Magnetic properties of κ-(MDTTTF)2AuI2 salt in the normal and superconducting states

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Magnetic properties of κ-(MDTTTF)2AuI2 salt in the

normal and superconducting states

P. Delhaes, J. Amiell, S. Flandrois, L. Ducasse, Agnès Fritsch, B. Hilti, C.W.

Mayer, J. Zambounis, G.C. Papavassiliou

To cite this version:

Magnetic

properties

of

_{03BA-(MDTTTF)2AuI2}

salt in

the normal

and

_{superconducting}

states

P. Delhaes

_(1),

J. Amiell

_(1),

S. Flandrois

_(1),

L. Ducasse

_(2),

A. Fritsch

_(2),

B. Hilti

(3),

C. W.

Mayer

(3),

J. Zambounis

₍₃₎

and G. C.

_{Papavassiliou}

₍₄₎

(1)

Centre de Recherche Paul Pascal, CNRS, Château Brivazac, Avenue A. Schweitzer, 33600

Pessac, France

(2)

Laboratoire de Chimie

Théorique,

Univ. de Bordeaux I, 33405 _Talence, France

(3)

Central Research Laboratories,

Ciba-Geigy

AG. 4002 Basel, Switzerland

(4)

Theoretical and

Physical Chemistry

Institute, National Hellenic Research _Foundation, 48 Vassileos Konstantinou, GR 11635 Athens, Greece

(Reçu

le 21 décembre 1989,

accepté

sous

_{forme définitive}

le 20

_{février 1990)}

Résumé. 2014 Nous

avons examiné les

_{propriétés magnétiques (RPE, magnétisme statique}

et

mesures de

magnétisation)

du nouveau

_{supraconducteur}

_{organique 03BA-(MDTTTF)2AuI2}

au-dessus et au-dessous de la

_température

de transition sous

_{pression atmosphérique (Tc ~}

4

_K).

La structure

_{électronique}

de ce sel et de deux

_composés

voisins

_{[03BA-(BEDT2Cu(SCN)2}

et

03BA-(BEDT)2Ag(CN)2H2O]

a été calculée en

_employant

la méthode de Huckel étendue.

L’analyse

des résultats

_{expérimentaux}

et

_théoriques

nous a conduits à proposer une

description

unifiée

indépendante

des entités moléculaires

présentes.

A

partir

des preuves

expérimentales

de

corrélations

_{électroniques significatives,}

le

_problème

de la

_compétition

entre les états à basse

température

AF et SC a été examiné.

_Cependant,

en l’absence d’aucune évidence

_{expérimentale}

d’un état AF,

l’origine

des

paires

d’électrons n’est pas éclaircie dans ces matériaux

quasiment

bidimensionnels.

Abstract. 2014 We

have

investigated

the

_{magnetic properties (ESR,}

static

susceptibility

and

magnetization experiments)

of the new

_{organic superconductor 03BA-(MDTTTF)2AuI2}

above and below the SC

_phase

transition _temperature

_{(Tc ~ 4 K).}

At ambient pressure the electronic

structures of this salt and of two related

compounds [03BA-(BEDT)2Cu(SCN)2

and

03BA-(BEDT)2Ag(CN)2, H2O

have been calculated

_using

an extended Huckel method. The

_analysis

of both

experimental

results and calculated data has led us to _propose a unified

picture

independently

of the _present molecular blocks.

_Finally

the

_problem

of the

_competition

between the SC and AF

low-temperature

states which is based on the

_experimental

evidences of electronic correlations has been examined. However, in the absence of any

experimental

evidence of an AF

state, the

origin

of the electron

_pairing

is still unclear in these

_quasi

2d materials. Classification

Physics

Abstracts 71.25P - 71.45L

Introduction.

The list of

_organic

and

_{metallo-organic}

molecules which

_give

_{superconducting}

radical-ion salts

(RIS)

is

_steadily

_expanding

_[1].

