Metal to non metal transition in Tl2Mo6Se6 under uniaxial stress: A study of the thermopower

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Solid State Communications,Vol. 82, No. 11, pp. 841-844, 1992. Printed in GreatBritain.

0038-1098/9255.00+.00 Pergamon Press Ltd

METAL TO NON METAL TRANSITION IN TIgMo~Se~ UNDER UNIAXIAL STRESS: A STUDY OF THE THERMOPOWER Y.T. Tseng, G.X. Tessema, M.J.Skove Department of Physics and Astronomy, Clemson University Clemson, SC 29634-1911.

Laboratolre

M. Potel and P. Gougeon de Chlmle Minerale B., Unlverslte de Rennes,

France

( Received January 31 , 1992 by P. Burlet )

We report a study of the effect of elastic uniaxlal stress (4) on the thermopower (S) of the compound Tl2Mo6Se6 . For T > 25 K, S is negative, and JS I increases with increasing 4. For T < 25 K, we observe two different regimes: for 4 < 0.50%, S behaves just as it does in the T > 25 K regime; but for 4 > 0.60%, ISI decreases and crosses zero, reaching positive several mV/K at 1% stress. The data support the speculation that the metallic and superconducting properties of this compound are strongly influenced by a small electron pocket at the zone boundary, with the depletion of the electron pocket (at about • - 0.5%) leading to the metal-to-nonmetal phase transition observed in most of the other linear chain M2Mo6X 6 compounds.

INTRODUCTION

clear, but could be due to a Peierls distortion and CDW formation; this speculation is also suggested by the s t r ~ anisotropy o ~ r v e d in optical r e f l e c ~ i t y tvj, resistivlty t'1 and critical field t~J measurements.

The metal to nonmetal phase transition observed in m o s t M2Mo6Se 6 (M - TI, In, Na .... etc.) has been a t t r i b u t e d to a Pelerls transition ~nd the formation of a charge density wave (CDW) [ ]. This speculation has not yet, however, b e e n s u p p o r t e d by either structural or non-linear transport studies. Among these compounds, the M - In or T1 compounds are exceptional in that they remain metallic at low temperatures. The T1 compound even becomes s u p e r c o n ~ q t l n g at the rather high temperature of 6.3 K L~j. According to the band structure c a l c u l a t i o ~ ~$ Kelly and Anderson and Nohl et al. (KANK) L~, J, the Fermi surface of these two compounds is made of two parts: two flat, nearly one-dimensional sheets perpendicular to the c axis near ±z/c from F to A and small ellipsoids at the zone boundaries centered around A. They predict from this calculated band structure that the small electron pocket at the zone boundary prevents the Peierls distortion and is responsible for the superconductivity observed in the T1 compound. They further speculate that the size of the electron pocket would be very sensitive to stress, so that an achievable stress would deplete these pockets and could lead to the formation of a periodic lattice distortion (PLD) and the formation of a CDW, with a gap opening at the position of the nearly one-dimensional band. In a previous paper, we reported the effect of uniaxial stress on the resistance [5] of this compound. We found that a stress induced metal-to-nonmetal phase transition occurs in the material when the strain exceeds 4 - 0.65 ± .05% and T < 25 K ( T_ 15 ± 3K ). The nature of the nonmetallic phase Is still not

RESULTS We present here further work, this time on the effect of uniaxial stress o on the thermopower (S). Note that we measure the strain 4 along the axis of the whlsker-like sample, but apply a uniaxial stress along this axis. Since Y - 400 ± 40 GPa, a measured strain of 1% implies a unlaxlal stress o - Y4 - 4.0 GPa, for example. We think these new results provide further evidence of the depletion of the electron pocket and support the idea that the low temperature metal-to-nonmetal phase transition is indeed prevented by the existence of this pocket and that stress-lnduced depletion of the pocket leads to the opening of a gap which may be a CDW or SDW gap. The samples of typical dimensions (2)x(0.04)x(0.01) m~ 3 were mounted on a quartz puller as described in ref. [5]. The two Cu T12Mo6Se 6 Junctions at the ends of the sample were thermally anchored to two copper blocks. A heater was wound around one of the copper blocks, and the current in the heater adjusted to give a temperature gradient of about 0.2 K. The thermal gradient was measured with a AuFe (0.07 at.%)-Cu thermocouple also anchored to the copper blocks. The temperature of the sample chamber was measured using a silicon diode placed near the sample. Figure i shows the effect of unlaxlal stress on S. Since the behavior below about 25 K is quite different than the behavior above 25

841

842

Tl2Mo6Se 6 UNDER UNIAXIAL STRESS -20

Vol. 82, No. 11

7

•~ . . ~ 0

T

=

t

/ T = 6.59 K

29.4 K

200

~

._.6

1

=5

o

O0

> -1 O0

-6O

-0 3 -80

0.0

0.4

0.8

1.2

e (%)

-200

2

300

0.0

0.2

0.4 e (%)

0.6

o.s

(a) Figure 2. A plot of S vs E at T - 6.59 K for e < 0.70% ( squares ). Notice the minimum in S and the change in sign at about 0.60%. LOgl0(R/l~ ) is also shown (triangles) on the left scale for comparison with the plezoresistance data.

