共查询到20条相似文献,搜索用时 31 毫秒
1.
Yi-Shan Huang You-Neng Cheng Jin-Duan Li Wen-Long Lu Hsueh-Chung Wang Chun-Hsing Chao 《Fullerenes, Nanotubes and Carbon Nanostructures》2013,21(4):573-585
Abstract A time-dependent x-ray scattering study is presented of the kinetics of the C60-based star polymer containing 6 urethane-connected polyether and the star model polymers containing 4, 3, and 2 urethane-connected polyether arms. By varying the temperature from 40°C to -40°C, the order-disorder phase transition (recrystallization) of the polymers has been observed. Two crystalline domains with the periodicities of 4.5±0.6 Å and 3.7±0.4 Å were detected. The domain sizes ranging from 70 Å to 250 Å, the transition temperatures from -15°C to 25°C and the maximum percentage of recrystallization from 13% to 34% increase as the number of polyether arms increases. The x-ray scattering peaks of the order phase grow up as I1 +I2(l - e?t/t0), where I1 and I2 are the initial intensity and the proportional intensity modulus, respectively. The relaxation time t0 of the order phase is varying as a power law with respect to the reduced temperature TR(=(T - TC)/TC): t0 = (35±10)·|TR|?1.42±0.07 seconds for the C60-based star polymer containing 6 urethane-connected polyether; t0 = (315 ± 64)·|TR|?1.68±0.08 seconds for the star model polymer containing 4 urethane-connected polyether arms. 相似文献
2.
An imbalanceQ in the quasiparticle populations of thek>k F andk F branches of a superconductor may be generated by the injection of a current through a tunnel barrier. This imbalance relaxes with a characteristic timeτ Q . The steady-state value ofQ gives rise to a quasiparticle potentialV that differs from the pair chemical potential, and which may be measured by a second tunnel junction. Tinkham's theory of these effects is briefly reviewed. Detailed experimental results are presented for tin. Most of the data are for injection voltages much greater than the energy gapΔ(T). Close to the transition temperatureT c ,Q relaxes by inelastic phonon scattering, andτ Q ph =(1.0±0.2)×10?10 Δ(0)/Δ(T) sec. This time is about one-half that estimated by Tinkham. At temperatures somewhat belowT c, elastic scattering also contributes to theQ relaxation, provided the superconducting energy gap is anisotropic. With a size-effect-limited mean free path of about 2800 Å, we findτ Q el ≈1.4×10?10 sec forT/T c <0.6, a value that is also in good agreement with Tinkham's theory. In a sample with a mean free path of 420 Å, and a greatly reduced anisotropy,τ Q el was increased to about 10?9 sec. Data were also taken for injection voltages close toΔ(T)/e. The degree of imbalance per unit injection current was greatly reduced, as predicted by the theory. Preliminary measurements were made on lead. At 4.2K (T/T c =0.58) the characteristic time was about 3×10?12 sec. The quasiparticle potential apparently increased steadily as the temperature was lowered, probably indicating that the characteristic time also increased. This result is not well understood. However, it is possible that recombination processes play a role in theQ relaxation in lead. 相似文献
3.
Roger Moret Pascale Launois Sylvain Ravy 《Fullerenes, Nanotubes and Carbon Nanostructures》2013,21(6):1287-1298
Abstract Single-crystal X-ray diffraction studies of the orientational ordering in solid C60 are reviewed. The temperature dependence of selected Bragg reflections was carefully examined, allowing to obtain original information on the first-order orientational ordering transition at To = 259K, the unusual behavior of both fundamental and superstructure reflections below To and the freezing in of the C60 reorientations at Tg ≈ 85K. The diffuse intensity due to orientational disorder of the C60 molecules at room temperature is found to be strongly modulated, both radially and azimuthally. showing that the molecular orientations are indeed correlated. The corresponding intensity distribution has been calculated within a mean-field theory for different microscopic models of inter molecular interactions. It should allow a better understanding of these interactions. 相似文献
4.
