We have investigated the excitation spectra of $j_{\mathrm{eff}}=\frac{1}{2}$ Mott insulator ${\mathrm{Na}}_2$${\mathrm{IrO}}_3$. Taking into account a relativistic multiplet structure of Ir ions, we have calculated the optical conductivity $\sigma \left(\omega \right)$ and resonant inelastic x-ray scattering (RIXS) spectra, which manifest different features from those of a canonical $j_{\mathrm{eff}}=\frac{1}{2}$ system ${\mathrm{Sr}}_2$${\mathrm{IrO}}_4$. Distinctly from the two-peak structure in ${\mathrm{Sr}}_2$${\mathrm{IrO}}_4$, $\sigma \left(\omega \right)$ in ${\mathrm{Na}}_2$${\mathrm{IrO}}_3$ has a broad single peak dominated by interband transitions from $j_{\mathrm{eff}}=\frac{3}{2}$ to $\frac{1}{2}$. RIXS spectra exhibit the spin-orbit (SO) exciton that has a two-peak structure arising from the crystal-field effect, and the magnon peak at energies much lower than in ${\mathrm{Sr}}_2$${\mathrm{IrO}}_4$. In addition, a small peak near the optical-absorption edge is found in RIXS spectra, originating from the coupling between the electron-hole ($e$-$h$) excitation and the SO exciton. Our findings corroborate the validity of the relativistic electronic structure and importance of both itinerant and local features in ${\mathrm{Na}}_2$${\mathrm{IrO}}_3$.

We present magnetotransport measurements performed on two-dimensional hole gases embedded in carbon doped $p$-type GaAs/AlGaAs heterostructures grown on [001] oriented substrates. A pronounced beating pattern in the Shubnikov–de Haas oscillations proves the presence of strong spin-orbit interaction in the device under study. We estimate the effective masses of spin-orbit-split subbands by measuring the temperature dependence of the Shubnikov–de Haas oscillations at different hole densities. While the lighter heavy-hole effective mass is not energy dependent, the heavier heavy-hole effective mass has a prominent energy dependence, indicating a strong spin-orbit induced nonparabolicity of the valence band. The measured effective masses show qualitative agreement with self-consistent numerical calculations.

Heterostructures of transition-metal oxides have emerged as a new route to engineer electronic systems with desired functionalities. Motivated by these developments, we study a two-orbital Hubbard model in a thin-film geometry confined along the cubic [001] direction using the dynamical mean-field theory. We contrast the results of two approximate impurity solvers (exact diagonalization and one-crossing approximation) to the results of the numerically exact continuous-time quantum Monte Carlo solver. Consistent with earlier studies, we find that the one-crossing approximation performs well in the insulating regime, while the advantage of the exact-diagonalization-based solver is more pronounced in the metallic regime. We then investigate various aspects of strongly correlated $e_g$-orbital systems in thin-film geometries. In particular, we show how the interfacial orbital polarization dies off quickly a few layers from the interface and how the film thickness affects the location of the interaction-driven Mott transition. In addition, we explore the changes in the electronic structure with varying carrier concentration and identify large variations of the orbital polarization in the strongly correlated regime.

The appearance of unusually large Au-Au bond distances in linear atomic chains (LACs) of Au nanowires is commonly attributed to the presence of atomic impurities. However, the origin of those contaminants is unknown. We present a study based on density functional theory calculations using quasistatic ($T=0$) and finite-temperature ab initio molecular-dynamics simulations of a possible route for the formation of atomic impurities in Au nanowires. This process starts with the adsorption of an O${}_2$ molecule followed by a CO molecule on Au LACs, leading to the formation of an intermediate O${}_2$CO complex. Upon thermal activation at finite temperatures, the complex is able to proceed to oxidation forming a CO${}_2$ molecule and leaving an atomic O impurity in the Au LAC.

It is shown that the current-induced torques between a ferromagnetic layer and an antiferromagnetic layer with a compensated interface vanish when the ferromagnet is aligned with an axis of spin-rotation symmetry of the antiferromagnet. For properly chosen geometries, this implies that the current-induced torque can stabilize the out-of-plane (or hard-axis) orientation of the ferromagnetic layer. This current-induced torque relies on phase-coherent transport, and we calculate the robustness of this torque to phase-breaking scattering. From this, it is shown that the torque is not linearly dependent on the applied current, but has an absolute maximum.

