To investigate the origin of a phase transition at $T_{\mathrm{Q}}=0.06$ K simultaneously occurring with a superconducting transition in a cage compound PrRh${}_2$Zn${}_{20}$, we carried out ultrasonic measurements on a single-crystalline sample. The transverse modulus ($C_{11}-C_{12}$)/2 is intimately coupled to the non-Kramers ground doublet ${\Gamma }_3$, and elastic softening is observed at low temperatures. Below $T_{\mathrm{Q}}$, the softening stops, suggesting the disappearance of quadrupole degrees of freedom. We clarified the negative quadrupole-quadrupole coupling constant and reentrant behavior of $T_{\mathrm{Q}}\left(H\right)$ in a magnetic field $H$. These results reveal that the phase transition at $T_{\mathrm{Q}}$ is antiferroquadrupolar ordering. The anisotropic magnetic field-temperature phase diagram is determined for $H\parallel \left[100\right]$, [110], and [111]. A magnetic-field-induced phase transition is newly found at high fields in all three field directions. We also observed ultrasonic dispersion at around 50 K owing to the rattling motion of Zn atoms at the 16$c$ site, and pointed out the strong electron-phonon coupling in PrRh${}_2$Zn${}_{20}$.

The metallic surface state of a topological insulator (TI) is not only topologically protected, but it exhibits a remarkable property of inducing an effective vector potential on curved surfaces. For an electron in the surface state of a spherical or a cylindrical TI (TI nanoparticle or nanowire), a pseudomagnetic monopole or a fictitious solenoid is effectively induced, encoding the geometry of the system. Here, by using a hyperbolic surface as an example, we demonstrate that as a consequence of this property stemming from its active spin degree of freedom, the surface state is by itself topologically protected.

Typical empirical bond-order potentials are short ranged and give ductile instead of brittle behavior for materials such as crystalline silicon or diamond. Screening functions can be used to increase the range of these potentials. We outline a general procedure to combine screening functions with bond-order potentials that does not require refitting any of the potential's properties. We use this approach to modify J. Tersoff's [Phys. Rev. B 39, 5566 (1989)], P. Erhart and K. Albe's [Phys. Rev. B 71, 35211 (2005)], and T. Kumagai et al.'s [Comput. Mater. Sci. 39, 457 (2007)] Si, C, and Si-C potentials. The resulting potential formulations correctly reproduce brittle materials’ response and give an improved description of amorphous phases.

We report the experimental determination of Rashba spin-orbit interaction (SOI) strength in two-dimensional electrons in a MgZnO/ZnO heterostructure using electrically detected electron spin resonance. The Rashba parameter is determined to be $7.0\times {10}^{-14}$ eV m, which is the second smallest value among prevalent semiconductor heterostructures, following Si/SiGe. A long transverse spin relaxation time was derived to show a maximum value of 27 ns, owing to weak SOI. Our study demonstrates that the ZnO heterostructure is a promising candidate for spintronic devices utilizing long spin coherence.

We study the reversal mechanism between opposite closed flux states of ferromagnetic nanorings driven by an azimuthal magnetic field. The reversal proceeds via the formation of ${360}^{\circ }$ domain walls, and we show that the role of interacting nucleation sites is essential for the process to take place. Such nucleation is seen to create domain walls with the right topological charge conditions for ${360}^{\circ }$ domain-wall formation. Given the symmetry of the system, we utilize an energetic description as a function of the azimuthal field magnitude, which clearly reveals the different stages of this reversal process. The annihilation of the ${360}^{\circ }$ domain walls that is necessary for the reversal process to complete is controlling the field value at the final stage of the process. Such a fundamental mechanism for ring reversal has several implications and will guide the design of the various data-storage-device proposals based on nanorings.

We theoretically investigate current-induced magnetization dynamics in chiral-lattice helimagnets. Spin-orbit coupling in noncentrosymmetric crystals induces a reactive spin-transfer torque that has not been previously considered. We demonstrate how the torque is governed by the crystal symmetry and acts as an effective magnetic field along the current direction in cubic B20-type crystals. The effects of the new torque are computed for current-induced dynamics of spin spirals and the Doppler shift of spin waves. In current-induced spin-spiral motion, the new torque tilts the spiral structure. The spin waves of the spiral structure are initially displaced by the new torque, while the dispersion relation is unaffected.

