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20 Organic Superconductors

Michael Lang Phys.-Institut der J.W. Goethe Univ. Frankfurt, Germany

Jens M¨uller Max–Planck–Institut f¨ur Chemische Physik fester Stoffe, Dresden, Germany

20.1 Introduction ............................................................................1155

20.2 Characteristics of Organic Charge-Transfer Conductors .....................................1157

20.2.1MolecularBuildingBlocks..........................................................1157

20.2.2StructuralAspects..................................................................1159

20.3 Normal-State Properties ..................................................................1162

20.3.1ElectronicStructure................................................................1162

20.3.2TransportandOpticalProperties....................................................1165

20.3.3ThermalandMagneticProperties....................................................1171

20.3.4AnionOrderingandGlassyPhenomena..............................................1175

20.3.5PhaseDiagrams....................................................................1179

20.4 Superconducting-State Properties .........................................................1182

20.4.1TheSuperconductingPhaseTransition...............................................1183

20.4.2SuperconductingParameters........................................................1189

20.4.3MixedState........................................................................1193

20.4.4Magnetic-Field-InducedSuperconductivity...........................................1197

20.4.5TheSuperconductingState:PairingMechanismandOrder-ParameterSymmetry .........1197

20.5 Epilogue ................................................................................1212

References..............................................................................1214

20.1 Introduction

Over the past 20 years the research on organic con-

ductors has developed into a most active branch of

modern condensed-matter physics. The main differ-

ence of molecular conductors compared to conven-

tional metals is that the former are made up of build-

ing blocks constructed from carbon atoms and their

combinations with other elements such as sulfur, se-

leniumor oxygen.As a result,when forminga crystal,

these molecular units preserve to a large extent their

specific features such as the molecular orbitals, the

ionization energy and the intramolecular vibrational

modes. Molecular materials thus have the potential

of providing a ﬂexible building-block system where

the physical properties can be tuned by small mod-

ifications in the arrangements and bridging of these

functionalunits. Incontrast to the molecular crystals

formed by weakly van der Waals-bond neutral enti-

ties,organic conductorsconsist of open-shell molec-

ular units which are the result of a partial oxidation

and reduction of the donor and acceptor molecules

in the crystal-growth process. It is the unpaired elec-

tron residing in the -molecular orbital (-hole) of

the donor unit whichis responsible for the electronic

properties of these charge-transfer salts. Due to a -

orbital overlap between adjacent molecules, the -

holes can delocalize throughout the crystal giving

rise to metallic conductivity.

1156 M. Lang and J. M¨uller

The unique chemistry of carbon, which, on the

one hand, provides a rich basis of potential organic

donor molecules, and the manifold possibilities of

combiningthem withcharge-compensatingacceptor

units, on the other hand, have enabled the synthe-

sisofanenormousnumberofconductingorganic

charge-transfer salts. The geometry of the build-

ing blocks and the way they are packed together

in the crystal determine the effective dimension-

ality of the electronic structure of the compound.

The planar shape of the TMTSF (tetramethyltetrase-

lenafulvalene) molecule, a derivative of the proto-

type TTF (tetrathiafulvalene), permits an infinite

stacking of these units in the crystal structure. As

a consequence, a significant intermolecular overlap

of the -orbitals occurs only along the stacking axis

giving rise to a quasi-one-dimensional conduction

band. In contrast, packing of the larger BEDT-TTF

(bisethylenedithio-tetrathiafulvalene) molecules of-

ten results in a quasi-two-dimensional electronic

structure.

These low-dimensional organic conductors con-

stitute a peculiar class of materials which has gained

strong interest among scientists due to the wealth

of interesting phenomena that have been observed

over the last two decades or so. Above all it was

the discovery of superconductivity in pressurized

(TMTSF)

by J´erome et al. in 1979 [1] which

created much excitement among both solid-state

chemists and physicists. Indeed, this finding was

a result of interdisciplinary efforts in synthesiz-

ing and characterizing organic conductors; activi-

ties which started up in the early 1950s by the pi-

oneering work of Akamatu et al. [2] and which re-

ceived a fresh impetus in 1964 thanks to the work of

Little [3]. He proposed that in a suitably designed

one-dimensional conductor embedded in a polar-

izable medium, superconductivity at high temper-

atures should be possible. Although it soon became

clear that the nature of the superconducting state

in the (TMTSF)

