Showing papers on "Spin-½ published in 1996"
TL;DR: In this paper, the interaction between spin waves and itinerant electrons is considerably enhanced in the vicinity of an interface between normal and ferromagnetic layers in metallic thin films, leading to a local increase of the Gilbert damping parameter which characterizes spin dynamics.
Abstract: The interaction between spin waves and itinerant electrons is considerably enhanced in the vicinity of an interface between normal and ferromagnetic layers in metallic thin films. This leads to a local increase of the Gilbert damping parameter which characterizes spin dynamics. When a dc current crosses this interface, stimulated emission of spin waves is predicted to take place. Beyond a certain critical current density, the spin damping becomes negative; a spontaneous precession of the magnetization is predicted to arise. This is the magnetic analog of the injection laser. An extra dc voltage appears across the interface, given by an expression similar to that for the Josephson voltage across a superconducting junction. \textcopyright{} 1996 The American Physical Society.
4,433 citations
TL;DR: In this paper, a model of the magnetization within these particles consisting of ferrimagnetically aligned core spins and a spin-glass-like surface layer is proposed, and the qualitative features of this model are reproduced by a numerical calculation of the spin distribution.
Abstract: Nickel ferrite nanoparticles exhibit anomalous magnetic properties at low temperatures: low magnetization with a large differential susceptibility at high fields, hysteresis loops which are open up to 160 kOe, time-dependent magnetization in 70 kOe applied field, and shifted hysteresis loops after field cooling. We propose a model of the magnetization within these particles consisting of ferrimagnetically aligned core spins and a spin-glass-like surface layer. We find that qualitative features of this model are reproduced by a numerical calculation of the spin distribution. Implications of this model for possible macroscopic quantum tunneling in these materials are discussed.
1,407 citations
01 Mar 1996
1,123 citations
TL;DR: Coulomb oscillations in vertical quantum dots containing a tunable number of electrons starting from zero are measured, as predicted by Hund’s rule, to favor the filling of parallel spins.
Abstract: We study atomiclike properties of artificial atoms by measuring Coulomb oscillations in vertical quantum dots containing a tunable number of electrons starting from zero. At zero magnetic field the energy needed to add electrons to a dot reveals a shell structure for a two-dimensional harmonic potential. As a function of magnetic field the current peaks shift in pairs, due to the filling of electrons into spin-degenerate single-particle states. When the magnetic field is sufficiently small, however, the pairing is modified, as predicted by Hund’s rule, to favor the filling of parallel spins. [S00319007(96)01418-4]
1,090 citations
26 May 1996
TL;DR: In this paper, the authors used algebraic Bethe Ansatz for solving integrable models and showed how it works in detail on the simplest example of spin 1/2 XXX magnetic chain.
Abstract: I study the technique of Algebraic Bethe Ansatz for solving integrable models and show how it works in detail on the simplest example of spin 1/2 XXX magnetic chain. Several other models are treated more superficially, only the specific details are given. Several parameters, appearing in these generalizations: spin $s$, anisotropy parameter $\ga$, shift $\om$ in the alternating chain, allow to include in our treatment most known examples of soliton theory, including relativistic model of Quantum Field Theory.
814 citations
TL;DR: In this article, a functional calculus is used to construct a quantum theory of geometry, where the fundamental excitations of quantum geometry are 1-dimensional, rather like polymers, and the 3-dimensional continuum geometry emerges only on coarse graining.
Abstract: A new functional calculus, developed recently for a fully non-perturbative treatment of quantum gravity, is used to begin a systematic construction of a quantum theory of geometry. Regulated operators corresponding to areas of 2-surfaces are introduced and shown to be self-adjoint on the underlying (kinematical) Hilbert space of states. It is shown that their spectra are {\it purely} discrete indicating that the underlying quantum geometry is far from what the continuum picture might suggest. Indeed, the fundamental excitations of quantum geometry are 1-dimensional, rather like polymers, and the 3-dimensional continuum geometry emerges only on coarse graining. The full Hilbert space admits an orthonormal decomposition into finite dimensional sub-spaces which can be interpreted as the spaces of states of spin systems. Using this property, the complete spectrum of the area operators is evaluated. The general framework constructed here will be used in a subsequent paper to discuss 3-dimensional geometric operators, e.g., the ones corresponding to volumes of regions.
