The extended dynamical mean-field theory has played an important role in the study of quantum phase transitions in heavy-fermion systems. In order to incorporate the physics of unconventional superconductivity, we develop a cluster version of the extended dynamical mean-field theory. In this approach, we show how magnetic order and superconductivity develop as a result of intersite spin-exchange interactions, and analyze in some detail the form of correlation functions. We also discuss the methods that can be used to solve the dynamical equations associated with this approach. Finally, we consider different settings in which our approach can be applied, including the periodic Anderson model for heavy-fermion systems.

Employing extensive cellular dynamical mean-field theory calculations with an exact diagonalization impurity solver, we investigate the ground-state phase diagrams and nonmagnetic metal-insulator transitions of the half-filled Hubbard model on two plaquette (the 1/5 depleted and checkerboard) square lattices. We identify three different insulators in the phase diagrams: dimer insulator, antiferromagnetic insulator, and plaquette insulator. We also demonstrate that the plaquette insulator is a novel fragile Mott insulator (FMI) which features a nontrivial one-dimensional irreducible representation of the $C_{4v}$ crystalline point group and cannot be adiabatically connected to any band insulator with time-reversal symmetry. Furthermore, we study the nonmagnetic quantum phase transitions from the metal to the FMI and find that this Mott metal-insulator transition is characterized by the splitting of the noninteracting bands due to interaction effects.

We present quasiparticle self-consistent $GW$ $(\mathrm{QS}GW)$ calculations of semimetallic bulk Bi. We go beyond the conventional $\mathrm{QS}GW$ method by including the spin-orbit coupling throughout the self-consistency cycle. This approach improves the description of the electron and the hole pockets considerably with respect to standard density functional theory (DFT), leading to excellent agreement with experiment. We employ this relativistic $\mathrm{QS}GW$ approach to conduct a study of the semimetal-to-semiconductor and the trivial-to-topological transitions that Bi experiences under strain. DFT predicts that an unphysically large strain is needed for such transitions. We show, by means of the relativistic $\mathrm{QS}GW$ description of the electronic structure, that an in-plane tensile strain of only 0.3% and a compressive strain of 0.4% are sufficient to cause the semimetal-to-semiconductor and the trivial-to-topological phase transitions, respectively. Thus, the required strain moves into a regime that is likely to be realizable in experiment, which opens up the possibility to explore bulklike topological behavior of pure Bi.

Using a combination of density functional theory and dynamical mean field theory we show that electric polarization and magnetism are strongly intertwined in $\mathrm{TMTTF}{}_2\text{−}X$ ($X$ $=$ $\mathrm{PF}{}_6,\mathrm{AsF}{}_6$, and $\mathrm{SbF}{}_6)$ organic crystals. Electronic correlations induce a charge-ordered state which, combined with the molecular dimerization, gives rise to a finite electronic polarization and to a ferroelectric state. The value of the electronic polarization is enhanced by the onset of antiferromagnetism showing a sizable magnetoelectric effect which predicts the multiferroic behavior of $\mathrm{TMTTF}{}_2\text{−}X$ compounds.

The photoluminescence (PL) properties of silicon nanocrystals (Si NCs) deposited on gold nanostructures have been studied regarding PL intensity and lifetime. In contrast to most investigations, which attempt to optimize the overlap between the plasmon resonance and the Si NCs' PL band, we chose much smaller gold nanoparticles to achieve an improved exciton-plasmon coupling. PL enhancements of up to 20 were observed near the plasmon resonance. In the maximum of the PL band at 710 nm, we determined a PL enhancement of $\sim 4$ while the average PL lifetime was found to increase from 45 to 66 $\mu \mathrm{s}$. The experimental observations were successfully explained with a model invoking bidirectional energy transfer between excitonic and plasmonic states, being operative in addition to the normal exciton recombination rate enhancement.

Iron doped ${\text{LiNbO}}_3$ crystals with different iron valence states are investigated. An extended x-ray absorption fine structure (EXAFS) spectroscopy study highlights evident changes in the local structure around iron that can be ascribed to the presence of small polarons. In particular, when a ${\text{Fe}}^{3+}$ replaced a Li ion, the oxygen octahedron shrinked with respect to the pure material, with an average iron-oxygen bond value very similar to that of ${\text{Fe}}_2{\text{O}}_3$ hematite. When adding an electron, it localizes at the Fe site in a configuration very close to the atomic Fe $d$ orbitals, inducing a relaxation of the oxygen cage. The same system was modelled by spin-polarized density functional theory (DFT). Several local as well as hybrid exchange-correlation functionals were probed on the bulk ${\text{LiNbO}}_3$ structural properties. The computation is then extended to the case of hematite and finally to the Fe defect in ${\text{LiNbO}}_3$. The calculations reproduced with good accuracy the large lattice relaxation of the oxygen ligands associated to the electronic capture at the Fe center that can be interpreted as due to the polaron formation. The calculations reproduce satisfactorily the available EXAFS data, and allow for the estimation of the polaron energies and the optical properties of the defect.

