Duplex materials constituted by 0.50 mol titania-doped yttria-stabilized zirconia (YSZ) and 0.50 mol titania-doped yttria-tetragonal zirconia polycrystalline (YTZP) solid solutions in the ZrO${}_2$-Y${}_2$O${}_3$-TiO${}_2$ ternary system can be obtained using different processing strategies. In this study, different amounts of TiO${}_2$ dopant and different sintering times have been used for the preparation of the duplex materials: Doping YSZ with 10 mol% of TiO${}_2$ and sintering in air for 8 h (10Ti8h) and doping YSZ with 15 mol% of TiO${}_2$ and sintering in air for 2 h (15Ti2h) are both successful routes to obtain duplex materials. If we compare the field emission scanning electron microscopy-energy dispersive x-ray analyses of each cubic and each tetragonal solid solution of the two duplex materials, we conclude that the composition of each phase is different from each other, as was expected. The total ionic conductivity of both duplex samples is strongly reduced with respect to that of YSZ, and this reduction increases with the Ti content. In addition, the activation energy for ionic migration in 10Ti8h and 15Ti2h is lower than that for YSZ but higher than that for YTZP, as expected in terms of the relative amount of both YSZ and YTZP fractions in the materials. X-ray absorption spectroscopy (XAS) results, both x-ray absorption near-edge structure and extended x-ray absorption fine structure, show that Ti is sixfold coordinated in both the cubic and tetragonal phases of the duplex materials, departing from the eightfold coordination expected if a simple substitution at the Zr sites would take place. The XAS results also point out that the ability of Ti to trap oxygen vacancies in the cubic phase increases as Ti content does, in agreement with the electrical conductivity behavior, which cannot be accounted for in terms of a dilution effect.

Recent highly idealized model studies of lubricated nanofriction for two crystalline sliding surfaces with an interposed thin solid crystalline lubricant layer showed that the overall relative velocity of the lubricant $v_{\mathrm{lub}}/v_{\mathrm{slider}}$ depends only on the ratio of the lattice spacings, and retains a strictly constant value even when system parameters are varied within a wide range. This peculiar “quantized” dynamical locking was understood as due to the sliding-induced motion of misfit dislocations, or soliton structures. So far the practical relevance of this concept to realistic sliding three-dimensional crystals has not been demonstrated. In this work, by means of classical molecular dynamics simulations and theoretical considerations, we realize a realistic three-dimensional crystal-lubricant-crystal geometry. Results show that the flux of lubricant particles associated with the advancing soliton lines gives rise here too to a quantized-velocity ratio. Moreover, depending on the interface lattice spacing mismatch, both forward and backward quantized motion of the lubricant is predicted. The persistence under realistic conditions of the dynamically pinned state and quantized sliding is further investigated by varying sliding speed, temperature, load, and lubricant film thickness. The possibilities of experimental observation of quantized sliding are also discussed.

Magnetic correlations of two isostructural quasi-one-dimensional (1D) antiferromagnetic spin-chain compounds Sr$M_2$V${}_2$O${}_8$ (M = Co, Mn) have been investigated by magnetization and powder neutron diffraction. Two different collinear antiferromagnetic (AFM) structures, characterized by the propagation vectors, $k=$ (0 0 1) and $k$ = (0 0 0), have been found below $\sim$5.2 and $\sim$42.2 K for the Co and Mn compounds, respectively. For the Mn compound, AFM chains (along the $c$ axis) order ferromagnetically within the $ab$ plane, whereas, for the Co compound, AFM chains order ferromagnetically/antiferromagnetically along the a/b direction. The critical exponent study confirms that the Co and Mn compounds belong to the Ising and Heisenberg universality classes, respectively. For both compounds, short-range spin-spin correlations are present over a wide temperature range above $T_N$. The reduced ordered moments at base temperature (1.5 K) indicate the presence of quantum fluctuations in both compounds due to the quasi-1D magnetic interactions.

We report a systematic study on the low-temperature thermal conductivity ($\kappa$) of $R_2$Ti${}_2$O${}_7$ ($R$ = Gd and Er) single crystals with different directions of magnetic field and heat current. It is found that the magnetic excitations mainly act as phonon scatterers rather than heat carriers, although these two materials have long-range magnetic orders at low temperatures. The low-$T$ $\kappa \left(H\right)$ isotherms of both compounds show rather complicated behaviors and have good correspondences with the magnetic transitions, where the $\kappa \left(H\right)$ curves show drastic dip- or steplike changes. In comparison, the field dependencies of $\kappa$ are more complicated in Gd${}_2$Ti${}_2$O${}_7$, due to the complexity of its low-$T$ phase diagram and field-induced magnetic transitions. These results demonstrate the significant coupling between spins and phonons in these materials and the ability of heat-transport properties probing the magnetic transitions.

