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1.
X-ray and resistivity measurements on YBa2Cu3O7−δ
(1-2-3) samples show that for the same but low oxygen concentration,δ⋍0·55, no superconducting transition down to 4·2 K is observed for the tetragonal phase samples while the orthorhombic phase
shows aT
c ∼ 31 K. The effect of oxygen concentration onT
c is isolated.T
c=91±1 K has, however, been observed continuously for the normal oxygen annealed samples,δ⋍0·07. The experimental results suggest strongly the necessity of the 1-2-3 compound to be in the orthorhombic phase for the
superconducting mechanism to be operative. 相似文献
2.
Alkali-ion conducting glasses/glass ceramics of the system Me2O-A12O3-SiO2 (Me=Li, Na) were applied as solid electrolytes in potentiometric gas sensors to detect CO2 in the presence of O2 at increased temperatures. The corresponding Me-Carbonates were utilized as auxiliary electrodes. Sensors using the direct
Au-glass contact as a kind of reference electrode (type I), as well as symmetrical sensors with carbonate phase at the reference
and measuring electrode (type II - for comparative measurements) were manufactured.
By applying Au as electrode metal, the theoretically expected EMF difference and the observed EMF difference of both sensor
types agree quite well with the expected values according to the Nernst equation between 500 and 600 °C (over four orders
of magnitude of CO2 partial pressure (10−5 – 10−1 bar) at constant O2 partial pressure (2.1×10−1 bar)). A long time stability of 120 days for sensors of type I with Li glasses has been observed, although evaporation of
carbonate phase (Li2CO3) was detected under the conditions of sensor application. Sensors of type I (with Li2CO3) show thermodynamically unexpected cross-sensitivities to H2O.
Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996 相似文献
3.
M. Weber P. Birrer F. N. Gygax B. Hitti E. Lippelt H. Maletta A. Schenck 《Hyperfine Interactions》1991,63(1-4):207-212
Possibleμ
+ sites have been identified by a comparison of measured spontaneous internal fields in YBa2Cu3O6+δ and REBa2Cu3O7−δ and of measured second moments and width of transverse field powder spectra with the corresponding calculated quantities.
In the YBa2Cu3O7 system only one possibleμ
+ site emerges at a distance of ∼1.05 A from a chain oxygen O(4) at the position (0.15(1), 0.44(1), 0.071(1)). In the system
YBa2Cu3O6 the only possibleμ
+ sites are near to an oxygen O(1) with z/c=0.133 and the same distance as above. The analysis leads also to improved valuesμ
Ho = 2.2(1)μ
B andμ
Cu = 0.67(4)μ
B and to a determination of the latters direction:μ
Cu ‖〈110〉. 相似文献
4.
R. Z. Bakhtizin Qikun Xue T. Sakurai T. Hashizume 《Journal of Experimental and Theoretical Physics》1997,84(5):1016-1021
Scanning tunneling microscopy is applied for the first time to an atomic-resolution investigation of the 4×2 and 4×6 phases
on a gallium-rich GaAs(001) surface obtained by molecular-beam epitaxy and migration-enhanced epitaxy. A unified structural
model is proposed with consideration of the results of experiments and first-principles calculations of the total energy.
In this model the 4×2 phase consists of two Ga dimers in the top layer and a Ga dimer in the third layer, and the 4×6 phase
is matched to periodically arranged Ga clusters at the corners of a 4×6 unit cell on top of the 4×2 phase.
Zh. éksp. Teor. Fiz. 111, 1858–1868 (May 1997) 相似文献
5.
S. Garcia L. Ghivelder S. Soriano I. Felner 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,51(3):307-313
The magnetic properties of the superconducting ruthenate-cuprate RuSr2Eu
1.4Ce0.6Cu2O10-δ (Ru-1222) have been studied by a
scaling analysis of the dc magnetization and ac susceptibility measurements.
Non-linear M(H) curves reveal the presence of nano-size clusters with a
net magnetic moment of ∼102 μB at 180 K, near the
deviation from a Curie-Weiss behavior. On cooling, no scaling was observed
down to 90 K, discarding the possibility of magnetic phase separation of
collinear ferromagnetic particles. We explain this result in terms of a
variable number of contributing particles, with a temperature dependent net
magnetic moment. For 70 K ≤T≤ 90 K the scaling plots evidence the
emergence of a system of non-interacting particles, which couple on further
cooling. The observed cluster-glass features are preserved down to the
lowest measured temperature (10 K); no signature of long-range order was
detected. The frequency shift of the peak in the real part of the ac
susceptibility does not follow the Vogel-Fulcher type dependence, as
previously reported. The puzzling temperature dependence of the coercive
field, HC(T), is correlated with the changes in the scaling factors. 相似文献
6.