Muon-spin rotation has been used to observe directly the spatial variation of the magnetic flux density near the ferromagnetic-superconducting interface in a permalloy-niobium trilayer. Above the superconducting transition temperature $T_c$ the profile of the induced magnetic flux density within the niobium layer has been determined. Below $T_c$ there is a significant reduction of the induced flux density, predominantly near the ferromagnetic-superconducting interfaces. We are uniquely able to determine the magnitude and spatial variation of this reduction in induced magnetization due to the presence of the Cooper pairs, yielding the magnitude and length scale associated with this phenomenon. Both are inconsistent with a simple Meissner screening and indicate the existence of another mechanism, the influence of which is localized within the vicinity of the ferromagnetic interface.

We derive a nonlinear differential equation for the gap parameter of a superfluid Fermi system by performing a suitable coarse graining of the Bogoliubov–de Gennes (BdG) equations throughout the BCS-BEC crossover, with the aim of replacing the time-consuming solution of the original BdG equations by the simpler solution of this novel equation. We perform a favorable numerical test on the validity of this new equation over most of the temperature-coupling phase diagram, by an explicit comparison with the full solution of the original BdG equations for an isolated vortex. We also show that the new equation reduces both to the Ginzburg-Landau equation for Cooper pairs in weak coupling close to the critical temperature and to the Gross-Pitaevskii equation for composite bosons in strong coupling at low temperature.

The pnictide phase ${\text{Ca}}_{11}{\text{Bi}}_{10-x}$ shows bulk superconductivity around 2.2 K in temperature-dependent resistivity and magnetic susceptibility data. The compound is a $p$-type metal with low carrier density of 6.5 × 10${}^{18}$ cm${}^{-3}$. ${\text{Ca}}_{11}{\text{Bi}}_{10-x}(x=0.12)$ was synthesized by the reaction of Ca metal with excess Bi and crystallizes in the tetragonal space group $I$4/mmm with $a$ = 12.2842(6) Å and $c$ = 17.866(4) Å. The structure of ${\text{Ca}}_{11}{\text{Bi}}_{10}$ contains three discrete units: isolated Bi atoms, dumbbells, and square planar rings of Bi surrounded by Ca atoms. Vacancies were found in the isolated Bi(1) atoms and square planar Bi${}_4$ units of the structure. The Ca${}_{11}$Bi${}_{10}$ system is the first member found to exhibit superconductivity among the intermetallic class $M_{11}{X}_{10}$ ($M$ = Ca, Sr, Ba; $X$ = Bi, Sb), suggesting that a broader family of Bi or Sb containing superconductors may exist. Electronic structure density functional theory calculations confirm the metallic nature of the compound with several steep Bi $p$-orbital bands crossing the Fermi level as well as a single flat band reaching the Fermi level upon the introduction of Bi vacancies.

Chiral fermions are responsible for low-temperature properties of vortices in fermionic condensates, both superconducting (charged) and superfluid (neutral). One of the most striking consequences of this fact is that the core of a single-quantum vortex collapses at low temperatures, $T\to 0$ (i.e., the Kramer-Pesch effect for superconductors), due to the presence of chiral quasiparticles in the vortex-core region. We show that the situation changes drastically for fermionic condensates confined in quasi-one-dimensional and quasi-two-dimensional geometries. Here quantum confinement breaks the chirality of in-core fermions. As a result, instead of the ultimate shrinking, the core of a single-quantum vortex extends at low temperatures, and the condensate profile surprisingly mimics the multiquantum vortex behavior. Our findings are relevant for nanoscale superconductors, such as recent metallic nanoislands on silicon, and also for ultracold superfluid Fermi gases in cigar-shaped and pancake-shaped atomic traps.

We present a femtosecond spectroscopic magneto-optical investigation of the coherent and incoherent spin dynamics in the antiferromagnetic dielectric ${\text{KNiF}}_3$. The pathways of the photoinduced energy flow to spins were controlled by tuning the pump photon energy. In particular, we demonstrate that laser pulses, with photon energy tuned to a nearly-zero-absorption region, excite the spin system without any signatures of heating of electrons or phonons. In this regime the ultrafast excitation of coherent spin waves is followed by a gradual increase of the spin temperature solely due to decoherence of the laser-generated magnons, as revealed by our simultaneous measurement of both the transversal and the longitudinal component of the spin dynamics.

