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Chapter Five Provided for non-commercial research and educational use only. Not for reproduction, distribution or commercial use. This chapter was originally published in the book Superconductivity in New Materials. The copy attached is provided by Elsevier for the author’s benefit and for the benefit of the author’s institution, for non-commercial research, and educational use. This includes without limitation use in instruction at your institution, distribution to specific colleagues, and providing a copy to your institution’s administrator. All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institution’s website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier's permissions site at: http://www.elsevier.com/locate/permissionusematerial From Denis Jérome, Organic Superconductivity: A Mouse may be of Service to a Lion. In: Z. Fisk and H. R. Ott editors, Superconductivity in New Materials. Elsevier 2011, p. 149. ISBN: 978-0-444-53425-5 © Copyright 2011 Elsevier B.V. Elsevier. Author's personal copy CHAPTER FIVE Organic Superconductivity: A Mouse may be of Service to a Lion Denis Jérome Laboratoire de Physique des Solides, UMR 8502, Universit�e Paris-Sud, 91405 Orsay, France Contents 1. Introduction 150 2. Superconductivity in Metals and Alloys (1911–1973) 151 2.1 The early days 151 2.2 Looking for high Tc 152 3. Organic Crystals Becoming Conducting (1974–1979) 154 3.1 1D charge transfer compounds 154 3.2 Peierls transition and Fröhlich conductivity 155 4. Organic Superconductivity and Phase Diagrams (1980–2004) 159 4.1 (TMTSF)2PF6: The prototype 1D superconductor 159 4.2 The ID generic phase diagram for (TM)2X materials 161 4.3 Transverse transport and deconfinement 164 4.4 A textbook example for the Wigner crystal, quarter-filled compounds 168 5. Organic Superconductivity and Recent Developments, (2005→2010) 169 5.1 Experimental investigations of the SC phase 170 5.1.1 The SC phase from transport measurements 170 5.1.2 Evidence for SC from magnetic data 171 5.1.3 The SC phase from calorimetric measurements 174 5.2 The SC phase diagram under pressure 174 5.2.1 The SDW-SC coexistence regime 174 5.2.2 The SC phase under pressure 178 5.3 Nature of SC 179 5.3.1 Role of nonmagnetic defects on SC 179 5.3.2 Spin part of the SC pairing 183 5.3.3 Beyond Pauli limit in low dimensional superconductors 184 5.4 The metallic phase of the (TM)2X diagram, control parameters of Q1D organic superconductivity 185 5.4.1 Contribution of transport toward the investigation of the metallic phase 185 Superconductivity in New Materials, Volume 04 Ó 2011 Elsevier B. V. ISSN 1572-0934, DOI 10.1016/S1572-0934(11)04005-4 All rights reserved. 149 Author's personal copy 150 Denis Jérome 5.4.2 Longitudinal transport 188 5.4.3 Transverse transport 194 6. Discussion 198 6.1 A charge pseudo-gap? 198 6.2 Antiferromagnetic fluctuations 199 6.3 What about paraconductive fluctuations? 201 6.4 Some theory 202 7. Conclusion 207 Acknowledgments 210 References 210 1. INTRODUCTION Superconductivity is a domain of condensed matter physics and materials science nearly 100 years old and yet still keeping busy an impress­ ive number of physicists and materials scientists. It is interesting to notice that two major achievements in superconductivity have been obtained by chance as this is often inherent to scientific discoveries. The first one is the discovery of the initial phenomenon of supercon­ ductivity by Gilles Holst in 1911 at Leiden by chance when he was involved in the measurement of the resistance of high purity metals at low tempera­ ture in the laboratory of Heike Kamerlingh Onnes who had succeeded 3 years before in liquifying helium. The second one is the discovery of superconductivity above 30 K by G. Bednorz and A. Müller in 1986 in copper oxides, also an unexpected discovery since oxides were studied at that time for their insulating and magnetic properties rather than for their conductivity. However, it is the prospect of using superconductivity in power trans­ mission, computer circuits, small magnetic fields detectors, etc., which maintained a very strong pressure on both theoreticians and materials scientists keeping in mind the synthesis of new conducting compounds in which superconductivity could be stabilized as close as possible to room temperature for an easier and cheaper application of the phenomenon (Fig. 5.1). With this idea in mind an international conference on Organic Super­ conductors was organized by W. A. Little at Hawaii in 1969, i.e., 10 years before the discovery of this phenomenon in organic conductors and at a time where conducting organic crystals were still unheard of. It is fair to recognize that the interest manifested by the whole