${\text{CaFe}}_2{\text{As}}_2$ single crystals under uniaxial pressure applied along the $c$ axis exhibit the coexistence of several structural phases at low temperatures. We show that the room-temperature tetragonal phase is stabilized at low temperatures for pressures above 0.06 GPa, and its weight fraction attains a maximum in the region where superconductivity is observed under applied uniaxial pressure. Simultaneous resistivity measurements strongly suggest that this phase is responsible for the superconductivity in ${\text{CaFe}}_2{\text{As}}_2$ found below 10 K in samples subjected to nonhydrostatic pressure conditions.

We measured the magnetic torque of the ring-shaped crystals of ${\text{TaSe}}_3$ by using piezoresistive cantilevers to investigate the superconducting topological properties. We measured three ring samples and we observed that the magnetic torque of the ring crystals oscillates with increasing external magnetic field. The magnetic periods of Sample A (radius: $37.9\mu \text{m}$), Sample B $\left(24.5\mu \text{m}\right)$, and Sample C $\left(15.6\mu \text{m}\right)$ were $2.16\pm 6.6\times {10}^{-2}\text{G}$, $4.69\pm 1.8\times {10}^{-1}\text{G}$, and $5.44\pm 3.8\times {10}^{-1}\text{G}$, respectively. We found that hundreds of vortices collectively and simultaneously penetrated the ring crystal. When such a phenomenon occurs, it is natural that the vortices in the ring crystal would be positioned along the circumference. From these results, we suggest that vortices exist as cylinder vortices in the rings, and this phenomenon provides unambiguous experimental evidence of the topological effect in a superconductor.

We present neutron-scattering measurements of the static structure factor, $S\left(Q\right)$, of helium confined in the porous media MCM-41 of pore diameter $47\pm 1.5\text{Å}$. The $S\left(Q\right)$ shows a transition from the liquid to an amorphous solid as temperature is decreased. No Bragg peaks are observed in the solid and the $S\left(Q\right)$ of the amorphous solid differs little from that of the liquid. On freezing, a small additional intensity in $S\left(Q\right)$ near the main peak of the liquid $S\left(Q\right)$ is observed. The $S\left(Q\right)$ is compared with simulations of freezing and melting in porous media. From the measurements of freezing, an approximate phase diagram is determined. A similar amorphous $S\left(Q\right)$ is observed in $34\text{Å}$ gelsil.

The parent compounds of iron-arsenide superconductors, $A{\text{Fe}}_2{\text{As}}_2$ $\left(A=\text{Ca},\text{Sr},\text{Ba}\right)$, undergo a tetragonal to orthorhombic structural transition at a temperature $T_{\text{TO}}$ in the range 135–205 K depending on the alkaline-earth element. Below $T_{\text{TO}}$ the free standing crystals split into equally populated structural domains, which mask intrinsic, in-plane, anisotropic properties of the materials. Here we demonstrate a way of mechanically detwinning ${\text{CaFe}}_2{\text{As}}_2$ and ${\text{BaFe}}_2{\text{As}}_2$. The detwinning is nearly complete, as demonstrated by polarized light imaging and synchrotron x-ray measurements, and reversible, with twin pattern restored after strain release. Electrical resistivity measurements in the twinned and detwinned states show that resistivity, $\rho$, decreases along the orthorhombic $a_o$ axis but increases along the orthorhombic $b_o$ axis in both compounds. Immediately below $T_{\text{TO}}$ the ratio ${\rho }_{bo}/{\rho }_{ao}=1.2$ and 1.5 for Ca and Ba compounds, respectively. Contrary to ${\text{CaFe}}_2{\text{As}}_2$, ${\text{BaFe}}_2{\text{As}}_2$ reveals an anisotropy in the nominally tetragonal phase, suggesting that either fluctuations play a larger role above $T_{\text{TO}}$ in ${\text{BaFe}}_2{\text{As}}_2$ than in ${\text{CaFe}}_2{\text{As}}_2$ or that there is a higher temperature crossover or phase transition.

We investigate combined effects of spin-orbit coupling and magnetic field in carbon nanotubes containing one or more bends along their length. We show how bends can be used to provide electrical control of confined spins, while spins confined in straight segments remain insensitive to electric fields. Device geometries that allow general rotation of single spins are presented and analyzed. In addition, capacitive coupling along bends provides coherent spin-spin interaction, including between otherwise disconnected nanotubes, completing a universal set of one- and two-qubit gates.

