organic superconductivity

Physica 109 & 1 lOB (1982) 1447-14@) 1447 North-Holland Publishing Company

O R G A N I C S U P E R C O N D U C T I V I T Y

D. J E R O M E *

Laboratoire de Physique des Solides, Universit~ Paris-Sud, 91405 Orsay, France

Superconductivity is observed at low temperature in the conducting organic salts of the cation radicals tetramethyltetraselenafulvalene and tetramethyltetrathiofulvalene, (TMTSF)2X and (TMT/T)2X, with Tc ~< 1.2 K and 3 K, respectively, where X is an inorganic anion. The superconducting properties are strongly influenced by the one-dimensional transport properties of these conductors.

The development of short-range superconducting order at temperatures much larger than the critical temperature manifests itself by a significant pseudo-gap in the quasi-particle density of states observed by electron tunneling spectroscopy up to 15 K. Moreover, paraconduction is the dominant conduction channel up to about 40 K.

Furthermore, preliminary junctions data indicate that the stabilization of superconduction at higher temperatures (i.e. 12 K in (TMTSF)2PF6 under I 1 kbar and possibly 26 K in (TMTSF)2CIO4 under ambient pressure) is likely to arise from chain cross-linking.

The (T, P) phase diagram of most organic superconductors displaying a phase transition line between a superconducting and a magnetic state, and the 1-D electron-gas theories suggest a dominant role of Coulomb interactions in the mechanism of Cooper pairs formation.

T h e d i scovery of supe rconduc t iv i ty in conden-

sed m a t t e r m a d e of o rganic molecu les is the

o u t c o m e of a conce r t ed work in i t i a ted by a sug-

ges t ion of L o n d o n [1] in 1950. Acco rd ing to

L o n d o n supe rconduc t iv i ty need not be l imi ted to

the class of inorgan ic conduc to r s but could

poss ib ly p lay an i m p o r t a n t ro le in cer ta in large

organic molecu les of b iochemica l in teres t .

Fo l lowing B C S ' s successful t heo ry of super-

conduc t iv i ty in meta l s based on the cohe ren t

mo t ion of e l ec t ron pai rs he ld t o g e t h e r by a

p h o n o n - m e d i a t e d a t t r ac t ive in te rac t ion [2], Li t t le

[3] p r o p o s e d an ex tens ion of that s ame pa i r ing

idea to e l ec t rons moving in m e d i a of high elec-

t r ical po la r izab i l i ty .

In the m o d e l of Li t t le , a large effect ive a t t rac-

t ion b e t w e e n the e l ec t rons moving a long an un-

s a tu r a t ed ca rbon chain is p r o v i d e d by the v i r tual

osc i l la t ion of charges on s ide -molecu les a t t a c h e d

to the chain. M o r e o v e r , this m o d e l of super-

conduc t iv i ty with organic molecu les sugges ted

the poss ib i l i ty of cr i t ical t e m p e r a t u r e s as large as

* Work supported in part by DGRST, contract no. 80.7.0165, CNRS-ATP program 1981-1982 and a NATO grant 171.80.

a mb ie n t t e m p e r a t u r e and m a r k e d t he r e fo re the

s tar t ing poin t of n u m e r o u s r e sea rches in the

field of conduc t ing organic mo lecu la r solids.

It was not unti l the ear ly sevent ies that organic

conduc to r s a p p e a r e d in the guise of radical -

ca t ion salts [4] and charge t ransfe r complexes

[5]; both sys tems conta in ing open shell p l ana r

molecu les be tw e e n which large i n t e r m o l e c u l a r

7r-wave funct ion over l aps are r e spons ib le for an

e lec t ron de loca l iza t ion . H o w e v e r , the s t acked

conf igura t ion of the p l ana r molecu les in the

crystals confines the mo t ion of the car r ie rs to a

single d i rec t ion in space since the ~--electrons '

in te rac t ion is weak in the d i rec t ions p e r p e n d i -

cu lar to the s tacking axis. It a p p e a r e d t he r e fo re

that the concep ts of "phys ics in one d i m e n s i o n "

f i t ted the desc r ip t ion of the e lec t ronic p r o p e r t i e s

of such unusua l ly an i so t rop ic conduc to r s [6].

A t an ear ly s tage Byschkov, G o r k o v and

Dzya losh insk i i [7] no t i ced that co r re la t ion func-

t ions of a di f ferent na tu r e b e c o m e equa l ly

d ive rgen t at low t e m p e r a t u r e for a o n e - d i m e n -

s ional ( I - D ) e lec t ron gas. M o r e specifically, they

showed that w h e n e v e r the e l e c t r o n - e l e c t r o n in-

t e rac t ion was a p p r o x i m a t e d by two coupl ing

0378-4363/82/0000--0000/$02.75 © 1982 N o r t h - H o l l a n d

ldd-S D. Jerome / Organic superconducticity

constants g~ and g, for momentum transfer 2k~ and I). respectively, both the Peierls (electron- hole) and the Cooper (electron-electron) pair- ings were equally divergent in the logarithmic

approximation, and led to a phase transition at finite temperature . The degeneracy between

superconducting and insulating ground states is however lifted by a more elaborate t reatment of

the 1-D problem. In the first-order renor- realization theory [8], superconductivity is the

most divergent instability at low tempera ture if g~>2ge: the triplet superconducting response function being more strongly divergent than the singlet response, for g~ > 0 . When g~ < 2g> spin density wave (SDW) and charge density wave (CDW) responses both diverge for g~ >(), whereas the C D W state is the only divergent state at g~ < O.

