Showing papers on "Spin-½ published in 1995"

Direct Observation of Transfer of Angular Momentum to Absorptive Particles from a Laser Beam with a Phase Singularity

Abstract: Black or reflective particles can be trapped in the dark central minimum of a doughnut laser beam produced using a high efficiency computer generated hologram. Such beams carry angular momentum due to the helical wave-front structure associated with the central phase singularity even when linearly polarized. Trapped absorptive particles spin due to absorption of this angular momentum transferred from the singularity beam. The direction of spin can be reversed by changing the sign of the singularity.

Anomalous Properties around Magnetic Instability in Heavy Electron Systems

Abstract: Spin fluctuations in heavy electron systems around their antiferromagnetic instability are discussed from a phenomenological point of view by using a sum rule for the dynamical susceptibility valid in the strong correlation limit. As a result the dynamical susceptibility takes the same form as in the standard spin fluctuation theory of weak itinerant antiferromagnetism, although the values for the parameters are substantially different from those in d-metals. The expression for the Neel temperature given in terms of the staggered magnetization at 0 K and the spin fluctuation parameters are successfully compared with experiment. Anomalous (non-Fermi liquid) behaviors reported for the specific heat and electrical resistivity in some heavy electron systems around their magnetic instabilities are explained in terms of this theory.

Projected shell model and high-spin spectroscopy

Abstract: Most of the nuclei in the nuclear chart are deformed except for those in the vicinity of the magic numbers. It is difficult to treat such nuclei within the framework of the standard (spherical) shell model. On the other hand, the necessity for a proper quantum mechanical treatment of high-spin states has been steadily growing ever since modern experimental techniques made it possible to measure the fine details of the high-spin states of heavy nuclei. The present article reviews an approach based on the angular momentum projection technique which was initiated in the late seventies for the purpose of carrying out shell model configuration mixing calculations efficiently. A large number of examples is presented with an emphasis on the physical interpretation of the numerical results. Computing time for the whole spectrum up to spin ≈ 40 of an axially symmetric rare-earth nucleus takes only a few minutes on a Mainframe, showing the efficiency of the method. Most of the present calculations were carried out on a Workstation, but computation on a modern PC also presents no problem, so that one can enjoy a genuine quantum mechanical analysis of high-spin data using a facility available everywhere. Detailed technical information which may be useful for programming purposes is given in an Appendix.

Spin blockades in linear and nonlinear transport through quantum dots.

Abstract: The transport properties of a quantum dot that is weakly coupled to leads are investigated by using the exact quantum states of a finite number of interacting electrons. It is shown that, in addition to the Coulomb blockade, spin selection rules strongly influence the low temperature transport and lead to experimentally observable effects. Transition probabilities between states that correspond to successive electron numbers vanish if the total spins differ by $|\ensuremath{\Delta}S|g\frac{1}{2}$. In nonlinear transport, this can lead to negative differential conductances. The linear conductance peaks are suppressed if transitions between successive ground states are forbidden.

Main Group Effective Nuclear Charges for Spin-Orbit Calculations

Abstract: The effective nuclear charges (Z.,ff), which are empirical parameters in an approximate spin-orbit Hamiltonian, are determined for main group elements in the second to fifth periods by using experimental results for the fine structure splittings (FSS) in II states of diatomic hydrides. All calculations use full valence multiconfiguration self-consistent field (MCSCF) wave functions with the effective core potential (ECP) basis sets proposed by Stevens et al., augmented by one set of polarization functions. These effective nuclear charges are tested by predicting the FSS in many diatomic molecules and are then applied to evaluate the relativistic potential energy curves of the methylene analogs AHz (A = C, Si, Ge, and Sn), as well as XHX and NaX (X = Br and 1). Disciplines Chemistry Comments Reprinted (adapted) with permission from Journal of Physical Chemistry 99 (1995): 12764, doi:10.1021/ j100034a013. Copyright 1995 American Chemical Society. This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/chem_pubs/297 12764 J. Phys. Chern. 1995, 99, 12764-12772 Main Group Effective Nuclear Charges for Spin-Orbit Calculations

Spin Gap Behavior of S=1/2 Quasi-Two-Dimensional System CaV4O9

Abstract: Spin susceptibility and spin-lattice relaxation rate of 51 V nuclear moments of CaV 4 O 9 with tetragonal structure have been measured. They exhibit characteristics of spin gap systems. This provides the first example of quasi-two-dimensional spin systems with spin gap.

