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First, organic conductors are described, then the structures and electronic properties of organic superconductors are discussed, illustrated with examples of typical compounds. The book deals in detail with theories of the mechanism of superconductivity, and more briefly with spin-density waves.
The design, principle, and synthesis of organic superconductors are also described.
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With increasing the distance between the ET dimers in Fig.
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According to this line of thought, higher T c is expected for the salt having alarger anion spacing. Topological structures of their Fermi surfaces studied by SdH, dHvA, and AMRO Table 2 [19,20,53,70,71,85—87] show that the area of the closed Fermi surface relative to the first Brillouin zone and cyclotron mass calculated from SdH oscillations are Fermi surface of 2 Fig. Upper critical magnetic field H c 2 : 2gave higher H c2 values for the magnetic field parallel to the two-dimensional plane than H Pauli based on a simple BCS model[ 88 , 89 ].
STM on 3also showed the same symmetry[ 92 ].
A recent specific heat measuremen on 2 and 3 was consistent with these results [ 93 ]. Inverse isotope effect: Inverse isotope effect has so far been observed for 1 [ 50 ],2[ 54 ], and 4[ 58 ], while normal isotope effect for3[ 56 ]. The reason of the observed isotope effects is not fully understood yet consistently. This tendency has been observed under hydrostatic pressure but not under uniaxial pressure see Sections, , Thus the phase diagram in Fig. It has long been predicted that the geometrical spin frustration of antiferromagnets caused by the spin correlation in particular spin geometry triangle, tetrahedral, Kagome Scheme 5 , etc.
So the spins of Ising system with AF interaction in the equilateral triangle spin lattice will not show any long-range order even at 0 K, and hence the phase, namely quantum spin liquid phase, has high degeneracy [ 97 ]. Such spin liquid state has only been predicted theoretically [ 98 ], and a variety of ideal materials have been designed and examined for long [99—].
Some inorganic materials reported as spin liquid candidates were eliminated owing to the spin ordering at extremely low temperatures, etc [—]. In both cases, a superconducting statereadily appeared nearly above 0.
Organic superconductors--new benchmarks.
It is very noteworthy that the spin liquid phase is neighboring to the superconducting state and its T c is fairly anisotropic as shown in Fig. T IM , which is a Mott insulator-metal transition temperature, indicates thatin comparison with the hydrostatic pressure results, the uniaxial method afforded: 1 a much higher T c value, 2 an increase of T c at the initial pressure region, 3 an anisotropic pressure dependence, and 4 superconducting phase remains at higher pressure.
TIM: a Mott insulator-metal transition temperature. The formation of the superlattice was suppressed by the pressure above 0. Under hydrostatic pressure it became two-dimensional metal down to low temperatures 2 GPa , however, it became superconductor under the uniaxial pressure along the a -axis 0.
Uniaxial strain increased T c anisotropically 6 K at 0. For the FeCl 4 salt, a relaxor ferroelectric behavior in the metallic state below 70 K[ ] and a firm nonlinear electrical transport associated with the negative resistance effect in the magnetic ordered state have been observed[ ]. Moreover it has been found that the FeCl 4 salt shows the field-induced superconducting transition under a magnetic field of 18—41 T applied exactly parallel to the conducting layers[ ]. Similar phenomena, namely AF, ferromagnetic, or field-induced superconductivity, have been observed in several inorganic solids such as Chevrel phase[ ] and heavy-fermion system[ ].
Numbers in bracket are the total members of each superconductor and the highest Tc.
In , superconducting phase wasobservedbelow 19 K for the potassium-doped compounds prepared by a vapor-solid reaction [ ],immediately after the isolation of macroscopic quantities of C 60 solid [ ]. Powder X-ray diffraction profile revealed that the composition of the superconducting phase is K 3 C 60 and the diffraction pattern can be indexed to be a face-centered cubic fcc structure [ ]. The superconductivity has been observed for many M 3 C 60 M: alkali metal , e. The T c varies monotonously with lattice constant, independently of the type of the alkali dopant Fig.