Interferometry of non-Abelian edge excitations is a useful tool in topological quantum computing. In this paper we present a theory of a non-Abelian edge-state interferometer in a three-dimensional topological insulator brought in proximity to an $s$-wave superconductor. The non-Abelian edge excitations in this system have the same statistics as in the previously studied 5/2 fractional quantum-Hall (FQH) effect and chiral $p$-wave superconductors. There are however crucial differences between the setup we consider and these systems, like the need for a converter between charged and neutral excitations and the neutrality of the non-Abelian excitations. These differences manifest themselves in a temperature scaling exponent of $-7/4$ for the conductance instead of $-3/2$ as in the 5/2 FQH effect.

The ability to predict the existence and crystal type of ordered structures of materials from their components is a major challenge of current materials research. Empirical methods use experimental data to construct structure maps and make predictions based on clustering of simple physical parameters. Their usefulness depends on the availability of reliable data over the entire parameter space. Recent development of high-throughput methods opens the possibility to enhance these empirical structure maps by ab initio calculations in regions of the parameter space where the experimental evidence is lacking or not well characterized. In this paper we construct enhanced maps for the binary alloys of hcp metals, where the experimental data leaves large regions of poorly characterized systems believed to be phase separating. In these enhanced maps, the clusters of noncompound-forming systems are much smaller than indicated by the empirical results alone.

A detailed examination of the Ehrlich-Schwöebel barrier that governs transport of molecules over step edges for small organic molecules shows that the nature of this barrier for molecular systems is far richer than has been previously understood. While such barriers for atomic systems can be represented by one numerical value, we show that step-edge energy barriers for molecular systems depend sensitively on the angle of approach to the step edge which, in turn, depends on the ability of the molecule to explore rotational degrees of freedom. Thus a multiplex of barriers can be obtained for edge descent depending on the angle of approach. Studies of seven aromatic molecules (small acenes, ${\text{C}}_{60}$, rubrene, diindenoperylene, and sexiphenyl) that cover a range of size, shape, and rotational freedom explore how the degree of molecular twisting and bending affects the value of the Ehrlich-Schwöebel barrier on a surface composed of itself (“self”-Schwöebel barriers) and of other organic molecules (“hetero”-Schwöebel barriers). Nonspherical small organic molecules exhibit a strong tendency to “log roll” over step edges at angles around $20°$ from parallel to the step edge as the lowest-energy descent mechanism. Rigid models of small organic molecules fail to capture conformational flexibility during edge descent, resulting in the production of considerably higher barriers. Intriguingly, while bending and twisting have only a small direct effect on the magnitude of the barrier, they play a crucial role on the angle of approach to the step edge which turns out to have a large impact on the barrier height. In addition, we quantify the importance of selecting a sufficiently representative potential model and of employing an appropriately conducted search mechanism. Finally, the broad scope of this study allows us to correlate Schwöebel barriers with the binding energy of the adsorbed molecule to the surface, obviating the need to undertake molecular simulations for molecules not studied here and making possible accurate predictions of self- and hetero-Ehrlich-Schwöebel barriers.

We investigate the parametrization and optimization of thermal conductivity in silicon-germanium alloy nanowires by the cluster-expansion technique. ${\text{Si}}_{1-x}{\text{Ge}}_x$ nanowires are of interest for thermoelectric applications and the reduction in lattice thermal conductivity $\left({\kappa }_L\right)$ is desired for enhancing the thermoelectric figure of merit. We seek the minimization of ${\kappa }_L$ with respect to arrangements of Si and Ge atoms in 1.5 nm diameter [111] ${\text{Si}}_{1-x}{\text{Ge}}_x$ nanowires, by obtaining ${\kappa }_L$ from equilibrium classical molecular-dynamics (MD) simulations via the Green-Kubo formalism, and parametrizing the results with a coarse-grained cluster expansion. Using genetic algorithm optimization with the coarse-grained cluster expansion, we are able to predict configurations that significantly decrease ${\kappa }_L$ as verified by subsequent MD simulations. Our results indicate that superlatticelike configurations with planes of Ge show drastically lowered ${\kappa }_L$.

We derive a closed equation of motion for the current density of an inhomogeneous quantum many-body system under the assumption that the time-dependent wave function can be described as a geometric deformation of the ground-state wave function. By describing the many-body system in terms of a single collective field we provide an alternative to traditional approaches, which emphasize one-particle orbitals. We refer to our approach as continuum mechanics for quantum many-body systems. In the linear response regime, the equation of motion for the displacement field becomes a linear fourth-order integrodifferential equation, whose only inputs are the one-particle density matrix and the pair-correlation function of the ground state. The complexity of this equation remains essentially unchanged as the number of particles increases. We show that our equation of motion is a Hermitian eigenvalue problem, which admits a complete set of orthonormal eigenfunctions under a scalar product that involves the ground-state density. Further, we show that the excitation energies derived from this approach satisfy a sum rule which guarantees the exactness of the integrated spectral strength. Our formulation becomes exact for systems consisting of a single particle and for any many-body system in the high-frequency limit. The theory is illustrated by explicit calculations for simple one- and two-particle systems.

