High-field magnetotransport of organic conductors (BEDT-TTF)2TlHg(XCN)4 With X = S and Se

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High-field magnetotransport of organic conductors (BEDT-TTF)2TlHg(XCN)4 With X = S and Se

M. Kartsovnik, D. Mashovets, D. Smirnov, V. Laukhin, A. Gilewski, N.

Kushch

To cite this version:

M. Kartsovnik, D. Mashovets, D. Smirnov, V. Laukhin, A. Gilewski, et al.. High-field magnetotrans-

port of organic conductors (BEDT-TTF)2TlHg(XCN)4 With X = S and Se. Journal de Physique I,

EDP Sciences, 1994, 4 (1), pp.159-166. �10.1051/jp1:1994104�. �jpa-00246888�

Classification Physics Abstracts

72.15G 73.20D 74.70K

High-field magnetotransport ^of organic conductors (BEDT- TTF)~TlHg(XCN)~ with X

=

S and Se

M. V. Kalrtsovnik (1.

*),

D. V. Mashovets

(2j,

D. V. Smirnov

(2),

V. N. Laukhin

(3),

A. Gilewski (4. 5) ^and N. D. Kushch

(3)

(1) Institute of Solid State Physics,

Chemogolovka,

MD, 142432 Russia (2) A. F. loffe

Physical

Technical Institute,

St,Petersburg,

194021 Russia

(3)Institute

of Chemical

Physics, Chemogolovka,

MD, 142432Russia (4) MAG-NET Laboratory, ul.

Gajowicka

95, 53-529 Wroclaw, Poland

(5) Intemational

Laboratory

of

Strong

Magnetic Fields and Low

Temperatures,

^ul. Gajowicka 95, 53,529 Wroclaw, Poland

(Received 26

July

1993, accepted 3

September

1993)

Abstract.

High

field

magnetoresistance

of

layered

_organic metals

(BEDT-TTF)2TlHg(XCN)4

with X

= S and Se has been studied

using pulsed

magnetic fields.

Sharp

_negative

slope

and strong

hysteresis

^{of the} magnetoresistance ^of (BEDT-TTF)2TlHg(SCN)4 ^{found between about} 11 and 27 T suggest a first,order phase transition to occur at these fields. Two different types ^of angular magnetoresistance oscillations

corresponding

most

likely

_tu different electronic _{states are} observed in the low- and high-field phases, respectively. in _contrast, the classical magnetoresistance ^{of the} sait with X

=

Se _is found _tu be

smoothly

_growing with the field increase. The obtained results _are compared ^{with those} reported ^earlier on the (BEDT-TIF)2MHg(SCN)4 ^salts with M

=

K and NH4 and discussed in the framework of the model proposed recently ^{for the} low-temperature

electronic state of

(BEDT-TTF)2TlHg(SCN)4.

Introduction.

The low-dimensional

organic

conductors

(BEDT-TTF)~MHg(SCN)4

with

polymedc

_aurons

MHg (SCN)j,

where M stands for

K, Tl,

Rb _or

NH~, [1-3]

have been

initially synthesized

as a

modification of _a

superconductor K-(BEDT-TTF)~Cu(NCS)~

which has the

superconducting

cdtical temperature, T~ ^=10.5 K, _one of the

highest

_{among organic} metals. It _tums Dut, however, that

only

the

compound

with M

=

NH~

becomes _a

superconductor

above 0.4 K

[4],

the others

undergoing

_a

phase

transition _at about T~ >

10 K which is reflected _{as a} smooth

hump

_in the resistivity versus temperature

dependence [2, 5, 6]. (As

_was

recently

shown, the

(*) Present address _: Department of Physics, Faculty of Sciences, Kyoto University, Kyoto 606-01,

Japan.

160 JOURNAL DE PHYSIQUE I N°

salts with M

=

K and Rb exhibit _a weak

superconductivity

below 200mK

[7, 8].)

The

magnetic

susceptibility

of

(BEDT-TTF)~KHg(SCN)~

_was found

[9]

to be

essentially anisotropic

below the

phase

transition temperature, T~.

Taking

into _account results of the band

structure calculation

Il predicting

the existence of

slightly warped

_open Fermi surface

(FS)

sheets in addition _ta the

quasi-2D cylindrical

ones, the

phase

transition

might

^{be attributed} ta a

spin-density-wave (SDW) instability typical

of _some

quasi-

ID

organic

conductors

loi.

NMR

studies of

(BEDT-TTF)~KHg(SCN)~ [l Il

revealed _some

changes

in the electronic system

below

T~,

^{however no direct}

signs

of _a

magnetic ordering

_were found. Further

investigation

should be made _ta

clarify

the _nature of the

low-temperature

state of these

compounds

and the

possible competition

between the

magnetic ordering

and

superconductivity.