Lattice dynamics and high-pressure phase transitions in $A\mathrm{W}{\mathrm{O}}_4$ ($A=\mathrm{Ba},\mathrm{Sr},\mathrm{Ca}$, and $\mathrm{Pb}$) have been investigated using inelastic neutron scattering experiments, ab initio density functional theory calculations, and extensive molecular dynamics simulations. The vibrational modes that are internal to $\mathrm{W}{\mathrm{O}}_4$ tetrahedra occur at the highest energies consistent with the relative stability of $\mathrm{W}{\mathrm{O}}_4$ tetrahedra. The neutron data and the ab initio calculations are found to be in excellent agreement. The neutron and structural data are used to develop and validate an interatomic potential model. The model is used for classical molecular dynamics simulations to study their response to high pressure. We have calculated the enthalpies of the scheelite and fergusonite phases as a function of pressure, which confirms that the scheelite to fergusonite transition is second order in nature. With increase in pressure, there is a gradual change in the $A{\mathrm{O}}_8$ polyhedra, while there is no apparent change in the $\mathrm{W}{\mathrm{O}}_4$ tetrahedra. We found that all the four tungstates amorphize at high pressure. This is in good agreement with available experimental observations which show amorphization at around 45 GPa in $\mathrm{BaW}{\mathrm{O}}_4$ and $40\mathrm{GPa}$ in $\mathrm{CaW}{\mathrm{O}}_4$. Further molecular dynamics simulations at high pressure and high temperature indicate that application of pressure at higher temperature hastens the process of amorphization. On amorphization, there is an abrupt increase in the coordination of the W atom while the bisdisphenoids around the $A$ atom are considerably distorted. The pair-correlation functions of the various atom pairs corroborate these observations. Our observations aid in predicting the pressure of amorphization in $\mathrm{SrW}{\mathrm{O}}_4$ and $\mathrm{PbW}{\mathrm{O}}_4$.

We investigate magnetic nanostructures with balanced gain and loss and show that such configurations can result in a new type of dynamics for magnetization. Using the simplest possible setup consisting of two coupled ferromagnetic films, one with loss and another one with a balanced amount of gain, we demonstrate the existence of an exceptional point where both the eigenfrequencies and eigenvectors become degenerate. This point corresponds to a particular value of the gain and loss parameter $\alpha ={\alpha }_c$. For $\alpha <{\alpha }_c$ the frequency spectrum is real, indicating stable dynamics, while for $\alpha >{\alpha }_c$ it is complex, signaling unstable dynamics which is, however, stabilized by nonlinearity.

We calculate spectra of magnetic excitations in the spin-spiral state of perovskite manganates. The spectra consist of several branches corresponding to different polarizations and different ways of diffraction from the static magnetic order. The easy-axis crystal-field anisotropy and the Dzyaloshinski-Moriya anisotropy lead to opening of gaps in Goldstone modes and to discontinuities in magnon dispersions. Comparing results of the calculation with available experimental data, we determine values of effective exchange parameters and anisotropies. To simplify calculations and to get a clearer physical insight into the structure of excitations, we use the $\sigma$-model-like effective-field theory instead of the conventional spin-wave approach to analyze the Heisenberg Hamiltonian and to derive the spectra.

Stoichiometric polycrystalline samples of $Ln\mathrm{F}{\mathrm{e}}_{2/3}\mathrm{M}{\mathrm{o}}_{1/3}{\mathrm{O}}_3(Ln=\mathrm{Nd},\mathrm{Pr},\mathrm{Ce},\mathrm{La})$ have been prepared by solid-state reaction and studied by means of x-ray and neutron powder diffraction as well as Mössbauer spectroscopy and magnetic measurements. All samples were found to be of single phase and to have Pnma symmetry with valence state +3 of Fe and Mo. It is demonstrated that the B-site cations of $Ln\mathrm{F}{\mathrm{e}}_{2/3}\mathrm{M}{\mathrm{o}}_{1/3}{\mathrm{O}}_3$ in accord with $Ln\mathrm{Fe}{\mathrm{O}}_3$ order in a G-type antiferromagnetic structure with the magnetic moments aligned along the $b$ axis. However, with significantly lower Néel temperatures than their $Ln\mathrm{Fe}{\mathrm{O}}_3$ parent compounds. The Fe-O-Fe bond lengths and bond angles and thus the magnitude of the antiferromagnetic superexchange interaction are found to systematically change with the ionic radius of Ln such that $T_N$ increases with increasing radius. Only the $\mathrm{CeF}{\mathrm{e}}_{2/3}\mathrm{M}{\mathrm{o}}_{1/3}{\mathrm{O}}_3$ compound experiences a low temperature spin reorientation from alignment along the $b$ axis to the $a$ axis.

