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1.
I. N. Askerzade 《Pramana》2003,61(3):611-616
Two-band Ginzburg-Landau (TB G–L) equations for a bulk MgB2 were solved analytically to determine the temperature dependence of surface critical magnetic fieldH
c3 (T). It is shown thatH
c3 (T) has the same temperature dependence with Hc2 (T), similar to the case of a single-band superconductor,H
c3 (T) = 166H
c2 (T). We use an elimination procedure for the decoupling of G–L equations of two-band superconductivity, which eases the calculations.
It is expected that the temperature dependence forH
c3 (T) gives positive curvature nearT
c
. 相似文献
2.
The upper critical field in MgB2 is investigated in the framework
of the two-gap Ginzburg-Landau theory. A variational solution of linearized
Ginzburg-Landau equations agrees well with the Landau level expansion
and demonstrates that spatial distributions of the gap functions are different
in the two bands and change with temperature. The temperature variation of the ratio of two gaps
is responsible for the upward temperature dependence of in-plane Hc2
as well as for the deviation of its out-of-plane behavior from the standard angular dependence.
The hexagonal in-plane modulations of Hc2 can change sign with decreasing temperature. 相似文献
3.
V. A. Gasparov N. S. Sidorov I. I. Zver’kova S. S. Khassanov M. P. Kulakov 《Journal of Experimental and Theoretical Physics》2005,101(1):98-106
We report on the synthesis and measurements of the temperature dependences of the resistivity ρ, the penetration depth λ, and the upper critical magnetic field Hc2, for polycrystalline samples of dodecaboride ZrB12 and diboride MgB2. We conclude that ZrB12 behaves as a simple metal in the normal state with the usual Bloch-Grüneisen temperature dependence of ρ(T) and with a rather low resistive Debye temperature TR = 280 K (to be compared to TR = 900 K for MgB2). The ρ(T) and λ(T) dependences for these samples reveal a superconducting transition in ZrB12 at Tc = 6.0 K. Although a clear exponential λ(T) dependence in MgB2 thin films and ceramic pellets was observed at low temperatures, this dependence was almost linear for ZrB12 below Tc/2. These features indicate an s-wave pairing state in MgB2, whereas a d-wave pairing state is possible in ZrB12. In disagreement with conventional theories, we found a linear temperature dependence, of Hc2(T) for ZrB12 (Hc2(0) = 0.15 T). 相似文献
4.
采用二带Ginziburg-Landau(G-L)理论模型计算了各向异性超导体的上临界磁场,发现上临界磁场Hc2的解析式中包含序参量的各向异性和有效质量的各向异性;上临界磁场ab-面内分量Hc|2|ab、c-轴方向分量Hc|2|ab与实验数据符合;上临界磁场比率在实验许可范围之内。 相似文献
5.
S. I. Krasnosvobodtsev A. V. Varlashkin A. I. Golovashkin N. P. Shabanova 《Physics of the Solid State》2005,47(9):1600-1604
The upper critical field H c 2 (H ‖ c) of the two-band superconductor MgB2 is studied as a function of the residual resistivity ρn. It is found that the superconductor follows the standard trend: the slope-dHc2/dT of the temperature dependence of Hc2(T) increases with the number of defects. The upper critical field in the clean limit is found, and direct estimations of the parameters of carriers in the 2D σ band (including the Fermi velocity and the coherence length) are made. The contribution of the electron scattering to the magnitude of Hc2 is determined, and the mean free path of electrons in samples with various defect concentrations is estimated. The density of states of σ electrons at the Fermi level is calculated using the dependence of the slope-dHc2/dT on ρn and a band structure model. It is impossible to estimate this density of states directly, because the upper critical field is determined by the carriers of one band, whereas the resistivity depends on the carriers in both bands. 相似文献
6.
