Physica 143 B I1986) 400-41;2 400 North-Holland, Amsterdam

HIGH FIELD MAGNETORESISTANCE OF (TMTSF)2CIO 4 AND (TMTSF)2PF 6. QUANTUM EFFECTS AND PHASE TRANSITIONS

Jean-P ie r re ULMET~ Ahmed KHMOU and Laurence BACHERE

Laboratoire de Physique des Solides e t Service des Champs Magn~tiques intenses, INSA, Avenue de Rangueil, 31077 Toulouse-Cedex, France

The t ransverse magnetores i s tance of (TMTSF)2CIO 4 and (TMTSF)2PF 6 has been invest igated in a high magnet ic Iield up to 32 T and at low tempera tu re down to 2.4 K. Both co,npounds have exhibited an

oscil latory behaviour and slope changes of the monotonic magnetores is tance a t t r ibuted to phase

transi t ions. The true nature of the Shubnikov-de Haas-like oscillations remains unknown but their

f requencies , respect ively close to 260 and 230 T for B//c* seem to indicate a longitudinal nesting

vector QL ~ (2 k F, 0, 0).

1. INTRODUCTION

The understanding of the behaviour o£ TMTSF

salts under high magnet ic field and low

t empera tu re conditions remains incomplete in

spite of numerous exper iments . Among these

compounds, (TMTSF)2CIO ~ and (TMTSF)2PF 6

part icularly have exhibited new and surprising

e f f ec t s such as the possibility of being in a

t empera tu re and field dependent spin-densi ty-wave 1

(SDW) state •

Recent theore t ica l works give a good

explanat ion of the very low temperature

exper iments by introducing a set of phase

t ransi t ions inside the SDW region oI the phase

diagram 2, 3. However the part located at higher

Iields and t empera tu res (roughly beyond 12 T and

2 K) has been less explored and many questions

arise to explain what actually happens in this

range.

In this ar t ic le we report some original results

which are hoped to contr ibute to the

understanding of the high Iield region.

2. RESULTS AND DISCUSSION

2.1. Magnetores is tance of (TMTSF)2CIO 4

The samples are classically mounted with four

contac ts , the current Howing along the needle

axis. They are slowly cooled from room

tempera ture down to 40 K to avoid cracks. From

40 K to 4.2 K tile cooling ra te does not exceed

0.3 K/minute to get a relaxed s ta te (R-state) .

The t ransverse magnetores i s tance is measured up

to 32 T for d i / / e ren t t empera tures down to Z.t~ ~

and for d i f fe rent angles !~ :: (B, b'). Ihe figure l

gives an example of such ~ magnetores~stance

taken at T = 4.2 K and for B//c ~.

~___P Po

40

30

20

10

0

b,

TMTSF 2 CIO 4

T = 4 2 K

B / / c * a , ~ / b~

a V

a a

~ 2'O 10 3'o B

FIGURE 1

Transverse magnetores i s tance ol (TMTSF)2CtO 4

for T = 4.2 K and B / / c * ,

0378 - 4363/86/$03.50 © Elsevier Science Publishers B.'v. (North-Holland Physics Publishing Division) and Yamada Science Foundation

J. -P. Ulmet et al. / High field magnetoresistance of ( TMTSF)2(3104 and ( TMTSF)2PF 6 401

The slope change occuring at a c r i t i ca l f ie ld

B c is assigned to the metal-SDW transi t ion # .

A f te r this point high frequency oscil lat ions

appear which have a good per iod ic i ty in I /B and

look very simi lar to Shubnikov-de Haas (SDH)

ones. This kind of magnetoresistance has already

been observed 5'6, however in our experiments we

obtain for the f i rst t ime two series of

osci l lat ions called "a" and "b". They have the

same frequency but their amplitudes behave quite

d i f fe rent ly wi th the temperature • below #.2 K

the two series are present but rather quickly,

between 4.2 K and 4.8 K) the "b" oscil lat ions

disappear whi le the "a" series remains present at

much higher temperatures (up to about 12 K).

The f ie ld positions of the peaks) BN) permi t to

calculate the fundamental f ie ld B F of the

series •

l IB N = l IB F x (N - N O )

where N is an integer and N O the phase of the

series.

For SDH-like osci l lat ions, the f ie ld B F is

re lated to the cross-section of the Fermi surface

(FS) : 2 ~ r e

S = ----h---- B F

The f igure 2 presents the angular var ia t ion of

B F and N O for the two series at T = 3 K . We

can not ice that the fundamental f ie ld of the two

series is close to 260 T in good agreement wi th

other determinat ions 5, 6. No signif icant var ia t ion

of this value wi th the temperature has been

found. The corresponding pockets of carr iers

represent about 3.# % of the Bri l louin zone

which seems possible wi th a longitudinal nesting

vector QL = ( 2 k F ) 0 , 0). This vector is precisely

the one predicted by recent theories.

