Skutterudites are promising materials for future thermoelectric applications. Whereas the skutterudite ${\text{CoSb}}_3$ is intensively studied, very few investigations for ${\text{FeSb}}_3$ are performed due to its metastable character and the comparably low decomposition temperature. In this work, single-phase ${\text{FeSb}}_3$ thin films were prepared by codeposition of Fe and Sb using molecular beam epitaxy at room temperature followed by postannealing. The transport properties of a Fe-Sb composition series were determined and reveal high power factors $S^2\sigma$ up to $14\mu \mathrm{W}/{\mathrm{K}}^2\mathrm{cm}$. Furthermore, the structural parameters, the electronic structure, and the transport parameters were calculated by density functional theory, giving excellent agreement with the experimental data.

Enhancement of the spin-orbit coupling in graphene may lead to various topological phenomena and also find applications in spintronics. Adatom adsorption has been proposed as an effective way to achieve the goal. In particular, great hope has been held for indium in strengthening the spin-orbit coupling and realizing the quantum spin Hall effect. To search for evidence of the spin-orbit coupling in graphene adsorbed with indium adatoms, we carry out extensive transport measurements. It is found that indium adatoms dope graphene and introduce significant Coulomb scattering. No signature of the spin-orbit coupling is observed in the weak localization magnetoconductance and nonlocal spin Hall effect. Possible explanations are discussed.

Optical antennas based on noble metal nanoparticles can increase the photoluminescence of quantum dots, but the exact strength of this enhancement depends on the brightness (i.e., the intrinsic quantum yield ${\eta }_i$) of the emitters. Here we perform temperature-dependent measurements on a system of PbS colloidal quantum dots coupled with Au ring arrays that bring quantitative insight into this phenomenon. We show that although the boost in photoluminescence is lower at cryogenic temperatures where the nanocrystals become very bright emitters, the spectral signature of this enhancement is remarkably independent of ${\eta }_i$. These observations remain true even at wavelengths where the losses by absorption in the metal nanoparticles considerably increase due to the excitation of localized plasmon resonances, in contradiction with standard theory that treats the emitters as a collection of two-level systems. We propose a mechanism in which the quantum dots are modeled as multilevel and inhomogeneously broadened emitters to account for these findings.

We report measurements of thermal conductivity $\left(\kappa \right)$ on single crystals of the quasi-one-dimensional $\text{spin}-\frac{1}{2}$ anitferromagnetic chain compound ${\mathrm{CuSb}}_2{\mathrm{O}}_6$ and its nonmagnetic analog ${\mathrm{ZnSb}}_2{\mathrm{O}}_6$ in the temperature range $5\mathrm{K}\le T\le 300\mathrm{K}$. A substantially lower $\kappa$ for ${\mathrm{CuSb}}_2{\mathrm{O}}_6$ compared to that of ${\mathrm{ZnSb}}_2{\mathrm{O}}_6$ has its origin in resonant phonon–spin scattering involving excitation of a two-level system $(\text{energy}\text{scale}\sim 100\mathrm{K})$ associated with the onset of local spin order at $T\le 150\mathrm{K}$. Anisotropy in $\kappa$ for the ${\mathrm{CuSb}}_2{\mathrm{O}}_6$ also emerges at $T\le 150$ K and might be associated with a magnetic contribution to heat transport. A magnetic mean free path comparable to that of other $\text{spin}-\frac{1}{2}$ linear-chain compounds $(\sim 0.3\mu \mathrm{m})$ is implied.

Magnetic small-angle neutron scattering (SANS) is a powerful tool for investigating nonuniform magnetization structures inside magnetic materials. Here, we consider a ferromagnetic medium with weakly inhomogeneous uniaxial magnetic anisotropy, saturation magnetization, and exchange stiffness, and derive, to second order in the amplitudes of the inhomogeneities, the micromagnetic solutions for the equilibrium magnetization textures. Further, we compute the corresponding magnetic SANS cross section up to the third order. For the special case of scattering geometry where the incident neutron beam is perpendicular to the applied magnetic field, twice the cross section along the direction orthogonal to both the field and the neutron beam cancels the cross section along the field direction in the second order. This cancellation does not depend on the defect shape and amplitudes of the exchange inhomogeneities. Hence, such a cross-section difference has only a third-order contribution in the amplitudes of the inhomogeneities. It provides a separate gateway for a deeper analysis of the sample's magnetic structure. We derive and analyze analytical expressions for the dependence of this difference on the scattering-vector magnitude for the case of spherical Gaussian inhomogeneities.