Despite the absence of long-range order development at low tempera ture in 1-D conductors, it is possible for large size correlations to build up and to trigger the occurrence of a phase transition in real materials, this being due to various sources of interchain coupling.

In an at tempt to understand the phase tran- sitions of quasi-one-dimensional (Q- l -D) conductors, two scales of temperatures are im-

portant. First, the tempera ture T~ at which three- dimensional long-range order occurs and where a phase transition is detected as in 3-D materials.

Secondly, the tempera ture Ti, such as Tt > T~, which characterizes the growth of the expectation value of the order parameter without any interehain coherence. As far as superconductivity is concerned the dimensionality of the order paramete r is two (amplitude and phase). The treatment of the superconducting phase transition of a Q - I - D conductor is thus equivalent to the ordering of a Heisenberg two-dimensional ferromagnet and the transition tempera ture is given by [9]

W~ ~- (al ia±) 1/2 , (1)

where o~l! and a j are respectively the intra and

interchain superconducting coupling strengths. ~ is related to the temperature T~ (or the amplitude of the pseudo-gap at the Fermi level of the quasi-particle density of states) and ~ is

given by the interchain Joscphson coupling t ~ / t

(11 and t being respectively the intra and interchain overlap integrals). A weak interchain

coupling can lead to a sizeable depression of the transition temperature by phase-fluctuations of the order parameter below T~.

In 1079, Bechgaard et al. [101 found that the

family of Q - I - D conducting salts based on the tetramethyltetraselenafulvalene (TMTSF) molecule, namely (TMTSF)eX with X ( N O , PF,,.

AsF,,, SbF, . . . . ) exhibits unusually large values of the conductivity at low temperature. Around 15 K, conductivities of the order of l() ~ (_Qcm) have been reported [10] for (TMTSF)ePF,, or (TMTSF)eNO~. The increase of the conductivity

at low tempera ture is however interrupted by the occurrence of a sharp transition towards an insulating ground state. As far as (TMTSF),PF,, is concerned the metal- insulator transition occurs at TM-I ~- 12 K under ambient pressure. A somewhat similar behaviour with high conductivity has been observed earlier m a charge transfer compound containing the TMTSF

molecule. Under a pressure in excess of Ill kbar [I 1], the conductivity of T M T S F - D M T C N Q (di- methyl TCNQ) reaches -~10s(,Qcm) i a t helium

temperature. Moreover, the low resistance is extraordinarily sensitive to the presence of defects, since k p ~3(t(lO/,t.(2cm per % of impurities [12] and to the application of a magnetic field perpendicular of the stacking axis [11], fig. 1,

As an interpretation for the unusual transport properties of T M T S F - D M T C N Q . the author suggested [11] the existence of a gradual growth below 30K of a fluctuating superconducting pairing between +kv and - k v electron states.

The determination by X-ray diffuse scattering experiments [13] of a band filling which is the lowest ever observed in highly conducting charge transfer complexes, namely a one-quarter filled

D. Jerome / Organic superconductivity 1449

0.50

~ 0.2 0 o

rY \ 0.1 l-.-.

n,," 0.05

T M TpS[1; DbMTeCNQ /

. . . . . . . . . . Ht~- 75 k ~ /

/ i

i

.H =0

10

q

o

i

50 o

i - -

"6

0.02 ...... . - 200 I I

5 10 510 100 3 0 0 Temperature (K)

Fig. 1. Tempera ture dependence of the longitudinal resistivity of T M T S F - D M T C N Q at 13 kbar without applied magnetic field and under a magnetic field of 75 kOe applied along the transverse direction.

band under ambient pressure, stimulated the elaboration of a series of organic conductors (TMTSF)2X in which the charge transfer is fixed ( p = 0 . 5 ) by the stoichiometry of the salt. (TMTSF)2X salts are cation-radical salts of TMTSF with inorganic anions X = N O 3 , BF4, PF6, AsF6, SbF~, TaF6, C104, ReO4, etc. in which a positive charge is shared between two organic molecules. The three-dimensional structure con- sists of a triclinic columnar arrangement of TMTSF molecules in the form of sheets of donor stacks separated by anion sheets [14], fig. 2. Along the stacking direction the unit cell con- tains two TMTSF molecules. The two-dimensional side view of the structure, as shown in fig. 2, could imply that the potential created on the TMTSF molecules by the anions on the left and on the right of a given stack cancel each other. However, a consideration of the full three- dimensional triclinic structure indicates that no such cancellation occurs, and therefore the periodicity of the potential seen by the delo-

calized charges on the TMTSF stack is fixed by the distance between the anions, namely a. Consequently, the conduction band becomes half-filled (instead of one-quarter filled as suggested by the stoichiometry and fig. 2. The existence of a weak dimerization of the average intermolecular distance is a sign of the a lattice periodicity along the stacking direction.

