We report a density functional study of the electronic structure and magnetism of ${\mathrm{Y}}_2{\mathrm{Ni}}_7$. The results show itinerant magnetism very similar to that in the weak itinerant ferromagnet ${\mathrm{Ni}}_3\mathrm{Al}$. The electropositive Y atoms in ${\mathrm{Y}}_2{\mathrm{Ni}}_7$ donate charge to the Ni host mostly in the form of $s$ electrons. The non-spin-polarized state shows a high density of states at the Fermi level, $N\left(E_F\right)$, due to flat bands. This leads to a ferromagnetic instability. However, there are also several much more dispersive bands crossing $E\left(F\right)$, which should promote the conductivity. Spin fluctuation effects appear to be comparable to or weaker than ${\mathrm{Ni}}_3\mathrm{Al}$, based on comparison with experimental data. ${\mathrm{Y}}_2{\mathrm{Ni}}_7$ provides a uniaxial analog to cubic ${\mathrm{Ni}}_3\mathrm{Al}$ for studying weak itinerant ferromagnetism, suggesting detailed measurements of its low temperature physical properties and spin fluctuations, as well as experiments under pressure.

We report on a study of electric resistivity and magnetic susceptibility measurements in electron irradiated ${\mathrm{SrTi}}_{0.987}{\mathrm{Nb}}_{0.013}{\mathrm{O}}_3$ single crystals. Pointlike defects, induced by electron irradiation, lead to an almost threefold enhancement of the residual resistivity, but barely affect the superconducting critical temperature $\left(T_c\right)$. The pertinence of Anderson's theorem provides strong evidence for a $s$-wave superconducting order parameter. Stronger scattering leads to a reduction of the effective coherence length $\left(\xi \right)$ and the deduced intrinsic coherence length $\left({\xi }_0\right)$ is close to the BCS coherence length $\left({\xi }_{\text{BCS}}\right)$. Combined with thermal conductivity data pointing to multiple nodeless gaps, the current results identify optimally doped ${\mathrm{SrTi}}_{1-x}{\mathrm{Nb}}_x{\mathrm{O}}_3$ as a multiband $s$-wave superconductor.

We study the equilibrium and dynamical properties of skyrmions in thin films of chiral magnets with oblique magnetic field. The shape of an individual skyrmion is noncircular and the skyrmion density decreases with the tilt angle from the normal of films. As a result, the interaction between two skyrmions depends on the relative angle between them in addition to their separation. The triangular lattice of skyrmions under a perpendicular magnetic field is distorted into a centered rectangular lattice for a tilted magnetic field. For a low skyrmion density, skyrmions form a chainlike structure. The dynamical response of the noncircular skyrmions depends on the direction of external currents.

A hallmark of materials with extremely large magnetoresistance (XMR) is the transformative turn-on temperature behavior: when the applied magnetic field $H$ is above certain value, the resistivity versus temperature $\rho \left(T\right)$ curve shows a minimum at a field dependent temperature $T^{*}$, which has been interpreted as a magnetic-field-driven metal-insulator transition or attributed to an electronic structure change. Here, we demonstrate that $\rho \left(T\right)$ curves with turn-on behavior in the newly discovered XMR material ${\mathrm{WTe}}_2$ can be scaled as $\text{MR}\sim {(H/{\rho }_0)}^m$ with $m\approx 2$ and ${\rho }_0$ being the resistivity at zero field. We obtained experimentally and also derived from the observed scaling the magnetic field dependence of the turn-on temperature $T^{*}\sim {(H-H_c)}^{\nu }$ with $\nu \approx 1/2$, which was earlier used as evidence for a predicted metal-insulator transition. The scaling also leads to a simple quantitative expression for the resistivity ${\rho }^{*}\approx 2{\rho }_0$ at the onset of the XMR behavior, which fits the data remarkably well. These results exclude the possible existence of a magnetic-field-driven metal-insulator transition or significant contribution of an electronic structure change to the low-temperature XMR in ${\mathrm{WTe}}_2$. This work resolves the origin of the turn-on behavior observed in several XMR materials and also provides a general route for a quantitative understanding of the temperature dependence of MR in both XMR and non-XMR materials.

