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Tl2Mo6Se6: Reentrant normal-superconductor-density wave phase transition
Synthetic Metals, 41-43 (1991) 3989-3992
3989
TI2M__gCr~: R EENTRANT NORMAL-SUPER(~ONDI.JCTOR-DEN$ITY WAVE PHASE TRANSITION
G.X. TESSEMA, Y.T. TSENG, M..I. SKOVE, E.P. STILLWELL Departement of Physics and Astronomy, Clemson University, Clemson S.C. 29634 (U.S.A.) R. BRUSETFI, P. MONCEAU C.R.T.B.T., C.N.R.S., Grenoble, France.
ABSTRACT We report experimental results showing that uniaxial stress (~) increases the resistance in the full range of temperature and leads to a metal to nonmetal phase transition. It suppresses T c. And, for ~ >--0.50 %, a peak appears at Tp = 15 + 3 K in d(log(R(e,T)))/d(1/T) vs I / T plots. We speculate a Peierls phase transition takes place at Tp with a fully developed gap Eg = 57 K below T = 10 K. We summarize our results in a phase diagram in the e vs T plane which, in the case of type-A samples exhibits a reentrant metal-superconductor- semiconductor characteristics.
INTRODUCTION TI2Mo6Se6 is unique among the general family of M2Mo6X 6 (M = TI, In, Na, ...etc and X = S, Se, Te ) quasi-onedimensional compounds in that it is the only one superconducting and at a rather high T c (= 6.5 K ) [1]. The compound is highly anisotropic as demonstrated from resistance, superconducting critical magnetic field He 2 and polarized optical reflectance measurements [2, 3, 4]. The calculated Fermi surface consists of two parts: sheets and ellipsoids. The nearly planar sheets are perpendicular to the chain axis c axis near ~r/c from r to A. The ellipsoids are centered at A and their existence is due to the energy overlap of the a2 bands at ~r/2 and the e bands at A. These electrons are believed to protect the structure from a Peierls distortion and are responsible for the superconductivity. Nohl et al predicted that stress might change the occupancy of these pockets leading to a Peierls distortion and formation of CDW [5]. We present a study of the effect of uniaxial stress on this compound.
RESULTS The needle like samples are mounted on a quartz puller described elsewhere [6] and the resistances are measured using the standard four probe technique. Figure 1 shows a typical behavior, both for type-A and type-B samples, of the R(~,T) vs T in a semi-log plot and for different values of e. The materials undergo a metal to nonmetal phase transition below a c dependent temperature Tm at which R(~) reaches a minimum. Tm increases linearly with ~. 0379-6779/91/$3.50
© Elsevier Sequoia/Printed in T h e N e t h e r l a n d s
3990
.----.
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,
.
,
80 120 1 O00/T (I/K)
40 I
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Figure 1_- A plot o f log(R(e~,T)/R 0 ) v,s 1000/T. T h e legends are given in the box; the n u m b e r s refer to the values of ~ in %. T h e insert shows plots of d ( L o g ( R ( 6 , T ' ) ) / d ( 1 0 0 0 / T ) for different values of 6. A broad peak is observed at T = 15 K. All the curves for (~ > 0.6 % converge to a stress independent value o f E
g
= 57 K at T < 10 K.
The insert in Fig. 1 shows a plot of the slope d(log(R(~,T)))/d(1000/T). A rather broad peak is observed at Tp = 15 K _+3 K and for ~ _>0.50 %. Tp appears to be independent of ~ above 0.70 % but the peak becomes better defined for higher 6. This slope saturates to a stress and T independent value of 57 K below about 10 K. The above results suggest that the prediction of Nohl et al might indeed be correct and a CDW might occur with a Peierls transition temperature Tp = 15 K + 3 K and an energy gap E g = 57 K which would lead to a ratio E / k T p = 3.4 which is surprisingly equal to the BCS ratio. Stuctural studies under stress and at low temperatures will be required to confirm the above speculations. However, we have observed nonlinear electrical conductance above a sample dependent threshold electric field [7]. The search for narrow band noise was not succesfull but broad band noise above a second threshold field is observed in some samples.
