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31.
R. Lü J.-L. Zhu X. Chen L. Chang 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,3(1):35-40
At low temperatures the Neel vector in a small antiferromagnetic particle can possess quantum coherence between the classically
degenerate minima. In some cases, the topological term in the magnetic action can lead to destructive interference between
the symmetry-related trajectories for the half-integer excess spin antiferromagnetic particle. By studying a macroscopic quantum
coherence problem of the Neel vector with biaxial crystal symmetry and a weak magnetic field applied along the hard axis,
we find that the quenching of tunnel splitting could take place in the system without Kramers' degeneracy. Both the Wentzel-Kramers-Brillouin
exponent and the pre-exponential factors are found exactly for the tunnel splitting. Results show that the tunnel splitting
oscillates with the weak applied magnetic field for both the integer and half-integer excess spin antiferromagnetic particles,
and vanishes at certain values of the field. All the calculations are performed based on the two sublattices model and the
instanton method in spin-coherent-state path integral.
Received: 24 July 1997 / Accepted: 30 September 1997 相似文献
32.
S.A. Mikhailov K. Ziegler 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,28(1):117-120
A floating Wigner crystal differs from the standard one by a spatial averaging over positions of the Wigner-crystal lattice.
It has the same internal structure as the fixed crystal, but contrary to it, takes into account rotational and/or translational
symmetry of the underlying jellium background. We study properties of a floating Wigner molecule in few-electron spin-polarized
quantum dots, and show that the floating solid has the lower energy than the standard Wigner crystal with fixed lattice points.
We also argue that internal rotational symmetry of individual dots can be broken in arrays of quantum dots, due to degenerate
ground states and inter-dot Coulomb coupling.
Received 12 September 2001 / Received in final form 24 April 2002 Published online 9 July 2002 相似文献
33.
Hiroaki Yamamoto Youji Kanie Masakazu Arakawa Keiji Taniguchi 《Applied Physics A: Materials Science & Processing》1990,50(6):577-581
The transmission coefficient and the resonance condition in the one-dimensional rectangular double-, triple-, quadruple-, and quintuple-barrier structures are derived theoretically under the assumption of the constant tunneling effective mass. It is found that the resonance energies are different from the eigenvalues in the quantum well due to coupling between wells in the multiple-barrier (much more than triple-barrier) structures. It is confirmed that the transmission spectrum is a Lorentzian near to energies of resonance. 相似文献
34.
J. Ahrens S. Altieri J. R. M. Annand H. -J. Arends R. Beck A. Braghieri N. d'Hose H. Dutz S. Goertz P. Grabmayr S. Hasegawa E. Heid H. Holvoet L. Van Hoorebeke N. Horikawa T. Iwata O. Jahn P. Jennewein R. Kondratiev J. Krimmer M. Lang B. Lannoy K. Livingston J. C. McGeorge W. Meyer A. Panzeri P. Pedroni T. Pinelli I. Preobrajenski G. Reicherz G. Rosner M. Rost T. Rostomyan D. Ryckbosch M. Schumacher B. Seitz G. Tamas A. Thomas R. van de Vyver Th. Walcher F. Zapadtka 《The European Physical Journal A - Hadrons and Nuclei》2005,26(1):135-140
The G asymmetry of the γp↦Nπ reaction has been measured for the first time for Eγ = 340±14 MeV. This observable, for which very scarce published data exist, plays an important role to disentangle the contributions
of the various nucleon resonances. The experiment, performed at the Mainz microtron MAMI, used a 4π-detector system, a linearly
polarized, tagged photon beam, and a longitudinally polarized proton target. 相似文献
35.
We report the effect of compressive strain on magnetic and magneto-electrical properties of lightly doped manganite La0.88Sr0.12MnO3 thin films. Films, having 5-60 nm thickness, were grown on (001) LaAlO3 and (001) SrTiO3 substrate by DC-magnetron sputtering. These films show a magnetoresistance as high as ∼65% at room temperature and insulator-metal transition temperature . Further, we demonstrate that a small variation in strain causes significant changes in their properties. We have discussed the possible origin of these features and compared with the reported literature. 相似文献
36.
I. K. Bensafa P. Achenbach M. Ases Antelo C. Ayerbe D. Baumann R. Böhm D. Bosnar E. Burtin X. Defaÿ N. D'Hose M. Ding M. O. Distler L. Doria H. Fonvieille J. M. Friedrich J. Friedrich J. García Llongo P. Janssens G. Jover Mañas M. Kohl G. Laveissière M. Lloyd M. Makek J. Marroncle H. Merkel P. Merle U. Müller L. Nungesser B. Pasquini R. Pérez Benito J. Pochodzalla M. Potokar G. Rosner S. Sánchez Majos M. Seimetz S. Širca T. Spitzenberg G. Tamas R. Van de Vyver L. Van Hoorebeke Th. Walcher M. Weis 《The European Physical Journal A - Hadrons and Nuclei》2007,32(1):69-75
The beam-helicity asymmetry has been measured simultaneously for the reactions
p→epγ and
p→epπ
0 in the Δ(1232)-resonance region at Q
2 = 0.35(GeV/c)2. The experiment was performed at MAMI with a longitudinally polarized beam and an out-of-plane detection of the proton. The
results are compared with calculations based on dispersion relations for virtual Compton scattering and with the MAID model
for pion electroproduction. There is an overall good agreement between experiment and theoretical calculations. The remaining
discrepancies may be ascribed to an imperfect parametrization of some γ
(*)
N→πN multipoles, mainly contributing to the non-resonant background. The beam-helicity asymmetry in both channels (γ and π
0) shows a good sensitivity to these multipoles and should allow future improvement in their parametrization. 相似文献
37.
