We present a systematic study of isolated hydrogen in diverse forms of ${\mathrm{ZrO}}_2$ (zirconia), both undoped and stabilized in the cubic phase by additions of transition-metal oxides $({\mathrm{Y}}_2{\mathrm{O}}_3,{\mathrm{Sc}}_2{\mathrm{O}}_3$, MgO, CaO). Hydrogen is modeled by using muonium as a pseudoisotope in muon-spin spectroscopy experiments. The muon study is also supplemented with first-principles calculations of the hydrogen states in scandia-stabilized zirconia by conventional density-functional theory (DFT) as well as a hybrid-functional approach which admixes a portion of exact exchange to the semilocal DFT exchange. The experimentally observable metastable states accessible by means of the muon implantation allowed us to probe two distinct hydrogen configurations predicted theoretically: an oxygen-bound configuration and a quasiatomic interstitial one with a large isotropic hyperfine constant. The neutral-oxygen-bound configuration is characterized by an electron spreading over the neighboring zirconium cations, forming a polaronic state with a vanishingly small hyperfine interaction at the muon. The atom-like interstitial muonium is observed also in all samples but with different fractions. The hyperfine interaction is isotropic in calcia-doped zirconia $[A_{\mathrm{iso}}=3.02\left(8\right)$ GHz], but slightly anisotropic in the nanograin yttria-doped zirconia $[A_{\mathrm{iso}}=2.1\left(1\right)$ GHz, $D=0.13\left(2\right)$ GHz] probably due to muons stopping close to the interface regions between the nanograins in the latter case.

The control of the spin-Seebeck current is still a challenging task for the development of spin caloritronic devices. Here, we construct a spin-Seebeck device by inserting a quantum dot (QD) between the metal lead and magnetic insulator. Using the slave-particle approach and noncrossing approximation, we find that the spin-Seebeck effect increases significantly when the energy level of the QD locates near the Fermi level of the metal lead due to the enhancement of spin flipping and occurrences of quantum resonance. Since this can be easily realized by applying a gate voltage in experiments, the spin-Seebeck device proposed here can also work as a thermovoltaic transistor. Moreover, the optimal correlation strength and the energy level position of the QD are discussed to maximize the spin-Seebeck current as required for applications in controllable spin caloritronic devices.

The fundamental electronic and optical properties of elongated InAs nanostructures embedded in quaternary InGaAlAs barrier are investigated by means of high-resolution optical spectroscopy and many-body atomistic tight-binding theory. These wire-like shaped, self-assembled nanostructures are known as quantum dashes and are typically formed during the molecular beam epitaxial growth on InP substrates. In this paper, we study properties of excitonic complexes confined in quantum dashes emitting in a broad spectral range from below 1.2 to 1.55 μm. We find peculiar trends for the biexciton and negative trion binding energies, with pronounced trion binding in smaller size quantum dashes. These experimental findings are then compared and qualitatively explained by atomistic theory. The theoretical analysis shows a fundamental role of correlation effects for the absolute values of excitonic binding energies. Eventually, we determine the bright exciton fine structure splitting (FSS), where both the experiment and theory predict a broad distribution of the splitting varying from below 50 to almost 180 μeV. We identify several key factors determining the FSS values in such nanostructures, including quantum dash size variation and composition fluctuations.

The outstanding magneto-optical properties of the nitrogen-vacancy (NV) center in diamond have stimulated the search for similar systems. We show here that NV triplet centers can also be generated in all the main SiC polytypes. We have identified by electron paramagnetic resonance spectroscopy and first-principles calculations the axial ${\mathrm{NV}}^{-}$ pairs in $3C,4H$, and $6H$ SiC, showing polytype and lattice site-specific magnetic and optical properties. We demonstrate very efficient room-temperature spin polarization of the ground state upon near infrared optical excitation for the NV center in $3C$ SiC and axial NV centers in the hexagonal $(4H,6H)$ polytypes; the signals of basal pairs are much lower in intensity. Axial NV centers in hexagonal SiC polytypes and thus constitute unidirectional ensembles which may be useful in nanosensing applications.

We study sharp peaks in the conductance-voltage characteristics of a double quantum dot and a quantum dot spin valve that are located around zero bias. The peaks share similarities with a Kondo peak but can be clearly distinguished, in particular as they occur at high temperatures. The underlying physical mechanism is a strong current suppression that is quenched in bias-voltage dependent ways by exchange interactions. Our theoretical results are based on the quantum master equation methodology, including the Born-Markov approximation and a numerically exact, hierarchical scheme, which we extend here to the spin-valve case. The comparison of exact and approximate results allows us to reveal the underlying physical mechanisms, the role of first-, second- and beyond-second-order processes and the robustness of the effect.

