Transport properties superconductance

Tajima N, Ebina-Tajima A, Tamura M, Nishio Y, Kajita K (2002) Effects of uniaxial strain on transport properties of organic conductor a-(BEDT-TTF)2l3 and discovery of superconductivity. J Phys Soc Jpn 71 1832-1835... 

A comprehensive report which focussed on the La2 xSrxCu O4-x/2+S ser es was published (139) in 1983 by this research group. In this broad review they reported the magnetic and electrical transport properties of these mixed-valent copper oxides in the temperature range 120-650 K. They concluded that the original semiconducting behavior in La2Cu04 transformed to semi-metallic behavior as the Cu3+ content increased with Sr-substitution. No experiments were conducted below 50 K, and therefore superconductivity was not observed. Three series of compounds, with 0.00 < x < 1.20 were... 

A substitution for Cu by Fe, Co or Mn in the Bi2Sr2CuOy phase produces a drastic change in the transport properties, in particular an electronic transition from superconducting to semiconducting for Fe or Co or to insulating for Mn. This indicates that the presence of 3d metal oxygen layers is not, by itself, sufficient for... 

The superconducting properties induced in the normal metal manifest themselves in many different ways, including energy-dependent transport properties and a modification of the local density of states. For instance, the conductance of a normal conductor connected to a superconducting electrode shows a striking re-entrant behavior [4]. At non-zero temperature and/or bias, the conductance of the normal metal is enhanced as compared to the normal-state. At zero temperature and zero bias, the expected conductance coincides with the normal-state value. The conductance has therefore a non-monotonous behavior. 

The metallic properties observed for the d7 compounds listed in Table I are also consistent with the Goodenough model. The rhodium-selenium system is of particular interest and demonstrates clearly the important relationships between structure and transport properties. Cations may be removed from the superconducting compound (Tc = 6°K), RhSe2. The pyrite structure is maintained as the eg band is gradually emptied, and at the composition Rh2/3Se2 (RhSe3), all cations are trivalent—i.e., have the configuration 4d6. It is not known yet if the ideal... 

Transport properties above Tc(x) Actually, it is not necessary to study the properties near T = 0 to rule out a pseudogap region beyond an xc in the superconducting range of x. If fig. (2) were true, the universal normal state anomalies would change to the pseudogap properties for any x for temperatures below T (x) and above Tc(x). The data does not sustain this point of view. 

Since proximity to the MIT controls the normal state transport properties it is natural to assume that it might also influence the superconductive properties. To include these effects in a model, we modified the the Morel-Anderson model for Tc, to span the entire metallic range of an alloy ... 

The investigation of transport properties under pressure has shown that the SDW ground state can be suppressed above a critical pressure of 25 kbar [160]. (TMTTF)2Br was thus found to retain a strong metallic character down to 1.2 K. Furthermore, some samples showed a slight drop of the resistance down to a nonzero value near 3.5 K which was ascribed to a possible signature of superconductivity [160]. 

Important further illuminations of the relationships between the structural and transport properties is provided by crystallographic studies under pressure. As yet, very limited information is available. However, from studies of (TMTSF)2PF6, the volume compressibility is estimated to be 0.5% kbar-1 (49). It, therefore, requires 6 kbar to compress (TMTSF)2PF6 into the same volume as (TMTSF)2C104, which is in rough agreement with the critical pressure for superconductivity in the PF6 compound. 

It is interesting to compare the transport properties of available metallic polymers with those of the underdoped High T superconductors at doping levels where k l 1. Eor example, when YBa2Cu307 8 is underdoped to values of 8 such that the resistivity increases as the temperature is lowered with Pr=p(1.4K)/ p(300 K) 2 (i.e. with temperature dependence similar to that found in the best metallic polymers), the disorder associated with the random occupation of the oxygen sites quenches the superconductivity [273]. 

The analyses of both deposits confirm that they contain the [TTF][Ni(dmit)2]2 phase both exhibit a metallic behaviour. A deeper study of the transport properties of the fibre-like films showed that superconductivity manifests by a broad drop of the resistance below 0.8 K. Although the transition is incomplete because of inter-fibre resistance contributions, Figure 4.38 shows that superconductivity is confirmed by the application of a magnetic field perpendicular to the plane of the film. 

Fig. 5.27. a-d Enlarged plot of transport properties of Cu20A1bo metallic glass, a) Resistivity [5.112], Below T= 15 K a magnetic field has been applied in order to suppress superconductivity b) The second derivative of the resistivity data in a) c) Thermopower [5.80] d) Hall coefficient [5.112]. All measurements have been performed with the samples well annealed... 

Recently Bednorz and Muller (1) reported possible superconductivity at 30 K in the metallic-oxide La-Ba-Cu-0 system. Subsequent work by Tanaka and co-workers (2) showed that the high temperature superconductivity is due to the solid solution La BaxCuO. y which forms in the K2NiFA-type structure. Making solid solutions with strontium instead of barium, Cava et. al. (3). were able to show bulk superconductivity at 38 K in La2 xSrxCuOA y. While solid solutions of the type La2.xMxCuOA y (M = Sr, Ba) have been known for some time, (4-5) magnetic and electron transport properties of these compounds had not previously been measured at low temperatures (i.e. <120 K). ... 

More recently, (6-7) several groups have observed superconductivity in the unsubstituted phase La2CuOA y at temperatures just below the metal to insulator phase transition. However, in this case the Meissner effect measurements were consistent with only a small fraction of the sample being in the superconducting state. While the magnetic and electron transport properties of La2CuOA y had been measured some time ago, (8) superconductivity was not previously observed. Thus, the presence or absence of superconductivity in samples of La2CuOA y is a sensitive function of the method of sample preparation. 

The system 8a La Cu Q chosen by Bednorz and Muller (1) for their first investigation of superconductivity was connected to the existence of metallic properties of the oxide BaLa CusOis+s recently isolated in Caen (2). However the method used for the synthesis did not lead to the expected oxide but to a mixture of phases. Fortunately, one of these latter phases was found to be at the origin of superconductivity. In fact the concerned oxide is a M liF -type oxide, La2-xBaxCu04-y, isolated and studied for its electron transport properties several years before in Caen (3-4). It was then shown by several authors (5-9) that the strontium isostructural oxide La2-xSrxCu04-y (10-11) and the calcium compound (3-4) were also superconductors with Tc values ranging form 20K to 40K. 

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