After the initial discoveries with TMTSF

raselenafulvalene)

and BEDT

_{(bisethylenedithiolotetrathiafulvalene)}

_symmetrical

donor

molecules it has

_recently

been evidenced that

unsymmetrical

ones, such as DMET

_(a

« hybrid »

between the first two

_molecules)

and MDTTTF

_{(methylenedithiotetrathiafulvalen}

e), [2],

are also ambient _pressure

_{superconductors.}

A characteristic feature of BEDT is its

propensity

to

_give

salts in several

_crystal

_{morphologies (a, 81}

y, 0, K,...

forms)

with,

in

particular,

a

_{K(BEDT)2-Cu(SCN)2}

salt which exhibits the

_{highest superconducting (SC)}

transition

_actually

known

_{(Tc ~}

10.5

_{K) [1].}

The recent

_discovery

of

_{(MDTTTF)2AuI2}

as a new molecular

_{superconductor (Tc= 4 K)}

is

_noteworthy

because of its

_similarity

of donor

packing

with this BEDT salt. In both cases we are in the presence of radical-cation dimers which are

_{orthogonally arranged}

to form a 2d structural network and therefore to

_give

rise to

an

_anisotropic

2d electronic

_system.

We have

_investigated

the

_{magnetic properties}

of this new

compound

for,

on the one

_hand,

_{fully characterizing}

the SC state, and on the other

hand,

looking

for a

_{possible competing antiferromagnetic (AF)}

state.

_Finally,

we will

_explain

the

experimental

results

_{by comparison}

with the other known

_K-type

_{compounds : Cu(SCN)2}

(Tc

= 10.5

K), 13 (Tc

= 3.5

K), [3] Hg2.S9Brs (Tc

=

4.3K), [4].

and

Ag(CN)2 (no

_apparent

SC

_{state) [5]}

BEDT salts but also

_{(DMET)2AuBr2 (Tc}

= 1.9

K), [6].

All these

compounds

constitute a

homeomorphic

_group with common

physical

features that we will discuss.

Experimental.

The MDT-TTF molecule

_(see

insert of

_{Fig. 1)}

has been

_{synthesized following}

a

_procedure

described

_previously

_{[2]. Crystals}

of

K-(MDTTTF)2AuI2

were _grown

_by

the classical electro-oxidation

_technique

at constant current

_{(I ~}

1-2

_uA)

in THF as a solvent and with

(nBu)4NAuI2

as the

supporting

electrolyte :

small black

_platelets

were obtained which

Fig.

1. - The

temperature

dependence

of the

_{paramagnetic susceptibility (full line)}

measured on a

crystallize

in the orthorhombic

_system

_(space

_group

_Pbmn).

The

_{centrosymmetric}

dimers lie in the ab

_{plane ; along}

the c-axis these sheets are

_{separated by AuI2 planes.}

These

crystals

exhibit a moderate room

_temperature

electrical

_conductivity

with a metallic

type

dependence

below 200

K,

[2].

ESR

_experiments

on

_{single crystals}

were carried out with an X

(and Q)

band Varian

_spectrometer

equipped

with an Oxford variable

_temperature

accessory. Measurements of static

susceptibility

on a

_{polycrystalline}

batch

_{(15 mg)}

were

performed

with a standard

Faraday

balance between 4 and 290 K in the

_presence

of a

magnetic

field

_strength

of 1.1 T.

_Besides,

for the low

temperature

data

_(1.8-15

_K),

we have used a

_SQUID

_magnetometer

_(SHE

_{company) working}

with a variable

magnetic

field

strength

(0

to 60

_kOe).

Results.