K. Raju S. Manjunathrao P. Venugopal Reddy 《Journal of Low Temperature Physics》2012,168(5-6):334-349
To understand the structural, electrical, magnetic, elastic and anelastic properties of La0.67?x Eu x Sr0.33MnO3 (0.30≤x≤0.39) manganites, a series of samples was prepared by citrate gel route. X-ray diffraction studies along with Rietveld analysis indicate the samples crystallize in single phase with Pnma space group. Studies on the variations of magnetization with temperature indicate that the Curie transition temperature (T C) decreases with increasing Eu content. Furthermore, Eu substitution is found to increase the electrical resistivity and significantly enhances the colossal magnetoresistance effect, while it is found to decrease the characteristic metal–insulator transition temperature (T P). On analyzing the electrical resistivity data, it has been concluded that the resistivity below T P can be explained based on electron–electron, electron–phonon and two magnon scattering processes, while that above T P, the adiabatic small polaron model is found to explain the observed behavior. Finally, the longitudinal modulus (L) and internal friction (Q ?1) have been measured and a dramatic change in L (T) is observed at T C, accompanied by a sharp peak in Q ?1 (T). Simultaneous occurrence of magnetic, transport and lattice anomalous behavior at T C indicate the presence of strong electron–phonon and spin–phonon interactions in these manganites. 相似文献
5.
Transport properties of antiferromagnetic superconductors with T
N<T
chave been investigated. Detailed numerical results are given for SmRh4B4 by using the following model. The paramagnetic phase (T
NT
c)is described by using Abrikosov-Gorkov theory of magnetic ions in an ordinary superconductor. In the AF phase (TT
N)the effects of the molecular field H
Q
(T) and the elastic scattering of conduction electrons from spin fluctuations are included. Expressions for H
Q
(T) and the scattering rate from spin fluctuations have been derived. The aim has been to see if properties are enhanced or depressed by the AF ordering occurring below T
N.It is found that whereas the electronic thermal conductivity K
s,nuclear spin relaxation rate R
s,and the longitudinal ultrasonic attenuation
s,are depressed by the AF ordering, the inverse of the magnetic penetration depth, [(T)]–1 is enhanced below T
N.The effect of all types of impurities is included in the study. Theoretical results for K
sand [(T)]–1 agree with the experimental values for SmRh4B4 (experimental data for other properties are not available in literature). In the above, T
Nis the Néel temperature and T
cis superconducting transition temperature. 相似文献
6.
V. S. Vikhnin 《Journal of Superconductivity and Novel Magnetism》1994,7(3):623-625
The influence onT c of dilatation centers (DC) as elastic defects near the surface or in thin films of oxide high-Tc superconductors is considered. The distribution function ofT c in this case was obtained. This is the Lorentzian with center shifted to largerT c values. The half width of theT c distribution βn (n is the concentration of DC) is larger than its center shift and corresponds to the increase ofT c fluctuations near the surface. As a result, a continuum percolation behavior with increasing critical temperature percolation levelT (c) near the surface appears. The inequalityβn>T (c)>T c initial is fulfilled. ForT>T (c) the quasi-2D Josephson media takes place where a finite superconductive domain withT c local >T (c) exists. The influence of DC considerably increases for strong DC such as off-center impurity ions. 相似文献
7.
The mechanical loss (Q
–1) and Young's modulus (E) were measured as a function of temperature in the range 100 K3 driven electrostatically in flexural vibration mode at resonance frequencies between 2–4 kHz. Q
–1(T) and E(T) curves show the following three phase transitions in BaTiO3: rhombohedral to orthorhombic (T
R-O), orthorhombic to tetragonal (T
O-T), and tetragonal to cubic (T
Curie). Each phase transition induces a very narrow mechanical loss peak of Q
–1 and a very sharp anomaly in the elastic modulus. Moreover, three other mechanical loss peaks are located below the phase transition temperatures in coarse grained specimens and these peaks seem to be associated with the motion of domain walls. These secondary peaks are not observed in fine grained undoped ceramics. The use of dopants such as Co or Nb produces an attenuation of the mechanical loss, a smoothing of elastic modulus anomalies, and a shifting of the phase transition temperatures. 相似文献
8.
