Showing papers on "Spin-½ published in 1990"

Topological Orders in Rigid States

Abstract: We study a new kind of ordering — topological order — in rigid states (the states with no local gapless excitations). We concentrate on characterization of the different topological orders. As an example we discuss in detail chiral spin states of 2 + 1 dimensional spin systems. Chiral spin states are described by the topological Chern-Simons theories in the continuum limit. We show that the topological orders can be characterized by a non-Abelian gauge structure over the moduli space which parametrizes a family of the model Hamiltonians supporting topologically ordered ground states. In 2 + 1 dimensions, the non-Abelian gauge structure determines possible fractional statistics of the quasi-particle excitations over the topologically ordered ground states. The dynamics of the low lying global excitations is shown to be independent of random spatial dependent perturbations. The ground state degeneracy and the non-Abelian gauge structures discussed in this paper are very robust, even against those perturbati...

Quantum Description of High‐Resolution NMR in Liquids

Abstract: 1 Elementary description of NMR 2 Epitome of quantum mechanics 3 Spin and magnetic moment 4 Quantum statistical mechanics 5 Quantum description of NMR 6 Generalities on 2D spectroscopy 7 J Spectroscopy 8 Shift correlation spectroscopy 9 Multiple quantum coherence and applications 10 Fundamentals of relaxation theory

Aharonov-Bohm scattering of particles with spin.

Abstract: The scattering of relativistic spin-one-half particles in an Aharonov-Bohm potential is considered. It is shown that earlier approaches to this problem have neglected a crucial delta function contribution to the potential. By formulating the problem with a source of finite radius which is then allowed to go to zero, it is established that this is the delta function alone that causes solutions that are singular at the origin to become relevant. The changes in the amplitude which arise from the inclusion of spin are seen to modify the cross section for the case of polarized beams. Finally, the calculated Aharonov-Bohm amplitude is shown to describe the scattering of particles with arbitrary spin in the c=\ensuremath{\infty} limit.

Pseudogaps and the spin-bag approach to high-Tc superconductivity.

Abstract: It is shown that antiferromagnetic spin fluctuations in a two-dimensional metal, such as heavily doped cuprate superconductors, lead to a pseudogap in the electronic spectrum. The spectral function evolves from one peak in the Fermi-liquid regime to two peaks, one for particles and one for holes. The self-energy of spin bags and their pairing interaction are calculated. These results are consistent with the corresponding results in the weakly doped ordered antiferromagnet.

Histories of adiabatic quantum transitions

Abstract: The way in which the transition amplitude to an initially unoccupied state increases to its exponentially small final value is studied in detail in the adiabatic approximation, for a 2-state quantum system. By transforming to a series of superadiabatic bases, clinging ever closer to the exact evolving state, it is shown that transition histories renormalize onto a universal one, in which the amplitude grows to its final value as an error function (rather than via large oscillations as in the ordinary adiabatic basis). The time for the universal transition is of order $\surd (\hslash /\delta)$ where $\delta $ is the small adiabatic (slowness) parameter. In perturbation theory the pre-exponential factor of the final amplitude renormalizes superadiabatically from the incorrect value ${\textstyle\frac{1}{3}}\pi $ (for the ordinary adiabatic basis) to the correct value unity. The various histories could be observed in spin experiments.

High-spin and low-spin states in Invar and related alloys.

Abstract: Total-energy band calculations that show the coexistence of a high-spin and low-spin state in fcc transition metals and alloys are presented. The energy difference between the two states is shown to be a function of the electron concentration and to vanish at 8.6. At larger electron concentrations the low-temperature state is the high-spin state, and the thermal expansion is shown to pause at a system-dependent characteristic temperature. At lower electron concentrations the low-temperature state is the low-spin state, and enhanced thermal expansion is expected. An analysis that leads to a qualitative understanding of the thermal properties of Invar and that implies a connection with martensitic transformations and spin glasses in related alloys is presented. For Invar a magnetic collapse from the high-spin to the low-spin state at a pressure of 55 kbar is predicted.

Boson-vortex-Skyrmion duality, spin-singlet fractional quantum Hall effect, and spin-1/2 anyon superconductivity.

Abstract: In two space dimensions we generalize the boson-vortex duality picture for spinless particles to boson-vortex-Skyrmion duality for spin-1/2 particles. The spin-singlet fractional quantum Hall effect and spin-singlet anyon superconductivity can be understood as the condensation of vortices and Skyrmions into various fractional quantum Hall states.

