共查询到20条相似文献,搜索用时 31 毫秒
1.
S.G. Mishra P.A. Sreeram 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,14(2):287-291
Specific heat versus temperature curves for various pressures, or magnetic fields (or some other external control parameter) have been seen to
cross at a point or in a very small range of temperatures in many correlated fermion systems. We show that this behavior is
related to the possibility of existence of a quantum critical point. Vicinity to a quantum critical point in these systems
leads to a crossover from quantum to classical fluctuation regime at some temperature . The temperature at which the curves cross turns out to be near this crossover temperature. We have discussed the case of
the normal phase of liquid Helium three and the heavy fermion systems CeAl3 and UBe13 in detail within the spin fluctuation theory, a theory which inherently contains a low energy scale which can be identified
with . When the crossover scale is a homogeneous function of these control parameters there is always crossing at a point. We also
mention other theories exhibiting a low energy scale near a quantum critical point and discuss this phenomenon in those theories.
Received 25 June 1999 相似文献
2.
N. Geier M. Weiss M. Moske K. Samwer 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(1):37-40
The thermal expansion of non-crystalline Zr65Al7.5Cu17.5 Ni10 has been studied in the range of the glass transition and in the undercooled liquid using a dilatometric device. The measuring
technique used permits reliable experimental results up to 40 K above the glass transition temperature. The linear thermal
expansion coefficient obtained is almost constant in the glassy state with a value of . It discontinuously increases at the glass transition temperature yielding a value of in the undercooled liquid. The results are compared with specific heat measurements of the amorphous material in this temperature
range and are interpreted in the framework of a cluster model.
Received 5 March 1999 and received in final form 11 June 1999 相似文献
3.
A. Bousseksou N. Negre M. Goiran L. Salmon J.-P. Tuchagues M.-L. Boillot K. Boukheddaden F. Varret 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(3):451-456
We report the first study of the effect of a high pulsed magnetic field on a spin transition complex in the solid state. The
high spin fraction was determined by optical reflectivity. Sizeable effects are observed for the well-known spin transition
solid Fe(Phen)2(NCS)2. In the hysteresis loop temperature range, an increase in the HS fraction is obtained, with an irreversible (reversible)
character in the ascending (descending) branch of the loop. The time dependence of the HS fraction provides information on
the kinetics of the spin-crossover process at the spin transition.
Received 23 February 1999 and Received in final form 8 June 1999 相似文献
4.
P. Launois R. Moret N.-R. de Souza J.A. Azamar-Barrios A. Pénicaud 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(3):445-450
The fulleride salt C60-tetraphenylphosphonium bromide is investigated as a function of temperature by single crystal X-ray diffuse scattering and
diffraction. At room temperature, the C60 orientational disorder is found to be more complex than previously expected. Moreover, a structural phase transition, due
to the C60 orientational ordering, is evidenced around 120 K. Its relation with the stabilization of a static Jahn-Teller effect is
discussed.
Received 3 November 1999 相似文献
5.
A. Goujon O. Roubeau F. Varret A. Dolbecq A. Bleuzen M. Verdaguer 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,14(1):115-124
We studied the photo-excitation process, the relaxation of the photo-excited state towards the stable state, and the photo-induced
magnetic properties of the Prussian blue analogue Rb0.52Co [Fe(CN)6]0.84, 2.3 H2O. Magnetic, M?ssbauer and reflectivity measurements have been performed during and after illumination. The efficiency of
the photo-excitation device is maximum at nm. The process, however, is severely hindered by bulk absorption of the light; it is rapidly completed at the surface of
the sample and then proceeds slowly in the bulk. Under the effect of photo-excitation the system turns from a dia- to a ferri-
magnet, with a value K, indicative of the transformation of the material due to the following optical electron transfer: Thermal relaxation towards the stable electronic state is observed in the 95-110 K temperature interval, obeying a self-accelerated
kinetics. At low temperature, a weak, non-exponential, relaxation is detected. These features are discussed in terms of co-operative
effects in the frontal process of photo- excitation. The peculiarities of a photo-excited state created below the magnetic
ordering temperature are discussed. A metastable magnetic state has been observed in low fields, denoted “Raw Photo-Induced
State" (RPI), with a magnetization curve in-between the field-cooled and zero-field-cooled curves.
Received 12 March 1999 and Received in final form 11 December 1999 相似文献
6.
