Showing papers on "Spin-½ published in 1998"
TL;DR: In this paper, the authors used 17O NMR to determine the spin susceptibility of the layered oxide superconductor Sr2RuO4 and showed no change in spin susceptibility on passing through the superconducting transition temperature.
Abstract: Superconductivity — one of the best understood many-body problems in physics — has again become a challenge following the discovery of unconventional superconducting materials: these include heavy-fermion1, organic2 and the high-transition-temperature copper oxide3 superconductors In conventional superconductors, the electrons form superconducting Cooper pairs in a spin-singlet state, which has zero total spin (S = 0) In principle, Cooper pairs can also form in a spin-triplet state (S = 1), analogous to the spin-triplet ‘p-wave’ state of paired neutral fermions in superfluid 3He (ref 4) At present, the heavy-fermion compound UPt3 is the only known spin-triplet superconductor5,6, although the layered oxide superconductor Sr2RuO4 (ref 7) is believed, on theoretical grounds8, to be a promising candidate The most direct means of identifying the spin state of Cooper pairs is from measurements of their spin susceptibility, which can be determined by the Knight shift (as probed by nuclear magnetic resonance (NMR)) Here we report Knight-shift measurements of Sr2RuO2 using 17O NMR Our results show no change in spin susceptibility on passing through the superconducting transition temperature, which provides the definitive identification of Sr2RuO4 as a spin-triplet superconductor
755 citations
TL;DR: In this article, it was shown that the spin orientation of the trapped alkali atoms cannot be regarded as a degree of freedom, and that the order parameter is a vector rather than scalar quantity.
Abstract: Bose–Einstein condensates — a low-temperature form of matter in which a macroscopic population of bosons occupies the quantum-mechanical ground state — have been demonstrated for weakly interacting, dilute gases of alkali-metal1,2,3 and hydrogen25 atoms. Magnetic traps are usually employed to confine the condensates, but have the drawback that spin flips in the atoms lead to untrapped states. For this reason, the spin orientation of the trapped alkali atoms cannot be regarded as a degree of freedom. Such condensates are therefore described by a scalar order parameter, like the spinless superfluid 4He. In contrast, a recently realized optical trap4 for sodium condensates confines atoms independently of their spin orientations. This offers the possibility of studying ‘spinor’ condensates in which spin comprises a degree of freedom, so that the order parameter is a vector rather than scalar quantity. Here we report the observation of equilibrium states of sodium spinor condensates in an optical trap. The freedom of spin orientation leads to the formation of spin domains in an external magnetic field, which can be either miscible or immiscible with one another.
744 citations
TL;DR: This work considers the various asymmetries, notably single spin asymmetry, that appear in leptoproduction as a consequence of the presence of time-reversal odd distribution functions, which could facilitate experimental searches for time-REVERSal odd phenomena.
Abstract: We consider the various asymmetries, notably single spin asymmetries, that appear in leptoproduction as a consequence of the presence of time-reversal odd distribution functions. This could facilitate experimental searches for time-reversal odd phenomena.
635 citations
TL;DR: In this article, a set of collective spin states is derived for a trapped Bose-Einstein condensate in which atoms have three internal hyperfine spins, and the time scale of spin mixing is predicted.
Abstract: A set of collective spin states is derived for a trapped Bose-Einstein condensate in which atoms have three internal hyperfine spins. These collective states minimize the interaction energy among condensate atoms, and they are characterized by strong spin correlations. We also examine the internal dynamics of an initially spin-polarized condensate. The time scale of spin mixing is predicted.
417 citations
TL;DR: In this paper, the magnetization of a conducting ferromagnet in the presence of a spin-polarized current is derived and the current effects enter in the form of a topological term in the Landau-Lifshitz equation.
Abstract: We derive a continuum equation for the magnetization of a conducting ferromagnet in the presence of a spin-polarized current. Current effects enter in the form of a topological term in the Landau-Lifshitz equation. In the stationary situation the problem maps onto the motion of a classical charged particle in the field of a magnetic monopole. The spatial dependence of the magnetization is calculated for a one-dimensional geometry and suggestions for experimental observation are made. We also consider time-dependent solutions and predict a spin-wave instability for large currents.
