Magnetoresistance Studies in the Low-Dimensional Organic Metal α-(ET)2TlHg(SeCN)4 under Pressure: Experiments and Simulation by Tight Binding Band Structure Calculations

(1)

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Magnetoresistance Studies in the Low-Dimensional Organic Metal α-(ET)2TlHg(SeCN)4 under Pressure:

Experiments and Simulation by Tight Binding Band Structure Calculations

P. Auban-Senzier, A. Audouard, V. Laukhin, R. Rousseau, E. Canadell, L.

Brossard, D. Jérome, N. Kushch

To cite this version:

P. Auban-Senzier, A. Audouard, V. Laukhin, R. Rousseau, E. Canadell, et al.. Magnetoresistance

Studies in the Low-Dimensional Organic Metal α-(ET)2TlHg(SeCN)4 under Pressure: Experiments

and Simulation by Tight Binding Band Structure Calculations. Journal de Physique I, EDP Sciences,

1995, 5 (10), pp.1301-1310. �10.1051/jp1:1995198�. �jpa-00247137�

(2)

J. Phys. I fronce 5

(1995)

1301-1310 OCTOBER1995, PAGE 1301

Classification

Physics Abstracts

72.15Gd 71.25Hc 71.25Pi

Magnetoresistance Studies in the Low-Dimensional Organic Metal

a-(ET)2TlHg(SeCN)4 under Pressure: Experiments and Simulation by Tight Binding Band Structure Calculations

P. Auban-Senzier

(~),

^A. Audouard

(~),

^V. ^N. ^Laukhin (~>~), ^R. Rousseau (~>5), ^E.

Canadell (~>6), ^L. Brossard

(~),

D. Jérome (~) ^and ^N. ^D. ^Kushch

(~)

(~ Laboratoire de Physique des

Solides(*),

Université de Paris-Sud, 91405 Orsay Cedex, France (~) Laboratoire de Physique des

Solides(**

et Service National des Champs Magnétiques Pulsés (***

),

INSA, Complexe Scientifique de Rangueil, 31077 Toulouse, France

(~) ^Institute of Chemical Physics _in

Chernogolovka,

Russian Academy of Sciences, Chernogolovka,

MD 142432, Russia

(~) ^Institut ^de ^Ciència ^de ^Materials ^de Barcelona, Campus de la U-A-B., 08193 Bellaterra, Spain

(~) ^Permanent ^address: Department of Chernistry, The University of Michigan, Ann Arbor, Michigan 48109, USA

(~) Permanent address: Laboratoire de Chimie

Théorique(****

), Université de Paris-Sud, 91405

Orsay Cedex, France.

(Received

27 March 1995, revised 19 May 1995, accepted 4

July1995)

Résumé. Des _mesures de magnétorésistance ont été effectuées dans le conducteur organique bidimensionnel

cv-(ET)2TlHg(SeCN)4

jusqu'à la température de 0A K et _sous pression hydrosta- tique. Une seule série d'oscillations Shubnikov-de Haas est observée dans la gamme de pression

comprise ^entre 3.5 et 11 kbar. La _masse cyclotron décroit lentement lorsque la pression _aug-

mente et la fréquence des oscillations augmente rapidement depuis la valeur de

(653

+

3)T

à

pression ambiante jusqu'à (790 +

3)T

à il kbar. Une modélisation basée _sur la méthode des liaisons fortes suggère _que la dépendance _en _pression de l'aire de l'orbite fermée de la surface de Fermi est due _au glissement, induit par la pression, des molécules des chaines _ne contenant qu'un type de donneurs. Au contraire de _son composé frère

o-(ET)2NH4Hg(SCN)4,

il présente des

oscillations lentes, de fréquence

(47+3)T,

à pression ambiante. Elles

ne sont pas observées entre 3.5 et 11 kbar et pourraient être

en relation _avec _un emboîtement des orbites ouvertes à pression ambiante qui pourrait ^détruire ^la supraconductivité dans le composé à base de sélénium.

