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1.
Mössbauer measurements at temperatures ranging from 12 K to 300 K have been carried out on highT c superconductor YBa2Cu3O6+δ samples doped with Sn (5,2 and 0.5 wt%). The temperature dependent recoilless fractionf(T) has a narrow and small drop nearby transition temperatureT c which is dependent on the Sn-concentration and has a drop between 200–220 K. This indicates the occurrence of phonon softening at these temperatures. The Debye temperature θD of the samples containing 5, 2 wt% Sn atoms was determined as (290±25) K and (320±13) K respectively. These results show that phonons may play an important role for electron pairing in novel oxide superconductor.  相似文献   

2.
A study of the influence of the hydrogen concentration cH on the superconducting transition temperature Tc and the d.c.-conductivity σ of Al-films has been performed. The samples were prepared by co-deposition of Al and H (H2) onto a sapphire substrate held at a temperature of about 5 K. Within the experimental errors, there is no difference between the behaviour of Al-H and Al-H2 respectively. σ shows a decrease with increasing cH and a metal-insulator transition at cH?0.68. The Tc values exhibit a maximum Tc ? 5.3 K for 0.22 ? cH ? 0.5. Annealing experiments result in a decrease of Tc and an increase of σ. An analysis of the annealing experiments leads us to the conclusion that the as-prepared Al-H (Al-H2) films have an amorphous structure, which transforms during annealing in a granular system of Al grains embedded in insulating AlH3. No H-specific effect has been detected.  相似文献   

3.
The unit cell parameters a and c of nonirradiated [N(C2H5)4]2ZnBr4 crystals in the temperature region 90–300 K and of samples irradiated with γ rays to doses of 106 and 5 × 106 R in the 270-to 300-K interval were measured using x-ray diffraction. The data obtained were used to derive the thermal expansion coefficients αa and αc. It is shown that the parameter a increases and the parameter c decreases with increasing temperature. In the vicinity of the phase transition (PT) at T = 285 K, the temperature dependences of a(T) and c(T) reveal anomalies in the form of jumps and the αa(T) and αc(T) curves have a maximum and a minimum, respectively. The heat capacity of nonirradiated and irradiated [N(C2H5)4]2ZnBr4 samples was measured by adiabatic calorimetry. A maximum was found in the C p(T) curve at T = 285 K. Both x-ray diffraction and heat capacity measurements showed that the PT temperature decreased after γ irradiation.  相似文献   

4.
Magnetization M(H,T) in magnetic fields H up to 90 kOe and at temperatures 2 K ≤ T < T c (where Tc is the superconducting transition temperature), along with magnetic susceptibility χ(T) in the normal state T c < T < 400 K for optimally oxygen-doped samples of YBa2Cu3O6.92 with varying degrees of defects in the crystal structure, are studied to determine the influence of structural inhomogeneity on the electron systems characteristics of cuprate superconductors. It is shown that the existence of structural inhomogeneity of samples leads to the manifestation of peculiarities appropriate to pseudogap regime in their properties.  相似文献   

5.
Bulk composites have been prepared based on one-dimensional fibers of natural chrisothil-asbestos with various internal diameters (d = 6–2.5 nm) filled with tin. The electrical and magnetic properties of quasi-one-dimensional Sn wires have been studied at low temperatures. The electrical properties have been measured at T = 300 K at a pressure P = 10 kbar. It has been found that the superconducting (SC) characteristics of the nanocomposites (critical temperature Tc and critical magnetic field Hc) increase as the Sn filament diameter decreases. The temperature spreading of the resistive SC transition also increases as the Sn filament diameter decreases, which is explained by the SC order parameter fluctuations. The size effects (the increase in critical temperature Tc and transition width ΔTc) in Sn nanofilaments are well described by the independent Aslamazov–Larkin and Langer–Ambegaokara fluctuation theories, which makes it possible to find the dependence of Tc of the diffuse SC transition on the nanowire diameter. Using the temperature and magnetic-field dependences of the magnetic moment M(T, H), it has been found that the superconductor–normal metal phase diagram of the Sn–asbestos nanocomposite has a wider region of the SC state in T and H as compared to the data for bulk Sn. The magnetic properties of chrisotil-asbestos fibers unfilled with Sn have been studied. It has been found that the Curie law is fulfilled and that the superparamagnetism is absent in such samples. The obtained results indicate the absence of magnetically ordered impurities (magnetite) in the chrisotil-asbestos matrix, which allowed one to not consider the problem of the interaction of the magnetic subsystem of the asbestos matrix and the superconducting subsystem of Sn nanowires.  相似文献   