We reveal that three-dimensional multiorbital topological superconductivity can be identified by a bulk measurement, i.e., the temperature dependence of nuclear magnetic relaxation (NMR) rates. Below a critical temperature $T_{\mathrm{c}}$, the NMR rate in the topological state exhibits an antipeak profile, which is opposite to the conventional $s$-wave state. This inversion coherence effect comes from a twist of order parameters with respect to orbital and spin degrees of freedom. Our self-consistent calculations in the model for ${\mathrm{Cu}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$ prove that the inverse coherence effect appears as a concave temperature dependence of the NMR rates. We propose that a time-reversal-invariant orbital-singlet spin-triplet topological superconductivity is characterized by the temperature dependence of the NMR rate.

Evolution of the ESR absorption in a strong-leg spin ladder magnet ${\left({\mathrm{C}}_7{\mathrm{H}}_{10}\text{N}\right)}_2{\mathrm{CuBr}}_4$ (abbreviated as DIMPY) is studied from 300 K to 400 mK. Temperature dependence of the ESR relaxation follows a staircase of crossovers between different relaxation regimes. We argue that the main mechanism of ESR line broadening in DIMPY is uniform Dzyaloshinskii-Moriya interaction $(\left|\mathbf{D}\right|=0.31$ K) with an effective longitudinal component along an exchange bond of Cu ions within the legs resulting from the low crystal symmetry of DIMPY and nontrivial orbital ordering. The same Dzyaloshinskii-Moriya interaction along with other weaker anisotropic spin-spin interactions results in the lifting of the triplet excitation degeneracy, revealed through the weak splitting of the ESR absorption at low temperatures.

Two-dimensional systems with $U\left(1\right)$ symmetry exhibit a peculiar phase, i.e., the Berezinskii-Kosterlitz-Thouless (BKT) phase. In particular situations, the BKT phase exists as an intermediate-temperature phase. There have been scenarios for the phase transitions at the two end points of the intermediate BKT phase; i.e., the phase transition at the low-temperature end point is a BKT transition and that at the high-temperature end point is either a BKT transition or a first-order transition. The present study gives a novel scenario, i.e., a second-order transition with a new critical universality and a BKT transition. We found that this new phase transition is realized in spin-crossover systems on a triangular lattice with an antiferromagnetic short-range interaction. At the low-temperature transition the elastic interaction plays as a ferromagnetic infinite-range interaction and encourages the breaking of $Z_2$ symmetry between high-spin-rich and low-spin-rich states.

We investigate the existence and origin of magneto-optical anisotropy in epitaxial hcp Co films. Our results show that a significant magneto-optical anisotropy exists in our samples and, more importantly, they reveal that its amplitude is directly correlated with epitaxial strain. We find a linear coefficient of $16.8\%$ magneto-optical anisotropy per every 1% epitaxial strain, which is in stark contrast to an isotropic magneto-optical coupling factor $Q$, a very frequent and common assumption in magneto-optics of metallic thin films and multilayers. In addition, the Co films exhibit a similar strain-induced increase of the magnetocrystalline anisotropy energy, evidencing the fact that both magneto-optical anisotropy and magnetocrystalline anisotropy are dependent on the modification of the spin-orbit coupling introduced by anisotropic lattice distortions.

Electron transport through a quantum dot coupled to superconducting leads shows a sharp conductance onset when a quantum dot orbital level crosses the superconducting coherence peak of one lead. We study superconducting single electron transistors in the weak coupling limit by connecting individual gold nanoparticles with aluminum leads formed by electromigration. We show that the transport features close to the conductance onset threshold can be accurately described by the quantum dot levels' hybridization with the leads, which is strongly enhanced by the divergent density of states at the superconducting gap edge. This highlights the importance of electron cotunneling effects in spectroscopies with superconducting probes.