At room temperature, under ambient pressure, the dimerization amounts to 0.08% and 0.027% in (TMTSF)2PF6 and (TMTSF)2CIO4, respectively [14, 15]. It is however interesting to notice that the sulfur analogue series of the (TMTSF)2X family exhibits a dimerization which, crudely speaking, is one order of magnitude larger, since it amounts to 0.6% at 300K in (TMT-FF)zBr [16] (the less dimerized salt of the

sulfur series). The (TMTSF)2X family exhibits a wide spread

of electronic properties, with the perrhenate (ReO4) and the tetrafluoroborate (BF4) becoming insulating at 180K and 40K, respectively [10, 17,18], the hexafluorate salts (PF6, AsF6, TaF6 . . . ) undergoing a metal to insulator transition at low temperature [10, 19] (TM-I 12 K) and the perchlorate (C104) remaining in a highly conducting state, with o-> 105(g2cm) -1 down to helium temperature [20].

The salient feature of the conductivity of a salt such as (TMTSF)2PF6 remaining highly conducting below 20 K is the discrepancy existing between the temperature dependence of the d.c. conductivity, increasing by x l00 between ambient and 20 K, and the optical conductivity increasing by no more than a factor 3.5 in the same temperature interval [21].

Contrasting with all other studied charge transfer Q-1-D molecular conductors, X-ray diffuse scattering investigations have failed to detect any appreciable precursor scattering with wave vector 2kv at T larger than TM-I [22] as expected if the insulating state observed below TM4 in (TMTSF)2PF6 were of a Peierls nature [23]. Instead, several magnetic measurements performed on the insulating state of (TMTSF)2

1451) D. Jerome / Organic superconductivity

C,~.~ ~'-.,

/ / " "X )

H3C S S I CH3 H3C Se Se CH 3 H3C~ S S I'~CH3 H3C ~Se'~xSe I~CH3

THTTF THTSF

Fig. 2. Two molecules, TMTSF a ,d TMTTF. leading to superconducting cation-radical salts. A side view of the (TMTSF):PF,. crystalline structure. The high conduction direction is along the stacking axis.

PF,~ or AsF~ have provided some clear-cut information as to the driving force of the insulating state in these salts. The 77Se (I = ~) NMR line of (TMTSF)2PF~ broadens inhomogeneously at 12K, and becomes too broad to be detected below 11 K [24]. A similar line broadening is also observed for the methyl-group protons [24, 25]. Since both sets of nuclei have spin ~, NMR shows the existence of strong local magnetic fields below TM-I (fig. 3).

The ESR line disappears abruptly in the insulating state below TMq [26], whereas hardly any change of the powder susceptibility is noticed with a Farady susceptibility technique

I271.

5

~2

oJ b

Z

f

b . . . . . .

'5e ,: ~, = 4.5 MHz)

/

- p ~ " - - - 4 -

-emperafure k: i

Fig. 3. 7~Se NMR signal amplitude vanishing below 12 K at the onset of the SDW state in (TMTSF)2PF6. The dashed curve shows the Curie lay,' temperature dependence which is expected in the absence of magnetic ordering.


These results point out that unlike "ITF-TCNQ and similar compounds exhibiting a Peirls insulating ground state, magnetism remains present in the insulating state of (TMTSF)2PF6, (AsF6).

Moreover, single crystal susceptibility measurements [28] have revealed a behaviour of the susceptibility of the insulating state analo- gous to that of an antiferromagnet. Below TM-I, the easy axis of the magnetization [29] lies close to the b*-axis of the crystal structure, whereas the a and c*-axes are the intermediate and hard axes, respectively. Field dependence studies of the magnetization show the existence of a spin- flop transition along b* exceeding ~-4.5 kOe (fig. 4).

The possibility for the occurrence of magnetic ordering, i.e. spin density waves (SDW) in low- dimensional conductors, has been suggested by

E

'~o

>-

_.J

g _z a. ct}

(TMTSF) 2 AsF 6

H=3KGouss

HIIc* ~ ' v \

v~vxv\

Htla o v-. . . . . . . . o "O\o % ~ @ ~ ~ o o

0

0

0.2

,"TO --001

?(£)

HIIb ~

\ \a

N, N

0.3 / / o

H~Pb" n o °~° I i 0 I 0 20 30

T E M P E R A T U R E (10 . 4 e m u / m o l e l

Fig. 4. Spin susceptibility of (TMTSF)2AsF6 as deduced from static measurements with field parallel to a, b* and c*. The magnetic field H = 3 kOe is less than the critical spin-flop field. (After ref. 29 and K. Mortensen, private communication.)

