The torque generated by the transfer of spin angular momentum from a spin-polarized current to a nanoscale ferromagnet can switch the orientation of the nanomagnet much more efficiently than a current-generated magnetic field and is therefore in development for use in next-generation magnetic random access memory (MRAM). Up to now, experiments have focused on spin-torque switching driven by simple square-wave current pulses. Here we present measurements showing that spin transfer from a microwave-frequency current pulse can produce a resonant excitation of a nanomagnet and improved switching characteristics in combination with a square current pulse. With the assistance of a microwave-frequency pulse, the switching time is reduced and achieves a narrower distribution than when driven by a square current pulse alone, and this can permit significant reductions in the integrated power required for switching. Resonantly excited switching may also enable alternative, more compact MRAM circuit architectures.

Dielectric and magnetic phenomena in spin- and charge-frustrated system $R{\text{Fe}}_2{\text{O}}_4$ ($R$ is a rare-earth metal ion) are studied. An electronic model for charge, spin, and orbital degrees in a pair of triangular-lattice planes is derived. We analyze this model by utilizing the mean-field approximation and the Monte Carlo simulation in a finite size cluster. A threefold-type charge-ordered structure with charge imbalance between the planes is stabilized in finite temperatures. This polar charge order is reinforced by spin ordering of Fe ions. This magnetodielectric phenomenon is caused by spin frustration and charge-spin coupling in the exchange interaction. We show cross-correlation effect in magnetic- and electric-field responses. Oxygen-deficiency effect as an impurity effect in a frustrated charge-spin coupled system is also examined.

Dislocation and grain-boundary melting are studied in three dimensions using the phase field crystal method. Isolated dislocations are found to melt radially outward from their core, as the localized excess elastic energy drives a power-law divergence in the melt radius. Dislocations within low angle to intermediate angle grain boundaries melt similarly until an angle-dependent first-order wetting transition occurs when neighboring melted regions coalesce. High angle boundaries are treated within a screening approximation, and issues related to ensembles, metastability, and grain size are discussed.

We present an algorithm for generating all derivative superstructures—for arbitrary parent structures and for any number of atom types. This algorithm enumerates superlattices and atomic configurations in a geometry-independent way. The key concept is to use the quotient group associated with each superlattice to determine all unique atomic configurations. The run time of the algorithm scales linearly with the number of unique structures found.

Using first-principles electronic structure calculations based on the $N\text{th}$ order muffin-tin orbital (NMTO)-downfolding technique, we derived the low-energy spin model for ${\text{CuTe}}_2{\text{O}}_5$. Our study reveals that this compound is a two-dimensional coupled spin-dimer system with the strongest Cu-Cu interaction mediated by two O-Te-O bridges. We checked the validity of our model by computing the magnetic susceptibility with quantum Monte Carlo technique and comparing it with available experimental data. We also present magnetization and specific-heat results which may be compared with future experimental investigations. Our derived model provides an alternative scenario to a recently proposed model for this compound [Phys. Rev. B 74, 174421 (2006)]. The situation needs to be settled in terms of further experimental investigations.

We report structural, transport, and thermal properties of carrier-tuned ${\text{Ba}}_8{\text{Ga}}_{16}{\text{Sn}}_{30}$ single crystals with the type-1 clathrate structure ($\beta$ phase), demonstrating that ${\text{Ba}}_8{\text{Ga}}_{16}{\text{Sn}}_{30}$ is a unique thermoelectric clathrate material wherein both the structure type and the carrier type are tunable. The results are compared with the properties of the better known type-8 structure ($\alpha$ phase) and of $A_8{\text{Ga}}_{16}{\text{Ge}}_{30}$ $\left(A=\text{Sr},\text{Eu}\right)$. Differential thermal analysis and powder x-ray diffraction show that both phases are stable up to their virtually identical melting point of $520\pm 3°\text{C}$. Refinements of single-crystal x-ray diffraction data indicate that the Ba(2) guest ion in the tetrakaidecahedron occupies the off-center $24k$ sites which are $0.43–0.44\text{Å}$ away from the centered $6d$ site. The temperature-linear coefficient of the specific heat is $29\text{mJ}/\text{mol}{\text{K}}^2$ for both $n$- and $p$-type carriers in the $\beta$ phase, four times larger than that for the $\alpha$ phase, suggesting contributions from tunneling of the Ba(2) guest ions between off-center minima. Analysis of specific heat with a soft-potential model (SPM) gives a characteristic energy of 20 K for the Ba(2) vibration, significantly lower than 50 K for the $\alpha$ phase and in fact the lowest among type-1 clathrates. The lattice thermal conductivities ${\kappa }_L$ for the $\beta$ phase with both charge carrier types are very similar and show a glasslike temperature dependence. This behavior in ${\kappa }_L\left(T\right)$ is also described using SPM analysis, where it is found that the coupling strength between guest modes and acoustic phonons for the $\beta$ phase is significantly larger than that for ${\text{Sr}}_8{\text{Ga}}_{16}{\text{Ge}}_{30}$.