The angular var ia t ion of B F fol lows a cosine

law indicat ing ei ther plane orbits or regular

tubes extending along the c* axis.

The most natural explanat ion for the two

series involves pockets of electrons and holes

having the same area) which is coherent wi th

the longitudinal nesting vector . The reason for

B F

400

3 0 0

200

1 0 0

'\ ,M,s ,3Kco f ' / /1

*~ a s e r i e s • 0 • b - -

/ , ~ . "--~. , ~ ~ . . ( e . 9 o ) ' ~ . , .

- 1 2 0 - 9 0 - 6 0 e

- .5

FIGURE 2

Angular dependence of B F and N O for "a" and "b" series) T = 3 K

the vanishing of the "b" series above 5 K is not

wel l established.

To come back to the metal-SDW transit ion)

the temperature p lot ted against the f ie ld B c

gives the boundary of the two states) already

published elsewhere 4. Above 5 K) the B f ie ld C

becomes impossible to determine (correlat ion wi th

the vanishing of the "b" series ?) as no clear

change in the magnetoresistance slope is visible

any more.

2.2. Magnetoresistance of (TMTSF)2PF 6

The crystals are always slowly cooled but the

cooling rate at low temperature is not c r i t i ca l

as no anion ordering is expected. The

magnetoresistance has been studied in the same

f ie ld range ( 0 - 3 2 T)) at low temperature down

to 3.8 K. A set of curves is given in f igure 3.

At high field (B > 18 T) SDH-like oscil lat ions

become visible 7 but less numerous) maybe due to

a f a s t e r damping towards low fields. Only one

ser ies is observed leading to a fundamen ta l field

B F = 230 T in the c* di rect ion. Here also) the

f rac t ion of the Brillouin zone area involved)

about 3 %) indica tes t ha t the pockets of car r ie rs

are probably due to a longitudinal nes t ing .

402 .L-/~ Ulmet e t al. / High f i e l d magne toresis tance o f ( T M T S F ) 9C10 . and ( T M T S F ) 4° t - .

could account [or both e i I ec t s .

ap/p

z,o

10

TMTSF_ PF~ O ~90 ~ ( B//c" 1

5K

~.6K

4gK

~2 K

3~K

J L liS ~ i i 5 ~0 20 25 ~0

ACKNOWLEDGEMENTS

We would like to thank !% Jerome,

A. Moradpour, M. Ribault (Laboratoire de Physique

des Solides, Orsay, France) [or their precious

collaboration.

REFERENCES

I. F. Takahashi, D. Jerome and K. Bechgaard, h

Physique Lett . t~3 (1982) L565.

2. M. Her i t i e r , G. Mon tambaux and P. Lederer,

] . Physique Le t t . t¢5 (19g/4) L9/,3.

FIGURE 3

Set of t ransverse magnetores i s tances ol

(TMTSF)2PF 6 for B//c ~

The angular study given in figure # exhibits a

slight deviation from the cosine law maybe

re la ted to a three-dimensional FS looking like a

warped tube.

B F

30G

20C

• % } 'x T : 5K /. \ /

'X. / . , , \

.,~.

. / ",

I I |

- 1 2 0 - 9 0 - 6 0

No

--.5

-1

0

3. M. Heritier~ G. ,Montambaux and P. Lederer, ]. Physique g e t t . #6 (1995) Lg31.

z~. ] . p . IJhnet, P. Auban and ~. Askenazy, Phys.

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5. P.M. Chaikin, M.Y. ( .'hoi, ].F. Kwak,

].S. Brooks, K.P. Martin, M. ,2 . Naughton,

E.M. Engler and R.L. Greene, Phys. Rev. Let t . 51 (1983) 2333.

6. ].P. I hnet , P. Auban and >. \ skenazy , Solid

State Commun. 52 (19g#) 547.

7. .J.P. Ulmet, P. Auban, A. Khmou, S. Askenazy,

71. Physique Let t . 46 (1985) L535.

FIGURE #

Angular dependence of B F and NO, T = 5 K (dotted line represents the cosine variation)

A st r ik ing phenomenon is observed between

about 4.2 K and 3.g K : we see at the same

t ime the quick vanishing of the osc i l la t ions and

the sa tu ra t ion of the magnetores is tance

prev ious ly large. A t rans i t i on towards an open FS