Low-temperature thermal conductivity measurements were performed on single crystals of $\mathrm{TlNi}{}_2\mathrm{Se}{}_2$, a nickel chalcogenide heavy-electron superconductor with $T_c\simeq 3.7$ K. In zero field, the residual electronic contribution at $T\to 0$ K (${\kappa }_0/T$) was well separated from the total thermal conductivity, which is less than 0.45% of its normal-state value. Such a tiny residual ${\kappa }_0/T$ is unlikely contributed by the nodal quasiparticles. The nodeless gap structure is supported by the very weak field dependence of ${\kappa }_0\left(H\right)/T$ in low magnetic fields. In the whole field range, ${\kappa }_0\left(H\right)/T$ exhibits an $\mathsf{S}$-shaped curve, as in the case of nickel pnictides $\mathrm{BaNi}{}_2\mathrm{As}{}_2$ and $\mathrm{SrNi}{}_2\mathrm{P}{}_2$. This common feature of nickel-based superconductors can be explained by multiple nodeless superconducting gaps.

Herbertsmithite ${\mathrm{ZnCu}}_3\left(\mathrm{OH}\right){}_6{\mathrm{Cl}}_2$ is a kagome lattice antiferromagnet with spin-1/2 and has been demonstrated to be a likely candidate of spin liquid by a number of recent experiments. The high-field magnetization of the kagome lattice is complicated due to the presence of a few percent of extra Cu impurities sitting on the interlayer metallic sites. To determine the magnetic ground state of the kagome lattice, we measured the magnetization of a single crystalline ${\mathrm{ZnCu}}_3\left(\mathrm{OH}\right){}_6{\mathrm{Cl}}_2$ using torque magnetometry down to the base temperatures 20 mK in intense magnetic field as high as 31 T. The high-field intrinsic magnetization from the kagome lattice turns out to be linear with magnetic field, and the magnetic susceptibility is independent of temperature at $20\text{mK}\le T\le 5$ K. Moreover, below 2 K, several field-induced anomalies are observed in between 7 T and 15 T.

When an in-plane field is applied to a clean interface superconductor, a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)-like phase is stabilized. This phase has a $\mathrm{U}\left(1\right)\times \mathrm{U}\left(1\right)$ symmetry and, in principle, this symmetry allows for flux carrying topological excitations different from Abrikosov vortices (which are the simplest defects associated with $S^1\to S^1$ maps). However, in practice, largely due to electromagnetic and other intercomponent interactions, such topological excitations are very rare in superconducting systems. Here, we demonstrate that a realistic microscopic theory for interface superconductors, such as $\mathrm{SrTiO}{}_3/\mathrm{LaAlO}{}_3$, predicts an unconventional magnetic response where the flux-carrying objects are skyrmions, characterized by homotopy invariants of $S^2\to S^2$ maps. Additionally, we show that this microscopic theory predicts that stable fractional vortices form near the boundary of these superconductors. It also predicts the appearance of type-1.5 superconductivity for some range of parameters. Central to these results is the assumption that the Rashba spin-orbit coupling is much larger than the superconducting gap.

Boundary conformal field theory is brought to bear on the study of topological insulating phases of non-Abelian anyonic chains. These phases display protected anyonic end modes. We consider spin-1/2 su(2)${}_k$ chains at any level $k$, focusing on the most prominent examples: the case $k=2$ describes Ising anyons (equivalent to Majorana fermions) and $k=3$ corresponds to Fibonacci anyons. The method we develop is quite general and rests on a deep connection between boundary conformal field theory and topological symmetry. This method tightly constrains the nature of the topological insulating phases of these chains for general $k$. Emergent anyons which arise at domain walls are shown to have the same braiding properties as the physical quasiparticles. This suggests a “solid-state” topological quantum computation scheme in which emergent anyons are braided by tuning the couplings of non-Abelian quasiparticles in a fixed network.