Time-Dependent Optimized Effective Potential

Abstract: Given an expression for the quantum mechanical action of an N -electron system as a functional of N time-dependent spin orbitals, we present a method of constructing the variationally best local time-dependent single-particle potentials which, when inserted in time-dependent singleparticle Schrodinger equations for the spin-up and spin-down electrons yield orbitals that make stationary. We also propose a simplification of this scheme leading to a time-dependent generalization of the static optimized effective potentials recently introduced by Krieger, Li, and Iafrate [Phys. Lett. A 146, 256 (1990)]. Owing to rapid experimental progress in the field of laser physics, ultrashort laser pulses of very high intensity have become available in recent years. The electric field produced in such pulses can reach the strength of the electric field caused by atomic nuclei. If an atomic system is placed in the focus of such a laser pulse, one observes a wealth of new phenomena [1] which cannot be explained by perturbation theory. The nonperturbative quantum mechanical description of interacting particles moving in a very strong time-dependent external field therefore has become a prominent problem of theoretical physics. Since its rigorous foundation by Runge and Gross [2], time-dependent density functional theory (TDDFT) [3 ‐ 5] is available as a method to deal with time-dependent many-particle problems of this kind. The central statement of TDDFT is that the time-dependent density of a system of interacting particles moving in an external potential can be calculated, in principle exactly, from a set of time-dependent single-particle equations which can be viewed as the time-dependent counterpart of the Kohn-Sham scheme. These singleparticle equations involve an exchange-correlation potential, , which is a functional of and has to be approximated in practice. An extension of TDDFT to spin-polarized systems has been proposed by Liu and Vosko [6]. Neglecting magnetic effects associated with the orbital motion of the electrons, they consider external time-dependent potentials acting on electrons with spin only. In this case, two different corresponding to the two spin orientations are needed which are functionals of the spin densities . Again, the key problem is to obtain good approximations of . To date, only a rather crude adiabatic approximation is available which adopts the functional form of the static exchangecorrelation potential. The purpose of this paper is to introduce a different approach to the construction of

Ab initio spin dynamics in magnets.

Abstract: A set of coupled equations of motion for the evaluation of spin dynamics in magnets is introduced. This adiabatic approach considers the orientation of the local magnetic moments to be slowly varying relative to their magnitudes. The method is implemented within the local density approximation and applied to $\ensuremath{\gamma}$-Fe, a frustrated system where we obtain new low energy magnetic configurations.

The Spin-Spin Correlation Function in the Two-Dimensional Ising Model in a Magnetic Field at $T=T_c$

Abstract: The form factor bootstrap approach is used to compute the exact contributions in the large distance expansion of the correlation function $ $ of the two-dimensional Ising model in a magnetic field at $T=T_c$. The matrix elements of the magnetization operator $\sigma(x)$ present a rich analytic structure induced by the (multi) scattering processes of the eight massive particles of the model. The spectral representation series has a fast rate of convergence and perfectly agrees with the numerical determination of the correlation function.

