共查询到20条相似文献,搜索用时 234 毫秒
1.
We investigate the transition temperature of layered superconductors by considering a stack ofL
z superconducting layers, separated by insulating material. We adopt a pairing Hamiltonian, invoke the variational principle and solve the resulting gap equations numerically. Our results confirm previous weak coupling and Ginzburg-Landau treatments and reveal a rise ofT
c withL
z and saturation at the bulk transition temperature. Thus, the rise ofT
c is traced back to a finite size effect, corresponding to a crossover from 2-d to 3-d superconductivity. The results also reveal a sizeable variation of the gap along the stack with pronounced variation at the ends. 相似文献
2.
Electronic plus phonon-exchange mechanism for high-temperature superconductivity in layered crystals
Sudhanshu S Jha 《Pramana》1987,29(6):L615-L625
A general mathematical formulation is developed for calculating the effective electron-electron interaction in layered crystals
like YBa2Cu3O7−δ, and for finding the resulting superconducting transition temperatureT
c in such systems within the framework of the conventional BCS pairing arising from various possible excitations in the medium.
This differs considerably from the usual case of an effective three-dimensional homogeneous system, and should be relevant
in the calculation ofT
c for the new class of high-T
c perovskites in which oxygen deficiencies in Cu-O layers and their distribution in the crystal play a crucial role. The explicit
form of the effective interactionV
jj(q
t,ω) in a given layerj in the unit cell of the crystal is found to be determined not only by the true polarization functionπ
j(q
t,ω) of that layer, but also of other layers. The exchange of electronic excitations of a nearby insulating layer by carriers
in a conducting layer thus becomes possible to get highT
c, with or without the usual phonon exchange. 相似文献
3.
A change in the effect of a frozen magnetic field parallel to the c-axis on rf power absorption, which is associated with the motion of Josephson vortices, is observed in the layered superconductor Bi2Sr2CaCu2O8 at a low temperature (~15 K). The effect is interpreted as a change in the interaction between an Abrikosov vortex and a Josephson vortex from attraction (at high temperatures) to repulsion (at low temperatures). It is found that the dynamics of the magnetic flux parallel to the ab plane of the single crystal becomes irreversible upon a transition of the superconductor to the layered state. Possible reasons behind the observed effect are considered, one of them being a manifestation of the second superconducting transition in the elementary-excitation spectrum of a d-type superconductor near the core of Abrikosov vortices. 相似文献
4.
L.?Ya?Vinnikov D.?é.?Bo?nagrov V.?N.?Zverev I.?S.?Veshchunov J.?Karpinski 《Journal of Experimental and Theoretical Physics》2009,109(2):280-285
The vortex lattice of the YBa2Cu4O8 high-temperature superconductor is studied in the basal plane of monocrystalline samples using the decoration technique in
a field interval of 40–600 Oe. Vortex lattice anisotropy (field-independent “compression” of a regular hexagonal vortex cell
in the poorly conducting direction by a factor of about 1.3) is detected. Resistivity anisotropy ρ
a
/ρ
b
measured at temperatures from T
c
to room temperature is 16–9. Possible reasons for the discrepancy between our results and the available data are discussed. 相似文献
5.
I. N. Askerzade 《Pramana》2003,61(6):1145-1149
The recently obtained analytical result [1] for renormalization of the jump of the heat capacity (C
S–C
N)/CN by anisotropy of the order parameter is applied to the layered superconductors. The graph of (C
S–C
N)/C
N vs. the anisotropy of the order parameter allows a direct determination of the gap anisotropy in MgB2 using available experimental data 相似文献
6.
The thermodynamics of the vortex lattice of high-temperature superconductors has been studied by solving the generalized Ginzburg-Landau equations derived microscopically. Our numerical simulation indicates that the structure of the vortex lattice is oblique at the temperature far away from the transition temperature Tc, where the mixed s-dx2-y2 state is expected to have the lowest energy. Whereas, very close to Tc, the dx2-y2 wave is slightly lower energetically, and a triangular vortex lattice recovers. The coexistence and the coupling between the s and d waves would account for the unusual dynamic behaviours such as the upward curvature of the upper critical field curve HC2(T), as observed in dc magnetization measurements on single-crystal YBa2Cu3O7 samples. 相似文献
7.