We derive the theory of the quantum (zero-temperature) superconductor to metal transition in disordered materials when the resistance of the normal metal near criticality is small compared to the quantum of resistivity. This can occur most readily in situations in which “Anderson’s theorem” does not apply. We explicitly study the transition in superconductor-metal composites, in an $s$-wave superconducting film in the presence of a magnetic field, and in a low-temperature disordered $d$-wave superconductor. Near the point of the transition, the distribution of the superconducting order parameter is highly inhomogeneous. To describe this situation we employ a procedure which is similar to that introduced by Mott for description of the temperature dependence of the variable-range hopping conduction. As the system approaches the point of the transition from the metal to the superconductor, the conductivity of the system diverges, and the Wiedemann-Franz law is violated. In the case of $d$-wave (or other exotic) superconductors we predict the existence of (at least) two sequential transitions as a function of increasing disorder: A $d$ wave to $s$ wave and then an $s$ wave to metal transition.

The low-temperature ferroelectric polarization of multiferroic ${\text{YMn}}_2{\text{O}}_5$ is completely reversed at a critical pressure of 10 kbar and the phase transition from the incommensurate to the commensurate magnetic phase is induced by pressures above 14 kbar. The high-pressure data correlate with thermal-expansion measurements, indicating a significant lattice strain at the low-temperature transition into the incommensurate phase. The results support the exchange striction model for the ferroelectricity in multiferroic ${R\text{Mn}}_2{\text{O}}_5$ compounds and they show the importance of magnetic frustration as well as the spin-lattice coupling.

We studied the electron pair emission from a Cu(111) surface upon photon absorption. We found that the energy sharing depends on the angle between the trajectories of the two emitted electrons. The angular distribution of the coincidence intensity displays a zone of reduced intensity, if the emission direction of one electron is fixed. We are able to observe the full extension and shape of this depletion zone. It has an angular extension of $\approx 1.2\text{rad}$, which is independent of the electron energy. This depletion zone is a manifestation of the exchange-correlation hole. The experimental results are discussed in connection with a detailed theoretical description.

Current-induced magnetic domain wall motion, induced by transfer of spin transfer effect due to exchange interaction, is expected to be useful for next generation high-density storages. Here we showed that efficient domain wall manipulation can be achieved by the introduction of Rashba spin-orbit interaction, which induces spin precession of conduction electron and acts as an effective magnetic field. Its effect on domain wall motion depends on the wall configuration. We found that the effect is significant for Bloch wall with the hard axis along the current, since the effective field works as $\beta$ or fieldlike term and removes the threshold current if in extrinsic pinning is absent. For Néel wall and Bloch wall with easy axis perpendicular to Rashba plane, the effective field induces a step motion of wall corresponding to a rotation of wall plane by the angle of approximately $\pi$ at current lower than intrinsic threshold. Rashba interaction would therefore be useful to assist efficient motion of domain walls at low current.

Measured intermodulation distortion (IMD) power at 1.5 GHz in a series of ${\text{YBa}}_2{\text{Y}}_3{\text{O}}_{7-\delta }$ stripline resonators of varying strip widths is compared to the predictions of two qualitatively distinct theories of the nonlinear Meissner effect. The stripline resonators are patterned from a single wafer to ensure uniformity of the material properties. According to the first theory [T. Dahm and D. J. Scalapino, Phys. Rev. B 60, 13125 (1999)], the IMD power is dominated by contributions from the strip edges, while according to the second theory [D. Agassi and D. E. Oates, Phys. Rev. B 72, 014538 (2005)] it is dominated by contributions from the body of the strip. The parameter-free comparison of the measured data with the theoretical predictions clearly favors the latter theory. We conclude that the nonlinear component of the penetration depth must be treated with nonlocal electrodynamics. The origins of this outcome are discussed briefly in the framework of a Green’s-function approach.

We study a spin $S$ quantum Heisenberg model on the Fe lattice of the rare-earth oxypnictide superconductors. Using both large $S$ and large $N$ methods, we show that this model exhibits a sequence of two phase transitions: from a high-temperature symmetric phase to a narrow region of intermediate “nematic” phase, and then to a low-temperature spin ordered phase. Identifying phases by their broken symmetries, these phases correspond precisely to the sequence of structural (tetragonal to monoclinic) and magnetic transitions that have been recently revealed in neutron-scattering studies of LaFeAsO. The structural transition can thus be identified with the existence of incipient (“fluctuating”) magnetic order.

