The previously accepted notion that the spontaneous magnetization of $\mathrm{S}{\mathrm{m}}_2\mathrm{F}{\mathrm{e}}_{17}$ lies in the basal plane of the crystal is true only approximately, and then only around room temperature. At low temperatures the magnetization, whose orientation is not fixed by the symmetry, is found to deviate from the basal plane by as much as ${10}^{\circ }$. The threefold symmetry axis is a hard direction; to magnetize the crystal in this direction, a magnetic field of about 9 T is required. The hard-axis magnetization arrives at saturation discontinuously, by way of a first-order phase transition. The behavior is a general one for trigonal ferromagnets where $K_1<0$ and the leading trigonal anisotropy constant is nonzero, $K_2^{\prime }\ne 0$. Although of universal occurrence, the first-order transition is only visible at low temperatures, where it is accompanied by a magnetization anomaly of sufficient size.

We investigate the effects of material anisotropy on the superheating field of layered superconductors. We provide an intuitive argument both for the existence of a superheating field, and its dependence on anisotropy, for $\kappa =\lambda /\xi$ (the ratio of magnetic to superconducting healing lengths) both large and small. On the one hand, the combination of our estimates with published results using a two-gap model for ${\mathrm{MgB}}_2$ suggests high anisotropy of the superheating field near zero temperature. On the other hand, within Ginzburg-Landau theory for a single gap, we see that the superheating field shows significant anisotropy only when the crystal anisotropy is large and the Ginzburg-Landau parameter $\kappa$ is small. We then conclude that only small anisotropies in the superheating field are expected for typical unconventional superconductors near the critical temperature. Using a generalized form of Ginzburg Landau theory, we do a quantitative calculation for the anisotropic superheating field by mapping the problem to the isotropic case, and present a phase diagram in terms of anisotropy and $\kappa$, showing type I, type II, or mixed behavior (within Ginzburg-Landau theory), and regions where each asymptotic solution is expected. We estimate anisotropies for a number of different materials, and discuss the importance of these results for radio-frequency cavities for particle accelerators.

We propose to extract the quasiparticle density of states (DOS) of the superconductor directly from the experimentally measured superconductor-insulator-superconductor junction tunneling data by applying the maximum entropy method to the nonlinear systems. It merits the advantage of model independence with minimum a priori assumptions. Various components of the proposed method have been carefully investigated, including the meaning of the targeting function, the mock function, as well as the role and the designation of the input parameters. The validity of the developed scheme is shown by two kinds of tests for systems with known DOS. As a preliminary application to a ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ sample with its critical temperature $T_c=89\mathrm{K}$, we extract the DOS from the measured intrinsic Josephson junction current data at temperatures of $T=4.2\mathrm{K}$, $45\mathrm{K}$, $55\mathrm{K}$, $95\mathrm{K}$, and $130\mathrm{K}$. The energy gap decreases with increasing temperature below $T_c$, while above $T_c$, a kind of energy gap survives, which provides an angle to investigate the pseudogap phenomenon in high-$T_c$ superconductors. The developed method itself might be a useful tool for future applications in various fields.

We present a microscopic study of superconductivity in ${\mathrm{OsB}}_2$, and discuss the origin and characteristic length scales of the superconducting state. From first-principles we show that ${\mathrm{OsB}}_2$ is characterized by three different Fermi sheets, and we prove that this fermiology complies with recent quantum-oscillation experiments. Using the found microscopic properties, and experimental data from the literature, we employ Ginzburg-Landau relations to reveal that ${\mathrm{OsB}}_2$ is a distinctly type-I superconductor with a very low Ginzburg-Landau parameter $\kappa$—a rare property among compound materials. We show that the found coherence length and penetration depth corroborate the measured thermodynamic critical field. Moreover, our calculation of the superconducting gap structure using anisotropic Eliashberg theory and ab initio calculated electron-phonon interaction as input reveals a single but anisotropic gap. The calculated gap spectrum is shown to give an excellent account for the unconventional behavior of the superfluid density of ${\mathrm{OsB}}_2$ measured in experiments as a function of temperature. This reveals that gap anisotropy can explain such behavior, observed in several compounds, which was previously attributed solely to a two-gap nature of superconductivity.

