共查询到20条相似文献,搜索用时 46 毫秒
1.
Shuai Jiang 《Solid State Communications》2007,143(3):136-139
We have synthesized NaxAy(H3O)zCoO2⋅wH2O (A=Mg, Ca, Sr and Ba) superconductors via ion exchange at ambient conditions. The extent of incorporation of alkaline-earth elements was found to be related to the similarity in size between the guest ion and the host one. In the case of A=Ca, about half of the sodium was successfully replaced by calcium. All the synthesized samples were shown to be superconducting with the Tc from 3.1 to 4.5 K. 相似文献
2.
The temperature dependences of the electrical resistivity of (Sn1?z Pbz)1?x InxTe alloys with different concentrations of lead (z=0–0.60) and indium (x=0.03–0.20) were studied at temperatures T=0.4–4.2 K in magnetic fields from zero to H=15 kOe. A resistivity drop of no less than three-four orders of magnitude was observed in this range of alloy compositions. Application of a magnetic field above a critical level resulted in a recovery of the sample resistivity to the original value. The observed resistivity drop is identified with a superconducting transition. The critical parameters of the superconducting transition (T c and H c2) were determined at the drop to one half the normal resistivity level. Experimental dependences of the critical supercon-ducting-transition temperature T c and of the second critical magnetic field H c2 on the contents of lead (z) and indium (x) were measured. The data obtained confirm a strong localization of the In impurity states and are evidence of the extrinsic nature of superconductivity in the class of materials under study. It was established that as the Pb content in (Sn1?z Pbz)1?x InxTe increases, T c and H c2 decrease as the Fermi level E F (fixed in the In impurity resonance band) leaves the Δ extremum and the superconductivity breaks down when E F leaves the LΣ saddle point in the valence-band energy spectrum. 相似文献
3.
Polycrystalline La2−xPrxCa2xBa2Cu4+2xOz (LPCaBCO) compounds with x=0.1-0.5 were synthesized by solid-state reaction method and studied by room temperature X-ray diffraction, dc resistivity, dc magnetization and iodometry. The superconducting transition temperatures in these tetragonal triple perovskite compounds increases from 32 to 62 K (Tconset values) with increasing dopant concentration. The mixing of rare earth La3+ and Pr3+/4+ ions at rare earth site (La3+) along with substitution of divalent Ca2+ results in the shrinkage of unit cell volume. The contraction of unit cell volume due to larger ion being substituted by smaller ions, gives rise to creation of pinning centres in the unit cell leading to increase in critical current density and flux pinning. 相似文献
4.
The specific heat of LaAl2 and (La1-xCex)Al2 (x ? 0.0064) has been measured between 0.3 and 5 K, both in the superconducting and in the normal state. For all samples the same values for the Debye temperature as well as for the electronic specific heat coefficient have been determined. LaAl2 shows an excellent BCS behavior. A remarkable excess specific heat at low temperatures due to the Kondo effect has been observed for all superconducting as well as for the normal conducting (La1-xCex) Al2 alloys. The specific heat jump ΔC at Tc depressed rapidly with increasing Ce concentration, allows the Kondo temperature TK ? 1 K to be determined. ΔC vanishes at finite temperatures. 相似文献
5.
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
6.
本文报道,通过对Y1-xCaxBa2Cu3-xMxO7-δ(M=Fe,Ni)体系样品的晶体结构、氧含量、正常态电阻率与温度的关系,以及超导转变温度等测量,并与YBa2Cu3-xMxO7-δ(M=Fe,Ni)体系进行比较,发现Y1-xCaxBa2Cu3-xFexO7-δ体系的Tc显著地高于相应x值的YBa2Cu3-xFexO7-δ体系,而Y1-xCaxBa2Cu3-xNixO7-δ体系则相反,Tc低于仅Ni替代的体系,表明Ca和Fe同时替代时两者引起的载流子浓度(nH)变化相互补偿,抑制了仅Fe替代时引起的nH和Tc急剧下降;而作Ca和Ni同时替代时主要的不是两者引起载流子浓度变化的相互补偿,Ca和Ni替代效应之间的关联较弱。作者认为,对Y1-xCaxBa2Cu3-xFexO7-δ体系属于CuO2平面外的元素替代,这时载流子浓度是决定Tc的主要因素;而对Y1-xCaxBa2Cu3-xNixO7-δ体系,由于Ni2+离子主要占据Cu(Ⅱ)位,它导致磁拆对效应,Ni2+离子的拆对效应是引起Tc下降的直接原因。
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7.
