superconducting and magnetic instabilities in(tmtsf)2x and (bedt-ttf)2conductors

Physica 143B (1986) 329-333 North-Holland, Amsterdam 329

SUPERCONDUCTING AND MAGNETIC INSTABILITIES IN(TMTSF)2X AND (BEDT-TTF)2CONDUCTORS

D.JEROME, F.CREUZET and C.BOURBONNAIS

Laboratoire de Physique des Solides, Universit~ Paris-Sud, 91405 ORSAY (France)

The experimental investigation of the antiferromagnetic ground states in the(TMTSF)pX series sup- ports the models predicting magnetic long range order driven either by interchain e~change of ID-2k F spin correlations or by 3D nesting of the Q-I-D Fermi surface. In the second situation orderlng occurs at a temperature below the one-particle dimensionality cross-over T x. We g{ve experimental evidences supporting the pressure and magnetic field dependence of T .

• . X We have also reviewed some necessary experimental requirements for the stab111satlon of bulk superconductivity at 8.1K in the Q-2-D strongly correlated conductor 8-(BEDT-TTF)213.

I. ORGANIC SUPERCONDUCTIVITY ABOVE 8 K

Almost three years after superconductivity has been discovered in the (TMTSF)2X series of organic conductors I with a maximum critical temperature of 1.25 K for (TMTSF)~CIO. under zero pressure 2, the organic molecule, ~EDT!TTF 3, has led to a new series of organic superconductors. Among the BEDT-TTF salts the first leading to superconductivity has been (BEDT-TTF)gReOA, with a T of 1.4 above 5 kbar 4. Later, in-1985, Professo~ Schegolev's group published a T of 7.4 K in the B-phase of the triiodine salt o~ BEDT-TTF 5, namely (BEDT-TTF)91 ~ called from now on B-(ET)213. A similar incre~sg of T was also announced 5y- a group in Japan 6. FurThermore, the last two groups have also reported in the same system a possible stabilization of superconductivity around 7 K at ambient pressure either after temperature cycling or after the release at ambient temperature of a previously applied pressure. However none of these works provided clear cut proofs for the stabilization of bulk superconductivity at 7 K under ambient pressure as either the drop of resistivity below T was not complete or no Meissner signal could dete~ted with a SQUID magnetometer7.

At Orsay we have tried to clarify the establishment of superconductivity in (ET)plq. We ma- naged to stabilize a sharp superconducTiNg transition under ambient pressure at 8.1 K following the hydrostatic pressure-temperature cycling pro- cedure displayed in figure (I) 8. The existence of an AC-susceptibility signal related to the transition exactly similar in amplitude to the one obtained under a pressure of 1.3 kbar where Meissner effect has been established by magneti- zation measurements 7 shows that the superconducting state stabilized at i atm is indeed a bulk superconductor 9. This high-T (B-H) state of B-(ET)213 is however metastatic since warming up to room temperature followed by a cooling down to helium temperature does not show superconductivity at 8.1 K, figure (2). Various experiments performed in temperature have established that the

i i i i i i i

R/lOSQcm

2** 13-1

1 0

Ptkb=r

I I I / 1 I I I 2 ~ 6 8 10 12 It.

Temperature (K}

FIGURE i

Superconducting transition at 8.1K in B-(BEDT-TTF)213 under i atm after sample preparation following the T-P path displayed in the insert.

sf f,

, , , , • , • ( d l

t -(el . . . . . . . . . . . . . . . . • (h i

lo / r ' ' "

• " ( a )

. ~

, " ~-(BEDT-TTF)213 °A

• A

• , , " A

FIGURE 2 AC susceptibility signal in B-(ET)213, under 1.6 kbar (a), after pressure release at low temperature (h), after annealing above 131 K (c),after annealing at 125 K (d).

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

330 D. Jerome et al. / Superconduct ing and magnetic instabilities

domain of stability of the B-H state at 1 atm extends up to 125 E. Without preliminary pressure-temperature cycling the phase of ~-(ET)2I 3 which is in thermal equilibrium at low temperature is the phase displaying superconductivity at 1.4 K or so(the 8-L phase) I0.

The stabilisation of superconductivity at 8.1K in $-(ET)2I ~ has been understood by suggesting 8 that the B-H and 8-L phases correspond respectively to the structure stable at room temperature and to the modulated phase observed below 200 K Ii.

We conclude by saying that ~-(ET)2I ~ is indeed a high T organic superconducto~ ~hen com-

to the C(TMTSF)2X series) in the absence pared of structural phase transition establishment below 2OO-185 K.

