Angle-Dependent Magnetoresistance Oscillations and Thermoelectric Power of (BO)2Cl(H2O)x

HAL Id: jpa-00247285

https://hal.archives-ouvertes.fr/jpa-00247285

Submitted on 1 Jan 1996

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Angle-Dependent Magnetoresistance Oscillations and Thermoelectric Power of (BO)2Cl(H2O)x

T. Mori, S. Ono, H. Mori, S. Tanaka

To cite this version:

T. Mori, S. Ono, H. Mori, S. Tanaka. Angle-Dependent Magnetoresistance Oscillations and Thermo- electric Power of (BO)2Cl(H2O)x. Journal de Physique I, EDP Sciences, 1996, 6 (12), pp.1849-1853.

�10.1051/jp1:1996192�. �jpa-00247285�

Angle Dependent Magnetoresistance Oscillations and Thermoelectric Power of (BO)2Cl(H20)x

T. Mari

(~>*),

^{S. Ono}

(~),

H.

Mori (~)

and S. Tanaka

(~)

(~

Department

of

Organic

and

Polymeric Materials, Tokyo

Institute of

Technology, O-okayama, Tokyo

152,

Japan

(~) International

Superconductivity Technology Center, Shinonome, Tokyo

132,

Japan

(Received

26

February1996,

revised 22

April

1996,

accepted

3 June

1996)

PACS.71.20.Rv

Polymers

and organic

compounds

PACS.îl.18.+y

Fermi surface: calculations and measurements; effective _{mass, g} factor PACS.71.20.-b Electron

density

of states anal band structure of

crystalline

solids

Abstract.

Magnetoresistance

of the t.ittle _organic

metal,

where BO is

bis[ethylenedioxy)- tetrathiafulvalene,

is

investigated.

The

angle dependent

oscillations indicate the existence of

cylindrical

Fermi surface, and trie observed

penodicity

is consistent with the calculated band

structure. The

amplitude

of the oscillation is very small in the direction where the Fermi surface

touches trie _zone

boundary.

Trie thermoelectric power is _zero in trie whole temperature _range in ail

directions, suggesting effectively

_a half-filled situation of trie band structure.

1. Introduction

Recently-synthesized organic metals, particularly

those based _on BEDT-TTF

(bis(ethylenedi- thio)tetrathiafulvalene) family donors,

exhibit two-dimensional

conducting properties owing

to their

loyer

structures

composed

of alternate

stacking

of _organic

conducting

^sheets and

insulating

_anion

layers-

The Fermi surface of these

organic

^metals are those

expected

for two-dimensional

conductors;

trie Fermi surface

basically

bas _a

cylindrical shape

with _some modifications

originating

in trie

crystal

structures. Trie

topology

of trie Fermi surface bas been calculated

by

_means of trie

simple tight-binding approximation,

and trie results bave been _in

good agreement

with trie observations of trie Shubnikov-de Haas and trie de Haas-van

Alphen

eoEects

iii

In addition

oscillating

structure was found in

angle dependent magnetoresistance

of

9-(BEDT-TTF)213

_12] and

fl-(BEDT-TTF)2IBr2

_13], and _was

interpreted by considering

trie

corrugation

of trie

cylindrical

Fermi surface [4]- ^{From trie}

positions

^{of these}

angle dependent magnetoresistance

oscillations

(ADMRO),

_{we cari} estimate trie

positions

of

kF, namely

trie

shape

of trie Fermi surface.

Organic metal, (BO)2Cl(H20)~ (BO bis(ethylenedioxy)tetrathiafulvalene),

that we will deal with in the

present report,

was first

reported by

Schweitzer et ai-

[si

^{On trie basis of trie}

X-ray

structure

analysis,

Schweitzer et ai. concluded trie

composition

to be

I:I, (BO)CIH20,

but _our observations of trie Shubnikov-de Haas and trie de Haas-van

Alphen effects,

followed

by

trie chemical

analysis,

indicated that trie

composition

bas to be 2:1,

(BO)2Cl(H20)~ _[6j, though (*)

Author for

correspondence je-mail: takehiko©o.cc.titech-ac.jp)

©

Les

Éditions

_de

_Physique

₁₉₉₆

1850 JOURNAL DE

PHYSIQUE

I N°12

C

H

8 b

é,"" ₄

Fig.

i- Definitions of

angles.

trie _exact content of

H20

could net be estimated-

Taking

trie 2:1

composition

into

account,

_we

can

regard

trie donor

arrangement

_as so-called

Ù-type,

where trie donor

arrangement

^{is similar}

to

Ù-(BEDT-TTF)213.

Because trie latter salt is _one of trie

compounds

in which ADMRO

was observed first

[2j,

^and

recently

a more detailed

study

of trie Fermi surface

by

_means of ADMRO bas been

reported I?i,

_we

applied

this method to trie

present compound.

Trie observed ADMRO is

basically

similar to

Ù-(BEDT-TTF)213

and agrees with trie calculated Fermi surface.