We present a systematic study of the electronic specific-heat jump ($\Delta C_{\mathrm{el}}$) at the superconducting transition temperature $T_c$ of ${\text{K}}_{1-x}$${\text{Na}}_x$${\text{Fe}}_2$${\text{As}}_2$. Both $T_c$ and $\Delta C_{\mathrm{el}}$ monotonously decrease with increasing $x$. The specific heat jump scales approximately with a power law, $\Delta C_{\mathrm{el}}\propto T_c^{\beta }$, with $\beta \approx 2$ determined by the impurity scattering rate, in contrast to most iron-pnictide superconductors, where the remarkable Bud'ko-Ni-Canfield (BNC) scaling $\Delta C_{\mathrm{el}}\propto T^3$ has been found. Both the $T$ dependence of $C_{\mathrm{el}}\left(T\right)$ in the superconducting state and the nearly quadratic scaling of $\Delta C_{\mathrm{el}}$ at $T_c$ are well described by the Eliashberg theory for a two-band $d$-wave superconductor with weak pair breaking due to nonmagnetic impurities. The disorder induced by the Na substitution significantly suppresses the small gaps, leading to gapless states in the slightly disordered superconductor, which results in a large observed residual Sommerfeld coefficient in the superconducting state for $x>0$.

We report on the atomic pair distribution function determination in the ternary compound CeRuSn using high-energy synchrotron x-ray diffraction. Above room temperature CeRuSn orders in a monoclinic structure that is related to the CeCoAl type of structure by a doubling of the $c$-axis parameter. Upon cooling a very broad hysteretic structural transition has been observed in CeRuSn leading to an ill-defined structure with a main component three times larger than the $c$ parameter. The pair distribution functions collected between 90 and 315 K suggest that the local symmetry type does not change significantly during the phase transformation. However, anomalously large atomic displacement factors suggest that a wide distribution of interatomic distances exists in the sample. Limited agreement between the data at 90 K and the best fit suggests that the structure with a three times larger $c$ lattice parameter with respect to the CeCoAl type can only be regarded as an approximation to the real crystal structure of CeRuSn at low temperatures.

We study the interplay of structural distortions in double perovskites ${\text{Sr}}_{2}M{\text{WO}}_6$ ($M=$ Zn, Ca, Mg) by means of first-principles calculations and group theoretical analysis. Structure relaxations of the cubic, tetragonal, and monoclinic phases show that the ground states of the three compounds are monoclinic, although the energy difference between the monoclinic and tetragonal structures is very small in the case of ${\text{Sr}}_2\text{Mg}{\text{WO}}_6$. The symmetry analysis of the distortions involved in the experimental and calculated low-temperature structures shows that the amplitude of two primary distortions associated to rigid rotations of the $MX$${}_6$ and WO${}_6$ octahedra are dominant, although the amplitude of a third mode related to deformations of the $MX$${}_6$ groups can not be neglected. The energy maps of the space spanned by the three relevant modes are calculated, and the couplings among the modes are evaluated, showing that the role of a hard secondary mode (in the Landau sense) coupled trilinearly to the two primary instabilities is crucial to stabilize the monoclinic ground state. Results suggest that the key role of the trilinear coupling among three modes could be rather common. A phenomenological theory including the effects of the chemical pressure is also developed. We find that the evolution of the stiffness constants in terms of the atomic substitution follows an accurate linear dependence and that the influence of quantum saturation of the order parameters could stabilize the tetragonal phase of ${\text{Sr}}_2\text{Mg}{\text{WO}}_6$.

The magnetic skyrmion, i.e., the nanometric swirling spin vortexlike object with the topolgical charge, is broadly observed in chiral-lattice cubic magnets, typically MnSi; where the cylindrical-shape skyrmions form the two-dimensional hexagonal crystal, in a very narrow temperature-magentic field window for bulk crystals but in a much wider one for tens of nanometer thin films under the perpendicular magnetic field applied. We have investigated the stability of two-dimensional skyrmion states emerging in epitaxial thin films of Mn${}_{1-x}$Fe${}_x$Si with various thicknesses $t$ and iron contents $x$ ($t=10$, 15, and 20 nm; $x=0$, 0.02, and 0.04) by changing the magnetic-field direction. Topological Hall effect arising from the skyrmions is critically suppressed in the course of tilting the applied magnetic field from the normal vector, indicating the collapse of the skyrmion state. Utilizing this observation, the stable region of the skyrmions, which depends on the film thickness relative to the helimagnetic period, can be mapped out in the temperature-magnetic field plane.