We present a microscopic theory of the chiral one-dimensional electron gas system localized on the sidewalls of magnetically doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$-family topological insulator nanoribbons in the quantum anomalous Hall effect (QAHE) regime. Our theory is based on a simple continuum model of sidewall states whose parameters are extracted from detailed ribbon and film geometry tight-binding model calculations. In contrast to the familiar case of the quantum Hall effect in semiconductor quantum wells, the number of microscopic chiral channels depends simply and systematically on the ribbon thickness and on the position of the Fermi level within the surface state gap. We use our theory to interpret recent transport experiments that exhibit nonzero longitudinal resistance in samples with accurately quantized Hall conductances.

We study the interplay of conductive and radiative heat transfer (RHT) in planar geometries and predict that temperature gradients induced by radiation can play a significant role on the behavior of RHT with respect to gap sizes, depending largely on geometric and material parameters and not so crucially on operating temperatures. Our findings exploit rigorous calculations based on a closed-form expression for the heat flux between two plates separated by vacuum gaps $d$ and subject to arbitrary temperature profiles, along with an approximate but accurate analytical treatment of coupled conduction-radiation in this geometry. We find that these effects can be prominent in typical materials (e.g., silica and sapphire) at separations of tens of nanometers, and can play an even larger role in metal oxides, which exhibit moderate conductivities and enhanced radiative properties. Broadly speaking, these predictions suggest that the impact of RHT on thermal conduction, and vice versa, could manifest itself as a limit on the possible magnitude of RHT at the nanoscale, which asymptotes to a constant (the conductive transfer rate when the gap is closed) instead of diverging at short separations.

In this paper, we investigate how nonlocal correlations affect, selectively, the physics of correlated electrons over different energy scales, from the Fermi level to the band edges. This goal is achieved by applying a diagrammatic extension of dynamical mean field theory (DMFT), the dynamical vertex approximation $\left(\mathrm{D}\mathrm{\Gamma }\mathrm{A}\right)$, to study several spectral and thermodynamic properties of the unfrustrated Hubbard model in two and three dimensions. Specifically, we focus first on the low-energy regime by computing the electronic scattering rate and the quasiparticle mass renormalization for decreasing temperatures at a fixed interaction strength. This way, we obtain a precise characterization of the several steps through which the Fermi-liquid physics is progressively destroyed by nonlocal correlations. Our study is then extended to a broader energy range, by analyzing the temperature behavior of the kinetic and potential energy, as well as of the corresponding energy distribution functions. Our findings allow us to identify a smooth but definite evolution of the nature of nonlocal correlations by increasing interaction: They either increase or decrease the kinetic energy w.r.t. DMFT depending on the interaction strength being weak or strong, respectively. This reflects the corresponding evolution of the ground state from a nesting-driven (Slater) to a superexchange-driven (Heisenberg) antiferromagnet (AF), whose fingerprints are, thus, recognizable in the spatial correlations of the paramagnetic phase. Finally, a critical analysis of our numerical results of the potential energy at the largest interaction allows us to identify possible procedures to improve the ladder-based algorithms adopted in the dynamical vertex approximation.

The ruby lattice is a four-valent lattice interpolating between honeycomb and triangular lattices. In this work we investigate the topological spin-liquid phases of a spin Hamiltonian with Kitaev interactions on the ruby lattice using exact diagonalization and perturbative methods. The latter interactions combined with the structure of the lattice yield a model with ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ gauge symmetry. We mapped out the phase diagram of the model and found gapped and gapless spin-liquid phases. While the low-energy sector of the gapped phase corresponds to the well-known topological color code model on a honeycomb lattice, the low-energy sector of the gapless phases is described by an effective spin model with three-body interactions on a triangular lattice. A gap is opened in the spectrum in small magnetic fields, where we showed that the ground state has a finite topological entanglement entropy. We argue that the gapped phases could be possibly described by exotic excitations, and their corresponding spectrum is richer than the Ising phase of the Kitaev model.

A realization of Majorana fermions is proposed in the frustrated magnets via the topological proximity effect. Specifically, we consider a theoretical model, where a topological insulator is coupled to a frustrated magnetic material through the spin exchange interaction. Using the renormalization group-based self-consistent mean-field approach, and calculating the self-energy correction due to the topological insulator, we find that the spin texture and the spin-momentum locking of the Dirac cone will be inherited by the spinons in the nearby frustrated magnets. This leads to a particular topological state of matter that supports the Majorana excitations. Unlike the conventional realization in superconductor systems, these Majorana fermions are the combination of spinons and antispinons, rather than electrons and holes. They can participate in the transport of spinons, leading to nontrivial properties of the spin transport.

