共查询到20条相似文献,搜索用时 875 毫秒
1.
L.M. Schmitt 《Applied Physics A: Materials Science & Processing》1999,68(5):553-558
We discuss the transmission coefficient τd in non-repetitive, one-dimensional, rectangular double-barrier structures without simplifications such as strongly attenuating
barriers, strong localization, or overall constant effective tunneling mass of the electron. For resonance τd=1, we obtain two non-approximative conditions which require different resonance energies of the tunneling electron than previously
reported in the literature. In fact, the resonance peaks are shifted to higher energy levels in the order of the width of
the peaks due to the effect of non-constant tunneling mass. We investigate the dependence of the resonance condition and the
shape of the resonance peaks in regard to perturbation of the electron energy, the gap width as well as the barrier width
and height. Resonance is stable for variation of the barrier width but sensitive for variation of the barrier height and the
gap width.
Received: 9 December 1998 / Accepted: 5 January 1999 / Published online: 31 March 1999 相似文献
2.
G. Montambaux 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(3):377-383
The stability of a Charge Density Wave (CDW) in a one-dimensional ring pierced by a Aharonov-Bohm flux is studied in a mean-field
picture. It is found that the stability depends on the parity of the number N of electrons. When the size of the ring becomes as small as the coherence length , the CDW gap increases for even N and decreases for odd N. Then when N is even, the CDW gap decreases with flux but it increases when N is odd. The variation of the BCS ratio with size and flux is also calculated. We derive the harmonics expansion of the persistent
current in a presence of a finite gap.
Received: 16 September 1997 / Received in final form: 12 November 1997 / Accepted: 13 November 1997 相似文献
3.
M. Dumm M. Dressel M. Nicklas P. Lunkenheimer A. Loidl M. Weiden F. Steglich B. Assmann H. Homborg P. Fulde 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(3):317-322
The magnetic susceptibility, using dc and electron spin resonance (ESR) methods, the specific heat, and the infrared properties
of the one-dimensional molecular semiconductors lithium phthalocyanine (LiPc) and the iodinated compound LiPcI have been investigated
for temperatures K. LiPc has a half-filled conduction band and is expected to be an organic metal. However, due to the strong Coulomb repulsion
the system is a one-dimensional Mott-Hubbard insulator with a Hubbard gap of 0.75 eV as inferred from optical measurements.
The localized electrons along the molecular stacks behave like a S = 1/2 antiferromagnetic spin chain. The spin susceptibility, as determined by ESR experiments, and the magnetic contribution
to the heat capacity show a Bonner-Fisher type of behavior with an exchange constant K. LiPcI is an intrinsic narrow-gap semiconductor with an optical gap of 0.43 eV. In ESR experiments it is silent, indicating
that all the unpaired electrons have been removed from the macrocycle via doping with iodine.
Received: 16 June 1998 / Accepted: 14 July 1998 相似文献
4.
I. Bâldea H. Köppel L.S. Cederbaum 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,3(4):507-515
We investigate the Peierls transition in finite chains by exact (Lanczos) diagonalization and within a seminumerical method
based on the factorization of the electron-phonon wave function (Adiabatic Ansatz, AA). AA can be applied for mesoscopic chains
up to micrometer sizes and its reliability can be checked self-consistently. Our study demonstrates the important role played
for finite systems by the tunneling in the double well potential. The chains are dimerized only if their size N exceeds a critical value Nc which increases with increasing phonon frequency. Quantum phonon fluctuations yield a broad transition region. This smooth Peierls transition contrasts not only to the sharp mean field transition, but also with the sharp RPA soft mode instability,
although RPA partially accounts for quantum phonon fluctuations. For weak coupling the dimerization disappears below micrometer
sizes; therefore, this effect could be detected experimentally in mesoscopic systems.
Received: 3 January 1998 / Revised: 13 March 1998 / Accepted: 3 April 1998 相似文献
5.
N. Vandewalle M. Ausloos 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(2):257-261
The resonance splitting in finite semiconductor superlattices which consist of a number of electric barriers is investigated.
It is found that (n-1)-fold splitting for n-barrier tunneling obtained in periodic superlattices of identical barriers no longer holds for superlattices which are periodically
juxtaposed with two different building barriers. In general, one resonant domain in the former splits into two resonant subdomains
in the latter, and splitting occurs each time when two new barriers are added. The results indicate that the resonance splitting
is determined not only by the structure but also by the parameters of building blocks.
