Compounds with intermediate-size transition metals such as Fe or Mn are close to the transition between charge-transfer systems and Mott-Hubbard systems. We study the optical conductivity $\sigma \left(\omega \right)$ of insulating layered LaSrFeO${}_4$ in the energy range $0.5–5.5$ eV from 15 K to 250 K by the use of spectroscopic ellipsometry in combination with transmittance measurements. A multipeak structure is observed in both ${\sigma }^a\left(\omega \right)$ and ${\sigma }^c\left(\omega \right)$. The layered structure gives rise to a pronounced anisotropy, thereby offering a means to disentangle Mott-Hubbard and charge-transfer absorption bands. We find strong evidence that the lowest dipole-allowed excitation in LaSrFeO${}_4$ is of Mott-Hubbard type. This rather unexpected result can be attributed to Fe $3d$-O $2p$ hybridization and in particular to the layered structure with the associated splitting of the $e_g$ level. In general, Mott-Hubbard absorption bands may show a strong dependence on temperature. This is not the case in LaSrFeO${}_4$, in agreement with the fact that spin-spin and orbital-orbital correlations between nearest neighbors do not vary strongly below room temperature in this compound with a high-spin $3d^5$ configuration and a Néel temperature of $T_N=366$ K.

Within the framework of ab initio time-dependent density functional theory (TD-DFT), we propose a static approximation to the exchange-correlation kernel based on the jellium-with-gap model. This kernel accounts for electron-hole interactions, and it is able to address both strongly bound excitons and weak excitonic effects. TD-DFT absorption spectra of several bulk materials (both semiconductor and insulators) are reproduced in very good agreement with the experiments and with a low computational cost.

We study the phase diagram of the Kondo lattice model with nearest-neighbor hopping in the square lattice by means of the variational Monte Carlo technique. Specifically, we analyze a wide class of variational wave functions that allow magnetic and superconducting order parameters, so as to assess the possibility that superconductivity might emerge close to the magnetic instability, as is often observed in heavy-fermion systems. Indeed, we do find evidence of $d$-wave superconductivity in the paramagnetic sector, i.e., when magnetic order is not allowed in the variational wave function. However, when magnetism is allowed, it completely covers the superconducting region, which thus disappears from the phase diagram.

We present calculations of the charge density profile, subband occupancy, and ellipsometry spectra of the electron gas at the LaAlO${}_3$/SrTiO${}_3$ interface. The calculations employ self-consistent Hartree and random phase approximations, a tight-binding parametrization of the band structure, and a model for the optical phonon of SrTiO${}_3$. The dependence of the spatial structure and occupancy of subbands on the magnitude of the polarization charge at the interface and the dielectric function is determined. The interface-confined subbands may be labeled by the symmetry ($xy$, $xz$, or $yz$) of the Ti $d$ orbitals from which they mainly derive. The $xy$-derived band nearest the interface contains the major proportion of the electronic charge, but a large number of more distant, slightly occupied $xy$-derived bands are also found. Depending on the magnitude of the polarization charge, zero, one, or two $xz/yz$-derived subbands are found. When present, these $xz/yz$ bands give the dominant contribution to the long-distance tail of the interface charge. The response to applied ac electric fields polarized parallel and perpendicular to the interface is calculated and the results are presented in terms of ellipsometry angles. Two features are found: a dip in the spectrum near the longitudinal optic (LO) feature of the SrTiO${}_3$ phonon and a peak at the higher energy. We show that the form and magnitude of the dip are related to the Drude response of carriers moving in the plane of the interface, while the peak arises from the plasmon excitation of the $xz$ and $yz$ electrons. The relation of the features of the subband occupancies and the in-plane conductivities is given.

Based on van der Waals density functional calculations, we have studied few-quintuple-layer (QL) films of Bi${}_2$Se${}_3$ and Bi${}_2$Te${}_3$. The separation between the QLs near the surface is found to have a large increase after relaxation, whereas, the separation between the inner QLs is smaller and approaches the bulk value as the thickness grows, showing a two-dimensional to three-dimensional structural crossover. Accordingly, the surface Dirac cone of the Bi${}_2$Se${}_3$ film is evidently gapped for small thicknesses (two to four QLs), and the gap is reduced and, finally, is closed with the increasing thickness, agreeing well with the experiments. We further studied the substrate effect by investigating the Bi${}_2$Se${}_3$/graphene system. It is found that the underlying graphene induces a giant thickness-dependent Rashba splitting and Dirac point shift. Because Bi${}_2$Te${}_3$ films have smaller relative inter-QL expansion and stronger spin-orbit coupling, the topological features start to appear in the film as thin as two QLs in good accord with the experiments.

