The harmonic Kanzaki-Krivoglaz-Khachaturyan model of strain-induced interaction is generalized to concentrated size-mismatched alloys and adapted to first-principles calculations. The configuration dependence of both Kanzaki forces and force constants is represented by real-space cluster expansions that can be constructed based on the calculated forces. The model is implemented for the fcc lattice and applied to ${\mathrm{Cu}}_{1-x}{\mathrm{Au}}_x$ and ${\mathrm{Fe}}_{1-x}{\mathrm{Pt}}_x$ alloys for concentrations $x=0.25$, 0.5, and 0.75. The asymmetry between the $3d$ and $5d$ elements leads to large quadratic terms in the occupation-number expansion of the Kanzaki forces and thereby to strongly non-pairwise long-range interaction. The main advantage of the full configuration-dependent lattice deformation model is its ability to capture this singular many-body interaction. The roles of ordering striction and anharmonicity in Cu-Au and Fe-Pt alloys are assessed. Although the harmonic force constants defined with respect to the unrelaxed lattice are unsuitable for the calculation of the vibrational entropies, the phonon spectra for ordered and disordered alloys are found to be in good agreement with experimental data. The model is further adapted to concentration wave analysis and Monte Carlo simulations by means of an auxiliary multiparametric real-space cluster expansion, which is used to find the ordering temperatures. Good agreement with experiment is found for all systems except ${\mathrm{CuAu}}_3$ (due to the known failure of the generalized gradient approximation) and ${\mathrm{FePt}}_3$, where the discrepancy is likely due to the neglect of magnetic disorder.

Heat-assisted recording is a promising technique to further increase the storage density in hard disks. Multilayer recording grains with graded Curie temperature is discussed to further assist the write process. Describing the correct magnetization dynamics of these grains, from room temperature to far above the Curie point, during a write process is required for the calculation of bit error rates. We present a coarse-grained approach based on the Landau-Lifshitz-Bloch (LLB) equation to model exchange-coupled grains with low computational effort. The required temperature-dependent material properties such as the zero-field equilibrium magnetization as well as the parallel and normal susceptibilities are obtained by atomistic Landau-Lifshitz-Gilbert simulations. Each grain is described with one magnetization vector. In order to mimic the atomistic exchange interaction between the grains a special treatment of the exchange field in the coarse-grained approach is presented. With the coarse-grained LLB model the switching probability of a recording grain consisting of two layers with graded Curie temperature is investigated in detail by calculating phase diagrams for different applied heat pulses and external magnetic fields.

Magnetic order and excitations in multiferroic ${\mathrm{DyMnO}}_3$ were studied by neutron scattering experiments using a single crystal prepared with an enriched ${}^{162}\mathrm{Dy}$ isotope. The ordering of Mn moments exhibits pronounced hysteresis arising from the interplay between Mn and Dy magnetism, which has a strong impact on the ferroelectric polarization. The magnon dispersion resembles that reported for ${\mathrm{TbMnO}}_3$. We identify the excitations at the magnetic zone center and near the zone boundary in the $b$ direction, which can possess electromagnon character. The lowest frequency of the zone-center magnons is in good agreement with a signal in a recent optical measurement, so this mode can be identified as the electromagnon coupled by the same Dzyaloshinskii-Moriya interaction as the static multiferroic phase.

We measured the pressure dependence of in-plane resistivity ${\rho }_{ab}$ in the recently discovered iron-based superconductor ${\mathrm{Ca}}_{10}\left({\mathrm{Ir}}_4{\mathrm{As}}_8\right){\left({\mathrm{Fe}}_{2-x}{\mathrm{Ir}}_x{\mathrm{As}}_2\right)}_5$, which shows a unique structural phase transition in the absence of magnetic ordering, with a superconducting transition temperature $T_{\mathrm{c}}=16$ K and a structural phase transition temperature $T_{\mathrm{s}}\simeq 100$ K at ambient pressure. $T_{\mathrm{c}}$ and $T_{\mathrm{s}}$ are suppressed on applying pressure and disappear at approximately 0.5 GPa, suggesting a relationship between superconductivity and structure. ${\mathrm{Ca}}_{10}\left({\mathrm{Ir}}_4{\mathrm{As}}_8\right){\left({\mathrm{Fe}}_{2-x}{\mathrm{Ir}}_x{\mathrm{As}}_2\right)}_5$ is a rather rare example in which the superconductivity appears only in a low-temperature ordered phase. The fact that the change in the crystal structure is directly linked with superconductivity suggests that the crystal structure as well as magnetism are important factors governing superconductivity in iron pnictides.

We numerically model experiments in which large-scale vortex rings—bundles of quantized vortex loops—are created in superfluid helium by a piston-cylinder arrangement. We show that the presence of a normal-fluid vortex ring together with the quantized vortices is essential to explain the coherence of these large-scale vortex structures at nonzero temperatures, as observed experimentally. Finally we argue that the interaction of superfluid and normal-fluid vortex bundles is relevant to recent investigations of superfluid turbulence.