MAGNETISM IN THE NORMAL STATE. - The static

_{susceptibility}

measured with the

_Faraday

balance was corrected for the core

_diamagnetic

_component

_calculated,

as

_usual,

from Pascal’s

constants. We

_neglect

a

possible

_presence of a small

_{Landau-type diamagnetism.}

The

temperature

dependence

of the

_{paramagnetic susceptibility}

is shown in

_figure

1. A weak

temperature

dependence,

more or less characteristic ôf Pauli

_{paramagnetism,}

is observed

with a flat maximum followed

_by

a small minimum around 20

K ; besides,

the

_extrapolated

absolute value

_{(,yp)o -}

6 x

10-4

emu. CGS

mole-1

is

_{quite high,}

as in most of the TTF

_type

radical-cation

salts,

indicating

the presence of

strong

electronic correlations

_(see

next

_part).

ESR

experiments

give

the

_g-value

and linewidth room

_temperature

_{anisotropy by}

_exploring

their

_{angular dependences}

on

_{single crystals.}

As indicated in

_figure

_2,

we detect a weak

anisotropy

for both line

_position

and width characteristics which

_correspond

to a Lorentzian

absorption

curve,

except

in the case when the

_platelet

is horizontal

_(the

c axis

aligned

with the static

_{magnetic field)}

where a

_{weakly Dysonian}

line is observed.

For this orthorhombic

_{crystal [2],}

after

_checking

that the

_principal

axes of these second rank

tensors are

_aligned

with the

_{crystallographic}

axis,

we confirm the

_{correspondence}

between the

extrema values of the linewidth and the

_g-factor

_values,

_[7].

It is well known that the

_principal

values of the

_g-factor

are molecular characteristics more or less constant in the TTF series

(g1 >

2.010,

g2 ~

2.007,

g3 ~ 2.003

_{respectively along}

the directions of the molecular

_long

axis,

short axis and normal to the

_{molecuiar planes) [8].}

In the

_present

case, we rather observe

mean values because of the _presence of more or less

orthogonalized

dimers ;

as evidenced

_by

the

_crystal

structure

_[2],

the two a and b

_{crystallographic}

directions are almost

equivalent

and the ESR

_signal

_appears to

_present

an axial

_symmetry.

This

_point

is confirmed

by

the tensor trace which has to be invariant for these radical-cation

_{species ) ( g > 2.0063 ).}

Concerning

the linewidth AH which is almost

_isotropic,

we have been

_looking

for a

frequency

dependence by using

a room

_temperature

_Q-band

_{spectrometer :}

a constant

linewidth has been observed. Therefore there is no

_apparent

_{frequency dependent exchange}

effect at 290 K in this

_compound

as found in classical

_magnetic

insulators.

The

_temperature

_dependences

of both the linewidth and the line

_position

for a

_{given crystal}

orientation

_{(Ho//a )}

are

_given

in

_figure

2. The

g-factor

is

_temperature

_independent

while the linewidth is

_increasing

when the

_temperature

is

_decreased,

almost

linearly

down to 100 K and with a

_strong

_{regime change}

around T* = 70 K.

Finally,

after a

_broadening

the resonance line is lost around 50 K and a new line is observed below 20 K which does not behave as a classical

AF resonance line. Its characteristics are

_independent

of the

magnetic

field direction :

g =

2.060,

OH = 90 gauss with an estimated

_intensity

about one _per cent of the intrinsic line. This is an extrinsic effect which could be

_associated

with the tri-iodide of MDTTTF

_prepared

Fig.

2. -

The temperature

dependences

of the ESR

g-factor

and linewidth

_(AH)

for a

_{given crystal}

orientation

_{(Ho//a )}

of the

(MDTTTF)2AuI2 compound.

The

_g-factor

and linewidth

(AH)

room temperature

principal

values are also

given. (In

the insert, the normalized resistance T

_dependence

published previously [2]).

Nevertheless,

the intrinsic behavior is the linewidth linear increase between 300 and 60

K ;

this is in contrast with the classical

_temperature

decrease in RIS which are one-chain

compounds.

This result has also been observed in other K

salts,

[1,

4,

9].