390 citations
TL;DR: In this article, a study of the elastic exciton-exciton Coulomb scattering in a semiconductor quantum well is presented, including the interexciton exchange of carriers and the spin degrees of freedom.
Abstract: A study of the elastic exciton-exciton Coulomb scattering in a semiconductor quantum well is presented, including the interexciton exchange of carriers and the spin degrees of freedom. The theoretical results show that electron-electron and hole-hole exchanges are the dominant mechanisms of interaction, while the classical direct term is negligible. The density-dependent homogeneous linewidth is calculated within the Born approximation and good agreement with the existing experimental data is obtained. Owing to the interexciton exchange of carriers, collisions lead to spin relaxation as actually observed in recent time-resolved photoluminescence experiments. [S0163-1829(98)06736-8].
345 citations
TL;DR: In this article, the running of the cosmological constant and Newton's constant at sub-Planckian energies, taking into account the effect of quantum fields with any spin between 0 and 2, was studied.
Abstract: We compute the running of the cosmological constant and Newton's constant at sub-Planckian energies, taking into account the effect of quantum fields with any spin between 0 and 2. We find that Newton's constant does not vary appreciably but the cosmological constant can change by many orders of magnitude when one goes from cosmological scales to typical elementary particle scales. In the extreme infrared, zero modes drive a positive cosmological constant to zero.
304 citations
TL;DR: In this article, the authors investigate a class of inhomogeneity corrections based on a new second-order gradient parameter, which is logically motivated by previous work on Taylor expanded exchange hole densities, and generate exchange correlation functionals more accurate than those containing first-order gradients only.
Abstract: Density-functional exchange–correlation approximations depending on spin densities and their gradients have proven remarkably accurate in recent thermochemical tests [e.g., A. D. Becke, J. Chem. Phys. 107, 8554 (1997)]. With the inherent limitations of first-order gradient corrections now in sight, however, we investigate here a class of inhomogeneity corrections based on a new second-order gradient parameter. The new parameter is logically motivated by previous work on Taylor expanded exchange hole densities, and generates exchange–correlation functionals more accurate than those containing first-order gradients only.
280 citations
TL;DR: In this article, the authors derived the quantum phase-noise limit to the sensitivity of a Mach-Zehnder interferometer in which the incident quantum particles enter via both input ports and showed that if the incident particles are entangled and correlated properly, then the phase sensitivity scales asymptotically like the Heisenberg-limited Ω(1/N), where N is the number of particles incident per unit time.
Abstract: I derive the quantum phase-noise limit to the sensitivity of a Mach-Zehnder interferometer in which the incident quantum particles enter via both input ports I show that if the incident particles are entangled and correlated properly, then the phase sensitivity scales asymptotically like the Heisenberg-limited $\ensuremath{\Delta}\ensuremath{\varphi}=O(1/N),$ for large $N,$ where $N$ is the number of particles incident per unit time (In a one-input-port device, the sensitivity can be at best $\ensuremath{\Delta}\ensuremath{\varphi}=1/\sqrt{N}$) My calculation applies to bosons or fermions of arbitrary integer or half-integer spin Applications to optical, atom-beam, and atom-laser gyroscopes are discussed---in particular, an atom-laser can be used to obtain the required entanglements for achieving this Heisenberg-limited sensitivity with atomic matter waves
277 citations
TL;DR: In this paper, the ground state is a spin-liquid state: the spin-spin correlation functions decay exponentially with distance and the correlation length never exceeds the interatomic distance, and the low energy excitations are singlet-singlet ones, with a finite spin gap.
Abstract: The quantum pyrochlore antiferromagnet is studied by perturbative expansions and exact diagonalization of small clusters. We find that the ground state is a spin-liquid state: The spin-spin correlation functions decay exponentially with distance and the correlation length never exceeds the interatomic distance. The calculated magnetic neutron diffraction cross section is in very good agreement with experiments performed on $\mathrm{Y}(\mathrm{Sc}){\mathrm{Mn}}_{2}$. The low energy excitations are singlet-singlet ones, with a finite spin gap.
