共查询到20条相似文献,搜索用时 31 毫秒
1.
2.
D. Vrel S. Dubois E.M. Heian N. Karnatak M.-F. Beaufort 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,33(1):31-39
A simple method to calculate thermal diffusivity in situ after a combustion synthesis reaction is presented. The combustion reaction was analyzed via time-resolved X-ray diffraction analysis and infrared thermography. Thermal diffusivity was estimated and used to calculate
temperature profiles based on temperature profiles one second earlier. For a sample of TiC formed from Ti and C, a value of
2.00×10-6±0.20×10-6 m2 s-1 was calculated for temperatures between 1000 and 1900 K. This method is rapid and can avoid some problems associated with
furnace-based measurements of thermal diffusivity, such as recrystallization and destruction of non-equilibrium phases.
Received 29 November 2002 / Received in final form 19 March 2003 Published online 23 May 2003
RID="a"
ID="a"e-mail: vrel@limhp.univ-paris13.fr
RID="b"
ID="b"UPR 1311
RID="c"
ID="c"UMR 6630 相似文献
3.
F. Uhlík Z. Slanina E. Ōsawa 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,16(1):349-352
The five isolated-pentagon-rule (IPR) satisfying isomers of C78, labeled 1-5, or according to symmetry as D3, C2v, C'
2v
, D3h, and D'
3h
, are computed. The cage geometries are optimized at the ab initio HF level with the standard 3-21G basis set (HF/3-21G). The separation energetics is then computed using the B3LYP density-functional
treatment in the standard 6-31G* basis set (B3LYP/6-31G*//HF/3-21G). Harmonic vibrational frequencies are calculated by the
SAM1 semiempirical method. The computed energies, structural and vibrational data are employed in the construction of isomeric
partition functions and evaluation of the relative Gibbs free energies. The results are converted into relative concentrations
for a wide temperature interval. The C'
2v
structure is the most populated throughout while the D3h species is negligible at all temperatures. The agreement between theory and experiment is reasonable, though some aspects
are still to be clarified.
Received 28 November 2000 相似文献
4.
M. Jourdan N. Blümer H. Adrian 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,33(1):25-30
Superconducting SrTiO
3 - δ
was obtained by annealing single crystalline SrTiO3 samples in ultra high vacuum. An analysis of the V
(
I
) characteristics revealed very small critical currents I
c
which can be traced back to an unavoidable doping inhomogeneity. R
(
T
) curves were measured for a range of magnetic fields B at I
≪
I
c
, thereby probing only the sample regions with the highest doping level. The resulting curves B
c2
(
T
) show upward curvature, both at small and strong doping. These results are discussed in the context of bipolaronic and conventional
superconductivity with Fermi surface anisotropy. We conclude that the special superconducting properties of SrTiO
3 - δ
can be related to its Fermi surface and compare this finding with properties of the recently discovered superconductor MgB2.
Received 4 December 2002 / Received in final form 10 March 2003 Published online 23 May 2003
RID="a"
ID="a"e-mail: jourdan@uni-mainz.de 相似文献
5.
R. Combescot X. Leyronas 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,23(2):159-163
We investigate a simple explanation for the high maximum gap to T
c
ratio found experimentally in high T
c
compounds. We ascribe this observation to the lowering of T
c
by boson scattering of electrons between parts of the Fermi surface with opposite sign for the order parameter. We study
the simplest possible model within this picture. Our quantitative results show that we can account for experiment for a rather
small value of the coupling constant, all the other ingredients of our model being already known to exist in these compounds.
A striking implication of this theory is the fairly high value of the critical temperature in the absence of boson scattering.
Received 12 March 2001 and Received in final form 25 May 2001 相似文献
6.
E.P. Borges 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,30(4):593-595
Comment on Eur. Phys. J. B 27, 445 (2002)
We analyze the distribution of success of musicians, comparing a stretched exponential (found by J.A. Davies [Eur. Phys.
J. B 27, 445 (2002)]) with a distribution of the family of the q-exponential (presenting an intermediate power-law regime with a crossover to an exponential tail). We find that both assumptions
yield comparable results, within the available range of data, hence a definite conclusion cannot yet be taken. But this example
joins many others that has been found to be fairly described by q-exponentials (or variations of it), which may be indicative that there is a (significantly large) class of systems described
by nonextensive statistical mechanics, from where q-exponentials naturally appear.
Received 17 October 2002 Published online 31 December 2002 相似文献
7.
Y. Piskunov D. Jérome P. Auban-Senzier P. Wzietek C. Bourbonnais U. Ammerhal G. Dhalenne A. Revcolevschi 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,24(4):443-456
63Cu-NMR measurements have been performed on two-leg hole-doped spin ladders Sr14-xCaxCu24O41 single crystals 0 ?
x
? 12 at several pressures up to the pressure domain where the stabilization of a superconducting ground state can be achieved.