2 +O2→H2 O in the pressure range 0.2 Torr≤ptot≤10 Torr on Pt(111) surface. At a catalyst temperature of T=700 K the equilibrium oxygen coverage θo is determined as a function of hydrogen partial pressure α. The experimentally obtained θo is modelled in a two step process considering the mass transport in the gas phase as well as the catalytic reaction on the
surface. In this pressure range the mass transport in the gas phase changes from molecular flow conditions to laminar flow,
inducing a strong modification of the gas phase present at the catalyst through different diffusivities of the reactants as
well as through desorbing reaction products from the catalyst. It is shown that these gas phase alterations have to be taken
into account for a proper modelling of the surface mechanism.
Simulation calculations allow one to identify the sequential hydrogen addition reaction as the main reaction path for water
production in this parameter range. Excellent agreement with previous investigations is obtained for the determined activation
energies of the water-producing reaction steps equal to Ef
H2O≥0.7 eV.
Received: 20 September 1998 / Revised version: 15 December 1998 相似文献
7.
In 1964 Davis and Adams established that the large increase of the thermal expansion and compressibility in the critical region
of the γ-to α-Ce phase transition occurs predominantly in the α phase. This provides strong evidence that a tricritical point
is realized in Ce. This also means that the aforementioned transition is not isomorphic and that α-Ce should have a distorted
fcc structure. A careful examination of Jayaraman’s data (1965) shows that a second-order transition line continues beyond
the tricritical point to the vicinity of a triple point on the melting curve. The phase boundary with the tricritical point
and the minimum of the melting curve are reconstructed within the framework of Landau theory.
Pis’ma Zh. éksp. Teor. Fiz. 67, No. 2, 111–117 (25 January 1998)
Published in English in the original Russian journal. Edited by Steve Torstveit. 相似文献
8.
V. A. Ivanov E. A. Ugolkova M. E. Zhuravlev 《Journal of Experimental and Theoretical Physics》1998,86(2):395-404
We have analyzed the electronic structure and superconducting properties of layered crystals based on BEDT-TTF salts (hereafter
denoted as ET), in which intramolecular interaction among electrons is important. For the case of realistic κ-packing of ET-molecules in a plane of ET2 dimers we have calculated the electron density of states. Using the calculated electronic structure, we have analyzed the
symmetry of the anisotropic superconducting pairing. The critical value of the effective attraction between electrons for
formation of a bound pair in an empty lattice has been estimated. The relation between the nodes of the d-type order parameter on the anisotropic Fermi surface and superconducting properties of the condensate is discussed in the
Bardeen-Cooper-Schrieffer (BCS) model. The results are in agreement with the known band parameters of the normal phase and
measurements of the magnetic field penetration depth, heat capacity as a function of temperature in the low-temperature range,
and NMR in κ-ET2X superconducting salts.
Zh. éksp. Teor. Fiz. 113, 715–733 (February 1998) 相似文献
9.
Anderson Rosa Kurelo Yuri Aparecido Opata Lincoln Brum de Leite Gusmão Pinheiro Alcione Roberto Jurelo Adilson Luiz Chinelatto 《Brazilian Journal of Physics》2011,41(1):1-6
We report on systematic conductivity fluctuation measurements on samples of Ho1 − x
Ce
x
Ba2Cu3O7 − δ
with x = 0.00, 0.05, and 0.10. The samples were produced by a standard solid-state reaction method, and the microstructure was analyzed
by X-ray diffraction. To identify power-law divergences of the conductivity, the results were analyzed in terms of the temperature
derivative of the resistivity dρ/dT and with the logarithmic derivative of the conductivity with respect to temperature − dln (Δσ)/dT. It was observed that the critical temperature decreases and that the transition width increases with increasing Ce doping.
The data showed the occurrence of a two-stage transition besides the pairing transition splitting, associated with Ce doping
and related with the occurrence of a phase separation. Above the critical temperature, the Gaussian and critical regimes were
observed. On approaching the zero resistance state, our results showed a power-law behavior that corresponds to a phase transition
from a paracoherent to a coherent state of the granular array. 相似文献
10.