Abstract. Magnetotransport measurements have been carried out in layered organic metal

o-(ET)2TlHg(SeCN)4

at temperatures down to 0A K and under hydrostatic _pressure. Only _one

(*)

(URA

CNRS

02)

(**

(ERS

CNRS 111) (***

(UMS

CNRS

819)

****

(URA

CNRS 506)

joLoNALDBmnsiQtJBL-T.3.N°moCrowaiw3 si

(3)

1302 JOURNAL DE PHYSIQUE I N°10

series of Shubnikov-de Haas oscillations is observed in the applied _pressure _range from 3.5 to 11 kbar. The cyclotron _mass decreases slowly with increasing _pressure and trie oscillation frequency increases rapidly from its ambient _pressure value of (653 +

3)T

_to

(790

+

3)T

_at 11 kbar. A model tight binding study suggests ^that the _pressure dependence of the _area of the closed part of trie Fermi surface is consistent with

a pressure induced

sliding

motion of trie molecules in the donor chairs containing only _one type of donors. Slow oscillations with frequency (47 +

3)T

bave also been observed at ambient pressure in contrast to its superconducting sister compound

o-(ET)2NH4Hg(SCN)4.

They _are not observed in trie applied _pressure _range and might be connected with _some nesting of the open orbits at ambient pressure which _may result in _a

destruction of superconductivity _in trie

Se.containing

compound.

1. Introduction

The members of trie isostructural

a-(ET)2MHg(XCN)4 family (ET=bis(ethylenedithio)tetra- thiafuIvalene; M=K, Tl,

Rh _or

NH4i

X=S _or

Se)

_are

quasi

two-dimensional

(2D) organic

conductors of great interest due to trie

variety

of their

low-temperature properties.

At _room temperature their Fermi surface

(FS),

which is

composed

of both

slightly warped quasi-2D cyhnder (dosed

noie

orbits)

and

quasi-ID

sheets

(open

electron

orbits) iii,

is

expected

to be very similar for ail of them [2].

Nevertheless, a-(ET)2MHg(SCN)4 compounds

with M

=

K, TI,

Rh

undergo

_a

magnetic

field- and

temperature-dependent phase

transition at Iow

(Tp

= 8÷10

K)

temperature

[3-5]

^into a

ground

state with 1)

anisotropic magnetic susceptibility

[6]

typical

of

magnetic ordering

_system with formation of spin

density

wave

(SDW), ii) complex magneto-oscillatory

behaviour and

iii)

anomalies in trie semidassical and

angular-dependent magnetoresistance (MR) [3,4, 7-13]. They

_even exibit trie _so called weak

superconductivity

at T _< 300 mK

[14,15].

On trie other hand trie

compound

with M

=

NH4

does not

undergo

trie

phase

transition at Tp ^and ^becomes a

superconductor

below IA K

[16,17],

whereas trie

selenium-containing

^sait

a-(ET)2TlHg(SeCN)4

is _a stable

organic

metal without _any

phase

transition down to at least 80 mK

[18,19].

Hydrostatic

_pressure

(P

> 3

kbar)

_suppresses trie

low-temperature phase

transition at

Tp

associated with trie

nesting

of trie _open orbits in

a-(ET)2MHg(SCN)4

with M

=

K, Tl,

Rh

restoring

trie

high-temperature

^metallic state

[17, 20].

^In contrast, _a moderate _pressure

(P

_> 2

kbar) destroys superconductivity

in

a-(ET)2NH4Hg(SCN)4

and

improves

trie

nesting

of trie

quasi-ID

_open sheets thus

producing

^small ^dosed ^orbits ^which

give

rise to slow magnetoresis-

tance oscillations with

frequency

around 45T at 2 kbar [21].

An

important problem

to solve is _to understand

why

trie

a-(ET)2TlHg(SeCN)4

sait does _not exibit

superconductivity

in _contrast to its sister

compound a-(ET)2NH4Hg(SCN)4.

In order to

bring

_some

enlightments

about this

question,

we bave studied trie effect of pressure on trie magnetotransport

properties

and FS of

Se.containing

sait

a-(ET)2TlHg(SeCN)4

_in connection with

tight binding

^FS calculations.

2.

Experimental

The details of trie electrochemical

synthesis

_as well _as trie

crystal

structure and _some of trie zero-field

properties

of trie

a-(ET)2TlHg(SeCN)4 compound

bave been

published

elsewhere [18].