We formulate a momentum-shell renormalization-group (RG) procedure that can be used in theories containing both bosons and fermions with a Fermi surface. We focus on boson-fermion couplings that are nearly forward scattering, i.e., involving small momentum transfer $\left(\vec{q}\approx 0\right)$ for the fermions. Special consideration is given to phase space constraints that result from the conservation of momentum and the imposition of ultraviolet cutoffs. For problems where the energy and momentum scale similarly (dynamic exponent $z=1$), we show that more than one formalism can be used and they give equivalent results. When the energy and momentum must scale differently $\left(z\ne 1\right)$, the procedures available are more limited but a consistent RG scheme can still be formulated. The approach is applicable to a variety of problems, such as itinerant-electron magnets and gauge fields interacting with fermions.

The development of ferromagnetism in Mn-doped ${\text{Bi}}_2{\text{Te}}_3$ is characterized through measurements on a series of single crystals with different Mn content. Scanning tunneling microscopy analysis shows that the Mn substitutes on the Bi sites, forming compounds of the type ${\text{Bi}}_{2-x}{\text{Mn}}_x{\text{Te}}_3$, and that the Mn substitutions are randomly distributed, not clustered. Mn doping first gives rise to local magnetic moments with Curie-like behavior, but by the compositions ${\text{Bi}}_{1.96}{\text{Mn}}_{0.04}{\text{Te}}_3$ and ${\text{Bi}}_{1.91}{\text{Mn}}_{0.09}{\text{Te}}_3$, a second-order ferromagnetic transition is observed, with $T_C\sim 9–12\text{K}$. The easy axis of magnetization in the ferromagnetic phase is perpendicular to the ${\text{Bi}}_2{\text{Te}}_3$ basal plane. Thermoelectric power and Hall effect measurements show that the Mn-doped ${\text{Bi}}_2{\text{Te}}_3$ crystals are $p$-type. Angle-resolved photoemission spectroscopy measurements show that the topological surface states that are present in pristine ${\text{Bi}}_2{\text{Te}}_3$ are also present at 15 K in ferromagnetic Mn-doped ${\text{Bi}}_{2-x}{\text{Mn}}_x{\text{Te}}_3$ and that the dispersion relations of the surface states are changed in a subtle fashion.

Magnetoelectric responses are a fundamental characteristic of materials that break time-reversal and inversion symmetries (notably multiferroics) and, remarkably, of “topological insulators” in which those symmetries are unbroken. Previous work has shown how to compute spin and lattice contributions to the magnetoelectric tensor. Here we solve the problem of orbital contributions by computing the frozen-lattice electronic polarization induced by a magnetic field. One part of this response (the “Chern-Simons term”) can appear even in time-reversal-symmetric materials and has been previously shown to be quantized in topological insulators. In general materials there are additional orbital contributions to all parts of the magnetoelectric tensor; these vanish in topological insulators by symmetry and also vanish in several simplified models without time reversal and inversion whose magnetoelectric couplings were studied before. We give two derivations of the response formula, one based on a uniform magnetic field and one based on extrapolation of a long-wavelength magnetic field, and discuss some of the consequences of this formula.

Shubnikov-de Haas (SdH) oscillations and angle-resolved photoemission spectroscopy (ARPES) are used to probe the Fermi surface of single crystals of ${\text{Bi}}_2{\text{Se}}_3$. We find that SdH and ARPES probes quantitatively agree on measurements of the effective mass and bulk band dispersion. In high carrier density samples, the two probes also agree in the exact position of the Fermi level $E_F$, but for lower carrier density samples discrepancies emerge in the position of $E_F$. In particular, SdH reveals a bulk three-dimensional Fermi surface for samples with carrier densities as low as ${10}^{17}{\text{cm}}^{-3}$. We suggest a simple mechanism to explain these differences and discuss consequences for existing and future transport studies of topological insulators.

