It has been widely observed that an increasing carrier density in a strongly coupled semiconductor microcavity (MC) alters the dispersion of cavity polaritons, below and above the condensation threshold. The interacting nature of cavity polaritons stems from their excitonic fraction being intrinsically subject to Coulomb interactions and the Pauli-blocking principle at high carrier densities. By means of injection-dependent photoluminescence studies performed nonresonantly on a GaN-based MC at various temperatures, it is shown that already below the condensation threshold saturation effects generally dominate over any energy variation in the excitonic resonance. This observation is in sharp contrast to the usually assumed picture in strongly coupled semiconductor MCs, where the impact of saturation is widely neglected. These experimental findings are confirmed by tracking the exciton emission properties of the bare MC active medium and those of a high-quality single GaN quantum well up to the Mott density. The systematic investigation of renormalization up to the polariton condensation threshold as a function of lattice temperature and exciton-cavity photon detuning is strongly hampered by photonic disorder. However, when overcoming the latter by averaging over a larger spot size, a behavior in agreement with a saturation-dominated polariton renormalization is revealed. Finally, a comparison with other inorganic material systems suggests that for correctly reproducing polariton renormalization, exciton saturation effects should be taken into account systematically.

We have systematically investigated relaxation of the surface photovoltage effect on the atomically controlled In/Si(111) surfaces with distinctive surface states and different amounts of the surface band bending. The temporal variations were traced in $real$ $time$ by time-resolved photoemission spectroscopy using soft x-ray synchrotron radiation. The relaxation is found to be temporally limited by two steps of the carrier transfer from the bulk to the surface: the tunneling process at a delay time $\le 100$ ns and the thermionic process on the following time scale ($\ge 100$ ns). Crossover of the two mechanisms can be understood by breakdown of the quantum tunneling regime by the increase in width of the space-charge layer during the relaxation.

We investigate with low-energy-electron diffraction (LEED) and angle-resolved photoelectron spectroscopy (ARPES) the effect of oxygen adsorbates on the phase transition of the Si(111)$4\times 1$-In surface. On this surface, oxygen was reported to increase the transition temperature (${\mathrm{T}}_{\mathrm{c}}$) into the distorted $8\times 2$ phase. We observed that while the ${\mathrm{T}}_{\mathrm{c}}$ into the $8\times 2$ phase increases up to 180 K, the substantial disorder is also induced on the surface. ARPES study reveals that the oxygen-induced $8\times 2$ phase has the same insulating band structure with that on the pristine surface at a lower temperature. This indicates clearly that the oxygen-induced $4\times 1$–$8\times 2$ transition is a consistent metal-insulator transition with that on the pristine surface. On the other hand, oxygen adsorbates slightly increase the band filling of the one-dimensional metallic bands of the $4\times 1$-In surface. This contrasts with the expectation of the hole doping and the increased ${\mathrm{T}}_{\mathrm{c}}$, thus cannot be explained by the doping effect of adsorbates. A further study of the oxygen-induced $8\times 2$ phase is requested to understand the microscopic mechanism of the adsorbate-induced elevation of the metal-insulator ${\mathrm{T}}_{\mathrm{c}}$.