A. M'chirgui 《Solid State Communications》2005,133(5):321-325
A series of polycrystalline samples of Mg1−xPbxB2 (0≤x≤0.10) were prepared by a solid state reaction method and their structure, superconducting transition temperature and transport properties were investigated by means of X-ray diffraction (XRD) and resistivity measurements. Mg1−xPbxB2 compounds were shown to adopt an isostructural AlB2-type hexagonal structure in a relatively small range of lead concentration, x≤0.01. The crystalline lattice constants were evaluated and were found to exhibit slight length compression as x increases. The superconducting transition temperature (Tc) steadily decreases with Pb doping. It is suggested that the mechanism of superconductivity reduction by lead doping can be attributed to the chemical pressure effect. 相似文献
8.
Measurements of the d.c. electrical conductivity σ and the superconducting transition temperature Tc in amorphous BixKr1?x mixtures as a function of the Bi concentration x give clear evidence for the occurence of an Anderson or Anderson-Mott type metal-insulator transition at xc = 0.55 ± 0.02. From our σ and Tc data we estimate two characteristic exponents which we compare with those derived from existing scaling theories for the metal-insulator transition in disordered systems. 相似文献
9.
A. N. Lykov 《JETP Letters》2001,73(10):549-551
Generation of coherent electromagnetic radiation by GdBa2Cu3O7?x superconducting films is observed in the frequency range 1–10 MHz at the temperature of boiling liquid nitrogen. The generation is caused by the feedback synchronization of the jumps of Abrikosov vortices produced by a low-frequency external magnetic field. This offers new possibilities for the application of high-T c superconductors in superconducting electronics. 相似文献
10.
The magnetic phase diagrams of Mn(Nb0.5Ta0.5)2O6 have been determined by means of neutron diffraction. Two spin-flop-transitions GxAz → AxGz → Gy have been detected in a magnetic field parallel to the x-direction. The first phase boundary increases as the temperature rises up to a triple point at 4.74 K and 20.5 kOe, whereas the second phase boundary decreases until another triple point at 4.55 K and 35.5 kOe is reached. In fields parallel to the z-direction a transition GxAz → GxGz has been revealed, the phase boundary meets the paramagnetic one in a triple point at 4.65 K and 55 kOe. 相似文献
11.
P. B. Mozhaev F. V. Komissinskii Z. G. Ivanov G. A. Ovsyannikov 《Physics of the Solid State》2000,42(9):1590-1595
Thin films of the Nd1+x Ba2?x Cu3Oy high-T c superconductor (NBCO) with different neodymium/barium ratios have been obtained by laser ablative cosputtering of targets with different elemental compositions. The films with excess neodymium (x>0) had a low surface particle density and were rough, but their critical temperature decreased with increasing x. Conversely, barium-rich films (x<0) exhibited independence of the superconducting properties of the film composition, with an appreciable number of particles observed on the surface. Substitution of Ba for Nd in NBCO thin films is apparently impeded, so that excess barium precipitates in the form of (Ba,Cu)Oz particles. The structure and superconducting properties of NBCO reveal a strong dependence on the conditions of film saturation by oxygen. 相似文献
12.