M. V. Indenbom L. S. Uspenskaya M. P. Kulakov I. K. Bdikin S. A. Zver’kov 《JETP Letters》2001,74(5):274-278
The influence of nonstoichiometry of the new high-temperature superconductor MgB2 on its critical temperature was studied by the direct magnetooptical observations of the penetration and trapping of magnetic flux. To preclude the possible influence of accidental factors, a special sample with transition from pure boron to the MgB2 with an excess of Mg was synthesized. In a narrow region near the unreacted boron, the magnetic-field trapping and screening disappear at a temperature 1.5 K higher than in the dominant stoichiometric region of the sample. 相似文献
7.
A theory of the thermodynamic properties of a two-band superconductor with a low carrier density is developed; it is based on a phonon superconductivity mechanism with a strong electron-phonon coupling. This theory can describe the variation of the critical temperature T c, the energy gaps Δ1 and Δ2, and the relative electronic specific heat jump (C S ? C N)/C N at T = T c with the carrier density in the compound MgB2 when substitutional impurities of various valences are introduced into this system. The values of T c, Δ1, and Δ2 are shown to decrease as this compound is doped by electrons and to remain constant (or almost constant) as it is doped by holes. This behavior follows from the mechanism of filling the σ and π energy bands, which overlap at the Fermi surface. The theory agrees qualitatively with experimental data. This agreement is found to be better when intra-and interband electron scattering by an impurity potential is taken into account. 相似文献
8.
An analytical formula is obtained for the temperature dependence of the anisotropy parameter of the upper critical field of a two-band superconductor in the context of the generalized Ginzburg-Landau theory for layered superconductors. The theoretical results are compared with the experimental data for new superconductor LiFeAs. 相似文献
9.
用两带Ginzburg-Landau(GL)理论分析了Lu2Fe3Si5的表面临界磁场,当超导体的表面与任一主晶面重合,且外磁场平行于超导体的表面时,比值Hc3/Hc2(HC2是Lu2Fe3Si5的上临界磁场)强烈依赖于温度. 当超导体的表面是bc平面,且
关键词:
两带超导体
GL理论
2Fe3Si5')" href="#">Lu2Fe3Si5
表面临界磁场 相似文献
10.
A. Kohen F. Giubileo Th. Proslier F. Bobba A. M. Cucolo W. Sacks Y. Noat A. Troianovski D. Roditchev 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,57(1):21-25
Using Scanning Tunneling Microscope at low temperature
we explore the superconducting phase diagram in the π-band of
the two-band superconductor MgB2. In this band the peculiar
shape of the local tunneling spectra and their dynamics in the
magnetic field reveal the complex character of the quasiparticle
density of states (DOS). The gap in the DOS is first rapidly
filled with states in raising the magnetic field up to 0.5 T and
then slowly approaches the normal state value: the gap is
observed up to 2 T. Such a change in the DOS dynamics suggests the
existence of two terms in the DOS of the π-band: a first one,
reflecting an intrinsic superconductivity in the band and a second
one, originating from an inter-band coupling to the σ-band.
Our findings allow a deeper understanding of the unique phase
diagram of MgB2. 相似文献
11.
A. Santoni U.B. Vetrella G. Celentano U. Gambardella A. Mancini 《Applied Physics A: Materials Science & Processing》2007,86(4):485-490
Superconducting MgB2 films were obtained by in-situ annealing of precursor multilayers deposited at low substrate temperature by sputtering from
a MgB2 stoichiometric target and by thermal evaporation of pure Mg. After an in-situ annealing at 500–600 °C, the films showed a
zero resistance critical temperature up to 31 K. The as-obtained MgB2 films were investigated by X-ray photoelectron spectroscopy (XPS) and X-ray auger electron spectroscopy (XAES). The electronic
structure was studied by monitoring the B 1s, Mg 2p, O 1s core-levels and the Mg KL2L3 Auger line. For comparison, the electronic structure of an MgB2 commercial superconducting sputtering target, of a not-annealed precursor film and of a sample obtained by direct sputtering
from the MgB2 target have also been investigated. Electron spectroscopy showed that in the superconducting systems the Mg KL2L3 Auger line kinetic energy position is always higher by about 0.9 eV with respect to the energy position of the same Auger
line measured in the non-superconducting samples.
