STM Spectroscopy on Anisotropic Superconductors

The superconducting phase of organic superconductors -(BEDT-TTF)₂Cu(NCS)₂ and (MDT-TTF)₂AuI₂ was investigated by the electron tunneling spectroscopy using low temperature STM. The tunneling differential conductance at the lateral surface of -(BEDT-TTF)₂Cu(NCS)₂ varies its shape depending on the tip direction. The in-plane anisotropy of the conductance is well explained by the d-wave symmetry with line nodes along the direction 45° from the c-axis. For (MDT-TTF)₂AuI₂, the tunneling conductance at T = 1.4 K shows the superconducting energy gap structure clearly. The finite conductance inside the gap edge suggests the gap anisotropy. The tunneling spectrum is explained by the d-wave pairing. The obtained gap ₀ = 2 meV (2₀/kT _c = 12) is larger than that of the weak coupling limit. The pseudogap structure is observed near T _c .     

The Metal-Insulator Transition and Superconductivity in Allotropes of Carbon Intercalated with Copper: Prediction and Experiment

Abstract

The complex of studies carried out on Cu_nC₆₀ samples, including SQUID and microwave measurements clearly support the existence of a superconducting phase of Cu_1,5 C₆₀ with T_c=120K. Heating a samples of pyrolitic graphite intercalated with copper and oxygen above 77K initiates a Metal - Insulator transition. We assume the same idea that the superconducting transition temperature T_c in Cu_n C₆₀ samples changes near the M -I transition.     

Surface Superconducting State of Heavy-Fermion Superconductor CeIrIn5 Probed by CeIrIn5-Ag Junctions

The CeIrIn₅-Ag junctions of about 2×10^–9 cm^–2 area have been made using microfabrication techniques, and the surface superconducting state of CeIrIn₅, which has two characteristic temperatures T ₀ and T _c, has been investigated, where T ₀ and T _c are the transition temperature to zero-resistivity state and the bulk, thermodynamic transition temperature, respectively. The temperature, below which superconducting anomalies are observed, varies from junction to junction, and yet it is always well above T _c=0.4 K. This result, together with no indication of transition at T _c, suggests that at least the surface of CeIrIn₅ is in the superconducting state above T _c. The data on the critical current I _c in superconducting anomalies point to the possibility to define a local transition temperature for each junction.     

Preparation and degradation behaviour of Bi-cuprate superconductors

The role of preparation conditions and the effect of addition of Pb in Bi-Sr-Ca-Cu-O (BSCCO) superconductor, on theT _c,J _c and grain orientation have been studied. Calcination at a temperature higher than the melting point of Bi₂O₃ after the prereaction at 800°C leads to formation ofc-axis oriented nearly single phase material. The presence of lead yields a nearly highT _c phase (2223) exhibiting a maximum transition temperatureT _c=110 K and ΔT _c=2 K. The addition of lead and grain orientation together result in an increase inJ _c by nearly two orders of magnitude. The degradation behaviour was studied in terms of changes inT _c,J _c and structure on exposure to atmosphere up to 250 days. Samples with 0.6 Pb were found to be superconducting withT _c=97 K even after 250 days. The highT _c phase was stable against degradation. Our studies indicate that the degradation of leaded BSCCO is a surface phenomenon rather than bulk phenomenon.     

Interplay of low and highT c phases in Bi-Sr-Ca-Cu-O superconductor

In the Bi-Sr-Ca-Cu-O system stringent conditions of heat-treatment lead to the formation of a mixture of both the low and highT _c phases and obtaining a single-phase material becomes extremely difficult. This study reports preparation of samples with single superconducting transitions at ∼ 75 K and ∼ 108 K; the compositions of which correspond ton=2,3 in the series Bi₂Sr₂Ca _n−1Cu _n O_{4 + 2n} . X-ray diffraction studies show that the lowerT _c material is a relatively pure phase while the higherT _c phase only co-exists with the lowerT _c phase. The most obvious effect of doping the system with lead is to make the reaction take place faster and thereby increase the volume fraction of the 110K phase.     