In trie

present

_paper we also

report

^trie measurement of

anisotropy

of trie thermoelectric _power, because it is also

closely

associated with trie

anisotropy

^of trie Fermi surface [8j-

2.

Experimental

The

crystals

in trie form of

hexagonal plates

_were obtained

by

trie electrochemical method

là, 6].

Four electrical contacts _were

placed

_{so as} trie current

patin

to be

perpendicular

to trie

conducting plane,

and trie

sample

_was mounted _{on a} holder with two

degrees

of rotational freedom- The definitions of trie

angles

_are shown _in

Figure

i- The latitudinal rotation à _was transferred from trie

top

^{of trie}

cryostat

with _a shaft and _a

pair

of bevel _gears, and computer controlled with _a

stepping

motor. Trie azimuthal rotation çi was achieved

by connecting

_a shaft _to trie

sample

holder at ₌ o°

position. Magnetic

field _up to 9 T _was

applied vertically-

The residual

resistivity

ratio of trie

sample

used _was 15. Trie

crystallographic

_{axes were} determined

by

means of _an

X-ray

four-circle diffractometer after trie measurements-

The measurement of trie thermoelectric _{power was carried out as} described in reference

[9j.

Trie energy band and trie Fermi surface _were calculated

by

trie

tight-bindiiig method,

where trie transfer

integrals

_were estimated from trie

overlap

of HOMO obtained

by

trie extended

Hückel molecular orbital calculation [6j-

3. Results and Discussion

The

sample

showed

large magnetoresistance

with considerable

amsotropy;

at 1-5 K trie resis- tance under _a field of 9 T _was

2-5, 1.9,

and 1-4 times

larger

than trie zero-field value for H _{jj a,} H jj b, and H

jj c,

respectively-

In addition clear

oscillating

structures _were observed _as shown

in

Figure

2. Trie ADMRO _{was very}

large

in trie _çi

= 90° direction

(H

jj

b),

^while _very small in trie _çi

= o° direction

(H

jj

ai.

Trie _same

tendency

bas been observed in

Ù-(BEDT-TTF)213

_1?i,

though

trie oscillation

amplitudes

of trie

present compound

_seem to be _more diminished-

This

phenomenon

is

possibly

associated with trie

shape

of the Fermi surface which is about to touch the _zone

boundary

_in this direction

(Fig. 3). Owing

to the

crystal symmetry, namely

from trie

a-ghde plane

of trie

P21la

_{space group,} trie _energy band is

degenerated

_on this XS

zone

boundary. Therefore,

_even if the Fermi surface _crosses this _zone

boundary,

the Fermi surface will _never

split

to

two,

and will

keep

the

large

connected

cylindrical geometry-

^It

is,

however, possible

that _a very small second-order eoEect _may obscure the connection of the

io

g~W4

8

,

7

6

~ ~

C

à

3 8 ~m75°

~

à

7

6

,

^#m90"

~ ~~

Î ~/

T=1.5K

-ioo o ioo

@ideg.

Fig.

2.

Angle dependent

magnetoresistance of

(BO)2Cl(H20)~.

Zero-field _resistance is 4 ohm.

corrugated cylindrical

Fermi surface _near the X point, and may

drastically

reduce the

amplitude

of ADMRO in this direction.

Actually

the calculated Fermi surface of

Ù-(BEDT-TTF)213

is

substantially

_away from the X

point,

but that of the

present compound

is very close to the X

point.

The _more reduced ADMRO of the

present compound

is

probably

associated with this difference of the Fermi surface.

The oscillations at

#

₌ 90°

(H

⁽

_b)

_are

slightly unsymmetrical

with

respect

to ₌ 0°. This is not

necessarily

related to the

misalignment

of the

sample,

but is attributable to _an mherent

phenomenon,

because the present

compound belongs

to monoclinic

system (fl

₌

98.0à°)

_[Si.

On the

contrary R-(BEDT-TTF)213

has

essentially

orthorhombic

symmetry except

trie low-

temperature slight

modulation

[10),

^{and trie observed ADMRO is}

practically symmetrical [7j.

Trie

peak angles

^of ADMRO should

satisfy

the

relation,

A tan =

7r/ckf _Î4j.

From trie intervals of trie observed

peaks, kF

_is estimated for each

#,

_as shown _m

Figure

3. In the

1852 JOURNAL DE

PHYSIQUE

I N°12

kb

o.7

~ S

_ .

>

uÎ

ka

~y ~

x

Ul

-0.4

Y r x s r

a) b)

Fig.

3.

(a) Energy

band structure of the HOMO band, and

(b)

Fermi surface [6]. ^{The closed} squares stand for kF values obtained from ADMRO.

- s

~

~

4

~

~

~ ~

à ^Ù~

'Z~

j

lia

( Î

₂

o

E

5

0 J00 200 300

Temperature (K) Fig.