275 citations
TL;DR: In this article, it was shown that the Brodsky-Lepage evolution equation for the leading twist spin 3/2 baryon distribution amplitude is completely integrable and reduces to the three-particle XXX-s=-1} Heisenberg spin chain.
Abstract: We show that Brodsky-Lepage evolution equation for the leading twist spin 3/2 baryon distribution amplitude is completely integrable and reduces to the three-particle XXX_{s=-1} Heisenberg spin chain. Trajectories of the anomalous dimensions are identified and calculated using the 1/N expansion. Extending this result, we prove integrability of the evolution equations for twist 3 quark-gluon operators in the large N_c limit and derive explicit expressions for the corresponding integrals of motion.
TL;DR: In this article, the spin asymmetries A1 and the spin structure functions g1 of the proton and the deuteron in the kinematic range 0.0008
Abstract: We present the final results of the spin asymmetries A1 and the spin structure functions g1 of the proton and the deuteron in the kinematic range 0.0008
TL;DR: In this paper, two-dimensional neutron-scattering imaging of YBa2Cu3O6.6 was presented, which revealed that the low-frequency magnetic excitations are virtually identical to those of similarly doped La2−xSrxCuO4.
Abstract: An important feature of the high-transition-temperature (high-Tc) copper oxide superconductors is the magnetism that results from the spins associated with the incomplete outer electronic shells (3d9) of the copper ions. Fluctuations of these spins give rise to magnetic excitations of the material, and might mediate the electron pairing that leads to superconductivity. If the mechanism for high-Tc superconductivity is the same for all copper oxide systems, their spin fluctuations should be universal. But so far, theopposite has seemed to be the case: neutron scattering data reveal clear differences between the spin fluctuations for two major classes of high-Tc materials, La2−xSrxCuO4 (1-3) and YBa2Cu3O7−x (4-6), whose respective building blocks are CuO2 layers and bilayers. Here we report two-dimensional neutron-scattering imaging of YBa2Cu3O6.6, which reveals that the low-frequency magnetic excitations are virtually identical to those of similarly doped La2−xSrxCuO4. Thus, the high-temperature (Tc ≲ 92 K) superconductivity of the former materials may be related to spatially coherent low-frequency spin excitations that were previously thought to be unique to the lower-Tc (<40 K) single-layer La2−xSrxCuO4 family.
TL;DR: In this paper, the low-energy magnetic excitations of nanographite ribbons with zigzag edges were investigated by a random phase approximation of the corresponding Hubbard model, which was used to derive an effective Heisenberg model with ladder structure.
Abstract: We consider the low-energy magnetic excitations of nanographite ribbons with zigzag edges. The zigzag ribbons possess almost flat bands at the Fermi level which cause a ferrimagnetic spin polarization localized at the edge sites. The spin wave mode of this magnetic state is investigated by a random phase approximation of the corresponding Hubbard model. This result is used to derive an effective Heisenberg model with ladder structure. Although this system has a spin gap (Haldane type), our analysis shows that the gap is small and the tendency towards ferrimagnetic correlation at the edges is strong.
TL;DR: The level spectrum of a single-walled carbon nanotube rope, studied by transport spectroscopy, shows Zeeman splitting in a magnetic field parallel to the tube axis as mentioned in this paper.
Abstract: The level spectrum of a single-walled carbon nanotube rope, studied by transport spectroscopy, shows Zeeman splitting in a magnetic field parallel to the tube axis. The pattern of splittings implies that the spin of the ground state alternates by 1/2 as consecutive electrons are added. Other aspects of the Coulomb blockade characteristics, including the current-voltage traces and peak heights, also show corresponding even-odd effects.
TL;DR: In this article, the spin density distribution in transition metal complexes is discussed in qualitative terms, taking into account the coexistence of spin delocalization and spin polarization mechanisms, with the help of numerical results for several complexes obtained from density functional calculations.
