We report on the spin dynamics in antiferromagnetic ($T_N=21$ K) chromium oxide $\beta$-CaCr${}_2$O${}_4$, which has been proposed recently as a good experimental realization of a spin-$3/2$ strongly frustrated zigzag ladder. Inelastic neutron-scattering experiments evidence unconventional magnetic excitations, reminiscent of a spin-(pseudo)gap behavior above $T_N$. We interpret this result as related to strong one-dimensional quantum fluctuations over a broad temperature range extending up to $~$80 K. Below $T_N$, spin dynamics are compatible with a model of weakly ferromagnetically coupled frustrated ladders.

We report an effect that occurs in semiconductors where internal electrical fields interact with a temperature gradient. Steady current vortices and a magnetic field develop in the system, even without external carrier injection. The effect is electrodynamic, energy dissipative, and fundamentally distinct from any previously described electrothermal effects. In bipolar structures the effective thermopower can be significantly modified by the vortices. Joule heating arising from the vortices reduces the thermal conductivity by an amount comparable to the electronic thermal conductivity.

Low-temperature magnetic force microscopy has been used to visualize spontaneous formation of vortex-antivortex pairs in hybrid ferromagnet/superconductor systems. Vortex-antivortex pairs are induced by the periodic stray field of the ferromagnet. We find general equilibrium conditions for which spontaneous vortex-antivortex pairs are formed during zero-field cooling of the hybrid ferromagnet/superconductor bilayers. Vortices can be generated by the ferromagnet domains in the absence of an external field and they are thermodynamically stable for values of the stray field and the period of the stripe magnetic domains that exceed a certain threshold.

By neutron scattering, we have observed a well-ordered magnetic vortex lattice (VL) in KFe${}_2$As${}_2$ single crystals. With the field along the $c$ axis, a nearly isotropic hexagonal VL is formed, with no symmetry transitions up to high fields, indicating a small anisotropy of the superconducting state around this axis. This rules out line nodes parallel to the $c$ axis, and thus $d$-wave or anisotropic $s$-wave pairing. However, the strong temperature dependence of the signal at $T\ll T_c$ shows that extremely small gap values exist; these may arise from nodal lines perpendicular to the $c$ axis.

In situ tensile tests in a transmission electron microscope were carried out on a “Gum Metal” $\beta$-titanium alloy. Conventional dislocation slip was observed to be the only mechanism occurring during the plastic deformation. The low mobility of screw dislocations was shown to be due to their core structure configuration. Nanometer-sized obstacles were also present but have a weaker effect on the dislocation mobility. The density of these obstacles and the variation in energy due to the core structure of screw dislocations were measured and compared to theoretical data in the literature.

We present a comprehensive computational study of the phase diagram of the frustrated $S=1/2$ Heisenberg antiferromagnet on the honeycomb lattice, with second-nearest $(J_2)$ and third-neighbor $(J_3)$ couplings. Using a combination of exact diagonalizations (EDs) of the original spin model, of the Hamiltonian projected into the nearest-neighbor short-range valence-bond basis, and of an effective quantum dimer model, as well as a self-consistent cluster mean-field theory, we determine the boundaries of several magnetically ordered phases in the region $J_2,J_3\in [0,1]$, and find a sizable magnetically disordered region in between. We characterize part of this magnetically disordered phase as a plaquette valence-bond crystal phase. At larger $J_2$, we locate a sizable region in which staggered valence-bond crystal correlations are found to be important, either due to genuine valence-bond crystal (VBC) ordering or as a consequence of magnetically ordered phases, which break lattice rotational symmetry. Furthermore, we find that a particular parameter-free Gutzwiller projected tight-binding wave function has remarkably accurate energies compared to finite-size extrapolated ED energies along the transition line from conventional Néel to plaquette VBC phases, a fact that points to possibly interesting critical behavior—such as a deconfined critical point—across this transition. We also comment on the relevance of this spin model to model the spin liquid region found in the half filled Hubbard model on the honeycomb lattice.

Stressed ferroelectric nanoparticles, less than 10 nm in diameter, are investigated in a nonpolar fluid. Measuring ferroelectric properties of the smallest achieved BaTiO${}_3$ ferroelectric nanoparticles allows for the determination of dipole moment and spontaneous polarization as a function of size and concentration; the results are significantly greater than in bulk BaTiO${}_3$. This is achieved by using a direct measurement of the displacement current density and either integrating over half a period or fitting the experimental results using the derivative of the Langevin function.

