Investigation of superconductivity in the single-walled carbon nanotubes

Superconductivity in the single-walled carbon nanotubes is investigated. First, effect of diameter increasing on the clean systems critical temperature, T_c, is calculated. Then effect of impurity doping on the reduction of critical temperature T_c, of single-walled carbon nanotubes, is discussed. Our calculations illustrate that metallic zigzag single-walled carbon nanotubes have higher T_c than armchair single-walled carbon nanotubes with approximately same diameters and T_c decreases by increasing diameter. This can explain why superconductivity could be found in the small diameter single-walled carbon nanotubes. We found for the impurity doped systems, impurity in the strong scattering regime can decrease T_c significantly while in the weak scattering regime T_c is not affected by impurity doping.     

Observation of a Kondo effect for a dilute alloy containing Sm impurities

Measurements of the depressions of the superconducting transition temperature T_c with Sm impurity concentration and the specific heat jump at T_c as a function of T_c, and the temperature dependences of the normal state specific heat and magnetic susceptibility are reported for the matrix impurity system (LaSm)Sn₃. The results constitute the first definitive evidence of a Kondo effect for a dilute alloy containing Sm impurities.     

Five-fold way to new high <Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> superconductors

Discovery of high T _c superconductivity in La_2?x Ba _x CuO₄ by Bednorz and Muller in 1986 was a breakthrough in the 75-year long search for new superconductors. Since then new high T _c superconductors, not involving copper, have also been discovered. Superconductivity in cuprates also inspired resonating valence bond (RVB) mechanism of superconductivity. In turn, RVB theory provided a new hope for finding new superconductors through a novel electronic mechanism. This article first reviews an electron correlation-based RVB mechanism and our own application of these ideas to some new noncuprate superconducting families. In the process we abstract, using available phenomenology and RVB theory, that there are five directions to search for new high T _c superconductors. We call them five-fold way. As the paths are reasonably exclusive and well-defined, they provide more guided opportunities, than before, for discovering new superconductors. The five-fold ways are (i) copper route, (ii) pressure route, (iii) diamond route, (iv) graphene route and (v) double RVB route. Copper route is the doped spin-½ Mott insulator route. In this route one synthesizes new spin-½ Mott insulators and dopes them chemically. In pressure route, doping is not external, but internal, a (chemical or external) pressure-induced self-doping suggested by organic ET-salts. In the diamond route we are inspired by superconductivity in boron-doped diamond and our theory. Here one creates impurity band Mott insulators in a band insulator template that enables superconductivity. Graphene route follows from our recent suggestion of superconductivity in doped graphene, a two-dimensional broadband metal with moderate electron correlations, compared to cuprates. Double RVB route follows from our recent theory of doped spin-1 Mott insulator for superconductivity in iron pnictide family.     

Initial suppression of Tc in optimally-doped Bi2Sr1.6RE0.4Cu1−xZnxO6+δ (RE = La,Pr, Nd,Sm, and Eu)

We have investigated the initial suppression of the superconducting transition temperature, T_c, by Zn substitution in Bi₂Sr_1.6RE_0.4CuO_6+δ (Bi(RE)-2201) with a single CuO₂ plane. We propose a phase diagram of Bi(RE)-2201 as a function of the degree of disorder outside the CuO₂ plane. Upon increasing the degree of disorder, while T_c decreased gradually, the pseudogap is significantly strengthened as a quadratic function of the degree of disorder. The initial suppression rate of T_c is well described by considering the enhancement of the pseudogap by the out-of-plane disorder.     

Superconductivity in two-band systems with a variable carrier density: The case of MgB<Subscript>2</Subscript>

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.     

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.     

Suppression of Tc by Zn impurity in the electron-type LaFe0.925−y Co0.075ZnyAsO system

The effect of non-magnetic Zn impurity on superconductivity in electron-type pnictide superconductor LaFe_0.925−yCo_0.075Zn_yAsO is studied systematically. The optimally doped LaFe_0.925Co_0.075AsO without Zn impurity exhibits superconductivity at T_c^mid of 13.2 K, where T_c^mid is defiend as the mid-point in the resistive transition. In the presence of Zn impurity, the superconducting transition temperature, T_c^mid, is severely suppressed. The result is consistent with the theoretic prediction on the effect of non-magnetic impurity in the scenario of s_± pairing, but it is in sharp contrast to the previous report on the effect of Zn impurity in the F-doped systems. The possible interpretation of the different effects of Zn impurity on superconductivity in different systems is discussed.     