For temperatures below 0.6 K the geometrically frustrated layered quantum antiferromagnet ${\mathrm{Cs}}_2{\mathrm{CuCl}}_4$ in a magnetic field perpendicular to the layers orders magnetically in a so-called cone state where the magnetic moments have a finite component in the field direction, whereas their projection onto the layers forms a spiral. Modeling this system by a two-dimensional spatially anisotropic quantum Heisenberg antiferromagnet with Dzyaloshinskii-Moriya interaction, we find that even for vanishing temperatures the usual spin-wave expansion is plagued by infrared divergencies which are due to the coupling between longitudinal and transverse spin fluctuations in the cone state. Similar divergencies appear also in the ground state of the interacting Bose gas in two and three dimensions. Using known results for the correlation functions of the interacting Bose gas, we present a nonperturbative expression for the dynamic structure factor in the cone state of ${\mathrm{Cs}}_2{\mathrm{CuCl}}_4$. We show that in this state the spectral line shape of spin fluctuations exhibits singular scattering continua which can be understood in terms of the well-known anomalous longitudinal fluctuations in the ground state of the two-dimensional Bose gas.

We have investigated the surface states of a potential mixed-valent topological insulator ${\mathrm{SmB}}_6$ based on first-principles density functional theory slab band structure analysis. We have found that metallic surface states are formed in the bulk band gap region, providing evidence for the topological insulating nature of ${\mathrm{SmB}}_6$. The obtained surface in-gap states are quite different from those in existing reports in that they are formed differently depending on the Sm or ${\mathrm{B}}_6$ surface termination, and are composed of mainly the Sm $4f$ state indicating the essentiality of including $f$ electrons in describing the surface states. We have obtained the spin chiral structures of the Fermi surfaces, which are also in accordance with the topological insulating nature of ${\mathrm{SmB}}_6$.

Due to interest in both solid-state-based quantum computing architectures and the application of quantum mechanical systems to nanomagnetometry, there has been considerable recent attention focused on understanding the microscopic dynamics of solid-state spin baths and their effects on the coherence of a controllable, coupled central electronic spin. Using a systematic approach based on the spatial statistics of the spin-bath constituents, we develop a detailed, purely analytic theory for the central-spin decoherence problem of a nitrogen-vacancy center electron coupled to its native 1.1% bath of ${}^{13}\mathrm{C}$ nuclear spins. Our theory reproduces the experimental and numerical results found in the literature, and provides a detailed theoretical description of the relevant decoherence profiles, their associated rates, corresponding electron spin-echo envelope modulations features, and an explicit analytic account of why the strength of an applied magnetic field has such a profound effect on the coherence time of the central spin.

The interplay between band gap renormalization and band filling (Burstein-Moss effect) in n-type wurtzite GaN is investigated. For a wide range of electron concentrations up to $1.6\times {10}^{20}{\mathrm{cm}}^{-3}$ spectroscopic ellipsometry and photoluminescence were used to determine the dependence of the band gap energy and the Fermi edge on electron density. The band gap renormalization is the dominating effect up to an electron density of about $9\times {10}^{18}{\mathrm{cm}}^{-3}$; at higher values the Burstein-Moss effect is stronger. Exciton screening, the Mott transition, and formation of Mahan excitons are discussed. A quantitative understanding of the near gap transition energies on electron density is obtained. Higher energy features in the dielectric functions up to $10\mathrm{eV}$ are not influenced by band gap renormalization.

Optical interband transitions in monolayer transition metal dichalcogenides such as ${\mathrm{WSe}}_2$ and ${\mathrm{MoS}}_2$ are governed by chiral selection rules. This allows efficient optical initialization of an electron in a specific K valley in momentum space. Here we probe the valley dynamics in monolayer ${\mathrm{WSe}}_2$ by monitoring the emission and polarization dynamics of the well-separated neutral excitons (bound electron-hole pairs) and charged excitons (trions) in photoluminescence. The neutral exciton photoluminescence intensity decay time is about 4 ps, whereas the trion emission occurs over several tens of ps. The trion polarization dynamics shows a partial, fast initial decay within tens of ps before reaching a stable polarization of $\approx 20\%$, for which a typical valley polarization decay time of the order of 1 ns can be inferred.