salt is of a fundamentally dif-

ferent type from what had been discussed in Little’s

model, this finding set the stage for the discovery of

a large number of conducting charge-transfer salts

including up to the latest count, more than 80 su-

perconductors.Many of these systems superconduct

even at ambient pressure with the highest T

value

of about 12 K received in the quasi-two-dimensional

BEDT-TTF-based systems. Besides superconductiv-

ity, the organic charge-transfer salts reveal a variety

of other interesting collective phenomena includ-

ing metal-to-insulator transitions, spin-Peierls and

density-wave states as well as phases with localized

charges and commensurate-type antiferromagnetic

order. These states have been found to depend most

sensitively on factors such as the acceptor ions, the

magnetic field or external pressure.

One of the key features underlying the above phe-

nomena is the low dimensionality of the materials.

The confinement of the carrier motion to one or

two spatial dimensions together with the low charge-

carrier concentration enhance the effect of the in-

teractions between the electrons. Another impor-

tant feature specific to these molecular systems is

a considerably strong coupling of the charge carri-

ers to the lattice degrees of freedom. For molecular

crystals, this coupling includes both the interactions

with high-frequency intramolecular modes and the

low-lying intermolecular vibrations.The present or-

ganic metals thus represent ideal model systems for

exploring the interplay of strong electron–electron

and electron–phonon interactions in reduced di-

mensions. In particular, the close proximity of su-

perconductivity to a magnetically-ordered state en-

countered not only in the organic materials but also

in other strongly-correlated electron systems such

as the heavy-fermion metals and high-temperature

superconductors continues to challenge our under-

standing of superconductivity. While for the latter

systems evidence is growing that, indeed, the mag-

netism and superconductivity are intimately related

to each other, the nature of the superconducting state

for the present materials is far from being under-

stood.

The discussion of the superconducting-state

properties for the present organic materials is com-

plicated by the existence of contradictory experi-

mental evidences. In some cases, this controversy

even encompasses results on the same quantitywhen

the measurements have been carried out by different

groups. While about one half of the data seem to

support an anisotropic superconducting state, with a

20 Organic Superconductors 1157

d-wave order parameter being the most favored one,

the other half are consistent with an order parameter

which is finite everywhere on the Fermi surface.

Closely related to the issue of superconductiv-

ity in the proximity to antiferromagnetic order is

the nature of the state above T

.Forthequasi-two-

dimensional organic conductors, for example, un-

usual metallic properties have been observed pos-

ing the question whether this phenomenon is related

to the equally unusual behavior seen in the other

class of quasi-two-dimensional superconductors,the

high-T

cuprates [4].

This article will give an overview on the normal-

state and superconducting-state properties of or-

ganic superconductors.There are a number ofreview

articles onthis subject [5–10],mostly focussing on ei-

ther the quasi-one-dimensional or two-dimensional

materials. The intention of the present review is

therefore to provide a discussion which covers as-

pects common to both families on the same footing.

Instead of reviewing the whole diversity of behav-

iors found among the various compounds with all

their structural and chemical modifications, we will

mainly focus on selected compounds of both fami-

lies. These are the most extensively studied and best

characterized (TMTSF)

X and (BEDT-TTF)

Xslats

whose properties are representative for a wide class

of materials. Special attention is paid to the more

recent developments including the controversial dis-

cussions of some aspects,in particular the discussion

on the nature and symmetry of the superconducting

state.