ZrB${}_2$ is a nonsuperconducting Pauli paramagnetic crystallizing in the AlB${}_2$ structure. V substitution for Zr in Zr${}_{1-x}$V${}_x$B${}_2$ (0.01 $\le$ $x$ $\le$ 0.1) at the few percentage level induces superconductivity with critical temperature reaching a maximum of 8.7 K for Zr${}_{0.96}$V${}_{0.04}$B${}_2$ near the solid solubility limit. Specific-heat and lower critical field temperature dependence results suggest the possibility of unconventional superconductivity possibly arising from multiband effects.

Crystal structures of metals are often treated as dense packing of atomic spheres. Face-centered cubic and hexagonal close-packed structures are favored in many metals. Long-period-stacking structures such as 9$R$ are sometimes formed. However, nonclose packed structures such as body centered cubic and $\omega$ are formed depending upon chemistry and process conditions. Even in metallic elements, it is a priori unknown how such close/nonclose packed structures are formed and what are their interrelationships. In the present study we show a simple algorithm for automated searching of the phase-transition pathway based upon first-principles calculations, which is applied to systematically pursue the evolution of crystal structures. Following the present algorithm, dynamical stability and interrelationships of different structures generated from a simple cubic structure are revealed for seven metallic elements. Effects of pressure are examined as well. The powerfulness of the automated method to investigate the nature of the phase transition and to predict as-yet-unknown metastable structures is demonstrated.

We demonstrate that the inclusion of van der Waals dispersive interaction sensibly improves the prediction of lattice constants by density functional theory in iron-chalcogenides (FeCh) superconductor compounds, namely FeSe and FeTe. We show how generalized gradient approximation (GGA) for the exchange correlation potential overestimates the out-of-plane lattice constants in both compounds when compared with experiments. In addition, GGA predicts a too weak bonding between the neutral FeCh layers, with a sensible underestimation of the bulk modulus. van der Waals corrected simulations completely solve both problems, reconciling theoretical results with experiments. These findings must be considered when dealing with theoretical predictions in FeCh compounds.

The intramolecular exchange interactions within the single-molecule magnet (SMM) “butterfly” molecule [Fe${}_3$Ln(${\mu }_3$-O)${}_2$(CCl${}_3$COO)${}_8$(H${}_2$O)(THF)${}_3$], where Ln(III) represents a lanthanide cation, are determined in a combined experimental [x-ray magnetic circular dichroism (XMCD) and vibrating sample magnetometer (VSM)] and theoretical work. Compounds with $\mathrm{Ln}=\mathrm{Gd}$ and Dy, which represent extreme cases where the rare earth presents single-ion isotropic and uniaxial anisotropy, on one hand, and with $\mathrm{Ln}=\mathrm{Lu}$ and Y(III) as pseudolanthanide substitutions that supply a nonmagnetic Ln reference case, on the other hand, are studied. The Dy single-ion uniaxial anisotropy is estimated from ab initio calculations. Low-temperature ($T\simeq 2.5$ K) hard x-ray XMCD at the Ln L${}_{2,3}$ edges and VSM measurements as a function of the field indicate that the Ln moment dominates the polarization of the molecule by the applied field. Within the $\{{\mathrm{Fe}}_3$LnO${}_2\}$ cluster the Ln-Fe${}_3$ subcluster interaction is determined to be antiferromagnetic in both Dy and Gd compounds, with values $J_{\mathrm{Dy}-{\mathrm{Fe}}_3}=-0.4$ K and $J_{\mathrm{Gd}-{\mathrm{Fe}}_3}=-0.25$ K, by fitting to spin Hamiltonian simulations that consider the competing effects of intracluster interactions and the external applied magnetic field. In the uniaxial anisotropic $\{{\mathrm{Fe}}_3$DyO${}_2\}$ case, a field-induced reorientation of the Fe${}_3$ and Dy spins from an antiparallel to a parallel orientation takes place at a threshold field (${\mu }_{0}H=4$ T). In contrast, in isotropic $\{{\mathrm{Fe}}_3$GdO${}_2\}$ this reorientation does not occur.

A one-dimensional model of coupled spin-1/2 spins and pseudospin-1/2 orbitals with nearest-neighbor interaction is rigorously shown to exhibit spin-orbital separation by means of a nonlocal unitary transformation. On an open chain, this transformation completely decouples the spins from the orbitals in such a way that the spins become paramagnetic while the orbitals form the soluble XXZ Heisenberg model. The nature of various correlations is discussed. The more general cases, which allow spin-orbital separation by the same method, are pointed out. A generalization for the orbital pseudospin greater than 1/2 is also discussed. Some qualitative connections are drawn with the recently observed spin-orbital separation in Sr${}_2$CuO${}_3$.