The extents of the cubic fluorite and pyrochlore regions in the ternary oxide system ZrO2-Gd2O3-TiO2 have been determined experimentally. AC impedance measurements of single phase fluorite and pyrochlore compositions show
high oxide ion conduction in compositions containing 20 mol% TiO2. Resolution of the bulk and grain boundary contributions to the total conductivity show that the bulk ionic conductivity
increases with Gd2O3 content up to the stoichiometric composition containing 33.33 mol% Gd2O3 and then decreases upon further addition of the aliovalent dopant. Trends in conductivity as a function of temperature and
dopant density are discussed.
Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997. 相似文献
11.
D. O. Soares-Pinto I. S. Oliveira M. S. Reis 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,62(3):337-340
In this work we report Monte Carlo simulations of a 2D
Ising model, in which the statistics of the Metropolis algorithm
is replaced by the nonextensive one. We compute the magnetization
and show that phase transitions are present for q ≠ 1. A
q - phase diagram (critical temperature vs. the entropic
parameter q) is built and exhibits some interesting features,
such as phases which are governed by the value of the entropic
index q. It is shown that such phases favors some energy levels
of magnetization states. It is also shown that the contribution
of the Tsallis cutoff is capital to the existence of phase
transitions. 相似文献
12.
A novel water-soluble solvatochromic molecule, 7-(dimethylamino)-2-fluorenesulfonate (2,7-DAFS), was prepared by a three-step
reaction from 2-nitrofluorene in good overall yield. The pH and solvent effects on the UV-VIS absorption and fluorescence
spectra of 2,7-DAFS have been studied. Protonation of the dimethylamino group switches the absorption from intramolecular
charge-transfer (ICT) to π → π* transition. The ground state pKa value of 2,7-DAFS was determined as 4.51. The fluorescence spectrum of the excited basic form, *(DAFS), shows a structureless
single band with a large Stokes shift, whereas that of the acidic form, *(+HDAFS), exhibits a structured band with a small Stokes shift. The emission intensities of the basic and acidic forms versus
pH/Ho plots show stretched sigmoidal curves and indicate that (1) the rate of deprotonation of *(+HDAFS) is comparable to the fluorescence decay of the species, and (2) the efficient proton-induced quenching of *(DAFS) fluorescence
occurs. The pKa* was estimated as −1.7 from the fluorescence titration curve. The fluorescence maximum of *(DAFS) is blue-shifted as the
polarity of solvent decreases. However, no clear dependency of the emission intensity and spectral half width, and thus fluorescence
quantum yield, on the solvent polarity was revealed. It appears that the fluorescence sensitivity of 2,7-DAFS is 15 ∼ 25 times
greater than the sensitivity of a widely utilized fluorescent probe, 5-(dimethylamino)-1-naphthalenesulfonate. This higher
sensitivity, together with the ease of derivatization, would provide the fluorene-based fluorescent molecules significant
advantages for a variety of applications. 相似文献
13.
The partial substitution of Sr in TlSr2CaCu2O7-δ with the rare-earth element Pr has the advantage to stablize the tetragonal phase with space group P4/mmm. The Tc value and the superconducting volume fraction increase with increasing Pr concentration up to x=0.5 (Tc=80 K) then decrease. Enhancement of superconductivity by Pr ions in the inherent “over-hole-doping” of the parent TlSr2CaCu2O7-δ compound suggests that the magnetic-pair-breaking effect is very small in this system. 相似文献
14.
W. Henggeler T. Chattopadhyay B. Roessli D. I. Zhigunov S. N. Barilo A. Furrer 《Zeitschrift für Physik B Condensed Matter》1995,99(1):465-467
We performed inelastic neutron scattering experiments to determine the dispersion of the Г
6
(1)
-Г
6
(2)
crystal field excitation in Nd2CuO4. Our results can be very well described within the random phase approximation model. This allows a direct determination of
the exchange coupling constants between the Nd ions. The superexchange interactions mediated by the oxygen and copper-oxygen
layers are determined to be significantly stronger than the in-plane exchange between the Nd ions. 相似文献
15.
It is shown that the phase diagram of a 2D metal undergoing a superconducting transition consists of regions of a normal phase
where the modulus of the order parameter is absent, an “anomalous normal” phase where the modulus of the order parameter is
different from zero but the phase of the order parameter is a random quantity, and a Berezinskii-Kosterlitz-Thouless phase.
The characteristic temperatures of transitions between the phases and the behavior of the chemical potential as a function
of the fermion density and temperature are found.
Pis’ma Zh. éksp. Teor. Fiz. 65, No. 2, 170–175 (25 January 1997) 相似文献
16.
