Showing papers on "Spin-½ published in 1994"
TL;DR: Theoretical ideas and experimental results concerning high-temperature superconductors are reviewed in this article, with special emphasis on calculations performed with the help of computers applied to models of strongly correlated electrons proposed to describe the two-dimensional Cu${\mathrm{O}}_{2} planes.
Abstract: Theoretical ideas and experimental results concerning high-temperature superconductors are reviewed Special emphasis is given to calculations performed with the help of computers applied to models of strongly correlated electrons proposed to describe the two-dimensional Cu${\mathrm{O}}_{2}$ planes The review also includes results using several analytical techniques The one- and three-band Hubbard models and the $t\ensuremath{-}J$ model are discussed, and their behavior compared against experiments when available The author found, among the conclusions of the review, that some experimentally observed unusual properties of the cuprates have a natural explanation through Hubbard-like models In particular, abnormal features like the mid-infrared band of the optical conductivity $\ensuremath{\sigma}(\ensuremath{\omega})$, the new states observed in the gap in photoemission experiments, the behavior of the spin correlations with doping, and the presence of phase separation in the copper oxide superconductors may be explained, at least in part, by these models Finally, the existence of superconductivity in Hubbard-like models is analyzed Some aspects of the recently proposed ideas to describe the cuprates as having a ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ superconducting condensate at low temperatures are discussed Numerical results favor this scenario over others It is concluded that computational techniques provide a useful, unbiased tool for studying the difficult regime where electrons are strongly interacting, and that considerable progress can be achieved by comparing numerical results against analytical predictions for the properties of these models Future directions of the active field of computational studies of correlated electrons are briefly discussed
2,262 citations
TL;DR: The theory for analytic energy derivatives of excited electronic states described by the equation-of-motion coupled cluster (EOM•CC) method has been generalized to treat cases in which reference and final states differ in the number of electrons as discussed by the authors.
Abstract: The theory for analytic energy derivatives of excited electronic states described by the equation‐of‐motion coupled cluster (EOM‐CC) method has been generalized to treat cases in which reference and final states differ in the number of electrons. While this work specializes to the sector of Fock space that corresponds to ionization of the reference, the approach can be trivially modified for electron attached final states. Unlike traditional coupled cluster methods that are based on single determinant reference functions, several electronic configurations are treated in a balanced way by EOM‐CC. Therefore, this quantum chemical approach is appropriate for problems that involve important nondynamic electron correlation effects. Furthermore, a fully spin adapted treatment of doublet electronic states is guaranteed when a spin restricted closed shell reference state is used—a desirable feature that is not easily achieved in standard coupled cluster approaches. The efficient implementation of analytic gradien...
532 citations
TL;DR: The master equation describing non-equilibrium one-dimensional problems like diffusion limited reactions or critical dynamics of classical spin systems can be written as a Schrodinger equation in which the wave function is the probability distribution and the Hamiltonian is that of a quantum chain with nearest neighbor interactions as discussed by the authors.
306 citations
TL;DR: It is shown how suitable boundary conditions, which do not frustrate N\'eel order, allow the study of samples with N=3p+1 spins, and a thorough study of these situations is done in parallel with the more conventional case N= 3p.
Abstract: Exact spectra of periodic samples are computed up to N=36. Evidence of an extensive set of low-lying levels, lower than the softest magnons, is exhibited. These low-lying quantum states are degenerated in the thermodynamic limit; their symmetries and dynamics as well as their finite-size scaling are strong arguments in favor of N\'eel order. It is shown that the N\'eel order parameter agrees with first-order spin-wave calculations. A simple explanation of the low-energy dynamics is given as well as the numerical determinations of the energies, order parameter, and spin susceptibilities of the studied samples. It is shown how suitable boundary conditions, which do not frustrate N\'eel order, allow the study of samples with N=3p+1 spins. A thorough study of these situations is done in parallel with the more conventional case N=3p.
278 citations
TL;DR: The results in the language of recently discussed scaling theories, give the fixed-point action, and show that at this fixed point the fermion-gauge-field interaction is marginal in d=2, but irrelevant at low energies in d\ensuremath{\ge}2.
Abstract: We calculate the fermion Green function and particle-hole susceptibilities for a degenerate two-dimensional fermion system with a singular gauge interaction. We show that this is a strong-coupling problem, with no small parameter other than the fermion spin degeneracy N. We consider two interactions, one arising in the context of the t-J model and the other in the theory of half-filled Landau level. For the fermion self-energy we show that the qualitative behavior found in the leading order of perturbation theory is preserved to all orders in the interaction. The susceptibility ${\mathrm{\ensuremath{\chi}}}_{\mathit{Q}}$ at a general wave vector Q\ensuremath{
e}2${\mathbf{p}}_{\mathbf{F}}$ retains the Fermi-liquid form. However, the 2${\mathit{p}}_{\mathit{F}}$ susceptibility ${\mathrm{\ensuremath{\chi}}}_{2\mathit{p}\mathit{F}}$ either diverges as T\ensuremath{\rightarrow}0 or remains finite but with nonanalytic wave-vector, frequency, and temperature dependence. We express our results in the language of recently discussed scaling theories, give the fixed-point action, and show that at this fixed point the fermion-gauge-field interaction is marginal in d=2, but irrelevant at low energies in d\ensuremath{\ge}2.
