The "Bechgaard salts", principally the organic
first drew attention because of the discovery of the first evidence of
superconductivity in a synthetic metal based on organic molecules. This
discovery took place in 1979 in the laboratoire de Physique des Solides
d'Orsay [1,2], in Denis Jérome's
team (also see in this site the page
devoted to the "20th Anniversary of the Discovery of Organic Superconductivity").
Unfortunately, critical temperatures obtained in molecular compounds did not reach the expectancies... The record (in 2002) is still held by a material of the compounds based on the BEDT-TTF molecule. It does not exceed about 12 K (i.e., about -261°C). In the case of the TMTSF-based superconductors family, Tc is even more disappointing: about 1,2K...
Nature has not shown much generosity in terms of superconducting performances, but it has largely compensated by providing the (TMTSF)2ClO4 and (TMTSF)2PF6 organic superconductors with amazing magnetic properties in high magnetic field, which have been keeping busy many research teams in condensed matter across the world, for years, and still continue to raise enigmas that theoreticians have not completely solved in 2002. The inexhaustible richness of the quantised magnetic phases induced by the magnetic field once led Paul Chaikin to declare that (TMTSF)2ClO4 is the most amazing material Man ever created!
The following section draws up a summary of a few theoretical models that have been worked out between 1984 and 1993 to try explaining the richness in experimental behaviours evidenced in Orsay and elsewhere in the world (principally in the USA and Japan). More details may be found in this web site, in the publications of our team on this subject. Particularly, the reader may refer to the introduction chapter of the 1993 habilitation dissertation (Calorimetric studies of the multicritical behaviours of the spin-density-waves phases in a molecular compound).
b- On the molecular and electronic structures of Bechgaard salts
Organic conductors (TMTSF)2X
have been synthesised in Denmark in Klaus Bechgaard's team .
They are since called the "Bechgaard salts". The X anion may
be quite diverse, however the richest salts as far as physical properties
are concerned contain either perchlorate (ClO4),
or hexafluorophosphate (PF6). The base
molecule is tetramethyltetraselenafulvalenium (TMTSF) (Fig.
1), though another family also exists where the Selenium atoms are
substituted with Carbon atoms. Then the organic molecule becomes tetramethyltetrathiofulvalenium
(TMTTF). The material grows as thin needle-like crystals.
Figures 2 and 3, artist's side
and top views, display how the organic molecules zigzag stacked in the
a direction (period: a=0.73nm). They form conducting chains
by overlapping the pi double bonds between Carbon atoms (C=C).
The chains are juxtaposed along the transverse direction b (period:
b=0.78nm). Between the chain planes, in the c direction,
are intercalated anions planes (period: c=1.33nm). The structure
stoichiometry is 2:1. This means that there are twice more TMTSF molecules
than anions in this structure. The counter-ion plays the role of an electron
acceptor, during a charge transfer. Half an electron hole remains in each
organic molecule. So the electronic states band is half filled, which
confers a metallic character to these materials. They are organic conductors,
in some way "synthetic metals" .
Due to this particular topography, electrons can move freely along the chains, in the a direction, with a certain hopping probability. The latter is quantified by a longitudinal "transfer integral" ta (in the framework of a "tight binding" model). This probability is 10 times greater than the one for the electron to hop from a conducting chain to another, in the b direction (integral tb), and 100 times greater than the probability to hop from a chain plane to its neighbour, through the anion planes (integral tc) .
In truth, this particular electronic structure implies first that these materials are electrical conductors, however conduction is not the same in all space directions (conduction is said "anisotropic"). In fact, it is experimentally observed that electrical conductivity is 100 times better in the a direction than in the transverse direction b, and 10000 times than in the c direction! This is the reason why these organic metals are called "quasi-one-dimensional" (Q1D) .
A second consequence results from the instability of the one-dimensional electron gas. It may be graphically visualised by examining the shape of the electronic system "Fermi surface". This surface of the reciprocal space (the space of wave vectors k), consists of two sheets with two opposite values for the "Fermi vector": that is to say, at +kF and -kF (Fig. 4).
This topography results from the quasi-one-dimensional structure of these crystals, though three-dimensional (figures 2 et 3). The nesting wave vector Q=(2kF, p/b, p/c) relates electronic states located at the Fermi level. The two sheets are almost totally superposable. If the temperature is sufficiently low, electronic state at the Fermi level condense into electron-hole pairs, in a similar way as fermion condensation into Cooper pairs that cause the superconducting state to appear.
Such a system can thus transform its metallic fundamental state into two very different states as far as their physical properties are concerned, however very similar due to the "many-body" physical mechanism from which they both result. Superconductivity is well known for (among other properties) its extraordinary ability to drive an electrical current without any loss. On the contrary, electron-hole condensations lead to a family of insulating fundamental states, such as charge density waves (CDW), or spin density waves (SDW). The latter correspond to a magnetic fundamental state. This an antiferromagnetic state, the modulation of which having the nesting wave vector Q as period.
Such a magnetic state is known to be incompatible with a superconducting state, because the presence of magnetism weaken the Cooper pairs of superconductivity (by breaking the time-reversal symmetry). Still, these two kinds of fundamental states can be observed in the same organic compound, by varying an external physical parameter such as temperature, pressure, or magnetic field. It is striking to see how the phase diagram of Bechgaard salts look like the one of the superconducting copper oxides (the so-called "high critical temperature superconductors", discovered a few years after the organic superconductors ): in both systems, SDW phases are in the vicinity of a superconducting phase. In this web site, we are only interested in the SDW phases induced by the magnetic field in (TMTSF)2ClO4 (which is the subject of our nanocalorimetry studies).
A summarised description of the theoretical models of the FISDW phases will be found in the following section: Theoretical models of the quantised SDW phases.
Conducteurs organiques: "sels de Bechgaard"