A. A. Shokri 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,50(3):475-481
Theoretical studies on spin-dependent transport in magnetic tunnel
heterostructures consisting of two diluted magnetic semiconductors
(DMS) separated by a nonmagnetic semiconductor (NMS) barrier, are
carried in the limit of coherent regime by including the effect of
angular dependence of the magnetizations in DMS. Based on
parabolic valence band effective mass approximation and
spontaneous magnetization of DMS electrodes, we obtain an
analytical expression of angular dependence of transmission for
DMS/NMS/DMS junctions. We also examine the dependence of spin
polarization and tunneling magnetoresistance (TMR) on barrier
thickness, temperature, applied voltage and the relative angle
between the magnetizations of two DMS layers in GaMnAs/GaAs/GaMnAs
heterostructures. We discuss the theoretical interpretation of
this variation. Our results show that TMR of more than 65% are
obtained at zero temperature, when one GaAs monolayer is used as a
tunnel barrier. It is also shown that the TMR decreases rapidly
with increasing barrier width and applied voltage; however at high
voltages and low thicknesses, the TMR first increases and then
decreases. Our calculations explain the main features of the
recent experimental observations and the application of the
predicted results may prove useful in designing nano spin-valve
devices. 相似文献
38.
G.-Y. Sun C.-X. Wu Y. Chen Z.-L. Yang 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,49(4):459-464
Using the Keldysh nonequilibrium Green function method, we
theoretically investigate the electron transport properties of a
quantum dot coupled to two ferromagnetic electrodes, with
inelastic electron-phonon interaction and spin flip scattering
present in the quantum dot. It is found that the electron-phonon
interaction reduces the current, induces new satellite polaronic
peaks in the differential conductance spectrum, and at the same
time leads to oscillatory tunneling magnetoresistance effect. Spin
flip scattering suppresses the zero-bias conductance peak and
splits it into two, with different behaviors for parallel and
anti-parallel magnetic configuration of the two electrodes.
Consequently, a negative tunneling magnetoresistance effect may
occur in the resonant tunneling region, with increasing spin flip
scattering rate. 相似文献
39.
Transmission resonances in magnetic-barrier structures 总被引:1,自引:0,他引:1
Y. Guo Z.-Q. Li B.-L. Gu Q. Sun J.-Z. Yu Y. Kawazoe 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,3(2):263-270
Quantum transport properties of electrons in simple magnetic-barrier (MB) structures and in finite MB superlattices are investigated
in detail. It is shown that there exists a transition of transmission resonances, i.e., from incomplete transmission resonances in simple MB structures consisting of unidentical blocks, to complete transmission
resonances in comparatively complex MB structures (, n is the number of barriers). In simple unidentical block arrangements in double- and triple-MB structures we can also obtain
complete transmission by properly adjusting parameters of the building blocks according to ky-value (ky is the wave vector in y direction). Strong suppression of the transmission and of the conductance is found in MB superlattices which are periodic
arrangements of two different blocks. The resonance splitting effect in finite MB superlattices is examined. It is confirmed
that the rule (i.e., for n-barrier tunneling the splitting would be (n-1)-fold) obtained in periodic electric superlattices can be extended to periodically arranged MB superlattices of identical
blocks through which electrons with tunnel, and it is no longer proper for electrons with k
y
<0 to tunnel.
Received: 18 August 1997 / Revised: 20 September 1997 / Accepted: 13 October 1997 相似文献
40.
The substitution of small contents of trivalent cations for manganese in the oxides Ln0.57Ca0.43Mn1−xMxO3 (Ln=Pr, Na) has been explored for M=Al, Ga, Fe, Cr, Sc, In. It is shown that similarly to Ba-doping, the M-doped Pr-manganites exhibit a great predisposition to ferromagnetism in a low magnetic field of 0.25 T, reaching ferromagnetic (FM) fractions of 85% for M=Ga and 100% for m=Cr, whereas in contrast only small FM fractions (∼8%) can be reached for the M-doped Nd-manganites. The great ability of both Pr and Nd manganites to exhibit ultrasharp steps at 2.5 K, at increasing magnetic field is also demonstrated. The different behaviors of the so-doped manganites are interpreted in terms of geometric effects (size of the A-sites cations and of the doping elements) and of electronic configurations and magnetic properties of the dopants. 相似文献