in a mixed-type GaAs quantum-well structure, in which an electron-hole bilayer forms through optical carrier injection. In this bilayer system, an interlayer correlation between holes and excitons can strongly enhance the hole tunneling rate, opening a new avenue for optical control of transport processes in semiconductors. Interlayer hole tunneling is characterized with respect to these excitonic correlations, as well as carrier densities, temperature, and magnetic field, providing valuable insights on the tunneling process.

We present a quantum model to calculate the dipole-dipole coupling between electronic excitations in the conduction band of semiconductor quantum wells. We demonstrate that the coupling depends on a characteristic length, related to the overlap between microscopic current densities associated with each electronic excitation. As a result of the coupling, a macroscopic polarization is established in the quantum wells, corresponding to one or few bright collective modes of the electron gas. Our model is applied to derive a sum rule and to investigate the interplay between tunnel coupling and Coulomb interaction in the absorption spectrum of a dense electron gas.

We have studied the properties of several representative one-dimensional structures—MgO nanowires, ${\mathrm{Fe}}_3{\mathrm{O}}_4$ hollow nanotubes, ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanowires, and $\mathrm{Mg}\mathrm{O}/{\mathrm{Fe}}_3{\mathrm{O}}_4$ core/shell nanotubes—by means of first-principles-based calculations. Each of these nanostructures reveals different electronic properties with novel electronic states due to the large surface/interface of the nanocylinders. Electronic states of the ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanowire are localized around small clusters of atoms, and its bands appear with almost no energy dispersion. Localization is not a direct consequence of structural disorder; instead, it seems to be induced by an enhanced charge transfer due to the undercoordination on the surface. The combined effect of the $\mathrm{Mg}\mathrm{O}/{\mathrm{Fe}}_3{\mathrm{O}}_4$ nanostructure shows that the MgO is well coated and even bulk-like states can be observed. However, the magnetite suffers important atomic reconstructions losing symmetries and increasing its atomic-like behavior. Effects of axial deformations on the properties and the relation of the results to potential technological applications are discussed.

We study the second-harmonic generation by a spherical dielectric nanoparticle covered by graphene. We demonstrate that a strong nonlinear response is caused by an induced surface current in the graphene nanoparticle illuminated by an external electromagnetic wave. We obtain analytical expressions for the field multipoles characterizing the second-harmonic radiation and analyze the dependence of intensity and directivity of the nonlinear scattering on the frequency and structure of the electromagnetic field, revealing the asymmetric radiation patterns due to constructive multipole interference for the resonantly enhanced second-harmonic generation.

We present a mechanism to endow the polariton gas with attractive interactions. The scheme relies on an exciton reservoir, which can be formed even if the excitons lie outside of the light cone. Such an attractive interaction should open different routes in the physics of quantum fluids, which we illustrate with a simple but nontrivial application whereby the polariton condensate enters the self-oscillation regime powered by the attraction from the reservoir. This results in pulsed emission in the GHz regime.

We study the entanglement between blocks of energy levels in 1D models for $s$-wave and $p$-wave superconductivity. The ground state entanglement entropy and entanglement spectrum (ES) of a block of $\ell$ levels around the Fermi point is obtained and related to its physical properties. In the superconducting phase at large coupling, the maximal entropy grows with the number of levels $L$ as $\frac{1}{2}ln\left(L\right)$. The number of levels presenting maximal entanglement is shown to estimate the number of Cooper pairs involved in pairing correlations. Moreover, the properties of the ES signal the presence of the Read-Green quantum phase transition in the $p+ip$ model, and of the Moore-Read line, which is difficult to characterize. This work establishes a link between physical properties of superconducting phases and quantum entanglement.

We consider the competition between the conventional $s$-wave and the triplet Balian-Werthamer or the $B$-phase pairings in doped three-dimensional narrow-gap semiconductors, such as ${\mathrm{Cu}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ${\mathrm{Sn}}_{1-x}{\mathrm{In}}_x\mathrm{Te}$. When the coupling constants of the two contending channels are comparable, we find a simultaneously time-reversal and parity violating $p+is$ state at low temperatures, which provides an example of a dynamic axionic state of matter. In contradistinction to the time-reversal invariant, topological $B$ phase, the $p+is$ state possesses gapped Majorana fermions as surface Andreev bound states, which give rise to an anomalous surface thermal Hall effect. The anomalous gravitational and electrodynamic responses of the $p+is$ state can be described by the $\theta$ vacuum structure, where $\theta \ne 0$ or $\pi$.

One-dimensional tight binding models such as the Aubry-André-Harper (AAH) model (with an on-site cosine potential) and the integrable Maryland model (with an on-site tangent potential) have been the subject of extensive theoretical research in localization studies. AAH can be directly mapped onto the two-dimensional (2D) Hofstadter model which manifests the integer quantum Hall topology on a lattice. However, such a connection needs to be made for the Maryland model (MM). Here we describe a generalized model that contains AAH and MM as the limiting cases with the MM lying precisely at a topological quantum phase transition (TQPT) point. A remarkable feature of this critical point is that the one-dimensional MM retains well defined energy gaps whereas the equivalent 2D model becomes gapless, signifying the 2D nature of the TQPT.