The valence and spin state evolution of Mn and Co on $\mathrm{TbM}{\mathrm{n}}_{1-x}\mathrm{C}{\mathrm{o}}_x{\mathrm{O}}_3$ series is precisely determined by means of soft and hard x-ray absorption spectroscopy (XAS) and $\mathrm{K}\beta$ x-ray emission spectroscopy (XES). Our results show the change from $\mathrm{M}{\mathrm{n}}^{3+}$ to $\mathrm{M}{\mathrm{n}}^{4+}$ both high-spin (HS) together with the evolution from $\mathrm{C}{\mathrm{o}}^{2+}$ HS to $\mathrm{C}{\mathrm{o}}^{3+}$ low-spin (LS) with increasing $x$. In addition, high energy resolution XAS spectra on the K pre-edge region are interpreted in terms of the strong charge transfer and hybridization effects along the series. These results correlate well with the spin values of Mn and Co atoms obtained from the $\mathrm{K}\beta$ XES data. In this paper, we determine that Co enters into the transition metal sublattice of $\mathrm{TbMn}{\mathrm{O}}_3$ as a divalent ion in HS state, destabilizing the Mn long-range magnetic order since very low doping compositions ($x\le 0.1$). Samples in the intermediate composition range ($0.4\le x\le 0.6$) adopt the crystal structure of a double perovskite with long-range ferromagnetic ordering which is due to $\mathrm{M}{\mathrm{n}}^{4+}$-O-$\mathrm{C}{\mathrm{o}}^{2+}$ superexchange interactions with both cations in HS configuration. Ferromagnetism vanishes for $x\ge 0.7$ due to the structural disorder that collapses the double perovskite structure. The spectroscopic techniques reveal the occurrence of $\mathrm{M}{\mathrm{n}}^{4+}$ HS and a fluctuating valence state $\mathrm{C}{\mathrm{o}}^{2+}$ HS/$\mathrm{C}{\mathrm{o}}^{3+}$ LS in this composition range. Disorder and competitive interactions lead to a magnetic glassy behavior in these samples.

We investigate the effect of Coulomb interactions on the electromagnetic response of three-dimensional Dirac and Weyl semimetals. In a calculation reminiscent of Schwinger's seminal work on quantum electrodynamics, we find three physically distinct effects for the anomalous magnetic moment of the relativisticlike quasiparticles in the semimetal. In the case of nonzero doping, the anomalous magnetic moment is finite at long wavelengths and typically orders of magnitude larger than Schwinger's result. We also find interesting effects of one of the three new Hamiltonian terms on the topological surface states at the interface between vacuum and a Weyl semimetal. We conclude that observation of these effects should be within experimental reach.

Pump-probe techniques with high temporal resolution allow one to drive a system of interest out of equilibrium and at the same time probe its properties. Recent advances in these techniques open the door to studying new, nonequilibrium phenomena such as Floquet topological insulators and superconductors. These advances also necessitate the development of theoretical tools for understanding the experimental findings and predicting new ones. In the present paper, we provide a theoretical foundation to understand the nonequilibrium behavior of a Dirac system. We present detailed numerical calculations and simple analytic results for the time evolution of a Dirac system irradiated by light. These results are framed by appealing to the recently revitalized notion of side bands [A. Farrell and T. Pereg-Barnea, Phys. Rev. Lett. 115, 106403 (2015); Phys. Rev. B 93, 045121 (2016)], extended to the case of nonperiodic drive where the fast oscillations are modified by an envelope function. We apply this formalism to the case of photocurrent generated by a second probe pulse. We find that, under the application of circularly polarized light, a Dirac point only ever splits into two copies of side bands. Meanwhile, the application of linearly polarized light leaves the Dirac point intact while producing side bands. In both cases the population of the side bands are time dependent through their nonlinear dependence on the envelope of the pump pulse. Our immediate interest in this work is in connection to time- and angle-resolved photoemission experiments, where we find excellent qualitative agreement between our results and those in the literature [Wang et al., Science 342, 453 (2013)]. However, our results are general and may prove useful beyond this particular application and should be relevant to other pump-probe experiments.

We study coherent transport and bound state formation of Bogoliubov quasiparticles in a high-mobility $\mathrm{I}{\mathrm{n}}_{0.75}\mathrm{G}{\mathrm{a}}_{0.25}\mathrm{As}$ two-dimensional electron gas (2DEG) coupled to a superconducting Nb electrode by means of a quantum point contact (QPC) as a tunable single-mode probe. Below the superconducting critical temperature of Nb, the QPC shows a single-channel conductance greater than the conductance quantum $2e^2/h$ at zero bias, which indicates the presence of Andreev-reflected quasiparticles, time-reversed states of the injected electron, returning back through the QPC. The marked sensitivity of the conductance enhancement to voltage bias and perpendicular magnetic field suggests a mechanism analogous to reflectionless tunneling—a hallmark of phase-coherent transport, with the boundary of the 2DEG cavity playing the role of scatterers. When the QPC transmission is reduced to the tunneling regime, the differential conductance vs bias voltage probes the single-particle density of states in the proximity area. Measured conductance spectra show a double peak within the superconducting gap of Nb, demonstrating the formation of Andreev bound states in the 2DEG. Both of these results, obtained in the open and closed geometries, underpin the coherent nature of quasiparticles, i.e., phase-coherent Andreev reflection at the InGaAs/Nb interface and coherent propagation in the ballistic 2DEG.

