Superconductivity in mesoscopic high-Tc superconducting particles

Based on the Hubbard model in the framework of non-phonon kinematical mechanism and taking into account the discreetness of an electronic energy spectrum, the superconducting critical temperature of a mesoscopic high-T_c sphere is analyzed as a function of doping and as a function of particle's radius. The critical temperature T_c is found to be an oscillating function of the radius of a particle. The size-dependent doping regime is revealed in high-T_c nanoparticles. Our analysis shows that each oscillation in T_c corresponds to the increase in a number of the energy levels in the sphere by 1. The amplitude of oscillations of T_c increases with decreasing R and can reach a value of 6 K for nanoparticles with sizes about 25 nm, in good agreement with experimental studies of YBa₂Cu₃O_7−δ nanoparticles.     

The spin exchange interaction effect on Tc equation of anisotropic impure superconductors

We study the influence of spin exchange interaction of impurity scattering on critical temperature of anisotropic impure superconductors. The model of random non-magnetic and magnetic impurity are revised to cover the effect of spin exchange interaction. The sign of magnitude of the second-order Born scattering has been changed after consideration of the spin exchange interaction that also effects the form of T_c equation. We can get the general T_c equation that can be described well by anisotropic impure superconductors and covers all models done before.     

Influence of Co, Ni and Zn substitution for Cu on the structural and superconducting properties of (Hg0.7Cr0.3)Sr2CuO4+δ

The effect of Co, Ni and Zn substitutions for Cu on the phase stability and superconducting properties of (Hg_0.7Cr_0.3)Sr₂CuO_4+δ was investigated. X-ray diffraction (XRD) revealed that both Co and Zn are soluble in the (Hg_0.7Cr_0.3)Sr₂CuO_4+δ material up to about 5% of the Cu content, whereas the solubility of Ni is extended up to 10%. Electrical resistivity and magnetic susceptibility measurements show that the value of the superconducting critical temperature T_c decreases linearly with the impurity content. The depression of T_c indicates that the suppression of the superconductivity in Co- and Ni-substituted samples is much stronger than that in Zn-substituted ones. The residual resistivity scales linearly with the doping level as expected from the impurity scattering due to disorder. Some possible explanations for the stronger suppression of T_c by the Co and Ni substitution than by Zn substitution are provided.     

Paramagnetic centers in single-walled carbon nanotubes encapsulated with palladium and their interaction with hydrogen at H/C ≥ 1.0

The nature of paramagnetic centers in a nanostructure based on single-walled carbon nanotubes (SWCNTs) encapsulated with Pd was studied by EPR spectroscopy at 77 and 293 K. It was found that strong charge-transfer π complexes of the (Pd-C _x ) type, which manifested themselves as a narrow resonance (ΔH = 6–8 G and g = 2.002 at T = 77 k), were formed in the Pd-SWCNT composite along with impurity centers (Fe₃O₄ nanoparticles within the nanotubes), which were responsible for a broad EPR signal (ΔH = 75 G and g = 2.065 at T = 293 K). These complexes were found to be predominant adsorption sites responsible for a high gravimetric density of hydrogen (H/C ≥ 1.0) within the single-walled carbon nanotubes.     

Anderson localization and the pseudogap in the energy spectrum of holes in PbTe: Tl under resonant scattering

This paper reports on a comprehensive study of the effect of additional doping with the Na acceptor impurity on the low-temperature resistivity of PbTe samples doped with Tl (2 at %), an impurity producing a band of resonant states within the valence-band spectrum. By additional doping with Na, we have shifted the Fermi level within the band of the resonant states of Tl in PbTe and varied the hole filling (k _h ) of the thallium impurity states. The larger part of the PbTe: (Tl,Na) samples transfers to the superconducting state with a critical temperature T _c = 0.4–2.3 K. The T _c (k _h ) relation obtained argues for the fact that, in the region of resonant states in PbTe: Tl, Anderson localization of holes and a pseudogap in the density of delocalized states are observed.     

Studies of chemical diffusion in La1-X SrXCoO3-δ

Recent investigations of superconductivity in carbon nanotubes have shown that a single-wall zig-zag nanotube can become superconducting at around 15?K. Theoretical studies of superconductivity in nanotubes using the traditional phonon exchange model, however, give a superconducting transition temperature T _c less than 1?K. To explain the observed higher critical temperature we explore the possibility of the plasmon exchange mechanism for superconductivity in nanotubes. We first calculate the effective interaction between electrons in a nanotube mediated by plasmon exchange and show that this interaction can become attractive. Using this attractive interaction in the modified Eliashberg theory for strong coupling superconductors, we then calculate the critical temperature T _c in a single-wall nanotube. Our theoretical results can explain the observed T _c in a single-wall nanotube. In particular, we find that T _c is sensitively dependent on the dielectric constant of the medium, the effective mass of the electrons and the radius of the nanotube. We then consider superconductivity in a bundle of single-wall nanotubes and find that bundling of nanotubes does not change the critical temperature significantly. Going beyond carbon nanotubes we show that in a metallic hollow nanowire T _c has some sort of oscillatory behaviour as a function of the surface number density of electrons.     