The theoretical modeling of metal-organic interfaces represents a formidable challenge, especially considering the delicate balance of various interaction mechanisms and the large size of the involved molecular species. In the present study, the energies of interface states, which are known to display a high sensitivity to the adsorption geometry and electronic structure of the deposited molecular species, have been used to test the suitability and reliability of current theoretical approaches. Two well-ordered overlayer structures (relaxed and compressed monolayers) of 1,4,5,8-naphthalene-tetracarboxylic acid dianhydride (NTCDA) on Ag(111) have been investigated using two-photon photoemission to derive precise interface-state energies for these closely related systems. The experimental values are reproduced by our density-functional theory (DFT) calculations with two approaches to treat dispersion interactions (semi-empirical correction DFT-D3 and parametrized functional optB88) and basis set approaches (localized numerical atomic orbitals, plane waves) with remarkable accuracy. Our results underline the trustworthiness and some of the limitations of current DFT-based methods regarding the description of geometric and electronic properties of metal-organic interfaces.

The structural and electronic properties of FeSe ultrathin layers on $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ have been investigated with a combination of scanning tunneling microscopy and spectroscopy and angle-resolved photoemission spectroscopy. The FeSe multilayers, which are predominantly 3–5 monolayers (MLs) thick, exhibit a hole pocket-like electron band at $\overline{\mathrm{\Gamma }}$ and a dumbbell-like feature at $\overline{M}$, similar to multilayers of FeSe on $\mathrm{SrTi}{\mathrm{O}}_3$. Moreover, the topological state of the $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ is preserved beneath the FeSe layer, as indicated by a heavily n-doped Dirac cone. Low temperature scanning tunneling spectroscopy does not exhibit a superconducting gap for any investigated thickness down to a temperature of 5 K.

Rhombohedral multilayer graphene is a physical realization of the chiral two-dimensional electron gas that can host zero-line modes (ZLMs), also known as kink states, when the local gap opened by inversion symmetry breaking potential changes sign in real space. Here we study how the variations in the local stacking coordination of multilayer graphene affects the formation of the ZLMs. Our analysis indicates that the valley Hall effect develops whenever an interlayer potential difference is able to open up a band gap in stacking faulted multilayer graphene, and that ZLMs can appear at the domain walls separating two distinct regions with imperfect rhombohedral stacking configurations. Based on a tight-binding formulation with distant hopping terms between carbon atoms, we first show that topologically distinct domains characterized by the valley Chern number are separated by a metallic region connecting AA and ${\mathrm{AA}}^{\prime }$ stacking line in the layer translation vector space. We find that gapless states appear at the interface between the two stacking faulted domains with different layer translation or with opposite perpendicular electric field if their valley Chern numbers are different.

We present a numerical path-integral iteration scheme for the low-dimensional reduced density matrix of a time-dependent quantum dissipative system. Our approach simultaneously accounts for the combined action of a microscopically modeled pure-dephasing-type coupling to a continuum of harmonic oscillators representing, e.g., phonons, and further environmental interactions inducing non-Hamiltonian dynamics in the inner system represented, e.g., by Lindblad-type dissipation or relaxation. Our formulation of the path-integral method allows for a numerically exact treatment of the coupling to the oscillator modes and moreover is general enough to provide a natural way to include Markovian processes that are sufficiently described by rate equations. We apply this new formalism to a model of a single semiconductor quantum dot which includes the coupling to longitudinal acoustic phonons for two cases: (a) external laser excitation taking into account a phenomenological radiative decay of the excited dot state and (b) a coupling of the quantum dot to a single mode of an optical cavity taking into account cavity photon losses.

The electronic and transport properties of bulk and monolayer ${\mathrm{CuSbS}}_2$ and ${\mathrm{CuSbSe}}_2$ are determined by using density functional theory and semiclassical Boltzmann transport theory, in order to investigate the role of interlayer coupling for the thermoelectric properties. The calculated band gaps of the bulk compounds are in agreement with experiments and significantly higher than those of the monolayers, which thus show lower Seebeck coefficients. Since also the electrical conductivity is lower, the monolayers are characterized by lower power factors. Therefore, interlayer coupling is found to be essential for the excellent thermoelectric response of ${\mathrm{CuSbS}}_2$ and ${\mathrm{CuSbSe}}_2$, even though it is weak.