We analyze the ground-state properties of mixtures consisting of scalar bosons and spin-$\frac{1}{2}$ fermions using a mean-field treatment of the local boson-fermion interaction on a simple cubic lattice. In the deep superfluid limit of the boson sector and the BCS regime of the fermion sector, we derive BCS-type equations to determine the phase diagram of the system. We find a competition between a charge density wave and a superconducting phase. In the opposite limit, we study the Mott-insulator-to-superfluid transition of the boson sector in the presence of a staggered density-induced alternating potential provided by the fermions, and determine the mean-field transition line. In the two-superfluids phase of the mixture, we restrict to nearest-neighbor-induced interactions between the fermions and consider the extended Hubbard model. We perform a mean-field analysis of the critical temperature for the formation of boson-assisted $s$-, extended $s^{-}$-, $d$-, and $p$-wave pairs at fermionic half-filling. We compare our results with a recent dynamical mean-field study [P. Anders et al., Phys. Rev. Lett. 109, 206401 (2012)].

The morphotropic phase boundary (MPB) has been utilized extensively in ferroelectrics and recently has attracted interest in ferromagnets [S. Yang, H. Bao, C. Zhou, Y. Wang, X. Ren, Y. Matsushita, Y. Katsuya, M. Tanaka, K. Kobayashi, X. Song, and J. Gao, Phys. Rev. Lett. 104, 197201 (2010); R. Bergstrom, M. Wuttig, J. Cullen, P. Zavalij, R. Briber, C. Dennis, V. O. Garlea, and M. Laver, Phys. Rev. Lett. 111, 017203 (2013)] for obtaining enhanced large field-induced strain. Here we report that the MPB can also lead to weakening (the inverse effect as compared to the known MPB materials) of field-induced strain, as exhibited in the Tb${}_{1-x}$Gd${}_x$Co${}_2$ system. With synchrotron x-ray diffractometry, the structure symmetry of TbCo${}_2$-rich compositions is detected to be rhombohedral below T${}_C$ and that of GdCo${}_2$-rich compositions is tetragonal. The MPB composition Tb${}_{0.1}$Gd${}_{0.9}$Co${}_2$, corresponding to the two phases (rhombohedral and tetragonal) of coexistence, shows the exotic minimum (near zero) magnetostriction as well as the largest magnetic susceptibility among all samples. Further analysis suggests that whether MPB can enhance or weaken magnetostriction is determined by the degree of magnetic ordering of two end members that form ferromagnetic MPBs, which was not considered previously. Our work not only reveals a new type of ferromagnetic MPB, but also provides a new recipe for designing functional high-susceptibility and low-strain magnetic materials.

Due to the large spin-orbital coupling in the layered $5d$-transition metal chalcogenides compound, the occurrence of superconductivity in doped Ir${}_{2-x}$Pd${}_x$Te${}_2$ offers a good chance to search for possible topological superconducting states in this system. We did comprehensive studies on the superconducting properties and electronic structures of single crystalline Ir${}_{0.95}$Pd${}_{0.05}$Te${}_2$ samples. The superconducting gap size, critical fields, and coherence length along different directions were experimentally determined. Macroscopic bulk measurements and microscopic low temperature scanning tunneling spectroscopy results suggest that Ir${}_{0.95}$Pd${}_{0.05}$Te${}_2$ possesses a BCS-like $s$-wave state. No sign of zero bias conductance peak was found in the vortex core at 0.4 K.

We study the mechanism of the $B2\to R$ martensitic transformation in the shape memory alloy AuZn by means of first-principles theory. Phonon anomalies in the TA${}_2$ acoustic branch along the $\Gamma$-$M$ [$\xi$,$\xi$,0] direction associated with a structural transformation are observed. The calculated Fermi surface of the $B2$ phase of AuZn reveals large portions nested with each other by a translation through a vector $\mathbf{q}=\frac{1}{3}[1,1,0]$ associated with the soft mode. In addition, we find that the $B2$ phase can be stabilized by pressure in the low-temperature limit. The energetic barrier for the $B2\to R$ transition is 2 mRy and appears to be near a critical point.

We study the fate of the Kondo effect with one-dimensional conduction baths at very low densities, such that the system explores the bottom of the conduction band. This can involve either finite low densities or a small number of fixed conduction electrons in a large system; i.e., the limit of large bath sizes can be taken with either fixed small density or with fixed number. We characterize the Kondo physics for such systems through the energy gain due to Kondo coupling, which is a general analog of the Kondo temperature scale, and through real-space profiles of densities and spin-spin correlation functions.