Received: 12 October 1997/ Revised and Accepted: 5 January 1998 相似文献
6.
W. Hanke O. Schmitt H. Endres R. Kleiner P. Müller 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):465-471
A quantitative analysis of a microscopic model for the intrinsic Josephson effect in high-temperature superconductors based
on interlayer tunneling is presented both within a mean-field BCS evaluation and a numerically essentially exact Quantum Monte-Carlo
study. The pairing correlations in the CuO2-planes are modelled by a 2D Hubbard model with attractive interaction, a model which accounts well for some of the observed
features such as the short planar coherence length. The stack of Hubbard planes is arranged on a torus, which is threaded
by a magnetic flux. The current perpendicular to the planes is calculated as a function of applied flux (i.e. the phase), and - after careful elimination of finite-size effects due to single-particle tunneling - found to display a
sinusoidal field dependence in accordance with interlayer Josephson tunneling. Studies of the temperature dependence of the
supercurrent reveal at best a mild elevation of the Josephson transition temperature compared to the planar Kosterlitz-Thouless
temperature. These and other results on the dependence of the model parameters are compared with a standard BCS evaluation.
Received: 24 February 1998 / Revised: 28 April 1998 / Accepted: 23 June 1998 相似文献
7.
S. Chakravarty 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(3):337-343
The interlayer tunneling mechanism of the cuprate high temperature superconductors involves a conversion of the confinement
kinetic energy of the electrons perpendicular to the CuO-planes (c-axis) in the normal state to the pair binding energy in the superconducting state. This mechanism is discussed and the arguments
are presented from the point of view of general principles. It is shown that recent measurements of the c-axis properties support the idea that the electrons substantially lower their c-axis kinetic energy upon entering the superconducting state, a change that is nearly impossible in any conventional mechanism.
The proper use of a c-axis conductivity sum rule is shown to resolve puzzles involving the penetration depth and the optical measurements.
Received: 5 January 1998 / Accepted: 17 March 1998 相似文献
8.
H. Lambaré P. Roche S. Balibar H.J. Maris O.A. Andreeva C. Guthmann K.O. Keshishev E. Rolley 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(3):381-391
We have studied the nucleation of bubbles in pure superfluid helium-4 at temperatures down to 65 mK. We have found that the
nucleation is a stochastic process, and that at temperatures below 600 mK the nucleation rate is independent of temperature.
These results are consistent with the assumption that the nucleation takes place via quantum tunneling.
Received: 15 November 1997 / Received in final form: 19 December 1997 / Accepted: 22 January 1998 相似文献
9.
J. Jersch F. Demming I. Fedotov K. Dickmann 《Applied Physics A: Materials Science & Processing》1999,68(6):637-641
Time-resolved dependence of the transient current through a ns laser pulse illuminated scanning tunneling microscope (STM)
tip/sample gap in tunneling mode and out of tunneling range is presented. A self-designed fast STM-preamplifier (bandwidth
35 MHz) allows one to resolve the fine structure of the transient signal as well as the observation of some effects that are
undetectable by using conventional low-band preamplifiers. The dependence of the threshold laser pulse intensity, which corresponds
to the beginning of electron emission from tip (in non-tunneling mode), as a function of the tip/sample distance was investigated.
At tip/sample distances from tunnel contact up to approximately 1 μm a linear dependence is found. This behavior is in good
agreement with the theory for field enhancement in a STM tip/sample system. In tunneling mode a ns (fast component) as well
as a μs (slow component) current response was found as a result of the laser pulse illumination. These data suggest the tip
bending to be an important factor in clarifying the thermal/mechanical mechanism of laser-assisted surface nanomodification.
Received: 4 May 1998 / Accepted: 29 January 1999 / Published online: 28 April 1999 相似文献
10.
Y. Torii N. Shiokawa T. Hirano T. Kuga Y. Shimizu H. Sasada 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1998,1(3):239-242
We utilized a blue-detuned Laguerre-Gaussian (doughnut) laser beam to trap cold rubidium atoms by optical dipole force. ”Pulsed”
polarization gradient cooling was applied to the trapped atoms to suppress the trap loss due to heating caused by random photon
scattering of the trapping light. In this trap about 108 atoms were initially captured and the trap lifetime was 1.5 s, which was consistent with losses due to background gas collisions.