The data reveal a marked decrease of the spin gap derived from Knight shift measurements upon Ca substitution and also under
pressure and confirm the onset of low lying spin excitations around P
c
as previously reported. The spin gap in Sr
2
Ca
12
Cu
24
O
41
is strongly reduced above 20 kbar. However, the data of an experiment performed at P
= 36 kbar where superconductivity has been detected at 6.7 K by an inductive technique have shown that a significant amount of
spin excitations remains gapped at 80 K when superconductivity sets in. The standard relaxation model with two and three-magnon
modes explains fairly well the activated relaxation data in the intermediate temperature regime corresponding to gapped spin
excitations using the spin gap data derived from Knight shift experiments. The data of Gaussian relaxation rates of heavily
doped samples support the limitation of the coherence length at low temperature by the average distance between doped holes.
We discuss the interplay between superconductivity and the spin gap and suggest that these new results support the exciting
prospect of superconductivity induced by the interladder tunneling of preformed pairs as long as the pressure remains lower
than the pressure corresponding to the maximum of the superconducting critical temperature.
Received 8 March 2001 and Received in final form 27 July 2001 相似文献
8.
U.D. Rapol A. Krishna V. Natarajan 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2003,23(2):185-188
We demonstrate a technique to measure hyperfine structure using a frequency-stabilized diode laser and an acousto-optic modulator
locked to the frequency difference between two hyperfine peaks. We use this technique to measure hyperfine intervals in the
5
P
3/2
state of 85Rb and obtain a precision of 20 kHz. We extract values for the magnetic-dipole coupling constant A
= 25.038(5) MHz and the electric-quadrupole coupling constant B
= 26.011(22) MHz. These values are a significant improvement over previous results.
Received 6 March 2003 Published online 15 April 2003 相似文献
9.
V. Koptev M. Nekipelov M. Büscher S. Dymov M. Hartmann V. Hejny H.R. Koch N. Lang S. Mikirtichyants H. Ohm A. Petrus Z. Rudy R. Schleichert A. Sibirtsev K. Sistemich H. Ströher K.-H. Watzlawik 《The European Physical Journal A - Hadrons and Nuclei》2003,17(2):235-240
Results of a first experiment on (K+p) and (K+d) correlations from proton-carbon (pC) and proton-deuteron (pd) interactions at beam energies above and much below the threshold for elementary kaon production in nucleon-nucleon reactions
( T
NN = 1580 MeV) are discussed. These data, obtained with the ANKE spectrometer at COSY-Jülich, provide first direct evidence
for K+ production via the two-step mechanism and an indication for a cluster mechanism. It is shown that both processes contribute
significantly in pC collisions at 1200 MeV, while they are strongly suppressed at 2300 MeV and also in pd-interactions at 1344 MeV. It is emphasized that the underlying kinematics can be exploited to distinguish between these reaction
mechanisms.
Received: 26 November 2002 / Accepted: 26 March 2003 / Published online: 20 May 2003 相似文献
10.
S. Broderick L. Degiorgi H.R. Ott J.L. Sarrao Z. Fisk 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,33(1):47-54
Magneto-optical data on EuB
6
, a ferromagnet with a Curie temperature T
C
∼ 15 K, are presented and discussed in detail. We have measured the polar Kerr rotation, covering a spectral range from the infrared
up to the ultraviolet, as a function of temperature between 1.5 and 20 K and in external magnetic fields between 0 and 10
T. The Kerr rotation in high fields and at low temperatures is enormous. Our observations, which implicitly reflect the large
magnetoresistive effects, are shown to discriminate between the spectroscopic response of localized and itinerant electronic
states. Our data analysis is based on the phenomenological Lorentz-Drude model, following from the classical dispersion theory
and appropriately extended to magneto-optical experiments.
Received 30 January 2003 / Received in final form 21 March 2003 Published online 23 May 2003
RID="a"
ID="a"e-mail: degiorgi@solid.phys.ethz.ch 相似文献
11.
P. Lejček S. Hofmann 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,31(3):439-441
The paper of L. Rubinovich and M. Polak [Eur. Phys. J. B 22, 267 (2001)] contains some controversial items. The notation “excess” in connection with various thermodynamic quantities
describing solute segregation on the basis of the Langmuir-McLean isotherm is used in a meaning contradictory to general thermodynamics.
The approach used to interpret the linear relationship between segregation entropy and enthalpy results in rather strange
conclusions. These items are discussed, and it is shown how a formulation with standard terms is in accordance with general
thermodynamics.