$E$-type perovskite magnet HoMnO${}_3$ is considered to be a prototype Mn-Mn exchange-striction driven multiferroic. We have grown orthorhombic HoMnO${}_3$ single crystals and studied the directional dependence of their electric polarization P. The direction of P is not along the crystallographic $a$ axis, as predicted, but along the $c$ axis. The magnetic state is incommensurate. The direction P along the $c$ axis is primarily induced by Ho-Mn exchange striction, and Mn-Mn striction in the incommensurate spin state leads to an antiferroelectric order along the $a$ axis. We predict the rotation of P with temperature in commensurate $E$-type $R$MnO${}_3$ ($R$ denotes magnetic rare earth).

We consider nuclear-spin dynamics in a two-electron double dot system near the intersection of the electron spin singlet $S$ and the lower energy component $T_{+}$ of the spin triplet. The electron spin interacts with nuclear spins and is influenced by the spin-orbit coupling. Our approach is based on a quantum description of the electron spin in combination with the coherent semiclassical dynamics of nuclear spins. We consider single and double Landau-Zener passages across the $S$-$T_{+}$ anticrossings. For linear sweeps, the electron dynamics is expressed in terms of parabolic cylinder functions. The dynamical nuclear polarization is described by two complex conjugate functions ${\Lambda }^{\pm }$ related to the integrals of the products of the singlet and triplet amplitudes ${\mathrm{c̃}}_S^{*}{\mathrm{c̃}}_{T_{+}}$ along the sweep. The real part $P$ of ${\Lambda }^{\pm }$ is related to the $S$-$T_{+}$ spin-transition probability, accumulates in the vicinity of the anticrossing, and for long linear passages coincides with the Landau-Zener probability $P_{\mathit{LZ}}=1-e^{-2\pi \gamma }$, where $\gamma$ is the Landau-Zener parameter. The imaginary part $Q$ of ${\Lambda }^{+}$ is specific for the nuclear-spin dynamics, accumulates during the whole sweep, and for $\gamma \gtrsim 1$ is typically an order of magnitude larger than $P$. $P$ and $Q$ also show critically different dependences on the shape and the duration of the sweep. $Q$ has a profound effect on the nuclear-spin dynamics, by (i) causing intensive shakeup processes among the nuclear spins and (ii) producing a high nuclear spin generation rate when the hyperfine and spin-orbit interactions are comparable in magnitude. Even in the absence of spin-orbit coupling, when the change in the the total angular momentum of nuclear spins is less than $\hslash$ per single Landau-Zener passage, the change in the global nuclear configuration might be considerably larger due to the nuclear-spin shakeups. We find analytical expressions for the back action of the nuclear reservoir represented via the change in the Overhauser fields the electron subsystem experiences.

The magnetoresistance of thin superconducting (SC) strips subject to a perpendicular magnetic field $B$ and low temperatures $T$ manifests a sequence of alternating SC-insulator transitions (SITs). We study this phenomenon within a quasi-one-dimensional (1D) model for the quantum dynamics of vortices in a line junction between coupled parallel SC wires, at parameters close to their SITs. Mapping the vortex system to 1D Fermions at a chemical potential dictated by $B$, we find that a quantum phase transition of the Ising type occurs at critical values of the vortex filling, from a SC phase near integer filling to an insulator near $1/2$ filling. For $T\to 0$, the resulting magnetoresistance $R(B)$ exhibits oscillations similar to the experimental observation.

