Structural phase transitions Superconductivity

The recent discoveries of an efficient synthesis [1] of Cfto and superconductivity in K3C60 and Rb3C6o [2] have generated great interest in the structural and electronic properties of these materials. It is known that at room temperature C o molecules are centered at fee Bravais lattice sites in both solid Cm and K3C60 (3l. Recently it was reported [4] that solid Cm undergoes a structural phase transition around 250 K from fee to the simple cubic Pa3 structure at low temperature. Both x-ray and NMR measurements [3] indicate that this transition is related to the orientational order of the Cm molecules. 

Being able to conduct diffraction studies at different temperatures is important when correlations between structural changes and other temperature-dependent physical properties are to be studied. Examples include the study of structures that undergo phase transitions. These may be structural phase transitions or other types of transitions such as magnetic order/disorder or onset of superconductivity transitions. 

Electronic and elastic properties of carbon nanotubes are actual in connection with perspectives of their applications in nanoelectronic devices [1] and in composite materials [2], A study of phase transitions in carbon nanotubes, including a possibility of a superconducting state [3], is also of much current interest This work is devoted to structural phase transitions controlled by an expansion of carbon nanotubes. It is considered on the example of armchair metallic (5,5) nanotube. 

As in polycrystalline pressed at-(BEDT-TTF)2l3, in pressed samples of Pp-(BEDT-l lF)2l3 a pressure-induced structural phase transition plays an important role. As a consequence of the structural phase transition in pp-(BEDT-TTF)2l3, the superconducting transition temperature is increas. This behavior re-emphasizes that organic superconductors might also be of interest for industrial applications, since polyciystalline materials are easier to use than single crystals. In addition, the discovery of bulk superconductivity in large pressed samples of crystallites of organic metals, of the typical diameter of 1 p.m and below, indicates that the observation of superconductivity in conducting polymers should be possible as well. 

Figure 7. At / = 0.75, pressure as a function of/./ = / e / > and for the normal and color superconducting quark phases. The dark solid lines represent two locally neutral phases (i) the neutral normal quark phase on the left, and (ii) the neutral gapless 2SC phase on the right. The appearance of the swallowtail structure is related to the first order type of the phase transition in quark matter.

It is interesting to notice that the three pressure surfaces in Figure 7 form a characteristic swallowtail structure. As one could see, the appearance of this structure is directly related to the fact that the phase transition between color superconducting and normal quark matter, which is driven by changing parameter //,. is of first order. In fact, one should expect the appearance of a similar swallowtail structure also in a self-consistent description of the hadron-quark phase transition. Such a description, however, is not available yet. 

This application will concentrate on the MBa2Cu307, 5 (123) system, where M = Y. Another designation for this system is YBCO. For this system, the optimum superconductivity is obtained for YBa2Cu307 5, when S = 0.3. This formulation has an orthorhombic (O) structure. Upon loss of oxygen a phase transition takes place to an insulator with a tetragonal (T) structure. Figure 4-43 shows the structures for the O and T-phases. 

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