Y-modified Pb(SnTi)O3 polycrystalline materials with a general formula (Pb (Sn (PYST) (x = 0, 0.05, 0.07, 0.1) have been synthesized using the standard mixed oxide method. Crystallographic and surface morphology
studies are carried out by the powder X-ray diffraction and scanning electron microscopy techniques. X-ray diffraction studies
of the samples showed the tetragonal structure of the unit cell. Uniform grain distributions with porosity throughout the
surface of the sample pellets are observed. The T
C (transition temperature) increases with increase in the mole fraction of the substituent (x). ɛ/
max is found to decrease with increase in x. The slope of lnσac∼ 1/T shows distinct variations for temperature below 410 K, 410 K to T
c and for T > T
c. The room temperature ac conductivity at 10 kHz lies in the range of 10−5–10−6 (ohm−1 m−1). 相似文献
9.
V. S. Vikhnin 《Journal of Superconductivity》1994,7(3):623-625
The influence onT
c
of dilatation centers (DC) as elastic defects near the surface or in thin films of oxide high-Tc superconductors is considered. The distribution function ofT
c
in this case was obtained. This is the Lorentzian with center shifted to largerT
c
values. The half width of theT
c
distribution n (n is the concentration of DC) is larger than its center shift and corresponds to the increase ofT
c
fluctuations near the surface. As a result, a continuum percolation behavior with increasing critical temperature percolation levelT
(c) near the surface appears. The inequalityn>T
(c)>T
c
initial
is fulfilled. ForT>T
(c) the quasi-2D Josephson media takes place where a finite superconductive domain withT
c
local
>T
(c) exists. The influence of DC considerably increases for strong DC such as off-center impurity ions. 相似文献
10.
11.
The role of pair breaking effects on the isotope effect coefficient is investigated using the generalized Abrikosov and Gorkov theory of superconductors. It is found that the pair breaking effects enhance the isotope effect coefficient. This provides a plausible explanation for the enhancement of the isotope effect coefficient as a function of impurity concentration in yttrium-, europium-, and lanthanum-based high-T
c superconductors. It may also explain the very large value of the isotope effect coefficient in Rb3C60 superconductors provided some intrinsic disorder is assumed. 相似文献
12.
T. Suzuki M. Nohara Y. Maeno T. Fujita I. Tanaka H. Kojima 《Journal of Superconductivity》1994,7(2):419-421
Novel softening has been found in the transverse elastic constant (C
11–C
12)/2 below 50 K in single-crystalline La1.86Sr0.14CuO4 (LSCO) by high-precision ultrasonic measurements in magnetic fieldsH along thec axis. With decreasing temperature, this lattice softening persists down to the superconducting transition temperatureT
c(H), which is reduced to 14 K by applying fields up to 14 T. BelowT
c(H) the softening turns to rapid hardening. This behavior indicates the presence of lattice instability of the orthorhombic (Bmab) structure in the normal state of LSCO, which disappears in the superconducting state. This is evidence for the intimate interplay between high-T
c
superconductivity and the lattice instability in LSCO. 相似文献
13.