We measured the microwave surface impedance of ${\mathrm{FeSe}}_{0.4}{\mathrm{Te}}_{0.6}$ single crystals with and without external magnetic fields. The superfluid density exhibited a quadratic temperature dependence, indicating a strong pair-breaking effect. The flux-flow resistivity behaved as ${\rho }_f(B\ll B_{\mathrm{c}2})/{\rho }_n=\alpha B/B_{\mathrm{c}2}$. The observed $\alpha$ value of $\approx 0.66$ was considerably smaller than that of other Fe-based materials $(\alpha \ge 1)$ and was attributed to a back-flow of superfluids remarkable in disordered superconductors. This is an observation of the back-flow phenomenon caused by an origin other than the vortex pinning in multiple-band systems.

We investigated the atomistic structure of metallic glasses subjected to thermomechanical creep deformation using high energy x-ray diffraction and molecular dynamics simulation. The experiments were performed in situ at high temperatures as a time dependent deformation in the elastic regime, and ex situ on samples quenched under stress. We show that all the anisotropic structure functions of the samples that have undergone thermomechanical creep can be scaled into a single curve, regardless of the magnitude of anelastic strain, stress level, and the sign of the stress, demonstrating universal behavior and pointing to a unique atomistic unit of anelastic deformation. The structural changes due to creep are strongly localized within the second nearest neighbors, involving only a small group of atoms.

We present a comprehensive density functional theory (DFT) -based study of different aspects of one vacancy and He impurity atom behavior at semicoherent interfaces between the low-solubility transition metals Cu and Nb. Such interfaces have not been previously modeled using DFT. A thorough analysis of the stability and mobility of the two types of defects at the interfaces and neighboring internal layers has been performed and the results have been compared to the equivalent cases in the pure metallic matrices. The different behavior of fcc and bcc metals on both sides of the interface has been specifically assessed. The modeling effort undertaken is the first attempt to study the stability and defect energetics of noncoherent Cu/Nb interfaces from first principles, in order to assess their potential use in radiation-resistant materials.

Nonequilibrium spin-dependent transport in magnetic tunnel junctions comprising a ferroelectric barrier is theoretically investigated. The exact solutions of the free electron Schrödinger equation for electron tunneling in the presence of interfacial screening are obtained by combining Bessel and Airy functions. We demonstrate that the spin transfer torque efficiency, and more generally the bias dependence of tunneling magneto- and electroresistance, can be controlled by switching the ferroelectric polarization of the barrier. In particular, the critical voltage at which the in-plane torque changes sign can be strongly enhanced or reduced depending on the direction of the ferroelectric polarization of the barrier. This effect provides a supplementary way to electrically control the current-driven dynamic states of the magnetization and related magnetic noise in spin transfer devices.

We predict experimentally accessible signatures for helical Majorana fermions in a topological superconductor by coupling to two quantum dots in the local moment regime (corresponding to spin-$1/2$ impurities). Taking into account Ruderman-Kittel-Kasuya-Yosida interactions mediated by bulk and edge modes, where the latter cause a long-range antiferromagnetic Ising coupling, we formulate and solve the low-energy theory for this two-impurity helical Majorana problem. In particular, we show that the long-time spin dynamics after a magnetic field quench displays weakly damped oscillations with universal quality factor.

Recent experiments announced an observation of topological superconductivity and Majorana quasiparticles in Shiba chains, consisting of an array of magnetic atoms deposited on top of a superconductor. In this work we study helical Shiba chains and generalize the microscopic theory of subgap energy bands to a regime where the decoupled magnetic impurity energy and the hybridization of different impurity states can be significant compared to the superconducting gap of the host material. From exact solutions of the Bogoliubov-de Gennes equation we extract expressions for the topological phase boundaries for arbitrary values of the superconducting coherence length. The subgap spectral problem can be formulated as a nonlinear matrix eigenvalue problem from which we obtain an analytical solution for energy bands in the long coherence length limit. Physical consequences and departures from the previously obtained results in the deep dilute impurity limit are discussed in detail.

The double perovskite ${\mathrm{La}}_2{\mathrm{NiTiO}}_6$ is identified as a three-dimensional $S=1$ quantum magnet. By means of density functional theory we demonstrate that this material is a high-spin $d$-electron system deep within the Heisenberg limit and establish that its paramagnetic Mott phase persists down to low temperatures (experimental Néel temperature $T_{\mathrm{N}}\sim 25\mathrm{K})$ not because of frustration effects but rather for strong local fluctuations of the magnetic order parameter. Our many-body calculations on an ab initio–derived multiorbital basis predict indeed a kinetic energy gain when entering the magnetically ordered phase. ${\mathrm{La}}_2{\mathrm{NiTiO}}_6$ emerges thus as a paradigmatic realization of a Hund's-coupling-driven Mott insulator. Its peculiar properties may turn out to be instrumental in the ongoing search for correlated topological states of matter.