6.
A complete analysis of magnetization curves is given for bulk sintered samples of Nb3Sn with low hysteresis and for finely powdered sintered and melted samples of Nb3Sn, Nb3Al, V3Si and V3Ga. The Ginzburg-Landau parameterκ and the penetration depthλ of weak magnetic fields is calculated from the temperature dependence of the thermodynamic and upper critical fieldsH c andH c2. The slope of theH c ?T curve is about 360 Oe/°K and approximately the same for all measured samples. There is an extremely high increase of the lower critical fieldH c1 if the grain size approachesλ. The occurence of field expulsion and partially reversible magnetization curves of sintered Nb3Sn samples with high sintering temperatures and high pore volumes is explained by means of connections between single grains which have lower transition temperatures.  相似文献   

7.
The Hall coefficients, RH, for A15 structure single crystal V3Si and polycrystal V25.25 at.% Si and Nb 26 at.% Sn have been measured as functions of temperature. The data between the superconducting transition temperature, Tc, and 80 K for the Nb-Sn show a small dip centred arounf 30 K which accompanies a cubic-to-tetragonal lattice distortion commencing at about 45 K. RH for the VSi samples, which are believed to be non-transforming, is temperature independent in the range 17–40 K and equal to 2.0 ± 0.2×10?10m3C?1. These result are discussed in relation to the effect of the tetragonal distortion on the band structures of these compounds.  相似文献   

8.
We report on syntheses and electron transport properties of polycrystalline samples of diborides (AB2) with different transition metals atoms (A=Zr, Nb, Ta). The temperature dependence of resistivity, ρ(T), and ac susceptibility of these samples reveal a superconducting transition of ZrB2 with T c =5.5 K, while NbB2 and TaB2 have been observed to be nonsuperconducting up to 0.37K. H c2(T) is linear in temperature below T c , leading to a rather low H c2(0)=0.1 T. At T close to T c , H c2(T) demonstrates a downward curvature. We conclude that these diborides, as well as MgB2 samples, behave like simple metals in the normal state with usual Bloch-Grüneisen temperature dependence of resistivity and with Debye temperatures 280, 460, and 440 K for ZrB2, NbB2, and MgB2, respectively, rather than T 2 and T 3, as previously reported for MgB2.  相似文献   

9.
The synthesis of high temperature superconducting phases in the NbGe, NbSn, VSi, VGe, VSn, NbC and MoC systems is described by method consisting in the thermolysis of volatile hydrides or organometallic compounds on resistively heated wires. For face-centred cubic NbC a higher transition temperature than previously reported was obtained. The A15 phase boundary of NbGe is extended towards the stoichiometric 3:1 composition, affording samples of Nb3 Ge with a Tc onset of 15.8°K.  相似文献   

10.
A significant monotonous decrease of Hc2(0) almost proportional to Tc has been observed after heavy ion irradiation of the A15 superconductor Nb3Sn at low temperature < 30 K and isochronal annealing up to 300 K has been measured. The effects are discussed in connection with the Goodman—Gorkov-relation.  相似文献   

11.
在16.0°K—20.3°K之间测量了Nb3Sn样品的热容量。Nb3Sn在临界温度附近的比热跳跃值ΔC=2.21(±5%)焦耳/克分子·度。样品的临界温度Tc=17.88°K,转变宽度ΔTc≈0.2°K。ΔC值利用热力学关系式确定了Nb3Sn在0°K时的热力学临界场H0=5300奥斯特。利用本文的结果和文献上关于热膨胀系数的跳跃值Δα及?T/?P值验证了热力学关系式。扼要地描述了比热测量装置.  相似文献   

12.
We studied the conductivity and superconducting transition temperature Tc of SnAr films. The films were prepared by condensing the SnAr mixture on a sapphire substrate held at 5 K. A plot of the conductivity as a function of Sn concentration shows a metal-insulator transition which agrees with a percolation model consisting of Sn clusters embedded in solid Ar. A drop of Tc is observed below the percolation threshold.  相似文献   

13.
Superhard superconducting samples with a critical temperature of Tc = 12.6 K are obtained when synthetic diamond powders that were preliminarily coated with a niobium film are sintered at a pressure of 7.7 GPa and a temperature of 1973 K. Superhard superconductors with Tc = 9.3 K are obtained when diamond and molybdenum powders are sintered at a pressure of 7.7 GPa and a temperature of 2173 K. Superconducting samples with Tc = 36.1–37.5 K have been obtained in the systems diamond-MgB2 and cubic boron nitride-MgB2.  相似文献   