The theory of transformation optics for light has fascinated the scientific community for almost one decade due to its revolutionary and groundbreaking implications. Because the propagation of electrons in condensed-matter systems is also described by a wave equation, one may also envision the extension of this idea to matter waves. Here, we suggest that graphene can be used as a platform to demonstrate a “tunnel” or “wormhole” for electrons. Based on an effective medium approach, we theoretically demonstrate that two properly designed graphene-based nanomaterials can effectively “annihilate” one another from an electronic point of view, and provide for the delocalization of the wave-function stationary states, similar to a wormhole.

Magnetic properties of the layered itinerant system ${\mathrm{Sr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{Co}}_2{\mathrm{P}}_2$ were investigated under a magnetic field up to 70 T. As for the exchange-enhanced Pauli paramagnetic metal $\mathrm{Sr}{\mathrm{Co}}_2{\mathrm{P}}_2$, the magnetization curve shows two characteristic anomalies. The high-field anomaly corresponds to an itinerant-electron metamagnetic transition (IEMT), while the low-field one is small and broad without any obvious hystereses just as the crossover phenomenon between different energy states. Such a successive transition in the magnetization curve cannot be explained by the conventional phenomenological theory for IEMT based upon the Landau expansion of the magnetic free energy, but by the extended Landau expansion theory dealing with two distinguishable energy states. In the systematical study on ${\mathrm{Sr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{Co}}_2{\mathrm{P}}_2$, furthermore, the metamagnetic transition field decreases and goes to zero as $x$ increases up to 0.5, indicating that the ferromagnetic quantum critical point exists at $x\sim 0.5$.

We propose a simple scheme to estimate the potential energy surface (PES) for which the accuracy can be easily controlled and improved. It is based on model selection within the framework of linear regression using the least absolute shrinkage and selection operator (LASSO) technique. Basis functions are selected from a systematic large set of candidate functions. The sparsity of the PES significantly reduces the computational cost of evaluating the energy and force in molecular dynamics simulations without losing accuracy. The usefulness of the scheme for describing the elemental metals Na and Mg is clearly demonstrated.

We explore with exact diagonalization the propagation of a single hole in four magnetic phases of the $t\text{−}J$-like Kitaev-Heisenberg model on a honeycomb lattice: the Néel antiferromagnetic, stripe, zigzag, and Kitaev spin-liquid phases. We find coherent propagation of spin-polaron quasiparticles in the antiferromagnetic phase by a mechanism similar to that in the $t\text{−}J$ model for high-$T_c$ cuprates. In the stripe and zigzag phases clear quasiparticles features appear in spectral functions of those propagators where holes are created and annihilated on one sublattice, while they remain largely hidden in those spectral functions that correspond to photoemission experiments. As the most surprising result, we find a totally incoherent spectral weight distribution for the spectral function of a hole moving in the Kitaev spin-liquid phase in the strong-coupling regime $t\gg J$ relevant for iridates. At intermediate coupling the finite systems calculation reveals a well-defined quasiparticle at the $\Gamma$ point; however, we find that the gapless spin excitations wipe out quasiparticles at finite momenta. Also for this more subtle case we conclude that in the thermodynamic limit the lightly doped Kitaev liquid phase does not support quasiparticle states in the neighborhood of $\Gamma$, and therefore yields a non-Fermi liquid, contrary to earlier suggestions based on slave-boson studies. These observations are supported by the presented study of the dynamic spin-structure factor in the Kitaev spin liquid regime.

Ternary intermetallics Ep-T-X, crystallizing in ordered variants of the ${\mathrm{BaAl}}_4$ structure type, have been investigated systematically with respect to their formation and stability. For this, a comprehensive overview of the ${\mathrm{BaAl}}_4$ derivative structure types including group-subgroup relations was established. Special emphasis was laid on compounds where inversion symmetry is missing in the respective crystal structures and where superconductivity is observed at low temperatures. ${\mathrm{EpTX}}_3$ compounds crystallize in the noncentrosymmetric ${\mathrm{BaNiSn}}_3$ structure type (space-group $I4mm$; $a\approx 0.4$ and $c\approx 1$ nm), an ordered ternary derivative of ${\mathrm{BaAl}}_4$. Superconductivity below 3 K was found for seven members of this series, as evidenced from heat capacity and electrical resistivity measurements. Although the Rashba-like spin-orbit coupling in noncentrosymmetric systems can enable a mixture of spin-singlet and spin-triplet pairs in the superconducting condensate, the experimental data basically indicate a predominant $s$-wave superconducting state in all of these compounds. For this family of compounds, fully relativistic density functional theory (DFT) calculations of the electronic structure and phonon properties were done. Despite the different size of spin-orbit coupling depending on the actual choice of elements for Ep, T, and X that result in different spin-orbit splittings of the Fermi surfaces, the experimental observation of a prevalent spin-singlet pairing in the superconducting phases of the ${\mathrm{EpTX}}_3$ compounds is supported.