The magnetic states of the noncentrosymmetric pressure-induced superconductor ${\mathrm{CeCoGe}}_3$ have been studied with magnetic susceptibility, muon spin relaxation ($\mu \mathrm{SR}$), single-crystal neutron diffraction, and inelastic neutron scattering (INS). ${\mathrm{CeCoGe}}_3$ exhibits three magnetic phase transitions at $T_{\mathrm{N}1}=21,T_{\mathrm{N}2}=12$, and $T_{\mathrm{N}3}=8\mathrm{K}$. The presence of long-range magnetic order below $T_{\mathrm{N}1}$ is revealed by the observation of oscillations of the asymmetry in the $\mu \mathrm{SR}$ spectra between 13 and 20 K and a sharp increase in the muon depolarization rate. Single-crystal neutron-diffraction measurements reveal magnetic Bragg peaks consistent with propagation vectors of $\mathbf{k}=(0,0,\frac{2}{3})$ between $T_{\mathrm{N}1}$ and $T_{\mathrm{N}2},\mathbf{k}=(0,0,\frac{5}{8})$ between $T_{\mathrm{N}2}$ and $T_{\mathrm{N}3}$ and $\mathbf{k}=(0,0,\frac{1}{2})$ below $T_{\mathrm{N}3}$. An increase in intensity of the (110) reflection between $T_{\mathrm{N}1}$ and $T_{\mathrm{N}3}$ also indicates a ferromagnetic component in these phases. These measurements are consistent with an equal moment two-up two-down magnetic structure below $T_{\mathrm{N}3}$ with a magnetic moment of $0.405\left(5\right){\mu }_{\mathrm{B}}/\mathrm{Ce}$. Above $T_{\mathrm{N}2}$, the results are consistent with an equal moment two-up one-down structure with a moment of $0.360\left(6\right){\mu }_{\mathrm{B}}/\mathrm{Ce}$. INS studies reveal two crystal-electric-field (CEF) excitations at $\sim 19$ and $\sim 27\mathrm{meV}$. From an analysis with a CEF model, the wave functions of the $J=\frac{5}{2}$ multiplet are evaluated along with a prediction for the magnitude and direction of the ground-state magnetic moment. Our model correctly predicts that the moments order along the $c$ axis, but the observed magnetic moment of $0.405\left(5\right){\mu }_{\mathrm{B}}$ is reduced compared to the predicted moment of $1.0{\mu }_{\mathrm{B}}$. This is ascribed to hybridization between the localized ${\mathrm{Ce}}^{3+}$ $f$ electrons and the conduction band. This suggests that ${\mathrm{CeCoGe}}_3$ has a degree of hybridization between that of ${\mathrm{CeRhGe}}_3$ and the noncentrosymmetric superconductor ${\mathrm{CeRhSi}}_3$.

Using a state-of-the art combination of density functional theory and impurity solver techniques, we present a complete and parameter-free picture of the Kondo effect in the high-spin ($S=3/2$) coordination complex known as manganese phthalocyanine adsorbed on the Pb(111) surface. We calculate the correlated electronic structure and corresponding tunnel spectrum and find an asymmetric Kondo resonance, as recently observed in experiments. Contrary to previous claims, the Kondo resonance stems from only one of three possible Kondo channels with origin in the Mn $3d$ orbitals, its peculiar asymmetric shape arising from the modulation of the hybridization due to a strong coupling to the organic ligand. The spectral signature of the second Kondo channel is strongly suppressed as the screening occurs via the formation of a many-body singlet with the organic part of the molecule. Finally, a spin-1/2 in the $3d$ shell remains completely unscreened due to the lack of hybridization of the corresponding orbital with the substrate, hence leading to a spin-3/2 underscreened Kondo effect.

The local nuclear and magnetic structure of wüstite, Fe${}_{1-x}$O, and the coupling between them, have been examined using reverse Monte Carlo refinements of variable-temperature neutron total scattering data. The results from this analysis suggest that the individual units in a tetrahedral defect cluster are connected along $\langle 110\rangle$ vectors into a Koch-Cohen-like arrangement, with the majority of octahedral vacancies concentrated near these defects. Bond valence calculations indicate a change in the charge distribution on the cations with the charge on the tetrahedral interstitials increasing on cooling. The magnetic structure is more complex than previously thought, corresponding to a noncollinear spin arrangement described by a superposition of a condensed spin wave on the established type-II antiferromagnetic ordering. This leads to an architecture with four groups of cations, each with different spin directions. The cations within the interstitial clusters appear to be weakly ferromagnetically coupled, and their spins are correlated to the spins of the octahedral cations closest to them. This work not only provides further insight into the local structure of wüstite but also enables a better understanding of the coupling between defect structures and magnetic and charge ordering in complex materials.