Spin-flip Raman scattering of electrons and heavy holes is studied for resonant excitation of neutral and charged excitons in a CdTe/Cd${}_{0.63}$Mg${}_{0.37}$Te quantum well. The spin-flip scattering is characterized by its dependence on the incident and scattered light polarization as well as on the magnetic field strength and orientation. Model schemes of electric-dipole-allowed spin-flip Raman processes in the exciton complexes are compared to the experimental observations, from which we find that lowering the exciton symmetry, time of carrier spin relaxation, and mixing between electron states and, respectively, light- and heavy-hole states play an essential role in the scattering. At the exciton resonance, anisotropic exchange interaction induces heavy-hole spin-flip scattering, while acoustic phonon interaction is mainly responsible for the electron spin-flip. In resonance with the positively and negatively charged excitons, anisotropic electron-hole exchange as well as mixed electron states allow spin-flip scattering. Variations in the resonant excitation energy and lattice temperature demonstrate that localization of resident electrons and holes controls the Raman process probability and is also responsible for symmetry reduction. We show that the intensity of the electron spin-flip scattering is strongly affected by the lifetime of the exciton complex, and in tilted magnetic fields it is affected by the angular dependence of the anisotropic electron-hole exchange interaction.

We investigate the adsorption sites of $3d$ transition metal (TM) adatoms by means of low-temperature scanning tunneling microscopy and spectroscopy. Co and Ni adatoms were adsorbed on two types of graphene on SiC(0001), i.e., pristine epitaxial monolayer graphene (MLG) and quasi-free-standing monolayer graphene (QFMLG). In the case of QFMLG, two stable adsorption sites are identified, while in the case of MLG, only one adsorption site is observed. Our experimental results reveal the decoupling efficiency as a crucial parameter for determining the adsorption site as well as the electronic properties of $3d$ transition metal atoms on graphene. Furthermore, we show that Co atoms adsorbed on QFMLG are strong scattering potentials for Dirac fermions and cause intervalley scattering in their vicinity.

Selection rules and interference effects in angle-resolved photoemission spectra (ARPES) from twisted graphene bilayers are studied within a long-wavelength theory for the electronic structure. Using a generic model for the interlayer coupling, we identify features in the calculated ARPES momentum distributions that are controlled by the singularities and topological character of its long-wavelength spectrum. We distinguish spectral features that are controlled by single-layer singularities in the spectrum, their modification by gauge potentials in each layer generated by the interlayer coupling, and energy-dependent interference effects that directly probe the interlayer coherence. The results demonstrate how the energy- and polarization-dependence of ARPES spectra can be used to characterize the interlayer coupling in twisted bilayer graphenes.

We calculate the optical ($\Lambda \gg \omega \gg T$) conductivity in clean graphene in the ultimate low-energy regime, when retardation effects of the electromagnetic interaction become important and when the full Lorentz symmetry emerges. In contrast to what happens with the short range or with the Coulomb long-range instantaneous interactions, the optical conductivity is now no longer equal to its noninteracting value, but acquires universal corrections in powers of the fine structure constant. The coefficient of the first order correction is computed, and found to be of order one. We also present the result for the conductivity in the large-$N$ limit, with $N$ as the number of Dirac fermions species, to the order $1/N^2$.

Hyperfine interaction (HFI) and spin-orbit coupling are two major sources that affect electron spin dynamics. Here we present a systematic study of the HFI and its role in organic spintronic applications. For electron spin dynamics in disordered $\pi$-conjugated organics, the HFI can be characterized by an effective magnetic field whose modular square is a weighted sum of contact and dipolar contributions. We determine the effective HFI fields of some common $\pi$-conjugated organics studied in the literature via first-principles calculations. Most of them are found to be less than 2 mT. While the H atoms are the major source of the HFI in organics containing only the C and H atoms, many organics contain other nuclear spins, such as Al and N in tris-(8-hydroxyquinoline) aluminum, that contribute to the total HFI. Consequently, the deuteration effect on the HFI in the latter may be much weaker than in the former. The HFI gives rise to multiple resonance peaks in electron spin resonance. In disordered organic solids, these individual resonances are unresolved, leading to a broad peak whose width is proportional to the effective HFI field. As electrons hop among adjacent organic molecules, they experience a randomly varying local HFI field, inducing electron spin relaxation and diffusion. This is analyzed rigorously based on master equations. Electron spin relaxation undergoes a crossover along the ratio between the electron hopping rate $\overline{\eta }$ and the Larmor frequency $\Omega$ of the HFI field. The spin relaxation rate increases (decreases) with $\overline{\eta }$ when $\overline{\eta }\ll \Omega$ ($\overline{\eta }\gg \Omega$). A coherent beating of electron spin at $\Omega$ is possible when the external field is small compared to the HFI. In this regime, the magnetic field is found to enhance the spin relaxation.