Incandescent sources that produce light from electrically heated filaments or films tend to feature low efficiencies and offer poor spectral and angular control. We demonstrate that a judicious nanostructuring of a SiC surface can focus thermal emission of a preselected spectral range to a well-defined height above the surface. SiC is known to support electromagnetic surface waves that afford the required thermal emission control. Here, we provide general design rules for this type of focusing element that can be extended to other material systems, such as metals supporting surface plasmon-polariton waves. These rules are verified using full-wave calculations of the spatial variation of thermal emission. The obtained results establish a foundation for developing more complex algorithms for the design of complex thermal lenses.

The decohering environment of a quantum bit is often described by the coupling to a large bath of spins. The quantum bit itself can be seen as a spin $S=\frac{1}{2}$ which is commonly called the central spin. The resulting central spin model describes an important mechanism of decoherence. We provide mathematically rigorous bounds for a persisting magnetization of the central spin in this model with and without magnetic field. In particular, we show that there is a well-defined limit of infinite number of bath spins. Only if the fraction of very weakly coupled bath spins tends to 100% does no magnetization persist.

In order to elucidate the origin of the temperature ($T$) dependence of spin-dependent tunneling conductance ($G$) of magnetic tunnel junctions (MTJs), we experimentally investigated the $T$ dependence of $G$ for the parallel and antiparallel magnetization alignments, $G_{\mathrm{P}}$ and $G_{\mathrm{AP}}$, of high-quality $\mathrm{C}{\mathrm{o}}_2\mathrm{MnSi}$ (CMS)/MgO/CMS MTJs having systematically varied spin polarizations ($P$) at 4.2 K by varying the Mn composition $\alpha$ in $\mathrm{C}{\mathrm{o}}_2\mathrm{M}{\mathrm{n}}_{\alpha }\mathrm{Si}$ electrodes that exhibited giant tunneling magnetoresistance ratios. Results showed that $G_{\mathrm{P}}$ normalized by its value at 4.2 K exhibited a notable, nonmonotonic $T$ dependence although its variation with $T$ was significantly smaller than that of $G_{\mathrm{AP}}$ normalized by its value at 4.2 K, indicating that an analysis of the experimental $G_{\mathrm{P}}\left(T\right)$ is critical to revealing the origin of the $T$ dependence of $G$. By analyzing the experimental $G_{\mathrm{P}}\left(T\right)$, we clarified that both spin-flip inelastic tunneling via a thermally excited magnon and spin-conserving elastic tunneling in which $P$ decays with increasing $T$ play key roles. The experimental $G_{\mathrm{AP}}\left(T\right)$, including its stronger $T$ dependence for higher $P$ at 4.2 K, was also consistently explained with this model. Our findings provide a unified picture for understanding the origin of the $T$ dependence of $G$ of MTJs with a wide range of $P$, including MTJs with high $P$ close to a half-metallic value.

We use scanning electron microscopy with polarization analysis to image deterministic, spin-orbit torque-driven magnetization reversal of in-plane magnetized CoFeB rectangles in zero applied magnetic field. The spin-orbit torque is generated by running a current through heavy metal microstrips, either Pt or Ta, upon which the CoFeB rectangles are deposited. We image the CoFeB magnetization before and after a current pulse to see the effect of spin-orbit torque on the magnetic nanostructure. The observed changes in magnetic structure can be complex, deviating significantly from a simple macrospin approximation, especially in larger elements. Overall, however, the directions of the magnetization reversal in the Pt and Ta devices are opposite, consistent with the opposite signs of the spin Hall angles of these materials. Our results elucidate the effects of current density, geometry, and magnetic domain structure on magnetization switching driven by spin-orbit torque.

The site-diluted compound ${\left({\mathrm{Yb}}_{1-x}{\mathrm{Lu}}_x\right)}_4{\mathrm{As}}_3$ is a scarce realization of the linear Heisenberg antiferromagnet partitioned into finite-size segments and is an ideal model compound for studying field-dependent effects of quenched disorder in the one-dimensional antiferromagnets. It differentiates from the systems studied so far in two aspects—the type of randomness and the nature of the energy gap in the pure sample. We have measured the specific heat of single-crystal ${\left({\mathrm{Yb}}_{1-x}{\mathrm{Lu}}_x\right)}_4{\mathrm{As}}_3$ in magnetic fields up to 19.5 T. The contribution $C_{\perp }$ arising from the magnetic subsystem in an applied magnetic field perpendicular to the chains is determined. Compared to pure ${\mathrm{Yb}}_4{\mathrm{As}}_3$, for which $C_{\perp }$ indicates a gap opening, for diluted systems a nonexponential decay is found at low temperatures which is consistent with the thermodynamic scaling of the specific heat established for a Bose-glass phase.