A combination of electronic-structure methodologies from density functional theory (DFT) through a tight-binding (TB) model to analytic bond-order potentials (BOPs) has been used to investigate structural trends within TCP phases, which we recently discussed using an empirical structure map [Acta Materialia 59, 749 (2011)]. First, DFT is used to calculate the structural energy differences across the elemental $4d$ and $5d$ transition metal series and the heats of formation of the binary alloys Mo-Re, Mo-Ru, Nb-Re, and Nb-Ru, where we show that the valence electron concentration stabilizes A15, $\sigma$, and $\chi$ phases but destabilizes $\mu$ and Laves phases. Second, a one-parameter canonical $d$-band TB model in combination with the structural energy difference theorem is found to reproduce the observed elemental DFT structural trends. The structural energy difference theorem is also used to rationalize the influence of the relative size differences on the stability of $\mu$ and Laves phases in binary systems. Third, analytic BOP theory using the TB bond integrals as input is shown to converge to the TB structural energy difference curves as the number of moments in the BOP expansion is increased. In order to provide a simple interpretation of these structural energy difference curves in terms of analytic response functions and the differences in the moments of the density of states (DOS), an expression is used for the difference in the band energy that is correct to first order in the Fermi energy differences. We find that the fourth-moment contribution separates the A15, $\sigma$, and $\chi$ phases from the $\mu$ and Laves phases in agreement with the empirical structure map due to difference in the bimodality of the corresponding DOS caused mainly by distortions in their coordination polyhedra from ideal Frank-Kasper polyhedra. Finally, it is shown that at least six moments are needed to predict the structural trend $\text{A15}\to \sigma \to \chi$.

A NMR anvil cell design is used for measuring the influence of high pressure on the electronic properties of the high-temperature superconductor YBa${}_2$Cu${}_4$O${}_8$ above the superconducting transition temperature $T_{\mathrm{c}}$. It is found that pressure increases the spin shift at all temperatures in such a way that the pseudogap feature has almost disappeared at 63 kbar. This change of the temperature-dependent spin susceptibility can be explained by a pressure-induced proportional decrease (factor of 2) of a temperature-dependent component, and an increase (factor of 9) of a temperature-independent component, contrary to the effects of increasing doping. The results demonstrate that one can use anvil cell NMR to investigate the tuning of the electronic properties of correlated electronic materials with pressure.

Frequency shifts and dissipation of a compound torsional oscillator induced by solid ${}^4\mathrm{He}$ samples containing ${}^3\mathrm{He}$ impurity concentrations ($x_3$ $=$ 0.3, 3, 6, 12, and 25 in units of 10${}^{-6}$) have been measured at two resonant mode frequencies ($f_1$ $=$ 493 and $f_2$ $=$ 1164 Hz) at temperatures ($T$) between 0.02 and 1.1 K. The fractional frequency shifts of the $f_1$ mode were much smaller than those of the $f_2$ mode. The observed frequency shifts continued to decrease as $T$ was increased above 0.3 K, and the conventional nonclassical rotation inertia fraction was not well defined in all samples with $x_3⩾$ 3 ppm. Temperatures where peaks in dissipation of the $f_2$ mode occurred were higher than those of the $f_1$ mode in all samples. The peak dissipation magnitudes of the $f_1$ mode were greater than those of the $f_2$ mode in all samples. The activation energy and the characteristic time (${\tau }_0$) were extracted for each sample from an Arrhenius plot between mode frequencies and inverse peak temperatures. The average activation energy among all samples was 430 mK, and ${\tau }_0$ ranged from 2$\times {10}^{-7}$ s to 5$\times {10}^{-5}$ s in samples with $x_3$ $=$ $0.3-25$ ppm. The characteristic time increased with increasing $x_3$. Observed temperature dependencies of dissipation were consistent with those expected from a simple Debye relaxation model if the dissipation peak magnitude was separately adjusted for each mode. Observed frequency shifts were greater than those expected from the model. The discrepancies between the observed and the model frequency shifts increased at the higher frequency mode.

We describe a new class of spin liquids with global SU(2) spin-rotation symmetry in spin-1/2 systems on the triangular lattice, which have real Majorana fermion excitations carrying spin $S=1$. The simplest translationally invariant mean-field state on the triangular lattice breaks time-reversal symmetry and is stable to fluctuations. It generically possesses gapless excitations along three Fermi lines in the Brillouin zone. These intersect at a single point where the excitations scale with a dynamic exponent $z=3$. An external magnetic field has no orbital coupling to the SU(2) spin-rotation-invariant fermion bilinears that can give rise to a transverse thermal conductivity, thus leading to the absence of a thermal Hall effect. The Zeeman coupling is found to gap out two-thirds of the $z=3$ excitations near the intersection point and this leads to a suppression of the low-temperature specific heat, the spin susceptibility, and the Wilson ratio. We also compute physical properties in the presence of weak disorder and discuss possible connections to recent experiments on organic insulators.