Uniaxial stress suppresses superconductivity in both type of T12Mo6S% samples. However, the details of the effect of E on Tc depend on the type of sample. As shown if Fig. 2a, Tc decreases monotonically in type-B compounds with a rate increasing with ~ until superconductivity is totally suppressed at about 0.50 %. Tc for type-A samples shows an initial increase at low E (< 0.35 %) wilh a rate ATe/Act = 0.18 K/GPa it reaches a maximum at ~ = 0.35 % and start decreasing steeply ( ZXTc/As = 2.5 K/GPa calculated using Young's modulus Y = 400 GPa +- 40 GPa measured by X.F. Chin et al [8]) before vanishing at about 6 = 0.55 % + 0.05 %. At higher 6 the material exhibits a non metallic behavior, possibly a semiconducting behavior with a fully developed CDW or SDW, as mentioned earlier in this report. The reported reentrant normal-superconducting-semiconducting behavior is best illustrated in Fig. 2b where log(R(T)) is plotted against 6 for a t . ~ - A sample. For T = 6.70 K R decreases as the strain is applied to the sample, R reaches a minimum at 6 = 0.10 % and increases into the nonmetallic regime.
3991
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.......
-:-'W TEK]
4
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0.2
0.4
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0.8
Strain dependence of resistance 104
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/
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1 02
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,"
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..7
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Figure 2~- Fig 2a. The effect of E on the T . T for type-B samples, ( full circles), decreases monotonously. Tc for type-A samples, (open circles), exhibits an initial rise to a broad maximum before it decreases steeply. The inset shows the effect of ~ on the resistive transition of a type-A sample. The value of e in % is 0.04), 0.12, 0.38, 0.46, 0.520 for the solid, the doted, dashed dotted, short dashed and long dashed lines respectively. The normalized resistance R N = R(E ,T)/R( ~, 7.51{) is shown vs T. Fig 2b: the ~ dependance of a type-A sample.
O u r i n t e r p r e t a t i o n o f t h e s e results is s u m m a r ; z e d in the p h a s e d i a g r a m s h o w n in F i g 3. A t low t e m p e r a t u r e s ( T < 10 K ) T I 2 M o 6 S e 6 can b e f o u n d in t h r e e possible states: n o r m a l metallic state, s u p e r c o n d u c t i n g s t a t e at e = 0 o r low stress (E _< 0 . 4 0 % ) a n d a dc;nsity w a v e stale ( C D W (xr S D W ) at h i g h e r c. T y p e A s a m p l e h a v e a n i n t e r e s t i n g r e e n t r a n t p h a s e diagram.
3992 0.8
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.
.
.
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T(K) Figure3 The suggestedpha~ diagram.The solid linesshowthe reentrarttnormal-superconducting-nonmetallic( CDW or SDW ) characterof type-Asamples.]'he broken linesshowthe behaviorof type-Bsamples.
ACKNOWLEDGEMENT This work was supported by NSF grant DMR-8822968.
REFERENCES 1. M. Potel, R. Chevrel, M. Sergent, J.C. Armici, M. Decroux and O. Fisher; J. of Solid State Chem. 35, 286, (1980). 2. J.C. Armici, M. Decroux, O Fisher, M. Potel, R. Chevrel, and M. Sergent, Solid State Commun., 33, 607, (1980). 3. R. Lepetit, P. Monceau, M. Potel, P. Gougeon and M. Sergent, J. of Low Temp. Physics, 56, Nos 3/4, (1984). 4. H.P. Geserich, G. Scbeiber, M. Durrler, M. Potel, M. Sergent and P. Monceau, Physica 148 B, 234 (1986). 5. H. Nohl, W. Klose and O.K. Anderson in" Superconductivity in Ternary Compounds"; O. Fisher and M. B. Maple, Eds; Springer Verlag: New York,
1982; Chapter 6.
6. D. R. Overcash, M.J. Skove, and E.P. Stillwell, Phys. Rev. 156, 570, (1969). 7. G.X. Tessema, Y.T. Tseng, M.J. Skove, E.P. Stillwell, R. Brusetti, P. Monceau, M. Potel, and P. Gougeon, submitted for publication. 8. X.F. Chen, M. J. Skove and G.X. Tessema, private communication.
3989
TI2M__gCr~: R EENTRANT NORMAL-SUPER(~ONDI.JCTOR-DEN$ITY WAVE PHASE TRANSITION
G.X. TESSEMA, Y.T. TSENG, M..I. SKOVE, E.P. STILLWELL Departement of Physics and Astronomy, Clemson University, Clemson S.C. 29634 (U.S.A.) R. BRUSETFI, P. MONCEAU C.R.T.B.T., C.N.R.S., Grenoble, France.