Trie samples were mounted using trie standard four-contact method with annealed platinum wires

(20

_/~m in

diarneter)

which _were

glued

to

sample by graphite

poste. ^Both

alternating

current

(10

_or 20 /~A, 77

Hz)

and

magnetic

field _were

applied along

trie b*-direction

(1.e.,

_per-

(4)

N°10 MAGNETORESISTANCE OF

o-(ET)2TlHg(SeCN)4

1303

11

3 T ₌ o_4 K

~

skbar

X ~

~

i

o

o 3 6 9 12 15

Bm

Fig. 1. -Low-temperature magnetic field

dependence

of trie longitudinal magnetoresistance

(IllBllb*)

of

a-(ET)2TlHg(SeCN)4

for diiferent _pressures.

pendicular

to

conducting ac-plane)

and trie

signal

across the

potential

contacts _was detected

by

lock-in

amplifier. Experiments

under

hydrostatic

_pressure were carried out

using

a non-

magnetic "piston-cylinder"

cell

(6

mm in inner

diarneter)

with _a silicon

polymer liquid

_as _a

pressure medium. Pressure up ^to 14 kbar _was

applied

_at _room temperature afterwards trie cell _was thermalized in _a ~He cryostat

providing

temperatures ^down to 0A K in _a 12 T _su-

perconducting

magnet. Trie temperature ^{of trie}

sample

_was varied between 0.4 K and 300 K and measured

by

^calibrated ^ruthenium

oxide,

carbon

glass

^and

platinum

_sensors

depending

on different temperature _ranges. ^A ^decrease ^of _pressure on

cooling

_was taken into account for

estimating

^trie low temperatures _pressure value

according

to

[22]. Magnetoresistance

_measure-

ments at ambient pressure were carried out with _a

slowly decreasing

(~J ^1.2

s) pulsed magnetic

fields

reaching

36 T at temperatures down to 80 mK and bave been described elsewhere

[19].

3.

Experimental

Results and Discussion

Both trie _room temperature

conductivity (dIn(a)/dP

= 0.12

kbar~~; a(P

=

0)

=

1000 ÷ 2000 Q~~

cm~~)

and trie residual

resistivity

ratio

(from

40 at 3.5 kbar _up to 60 at 11

kbar)

of trie

sample

increase with

increasing

_pressure. Trie low temperature

magnetic

field

dependence

of trie

magnetoresistance

at 0.4 K is

displayed

in

Figure

1 for three diiferent

hy-

drostatic _pressures.

Only

_one series of Shubnikov-de Haas

(SdH)

oscillations is observed. Trie semiclassical

magnetoresistance (Rsc Ru) /Ro (where Rsc

and

Ro

_are trie

field-dependent

semiclassical part ^{of trie} ^resistance and trie zero-field

resistance, respectively,

_at trie

given

_pres-

sure)

increases with

increasing

_pressure while trie SdH oscillation

amplitude dearly

decreases.

Trie

oscillatory

part ^{of trie} magnetoresistance bave been calculated _as

(R Rsc)/Rsc,

where R is trie

sample

resistance. Trie data deduced from

Figure

1 above 5 T is shown in

Figure

2. Clear SdH oscillations become visible

starting

from

~J 5 T _at T < 1

K,

_as it is trie _case _at

ambient pressure

[18,19].

As

displayed

in

Figure

3, Fourier transforms reveal _a fundamental

frequency

and _a well- pronounced second harmonic. We would like to

emphasize

trie strong _pressure

dependence

of trie fundamental

amplitude,

while trie

amplitude

of trie second harmonic _is

roughly independent

(5)

1304 JOURNAL DE PHYSIQUE1 NO10

T=0.4K

f

o

'É (R-R~JJR~=o.05

7.5

11 kbar

o.08 o.12 o.16 o.20

1/B~r-i)

Fig. 2. Oscillatory part of trie magnetoresistance deduced from Figure 1. Rsc

is the semiclassical part of the resistance

(see text).

on pressure.

Trie measured SdH fundamental

frequency, F,

increases

linearly

from trie ambient _pressure value of

(653

+

3)T

_[19] to

(790

+

3)T

at 11 kbar

(Fig. 4a).