PACS 74.25Jb; 74.78.Bz; 74.70.Ad 相似文献
12.
Early work on the iron-arsenide compounds supported the view, that a reduced dimensionality might be a necessary prerequisite
for high-T
c superconductivity. Later, however, it was found that the zero-temperature upper critical magnetic field, H
c2(0), for the 122 iron pnictides is in fact rather isotropic. Here, we report measurements of the temperature dependence of
the electrical resistivity, ρ(T), in Ba0.5K0.5Fe2As2 and Ba0.68K0.32Fe2As2 single crystals in zero magnetic field and in Ba0.68K0.32Fe2As2 in static and pulsed magnetic fields up to 60 T. We find that the resistivity of both compounds in zero field is well described
by an exponential term due to inter-sheet umklapp electron-phonon scattering between light electrons around the M point to heavy hole sheets at the Γ point in reciprocal space.
From our data, we construct an H-T phase diagram for the inter-plane (H | c) and in-plane (H | ab) directions for Ba0.68K0.32Fe2As2. Contrary to published data for 122 underdoped FeAs compounds, we find that H
c2(T) is in fact anisotropic in optimally doped samples down to low temperatures. The anisotropy parameter, γ = H
c2
ab
/H
c2
c
, is about 2.2 at T
c
. For both field orientations we find a concave curvature of the H
c2 lines with decreasing anisotropy and saturation towards lower temperature. Taking into account Pauli spin paramagnetism,
we perfectly can describe H
c2 and its anisotropy. 相似文献
13.
N. V. Anshukova B. M. Bulychev A. I. Golovashkin L. I. Ivanova I. B. Krynetskii A. P. Rusakov 《Physics of the Solid State》2003,45(1):6-10
The thermal expansion coefficient α(T) of MgB2 was measured at low temperatures both in a zero magnetic field and at H=36 kOe. As in the oxide HTSCs, a region of anomalous (negative) thermal expansion and a strong effect of magnetic field on α(T) were revealed. The results obtained indicate the anomalous properties of MgB2 and the oxide HTSCs to follow a common pattern. 相似文献
14.
M. E. Palistrant I. D. Chebotar V. A. Ursu 《Journal of Experimental and Theoretical Physics》2009,109(2):227-236
A theory describing the magnetic properties of a two-band superconductor with a varying charge carrier density is constructed.
The upper critical field H
c2(ab) parallel to the ab plane and field H
c2(c) parallel to the c axis are determined in the entire temperature range 0 < T < T
c
. A considerable increase in upper critical field H
c2(ab) as compared to H
c2(c) because of strong anisotropy of the system is detected. Anisotropy of coefficient γ
H
= H
c2(ab) / H
c2(c) is obtained as a function of temperature for pure MgB2 and as a function of the chemical potential in the case when Mg and B atoms are replaced with other chemical elements. A
correlation between the variation in the superconducting transition temperature upon an increase in the chemical potential
and critical magnetic fields H
c2(ab) and H
c2(c) is observed. The effect of doping on magnetic anisotropy is also determined. 相似文献
15.
M. Bonura A. Agliolo Gallitto M. Li Vigni C. Ferdeghini C. Tarantini 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,63(2):165-177
We report on the microwave surface resistance of two
polycrystalline Mg11 B2 samples; one consists of
pristine material, the other has been irradiated at very high neutron
fluence. It has already been reported that in the strongly irradiated
sample the two gaps merge into a single value. The mw surface resistance
has been measured in the linear regime as a function of the temperature and the DC magnetic
field, at increasing and decreasing fields. The results obtained in the
strongly irradiated sample are quite well justified in the framework of a
generalized Coffey and Clem model, in which we take into account the field
distribution inside the sample due to the critical state. The results
obtained in the pristine sample show several anomalies, especially at low
temperatures, which cannot be justified in the framework of standard
models for the fluxon dynamics. Only at temperatures near Tc and for
magnetic fields greater than 0.5Hc2(T) the experimental data can quantitatively be
accounted for by the Coffey and Clem model, provided that the
upper-critical-field anisotropy is taken into due account. 相似文献
16.
