Despite considerable research efforts, a clear understanding of superconductivity in Ba${}_{0.5}$K${}_{0.5}$BiO${}_3$ has been elusive. Recent studies showed that although electron-correlation effects in this compound can significantly increase the electron-phonon coupling, they do not reproduce the measured Eliashberg spectral function (${\alpha }^{2}F$). We show that the oxygen octahedra tilts in Ba${}_{0.5}$K${}_{0.5}$BiO${}_3$ increase ${\alpha }^{2}F$ in the range of frequencies near 30 meV, even on the level of the generalized gradient approximation. This increase in ${\alpha }^{2}F$ changes its shape to provide better agreement with experiment and results in a 50–60% increase of the average electron-phonon coupling strength $\lambda$. We use the Wannier interpolation technique to determine the electron-phonon coupling with high precision.

The determinant quantum Monte Carlo (DQMC) method is used to study the effect of nonzero hopping $t_f$ in the “localized” $f$ band of the periodic Anderson model (PAM) in two dimensions. The low-temperature properties are determined in the plane of interband hybridization $V$ and $t_f$ at fixed $U_f$ and half filling, including the case when the sign of $t_f$ is opposite to that of the conduction band $t_d$. For $t_f$ and $t_d$ of the same sign and when $t_f/t_d>{(V/4t_d)}^2$, the noninteracting system is metallic. We show that a remnant of the band insulator to metal line at $U_f=0$ persists in the interacting system, manifesting itself as a maximal tendency toward antiferromagnetic correlations at low temperature. In this “optimal” $t_f$ region, short-range (e.g., near-neighbor) and long-range spin correlations develop at similar temperatures and have comparable magnitude. Both observations are in stark contrast to the situation in the widely studied PAM ($t_f=0$) and single-band Hubbard model, where short-range correlations are stronger and develop at higher temperature. The effect that finite $t_f$ has on Kondo screening is investigated by considering the evolution of the local density of states for selected $t_f$ as a function of $V$. We use mean-field theory as a tool to discriminate those aspects of the physics that are genuinely many-body in character.

Nonlinear optics of semiconductors is an important field of fundamental and applied research, but surprisingly the role of excitons in the coherent processes leading to harmonics generation has remained essentially unexplored. Here we report results of a comprehensive experimental and theoretical study of the three-photon process of optical second-harmonic generation (SHG) involving the exciton resonances of the noncentrosymmetric hexagonal wide-band-gap semiconductor ZnO in the photon energy range of $3.2$–$3.5$ eV. Resonant crystallographic SHG is observed for the $1s(A,B)$, $2s(A,B)$, $2p(A,B)$, and $1s\left(C\right)$ excitons. We show that SHG signals at these exciton resonances are induced by the application of a magnetic field when the incident and the SHG light wave vectors are along the crystal $z$ axis where the crystallographic SHG response vanishes. A microscopic theory of SHG through excitons is developed, which shows that the nonlinear interaction of coherent light with excitons has to be considered beyond the electric-dipole approximation. Depending on the particular symmetry of the exciton states SHG can originate from the electric- and magnetic-field-induced perturbations of the excitons due to the Stark effect, the spin as well as orbital Zeeman effects, or the magneto-Stark effect. The importance of each mechanism is analyzed and discussed by confronting experimental data and theoretical results for the dependences of the SHG signals on photon energy, magnetic field, electric field, crystal temperature, and light polarization. Good agreement is obtained between experiment and theory proving the validity of our approach to the complex problem of nonlinear interaction of light with ZnO excitons. This general approach can be applied also to other semiconductors.

With its combination of favorable band gap, high absorption coefficient, material abundance, and low cost, iron pyrite, FeS${}_2$, has received a great deal of attention over the past decades as a promising material for photovoltaic applications such as solar cells and photoelectrochemical cells. Devices made from pyrite, however, exhibit open circuit voltages significantly lower than predicted, and despite a recent resurgence of interest in the material, there currently exists no widely accepted explanation for this disappointing behavior. In this paper, we show that phonons, which have been largely overlooked in previous efforts, may play a significant role. Using fully self-consistent $GW$ calculations, we demonstrate that a phonon mode related to the oscillation of the sulfur-sulfur bond distance in the pyrite structure is strongly coupled to the energy of the conduction-band minimum, leading to an ultrafast ($\approx$100 fs) oscillation in the band gap. Depending on the coherency of the phonons, we predict that this effect can cause changes of up to $\pm$0.3 eV relative to the accepted FeS${}_2$ band gap at room temperature. Harnessing this effect via temperature or irradiation with infrared light could open up numerous possibilities for novel devices such as ultrafast switches and adaptive solar absorbers.