611 citations
Posted Content•
23 Feb 1996TL;DR: In this article, a functional calculus is used to construct a quantum theory of geometry, where the fundamental excitations of quantum geometry are 1-dimensional, rather like polymers, and the 3-dimensional continuum geometry emerges only on coarse graining.
Abstract: A new functional calculus, developed recently for a fully non-perturbative treatment of quantum gravity, is used to begin a systematic construction of a quantum theory of geometry. Regulated operators corresponding to areas of 2-surfaces are introduced and shown to be self-adjoint on the underlying (kinematical) Hilbert space of states. It is shown that their spectra are {\it purely} discrete indicating that the underlying quantum geometry is far from what the continuum picture might suggest. Indeed, the fundamental excitations of quantum geometry are 1-dimensional, rather like polymers, and the 3-dimensional continuum geometry emerges only on coarse graining. The full Hilbert space admits an orthonormal decomposition into finite dimensional sub-spaces which can be interpreted as the spaces of states of spin systems. Using this property, the complete spectrum of the area operators is evaluated. The general framework constructed here will be used in a subsequent paper to discuss 3-dimensional geometric operators, e.g., the ones corresponding to volumes of regions.
525 citations
Book•
01 Jan 1996
TL;DR: The Seiberg-Witten Moduli Space as discussed by the authors was used for the Dirac Operator and its spin groups in the early 20th century, and it was used in the creation of spin bundles and spindles.
Abstract: 1Introduction12Clifford Algebras and Spin Groups53Spin Bundles and the Dirac Operator234The Seiberg-Witten Moduli Space555Curvature Identities and Bounds696The Seiberg-Witten Invariant877Invariants of Kahler Surfaces109Bibliography127
456 citations
TL;DR: Two-dimensional magic-angle spinning (triple quantum, single quantum) correlation pulse sequences and phase cycles based on the technique of Frydman and Harwood for the reconstruction of the isotropic spectrum of half-integer spin quadrupolar nuclei broadened to second-order are described.
381 citations
TL;DR: This work studies resonant tunneling through a single-level quantum dot in the presence of strong Coulomb repulsion beyond the perturbative regime and predicts that the sign of the zero-bias anomaly depends on the level position relative to the Fermi level of the leads.
Abstract: We study resonant tunneling through a single-level quantum dot in the presence of strong Coulomb repulsion beyond the perturbative regime. The level is either spin degenerate or can be split by a magnetic field. Furthermore we discuss the influence of a bosonic environment. Using a real-time diagrammatic formulation, we calculate transition rates, the spectral density, and the nonlinear I-V characteristic. The spectral density shows a multiplet of Kondo peaks split by the transport voltage and the boson frequencies and shifted by the magnetic field. This leads to zero-bias anomalies in the differential conductance, which agree well with recent experimental results for the electron transport through single-charge traps. Furthermore, we predict that the sign of the zero-bias anomaly depends on the level position relative to the Fermi level of the leads. \textcopyright{} 1996 The American Physical Society.
TL;DR: In this paper, the static and dynamical properties of weakly coupled antiferromagnetic spin chains are treated using a mean-field approximation for the interchain coupling and exact results for the resulting effective one-dimensional problem.
Abstract: Static and dynamical properties of weakly coupled antiferromagnetic spin chains are treated using a mean-field approximation for the interchain coupling and exact results for the resulting effective one-dimensional problem. Results for staggered magnetization, N\'eel temperature, and spin wave excitations are in agreement with experiments on ${\mathrm{KCuF}}_{3}$. The existence of a narrow longitudinal mode is predicted. The results are in agreement with general scaling arguments, contrary to spin wave theory.
TL;DR: In this paper, the existence of resonant spin tunnelling between degenerate excited levels of opposite spin orientation was inferred from magnetic-field values Bn = nB1, where B1? 5 kG.
