28,779 research outputs found
Probing the Role of the Barrier Layer in Magnetic Tunnel Junction Transport
Magnetic tunnel junctions with a ferrimagnetic barrier layer have been
studied to understand the role of the barrier layer in the tunneling process -
a factor that has been largely overlooked until recently. Epitaxial oxide
junctions of highly spin polarized La0.7Sr0.3MnO3 and Fe3O4 electrodes with
magnetic NiMn2O4 (NMO) insulating barrier layers provide a magnetic tunnel
junction system in which we can probe the effect of the barrier by comparing
junction behavior above and below the Curie temperature of the barrier layer.
When the barrier is paramagnetic, the spin polarized transport is dominated by
interface scattering and surface spin waves; however, when the barrier is
ferrimagnetic, spin flip scattering due to spin waves within the NMO barrier
dominates the transport.Comment: 10 pages, 3 figure
Markov quantum fields on a manifold
We study scalar quantum field theory on a compact manifold. The free theory
is defined in terms of functional integrals. For positive mass it is shown to
have the Markov property in the sense of Nelson. This property is used to
establish a reflection positivity result when the manifold has a reflection
symmetry. In dimension d=2 we use the Markov property to establish a sewing
operation for manifolds with boundary circles. Also in d=2 the Markov property
is proved for interacting fields.Comment: 14 pages, 1 figure, Late
The Divine Clockwork: Bohr's correspondence principle and Nelson's stochastic mechanics for the atomic elliptic state
We consider the Bohr correspondence limit of the Schrodinger wave function
for an atomic elliptic state. We analyse this limit in the context of Nelson's
stochastic mechanics, exposing an underlying deterministic dynamical system in
which trajectories converge to Keplerian motion on an ellipse. This solves the
long standing problem of obtaining Kepler's laws of planetary motion in a
quantum mechanical setting. In this quantum mechanical setting, local mild
instabilities occur in the Kelperian orbit for eccentricities greater than
1/\sqrt{2} which do not occur classically.Comment: 42 pages, 18 figures, with typos corrected, updated abstract and
updated section 6.
Spin-charge-lattice coupling near the metal-insulator transition in Ca3Ru2O7
We report x-ray scattering studies of the c-axis lattice parameter in
Ca3Ru2O7 as a function of temperature and magnetic field. These structural
studies complement published transport and magnetization data, and therefore
elucidate the spin-charge-lattice coupling near the metal-insulator transition.
Strong anisotropy of the structural change for field applied along orthogonal
in-plane directions is observed. Competition between a spin-polarized phase
that does not couple to the lattice, and an antiferromagnetic metallic phase,
which does, gives rise to rich behavior for B b.Comment: 6 pages, 4 figures, to appear in Phys. Rev.
Transition amplitudes and sewing properties for bosons on the Riemann sphere
We consider scalar quantum fields on the sphere, both massive and massless.
In the massive case we show that the correlation functions define amplitudes
which are trace class operators between tensor products of a fixed Hilbert
space. We also establish certain sewing properties between these operators. In
the massless case we consider exponential fields and have a conformal field
theory. In this case the amplitudes are only bilinear forms but still we
establish sewing properties. Our results are obtained in a functional integral
framework.Comment: 33 page
Pairing of Parafermions of Order 2: Seniority Model
As generalizations of the fermion seniority model, four multi-mode
Hamiltonians are considered to investigate some of the consequences of the
pairing of parafermions of order two. 2-particle and 4-particle states are
explicitly constructed for H_A = - G A^+ A with A^+}= 1/2 Sum c_{m}^+ c_{-m}^+
and the distinct H_C = - G C^+ C with C^+}= 1/2 Sum c_{-m}^+ c_{m}^+, and for
the time-reversal invariant H_(-)= -G (A^+ - C^+)(A-C) and H_(+) = -G
(A^+dagger + C^+)(A+C), which has no analogue in the fermion case. The spectra
and degeneracies are compared with those of the usual fermion seniority model.Comment: 18 pages, no figures, no macro
Properties of Resonating-Valence-Bond Spin Liquids and Critical Dimer Models
We use Monte Carlo simulations to study properties of Anderson's
resonating-valence-bond (RVB) spin-liquid state on the square lattice (i.e.,
the equal superposition of all pairing of spins into nearest-neighbor singlet
pairs) and compare with the classical dimer model (CDM). The latter system also
corresponds to the ground state of the Rokhsar-Kivelson quantum dimer model at
its critical point. We find that although spin-spin correlations decay
exponentially in the RVB, four-spin valence-bond-solid (VBS) correlations are
critical, qualitatively like the well-known dimer-dimer correlations of the
CDM, but decaying more slowly (as with , compared with
for the CDM). We also compute the distribution of monomer (defect) pair
separations, which decay by a larger exponent in the RVB than in the CDM. We
further study both models in their different winding number sectors and
evaluate the relative weights of different sectors. Like the CDM, all the
observed RVB behaviors can be understood in the framework of a mapping to a
"height" model characterized by a gradient-squared stiffness constant . Four
independent measurements consistently show a value , with the same kinds of numerical evaluations of give
results in agreement with the rigorously known value . The
background of a nonzero winding number gradient introduces spatial
anisotropies and an increase in the effective K, both of which can be
understood as a consequence of anharmonic terms in the height-model free
energy, which are of relevance to the recently proposed scenario of "Cantor
deconfinement" in extended quantum dimer models. We also study ensembles in
which fourth-neighbor (bipartite) bonds are allowed, at a density controlled by
a tunable fugacity, resulting (as expected) in a smooth reduction of K.Comment: 26 pages, 21 figures. v3: final versio
Orbital Correlations in Doped Manganites
We review our recent x-ray scattering studies of charge and orbital order in
doped manganites, with specific emphasis on the role of orbital correlations in
Pr_1-xCa_xMnO_3. For x=0.25, we find an orbital structure indistinguishable
from the undoped structure with long range orbital order at low temperatures.
For dopings 0.3<x<0.5, we find scattering consistent with a charge and
orbitally ordered CE-type structure. While in each case the charge order peaks
are resolution limited, the orbital order exhibits only short range
correlations. We report the doping dependence of the correlation length and
discuss the connection between the orbital correlations and the finite magnetic
correlation length observed on the Mn^3+ sublattice with neutron scattering
techniques. The physical origin of these domains, which appear to be isotropic,
remains unclear. We find that weak orbital correlations persist well above the
phase transitions, with a correlation length of 1-2 lattice constants at high
temperatures. Significantly, we observe similar correlations at high
temperatures in La_0.7Ca_0.3MnO_3, which does not have an orbitally ordered
ground state, and we conclude that such correlations are robust to variations
in the relative strength of the electron-phonon coupling.Comment: 22 pagegs, 7 figure
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