Accurate relative energetic stabilities between the tetrahedrally coordinated (zinc-blende or wurtzite) and octahedrally coordinated (rock-salt) phases of MgO, ZnO, GaN, and MnO are obtained by first-principles calculations within the framework of adiabatic connection fluctuation-dissipation theorem (ACFDT) and with the random phase approximation (RPA) to the correlation energy. The RPA-ACFDT correctly recovers the rock-salt structure of MnO as the ground-state phase, as observed experimentally, whereas previous density and hybrid functional methods obtained the wrong energy ordering. Even though standard density functionals give the correct ordering of the non-transition-metal compounds, significant quantitative changes occur also for MgO and ZnO. We conclude that the RPA can serve as an important benchmark for structural preferences in polymorphic materials. The present study suggests that density functional predictions for open $d$-shell materials such as transition metal compounds might be more prone to erroneous structure prediction than commonly expected.

Recent experimental evidence that fluorinated graphene creates local magnetic moments around F adatoms has not been supported by semilocal density-functional theory (DFT) calculations where the adsorption of an F adatom induces no magnetic moment in graphene. Here, we show that such an incorrect prediction of the nonmagnetic ground state is due to the self-interaction error inherent in semilocal exchange-correlation functionals. The present hybrid DFT calculation for an F adatom on graphene predicts not only a spin-polarized ground state with a spin moment of $\sim 1$ ${\mu }_{\mathrm{B}}$, but also a long-range spin polarization caused by the bipartite nature of the graphene lattice as well as the induced spin polarization of the graphene states. The results provide support for the experimental observations of local magnetic moments in fluorinated graphene.

We report on the growth and characterization of single crystals of the iron chalcogenide superconductor FeCr${}_{0.02}$Se. There is an enhancement of the superconducting transition temperature ($T_c$) as compared to the $T_c$ of the single crystals of the parent compound Fe${}_{1+x}$Se by about 25$\%$. The superconducting parameters such as the critical fields, coherence length, penetration depth, and the Ginzburg-Landau parameter have been estimated for these single crystals. Analysis of the critical current data suggests a fluctuation in the electronic mean free path induced ($\delta l$) pinning mechanism in this material. Thermally activated transport across the superconducting transition in the presence of external magnetic fields suggests a crossover from a single vortex pinning regime at low fields to a collective flux creep regime at higher magnetic fields. The nature of charge carriers in the normal state estimated from the Hall effect and thermal transport measurements could provide crucial information on the mechanism of superconductivity in Fe-based materials.

We investigate temperature and fluence dependent dynamics of the photoexcited quasiparticle relaxation and low-energy electronic structure in electron-doped 1111-structure Sm(Fe${}_{0.93}$Co${}_{0.07}$)AsO single crystal. We find that the behavior is qualitatively identical to the 122-structure Ba(Fe,Co)${}_2$As${}_2$ including the presence of a normal state pseudogap and a marked twofold symmetry breaking in the tetragonal phase that we relate to the electronic nematicity. The twofold symmetry breaking appears to be a general feature of the electron-doped iron pnictides.

Using detector tomography, we investigate the detection mechanism in NbN-based superconducting single photon detectors (SSPDs). We demonstrate that the detection probability uniquely depends on a particular linear combination of bias current and energy, for a large variation of bias currents, input energies and detection probabilities, producing a universal detection curve. We obtain this result by studying multiphoton excitations in a nanodetector with a sparsity-based tomographic method that allows factoring out of the optical absorption. We discuss the implication of our model system for the understanding of meander-type SSPDs.

