Robust superconducting state in the low-quasiparticle-density organic metals β^{″}-(BEDT-TTF) _ {4}[(H_ {3} O) M (C_ {2} O_ {4}) _ {3}]∙ Y: superconductivity due to … (original) (raw)
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Physical Review B, 2005
We report magnetotransport measurements on the quasi-two-dimensional charge-transfer salts Љ-͑BEDT-TTF͒ 4 ͓͑H 3 O͒M͑C 2 O 4 ͒ 3 ͔Y, with Y =C 6 H 5 NO 2 and C 6 H 5 CN using magnetic fields of up to 45 T and temperatures down to 0.5 K. A surprisingly robust superconducting state with an in-plane upper critical field B c2ʈ Ϸ 33 T, comparable to the highest critical field of any BEDT-TTF superconductor, and critical temperature T c Ϸ 7 K is observed when M = Ga and Y =C 6 H 5 NO 2. The presence of magnetic M ions reduces the in-plane upper critical field to Ϸ18 T for M = Cr and Y =C 6 H 5 NO 2 and M = Fe and Y =C 6 H 5 CN. Prominent Shubnikov-de Haas oscillations are observed at low temperatures and high magnetic fields, showing that the superconducting salts possess Fermi surfaces with one or two small quasi-two-dimensional pockets, their total area comprising Շ6% of the room-temperature Brillouin zone; the quasiparticle effective masses were found to be enhanced when the ion M was magnetic ͑Fe or Cr͒. The low effective masses and quasiparticle densities, and the systematic variation of the properties of the Љ-͑BEDT-TTF͒ 4 ͓͑H 3 O͒M͑C 2 O 4 ͒ 3 ͔Y salts with unit-cell volume points to the possibility of a superconducting groundstate with a charge-fluctuation-mediated superconductivity mechanism such as that proposed by Merino and McKenzie ͓Phys. Rev. Lett. 87, 237002 ͑2001͔͒, rather than the spin-fluctuation mechanism appropriate for the-͑BEDT-TTF͒ 2 X salts.
Superconducting fluctuations in organic molecular metals enhanced by Mott criticality
Scientific Reports, 2013
Unconventional superconductivity typically occurs in materials in which a small change of a parameter such as bandwidth or doping leads to antiferromagnetic or Mott insulating phases. As such competing phases are approached, the properties of the superconductor often become increasingly exotic. For example, in organic superconductors and underdoped high-T c cuprate superconductors a fluctuating superconducting state persists to temperatures significantly above T c . By studying alloys of quasi-two-dimensional organic molecular metals in the k-(BEDT-TTF) 2 X family, we reveal how the Nernst effect, a sensitive probe of superconducting phase fluctuations, evolves in the regime of extreme Mott criticality. We find strong evidence that, as the phase diagram is traversed through superconductivity towards the Mott state, the temperature scale for superconducting fluctuations increases dramatically, eventually approaching the temperature at which quasiparticles become identifiable at all. I ntriguing unconventional superconducting states, including those occuring in the high-T c cuprates, pnictides, heavy Fermion superconductors and quasi-two-dimensional (Q2D) organic molecular superconductors, often compete with Coulomb-interaction driven antiferromagnetic or Mott insulating states 1,2 . In many cases, the parameter controlling the competition between the magnetic insulating and superconducting states is band filling; for example, high-T c superconductivity occurs when the Mott insulating parent compounds such as La 2 CuO 4 are doped with acceptors such as Sr on the La sites 1,2 . In the case of organic molecular superconductors, however, the first order Mott-superconductivity phase boundary is traversed by tuning the ratio, t/U, of the electronic bandwidth to the strength of Coulomb repulsions 3 . In the classic case of the Q2D k-(BEDT-TTF) 2 X family of organic molecular metals, t/U can be adjusted by varying the lattice spacing (and therefore the intermolecular hopping energy scale), either continuously by applying hydrostatic pressure to a Mott-insulating parent compound such as X 5 Cu[N(CN) 2 ]Cl, or sampling discretely by varying the anion volume. [In fact, in k-(BEDT-TTF) 2 X, there is a degree of frustration determined by a second hopping energy scale, t9, with important consequences for proximity to and stability of the Mott state 4-6 . However, in the following discussion, the key physics is encapsulated in the ratio t/U.] The extraordinary degree of control over the fundamental competing energy scales makes k-(BEDT-TTF) 2 X a particularly appealing experimental environment for studying strong correlations in Q2D 7 .
Some electronic properties of the organic superconductor β - (BEDT-TTF )2I3
Journal de Physique, 1989
2014 Nous avons mesuré de 300 K à 4,2 K la magnétorésistance, la susceptibilité et l'anisotropie magnétique de monocristaux du supraconducteur organique bi-dimensionnel 03B2L-(BEDT-TTF)2I3. A partir de la magnétorésistance suivant c*, nous avons calculé une valeur de 0,5 meV pour l'intégrale de transfert tc dans un modèle de liaisons fortes. De plus, ces mesures indiquent que l'anisotropie de conductivité dans le plan a-b à basse température est d'un facteur cinq. La faible valeur du temps de relaxation (0,5 ps à 9 K) indique que le transport électronique entre plans devient incohérent, ce qui peut affecter la température de transition supraconductrice. La densité d'états dérivée de la susceptibilité de spin est en bon accord avec celle obtenue par chaleur spécifique. Lorsque le champ magnétique est dirigé perpendiculairement aux plans conducteurs, l'anisotropie magnétique provient en partie d'un diamagnétisme des électrons de conduction qui varie avec la température.