Preparation of Bi,Pb-Sr-Ca-Cu-O superconductor by the sol-gel method

The high -T _c superconducting phase in the Bi, Pb-Sr-Ca-Cu-O system has been prepared by the sol-gel method. The formation process of the low-T _c phase as well as the high-T _c phase has been examined by thermal analysis, infra-red measurements and X-ray diffraction, and compared with the conventional solid-state reaction. At a low heat-treatment temperature of 400 ° C, Bi₂O₃, CaCO₃ and CuO are precipitated from the gel. These oxides react with each other to form several double oxides above 600 ° C and yield the low-T _c phase as a main product at 800 ° C. The heat treatment at higher temperatures effectively leads to the formation of the high-T _c phase. In particular, the high-T _c phase is most predominantly formed in the specimen heat treated at 845 ° C for 48 h after calcination at 800 ° C for 12 h, which is reflected in the critical temperature of 104 K. The temperature at which the low-T _c phase is formed is lower in the sol-gel-derived sample than in the specimen prepared by the solid-state reaction. Further-more, the sol-gel method brought about a larger amount of the high-T _c phase than the solid-state reaction when the conditions for the heat treatment were identical. These facts clearly indicate the superiority of the sol-gel method for the formation of both high-T _c and low-T _c Bi, Pb-Sr-Ca-Cu-O superconducting phases.     

Elastic properties of the heavy fermion superconductor URu2Si2

Measurements of the elastic constants in the normal and superconducting state on a high quality URu₂Si₂ single crystal exhibit large anomalies in thec ₁₁ mode versus temperature around 30 K andT _c. No such anomalies are visible in the c₃₃ mode and in the shear elastic constants. We describe this behaviour with the anisotropic Grüneisen parameter coupling. Using the c₁₁ anomaly a B-T phase diagram is determined which clearly shows the single phase bulk properties of superconducting URu₂Si₂. The magnetic field dependences of the shear waves in the superconducting and normal state can be interpreted quantitatively with a Lorentz force mechanism.     

Pressure Effect on BDA-TTP Conductors

The pressure-induced conducting behavior of the charge-transfer salts (BDA-TTP)₂X [BDA-TTP = 2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene; X = ClO₄·H₂O, ClO₄, BF₄·H₂O, BF₄, and I₃] has been investigated. With increasing pressure, the transition temperature to insulator observed in the ClO₄·H₂O, ClO₄, BF₄·H₂O, and BF₄ salts increases. On the other hand, the resistance of the I₃ salt as a function of temperature at 10.0 kbar shows a drop attributable to a superconducting transition with an onset at 4.5 K. The superconducting transition temperature (T_c) can reach 8.2 K by applying a pressure of 10.3 kbar, but gradually decreases with further increases in the pressure.     

Superconductivity of Ba1 ? xKxBiO3 (0.35 < x < 1) Synthesized by the High Pressure and High Temperature Technique

Ba_{1 – x} K _x BiO₃ (BKBO) samples with 0.35 < x < 1 were synthesized by the high pressure and high temperature technique. XRD analysis showed that the BKBO samples were single phase for the whole range of the potassium doping concentration. The change of superconducting transition temperature, T _c, as well as lattice parameters have been investigated upon doping concentration. As the K doping concentration (x) increases from x = 0.37, T _c decreases from 30.4 K to almost zero at x = 0.74. However, in some BKBO samples without including any barium in the starting composition (x = 1), which is denoted as KBO samples, superconductivity is observed with T _c as high as 9 K with partial substitutions of Bi at the K site. Depending on the synthesis condition of the KBO samples, T _c and lattice parameters were different from sample to sample. Compared with other superconducting bismuthates, the evolution of T _c by potassium doping in the cubic BKBO system is discussed in terms of its electronic band structure.     

Fluctuations of the Order Parameter''s Phase in Layered Superconductors

The influence of the quantum fluctuations of the order parameter's phase on the critical temperature T _c is studied for a Josephson coupled layered superconductor. Two characteristic critical temperatures exist for a system, namely the superconducting critical temperature T ⁽²⁾ _c for a single layer estimated by the mean-field theory and the transition temperature for the outset or the superconducting phase coherence T* _c . The true critical temperature T _c is shown to vary inside the intervals T* _c T _c T ⁽²⁾ _c . For a strong quantum phase fluctuation limit, the superconducting layers become decoupled.     