Magnetic scaling in cuprate superconductors

Abstract: We determine the magnetic phase diagram for the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$ and ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ systems from various NMR experiments. We discuss the possible interpretation of NMR and neutron scattering experiments in these systems in terms of both the nonlinear \ensuremath{\sigma} model of nearly localized spins and a nearly antiferromagnetic Fermi liquid description of magnetically coupled quasiparticles. We show for both the 2:1:4 and 1:2:3 systems that bulk properties, such as the spin susceptibiltiy, and probes at the antiferromagnetic wave vector (\ensuremath{\pi},\ensuremath{\pi}), such as $^{63}$${\mathit{T}}_{1}$, the $^{63}\mathrm{Cu}$ spin-lattice relaxation time, both display a crossover at a temperature ${\mathit{T}}_{\mathrm{cr}}$, which increases linearly with decreasing hole concentration, from a nonuniversal regime to a z=1 scaling regime characterized by spin pseudogap behavior. We pursue the consequences of the ansatz that ${\mathit{T}}_{\mathrm{cr}}$ corresponds to a fixed value of the antiferromagnetic correlation length \ensuremath{\xi} and show how this enables one to extract the magnitude and temperature dependence of \ensuremath{\xi} from measurements of ${\mathit{T}}_{1}$ alone. We show that like ${\mathit{T}}_{\mathrm{cr}}$, the temperature ${\mathit{T}}_{\mathrm{*}}$ which marks a crossover at low temperatures from the z=1 scaling regime to a quantum disordered regime, exhibits the same dependence on doping for the 2:1:4 and 1:2:3 systems, and so arrive at a unified description of magnetic behavior in the cuprates, in which the determining factor is the planar hole concentration.

Absence of superconductivity in the doped antiferromagnetic spin-ladder compound (La,Sr)CuO2.5

Abstract: A SPIN-½ Heisenberg antiferromagnetic ladder is a model system comprising parallel chains of interacting elemental spins. Spin ladders having an even number of chains have been predicted1a¤-6 to exhibit interesting dynamics, including superconductivity, when unoccupied spin sites are introduced along the chain (that is, when the chains are doped with holes). Spin-ladder models thus provide a novel potential mechanism for high-temperature superconductivity in real materials. But unfortunately materials with spin-ladder structure are quite rare4a-10, and the few known compounds have not previously been doped successfully with holes. Here we report the high-pressure synthesis of a new hole-doped two-chain ladder compound, La1a¤-xSrxCuO2.5. We have observed a marked insulator-to-metal transition in this compound with increased doping, but no superconducting transition down to 5 K. The absence of superconductivity in this material must be reconciled with theoretical predictions based on ideal hole-doped spin-ladder models.

Single spin asymmetry for $p^{\uparrow}p \to πX$ in perturbative QCD

Abstract: Within the QCD-improved parton model and assuming the factorization theorem to hold in the helicity basis and for higher twist contributions, we show how non zero single spin asymmetries in hadron-hadron high energy and moderately large $p_T$ inclusive processes can be obtained, even in massless perturbative QCD, provided the quark intrinsic motion is taken into account. A simple model is constructed which reproduces the main features of the data on the single spin asymmetry observed in inclusive pion production in $p\,p$ collisions.

Fractional quantum Hall effect around nu =3/2: Composite fermions with a spin.

Abstract: Angular dependent magnetotransport measurements on the fractional quantum Hall (FQHE) states around Landau level filling factor $\ensuremath{ u}=\frac{3}{2}$ are explained very effectively in terms of composite fermions (CFs) with a spin. The disappearance and reappearance of FQHE states as well as their spin polarization is deduced from a simple "Landau level" fan diagram for CFs. While the "Landau splitting" scales with effective magnetic field, with its origin at $\ensuremath{ u}=\frac{3}{2}$, the spin-splitting scales with total external magnetic field having its origin at $B=0$. The $g$ factor of a CF is largely the $g$ factor of the electron.

Enhanced spin interactions in digital magnetic heterostructures.

Abstract: A new heterostructure is developed in which fractional monolayers of magnetic ions are introduced ``digitally'' within a semiconductor quantum well. Rearranging moments within two-dimensional (2D) planes provides additional control over their interactions, yielding tunable two-level electronic systems in which spin splittings are significantly larger than inhomogeneous linewidths in modest magnetic fields. Femtosecond-resolved electronic and magnetic spectroscopies reveal spin-flip scattering dependent only on the Zeeman energy but not the local magnetic environment, and long-lived dynamic magnetizations.

Low-Lying Eigenstates of the One-Dimensional Heisenberg Ferromagnet for any Magnetization and Momentum.

Abstract: I present the exact dispersion relations for certain low-lying states of the one-dimensional Heisenberg ferromagnet. These states are bound complexes of $M$ overturned spins, and in fact are the states with the lowest energy for given values of the total spin, $z$ component of spin and momentum.