Y. J. Wang C.-H. Lam X. Zhang O. K.C. Tsui 《The European physical journal. Special topics》2007,141(1):181-187
According to classical theory of phase transition,
fluctuations in systems with low dimensions are so violent that the
phase boundary between unstable and metastable states would be
smeared. In this experiment, we measure the growth of surface
fluctuations on an unstable polymer film with a thickness, h0 =
5.1 nm, which is much less than the spinodal thickness, h
sp (= 243 nm) thereby the film is in the very deep unstable
region. We find the film to show rupturing behavior markedly
different from that of an unstable film. Specifically, nucleation of
holes – a characteristic rupturing feature of metastable films –
is prominent, which is surprising for a film in the very deep
unstable region even provision is given to thin films being
two-dimensional and hence are susceptible to broadening of the phase
boundary by fluctuations. Monte Carlo simulation shows that the
nucleated holes can be caused by stochastic thermal fluctuations.
Our result thus confirms the broadening of the phase boundary in
thin films by fluctuations to be extremely large. As a consequence,
the phase behavior of thin films cannot be predicted by the
mean-field calculated phase boundary, which however has been the
general practice so far. 相似文献
17.
A novel design method for switching devices is demonstrated based on waveguide structures with Y-branches and abrupt waveguide
changes as key elements. A combination of BPM calculation and eigenmode analysis is shown to be a promising tool to obtain
deep insight into the physical mode behaviour. Based on the analysis, a mode converter to match the eigenmodes in amplitudes
and phase can be developed and introduced as a correcting device. This method is demonstrated for the crosstalk compensation
of a digital switch (DOS) and a Δβ-reversal switch. A crosstalk of about −20 dB can be reduced to −40 dB by this method.
This revised version was published online in November 2006 with corrections to the Cover Date. 相似文献
18.
X. Wu Z. Wu 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,50(4):521-526
First-principles calculations based on density functional
theory were used to study the high-pressure phases of both ZnF2 and
CdF2. We found that the sequence of the pressure-induced phase
transitions is: Rutile (P42/mnm) ↦ CaCl2 (Pnnm) ↦ PdF2
(Pa-3) and CaF2 (Fm3m) ↦ PbCl2 (Pnma) ↦ Ni2In
(P63/mmc) for ZnF2 and CdF2 respectively. In ZnF2 the
behavior of the ground-state total energy, of the Gibbs free energy and of
the lattice constant vs. pressure shown that the phase transition at 4 GPa
from the rutile-type phase to the CaCl2-type phase is a second-order
phase transition. The mechanism of the structural change was also revealed
by the transition from the PbCl2-type phase to the Ni2In-type
phase in CdF2. Moreover, the high-pressure behavior of divalent metal
fluorides was compared and discussed. 相似文献
19.
M. Hinczewski A. N. Berker 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,51(4):461-472
The anisotropic d=3 tJ model is studied by
renormalization-group theory, yielding the evolution of the system
as interplane coupling is varied from the isotropic
three-dimensional to quasi-two-dimensional regimes.
Finite-temperature phase diagrams, chemical potential shifts, and
in-plane and interplane kinetic energies and antiferromagnetic
correlations are calculated for the entire range of electron
densities. We find that the novel τ phase, seen in earlier
studies of the isotropic d=3 tJ model, persists even for
strong anisotropy. While the τ phase appears at low
temperatures at 30–35% hole doping away from 〈
ni〉=1, at smaller hole dopings we see a complex lamellar
structure of antiferromagnetic and disordered regions, with a
suppressed chemical potential shift, a possible marker of
incommensurate ordering in the form of microscopic stripes. An
investigation of the renormalization-group flows for the isotropic
two-dimensional tJ model also shows a clear pre-signature of the
τ phase, which in fact appears with finite transition
temperatures upon addition of the smallest interplane coupling. 相似文献
20.
A differential thermal analysis in vacuum and studies of the coefficients of electrical conductivity and thermal conductivity,
and of the thermal emf are made in Ag2Te in the neighborhood of the structural phase transition. It is shown that these data can be used to calculate the switching
factor L(T), determine the region in which the phases coexist within the transition, and calculate the thermodynamic parameters. Prior
to and after the main phase transition, additional displacement transitions are observed. It is found that the phase transition
takes place roughly according to the scheme α
385 K→α′405 K→β′420 K→β
440 K. The specific heat C
p, changes in entropy ΔS and transition enthalpy ΔH, as well as the minimum phase fluctuation volume V, and the heat Q of the phase transition are determined. It is shown that excesses of Te and Ag have almost no effect on the transition temperatures
T
0, but have a substantial effect on the thermodynamic parameters.
Fiz. Tverd. Tela (St. Petersburg) 40, 1693–1697 (September 1998) 相似文献