This trap can readily be applied to atom guiding, compression, and evaporative cooling.
Received: 10 July 1997 / Received in final form: 5 January 1998 / Accepted: 16 January 1998 相似文献
11.
M. Barat J.C. Brenot H. Dunet J.A. Fayeton Y.J. Picard 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1998,1(3):271-276
An experimental study of the dissociation of the heteronuclear Na ion by collisions with He and H2 targets at a collision energy of 2.4 keV is presented. The use of a technique based on the measurement of the velocity vectors
of the fragments allows investigating in detail the two basic dissociation mechanisms: impulsive and electronic. In the present
asymmetric case, one is able to assess the relative role of close encounters with each of the Na+ and atomic cores of the molecule. Electronic transitions at a surface crossing are proposed to explain processes which happens
in close collisions between the core and the He target.
Received: 3 October 1997 / Revised: 1 December 1997 / Accepted: 16 January 1998 相似文献
12.
P.L. Krapivsky J.F.F. Mendes S. Redner 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(4):401-404
We investigate submonolayer epitaxial growth with a fixed monomer flux and irreversible aggregation of adatom islands due
to an effective island diffusion, with a diffusivity for an mass k island proportional to . For , there is a steady state, while for , continuously evolving logarithmic islanding occurs in which the island density grows extremely slowly, as . In the latter regime, the island size distribution exhibits complex, but universal, multiple-scale mass dependence which
we account for theoretically.
Received: 3 June 1998 / Accepted: 13 July 1998 相似文献
13.
C. Waldtmann H.-U. Everts B. Bernu C. Lhuillier P. Sindzingre P. Lecheminant L. Pierre 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(4):501-507
We study the exact low energy spectra of the spin 1/2 Heisenberg antiferromagnet on small samples of the kagomé lattice of up to N=36 sites. In agreement with the conclusions of previous authors, we find that these low energy spectra contradict the hypothesis
of Néel type long range order. Certainly, the ground state of this system is a spin liquid, but its properties are rather
unusual. The magnetic () excitations are separated from the ground state by a gap. However, this gap is filled with nonmagnetic () excitations. In the thermodynamic limit the spectrum of these nonmagnetic excitations will presumably develop into a gapless
continuum adjacent to the ground state. Surprisingly, the eigenstates of samples with an odd number of sites, i.e. samples with an unsaturated spin, exhibit symmetries which could support long range chiral order. We do not know if these
states will be true thermodynamic states or only metastable ones. In any case, the low energy properties of the spin 1/2 Heisenberg
antiferromagnet on the kagomé lattice clearly distinguish this system from either a short range RVB spin liquid or a standard chiral spin liquid. Presumably
they are facets of a generically new state of frustrated two-dimensional quantum antiferromagnets.
Received: 27 November 1997 / Accepted: 29 January 1998 相似文献
14.
B. Reusch R. Blümel 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1998,3(2):123-127
Numerical simulations of the equations of motion of 300 charged particles confined to a plane with an additional magnetic
field orthogonal to the plane reproduce recently observed self-organization of non-neutral plasmas into a small number of
interacting vortices. In the presence of damping we observe crystallized vortices, i.e. vortices with regular internal structure. We also observe crystallized vortex crystals, i.e. geometric patterns of crystallized vortices. Fractal vortex arrangements are investigated and found to be stable. Our results
are relevant for quantum dots and artificial atoms.
Received: 24 February 1998 / Revised: 4 March 1998 / Accepted: 4 May
1998 相似文献
15.
N. Demoncy O. Stéphan N. Brun C. Colliex A. Loiseau H. Pascard 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(2):147-157
Various filled carbon nanotubes have recently been successfully produced by the arc-discharge method by doping a 99.4% graphite
anode with a transition metal like Cr, Ni, a rare earth like Yb, Dy, or a covalent element like S, Ge. In this work, the structural
characteristics of these encapsulated nanowires were studied by High Resolution Transmission Electron Microscopy and their
chemical composition was investigated using Electron Energy-Loss Spectroscopy with high spatial resolution: this analysis
mode provides elemental concentration profiles across or along the filled nanotubes. Except in the case of Ge for which only
pure Ge fillings were identified, surprising amounts of sulfur, which was present as an impurity ( 0.25%) in the graphite rods, were found within numerous filling materials. When using high purity carbon rods, no filled
nanotube was obtained. We chose the case of Cr to clearly evidence that the addition of sulfur in catalytic quantity is responsible
for the formation of filled nanotubes, including sulfur free encapsulated nanowires. A growth mechanism based on a catalytic
process involving three elements, i.e. carbon, a metal and sulfur, and taking into account the experimental results is proposed.