We have studied the temperature dependence of the upper critical fields ${\mu }_0{H}_{c2}$ of $K_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_{2-z}{S}_z$ single crystals up to 60 T. The ${\mu }_0{H}_{c2}$ for $H\parallel ab$ and $H\parallel c$ decrease with increasing sulfur content. The detailed analysis using Werthamer-Helfand-Hohenberg theory including the Pauli spin-paramagnetic effect shows that ${\mu }_0{H}_{c2}$ for $H\parallel ab$ is dominated by the spin-paramagnetic effect, which diminishes with higher S content, whereas ${\mu }_0{H}_{c2}$ for $H\parallel c$ shows a linear temperature dependence with an upturn at high fields. The latter observation can be ascribed to multiband effects that become weaker for higher S content. This results in an enhanced anisotropy of ${\mu }_0{H}_{c2}$ for high S content due to the different trends of the spin-paramagnetic and multiband effect for $H\parallel ab$ and $H\parallel c$, respectively.

We study the nonequilibrium dynamics of an electronic model of competition between an unconventional charge density wave (a bond density wave) and $d$-wave superconductivity. In a time-dependent Hartree-Fock+BCS approximation, the dynamics reduces to the equations of motion of operators realizing the generators of SU(4) at each pair of momenta, $(\mathit{k},-\mathit{k})$, in the Brillouin zone. We also study the nonequilibrium dynamics of a quantum generalization of a O(6) nonlinear $\sigma$ model of competing orders in the underdoped cuprates [Hayward et al., Science 343, 1336 (2014)]. We obtain results, in the large $N$ limit of a $O\left(N\right)$ model, on the time dependence of correlation functions following a pulse disturbance. We compare our numerical studies with recent picosecond optical experiments. We find that, generically, the oscillatory responses in our models share various qualitative features with the experiments.

A nuclear capture reaction of a single neutron by ultracold superfluid ${}^3\mathrm{He}$ results in a rapid overheating followed by the expansion and subsequent cooling of the hot subregion, in a certain analogy with the big bang of the early universe. It was shown in a Grenoble experiment that a significant part of the energy released during the nuclear reaction was not converted into heat even after several seconds. It was thought that the missing energy was stored in a tangle of quantized vortex lines. This explanation, however, contradicts the expected lifetime of a bulk vortex tangle, ${10}^{-5}$–${10}^{-4}$ s, which is much shorter than the observed time delay of seconds. In this paper we propose a scenario that resolves the contradiction: the vortex tangle, created by the hot spot, emits isolated vortex loops that take with them a significant part of the tangle's energy. These loops quickly reach the container walls. The dilute ensemble of vortex loops attached to the walls can survive for a long time, while the remaining bulk vortex tangle decays quickly.

Layered molybdenum dichalchogenides are semiconductors whose gap is controlled by delicate interlayer interactions. The gap tends to drop together with the interlayer distance, suggesting collapse and metallization under pressure. We predict, based on first-principles calculations, that layered semiconductors ${2\mathrm{H}}_c$-${\mathrm{MoSe}}_2$ and ${2\mathrm{H}}_c$-${\mathrm{MoTe}}_2$ should undergo metallization at pressures between 28 and 40 GPa (${\mathrm{MoSe}}_2$) and 13 and 19 GPa (${\mathrm{MoTe}}_2$). Unlike ${\mathrm{MoS}}_2$ where a ${2\mathrm{H}}_c$ $\to$ ${2\mathrm{H}}_a$ layer-sliding transition is known to take place, these two materials appear to preserve the original ${2\mathrm{H}}_c$ layered structure at least up to 100 GPa and to increasingly resist lubric layer sliding under pressure. Similar to metallized ${\mathrm{MoS}}_2$, they are predicted to exhibit a low density of states at the Fermi level, and presumably very modest superconducting temperatures, if any. We also study the $\beta$-${\mathrm{MoTe}}_2$ structure, metastable with a higher enthalpy than ${2\mathrm{H}}_c$-${\mathrm{MoTe}}_2$. Despite its ready semimetallic and (weakly) superconducting character already at zero pressure, metallicity is not expected to increase dramatically with pressure.

Detailed electronic structure calculations of picene clusters doped by potassium modeling the crystalline $\mathrm{K}{}_3\mathrm{picene}$ structure show that while two electrons are completely transferred from potassium atoms to the lowest-energy unoccupied molecular orbital of pristine picene, the third one remains closely attached to both material components. Multiconfigurational analysis is necessary to show that many structures of almost degenerate total energies compete to define the cluster ground state. Our results prove that the $4s$ orbital of potassium should be included in any interaction model describing the material. We propose a quarter-filled two-orbital model as the most simple model capable of describing the electronic structure of K-intercalated picene. Precise solutions obtained by a development of the Lanczos method show low-energy electronic excitations involving orbitals located at different positions. Consequently, metallic transport is possible in spite of the clear dominance of interaction over hopping.