A number of

puzzling magnetoresistance

anomalies have been

repolrted

for the

compounds

with K

[5, 9, 12-14]

and Tl

[2, 15-17] indicating surprisingly

strong effect of

magnetic

field _on their transpolrt

properties.

Detailed studies of

angular-dependent oscillatory magnetoresistance

of

(BEDT-TTF)~TlHg(SCN)~ [15, 17]

_grue evidence of _a commensurate

superstructure

ansing ^{in the} electronic system ^below T~. ^The superstructure wave vector component,

Qa, Perpendicular

to the

plane

of the open FS sheets was found _to be close ta the distance

between the

sheets, 2k~,

thus

supporting

the idea of the

Peierls-type density

wave

formation

[17].

As _was shown _in

[2, 17], (BEDT-TTF )~TlHg (SCN

_)~ shares

essentially

ail the

magnetoresi-

stance features with the _most

popular (BEDT-TTF ~KHg(SCN)~

under

magnetic

fields up to

14 T. Since _a number of

important

anomalies have been observed in the latter

compound

in the

fields above 15÷17T, it would be aise

interesting

_{to go} to the

higher

fields for

(BEDT-TTF )2TlHg (SCN

)4.

Here, we present ^{the results of} the

high-field magnetoresistance

studies of

(BEDT-TTF)~TlHg(SCN)~.

In order _ta

distinguish

the

specific

features related ta the low-

temperature state of the

compound,

we have aise studied _{a new}

conductor,

(BEDT-TTF )~TlHg (SeCN

)~, ^which

only

differs from the former _in

substituting

the S _atom

by

Se in the anion and does not exhibit the

low-temperature phase

transition

[18].

Experiment.

Single crystals

of

(BEDT-TTF )~TlHg (SCN

_{)~ and}

(BEDT-TTF )~TlHg(SeCN

_)~ were obtained

by electro-crystallization

as described elsewhere

[2, 18].

The

sample

resistance _was measured

perpendicular

to the

crystal highly-conducting ac-plane

in

pulsed magnetic

fields up ^to 35 T at temperatures 1.5 _÷ 4.2 K. The

samples

were

glued

to 20

p~m-diameter plantinum

wires

by

a

graphite

_paste. TO avoid vibrations

during

the field

puises,

the

samples

were fixed _{ta a}

sample

holder

by

_a silicon

polymer liquid.

The measurements were

perforrned

in

puise

magnets in the

Laboratory

of Low

Temperature

Transport

Phenomena

(LLTTP)

of the A.I. Ioffe

Physico-Technical Institute,

St.

-Petersburg,

Russia, and in the Intemational

Laboratory

^of

Strong Magnetic

^Fields and Low

Temperatures

(ILSMFLT), Wroclaw,

Poland.

In

LLTTP,

the magnetic ^fields

perpendicular

ta the

ac-plane

were obtained with either 5 _ms total

puise

duration

(referred

below _{as a}

short-puise regime)

or 2.5 ms-rise and 25 ms-decrease

(a long-puise regime).

The

signal

from the

crystal

was

amplified by

a broad-band

amplifier

and

transferred _{ta an}

analog-to-digit

converter

(ADC)

with _p~s gate ^{and 20} p~s conversion lime.

In

ILSMFLT,

_magnetic fields _were

generated by

_a 40T

puise

magnet ^{with the}

puise

duration of 10 _ms. The

crystals

^{could be rotated} with respect ^{to the}

magnetic

field in _a

plane perpendicular

ta the

ac-plane.

The Shubnikov,de Haas

(SdH)

oscillations _were studied with a

de,method using an

analogue

second-denvative

technique

with _a broad-band

amplifier.

The classical

magnetoresistance

was measured

by

a standard

ac-technique

with a narrow-band

amplifier ~f

₌ ⁵⁰ kHz

).

The

amphfied signais

were

processed by

ADC with the

digitizing frequency

of 300 kHz.

Results.

MAGNETORESISTANCE

(MR)

FIELD DEPENDENCES. H L

ac-plane. Figure

_represents

typi-

cal MR _traces obtained for the

(BEDT-TTF )~TlHg (SCN

_)~

crystals

at different _«

sholrt-puise

_»

sweeps ^at 1.6 K and 4.2 K. The

following _impolrtant

features of the classical part ^{of MR} are to be

pointed

eut.

w

loi ~

j

^{<0 6K}

50

20

<o

o

o io Jo ào

~, ào

20 H~

~ l 6K

io

4.2K

o io 20 ào

H,

Fig.

l.