A quantum fluids approach to frustrated Heisenberg models

Abstract: Emphasising the close analogies between antiferromagnetism and neutral superfluidity, the authors develop a gauge-invariant quantum fluids description of non-bipartite Heisenberg systems. The antiferromagnet is treated as a spin superfluid with a rotational gauge invariance associated with the continuity of spin flow. They show how an extended Schwinger boson approach naturally incorporates the Onsager reaction fields generated by spin fluctuations, and correctly reproduces the semiclassical behaviour of spin wave theory in the large-S limit. The important short-wavelength physics of fluctuation-stabilised order is also captured by this description. For two-dimensional helimagnets at small S, the method predicts that the twist will survive the loss of sublattice magnetisation, closely analogous to the biaxial-uniaxial transition of nematic liquid crystals.

Mechanism for large neutrino magnetic moments.

Abstract: A general mechanism, based on very simple considerations of spin, is proposed, which has the effect of enhancing the neutrino magnetic dipole moment relative to the neutrino mass

Ground state and electron-magnon interaction in an itinerant ferromagnet: half-metallic ferromagnets

Abstract: Electron and spin Green functions of a Hubbard ferromagnet are calculated, both starting from the Stoner ground state and for a ferromagnet with Hubbard subbands. The temperature dependences of the spin-wave stiffness and damping, the magnetisation, the local moment on a site and the thermodynamic properties are investigated. The role of non-quasiparticle contributions, described by branch cuts of electron Green functions, is discussed. The non-quasiparticle ('ferrospinon') correction to the linear term in the specific heat is obtained. Experimental data on 'half-metallic' ferromagnets (in particular, spin polarisation and longitudinal nuclear relaxation rate) are analysed.

Test of the linearity of quantum mechanics in an atomic system with a hydrogen maser.

Abstract: We extend a nonlinear generalization of quantum mechanics, recently formulated by Weinberg, to systems of composite spin, such as atoms. We have used hydrogen (H) masers to set a limit of 3.7\ifmmode\times\else\texttimes\fi{}${10}^{20}$ eV (8.9 \ensuremath{\mu} Hz) on the magnitude of a nonlinear correction to the quantum mechanics of atomic spins. This result is of comparable magnitude to the limit recently set in a single-valued (nuclear) spin system. In the absence of nonlinear effects, the experiment provides a new and stringent test of H-maser clock performance and the applicability of standard maser theory.

Cyclic quantum evolution and Aharonov-Anandan geometric phases in SU(2) spin-coherent states

Abstract: We show that cyclic quantum evolution can be realized and the Aharonov-Anandan (AA) geometric phase can be determined for any spin-j system driven by periodic fields Two methods are extended for the study of this problem: the generalized spin-coherent-state technique and the Floquet quasienergy approach Using the former approach, we have developed a generalized Bloch-sphere model and presented a SU(2) Lie-group formulation of the AA geometric phase in the spin-coherent state We show that the AA phase is equal to j times the solid angle enclosed by the trajectory traced out by the tip of a generalized Bloch vector General analytic formulas are obtained for the Bloch vector trajectory and the AA geometric phase in terms of external physical parameters In addition to these findings, we have also approached the same problem from an alternative but complementary point of view without recourse to the concept of coherent-state terminology Here we first determine the Floquet quasienergy eigenvalues and eigenvectors for the spin-j system driven by periodic fields This in turn allows the construction of the time-evolution propagator, the total wave function, and the AA geometric phase in a more general fashion

Inconsistency of spin 4-spin 2 gauge field couplings

Abstract: The authors invert the usual problem of coupling higher spin gauge fields to gravity by treating (linearised) gravity as the 'matter field' source of spin 4 gauge theory. This is motivated by the existence of the conserved gravitational four-index Bel-Robinson tensor as a possible current for the spin 4 field. They first derive this tensor as a Noether current, thereby linking it to a novel invariance of spin 2. It is then shown that, as usual for higher spins, consistency does not survive beyond lowest-order cubic coupling.