224 citations
219 citations
IBM1
TL;DR: In this article, two types of patterned, unshielded giant magneto-resistance (GMR) spin valve sensors have been fabricated: nano-layered NiFe/Co/Cu/co/NiFe and simpler NiFe /Cu/Co spin valves.
Abstract: Two types of patterned, unshielded Giant MagnetoResistance (GMR) spin valve sensors have been fabricated: nano-layered NiFe/Co/Cu/Co/NiFe and simpler NiFe/Cu/Co spin valves. GMR values of 7.6% for /spl Delta/H=10 Oe were measured for the nano-layered structures on coupons. Transfer curves in uniform fields were obtained and were in agreement with theoretical expectations. The sensors were highly linear and well biased. Optimum biasing of the free layer in the spin valve sensor has new features over that in AMR sensors. These were explored in shielded as well as unshielded spin valves using micromagnetic simulation. >
217 citations
204 citations
TL;DR: In this paper, the spin purity of states described by the coupled-cluster singles and doubles (CCSD) approximation based on unrestricted and restricted open-shell Hartree-Fock (UHF and ROHF) determinants is investigated.
Abstract: The spin purity of states described by the coupled‐cluster singles and doubles (CCSD) approximation based on unrestricted and restricted open‐shell Hartree–Fock (UHF and ROHF) determinants is investigated. By contracting matrix elements of the S2 operator with reduced one‐ and two‐particle density matrices, the extent of spin contamination in ROHF‐CCSD and UHF‐CCSD solutions is quantitatively characterized. Results are presented for a representative set of molecules in doublet electronic states with UHF values of S2 that range from 0.76 to 1.18. In these examples, ROHF‐CCSD and UHF‐CCSD expectation values of S2 are similar, with both less than 0.015 above the nominally exact value of 3/4. This finding illustrates the characteristic insensitivity of CC methods with respect to the choice of orbitals. Finally, the negligible differences between UHF‐CCSD values of S2 and those corresponding to a spin eigenfunction suggests that modest reference state spin contamination does not represent a serious problem whe...
202 citations
01 Jan 1994
TL;DR: In this article, Lorentz invariance and Poincare invariance of free particles with spin and interacting fields are discussed. But they do not consider the effect of massless vector fields.
Abstract: Introduction 1. Lorentz and Poincare invariance 2. Relativistic quantum mechanics of free particles 3. Interacting fields 4. Particles with spin 5. Explorations 6. Renormalization 7. Massive and massless vector fields 8. Unitarity Appendices.
199 citations
TL;DR: In this article, the Kondo lattice model with Hund's ferromagnetic spin coupling was investigated as a microscopic model of the perovskite-type 3d transition-metal oxide La${1-x}$Sr$_x$MnO$_3.
Abstract: The Kondo lattice model with Hund's ferromagnetic spin coupling is investigated as a microscopic model of the perovskite-type 3d transition-metal oxide La$_{1-x}$Sr$_x$MnO$_3$. In the classical spin limit $S=\infty$ and the infinite-dimensional limit $D=\infty$, the one-body Green's function is calculated exactly. Transport properties of the system in the presence of magnetic fields are calculated. The giant magnetoresistance of this model, which is in a good agreement with the experimental data of La$_{1-x}$Sr$_x$MnO$_3$, is explained by the spin disorder scattering process.
01 Jan 1994
TL;DR: In this article, the neutrino spin precession effects in the magnetic field of the Sun are considered as an explanation of the outcome of Davis' solar neutrinos experiments. But, the value for the Neutrino magnetic moment evaluated under these theoretically anomalous circumstances is still so small that only the largest possible estimate for magnetic field strength in the Sun's interior would cause the required effect.
Abstract: Neutrino spin precession effects in the magnetic field of the Sun are considered as an explanation of the outcome of Davis' solar neutrino experiments. Theoretically, it is possible to account for a neutrino magnetic moment only as the result of the interaction of the electromagnetic field with charged particles into which the neutrino can transform virtually. The currently accepted theory of weak interactions (the two component neutrino andV-A interactions) forbids a resulting magnetic moment interaction with the electromagnetic field for all such virtual processes. Modifications of this theory are considered to find out whether an appreciable precession effect is permitted within the experimentally established limits. It is found that the value for the neutrino magnetic moment evaluated under these theoretically anomalous circumstances is still so small that only the largest possible estimate for the magnetic field strength in the Sun's interior would cause the required effect.