On the Presence of 1D-Like Stripes in Superconducting Epitaxial La2-xSrxCuO4 thin Films

A large positive magnetoresistance (up to tens of percents) is observed in both underdoped (x < 0.15) and overdoped superconducting La_{2− x}Sr_xCuO₄ (LSCO) epitaxial thin films, at temperatures far above the superconducting critical temperature T _c . In the underdoped regime, this magnetoresistance cannot be described by the Kohler rule and is due to the influence of superconducting fluctuations. On the other hand, in the overdoped regime, the Kohler rule does not seem to be violated. The strong magnetoresistance above T _c can be related to the preformed superconducting pairs existing well above T _c but forming a phase coherent superconducting state below T _c . The observations support the idea of a close relation between the pseudogap and the superconducting gap and provide evidence for the presence of pre-pairs above T _c . Both the observed fluctuations and the observed magnetoresistance are in accordance with the existence of 1D-like stripes. These results are further supported by recent high magnetic field measurements (up to 55 T) of the transverse magnetoconductivity σ _xy, which goes to zero for T→0 K.     

Reproducibility ofT c in a Bi2Sr2CaCu208 superconductor

The reproducibility ofT _c in superconductive Bi₂Sr₂CaCu₂O₈ is known to be very poor. In our study, the reproducibility was found to depend greatly on preparation conditions. DTA, TGA and powder X-ray diffraction methods were used to study the Bi₂Sr₂CaCu₂O₈ superconductors prepared by different routes. Resistivity and diamagnetic susceptibility were taken as the measure of superconductivity. It was found that those superconductors prepared from the oxides/carbonates (one-step) process resulted in scatteredT _c data which were less reproducible. The two-step synthesis from precalcined precursors of Bi-Sr-O and Sr-Ca-Cu-0 containing mixtures, resulted in a much improved reproducibility with the predominantT _c = 80 K superconducting phase (more than 95%). It was also found that the amount ofT _c = 115 K phase tends to decrease after repeated pulverization and sintering, leading to a single 80 K phase state. Compaction of the pulverized powder at a pressure > 4.5 ton cm^–2 was found to induce a preferred (00L) orientation during sintering.     

Preparation of high-T c superconducting film in the system of Bi-Sr-Ca-Cu-O from sol-gel process and its properties

High-T _c superconducting Bi₂Sr₂CaCu₂O_x films withT _c ^off =80 K were prepared by the dipping method of sol-gel processing using inorganic salts. The influence of the preparation conditions on the superconducting properties of the derived material is reviewed. Bi, Sr, Ca and Cu nitrates were used as raw materials. Glycerol was used as solvent. The thickness of films made by the dip method was about 0.5 m. The films were crystallized by heat-treatment at 830°C for 10 min.T _c ^off of films was 80 K andJ _c at 77 K was more than 8 kA cm^–2. Synthesis of high-T _c superconducting films was very easy and the crystallization of films was possible with a relatively low heat-treatment temperature.     

Superconductivity at 103 K in CdBa2(Ca0.7Y0.3)Cu2Oy

A cadmium analogue of the mercury system with nominal composition CdBa₂(Ca_1–xY_x)Cu₂O_y has been synthesized. Thex=0 samples contain about 12 vol.% of the 1212 phase but are not superconducting. Thex=0.3 samples are superconducting atT _on = 103 K. The EDX analysis of 18 microcrystals shows a broad cationic distribution of the different components. The observed broad superconducting transition is attributed to the variousT _c of the different microcrystals.     