Magnetic anisotropy of an extended double exchange model for layered manganites: A Monte Carlo study
Hongsuk Yi Sangsan Lee 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,31(4):449-452
The magnetic ground state properties of layered manganites are investigated on the basis of an anisotropic double exchange
model using a Monte Carlo technique. The temperature dependence of magnetization and spin-spin correlations are calculated
in a highly anisotropic hopping integral t
c/t
ab regime. The ferromagnetic ordering temperature (Tc) is suppressed by introducing t
c/t
ab, and eventually a layered ferromagnetic structure appears along the c-axis, but there are block-walls. The significant change of magnetic anisotropy is also observed for the antiferromagnetic
superexchange integral J
c/t
ab. We discuss the connection of these results to the magnetic anisotropy observed in the La1.4Sr1.6Mn2O7. 相似文献
8.
K Muraleedharan 《Pramana》1989,32(1):73-75
It is shown that the lattice parametersa, b andc in the superconducting perovskites RBa2Cu3O
x
are inter-related and well correlated to the total oxygen content. The correlation shows a universal trend. 相似文献
9.
G. Kalai Selvan M. Kanagaraj S. Esakki Muthu Rajveer Jha V. P. S. Awana S. Arumugam 《固体物理学:研究快报》2013,7(7):510-513
We investigate the external hydrostatic pressure effect on the superconducting transition temperature (Tc) of new layered superconductors Bi4O4S3 and NdO0.5F0.5BiS2. Though the Tc is found to have a moderate decrease from 4.8 K to 4.3 K (dTconset/dP = –0.28 K/GPa) for Bi4O4S3 superconductor, the same increases from 4.6 K to 5 K (dTconset/dP = 0.44 K/GPa) up to 1.31 GPa followed by a sudden decrease from 5 K to 4.7 K up to 1.75 GPa for NdO0.5F0.5BiS2 superconductor. The variation of Tc in these systems may be correlated to an increase or decrease of the charge carriers in the density of states under externally applied pressure. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
10.
In an investigation of the resistivity anisotropy of YBa2Cu3O7−x
single crystals with suboptimal oxygen content it is observed that the superconducting transition for the component ρc of the resistivity tensor is shifted to lower temperatures with respect to the transition for the component ρab. A similar shift is also observed for the transition in the temperature dependence of the dynamic magnetic susceptibility.
Pis’ma Zh. éksp. Teor. Fiz. 68, No. 4, 309–313 (25 August 1998) 相似文献
11.
采用脉冲激光沉积技术,在单晶SrTiO3基底上外延生长了一系列名义结构为p×(NdBa2Cu3O7-δ(m)/YBa2Cu3O7-δ(n))的多层膜和准多层膜(单元层NdBa2Cu3O7-δ较厚而YBa2Cu3O7-δ呈非连贯的岛状分布,m,n为激光脉冲数,p为重复周期).样品的超导转变温度在87—91 K范围,具体大小取决于不同的调制结构,多层膜的重复周期越大,层状界面越多,超导转变温度就越低.磁传输测量表明,准多层的样品不仅具有较高的超导转变温度,而且具有较强的磁通钉扎性能,77K零场下的临界电流密度高达4×106 A/cm2,显示出良好的应用前景.
关键词:
2Cu3O7-δ')" href="#">NdBa2Cu3O7-δ
多层膜
磁通钉扎
临界电流密度 相似文献
12.
R Srinivasan V Sankaranarayanan N P Raju S Natarajan U V Varadaraju G V Subba Rao 《Pramana》1987,29(2):L225-L230
The absolute thermopower of single phase YBa2Cu3O7 and Y0.8Er0.2Ba2Cu3O7 has been measured in the range 250 K to the superconducting transition temperature. It is found that these compounds show
a large enhancement of thermopower in the range 150 K down toT
c. This enhancement shows a steep exponential drop as the temperature increases from the transition temperature. The temperature
variation of the enhancement is too steep to be accounted for by electron-phonon or electron-local structural excitation mechanisms. 相似文献
13.
We report the observation of a unique tricritical superconducting transition behaviour (a peak in the R-T curve below Tc) in Au(10Å)/ Ge(13Å) alternating ultra-thin layered sandwich films. A first transition from the normal to the superconducting state occurs at around 2K, a second (back to the normal state) at about 1.8K, and finally a third at about 1.7K (again to the superconducting state). The tricritical transition appearance is accompanied by a resistance anomaly and an electromotive force anomaly in the superconducting transition regime. The Hc2 behaviour is strongly anisotropic. From an analysis of the Hc2 anisotropy, the system seems to be a three dimensional coupled-layer superconductor. 相似文献
14.