The interplay between interference effects and electron-electron interactions in electron transport through an interacting double quantum dot system is investigated using a hierarchical quantum master equation approach which becomes exact if carried to infinite order and converges well if the temperature is not too low. Decoherence due to electron-electron interactions is found to give rise to pronounced negative differential resistance, enhanced broadening of structures in current-voltage characteristics, and an inversion of the electronic population. Dependence on gate voltage is shown to be a useful method of distinguishing decoherence-induced phenomena from effects induced by other mechanisms such as the presence of a blocking state. Comparison of results obtained by the hierarchical quantum master equation approach to those obtained from the Born-Markov approximation to the Nakajima-Zwanzig equation and from the noncrossing approximation to the nonequilibrium Green's function reveals the importance of an interdot coupling that originates from the energy dependence of the conduction bands in the leads and the need for a systematic perturbative expansion.

We calculate the nonequilibrium conductance of a system of two capacitively coupled quantum dots, each one connected to its own pair of conducting leads. The system has been used recently to perform pseudospin spectroscopy by controlling independently the voltages of the four leads. The pseudospin is defined by the orbital occupation of one or the other dot. Starting from the SU(4) symmetric point of spin and pseudospin degeneracy in the Kondo regime, for an odd number of electrons in the system, we show how the conductance through each dot varies as the symmetry is reduced to SU(2) by a pseudo-Zeeman splitting, and as bias voltages are applied to any of the dots. We analyze the expected behavior of the system in general, and predict characteristic fingerprint features of the SU(4) $\to$ SU(2) crossover that have not been observed so far.

We report Shubnikov-de Haas oscillation measurements within the high magnetic field (${\mu }_{0}H>39$ T) magnetically polarized regime of URu${}_2$Si${}_2$, made possible using mesoscopic samples prepared by means of focused ion beam lithography. A significant change in the Fermi surface topology relative to the “hidden-order” phase is observed, signaling a transformation into a high magnetic field regime in which $5f$-electrons are removed from the Fermi surface. URu${}_2$Si${}_2$ is therefore a rare example of an actinide compound in which a transformation of $5f$-electrons can be directly observed at low temperatures, setting the stage for the unconventional ordering and high magnetic field quantum criticality in this material.

We study the phase diagram of the frustrated $XY$ model on the honeycomb lattice by using accurate correlated wave functions and variational Monte Carlo simulations. Our results suggest that a spin-liquid state is energetically favorable in the region of intermediate frustration, intervening between two magnetically ordered phases. The latter ones are represented by classically ordered states supplemented with a long-range Jastrow factor, which includes relevant correlations and dramatically improves the description provided by the purely classical solution of the model. The construction of the spin-liquid state is based on a decomposition of the underlying bosonic particles in terms of spin-$1/2$ fermions (partons), with a Gutzwiller projection enforcing no single occupancy, as well as a long-range Jastrow factor.

We reveal several distinct regimes of the relaxation dynamics of a small quantum system coupled to an environment within the plane of the dissipation strength and the reservoir temperature. This is achieved by discriminating between coherent dynamics with damped oscillatory behavior on all time scales, partially coherent behavior being nonmonotonic at intermediate times but monotonic at large ones, and purely monotonic incoherent decay. Surprisingly, elevated temperature can render the system “more coherent” by inducing a transition from the partially coherent to the coherent regime. This provides a refined view on the relaxation dynamics of open quantum systems.

The frequency splitting between the dip and the peak of the resistively detected nuclear magnetic resonance dispersive line shape has been measured in the quantum Hall effect regime as a function of filling factor, carrier density, and nuclear isotope. The splitting increases as the filling factor tends to $\nu =1$ and is proportional to the hyperfine coupling, similar to the usual Knight shift versus $\nu$ dependence. The peak frequency shifts linearly with magnetic field throughout the studied filling factor range and matches the unshifted substrate signal, detected by classical NMR. Thus, the evolution of the splitting is entirely due to the changing Knight shift of the dip feature. The nuclear spin relaxation time $T_1$ is extremely long ($\sim$hours) at precisely the peak frequency. These results are consistent with the local formation of a $\nu =2$ phase due to the existence of spin singlet $D^{-}$ complexes.