The “$d$-wave” symmetry of the superconducting order in the cuprate high temperature superconductors is a well established fact [J. Tsuei and J. R. Kirtley, Rev. Mod. Phys. 72, 969 (2000) and D. J. Vanharlingen, Rev. Mod. Phys. 67, 515 (1995)], and one which identifies them as “unconventional.” However, in macroscopic contexts—including many potential applications (i.e., superconducting “wires”)—the material is a composite of randomly oriented superconducting grains in a metallic matrix, in which Josephson coupling between grains mediates the onset of long-range phase coherence. [See, e.g., D. C. Larbalestier et al., Nat. Mater. 13, 375 (2014), A. P. Malozemoff, MRS Bull. 36, 601 (2011), and K. Heine et al., Appl. Phys. Lett. 55, 2441 (1989)] Here we analyze the physics at length scales that are large compared to the size of such grains, and in particular the macroscopic character of the long-range order that emerges. While $XY$-superconducting glass order and macroscopic $d$-wave superconductivity may be possible, we show that under many circumstances—especially when the $d$-wave superconducting grains are embedded in a metallic matrix—the most likely order has global $s$-wave symmetry.

Nonresonant inelastic x-ray scattering (NIXS) is a possible tool to detect charge excitations in electron systems. In addition, multipole transitions at high-momentum-transfer regions open a new possibility to determine orbital-selective electronic excitations in multiorbital itinerant $3d$ electron systems. As a theoretical example, we choose the antiferromagnetic state of iron arsenides and demonstrate that the orbital-selective excitations are detectable by choosing appropriate momentum transfer in NIXS. We propose that both NIXS and resonant inelastic x-ray scattering are complementary to each other for fully understanding the nature of orbital excitations in multiorbital itinerant electron systems.

We present a comprehensive study of the thermodynamic and electronic properties of intrinsic point defects in the solar energy conversion materials $\mathrm{C}{\mathrm{u}}_2\mathrm{ZnSnS}{\mathrm{e}}_4$ and $\mathrm{CuInS}{\mathrm{e}}_2$ based on the screened-exchange hybrid density functional theory. A comparison between the defect transition levels for $\mathrm{C}{\mathrm{u}}_2\mathrm{ZnSnS}{\mathrm{e}}_4$ and $\mathrm{CuInS}{\mathrm{e}}_2$ reveals that in $\mathrm{C}{\mathrm{u}}_2\mathrm{ZnSnS}{\mathrm{e}}_4$, the $\mathrm{S}{\mathrm{n}}_{\mathrm{Cu}}$ and $\mathrm{S}{\mathrm{n}}_{\mathrm{Zn}}$ antisite defects can be recombination centers with defect states close to midgap, while the $\mathrm{I}{\mathrm{n}}_{\mathrm{Cu}}$ antisite defect has a shallow defect level in $\mathrm{CuInS}{\mathrm{e}}_2$. The resultant higher Shockley-Read-Hall recombination rate in $\mathrm{C}{\mathrm{u}}_2\mathrm{ZnSnS}{\mathrm{e}}_4$ reduces the steady-state concentration of minority carriers and quasi-Fermi level separation under illumination. This may explain the origin of the low open-circuit voltage values for $\mathrm{C}{\mathrm{u}}_2\mathrm{ZnSnS}{\mathrm{e}}_4$ solar cells compared to $\mathrm{CuInS}{\mathrm{e}}_2$ solar cells.

We report on the ground- and excited-state properties of ${\mathrm{Fe}}^{3+}$ centers in hydrothermally and chemical-vapor-transport grown single ZnO crystals studied by continuous-wave electron-paramagnetic resonance (EPR) under dark and laser-illuminated conditions, pulsed-EPR and magneto-photoluminescence. By use of EPR experiments, the fine-structure parameters of the ${\mathrm{Fe}}^{3+}$ spin Hamiltonian are determined. Three types of charge-compensated ${\mathrm{Fe}}^{3+}$ centers are identified and the charge conversion from ${\mathrm{Fe}}^{2+}$ to ${\mathrm{Fe}}^{3+}$ is highlighted. The magneto-optical studies of the Zeeman components of the spin-forbidden electric-dipole transitions from excited ${}^{4}T_1\left(G\right)$ to ground ${}^{6}A_1\left({}^{6}S\right)$ states of the ${\mathrm{Fe}}^{3+}$ center indicate the trigonal symmetry of the fine structure of the lowest ${\mathrm{\Gamma }}_8\left({}^{4}T_1\right)$ excited state. The energy positions of the Zeeman components are measured in the external magnetic field of 8 T rotated in $\left(1\overline{2}10\right)$ and (0001) crystal planes. The angular variation of the Zeeman lines exhibits two magnetically nonequivalent ${\mathrm{Fe}}^{3+}$ centers. These features result from the contribution of high-rank Zeeman terms of dimension $\mathbf{B}{\mathbf{J}}^3$ in the spin Hamiltonian. For the electron spin $S=5/2$ system of the trigonal ${\mathrm{Fe}}^{3+}$ ion, we further demonstrate the tuning of one-photon Rabi oscillations by means of electron spin-echo measurements.