We present a combined density functional perturbation theory and inelastic neutron scattering study of the lattice dynamical properties of YNi${}_2$B${}_2$C. In general, very good agreement was found between theory and experiment for both phonon energies and line widths. Our analysis reveals that the strong coupling of certain low energy modes is linked to the presence of large displacements of the light atoms, i.e., B and C, which is unusual in view of the rather low phonon energies. Specific modes exhibiting a strong coupling to the electronic quasiparticles were investigated as a function of temperature. Their energies and line widths showed marked changes on cooling from room temperature to just above the superconducting transition at $T_c$ = 15.2 K. Calculations simulating the effects of temperature allow us to model the observed temperature dependence qualitatively.

We present a theory describing the trapping of a quasiparticle in a prototypical Josephson junction, a single-channel superconducting weak link. We calculate the trapping and untrapping rates associated to absorption and emission of both photons and phonons. We show that the presence of an electromagnetic mode with frequency smaller than the gap gives rise to a rather abrupt transition between a fast-relaxation regime dominated by coupling to photons and a slow-relaxation regime dominated by coupling to phonons. This conclusion is illustrated by the analysis of a recent experiment [Zgirski, Bretheau, Le Masne, Pothier, Esteve, and Urbina, Phys. Rev. Lett. 106, 257003 (2011)] measuring the dynamics of quasiparticle trapping in a superconducting atomic contact coupled to a Josephson junction. With realistic parameters, the theory provides a semiquantitative description of the experimental results.

In this work we have determined the magnetic structure of $R_2$CoGa${}_8$ ($R=$ Gd, Tb, and Dy) intermetallic compounds using x-ray resonant magnetic scattering in order to study the evolution of the anisotropic magnetic properties along the series for $R=$ Gd-Tm. The three compounds have a commensurate antiferromagnetic spin structure with a magnetic propagation vector $\vec{\tau }=$ $(\frac{1}{2},\frac{1}{2},\frac{1}{2})$ and a Néel temperature of approximately 20, 28.5, and 15.2 K for $R=$ Gd, Tb, and Dy, respectively. The critical exponent $\beta$ obtained from the temperature dependence of the magnetic peaks suggest a three-dimensional universality class for the three compounds. Comparing the simulated and integrated intensities we conclude that the magnetic moment direction is in the $ab$ plane for the Gd${}_2$CoGa${}_8$ compound and parallel to the $c$ axis for the Tb${}_2$CoGa${}_8$ and Dy${}_2$CoGa${}_8$ compounds. The evolution of the magnetic properties of the $R_2$CoGa${}_8$ series for $R=$ Gd-Tm is discussed taking into account the indirect Ruderman-Kittel-Kasuya-Yoshida interaction and crystalline-electric field effects. The comparison between the reported magnetic properties of the Ga-based compounds with those for the In-based isostructural family reveals differences in their exchange couplings that contribute to the understanding of the role of the $f$-electron magnetism in these classes of materials.

We investigate the electron spin relaxation due to the D'yakonov-Perel' and Elliot-Yafet mechanisms, including the intra- and intervalley processes in monolayer MoS${}_2$ in the absence of the external electric field. We construct the effective Hamiltonian for the conduction band using the Löwdin partition method from the anisotropic two-band Hamiltonian with the intrinsic spin-orbit coupling of the conduction band included. The spin-orbit coupling of the conduction band induces the intra- and intervalley D'yakonov-Perel' relaxation for in-plane spins. In addition, the Elliot-Yafet spin relaxation also takes place for in-plane spins due to the interband spin mixing. We find that the D'yakonov-Perel' mechanism dominates the in-plane spin relaxation. In the framework of this mechanism, the intravalley process is shown to play a more important role at low temperature, whereas the intervalley one becomes more important at high temperature. At the temperature in between, the leading process of the in-plane spin relaxation changes from the intervalley one to intravalley one as the electron density increases. Moreover, we find that the intravalley process is dominated by the electron-electron Coulomb scattering even with high impurity density since the dominant term in the spin-orbit coupling is isotropic, which does not lead to the spin relaxation together with the electron-impurity scattering. This is very different from previous studies in semiconductors and graphene. In addition to the in-plane spin relaxation, we also study the out-of-plane one in the presence of an in-plane magnetic field. The out-of-plane spin relaxation time is found to decrease with the increase of the in-plane magnetic field.