20.2 Characteristics of Organic

Charge-Transfer Conductors

20.2.1 Molecular Building Blocks

The prerequisites of forming conducting molecular

solids are essentially (i) the creationof unpairedelec-

trons and (ii) their delocalization throughout the

crystal. For the organic superconductors discussed

in this article, condition (i) is satisfied by a partial

transfer of charge between the two constituent parts

of a charge-transfer complex: an organic electron-

donor molecule D is combined with an,in most cases

inorganic, electron-acceptor complex X according to

the reaction [D

]+[X

] → [D

]

+ı

+[X

]

−ı

,where

m and n are integers. Since the -electrons of the

D molecule with orbits extending perpendicularly to

the planes of the molecules have low binding ener-

gies, much lower than those of the -electrons, can

easily be excited which can be seen in the charge-

transfer process. Accordingly, the so created charge

carriers have a -electron (hole) character. In most

cases, the negatively charged anions [X

]

−ı

adopt

a closed-shell configuration and thus do not con-

tribute to the electrical conductivity.When the crys-

tal is formed, a delocalization of the charge carri-

ers (ii) may be obtained by a dense packing of the

donor molecules. As a result, the -orbitals of the

partially filled outer molecular shells overlap and an

electronic band structure is formed.

The conductivity of solids depends on both the

number of free carriers and their mobility in the

crystal. In organic charge-transfer salts, the carrier

concentration is determined by the electronegativ-

ity of the donor, the electron affinity of the acceptor

molecule and the chemical bonds. A high mobility

is obtained by a relatively large bandwidth which is

the result of a considerably strong overlap of the -

orbitals from adjacent molecules.

Organic superconductors have been derived

from a variety of different organic electron-donor

molecules, where most of them are derivatives of

the archetype TTF molecule. Its combination with

the electron acceptor TCNQ (tetracyanoquinodi-

methane) in 1973 led to the synthesis of the first

quasi-one-dimensional (quasi-1D) organic conduc-

tor TTF-TCNQ [11, 12]. This material and its deri-

vatives served for a long time as model systems for

exploring the physical properties of quasi-1D con-

ductors.

In the vast majority of D

salts, the donor–

acceptor molecular ratio m: n isfixedto2:1,i.e.,two

donor molecules transfer one electron to the accep-

tor X. Organic and organometallic chemistry have

provided an enormous number of donor molecules

which may serve as building blocks for organic con-

TMTTF stands for tetramethyltetrathiafulvalene.

1158 M. Lang and J. M¨uller

Fig. 20.1. Principal structures of donor

molecules that furnish organic super-

conductors

ductors and superconductors. The main examples

are the TTF derivatives TMTSF, TMTTF

, BEDT-TTF

(commonly abbreviated as ET) and BEDT-TSF

(or

simply BETS), see Fig. 20.1.The TMTSF differs from

TMTTF in that the four sulfur atoms ofthe latter have

been replaced by selenium. In the same way, BEDT-

TSF is derived from BEDT-TTF. It is interesting to

note that organic superconductors have been synthe-

sized not only by using these symmetric molecules,

but also on the basis of asymmetric complexes.These

are the DMET (dimethylethylenedithio-diselen-

edithiafulvalene) [13,14],which is a hybrid ofTMTSF

and BEDT-TTF, MDT-TTF (methylenedithio-TTF)

[13, 14] and BEDO-TTF (bisethylenedithioxy-TTF)

[15], see [5] for an overview.

TheTMTSF molecule providesthebasisfor the so-

called Bechgaard salts (TMTSF)

X whichform with a

variety of inorganic monovalent acceptor molecules

X. Indeed, it was (TMTSF)

where in 1979 super-

conductivity had been observed for the first time

in an organic compound [1]. At ambient-pressure

conditions, the material was found to undergo a

metal-to-insulator transition around 12 K which had

been identified as a spin-density-wave (SDW) order-

ing [6]. By the application of hydrostatic pressure of

12 kbar the SDW instability can be suppressed and

superconductivity forms below T

=0.9 K [1, 16].

AreplacementofPF

by ClO

has resulted in the

first and – till now – only member of the quasi-

1D salts which becomes superconducting at ambient

pressure [17].