We have benchmarked long range behavior of seven different van der Waals functionals comparing them with our ACF-RPA correlation calculations for graphene on a Ag(111) system. Correlation given by the second version of van der Waals density functional vdW-DF2 agrees remarkably well with our random phase approximation (RPA) calculation in the long range region. In the intermediate and shorter range regions combining vdW-DF2 correlation with proper exchange functional becomes important. We compared the results of the van der Waals functionals in this region to the previous RPA calculations and to some extent to experimental observations, and calculated that the combined vdW-DF2(C09x) or rev-vdW-DF2 functionals show satisfactory behavior.

We have investigated the dynamics of spin-state crossover (SSC) for perovskite ${\mathrm{LaCoO}}_3$ through optical phonons by means of infrared and inelastic x-ray spectroscopy. Upon thermally induced SSC, anomalously dispersionless Co-O bond stretching phonons coupled to the thermally excited spin state have been identified. The enhanced spin-state fluctuation irrespective of the presence of a clear charge gap suggests the emergence of complex spin-state disproportionation involving low-, intermediate-, and high-spin states due to the strong correlation among thermally activated spin states.

We demonstrate that the anyon statistics and three-loop statistics of various $2d$ and $3d$ topological phases can be derived using semiclassical nonlinear sigma model field theories with a topological $\Theta$ term. In our formalism, the braiding statistics has a natural geometric meaning: The braiding process of anyons or loops leads to a nontrivial field configuration in the space-time, which will contribute a braiding phase factor due to the $\Theta$ term.

We probe $d$-band quantum-well (QW) states in Pd nanofilms grown on Cu(001) using first- principles density functional theory (DFT) calculations combined with scanning tunneling spectroscopy (STS) experiments. This study reveals that QW states occur in the Pd overlayer films over a strikingly large film thickness and in a large binding energy range. The magnetic quantum number resolved orbital characters of these states are identified unambiguously by our DFT calculations. Calculations also demonstrate oscillatory multilayer relaxation and $d$-derived quantum size oscillation of the electronic density of states at the Fermi energy. The pseudomorphic growth, well-defined interface, and STS with single-layer thickness resolution allow us to probe individual QW states arising from the electron confinement along the growth axis of Pd films and to extract an accurate dispersion of the $({\Delta }_5$-type) $d$ electronic band, as these states are laterally highly localized and give rise to distinct and sharp peaks in the tunneling spectra.

Current-induced electron spin polarization is shown to produce nuclear hyperpolarization through dynamic nuclear polarization. Saturated fields of several millitesla are generated upon the application of an electric field over a time scale of 100 s in InGaAs epilayers and measured using optical Larmor magnetometry. We show that, in contrast to previous demonstrations of current-induced dynamic nuclear polarization, the direction of the current relative to the crystal axis and external magnetic field may be used to control the magnitude and direction of the saturation nuclear field.

We use muon spin relaxation $\left(\mu \mathrm{SR}\right)$ to investigate the magnetic properties of a bulk form diluted ferromagnetic semiconductor (DFS) ${\mathrm{Li}}_{1.15}({\mathrm{Zn}}_{0.9}\mathrm{Mn}{}_{0.1})\mathrm{P}$ with $T_C\sim 22$ K. $\mu \mathrm{SR}$ results confirm the gradual development of ferromagnetic ordering below $T_C$ with a nearly 100% magnetic ordered volume. Despite its low carrier density, the relation between static internal field and Curie temperature observed for Li(Zn,Mn)P is consistent with the trend found in (Ga,Mn)As and other bulk DFSs, indicating these systems share a common mechanism for the ferromagnetic exchange interaction. ${\mathrm{Li}}_{1+y}\left({\mathrm{Zn}}_{1-x}{\mathrm{Mn}}_x\right)\mathrm{P}$ has the advantage of decoupled carrier and spin doping, where ${\mathrm{Mn}}^{2+}$ substitution for ${\mathrm{Zn}}^{2+}$ introduces spins and ${\mathrm{Li}}^{+}$ off-stoichiometry provides carriers. This advantage enables us to investigate the influence of overdoped Li on the ferromagnetic ordered state. Overdoping Li suppresses both $T_C$ and saturation moments for a certain amount of spins, which indicates that more carriers are detrimental to the ferromagnetic exchange interaction, and that a delicate balance between charge and spin densities is required to achieve highest $T_C$.

