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1.
E. Buixaderas V. Porokhonskyy A. Paskhin M. Savinov J. Petzelt 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,30(3):319-329
Barium sodium niobate (BNN) single crystals are studied by IR spectroscopy, time-domain THz transmission spectroscopy, HF
coaxial wave-guide technique and LF dielectric spectroscopy to cover the frequency range 102-1014 Hz in a wide temperature interval. The dielectric response parallel and perpendicular to the polar c-axis is discussed. The ferroelectric transition at T
c
= 830 K is driven by a relaxational soft mode coupled with another central-mode type relaxation which both gradually disappear
on cooling in the ferroelectric phase. Below T
i
the parameters of the expected IR active amplitudon were estimated. The low-temperature permittivity increase on cooling
for the
field direction has been explained by an incipient proper ferroelectric-ferroelastic transition driven by an IR and Raman
active B2-symmetry soft mode.
Received 24 August 2002 Published online 19 December 2002
RID="a"
ID="a"e-mail: buixader@fzu.cz 相似文献
2.
T. Vuletić P. Auban-Senzier C. Pasquier S. Tomić D. Jérome M. Héritier K. Bechgaard 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,25(3):319-331
The phase diagram of the organic superconductor (TMTSF)2PF6has been revisited using transport measurements with an improved control of the applied pressure. We have found a 0.8 kbar
wide pressure domain below the critical point (9.43 kbar, 1.2 K) for the stabilisation of the superconducting ground state
featuring a coexistence regime between spin density wave (SDW) and superconductivity (SC). The inhomogeneous character of
the said pressure domain is supported by the analysis of the resistivity between T
SDW
and T
SC
and the superconducting critical current. The onset temperature T
SC
is practically constant ( 1.20±0.01 K) in this region where only the SC/SDW domain proportion below T
SC
is increasing under pressure. An homogeneous superconducting state is recovered above the critical pressure with T
SC
falling at increasing pressure. We propose a model comparing the free energy of a phase exhibiting a segregation between
SDW and SC domains and the free energy of homogeneous phases which explains fairly well our experimental findings.
Received 3 September 2001 and Received in final form 9 November 2001 相似文献
3.
F. Fauth E. Suard V. Caignaert B. Domengès I. Mirebeau L. Keller 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,21(2):163-174
The oxygen deficient cobaltites LnBaCo2O5 (Ln = Tb, Dy, Ho) exhibit two successive crystallographic transitions at T
N
∼340 K and at T
CO
∼210 K. Whereas the first transition (P4/mmm to Pmmm) is related to the long-range antiferromagnetic ordering of the Co ions
(spin ordering), the second transition (Pmmm to Pmmb) corresponds to the long-range ordering of the Co2+ and Co3+ species (charge ordering) occurring in 1:1 ratio in the structure. The charge ordered (CO) state was directly evidenced by
the observation of additional superstructure peaks using neutron and electron diffraction techniques. The CO state was also
confirmed indirectly from refinement of high resolution neutron diffraction data as well as from resistivity and DSC measurements.
From the refined saturated magnetic moment values only, ∼3.7
and ∼2.7
, the electronic configuration of the Co ions in LnBaCo2O5 remains conjectural. Two pictures, with Co3+ ions either in intermediate spin state ( t
5
2g
e
1
g
) or in high spin state ( t
4
2g
e
2
g
), describe equally well our experimental data. In both cases, the observed magnetic structure can be explained using the
qualitative Goodenough-Kanamori rules for superexchange. Finally, in contrast to the parent Ln = Y compound [Vogt et al. , Phys. Rev. Lett. 84, 2969 (2000)], we do not report any spin transition in LnBaCo2O5 (Ln = Tb, Dy, Ho).
Received 13 December 2000 相似文献
4.
Govind A. Pratap Ajay R.S. Tripathi 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,23(2):153-158
In the present paper, we study the magnetic properties of bilayer cuprate antiferromagnets. In order to evaluate the expressions
for spin-wave dispersion, sublattice magnetization, Néel temperature and the magnetic contribution to the specific heat, the
double time Green's function technique has been employed in the random phase approximation (RPA). The spin wave dispersion
curve for a bilayer antiferromagnetic system is found to consist of one acoustic and one optic branch. The “optical magnon
gap” has been attributed solely to the intra-bilayer exchange coupling (J
⊥
) as its magnitude does not change significantly with the inter-bilayer exchange coupling (Jz). However Jz is essential to obtain the acoustic mode contribution to the magnetization. The numerical calculations show that the Néel
temperature (T
N
) of the bilayer antiferromagnetic system increases with the Jz and a small change in Jz gives rise to a large change in the Néel temperature of the system. The magnetic specific heat of the system follows a T2 behaviour but in the presence of Jz it varies faster than T2.