Abstract: The spin density distribution in transition metal complexes is discussed in qualitative terms, taking into account the coexistence of spin delocalization and spin polarization mechanisms, with the help of numerical results for several complexes obtained from density functional calculations. The covalent character of the metal-ligand bonds as well as the σ- or π-characteristics of the partially filled d orbitals must be taken into account to qualitatively predict the sign of the spin density at a particular atom within a ligand. The same patterns can be applied to binuclear complexes and can be helpful in determining the ferro- or antiferromagnetic character of the exchange coupling between two paramagnetic ions when the energy gap between the partially occupied molecular orbitals is small. An attempt is made to establish a link between the qualitative-Hay-Thibeault-Hoffmann model of exchange coupling and the of spin polarization model.
TL;DR: In this article, a variational formulation of the time-dependent linear response based on the Sternheimer method is developed in order to make practical ab initio calculations of dynamical spin susceptibilities of solids.
Abstract: A variational formulation of the time-dependent linear response based on the Sternheimer method is developed in order to make practical ab initio calculations of dynamical spin susceptibilities of solids. Using gradient density functional and a muffin-tin-orbital representation, the efficiency of the approach is demonstrated by applications to selected magnetic and strongly paramagnetic metals. The results are found to be consistent with experiment and are compared with previous theoretical calculations.
TL;DR: In this paper, it is shown how to construct the non-euclidean geometry of space-time from the information carried by neutral particles, and the algebraic method is extended to obtain solutions of Einstein's gravitational field equations for empty space, with a cosmological term.
Abstract: It is possible to construct the non-euclidean geometry of space-time from the information carried by neutral particles Points are identified with the quantal events in which photons or neutrinos are created and annihilated, and represented by the relativistic density matrices of particles immediately after creation or before annihilation From these, matrices representing subspaces in any number of dimensions are constructed, and the metric and curvature tensors are derived by an elementary algebraic method; these are similar in all respects to those of Riemannian geometry The algebraic method is extended to obtain solutions of Einstein’s gravitational field equations for empty space, with a cosmological term General relativity and quantum theory are unified by the quantal embedding of non-euclidean space-time, and the derivation of a generalisation, consistent with Einstein"s equations, of the special relativistic wave equations of particles of any spin within representations of SO(3) ⊗ SO(4; 2) There are some novel results concerning the dependence of the scale of space-time on properties of the particles by means of which it is observed, and the gauge groups associated with gravitation
TL;DR: In this paper, the authors performed inelastic neutron scattering on a single crystal sample of Sr14Cu24O41 to study the spin dynamics of the Cu2O3 spin ladder layers, and CuO2 chains.
Abstract: We have performed inelastic neutron scattering on a single crystal sample of Sr14Cu24O41 to study the spin dynamics of the Cu2O3 spin ladder layers, and CuO2 chains. Data collected with incident energies of 50 meV, 200 meV, 350 meV and 500 meV are best fitted with a dispersion with a spin gap of 32.5+/-0.1 meV and a maximum of 193.52.4 meV, consistent with a coupling along the ladders, J|| = 130 meV and a rung coupling J^=72 meV. We find that excitations with an energy transfer of approximately 11.5 meV can be described solely in terms of a dimer chain with an antiferromagnetic intra-dimer coupling, J1 = 11.2 meV, between next-nearest-neighbour Cu ions and a ferromagnetic inter-dimer coupling, J2 = -1.1 meV. The dimers are separated by two Cu ions providing a periodicity for the dimer chain of five units.
TL;DR: In this paper, a perturbative renormalization-group analysis reveals that at half-filling the model scales onto an exactly soluble Gross-Neveu model for arbitrary finite-ranged interactions, provided they are sufficiently weak.