Slater [30]. Quite generally the stabilization of a SDW state implies the predominance of the repulsive electron-electron over the electron- phonon interactions. Therefore, in a SDW state the exchange part of the electron potential cou- ples the states at + k F and --kv and opens an exchange gap at the Fermi level which trans- forms the Q-1-D conductor at high temperature into an insulator below TM-I.

At T < TMq, on the one hand SDW modes couple to the nuclear spins enhancing the nuclear spin lattice relaxation [24], while on the other hand they shift the ESR far away from the nearly free ESR line position observed in the conducting regime [31]. An activation energy of ~30 K is derived from the temperature dependence of the susceptibility with Hllb* in (TMTSF)2AsF6 [29], in fair agreement with the band gap of ~66 K observed in electron tunneling experiments with (TMTSF)zPF6 under ambient pressure [32]. No such activated regime is however detected in the temperature dependence of the conductivity at low temperature in the absence of a magnetic field [33]. An increase of the helium temperature conductivity at high frequency above ~109Hz has been interpreted in terms of the response of a collective pinned SDW mode to a high- frequency excitation [34].

The (TMTSF)2C104 salt is the only salt of the (TMTSF)2X family in which an insulating state does not take place at low temperature under ambient pressure. The resistance of (TMTSF)2CIO4 exhibits a continuous fall by about two orders of magnitude between room temperature and 5 K [20]. Below 5 K a slight upturn of the resistance is however observed in good-quality (low-resistance) crystals [19]. The resistance at 1.2 K can become two or four times larger than the value at 5 K. It is the result of high pressure work which stimulated to a large extent the interest in conducting charge transfer compounds. As far as TYF-TCNQ is concerned, high pressure has revealed commensurability effects in the vicinity of 19kbar [35] when the wave length of the CDW becomes commensurate with

14. = D. Jerotn e / Organic superconductivity

the underlying lattice with concomitant con-

sequences on the pinning of the conductivity of

Fr6hlich fluctuating origin [35]. In the

(TMTSF)2X family, high pressure suppresses the insulating state, fig. 5. Above 6 kbar or so. the

SDW state of (TMTSF)2PF6 or (TMTSF)eAsF~ is

very quickly removed by the hydrostatic pres-

sure. The plot of the logarithm of the transition

versus the applied pressure shows (fig. 6) that a

law such as

TSDW ~ A exp ( k / P , ~ - P

would apply (with Pc = 11.5 kbar) between 1 atm.

and 8 kbar. Above 8.5 kbar the ground state of

(TMTSF)2PF6 is superconducting [36]. The resis-

tivity under 12 kbar drops to zero below 0.9 K (fig.

7), whereas above 0.9 K the constant rate increase

observed as from helium temperature is in striking

contrast with the behaviour of the resistance

limited by impurities and lattice defects in ordinary

metals. In the pressure regime where SDW and

superconductivity coexist with a common phase

transition line, the phase diagram is rather com-

plex [37]. In fact, a detailed pressure investigation

performed on the (TMTSF)2AsF6 phase diagram shows that superconductivity is re-entrant under

- 16

• 8 7

4

• 12 K Zero plessure ~ • • q ,q • • • 12 k bar pressu re

o

T c 1 0 210 30 40

TEMPERATURE IKI

Fig. 5. Temperature dependence of the resistance of (TMTSF)zPF6 under ambient pressure (showing a metal semiconductor transition at 12K) and under 12kbar with a highly conducting state stable down to very low temperature.

" ,ITMTSF/ PF

i ~il D Superconductor I

1 ~nsu la t0 r

0.5 •.

3L -. ~,upe>-,snuucfoP "a-

0.1 0 a (:, is 30 2~ 28

[:[ ~SSd[ e / { K L,:2r !

Fig. 6. The phase diagram (temperature versus pressure) of (TMTSF):PF6. The various points on the superconducting transition line come from the data obtained at Orsay A, Bell I,abs. • and IBM O.

the SDW state at low temperature in that pressure

regime [38].

As shown in fig. 6, the critical temperature for

superconduction is strongly pressure dependent: a l n T J O P ~ - 7 % kbar ~ at P - l l kba r [39].

However. the pressure dependences of the band

parameters cannot explain this large pressure

coefficient. The ambient temperature Seebeck

coefficient which does not vary significantly be-

tween 1 atm and 12 kbar and the measurement of

the 300 K conductivity anisotropy tr,/o'l ~ ( t i t , ) :

which has been successful without sample break-

ing in (TMTSF)2NOs up to 12 kbar [40] show that neither t41 nor t± increase by more than 10% or so

under a pressure of 12 kbar.

The superconducting state of (TMTSF)2X salts

is characterized by a diamagnetic state [411 and magnetic flux expulsion [42], with a field dependence of the magnetization which reveals a type- II behaviour.

Magnetization curves in very low fields indicate that field penetration into the superconducting state occurs already at a very low amplitude of the perpendicular applied field. In (TMTSF)2C104, the only salt of the (TMTSF)zX


0.75

0.5

0.2!