Band structure, phonon scattering, and the performance limit of single-walled carbon nanotube transistors

Semiconducting single-walled carbon nanotubes are studied in the diffusive transport regime. The peak mobility is found to scale with the square of the nanotube diameter and inversely with temperature. The maximum conductance, corrected for the contacts, is linear in the diameter and inverse temperature. These results are in good agreement with theoretical predictions for acoustic phonon scattering in combination with the unusual band structure of nanotubes. These measurements set the upper bound for the performance of nanotube transistors operating in the diffusive regime.     

Cyclotron resonance study of conduction electrons in n-type indium antimonide under a strong magnetic field—I: Thermal equilibrium case

The far IR cyclotron resonance of conduction electrons is investigated in n-type indium antimonide in the quantum regimes, ?ω_ck_BT and ?ω_c?k_BT. The resonance peak position, half width, and the degree asymmetry in the line shape are studied as a function of temperature. In analyzing the experimental data, the three band model has been employed together with modern theoretical results of electron scattering by ionized impurities in the presence of a strong magnetic field. It is found that, for example for an experiment at 84 μm, the Une width depends very little on temperature between 4.2 and 45 K where the ionized impurity scattering is dominant, and increases rapidly with temperature above 45 K where the onset of phonon scattering is expected. Further details of the ionized impurity scattering were investigated by using three different laser wavelengths 84, 119 and 172μm. The line width at the phonon-limited temperature region depends very little on magnetic field and sample. The temperature dependence of the band gap was also determined by analysis of the resonance peak shift.     

Superconducting—normal phase boundaries of (La,Th)Ce systems

The dependence of the superconducting transition temperature T_c on Ce impurity concentration n is reported for various ($\text{La, Th}$)Ce systems. These results document for the first time the evolution of the superconducting— normal phase boundary (T_c vs n curve) as an impurity undergoes magnetic- nonmagnetic transition which for Ce impurities in ($\text{La, Th}$)Ce systems proceeds with increasing Th concentration. Observed for only the second time, re-entrant T_c vs n curves are reported for two La-rich La, Th host compositions.     

Doping effect on the carrier scattering in iron-pnictide superconductors studied by charge transport

In order to reveal the role of “carrier doping” in the iron-based superconductors, we investigated the transport properties of the oxygen-deficient iron-arsenides LnFeAsO_1−y (Ln=La, Ce, Pr and Nd) over a wide doping range. We found that the effect of “doping” in this system is mainly on the carrier scattering rather than carrier density, quite distinct from that in high-T_c cuprates. In the case of La system with lower T_c, the low temperature resistivity is dominated by T² term and fairly large magnetoresistance is observed. On the other hand, in the Nd system with higher T_c, carriers are subject to stronger scattering showing nearly T-linear resistivity and small magnetoresistance. Such strong scattering appears intimately correlated with high-T_c superconductivity in the iron-based system.     

Influence of defects on the phase transition in squaric acid (H2C4O4)

We have investigated the influence of defects induced by chromium and deuterium doping on the phase transition in solid squaric acid (H₂C₄O₄) by high resolution ¹³C NMR. Deuterium doping alone is observed to increase the phase transition temperature T_c linearly with the ²H concentration, whereas chromium defects, destroy the local order and lead to clusters of the high temperature phase. Correspondingly the critical temperature decreases with increasing chromium doping and the critical temperature region is smeared out.     

Spin-orbit effect on Tc of anisotropic superconductors

It is shown that spin-orbit impurity scattering brings about only a negligible change on the transition temperature T_c for anisotropic superconductors with an ellipsoidal Fermi surface.     

Influence of impurities on the band ferromagnetism

The influence of non-magnetic impurities on band ferromagnetism for an electron gas withδ-function interaction is investigated in the framework of the quantum field theory applied to phase transitions. The Hartree-Fock approximation is used for non-zero temperatures. An equation for the magnetizationM and the critical temperatureT F_c is obtained. It is found that the transition temperature is lowered by increasing impurity concentration. There exists a critical concentration which vanishes the ferromagnetism.     