We report density functional theory calculations for the parent compound LaFeAsO of the recently discovered 26 K Fe-based superconductor ${\text{LaFeAsO}}_{1-x}{\text{F}}_x$. We find that the ground state is an ordered antiferromagnet, with staggered moment of about $2.3{\mu }_B$, on the border with the Mott insulating state. We fit the bands crossing the Fermi surface, derived from Fe and As, to a tight-binding Hamiltonian using maximally localized Wannier functions on $\text{Fe}3d$ and $\text{As}4p$ orbitals. The model Hamiltonian accurately describes the Fermi surface obtained via first-principles calculations. Due to the evident proximity of superconductivity to antiferromagnetism and the Mott transition, we suggest that the system may be an analog of the electron-doped cuprates, where antiferromagnetism and superconductivity coexist.

We study the $\nu =\frac{2}{k+2}$ quantum Hall states which are particle-hole conjugates of the $\nu =\frac{2}{k+2}$ Read-Rezayi states. We find that equilibration between the different modes at the edge of such a state leads to an emergent $\mathrm{SU}{\left(2\right)}_k$ algebra in the counter-propagating neutral sector. Heat flow along the edges of these states will be in the opposite direction of charge flow. In the $k=3$ case, which may be relevant to $\nu =2+\frac{2}{5}$, the thermal Hall conductance and the exponents associated with quasiparticle and electron tunneling distinguish this state from competing states such as the hierarchy and/or Jain state.

We establish a theoretical framework that provides a bridge between the microscopic to macroscopic descriptions of the radiation force (RF) under a quantum resonance condition. By using this framework, we derive an explicit analytical expression to clearly demonstrate the properties of the resonant RF on nanostructures and related novel phenomena. (i) For a single nano-object the RF drastically changes with the size, shape, and quality of a nano-object due to the spatial correlations of the internal radiation field and the matter-excited states. This property is highly advantageous in the selective manipulation of quantum properties of nano-objects. (ii) For multiple nano-objects an attractive (repulsive) interobject radiation force (IRF) arises between nano-objects under the optical excitation of a particular coupled state of their spatially separated polaritons, and we term this state as “polaritonic molecule.” This IRF can be enhanced even between the nano-objects that have a large spatial separation if there exist intermediate nano-objects even with very weak induced polarizations, and this effect is termed as “superinterobject radiation force.” In addition, we clarify that a “negative dissipative force” arises when the electronic polarization in a particular nano-object is inverted by a photomediated interaction. Since the resonant RF and IRF depend on many degrees of freedom of both nanostructures and light, they will provide a great variety of optical control methods for the collective dynamics of nanocomposite materials.

We study the role of particle-hole symmetry on the universality class of various quantum phase transitions corresponding to the onset of superfluidity at zero temperature of bosons in a quenched random medium. To obtain a model with an exact particle-hole symmetry it is necessary to use the Josephson junction array, or quantum rotor, Hamiltonian, which may include disorder in both the site energies and the Josephson couplings between wave function phase operators at different sites. The functional integral formulation of this problem in $d$ spatial dimensions yields a $(d+1)$-dimensional classical $XY$ model with extended disorder, constant along the extra imaginary time dimension—the so-called random rod problem. Particle-hole symmetry may then be broken by adding nonzero site energies, which may be uniform or site dependent. We may distinguish three cases: (i) exact particle-hole symmetry, in which the site energies all vanish; (ii) statistical particle-hole symmetry, in which the site energy distribution is symmetric about zero, vanishing on average; and (iii) complete absence of particle-hole symmetry in which the distribution is generic. We explore in each case the nature of the excitations in the nonsuperfluid Mott insulating and Bose glass phases. We show, in particular, that, since the boundary of the Mott phase can be derived exactly in terms of that for the pure, nondisordered system, there can be no direct Mott-superfluid transition. Recent Monte Carlo data to the contrary can be explained in terms of rare region effects that are inaccessible to finite systems. We find also that the Bose glass compressibility, which has the interpretation of a temporal spin stiffness or superfluid density, is positive in cases (ii) and (iii), but that it vanishes with an essential singularity as full particle-hole symmetry is restored. We then focus on the critical point and discuss the relevance of type (ii) particle-hole symmetry-breaking perturbations to the random rod critical behavior, identifying a nontrivial crossover exponent. This exponent cannot be calculated exactly but is argued to be positive and the perturbation therefore relevant. We argue next that a perturbation of type (iii) is irrelevant to the resulting type (ii) critical behavior: The statistical symmetry is restored on large scales close to the critical point, and case (ii) therefore describes the dirty boson fixed point. Using various duality transformations we verify all of these ideas in one dimension. To study higher dimensions, we attempt, with partial success, to generalize the Dorogovtsev–Cardy–Boyanovsky double-epsilon expansion technique to this problem. We find that when the dimension of time ${ϵ}_{\tau }<{ϵ}_{\tau }^c\simeq \frac{8}{29}$ is sufficiently small a type (ii) symmetry-breaking perturbation is irrelevant, but that for sufficiently large ${ϵ}_{\tau }>{ϵ}_{\tau }^c$ particle-hole asymmetry is a relevant perturbation and a new stable fixed point appears. Furthermore, for ${ϵ}_{\tau }>{ϵ}_{\tau }^{c2}\approx \frac{2}{3}$, this fixed point is stable also to perturbations of type (iii): at $ϵ={ϵ}_{\tau }^{c2}$ the generic type (iii) fixed point merges with the new fixed point. We speculate, therefore, that this new fixed point becomes the dirty boson fixed point when ${ϵ}_{\tau }=1$. We point out, however, that ${ϵ}_{\tau }=1$ may be quite special. Thus, although the qualitative renormalization group flow picture the double-epsilon expansion technique provides is quite compelling, one should remain wary of applying it quantitatively to the dirty boson problem.