ABSTRACT We report experimental results showing that uniaxial stress (~) increases the resistance in the full range of temperature and leads to a metal to nonmetal phase transition. It suppresses T c. And, for ~ >--0.50 %, a peak appears at Tp = 15 + 3 K in d(log(R(e,T)))/d(1/T) vs I / T plots. We speculate a Peierls phase transition takes place at Tp with a fully developed gap Eg = 57 K below T = 10 K. We summarize our results in a phase diagram in the e vs T plane which, in the case of type-A samples exhibits a reentrant metal-superconductor- semiconductor characteristics.
INTRODUCTION TI2Mo6Se6 is unique among the general family of M2Mo6X 6 (M = TI, In, Na, ...etc and X = S, Se, Te ) quasi-onedimensional compounds in that it is the only one superconducting and at a rather high T c (= 6.5 K ) [1]. The compound is highly anisotropic as demonstrated from resistance, superconducting critical magnetic field He 2 and polarized optical reflectance measurements [2, 3, 4]. The calculated Fermi surface consists of two parts: sheets and ellipsoids. The nearly planar sheets are perpendicular to the chain axis c axis near ~r/c from r to A. The ellipsoids are centered at A and their existence is due to the energy overlap of the a2 bands at ~r/2 and the e bands at A. These electrons are believed to protect the structure from a Peierls distortion and are responsible for the superconductivity. Nohl et al predicted that stress might change the occupancy of these pockets leading to a Peierls distortion and formation of CDW [5]. We present a study of the effect of uniaxial stress on this compound.
RESULTS The needle like samples are mounted on a quartz puller described elsewhere [6] and the resistances are measured using the standard four probe technique. Figure 1 shows a typical behavior, both for type-A and type-B samples, of the R(~,T) vs T in a semi-log plot and for different values of e. The materials undergo a metal to nonmetal phase transition below a c dependent temperature Tm at which R(~) reaches a minimum. Tm increases linearly with ~. 0379-6779/91/$3.50
© Elsevier Sequoia/Printed in T h e N e t h e r l a n d s
3990
.----.
2
o
• []
0.70 0.65
.
ooo/
/"~--f/J /T J-" ~ _ _ / / - - / / ~(//,Ar
fj
1
0.04
Of.
o
O. O0
--1
I
f
0
30
I
I
60
I
I
90
'M.
,
I
,
,
.
,
80 120 1 O00/T (I/K)
40 I
1 20
I O00/T (I/K)
I
I
1 50
I
1 80
Figure 1_- A plot o f log(R(e~,T)/R 0 ) v,s 1000/T. T h e legends are given in the box; the n u m b e r s refer to the values of ~ in %. T h e insert shows plots of d ( L o g ( R ( 6 , T ' ) ) / d ( 1 0 0 0 / T ) for different values of 6. A broad peak is observed at T = 15 K. All the curves for (~ > 0.6 % converge to a stress independent value o f E
g
= 57 K at T < 10 K.
The insert in Fig. 1 shows a plot of the slope d(log(R(~,T)))/d(1000/T). A rather broad peak is observed at Tp = 15 K _+3 K and for ~ _>0.50 %. Tp appears to be independent of ~ above 0.70 % but the peak becomes better defined for higher 6. This slope saturates to a stress and T independent value of 57 K below about 10 K. The above results suggest that the prediction of Nohl et al might indeed be correct and a CDW might occur with a Peierls transition temperature Tp = 15 K + 3 K and an energy gap E g = 57 K which would lead to a ratio E / k T p = 3.4 which is surprisingly equal to the BCS ratio. Stuctural studies under stress and at low temperatures will be required to confirm the above speculations. However, we have observed nonlinear electrical conductance above a sample dependent threshold electric field [7]. The search for narrow band noise was not succesfull but broad band noise above a second threshold field is observed in some samples.