As we will

_explain

in

the next

_part,

this effect is associated with the intermediate values observed for these

_g-factor

components

because

_they

result from the back and forth

_spin

motion between the two kinds of

dimers.

MAGNETISM IN THE SUPERCONDUCTING STATE. -

Magnetization

measurements on

_powder

samples

have been carried out at _{ambient pressure} versus

magnetic

field

_(H)

and

_temperature

Fig.

3. -

Magnetic

field

dependences

of the

magnetization

at different temperatures

(in

the insert, the

temperature

dependences exhibiting

the

_{diamagnetic shielding}

and Meissner effects under a weak

magnetic field).

superconductor

with a transition

_temperature

_{T, =}

4 K. These

magnetic

field isothermal

dependences

have demonstrated that :

- at T >

Te

there is no

_apparent

_{antiferromagnetic}

_{state, which could be detected}

_by

AF

resonance, the

anisotropy

of the static

susceptibility,

or even

_by

the

_analysis

of d.c. electrical

- at T «

T,

there is a

_diamagnetic

state as

_{predicted by}

the classical SC

_{theory (the bump}

observed around 2 K should be due to the

_anisotropic

effects

_present

in these

_layered

structure

_type

_{superconductors)..}

To confirm this observation of a

_{diamagnetic ground}

state, we have carried out the

temperature

dependences

in the presence of a low

_magnetic

field

_{strength (see}

insert of

Fig. 3).

The

_sample

is first cooled in the absence

_{of any magnetic}

field down to 1.8

_K,

then the field is

_applied

and the

_{magnetization}

is _{measured upon}

_{heating :}

the

_{diamagnetic shielding}

is observed. When the

_sample

is

_subsequently

cooled under

_magnetic

field the Meissner effect is also detected.

Our observations are _very similar to those

_{already reported}

on

_organic

SC

_{[11, 12].}

From the saturation value of the Meissner effect when the

temperature

is

_going

down to zero

Kelvin,

we calculate the effective fraction of

_{complete diamagnetism MMeiss =}

7 %

_MdiD

a rather low

_value,

nevertheless

_quite,

usual in these materials

_[12].

The measurement of

M-H static

_{hysteresis loop}

has been

_performed

at 1.6 K

_{(Fig. 4).}

This

_experiment

allows us to

estimate the critical current

_{density Ii a}

/1:

as

_{predicted by}

Bean’s model

_[13],

where

D

AM is the difference in

_{magnetization}

at a fixed value for

_increasing

and

_decreasing

fields,

and D is the mean

_grain

diameter.

_Estimating

D =

10-4

m and H = 500 Oersted we obtain a

quite

low but

_acceptable

value

_{Ic ~}

3 700

_A/cm2.

The

last

_piece

of information

_{provided by}

this

_experiment

is about the derivation of the lower critical field

_Hcl

which is defined as the field where the flux starts to

_penetrate

into the

sample

and thus

_corresponds

to the first deviation from

_linearity

in the

_{magnetization}

curve : the

_apparent

mean value is

Hcl :5

50

Oersted,

a rather

_{high figure (see Fig. 4).}

Fig.

4. - M. H.

_Hysteresis

_loop

at constant _temperature

_{(T =}

1.6

_K)

obtained with a succession of

From the

_magnetic

field

_dependence

observed at 2 K

_(Fig.

_3),

we can also estimate the upper critical field value

fic2

==15 kOersted which is in

_rough

_agreement

with the estimated

Pauli limit. These results are similar to those observed

on 6

and

_K-phases

superconductors,

[1,

12].

These

_compounds

are

_anisotropic

_type

II

_SC,

well described

_by

the

_{Ginzburg-Landau}

theory ;

however to

_apply

this

_theory

it will be necessary to work on

_{single crystals, [14].}

Actually,

with the

_present

results

_only

mean

approximate

values for the critical fields are

given,

and

_they

do not allow us to

_give

a valuable estimate of

the

coherent

_lengths.