We reinvestigate the pressure dependence of the crystal structure and antiferromagnetic phase transition in ${\mathrm{MnTe}}_2$ using the rigorous and reliable tool of high-pressure neutron powder diffraction. First-principles density functional theory calculations are carried out in order to gain microscopic insight. The measured Néel temperature of ${\mathrm{MnTe}}_2$ is found to show unusually large pressure dependence of $12\text{K}{\text{GPa}}^{-1}$. This gives rise to a large violation of Bloch's rule given by $\alpha =\frac{dlog{T}_N}{dlogV}=-\frac{10}{3}\approx -3.3$, to an $\alpha$ value of $-6.0\pm 0.1$ for ${\mathrm{MnTe}}_2$. The ab initio calculation of the electronic structure and the magnetic exchange interactions in ${\mathrm{MnTe}}_2$ for the measured crystal structures at different pressures indicates the pressure dependence of the Neél temperature $\alpha$ is $-5.61$, in close agreement with experimental findings. The microscopic origin of this behavior turns out to be dictated by the distance dependence of the cation-anion hopping interaction strength.

We have performed dielectric measurements and neutron diffraction experiments on the double perovskite ${\text{In}}_2{\text{NiMnO}}_6$. A ferroelectric polarization, $P\simeq 30\mu \mathrm{C}/{\mathrm{m}}^2,$ is observed in a polycrystalline sample below $T_N=$ 26 K where a magnetic phase transition occurs. The neutron diffraction experiment demonstrates that a complex noncollinear magnetic structure with “cycloidal” and “proper screw” components appears below $T_N$, which has the incommensurate propagation vector $\mathit{k}=(k_a,0,k_c;k_a\simeq 0.274,k_c\simeq -0.0893)$. The established magnetic point group ${21}^{\prime }$ implies that the macroscopic ferroelectric polarization is along the monoclinic $b$ axis. Recent theories based on the inverse Dzyaloshinskii-Moriya effect allow us to specify two distinct contributions to the polarization of ${\text{In}}_2{\text{NiMnO}}_6$. One of them is associated with the cycloidal component, ${\mathit{p}}_1\propto {\mathit{r}}_{ij}\times {({\mathit{S}}_i\times {\mathit{S}}_j)}_{\perp }$, and the other with the proper screw component, ${\mathit{p}}_2\propto [{\mathit{r}}_{ij}·{({\mathit{S}}_i\times {\mathit{S}}_j)}_{\left|\right|}]\mathit{A}$. The latter is explained by coupling between spin helicity and “ferriaxial” crystal rotation with macroscopic ferroaxial vector $\mathit{A}$, characteristic of the $B$-site ordered perovskite systems with out-of-plane octahedral tilting.

We analyze the impact of growth conditions on the asymmetric magnetic bubble expansion under an in-plane field in ultrathin Pt/Co/Pt films. Specifically, using sputter deposition, we vary the Ar pressure during the growth of the top Pt layer. This induces a large change in the interfacial structure as evidenced by a factor three change in the effective perpendicular magnetic anisotropy. Strikingly, a discrepancy between the current theory for domain-wall propagation based on a simple domain-wall energy density and our experimental results is found. This calls for further theoretical development of domain-wall creep under in-plane fields and varying structural asymmetry.

${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, a leading-candidate spin-triplet superconductor and a highly anisotropic quasi-two-dimensional type II superconductor, provides a unique opportunity to study unconventional as well as conventional vortex phases. To investigate its vortex-matter phases, we studied the ac susceptibility of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ for fields parallel to the ${\mathrm{RuO}}_2$ plane by adapting two different thermal processes: In addition to the ordinary field sweep (FS) process, we employed the “each-point field-cooling” (EPFC) process, in which the superconductivity is once thermally destroyed before collecting the data. We find that the ac susceptibility signal substantially changes with the EPFC process. This result indicates that we succeed in inducing new metastable vortex states via the EPFC process. We also find a new field scale $H^{*1}$, below which the FS and EPFC processes provide the same value of the ac susceptibility. This new field scale suggests a liquidlike vortex state in the low-field region.