Received 14 January 2002 / Received in final form 27 September 2002 Published online 14 February 2003
RID="a"
ID="a"e-mail: lejcekp@fzu.cz 相似文献
12.
Large spin systems as given by magnetic macromolecules or two-dimensional spin arrays rule out an exact diagonalization of
the Hamiltonian. Nevertheless, it is possible to derive upper and lower bounds of the minimal energies, i.e. the smallest energies for a given total spin S. The energy bounds are derived under additional assumptions on the topology of the coupling between the spins. The upper
bound follows from “n-cyclicity", which roughly means that the graph of interactions can be wrapped round a ring with n vertices. The lower bound improves earlier results and follows from “n-homogeneity", i.e. from the assumption that the set of spins can be decomposed into n subsets where the interactions inside and between spins of different subsets fulfill certain homogeneity conditions. Many
Heisenberg spin systems comply with both concepts such that both bounds are available. By investigating small systems which
can be numerically diagonalized we find that the upper bounds are considerably closer to the true minimal energies than the
lower ones.
Received 22 October 2002 / Received in final form 4 April 2003 Published online 20 June 2003
RID="a"
ID="a"e-mail: jschnack@uos.de 相似文献
13.
V.P. Zhdanov B. Kasemo 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,27(4):541-547
We present Monte Carlo simulations of the formation of (1×2) islands in the case of the 2
A
+
B
2
→ 2
AB reaction occurring via the Langmuir-Hinshelwood mechanism on a square lattice under steady-state conditions. The model employed takes into account
the effect of anisotropic lateral B-B interactions on the rates of B diffusion and elementary reaction events. The results obtained with qualitatively realistic ratio of the rate of elementary
reaction steps indicate that the average island size depends on the details of diffusion and reaction dynamics in a similar
way as in the earlier studied case of the simplest A
+
B reaction running via the Eley-Rideal mechanism.
Received 4 January 2002 and Received in final form 2 April 2002
Published online 25 June 2002 相似文献
14.
U. Löring B.Ch. Metsch H.R. Petry 《The European Physical Journal A - Hadrons and Nuclei》2001,10(4):395-446
This is the second of a series of three papers treating light-baryon resonances up to 3 GeV within a relativistically covariant
quark model based on the three-fermion Bethe-Salpeter equation with instantaneous two- and three-body forces. In this paper
we apply the covariant Salpeter framework (which we developed in the first paper, U. L?ring, K. Kretzschmar, B.Ch. Metsch,
H.R. Petry, Eur. Phys. J. A 10, 309 (2001)) to specific quark model calculations. Quark confinement is realized by a linearly rising three-body string potential
with appropriate spinorial structures in Dirac space. To describe the hyperfine structure of the baryon spectrum we adopt
't Hooft's residual interaction based on QCD-instanton effects and demonstrate that the alternative one-gluon exchange is
disfavored on phenomenological grounds. Our fully relativistic framework allows to investigate the effects of the full Dirac
structures of residual and confinement forces on the structure of the mass spectrum. In the present paper we present a detailed
analysis of the complete non-strange-baryon spectrum and show that several prominent features of the nucleon spectrum such
as, e.g., the Roper resonance and approximate “parity doublets” can be uniformly explained due to a specific interplay of relativistic
effects, the confinement potential and 't Hooft's force. The results for the spectrum of strange baryons will be discussed
in a subsequent paper, see U. L?ring, B.Ch. Metsch, H.R. Petry, this issue, p. 447.
Received: 27 March 2001 / Accepted: 17 April 2001 相似文献
15.
S.V. Dordevic D.N. Basov R.C. Dynes B. Ruzicka V. Vescoli L. Degiorgi H. Berger R. Gaál L. Forró E. Bucher 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,33(1):15-23
We present a comprehensive analysis of the optical constants of the two-dimensional dichalcogenide materials 2
H
-
TaSe
2
and 2
H
-
NbSe
2
, in an attempt to address the physics of two-dimensional correlated systems. The title compounds were studied over several
decades in frequency, from the far-infrared to the ultraviolet. Measurements with linearly polarized light have allowed us
to obtain both the in-plane and out-of-plane components of the conductivity tensor. Although the electromagnetic response
of dichalcogenides is strongly anisotropic, both the in-plane and out-of-plane components of the conductivity tensor share
many common features, including the presence of a well-defined metallic component, as well as a “mid-infrared band”. We discuss
the implications of these results in the context of the spectroscopic results of other classes of low-dimensional conductors
such as the high-temperature superconducting cuprates. In particular, the analysis of the redistribution of the spectral weight
as a function of temperature, as well as the behavior of the quasiparticles relaxation rate, points to significant distinctions
between the charge dynamics of dichalcogenides and other classes of low dimensional conductors.