A combined experimental and theoretical study has been carried out on the synthesis and characterization of tungsten hydride at high pressures. We confirm the synthesis of tungsten monohydride at above 25 GPa and 300 K. At higher pressures, hydrogen content is found to increase and ab-initio calculations reveal the formation of overstoichiometric tungsten hydride WH${}_n$ with $n\sim 1\frac{1}{3}$. Synchrotron x-ray diffraction and transmission electron microscopy (TEM) measurements demonstrate the formation of a nanocrystalline microstructure upon hydrogenation. TEM micrographs show elongated grains with diameters of $\sim$20 nm, a structure similar to nanocrystalline diamond.

In this work the inhomogeneous (zero-phonon line) and homogeneous linewidths and the permanent electric dipole moment change (averaged value of all dipole orientations) for the Ce $4f$-$5d$ transition in Y${}_2$SiO${}_5$ were measured in order to investigate the possibility for using Ce as a sensor to detect the hyperfine state of a spatially close-lying Pr or Eu ion. The experiments were carried out on Ce doped or Ce-Pr co-doped single Y${}_2$SiO${}_5$ crystals. The homogeneous linewidth is essentially limited by the excited state lifetime. Based on the linewidth measurements, the oscillator strength, absorption cross section, and saturation intensity were calculated to be about 6.2$(\pm 1.7)\times {10}^{-7}$, $4.5(\pm 1.3)\times {10}^{-19}$m${}^2$, and $1.4(\pm 0.4)\times {10}^7$W/m${}^2$, respectively. The difference in permanent dipole moment, $△{\mu }_{\mathrm{Ce}}$, between the ground and excited states of the Ce ion was measured as $9.6(\pm 5.3)\times {10}^{-30}$C  m. These measurements indicate that Ce is a promising readout ion to probe a single-ion qubit state for the quantum computing scheme using rare-earth ions.

The dynamical behavior of many-body systems is often richer than what can be anticipated from their static properties. Here we show that in closed quantum systems this becomes evident by considering the statistics of time-integrated observables. In particular, the analytic properties of their generating functions, as estimated by full counting statistics (FCS), allow one to identify FCS phases, i.e., phases with specific fluctuation properties of time-integrated observables, and to locate transitions between these phases. We discuss in detail the case of the quantum Ising chain in a transverse field. We show that this model displays a continuum of full counting statistics transitions, of which the static transition is just an end point. These singularities are not a consequence of particular choices of initial conditions or other external nonequilibrium protocols such as quenches in coupling constants. They can be probed generically through quantum jump statistics of an associated open problem, and for the case of the quantum Ising chain we outline a possible experimental realization of this scheme by digital quantum simulation with cold ions.

Collinear and noncollinear spin structures of wurtzite phase CoO often appearing in nanosized samples are investigated using first-principles density functional theory calculations. We examined the total energy of several different spin configurations, electronic structure, and the effective magnetic coupling strengths. It is shown that the AF3-type antiferromagnetic ordering is energetically most stable among possible collinear configurations. Further, we found that a spiral spin order can be stabilized by including the relativistic spin-orbit coupling and the noncollinearity of spin direction. Our result suggests that a noncollinear spin ground state can be observed in the transition-metal-oxide nanostructures, which adds an interesting aspect to the nanomagnetism study.

We use an efficient general hybridization-expansion continuous-time quantum Monte Carlo impurity solver (Krylov approach) to study orbital and spin ordering phenomena in strongly correlated systems within the local-density approximation plus dynamical mean-field theory approach. This allows us to include often-neglected interaction terms, to study models with large basis sets, to consider crystals with low-symmetry distortions, and to reach the very low experimental temperatures. We use this solver to study ordering phenomena in a selection of exemplary low-symmetry transition-metal oxides. For the rare-earth manganites, we show that including spin-flip and pair-hopping terms does not affect the Kugel-Khomskii orbital-order melting transition. For LaMnO${}_3$, we find that the commonly used two-band model with classical $t_{2g}$ spin gives a good description of the $e_g$ electrons when compared with the full five-orbital Hubbard model. Surprisingly not only the occupied orbital but also the $e_g$ spectral matrix is well reproduced. For the $d^1$ perovskites CaVO${}_3$ and YTiO${}_3$ we show that spin-flip and pair-hopping terms only weakly affect orbital fluctuations. Moreover, for the Mott insulator YTiO${}_3$ we can study the ferromagnetic polarization to very low temperatures, finding a transition temperature in remarkably good agreement with experiments.