Received 13 July 2000 and Received in final form 14 May 2001 相似文献
5.
V. Mohaček-Grošev D. Kirin 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,20(1):85-90
Methyl-mercury(II) halides CH3HgX (X = Cl, Br and I) were studied by means of temperature dependent Raman spectroscopy from 10 K to 410 K. In addition to
the previously reported soft phonons, new changes in the low frequency spectra were observed at T
≈ 70 K in CH3HgBr and at T
≈ 100 K in CH3HgI. The bandwidths of the two internal modes in CH3HgBr, the CH3 symmetric stretching band and the C-Hg-Br bending band, rise towards a local maximum at T
≈ 50 K as the temperature is raised from 10 K to 300 K. On the other hand the bandwidths of the two corresponding modes in CH3HgI crystals monotonously increase with temperature, obeying an Arrhenius law. Besides the three phonon modes present in the
Raman spectra of CH3HgCl at room temperature, the fourth phonon band that has been observed at temperatures below 245 K might correspond to the
freezing of methyl librations. The huge bandwidth of the C-Hg-Br bending mode could suggest the presence of additional weak
bonding of a mercury atom with bromine atoms from other molecules, thus inducing positional disorder.
Received 19 November 1999 and Received in final form 10 November 2000 相似文献
6.
T.J. Smith H. Capellmann R.K. Kremer K-U. Neumann K.R.A. Ziebeck 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,21(3):341-347
A polarised neutron scattering investigation has been carried out on a powder sample of CuGeO3 within the temperature range of 1.5 K to 600 K. The magnetic scattering has been separated from all other contributions by
using polarised neutrons and polarisation analysis and placed onto an absolute scale. At low temperatures the long wavelength
components of the paramagnetic response are suppressed consistent with the formation of Cu dimers in which the magnetic moments
are correlated antiferromagnetically. This form of the scattering persists to temperatures well above the dimerisation temperature
T
sp
∼ 14 K. However as the temperature is raised the intensity of the long wavelength spin fluctuations increases and above ∼150 K they are the dominant feature in the wave vector dependence of the response. At all temperatures the observed scattering
extrapolates smoothly to the Q
= 0 value given by the uniform susceptibility. Consequently the thermal variation of the uniform susceptibility arises from the
evolution of the long wavelength magnetic fluctuations. At large wave vectors the energy dependence of the scattering revealed
that the response occurs below 16 meV in agreement with the reported maximum magnetic excitation energy at the zone boundary
in the ground state. However the total magnetic scattering is significantly less than that expected for a local moment system
suggesting that the spectrum of thermal and quantum fluctuations overlap.
Received 30 May 2000 and Received in final form 22 March 2001 相似文献
7.
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
8.
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
9.
H. Paulsen H. Grünsteudel W. Meyer-Klaucke M. Gerdan H.F. Grünsteudel A.I. Chumakov R. Rüffer H. Winkler H. Toftlund A.X. Trautwein 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,23(4):463-472
The temperature-induced spin crossover of iron(II) in the [
Fe
(
tpa
)(
NCS
)
2
] complex has been investigated by nuclear forward scattering (NFS), nuclear inelastic scattering (NIS), extended X-ray absorption
fine structure (EXAFS) spectroscopy, conventional M?ssbauer spectroscopy (MS) and by measurements of the magnetic susceptibility
(SQUID). The various measurements consistently show that the transition is complete and abrupt and exhibits a hysteresis between
102 and 110 K. The dependence of the hyperfine parameters of the high-spin (HS) and of the low-spin (LS) phase on temperature
is gradual while the effective thickness (determined by the Lamb-M?ssbauer factor f
LM
) shows a step at the transition temperature. This step could be identified clearly because the effective thickness is measured
directly by NFS. The Lamb-M?ssbauer factor, the Debye temperature and the mean-square displacement of iron(II) could be determined
for the HS and for the LS phase. When comparing the NIS data with the results from density functional theory (DFT), the Fe-N
stretching vibrations of both LS and HS phases could be unambiguously identified and the f
LM
could be factorized for both phases into a lattice and a molecular part. The structural information from EXAFS and DFT geometry
optimization are in reasonable agreement.
