Avalanches of the magnetization, that is to say an abrupt reversal of the magnetization at a given field, have been previously reported in the spin-ice compound ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$. This out-of-equilibrium process, induced by magnetothermal heating, is quite usual in low-temperature magnetization studies. A key point is to determine the physical origin of the avalanche process. In particular, in spin-ice compounds, the origin of the avalanches might be related to the monopole physics inherent to the system. We have performed a detailed study of the avalanche phenomena in three single crystals, with the field oriented along the [111] direction, perpendicular to [111], and along the [100] directions. We have measured the changing magnetization during the avalanches and conclude that avalanches in spin ice are quite slow compared to the avalanches reported in other systems such as molecular magnets. Our measurements show that the avalanches trigger after a delay of about 500 ms and that the reversal of the magnetization then occurs in a few hundreds of milliseconds. These features suggest an unusual propagation of the reversal, which might be due to the monopole motion. The avalanche fields seem to be reproducible in a given direction for different samples, but they strongly depend on the initial state of magnetization and on how the initial state was achieved.

Alkali-doped aromatic compounds have shown evidence of metallic and superconducting phases whose precise nature is still mysterious. In potassium and rubidium-doped phenanthrene, superconducting temperatures around 5 K have been detected, but such basic elements as the stoichiometry, crystal structure, and electronic bands are still speculative. We seek to predict the crystal structure of ${\mathrm{M}}_3$-phenanthrene (M = K, Rb) using ab initio evolutionary simulation in conjunction with density functional theory (DFT), and find metal but also insulator phases with distinct structures. The original $P{2}_1$ herringbone structure of the pristine molecular crystal is generally abandoned in favor of different packing and chemical motifs. The metallic phases are frankly ionic with three electrons acquired by each molecule. In the nonmagnetic insulating phases the alkalis coalesce reducing the donated charge from three to two per phenanthrene molecule. A similar search for ${\mathrm{K}}_3$-picene yields an old and a new structure, with unlike potassium positions and different electronic bands, but both metallic retaining the face-to-edge herringbone structure and the $P{2}_1$ symmetry of pristine picene. Both the new ${\mathrm{K}}_3$-picene and the best metallic ${\mathrm{M}}_3$-phenanthrene are further found to undergo a spontaneous transition from metal to antiferromagnetic insulator when spin polarization is allowed, a transition which is not necessarily real, but which underlines the necessity to include correlations beyond DFT. Features of the metallic phases that may be relevant to phonon-driven superconductivity are underlined.

We investigate the entanglement spreading in the anisotropic spin-$1/2$ Heisenberg $\left(XXZ\right)$ chain after a geometric quench. This corresponds to a sudden change of the geometry of the chain or, in the equivalent language of interacting fermions confined in a box trap, to a sudden increase of the trap size. The entanglement dynamics after the quench is associated with the ballistic propagation of a magnetization wave front. At the free fermion point ($XX$ chain), the von Neumann entropy $S_A$ exhibits several intriguing dynamical regimes. Specifically, at short times a logarithmic increase is observed, similar to local quenches. This is accurately described by an analytic formula that we derive from heuristic arguments. At intermediate times partial revivals of the short-time dynamics are superposed with a power-law increase $S_A\sim t^{\alpha }$, with $\alpha <1$. Finally, at very long times a steady state develops with constant entanglement entropy, apart from oscillations. As expected, since the model is integrable, we find that the steady state is nonthermal, although it exhibits extensive entanglement entropy. We also investigate the entanglement dynamics after the quench from a finite to the infinite chain (sudden expansion). While at long times the entanglement vanishes, we demonstrate that its relaxation dynamics exhibits a number of scaling properties. Finally, we discuss the short-time entanglement dynamics in the $XXZ$ chain in the gapless phase. The same formula that describes the time dependence for the $XX$ chain remains valid in the whole gapless phase.

We present numerically exact results for the spectral function of a single charge carrier that is injected into an Ising chain at finite temperature $T$. Both ferromagnetic and antiferromagnetic coupling between the Ising spins are considered. The interaction between the carrier and the Ising spins is assumed to be on-site and of Ising type, as well. We find that the carrier's spectral function exhibits a distinctive fine structure of resonances that are due to the temporary entrapping of the carrier inside small magnetic domains. The connection to models of disordered binary alloys where similar effects can occur is explained. We use these results to construct an accurate (quasi)analytic approximation for low and medium-$T$ spectral functions.

Enhancement of light-matter interaction of a quantum emitter with subwavelength quasiperiodic metamaterials is proposed and demonstrated. The quasiperiodic metamaterials consist of subwavelength metal-dielectric multilayers, which are arranged into a Fibonacci lattice. The influence of Fibonacci metamaterials (FM) on the dipole emission is analyzed with a semiclassical model. The local density of states near FM is evaluated and a characteristic mode in higher wave numbers is revealed; a strong enhancement of the decay rate was predicted. A lifetime measurement is carried out and a reduction of lifetime of quantum dots on the surface of FM is observed. The enhancement of light-matter interaction arises from the localized latticelike state inherent for self-similar quasiperiodic order.