Received 28 October 2002 / Received in final form 10 March 2003 Published online 23 May 2003
RID="a"
ID="a"e-mail: degiorgi@solid.phys.ethz.ch 相似文献
16.
M.P. Lissitski A. Naddeo C. Nappi A. Tagliacozzo V.N. Gubankov R. Monaco M. Russo 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,32(3):309-314
Measurements of differential resistance in a superconductor-degenerate semiconductor junction Nb
-
n
+ +
GaAs at T
= 1.6 K show close similarity to those for a conventional superconductor-insulator- normal metal junction, except for the position
of the minimum which is located at 3.6 meV. Using a simple model for the charge screening at the Schottky barrier, we give
an argument why this minimum is by far displaced with respect to the superconducting gap energy ( Δ
g
= 1.5 meV for bulk Nb). We argue that a rebuilding of the density of states takes place at the barrier, due to the imperfect metal
screening in the degenerate semiconductor. Energy states close to the degenerate semiconductor Fermi energy are depleted at
the barrier and are not available for tunneling, up to an energy Eg which adds to the superconducting gap Δ
g
.
Received 11 November 2002 / Received in final form 21 February 2003 Published online 11 April 2003
RID="a"
ID="a"e-mail: c.nappi@cib.na.cnr.it 相似文献
17.
G. Li S.-J. Feng F. Liu Y. Yang R.-K. Zheng T. Qian X.-Y. Guo X.-G. Li 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,32(1):5-11
The magnetic transport properties have been measured for La0.67-xYxCa0.33MnO3 ( 0 ⩽
x
⩽ 0.14) system. It was found that the transition temperature T
p
almost linearly moves to higher temperature as H increases. Electron spin resonance confirms that above T
p
, there exist ferromagnetic clusters. From the magnetic polaron point of view, the shift of T
p
vs. H was understood, and it was estimated that the size of the magnetic polaron is of 9.7 ∼ 15.4 ? which is consistent with the magnetic correlation length revealed by the small-angle neutron-scattering technique. The
transport properties at temperatures higher than T
p
conform to the variable-range hopping mechanism.
Received 27 August 2002 / Received in final form 2 December 2002 Published online 14 March 2003 相似文献
18.
H. Lippert V. Stert L. Hesse C.P. Schulz W. Radloff I.V. Hertel 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2002,20(3):445-448
The H atom transfer reaction in electronically excited indole(NH
3
)
n
clusters is studied in pump-probe experiments with femtosecond laser pulses. By applying different probe photon energies
we are able to detect the dissociation products (NH
3
)
n - 1
NH
4
for n
= 1-6. Furthermore we show that the analysis of the corresponding ion signals is not distorted by contributions from larger cluster
ions due to evaporation of NH
3
molecules. The formation times of the products are ca. 140ps for n
= 2-4 and about 80ps for n
= 5, 6.
Received 30 April 2002 / Received in final form 29 May 2002 Published online 13 September 2002 相似文献
19.
B. Schmidt V. Yushankhai L. Siurakshina P. Thalmeier 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,32(1):43-47
A pronounced Curie-like upturn of the magnetic susceptibility χ(
T
) of the quasi one-dimensional spin chain compound Ba2V3O9 has been found recently [#!kaul:02!#]. Frequently this is taken as a signature for a staggered field mechanism due to the
presence of g-factor anisotropy and Dzyaloshinskii-Moriya interaction. We calculate this contribution within a realistic structure of vanadium
3
d- and oxygen 2
p-orbitals and conclude that this mechanism is far too small to explain experimental results. We propose that the Curie term
is rather due to a segmentation of spin chains caused by broken magnetic bonds which leads to uncompensated S
= ? spins of segments with odd numbers of spins. Using the finite-temperature Lanczos method we calculate their effective moment
and show that ∼ 1% of broken magnetic bonds is sufficient to reproduce the anomalous low-T behavior of χ(
T
) in Ba2V3O9.
Received 19 December 2002 / Received in final form 29 January 2003 Published online 14 March 2003 相似文献
20.
N. Beverini E. Maccioni F. Sorrentino V. Baraulia M. Coca 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2003,23(2):223-228
We have measured the ratio between the transition probabilities of two forbidden lines in Ca: the intercombination line at
657.3 nm and the E
2 line at 457.5 nm. The value we obtained for this ratio is (54.1±2.4). Combined with the most accurate value available in literature for the lifetime of the
3
P
1
level, our measurement gives a value of (54.4±4.0) s-1 for the
1
S
0
↦
1
D
2
transition probability, significantly more accurate than the previous determinations.
Received 17 June 2002 / Received in final form 9 December 2002 Published online 4 March 2003 相似文献