Electronic states associated with a chain of magnetic adatoms on the surface of an ordinary $s$-wave superconductor have been shown theoretically to form a one-dimensional (1D) topological phase with unpaired Majorana fermions bound to its ends. In a simple 1D effective model the system exhibits an interesting self-organization property: The pitch of the spiral formed by the adatom magnetic moments tends to adjust itself so that electronically the chain remains in the topological phase whenever such a state is physically accessible. Here we examine the physics underlying this self-organization property in the framework of a more realistic two-dimensional model of a superconducting surface coupled to a 1D chain of magnetic adatoms. Treating both the superconducting order and the magnetic moments self-consistently, we find that the system retains its self-organization property, even if the topological phase extends over a somewhat smaller portion of the phase diagram compared to the 1D model. We also study the effect of imperfections and find that, once established, the topological phase survives moderate levels of disorder.

Using first-principles calculations we investigate the impact of intrinsic defects and small polarons on the electrical and optical properties of ${\mathrm{SrTiO}}_3$. We pay special attention to the seemingly contradictory role of oxygen vacancies as shallow donor and source of deep-level luminescence, as reported in the literature. We find that oxygen vacancies are double donors, and that one electron is easily ionized, explaining the shallow donor behavior. The second electron is trapped in the form of a small polaron, and this additional binding energy explains the behavior as a deep center that gives rise to blue luminescence. At low temperatures, holes become self-trapped, and recombination of free electrons with self-trapped holes gives rise to green luminescence. These results explain the intricate interplay between the observed green and blue luminescence in ${\mathrm{SrTiO}}_3$, and form a framework for interpreting similar phenomena in other complex oxides.

The nitrogen split interstitial defect introduced by high-energy particle irradiation in $n$-type GaN has been investigated by very high (up to 324 GHz) frequency electron paramagnetic resonance (EPR) and $Q$-band electron nuclear double resonance (ENDOR) spectroscopy. The increased resolution of the EPR spectra at 324 GHz has allowed us to determine the $g$-tensor anisotropy, which is not resolved at $X$ or $Q$ band. The good agreement of the principal values $g_{xx}=1.9966,g_{yy}=2.0016$, and $g_{zz}=2.0036$ with the theoretically predicted $g$ tensor confirm the $\left(\mathrm{N}\text{−}\mathrm{N}\right){}_{\mathrm{N}}{}^0$ defect model. The hyperfine interactions of this defect have been studied by $Q$-band ENDOR. We observed well-resolved ENDOR lines with distant Ga atoms from which the quadrupole coupling constants and the electrical field gradients were determined and discussed with the help of theoretical values. The observation of ENDOR spectra of the central N and Ga atoms predicted in the 20–90-MHz range required the use of field-frequency ENDOR due to the large linewidth of the ENDOR lines. Our results confirm the importance of the nitrogen split interstitial in particle irradiated GaN similar to the case of diamond and silicon carbide in which the stable configuration at room temperature of the carbon interstitials is also the split interstitial configuration.

We develop a theory for the dynamics of the density matrix describing a multimode polariton condensate. In such a condensate several single-particle orbitals become highly occupied, due to stimulated scattering from reservoirs of high-energy excitons. A generic few-parameter model for the system leads to a Lindblad equation which includes saturable pumping, decay, and condensate interactions. We show how this theory can be used to obtain the population distributions, and the time-dependent first- and second-order coherence functions, in such a multimode condensate. As a specific application, we consider a polaritonic Josephson junction, formed from a double-well potential. We obtain the population distributions, emission line shapes, and widths (first-order coherence functions), and predict the dephasing time of the Josephson oscillations.