We present a single-crystal neutron scattering study of ${\text{BaMn}}_2{\text{Bi}}_2$, a recently synthesized material with the same ${\text{ThCr}}_2{\text{Si}}_2$-type structure found in several Fe-based unconventional superconducting materials. We show long-range magnetic order, in the form of a $G$-type antiferromagnetic structure, exists up to 390 K with an indication of a structural transition at 100 K. Utilizing inelastic neutron scattering, we observe a spin gap of 16 meV, with spin waves extending up to 55 meV. We find these magnetic excitations are well fit to a $J_1$-$J_2$-$J_c$ Heisenberg model and present values for the exchange interactions. The spin-wave spectrum appears to be unchanged by the 100 K structural phase transition.

Symmetry-protected topological (SPT) states are short-range entangled states with a symmetry $G$. They belong to a new class of quantum states of matter which are classified by the group cohomology ${\mathcal{H}}^{d+1}(G,\mathbb{R}/\mathbb{Z})$ in $d$-dimensional space. In this paper, we propose a class of symmetry-protected topological invariants that may allow us to fully characterize SPT states with a symmetry group $G$ [i.e., allow us to measure the cocycles in ${\mathcal{H}}^{d+1}(G,\mathbb{R}/\mathbb{Z})$ that characterize the SPT states]. We give an explicit and detailed construction of symmetry-protected topological invariants for $2+1$D SPT states. Such a construction can be directly generalized to other dimensions.

We show that networks of superconducting topological nanowires can realize the physics of exactly solvable Kitaev spin models on trivalent lattices. This connection arises from the low-energy theory of both systems being described by a tight-binding model of Majorana modes. In Kitaev spin models the Majorana description provides a convenient representation to solve the model, whereas in an array of Josephson junctions of topological nanowires it arises from localized physical Majorana modes tunneling between the wire ends. We explicitly show that an array of junctions of three wires—a setup relevant to topological quantum computing with nanowires—can realize the Yao-Kivelson model, a variant of Kitaev spin models on a decorated honeycomb lattice. Employing properties of the latter, we show that the network can be constructed to give rise to two-dimensional collective topological states characterized by Chern numbers $\nu =0,\pm 1$, and $\pm 2$, and that defects in the array can be associated with vortex-like quasiparticle excitations. In addition we show that the collective states are stable in the presence of disorder and superconducting phase fluctuations. When the network is operated as a quantum information processor, the connection to Kitaev spin models implies that decoherence mechanisms can in general be understood in terms of proliferation of the vortex-like quasiparticles.

We study the formation of second-harmonic double-resonance cones in hyperbolic metamaterials. An electric dipole on the surface of the structure induces second-harmonic light to propagate into two distinct volume plasmon-polariton channels: a signal that propagates within its own peculiar resonance cone and a phase-locked signal that is trapped under the pump's resonance cone. Metamaterial dispersion and birefringence induce a large angular divergence between the two volume plasmon polaritons, making these structures suitable for subwavelength second- and higher-harmonic imaging microscopy.

We present a field theoretical model of anomalous transport in Weyl semimetals. We calculate the chiral magnetic and chiral vortical effect in the electric, axial (valley), and energy current. Our findings coincide with the results of a recent analysis using kinetic theory in the bulk of the material. We point out that the kinetic currents have to be identified with the covariant currents in quantum field theory. These currents are anomalous and the CME appears as anomalous charge creation/annihilation at the edges of the Weyl semimetal. We discuss a possible simultaneous experimental test of the chiral magnetic and the chiral vortical effect sensitive to the temperature dependence induced by the gravitational contribution to the axial anomaly.

Using time-resolved luminescence, we report on the recombination lifetime of excitons bound to nitrogen isoelectronic centers (ICs) in $\delta$-doped GaP. For all ICs considered (A line and ${\text{NN}}_i$, with $i=1,3,4,6$), the recombination lifetime is identical for all transitions composing the fine structure, indicating a fast transfer between all eight excitonic states. From 5 to 60 K, the lifetime decreases by three orders of magnitude and is characterized by three distinct regimes successively dominated by (1) transfers between thermally mixed bright and dark states, (2) hole escape for $i\le 4$ or exciton escape for $i\ge 6$ and the A line, and (3) exciton capture through hopping from single nitrogen centers to nitrogen dyads. Improving on previous population balance models to include both light- and heavy-hole states, we accurately reproduce this temperature evolution of the lifetime and quantify the radiative lifetime of all transitions, the energy position of dark states, and the exciton capture time.