R(T)/R(L,.6 K) ,/,

./" ( TMTSF )2 PF6

, / i

51

¢/3 I - -

z D 5

r f i--

tY <

5

4.9

48

5 10 T ( K )

~""Z N'OBIUM I----

),--__

' ' ' I ' ' ' ' I ' ' ' '

Tempero fu re ( K )

t _ i - - ~ 47 2 3 /. 0.1

(TMTSF) 2 P~

0.5 1 T (K)

I00

50 z

< t ~

¢Y , <

150

100

Fig. 7. Supraconductive transition in (TMTSF)2PF6 under a pressure of 12 kbar, detected by resistance measurements (left) and a.c.-susceptibility with a comparison with a Niobium marker sample.

series in which the superconducting ground state is stable under ambient pressure, the lower critical field Hc~ is likely to be smaller than 50 mGs at 50m K [43], fig. 8. Easy field penetration is however expected for loosely coupled chains. The upper critical field He2 is measured by the field at which the resistivity recovers its value just above To. The angular dependence reveals a very pronounced anisotropy of the critical field in the plane perpendicular to the stacking axis. For example, in (TMTSF)2AsF6 the ratio HD*//411c* is equal to 15 ± 2 under 9.5 and 11 kbar

c 2 / ~ c 2

[38], fig. 9. This anisotropy of Hc~ reveals a band parameter anisotropy along the transverse direc- tions. Within the framework of the Landau- Ginzburg theory for the superconducting state below T~ with open Fermi surfaces, the anisotropy of the coherence length and band parameter are equal [44]. Therefore, as far as (TMTSF)2AsF6 is concerned t~*/t~" = 1 5 - 2 . The accurate measure of the anisotropy between the stacking axis direction and the transverse direc-

tions is hard to reach via critical fields experiments, since it requires an alignment of the sample with the external field which is very difficult to achieve with small crystals. Optical reflectance experiments have been probably more reliable. The anisotropy of the plasma edge leads to t~/t~" = 30 in (TMTSF)2PF6 at 25 K under ambient pressure [21]. Consequently, the band parameters of the (TMTSF)2PF6 and (TMTSF)2AsF6 structures are within the ratios tf : t~*: t~* = 1, 1/30, 1/450, respectively, with an anisotropy independent of pressure [40].

Superconductivity has been found in several other members of the (TMTSF)2X family, namely X = TaF6, SbF6, ReO4 [19, 18]. In ad- dition, recent experiments show that superconductivity can be found in the sulfur analogue series, (TMTI'F)2X, with X = Br [45].

At 22 kbar, (TMTYF)2Br undergoes a metal to insulator transition at 5 K, with a resistance ratio R(300 K)/R(5 K ) = 16, but below 25 kbar a transition towards a superconducting state is obser-

1454 D. Jerome / Organic superconductiuitv

S E IW

! i

( T M T S e F ) 2 QO~

[

- . 5_._0 m~

f ~ o

,500.__inK

• ------------- .600m K

~ - , ~ 7 0 0 m K __

~ _ . 8 0 O r a l "

p ' ~ ' ~ . ~ 2 o o _ _ ~

~ ° ~ ° ~ 1 K

0 5 1 0 "15 H ( O e )

Fig. 8. Field dependence of the magnetization in (TMTSF)2CIO4 at various temperatures as measured by a SQUID technique (after ref. 43). The dashed line represents the variation of the diamagnetic shielding versus magnetic field at T = 0.2 K.

ved at 3 K , as shown by a sharp drop of the

resis tance and the effect of the magnet ic field, fig.

10. Therefore , superconduct iv i ty of organic

molecules is not restr icted to a single family of

molecules con ta in ing se len ium he te roa toms. It is

likely to be a very genera l proper ty which can be

observed in most 2 to 1 cat ion radical salts. The

superconduct iv i ty of organic conductors has also

been s tudied via tunne l ing spectroscopy, a tech-

n ique which has been widely used to derive

quant i t ies such as the gap in the quasi-part ic le

densi ty of states in metall ic superconductors .

As far as (TMTSF)2PF6 is concerned the

Schottky barr ier t echn ique has been used with

some success [46]. Schottky junc t ions are pre-

200(

1500

I

1000

500

\ \

+

L

\

Sample I " . H//c

TMTSF) 2 As F 6

P= 11 kbo r

SampLe 2

H fib

÷ +

+

- , , \

• . . " \ . . . . i . . . . L • 2 - e 4 - . , . , , ,

0.5 Tc/ (K) 1 15

20

15

10c~

u

big. 9. Temperature dependence of the He: critical lield of (TMTSF)2AsF,, tinder 1i kbar derived from resistive measurements (after ref. 3S) with H along b* or c':.

pared by evapora t ing te l lur ium doped GaSb

( N d > 3 x l0 lscm 3) on a natura l face of a

(TMTSF)2PF~ single crystal.

