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951.
L.?Marié J.?Burguete F.?DaviaudEmail author J.?Léorat 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,33(4):469-485
A numerical study of the magnetic induction equation has
been performed on von Kármán type flows. These flows are
generated by two co-axial counter-rotating propellers in
cylindrical containers. Such devices are currently used in the
von Kármán sodium (VKS) experiment designed to study dynamo
action in an unconstrained flow. The mean velocity fields have
been measured for different configurations and are introduced in
a periodic cylindrical kinematic dynamo code. Depending on the
driving configuration, on the poloidal to toroidal flow ratio
and on the conductivity of boundaries, some flows are observed
to sustain growing magnetic fields for magnetic Reynolds numbers
accessible to a sodium experiment. The response of the flow to
an external magnetic field has also been studied: The results
are in excellent agreement with experimental results in the
single propeller case but can differ in the two propellers case. 相似文献
952.
A. Pautrat A. Daignere C. Goupil Ch. Simon B. Andrzejewski A.I. Rykov S. Tajima 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,33(3):279-284
We report complex impedance measurements in an untwinned YBaCuO crystal. Our broad frequency range covers both the quasi static
response and the resistive response of the vortex lattice. It allow us to characterize the irreversibility line without the
need of any frequency dependent pinning parameters. We confirm the validity of the two modes model of vortex dynamic, and
extract both the surface critical current and the flux flow resistivity around the first order transition Tm. This latter is identified by the abrupt loss of pinning and by an unexpected step of (T) at Tm.
Received 22 November 2002 / Received in final form 17 February 2003 Published online 20 June 2003
RID="a"
ID="a"e-mail: alain.pautrat@ismra.fr
RID="b"
ID="b"UMR 6508 associée au CNRS 相似文献
953.
S.A. Blundell M.D. Núñez-Regueiro 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,31(4):453-456
From the recently determined structure of the delafossite YCuO2.5, we argue that the Cu-O network has nearly independent Δ chains but with different interactions between the s = 1/2 spins. Motivated by this observation, we study the Δ chain for different ratios of the base-base and base-vertex interactions,
J
bb/J
bv. By exact diagonalization and extrapolation, we show that the elementary excitation spectrum is the same for total spins
S
tot = 0 and 1, but not for S
tot = 2, and has a gap only in the interval 0.4874(1) ⩽ J
bb/J
bv ⩽ 1.53(1). The gap, known to be dispersionless for J
bb = J
bv, is found to acquire increasing k-dependence as J
bb/J
bv moves away from unity.
Received 29 October 2002 / Received in final form 14 January 2003 Published online 6 March 2003
RID="a"
ID="a"e-mail: sblundell@cea.fr
RID="b"
ID="b"e-mail: nunezreg@lps.u-psud.fr 相似文献
954.
O. N. Bakharev N. Zelders H. B. Brom A. Schnepf H. Schnöckel L. Jos de Jongh 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2003,24(1-3):101-104
We present
69,71Ga-NMR experiments on
microcrystalline samples of the recently discovered
supramolecular compound Ga
84
[
N
(
SiMe
3
)
2
]
20
Li
6
Br
2
(
thf
)
20
.
2
toluene, which is composed of
ligand-coordinated Ga84 metal clusters,
packed together in a fully ordered crystalline matrix. The
compound is highly conducting and even shows superconductivity
below T
c
~ 7.2 K. Our preliminary
results between 10-300 K show a metallic-like behavior: the
nuclear spin-lattice relaxation rate T
1
-1
follows the Korringa law
69
(
T
1
T
)
-1
= 0.36 s
-1
K
-1
, but with a relaxation rate approximately three
times smaller than in bulk -Ga
metal. No quantum-size effects are observed, the Korringa law
being followed down to 10 K, whereas the quantum-gaps for
individual clusters should amount to ~
10
3
K. These results therefore suggest a transport process based on
intermolecular charge transfer, similar as in alkali-doped
fullerenes and silicon-clathrates. 相似文献
955.
A recent theoretical estimation indicated that the NM/FI/FI/NM double spin-filter junction (DSFJ, here the NM and FI represent the nonmagnetic electrode and the ferromagnetic insulator (semiconductor) spacer, respectively) could have very high tunneling magnetoresistance (TMR) at zero bias. To meet the requirement in research and application of the magnetoresistance devices, we have calculated the dependences of tunneling magnetoresistance of DSFJ on the bias (voltage), the thicknesses of ferromagnetic insulators (semiconductors) and the average barrier height. Our results show that except its very high value, the TMR of DSFJ does not decrease monotonously and rapidly with rising bias, but increase slowly at first and decrease then after having reached a maximum value. This feature is in distinct contrast to the ordinary magnetic tunnel junction FM/NI/FM (FM and NI denote the ferromagnetic electrode and the nonmagnetic insulator (semiconductor) spacer, respectively), and is of benefit to the use of DSFJ as a magnetoresistance device. 相似文献
956.
We study the pure point spectrum of the energy operator H(P
) of a many-particle charged quantum system in a homogeneous magnetic field based on the results in our previous work under fixation of the sum P
of the pseudomomentum components of the system. We prove that the discrete spectrum H(P
) of a short-range system is infinite under some conditions (which, for example, hold for a system of two oppositely charged particles) even in the case of a finitely supported potential. For a long-range system of the type of a (+)-ion of an atom (including the ion), the discrete spectrum is infinite. 相似文献
957.
Franç ois Germinet Abel Klein 《Proceedings of the American Mathematical Society》2003,131(3):911-920
We study the decay at large distances of operator kernels of functions of generalized Schrödinger operators, a class of semibounded second order partial differential operators of mathematical physics, which includes the Schrödinger operator, the magnetic Schrödinger operator, and the classical wave operators (i.e., acoustic operator, Maxwell operator, and other second order partial differential operators associated with classical wave equations). We derive an improved Combes-Thomas estimate, obtaining an explicit lower bound on the rate of exponential decay of the operator kernel of the resolvent. We prove that for slowly decreasing smooth functions the operator kernels decay faster than any polynomial.
958.
In [9] we introduced the notion of equilibrium surface current JdSx on the closed C smooth surfaces in R
3 as a generalization of electric solenoid, and proved their existence. We here show an algorithm for JdSx starting from a given surface current J dSx on . 相似文献
959.
The influence of treatment in a magnetic field at low temperatures on the nonlinear optical properties of NiI2-doped CdI2 single crystals is investigated. The impurity ions in the interlayer space or more complex centers into which these ions enter can align on exposure to a magnetic field due to the interaction with the membrane vibrations of the lattice and create residual polarization in a specimen. This polarization causes redistribution of the electric charge on the bonds and improves the nonlinear optical properties of single crystals. 相似文献
960.
We have obtained theoretically an expression for the nuclear free precession signal in an inhomogeneously broadened spin system with a complex structure of the NMR spectrum. It is shown that in this case in the nuclear precession signal there appear maxima the moments of whose formation depend on the detuning of the pulse carrier frequency from the central frequencies of the NMR lines, their widths, and the Rabi frequency. The theoretical results are in good agreement with the experimental data on the observation of proton free precession in toluene. We have constructed a physical model explaining the appearance of maxima in the free precession signal of the spin system. 相似文献