In this paper, we want to emphasize the fact that many experimental properties of ceramics can be explained by the existence of a core-shell structure of the grains, particularly at small sizes. In this framework, we have studied BaTiO${}_3$ (BT) ceramics constituted of core-shell nanoparticles, nanowires, or nanoplanes by using ab initio derived effective Hamiltonian calculations whose application range is for large values of shell thickness and low values of shell permittivity. Many differences and new features compared to the situation of nanodots are induced by the core-shell structure. For instance, phase sequences are different; there is also a coexistence of vortices found by Naumov, Bellaiche, and Fu [I. I. Naumov, L. Bellaiche, and H. Fu, Nature (London) 432, 737 (2004)] in the case of isolated dots with a homogeneous polarization, a transition from cubic paraelectric phase towards nonpolar rhombohedral phase, anomalies in dielectric permittivity associated with the onset of toroidal moments, etc. Afterwards, we compare these results with those obtained by the Landau theory of core-shell ceramics we have recently published. However, the ab initio calculations fail to capture the physics at small shell thickness and/or high shell permittivity, whereas the Landau theory fails to predict the peculiar properties of the phases in which vortices exist. Therefore, in a tentative way to build a global theory, we have constructed a Landau potential using both the polarization and the toroidal moment as competing order parameters, which allows us to propose a phase diagram, whatever the thickness and permittivity of the shell are.

We clarify the existence of several magnetization plateaus for the kagome $S=\frac{1}{2}$ antiferromagnetic Heisenberg model in a magnetic field. Using approximate or exact localized magnon eigenstates, we are able to describe in a similar manner the plateau states that occur for magnetization per site $m=\frac{1}{3}$, $\frac{5}{9}$, and $\frac{7}{9}$ of the saturation value. These results are confirmed using large-scale exact diagonalization on lattices up to 63 sites.

Honeycomb structures formed by the growth of perovskite $5d$ transition metal oxide heterostructures along the (111) direction in a $t_{2g}^5$ configuration can give rise to topological ground states characterized by a topological index $\nu =1$, as reported by Xiao et al. [Nat. Commun. 2, 596 (2011)]. Using a combination of a tight binding model and ab initio calculations we study the multilayers ${\left({\text{SrTiO}}_3\right)}_7/{\left({\text{SrIrO}}_3\right)}_2$ and ${\left({\text{KTaO}}_3\right)}_7/{\left({\text{KPtO}}_3\right)}_2$ as a function of parity asymmetry, on-site interaction $U$, and uniaxial strain and determine the nature and evolution of the gap. According to our density functional theory calculations, ${\left({\text{SrTiO}}_3\right)}_7/{\left({\text{SrIrO}}_3\right)}_2$ is found to be a topological semimetal whereas ${\left({\text{KTaO}}_3\right)}_7/{\left({\text{KPtO}}_3\right)}_2$ is found to present a topological insulating phase that can be understood as the high $U$ limit of the previous one, that can be driven to a trivial insulating phase by a perpendicular external electric field.

Applying the local density and dynamical mean field approximations to paramagnetic $\gamma$-iron we revisit the problem of the theoretical description of its magnetic properties in a wide temperature range. We show that contrary to $\alpha$-iron, the frequency dependence of the electronic self-energy has a quasiparticle form for both $t_{2g}$ and $e_g$ states. In the temperature range $T=1200$–1500 K, where $\gamma$-iron exists in nature, this substance can be nevertheless characterized by temperature-dependent effective local moments, which yield relatively narrow peaks in the real part of the local magnetic susceptibility as a function of frequency. At the same time, at low temperatures $\gamma$-iron (which is realized in precipitates) is better described in terms of the itinerant picture. In particular, the nesting features of the Fermi surfaces yield the maximum of the static magnetic susceptibility at the incommensurate wave vector ${\mathbf{q}}_{\max }$ belonging in the direction ${\mathbf{q}}_{\mathrm{X}}-{\mathbf{q}}_{\mathrm{W}}$ (${\mathbf{q}}_{\mathrm{X}}\equiv (2\pi /a)(1,0,0),{\mathbf{q}}_{\mathrm{W}}\equiv (2\pi /a)(1,1/2,0)$, $a$ is a lattice parameter) in agreement with the experimental data. This state is found, however, to compete closely with the states characterized by magnetic wave vectors along the directions ${\mathbf{q}}_{\mathrm{X}}-{\mathbf{q}}_{\mathrm{L}}-{\mathbf{q}}_{\mathrm{K}}$, where ${\mathbf{q}}_{\mathrm{L}}\equiv (2\pi /a)(1/2,1/2,1/2)$, ${\mathbf{q}}_{\mathrm{K}}\equiv (2\pi /a)(3/4,3/4,0)$. From the analysis of the uniform magnetic susceptibility we find that contrary to $\alpha$-iron, the Curie-Weiss law is not fulfilled in a broad temperature range, although the inverse susceptibility is nearly linear in the moderate-temperature region (1200–1500 K). The nonlinearity of the inverse uniform magnetic susceptibility in a broader temperature range is due to the density of states peak located close to the Fermi level. The effective exchange integrals in the paramagnetic phase are estimated on the base of momentum-dependent susceptibility.