Uniaxial stress suppresses superconductivity in both type of T12Mo6S% samples. However, the details of the effect of E on Tc depend on the type of sample. As shown if Fig. 2a, Tc decreases monotonically in type-B compounds with a rate increasing with ~ until superconductivity is totally suppressed at about 0.50 %. Tc for type-A samples shows an initial increase at low E (< 0.35 %) wilh a rate ATe/Act = 0.18 K/GPa it reaches a maximum at ~ = 0.35 % and start decreasing steeply ( ZXTc/As = 2.5 K/GPa calculated using Young's modulus Y = 400 GPa +- 40 GPa measured by X.F. Chin et al [8]) before vanishing at about 6 = 0.55 % + 0.05 %. At higher 6 the material exhibits a non metallic behavior, possibly a semiconducting behavior with a fully developed CDW or SDW, as mentioned earlier in this report. The reported reentrant normal-superconducting-semiconducting behavior is best illustrated in Fig. 2b where log(R(T)) is plotted against 6 for a t . ~ - A sample. For T = 6.70 K R decreases as the strain is applied to the sample, R reaches a minimum at 6 = 0.10 % and increases into the nonmetallic regime.
3991
0
.......
-:-'W TEK]
4
~
0.0
0.2
0.4
0.6
stroin ( Z )
0.8
Strain dependence of resistance 104
, /,
/
,
10~
1 02
,
,"
"-"'\
1 01 -0.001
..7
i 0,001
i 0.003
STUN (Z}
i 0.00.5
0.007
Figure 2~- Fig 2a. The effect of E on the T . T for type-B samples, ( full circles), decreases monotonously. Tc for type-A samples, (open circles), exhibits an initial rise to a broad maximum before it decreases steeply. The inset shows the effect of ~ on the resistive transition of a type-A sample. The value of e in % is 0.04), 0.12, 0.38, 0.46, 0.520 for the solid, the doted, dashed dotted, short dashed and long dashed lines respectively. The normalized resistance R N = R(E ,T)/R( ~, 7.51{) is shown vs T. Fig 2b: the ~ dependance of a type-A sample.
O u r i n t e r p r e t a t i o n o f t h e s e results is s u m m a r ; z e d in the p h a s e d i a g r a m s h o w n in F i g 3. A t low t e m p e r a t u r e s ( T < 10 K ) T I 2 M o 6 S e 6 can b e f o u n d in t h r e e possible states: n o r m a l metallic state, s u p e r c o n d u c t i n g s t a t e at e = 0 o r low stress (E _< 0 . 4 0 % ) a n d a dc;nsity w a v e stale ( C D W (xr S D W ) at h i g h e r c. T y p e A s a m p l e h a v e a n i n t e r e s t i n g r e e n t r a n t p h a s e diagram.
3992 0.8
.
.
.
.
Semiconducting ( CDWor SDW)
0.6 ~
~
._c0.4 oi...
/
-----~ \
.4_-,
~ 0.2
~I
normal
L
o.o
,
5
,
6
, ' ,
;,
7
,
,
8
,
9
,
10
T(K) Figure3 The suggestedpha~ diagram.The solid linesshowthe reentrarttnormal-superconducting-nonmetallic( CDW or SDW ) characterof type-Asamples.]'he broken linesshowthe behaviorof type-Bsamples.
ACKNOWLEDGEMENT This work was supported by NSF grant DMR-8822968.
REFERENCES 1. M. Potel, R. Chevrel, M. Sergent, J.C. Armici, M. Decroux and O. Fisher; J. of Solid State Chem. 35, 286, (1980). 2. J.C. Armici, M. Decroux, O Fisher, M. Potel, R. Chevrel, and M. Sergent, Solid State Commun., 33, 607, (1980). 3. R. Lepetit, P. Monceau, M. Potel, P. Gougeon and M. Sergent, J. of Low Temp. Physics, 56, Nos 3/4, (1984). 4. H.P. Geserich, G. Scbeiber, M. Durrler, M. Potel, M. Sergent and P. Monceau, Physica 148 B, 234 (1986). 5. H. Nohl, W. Klose and O.K. Anderson in" Superconductivity in Ternary Compounds"; O. Fisher and M. B. Maple, Eds; Springer Verlag: New York,
1982; Chapter 6.
6. D. R. Overcash, M.J. Skove, and E.P. Stillwell, Phys. Rev. 156, 570, (1969). 7. G.X. Tessema, Y.T. Tseng, M.J. Skove, E.P. Stillwell, R. Brusetti, P. Monceau, M. Potel, and P. Gougeon, submitted for publication. 8. X.F. Chen, M. J. Skove and G.X. Tessema, private communication.