Y. Mizuguchi F. Tomioka S. Tsuda T. Yamaguchi Y. Takano 《Physica C: Superconductivity and its Applications》2009,469(15-20):1027-1029
Tetragonal FeSe is a superconductor with a transition temperature Tc of 8 K and shows a huge enhancement of Tc with applying pressure. Tetragonal FeTe has a structure very analogous to superconducting FeSe, but does not show superconducting transition. We investigated the pressure effect of resistivity on FeTe. The resistivity at room temperature decreased with increasing pressure. An anomaly in resistivity around 80 K shifted towards a lower temperature with increasing pressure. 相似文献
15.
Abstract Transport phenomena of aligned polycrystalline Y1Ba2Cu3O7?δ samples in weak magnetic fields near the vortex-glass transition temperature Tg have been studied. In YBCO polycrystal a fully superconducting state develops via an “intermediate phase”. The non-ohmic dissipation in this “intermediate phase” is examined in terms of both the thermal activated flux-flow (TAFF) model and vortex-glass transition model. Our experimental results in low magnetic fields can better be explained by a vortex glass transition model rather than a TAFF model. We have reinterpreted the non-ohmic dissipative region below Tc , i.e., the so-called “intermediate phase” in terms of a vortex-glass phase. 相似文献
16.
J. Yuan H.B. Wang I. Iguchi S. Arisawa A. Ishii T. Hatano 《Physica C: Superconductivity and its Applications》2009,469(15-20):1140-1142
We measured the in-plane magnetoresistance of Pr0.9LaCe0.1CuO4 (PLCCO) epitaxial thin films under various magnetic fields H applied parallel to the tetragonal c-axis. The measurements were performed at the superconducting state as well as the normal state. As the magnetic field is between the low critical field Hc1 and upper critical field Hc2, a critical scaling behavior of electrical resistivity is found. We analyze the electrical transport properties and show the magnetic field H dependence of glass transition temperature Tg and the characteristic temperature T* for the PLCCO film, which may shed some light on vortex behavior in electron-doped superconductors. 相似文献
17.
H. Küpfer I. Apfelstedt W. Schauer R. Flükiger R. Meier-Hirmer H. Wühl H. Scheurer 《Zeitschrift für Physik B Condensed Matter》1987,69(2-3):167-171
The influence of fast neutron irradiation on the superconducting transition temperature,T
c
, the transition width, upper critical field,B
c2
, and critical current density,j
c
, has been investigated in YBa2Cu3O7 up to a fluence of 1019 cm–2 (E>1 MeV). TheT
c
degradation with fluence is slightly less than in PbMo6S8, but larger than in A 15 compounds. The irradiation induced increase of the normal state resistivity is accompanied by a remarkable decrease of both the intergrainj
c
and the superconducting volume fraction. 相似文献
18.
Abstract We have studied the effects of fast neutron (E>0.1 MeV) irradiation at reactor (~ 360 K) and low (~ 20 K) temperatures on the superconducting properties of polycrystalline orthorhombic YBa2Cu3O7?y . Measurements were made on the superconducting critical temperature Tc , critical current Jc , Meissner effect and magnetic field dependence of Jc . The Tc drops by an irradiation at reactor temperature and Jc increases with increasing fluence. On the other hand with the irradiation at low temperature, Tc rises and Jc increases. Results of observation of Meissner effect and the magnetic field dependence of Jc are consistent with the behavior of Tc and Jc . 相似文献
19.
The structural, elastic and electronic properties of ReN are investigated by first-principles calculations based on density functional theory (DFT). The most stable structure of ReN is a NiAs-like structure, belonging to space group P63/mmc with a=2.7472 and c=5.8180 Å. ReN is a metallic ultra-incompressible solid and it exhibits low elastic anisotropy. Its linear incompressibility along the c-axis exceeds that of diamond. Its ultra-incompressibility is attributed to the high valence electron density and strong covalence bondings. Our results indicate that ReN can be used as a potential ultra-incompressible conductor. In particular, a superconducting transition temperature is predicted as Tc≈4.8 K for NiAs-like ReN, which agrees well with the available experimental value. 相似文献
20.
Zhang Dian-lin Duan Hong-min Ma Bei-hai Lu Li Lin Shu-yuan 《Physica C: Superconductivity and its Applications》1988,156(5)
We have determined the superconducting transition in YBa2Cu3O7−δ single crystal by measurements of thermopower. Two transitions were observed in both the ab-plane and the c-direction. The high temperature transition may have a close relation to the surface shell of the crystal. 相似文献