We investigate the behavior of the Higgs (amplitude) mode when superconductivity emerges on a preexisting charge-density-wave state. We show that the weak overdamped square-root singularity of the amplitude fluctuations in a standard BCS superconductor is converted in a sharp, undamped power-law divergence in the coexisting state, reminiscent of the Higgs behavior in Lorentz-invariant theories. This effect reflects in a strong superconducting resonance in the Raman spectra, both for an electronic and a phononic mechanism leading to the Raman visibility of the Higgs. In the latter case, our results are relevant to the interpretation of the Raman spectra measured experimentally in ${\mathrm{NbSe}}_2$.

We present a polarization-dependent infrared reflectivity study of the spin-ladder compound ${\mathrm{Sr}}_{2.5}{\mathrm{Ca}}_{11.5}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ under pressure. The optical response is strongly anisotropic, with the highest reflectivity along the ladders/chains $\left(\mathbf{E}\parallel c\right)$ revealing a metallic character. For the polarization direction perpendicular to the ladder plane, an insulating behavior is observed. With increasing pressure the optical conductivity for $\mathbf{E}\parallel c$ shows a strong increase, which is most pronounced below $2000{\mathrm{cm}}^{-1}$. According to the spectral weight analysis of the $\mathbf{E}\parallel c$ optical conductivity the hole concentration in the ladders increases with increasing pressure and tends to saturate at high pressure. At $\sim 7.5$ GPa the number of holes per Cu atom in the ladders has increased by $\Delta \delta =0.09(\pm 0.01)$, and the Cu valence in the ladders has reached the value $+2.33$. The optical data suggest that ${\mathrm{Sr}}_{2.5}{\mathrm{Ca}}_{11.5}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ remains electronically highly anisotropic up to high pressure, also at low temperatures.

We consider recent experiments on wide superconductor–quantum spin Hall insulator (QSHI)–superconductor Josephson junctions, which have shown preliminary evidence of proximity-induced superconductivity at the edge modes of the QSHI system based on an approximate analysis of the observed Fraunhofer spectra of the Josephson critical current as a function of the applied magnetic field. Using a completely independent exact numerical method involving a nonlinear constrained numerical optimization, we calculate the supercurrent profiles, comparing our results quantitatively with the experimental Fraunhofer patterns in both HgCdTe and InAs-GaSb based QSHI Josephson junctions. Our results show good qualitative agreement with the experiments, verifying that the current distribution in the two-dimensional sample indeed has peaks at the sample edges when the system is in the QSHI phase, thus supporting the interpretation that superconductivity has indeed been induced in the QSHI edge modes. On the other hand, our numerical work clearly demonstrates that it will be very difficult, if not impossible, to obtain detailed quantitative information about the supercurrent distribution just from the analysis of the Josephson Fraunhofer spectra, and therefore, conclusions regarding the precise width of the edge modes or their topological nature are most likely premature at this stage.

We solve nonequilibrium dynamical mean-field theory (DMFT) using matrix product states (MPS). This allows us to treat much larger bath sizes and by that reach substantially longer times (factor $\sim 2–3$) than with exact diagonalization. We show that the star geometry of the underlying impurity problem can have substantially better entanglement properties than the previously favored chain geometry. This has immense consequences for the efficiency of an MPS-based description of general impurity problems: in the case of equilibrium DMFT, it leads to an orders-of-magnitude speedup. We introduce an approximation for the two-time hybridization function that uses time-translational invariance, which can be observed after a certain relaxation time after a quench to a time-independent Hamiltonian.

The dual-fermion approach offers a way to perform diagrammatic expansion around the dynamical mean field theory. Using this formalism, the influence of antiferromagnetic fluctuations on the self-energy is taken into account through ladder-type diagrams in the particle-hole channel. The resulting phase diagram for the (quasi-)two-dimensional Hubbard model exhibits antiferromagnetism and $d$-wave superconductivity. Furthermore, a uniform charge instability, i.e., phase separation, is obtained in the low-doping regime around the Mott insulator. We also examine spin/charge density wave fluctuations including $d$-wave symmetry. The model exhibits a tendency towards an unconventional charge density wave, but no divergence of the susceptibility is found.

We study by microphotoluminescence the polarization properties of single zinc-blende GaN/AlN quantum dots fabricated by two growth processes: the droplet epitaxy technique and the Stranski-Krastanov growth mode. A statistical analysis of their polarization properties both at the mesoscopic and nanoscopic scales indicates that it is possible to modify the phase and amplitude of their valence band mixing by switching on and off the influence of AlN antiphase domains through the growth process. $k.p$ calculations show that the strain effects and quantum dot geometries resulting from the growth process play a major role in shaping their polarization properties.