ELECTRONIC STRUCTURE OF

_{K-(MDTTTF)2AuI2}

AND RELATED COMPOUNDS.

The electronic structure of the title

_compound

is

_compared

to the band structure character-istics of two other

_K-type

_salts,

_{namely K-(BEDT)2Cu(SCN)2}

and

_{K-(BEDT)2Ag(CN)2H20.}

Tight-binding

calculations _{based upon the Extended Hückel}

_theory

have been

_performed.

For

K-(BEDT)2Cu(SCN)2

and

_{K-(BEDT)2Ag(CN)2H20,}

these structures have

_already

been

determined

_{[1, 5].}

As some technical differences may be found between the authors

(simple

vs

double basis set, values of the

_parameters),

in _{this paper} we _compare the results obtained within the same

_{approximations.}

The transfer

_integrals

are evaluated within the dimer

_splitting

_{approximation [15] ;}

they

involve the

_coupling

between the HOMO

_{(highest occupied}

molecular

_orbital)

of each

organic

molecule. For

_parallel

molecules,

the transfer

_{integrals couple}

pz atomic orbitals

(AO)

located on each monomer while p, AO on one more monomer

_{and py}

AO on the other

monomer are

_{coupled through}

the transfer

_integrals

between

perpendicular

molecules

(see

Fig.

5,

for the transfer

_{integral labelling}

and the definition of the reference

axes).

The transfer

integral

values are

_given

in table 1 : the

_symmetry

_groups of this

_AuI2

salt on the one

_hand,

and the BEDT

_compounds

on the other hand are différent and this

_explains

that the number

of

_equivalent

transfer

_integrals

is reduced in the former case.

Fig.

5. -

Projection

of the K structure in the

(b . x ) plane (x

is the a axis for Au and

Ag

salts and the c

axis for

_{Cu) giving}

the transfer

_{integral labelling.}

The transfer

_integrals

are

_{comparable, especially}

those relative to the BEDT salts. The MDTTTF salt exhibits a smaller dimerization

_parameter

_{S1 /S2.}

The BEDT different

molecular

_symmetry

does not lead to

_qualitative

differences

_{(compare 111}

and

_tI3,

tl2 and

tI4).

The

_resulting

band structures are similar to those

_{given previously}

so that

_orily

the

Table 1.

_{Transfer integrals expressed}

in meV.

boundaries,

where the contact of the two circular surfaces

_(which

is observed in MDTTTF

salt)

is not allowed in the two BEDT salts

_by

their lower

_crystal

_symmetry.

The

_density

of states was calculated and is shown for the three salts in

figure

7. The results evidence that there exist two Van Hove-like

_{singularities.}

The

_larger

one, which is found at

the lower energy for MDTTTF and below the Fermi

position

for the BEDT salts

_corresponds

to a flat band around the center of the Brillouin zone. The second

_{singularity corresponds}

to a small

_{dispersion along}

the M- Y direction. The

_density

of states at the Fermi level for these

Fig.

7.

_{- Density}

of states

_{N (E)}

versus _{energy (E)} of the two upper bands for the three considered

Table II.

_{- Summary}

_{of physical}

_properties

_for

three

_{K-phase compounds.}

half-filled band

_systems,

_{N (EF)}

is

_given

in table II : it is

_quite

similar for the three

compounds.

It has to be noticed that

_{N (EF)}

does not lie in the

_region

of the

_singularity

of

density

of states but rather on _{the upper}

_wing.

_Moreover,

if one assumes that the transfer

integrals might

increase at low

_temperatures

it is found that

_EF

will shift to a

_{higher position}

while the

_{singularities}

will stand more or less at the same _energy, thus

_implying

a decrease of

N(EF).

Interprétation

and comments.

