Superconducting state, instability

Recently another member of the a-phase family has successfully been synthesized [144]. a-(ET)2KHg(SeCN)4 behaves in many respects similar to a-(ET)2TlHg(SeCN)4. It neither becomes superconducting nor does it show a ground state instability, i. e., no resistivity hump around 10 K is observed. The band-structure parameters obtained from SdH measurements are almost the same for both materials. The SdH frequency follows the l/cos0 dependence with Fq S3 670 T. The effective cyclotron mass is Hc 2.0. No mass increase up to 23 T was observed [144]. 

It has now become apparent that f-band superconductors can be understood in a normal way, like d-band ones" we have just to think about density of states, bandwidth lattice unstabilities and electron correlations to interpret it. This was clearly summarized by Smith after the discovery of superconductivity of Pa and Am. Superconductivity of Am is made by the J = 0 ground state of its six 5 f electrons as was stressed by Johansson , this could also be the case for stabilized trivalent europium metal. The importance of lattice instabilities (like in A-15 high Tc superconductors) was put forward by Fournier who showed that the very large 6T/8p slope for U was mainly due to a very targe change in the electron-phonon coupling associated with the low temperature phase transition. 

An alternative way to clarify the nature of this state is to test its stability with respect to a metal-insulator transition. This has received a lot of theoretical attention recently. The JT singlet ground state makes these compounds free from the tendency towards a magnetic instability observed in so many Mott insulators. In fact, their ground state does not break any symmetry and Capone et al. explained [43] that it then has a zero entropy, which makes a direct connection with a metal impossible (it would violate the Luttinger theorem). These authors predict that the only way to go from the insulator to the metal would be through an exotic superconducting phase or a first-order transition. 

Almost two decades ago theoreticians predicted that in a quasi-one-dimensional metal, characterized as having a one-dimensional gas of weakly interacting electrons, instabilities could arise leading to transitions to various ground states, such as charge density wave (CDW), spin density wave (SDW), or superconducting (61). Previously, it had been predicted that electron-phonon coupling in a one-dimensional... 

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