We realize a nanoscale-area Mach-Zehnder interferometer with co-propagating quantum Hall spin-resolved edge states and demonstrate the persistence of gate-controlled quantum interference oscillations, as a function of an applied magnetic field, at relatively large temperatures. Arrays of top-gate magnetic nanofingers are used to induce a resonant charge transfer between the pair of spin-resolved edge states. To account for the pattern of oscillations measured as a function of magnetic field and gate voltage, we have developed a simple theoretical model which satisfactorily reproduces the data.

Quantum point contacts (QPCs) and quantum dots (QDs), two elementary building blocks of semiconducting nanodevices, both exhibit famously anomalous conductance features: the 0.7 anomaly in the former case, the Kondo effect in the latter. For both the 0.7 anomaly and the Kondo effect, the conductance shows a remarkably similar low-energy dependence on temperature $T$, source-drain voltage $V_{\mathrm{sd}}$, and magnetic field $B$. In a recent publication [F. Bauer et al., Nature (London) 501, 73 (2013)], we argued that the reason for these similarities is that both a QPC and a Kondo QD (KQD) feature spin fluctuations that are induced by the sample geometry, confined in a small spatial regime, and enhanced by interactions. Here, we further explore this notion experimentally and theoretically by studying the geometric crossover between a QD and a QPC, focusing on the $B$-field dependence of the conductance. We introduce a one-dimensional model with local interactions that reproduces the essential features of the experiments, including a smooth transition between a KQD and a QPC with 0.7 anomaly. We find that in both cases the anomalously strong negative magnetoconductance goes hand in hand with strongly enhanced local spin fluctuations. Our experimental observations include, in addition to the Kondo effect in a QD and the 0.7 anomaly in a QPC, Fano interference effects in a regime of coexistence between QD and QPC physics, and Fabry-Perot-type resonances on the conductance plateaus of a clean QPC. We argue that Fabry-Perot-type resonances occur generically if the electrostatic potential of the QPC generates a flatter-than-parabolic barrier top.

We study x-ray absorption spectra of azobenzene-functionalized self-assembled monolayers (SAMs), investigating excitations from the nitrogen $K$ edge. Azobenzene with H-termination and functionalized with ${\mathrm{CF}}_3$ groups is considered. The Bethe-Salpeter equation is employed to compute the spectra, including excitonic effects, and to determine the character of the near-edge resonances. Our results indicate that core-edge excitations are intense and strongly bound: their binding energies range from about 6 to 4 eV, going from isolated molecules to densely-packed SAMs. Electron-hole correlation rules these excitations, while the exchange interaction plays a negligible role.

We have characterized the structure of 176 different single-layer graphene grain boundaries grown with chemical vapor deposition using $>1000$ experimental high-resolution transmission electron microscopy images using a semiautomated structure processing routine. We introduce an algorithm for generating grain boundary structures for a class of hexagonal two-dimensional materials and use this algorithm and molecular dynamics to simulate the structure of $>79000$ linear graphene grain boundaries covering 4122 unique orientations distributed over the entire parameter space. The dislocation content and structural properties are extracted from all experimental and simulated boundaries, and various trends are explored. We find excellent agreement between the simulated and experimentally observed grain boundaries. Our analysis demonstrates the power of a statistically significant number of measurements as opposed to a small number of observations in atomic science.