TL;DR: The dynamics of spin-boson systems at very low temperatures has been studied using a real-time path-integral simulation technique, which combines a stochastic Monte Carlo sampling over the quantum fluctuations with an exact treatment of the quasiclassical degrees of freedom.
Abstract: The dynamics of spin-boson systems at very low temperatures has been studied using a real-time path-integral simulation technique, which combines a stochastic Monte Carlo sampling over the quantum fluctuations with an exact treatment of the quasiclassical degrees of freedom. To a large degree, this special technique circumvents the dynamical sign problem and allows the dynamics to be studied directly up to long real times in a numerically exact manner. This method has been applied to two important problems: (1) crossover from nonadiabatic to adiabatic behavior in electron-transfer reactions, (2) the zero-temperature dynamics in the antiferromagnetic Kondo region 1/2K1, where K is Kondo's parameter.
TL;DR: A Hartree-Fock approach to the CSTE's of the ferromagnetic incompressible ground state, which occurs in the quantum Hall effect at Landau-level filling factor [nu]=1, finds for Coulomb interactions that the charged spin-texture excitation energies are always smaller than theexcitation energies of localized spin 1/2 quasiparticles and quasiholes.
Abstract: We develop a Hartree-Fock approach to the charged spin-texture excitations (CSTE's) of the ferromagnetic incompressible ground state, which occurs in the quantum Hall effect at Landau-level filling factor [nu]=1. The CSTE's are the appropriate generalization of skyrmions to the situation when there is a nonzero Zeeman coupling. We find for Coulomb interactions that the charged spin-texture excitation energies are always smaller than the excitation energies of localized spin 1/2 quasiparticles and quasiholes. However, the amount by which the energy is lowered is quite small for typical experimental situations. The net spin of the CSTE's is always much larger than 1/2, suggesting that adding or removing charge from a filled Landau level rapidly degrades its spin polarization.
Book•
01 Jan 1994
TL;DR: In this article, the authors introduce basic concepts of RPA, RPA-based line broadening and the decay of oscillations, including spin modes and surface modes, as well as thermal effects.
Abstract: Preface 1. Introduction 2. Basic concepts 3. Theoretical tools 4. RPA 5. Dipole oscillations 6. Surface modes 7. Compressional modes 8. Spin modes 9. Line broadening and the decay of oscillations 10. Thermal effects Appendices References Index.
Journal Article•
TL;DR: It is shown that a magnetic field oriented parallel to the layers induces a highly collective commensurate-incommensurate phase transition in the magnetic order.
Abstract: Double-layer quantum Hall systems have interesting properties associated with interlayer correlations. At \ensuremath{
u}=1/m where m is an odd integer they exhibit spontaneous symmetry breaking equivalent to that of spin 1/2 easy-plane ferromagents, with the layer degree of freedom playing the role of spin. We explore the rich variety of quantum and finite temperature phase transitions in these systems. In particular, we show that a magnetic field oriented parallel to the layers induces a highly collective commensurate-incommensurate phase transition in the magnetic order.
TL;DR: In this paper, a spin valve configuration is presented in which an unpinned ferromagnetic film is separated from exchange-pinned magnetagnetic films on either side by two nonmagnetic spacers, thereby creating a symmetric spin valve structure.
Abstract: A spin valve configuration is presented in which an unpinned ferromagnetic film is separated from exchange-pinned ferromagnetic films on either side by two nonmagnetic spacers, thereby creating a symmetric spin valve structure. The symmetric spin valve is shown to increase the magnetoresistance by 50% over the values of individual spin valves. The increase is attributed to a reduction of spin-independent outer boundary scattering and doubling the number of spin-dependent scattering interfaces. The magnetoresistance and coupling fields of the spin valves that comprise the symmetric structure have been measured as a function of Cu spacer thickness. Oscillatory antiferromagnetic to ferromagnetic coupling was observed in standard spin valves, which had /spl Delta/R/R as high as 13%. >
Posted Content•
TL;DR: In this paper, complete solutions of the spin generalization of the elliptic Calogero Moser systems are constructed in terms of Riemann theta-functions and analoguous constructions for the trigonometric and rational cases are also presented.
Abstract: The complete solutions of the spin generalization of the elliptic Calogero Moser systems are constructed. They are expressed in terms of Riemann theta-functions. The analoguous constructions for the trigonometric and rational cases are also presented.
TL;DR: In this paper, the first three papers on the Glauber evolution of Ising spin systems with Kac potentials have been published, and all of them are related to our work.
