Superconductor-insulator transition

Summary. Various experimental observations of the superconductor-insulator transition are described and compared with two theoretical models one based on boson-vortex duality and the other where the superconducting fluctuations at low temperatures in the magnetic field are calculated. The latter shows that the superconducting fluctuations in dirty but homogeneous superconductor act as grains in a granular superconductor. 

The examples discussed so far are all magnetic quantum phase transitions. Our last example in this section on quantum-to-classical mapping is a quite different transition, viz. the superconductor-insulator transition in two-dimensional dirty boson systems. Experimentally, this transition can be realized in helium absorbed in a porous medium or in granular superconducting films as an example. 

The minimal model for describing the superconductor-insulator transition in the general case of both charge and phase fluctuations being relevant is the boson Hubbard model with a random local chemical potential.The Hamiltonian (defined on a square lattice) takes the form... 

Superconductor-Insulator Transition in Two-Dimensional Dirty Boson Systems. 

Kobayashi H, Sato A, Arai E, Akutsu Ft, Kobayashi A, Cassoux P (1997) Superconductor-to-insulator transition in an organic metal incorporating magnetic anions , ,-(BETS)2(FexGai x)... 

High-Temperature Superconductors and the Metal-Insulator Transition... 

The entire area has experienced a rebirth in the last few years, for several reasons. The most important perhaps is the realization that cuprate superconductors are not far from a largely correlation driven metal-insulator transition. Another is the appreciation that the transition is of wide occurrence, some recent examples being the superconductor-insulator, the quantum Hall fluid-insulator, and the stable quasicrystal metal-insulator transitions. Hopefully, the next few years should see considerable progress in the experimental and theoretical description of this basic electronic transition. 

The second family of superconducting materials is based on cation-radical salts of another donor, bis(ethylenedithio)-TTF (abbreviated BEDT-TTF or ET), which exhibit two-dimensional network mostly due to inter-stack S S interactions [100]. Another important feature of BEDT-TTF salts is their tendency to give polymorphs. For instance, (BEDT-TTF)2I3 salt affords four polymorphs (a, p, 0, and k phases), of which only the first undergoes metal-insulator transition, while the others are superconductors at ambient pressure (see Chapter 10 of this book). It is quite surprising that of all numerous BEDT-TTF structural analogs synthesized to date, only two salts of unsymmetrical derivatives, DMET [101] and MDT-TTF [102], led to superconductors. 

Organic conductors could be categorized in terms of the conduction mechanism. One-dimensional conductors seem to be sensitive to defects and disorders and become insulators at low temperatures due to the metal-insulator transitions even when the materials are metals at room temperature. On the other hand, more-than-one-dimensional conductors seem to be relatively insensitive to defects and disorders. Superconductors are found in this class of compounds. Taking the foregoing features into account, more-than-one-dimensional conductors have an advantage over one-dimensional conductors when they are to be used as the active elements of the conductive LB films. Actually, the active parts of the two metallic LB films belong to the more-than-one-dimensional conductors. 

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