Received 19 June 2001 相似文献
10.
V.P. Dyakonov M.T. Borowiec A. Jedrzejczak M. Górska N.A. Doroshenko E. Zubov Yu. Bukhantsev T. Zayarnyuk H. Szymczak 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,29(1):9-14
Magnetic phase transition in the CsDyW2O8 magnet has been studied by means of low temperature specific heat C
(
T
) measurements. The magnetic ordering temperature of the Dy3+ sublattice was established to be 1.34 K. The experimental results indicate on the antiferromagnetic character of interactions
between Dy3+ ions. The behavior of the C
(
T
) dependencies above and below T
N
is discussed in frames of different theoretical models. The measurements data on temperature and field dependencies of magnetization
are used to calculate the exchange and dipole-dipole interactions energy and to determine the possible magnetic structure
of the ground state.
Received 7 January 2002 / Received in final form 15 May 2002 Published online 7 September 2002 相似文献
11.
The shear-induced ordering of lamellar and gyroid structures of a nonionic surfactant C16E7/D2O system in a Couette shear cell ( 0.001 < < 10 s-1, : shear rate) has been investigated by using a small angle neutron scattering technique. In the lamellar phase, the steady
shear flow having > 0.01 s-1 suppresses undulation fluctuations of lamellae (Maxwell effect). This suppression of fluctuations brings two effects; 1)
shear-induced lamellae ordering toward a parallel orientation and 2) obstruction of a lamellar↦gyroid transition. It is quite
interesting to note that there is a characteristic shear rate range ( 0.01 < < 0.3 s-1), where both effects take place. We have also investigated the shear effects on the gyroid phase. Below the characteristic
shear rate range, the gyroid structure keeps three-dimensional network lattice, while above the characteristic shear rate
range, the gyroid structure transforms to the parallel orientation lamellae (shear-induced gyroid-lamellar transition). Thus
the shear flow having the characteristic shear rate plays very important roles in shear ordering phenomena.
Received 26 June 2000 and Received in final form 12 January 2001 相似文献
12.
E. Bertin P. Bonville J.-P. Bouchaud J.A. Hodges J.P. Sanchez P. Vulliet 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,27(3):347-354
Using 155Gd M?ssbauer spectroscopy down to 27 mK, we show that, in the geometrically frustrated pyrochlore Gd2Sn2O7, the Gd3+ hyperfine levels are populated out of equilibrium. From this, we deduce that the hyperfine field, and the correlated Gd3+ moments which produce this field, continue to fluctuate as T
↦ 0. With a model of a spin 1/2 system experiencing a magnetic field which reverses randomly in time, we obtain an analytical
expression for the steady state probability distribution of the level populations. This distribution is a simple function
of the ratio of the nuclear spin relaxation time to the average electronic spin-flip time. In Gd2Sn2O7, we find the two time scales are of the same order of magnitude. We discuss the mechanism giving rise to the nuclear spin
relaxation and the influence of the electronic spin fluctuations on the hyperfine specific heat. The corresponding low temperature
measurements in Gd2Ti2O7 are presented and discussed.
Received 17 October 2001 Published online 6 June 2002 相似文献
13.
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
14.
E. Buixaderas S. Kamba J. Petzelt M. Savinov N.N. Kolpakova 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,19(1):9-16
The infrared reflectivity of Cd2Nb2O7 single crystal was studied in the temperature interval of 10-540 K, together with complementary dielectric measurements.
A ferroelectric soft mode was revealed above the ferroelectric phase transition at T
c = 196 K coupled with a central-mode type dispersion in the near-millimetre range. This proves the mixed displacive and order-disorder
nature of the transition. Below T
c many new modes were detected due to lowering of the symmetry, especially below the previously suggested incommensurate transition
at 85 K. Discussion of the possible phase transitions based on symmetry considerations is presented with the conclusion that
the ferroelectric transition is proper with the F1u symmetry of the order parameter, whereas the intermediate ferroelastic transition is improper and triggered by the coupling
with the ferroelectric order parameter.
Received 17 July 2000 相似文献
15.
L. Martín-Carrón A. de Andrés 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,22(1):11-16
We have studied the behavior of the Raman phonons of a stoichiometric LaMnO3 single crystal as a function of temperature in the range between 77 K and 900 K. We focus on the three main phonon peaks
of the Pbnm structure, related to the tilt, antisymmetric stretching (Jahn-Teller mode) and stretching modes of Mn-O octahedra.