To explore the driving mechanisms of the metal-insulator transition (MIT) and the structural transition in VO${}_2$, we have investigated phonon dispersions of rutile VO${}_2$ ($R$-VO${}_2$) in the density functional theory (DFT) and the DFT$+U$ ($U$: Coulomb correlation) band calculations. We have found that the phonon softening instabilities occur in both cases, but the softened phonon mode only in the DFT$+U$ describes properly both the MIT and the structural transition from $R$-VO${}_2$ to monoclinic VO${}_2$ (M${}_1$-VO${}_2$). The present ab initio phonon dispersion calculations clearly demonstrate that the Coulomb correlation effect plays an essential role of assisting the Peierls transition in $R$-VO${}_2$ and producing the spin-Peierls ground state in M${}_1$-VO${}_2$.

We present an experimental study of electron-electron exchange interaction in self-assembled CdTe/ZnTe quantum dots based on the photoluminescence measurements. The character and strength of this interaction are obtained by simultaneous observation of various recombination channels of a doubly negatively charged exciton X${}^{2-}$, including previously unrecognized emission lines related to the electron-singlet configuration in the final state. A typical value of the electron singlet-triplet splitting, which corresponds to the exchange integral of electron-electron interaction, has been determined as 20.4 meV with a spread of 1.4 meV across the wide population of quantum dots. We also evidence an unexpected decrease of energy difference between the singlet and triplet states under a magnetic field in Faraday geometry.

We show the growth of high-quality epitaxial graphene on 6$H$-SiC with Raman signatures comparable to exfoliated flakes. We ascribe the remaining low-quality transport properties to the strong electron-phonon coupling to two low-energy phonon modes at 70 and 16 meV. The coupling of these modes is enhanced by the defects present in the SiC substrate and buffer layer. Measurements of the mobility versus carrier concentration show a square-root dependence, corroborating the importance of surface phonon scattering in the limited mobility of graphene on SiC.

Electron-electron interactions are responsible for a correction to the conductance of a diffusive metal, the “Altshuler-Aronov correction” $\delta G_{\mathrm{AA}}$. Here, we study the counterpart of this correction for a ballistic conductor, in which the electron motion is governed by chaotic classical dynamics. In the ballistic conductance, the Ehrenfest time ${\tau }_{\mathrm{E}}$ enters as an additional time scale that determines the magnitude of quantum interference effects. The Ehrenfest time effectively poses a short-time threshold for the trajectories contributing to the interaction correction. As a consequence, $\delta G_{\mathrm{AA}}$ becomes exponentially suppressed if the Ehrenfest time is larger than the dwell time or the inverse temperature. We discuss the explicit dependence on Ehrenfest time in quasi-one- and two-dimensional antidot arrays. For strong interactions, the sign of $\delta G_{\mathrm{AA}}$ may change as a function of temperature for temperatures in the vicinity of $\hslash /{\tau }_{\mathrm{E}}$.

We develop a technique based on density functional theory and the projector augmented wave method in order to obtain the x-ray absorption cross section at a general edge, both in the electric dipole and quadrupole approximations. The method is a generalization of Taillefumier et al., [Phys. Rev. B 66, 195107 (2002)]. We apply the method to the calculation of the Cu $L$${}_{2,3}$ edges in fcc copper and cuprite (Cu${}_2$O), and to the S $L$${}_{2,3}$ edges in molybdenite (2H-MoS${}_2$). The role of core-hole effects, modeled in a supercell approach, as well as the decomposition of the spectrum into different angular momentum channels are studied in detail. In copper we find that the best agreement with experimental data is obtained when core-hole effects are neglected. On the contrary, core-hole effects need to be included both in Cu${}_2$O and 2H-MoS${}_2$. Finally we show that a non-negligible component of S $L$${}_{2,3}$ edges in 2H-MoS${}_2$ involves transition to states with $s$ character at all energy scales. The inclusion of this angular momentum channel is mandatory to correctly describe the angular dependence of the measured spectra. We believe that transitions to $s$ character states are quantitatively significant at the $L$${}_{2,3}$ edges of third row elements from Al to Ar.