Large-scale atomistic simulations considering a 5 nm in radius H-passivated Si nanosphere that impacts with relatively low energies onto a H-passivated Si substrate reveal a transition between two fundamental collision modes. At impacting speeds of less than $\sim 1000\text{m}/\text{s}$ particle-reflection dominates. At increased speeds the partial onset in the nanosphere of a $\beta$-tin phase on the approach followed by $a\text{-Si}$ phase on the recoil is an efficient dissipative route that promotes particle capture. In spite of significant deformation, the integrity of the deposited nanosphere is retained. Our result explains the efficient fabrication of nanoparticulate films by hypersonic impaction, where the nanoparticle impact velocities equal $1000–2000\text{m}/\text{s}$.

We present analysis of the interacting quantum wire problem in the presence of magnetic field and spin-orbital interaction. We show that an interesting interplay of Zeeman and spin-orbit terms, facilitated by the electron-electron interaction, results in the spin-density wave (SDW) state when the magnetic-field and spin-orbit axes are orthogonal. We show that this instability is enhanced in a closely related problem of Heisenberg spin chain with asymmetric uniform Dzyaloshinskii-Moriya (DM) interaction. Magnetic field in the direction perpendicular to the DM anisotropy axis results in staggered long-range magnetic order along the orthogonal to the applied field direction. We explore consequences of the uniform DM interaction for the electron-spin-resonance (ESR) measurements, and point out that they provide way to probe right- and left-moving excitations of the spin chain separately.

$\text{Cr}\left[\left({\text{H}}_3\text{N-}{\left({\text{CH}}_2\right)}_2{\text{-PO}}_3\right)\left(\text{Cl}\right)\left({\text{H}}_2\text{O}\right)\right]$ represents a rare example of a polar organic-inorganic hybrid material that exhibits a canted antiferromagnetic order below $T_N=5.5\text{K}$. The unusual coexistence of a polar crystal structure and magnetic order triggered our investigation of the magnetodielectric coupling. The coupling is evidenced by an anomaly in the temperature dependence of the dielectric constant $\epsilon$ below the Néel temperature. The magnetocapacitance is enhanced by one order of magnitude below $T_N$. The main characteristics of the magnetodielectic response are interpreted by Landau theoretical coupling terms.

We report magnetization and specific-heat measurements on polycrystalline samples of $\text{BaCdVO}{\left({\text{PO}}_4\right)}_2$ and show that this compound is a $S=1/2$ frustrated square lattice with ferromagnetic nearest-neighbor $\left(J_1\right)$ and antiferromagnetic next-nearest-neighbor $\left(J_2\right)$ interactions. The coupling constants $J_1\simeq -3.6\text{K}$ and $J_2\simeq 3.2\text{K}$ are determined from a fitting of the susceptibility data and confirmed by an analysis of the saturation field $\left({\mu }_0{H}_s\simeq 4.2\text{T}\right)$, the specific heat, and the magnetic entropy. $\text{BaCdVO}{\left({\text{PO}}_4\right)}_2$ undergoes magnetic ordering at about 1 K, likely toward a columnar antiferromagnetic state. We find that $\text{BaCdVO}{\left({\text{PO}}_4\right)}_2$ with the frustration ratio $\alpha =J_2/J_1\simeq -0.9$ is closer to a critical (quantum spin liquid) region of the frustrated square lattice than any of the previously reported compounds. Positive curvature of the magnetization curve is observed in agreement with recent theoretical predictions for high-field properties of the frustrated square lattice close to the critical regime.

We propose a device for studying the Fermi-Hubbard model with long-range Coulomb interactions using an array of coupled quantum dots defined in a semiconductor two-dimensional electron-gas system. Bands above the lowest energy band are used to form the Hubbard model, so that a high average electron density may be used to implement the device. We find that depending on the average electron density, the system is well described by a one- or two-band Hubbard model. Our device design enables the control of the ratio of the Coulomb interaction to the kinetic energy of the electrons independently to the filling of the quantum dots, such that a large portion of the Hubbard phase diagram may be probed. Estimates of the Hubbard parameters suggest that a metal-Mott insulator quantum phase transition and a $d$-wave superconducting phase should be observable using current technologies.

We study the model of a molecular switch comprised of a molecule with a soft vibrational degree of freedom coupled to metallic leads. In the presence of strong electron-ion interaction, different charge states of the molecule correspond to substantially different ionic configurations, which can lead to very slow switching between energetically close configurations (Franck-Condon blockade). Application of transport voltage, however, can drive the molecule far out of thermal equilibrium and thus dramatically accelerate the switching. The tunneling electrons play the role of a heat bath with an effective temperature dependent on the applied transport voltage. Including the transport-induced “heating” self-consistently, we determine the stationary current-voltage characteristics of the device and the switching dynamics for symmetric and asymmetric devices. We also study the effects of an extra dissipative environment and demonstrate that it can lead to enhanced nonlinearities in the transport properties of the device and dramatically suppress the switching dynamics.