Magnetoresistance

traces for the Tl-(SCN) sait obtained from _a series of _successive field puises

at 1.6 K and 4.2 K. H 1 ac. The upper curve of each _set

corresponds

tu the

upward

field sweeps while the others represent ^{the downward} sweeps from different H~~i~~. Inset:

Magnetoresistance

hysteresis at

« long _» and _« short _» puises. Curve1 field up, H~~i~~ ⁼ 35 T _curve 2 field down,

long,puise

regime, H~~i~~ = 30 T _curve 3 field down, short-puise _{regime, H~~i~~}

=

30 T.

l.

Very high

MR in the fields up ^tu 13 T is consistent with that observed earher

[2].

At

higher fields,

MR stalrts ta decrease

gradually

and then

draps rapidly

^{in the interval between 20}

and 23 T. Above 27 T it becomes

practically field-independent being

at least 6 times smaller

thon the maximum MR. A very similar behavior has been

repolrted by

^several groups for

(BEDT-TTF)~KHg(SCN)~ (see

_e-g-

[12, 19, 20]).

162 JOURNAL DE PHYSIQUE I N°

2. At H

~

H~

- II T _a considerable difference _anses between the MR _traces

registered

at up and down sweeps of the field

(a

small difference between the _curves at lower fields _cames

most

likely

from a

slight over-heating

of the

sample during

the

puises,

it should be aise taken

into account that the accuracy of the measurements under

high-field puises

is

being

^{reduced in}

their low-field

parts).

Trie downward MR traces become lower _at

increasing

the maximum value of the

puise,

_{H~~j~~, up} to H~~j~~ ⁼

H~

=

(27

_±

1)

T. Further _increase of

H~~j~~, up to 35 T, does not affect the

position

of the downward MR _curve and _no

hysteresis

is observed

above

H~.

As for the

upward MR,

it remains

reproducible

for ail the

puises.

3. The

magnitude

of the

hysteresis, àR(H)mR~~(H)-R~~~~(H),

_increases

rapidly

on

cooling

the

sample

from 4.2 K _to 1.6 K. At the _same time the

changes

in the cntical fields,

H~

and

H~,

do non exceed 10 fb within this temperature range.

4.

Finally,

the

hysteresis magnitude,

àR

(H),

_{appears to}

depend

_on the

puise

duration. As is

seen in the inset in

figure1,

the

sholrtening

of the

field-decay

_time from 25 _{ms to} 2.5 _ms («

long

» and _« sholrt _»

puises, respectively,

with the _same H~~j~~

value)

leads _{tu a}

significant

increase of àR

(H).

As _concems the SdH

oscillations,

_we have observed both the fundamental and second

harmomcs

(Fig. 2)

with the

frequences

670 T and 340 T,

respectively,

in agreement ^{with the}

previous results[16, 17].

Like in

(BEDT-TTF)~KHg(SCN)~,

the second harmonic is

enorrnously

strong above 18 T,

although

somewhat weaker than _in the latter

compound,

and

drops rapidly

at about

HE.

aoo

~ oi

ôoo

-~

~

C©

~n3

200 0

.03

1/H, T~~

Fig. ^2.

splittmg of the oscillations, I.e. _strong second arrnonic is most ronounced

between about

18

and 27 T.

In contrast tu

(BEDT-TTF)~TlHg(SCN)~,

the

Se-contaimng analog (BEDT-

TTF)~TlHg(SeCN)~

exhibits rather smooth

growth

of the classical MR

(Fig. 3).

No _traces of either negative ^MR contribution _or

hysteresis

^have been found. The SdH oscillations _are in

perfect

_agreement with those observed in

[21]

in

steady magnetic

^field H~14T, with

R(H)

@

5 _©

j 6~

j

Î E

4

~

o soc iooo isoo éooo

F, T 3

2

~0

_{10 20} 30

H, T

Fig.

3.

Magnetoresistance

of the Tl-(SeCN) sait at T

=

1.6 K, H _{1 ac.} lnset _: Fourier spectrum ^{of the} SdH oscillations.

F

m 650 T, _{m =}

(1.95

_± 0.05

)

_{mo and}

T~

> 0.6 K. Both the fundamental and second harrnonic components grow up

monotonically

with

increasing

the field their

ratio, A(2 F)/A(F),

non

exceeding

_a few

percent

at 1.6 K

(see

inset in

Fig. 3).