physics community after the meeting has stimulated thinking and the research of new materials. Author's personal copy Organic Superconductivity 151 Hg(1223) T1BaSrCuO 100 YBaCuO Sm(OF)FeAs Cs3C60 LaBa(Sr)CuO SrFe2As2 Rb3C60 La(OF)FeAs Nb3Ge Nb Sn PuCoGa NbN 3 5 Ba(FeCo) As K3C60 2 2 Chevrel κ(ET) Cu(N(CN) )Cl (K) NbC V Si 2 2 c 3 BaPbBiO T 10 Pb κ(ET) Cu(SCN) β (ET) I 2 2 Nb H 2 3 β(ET) AuI Hg 2 2 LaOFeP β (ET)IBr2 In UPd Al (TMTSF)2ClO4 2 3 (TMTSF)2PF6 1 CeCu2Si2 1900 1920 1940 1960 1980 2000 2020 Year Figure 5.1 The figure displays the evolution of Tc in materials according to the date of the discovery of their superconducting properties. It clearly shows a renewed effort in superconductivity at the beginning of the eighties. 2. SUPERCONDUCTIVITY IN METALS AND ALLOYS (1911–1973) 2.1 The early days Superconductivity (a new state of matter) is one of the major discoveries of the twentieth century in physics. From an experimental point of view the finding by G. Holst in the laboratory of H. Kamerlingh Onnes [1] in 1911 of a current traveling without resistance through a metal cooled at very low temperature paved the way to a very large number of industrial applications. And this, long before the proposal of a satisfactory theoretical framework by Bardeen, Cooper, and Schrieffer (BCS) [2] in 1957 explaining both the zero resistance state and the magnetic flux expulsion (the Meissner effect). Furthermore, the BCS theory has also been the paradigm of modern physical theories based on the importance of the quantum nature at low temperature since it relies on the establishment of a new long-range ordered state described by the BCS wave function. BCS emphasized the existence of Author's personal copy 152 Denis Jérome a two-body attractive interaction between charge carriers (either electrons or holes) being a prerequisite for a Bose condensation of electron pairs into the superconducting state. This net attractive coupling in spite of the Coulomb repulsion between carriers of the same sign relies closely on an attraction between electrons mediated by their interaction with excitations of the lattice, namely, the phonons. Consequently, a major achievement of the BCS theory has been under­ standing the ionic mass dependence of the critical temperature, i.e., −1/2 (Tc α M ) also called the isotope effect. Although Fröhlich had proposed in 1954 a model for superconductivity based on the involvement of the lattice [3], this theory was unable to account for the superconductivity of metals but turned out to be relevant subsequently for the interpretation of some of the transport properties of 1D organic compounds to be encoun­ tered later in this presentation, vide infra. 2.2 Looking for high Tc At the beginning of the 60s, searching for new materials exhibiting the highest possible values of superconducting Tc was already a strong motiva­ tion in materials sciences and the term high temperature superconductor was already commonly used referring to the intermetallic compounds of the A15 structure, namely, (Nb3Sn or V3Si) [4]. The hidden 1D nature of the A15 structure provides an enhancement of the density of states at the Fermi level lying close to the van-Hove singularity of the density of states at the band edges of the 1D d-band. Within the BCS formalism large Tc could be expected. They were actually observed (17–23 K) but an upper limit was found to increase Tc since the large value of N(EF) also makes the structure unstable against a cubic to tetragonal Jahn–Teller band distortion [5, 6]. The theory [7] showed that Tc is maximized in the compounds Nb3Sn and V3Si. In this context at the beginning of the 80s some papers based on metallur­ gical considerations regarded 25–30 K as the highest possible value for Tc [8]. However, at that time, attempts to increase Tc were still based on the phonon-mediated BCS theory and its strong coupling extension [9, 10]. Expending the successful idea of the isotope effect of the BCS theory other models were proposed in which excitations of the lattice responsible for the electron pairing had been replaced by higher energy excitations, namely, electronic excitations, with the hope of new materials with Tc higher than those explained by the BCS theory. Consequently, the small electronic mass me of the polarizable medium would lead to an Author's personal copy Organic Superconductivity 153 1/2 enhancement of Tc of the order of (M/me) times the value which is observed in a conventional superconductor, admittedly a huge factor. V. L. Ginzburg [11, 12] considered in 1964 the possibility for the paring of electrons in metal layers sandwiched between polarizable dielectrics through virtual excitations at high energy.
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