We use bosonic field theories and the infinite system density matrix renormalization group method to study infinite strips of fractional quantum Hall states starting from microscopic Hamiltonians. Finite-entanglement scaling allows us to accurately measure chiral central charge, edge-mode exponents, and momenta without finite-size errors. We analyze states in the first and second levels of the standard hierarchy and compare our results to predictions of the chiral Luttinger liquid theory. The results confirm the universality of scaling exponents in chiral edges and demonstrate that renormalization is subject to universal relations in the nonchiral case. We prove a generalized Luttinger theorem involving all singularities in the momentum-resolved density, which naturally arises when mapping Landau levels on a cylinder to a fermion chain and deepens our understanding of non-Fermi liquids in one dimension.

One-dimensional wires with Rashba spin-orbit coupling, magnetic field, and strong electron-electron interactions are described by a spiral Luttinger liquid model. We develop a theory to investigate the tunneling density of states into a spiral Luttinger liquid under the proximity effect with superconductors. This approach provides a way to disentangle the delicate interplay between superconducting correlations and strong electron interactions. If the wire-superconductor boundary is dominated by Andreev reflection, we find that in the vicinity of the interface the zero-bias tunneling anomaly reveals a power law enhancement with the unusual exponent. Far away from the interface strong correlations inherent to the Luttinger liquid prevail and restore conventional suppression of the tunneling density of states at the Fermi level, which acquire, however, a Friedel-like oscillatory envelope with the period renormalized by the strength of the interaction.

The interplay between various many-body effects in a quantum dot attached to two normal and one superconducting lead is considered in the limit of a large superconducting gap. By the proximity effect the superconducting lead induces pairing correlations on the quantum dot. In the subgap region one observes the anomalous tunneling via direct and crossed Andreev scattering, whereas the usual single particle electronic transfer is suppressed. The interactions of electrons on the dot leading to such phenomena as the Coulomb blockade and the Kondo effect severely modify the currents flowing in the system. In particular: (i) They prevent the existence of the negative differential conductance observed for a noninteracting quantum dot over the whole range of voltages, (ii) affect the distribution of the currents as a function of the applied voltage, and (iii) lead to the appearance of an additional low bias feature due to the formation of the Abrikosov-Suhl resonance. The nonlocal correlations in the Coulomb blockade regime are most pronounced for the particle-hole symmetric dot and thus can be easily tuned by means of gate voltage. They are observed even in the Kondo regime and dominate the behavior close to the Abrikosov-Suhl resonance showing convincingly that Kondo correlations do not destroy subtle entanglement between electrons.

Photoemission of a molecular junction which is sandwiched between two Au nanospheres is studied theoretically. The Au spheres form a nanoresonator in which plasmon hybrid levels may strongly couple to the radiative transition in the molecular junction. To describe the resulting enhanced electroluminescence, a density matrix approach is introduced which is valid for the regime of sequential charge transmission through the molecule. The simulations account for energy exchange coupling between the molecule and the nanoresonator plasmon excitations, for nonradiative and radiative decay and for vibrational relaxation. Frequency-resolved photon emission spectra are also computed. The effects of varied molecular charging and vibrational reorganization energies are demonstrated. By changing the resonance condition between the plasmon hybrid level and the molecular transition, a selective enhancement of different sections of the vibrational progression in the emission spectrum becomes possible. In any case, electroluminescence spectra are enlarged by more than three orders of magnitude.

We demonstrate a general procedure for determining the conduction channels of quantum conductors from shot noise measurements. This numerical approach allows multichannel analysis which was previously limited to superconductors. Channel analysis of Ag and Au atomic contacts reveals a remarkable behavior in which the channels fully open one by one with increasing conductance. These results allow us to unambiguously distinguish between free-electron and tight-binding descriptions for the conductance of monovalent contacts. Furthermore, the channel resolution uncovers the presence of tunneling channels in parallel to the conductance through the main contact and provides a means for distinguishing between the contact conductance and tunneling contributions. Finally, unique channel distributions were found for Al and Pt contacts reflecting their distinct valence orbital structures.