4. Thermoelectric _power of

(BO)2Cl(H20)~.

directions close to H (

a

(-4à°

_{< <}

4à°),

ADMRO is not clear

enough

to

give kF

_{m a} reasonable _accuracy. The extracted

kF

is in

agreement

with the calculated Fermi surface.

In order to further demonstrate the

similanty

to

Ù-(BEDT-TTF)213,

_we bave measured thermoeletric _{power m} trie two

independent

directions within trie

conducting plane,

as showu iii

Figure

4. The thermoelectflic power is very close to _{zero over} trie whole measured

temperature

range

irrespective

of trie directions.

However,

this bas also been observed in

Ù-(BEDT-TTF)213 [11]. Although formally

trie douor HOMO band is

regarded

_as

quarter-filled,

_{we cau suppose}

that the _upper band is half-filled iii trie _sense that trie cross-section of trie Fermi surface _is half of trie first Brillouiu _zone.

Accordiug

to trie

simple

bond

picture,

_a half-filled bond may result

m zero thermoelectric _power.

This is _a naive

explanation,

but _we should rather

emphasize

the difference from other

fl-

aud

~-phase compouuds.

lu the latter

compouuds

the half-filled character of the _upper baud has beeu

poiuted

Dut _{as a} result of the

stroug

dimerizatiou of the douer molecules. These

compouuds,

however, show

large

thermoelectric _power,

typically

20 30

pV/K

at room tem-

perature [8],

^{aud other}

physical properties

aise have

suggested

trie

importance

of the correlatiou effect

[12).

^The preseut

Ù-phase

_is

probably

the

opposite limit;

there _{is no}

dimerizatiou,

this _ex-

actly

_cames from the

symmetry

^{of the}

crystal structure,

aud the effect of correlatiou is the least

important

_amoug the BEDT-TTF

type layered

structures. The importance of the correlatiou effect _seems to be

proportioual

to the

degree

^of dimerizatiou. We _cau therefore

regard

that

because correlatiou is uot

important

and because valve baud

picture

_{is a}

good approximation,

thermoelectric _power of the

Ù-phase

becomes _zero.

lu conclusion _we have showu that

basically

ADMRO of the preseut

compouud

has _a close resemblauce to that of

Ù-(BEDT-TTF)213,

aud agrees with the

cyliudrical

Fermi surface _ex-

pected

from trie baud calculatiou. Some differeuces _are

slight uusymmetry

comiug from trie

crystal symmetry,

^{aud trie influence of trie Fermi surface which touches trie} zone

bouudary.

lu

view of trie ADMRO

together

with trie thermoelectric _{power, we} bave discussed

comparative uuimportauce

of trie correlatiou iii trie

Ù-phase.

Acknowledgments

The authors _are

grateful

to Mr. N. Mizutaui at Iustitute for Molecular Science for mauufac- turiug a

cryostat

with two

degrees

of rotatioual freedom.

References

iii

For _a receut review, ^see Wosuiza

J.,

Int. J. Med.

Phys.

B 7

(1993)

2707.

[2]

Kajita K.,

Nishio

Y.,

Takahasi

T.,

Sasaki

W.,

Kato

R., Kobayashi

H. aud

Iye Y.,

Solid State Commun. 70

(1989)

l189.

[3] Kartsovuik

M.V.,

Kououovich

P-A-,

Laukhiu V.N. aud

Schegolev I.F.,

Pis'ma Zh.

Eksp.

Teor. Fiz. 48

(1988)

498.

[4]

Yamaji K.,

J.

Phys.

Soc.

Jpn

58

(1989)

1520.

[si

^Schweitzer D., Kahlich

S.,

Heiueu

I.,

Lau

S-E-,

Nuber

B.,

Keller

H-J-,

Wiuzer K. aud

Helberg H-W-, Synth.

Met. 56

(1993)

2827.

[6] Mori

T.,

Oshima

K.,

Okuuo

H.,

Ilato

K.,

Mari H. aud Tauaka

S., Phys.

Reu. B 51

(199à)

lll10.

[7]

Klepper

S-J-. Athas G-J-, Brooks

J-S-,

Tokumoto

M.,

Kinoshita

T.,

Tamura N. and Ki- noshita

M., Synth.

Met. 70

(199à)

835.

[8] Mori T. and Inokuchi H., J.

Phys.

Soc.

Jpn

57

(1988)

3674.

[9) Mon

T.,

Inokuchi

H., Ilobayashi A.,

Kato R. and

Kobayashi H., Phys.

Reu. B 38

(1988)

à913.

[10] Kobayashi H.,

Ilato

R., Kobayashi A.,

Nishio

Y., Kajita

K. aud Sasaki

W.,

Chem. Lett.

1986,

833.

[1ii

Tamura

M.,

Dr.

Thesis,

The

Uuiversity

of

Tokyo (199à).

[12] Miyagawa

K., Kawamoto

A.,

Nakazawa Y. and Kanoda

K., Phys.

Reu. Lett. 75

(1995)

l174.