Since B-(ET)2I 3 is a material showing superconductivity at a temperature about 5 or 6 times the T' observed in the (TMTSF)~X series it is partiCUlarly interesting to perform a comparati- ve study of the normal state properties of the two families. First, both series are half-filled band conductors. Secondly, optical reflectance data suggest that the I-D character is somewhat more pronounced for (TMTSF)oX than for ~-(ET)~Iq. The Fermi surface of (TMTSF~X is open(t /t,~10) ~ whereas the band a n l s o t r o p y , t / t b 2-3, 12,13 of ~-(ET) I_ indicates tflat the FS is like- ly to be very n~a~ the border between open and close . In spite of a marked 2-D character, the nuclear spin relaxation behaviour of ~-(ET)2I 3 reveals a spectacular enhancement bound to the superconducting transition which has been related to low dimensionality effects 14 and is in agreement with band anisotropy t /t b = 2 and t, /t > 50. The depression of the F~R oscilla- D c 13 tor strength and the Faraday susceptibility 15 show that Coulomb correlations play a dominant role in $ (ET)213. The enhancement of the uniform susceptibillty above the bare band value amounts to 3-4 and 2 for B-(ET)213 and (TMTSF) 2 CIO, respectively.

~n spite of large Coulomb correlations, no magnetic ordering has yet been observed in -(ET)21 R. Instead, it is precisely with such a

material-that a significant increase of the superconducting transition has been achieved. This fact shows that Coulomb interactinns and 3D kinetic couplings are two important factors gover- ning the onset of instabilities at low temperature in Q-ID organic conductors.

2. COULOMB AND 3-D KINETIC COUPLINGS IN(TMTS~ 2 X: PRECURSOR REGIME

In the last few years NMR relaxation studies performed on IH and 77Se nuclei in (TMTSF!2CIO 4 have clearly revealed a deviation to Korrlnga- like relaxation arising below 25-30 K 16. This feature has been attributed to the onset of one dimensional magnetic cooperative phenomena above a cross-over region between I-D and 3-D behaviour located at ~ ~ 8K in (TMTSF) 2CIO 4 in a magnetic

field of 31.9 kOe. As 25 K is also the temperature where CIO 4 anions order in (TMTSF)2CIO 4 it was important to perform a similar NMR study in a system where no such lattice modification could be detected at low temperature,namely

(TMTSF) 2PF 6 • The results obtained in (TMTSF)2PF 6 are

summarized in figure 3.

~TI -I (:-~)

150 i ~2s~;. Ip/T.~ 8.7K

/

\ " 2"" .~.'~ s sko

50, ~

I I I T0 20 30 20 5'o ~?~

FIGURE 3 77Se s p i n - l a t t i c e r e l a x a t i o n r a t e in(TNTSF)2CI04, do t t ed l i n e s and in (TMTSF) 2PF6, cont inuous lines, H = 3 T//b'

Comparisons between the behaviours of (TMTSF)2PF 6 under pressure (P>8 kbar) and (TMTSF)2ClO,reveals quitesimilar temperature dependence of 4 Ty 1

in the low temperature domain which allow us to rule out the effect of anion ordering on the behaviour of the relaxation rate and strongly suppor=s the model of Bourbonnais 18 in which the I-D to 3-D cross-over T can be depressed from the bare-band value atX~80 K down to the 8 K temperature region by strong intra-chain particle- particle correlations.

Theoreticians have proposed two possibilities for the onset of antiferromagnetic long-range order (AFLRO) observed at low temperature in (TMTSF)~X.

(i)A strongly correlated antiferromagnetic phase stabilized by inter-chain exchange interaction (IEX) between ID-2k F spin correlationsl9 of ad- jacent chains leading to a modulation wave-vector not related to the actual shape of the Fermi surface.

(ii)An AF phase induced by nesting properties of the open (but warped)Fermi surface (NAF) exhibi-

t ing a transverse wave vector such as T = (2kF' b::/4' xc::).

D. Jerome et al. / Superconducting and magnetic instabilities 331

One can summarize very crudely the theoretical picture, saying that IEX-AFLRO sets in at TN_IE X when the renormalized transverse overlap inlegral ~. is not pertinent in the temperature region o~ the transition,(i.e.T~ ~..>T ) while in the opposite situation the 3~I~-A~LRO takes over T >T . We shall now review some experimental x..N-NAF

evlaences (NMR and transport properties)supporting the above-mentioned theoretical models.