We present results from x-ray diffraction experiments and density functional theory calculations which provide a fully consistent picture of the high-pressure phosphorus-IV phase (P-IV). P-IV has an incommensurately-modulated crystal structure described by the four-dimensional superspace group $Cmmm\left(00\gamma \right)s00$. Electronic structure calculations using a three-dimensional commensurate approximant to this structure give excellent agreement with experiment for the structural parameters and their variation with pressure. Density functional perturbation theory shows a phonon instability at the incommensurate wave vector, related to the opening of a pseudogap at the Fermi surface, showing that the atomic motions comprising the incommensuration occur to eliminate Fermi-surface nesting. Unusually, the pseudogap opens not at the incommensurate wave vector $\left(00\gamma \right)$ itself, but at two larger wave vectors corresponding to strong reflections in the diffraction pattern.

We present a multiscale modeling approach that can simulate multimillion atoms effectively via density-functional theory. The method is based on the framework of the quasicontinuum (QC) approach with orbital-free density-functional theory (OFDFT) as its sole energetics formulation. The local QC part is formulated by the Cauchy-Born hypothesis with OFDFT calculations for strain energy and stress. The nonlocal QC part is treated by an OFDFT-based embedding approach, which couples OFDFT nonlocal atoms to local region atoms. The method—QCDFT—is applied to a nanoindentation study of an Al thin film, and the results are compared to a conventional QC approach. The results suggest that QCDFT represents a new direction for the quantum simulation of materials at length scales that are relevant to experiments.

We report on an investigation of the double-layer manganite series ${\left({\text{La}}_{1-z}{\text{Pr}}_z\right)}_{1.2}{\text{Sr}}_{1.8}{\text{Mn}}_2{\text{O}}_7$ $\left(0\le z\le 1\right)$, carried out on single crystals by means of both macroscopic magnetometry and local probes of magnetism [muon-spin rotation $\left(\mu \text{SR}\right)$, ${}^{55}\text{M}\text{n}$ NMR]. Muons and NMR demonstrate an antiferromagnetically ordered ground state at nonferromagnetic compositions $\left(z\ge 0.6\right)$, while more moderate Pr substitutions $\left(0.2\le z\le 0.4\right)$ induce a spin reorientation transition within the ferromagnetic phase. A large magnetic susceptibility is detected at $T_{C,N}<T<250\text{K}$ at all compositions. From ${}^{55}\text{M}\text{n}$ NMR spectroscopy, such a response is unambiguously assigned to the intergrowth of a ferromagnetic pseudocubic phase ${\left({\text{La}}_{1-z}{\text{Pr}}_z\right)}_{1-x}{\text{Sr}}_x{\text{MnO}}_3$, with an overall volume fraction estimated as 0.5%–0.7% from magnetometry. Evidence is provided for the coupling of the magnetic moments of these inclusions with the magnetic moments of the surrounding ${\left({\text{La}}_{1-z}{\text{Pr}}_z\right)}_{1.2}{\text{Sr}}_{1.8}{\text{Mn}}_2{\text{O}}_7$ phase, as in the case of finely dispersed impurities. We argue that the ubiquitous intergrowth phase may play a role in the marked first-order character of the magnetic transition and the metamagnetic properties above $T_c$ reported for double-layer manganites.

We report high-resolution resonant two-color laser spectroscopy on single self-assembled InGaAs quantum dots. The negatively charged exciton in a quantum dot can decay radiatively into both electron-spin ground states, the states with an electron in the quantum dot with its spin parallel or antiparallel to an applied magnetic field. The two decay paths can be used for optical spin alignment via optical pumping. We apply two laser fields at two different colors resonant with the two Zeeman split optical transitions to study the properties of the spin of the resident electron in the quantum dot in both the Faraday and Voigt geometries. The resonant two-color signal is monitored as a function of the laser power as well as the applied magnetic and electric field allowing us to determine spin decay and dephasing rates. We find the rate at which the optical spin alignment can be performed depending on the direction and the magnitude of the applied magnetic field. Finally we demonstrate the feasibility of performing coherent all-optical spin manipulation of an electron spin in a quantum dot.