The generation of photocurrents due to coupling of electrons to both classical and quantized electromagnetic fields in thin semiconductor films is described within the framework of the nonequilibrium Green's function formalism. For the coherent coupling to classical fields corresponding to single field operator averages, an effective two-time intraband self-energy is derived from a band decoupling procedure. The evaluation of coherent photogeneration is performed self-consistently with the propagation of the fields by using for the latter a transfer matrix formalism with an extinction coefficient derived from the electronic Green's functions. For the “incoherent” coupling to fluctuations of the quantized fields, which need to be considered for the inclusion of spontaneous emission, the first self-consistent Born self-energy is used, with full spatial resolution in the photon Green's functions. These are obtained from the numerical solution of Dyson and Keldysh equations including a nonlocal photon self-energy based on the same interband polarization function as used for the coherent case. A comparison of the spectral and integral photocurrent generation pattern reveals a close agreement between coherent and incoherent coupling for the case of an ultrathin, selectively contacted absorber layer at short circuit conditions.

We study one-dimensional wires with spin-orbit coupling. We show that in the presence of Zeeman field and strong electron-electron interaction a clean wire may form fractional helical liquid states with phenomenology similar to fractional quantum Hall liquids. Most notably, the wire's two-terminal conductance is predicted to show fractional quantized conductance plateaus at low electron density. When the system is proximity coupled to a superconductor, fractional Majorana bound states may be stabilized, giving rise to a one-dimensional analog of non-Abelian anyons. We discuss how disorder destabilizes these fractional phases. Possible experimental realizations of similar states in double wire systems are discussed.

The theory of the electron relaxation in simple metals excited by an ultrashort optical pump is developed on the basis of the solution of the linearized Boltzmann kinetic equation. The kinetic equation includes both the electron-electron and the electron-phonon collision integrals and assumes that Fermi liquid theory is applicable for the description of a simple metal. The widely used two-temperature model follows from the theory as the limiting case when the thermalization due to the electron-electron collisions is fast with respect to the electron-phonon relaxation. It is demonstrated that the energy relaxation has two consecutive processes. The first and most important step describes the emission of phonons by the photoexcited electrons. It leads to the relaxation of 90% of the energy before the electrons become thermalized among themselves. The second step describes electron-phonon thermalization and may be described by the two-temperature model. The second stage is difficult to observe experimentally because it involves the transfer of only a small amount of energy from electrons. Thus the theory explains why the divergence of the relaxation time at low temperatures has never been observed experimentally.

Microwave-induced resistance oscillations (MIRO) have been extensively studied for more than a decade but, until now, have remained unique to GaAs/AlGaAs-based 2D electron systems. Here, we report on the observation of MIRO in a 2D hole gas hosted in Ge/SiGe quantum well. Our findings confirm that MIRO is a universal phenomenon and demonstrate that microwave photoresistance can be utilized to probe the energy spectrum and the correlation effects of 2D holes in Ge/SiGe quantum wells.

Topological insulators support metallic surface states whose existence is protected by the bulk band structure. It was predicted early that the topology of the surface-state Fermi contour should depend on several factors, such as the surface orientation and termination, and this raises the question to what degree a given surface state is protected by the bulk electronic structure upon structural changes. Using tight-binding calculations, we explore this question for the prototypical topological insulator ${\mathrm{Bi}}_{1\text{−}x}$${\mathrm{Sb}}_x$, studying different terminations of the (111) and (110) surfaces. We also consider the implications of the topological protection for (110) surfaces for the semimetals Bi and Sb.

The thermoelectric properties in one- and two-dimensional silicon and germanium structures have been investigated using first-principles density functional techniques and linear response for the thermal and electrical transport. We have considered here the two-dimensional silicene and germanene, together with nanoribbons of different widths. For the nano ribbons, we have also investigated the possibility of nano structuring these systems by mixing silicon and germanium. We found that the figure of merit at room temperature of these systems is remarkably high, up to 2.5.

We study the zero-temperature phase diagram of two-dimensional hard-core bosons on a square lattice with nearest-neighbor and plaquette (ring-exchange) hoppings, at arbitrary densities, by means of a hierarchical mean-field theory. In the frustrated regime, where quantum Monte Carlo suffers from a sign problem, we find a rich phase diagram where exotic states with nonzero chirality emerge. Among them, novel insulating phases, characterized by nonzero bond-chirality and plaquette order, are found over a large region of the parameter space. In the unfrustrated regime, the hierarchical mean-field approach improves over the standard mean-field treatment as it is able to capture the transition from a superfluid to a valence bond state upon increasing the strength of the ring-exchange term, in qualitative agreement with quantum Monte Carlo results.