Spin-rotation coupling, which is responsible for angular momentum conversion between the electron spin and rotational deformations of elastic media, is exploited for generating spin current. This method requires neither magnetic moments nor spin-orbit interaction. The spin current generated in nonmagnets is calculated in the presence of surface acoustic waves. We solve the spin diffusion equation, extended to include spin-rotation coupling, and find that larger spin currents can be obtained in materials with longer spin lifetimes. Spin accumulation induced on the surface is predicted to be detectable by time-resolved Kerr spectroscopy.

Single crystals of PbZrO${}_3$ have been studied by Brillouin light scattering, birefringence, and piezoelectric coefficient measurements. The structural phase transition at $T_c$ is shown to be related to the softening of a longitudinal acoustic (LA) mode, whereas precursor effects are deduced from a central peak in the light scattering spectrum, finite birefringence, and piezoelectric coefficient above the structural instability. An intermediate phase in the vicinity of $T_c$ is apparent where a coexistence of the LA mode with a high frequency mode appears, and in addition the transverse acoustic (TA) mode becomes visible.

Excitons are electron-hole pairs appearing below the band gap in insulators and semiconductors. They are vital to photovoltaics, but are hard to obtain with time-dependent density-functional theory (TDDFT), since most standard exchange-correlation (xc) functionals lack the proper long-range behavior. Furthermore, optical spectra of bulk solids calculated with TDDFT often lack the required resolution to distinguish discrete, weakly bound excitons from the continuum. We adapt the Casida equation formalism for molecular excitations to periodic solids, which allows us to obtain exciton binding energies directly. We calculate exciton binding energies for both small- and large-gap semiconductors and insulators, study the recently proposed bootstrap xc kernel [S. Sharma et al., Phys. Rev. Lett. 107, 186401 (2011)], and extend the formalism to triplet excitons.

We present a theoretical analysis of catalyzed nanowire growth based on the material balance in a droplet within one monolayer growth cycle. Pulsed supersaturation and nucleation probability density are shown to originate from the material balance under rather general assumptions. We calculate explicitly the time-dependent nucleation probability as a function of nanowire radius and growth conditions. For small nanowire radii, the timescale hierarchy of different growth steps is demonstrated, leading to a temporal anticorrelation of nucleation events. Numerical analysis is performed in the case of Au-catalyzed GaAs nanowires, where the nucleation probabilities are mapped out as functions of nanowire radius at different conditions. The transition from deltalike to Poissonian temporal distribution of nucleation events is discussed depending on relevant parameters. We speculate that the self-regulated narrowing of nucleation probabilities suppresses random broadening of nanowire length distributions. This focusing effect is specific for nucleation in nanovolumes and might be used for tailoring the size spectra of different nano-objects.

Recent experimental progress in coupling nanoscale conductors to superconducting microwave cavities has opened up for transport investigations of the deep quantum limit of light-matter interactions, with tunneling electrons strongly coupled to individual cavity photons. We have investigated theoretically the most basic cavity-conductor system with strong, single photon induced nonlocal transport effects: two spatially separated double quantum dots (DQDs) resonantly coupled to the fundamental cavity mode. The system, described by a generalized Tavis-Cummings model, is investigated within a quantum master equation formalism, allowing us to account for both the electronic transport properties through the DQDs as well as the coherent, nonequilibrium cavity photon state. We find sizable nonlocally induced current and current cross-correlations mediated by individual photons. From a full statistical description of the electron transport we further reveal a dynamical channel blockade in one DQD lifted by photon emission due to tunneling through the other DQD. Moreover, large entanglement between the orbital states of electrons in the two DQDs is found for small DQD-lead temperatures.