Abstract: This is the first of three papers on the Glauber evolution of Ising spin systems with Kac potentials. We begin with the analysis of the mesoscopic limit, where space scales like the diverging range, gamma -1, of the interaction while time is kept finite: we prove that in this limit the magnetization density converges to the solution of a deterministic, nonlinear, nonlocal evolution equation. We also show that the long time behaviour of this equation describes correctly the evolution of the spin system till times which diverge as gamma to 0 but are small in units log gamma -1. In this time regime we can give a very precise description of the evolution and a sharp characterization of the spin trajectories. As an application of the general theory, we then prove that for ferromagnetic interactions, in the absence of external magnetic fields and below the critical temperature, on a suitable macroscopic limit, an interface between two stable phases moves by mean curvature. All the proofs are consequence of sharp estimates on special correlation functions, the v-functions, whose analysis is reminiscent of the cluster expansion in equilibrium statistical mechanics.
TL;DR: A femtosecond-resolved Faraday spectroscopy has been developed to directly monitor spin dynamics in magnetically tunable semiconductor quantum wells.
Abstract: A femtosecond-resolved Faraday spectroscopy has been developed to directly monitor spin dynamics in magnetically tunable semiconductor quantum wells. Tunable terahertz quantum beating of the optical polarization is observed from coherent excitation of the spin states Zeeman split by a single ultrathin magnetic tunneling barrier. Subsequent spin-flip scattering of a photoinjected spin-polarized excitons deposits a magnetic ``imprint'' in the barrier which is orientation dependent and persists for orders of magnitude longer than the carrier lifetime.
TL;DR: In this article, a special correlation function in the isotropic spin-1 2 Heisenberg antiferromagnet is considered, i.e., the probability of finding a ferromagnetic string of (adjacent) spins in the ground state.
TL;DR: The resistivity of magnetic multilayers is generally smaller when the magnetizations of successive layers are parallel, which is the so-called giant magnetoresistance or spin-valve effect, but scientists have been able to reverse this and to obtain a smaller resistivity for an antiparallel arrangement by intercalating thin Cr layers within half of the Fe layers in Fe/Cu multilayer.
Abstract: The resistivity of magnetic multilayers is generally smaller when the magnetizations of successive layers are parallel, which is the so-called giant magnetoresistance or spin-valve effect. %'e have been able to reverse this eA'ect and to obtain a smaller resistivity for an antiparallel arrangement by intercalating thin Cr layers within half of the Fe layers in Fe/Cu multilayers. This inverse spin-valve elfect is due to the inverse spin asymmetries of the electron scattering in successive Fe layers with and without Cr. This is a confirmation of the fundamental mechanism of the giant magnetoresistance.
TL;DR: The muon depolarization function can be analyzed to show that the temperature dependence of the strongly nonexponential form of the local spin autocorrelation function in these canonical alloys is similar to that observed in numerical simulations on Ising spin glasses.
Abstract: Muon spin depolarization has been studied in moderately concentrated AgMn and AuFe alloys from the freezing temperature T-g up to 300 K. The muon depolarization function can be analyzed to show that the temperature dependence of the strongly nonexponential form of the local spin autocorrelation function in these canonical alloys is similar to that observed in numerical simulations on Ising spin glasses. The dynamic behavior above T-g appears to be an intrinsic precursor to spin glass freezing
TL;DR: In this paper, frustrated, two-dimensional, quantum antiferromagnets in the vicinity of a quantum transition from a non-collinear, magnetically ordered ground state to a quantum-disordered phase are described.
TL;DR: In this paper, the spin fluctuation mechanism for the high temperature superconductivity in cuprates is investigated with a parametrization of the spin fluctuations spectra in terms of the self-consistent renormalization theory of spin fluctuations.
Abstract: The spin fluctuation mechanism for the high temperature superconductivity in cuprates is investigated with a parametrization of the spin fluctuation spectra in terms of the self-consistent renormalization theory of spin fluctuations. The doping concentration dependence of the transition temperature in the best studied cuprates is explained in terms of the parameter values estimated from the normal state experiments. We find that the larger the amplitude and the energy spread of spin fluctuations, the higher T c . In this context the cuprates have much favorable properties compared with some other antiferromagnetic spin fluctuation systems whose parameter values are estimated for comparison.
TL;DR: The results indicate that spin-polarized triplet helium is a promising candidate for Bose-Einstein condensation.
Abstract: We consider the decay kinetics of a trapped spin-polarized gas of metastable triplet helium 4He(2'S) at ultralow temperatures. The sample lifetime is found to be determined by spin relaxation and Penning ionization, both induced by spin-dipole interaction in pair collisions. The rates of these processes are calculated. The Penning ionization proves to be 5 orders of magnitude slower than in the unpolarized case. The results indicate that spin-polarized triplet helium is a promising candidate for Bose-Einstein condensation.