Abstract: Zero-field?cooled magnetization, magnetic relaxation, and a.c. susceptibility measurements have been performed on a Mn12-Ac oriented powdered sample as a function of temperature, field, and orientation. Magnetization jumps and susceptibility peaks have been observed about magnetic-field values Bn = nB1, where B1 ? 5 kG. These anomalies are due to the existence of relaxation rate maxima near Bn. From these experimental results we infer the existence of resonant spin tunnelling between degenerate excited levels of opposite spin orientation.
TL;DR: In this paper, the authors show that spin projection can seriously degrade the quality of potential energy surfaces calculated by density functional methods, just as spin projection yields poor results for Hartree-Fock potential energy surface.
Abstract: Spin unrestricted calculations using density functional theory can yield wave functions with spin contamination. In conventional post Hartree–Fock calculations (such as Mo/ller–Plesset perturbation theory), spin projection can ameliorate some of the problems caused by spin contamination. However, spin projection can seriously degrade the quality of potential energy surfaces calculated by density functional methods, just as spin projection can yield poor results for Hartree–Fock potential energy surfaces.
TL;DR: A model, based on the Larmor-precession-induced deviation of the conduction electron spin direction during domain-wall traversal is developed, which is possible to account for the amplitude of the measured magnetoresistive effect.
Abstract: By combining parallel and transverse magnetoresistance measurements on thin films of Co and Ni, the contribution of spin scattering at the domain walls is separated from the anisotropic magnetoresistance (AMR). A model, based on the Larmor-precession-induced deviation of the conduction electron spin direction during domain-wall traversal is developed. By using a scattering probability which varies with the cosine of the angle between the carrier spin and the local exchange field (as used for giant magnetoresistance systems) it is possible to account for the amplitude of the measured magnetoresistive effect. \textcopyright{} 1996 The American Physical Society.
TL;DR: In this article, the ground state properties of frustrated quantum Heisenberg antiferromagnet on the square lattice (J 1 - J 2 model) were investigated using exact diagonalization of finite clusters with 16, 20, 32, and 36 sites.
Abstract: We have performed a numerical investigation of the ground state properties of the frustrated quantum Heisenberg antiferromagnet on the square lattice (J 1 - J 2 model), using exact diagonalization of finite clusters with 16, 20, 32, and 36 sites. Using a finite-size scaling analysis we obtain results for a number of physical properties : magnetic order parameters, ground state energy, and magnetic susceptibility (at q = 0). In order to assess the reliability of our calculations, we also investigate regions of parameter space with well-established magnetic order, in particular the non-frustrated case J 2 0.68. An error analysis indicates uncertainties of order ±0.04 in the location of these critical values of J 2 . There thus is a region in parameter space without any form of magnetic order. For the unfrustrated case the results for order parameter, ground state energy, and susceptibility agree with series expansions and quantum Monte Carlo calculations to within a percent or better. Including the 16 site cluster, or analyzing the independently calculated magnetic susceptibility we also find a nonmagnetic region, but with modified values for the range of existence of the nonmagnetic region. From the leading finite-size corrections we also obtain results for the spin-wave velocity and the spin stiffness. The spin-wave velocity remains finite at the magnetic-nonmagnetic transition, as expected from the nonlinear sigma model analogy.
TL;DR: In this article, a natural statistical ensemble of 2J points on the unit sphere can be associated, via the Majorana representation, with a random quantum state of spin J, and an exact expression is obtained here for the general k point correlation function in this ensemble.
Abstract: A natural statistical ensemble of 2J points on the unit sphere can be associated, via the Majorana representation, with a random quantum state of spin J, and an exact expression is obtained here for the general k point correlation function in this ensemble. The pair correlation in the large-J limit takes the relatively simple form where and is the angular separation of the pair of points on the sphere. It appears (from the numerical work of others) that, in this limit, these statistics are typical of the zero points of analytic functions associated with chaotic quantum dynamical systems.