AFLRO corresponding to two different experimental conditions can be achieved at low temperature in (TMTSF)2PF 6 and (TMTSF)2CIO41

(i) At atmospheric pressure(T N = 12K) or under pressure as long as P<8 Kbar.- [ii)Under pressure (P>8 Kbar for (TMTSF)2PF6) and large magnetic fields // e ~:.

Figures (4) et (5) show the behaviour of the resistivity above T. for the two situations where AFLRO can be stabilized at low temperature 21. Under 7 kbar fig.(4), AFLRO resistive transition at 5 K is very sharp (the same sharpness as at i bar) with practically no resistive precursor regime observed above TN(in zero magnetic field). The AFLRO state existin~ at high field (H=8.8 T, P = 9 kbar) below 1.8 K is announced by a broad resistive precursor regime starting at 20 K and possibly above, fig.5.

Comparatively, the relaxation rate reveals a different behaviour. An extended precursor regime is observed below 30 K (see data at 5.5 kbar in fig.(3) with T N = 8.7 K 17). This is in striking contrast with the behaviour of the resistivity at zero field on fig.4. Notice that the NMR data have been obtained in a field of 3 T and not in zero field. However, we feel that their comparison with the zero field resistive T-dependence in fig.(4) is meaningful since the transverse magnetoresistance is known to be very small when the magnetic field is aligned along the b'-direction 22.

NMR data related to the establishment of the AFLRO stabilized under high magnetic field are shown on the composite figure 6~:. The relaxation rate is nearly independent of the field orienta- tion down to 8-10 K. At lower temperatures for fields parallel to b' a Korringa-like behaviour is recovered below the dimensionality cross-over T_%5 K whereas the divergence of I/T I for fields /~c:: is the signature of the onset of AFLRO at 1.8K(H = 7.4 T//e~:).

Summarizing the previous data, we notice that the onset of AFLRO stabilized under high magnetic field occurs at T N small~r than the cross-over

::This figure is based on(TMTSF)2CIO 4 NMR as the (TMTSF)pPF ~ NMR investigation under pressure has not yet-be~n completed at high fields //c;: . Ho- wever, it is known from figure (3) that qualita- tively the same T-dependence of T? I exists for

I CIO 4 salts at low pressure and PF 6 salts under pressure.

R(T)/R (?OK)

1.5 TMTSF 2 PF 6 P=Tkbar

.5 L/or

0 20 40 TIK

FIGURE 4 T-dependence of the resistivity of(TMTSF)2PF 6 under 7 kbar in zero and 7 T//c:: applied ~ield.

temperature T which can be detected either on resistive or ~n relaxation experiments(in PF 6 at 9 kbar under 8.8 T//c:: TN=I.8 K and T~2.5~ fig.(5), in CIO& at i bar under 7.4 T//c ~ T N = 1.8 K and T ~ 4 K, fig.6). On the other hand, whenever t~e AFLRO state is stable in zero applied magnetic field, PF. at 5.5 kbar, fig.(3) or PF 6 at 7 kbar, fig~) no cross-over temperature is seen in the experimental data. We have argued in referencel7 that the broad minimum of T? I around 25 K followed by a strong enhancement at lower temperature is not the signature of a dimensionality cross-over but rather marks the onset of a power law temperature dependence of strongly developed I-D spin correlations.

,0 b .25K

~. ~ TMTS~P~

0 20 gO 60 T/K

FIGURE 5 T-dependence of the resistivity of(TMTSF)2PF 6 under 9 kbar in various magnetic fields.

332 D. Jerome et al. / Superconducting and magnetic instabilities

3. TWO AFLRO STATES IN(TMTSF)2X:EXPERIMENTAL EVIDENCES

Transport properties and NMR investigations do support the theoretical model leading to IEX or NAF antiferromagnetic long range orders in (TMTSF)2X compounds. Well-defined Shubnikov-de Haas os~illations have been reported in (TMTSF) 2 PF 6 below 9 K at ambient pressure 23. The frequency of these oscillations 229 T for H//c:: corresponds to rather large F.S.cross sections (%3% of the Brillouin zone area). Such cross sections require a choice of wave-vector in the spin modulated state which does not optimize nesting properties of the Fermi surface. Further- more, below %5 K the transverse magnetoresistance saturates at high fields and the oscillations vanish 23. We take this fact as an evidence for a possible change in the topology of the semimetallic F.S.of the AFLRO state arising around 5 K. Below 5 K the size of the remaining electrons or ho~es pockets should be much smaller than at T~ K, implying the existence of a wave-vector in the AFLRO with non zero transverse components. Indeed, two recent analysis of IH NMR rotation NMR pattern of (TMTSF)oPF624,25 lead to a wave- vector ~ = (2kF, 0.24 ~:c, Oc:C) at 4.2 K and 1.2 K. Consequently a IEX-NAF transition is li- kely to occur in (TMTSF)2PF 6 in the vicinity of T ~5 K at ambient pressure with a concomitant x% . . .