Electron spin-lattice relaxation anomaly and the local pseudo freeze-out of impurity dynamics in H-bonded ferroelectrics

The strong decrease in the electron spin-lattice relaxation rate at the ferroelectric transition temperature T_c and the simultaneous increase in the transverse spin-spin relaxation rate can be both understood in terms of the local “spontaneous freeze-out” model of impurity dynamics recently proposed to explain the spontaneous dynamic symmetry breaking observed far above T_c in the EPR spectra of H-bonded ferroelectrics doped with paramagnetic impurities.     

The Hubbard model and pressure effects of YBa2Cu3O7−δ superconductors

We apply a mean field approach to the extended Hubbard model on a square lattice to the YBa₂Cu₃O_7−δ family of superconductors under pressure. The parameters of the tight-binding band are taken from experiments, and the coupling strength U and V are estimated by the zero pressure phase diagram (T_c×n_h). This scheme yields the non-trivial dependence of the superconductor critical temperature T_c as a function of the hole concentration n_h in the CuO₂ plane. With the assumption that the pressure P modifies the potential V and the on-plane hole content n_h, we can distinguish the charge transfer and the intrinsic contribution to T_c(P). We show that the changes on T_c(P) for the YBa₂Cu₃O₇ optimally doped compound at low pressures are almost entirely due to the intrinsic term.     

High-resolution photoemission study of FeSr2YCu2O7+δ

We have performed high-resolution photoemission spectroscopy (PES) on FeSr₂YCu₂O_7+δ, of which superconductivity of T_c=49 K was recently reported. We clearly observed opening of a d-wave-like superconducting gap and estimated the maximum gap value (Δ_max) to be 10 meV at 15 K. This gap value gives 2Δ_max/k_BT_c∼5, suggesting a strong-coupling nature of superconductivity in FeSr₂YCu₂O_7+δ. Comparative PES study with superconducting and insulating samples shows that the valence band is rigidly shifted as a function of doping without evolution of additional states within the insulating gap.     

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.     

Mechanism of carrier generation and the origin of the pseudogap and 60 K phases in YBCO

The mechanism of hole carrier generation is considered in the framework of a model assuming the formation of negative U centers (NUCs) in HTSC materials under doping. The calculated dependences of carrier concentration on the doping level and temperature are in quantitative agreement with experiment. An explanation is proposed for the pseudogap and 60 K phases in YBa₂Cu₃O_6+δ. It is assumed that a pseudogap is of superconducting origin and arises at temperature T* > T_c∞ > T_c in small nonpercolating clusters as a result of strong fluctuations in the occupancy of NUCs (T_c∞ and T_c are the superconducting transition temperatures of an infinitely large and finite NUC clusters, respectively). The T*(δ) and T_c(δ) dependences calculated for YBa₂Cu₃O_6+δ correlate with experimental dependences. In accordance with the model, the region between T*(δ) and T_c(δ) is the range of fluctuations in which finite nonpercolation clusters fluctuate between the superconducting and normal states due to NUC occupancy fluctuations.     

Mössbauer study of Tl containing highT c superconductors

Mössbauer spectroscopy, X-ray diffractometry, ESR and electrical resistivity measurements, were used to study highT _c (above 100 K) superconducting materials in order to get information about the effect of the preparation circumstances as well as about the anomalous behaviour in⁵⁷Fe and¹¹⁹Sn doped TlBaCaCu(⁵⁷Fe)O_4.5+δ and TlBaCuCu(¹¹⁹Sn)O_4.5+δ superconductors. We have found that the Mössbauer parameters strongly depend on the preparation circumstances. In many cases the presence of Mössbauer lines of nonsuperconducting oxide phases indicated that the iron and tin could not entirely built in into the lattice of superconductor. Four valence state tin and four valence state iron sites were identified in the superconducting phases. We have found anomalous temperature dependent changes in the⁵⁷Fe Mössbauer spectra of TlBaCaCu(⁵⁷Fe)O_4.5+δ samples around theT _c between 105–135 K. In this temperature range the total area of the spectrum temporary increases.     

Possible role of oxygen monomers reordering in the photoinduced charge transfer in RBa2Cu3O6+x material

The model of photoassisted oxygen ordering assumes that the enhancement of the conducting properties of RBa₂Cu₃O_6+x material during the light illumination, is the consequence of the CuO_x plane oxygen atoms reordering into long chain fragments, which are known to be better hole dopants than the short ones. Some experimental results suggest that this process is performed mainly through the reaccomodation of the oxygen monomer units (isolated oxygen ions). In the present work concentration of oxygen monomers in the CuO_x planes, which is assumed to be equal to the concentration of holes transferred out of the chain planes during the light illumination, is calculated as a function of oxygen content x, and δT_c (photoinduced enhancement of the superconducting critical temperature T_c) was estimated for different oxygen concentrations. Numerically found values of the δT_c are shown to be in good agreement with experimental findings.     