Apparently, the strong tendency of Bechgaard’s

salts to undergo a metal–insulator transition, inher-

ent to quasi-1D electron systems, counteracts with

their ability to become superconducting. In order to

achieve ambient-pressuresuperconductivityin these

organic complexes with possibly even higher transi-

tion temperatures it is necessary to increase their

dimensionality. According to this strategy, in 1982

the first metallic compound based on the new elec-

tron donormoleculeBEDT-TTF [18],cf.Fig.20.1,was

synthesized by Saito et al. [19]. The underlying idea

was to enhance the overlap between -orbitals of

adjacent molecules by enlarging the -electron sys-

tem on each molecule. This has been accomplished

by adding rings of carbon and sulfur atoms at the

outer ends of the TTF skeleton. In contrast to the

Bechgaard salts where the donor molecules form in-

finite stacks, steric effects specific to the BEDT-TTF

molecules prevent such an infinite face-to-face stack-

ing in the (BEDT-TTF)

X salts. As a consequence,

the side-by-side overlap between -orbitals of adja-

cent molecules becomes stronger and, in some cases,

comparable to the face-to-face interaction resulting

in a quasi-2Delectronic structureof the BEDT-TTF-

based salts. The combination of BEDT-TTF with the

monovalent anion X = Cu(NCS)

achieved in 1988

led to the discovery of the first ambient-pressure su-

perconductor in this class of materials, the second

generation of organic superconductors, with transi-

tion temperatures in the range of 10 K [20].

The quasi-1D and quasi-2D systems tend to grow

in needle-like and plate-like shapes, with the latter

usually showing well developed smooth and shiny

metallic surfaces.The synthesis of organicmolecular

conductors can be divided into two steps:(1) the syn-

thesis of the neutral donor and acceptor molecules

and (2) the oxidation and reduction of the donors

BEDT-TSF stands for bisethylenedithio-tetraselenafulvalene.

20 Organic Superconductors 1159

and acceptors to radical cations and radical anions.

For step (1) several independent methods have been

discussed and used even for selected donor and ac-

ceptormolecules,seee.g.[18].Conversely,toperform

step (2), the redox process, the electrocrystallization

technique has proved to be the method of choice for

synthesizing high-quality crystals. For details on the

chemical synthesis and crystal growth techniques,

the reader is referred to [5,18,21] and references cited

therein.

20.2.2 Structural Aspects

Figure 20.2 shows exemplarily the crystal structure

of (TMTSF)

viewed almost perpendicular to the

stacking axis (a-axis); the direction of highest con-

ductivity.All members of the (TM)

Xfamily,where

TM stands for TMTSF and TMTTF,are isostructural

with triclinic symmetry. The conducting stacks are

separated by the anions which are located at in-

version centers of the lattice. The donor molecules

which are nearly planar and almost perpendicular to

the chain axis are arranged in a zigzag-type manner

alongthe chains with two slightlydifferentintrachain

distances corresponding to a weak dimerization.The

fairly close Se ···Se distance of 3.87 Å along the b-

axis being smaller than the sum of the van der Waals

radii of 4 Å results in a weak interchain overlap [6]

and thus a weakly 2D electronic character.An impor-

tant structural aspect of the (TM)

Xcompoundsis

related to the symmetry of the anion X. Where these

anions are centrosymmetric such as the octahedral

X=PF

,AsF

,SbF

or TaF

complexes, their orien-

tation is fixed in the structure. In contrast, (TM)

salts formed with non-centrosymmetric tetrahedral

anions such as ClO

or FeO

undergo a structural

Fig. 20.2. Crystal structure of (TMTSF)

(side-viewed

tilted).The axis with the highest conductivity is the vertical



-axis,the one with the lowest conductivity the horizontal

c-axis. a



is the projection of the a-axis onto the direction

perpendicular to the bc-plane

transition from a disordered high-temperature state

to an ordered low-temperature phase [22].

While all members of the (TM)

Xfamilysharethe

same crystal structure, the rather loose intra-stack

coupling ofthe (BEDT-TTF)

Xsaltsgivesrisetoava-

riety of polymorphic phases (packing motifs) which

are distinguished by Greek characters; the most im-

portant amongst them are the ˛, ˇ,and-phases,

see e.g. [23] for a comprehensive review of the struc-

tural properties of these salts. In some cases, as for

instance realized in the compound with the linear

anion X = I

, various structural modifications ex-

ist even for the same anion, cf. Fig. 20.3. The ˛-type

Fig. 20.3. Schematic packing motifs of

the ET molecules in the ˛, ˇ and -

phases of (ET)

viewed along the long

axis of the ET molecules