We present a model of the electronic properties of monolayer transition-metal dichalcogenides based on a tight-binding approach which includes the effects of strain and curvature of the crystal lattice. Mechanical deformations of the lattice offer a powerful route for tuning the electronic structure of the transition-metal dichalcogenides, as changes to bond lengths lead directly to corrections in the electronic Hamiltonian while curvature of the crystal lattice mixes the orbital structure of the electronic Bloch bands. We first present an effective low-energy Hamiltonian describing the electronic properties near the K point in the Brillouin zone, then present the corrections to this Hamiltonian due to arbitrary mechanical deformations and curvature in a way which treats both effects on an equal footing. This analysis finds that local area variations of the lattice allow for tuning of the band gap and effective masses, while the application of uniaxial strain decreases the magnitude of the direct band gap at the K point. Additionally, strain induced bond length modifications create a fictitious gauge field with a coupling strength that is smaller than that seen in related materials like graphene. We also find that curvature of the lattice leads to the appearance of both an effective in-plane magnetic field which couples to spin degrees of freedom and a Rashba-like spin-orbit coupling due to broken mirror inversion symmetry.

We theoretically examine a scheme for projectively reading out the parity state of a pair of Majorana bound states (MBSs) using a tunnel-coupled quantum dot. The dot is coupled to one end of the topological wire but isolated from any reservoir and is capacitively coupled to a charge sensor for measurement. The combined parity of the MBS-dot system is conserved, and charge transfer between the MBS and dot only occurs through resonant tunneling. Resonance is controlled by the dot potential through a local gate and by the MBS energy splitting due to the overlap of the MBS pair wave functions. The latter splitting can be tuned from zero (topologically protected regime) to a finite value by gate-driven shortening of the topological wire. Simulations show that the oscillatory nature of the MBS splitting is not a fundamental obstacle to readout but requires precise gate control of the MBS spatial position and dot potential. With experimentally realistic parameters, we find that high-fidelity parity readout is achievable given nanometer-scale spatial control of the MBS and that there is a trade-off between required precisions of temporal and spatial control. Use of the scheme to measure the MBS splitting versus separation would present a clear signature of topological order and could be used to test the robustness of this order to spatial motion, a key requirement in certain schemes for scalable topological qubits. We show how the scheme can be extended to distinguish valid parity measurements from invalid ones due to gate calibration errors.

We study the quantum phase diagram of a system of hard-core bosons on the kagome lattice with nearest-neighbor repulsive interactions, for arbitrary densities, by means of the hierarchical mean-field theory and exact diagonalization techniques. This system is isomorphic to the spin $S=1/2$ XXZ model in presence of an external magnetic field, a paradigmatic example of frustrated quantum magnetism. In the nonfrustrated regime, we find two crystal phases at densities 1/3 and 2/3 that melt into a superfluid phase when increasing the hopping amplitude, in semiquantitative agreement with quantum Monte Carlo computations. In the frustrated regime and away from half-filling, we find a series of plateaux with densities commensurate with powers of 1/3. The broader density plateaux (at densities 1/3 and 2/3) are remnants of the classical degeneracy in the Ising limit. For densities near half-filling, this staircase of crystal phases melts into a superfluid, which displays finite chiral currents when computed with clusters having an odd number of sites. Both the staircase of crystal phases and the superfluid phase prevail in the noninteracting limit, suggesting that the lowest dispersionless single-particle band may be at the root of this phenomenon.

We investigated experimentally as well as theoretically the ultrafast response of the wave function of the conduction band (CB) of Si and Zr to a near-infrared laser field using extreme ultraviolet (XUV) absorption spectroscopy in the spectral range of 80–220 eV. The measured dynamics of the XUV transmission demonstrates that the wave function of the CB follows the electric field of the dressing laser pulse. In these terms, laser dressing was earlier mainly studied on gases. Measurements with two-femtosecond and 200-attosecond temporal steps were performed in the vicinity of the $\mathrm{Si}{L}_{2,3}$ edge near 100 eV, the $\mathrm{Si}{L}_1$ edge near 150 eV, and the $\mathrm{Zr}{M}_{4,5}$ edge near 180 eV. The observed changes were dependent on the core states being excited by the XUV probe pulse. At the $2p$ to CB transitions of Si, the XUV transmission increased via the effect of the dressing laser pulse, while at the $2s$ to CB transition of Si and the $3d$ to CB transition of Zr, the XUV transmission decreased. Furthermore, beats between the transition from $2p_{1/2}$ and $2p_{3/2}$ levels of Si and from $3d_{3/2}$ and $3d_{5/2}$ levels of Zr were observed with 20.7 fs and 3.6 fs periods.