Shigeki Miyasaka Akira Takemori Shin-nosuke Suzuki 《Journal of Physics and Chemistry of Solids》2011,72(5):414-417
We have investigated the relation between the temperature-dependence of resistivity and superconducting transition temperature Tc in RFeP1−xAsxO0.90F0.10 (x=0-1.0) (R=La and Nd). In contrast to the linear change of the crystal structure with increasing x, the temperature dependence of resistivity and Tc show non-monotonous x-dependence. When the As concentration x is increased, the temperature-dependence of resistivity changes from T2 to T-linear, and Tc distinctly increases in all the La compounds and the Nd ones with x<0.60. The results indicate that the substitution of As for P induces the spin fluctuation and resultantly enhances Tc. On the other hand, we could not find any relation between the temperature-dependence of resistivity and Tc in the Nd samples with x>0.60. This may suggest the existence of other parameters for determining Tc besides the antiferromagnetic correlation in this system. 相似文献
13.
(Tl0.5Pb0.5)Sr2Ca(Cu2−xMx)O7 (M=Co, Ni and Zn) have been synthesized and investigated by means of X-ray diffraction, scanning electron microscope, electrical resistivity and magnetic susceptibility measurements. X-ray diffraction patterns show that all studied samples contain the nearly single ‘1212’ phase. They crystallize in a tetragonal unit cell with a=3.8028-3.8040 Å and c=12.0748-12.1558 Å. In (Tl0.5Pb0.5)Sr2Ca(Cu2−xMx)O7 system (M=Co or Ni), the superconducting critical temperature Tc decreases linearly with both Co and Ni concentrations and the rate of Tc decrease is around −6.5 and −7.0 K/at%, respectively. For (Tl0.5Pb0.5)Sr2Ca (Cu2−xZnx)O7 system, the dependence of Tc on the Zn dopant concentration deviates from a linear behavior and the Zn substitution suppresses Tc much less (−2.5 K/at%) than the Co and Ni substitutions. The suppression in Tc in Co and Ni doped samples are attributed to the magnetic pair-breaking mechanism and the reduction in the carrier concentration. The suppression of Tc in Zn doped samples is not caused by the reduction in carrier concentration which should remain constant, but rather due to nonmagnetic pair-breaking mechanism induced by disorder as well as the filling of the local Cu dx2−y2 state due to the full d band of Zn ions. 相似文献
14.
Experimental data on the superconducting transition temperature, and low temperature specific heat, together with X-ray investigations, are reported for binary and pseudo-binary compounds of Ti with the A 15-type structure. A “true” relative maximum of the coefficient of the electronic specific heat, γ, as well as the superconducting transition temperature, Tc, occurs in the Ti3Ir?xPtx system near x = 0.2. Ti3Ir0.8Pt0.2 shows the lowest Debye temperature, θ0 , of all A 15-type compounds known to date. The anomalous temperature dependence of the lattice specific heat may be reproduced by a model phonon spectrum similar to that of Nb3Sn. 相似文献
15.
16.
A theory of superconductivity for transition-metal alloys is presented by using the coherent potential approximation. Expressions for the superconducting order parameter and transition temperature of transition-metal alloys AxB1?x in terms of the parameters characterizing the pure metals are given. 相似文献
17.
Chang Liu Takeshi Kondo A.D. Palczewski G.D. Samolyuk Y. Lee M.E. Tillman Ni Ni E.D. Mun R. Gordon A.F. Santander-Syro S.L. Bud’ko J.L. McChesney E. Rotenberg A.V. Fedorov T. Valla O. Copie M.A. Tanatar C. Martin B.N. Harmon P.C. Canfield A. Kaminski 《Physica C: Superconductivity and its Applications》2009,469(9-12):491-497
We present an overview of the electronic properties of iron arsenic high temperature superconductors with emphasis on low energy band dispersion, Fermi surface and superconducting gap. ARPES data is compared with full-potential linearized plane wave (FLAPW) calculations. We focus on single layer NdFeAsO0.9F0.1 (R1111) and two layer Ba1?xKxFe2As2 (B122) compounds. We find general similarities between experimental data and calculations in terms of character of Fermi surface pockets, and overall band dispersion. We also find a number of differences in details of the shape and size of the Fermi surfaces as well as the exact energy location of the bands, which indicate that magnetic interaction and ordering significantly affects the electronic properties of these materials. The Fermi surface consists of several hole pockets centered at Γ and electron pockets located in zone corners. The size and shape of the Fermi surface changes significantly with doping. Emergence of a coherent peak below the critical temperature Tc and diminished spectral weight at the chemical potential above Tc closely resembles the spectral characteristics of the cuprates, however the nodeless superconducting gap clearly excludes the possibility of d-wave order parameter. Instead it points to s-wave or extended s-wave symmetry of the order parameter. 相似文献
18.