The integrated intensity of the electronic background between 200 and 700 cm-1 observed in the Raman spectra of the YBa2Cu3O7-x 1-2-3 system exciting at 1.16 eV shows three changes with temperature. This electronic background is due to the scattering
between quasi-degenerate bands at the Fermi level with apical oxygen character which have neither CuO2 nor chain character. One of these transitions coincides with the appearance of superconductivity in the system. This indicates
that even if the opening of the gap in the apical oxygen electronic levels is not clearly observed, the interband transition
between these two levels is coupled to the superconducting order parameter. The other two changes of the background intensity
occur at temperatures greater thanT
c
. The one at lower temperature is proportional toT
c
and coincides probably with the appearance of self-organized electronic nanostructures already observed by EXAFS measurements
in other HTc material systems (La-Sr-Cu-O, Ba-Sr-Ca-Cu-O). The latter transition is observed at TD>- Tc (where TD decreases as Tc increases). This temperature coincides with the opening of a pseudogap that has been detected by several experimental techniques
in underdoped High-T,. systems. The possibility to observe the opening of a pseudogap in the density of states by means of
Raman scattering is analyzed in terms of different theoretical models that have been postulated to explain superconductivity
in cuprates. 相似文献
14.
K. K. Singh T. Pradeep D. E. Morris L. Chen J. V. Acrivos 《Fullerenes, Nanotubes and Carbon Nanostructures》2013,21(3):583-598
Abstract The electron spin resonance (esr) of C60.Ox and C60.HMTTEF (hexamethylentetratellurafulvalene) has been investigated at 9.36 GHz as a function of temperature. T (298 T 4 K). C60.Ox shows an esr absorption of equal ‘g’ value to that of C60 exposed to O2 and light but is more intense. The C60 in its pure form is in a singlet state. The impurity sites introduced by O2 produce the esr absorption. The Curie - Weiss plots of inverse esr absorption intensity versus temperature indicate an antiferromagnetic TH = 50 for air exposed C60 and 90K for C60Ox. From the esr intensity at room temperature, the calculated number of free spins (S = 1/2) is =1/3 per mole of C60-Ox. In case of C60.HMTTEF, there is a very weak esr absorption at room temperature suggesting that the room temperature form is diamagnetic with very small charge transfer between C60 and HMTTEF. This conclusion is consistent with the structure and magnetic susceptibility of this cocrystal. As the temperature is lowered, the equilibrium: AD <=> A?? + D?+ is displaced towards the formation of free radical species, A?? and D?+. 相似文献
15.
AbstractWe report growth and characterization of heavily boron-doped 3C-SiC and 6H-SiC and Al-doped 3C-SiC. Both 3C-SiC:B and 6H-SiC:B reveal type-I superconductivity with a critical temperature Tc=1.5 K. On the other hand, Al-doped 3C-SiC (3C-SiC:Al) shows type-II superconductivity with Tc=1.4 K. Both SiC:Al and SiC:B exhibit zero resistivity and diamagnetic susceptibility below Tc with effective hole-carrier concentration n higher than 1020 cm?3. We interpret the different superconducting behavior in carrier-doped p-type semiconductors SiC:Al, SiC:B, Si:B and C:B in terms of the different ionization energies of their acceptors. 相似文献
16.
N. Y. Korotkov K. V. Frolov I. S. Lyubutin E. P. Khlybov V. M. Pudalov 《Journal of Superconductivity and Novel Magnetism》2013,26(9):2877-2879
The 57Fe-Mössbauer spectroscopy measurements on the polycrystalline superconductor GdO0.88F0.12FeAs with the bulk critical temperature T C=53 K were carried out at temperatures between 5 and 300 K. Two nonequivalent Fe2+ sites were revealed in the Fe-As superconducting planes. Iron ions show clear nonmagnetic behavior in all temperature ranges. There were no observed significant changes in the hyperfine Mössbauer parameters below and above T C. The magnetic component appearing below 75 K indicates the presence of a small fraction of impurity phase FeAs. 相似文献
17.