Superconductivity in transition metal-germanium systems

The variation in the superconducting properties of various binary alloys of the transition metal-germanium system was surveyed by studying sputter-deposited samples prepared under various conditions. Radiofrequency cosputtering was used to form samples of the form T _x Ge _1–x , where T represents a transition metal, Y, La, Ti, Zr, Hf, V, Nb, Ta, Mo, W, and Re. The primary interest has been to study the formation of A-15 compounds. While sputtered V ₃ Ge hadT _cs close to bulk sample values, Nb ₃ Ge (T _c=21.7 K) and Mo ₃ Ge (T _c=5.7 K) showed a considerable enhancement inT _c over the bulk values. The formation of the metastable A - 15 modification of Ta ₃ Ge, Ti ₃ Ge, Zr ₃ Ge, and Hf ₃ Ge was attempted, without success. Superconductivity was observed in samples of ZrGe ₂ , HfGe ₂ , YGe ₂ , and LaGe ₂ , the highestT _c being that of ZrGe ₂ (T _c=8.0 K). Annealing studies indicated that superconducting ZrGe₂ and HfGe₂ were metastable, depending on the conditions of synthesis. Superconductivity was also observed in mixtures of semiconducting Ge and metallic NbGe ₂ , the highestT _c observed being 16 K. Several explanations are considered to account for the observedT _cbehavior among the various alloys.Supported in part by the National Science Foundation.     

Synthesis and Optical Studies of Superconducting MgB2 Thin Films

Synthesis and optical transmission of MgB₂ thin films on optically transparent glass are reported. In the 400–1000 nm regime as deposited films show high metallic reflectivity and very little transmission. After deposition, the films were annealed ex situ and rendered superconducting with T _c of 38 K, approaching that of the bulk material. The reaction conditions where quite soft ∼10 min at 550°C. The optical absorption coefficient, α and photon energy, E followed a Tauc-type behavior, = _T (E - E_g )(\alpha E)^{1/2} = \beta _T (E - E_{\rm g} ). The band gap (E _g) was observed to peak at 2.5 eV; but, the slope parameter β _T behaved monotonically with reaction temperature. Our results indicate that an intermediate semiconducting phase is produced before the formation of the superconducting phase; also optical measurements provide valuable information in monitoring the synthesis of MgB₂ from its metallic constituents. In addition these films have interesting optical properties that may be integrated into optoelectronics.     

Growth of highT c Bi-Sr-Ca-Cu-O thick films

Thick films of Bi-Sr-Ca-Cu-O were deposited on (100) MgO substrates by screen-printing technique with the starting composition 1112. To attain the superconducting state, the films were subjected to two-step heat-treatment. R-T and XRD have been studied for films annealed at different durations of the second step. InitiallyT _c (R=0) increased from 77 to 103 K as the annealing duration was increased after whichT _c decreased. Kinetics of the growth of highT _c phase is discussed in the light of our results.     

Pressure- and Field-induced Non-Fermi-liquid Behaviors in a Heavy-Fermion Antiferromagnet Ce7Ni3

We review recent investigations of the heavy-fermion antiferromagnet Ce₇Ni₃ with a geometrically frustrated structure under hydrostatic pressures and magnetic fields. Below T _N1=1.9 K, an incommensurate spin-density-wave (SDW) develops, and another antiferromagnetic transition occurs at T _N2 = 0.7 K. At a rather low critical pressure (P _c ), P _c = 0.39 GPa, both T _N1 and T _N2 vanish, and the specific heat divided by temperature (C/T) exhibits –lnT dependence (i.e., non-Fermi liquid behavior). At P = 0.43 GPa > P _c , a T-independent behavior of C/T, i.e., Fermi-liquid behavior, recovers below 0.2 K. However, the magnetic susceptibility continues to increase down to 0.09 K at 0.43 GPa, which is not consistent with a conventional Fermi-liquid theory. On the other hand, upon applying magnetic fields B along the hexagonal c axis, T _N1 decreases and vanishes at 0.3 T. Magnetoresistance, specific-heat, and magnetization measurements reveal that a field-induced magnetic (FIM) phase appears in a B–T region for B _{∥ c} > 0.7 T and T < 0.5 K. Neutron diffraction experiments indicate that the magnetic unit cell in the c plane for the FIM phase is treble that of the chemical unit cell. Moreover, the intensity of the magnetic reflection remains even in the region between the FIM phase and SDW phase. This observation indicates the presence of large spin fluctuations in the c-plane associated with the magnetic frustration, which should be responsible for the non-Fermi-liquid behavior of Ce₇Ni₃.