ANGULAR MAGNETORESISTANCE

osciLLATioNs.-Initially,

the

angular

MR oscillations

(AMRO)

found in

(BEDT-TTF)~KHg(SCN)~

at low temperatures

[12]

_were considered _tu have the _same

engin

as in

quasi-2D

metals

fl-(BEDT-TTF )~IBr~ [22]

and

0-(BEDT-TTF)~I~

[23],

1-e- associated with closed orbits

along

the

cylindrical

FS.

However, sharp

^minima rather than maxima have been noted _to be characteristic points ^of the MR

angular dependence.

As

was

suggested

in

[24],

the AMRO _reverse their

phase

around T~ ^or

H~,

_i-e- the MR minima _are transforrned into _maxima and vice-versa. The _reasons for such _an

abrupt change

of the oscillation

phase

are non clear.

On the other hand, an

analysis

of AMRO _in the

low-temperature

state of

(BEDT-TTF )~TlHg (SCN

_)~

[15]

showed that

they

must have the _nature different from that in

p-(BEDT-TTF)~IBr~ being

associated with the existence of _a

specific plane

in the electronic

system,

most

hkely,

_a

plane

of _an open FS.

Basing

_on the oscillation

parameters,

it _was

proposed [17]

that _a

Peierls-type

commensurate superstructure ^arises below T~ ^thus

changing essentially

the electronic band structure. In this _case, the

low-temperature-low-field

AMRO

relevant

inherently

to the _new superstructure ^should not be related to those

existing

above T~ and/or

H~.

In order _to compare the AMRO _in the low- and

high-field

states of

(BEDT-

TTF)~TlHg(SCN)~,

we have measured its

magnetoresistance

at different field orientations.

Figure

4 demonstrates the

angular dependences

of the magnetoresistance at

rotating

the

sample perpendicular

to the

highly-conducting ac-plane

^for two

fields,

13 T and 30 T. As _is seen in the

inset in

figure

4, there exist celrtain orientations for which a considerable

growth

of MR _occurs

164 JOURNAL DE PHYSIQUE I N°

300

f

200 ~=3~

p ^iOO

~

o

iO 20 30

w j H, T

E

_Î

~

_Î

i

~y~

~ Î,

~Y

'( /

~

"-~

20 40 60 80

p,

degrees

Fig. 4. Angular dependences of magnetoresistance of the Tl-(SCN) sait _at (1) 13 T _; (2) 30 T. Field rotation

perpendicular

tu the ac-plane, _q~ is the angle between H and the b*-axis, T

= 4.2 K. lnset Magnetoresistance field dependences for trie field orientations _near the first minimum and maximum _in

the _curve 2.

at

high fields,

instead of the

negative slope

charactenstic of most _common orientations. These celrtain field directions

correspond

to

sharp

MR maxima _at the

high-field angular dependence (curve 2, Fig. 4).

The

high-field

AMRO appear ^to be

periodic

in _{tan q~ as} well _as the low-field

unes. It is impolrtant tu note that,

despite

the apparent closeness of the

positions

of the

high-

field MR maxima

(downward

_{arrows in}

Fig.

4 at _q~

m

34°,

63° and

72°)

and the low-field MR

minima

(upward

_arrows at

~D =

26° and

65°),

the difference between them _{is out} of the

expenmental

_{error range.} The _{same can} be said about the oscillation

penods

:

although

it is difficult _to estimate them

correctly

due to lack of

experimental data,

_{one can see} that, _anyway, the low-field AMRO have _a

larger period

then the

high-field

ones.

Discussion.

Basing

_on the model

proposed recently

for the

groundstate

^of

(BEDT-TTF )~TlHg (SCN

_)~

[17],

one can understand the obtained results _as follows.

The low-field

(below H~)

magnetotranspolrt ^{is determined}

by

the FS

magnetic

^breakdown network forrned from the initial

cylinders

under _a

periodic density-wave potential.

This FS includes _new open sheets which may be

responsible

for

high

MR _at H

~ l1 T. The

relatively complicated

SdH

oscillations, including

the

enormously

strong second

harmomc,

_are due to the breakdown between the open sheets and small

cylindrical pockets [18].

The _new open

sheets _are aise _consistent with the low-field AMRO.

In the

high-field

_range, 1-e- above

H~,

the classical pan ^{of MR tends} to a constant value at

H L ac and the SdH oscillations demonstrate

extremely

_strong fundamental

frequency,

670

T,

and

only

_a small second harmonic contribution consistent with that

expected

from the Lifshits-

Kosevich formula. This _is very similar to the _case of the

Tl(SeCN)-sait (Fig. 3)

and

(BEDT-TTF )~NH~Hg(SCN)~ [25]

which do _non

undergo

the

phase

transition. Hence _we may

conclude that strong

magnetic

field restores the

high-temperature

electronic state in

(BEDT-TTF)~MHg(SCN)~

with M

= Tl and K with the FS

including

the

quasi-ID planes perpendicular

to the a-direction and

quasi-2D cylinders.