We will focus our discussion on three related

_{points :}

the enhanced

_paramagnetic

state, the ESR linewidth

_T-dependence

and the occurrence of the SC state. In order to introduce these

comments we have summarized in table II the electron band

_parameters

and the main

experimental

characteristics for the MDTTTF

_compound

and also for the other two BEDT salts which have been

_{carefully investigated.}

The

_quasi

2d electronic structure _may be modelled

by

two transfer

_{integrals :}

one denoted

(tll)

is an _{average of the interactions between}

_parallel

dimers while the

_{other (t1.)}

involves

perpendicular

molecules

_{(see Fig. 5).}

This

_simple

model allows us to evaluate the effective

electronic

_anisotropy

as shown in table II. The 2d electronic character increases from

MDTTTF to BEDT salts as a _{consequence of} a smaller dimerisation

_parameter

and a

weaker tIl

in the former case ; however in every case we are in the presence of

_quite

narrow

electronic bands

_(thé

total bandwidth is about 4 = 0.30 - 0.34

eV).

PARAMAGNETIC STATE AND EVIDENCE FOR ELECTRONIC CORRELATIONS. - From the

comparison

between the

_quasi

constant

_{experimantal paramagnetism}

and the calculated Pauli

paramagnetism (Pauli aN(EF))

we calculate the enhancement factor

(see

Fig. 1

and Tab.

_II),

which is 2.6 for the

_present

_compound

and around 2.0 for the

_{corresponding}

BEDT salts. As

_currently

admitted

_[16]

this is due to the electronic correlations which are evidenced

by

the presence of

charge

transfer bands in

_{optical absorption}

_spectroscopy

_[17].

Indeed a

first

_neighbor

site

_(V)

Coulomb interactions in the atomic limit of the extended Hubbard

model,

an

_{approximation}

which overestimates U and

_[17].

These

_parameters,

which are

essentially

molecular

_{characteristics,}

are estimated to be about 1-1.2 eV and 0.4 eV

respectively

in these

_compounds.

If we _{compare with the calculated} bandwidth

_{(Tab. II)}

we conclude that 4 = V U i.e. the electronic correlations should be dominant in these narrow electronic band

_systems

even

_if,

_up to now, we have not been able to _{detect any}

_apparent

AF

state near the _present SC state.

ESR LINEWIDTH T-DEPENDENCE. - This behavior has been the

_subject

of a

_long

_controversy

in

_quasi

Id

_organic

conductors because no firm

theory really

exists.

Two main

_problems

are

_{present :}

on the one

hand,

the relaxation mechanism and therefore the linewidth value which are

functions

of the electronic

_{dimensionality,}

and,

on the other

hand,

their

_temperature

_dependences,

either an increase or a decrease of the linewidth is observed with the

temperature

decrease.

Based upon the

experimental

facts a

_general

rule can be established which suffers very

scarce

_{exceptions :}

i)

For radical ion

salts,

or one-chain

compounds,

the linewidths are

_decreasing

with

temperature.

ii)

For

_charge

transfer

_complexes

such as

_TTF-TCNQ,

or two-chain

_compounds,

the linewidths are

_{generally increasing}

when the

_experimental

_temperature

is

_going

down in the absence of any

phase

transition

_[19].

The first situation is classical and reminiscent of a mechanism for the

_{spin-relaxation}

process

through

the lattice

scattering

and the associated motional

narrowing

effect which is

present

in classical 3d metals

_[20].

It has been demonstrated

_by

Elliott

_{[21] and}

Yafet

_[22]

that the linewidth and the

_g-factor

values are

_proportional

to the

_{spin-orbit coupling ;}

these two

_quantities

are related

_by

Elliott’s relation :

where

_Ag

is the

_{g-factor anisotropy}

and _{Te- ph} is the usual electrical

_conductivity

relaxation

time ;

it follows that in a metallic state AH and the electrical

_resistivity

must follow a similar

T-dependence.

As

_pointed

out

_by

Tomkiewicz et al.