We have studied the amplified emission properties of nanoislands with CdSe quantum dots in ZnSe/CdSe/ZnSe heterostructures surrounded by metallic antennas. It has been found that variations of the optical antenna length give rise to periodic amplification of the integral emission intensity. The period of the discovered oscillations corresponds to the wavelength of the surface plasmon-polariton mode propagating in the metallic antenna. The nature of observed periodicity was confirmed by results of numerical simulations for linear antennas. It has been established that the velocity of surface polaritons depends not only on the parameters of the dielectric constants of the metal and of the semiconductor substrate but also on the width of the metallic antenna. The influence of antenna antisymmetry (its helicity) on selective amplification of the degree of circular polarization of photoexcitation has been investigated. We found that plasmon-polariton standing waves induced in $S$-type (curved) antennas by circularly polarized light, which was used for quantum dot photoexcitation, result in enhanced polarization selectivity of the quantum dot emission. The selectivity of the polarization of photoexcitation is a periodic function of the helical antenna length.

We have studied the metal–insulator-like transition in pseudo-one-dimensional systems, i.e., lithium and beryllium rings, through the ab initio density matrix renormalization group (DMRG) method. Performing accurate calculations for different interatomic distances and using quantum information theory, we investigated the changes occurring in the wave function between a metallic-like state and an insulating state built from free atoms. We also discuss entanglement and relevant excitations among the molecular orbitals in the Li and Be rings and show that the transition bond length can be detected using orbital entropy functions. Also, the effect of different orbital bases on the effectiveness of the DMRG procedure is analyzed comparing the convergence behavior.

The spin-boson model, describing a two-level system coupled to a bath of harmonic oscillators, is a generic model for quantum dissipation, with manifold applications. It has also been studied as a simple example for an impurity quantum phase transition. Here, we present a detailed study of a U(1)-symmetric two-bath spin-boson model, where two different components of an SU(2) spin $\frac{1}{2}$ are coupled to separate dissipative baths. Nontrivial physics arises from the competition of the two dissipation channels, resulting in a variety of phases and quantum phase transitions. We employ a combination of analytical and numerical techniques to determine the properties of both the stable phases and the quantum critical points. In particular, we find a critical intermediate-coupling phase which is bounded by a continuous quantum phase transition which violates the quantum-to-classical correspondence.

Spin-orbit interactions in two-dimensional electron liquids are responsible for many interesting transport phenomena in which particle currents are converted to spin polarizations and spin currents and vice versa. Prime examples are the spin Hall effect, the Edelstein effect, and their inverses. By similar mechanisms, it is also possible to partially convert an optically induced electron-hole density wave to a spin density wave and vice versa. In this paper, we present a unified theoretical treatment of these effects based on quantum kinetic equations that include not only the intrinsic spin-orbit coupling from the band structure of the host material, but also the spin-orbit coupling due to an external electric field and a random impurity potential. The drift-diffusion equations we derive in the diffusive regime are applicable to a broad variety of experimental situations, both homogeneous and nonhomogeneous, and include on equal footing “skew scattering” and “side jump” from electron-impurity collisions. As a demonstration of the strength and usefulness of the theory we apply it to the study of several effects of current experimental interest: the inverse Edelstein effect, the spin-current swapping effect, and the partial conversion of an electron-hole density wave to a spin density wave in a two-dimensional electron gas with Rashba and Dresselhaus spin-orbit couplings, subject to an electric field.

We report electrical transport measurements in a tilted magnetic field on a high-mobility two-dimensional electron system (2DES) confined at the MgZnO/ZnO heterointerface. The observation of multiple crossing events of spin-resolved Landau levels (LLs) enables the mapping of the sequence of electronic states. We further measure the renormalization of electron spin susceptibility at zero field and the susceptibility dependence on the electron spin polarization. The latter manifests the deviation from the Pauli spin susceptibility. As a result, the crossing of spin-resolved LLs shifts to smaller tilt angles and the first Landau level coincidence event is absent even when the magnetic field has only a component perpendicular to the 2DES plane.

We theoretically investigate the full time evolution of a nonequilibrium double quantum dot structure from initial conditions corresponding to different product states (no entanglement between dot and lead) to a nonequilibrium steady state. The structure is described by a two-level spinless Anderson model where the levels are coupled to two leads held at different chemical potentials. The problem is solved by a numerically exact hierarchical master equation technique and the results are compared to approximate ones obtained from Born-Markov theory. The methods allow us to study the time evolution up to times of order ${10}^4$ of the bare hybridization time, enabling eludication of the role of the initial state on the transient dynamics, coherent charge oscillations and an interaction-induced renormalization of energy levels. We find that when the system carries a single electron on average the formation of the steady state is strongly influenced by the coherence between the dots. The latter can be sizeable and, indeed, larger in the presence of a bias voltage than it is in equilibrium. Moreover, the interdot coherence is shown to lead to a pronounced difference in the population of the dots.

The magnetoelectricity of garnets is considered by means of a symmetry and quantum mechanical combined analysis. It is shown that the magnetoelectric effect is not realized in most garnets although the necessary condition of the crystal magnetic structure antisymmetry in them is held at low temperatures. Nevertheless, the effect can be observed in some garnets as well as other odd effects, namely, the piezomagnetic effect, the magnetic field evoked piezoelectric one, etc. It is also discovered that magnetic fields can induce specific antiferroelectric structures in garnet crystals and produce electric polarization in epitaxial films. The polarization can also be caused in a bulk crystal by an inhomogeneous magnetic field.