After pressurizat ion and cooling, the differen-

tial resis tance of the junc t ion is s tudied versus

0 10 20 30 l,O 50 0.55 . . . . " ~" ' ' /

1 y - . , , , j j 0.7

[ / H = O , , , ~ ,

0 50 100 150 200 250 300 Temperal'ure ( K )

Fig. 10. Superconductivity in (TMTTF)zBr under a pressure of 25 kbar. The magnetic field is applied along the transverse direction. The finite resistance in the superconducting state is due to non-superconducting breaks in the sample (after ref. 45).


the bias voltage. The large resistance maximum at zero bias, together with the two minima observed on each sides of the characteristics, point out the existence of a gap in the quasi- particle density of states, fig. 11. The recording on fig. 11 has been performed under 11 kbar, at T = 50inK. Following the traditional interpretation of the tunneling data [47] the pairing energy 2A in fig. 11 amounts to 3.6 mV (40 K), i.e. about 40 times the critical temperature for (TMTSF)zPF6 at 11 kbar.

At higher temperatures, namely T > T3, the amplitude of the zero bias resistance maximum decreases steadily, but remains clearly visible up to about 15 K. A large magnetic field applied in a direction perpendicular to the stacking axis removes the large zero-bias resistance peak, fig. 12.

Results such as those displayed in fig. 1 l have

50-

Silver daq

' ~ . ~ n,'Ga. S ~ . 40

I .¢III I /11111/~, ,E ( TSF 2-PF ~ 1 1 1 1 1 1 1 1 1 1 1 i

Gold wire 30

\

10

I I -20 -10

dVldl /(kIl)

P : 11 k b a r

T : 0.05 K

V l(mVl I I t -

10 20

Fig. 11. The tunneling characteristics of the (TMTSF)2PF6- N/GaSb Schottky barrier at T = 50 m K under P = 11 kbar.

.5,

(Rmax / Rmln)- 1

1

\ "4,

\ \

I 5

(RmaxlRmin) -1

% \

\ \

%x

T :50 ink

"e ,..

lal

T /(K) I !

10 15

Ib)

H /(k E~) I I I ~_ 8 16 24

Fig, 12. Tempera ture dependence of the zero-bias tunnel resistance with a modulat ion current of 10 -7 A and magnetic field dependence at T = 50 m K with a modulat ion current of 5 x 10-9 A.

been interpreted by the opening of a gap in the quasi-particle density of states due to the formation of superconducting pairs in (TMTSF)2PF6. Electron tunneling data suggest that the gap of 2A = 40 K is not related to the establishment of a 3-D superconducting state below T3, but rather to the onset of very large intrachain pairing up to 7"l.

Since between Tz and T3 only the expectation value of the amplitude of the order parameter is large but the phase itself is still fluctuating, it is probably more appropriate to speak of a pseudo- gap of amplitude 2A rather than of a real gap [48].

Schottky junctions performed with (TMTSF)2C104 [49] exhibit a d V/dl characteristic with a pseudo-gap 2A ~ 4 mV (45 K).

1456 D. Jerome / Organic superconductieitv

In a large number of junctions the tunneling data display a behaviour different from that shown in fig. I1. Instead of a zero-bias resistance maximum with two resistance minima separated by 2A, typical of electron tunneling between a superconductor and a conductor in the normal state, i.e. S - I - N tunneling, a minimum of resistance at zero bias is observed quite often with two sets of resistance anomalies separated by 2,1 and 4,1 [50]. This is illustrated by the state 2 of a junction in fig. 13. State 2 can be understood in terms of a junction exhibiting S-I S quasi-par-

ticle tunneling between two superconducting domains of the organic superconductor,

separated by a weak insulating barrier. The resistance minimum at V = 0 shows that a superconducting current is able to flow between two superconducting domains. The application of a magnetic field of a few kOe suppresses the superconducting current and decreases the amplitude of the anomalies at +2,1.

In one junction of relatively low resistance

I S-I {tiinLS

I . . . . . . . l" " ~ 1 (fhick) S

0 A 2Z~

T=~,2K ~2Ad

(TMTSF)2PF 6 P = 11 Kbar

~ ° . . m V

S T A T E 1

.2A~

4 4 -g-2

~ _ _ _ _ T : I [JK

L mV

STATE

Fig. 13. The two states observed with (TMTSF)~PFe,-GaSb junctions. State 1 corresponds to the tunneling of electrons through a S-1-N contact. The distance 2A between resistance minima provides approximately the amplitude of the pseudo- gap of the I-D fluctuating superconductor. State 2 shows resistance extrema at +,.1 and ±2A corresponding to S-I -N and S--I.-S tunneling occurring in parallel at the junction. The large resistance dip at zero bias is the sign of a Josephson current flowing through the junction. This resistance (tip is suppressed by the application of a magnetic field.