As F is

directly proportional

to trie extremal cross-section _area of trie closed orbit in

k-space,

this indicates that trie _area of trie

quasi-2D

part ^{of FS} ^bas ^increased

by

^21% at 11 kbar. Same

(d(In(F)) /dP

= 0.02

kbar~~)

and

slightly higher (d(In(F))/dP

= 0.03

kbar~l)

_pressure

dependences

bave been observed in trie

sulphur-analog compounds o-(ET)2KHg(SCN)4

(20] and

a-(ET)2NH4Hg(SCN)4 _(21],

respectively.

It is worthwhile

noticing

that trie

phase

factor _~i

entering

^trie

Onsager

^relation

1/B~

=

(n +'i) IF (where

B~ is trie field value _at the maximum of the oscillation

corresponding

to the Landau Ievel

n)

is close to 0 _up to 3.5 kbar and increases under

higher

_pressure _16'i

~J 0.2

and 0.3 for 5 and 7.5

kbar, respectively).

^At 11

kbar,

_~i is dillicult _to estimate due to trie relative increase in trie

anharmonicity

of trie oscillations

(see Fig. 3).

Trie

cyclotron

_mass

associated with this orbit _was derived

by fitting

trie temperature

dependence

of trie Fourier transform

amplitude,

measured in trie temperature _range ^0.4 ^÷3.5 K at each _pressure

value,

to trie Lifschitz-Kosevich

IL-K)

formula [23]. ^Trie

cyclotron

_mass

slowly

decreases with increasing

pressure from _me

=

il.8

+

0.05)

_mo

(for

B _< 12

T)

_[19]

(where

_mû is trie bare electron

mass)

or me =

(2.05 +0.05)

_mû _[24] at ambient _pressure clown to _me =

(1.6

+

0.05)

_mû at 11 kbar

(Fig. 4b).

Trie

Dingle

temperature ^is ⁱⁿ ^trie _range 1÷1.6 K. It _can be noticed that much

Iarger pressure-induced

decrease of trie

cyclotron

_mass related _to closed orbits bave been

reported

in both

~-(ET)2Cu(SCN)2

_[26] ^and

a-(ET)2MHg(SCN)4 (where

M ₌

K, TI,

NH4 and

Rh) _[27],

while _no

pressure-dependence

bas been observed for

a-(ET)2NH4Hg(SCN)4 [21].

Trie

generally large

measured values of trie

cyclotron

_mass

(me

= 3.5 _mo and _me ₌ 2.1 _mû for

(6)

o-(ET)2TlHg(SeCN)4

1305

T=0.4K

~

ÀÎ

à

ù n

j

5 kbar

o

~f

fl

)

_7.5

Î

~

11 kbar

O 1000 2000 300O

kequency _~T~

Fig. 3. Fourier spectrum of trie Shubnikov-de Haas oscillations displayed in Figure 2.

790 .2

)

^~~~

(

710 ^~'~

é

~ ~

(

N- 670

.o

630 ^~

~.~

~

Î

18

1.5

0 2 4 6 8 10 12

pressure

(kbar)

Fig. 4. Pressure dependence of

a)

trie Shubnikov-de Haas oscillation frequency and

b)

cyclotron

mass. Ambient pressure points _are ^taken ^from [19]

(diamond)

and [24]

(square).

_mo _is the bare

electron _mass.

(7)

~-(ET)2Cu(SCN)2

_[26] and

o-(ET)2MHg(SCN)4 [27], respectively,

at ambient

pressure)

bave been ascribed to strong

electron-phonon

interactions [27] or electron correlations

[26].

^A more

detailed

knowledge

of trie _pressure

dependence

of these interactions is

required

to

interpret

trie pressure

dependence

of trie

cyclotron

_mass.

In order to

provide

_some

understauding

of the above

reported

_pressure

dependeuce

of the FS area, the

following

theoretical

study

of the FS _was carried out. Since the

crystal

structure of

a-(ET)2TlHg(SeCN)4 (like

_any of trie

a-(ET)2MHg(XCN)4 phases)

bas _not beeu determined under _pressure, _a direct calculatiou of trie FS uuder _pressure _is uot yet

possible.