R.S. Gonnelli A. CalzolariD. Daghero L. NataleG.A. Ummarino V.A. StepanovM. Ferretti 《Journal of Physics and Chemistry of Solids》2002,63(12):2369-2373
We studied the doping dependence of the superconducting gap in La2−xSrxCuO4 (LSCO) by means of Andreev reflection measurements in Au/LSCO point-contact junctions. The Andreev reflection features were found to disappear at TcA close to the bulk Tc. The fit of the conductance curves with the BTK-Tanaka-Kashiwaya model gives good results if a (s+d)-wave gap symmetry is used. The low-temperature dominant isotropic gap component Δs follows very well the Tc vs. x curve, while the gap-like features observed by angle-resolved photoemission spectroscopy and tunneling scale with T∗. This confirms the different origin of these two energy scales at T<Tc. 相似文献
19.
S. Harada Y. Inada G.-Q. Zheng 《Physica C: Superconductivity and its Applications》2010,470(20):1089-1091
We report the 11B and 195Pt NMR measurements in non-centrosymmetric superconductors Li2(Pd1−xPtx)3B (x = 0.0, 0.2, 0.5, 1.0). From the measurements of spin–lattice relaxation time (T1), we found that there was a coherence peak (CP) just below superconducting transition temperature (Tc) for x = 0–0.5 but no CP in x = 1. We demonstrated that the system for x = 0–0.5 were BCS superconductors but there existed line node in the superconducting gap for x = 1.0. The 195Pt Knight Shift in x = 0.2 decreased below Tc, indicating spin-singlet state. The results showed that BCS superconducting state evolves into an exotic state with line-nodes in the gap function when x is increased, as the spin–orbit coupling is enhanced. 相似文献
20.
The (MgB2)2−xCux (x=0-0.5) superconducting system was prepared by a solid-state reaction technique. Microstructural evolution and transport properties including resistivity versus temperature up to a magnetic field of 6 T, activation energy, thermoelectric power and Fermi energy, EF, and the corresponding velocity, VF, values of the samples prepared were also investigated. The XRD analysis showed a multiphase formation and no detectable solution of Cu in MgB2. Two different impurity phases, MgCu2 and CuB24, have been identified and their peak intensity increased when the Cu concentration increased. The temperature dependence of the resistivity of the samples showed a metallic behavior down to Tc. But, for the Cu concentrations above 0.3 the superconducting phase transition completely disappeared. The magnetic field strongly affects the electrical properties. For x=0.0 samples, the transition is found to be sharp, ΔT∼1 K, but it becomes broader with increasing magnetic field and Cu concentration. The calculated values of carrier concentration, n, of the samples are showed a sharp decrease with increasing Cu content. For x=0.0 sample the n was calculated to be 12×1021 cm−3, but for the x=0.5 sample it decreased to 1.3×1021 cm−3. We found that the activation energy, U(B), decreased sharply with increasing magnetic field. According to thermoelectric power and Fermi energy, EF, calculations the decrease of the carrier concentration by the additions of Cu into MgB2 gives a decrease in EF and this could be attributed to a shift of the Fermi level towards the top of the σ-hole band. 相似文献