We investigate spin entanglement in many-electron systems within the framework of density functional theory. We show that the entanglement length, which is extracted from the spatial dependence of the local concurrence, is a sensitive indicator of atomic shells and reveals the character, covalent or metallic, of chemical bonds. These findings shed light on the remarkable success of modern density functionals, which tacitly employ the entanglement length as a variable. This opens the way to further research on entanglement-based functionals.

Laser-induced ultrafast (fs) magnetization experiments in antiferromagnets have recently attracted large attention, paving the road for inherently fast spin dynamics in the THz regime without invoking stray fields. The technical importance is emphasized by the rising new research field of antiferromagnetic (AFM) spintronics, where superexchange-dominated strongly correlated compounds provide an interesting materials playground. An intriguing question is whether the Coulomb interaction may be a key to control AFM order on ultrafast time scales. Therefore, we study (de)magnetization processes in a time-dependent multiorbital Hubbard model, focusing on $t_{2g}$ electrons in a wider doping range. Depending on filling, we reveal large-amplitude spin oscillations via interaction quenches from the antiferromagnetic or paramagnetic state. Nonequilibrium ultrafast spin-orientation effects in prominent correlated transition-metal oxides are therefrom predicted.

We study the temperature dependent magnetic susceptibility of a strained graphene quantum dot using the determinant quantum Monte Carlo method. Within the Hubbard model on a honeycomb lattice, our unbiased numerical results show that a relative small interaction $U$ may lead to an edge ferromagneticlike behavior in the strained graphene quantum dot. Around half-filling, the ferromagnetic fluctuations at the zigzag edge are strengthened both by the on-site Coulomb interaction and the strain, especially in the low temperature region.

Diffusion of a Ga adatom at the As-rich, low temperature $c(4\times 4)$ reconstructions of a GaAs(001) surface is analyzed. We use a known energy landscape for the motion of a Ga adatom at two different $\alpha$ and $\beta$ surface phases to calculate diffusion tensor by means of the variational approach. A diffusion coefficient describes the character of low density adatom system motion at the surface. The resulting expressions allow us to identify the main paths of an adatom diffusion and to calculate an activation energy of this process. It is shown that diffusion at the $\alpha$ surface is slower and more anisotropic than for the $\beta$ surface.

We show that a class of weak three-dimensional topological insulators feature one-dimensional Dirac electrons on their surfaces. Their hallmark is a linelike energy dispersion along certain directions of the surface Brillouin zone. These one-dimensional Dirac line degeneracies are topologically protected by a symmetry that we refer to as in-plane time-reversal invariance. We show how this invariance leads to Dirac lines in the surface spectrum of stacked Kane-Mele systems and more general models for weak three-dimensional topological insulators.

The superconducting state of the caged type compound ${\mathrm{Lu}}_5{\mathrm{Rh}}_6{\mathrm{Sn}}_{18}$ has been investigated by using magnetization, heat capacity, and muon spin relaxation or rotation $\left(\mu \mathrm{SR}\right)$ measurements, and the results interpreted on the basis of the group theoretical classifications of the possible pairing symmetries and a simple model of the resulting quasiparticle spectra. Our zero-field $\mu \mathrm{SR}$ measurements clearly reveal the spontaneous appearance of an internal magnetic field below the transition temperature, which indicates that the superconducting state in this material is characterized by broken time-reversal symmetry. Further, the analysis of the temperature dependence of the magnetic penetration depth measured using the transverse-field $\mu \mathrm{SR}$ measurements suggests an isotropic $s-\text{wave}$ character for the superconducting gap. This is in agreement with the heat capacity behavior, and we show that it can be interpreted in terms of a nonunitary triplet state with point nodes and an open Fermi surface.

We present neutron inelastic scattering measurements of the low-energy phonons in single crystal ${\mathrm{BiFeO}}_3$. The dispersions of the three acoustic phonon modes (LA along [100], ${\mathrm{TA}}_1$ along [010], and ${\mathrm{TA}}_2$ along $\left[1\overline{1}0\right])$ and two low-energy optic phonon modes (LO and ${\mathrm{TO}}_1)$ have been mapped out between $300$ and $700\mathrm{K}$. Elastic constants are extracted from the phonon measurements. The energy linewidths of both TA phonons at the zone boundary clearly broaden when the system is warmed toward the magnetic ordering temperature $T_N=640\mathrm{K}$. This suggests that the magnetic order and low-energy lattice dynamics in this multiferroic material are coupled.