Superconducting properties and structure of ion bombarded Nb layers

The influence of ion bombardment on the superconducting transition temperature T_c and the structure of thin evaporated niobium layers has been investigated as a function of ion species and layer purity. Irradiation through pure layers with neon ions and fluences of typically 1016 ions/cm² leads to relatively small T_c decreases (δT_c × 0.5 K), while in oxygen contaminated layers larger effects depending on oxygen concentration are observed. Homogeneous implantation of chemically active impurities (nitrogen, oxygen) also drastically depresses T_c reaching the detection limit of 1.2K at a concentration of 15 at. %N. The T_c depressions correlate with a lattice parameter expansion of the Nb bcc structure at a rate of about 0.1 %per 1 at. % impurity.     

c-axis penetration depth in Bi2Sr2CaCu2O8+δ single crystals measured by ac-susceptibility and cavity perturbation technique

The c-axis penetration depth Δλ_c in Bi₂Sr₂CaCu₂O_8+δ (BSCCO) single crystals as a function of temperature has been determined using two techniques, namely, measurements of the ac-susceptibility at a frequency of 100 kHz and the surface impedance at 9.4 GHz. Both techniques yield an almost linear function Δλ_c(T)∝T in the temperature range T<0.5T _c. Electrodynamic analysis of the impedance anisotropy has allowed us to estimate λ_c(0)≈50 µm in BSCCO crystals overdoped with oxygen (T _c≈84 K) and λ_c(0)≈150 µm at the optimal doping level (T _c≈90 K).     

高温超导体低温、高压下的物理性质的研究

 在3~20 GPa压力范围内，测量了含氧量较低的YBa₂Cu₃O_7-δ（δ=0.46）单晶压力增强效应（dT_c/dp=4.9KGPa^-1）；YBa₂Cu₃O₇（T_c0=90 K）单晶在压力下临界电流密度随压力变化；外磁场H=30 kO_e时，T_c与磁场、压力关系；压力达16.5 GPa下，Bi₂Sr₂CaCu₂O_x单晶T_c(p)关系（dT_c/dp=-0.4 KGPa^-1）。发现Y系高温超导体的温度压力导数dT_c/dp与T_c0中间呈dT_c/dp=b-mT_c0线性关系（b、m为常数）。结合压力下Y系超导体结构相变和含氧量对T_c影响，分析这类超导体T_c有很强的正压力效应的原因。把实验结果同几种超导电性微观理论模型进行了分析和比较。     

Study of magnetic properties of melt textured YBa2Cu3O7−δ with Pt,Ce and Ag dopants

Bulk YBa₂Cu₃O_7−δ samples, doped with PtO₂, Ag₂O, CeO₂ or with a starting mixture of nanosized Y₂O₃, have been prepared by the top seeded melt texturing process. Magnetic measurements have been performed on these samples using VSM and SQUID magnetometer. Critical current densities as a function of the applied magnetic field J_c(B), temperature and field dependent normalized relaxation rates S(T) and S(B), as well as E(J) relationships have been derived. The effects of different dopants on these results have been investigated.     

The behaviour of an isolated impurity near a phase transition in a spherical-like model

The behaviour of an isolated impurity in a crystal undergoing a displacive phase transition is investigated in an exactly soluble spherical-like model. We find, depending on the parameter of the impurity, either a global phase transition at the bulkT _c or a freezing-out of local order at a temperatureT _c ^loc >T _c driven by a soft local mode. We furthermore discuss the dynamic autocorrelation function of the impurity by introducing a phenomenological damping.Work Supported by The Swiss National Science Foundation     

Stripes, electron fractionalization, and ARPES

Although structurally the high temperature superconductors are quasi-two-dimensional, there is both theoretical and experimental evidence of a substantial range of temperatures in which ‘stripe’ correlations make the electronic structure locally quasi-one-dimensional. We consider an array of Josephson coupled, spin gapped one dimensional electron gases as a model of the high temperature superconductors. For temperatures above T_c, this system exhibits electron fractionalization, yielding a single particle spectral response which is sharp as a function of momentum, but broad as a function of energy. For temperatures below the spin gap but above T_c, there are enhanced one-dimensional superconducting fluctuations and pseudogap phenomena. Pair tunneling induces a crossover to three-dimensional physics as T_c is approached. Below T_c, solitons are confined in multiplets with quantum numbers which are simply related to the electron, and a coherent piece of the single particle spectral function appears. The weight of this coherent piece vanishes in the neighborhood of T_c in proportion to a positive power of the interchain superfluid density. This behavior is highly reminiscent of recent ARPES measurements on the high temperature superconductors.