We use polarization-resolved near-field measurements, in conjunction with electromagnetic theory, to separate and quantify the electric and magnetic optical response of subwavelength holes in thick gold films. Using 1550 nm light, we determine the amplitudes of the electric and magnetic polarizabilities of holes with diameters ranging from 600 to 1000 nm. Additionally, we study the scattered field distributions that arise from the interactions of the holes with surface plasmon polaritons, and show that forward-backward scattering ratios as high as $2.5:1$ are possible. Our study provides experimental access and theoretical understanding of the full electromagnetic polarizability that describes the optical response of metallic holes at telecom wavelengths, which is a prototypical structure in currently explored optical signal processing and sensing devices.

Berry phases defined by the one-dimensional momentum as a parameter show $Z_2$ quantization due to the inversion symmetry combined with the time reversal, or existence of the reflection plane, which also protects the massless Dirac cones with continuous parameters. This is the symmetry protected $Z_2$ quantization. These ideas are applied in the fermionic Shastry-Sutherland model, which has a rich phase diagram, including phases with massless Dirac fermions, a quadratic band crossing point, and a pseudospin-1 Weyl fermion. We also demonstrate that the $Z_2$ Berry phases determine the existence of edge states as the bulk-edge correspondence of the massless Dirac fermion systems.

We theoretically study the effect of a nonmagnetic impurity in heavy fermion superconductor CeCoIn${}_5$ within a coherent three-dimensional Anderson lattice model and the $T$-matrix approximation approach. By considering two known possible pairing symmetry candidates $d_{x^2-y^2}$ and $d_{xy}$, we find that although both total density of states exhibit a similar V-shaped gaplike feature, only $d_{x^2-y^2}$-wave pairing symmetry gives rise to robust intragap impurity resonance states reflected by the resonance peaks near the Fermi energy in the local density of states. These features can be readily probed by scanning tunneling microscopy experiments, and are proposed to shed light on the pairing symmetry and provide hints on the microscopic mechanism of unconventional superconductivity in the Ce-based heavy fermion superconductors.

We study the dynamics of polariton condensate wave trains that propagate along a quasi-one-dimensional waveguide. Through the application of tuneable potential barriers the propagation can be reflected and multiple reflections used to confine and store a propagating state. Energy-relaxation processes allow the delayed relaxation into a long-living coherent ground state. Aside from the potential routing of polariton condensate signals, the system forms an AND-type logic gate compatible with incoherent inputs.

Pt is known to show spontaneous formation of monatomic chains upon breaking a metallic contact. From model calculations, these chains are expected to be spin polarized. However, direct experimental evidence for or against magnetism is lacking. Here, we investigate shot noise as a potential source of information on the magnetic state of Pt atomic chains. We observe a remarkable structure in the distribution of measured shot-noise levels, where the data appear to be confined to the region of nonmagnetic states. While this suggests a nonmagnetic ground state for the Pt atomic chains, from calculations we find that the magnetism in Pt chains is due to “actor” electron channels, which contribute very little to ballistic conductance and noise. On the other hand, there are weakly polarized “spectator” channels, which carry most of the current and are only slightly modified by the magnetic state.

A parabolic quantum dot (QD) as realized by biasing nanostructured gates on bilayer graphene is investigated in the presence of electron-electron interaction. The energy spectrum and the phase diagram reveal unexpected transitions as a function of a magnetic field. For example, in contrast to semiconductor QDs, we find a valley transition rather than only the usual singlet-triplet transition in the ground state of the interacting system. The origin of these features can be traced to the valley degree of freedom in bilayer graphene. These transitions have important consequences for cyclotron resonance experiments.

We theoretically investigate the spectral properties and the spatial dependence of the local density of states (LDOS) in disordered two-dimensional electron gases (2DEG) in the quantum Hall regime, taking into account the combined presence of electrostatic disorder, random Rashba spin-orbit interaction, and finite Zeeman coupling. To this purpose, we extend a coherent-state Green's function formalism previously proposed for spinless 2DEG in the presence of smooth arbitrary disorder, that here incorporates the nontrivial coupling between the orbital and spin degrees of freedom into the electronic drift states. The formalism allows us to obtain analytical and controlled nonperturbative expressions of the energy spectrum in arbitrary locally flat disorder potentials with both random electric fields and Rashba coupling. As an illustration of this theory, we derive analytical microscopic expressions for the LDOS in different temperature regimes which can be used as a starting point to interpret scanning tunneling spectroscopy data at high magnetic fields. In this context, we study the spatial dependence and linewidth of the LDOS peaks and explain an experimentally noticed correlation between the spatial dispersion of the spin-orbit splitting and the local extrema of the potential landscape.