15 Apr 1996
TL;DR: In this article, a transformation to a moving frame (the Eckardt frame) is used to study the quantum states of interacting electrons in parabolic quantum dots in the presence of a perpendicular magnetic field.
Abstract: A transformation to a moving frame (the Eckardt frame) is used to study the quantum states of interacting electrons in parabolic quantum dots in the presence of a perpendicular magnetic field. The approach is motivated by examining ground-state pair-correlation functions obtained by exact diagonalization. The main results concern the physical nature of the electron states and the origin of magic numbers. Some of the states are found to be localized about a single minimum of the potential energy. They have well-defined symmetry and are physically analogous to molecules. They are treated approximately by antisymmetrizing Eckardt frame rotational-vibrational states. This approach leads to selection rules that predict all the magic angular momentum and spin combinations found in previous numerical work. In addition, it enables the ground-state energy and low-lying excitations of the molecular states to be calculated to high accuracy. Analytic results for three electrons agree very well with the results of exact diagonalization. States that are not localized about a single minimum are also studied. They do not have distinct spatial symmetry and occur only when selection rules and conservation laws allow tunneling between states localized on different minima. These states appear to be small system precursors of fractional quantum Hall liquids. \textcopyright{} 1996 The American Physical Society.
TL;DR: In this article, a spin-fluctuation theory where the modes are coupled by interatomic exchange interactions is employed for the finite-temperature properties of ferromagnetic Fe, Co, and Ni.
Abstract: Finite-temperature properties are modeled for the itinerant-electron ferromagnets Fe, Co, and Ni by employing a spin-fluctuation theory where the modes are coupled by interatomic exchange interactions. Our method is based on the density functional theory using the local density approximation. The latter yields all parameters derived from constrained ground-state properties of noncollinear spin configurations to calculate ab initio the Curie temperatures, the magnetic susceptibilities, and, furthermore, the hcp-fcc phase transition of Co. Our results are in fair agreement with experimental data.
Posted Content•
TL;DR: In this article, the authors used algebraic Bethe Ansatz for solving integrable models and showed how it works in detail on the simplest example of spin 1/2 XXX magnetic chain.
Abstract: I study the technique of Algebraic Bethe Ansatz for solving integrable models and show how it works in detail on the simplest example of spin 1/2 XXX magnetic chain. Several other models are treated more superficially, only the specific details are given. Several parameters, appearing in these generalizations: spin $s$, anisotropy parameter $\ga$, shift $\om$ in the alternating chain, allow to include in our treatment most known examples of soliton theory, including relativistic model of Quantum Field Theory.
TL;DR: The calculated expectation values for the ground-state moments of the actinide ions in intermediate coupling show that the spin-orbit-induced operators, such as the magnetic-dipole term, differ strongly from their Hund's rule ground- state values.
Abstract: Sum rules for magnetic x-ray dichroism, relating the signals of the spin-orbit split core level absorption edges to the ground-state spin and orbital operators, are expressed in jj-coupled operators. These sum rules can be used in the region of intermediate coupling by taking into account the cross term between the j=l+/-1/2 ground-state levels and are, therefore, particularly useful in the study of actinides. The calculated expectation values for the ground-state moments of the actinide ions in intermediate coupling show that the spin-orbit-induced operators, such as the magnetic-dipole term, differ strongly from their Hund's rule ground-state values. We also prove the general rule that, when there is a perturbing interaction which is weak compared to the spin-orbit interaction, the ratio of operators with the same total moment remains constant. This rendition is usually fulfilled for the crystal-field interaction in the lanthanides and actinides. The values of the ground-state moments as a function off count give rise to an interesting trend in the dichroism of the spin-orbit split-core edges. The branching ratio of the 3d and 4d circular dichroism signal gradually increases from nearly zero for 5f(1) to similar to 0.4 for 5f(5) and is close to unity for a more than half-filled shell. The unusual behavior of the branching ratio can be related to the higher (lower) value of the magnetic dipole term, Ti, for a less (more) than half-filled shell of the actinides in the presence of spin-orbit interaction. Uranium compounds will have a much larger value of T-z than the corresponding 4f compounds. Its precise value can be used as a measure for the f count.