The self-energy functional theory (SFT) is generalized to describe the real-time dynamics of correlated lattice-fermion models far from thermal equilibrium. This is achieved by starting from a reformulation of the original equilibrium theory in terms of double-time Green's functions on the Keldysh-Matsubara contour. With the help of a generalized Luttinger-Ward functional, we construct a functional $\widehat{\Omega }\left[\Sigma \right]$ which is stationary at the physical (nonequilibrium) self-energy $\Sigma$ and which yields the grand potential of the initial thermal state $\Omega$ at the physical point. Nonperturbative approximations can be defined by specifying a reference system that serves to generate trial self-energies. These self-energies are varied by varying the reference system's one-particle parameters on the Keldysh-Matsubara contour. In the case of thermal equilibrium, this approach reduces to the conventional SFT. Contrary to the equilibrium theory, however, “unphysical” variations, i.e., variations that are different on the upper and the lower branches of the Keldysh contour, must be considered to fix the time dependence of the optimal physical parameters via the variational principle. Functional derivatives in the nonequilibrium SFT Euler equation are carried out analytically to derive conditional equations for the variational parameters that are accessible to a numerical evaluation via a time-propagation scheme. Approximations constructed by means of the nonequilibrium SFT are shown to be inherently causal, internally consistent, and to respect macroscopic conservation laws resulting from gauge symmetries of the Hamiltonian. This comprises the nonequilibrium dynamical mean-field theory but also dynamical-impurity and variational-cluster approximations that are specified by reference systems with a finite number of degrees of freedom. In this way, nonperturbative and consistent approximations can be set up, the numerical evaluation of which is accessible to an exact-diagonalization approach.

Residual resistivity of FeGe under pressure exhibits an anomaly at a critical pressure $p_c\approx 19$ GPa whose origin is not understood. In previous theoretical work, it has been suggested that at the critical pressure a minority spin band starts to become occupied in FeGe. Here we provide further support for this scenario by studying, within the standard Boltzmann-type transport theory combined with a $T$-matrix treatment of scattering on dilute point defects, how the residual resistivity of a helical ferromagnet changes with exchange splitting.

We propose that a combination of the semiclassical approximation with Monte Carlo simulations can be an efficient and reliable impurity solver for dynamical mean field theory equations and their cluster extensions with large cluster sizes. In order to show the reliability of the method, we consider two test cases: (i) the single-band Hubbard model within the dynamical cluster approximation with four- and eight-site clusters and (ii) the anisotropic two-orbital Hubbard model with orbitals of different bandwidth within the single-site dynamical mean field theory. We compare the critical interaction $U_c/t$ with those obtained from solving the dynamical mean field equations with the continuous-time and Hirsch-Fye quantum Monte Carlo approaches. In both test cases we observe reasonable values of the metal-insulator critical interaction strength $U_c/t$ and the nature of Mott physics in the self-energy behavior. While some details of the spectral functions cannot be captured by the semiclassical approximation due to the freezing of dynamical fluctuations, the main features are reproduced by the approach.

We investigate experimentally transport in gated microsctructures containing a band-inverted HgTe/Hg${}_{0.3}$Cd${}_{0.7}$Te quantum well. Measurements of nonlocal resistances using many contacts prove that in the depletion regime the current is carried by the edge channels, as expected for a two-dimensional topological insulator. However, high and nonquantized values of channel resistances show that the topological protection length (i.e., the distance on which the carriers in helical edge channels propagate without backscattering) is much shorter than the channel length, which is $\sim$100 $\mu$m. The weak temperature dependence of the resistance and the presence of temperature dependent reproducible quasiperiodic resistance fluctuations can be qualitatively explained by the presence of charge puddles in the well, to which the electrons from the edge channels are tunnel-coupled.

This work investigates the $\beta$-Si${}_3$N${}_4$(0001)/Si(111) interface based on a model with fully saturated interface bonds. The charge transfer at the interface and band alignment are calculated. The band alignment is corrected by $G{W}_0$ calculations. Furthermore, we investigate how substitutional phosphorus defects affect the electronic structure of the interface, in particular how they saturate the interface states and modify the valence band offsets.