Optical second harmonic generation (SHG) spectra of LaAlO${}_3$/SrTiO${}_3$, LaGaO${}_3$/SrTiO${}_3$, and NdGaO${}_3$/SrTiO${}_3$ interfaces with SrTiO${}_3$ substrates have been measured up to 4.2 eV as a function of film thickness and temperature. This spectral range is characterized by two-photon transitions from the valence to conduction bands of SrTiO${}_3$ which are classified with a model based on symmetry, selection rules, and atomic orbital overlaps. This model is further confirmed in spectral measurements as a function of film thickness, material overlayer, and temperature. SHG enhancement at low temperature indicates an increase of the interfacial polarity with decreasing temperature. This confirms the relation between SHG and the spatial translation of Ti ions which are more prone to move at lower temperatures because of the quantum-paraelectric nature of SrTiO${}_3$. We finally show that SHG spectroscopy captures structural details of the interfaces. In particular, we find evidence for proximity effects such as LaAlO${}_3$-induced distortions of the TiO${}_6$ octahedra or bond buckling. We also observe a correlation between SHG and the in-plane lattice mismatch between the SrTiO${}_3$ substrate and the LaAlO${}_3$, LaGaO${}_3$, and NdGaO${}_3$ overlayers.

We report a complete structural study of CoF${}_2$ under pressure. Its crystal structure and vibrational and electronic properties have been studied both theoretically and experimentally using first-principles density functional theory (DFT) methods, x-ray diffraction, x-ray absorption at Co $K$-edge experiments, Raman spectroscopy, and optical absorption in the 0–80 GPa range. We have determined the structural phase-transition sequence in CoF${}_2$ and corresponding transition pressures. The results are similar to other transition-metal difluorides such as FeF${}_2$ but different to ZnF${}_2$ and MgF${}_2$, despite that the Co${}^{2+}$ size (ionic radius) is similar to Zn${}^{2+}$ and Mg${}^{2+}$. We found that the complete phase-transition sequence is tetragonal rutile ($P{4}_2/mnm$) $\to$ CaCl${}_2$ type (orthorhombic $Pnnm$) $\to$ distorted PdF${}_2$ (orthorhombic $Pbca$)+PdF${}_2$ (cubic $Pa\overline{3}$) in coexistence $\to$ fluorite (cubic $Fm\overline{3}m$) $\to$ cotunnite (orthorhombic $Pnma$). It was observed that the structural phase transition to the fluorite at 15 GPa involves a drastic change of coordination from sixfold octahedral to eightfold cubic with important modifications in the vibrational and electronic properties. We show that the stabilization of this high-pressure cubic phase is possible under nonhydrostatic conditions since ideal hydrostaticity would stabilize the distorted-fluorite structure (tetragonal $I4/mmm$) instead. Although the first rutile $\to$ CaCl${}_2$-type second-order phase transition is subtle by Raman spectroscopy, it was possible to define it through the broadening of the $E_g$ Raman mode which is split in the CaCl${}_2$-type phase. First-principles DFT calculations are in fair agreement with the experimental Raman mode frequencies, thus providing an accurate description for all vibrational modes and elastic properties of CoF${}_2$ as a function of pressure.

Electrical transport and magnetic properties of ${\mathrm{CaFe}}_2{\mathrm{O}}_4$ have been studied at pressures up to 70 GPa using ${}^{57}\mathrm{Fe}$ Mössbauer spectroscopy (MS), Raman spectroscopy, and electrical resistance measurements. These studies have shown the onset of the Mott transition (MT) at a pressure of around 50 GPa, leading to the collapse of ${\mathrm{Fe}}_{3+}$ magnetic moments and to the insulator-metal (IM) transition. The observed onset of the MT corroborates with the recently reported isostructural transition accompanied by a 12% decrease in the Fe polyhedral volume. An analysis of the alterations of the electrical transport, magnetic, and structural properties with pressure increase and at the transition range suggests that the coinciding IM transition, magnetic moment, and volume collapse at around 50 GPa are caused by the closure of the Hubbard gap driven by the high-spin to low-spin (HS-LS) transition. At that, since MS did not reveal any evidence of a preceding LS state, it could be inferred that the HS-LS transition immediately leads to an IM transition and complete collapse of magnetism.