Abstract: We revisit the problem of interacting electrons hopping on a two-leg ladder. A perturbative renormalization-group analysis reveals that at half-filling the model scales onto an exactly soluble Gross-Neveu model for arbitrary finite-ranged interactions, provided they are sufficiently weak. The Gross-Neveu model has an enormous global SO(8) symmetry, manifest in terms of eight real Fermion fields that, however, are highly nonlocal in terms of the electron operators. For generic repulsive interactions, the two-leg ladder exhibits a Mott insulating phase at half-filling with $d$-wave pairing correlations. Integrability of the Gross-Neveu model is employed to extract the exact energies, degeneracies, and quantum numbers of all the low-energy excited states, which fall into degenerate SO(8) multiplets. One SO(8) vector includes two charged Cooper pair excitations, a neutral $s=1$ triplet of magnons, and three other neutral $s=0$ particle-hole excitations. A triality symmetry relates these eight two-particle excitations to two other degenerate octets, which are comprised of single-electron-like excitations. In addition to these 24 degenerate ``particle'' states costing an energy (mass) $m$ to create, there is a 28-dimensional antisymmetric tensor multiplet of ``bound'' states with energy $\sqrt{3}m.$ Doping away from half-filling liberates the Cooper pairs, leading to quasi-long-range $d$-wave pair field correlations, but maintaining a gap to spin and single-electron excitations. For very low doping levels, integrability allows one to extract exact values for these energy gaps. Enlarging the space of interactions to include attractive interactions reveals that there are four robust phases possible for the weak coupling two-leg ladder. While each of the four phases has a (different) SO(8) symmetry, they are shown to all share a common SO(5) symmetry---the one recently proposed by Zhang as a unifying feature of magnetism and superconductivity in the cuprates.
TL;DR: The spin dynamical behavior of these two systems is similar at low temperatures but drastically different at temperatures around T-C, while the formation of spin clusters of size (similar to 20 Angstrom) dominates the spin dynamics of the 197.9 K sample close to T -C, the paramagnetic to ferromagnetic transition for the 300.9 k sample is mon conventional as discussed by the authors.
Abstract: Neutron scattering was used to study the ferromagnetic manganites Nd0.7Sr0.3MnO3 (T-C = 197.9 K) and PT0.63Sr0.37MnO3 (T-C = 300.9 K). The spin dynamical behavior of these two systems is similar at low temperatures but drastically different at temperatures around T-C. While the formation of spin clusters of size (similar to 20 Angstrom) dominates the spin dynamics of the 197.9 K sample close to T-C, the paramagnetic to ferromagnetic transition for the 300.9 K sample is mon conventional. These results, combined with seemingly inconsistent earlier reports, reveal clear systematics in the spin dynamics of the manganites.
TL;DR: In this paper, the nodal liquid, a novel zero-temperature quantum phase obtained by quantum disordering a d-wave superconductor, was introduced and studied, which has numerous remarkable properties which lead to suggest it as an explanation of the pseudo-gap state in under-doped high temperature superconductors.
Abstract: We introduce and study the nodal liquid, a novel zero-temperature quantum phase obtained by quantum-disordering a d-wave superconductor. It has numerous remarkable properties which lead us to suggest it as an explanation of the pseudo-gap state in underdoped high-temperature superconductors. In the absence of impurities, these include power-law magnetic order, a T-linear spin susceptibility, nontrivial thermal conductivity, and two- and one-particle charge gaps, the latter evidenced, e.g. in transport and electron photoemission (which exhibits pronounced fourfold anisotropy inherited from the d-wave quasiparticles). We use a (2+1)-dimensional duality transformation to derive an effective field theory for this phase. The theory is comprised of gapless neutral Dirac particles living at the former d-wave nodes, weakly coupled to the fluctuating gauge field of a dual Ginzburg–Landau theory. The nodal liquid interpolates naturally between the d-wave superconductor and the insulating antiferromagnet, and our effective field theory is powerful enough to permit a detailed analysis of a panoply of interesting phenomena, including charge ordering, antiferromagnetism, and d-wave superconductivity. We also discuss the zero-temperature quantum phase transitions which separate the nodal liquid from various ordered phases.
TL;DR: In this paper, the first moments of the proton, deuteron, and neutron structure functions were analyzed and the Bjorken sum rule was found to be in agreement with the theoretical prediction at the level of 10%.
Abstract: We present a next-to-leading order QCD analysis of the presently available data on the spin structure function [Formula Presented] including the final data from the Spin Muon Collaboration. We present results for the first moments of the proton, deuteron, and neutron structure functions, and determine singlet and nonsinglet parton distributions in two factorization schemes. We also test the Bjorken sum rule and find agreement with the theoretical prediction at the level of 10%. © 1998 The American Physical Society.
