795 research outputs found
Diffuse transport and spin accumulation in a Rashba two-dimensional electron gas
The Rashba Hamiltonian describes the splitting of the conduction band as a
result of spin-orbit coupling in the presence of an asymmetric confinement
potential and is commonly used to model the electronic structure of confined
narrow-gap semiconductors. Due to the mixing of spin states some care has to be
exercised in the calculation of transport properties. We derive the diffusive
conductance tensor for a disordered two-dimensional electron gas with
spin-orbit interaction and show that the applied bias induces a spin
accumulation, but that the electric current is not spin-polarized.Comment: REVTeX4 format, 5 page
Neutron scattering from a coordination polymer quantum paramagnet
Inelastic neutron scattering measurements are reported for a powder sample of
the spin-1/2 quantum paramagnet . Magnetic neutron
scattering is identified above an energy gap of 1.9 meV. Analysis of the sharp
spectral maximum at the onset indicates that the material is magnetically
quasi-one-dimensional. Consideration of the wave vector dependence of the
scattering and polymeric structure further identifies the material as a
two-legged spin-1/2 ladder. Detailed comparison of the data to various models
of magnetism in this material based on the single mode approximation and the
continuous unitary transformation are presented. The latter theory provides an
excellent account of the data with leg exchange meV and
rung exchange meV.Comment: 10 pages, 11 figures, 1 tabl
RPA(D) and HRPA(D): <sup>13</sup>C-<sup>13</sup>C Spin-Spin Coupling Constants for Saturated Cycloalkanes
Ballistic spin-polarized transport and Rashba spin precession in semiconductor nanowires
We present numerical calculations of the ballistic spin-transport properties
of quasi-one-dimensional wires in the presence of the spin-orbit (Rashba)
interaction. A tight-binding analog of the Rashba Hamiltonian which models the
Rashba effect is used. By varying the robustness of the Rashba coupling and the
width of the wire, weak and strong coupling regimes are identified. Perfect
electron spin-modulation is found for the former regime, regardless of the
incident Fermi energy and mode number. In the latter however, the
spin-conductance has a strong energy dependence due to a nontrivial subband
intermixing induced by the strong Rashba coupling. This would imply a strong
suppression of the spin-modulation at higher temperatures and source-drain
voltages. The results may be of relevance for the implementation of
quasi-one-dimensional spin transistor devices.Comment: 19 pages (incl. 9 figures). To be published in PR
Spin gap in the Quasi-One-Dimensional S=1/2 Antiferromagnet: Cu2(1,4-diazacycloheptane)2Cl4
Cu_{2}(1,4-diazacycloheptane)_{2}Cl_{4} contains double chains of spin 1/2
Cu^{2+} ions. We report ac susceptibility, specific heat, and inelastic neutron
scattering measurements on this material. The magnetic susceptibility,
, shows a rounded maximum at T = 8 K indicative of a low dimensional
antiferromagnet with no zero field magnetic phase transition. We compare the
data to exact diagonalization results for various one dimensional
spin Hamiltonians and find excellent agreement for a spin ladder with
intra-rung coupling meV and two mutually frustrating
inter-rung interactions: meV and meV. The
specific heat in zero field is exponentially activated with an activation
energy meV. A spin gap is also found through inelastic
neutron scattering on powder samples which identify a band of magnetic
excitations for meV. Using sum-rules we derive an
expression for the dynamic spin correlation function associated with
non-interacting propagating triplets in a spin ladder. The van-Hove
singularities of such a model are not observed in our scattering data
indicating that magnetic excitations in Cu_{2}(1,4-diazacycloheptane)_{2}Cl_{4}
are more complicated. For magnetic fields above T specific
heat data versus temperature show anomalies indicating a phase transition to an
ordered state below T = 1 K.Comment: 9 pages, 8 postscript figures, LaTeX, Submitted to PRB 8/4/97, e-mail
Comments to [email protected]
Coupled Ladders in a Magnetic Field
We investigate the phase transitions in two-leg ladders systems in the
incommensurate phase, for which the gap is destroyed by a magnetic field
() and the ladder is not yet totally saturated (). We
compute quantitatively the correlation functions as a function of the magnetic
field for an isolated strong coupling ladder and use
it to study the phase transition occuring in a three dimensional array of
antiferromagnetically coupled ladders. The three dimensional ordering is in the
universality class of Bose condensation of hard core bosons. We compute the
critical temperature as well as various physical quantities such as
the NMR relaxations rate. has an unusual camel-like shape with a local
minimum at and behaves as for
. We discuss the experimental consequences for compounds such as
Cu_2(C_5H_{12}N_2)_2Cl_4Comment: 11 pages; some misprints corrected + one reference added; to appear
in PR
Theoretical analysis of the experiments on the double-spin-chain compound -- KCuCl
We have analyzed the experimental susceptibility data of KCuCl and found
that the data are well-explained by the double-spin-chain models with strong
antiferromagnetic dimerization. Large quantum Monte Carlo calculations were
performed for the first time in the spin systems with frustration. This was
made possible by removing the negative-sign problem with the use of the dimer
basis that has the spin-reversal symmetry. The numerical data agree with the
experimental data within 1% relative errors in the whole temperature region. We
also present a theoretical estimate for the dispersion relation and compare it
with the recent neutron-scattering experiment. Finally, the magnitude of each
interaction bond is predicted.Comment: 4 pages, REVTeX, 5 figures in eps-file
Spin-polarized transport in inhomogeneous magnetic semiconductors: theory of magnetic/nonmagnetic p-n junctions
A theory of spin-polarized transport in inhomogeneous magnetic semiconductors
is developed and applied to magnetic/nonmagnetic p-n junctions. Several
phenomena with possible spintronic applications are predicted, including
spinvoltaic effect, spin valve effect, and giant magnetoresistance. It is
demonstrated that only nonequilibrium spin can be injected across the
space-charge region of a p-n junction, so that there is no spin injection (or
extraction) at low bias.Comment: Minor Revisions. To appear in Phys. Rev. Let
Lattice Instability in the Spin-Ladder System under Magnetic Field
We study theoretically the lattice instability in the spin gap systems under
magnetic field. With the magnetic field larger than a critical value h_{c1},
the spin gap is collapsed and the magnetization arises. We found that the
lattice distortion occurs in the spin-ladder at an incommensurate wavevector
corresponding to the magnetization, while it does not occur in the Haldane
system. At low temperatures the magnetization curve shows a first order phase
transition with this lattice distortion.Comment: 10 pages, REVTEX, 2 figures(ps file), minor change
Mesoscopic Stern-Gerlach device to polarize spin currents
Spin preparation and spin detection are fundamental problems in spintronics
and in several solid state proposals for quantum information processing. Here
we propose the mesoscopic equivalent of an optical polarizing beam splitter
(PBS). This interferometric device uses non-dispersive phases (Aharonov-Bohm
and Rashba) in order to separate spin up and spin down carriers into distinct
outputs and thus it is analogous to a Stern-Gerlach apparatus. It can be used
both as a spin preparation device and as a spin measuring device by converting
spin into charge (orbital) degrees of freedom. An important feature of the
proposed spin polarizer is that no ferromagnetic contacts are used.Comment: Updated to the published versio
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