( ~ 1 0 ~ ) at high temperature , the zero voltage contact resistance dropped to zero below 12 K, fig. 14. This result seems to indicate that a superconducting short circuit sets in at the bar-

rier. As a matter of fact, the strong tendency of (TMTSF)ePF<, towards superconductivity can be stabilized by allowing electron pairs to jump

from one chain to its neighbour through local interchain bridges.

The theory of the superconductivity in I-D conductors in the presence of randomly dis-

tributed interchain cross-links [5l] shows that a

concentration of 4% per molecule of cross-links

(with t~ (local)-41) is large enough to raise T~, given by eq. (1), up to the neighbourhood of T~.

The temperature of 12K, below which the (TMTSF)2PF<~-GaSb contact becomes superconducting, is very close to the 1-D mean-field temperature derived from the weak coupling limit BCS relation, namely 2A/kT<,-- 3.5. This agreement is probably not very significant, hut order of magnitude wise these tunneling data

show that the onset of superconducting pairing along each chain occurs in a temperature domain

which is at least ten times larger than the ten> perature where 3-D ordering takes place in the bulk of a crystal.

Cross-linking can be achieved by the diffusion of foreign atoms (for example antimony) between organic stacks at the interface between the

Si- #(TMTSF}?PF s -QQSb contcl[t , / P = llkb(~F

0 50 100 150

Fig. 14. The zero-bias resistance of a (TMTSF)zPF~,-GaSb contact (resistance at the centre of the clip in the state 2 of fig. 13) going to zero below 12K which is interpreted by the three-dimensional stabilization of a superconducting state via chain cross-linking in the vicinity of the barrier.


(TMTSF)2PF6 crystal and the evaporated Ga-Sb layer [50]. Point contact junctions with (TMTSF)2CIO4 display similar effects [49]. The large zero-bias resistance dip is strongly affected by microwave irradiation, as shown in fig. 15, at T = 3.9 K, i.e. a temperature which is three times bigger than the critical temperature of the bulk of sample. Data in fig. 15 provide serious in- dications in favour of the existence of a Joseph- son effect. Several studies show that (TMTSF)2CIO4 junction characteristics are dras- tically affected by radio-frequency or microwave

irradiations up to about 26K or so [49]. Moreover, a TI 'F -TCNQ test junction with similar characteristics does not exhibit any sen- sitivity to a microwave irradiation up to a power level as large as 10 mW, at temperatures between 53 and 35 K.

T=3.9K '~ I \\P=l'uW

I i i . . . . . ~ i

o

T~_ //~ jdv P= o ~W b) d-T P= 50 HW

T = 25K [ / ~ j P : l O 0 ~W

i J ~ i i

L.'O - 20 -12 L, V

i i i i i i )

L, 12 2C ;O (mvl

Fig. 15. Differential resistance of a (TMTSF)2CIO4-chrysocal point contact junction located in an EPR microwave cavity (u = 9.6 GHz), irradiation at 3.9 K (a) and at 25 K (b).

Thermal conductivity experiments performed on (TMTSF)2PF6 at 12kbar [52] show a significant drop in the conductance, beginning around 40 K. The electronic contribution to the thermal conductivity predominates over the phonon contribution since the Wiedeman-Franz law is fairly well obeyed, both or and K increasing with the same rate under pressure at high temperature. The behaviour of the thermal conductivity at low temperature is thus an additional argument in favour of a pseudo-gap opening below 40 K. In the presence of a gap (or pseudo- gap) at the Fermi level, the density of particles able to contribute to the conduction is likely to decrease with decreasing temperatures. This is, however, in striking contrast with the huge enhancement of the conduction observed experimentally at low temperature either in (TMTSF)2PF6 under pressure [44] or even in (TMTSF)2CIO4 at ambient pressure [20]. As was suggested following the study of TMTSF- DMTCNQ, the formation of fluctuating pairs can contribute to the conduction even at temperatures much larger than 7"3 in Q-1-D conductors [11, 44].

The excess conduction (or paraconduction) is proportional to ([AI2), i.e. it becomes negligible above Tc in most 3-D superconductors. But the problem is quite different whenever the volume of a fluctuating domain is reduced by a factor (d,/~:02, as in the 1-D fluctuation regime (~:± < d±) of a Q-1-D superconductor [44].

The paraconductivity of a Q-1-D superconductor worked out by Schulz [53] gives the following result:

rre- ~: ~0~e 3/2 °'i = ~ lit ) , (2)

where e = ( T - T1)IT1 and ~ll(O) is the zero temperature Landau-Ginzburg coherence length. Besides supercurrents along the chains in the fluctuating regime, transverse supercurrents are likely to occur between adjacent chains with well-developed order parameters. As the likeli-

1458 D. Jerome / Organic superconductivity

hood of this occurrence decreases with increasing temperature , o'" decreases in tempera ture faster than o- I, i.e. or" ~ e s/:. Consequently, the aniso-

tropy of paraconductivity varies linearly with tempera ture above TI. With scll(0)/d~ ~> 50, o- I can easily exceed 104(,Ocm) I at temperatures as large as 25 K.