A. R. Chatto R. A. M. Lee R. V. Duncan D. L. Goodstein 《Journal of Low Temperature Physics》2007,148(5-6):519-526
When a heat flux Q is applied downward through a sample of 4He near the lambda transition, the helium self organizes such that the gradient in temperature matches the gravity-induced
gradient in T
λ
. All the helium in the sample is then at the same reduced temperature
and the helium is said to be in the Self-Organized Critical (SOC) state. We have made the first measurements of the 4He SOC state specific heat, C
∇
T
(T(Q)). There is no measurable difference between C
∇
T
and the static zero-gravity 4He specific heat for temperatures between 650 and 250 nK below T
λ
. Closer to T
λ
, the specific heat is depressed and reaches a maximum value at 50 nK below T
λ
. This depression is similar to that predicted theoretically as reported by R. Haussmann (Phys. Rev. B 60, 12349, 1999). Contrary to the expectations of theory, however, we see another depression far below T
λ
. In addition, over the heat flux range of 30 nW/cm2 to 13 μW/cm2, we have made improved measurements of the speed of a recently discovered propagating thermal mode, which travels only upstream
against the nominal heat flux of the SOC state. We are able to accurately predict the speed of this wave by treating the helium
of SOC state as a traditional fluid with a temperature dependent thermal conductivity. 相似文献
18.
J. Taniguchi H. Ichida Y. Aoki S. Fukazawa M. Hieda M. Suzuki 《Journal of Low Temperature Physics》2007,148(5-6):791-795
We have performed torsional oscillator and ultrasonic measurements for 4He confined in 2D nano-porous substrate, hectorite. For 4He films, the resonance frequency of the torsional oscillator increases rapidly just below a superfluid transition temperature
(T
C
), and then becomes constant at the lower temperature. In contrast, the sound velocity below T
C
increases monotonically with decreasing temperature. For liquid 4He the sound velocity has a small bend at T
b
of about 0.9 K. Its increase below T
b
is enhanced as the pressure is increased. 相似文献
19.
《Materials Science & Technology》2013,29(5):378-384
The available data on secondary creep in creep tests and on maximum stress in hot deformation tests obtained for δ and α ferrite have been reviewed and expressed in the form of the equation ? = K(σ/E)Pc sinh(Vσ/RT) exp(—Qc/RT) with pc = 2·5, V = 0·0013 m3 mol?1, and K = 2·6 × 1017 s?1, E and Qc being temperature dependent functions. The approximate range of validity is Z = ? exp (Qc/RT) ≈ 101—1011 s?1. Together with the corresponding equation for austenite the data can be used to predict the viscoplastic behaviour of austenite—ferrite mixtures.MST/1538 相似文献
20.
L. E. DeLong M. Tovar L. D. Woolf M. B. Maple D. C. Johnston J. Keller 《Journal of Low Temperature Physics》1980,38(1-2):119-147
The results of an investigation of the superconducting and normal state physical properties of (LaNd)Sn3 alloys are reported. Superconducting state data are presented describing the depression of the superconducting transition temperature T
cwith Nd impurity concentration, the reduction of the reduced jump in heat capacity C/C
0 with decreasing reduced superconducting transition temperature T
c/Tc0, and the pressure dependence of T
c. Normal state data for the temperature dependences of the magnetic susceptibility and heat capacity demonstrate the importance of crystalline electric field effects in the physical behavior of (LaNd)Sn3 alloys and indicate that Nd interimpurity magnetic interactions and possibly a small amount of Nd-Sn antisite disorder are manifested in the experimental results. Theories based on an isotropic exchange depairing model are unable to give a satisfactory account of all of the experimental data, suggesting that the physical properties of(LaNd)Sn3 alloys may be modified by processes involving the orbital exchange scattering of host conduction electrons by the Nd impurities.Supported by the U.S. Department of Energy under Contract No. Ey-76-S-03-0034-PA227-3.Fellow of the Consejo Nacional de Investigaciones, Cientificas y Tecnicas, de la Republica Argentina; leave from Centro Atomico Bariloche, Bariloche, Argentina. 相似文献