The magnetic and coherent transport properties of small-width zigzag graphene nanoribbons (ZGNRs) with monohydrogen edge passivation are investigated as a function of random edge-vacancy disorder and ribbon length. Results from noninteracting tight-binding models with (i) nearest and (ii) up to third nearest neighbor hopping are compared against those obtained from an extended mean-field Hubbard model for edge-defected ZGNRs (length = 48.02 $\AA$ and width = 9.24 $\AA$). Through ensemble averaging, a persistent magnetism and Hubbard-$U$ (i.e., spin-generated) conductance gap is found irrespective of the extent of random edge-vacancy disorder. At longer device lengths (up to 144.1 $\AA$) and at high disorder (42.5%), gaps open in the noninteracting model systems, whereas the gap in the Hubbard-calculated systems becomes spin dependent. In all cases, the conductance gaps increase as a function of increasing system length, although the gaps in the Hubbard systems remain smaller due to increased robustness against edge disorder. The continuance of the magnetic state and gap robustness in the ensemble-averaged Hubbard results indicates a complex interplay between the kinetics, disorder, system size, and spin interaction. Such findings may serve to reinform previous studies that have used noninteracting models to investigate disorder in ZGNRs.

We employ first-principles techniques tailored to properly describe semiconductors (semilocal exchange potential added to the exchange-correlation functional), to obtain the electronic band structures of both the zinc-blende and wurtzite phases of GaAs, GaSb, InAs, and InSb. We extract the spin-orbit fields for the relevant valence and conduction bands at the zone center, by fitting the spin splittings resulting from the lack of space inversion symmetry of these bulk crystal structures, to known functional forms—third-order polynomials. We also determine the orientations of the spin-orbit vector fields (for conduction bands) and the average spins (valence bands) in the momentum space. We describe the dependence of the spin-orbit parameters on the cation and anion atomic weights. These results should be useful for spin transport, spin relaxation, and spin optical orientation modeling of semiconductor heterostructures, as well as for realistic studies of semiconductor-based Majorana nanowires, for which accurate values of spin-orbit couplings are needed.

The evolution of the electronic structure of BiTeI, a layered semiconductor with a van der Waals gap, under compression is studied by employing semilocal and dispersion-corrected density-functional calculations. Comparative analysis of the results of these calculations shows that the band-gap energy of BiTeI decreases till it attains a minimum value of zero at a critical pressure, after which it increases again. The critical pressure corresponding to the closure of the band gap is calculated, at which BiTeI becomes a topological insulator. Comparison of the critical pressure to the pressure at which BiTeI undergoes a structural phase transition indicates that the closure of the band gap would not be hindered by a structural transformation. Moreover, the band-gap pressure coefficients of BiTeI are computed, and an expression of the critical pressure is devised in terms of these coefficients. Our findings indicate that the semilocal and dispersion-corrected approaches are in conflict about the compressibility of BiTeI, which result in overestimation and underestimation, respectively. Nevertheless, the effect of pressure on the atomic structure of BiTeI is found to be manifested primarily as the reduction of the width of the van der Waals gap according to both approaches, which also yield consistent predictions concerning the interlayer metallic bonding in BiTeI under compression. It is consequently shown that the calculated band-gap energies follow qualitatively and quantitatively the same trend within the two approximations employed here, and the transition to the zero-gap state occurs at the same critical width of the van der Waals gap.

We develop a methodology to incorporate nonlocal optical response of the free electron gas due to quantum-interaction effects in metal components of periodic two-dimensional plasmonic crystals and study the impact of spatial dispersion on promising building blocks for photonic circuits. Within the framework of the hydrodynamic model, we observe significant changes with respect to the commonly employed local-response approximation, but also in comparison with homogeneous metal films where nonlocal effects have previously been considered. Notable are the emergence of a contribution from nonlocality at normal incidence and the surprisingly large structural parameters at which finite blueshifts are observable, which we attribute to diffraction that offers nonvanishing in-plane wave vector components and increases the penetration depth of longitudinal (nonlocal) modes.