Some electronic properties of the organic superconductor &# 946;-(BEDT-TTF) 2I3
2011
2014 Nous avons mesuré de 300 K à 4,2 K la magnétorésistance, la susceptibilité et l'anisotropie magnétique de monocristaux du supraconducteur organique bi-dimensionnel 03B2L-(BEDT-TTF)2I3. A partir de la magnétorésistance suivant c*, nous avons calculé une valeur de 0,5 meV pour l'intégrale de transfert tc dans un modèle de liaisons fortes. De plus, ces mesures indiquent que l'anisotropie de conductivité dans le plan a-b à basse température est d'un facteur cinq. La faible valeur du temps de relaxation (0,5 ps à 9 K) indique que le transport électronique entre plans devient incohérent, ce qui peut affecter la température de transition supraconductrice. La densité d'états dérivée de la susceptibilité de spin est en bon accord avec celle obtenue par chaleur spécifique. Lorsque le champ magnétique est dirigé perpendiculairement aux plans conducteurs, l'anisotropie magnétique provient en partie d'un diamagnétisme des électrons de conduction qui varie avec la température.
Contrasted structural properties of organic superconductors
Synthetic Metals, 1987
We present structural studies of organic conductors and superconductors of the (TMTSF)2X and (BEDT-TTF)2X families. These materials undergo a large variety of structural phase transitions like ordering of non-centrosymmetric anions Xin the (TMTSF)2X salts and commensurate ((BEDT-TTF)2Re04) or incommensurate (8(BEDT-TTF)213, (TMTSF)2SCN) lattice modulations. We shall pay special attention on new results concerning the pressure induced anion-ordering transformation in (TMTSF)2ReO 4 and the two competing phase transitions of (TMTSF)2PF202.
Artificial control of intermolecular distance of organic superconductors: α-type BEDT–TTF compounds
Current Applied Physics, 2001
We developed the uniaxial strain method to arti®cially control the electronic properties of organic conductors by reducing the intermolecular distance along a desired direction without changing those along others. Using this method, we were able to induce and enhance the superconductivity in two-dimensional (2D) organic conductors, a-BEDT±TTF 2 KHgSCN 4 and its iso-structural compound having NH 4 instead of K, respectively. We found that these two compounds show essentially the same properties if their lattice parameters are reduced along appropriate directions by the uniaxial strain method, while they show apparently dierent properties under ambient and hydrostatic pressures.
Correlations, dimensionality and instabilities in organic superconductors
Physica B: Condensed Matter, 1995
We discuss the role of Coulombic repulsion in organic superconductors exhibiting quasi-one-dimensional transport properties. A recent investigation in high magnetic fields shows that the charge localization occurring at low temperature in a 1-D half-fillled band is suppressed by the 1-D to 2-D (3-D) cross-over in selenium compounds. However, a localization around 30 K in the non-ordered phase can be reactivated by the application of a high transverse magnetic field. This phenomenon provides an interpretation for the large transverse magnetoresistance observed in quasi-l-D conductors with open Fermi surfaces. The intermediate (strong) coupling limit is valid for the spin degree of freedom whereas the charge is governed by the strength of the Umklapp scattering varying by a large factor from sulfur to selenium compounds and under pressure.
Microscopic Theory of Organic Superconductors
Journal of Superconductivity, 2000
A microscopic theory of organic superconductors based on the concept of partial electron dielectrization is developed from first principles. Self-consistent equations for the superconducting order parameter (⌬) and insulating order parameter (D) are derived using a Green's function technique and equation of motion method. The theory is applied to explain the experimental results in the two-dimensional organic superconductor k-(BEDT-TTF) 2 Cu(NCS) 2 . The present model explains coexistence of spin density wave (SDW) state and superconductivity state in the system. The behavior of superconducting order parameter (⌬), insulating order parameter (D), specific heat, density of state, free energy, and critical field is also studied for the system k-(BEDT-TTF) 2 Cu(NCS) 2 .
Physical Review B, 2004
We give a theoretical analysis of published experimental studies of the effects of impurities and disorder on the superconducting transition temperature, Tc, of the organic molecular crystals κ-(BEDT-TTF)2X (where X=Cu[N(CN)2]Br and Cu(NCS)2 and BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene) and β-(BEDT-TTF)2X (for X=I3 and IBr2). The Abrikosov-Gorkov (AG) formula describes the suppression of Tc both by magnetic impurities in singlet superconductors, including s-wave superconductors and by non-magnetic impurities in a non-s-wave superconductor. We show that various sources of disorder (alloying anions, fast electron irradiation, disorder accidentally produced during fabrication and cooling rate induced disorder) lead to the suppression of Tc as described by the AG formula. This is confirmed by the excellent fit to the data, the fact that these materials are in the clean limit and the excellent agreement between the value of the interlayer hopping integral, t ⊥ , calculated from this fit and the value of t ⊥ found from angular-dependant magnetoresistance and quantum oscillation experiments. There are only two scenarios consistent with the current state of experimental knowledge. If the disorder induced by all of the four methods considered in this paper is, as seems most likely, non-magnetic then the pairing state cannot be s-wave. We show that published measurements of the cooling rate dependence of the magnetisation are inconsistent with paramagnetic impurities. Triplet pairing is ruled out by NMR and upper critical field experiments. Thus if the disorder is non-magnetic then this implies that l ≥ 2, in which case Occam's razor suggests that d-wave pairing is realised in both β-(BEDT-TTF)2X and κ-(BEDT-TTF)2X. However, particularly given the proximity of these materials to an antiferromagnetic Mott transition, it is possible that the disorder leads to the formation of local magnetic moments via some novel mechanism. Thus we conclude that either β-(BEDT-TTF)2X and κ-(BEDT-TTF)2X are d-wave superconductors or else they display a novel mechanism for the formation of localised moments, possibly related to the competition between the antiferromagnetic and superconducting grounds states. We suggest systematic experiments to differentiate between these two scenarios.