Exchange integrals between nearest-neighbor (NN) transition metal ions in II-VI diluted magnetic semiconductors (DMSs) are calculated within a local superexchange model, which includes orbital-dependent transfer, on-site Coulomb repulsion and Hund's exchange between $3d$ electrons, and ligand field effects. This extended model gives a quantitative account for the available experimental data on the NN exchange constants in all II-VI DMS family (wurtzite and zinc-blende) doped by cobalt or manganese. As expected, all obtained exchange integrals are antiferromagnetic. Remarkably, the model input parameters are taken directly from the photoemission spectroscopy. We show that in the case of Co-doped compounds, as compared to Mn-doped ones, the exchange process has at least two salient features. The first one is that the electron transfer between NN $\mathrm{Co}{}^{2+} 3d$ orbitals strongly depends on their symmetry positions in the crystal lattice. The second one is related to a peculiar virtual process, involving empty and occupied $\mathrm{Co}{}^{2+} 3d$ orbitals, which leads to an additional ferromagnetic contribution to the exchange constant. We argue that our systematic study of the superexchange opens a pathway toward an understanding of other exchange mechanisms occurring in DMSs.

We theoretically analyze the relaxation of paraexcitons in cuprous oxide due to exciton-phonon and exciton-exciton scattering. Particular attention is paid to the evolution of the distribution function as well as to the cooling process of the exciton gas. The results underline the importance of interexcitonic collisions at moderate and higher densities which prevent the formation of the typical bottleneck and accelerate the thermalization at ultralow temperatures significantly. Furthermore, we discuss the impact of strain on the exciton-phonon coupling and show that the overall cooling process of the excitons benefits from strain induced effects. However, for very low lattice temperatures ($T\ll 1.0\mathrm{K}$), the process of thermalization is slow, and the excitons might not reach the lattice temperature within their finite lifetime.

A theory for the collective plasma excitations in a linear periodic array of spherical two-dimensional electron gases (S2DEGs) is presented. This is a simple model for an ultra thin and narrow microribbon of fullerenes or metallic shells. Coulomb coupling between electrons located on the same sphere and on different spheres is included in the random-phase approximation. Electron hopping between spheres is neglected in these calculations. The resulting plasmon-dispersion equation is solved numerically. Results are presented for a superlattice of single-wall S2DEGs as a function of the wave vector. The plasmon dispersions are obtained for different spherical separations. We show that the one-dimensional translational symmetry of the lattice is maintained in the plasmon spectrum. Additionally, we compare the plasmon dispersion when the superlatice direction is parallel or perpendicular to the axis of quantization. However, because of anisotropy in the Coulomb matrix elements, there is anticrossing in the plasmon dispersion only in the direction perpendicular to the quantization axis. The S2DEG may serve as a simple model for fullerenes, when their energy bands are far apart.

A Josephson bijunction made of three superconductors connected by a quantum dot is considered in the regime where the dot carries a magnetic moment. In the range of parameters where such a dot, if inserted in a two-terminal Josephson junction, creates a $\pi$-shift of the phase, the bijunction forming a triangular unit is frustrated. This frustration is studied within both a phenomenological and a microscopic model. Frustration stabilizes a phase vortex centered on the dot, with two degenerate states carrying opposite vorticities, independently of the direction of the magnetic moment. Embedding the bijunction in a superconducting loop allows one to create a tunable “$\varphi$”-junction whose equilibrium phase can take any value. For large enough inductance, it generates noninteger spontaneous flux. Multiloop configurations are also studied.

The Anderson lattice model is used to explain the principal features of the heavy fermion compound ${\mathrm{UGe}}_2$ by means of the generalized Gutzwiller approach (the statistically consistent Gutzwiller approximation method). This microscopic approach successfully reproduces the magnetic and electronic properties of this material, in qualitative agreement with experimental findings from magnetization measurements, neutron scattering, and de Haas–van Alphen oscillations. Most importantly, it explains the appearance, sequence, character, and evolution in an applied magnetic field of the observed in ${\mathrm{UGe}}_2$ ferromagnetic and paramagnetic phases as an effect of a competition between the $f-f$ electron Coulomb interaction energy and $f$-conduction electron hybridization.

The $Z_2$ topological order in $Z_2$ spin liquid and in the exactly solvable Kitaev toric code model is the simplest topological order for 2 + 1D bosonic systems. More general 2 + 1D bosonic topologically ordered states can be constructed via exactly solvable string-net models. However, the most important topologically ordered phases of matter are arguably the fermionic fractional quantum Hall states. Topological phases of matter for fermion systems are strictly richer than their bosonic counterparts because locality has different meanings for the two kinds of systems. In this paper, we describe a simple fermionic version of the toric code model to illustrate many salient features of fermionic exactly solvable models and fermionic topologically ordered states.