We present a comprehensive theoretical study of the second harmonic generation (SHG) in metamaterials consisting of arbitrary distributions of cylindrical nanowires made of centrosymmetric materials. The electromagnetic field at both the fundamental frequency (FF) and second harmonic (SH), as well as the total cross section, the absorption cross section, and the scattering cross section, are calculated by means of a numerical algorithm based on the multiple scattering method. Our algorithm fully describes the nonlinear optical response of the metamaterial by incorporating the contributions of both the surface and bulk nonlinear polarizations and can be applied to both $s$- and $p$-polarized incident waves. We use this numerical method to investigate the SHG in a series of particular cases of practical interest, namely, a single metallic cylinder, chains of metallic cylinders, and periodic and random distributions of such cylinders. In particular, we study the relation between the local field enhancement, via the excitation of surface plasmon-polariton modes, and the amount of energy absorbed or scattered in the far-field, at the FF and the SH.

We report on the experimental observation of the quantum oscillations in microwave magnetoabsorption of a high-mobility two-dimensional electron gas induced by Landau quantization. Using original resonance-cavity technique, we observe two kinds of oscillations in the magnetoabsorption originating from inter-Landau-level and intra-Landau-level transitions. The experimental observations are in full accordance with theoretical predictions. Presented theory also explains why similar quantum oscillations are not observed in transmission and reflection experiments on high-mobility structures despite of very strong effect of microwaves on the dc resistance in the same samples.

The magnetic structure for the recently synthesized iron-arsenide compound ${\text{CaFe}}_4{\text{As}}_3$ has been studied by neutron-powder diffraction. Long-range magnetic order is detected below 85 K, with an incommensurate modulation described by the propagation vector $k=\left(0,\delta ,0\right)$, $\delta \sim 0.39$. Below $\sim 25\text{K}$, our measurements detect a first-order phase transition where $\delta$ locks into the commensurate value $\frac{3}{8}$. A model of the magnetic structure is proposed for both temperature regimes, based on Rietveld refinements of the powder data and symmetry considerations. The structures correspond to longitudinal spin-density waves with magnetic moments directed along the $b$ axis. A Landau analysis captures the change in thermodynamic quantities observed at the two magnetic transitions, in particular, the drop in resistivity at the lock-in transition.

We study a composite quantum quench of the energy gap and the interactions in the interacting ${\varphi }^4$ model using a self-consistent approximation. First we review results for free theories where a quantum quench of the energy gap or mass leads for long times to stationary behavior with thermal characteristics. An exception to this rule is the $2d$ case with zero mass after the quench. In the composite quench, however, we find that the effect of the interactions in our approximation is simply to effectively change the value of the mass. This means on the one hand that the interacting model also exhibits the same stationary behavior and on the other hand that this is now true even for the massless $2d$ case.

A quantum critical endpoint related to a metamagnetic transition causes distinct signatures in the thermodynamic quantities of a compound. We argue that, irrespective of the microscopic details of the considered material, the diverging differential susceptibility combined with the Ising symmetry of the endpoint give rise to a number of characteristic metamagnetic phenomena. In the presence of a magnetoelastic coupling, one finds a correspondence of susceptibility, magnetostriction, and compressibility and, as a result, a pronounced crystal softening, a diverging Grüneisen parameter, a sign change of thermal expansion $\alpha \left(H\right)$, and a minimum in the specific-heat coefficient $\gamma \left(H\right)$. We illustrate these signatures and their relation on the metamagnetic crossover at 8 T in the prototypical heavy-fermion system ${\text{CeRu}}_2{\text{Si}}_2$.

We study the dc Josephson effect in a diffusive multilayered ${\text{SF}}^{\prime }{\text{FF}}^{\prime }\text{S}$ structure, where S is a superconductor and F and ${\text{F}}^{\prime }$ are different ferromagnets. We assume that the exchange energies in the ${\text{F}}^{\prime }$ and F layers are different ($h$ and $H$, respectively) and the middle F layer consists of two layers with parallel or antiparallel magnetization vectors $M$. The $M$ vectors in the left and right ${\text{F}}^{\prime }$ layers are generally not collinear to those in the F layer. In the limit of a weak proximity effect we use a linearized Usadel equation. Solving this equation, we calculate the Josephson critical current for arbitrary temperatures, arbitrary thicknesses of the ${\text{F}}^{\prime }$ and F layers ($L_h$ and $L_H$) in the case of parallel and antiparallel $M$ orientations in the F layer. The part of the critical current $I_{c\text{SR}}$ formed by the short-range singlet and $S=0$ triplet condensate components decays on a short length ${\xi }_H=\sqrt{D/H}$, whereas the part $I_{c\text{LR}}$ due to the long-range triplet $\left|S\right|=1$ component decreases with increasing $L_H$ on the length ${\xi }_N=\sqrt{D/\pi T}$. Our results are in a qualitative agreement with the experiment [T. S. Khaire, M. A. Khasawneh, W. P. Pratt, Jr., and N. O. Birge, Phys. Rev. Lett. 104, 137002 (2010)].