^Under ^such

circumstances trie easiest way ^to tackle this

problem

is

by usiug

trie well kuown

tight-binding equations

for trie _energy _vs.

k-depeudence

_as _a function of trie diiferent transfer

integrals

of trie lattice [25]. ^Trie pressure eifect _can then be modeled

by systematically chauging

trie diiferent

transfer

integrals.

Once determined what

changes

_are consistent with trie observed iucrease of trie dosed

portion

of the

FS,

_oue _cau Iook at what structural

changes

_can Iead to these

variations of trie individual transfer

integrals

and discuss their likeliness.

Accordiug

to diiferent

luagnetoresistance

studies trie _area of trie dosed portion of trie FS is

remarkably

similar (~J 16% ^{for ail} ^{of trie}

a-(ET)2MHg(XCN)4 phases. Except

for _some subtle

diiferences,

trie calculated FS _[2] for trie salts with M

=

K, NH4,

Tl

/

X

= S and M

= Tl

/

X

= Se _are also _very similar.

However,

for trie sait with M

= K

/

X

= S trie

crystal

structures at both 298 K aud 104 K _are known

[ii.

In view of these two observations trie trausfer

iutegrals

calculated for trie low temperature structure of

o-(ET)2KHg(SCN)4

should

provide

trie best

starting point

for _our model

study (1.e.,

trie best

approximation

to trie low temperature ^and ambient _pressure

structure).

In

addition,

this sait offers also trie

possibility

of

studying

trie eifect of thermal contraction _on trie FS

which,

in

principle,

could be related to the eifect of pressure. Thus _we carried out 1) a

complete

extended Hückel

tight bindiug

calculatiou [28]

of trie FS'S of

a-(ET)2KHg(SCN)4

at 298 K and 104 K and then

ii)

trie model

tight binding study,

_on trie basis of trie transfer

integrals

calculated for trie 104 K structure.

Double-(

type ^orbitals ^for C aud S _were used

everywhere.

Trie

H~~ values,

_as well _as trie expouents and

weighting

coefficients of trie double

( orbitals,

_were taken from

[29].

^A

modified

Wolfsberg-Helmholz

formula _was used _to calculate trie

nondiagonal H~~

^values [30]

in trie

complete

calculations. Trie different transfer

integrals

_are defined in

Figure

5, which is

a representation ^{of trie} ^slab ^where ^trie ^donor ^molecules are viewed

approximately along

their

long

axis. Three of these transfer

integrals (ci

_{to c3} will be referred to _as "intrachain transfer

integrals"

and trie

remaining

^four

(pi

to

p4)

_as "interchain transfer

integrals" _[31].

As _cari be

seen in

Figure

5, ^trie ^donor ^lattice ^contains two different types ^of ^donor ^chains

(I

^and

II).

Chain type ^I ^contains

only

_one type ^of donors

(A)

whereas chain II contains two different _ones

(B

and

C).

Trie

complete

calculation of trie FS'S for

a-(ET)2KHg(SCN)4

at 298 K and 104 K lead to surfaces which contain trie usual

warped

ID and dosed 2D features [1,

2,32].

Trie

striking

result however is that trie _area of trie dosed

portion

almost does not

change.

Since trie transfer

integrals

increase with temperature

lowering,

this result is at first

sight surprising

when _com-

pared

with trie present

experimental

data.

Obviously,

_pressure and thermal contraction must

bring

in different structural

changes

to trie lattice.

Let _us _now tutu to trie model

tight binding

calculations. For this

study

trie FS _was recalcu- lated for

changes

of _up to 20% of trie initial values

of1)

_every individual transfer

integral

and

ii)

selected _groups of these

integrals.

Trie main results _are trie

following:

a)

An increase of ail both intrachain and interchain transfer

integrals by

trie _same amount leaves trie _area of trie dosed part ^of trie FS

unchanged.

b)

An increase of ail trie interchain transfer

integrals (p-type)

leads to an increase of trie dosed part of trie FS. For instance, increases of10% and 20% of these transfer

iutegrals

^lead

(8)

cv-(ET)2TlHg(SeCN)4

1307

a Q

C~

g~~ /~

~~kp ~§~ '

_ p.._

#Î~

j~3

#~~

~~j~2 '~ ~~~i

)~3

G~~ ~Î

~~Î ~$~

_~

#Î~ i~~

~§ ~

C

chain chain

I II

Fig. 5. Perspective _view of _an ET layer of _cv-

(ET)2 MHg(XCN)4

where trie different transfer integrals

are defined [31]. ^Each ^molecule _is viewed approximately along its central C ₌ C bond and the hydrogen

atoms _are not shown for simplicity.

to increases of 10.4% _and 18.8% of trie _area,

respectively.