Following the previous estimate, the paraconductivity is largely dominant at helium tem-

perature and consequently is likely to be suppressed by (i) the application of a magnetic field along a transverse direction, and (ii) a low concentration of magnetic defects created by irradiation [12].

In these concluding paragraphs we summarize the most significant experimental results of organic superconductivity and at tempt to suggest some guidelines towards an interpretation of the phenomena.

The properties of the superconductivity which occurs in the conducting cation-radical salts

(TMTSF)2X and (TMTI'F)2X is strongly influenced by low dimensionality. In this respect the tunneling data have answered crucial key questions. Short-range superconducting pairing takes place along the chains (without interchain coherence) at temperatures as high as 15 K. The impact of this short-range pairing manifests itself on the quasi-particle density of states by the

observation of a pseudo-gap at the Fermi level below 15 K or so in (TMTSF)2PF6 under pressure. Moreover, the ratio 2A/kT~ of about 40 for (TMTSF)2PF,~ under 11 kbar and (TMTSF)2CIO4 under ambient pressure indicates that the actual critical tempera ture T~, below which both a zero resistance state and magnetic flux expulsion are observed, is not determined solely by the strength of the superconducting pairing within each chain. The superconducting phase transition occurs when coherence is established between the phases of the order parameters on adjacent chains. As far as superconductivity is concerned, the transverse coup- lings which matter most are on the one hand the so-called transverse tunneling coupling t~ which

allows an electron to travel from one chain to its neighbour, and on the other hand the Josephson

coupling t2/tll which can transfer an electron pair between the chains,

Admittedly the transverse coupling ill the (TMTSF)2X structure is stronger along the b :~ direction which corresponds to short Se-Se interchain distances than along c*. However. the anisotropy of the plasma frequency edge between the a and b* axes leads to a longitudinal coupling t~ about 30 times bigger than t~* [21].

Several experiments have shown that one may be able to take advantage of the existence of strong superconducting fluctuations at high temperature . The observation of a contact be-

tween (TMTSF)2PF~ and an evaporated Ga-Sb lilm becoming superconducting below 12 K has

been interpreted as a local increase of the 3-D ordering tempera ture at the interface through randomly dittused interchain cross-links. The response of (TMTSF)~CIO4-GaSb junctions to a

weak 10 GHz irradiation suggests that the stabilization of a superconducting state via enhanced

interchain coupling is potentially feasible up to 26 K, or even to higher temperatures!

The high-temperature I-D superconducting ordering controls the behaviour of the transport propert ies below 40 K:

(i) the conduction of pairing origin which becomes dominant over the single particle conductivity below about 40 K. can be suppressed by the application of a magnetic field or by irradiation: and

(ii) the thermal conduction coming from the

electrons starts to drop significantly below 4(1 K. The guide towards an interpretation is there-

fore the pronounced 1-D behaviour accompanied by a large fluctuation temperature regime observed between T~ and ~IOT~. A major experimental piece of information is the existence of a SDW state at low temperature next to superconductivity. It is very likely that the stability of the dielectric SDW state is favoured by the presence of Umklapp scattering of two electrons from one side of the Fermi surface to


the other. All theories seem to agree on the important role played by Umklapp scattering on the nature of the ground state [54, 55].

Under ambient pressure the dimerization along the a-axis (and consequently the amplitude of the Umklapp scattering) appears to be smaller in (TMTSF)2CIO4, where superconductivity is observed, than in (TMTSF)zPF6, where a dielectric SDW state is the stable ground state. However, it is only with a structure determination of (TMTSF)2PF6 under pressure that the role of Umklapp scattering will be more firmly established. Is the interaction between the electrons leading to the superconducting pairing mediated by the phonons or not? We tend to favour a non-phonon mediated interaction for the following reasons: (i) the interplay between superconductivity and a magnetic state which suggests the presence of dominant Coulomb interactions; (ii) the enhancement of the spin susceptibility at high temperature and its non- activated temperature dependence which agrees with a positive gl; and (iii) the observation of (weak) 1-D 4kv X-ray diffuse scattering at high temperature in (TMTSF)zPF6, (TMTSF)2CIO4 and ReO4 [56].

However, the role played by the retarded etectron-phonon interactions versus the non- retarded electron-electron interactions awaits further investigations. Finally, the superconductivity of organic conductors appears as a new opening in condensed matter physics. The dis- covery of superconductivity in sulfur-based organic molecules shows that this phenomenon is probably not restricted to a small number of molecules.

The existence of high-temperature fluctuations and the possibility of Tc enhancement via chain cross-linking open up vast domains of investigation for the near future.

Acknowledgments

t wish to acknowledge the very fruitful cooperation which I have had (and still have)

with many of my colleagues: K. Bechgaard at Copenhagen, J.M. Fabre and L. Giral at Mont- pellier as well as the members of experimental groups at Orsay working on organic conductors. I am very grateful to J. Friedel, S. Barigi6 and H.J. Schulz who have strongly influenced the development of the theory in the field of one- dimensional conductors and superconductors.

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organic superconductivity

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