T. McAvoy J. Zhang C. Waldfried D.N. McIlroy P.A. Dowben O. Zeybek T. Bertrams S.D. Barrett 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,14(4):747-755
The experimental band structure of Mo(112) and the effects by temperature and adsorbate are presented. A surface resonance,
identified as crossing the Fermi level at about 1/3 from to of surface Brillouin zone, was observed to be very sensitive to both contamination and temperature. We find evidence of adsorbate
and temperature induced reconstruction of the Mo(112) surface. Examination of low-energy electron diffraction (LEED) and scanning
tunneling microscopy (STM) data provides evidence for an adsorbate induced reconstruction of the Mo(112) surface with periodicities
consistent with the Fermi level crossing of the surface resonance. The reconstruction is found to occur at coverages as low
as 0.03 Langmuirs of oxygen or carbon. The reconstruction and/or adsorbate affects the density of states and bands near the
Fermi level of a
1 symmetry.
Received 3 March 1999 and Received in final form 1 October 1999 相似文献
7.
W. Westerburg F. Martin P.J.M. van Bentum J.A.A.J. Perenboom G. Jakob 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,14(3):509-513
We measured the temperature dependence of the linear high field Hall resistivity of ( K) and ( K) thin films in the temperature range from 4 K up to 360 K in magnetic fields up to 20 T. At low temperatures we find a
charge-carrier density of 1.3 and 1.4 holes per unit cell for the Ca- and Sr-doped compound, respectively. In this temperature
range electron-magnon scattering contributes to the longitudinal resistivity. At the ferromagnetic transition temperature
a dramatic drop in the number of charge-carriers n down to 0.6 holes per unit cell, accompanied by an increase in unit cell volume, is observed. Corrections of the Hall data
due to a non saturated magnetic state will lead a more pronounced charge-carrier density collapse.
Received 22 July 1999 and Received in final form 7 October 1999 相似文献
8.
M. Pinterić M. Miljak N. Biškup O. Milat I. Aviani S. Tomić D. Schweitzer W. Strunz I. Heinen 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(2):217-225
We report a detailed characterization of the magnetism and AC transport in single crystals of the organic conductor -(BEDT-TTF)2Cu[N(CN)2]Cl by means of magnetic anisotropy measurements and low-frequency dielectric spectroscopy. Magnetic anisotropy obeys Curie-Weiss
law with negative Curie-Weiss temperature in the temperature range 300 K-70 K. An antiferromagnetic transition with concomitant
canted antiferromagnetic state is established at 22 K. A large hysteresis in the spin-flop transition and magnetic field reversal
of the weak ferromagnetic magnetization are documented for the first time. A broad dielectric relaxation mode of moderate
strength () emerges at 32 K, and weakens with temperature. The mean relaxation time, much larger than that expected for single-particle
excitations, is thermally activated in a manner similar to the DC conductivity and saturates below 22 K. These features suggest
the origin of the broad relaxation as an intrinsic property of the weak ferromagnetic ground state. We propose a charged domain
wall in a random ferromagnetic domain structure as the relaxation entity. We argue that the observed features might be well
described if Dzyaloshinsky-Moriya interaction is taken into account. A Debye relaxation with similar temperature dependence
was also observed and seems to be related to an additional ferromagnetic-like, most probably, field-induced phase. We tentatively
associate this phase, whose tiny contribution was sample dependent, with a Cu2+ magnetic subsystem.
Received 15 June 1998 and Received in final form 1 February 1999 相似文献
9.
10.
L.V. Butov 《Solid State Communications》2003,127(2):89-98
Bound electron-hole pairs—excitons—are Bose particles with small mass. Exciton Bose-Einstein condensation is expected to occur at a few degrees Kelvin—a temperature many orders of magnitude higher than for atoms. Experimentally, an exciton temperature well below 1 K is achieved in coupled quantum well (CQW) semiconductor nanostructures. In this contribution, we review briefly experiments that signal exciton condensation in CQWs: a strong enhancement of the indirect exciton mobility consistent with the onset of exciton superfluidity, a strong enhancement of the radiative decay rate of the indirect excitons consistent with exciton condensate superradiance, strong fluctuations of the indirect exciton emission consistent with critical fluctuations near the phase transition, and a strong enhancement of the exciton scattering rate with increasing concentration of the indirect excitons revealing bosonic stimulation of exciton scattering. Novel experiments with exciton condensation in potential traps, pattern formation in exciton system and macroscopically ordered exciton state will also be reviewed briefly. 相似文献
11.