Magnetostriction and thermal expansion of the spin-ladder compound piperidinium copper bromide ${\left({\text{C}}_5{\text{H}}_{12}\text{N}\right)}_2{\text{CuBr}}_4$ are analyzed in detail. We find perfect agreement between experiments and the theory of a two-leg spin-ladder Hamiltonian for more than a decade in temperature and in a wide range of magnetic fields. Relating the magnetostriction along different crystallographic directions to two static spin-spin correlation functions, which we compute with quantum Monte Carlo, allows us to reconstruct the magnetoelastic couplings of ${\left({\text{C}}_5{\text{H}}_{12}\text{N}\right)}_2{\text{CuBr}}_4$. We especially focus on the quantum critical behavior near the two critical magnetic fields $H_{c1}$ and $H_{c2}$, which is characterized by strong singularities rooted in the low dimensionality of the critical spin system. Extending our discussion in Lorenz [ et al. Phys. Rev. Lett. 100, 067208 (2008)], we show explicitly that the thermal expansion near the upper critical field $H_{c2}$ is quantitatively described by a parameter-free theory of one-dimensional, nonrelativistic fermions. We also point out that there exists a singular quantum critical correction to the elastic moduli. This correction is proportional to the magnetic susceptibility $\chi$, which diverges as $\chi \sim 1/\sqrt{T}$ at the critical fields and thus leads to a strong softening of the crystal.

The electronic structure of hexagonal potassium tungsten bronze ${\text{K}}_{0.25}{\text{WO}}_3$ has been investigated by high-resolution angle-resolved photoemission spectroscopy (ARPES). The experimentally determined band structure resolves the long-standing puzzles concerning the anomalous transport properties in this hexagonal bronze. We find that the ARPES-derived Fermi surface is the consequence of hidden one-dimensional (1D) bands, in good agreement with the calculated Fermi surface. These results indicate that the high-temperature anomaly in the electrical resistivity originates in the possible charge-density-wave formation associated with the hidden 1D Fermi surfaces.

One of the most promising candidate ground states for the quantum antiferromagnetic Heisenberg model on the kagome lattice is the valence bond solid (VBS) with a 36-site unit cell. We present a theory of triplet excitation spectra about this ground state using bond operator formalism. In particular we obtain dispersions of all 18 triplet modes in the reduced Brillouin zone. In the bond operator mean-field theory, it is found that a large number of triplet modes are nondispersive. In particular, the lowest triplet excitation is nondispersive and degenerate with a dispersive mode at the zone center. Away from the zone center, the lowest triplet is separated from two other flat modes by a small energy gap. Quantum fluctuations are considered by taking into account scattering processes of two triplets and their bound-state formation, which leads to a downward renormalization of the lowest spin triplet gap. The dispersion of the lowest triplet excitation in the VBS state is compared with the dispersive lower bound of the triplet continuum expected in competing spin liquid phases. Implications to future neutron-scattering experiments are discussed.

Photoluminescence upconversion removes energy from the medium. It has been applied extensively for laser cooling of atomic vapors and more recently also to condensed matter, such as dye solutions and glasses. The application to semiconductors has also been proposed, but so far, clear evidence is missing. We present a detailed experimental study of photoluminescence upconversion in GaAs quantum wells. We study the conversion efficiency over a wide temperature range as a function of the laser detuning. The best results are obtained when the laser detuning is comparable to the thermal energy, $2k_{B}T$.