Trie transfer

integrals

_pi and _p4

(see Fig. 5) play

^trie dominant rote _on this _increase.

c)

An increase of ail trie intrachain transfer

integrals (c-type)

leads to _a decrease of trie dosed part ^of ^trie ^FS. ^For instance, iucreases of10% and 20% of these transfer

integrals

^lead

to decreases of 9.9% and 17.8% of trie _area,

respectively.

Such decreases _are

mostly imposed by

trie transfer

integral

_c2 atone.

With these results in mind it is easy ^to understand the almost nil eifect of thermal contraction

on the _area of trie dosed part of trie FS. As

mentioned,

thermal contraction leads to _an increase of both interchain aud iutrachain trànsfer

integrals

_so that trie _two contributions cancel each

other. More important, these results also suggest ^how ^such au area increase _as that

reported

here under _pressure

(21%

at ii

kbar)

_can be reached.

Accordiug

to trie results in

(c)

_a decrease of _c2 should lead to _an increases of trie _area of trie dosed

portion

of trie FS. This is

actually

trie _case: _a decrease of10% in _c2 leads _to _an increase of10.1% _in trie _area. A decrease of

a and _c

crystallographic

parameters ^leads to interchain transfer

integrals

increase.

Thus,

if trie intrachain transfer

integrals

_can be

kept

almost constant, increases of around 20% of trie interchain transfer

integrals

will lead _to the observed 21% _area increase at 11 kbar. In fact, it is

quite

_easy to _see how _c2 _can be

kept

almost constant.

Indeed,

the calculations for _a

pair

of A molecules involved in interaction _c2

(see Fig. 5)

show that:

a)

If trie distance between

planes

of trie _two A donors decreases then _c2 _increases. For instance, _a decrease of trie

interplanar

distance of 0.05

À

_leads _to

an increase of _c2 of about

7%.

b)

If there _is _a

sliding

of _one donor towards trie other

along

trie short _axis of trie molecules then _c2 decreases. A

sliding

of about 0.08

À

_leads _to

a decrease in _c2 of about 7%. In fact,

(9)

2

~

~ G

o ~

à

c 0.12

l$ lÎ

'ôi ~

E

£ 0.06

3 4 5 6 7 8 9

ce

,

n

él _o 2

ce

'g

° P =1

ban

T = 0.8 K

-1

O.lO 0.15 0.20

llB ~T^"~

Fig. 6. Oscillatory part ^of ^the ^ambient _pressure longitudinal magnetoresistance versus inverse magnetic field. The insert displays trie _inverse magnetic ^field at trie slow oscillations extremum _versus

Landau level index

n at T

= 0.8 K. Black and white dots which refers to trie _arrows, depict maxima and minima of trie slow oscillations, respectively.

trie

large

^diiference between trie _two transfer

integrals along

chain I

ici

= -0.011 eV and c2 = 0.140 eV in trie 104 K structure of

a-(ET)2 KHg(SCN)4)

is

largely

due to trie different

degree

of

sliding

associated with the two

pairs

of A molecules. The

slidiug

of the donors towards each other

along

trie short molecular _axis is arouud 0.7

À larger

ⁱⁿ _ci ^than in _c2

[32].

Thus,

_even if _pressure increases _c2

by decreasiug

trie iutermolecular

spacing,

_a

compensation

of this increase may be

provided quite easily by

small

sliding

motions of trie douors. It _seems quite

likely

that trie _increase of 21% of trie _area of trie closed

portion

of trie FS _cau

simply

arise from _au increase of around 20% of trie interchain transfer

iutergrals (and maiuly

_pi and

p4).