We report inelastic neutron-scattering measurements of crystal-field transitions in PrFeAsO, PrFeAsO${}_{0.87}$F${}_{0.13}$, and NdFeAsO${}_{0.85}$F${}_{0.15}$. Doping with fluorine produces additional crystal-field excitations, providing evidence that there are two distinct charge environments around the rare-earth ions, with probabilities that are consistent with a random distribution of dopants on the oxygen sites. The 4$f$ electrons of the Pr${}^{3+}$ and Nd${}^{3+}$ ions have nonmagnetic and magnetic ground states, respectively, indicating that the enhancement of $T_c$ compared to LaFeAsO${}_{1-x}$F${}_x$ is not due to rare-earth magnetism.

In strongly anisotropic layered superconductors in tilted magnetic fields, the Josephson vortex lattice coexists with the lattice of pancake vortices. Due to the interaction between them, the dissipation of the Josephson vortex lattice is very sensitive to the presence of the pancake vortices. If the $c$-axis magnetic field is smaller than the corresponding lower critical field, the pancake stacks are not formed but the individual pancakes may exist in the fluctuational regime either near the surface in large-size samples or in the central region for small-size mesas. We calculate the contribution of such fluctuating pancake vortices to the $c$-axis conductivity of the Josephson vortex lattice and compare the theoretical results with measurements on small mesas fabricated out of Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+\delta }$ crystals. A fingerprint of fluctuating pancakes is a characteristic exponential dependence of the $c$-axis conductivity observed experimentally. Our results provide strong evidence of the existence of the fluctuating pancakes and their influence on the Josephson vortex lattice dissipation.

The folding of paper, hide, and woven fabric has been used for millennia to achieve enhanced articulation, curvature, and visual appeal for intrinsically flat, two-dimensional materials. For graphene, an ideal two-dimensional material, folding may transform it to complex shapes with new and distinct properties. Here, we present experimental results that folded structures in graphene, termed grafold, exist, and their formations can be controlled by introducing anisotropic surface curvature during graphene synthesis or transfer processes. Using pseudopotential-density-functional-theory calculations, we also show that double folding modifies the electronic band structure of graphene. Furthermore, we demonstrate the intercalation of ${\mathrm{C}}_{60}$ into the grafolds. Intercalation or functionalization of the chemically reactive folds further expands grafold's mechanical, chemical, optical, and electronic diversity.

The homoepitaxial growth of $m$-plane $(1\overline{1}00)$ GaN was investigated by plasma-assisted molecular beam epitaxy under nitrogen-rich conditions. The surface morphologies as a function of sample miscut were studied, providing evidence for a strong growth anisotropy that is a consequence of the anisotropy of Ga adatom diffusion barriers on the $m$-plane surface recently calculated ab initio [Lymperakis and Neugebauer, Phys. Rev. B 79, 241308(R) (2009)]. We found that substrate miscut toward $[000\overline{1}]$ implies a step flow toward $\langle 11\overline{2}\overline{6}\rangle$ while substrate miscut toward $[0001]$ causes formation of atomic steps either perpendicular or parallel to the $[0001]$ direction, under N-rich conditions at 730 ${}^{°}$C. We describe the growth conditions for achieving atomically flat $m$-plane GaN layers with parallel atomic steps.

Surface plasmons on nanoparticles provide numerous ways to alternate and manipulate light at nanoscale dimensions, which have the potential to perfect the novel electronic and optical devices. Here, we demonstrate that plasmon-assisted mass sensing can be achieved in a hybrid nanocrystal coupled to a nanomechanical resonator. Compared with the traditional mass measurements, we use an all-optical technique rather than electrical circuitry, which may avoid the heat effect, to realize an ultrasensitive mass detection. The plasmon-assisted mass measurement proposed here may eventually enable sensitive measurement applications such as on-chip detection and analysis as well as identification of compounds.

We report on the observation of bulk superconductivity from dc magnetization measurements in a cylindrical single crystal of Cu${}_x$Bi${}_2$Se${}_3$. The magnitude of the magnetization in the Meissner state is very small, and the magnetic-field dependence of the magnetization just above the lower critical field $H_{\mathrm{c}1}$ is very different from those of usual type-II superconductors. We studied the character of the vortex state theoretically in a spin-triplet pairing superconductor and compared it with the experimental results. The results showed that the superconductivity observed in Cu${}_x$Bi${}_2$Se${}_3$ is consistent with the spin-triplet pairing superconductivity with odd parity. We also observed a rapid relaxation phenomenon of the superconducting diamagnetism.