TL;DR: In this paper, the form of the spectral curve for 4d N = 2 supersymmetric Yang-Mills theory with matter fields in the fundamental representation of the gauge group suggests that its 1 d integrable counterpart should be looked for among (inhomogeneous) sl (2) spin chains with the length of the chain being equal to the number of colours N c.
TL;DR: In this paper, a magnet in the form of a spinning-top can float stably above a repelling magnetic base, and the principal mechanism of stability is static equilibrium in a potential energy field E, arising dynamically from the adiabatic coupling of the spin with the magnetic field B of the base and involving the magnitude B of this field.
Abstract: A magnet in the form of a spinning-top can float stably above a repelling magnetic base. The principal mechanism of stability is static equilibrium in a potential energy field E, arising dynamically from the adiabatic coupling of the spin with the magnetic field B of the base and involving the magnitude B of this field. E is close to a harmonic potential, that is, one whose Laplacian is zero, for which Earnshaw's theorem would forbid stable equilibrium. Therefore its minimum is very shallow, and requires the mass of the top to be adjusted delicately so that it hangs within a small interval of height. The stability interval is increased by a post-adiabatic dynamic coupling of the velocity of the top to B, through an effective `geometric magnetic field' constructed from the spatial derivatives of B; this effect gets stronger as the top is spun faster. The device is analogous to several traps for microscopic particles.
TL;DR: In this paper, it was shown that top-quark pairs are produced in an essentially unique spin configuration in polarized e + e − colliders at all energies above the threshold region.
TL;DR: In this article, the correlation of the spins of top quarks and antiquarks at the Tevatron and the LHC has been studied, and it has been shown that the top quark decays before its spin flips.
TL;DR: The spin dependence of the reflection probabilities is strong enough to give a large giant magnetoresistance even if there is no spin-dependent defect scattering as mentioned in this paper, and the calculated reflection amplitudes determine the strength of the oscillatory exchange coupling.
Abstract: First‐principles calculations of transmission and reflection from Ag/Fe, Au/Fe, Cu/Co, and Cu/Ni interfaces show very strong spin dependence that differs significantly from expectations based on free electron approximations The results can be used to understand both the giant magnetoresistance and the oscillatory exchange coupling observed in magnetic multilayers of these materials The spin dependence of the reflection probabilities is strong enough to give a large giant magnetoresistance even if there is no spin‐dependent defect scattering The calculated reflection amplitudes determine the strength of the oscillatory exchange coupling
TL;DR: In this paper, the spin-dependent next-to-leading order splitting functions are calculated in the light-cone gauge and the results for different prescriptions for the Dirac matrix are given.
Abstract: We present a complete description of the calculation of the spin-dependent next-to-leading order splitting functions. The calculation is performed in the light-cone gauge. We give results for different prescriptions for the Dirac matrix $\gamma_5$ in $d=4-2 \epsilon$ dimensions and provide the link to the results in dimensional reduction.
TL;DR: In this paper, a simple method of correcting the mixed spin energies resulting from unrestricted Hartree-Fock or density-functional theory calculations and removing the foreign spin components is presented, which allows for elimination of higher-multiplet components from the given mixed spin state solution by performing unrestricted calculations at the same fixed geometry for the higher multiplets and the state under consideration.
Abstract: The unrestricted Hartree-Fock and unrestricted Kohn-Sham calculations generally result in spin-contaminated solutions. Moreover, the energies from these calculations cannot be directly compared with the results of corresponding restricted calculations since the latter yield higher energies due to restrictions imposed on the form of the wave function. We present here a simple method of correcting the mixed spin energies resulting from unrestricted Hartree-Fock or density-functional theory calculations and removing the foreign spin components. The method allows for elimination of higher-multiplet components from the given mixed spin state solution by performing unrestricted calculations at the same fixed geometry for the higher multiplets and the state under consideration. The performance of the method is illustrated with several examples of density-functional calculations of radical species. The current method is also variational in nature and can be further extended in a self-consistent field fashion. \textcopyright{} 1996 The American Physical Society.