The confinement of the two-dimensional electron gas (2DEG), preferential occupancy of the Ti 3$d$ orbital, and strong spin-orbit coupling at the LaAlO${}_3$/SrTiO${}_3$ interface play a significant role in its emerging properties. Here we report a fourfold oscillation in the anisotropic magnetoresistance (AMR) and planar Hall effect (PHE) at the LaAlO${}_3$/SrTiO${}_3$ heterointerface. We evaluate the carrier confinement effects on the AMR and find that the fourfold oscillation appears only for the case of 2DEG systems while it is twofold for the three-dimensional system. The AMR behavior is further found to be sensitive to applied electric field, emphasizing the significance of spin-orbit coupling at the interface. These confinement effects suggest that the magnetic interactions are predominant at the interface, and the gate electric field modulation of AMR shows the tunability of magnetic interactions. As the fourfold oscillation fits well to the phenomenological model for a cubic symmetry system, this suggests that the origin of the oscillation may be linked to the anisotropy in magnetic scattering arising from the interaction of electrons with the localized magnetic moments coupled to the crystal symmetry. The observed large PHE further indicates the in-plane nature of magnetic ordering that arises from the in-plane Ti 3$d_{xy}$ orbitals.

We report strongly momentum-dependent short-ranged charge screening dynamics in CE-type charge, orbital, and spin ordered La${}_{0.5}$Sr${}_{1.5}$MnO${}_4$, based on Mn $K$-edge resonant inelastic x-ray scattering data. Through a comparison with theoretical calculations, we show that the observed momentum dependence reflects highly localized, nearest-neighbor screening of the transient local charge perturbation in this compound with an excitonlike screening cloud, rather than delocalized screening. The size of the screening cloud is estimated to be about 0.4–0.5 interatomic distances.

We have measured the zero-bias peak in differential conductance in a hole quantum dot. We have scaled the experimental data with applied bias and compared to real-time renormalization group calculations of the differential conductance as a function of source-drain bias in the limit of zero temperature and at finite temperatures. The experimental data show deviations from the $T=0$ calculations at low bias, but are in very good agreement with the finite-$T$ calculations. The Kondo temperature $T_K$ extracted from the data using $T=0$ calculations, and from the peak width at $\frac{2}{3}$ maximum, is significantly higher than that obtained from finite-$T$ calculations.

The possibility of temporally and spatially coherent thermal emission has been demonstrated utilizing stacked gratings. We demonstrate that the metallic grating with narrow air slit behaves like a homogeneous slab with large permittivity and small permeability and find that the interaction between the metallic grating and the Bragg grating gives rise to impendence matching at wavelengths located in the photonic band gap of the Bragg grating, which enables the stacked gratings to perform high emission with ultranarrow spectrum and antenna-like spatial response. This paves the way towards the design of a novel infrared source platform for applications such as thermal analysis, imaging, security, biosensing, and medical diagnoses.

The adsorption of benzene and C60 on graphene and boron nitride is studied using density functional theory with the van der Waals density functional (vdW-DF). By comparing these systems we can systematically investigate their adsorption nature and differences between the two functional versions vdW-DF1 and vdW-DF2. The bigger size of the C60 molecule makes it bind stronger to the surface than benzene, yet the interfaces between the molecules and the sheets are similar in nature. The binding separation is more sensitive to the exchange variant used in vdW-DF than to the correlation version. This result is related to the exchange and correlation components of the potential energy curve. We show that a moderate dipole forms for C60 on graphene, unlike for the other adsorption systems. We find that the corrugation (at the atomic scale) is very sensitive to the variant or version of vdW-DF used, in particular, the exchange. Further, we show that this sensitivity arises indirectly through the shift in binding separation caused by changing the vdW-DF variant. Based on our results, we suggest a concerted theory-experiment approach to assess the exchange and correlation contributions to physisorption. Using DFT calculations, the corrugation can be linked to the optimal separation, allowing us to extract the exchange-correlation part of the adsorption energy. Molecules with the same interfaces to the surface, but different geometries, can in turn cast light on the role of van der Waals forces.

Hydroxyl salts of the type $M_2$(OH)${}_{3}X$, and $M$(OH)$X$, where $M$ represents a transition metal ion and $X$ represents a halogen ion, widely exist as minerals and were recently reported to be geometrically frustrated magnets. Here, we report the finding of ferroelectric response in them. First, we observed strong magnetic-lattice-dielectric couplings in all of them as witnessed during their magnetic transitions at low temperatures. Secondly, we identified apparent ferroelectric responses in the deuterated hydroxyl salts of high crystal symmetries, i.e., rhombohedral Co${}_2$(OD)${}_3$Cl and Co${}_2$(OD)${}_3$Br, at high temperatures of 220–230 K through an isotope effect. The present work shows that multiferroicity may be a potentially universal phenomenon in magnetic hydroxyl salts. Meanwhile, it provides the first link between magnetic geometric frustration and hydrogen-bonded soft-mode ferroelectrics.