TL;DR: Using coupled-mode theory, a theoretical model shows that PMD can be reduced effectively by use of frequency-modulated spin profiles.
Abstract: Using coupled-mode theory, we develop a theoretical model to analyze the effects of fiber spin profiles on polarization mode dispersion (PMD). Constant, sinusoidal, and frequency-modulated spin profiles are examined, and phase-matching conditions are analyzed. Our analysis shows that PMD can be reduced effectively by use of frequency-modulated spin profiles.
TL;DR: In this article, a general formulation for constructing covariant helicity-coupling amplitudes involving two-body decays with arbitrary integer spins is given, and the decay amplitudes are given exclusively in terms of both definite orbital angular momentum and total intrinsic spin.
Abstract: A general formulation is given for constructing covariant helicity-coupling amplitudes involving two-body decays with arbitrary integer spins. The decay amplitudes are given exclusively in terms of both definite orbital angular momentum and total intrinsic spin. A systematic method is developed for calculating the energy and momentum dependence of daughter particles in the decay amplitudes, and a general formula for arbitrary integer spins is given. A number of illustrative examples is worked out, among which is that of the Higgs boson decay into two gauge bosons.
TL;DR: In this article, the authors show the embedding of the N = 8 AdS supergravity equations of motion in the full system at the linearized level and discuss the implications for the embedded theory.
Abstract: The product of two N = 8 supersingletons yields an infinite tower of massless states of higher spin in four dimensional anti de Sitter space. All the states with spin s ≥ 1 correspond to generators of Vasiliev's super higher spin algebra shsE(8|4) which contains the D = 4,N = 8 anti de Sitter superalgebra OSp(8|4). Gauging the higher spin algebra and introducing a matter multiplet in a quasi-adjoint representation leads to a consistent and fully nonlinear equations of motion as shown sometime ago by Vasiliev. We show the embedding of the N = 8 AdS supergravity equations of motion in the full system at the linearized level and discuss the implications for the embedding of the interacting theory. We furthermore speculate that the boundary N = 8 singleton field theory yields the dynamics of the N = 8 AdS supergravity in the bulk, including all higher spin massless fields, in an unbroken phase of M-theory.
TL;DR: In this article, the authors considered several single spin asymmetries in p↑p and p ↑p processes as higher twist QCD contributions, taking into account spin and intrinsic k ⊥ effects in the quark distribution functions.
TL;DR: In this article, Monte Carlo simulations including both nearest-neighbour and long-range interactions give a description of the observed relaxation curves which is consistent with the thermal spin equilibrium, and are directly observable by comparison of the full and unperturbed relaxation curves with curves for which the short-range correlations were destroyed using an appropriate irradiation technique.
Journal Article•
TL;DR: In this article, the nodal liquid, a novel zero-temperature quantum phase obtained by quantum disordering a d-wave superconductor, was introduced and studied, which has numerous remarkable properties which lead to suggest it as an explanation of the pseudo-gap state in under-doped high temperature superconductors.
Abstract: We introduce and study the nodal liquid, a novel zero-temperature quantum phase obtained by quantum-disordering a d-wave superconductor. It has numerous remarkable properties which lead us to suggest it as an explanation of the pseudo-gap state in underdoped high-temperature superconductors. In the absence of impurities, these include power-law magnetic order, a T-linear spin susceptibility, nontrivial thermal conductivity, and two- and one-particle charge gaps, the latter evidenced, e.g. in transport and electron photoemission (which exhibits pronounced fourfold anisotropy inherited from the d-wave quasiparticles). We use a (2+1)-dimensional duality transformation to derive an effective field theory for this phase. The theory is comprised of gapless neutral Dirac particles living at the former d-wave nodes, weakly coupled to the fluctuating gauge field of a dual Ginzburg–Landau theory. The nodal liquid interpolates naturally between the d-wave superconductor and the insulating antiferromagnet, and our effective field theory is powerful enough to permit a detailed analysis of a panoply of interesting phenomena, including charge ordering, antiferromagnetism, and d-wave superconductivity. We also discuss the zero-temperature quantum phase transitions which separate the nodal liquid from various ordered phases.