var~atlon of the magnetic modulatlon wave-vector. Furthermore, T should be increasing under

• . x.

pressure sznee it zs related to the bare band interchain coupling t±18. Actually, NMR data in fig.(3) indicate values of T in the neighbour- hood of 8 and 12 K at 8 and Xll kbar respectively. Figure (7) provides a rough sketch of the pressure dependence of T and T N in materials such as (TMTSF)2PF 6. . x

The rapid suppresszon of the AFLRO state under pressure occurs at a critical pressure above which the 3D-character of the band structure is dominant. The nesting of the Fermi surface is thus not good enough to support the stabilization of an AFLRO even with a transverse wave- vector in the modulated state.

However, in sufficiently large transverse magnetic field Gor'kov and Lebed 26 have shown that the electrons in this Q-I-D conductor are one dimensionalized by the effect of the field. Hence, the conductor becomes unstable above a certain threshold field against the formation of a se- quence of phase transitions between different AFLRO states characterized by a transverse wave vector 27. Transport properties 28 and magneto-ca- lorim~trie data29, 30 are consistent with the existence of transverse modulation wave-vectors(i.e. AFLRO of the NAF kind) for (TMTSF)2CI04 and (TMTSF)2PFg(under pressure) in the T-H domain, T<3 K and H<IO T. Outside the latter T-H region the detection of large pockets of carriers in (TMTSF)2CIO 4 and (TMTSF)oPF ~ by magneto-transport measurements 31 is in goSd ~greement with a lon- gitudinal modulation wave-vector which is no

( s "1}

80

60

20

If.~-TN =l.SK

I I I I I I I 0 I0 20 30 ~K

FIGURE 6 T-dependence of 77Se T -1 in (TMTSF)?CIOz~ a t H = 6.4 T // b' and H I__ 7.4 T//e:: b~low~5 K.

//Tx

) Pressure

FIGURE 7 Typical T-P phase diagram for (TMTSF)2PF6-1ike compounds. The shaded area near P refers to a narrow region where coexistence b~tween AFLRO and superconductivity is possible.

longer the one ~iving the best nesting of the Fermi surface 3~. These features can be understood by a model which is sketched in figure 8 for (TMTSF)2CIO~, but the same picture should apply to (TMTSF72PF 6 under pressure as well• As the net effect of the magnetic field is to diminish the effective interchain kinetic cou-

plzng (t± >t./H following Gor'kov 33)we can expect T to decrease at large f~elds and possl- bly cros~ the T critical temperature. At higher fields t~eN~tuation which is recovered is similar to the IEX-AFLRO of (TMTSF)2PF 6 between 12 and 5 K at ambient pressure. This seems to be a natural interpretation of the very high field data.

4. CONCLUSION

The comparison between theory and experiment~ has emphasized several features which can be su~ marized as follows:

D. Jerome et al. / Superconducting and magnetic instabilities 333

10"ti/K 1D-regime

I'"-.zx TMTSF21 bar Cl04 ~., TNj.-" "~

5, 3 D- regime ~

\X" AFLRO . , / / " . IEX

c .....,...7~ / 3 p N AIIF - , o 6 lb f5 H/'[

FIGURE 8 Typical T-H phase diagram for (TMTSF)pCIOA-Iike compounds (or(TMTSF), PF~ under pressure): T is

L . 0 . X

depressed under magnetlc fzeld. The IEX state becomes stable at very large fields.

There exists in the (TMTSF)2X series, a number of experimental indications suggesting two possl- bilities for antiferromagnetic ordering. -a) a semimetallic AFLRO state stabilized by in-

terchain exchange interaction with large pockets of carriers (~3% BZ).

-b) a quasi-semiconducting AF state with very few carriers ( < 1% BZ) driven by the nesting properties of the 3D Fermi surface. The IEX and NAF states are respectively sta-

ble above and below the renormalized temperature regime T establishing the separation between ID

X . < • (T>T) and 3D physzcs(T T ). Slnce T can be va-

• X . X X . .

rzed elther by pressure or by the applzcatzon of a large magnetic field IEX-NAF transitions are expected in a generalized T-P-H phase diagram of (TMTSF)pX.