Extension of the Local-Spin-Density Exchange-Correlation Approximation to Multiplet States

Abstract: We investigate a simple generalization of the local-spin-density exchange-correlation approximation of density-functional theory from single to multi-determinantal states. The method is explicitly spin independent and trivially preserves multiplet spin degeneracies. Tests on multiplet splittings in a variety of low-lying configurations of first-row atoms and ions are presented.

Correlated few-electron states in vertical double-quantum-dot systems

Abstract: The electronic properties of semiconductor, vertical, double quantum dot systems with few electrons are investigated by means of analytic, configuration-interaction, and mean-field methods. The combined effect of a high magnetic field, electrostatic confinement, and inter-dot coupling, induces a new class of few-electron ground states absent in single quantum dots. In particular, the role played by the isospin (or quantum dot index) in determining the appearance of new ground states is analyzed and compared with the role played by the standard spin.

Large deviations for Langevin spin glass dynamics

Abstract: We study the asymptotic behaviour of asymmetrical spin glass dynamics in a Sherrington-Kirkpatrick model as proposed by Sompolinsky-Zippelius. We prove that the annealed law of the empirical measure on path space of these dynamics satisfy a large deviation principle in the high temperature regime. We study the rate function of this large deviation principle and prove that it achieves its minimum value at a unique probability measureQ which is not markovian. We deduce that the quenched law of the empirical measure converges to δ Q . Extending then the preceeding results to replicated dynamics, we investigate the quenched behavior of a single spin. We get quenched convergence toQ in the case of a symmetric initial law and even potential for the free spin.

Dynamics of Spin Organization in Diluted Magnetic Semiconductors.

Abstract: It is shown that spin-spin interactions rather than spin-lattice coupling control the dynamics of the magnetization formation visible in time-resolved luminescence and SQUID magnetometry of bulk and layered diluted magnetic semiconductors. A numerical solution of a nonlinear Schr\"odinger equation is applied to find the relation between formation time of the exciton magnetic polaron and relaxation time of the spin subsystem.

Monte Carlo method for spin models with long-range interactions

Abstract: We introduce a Monte Carlo method for the simulation of spin models with ferromagnetic long-range interactions in which the amount of time per spin-flip operation is independent of the system size, in spite of the fact that the interactions between each spin and all other spins are taken into account. We work out two algorithms for the q-state Potts model and discuss the generalization to systems with other interactions and to O(n) models. We illustrate the method with a simulation of the mean-field Ising model, for which we have also analytically calculated the leading finite-size correction to the dimensionless amplitude ratio 2/ at the critical temperature.

All‐optical picosecond switching of a quantum well etalon using spin‐polarization relaxation

Abstract: All‐optical picosecond gate operation is demonstrated using the spin relaxation in a quantum well etalon. Electron spin‐polarization photoexcited by circularly polarized light in GaAs quantum wells decays in picoseconds. With the use of the spin relaxation in a quantum well etalon, all‐optical full switching is achieved with a decay time of as fast as 7 ps, by adopting a simple optical differential method, which uses only a quarter‐wave plate and a polarizer in addition to the conventional all‐optical switching setup.

Two-band singlet-hole model for the copper oxide plane.

Abstract: An effective Hubbard model for one-hole {ital d}-like states and two-hole singlet states is derived from the original {ital p}-{ital d} model to describe the low-energy electronic spectrum of the CuO{sub 2} plane in cuprates. By using the projection technique for the two-time matrix Green`s function in terms of Hubbard operators a two-band spectrum for {ital d}-like holes and singlets as well as the density of states is calculated. It is found that the hybridization between {ital d}-like holes and singlets results in a substantial renormalization of the spectrum. In addition, the dispersion relation depends strongly on the antiferromagnetic short-range spin correlations in the spin-singlet state: For large spin correlations at small doping values one finds a next-nearest-neighbor dispersion. With doping, by decreasing the spin correlations, the dispersion changes to an ordinary nearest-neighbor one.