14.
In tunneling experiments with high-quality single crystals of a single-layer cuprate superconductor Bi2Sr2CuO6+δ using the break junction and point-contact techniques at T<T c, the coexistence of the superconducting-state gap and the normal-state gap was observed. The values of the superconducting energy gap 2Δp?p are in the range from 13.4 to 15 meV (Δp?p=6.7–7.5 meV). The values of 2Δp?p are similar for two samples with T c=4 K and for two samples with T c=9–10 K and are independent of the carrier concentration. The normal-state gap, with the magnitude approximately equal to 50 meV, persists at T<T c and in the magnetic field H?H c2 up to 28 T. After the transition of the sample to the normal state, the intensity of the tunneling conductance rapidly decreases with increasing magnetic field strength and temperature. The observed large broadening of the tunneling spectra and large zero-bias conductances can be caused by a strong angular dependence of the superconducting gap. The tunneling results are in full agreement with the data of the angle-resolved photoemission spectroscopy measurements.  相似文献   

15.
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   

16.
The effect of disorder induced by neutron irradiation in a nuclear reactor (thermal neutron fluence 1×1019cm?2) on the superconducting transition temperature T c and the upper critical field H c2 of polycrystalline MgB2 samples was investigated. Despite the appreciable radiation-induced distortions (more than ten displacements per atom), the initial crystal structure (C32) was retained. The temperature T c decreased from 38 to 5 K upon irradiation and was practically completely restored after the subsequent annealing at a temperature of 70°C. A weak change in the dH c2/dT derivative upon irradiation is explained by the fact that the irradiated samples are described by the “pure” limit of the theory of disordered superconductors. The suppression of T c upon disordering may be due to the isotropization of the originally anisotropic (or multicomponent) superconducting gap or to a decrease in the density of electronic states at the Fermi level.  相似文献   

17.
We report on the structural, frequency dependent ac susceptibility, dc magnetization and magnetoresistance (MR) measurements on polycrystalline samples of La0.9Ca0.1Mn1−yCryO3 (y=0, 0.1 and 0.2) prepared by sol-gel technique. For y=0, a paramagnetic to ferromagnetic transition is observed at Tc=136 K. Both for y=0.1 and 0.2, Tc increases from 136 to 180 K. For y=0, the imaginary part of the ac susceptibility shows a broad transition at Tf<Tc which does not depend very much on the frequency. However, for y=0.1 and 0.2, the frequency dependence resembles that of a spin glass. Though all the three samples show a semi-conducting behavior between 300 and 5 K, a negative MR is observed corresponding to Tc and Tf. The value of MR decreases for the Cr substituted samples.  相似文献   

18.
The superconducting transition temperature (Tc) and the temperature dependence of the normal state resistivity of the Ti1?xSbx system between Tc and 300 K have been studied. The Tc values are found to depend on the heat treatment of the samples. Below 40 K, all alloys show a T2 dependence of the resistivity. However, the sample with x = 0.53 is not superconducting and shows a different behaviour of the resistivity.  相似文献   

19.
The dependence of the superconducting transition temperature Tc on Ce impurity concentration is reported for the (La0.80Y0.20)Ce system. Susceptibility and resistivity measurements on samples with about 0.85 at. % Ce show unique behavior which must be interpreted as a superposition of three transition curves: a first transition at 0.55 K from the normal to the superconducting state, a second one at 0.27 K back to the normal state, and finally a third one at about 0.05 K again to the superconducting state. The experimental evidence of this third transition (Tc3) is of special importance for the decision about the relevant physical mechanism of pair-breaking at low temperature (T ? TK).  相似文献   

20.
The electrical resistance of a linear chain metal Nb3Te4 were measured from 1.3 to 320 K. The residual resistance ratio R(300 K)R(4.2 K) is about 3. Nb3Te4 shows an anomaly in the resistivity vs temperature at about 80 K, suggesting an occurrence of a charge-density-wave transition. The transverse and longitudinal magnetoresistance at 4.2 K are proportional to the magnetic field in the range of 2–58 kOe. In the superconducting region close to the transition temperature Tc, the critical magnetic field Hc2 is proportional to δT=Tc?T. The angular dependence of Hc2 fits well with the fluxoid model of the Ginzburg-Landau theory. The ratio of the critical fields parallel and perpendicular to the chain direction is 4.8.  相似文献   

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