17.
V. P.S. Awana A. Vajpayee M. Mudgel V. Ganesan A. M. Awasthi G. L. Bhalla H. Kishan 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,62(3):281-294
We report synthesis, structure/micro-structure, resistivity under magnetic
field [ρ(T)H], Raman spectra, thermoelectric power S(T), thermal conductivity
κ(T), and magnetization of ambient pressure argon annealed
polycrystalline bulk samples of MgB2, processed under identical
conditions. The compound crystallizes in hexagonal structure with space
group P6/mmm. Transmission electron microscopy (TEM) reveals electron micrographs
showing various types of defect features along with the presence of 3–4 nm
thick amorphous layers forming the grain boundaries of otherwise crystalline
MgB2. Raman spectra of the compound at room temperature exhibited
characteristic phonon peak at 600 cm-1. Superconductivity is observed
at 37.2 K by magnetic susceptibility χ(T), resistivity ρ(T), thermoelectric power
S(T), and thermal conductivity κ(T) measurements. The power law fitting
of ρ(T) give rise to Debye temperature (ΘD) at 1400 K which is
found consistent with the theoretical fitting of S(T), exhibiting Θ
D of 1410 K and carrier density of 3.81 × 1028/m3. Thermal
conductivity κ(T) shows a jump at 38 K, i.e., at Tc, which was
missing in some earlier reports. Critical current density (Jc) of up to
105 A/cm2 in 1–2 T (Tesla) fields at temperatures (T) of up to 10 K
is seen from magnetization measurements. The irreversibility field, defined
as the field related to merging of M(H) loops is found to be 78, 68 and 42 kOe
at 4, 10 and 20 K respectively. The superconducting performance parameters
viz. irreversibility field (Hirr) and critical current density
Jc(H) of the studied MgB2 are improved profoundly with addition of
nano-SiC and nano-diamond. The physical property parameters measured for
polycrystalline MgB2 are compared with earlier reports and a
consolidated insight of various physical properties is presented. 相似文献
18.
M. Putti E. Galleani d'Agliano D. Marrè F. Napoli M. Tassisto P. Manfrinetti A. Palenzona C. Rizzuto S. Massidda 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,25(4):439-443
We report measurements of the resistivity, ρ, and the Seebeck coefficient, S
, of a MgB2 sintered sample, and compare S with theoretical calculations based on precise electronic structure calculations. ρ is fitted well by a generalized Bloch-Grüneisen equation with a Debye temperature Θ
R
of 1050 K. S is given by the sum of a diffusive and a phonon drag term and the behavior in the temperature region T
c
<
T
< 0.1Θ
R
follows the relationship AT+BT3. The phonon drag term indicates a strong electron-phonon interaction. The diffusive term, compared with calculations, suggests
that σ bands give the main contribution to the Seebeck effect.
Received 16 November 2001 and Received in final form 21 December 2001 相似文献
19.
A. G. Lebed 《JETP Letters》2012,94(9):689-692
We theoretically determine the Ginzburg-Landau slopes of the anisotropic upper critical magnetic field in a quasi-one-dimensional
superconductor and correct the previous works on this issue. By using the experimentally measured values of the Ginzburg-Landau
slopes in the superconductor (TMTSF)ClO4, we determine band parameters of its electron spectrum. Our main result is that the so-called quantum dimensional crossover
has to happen in this material in magnetic fields, H = 3–8 T, which are much lower than the previously assumed. We discuss how this fact influences metallic and superconducting
properties of the (TMTSF)2ClO4. 相似文献
20.
The upper critical field of clean MgB2 is investigated
using the two-band layered Ginzburg--Landau (GL) theory. The
calculated results are fitted to the experimental data of clean
MgB2 crystal very well in a broad temperature range. Based
on the GL theory for clean superconductors, a phenomenological
theory for dirty superconductor is proposed. Selecting appropriate
parameters, two-band layered GL theory is successfully applied to
the crystal of Mg(B1-xCx)2 and the neutron
irradiation samples of MgB2. 相似文献