We investigate charge transfer in prototypical molecular donor-acceptor compounds using hybrid density functional theory (DFT) and the $GW$ approximation at the perturbative level ($G_0{W}_0$) and at full self-consistency (sc-$GW$). For the systems considered here, no charge transfer should be expected at large intermolecular separation according to photoemission experiments and accurate quantum-chemistry calculations. The capability of hybrid exchange-correlation functionals of reproducing this feature depends critically on the fraction of exact exchange $\alpha$, as for small values of $\alpha$ spurious fractional charge transfer is observed between the donor and the acceptor. $G_0{W}_0$ based on hybrid DFT yields the correct alignment of the frontier orbitals for all values of $\alpha$. However, $G_0{W}_0$ has no capacity to alter the ground-state properties of the system because of its perturbative nature. The electron density in donor-acceptor compounds thus remains incorrect for small $\alpha$ values. In sc-$GW$, where the Green's function is obtained from the iterative solution of the Dyson equation, the electron density is updated and reflects the correct description of the level alignment at the $GW$ level, demonstrating the importance of self-consistent many-body approaches for the description of ground- and excited-state properties in donor-acceptor systems.

We consider a heterostructure consisting of a normal metal and a superconductor separated by a spin-active interface. At finite-bias voltages, spin-filtering and spin-mixing effects at the interface allow for an induced magnetization (spin imbalance) on the superconducting side of the junction, which relaxes to zero in the bulk. Such interfaces are also known to host a pair of in-gap Andreev bound states which were recently observed experimentally. We show that these states are responsible for the dominant contribution to the induced spin imbalance close to the interface. Motivated by recent experiments on spin-charge density separation in superconducting aluminum wires, we propose an alternative way to observe spin imbalance without applying an external magnetic field. We also suggest that the peculiar dependence of the spin imbalance on the applied bias voltage permits an indirect bound-state spectroscopy.

The newly emerging monolayer phosphorene was recently predicted to be a promising thermoelectric material. In this work, we propose to further enhance the thermoelectric performance of phosphorene using the strain-induced band convergence. The effect of the uniaxial strain on the thermoelectric properties of phosphorene was investigated by using the first-principles calculations combined with the semiclassical Boltzmann theory. When the zigzag-direction strain is applied, the Seebeck coefficient and electrical conductivity in the zigzag direction can simultaneously be greatly enhanced at the critical strain of 5%, at which the band convergence is achieved. The largest $ZT$ value of 1.65 at 300 K is then conservatively estimated by using the bulk lattice thermal conductivity. When the armchair-direction strain of 8% is applied, the room-temperature $ZT$ value can reach 2.12 in the armchair direction of phosphorene. Our results indicate that strain-induced band convergence could be an effective method to enhance the thermoelectric performance of phosphorene.

By carrying out simultaneous longitudinal and Hall measurements in graphene, we find that the $1/f$ noise for the charge carrier density is negatively correlated to that of mobility, with a governing behavior that differs significantly from the relation between their mean values. The correlation in the noise data can be quantitatively explained by a single-parameter theory whose underlying physics is the trapping and detrapping of the fluctuating charge carriers by the oppositely charged Coulomb scattering centers. This can alter the effective density of long-range scattering centers in a transient manner, with the consequent fluctuating effect on the mobility.

Metal borohydrides are intensively studied because of their potential applications as versatile hydrogen storage. The relation between the formation enthalpy and the Pauling electronegativity established for these materials [Phys. Rev. B 74, 045126 (2006)] led to the idea of developing mixed-cation compounds that may provide a route for tuning the thermodynamic stability of metal borohydrides. We report a systematic ab initio investigation of the single-metallic and bimetallic borohydrides, and via an examination of the Born effective charges we provide insight into the physical mechanism determining their stabilities. We show that the decreasing stability of metal borohydrides follows the increasing polarizing ability of the cationic bonding component, expressed as the square root of the cation's dynamical charge divided by its radius in coordinated anion polyhedra around the cation. The charge-to-size ratio thus provides a simple yet physically sound measure of the stabilities of metal borohydrides that can be obtained in relatively simple calculations.

The random phase approximation (RPA) to the correlation energy is among the most promising methods to obtain accurate correlation energy differences from diagrammatic perturbation theory at modest computational cost. We show here that a cubic system size scaling can be readily obtained, which dramatically reduces the computation time by one to two orders of magnitude for large systems. Furthermore, the scaling with respect to the number of $k$ points used to sample the Brillouin zone can be reduced to linear order. In combination, this allows accurate and very well-converged single-point RPA calculations, with a time complexity that is roughly on par or better than for self-consistent Hartree-Fock and hybrid-functional calculations. The present implementation enables new applications. Here, we apply the RPA to determine the energy difference between diamond Si and $\beta$-tin Si, the energetics of the Si self-interstitial defect and the Si vacancy, the latter with up to 256 atom supercells. We show that the RPA predicts Si interstitial and vacancy energies in excellent agreement with experiment. Si self-interstitial diffusion barriers are also in good agreement with experiment, as opposed to previous calculations based on hybrid functionals or range-separated RPA variants.