Proton-neutron symmetry in boson models of nuclear structure

Abstract: The present review deals with nuclear models which consider collective proton and neutron motion separately in terms of bosons. Extensions of the geometric Bohr-Mottelson model are sketched, followed by a more extensive discussion of the algebraic interacting boson approximation (IBA-2). In the latter the concept of F spin plays a central role, and detailed attention is paid to its tensorial properties. A general projection method is presented whereby any IBA-2 calculation for states of maximal F spin can be transformed into a simpler one in IBA-1, where the proton-neutron distinction is ignored. Data are presented on F-spin multiplets, and global fits with the IBA are discussed. A major portion of the review is concerned with M1 transitions. Strong M1s in deformed and spherical nuclei, with emphasis on the latter, are discussed as resulting from a change in proton-neutron symmetry. Weak M1s are considered as reflecting such symmetry changes in small amplitudes of the wave function. Attention is given to one-fluid boson models (IBA-1, geometric) simulating these two-fluid effects.

Superconductivity and other macroscopic quantum phenomena

Abstract: As first suggested by Fritz London, superconductivity and superfluid flow in liquid helium are macroscopic quantum phenomena. They depend on the fact that the energy states of even macroscopic objects, although closely spaced, are discrete, and on the statistical mechanics of systems made up of identical particles. The electrons in a superconducting metal, with a spin of one‐half, obey Fermi‐Dirac statistics and the exclusion principle. Helium atoms of isotopic mass 4 obey Einstein‐Bose statistics, in which there can be many particles in the same quantum state, as is the case with photons, the quanta of radiation, if they are regarded as particles.

Nonhydrodynamic spin transport in superfluid 3He.

Abstract: We report the observation of two kinds of novel nonhydrodynamic spin-transport phenomena of quasiparticles in superfluid 3 He-B. We find a drastic low-temperature depression of the transverse quasiparticle spin-diffusion coefficient. In addition, we have done the first measurements of the Leggett-Takagi spin-relaxation time far outside the hydrodynamic regime. The observations of spin diffusion are shown to be in quantitative agreement with a kinetic-equation approach to quasiparticle spin dynamics

Spin reorientation maneuver for spinning spacecraft

Abstract: A method for controlling reorientation of a spacecraft's spin from a minor axis spin bias to a desired major axis spin after spin transition. A control system monitors rotational rates about the principal axes to detect a separatrix crossing of a polhode path therein. Controlled thruster firings resulting from spin rate information successively decrease and increase a characteristic parameter and capture the spacecraft during a spin transition to a desired major axis bias orientation. It is possible to monitor only ω1 and ω3 in an alternate embodiment.

Neutron scattering study of YBa2Cu3O6+x single crystals

Abstract: Inelastic neutron scattering studies of YBa 2 Cu 3 O 6+ x single crystals are reported. In the pure AF phase ( x = 0.15, T N = 410 K) the 2D-character of the spin wave spectrum and the exchange parameters have been established. A small amount of p holes in CuO 2 planes ( x = 0.37, T N = 180 K,m n h = 1.8%) strongly modifies the spin dynamics at low q : strong damping of in-plane spin excitations and renormalization of the spin wave velocity. In the superconducting state ( x = 0.45, T c = 34.8 K) we have found dynamical magnetic correlations and an anomalous decrease of the intensity at low energy in the vicinity of T c .

Non-Factorizable Separable Systems and Higher-Order Symmetries of the Dirac Operator

Abstract: It is shown that there exist separable systems for the Dirac operator on four-dimensional lorentzian spin manifolds that are not factorizable in the sense of Miller. The symmetry operators associated to these new separable systems are of higher order than the Dirac operator. They are characterized in the second-order case in terms of quadratic first integrals of the geodesic flow satisfying additional invariant conditions.

Three-dimensional nuclear magnetic resonance approach to multiple-quantum-filtered correlated spectroscopy and its application to proteins

Abstract: The elucidation of protein structures using NMR spectroscopy relies on the identification of amino acid spin systems. This has been greatly facilitated by the introduction of multiple-quantum-filtered correlation spectroscopy (MQF-COSY). This communication describes a three-dimensional (3D) experiment which performs a straightforward time-saving acquisition of the usual double quantum filter, triple quantum filter, {hor ellipsis}, n-quantum filter COSY 2D spectra and its application to a protein.

On quantum holonomy for mixed states

Abstract: There is a natural connection and parallel transport on the Hilbert tensor product ℋ⊗ℋ (or, equivalently, the space of Hilbert-Schmidt operators), the elements of which represent density matrices in ℋ up to unitary operators. We postulate a time evolution equation, which leads to this connection after extracting a proper ‘dynamical’ unitary phase. As an example, we compute the holonomy of a loop of temperature states for the spin in a rotating magnetic field.