_[23]

in an ideal Id electronic

_system

this

relaxation mechanism cannot exist. This is the reason

_why

the observed linewidths are several orders of

_magnitude

lower than the

_expected

ones.

In

_partial

2d conductors where transverse electronic interactions are

_present,

the

dimensionality change

is

_empirically

accounted for

_by

a

_correcting

factor which allows us to

_explain

the linewidth

_broadening

and the similar

_T-dependence

characteristic of one chain

_{compounds [26].}

In order to

_explain

the

_opposite

behavior observed in

TTF-TCNQ

and related

_compounds

Weger

has

_proposed

the

_following

argument

[25] :

the measure of the 2d character is

In the case where the

_{longitudinal conductivity}

is coherent

_(this

is the case for these

compounds

at least below room

temperature)

and the transverse

_conductivity

is

_diffusive,

Weger

has shown that the

_probability

for

_hopping

between

_neighboring

chain is :

This derivation assumes that the two chains

(or

dimers)

are

_degenerate

in energy

_(but

their ESR

_signal

can

_present

a different

g-factor).

The

_{application of (3)}

modifies the

_{Elliott-Weger}

relation

(2) :

In this situation the linewidth increases

_{with t 1.}

value and the associated electronic

dimensionality,

but also follows the electrical

_conductivity

_temperature

_{dependence (indeed}

a coexistence of the two mechanisms can also be

_empirically

considered in order to _{fit every}

experimental

situation

_[24]).

This

_approach

is in favor of our and other

_experimental

observations on K-salts :

- The linewidths for the

K-phases (AH =

60 - 100 gauss at 290

K)

are in

_{general larger}

than for the other

_phases

which are more Id

type

compounds

(OH

= 10 - 20

_{gauss) [8].}

-

They

are

_{linearly increasing}

between room

_temperature

and T* where a

_{regime change}

occurs

_(see

Tab.

_II).

This is

_only

in

_qualitative

_agreement

with the electrical

_{conductivity T-dependence}

which appears to be rather

complex

in this MDTTTF salt

_(see

insert of

_{Fig. 2) ;}

however it shows that

_Weger’s

_argument

can be extended to these

_quasi

2d electronic

systems.

It appears

equivalent

to be in the presence of one kind of donors

_{(or acceptors) presenting}

two different

_{crystallographic positions}

in the lattice or to have both donor and

_acceptor

molecules in

_charge

transfer

_complexes.

Besides,

the presence of a

_{sharp regime}

_change

in the linewidth

_T-dependence

means that around T* the

_spin

relaxation mechanisms are

_strongly

modified. As

already

discussed,

the

linewidth

_broadening

could be attributed to the onset of an AF state but in the absence of _any other

_experimental

_evidence,

this

_hypothesis

cannot be maintained.

Other

_{possible suggestions}

to

_explain

this

_general

behavior are either a diffuse to coherent

transition of one

_component

of the transverse electrical

_{conductivity [25] occurring}

in these salts or even a structural

_change.

OCCURRENCE OF A SUPERCONDUCTING STATE. -

K-(MDTTTF)2AuI2

is an

_anisotropic

_type

II

_{superconductor}

like the other

_/3

and K

_{superconducting}

BEDT salts.

The main

_experimental

fact is that all these

_{K-phase compounds}

are low

_temperature

superconductors

except

one of them :

_{(BEDT)2Ag(CN)2H20 [5].}

Nevertheless for technical

reasons the low

_temperature

electrical

_conductivity

has not been

_{completely investigated [27],}

but we can guess that the SC state should be

_present

(if

some disorder effect associated with the presence of molecules of water does not _suppress

_it).

It _{is necessary} therefore to look for a

general origin,

whatever the molecular characteristics of the

_symmetrical

or

_{unsymmetrical}

molecules involved.