We present a rigorous microscopic theory of the extrinsic spin Hall effect in disordered graphene based on a nonperturbative quantum diagrammatic treatment incorporating skew scattering and anomalous (impurity-concentration-independent) quantum corrections on equal footing. The leading skew-scattering contribution to the spin Hall conductivity is shown to quantitatively agree with Boltzmann transport theory over a wide range of parameters. Our self-consistent approach, where all topologically equivalent noncrossing diagrams are resummed, unveils that the skewness generated by spin-orbit-active impurities deeply influences the anomalous component of the spin Hall conductivity, even in the weak-scattering regime. This seemingly counterintuitive result is explained by the rich sublattice structure of scattering potentials in graphene, for which traditional Gaussian disorder approximations fail to capture the intricate correlations between skew scattering and side jumps generated through diffusion. Finally, we assess the role of quantum interference corrections by evaluating an important subclass of crossing diagrams recently considered in the context of the anomalous Hall effect, the $X$ and $\mathrm{\Psi }$ diagrams [A. Ado et al., Europhys. Lett. 111, 37004 (2015)]. We show that $\mathrm{\Psi }$ diagrams, encoding quantum coherent skew scattering, display a strong Fermi energy dependence, dominating the anomalous spin Hall component away from the Dirac point. Our findings have direct implications for nonlocal transport experiments in spin-orbit-coupled graphene systems.

Competing orders widely exist in many material systems, such as superconductivity, magnetism, and ferroelectricity; $\mathrm{LaCrS}{\mathrm{b}}_3$ is a highly anisotropic magnetic material in which the spins are aligned ferromagnetically in one direction and canted antiferromagnetically in another in the Cr-Sb chains. Hole doping with $\mathrm{S}{\mathrm{r}}^{2+}$ and $\mathrm{C}{\mathrm{a}}^{2+}$ in the La site suppresses the antiferromagnetic correlations and transforms the anisotropic magnetic order into a ferromagnetic lattice in all directions. First-principles density functional theory calculations show that the canted magnetic order becomes energetically less favorable compared to the FM order upon hole doping. Doping in the La site is an effective approach to modulate the competing orders in $\mathrm{LaCrS}{\mathrm{b}}_3$.

Crystal and magnetic structures of the high-pressure stabilized perovskite phase of ${\mathrm{TlMnO}}_3$ have been studied by neutron powder diffraction. The crystal structure involves two types of primary structural distortions: $a^{+}{b}^{-}{b}^{-}$ octahedral tilting and antiferrodistortive type of orbital ordering, whose common action reduces the symmetry down to triclinic $P\overline{1}$. The orbital pattern and the way it is combined with the octahedral tilting are different from the family of ${\mathrm{LnMnO}}_3$ (Ln = lanthanide or Y) manganites who share with ${\mathrm{TlMnO}}_3$ the same tilting scheme. The experimentally determined magnetic structure with the $\mathit{k}=(1/2,0,1/2)$ propagation vector and $P_S\overline{1}$ symmetry implies anisotropic exchange interactions with a ferromagnetic coupling within the $(1,0,\overline{1})$ planes and an antiferromagnetic one between them (A type). The spins in the primary magnetic mode were found to be confined close to the $(1,0,\overline{1})$ plane, which underlines the predominant role of the single ion anisotropy with the local easy axes of ${\mathrm{Mn}}^{3+}$ following the Jahn-Teller distortions of the octahedra. In spite of the same octahedral tilting scheme in the perovskite structures of both ${\mathrm{LnMnO}}_3$ and ${\mathrm{TlMnO}}_3$ manganites, a coupling of the secondary ferromagnetic component to the primary A-type spin configuration through antisymmetric exchange interaction is allowed in the former and forbidden in the latter cases.

We study theoretically the coupling of electric charge and spin polarization in an equilibrium and nonequilibrium electric transport across a two-dimensional Josephson configuration comprised of disordered surface channels of a three-dimensional topological insulator. In the equilibrium state of the system, we predict the Edelstein effect, which is much more pronounced than its counterpart in conventional spin-orbit coupled materials. Employing a quasiclassical Keldysh technique, we demonstrate that the ground state of the system can be shifted experimentally into arbitrary macroscopic superconducting phase differences other than the standard “0” or “$\pi$,” constituting a ${\varphi }_0$ junction, solely by modulating a quasiparticle flow injection into the junction. We propose a feasible experiment in which the quasiparticles are injected into the topological insulator surface by means of a normal electrode and voltage gradient so that oppositely oriented stationary spin densities can be developed along the interfaces and allow for direct use of the spin-momentum locking nature of Dirac fermions in the surface channels. The ${\varphi }_0$ state is proportional to the voltage difference applied between the injector electrode and superconducting terminals that calibrates the injection rate of particles and, therefore, the ${\varphi }_0$ shift.