A. Cantarero J. Martinez-Pastor A. Segura A. Chevy 《Applied Physics A: Materials Science & Processing》1988,45(2):125-132
The absorption coefficient of bismuth sulfide single crystals has been measured through more than four orders of magnitude
and in the range of energies from 1.25 to 1.70 eV. A detailed study as a function of temperature has been carried out from
29 to 300 K. An Urbach tail for low values of absorption has been found. This tail and its temperature evolution fit the expression
for ionic materials. An excitonic region appears at low temperature and the shape of the exciton peak is Gaussian, which corresponds
to a strong exciton-phonon coupling. The exciton binding energy is estimated (28±3 meV) and then the energy gap at 29 K is
obtained (E
g
=1.523±0.003 eV). The fundamental electronic transition has been found to be a strongly anisotropic allowed direct transition.
From reflectivity measurements a localized level at 1.361 eV at 29 K has been found. The change of the gap with temperature
is interpreted through an electron-phonon mechanism. 相似文献
12.
C. Buschhaus R. Desquiotz K. Eichhorn M. Hofmann K. Hümmer V. Illich M. Kelemen S. Tarragona Auga T. Wokrina A. Zitsch E. Dormann 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(1):57-66
Crystal growth and X-ray structural analysis are reported for the new quasi-one-dimensional organic conductor perylene hexafluoroantimonate.
Due to deviation from exact 4:3 stoichiometry and anion-chain disorder the three-dimensional Peierls transition is suppressed
to temperatures below 30 K in spite of a molecular field transition temperature of 330 K. Anisotropy of the microwave conductivity
exceeds 3000:1 at room temperature. Curie paramagnetic defects with comparatively small electron spin resonance line width
predominate at low temperature. Thermally activated paramagnetic defects, giving rise to pronounced Overhauser shift, are
separated in the 30-100 K range. They are explained by neutral Perylene intra-stack defects, whose concentration can be reduced
by both aging and annealing. The angular and temperature dependence of the conduction electron spin resonance line is analysed
in detail, exhibiting the influence of intra-stack dipole interaction growing with temperature. Restricted diffusion of the
conduction electron spins with diffusion coefficient of about 6 cm2/s parallel to the stacking direction is detected by fixed gradient pulsed electron spin resonance at room temperature.
Received 17 July 1998 相似文献
13.
J. Kästner W. Petry S.M. Shapiro A. Zheludev J. Neuhaus Th. Roessel E.F. Wassermann H. Bach 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(4):641-648
The acoustic phonon dispersions of two Invar crystals , one ordered with the () structure, the other disordered fcc, have been investigated between 3.4 K and 470 K by inelastic and elastic neutron scattering.
For the ordered crystal, pronounced softening of the whole
phonon branch is observed on cooling below the Curie temperature. Particularly strong phonon softening at the M-point zone
boundary of the structure leads to a displacive, antiferrodistortive phase transition at low temperatures. For the disordered crystal, much
weaker softening of the
phonons is observed and restricted to the region near the Brillouin zone center, where increasing elastic scattering with
decreasing temperature indicates the growth of local tetragonal strain. This strain is considered as a typical precursor of
the transformation to bct martensite. Specific heat measurements, performed at low temperatures on both crystals confirm the
neutron scattering results and reveal considerable enhancement of the low energy phonon density of states in the ordered crystal.
Received 18 January 1999 相似文献
14.
Pb diffusion in α-Zr matrix between 823 and 1123 K was measured using heavy ion Rutherford backscattering spectrometry (HIRBS)
technique. A deviation from the Arrhenius law was observed, with two different regions. At low temperatures the activation
energy Q is close to the expected value for a substitutional diffuser but the pre-exponential factor D0 is higher than expected. Close to the phase transition temperature the opposite occurs, with a low Q value. This behavior
is similar to the one observed for Hf and self-diffusion in α-Zr.
Received: 29 September 1998 / Accepted: 29 January 1999 / Published online: 28 April 1999 相似文献
15.