Decoherence of a center spin or qubit in a spin bath is essentially determined by the many-body bath evolution. We develop a cluster-correlation expansion (CCE) theory for the spin-bath dynamics relevant to the qubit decoherence problem. A cluster-correlation term is recursively defined as the evolution of a group of bath spins divided by the cluster correlations of all the subgroups. This correlation accounts for the authentic (nonfactorizable) collective excitations within a given group. The bath propagator is the product of all possible cluster correlation terms. For a finite-time evolution as in the qubit decoherence problem, a convergent result can be obtained by truncating the expansion up to a certain cluster size. The two-spin cluster truncation of the CCE corresponds to the pair-correlation approximation developed previously [W. Yao et al., Phys. Rev. B 74, 195301 (2006)]. In terms of the standard linked cluster expansion, a cluster-correlation term is the infinite summation of all the connected diagrams with all and only the spins in the group flip-flopped, and thus the expansion is exact whenever convergence occurs. When the individual contribution of each higher-order correlation term to the decoherence is small (while all the terms combined in product could still contribute substantially), as the usual case for relatively large baths, where the decoherence could complete well within the bath spin flip-flop time, the CCE coincides with the cluster expansion [W. M. Witzel and S. Das Sarma, Phys. Rev. B 74, 035322 (2006)]. For small baths, however, the qubit decoherence may not complete within the bath spin flip-flop time scale and thus individual higher-order cluster correlations could become significant. In such cases, only the CCE converges to the exact coherent dynamics of multispin clusters. We check the accuracy of the CCE in an exactly solvable spin-chain model.

Magnetic $3d$ ions doped into wide-gap oxides show signatures of room-temperature ferromagnetism, although their concentration is two orders of magnitude smaller than that in conventional magnets. The prototype of these exceptional materials is Co-doped ZnO, for which an explanation of the room-temperature ferromagnetism is still elusive. Here we demonstrate that magnetism originates from ${\text{Co}}^{2+}$ oxygen-vacancy pairs with a partially filled level close to the ZnO conduction-band minimum. The magnetic interaction between these pairs is sufficiently long ranged to cause percolation at moderate concentrations. However, magnetically correlated clusters large enough to show hysteresis at room temperature already form below the percolation threshold and explain the current experimental findings. Our work demonstrates that the magnetism in ZnO:Co is entirely governed by intrinsic defects and a phase diagram is presented. This suggests a recipe for tailoring the magnetic properties of spintronics materials by controlling their intrinsic defects.

Femtosecond excitation of ${\text{SrRuO}}_3$ nanolayers in a ${\text{SrRuO}}_3/{\text{SrTiO}}_3$ superlattice quenches their ferromagnetism, resulting in magnetostriction. The buildup of mechanical stress is observed in real time by mapping lattice motions via ultrafast x-ray diffraction. A rise time of 500 fs is found for a wide range of excitation wavelengths. In the ferromagnetic phase, phonon-mediated $\left({\sigma }_{\text{ph}}\right)$ and magnetostrictive stress $\left({\sigma }_M\right)$ components display similar strengths but opposite signs. The amplitude of ${\sigma }_M$ follows the temperature dependent magnetization $M^2\left(T\right)$ whereas the strength of ${\sigma }_{\text{ph}}$ is determined by the amount of deposited energy.

Temperature dependences of the contact angle $\theta \left(T\right)$ between (i) specially grown tilt grain boundaries (GBs) in Al and the Zn-rich melt; (ii) tilt GBs in Zn and the Al-rich melt; and (iii) tilt GBs in Zn and the Al-based solid solution ${\left(\text{Al}\right)}^{″}$ were measured using scanning electron microscopy and light microscopy. $\theta$ decreases with increasing $T$ in all cases and reaches zero (complete wetting) at a certain temperature $T_w$ in cases (i) and (ii). The wetting transformation for Al GBs is discontinuous (first order): $\theta \left(T\right)$ dependence is convex, $d\theta /dT$ has a break at $T_w$, and $\theta \sim {\left[\left(T-T_w\right)/T_w\right]}^{1/2}$. The wetting transformation for Zn GBs is continuous: $\theta \left(T\right)$ dependence is concave, $d\theta /dT$ is continuous at $T_w$, and $\theta \sim {\left[\left(T-T_w\right)/T_w\right]}^{3/2}$. For the Zn GBs in contact with a second solid phase ${\left(\text{Al}\right)}^{″}$, $\theta \left(T\right)$ dependence is concave and $\theta \sim {\left[\left(T-T_w\right)/T_w\right]}^{3/2}$ for the extrapolated $T_w$. The observed change from the discontinuous wetting transition for GBs in a metal with a higher melting point (Al) to the continuous one for GBs in a metal with a lower melting point (Zn) is explained using the approach proposed in [Pandit et al., Phys. Rev. B 26, 5112 (1982)]. The validity of this approach and critical exponent of 3/2 may indicate that GB wetting in the Zn-rich alloys is governed by the long-range forces.