Keeping the C 60 valence invariant —3 , the intercalation of NH 3 molecules e.
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Changing the valence in cubic system also has a pronounced effect on T c. Tc as a function of volume occupied per C— in cubic M3C60 M: alkali metal. In , A15 or body-centered cubic bcc Cs 3 C 60 phase, which shows the bulk superconductivity under an applied hydrostatic pressure, was obtained together with a small amount of by-products of body-centered orthorhombic bco and fcc phases, by a solution process in liquid methylamine [ ]. Interestingly, the lattice contraction with respect to pressure results in the increase in T c up to around 0.
The trend in the initial pressure range is not explicable within the simple BCS theory. Very recently, it has been found that the fcc phase also shows an AF ordering at 2. We note that T c of the both phases follows the universal relationship for M 3 C 60 superconductors in the vicinity of the Mott boundary, as seen in Fig.
Doping of alkalimetals in picene Scheme 8 , that has a wider band gap 3. Considering the fact that Ca 1. At present, although the crystal structures of thedoped compounds are unclear, the refined lattice parametersare indicative of the deformation of the herringbone structureof pristine picene and the intercalation of dopants withinthe two-dimensional picene layers.
Among them, phenanthrene-based superconductors shows an enhancement of T c with increasing pressure, which is indicative of the non-BCS behavior. So far 8 aromatic hydrocarbon superconductors were prepared with the highest T c of 33 K at AP. In , first superconducting carbon nanotubes were discovered for ropes of single-walled carbon nanotubes SWNTs with diameters of the order of 1. The drop in magnetic susceptibility is more gradual than expected for three-dimensional superconductors, and superconducting gap estimated from the I - V plot shows the temperature dependency characteristic of one-dimensional fluctuations.
It is apparent that the isolation of carbon nanotubes from each other is responsible for the realization of the almost ideal one-dimensional system. We note that this superconducting system is classified into single component superconductors, contrary to the C 60 and graphite vide infra based superconductors. In , these efforts culminated in the discovery of CaC 6 with T c as high as In other alkaline-earth metal doped GICs, the superconducting phase was observed below 1. The apparent reduction of T c strongly suggests that the interlayer states of graphite have an impact on the electronic state of GIC, which was supported by theoretical calculations [ ].
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A typical sp 3 covalent system,diamond, is an electrical insulator with a wide band gap of 5. Doped boron introduces an acceptor level with a hole bindingenergy of 0. The T c varies between 1 and 10 K with the doping level[ ]. Besides numerous studies on multi-component CT solids, several single-component organic conductors have been developed.
Even though pentacene is known to be the first organic metal semimetal showing a decrease of resistivity down to ca. Electric conductivity increases by the enhancement of intermolecular interactions by appropriate use of hetero-atomic contacts. Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.
Materials Viewpoint of Organic Superconductors | Journal of the Physical Society of Japan
Help us write another book on this subject and reach those readers. Login to your personal dashboard for more detailed statistics on your publications. Edited by Alexander Gabovich. Alves, B. We are IntechOpen, the world's leading publisher of Open Access books. Built by scientists, for scientists. Our readership spans scientists, professors, researchers, librarians, and students, as well as business professionals. Downloaded: Introduction Since the first observation of superconductivity by K. Structures and properties 3. Superconductors based on electron donors 3.
Table 1. Table 2.
Polyaromatic hydrocarbon superconductors Doping of alkalimetals in picene Scheme 8 , that has a wider band gap 3. Carbon nanotubes, graphite, and diamond superconductors In , first superconducting carbon nanotubes were discovered for ropes of single-walled carbon nanotubes SWNTs with diameters of the order of 1. More Print chapter. How to cite and reference Link to this chapter Copy to clipboard. Available from:.