E. Chappel M.D. Núñez-Regueiro F. Dupont G. Chouteau C. Darie A. Sulpice 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,17(4):609-614
Magnetisation measurements up to 23 T and submillimeter wave ESR in the frequency region 48-380 GHz have been performed on
NaNiO2 powders at low temperature. Also typical spectra above the Néel temperature are shown. At 4 K the magnetisation shows a spin-flop transition at 1.8 T and saturation at 10 T. /Ni confirms the low spin state of the Ni3+ ions. The susceptibility exhibits a maximum at K with and K. NaNiO2 is an A-type antiferromagnet: we derive K and K for the interactions between Ni ions within and between adjacent layers, respectively. The AFMR spectra yield an energy
gap of 52.5 GHz, in agreement with the spin-flop value derived from the magnetisation. The anisotropy of the g factor observed at 100 K with can be attributed to the Jahn-Teller effect for Ni3+ ions in the low spin state, which stabilises the occupation. We finally comment on the isomorphic controversial Li1-xNi1+xO2 compound.
Received 9 March 2000 and Received in final form 13 July 2000. 相似文献
16.
J. Kästner J. Neuhaus E. F. Wassermann W. Petry B. Hennion H. Bach 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(1):75-81
In a previous neutron scattering study, we had observed that the TA phonon softening in L12-ordered ferromagnetic Fe72Pt28 Invar is pronounced at the zone boundary M-point and leads to an antiferrodistortive phase transition at low temperatures.
Here, we report on similar neutron scattering investigations on two ordered crystals with higher Fe content to investigate
the relation between the TA phonon softening and the martensitic transformation, which occurs in Fe-rich ordered Fe-Pt. We find that the TA phonon softening, especially at the M-point zone boundary, does not depend on the composition of the investigated crystals.
In Fe74.5Pt25.5, however, the antiferrodistortive phase transition temperature is enhanced due to tetragonal strain preceding the martensitic
transition. In Fe77Pt23 a precursor driven premartensitic phase transition is not observed. The structure of the martensite is, however, influenced
by the soft mode lattice instability of the austenite. This would explain the origin of structural details found previously
for Fe3Pt thermoelastic martensite.
Received 18 January 1999 and Received in final form 11 March 1999 相似文献
17.
18.
T. Garel H. Orland E. Orlandini 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(2):261-268
We consider polymers made of magnetic monomers (Ising or Heisenberg-like) in a good solvent. These polymers are modeled as
self-avoiding walks on a cubic lattice, and the ferromagnetic interaction between the spins carried by the monomers is short-ranged
in space. At low temperature, these polymers undergo a magnetic induced first order collapse transition, that we study at
the mean field level. Contrasting with an ordinary point, there is a strong jump in the polymer density, as well as in its magnetization. In the presence of a magnetic field,
the collapse temperature increases, while the discontinuities decrease. Beyond a multicritical point, the transition becomes
second order and -like. Monte Carlo simulations for the Ising case are in qualitative agreement with these results.
Received 11 February 1999 相似文献
19.
Using small-angle X-ray scattering, the repeat distance vs. temperature is measured for a homologous series of multilamellar vesicles of lecithins with varying acyl chain length in
excess water condition around the lipid main transition. A systematic chain length dependence is found which is in accordance
with a bending rigidity renormalization and critical unbinding of the lamellae close to the transition, as previously suggested
in H?nger et al. [Phys. Rev. Lett. 72, 3911 (1994)].
Received 13 January 1999 and Received in final form 6 September 1999 相似文献
20.
S. Zherlitsyn B. Lüthi V. Gusakov B. Wolf F. Ritter D. Wichert S. Barilo S. Shiryaev C. Escribe-Filippini J.L. Tholence 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,16(1):59-66
We report results of the ultrasonic investigation of Ba1-xKxBiO3 superconducting (SC) single crystals for two potassium concentrations and in a wide temperature range including the normal and the SC states. An instability of the crystal lattice that develops above
the superconducting phase transition leads to a softening of both the transverse c44 and the longitudinal c11 modes at temperatures between 200 K and 50 K. In the case of Ba0.65K0.35BiO3 a pronounced hysteresis was discovered. Low temperature X-ray powder diffraction analysis does not reveal any change of the
cubic structure in the samples within a resolution of our X-ray technique. The softening of the elastic moduli, the hysteresis,
the maximum in the attenuation of sound along with the possible short- (or long-) range structural distortion can be explained
qualitatively in a simple model by assuming a coupling of the acoustic modes with the anharmonic oscillations of BiO6 octahedra. Some weak anomalies are discovered in the velocity of the longitudinal sound in the vicinity of the superconducting
phase transition.
Received 25 June 1999 and Received in final form 14 February 2000 相似文献