In a recent _{review the alternative ways for the} mechanisms of

_{superconductivity}

in

_layered

structures have been

_carefully

examined

_by

Friedel

_[28]

and some

_arguments

used for the

perovskite high Tc superconductors

can be invoked.

As in copper oxides

[29]

the presence of

large

Van Hove anomalies in the

_density

of states

But it turns out that in the

_present

electronic situation our _energy band

,

calculation

_(Fig.

7)

does not exhibit this

_{coincidence ;}

we have found that

N (EF) is quite

constant in this series of salts

_(Tab.

_II)

and almost identical to the values also

_proposed

for the

/3

phase

BEDT

_{superconductors}

_[1].

The different

_Tc

values which have been observed are therefore due to a

_change

in the

_{pairing coupling}

mechanism which is

_subject

to discussions in these materials

[28].

It has been

_proposed,

for the first series of

_organic

_SC,

_namely

the TMTSF

salts,

that the

origin

of the BCS

_type

_coupling

mechanism is the electron-electron

_repulsion

via the

antiferromagnetic

fluctuations which are

_present

in these

quasi

Id materials

[30].

Indeed,

for a

_given

stoichiometric

_phase

in a

thermodynamic phase diagram,

both SC and AF states have been

observed,

which are connected and

mutually

exclusive.

In the

_present

series of

_compounds

which are

_anisotropic

2d electronic

_systems

(the

Fermi surfaces

_presented

in

_Fig.

6 are almost

closed)

we have observed the presence of sizeable electron-electron correlations on the one

_hand,

but no

_experimental

evidence of an AF state

on the other

hand.

This is a crucial

_point

which has to be

elucidated,

because theoretical 2d Hubbard models show that the

_competition

between AF and SC states

_{[31 ] is}

in favour of the SC state for a half-filled band. Even a recent

_{study [32]}

claim the

_possibility

of

superconduc-tivity

without the

_proximity

of an AF state. One

_experimental

way to answer this

_question

would be to

_play

with the band

_filling

factor as in

_{high Tc superconductors}

which is fixed

by

the

(2-1)

salt

stoichiometry

in all these

_organic

conductors.

Conclusion.

We have

_investigated

the

magnetic properties

of

_{K-(MDTTTF)2AuI2}

in the normal and the SC states. We have shown in the

_high

_temperature

_phase

the presence of rather

strong

electronic interactions in these narrow band

_compounds.

The ESR linewidth

T-dependence,

which does not

_obey

the usual Elliott

_relation,

has been

_{explained using}

an

_argument

already

proposed by

Weger

[25]

for the two-chain

_compounds

i.e.

_charge

transfer

_complexes

(TTF-TCNQ

type).

Besides we have

pointed

out that the sudden

_broadening

of this linewidth

around T* is an intrinsic behavior for this class of

_compounds.

We have also confirmed that this

_compound

is a volume second kind

_{superconductor}

below

4 K and more

_generally

that almost all the

_{K-phase compounds}

present

a similar behavior. A

unified

_picture

of these salts has been

tentatively

developed

based upon the

magnetic

properties

and the electronic band calculations which

_{infortunately}

do not take the electronic interactions into account. For a

_{general description}

of the

_physics

of

_anisotropic

2d electronic

systems

we have examined the

_{possible competition}

between SC and AF states but we have no evidence of even a hidden AF state. The

_origin

of the electron

_pairing,

in these

_materials,

remains therefore

_subject

to

_controversy.

Acknowledgements.

We thank Dr. M. Kurmoo for the

_{K-(BEDT)2Ag(CN)2, H20 crystal}

data and an

interesting

discussion,

Dr. M. Kinoshita for the

_{K-(BEDT)2Cu(SCN)2 crystal}

data,

and Mrs. B.

_Agricole

for

_helpful

technical assistance.

Note added in

_{proof :}

In a _very recent

_publication

the structural and electronic

_properties

of the

_{K-phase organic}

conductors have also been discussed in order to understand the occurrence of the

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