The phonon frequencies show a strong softening that can be fit taking into account their renormalization because of three
phonon anharmonic effects in the pseudoharmonic approximation. Thermal expansion effects, in particular the variation of Mn-O
bond lengths with temperature, are not relevant above 300 K. On the contrary, phonon width behavior deviates from the three
phonon scattering processes well bellow T
c
. The correlation between the magnitude of the cooperative Jahn-Teller distortion, that disappears at 800 K, and the amplitude
of the Raman phonons in the orthorhombic phase is shown. Nevertheless, Pbnm phonons are still observable above this temperature.
Phonon width and intensity behavior around T
c
can be explained by local melting of the orbital order that begins quite below T
c
and by fluctuations of the regular Mn-O octahedra that correspond to dynamic Jahn-Teller distortions.
Received 25 January 2001 and Received in final form 14 March 2001 相似文献
16.
E. Ercolessi G. Morandi M. Roncaglia 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,32(4):489-493
Recent neutron scattering experiments on CsNiCl3 reveal some features that are not well described by the standard nonlinear σ model, nor by numerical simulations, for isolated S
= 1 spin chains. In particular, in real systems at the antiferromagnetic point of the Brillouin zone, the intensity of the continuum
of multiparticle excitations, at T
= 6 K, is about 5 times greater than predicted. Also, the spin gap is higher and the correlation length is smaller than predicted.
We propose a theoretical scenario where the interchain interaction is approximated by an effective staggered magnetic field,
and that yields a correct prediction for the observed quantities.
Received 2 October 2002 / Received in final form 19 March 2003 Published online 7 May 2003 相似文献
17.
J. Schulenburg J.S. Flynn D.D. Betts J. Richter 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,21(2):191-204
In this paper finite bcc lattices are defined by a triple of vectors in two different ways - upper triangular lattice form
and compact form. In Appendix A are lists of some 260 distinct and useful bcc lattices of 9 to 32 vertices. The energy and
magnetization of the S = 1/2 XY ferromagnet have been computed on these bcc lattices in the lowest states for S
z = 0, 1/2, 1 and 3/2. These data are studied statistically to fit the first three terms of the appropriate finite lattice
scaling equations. Our estimates of the T = 0 energy and magnetization agree very well with spin wave and series expansion estimates.
Received 1st August 2000 and Received in final form 22 December 2000 相似文献
18.
V.A. Pashchenko A. Sulpice F. Mila P. Millet A. Stepanov P. Wyder 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,21(4):473-476
We report on the magnetic susceptibility and electron spin resonance measurements on polycrystalline samples of the vanadium
oxide VOSb2O4, a quasi-one-dimensional S
= 1/2 Heisenberg system. The susceptibility vanishes at zero temperature, but there is no cusp at the onset of the susceptibility
drop, and the ESR linewidth exhibits an increase characteristic of a phase transition at a much lower temperature. We show
that this behaviour is consistent with the formation of a pseudo-gap in a spin-Peierls system in the adiabatic limit.
Received 7 February 2001 and Received in final form 24 April 2001 相似文献
19.
G. Yang K. Maki 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,21(1):61-65
We study theoretically the effect of impurity scattering in f-wave (or E2u) superconductors. The quasi-particle density of states of f-wave superconductor is very similar to the one for d-wave superconductor as in hole-doped high T
c
cuprates. Also in spite of anisotropy in Δ(
), both the reduced superfluid density and the reduced electronic thermal conductivity is completely isotropic.
Received 11 October 2000 相似文献
20.
R.S. Gonnelli A. Calzolari D. Daghero L. Natale G.A. Ummarino V.A. Stepanov M. Ferretti 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,22(4):411-414
We present new Au/La2-xSrxCuO4 (LSCO) point-contact conductance measures as a function of voltage and temperature in samples with 0.08 ?
x
? 0.2. Andreev reflection features disappear at about the bulk T
c
, giving no evidence of gap for T
>
T
c
. The fit of the normalized conductance at any T
<
T
c
supports a (s
+
d)-wave symmetry of the gap, whose dominant low-T
s component follows the T
c
(
x
) curve in contrast with recent angle-resolved photoemission spectroscopy and quasiparticle tunneling data. These results prove
the separation between pseudogap and phase-coherence superconducting gap in LSCO at x
0.2.
Received 14 June 2001 相似文献