We report on a comparative study of spin Hall related effects and magnetoresistance in YIG$|$Pt and YIG$|$Ta bilayers. These combined measurements allow to estimate the characteristic transport parameters of both Pt and Ta layers juxtaposed to yttrium iron garnet (YIG): the spin mixing conductance $G_{\uparrow \downarrow }$ at the YIG$|$normal metal interface, the spin Hall angle ${\Theta }_{\mathrm{SH}}$, and the spin diffusion length ${\lambda }_{\mathrm{sd}}$ in the normal metal. The inverse spin Hall voltages generated in Pt and Ta by the pure spin current pumped from YIG excited at resonance confirm the opposite signs of spin Hall angles in these two materials. Moreover, from the dependence of the inverse spin Hall voltage on the Ta thickness, we extract the spin diffusion length in Ta, found to be ${\lambda }_{\mathrm{sd}}^{\mathrm{Ta}}=1.8\pm 0.7$ nm. Both the YIG$|$Pt and YIG$|$Ta systems display a similar variation of resistance upon magnetic field orientation, which can be explained in the recently developed framework of spin Hall magnetoresistance.

We study the Landau-Zener dynamics of a tunneling spin coupled to a torsional resonator. For strong spin-phonon coupling, when the oscillator frequency is large compared to the tunnel splitting, the system exhibits multiple Landau-Zener transitions. Entanglement of spin and mechanical angular momentum results in abrupt changes of oscillator dynamics, which coincide in time with spin transitions. We show that a large number of spins on a single oscillator coupled only through the in-phase phonon field behaves as a single large spin, greatly enhancing the spin-phonon coupling. We compare purely quantum and semiclassical dynamics of the system and discuss their experimental realizations. An experiment is proposed in which the field sweep is used to read out the exact quantum state of the mechanical resonator.

Quantum turbulence accompanying thermal counterflow in superfluid ${}^4$He was recently visualized by the Maryland group, using micrometer-sized tracer particles of solid hydrogen [Paoletti, Fiorito, Sreenivasan, and Lathrop, J. Phys. Soc. Jpn. 77, 111007 (2008)]. In order to understand the observations, we formulate the coupled dynamics of fine particles and quantized vortices, in the presence of a relative motion of the normal and superfluid components. Numerical simulations based on this formulation are shown to agree reasonably well with experimental observations of the velocity distributions of the tracer particles in thermal counterflow.

We demonstrate that the vortex state in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase may be very different depending on the field orientation relative to the crystalline axes. We numerically calculate the upper critical field near the tricritical point taking into account the modulation of the order parameter along the magnetic field as well as the higher Landau levels. For $s$-wave superconductors with the anisotropy described by an elliptical Fermi surface, we propose a general scheme for the analysis of the angular dependence of the upper critical field at all temperatures on the basis of an exact solution for the order parameter. Our results show that the transitions (with tilting magnetic field) between different types of mixed states may be a salient feature of the FFLO phase.

We observe an unusual combination of normal and superconducting state properties without any signature of strong spin fluctuations in single-crystal Ir${}_3$Te${}_8$. The electrical resistivity does not saturate by 700 K but exhibits a low-resistivity ratio, and it exhibits two extended linear regimes (approximately 20–330 and 370–700 K) with the same slope, separated by a small hysteretic interval marking a strong first-order phase transition from cubic to rhombohedral lattice symmetry at $T_{\mathrm{S}}$ = 350 K. The electronic heat-capacity coefficient $\left(11\mathrm{mJ}{\mathrm{mol}}^{-1}{\mathrm{K}}^{-2}\right)$ is consistent with a net diamagnetic, rather than a Pauli paramagnetic, normal state that yields to superconductivity below a critical temperature $T_{\mathrm{C}}$ = 1.8 K. The size of the heat-capacity jump near $T_{\mathrm{C}}$ indicates bulk superconductivity.