500 o -

,

q

ol\

~ r.n

.-

T = 6.39 K

o

300 " ' . ,° . . . ,

rl,,, °

°

,o

.%~-

t

....

100

- I 0G o.o

_/" i

o.4

o.8

,.~

e(%)

(b)

Figure i. Stress dependence of the Seebeck coeficient: (a) corresponds to T > 25 K. S decreases monotonically with ~ up to 1.2 %. (b) shows a plot for T - 6.59 K; S becomes positive and increases by several orders of magnitude for ¢ ~ 0.70 %. The inset in lb. shows a plot of S vs T.

scale of fig. 2. The minimum in S and the change ~n sign occur only for T ~ 25 K. Smi n decreases with temperature but the value of ( e m l - ) at which this minimum occurs shows very lltt~e change with temperature. Both Sml n and e_in show some sample dependence, however. F~gure 3a shows a 3D plot of the change in thermopower (S(e) - S(~-O)) as a function of and T. The projection in the (S, T) plane for some different, arbitrarily selected, values of is shown in Fig 3b. It is interesting to note that (S(~) - S(e-0)), for this particular sample, shows a minimum at around 7 K which is close to the zero strain superconducting transition temperature. The implication of this result will be discussed later. DISCUSSION The large zero-strain thermopower of TI2Mo6Se 6 obtained by Mori et al. and Brusetti et al. has been explained within the framework of the two band model of KANK [3'4]. Unusual electron scattering processes, strongly ent on the energy, as proposed by Mort ~vj for the transition metals have been invoked. According to this model, the large zero strain thermopower in this compound is due mainly to electrons scattered from the large ID band to the 3D band (the electron pocket). The resulting thermopower would then be:

?~R~nd

K, we present the data above and below 25 K separately. Above 25 K (Fig. la), the magnitude of S increases with increasing ~ while remaining negative (electron-llke). Below 25 K (Fig. ib), S is small and remains negative as long as ~ 0.60%; at higher values of e, S becomes positive and increases by several orders of magnitude (a few mV/K at 1% strain). The inset in Figure Ib shows the temperature dependence of S below i00 K. This is in a g r e e ~ g t with the ea~er results of Morl et al t°j and Brusettl et al L~J. A close scrutiny of the ~ < 0.70%, T < 25 K data is shown in Fig. 2; at a given T and for e < 0.60%, S decreases until it reaches a minimum Sml n before turning up to change sign rapidly and become hole-like. The resistance R on the other hand increases smoothly by several orders of magnitudes as shown on the left hand

S - -(~2k2/6e).(T/A) where A - ~0 - ef' ~o is the energy at the bottom of the 3D band and ~f is the energy at the Fermi surface. Any change in A would therefore have a strong influence on S. Perhaps the rise in the absolute value of S under stress at high temperature could be explained using this same model. As the electron pocket is depleted A would decrease and lead to an increase in the magnitude of S. However this leads to a contradiction with the sign of the measured plezoresistance [5] ( (Ap)/p 0 - (p(~)p(0))/(p(0))) since a decreasing A would lead to

Vol. 82, No. 11

T12Mo6Se6 UNDER UNIAXIAL STRESS

25 K, shows that the product So deceases with stress, perhaps indicating that of the two (or four, if one includes S 1 and S 3) competing

F

~

(a)

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>~-0.2

/

. . . _

rA -0.4

•

e = 0.4~

-0.6 5

10

15 T (K)

20

843

25

(b)

Figure 3. (a) 3D plot of the T and e dependence of S(~) - S(~ - 0) below 26 K. A projection in the (S,T) plane is shown in fig 3b for ~ - 0.30, 0.40 and 0.50%. The minimum may result from a competition between an enhanced phonon drag effect and the onset of superconductivity.

a decrease of the so-called anomalous scattering process and therefore to a negative (Ap)/p 0 rather than the observed large positive value. This contradiction can be lifted if one considers the two bands with a net conductance a - Ol + o 3 and a Seebeck coefficient S - (OlS +o3~3)/( oI + o3). Here t h e s u b s c r i p t s 1 an~ 3 refer to the ID band and the 3D electron pocket respectively. The depletion of the electron pocket would have two competing effects: o 3 would decrease because the number of electrons in the pocket decreases; and o I would increase because the relaxation time increases. The plezoreslstance results reported in reference 5 suggest that the overall conductance would be dominated by the drop in o 3. One way to verify this speculation is to plot the product Sxo vs e as in Fig.4. Such a plot at some characteristic temperatures (51 K, 81 K and 138 K) higher than

