Superconductivity under ambient pressure

Among the M(dmit)2-based superconductors, a-(EDT-TTF)[Ni(dmit)2] is also of donor-acceptor type and has two outstanding features it is the only one to contain a 1 1 molar ratio of donor and acceptor units and to exhibit superconductivity at ambient pressure [32]. It was found to be superconductive below 1.3 K under ambient pressure (Fig. 3). 

The principal crystal structure of the so-called a phase of (ET)2AT shown in Fig. 2.16 and some superconducting properties had already been known shortly after the first observation of ET-based superconductors [145]. More especially, the discovery of the annealed phase at-(ET)2ls with a drastically increased Tc 8 K initiated interest in this phase [276]. In these investigations it was found that annealing at-(ET)2l3 above 70 °C for several days transformed this phase into a /Jn-related phase of (ET)2l3 (see Sect. 2.3) which is stable under ambient pressure and room temperature. 

The radical cation salts (BEDT-TTF)2l3 have drawn much attention, since one of them was found to be an organic superconductor. This series of salts can appear in many modifications, known as the a-, p-, 0-, K-phases. Under ambient pressure, the a-(BEDT-TTF)2l3 phase undergoes a metal-insulator phase transition at 135 K [1], while the p-, 0-, and K- (BEDT-TTF)2l3 become superconductors below -- 1.3 K, 3.6 K and 3.6 K, respectively [2-5]. After some particular pressure and temperature processing, the P-phase shows superconductivity at ambient pressure, up to a temperature as high as 8.1 K [6]. Unlike these phases, which are usually synthesized by an electrochemical method, two new phases, called p j - and -(BEDT-TTF)2l3, were synthesized recently by D. Zhu and co-workers by a diffusion method [7,8]. 

The term high Tc snperconductivity is unfortunate since the decisive difference is not the critical temperature, bnt the type of system where superconductivity is found, for example, in oxides. In this sense, the cuprates were not the first ones. Rednced SrTiOj was known long before 1986, although with a very low critical temperature. Johnston et al. discovered superconductivity in ternary titanium oxides where the oxidation state of Ti is below -1-4. Sleight et al. found superconductivity in BaBiOj, where some Pb + has replaced Bi ions. In both cases, the critical temperature was above 10 K. However, the highest measured critical temperatures are still in cuprates 134 K under ambient pressure and 164 K under high-pressure conditions. 

The molecule BEDT-TTF (bis-ethylenedithia tetrathiofulvalene Figure 18.11) with acceptor molecule Cu(NSC)2, was found to be conducting and even superconducting at a quite high critical temperature (Tc = 10.4 K) under ambient pressure. An insulator-metal transition temperature is observed at 135 K at ambient pressure. 

The superconducting transition temperatures of boron and selected borides are listed in Table 4.2-30. Boron itself, which is an isolator at ambient pressure, was recently found to become superconducting under high pressure [2.129]. The boron atoms in transition metal borides can form chains, nets, and three-dimensional networks. In this respect the borides differ markedly from other interstitial compounds such as carbides... 

Since then seven further superconducting (at low temperatures and/or under high pressures) dmit-based complexes with Ni or Pd as metal, synthesized via electrocrystallization, have become known. The donor-acceptor complex a-(EDT-TTF)[Ni(dmit)2]2 is the first ambient pressure superconductor with Tc = 1.3 K.257 ... 

Intriguing forms of competition between superconductivity and ferromagnetism have recently been reported for the elements carbon and iron, where the two cooperative phenomena are related to different crystallographic structures. As discussed in section 1.1 pristine C60 consisting of dominantly van der Waals bounded molecules becomes superconducting if doped by electrons or holes. On the other hand, under sufficiently high pressure and temperature, a layered rhombohedral structure of Cft) forms where, within the layers, the C60 molecules are covalently bound. This phase is metastable at room temperature and ambient pressure. It shows a spontaneous magnetization, which is assumed to be based on unpaired electrons created by structure defects (Makarova et al. 2001 Xu and Scuseria 1995). 

Some homoleptic unsymmetrical (dmit/mnt, dmit/tdas) dithiolene nickel complex-based D-A compounds with D = TTF and EDT-TTF also exhibit metal-like conductivity (see Table I) (101). Their molecular structure is shown in Scheme 3. The unsymmetrical tetraalkylammonium salts [MLjLJ- (M = Ni, Pd, Pt) have been prepared by ligand exchange reaction between tetraalkylammonium salts of MLj and ML21 (128, 129) and the D-A compounds have been synthesized by electrooxidation. Among these complexes, only the Ni derivatives exhibit metallic-like properties, namely, TTF[Ni(dmit)(mnt)] (metallic down to --30 K), a-EDT-TTF[Ni(dmit)(mnt)] (metallic down to 30 K), TTF[Ni(dmit)(tdas)] (metallic down to 4.2 K), and EDT-TTF[Ni(dmit)(tdas)] (metallic down to --50 K) (see Table I). The complex ot-EDT-TTF-[Ni(dmit)(mnt)J is isostructural (130) to a-EDT-TTF[Ni(dmit)2)] [ambient pressure superconductor, Section II.B.2 (124)]. Under pressure, conductivity measurements up to 18 kbar show a monotonous decrease of the resistivity but do not reveal any superconducting transition (101). 

In most cases, superconductivity in dmit-based systems requires application of pressure, which, however, does not mean that all of these systems are under a generalized curse. Indeed, a-(EDT-TTF)[Ni(dmit)2] undergoes a superconducting transition at ambient pressure. 

Figure 20 Superconducting transition of (TMTSF)2PF6 under 8 kbar and k - (ET)2Cu(SCN)2 at ambient pressure.

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