The Ginzburg-Landau expansion and the slope of the upper critical field in superconductors with anisotropic normal-impurity scattering

We carry out the Ginzburg-Landau expansion for superconductors with anisotropic s and d pairing in the presence of anisotropic normal-impurity scattering, which enhances the stability of d pairing with respect to disordering. We find that the slope of the curve of the upper critical field, |dH _c2/dT|_{T c}, in superconductors with d pairing behaves nonlinearly as disorder grows: at low scattering anisotropy the slope rapidly decreases with increasing impurity concentration, then gradually but nonlinearly increases with concentration, reaches its maximum, and then rapidly decreases, vanishing at the critical impurity concentration. In superconductors with anisotropic s pairing, |dH _c2/dT|_{T c} always increases with impurity concentration, finally reaching the familiar asymptotic value characteristic of the isotropic case, irrespective of whether there is anisotropic impurity scattering. Zh. éksp. Teor. Fiz. 112, 2124–2133 (December 1997)     

Effect of sorbic acid doping on flux pinning in bulk MgB2 with the percolation model

In this paper, we study the doping effect of sorbic acid (C₆H₈O₂), from 0 to 20 wt.% of the total MgB₂, on critical temperature (T_c), critical current density (J_c), irreversibility field (H_irr) and crystalline structure. The XRD patterns of samples show a slightly decrease in a-axis lattice parameter for doped samples, due to the partial substitution of carbon at boron site. On the other hand, we investigate the influence of doping on the behavior of flux pinning and J_c(B) in the framework of percolation theory and it is found that the J_c(B) behavior could be well fitted in high field region. The two key parameters, anisotropy and percolation threshold, play very important roles. It is believed that the enhancement of J_c is due to the reduction of anisotropy in high field region.     

Calculating thermopower due to fluctuation-assisted tunnelling with application to carbon nanotube ropes

We show how Sheng's theory of fluctuation-assisted tunnelling can be used to calculate thermopower due to the presence of quantum mechanical tunnelling barriers. For a hole-like situation, this calculated thermopower behaviour increases approximately linearly with increasing temperature (commencing from zero in the zero temperature limit) until a knee is reached, after which the thermopower continues to increase linearly but with a reduced slope. In Sheng's theory, the amplitude for the barrier fluctuations increases with increasing temperature and the knee occurs when the thermal fluctuations become comparable to the magnitude of the tunnelling barriers. Interestingly, this thermopower behaviour compares quite favourably with that measured for systems of metallic single-walled carbon nanotubes (SWCNs) and, in particular, the knee seen in the temperature dependence of the carbon nanotube thermopower data at T∼100 K arises naturally in our calculation. For a system of carbon nanotubes, tunnelling barriers could occur either between individual carbon nanotubes or else due to defects within the carbon nanotubes.     

杂质对两能带超导体性质的影响

本文中用一个简单的两能带模型讨论了费米面复连通性对超导体性质的影响。计算表明,它的效应与各向异性类似。利用Абрикосов和Горъков所发展的格林函数方法讨论了杂质散射的影响。有杂质时只有一个能隙,且大于纯超导体两个能隙中较小的一个。加杂后转变温度,临界磁场及其在T=0°K附近的温度系数下降,但转变点比热跳跃的相对值增加。低掺杂时在实验中同时测量这些量的变化可以定量地检验理论计算。高掺杂两能带超导体的性质与单能带情形完全相同。此外,利用本文的结果讨论了Pb膜的隧道效应实验和In-Sn,In-Pb系统中的转变温度反常。     

Enhancement of superconductivity in disordered films by parallel magnetic field

We show that the superconducting transition temperature T _c (H) of a very thin highly disordered film with strong spin-orbital scattering can be increased by a parallel magnetic field H. This effect is due to the polarization of magnetic impurity spins, which reduces the full exchange scattering rate of electrons; the largest effect is predicted for spin-1/2 impurities. Moreover, for some range of magnetic impurity concentrations, the phenomenon of superconductivity induced by magnetic field is predicted: the superconducting transition temperature T _c (H) is found to be nonzero in the range of magnetic fields 0 < H* ≤ H ≤ H _c .     

柠檬酸掺杂的MgB2超导体钉扎机理的研究

本文研究了柠檬酸掺杂的MgB₂超导材料的J_c-B行为及其钉扎机理.在纯MgB₂多晶样品中,δT_c钉扎起主要作用,而在掺杂的样品中,则是δl钉扎和δT_c钉扎共同作用,并且δl钉扎机理占主要作用,其贡献比重随着掺杂量的增加而增加.从J_c-B行为和钉扎行为的分析都可以得到 关键词： 柠檬酸 2')" href="#">MgB₂ T_c钉扎')" href="#">δT_c钉扎 l钉扎')" href="#">δl钉扎     

Experimental determination of the critical exponent η for nickel

The first unambiguous experimental determination of the critical exponent η, describing the critical point line shape of the spin correlation function, is presented for a three dimensional magnetic system. As a consequence of a finite value of η the critical scattering displays a maximum at T_max(q) above the Curie temperature T_c. The shift of the maximum with increasing scattering vector q was observed by a neutron small-angle experiment, yielding η=0.041±0.009 for a nickel single-crystal.