Such _au iucrease under 11 kbar _seems to be

quite possible

in view of trie _increases of12% and

8%, respectively

in pi and p4

brought

about

by just

^trie thermal contraction betweeu 298 K and 104 K in

a-(ET)2KHg(SCN)4

In

addition,

20% is

probably

_an _upper limit for trie iucrease

in trie interchain transfer

integrals

because for _uot _very

large slidiugs

of trie

pairs

of molecules

in c2 trie transfer

integral

_can

eifectively

decrease and thus contribute to trie _increase of trie

area.

Therefore,

trie _pressure

dependence

of trie _area of trie dosed part ^of trie FS _seems _to be

reasonable and consistent with _a pressure iuduced

sliding

motion of trie molecules in trie donor chains

(type I) containiug only

_one type ^of ^molecules

IA).

Structural work uuder _pressure is

highly

desirable _to _test this

suggestion.

Besides the above discussed fast SdH

oscillations,

slow

frequency

oscillations bave been observed _at ambieut _pressure, _as it _was

already

noted in [19].

They

_can be _more

clearly

evideuced in trie field range between 5 aud 14 T below 1.5 K and manifest _as _some

amplitude

modulation of the fast oscillations

(Fig. 6).

Trie inverse

magnetic

fields

corresponding

to their extrema

(cf.

_arrows in

Fig. 6)

_are

plotted

_ueTsus _n in the insert of

Figure

6 for trie 0.8 K

experiment.

^An estimation of trie

frequency

of trie slow oscillations from trie

slope

in trie insert

(10)

a-(ET)2TlHg(SeCN)4

1309

of

Figure

6

gives

trie value of

(47+ 3)T

that

corresponds

to 1.1% of trie _area of trie first Brillouin

zone [33]. ^{On trie} contrary, we bave _Dot observed _any slow oscillations _on trie

samples

measured under _pressure in trie _range 3.5-11 kbar.

Assuming

these slow

frequency

oscillations _may be

described,

at ambient _pressure, within trie SdH

model, they

could be associated with _a small dosed orbit due to some

nesting

of

quasi-ID

sheets of FS in trie

a-(ET)2TlHg(SeCN)4.

In contrast, ^trie sister

compound

_a-

(ET)2NH4Hg(SCN)4

_as noted

above, displays

_very similar slow oscillations which appear

only

under moderate _pressure

(P

> 2

kbar),

trie

superconducting

state vanishes _as weII. Trie fact that _pressure _can

change

^trie

nesting

conditions in

organic

^conductors is known _to _occur in both

quasi-ID

_[34] and

quasi-2D [17,

20]

organic

conductors. We _may believe _now that

superconduc- tivity

_occurs in

a-(ET)2NH4Hg(SCN)4

since _no

density

_wave state _occurs at Iow temperature and ambient pressure

[21]. Although

_no

sign

^of

density

_wave condensation appears in either trie temperature

dependence

of trie

resistivity

_or trie

angle-dependent magnetoresistance

_os-

cillations data

[24, 35],

it bas been found that _no

superconductivity

is observed _at ambient pressure down to 80

mK,

which _may be due to _some

nesting

of trie

quasi-ID

sheets of trie FS

in

a-(ET)2TIHg(SeCN)4. Experiments

at

temperatures

Iower than 0A K and in the _pressure range below 3.5 kbar _are needed in order

ii)

to determine the electronic

ground

state and

iii)

to

study

trie

pressure-induced disappearence

of trie slow oscillations.

Acknowledgments

We would Iike to thank Prof. S.

Askenazy

^and ^Prof. E-B-

Yagubskii

for their interest and encouragements. We also

acknowledge

J. M.

Broto,

H. Rakoto and F. Goze for

experimental help

and P. Pari and G. Cotte for their contribution to trie

low-temperature experimental

device at ambient _pressure. VNL is

grateful

to CNRS for the

provision

of his stay ⁱⁿ ^Toulouse

by

Contract No. ID

41/31.

E-C- would Iike _to thank the CNRS for _a sabbatical which made trie stay at ICMAB

possible.

R-R- would like _to thank trie NSERC of Canada for _a

postgraduate

research

fellowship.

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in the numencal values used for such

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T)

measured at 2kbar

m

a-(ET)2NH4Hg(SCN)4

corresponds

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