The-normal conducting state above a IEX transition is dominated by strongly developed I-D correlations exhibiting at the same time an isotro- pic magnetic character (observed by the enhancement of I/T I below 30 K) and an anisotropic superconducting character (giving rise to the large conductivity at low temperature, the anomalous- ly large and anisotropic magnetoresistance and the strong DC collective mode in the far-infrared regime 13). When T is large, typically larger

X

than 5 K, a superconductzng state may become stable at low temperature, for instance in(TMTSF) 2 CI04, and the divergence of the. ID-2k_F correla-. . tions stops at 8-6 K. Under hzgh magnetlc fzeld however, the superconducting state is destroyed by the pair-breaking phenomenon and an antiferromagnetic state of the NAF nature can be stabilized. Coming back for the conclusion to (ET)213 we may wonder why no AF ordering has yet been observed in this conductor in spite of the large coulombic effects?,The answer li~s probably in the

amplitude of transverse kinetic coupling larger in(ET)21 R than in (TMTSF)2X preventing the onset of one-dimensional IEX ana even 3D-NAF states. Examination of 6-(ET)213 NMR data allows the lo- cation of T around 15 Kwhere assuperconductivity occurs at 8XK 14. We wish to conclude by saying that x 2 commensurability, strong coulombic repul- sions and moderate one dimensionality may all be factors influencing favourably the quest for high T c superconductivity in organic conductors.

I. D.J~rome et al, J.Physique Lett 41 (1980)L-95 2. K.Bechgaard et al, Phys. Rev.Lett 46, (1981)

852. 3. M.Mizuno et al, J.Chem. Soc. Chem. Comm.(1978)lS. 4. SSP.Parkin et al, Phys.Rev.Lett.50,(1983)270. 5. V.N.Laukhin et al, Soviet Physics JETP Lett

41, (1985) 81. 6. K.Murata et al, J.Phys. Soc. Japan 54 (1985)

1236. 7. H.Veith et al, Solid State Comm.56(1985)IO15. 8. F.Creuzet et al, J.Physique Lett 46, (1985)

L-IO79. 9. F.Creuzet et al, Europhys.Lett i, (1986) 461. lO.E.B.Yagubski'iet al, Sov.Phys.JETP Lett 39

(1984) 12. ii. T.J.Emge et al, Phy~.Rev. B30 (1984)6780 and

J.P.Pouget (private comm) has observed the onset of a similar incon~ensurate structure at 185 K. H.Tajima et al, Solid State Cormn 56(1985) 159. C.S.Jacobsen, Thesis, University of Copen- hagen 1986. F.Creuzet et al, Europhys.Lett 1(1986)467. B.Rothaemel et al, (ICSM 86 conference and submitted to Phys.Rev.) C.Bourbonnais et al, J.Physique Lett 45 (1984) L-755. C.Bourbonnais et al, submitted to Phys.Rev. (1986). C.Bourbonnais, Mol. Cryst.Liq. Cryst.l19(1985) ii. V.J.Emery, Phys.Rev. B 14 (1976) 2989. V.J.Emery et al, Phys.Rev.Lett.48 (1982)1039. B.Piveteau, Thesis Orsay (1986). K.Murata et al, Mol. Cryst.Liq. Cryst.l19(1985) 131. J.P.Ulmet et al, Solid State Co~m.J.Physique Lett 46. (1985) L-535. J.M.Delrieu et al, J.Physique 47(1986)839. T.Takahashi et al, J.Phys. Soe. Japan 55(1986). L.P.Gor'kov et al, J.Physique Lett 45 (1984) L-433. M.H~ritier et al, J.Physique Lett 45 (1984)° L-433. M.Ribault et al, J.Physique Lett 45 (1984) L-935 F.Pesty et al, Phys.Rev.Lett 55(1985)2495. M.J.Naughton et al,Phys.Rev.Lett.55(1985)969. J.P.Ulmet et al, Phys.Lett 98 A(1983)457. Large frequency Oscillations have also been detected by Ribault et al ref 2~.

33. L.P.Gor'kov, Sov.Phys.USP 27 (1984) 809.

12. ]3.

14. 15.

16.

17.

18.

19. 20. 21. 22.

23.

2~. 25. 26.

27.

28.

29. 30. 31. 32.

superconducting and magnetic instabilities in(tmtsf)2x and (bedt-ttf)2conductors

Documents