terms, the change in o 3 is the dominant term. Of course one can consider several other alternatives but in the absence of sufficient experimental data (magnetothermopower, Hall effect ...etc) such a discussion seems futile. At this point, it might be sufficient to point out that the thermopower vs strain results emphasize once again the considerable role played by that small electron pocket in determining the transport properties of this compound. Below about 25 K, an additional effect must be brought into the picture to explain the negative peak, the change in sign in S, and the large semlconducting like thermopower for e z 0.75%. Once the electron pocket is removed or reduced to a certain critical size, TI2Mo6Se 6 behaves as the other compounds of the same family and shows a metal-to-nonmetal phase transition with a p type thermopower in the semiconductlng phase. This could explain the large positive thermopower observed when T < 25 K and ~ z 0.75% and is in agreement with plezoreslstance measurements, which show that in this region the resistance is orders of magnitude higher than at zero stress. The large negative peak near T m - 8 K shown in Fig. 3 suggests that S in the region T < 25 K and • S 0.55% might be dominated by an Ntype phonon-drag process. The rise in the photon drag term wlth ~ could be a sign of enhanced electron-photon interaction. The sudden turn around at T m and the drop in S could be due to the onset of superconductivity which vanishes only when e ~ 0.60% [5]. T~ increases weakly with e Just as Emi n. This mlght also explain the reentrant T c vs o phase ~ g r a m in type -A samples we reported earlier --- as a result of competition between the decreasing density of states and the increasingly enhanced elecron-phonon interaction. In discussing this result one should also note that Hall effect data by Brusettl et al. [ll] have shown that there might be both electron and hole carriers in TI2Mo6Se 6 at zero stress. The competition between these carriers should also be taken into account as it could also contribute to the pronounced minima in S. Since ~ would affect the

-0.05 . -0.10

~

-0.15

~ -0.20 •

T=81 K

•

T=51K

-0.25 0.0

0;3

0.6 e

0.9

(%)

Figure 4. a.S versus e at 3 characteristic temperatures, 51 K, 81 K and 138 K.

high

844

T12Mo6Se6 UNDER UNIAXIAL STRESS

number of carriers as well as the mobilitles of both type of carriers a detailed study of the plezoreslstance in a magnetic field as well as the strain dependence of the Hall measurement should give valuable information. CONCLUSION The thermopower vs stress measurement provides further evidence that uniaxial stress leads to the depletion of the electron pocket and that this process triggers a metal-tononmetal phase transition at low temperature. The measurements indicate that the transition temperature is below 25± 3 K. In spite of the difficulty of identifying the non-metalic phase as a CDW state (there is no structural evidence for a CDW nor have we found systematically a threshold field at the onset of the observed non-llnear conductance), we believe that we have further evidence to support the likelihood of KANK's prediction of CDW formation in this compound under stress. We conclude by emphasizing the need for a structural study under stress to decisively confirm the predictions. Acknowledgements. We thank P. Monceau for his valuable comments and suggestions and R. Brusetti for communicating his unpublished Hall results. The samples for this study were prepared at the Laboratolre de Chlmle Minerale B., Universlte de Rennes I, Rennes France. This work is supported by NSF Grant No. DMR-8822968.

Vol. 82, No. 11

References i- J.M. Tarascon, ~.J. DiSalvo and J.V. Waszak, Solid State Commun. 52, 227 (1981). 2- R. Lepetit, P. Moneeau, M. Potel, P. Gougeon, and M. Sergent, J. Low Temp. Phys., 56, Nos. 3/4 (1984). 3- P.J. Kelly and O.K. Andersen, Proc. IV Conf. Superconductivity in d- and f- Band Metals (Karlsruhe 1982) Edited by W. Buckel and W. Weber [Kernforschungszentrum Karlsruhe GmbH] p. 137 4- H. Nohl, W. Klose and O.K. Andersen in " Superconductivity in Ternary Compounds", Edited by O. Fisher and M.B. Maple ( Springer Verlag 1982) p. 165. 5- G.X. Tessema, Y.T. Tseng, M.J. Skove, E.P. Stillwell, R. Brusetti, P. Monceau, M. Potel and P. Gougeon, Phys. Rev B, 43, 3434, (1991). 6- H.P. Geserich, G. Scheiber,M. Durrler, M. Potel, M. Sergent and P. Monceau, Physica 148B, 234 (1986). 7- J.C. Armlcl, M Decroux, O Fisher, M. Potel, R. Chevrel, Solid State Commun. 33, 607 (1980). 8- T. Mori, Y. Yokogawa, A. Kobayashi, Y. Sasaki, and H. Kobayashi, Solid State Commun. 49, 249 (1984). 9- R. Brusetti, P. Monceau, M. Potel, P. Gougeon and M. Sergent, Solid State Commun. 66, 181 (1988). I0- N.F. Mott, Proc. Roy. Soc. A 156, 368 (1936). - N.F. Mort and H. Jones, in "The Theory of the Properties of Metals and Alloys" Dover Publications, Inc. (1958). Ii- R. Brusetti, private communications