Valence- and conduction-band edges of ultrathin oxides (${\text{SiO}}_2$, ${\text{HfO}}_2$, ${\text{Hf}}_{0.7}{\text{Si}}_{0.3}{\text{O}}_2$, ${\text{ZrO}}_2$, and ${\text{Al}}_2{\text{O}}_3$) grown on a silicon substrate have been measured using ultraviolet photoemission and inverse photoemission spectroscopies in the same UHV chamber. The combination of these two techniques has enabled the direct determination of the oxide energy gaps as well as the offsets of the oxide valence- and conduction-band edges from those of the silicon substrate. These results are supplemented with synchrotron x-ray photoemission spectroscopy measurements allowing further characterization of the oxide composition and the evaluation of the silicon substrate contribution to the spectra. The electron affinity has also been systematically measured on the same samples. We find reasonably good agreement with earlier experiments where assumptions regarding energy-gap values were needed to establish the conduction-band offsets. The systematics of our photoemission and inverse photoemission results on different ultrathin films provide a comprehensive comparison of these related systems.

We have studied dilute liquid mixtures of ${}^3\text{H}\text{e}$ in ${}^4\text{H}\text{e}$ at millikelvin temperatures to find the maximum solubility in the zero-temperature limit, covering for the first time the whole pressure range of the liquid phase. Injecting pure ${}^4\text{H}\text{e}$ into the sample cell through a superleak made it possible to pressurize the mixture up to the melting curve at low temperatures, unattainable through ordinary capillaries due to the minimum of the melting pressure—which would block any customary filling lines at around 1 K. The possibility to selectively drain ${}^4\text{H}\text{e}$ out of the cell through the superleak enabled us to reversibly cover the full span of pressures with a given amount of ${}^3\text{H}\text{e}$ in the system and to add ${}^3\text{H}\text{e}$ into the sample volume through an ordinary filling line without the necessity to warm up in between, thus probing the concentration range in small steps in one continuous run in an unprecedented way. Here we report the results on the pressure and temperature dependence of the saturation concentration of the mixture, based on the response of an oscillating quartz tuning fork immersed in the helium mixture. Our data generally agree with earlier results, but suggest that above the solubility maximum around 10 bar the decrease in the saturation concentration as a function of pressure is not as steep as observed in earlier capacitive concentration measurements.

We derive and analyze the $f$-sum rule for a two-dimensional system of interacting electrons whose behavior is described by the Dirac equation. We apply the sum rule to analyze the spectral weight transfer in graphene within different approximations discussed in the literature. We find that the sum rule is generically dominated by interband transitions while other excitations produce subleading behavior. The $f$-sum rule provides strong constraints for theories of interacting electrons in graphene.

We discuss the polar Kerr effect in a chiral $p$-wave ($p_x+i{p}_y$-wave) superconductor. It is found that the off-diagonal component of a current-current correlation function is induced by impurity scattering in the chiral $p$-wave condensate, and a nonzero Hall conductivity is obtained using the Kubo formula. We estimate the Kerr rotation angle by using this impurity-induced Hall conductivity and compare it with experimental results [J. Xia et al., Phys. Rev. Lett. 97, 167002 (2006)].

First-principles calculations of magnetic and, to a lesser extent, electronic properties of the LaFeAsO-based superconductors show substantial apparent controversy, as opposed to most weakly or strongly correlated materials. Not only do different reports disagree about quantitative values but there is also a schism in terms of interpreting the basic physics of the magnetic interactions in this system. In this paper, we present a systematic analysis using four different first-principles methods and show that while there is an unusual sensitivity to computational details, well-converged full-potential all-electron results are fully consistent among themselves. What makes results so sensitive and the system so different from simple local magnetic moments interacting via basic superexchange mechanisms is the itinerant character of the calculated magnetic ground state, where very soft magnetic moments and long-range interactions are characterized by a particular structure in the reciprocal (as opposed to real) space. Therefore, unravelling the magnetic interactions in their full richness remains a challenging but utterly important task.

We show that Anderson localization in quasi-one-dimensional conductors with ballistic electron dynamics, such as an array of ballistic chaotic cavities connected via ballistic contacts, can be understood in terms of classical electron trajectories only. At large length scales, an exponential proliferation of trajectories of nearly identical classical action generates an abundance of interference terms, which eventually leads to a suppression of transport coefficients. We quantitatively describe this mechanism in two different ways: the explicit description of transition probabilities in terms of interfering trajectories, and an hierarchical integration over fluctuations in the classical phase space of the array cavities.