Received: 20 January 1998 / Received in final form and accepted: 9 April 1998 相似文献
16.
J. Dietel T. Koschny W. Apel W. Weller 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):439-445
We reconsider the theory of the half-filled lowest Landau level using the Chern-Simons formulation and study the grand-canonical
potential in the random-phase approximation (RPA). Calculating the unperturbed response functions for current- and charge-density
exactly, without any expansion with respect to frequency or wave vector, we find that the integral for the ground-state energy
converges rapidly (algebraically) at large wave vectors k, but exhibits a logarithmic divergence at small k. This divergence originates in the k-2 singularity of the Chern-Simons interaction and it is already present in lowest-order perturbation theory. A similar divergence
appears in the chemical potential. Beyond the RPA, we identify diagrams for the grand-canonical potential (ladder-type, maximally
crossed, or a combination of both) which diverge with powers of the logarithm. We expand our result for the RPA ground-state
energy in the strength of the Coulomb interaction. The linear term is finite and its value compares well with numerical simulations
of interacting electrons in the lowest Landau level.
Received: 19 February 1998 / Revised: 25 March 1998 / Accepted: 17 April 1998 相似文献
17.
The product of an axialvector and a scalar current and its relation to the chiral-odd distribution function h
1 is discussed in the framework of the renormalon approach. Using a bag-model calculation for h
1, we calculate its intrinsic uncertainty due to renormalon poles. The result is given as a function of Bjorken-x as well as for the first moments separately.
Received: 9 November 1998 / Revised version: 22 January 1999 相似文献
18.
X.L. Lei H.L. Cui 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(4):513-518
We suggest a balance-equation approach to hot-electron transport in a single arbitrary energy band subject to an intense radiation
field of terahertz (THz) frequency, including all the multiphoton emission and absorption processes and taking account of
realistic scatterings due to impurities and phonons. This approach, which allows one to calculate THz-driving, time-averaging
transport based on a set of time-independent equations, provides a convenient method to study the effect of an intense THz
electric field on carrier transport in a nonparabolic energy band. As an example, these fully three-dimensional, acceleration-
and energy-balance equations are applied to the discussion of superlattice miniband transport at lattice temperature T=77 and 300 K driven by the THz radiation field of varying strengths. It is shown that the current through a dc biased miniband
superlattice is greatly reduced by the irradiation of an intense THz electric field.
Received: 23 January 1998 / Revised: 31 March 1998 / Accepted: 20 April 1998 相似文献
19.
I. Tifrea M. Crisan 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,3(2):175-178
We derive the jump in the specific heat at T=T
c
for a superconductor in a non-Fermi liquid model. We took into consideration the two possible limits in this problem: the
spin-charge separation model for a Fermi liquid and the usual non-Fermi liquid model which satisfies the homogeneity relation
for the spectral function , ). We also derive the order parameter behavior for these two cases in the vecinity of the critical temperature.
Received: 25 January 1998 / Revised: 25 March 1998 / Accepted: 25 March 1998 相似文献
20.
R. Cressely V. Hartmann 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(1):57-62
In this experimental work we carefully investigate the rheological behaviour and in particular the shear thickening exhibited
by aqueous micellar solutions of CTAB with NaSal as added counterion. We are particularly interested in the evolution of the
critical shear rate (at which shear thickening occurs) versus
C
D
, the surfactant concentration. We show that , at fixed salt concentration C
S
, increases with C
D
following a power law evolution with a positive exponent of + 5.8. On the other hand we show that if the ratio C
D
/ C
S
is fixed, decreases with C
D
with a negative exponent of -2.0. Nevertheless investigations of the zero shear viscosity indicate that in all situations (implying variation of the surfactant concentration C
D
, or the salt concentration C
S
or the temperature) is a decreasing function of the length of the micelles. All these evolutions are compatible with a gelation mechanism which
could possibly be associated with entanglement effects of large interacting flowing structures.
Received: 3 March 1998 / Revised: 16 June 1998 / Accepted: 3 July 1998 相似文献