We study the effect of disorder on massless, spinful Dirac fermions in two spatial dimensions with attractive interactions, and show that the combination of disorder and attractive interactions is deadly to the Dirac semimetal phase. First, we derive the zero temperature phase diagram of a clean Dirac fermion system with tunable doping level ($\mu$) and attraction strength ($g$). We show that it contains two phases: a superconductor and a Dirac semimetal. Then we add disorder and show that arbitrarily weak disorder destroys the Dirac semimetal, turning it into a superconductor instead. Thus for Dirac fermions near charge neutrality, disorder actually assists superconductivity. We discuss the strength of the superconductivity for both long range and short range disorder. For long range disorder, the superconductivity is exponentially weak in the disorder strength. For short range disorder, a uniform mean field analysis predicts that superconductivity should be doubly exponentially weak in the disorder strength. However, a more careful treatment of mesoscopic fluctuations suggests that locally superconducting puddles should form at a much higher temperature, and should establish global phase coherence at a temperature that is only exponentially small in weak disorder. Thus, mesoscopic fluctuations exponentially enhance the superconducting critical temperature. We also discuss the effect of disorder on the quantum critical point of the clean system, building in the effect of disorder through a replica field theory. We show that disorder is a relevant perturbation to the supersymmetric quantum critical point. We expect that in the presence of attractive interactions, the flow away from the critical point ends up in the superconducting phase, although firm conclusions cannot be drawn since the renormalization group analysis flows to strong coupling. We argue that although we expect the quantum critical point to get buried under a superconducting phase, signatures of the critical point may be visible in the finite temperature quantum critical regime. Our results have implications for experiments on proximity induced superconductivity in Dirac fermion systems, where they imply an enormous disorder enhancement of the superconducting susceptibility. As a result, the proximity induced superconductivity in dirty systems is expected to be much stronger than that in clean systems at the Dirac point.

The spin dynamics of the layered square-lattice vanadate Pb${}_2$VO(PO${}_4$)${}_2$ is investigated by electron spin resonance (ESR) at various magnetic fields and at temperatures above magnetic ordering. The linewidth divergence towards low temperatures seems to agree with isotropic Heisenberg-type spin exchange suggesting that the spin relaxation in this quasi-two-dimensional compound is governed by low-dimensional quantum fluctuations. However, a weak easy-plane anisotropy of the $g$ factor points to the presence of a planar XY type of exchange. Indeed, we found that the linewidth divergence is described best by XY-like spin fluctuations which requires a single parameter only. Therefore, ESR-probed spin dynamics could establish Pb${}_2$VO(PO${}_4$)${}_2$ as the first frustrated square-lattice system with XY-inherent spin topological fluctuations.

We report the magnetic and superconducting properties of locally noncentrosymmetric SrPtAs obtained by muon-spin-rotation/relaxation ($\mu$SR) measurements. Zero-field $\mu$SR reveals the occurrence of small spontaneous static magnetic fields with the onset of superconductivity. This finding suggests that the superconducting state of SrPtAs breaks time-reversal symmetry. The superfluid density as determined by transverse field $\mu$SR is nearly flat approaching $T=0$ K proving the absence of extended nodes in the gap function. By symmetry, several superconducting states supporting time-reversal-symmetry breaking in SrPtAs are allowed. Out of these, a dominantly $d+id$ (chiral $d$-wave) order parameter is most consistent with our experimental data.

In Phys. Rev. Lett. 109, 245502 (2012), a method for retrieving the object's three-dimensional potential distribution by inverting the dynamical scattering was presented and validated by a reconstruction from simulated atomic resolution transmission electron microscopy (TEM) images. In this paper, an extension to ptychography and scanning confocal electron microscopy is demonstrated and validated with simulations. Ultimately, this will make it possible to operate the microscope in the mode that yields the best reconstruction instead of accommodating the microscope settings to the linear approximation to the specimen-electron interaction used in most reconstruction algorithms. Furthermore, simultaneous estimation of the object and the unknown defocus from simulated atomic resolution TEM images is attained. This is an important step towards experimental reconstructions.