We revisit the problem of electrons on a square lattice below half-filling close to an Ising-nematic quantum critical point. For Fermi surfaces with sufficiently strong antinodal nesting, the static nematic susceptibility is maximal at the antinodal nesting wave vector within a simple random phase approximation calculation. We present a detailed analysis of the nematic susceptibility within Eliashberg theory and show that the strong interaction between fermions in the antinodal regions shifts the maximum of the nematic susceptibility to slightly larger wave vectors. The corresponding order is akin to the incommensurate charge-density wave with $d$-wave form factor found recently in some underdoped cuprate materials. At sufficiently high temperatures around $T/t\sim 0.1$, nematic fluctuations are strongest at zero wave vector.

Employing a two-band model of Weyl semimetal, the existence of the chiral magnetic effect (CME) is established within the linear-response theory. The crucial role played by the limiting procedure in deriving correct transport properties is clarified. Besides, in contrast to the prediction based on linearized effective models, the value of the CME coefficient in the uniform limit shows a nontrivial dependence on various model parameters. Even when these parameters are away from the region of the linearized models, such that the concept of chirality may not be appropriate, this effect still exists. This implies that the Berry curvature, rather than the chiral anomaly, provides a better understanding of this effect.

We report the directly measured electronic structure of exfoliated monolayer molybdenum disulfide $\left(\mathrm{Mo}{\mathrm{S}}_2\right)$ using micrometer-scale angle-resolved photoemission spectroscopy. Measurements of both suspended and supported monolayer $\mathrm{Mo}{\mathrm{S}}_2$ elucidate the effects of interaction with a substrate. A suggested relaxation of the in-plane lattice constant is found for both suspended and supported monolayer $\mathrm{Mo}{\mathrm{S}}_2$ crystals. For suspended $\mathrm{Mo}{\mathrm{S}}_2$, a careful investigation of the measured uppermost valence band gives an effective mass at $\overline{\Gamma }$ and $\overline{K}$ of $2.00m_0$ and $0.43m_0$, respectively. We also measure an increase in the band linewidth from the midpoint of $\overline{\Gamma K}$ to the vicinity of $\overline{K}$ and briefly discuss its possible origin.

We study a Haldane model with nearest neighbor interactions. We find one-dimensional-like collective modes arising due to the interplay of pseudospin and valley degrees of freedom. In the large-band-gap or moderate interaction limit, these excitations are low energy modes lying in the band gap. The dispersion relations are qualitatively different in the trivial insulator phase and Chern insulator phase, thus can be used to identify the topology of the Haldane model with the bulk property. We also discuss how to detect these modes in cold atom systems. An Abelian gauge theory will emerge when a physical current-current interaction is introduced into the Haldane model or the Kane-Mele model.

We demonstrate gate control of the carrier distribution in $k$-space in bilayer and monolayer ${\mathrm{MoS}}_2$ devices, probed by microphotoluminescence spectroscopy on a contacted single flake. The characteristic emission lines of the neutral and the negatively charged exciton act as a sensor for electron depletion/agglomeration via gate voltage. Gate-induced carrier depletion enhances the sensitivity to defects in monolayers, whereas in bilayers the indirect transition becomes more weight. The specific band structure of bilayers results in a thermal dissociation of trions at 200 K and above, in contrast to monolayers where trion emission is observed up to room temperature. We show that these findings are a consequence of a bias-driven redistribution of charge carriers between the different band minima.

A roadblock in utilizing III-V semiconductors for scaled-down electronic devices is their poor dopant activation. As a first step to unravel the dopant behavior in InGaAs, we studied the tendency for dopant formation computationally using two approaches: ab initio and semiempirical methods. We studied a number of structural possibilities, such as the impact of local sites and local and global environments. We will show that the dopant we considered here, Si, has discrete preferences for certain sites and the nature of its surroundings. Substitutional defects are clearly preferred over interstitial locations. We shall show that cation ordering has an impact on dopant energetics. Critically, for large-scale simulations of dopant diffusion in InGaAs alloys, we also present a parameterization of the Abell-Tersoff semiempirical potential for pairwise interactions between silicon atoms and each of the elements constituting InGaAs. In the absence of experimental data, reference parameters for estimating the Tersoff values were obtained using ab initio pseudopotential calculations (density functional theory and generalized gradient approximations). These sets of Tersoff parameters were optimized to describe the bulk structural properties of the mostly theoretical alloys Si-As, Si-Ga, and Si-In. We demonstrate the transferability of these parameters by predicting formation energies of extrinsic point “defects” of Si on a variety of sites in ternary InGaAs alloys with different local compositional configurations, both random and ordered. Tersoff model predictions of the extrinsic “substitution energy” of a Si dopant on a cationic lattice site were found to be independent of the composition of the dopant's second nearest neighbors, but were affected by the strain induced by a local arrangement of In and Ga cationic atoms. This finding is important since common deposition processes used to create InGaAs may lead to specifically ordered patterns within the cation sublattice.