This _{is in} agreement with the

phase diagram proposed

^for

(BEDT-TTF )~KHg (SCN

_)~ l

9, 20].

^{In this} _case, the

conductivity

under

magnetic

field

perpendicular

to the

ac-plane

should be dominated

by

the FS

cylinders

that leads to the saturation of the MR _{to a} value much lower than in the low-field

phase.

Thus, the MR kink must manifest the

phase

transition between the

low-field-low-temperature

state and the

high-field/high-temperature

_one. The

hysteresis

indicates that _we deal with the first order transition. It is

interesting

_{to note} the strong ^{effect of} temperature on the

hysteresis magnitude, àR(H),

whereas its

edges, H~

and

H~,

remain almost

unchanged

within the studied

temperature ^interval.

Since the

magnetic

breakdown network does not exist any more in the

high-field phase

and the FS is

expected

_to be the _{same as} above

T~,

^{the SdH effect is} contributed

by

semi-classical

orbits _on the

quasi-2D cylinder.

This leads to the

simplification

of the oscillation spectrum ^and considerable enhancement of their

amplitude.

As is _{seen, in}

palrticular,

in

figure1,

the oscillation

amplitude (norrnalized

_to the

background MR)

is

approximately

the _{same as} for the

Tl(SeCN)-sait (Fig. 3).

The

angular

oscillations of the classical MR aise _{tum out to} be _in agreement with the above consideration.

Indeed,

while the low-field AMRO

(Fig. 4, curve1)

_are associated with the open FS

[15, 17],

their behavior in

high

fields

(Fig. 4,

_curve

2) corresponds perfectly

to the FS

cylinder.

An

analogous

transition from the _«

plane

» AMRO at T _~ T~ ^{to the} «

cylindric

_{» ones}

above T~ ^{has been} very

recently

^observed and

investigated

_on

(BEDT-TTF )~KHg (SCN

_)~

[26].

The

comparison

between the MR behavior

(BEDT-TTF )~TlHg (SCN

_)~ and

(BEDT-TTF )~KHg(SCN)~

shows

clearly

the

identity

of their electronic

propelrties.

The

only quantitative

difference is that the charactenstic

fields, H~

and

H~,

_are somewhat

higher

in the

Tl-contaimng compound.

Summanzing,

_we have studied the

high-field magnetotranspolrt

of

(BEDT- TTF)2TlHg(XCN)4

with X

=

S and Se. The MR behavior of

(BEDT-TTF)2 TlHg(SCN)4

has

been shown _to share ail the characteristic features with its

K-containing analog.

The

comparison

of the MR of _two latter

compounds

^with the

Tl(SeCN)-sait

supports

strongly

the idea that their

high-field

electronic _{states are} identical _to the

high-temperature

ones. The classical MR _as well _as the

angular

and SdH oscillations _can be understood within the model

proposed

ⁱⁿ

[17]

^for the

low-temperature

electronic state of

(BEDT-TTF)~TlHg(SCN)4.

The MR kink

corresponds

most

hkely

to the fisrt order

phase

^{transition between} the _« low-

temperature-low-field»

and the

«high-temperature/high-field

_» states. Of _course, fulrther

investigations

_are to be done to

clarify

the mechanism

responsible

for the

phase

transition and detalize the nature of the _«

low-temperature-low-field

_» state in these

compounds.

Acknowledgements.

We are thankful _to N. E. Alekseevskii and T. Palewski for their kind permission to

perform

a

part ^{of the} measurements in the Intemational

Laboratory

of

Strong Magnetic

Fields and Low

Temperatures

in

Wroclaw,

I. F.

Schegolev,

E. B.

Yagubskii

and R. Parfeniev for their _interest and

encouragement.

^{The work} was

partly supported by

the Intemational Soros Foundation

awarded

by

the Amencan

Physical Society. M.V.K.,

V.N.L. and N-D-K-

aknowledge

the

Russian Foundation for Fundamental Research Grant No. 93-02-14246.

166 JOURNAL DE

PHYSIQUE

I N°

Refereuces

[1] Oshima M., Mon H., Saito G. and Oshima K., Chem. Lent. (1989) l159.

[2] Kushch N. D., Buravov L.1., Kartsovnik M. V., Laukhin V. N., Pesotskii S.1., Shibaeva R. P.,

Rozenberg

L. P.,

Yagubskii

E. B, and Zvarikina A. V.,

Synth.

Menais 46

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