Upper critical field of the pressure induced superconductor BaMo6S8

Phystca C 157 ( 1989) 247-250 North-Holland, Amsterdam

UPPER CRITICAL FIELD OF THE PRESSURE

INDUCED

SUPERCONDUCTOR

BaMo&

YAO Yu Shu ‘, R.P. GUERTIN 2, D.W. CAPONE II 3 and D.G. HINKS 4 ’ InstrtuteofPhysrcs, Bezpzg, P.R Chrno ’ Tufts Unlverslty, Medford, MA 02155, USA 3 Supercon, Inc., Shrewsbury MA 01545, USA ’ Argonne Natronal Laboratory, Argonne, IL 60439, USA Recerved 10 January 1989

Measurements of the upper critrcal magnetic field, H,,, are presented at two quasrhydrostatrc hrgh pressures for superconductmg BaMo,Ss, which IS semrconductmg at ambient pressure. For P=4.5 GPa, H,,( T=O) reaches 25 T, whtch IS consistent wrth the large upper cntrcal fields of other dtvalent Chevrel phase superconducting systems, mcludmg the rsomorphtc pressure Induced superconductor, EuMo.&. However, the anomalous features in Hc2 (T, P) for EuMo& due to the Eu magnetrc moment are absent m the data for BaMo&.

1. Introduction

The physical properties of the Chevrel phase [ 1 ] systems have long been known to be sensitive to applied high pressure. Perhaps the most dramatic manifestation of the influence of pressure is the appearance of superconductivity at high pressure in EuMo,S, [ 21 and BaMo& [ 31. These are both semiconductors at low temperatures and ambient pressures due to a rhombohedral-to&clinic structural transition which takes place near 100 K and which accompanies a transition from the metallic to the semiconducting state [4]. Presumably a gap in the Fermi surface opens in the tnclinic phase, causing the semiconducting ground state [ 5 1. The existence of superconductivity of BaMo& at high pressure was originally called into question [ 61, but in a recent paper [ 71 we demonstrated zero resistivity m BaMo& for P> 5.0 GPa (50 kbar) and T<3 K in a series of measurements carried out in quasihydrostatic pressures (QHP) up to 7.0 GPa. The results showed that the properties for superconducting BaMo& were consistent with the other divalent superconducting Chevrel phases when the temperature and extent of the rhombohedral-to-triclinic structural transition was taken into account [ 81. In this paper we follow through on the work

presented m ref. [ 71 by presenting the results of electrical resistivity measurements on BaMo.& taken under reasonably extreme conditions: temperature, 2 K< Tc 15 K, magnetic field, 0 T
2. Experimental details The BaMo& sample was prepared usmg methods discussed in previous papers [ 11. Particular attention was paid to preventing the inadvertant introduction of oxygen into the system. Oxygen can strongly alter the superconducting transition temperature, T,, and its pressure dependence [ 111. The small quasthydrostattc self-locking pressure

0921-4534/89/%03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )

248

Yao Yu Shu et al / Upper crltlcalfield of

the pressure mduced superconductor BaMo&,

clamp device was built specifically for this experiment. The overall dimensions of the clamp, which was made entirely of hardened beryllmm copper and tungsten carbide, was about 3.2 x 7.5 cm2. The high pressure cell was located between two tapered tungsten carbide anvils. Lead access was with 0.002” Pt wires fed through small grooves in pyrophylite collar, and the pressure transmittmg medium was soft boron nitride. The working volume of the high pressure cell was less than 1 mm2. The sintered polycrystalline sample of BaMo& had dimensions approximately 0.75X0.015X0.015 mm3. The resistivity of the sample was determined using a Lmear Research LR400 self-balancing ac resistance bridge, and the magnetic field was produced by the 15 T superconducting solenoid facility located at the Francis Bitter National Magnet Laboratory, MIT. In a previous paper [ 71 we showed that the onset for superconductivity m BaMo& was at about 2.0 GPa (QHP) and that the maximum T,onset ( 12 K) occurred for P- 4.0 GPa. For higher pressures T,decreased, m a manner similar to SnMo$& and EuMo&, the latter being also a pressure induced superconductor at lower pressures. In addition, zero resistivity was demonstrated in BaMo& for the first time to the hmit of the measurement techmque, about lo-’ n-cm. For P=5.6 GPa, for example, T:Onset) was about 10 K and zero resistivity occurred for Tc 3 K. The width of the transition was attributed to the pressure gradient m the high pressure cell. Similar broadening under QHP conditions is observed in all Chevrel phase superconductors. In the present work, the effective high pressure area was somewhat smaller, and as a result, the pressure gradients were larger. However, withm the range of the measurement capability, zero resistivity was observed for P~6.0 GPa for T-c2K. In the high field measurements, the onset T,was used to calibrate roughly the pressure in the cell, using the experimental results from ref. [ 7 1. This pressure correlated well with the applied force dunng cell loading, and the uncertainty in the quoted pressures is about -t 1 GPa.

3. Experimental results In fig. 1 we show the resistance, R (in mR) vs temperature of the BaMo,S, sample for P= 6.0 GPa

100

r t

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40-

, 0

I

5

_+

,,,,,,,,,,,,,.f

10

15

20

T (K)

Fig. 1 Electrical resistance vs temperature for several applied magnetic fields for BaMo& under a quaslhydrostatlc pressure of 6.0 GPa. The mset shows a complete transition to zero reststlvlty m zero apphed field

QHP for several applied magnetic fields 0 T < H< 8 T. The inset shows the full R vs T transition and a straight line extrapolation method used to as a secondary definition of T,.The onset T,was defined as the temperature at which R first deviates from the straight line extrapolation to T=O K from the normal resistive state, e.g., 10 K for P=6.0 GPa and H=O. The effect of applied field is clearly seen to decrease T,,although slowly, so it is clear that the critical fields are large, as expected for a divalent Chevrel phase superconductor. In fig. 2 we show similar data for a lower applied pressure, Pz4.5 GPa. Note that the normal resistance is larger than for the data of fig. 1, which may reflect geometric effects. For the P= 4.5 GPa data, a full transition to zero resistivity is not observed, even in zero applied field. We attribute thts to increased non-hydrostatic behavior due to the small effective high pressure area and the finite size of the sample. The critical field vs temperature data obtained for the p vs T measurements are shown in fig. 3. Here the open data points represent onset T,and the closed data points represent the straight line extrapolation, as defined in the inset of fig. 1. The solid and dashed lines are fits of the dirty hmit WHH theory [9] of Hc2(T) vs T,using T,(H=O) and dHc2/dTIHEo as fitting parameters. The extrapolated results for T, onsetsgiveHc2(T=O)=25Tand9TfortheP=6.0 GPa and P~4.5 GPa data, respectively. These large

Yao Yu Shu et al / Upper crrtlcalfield of the pressure rnduced superconductor BaMoaa

249

4. Discussion

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180-

4 ,0” , 0

2

4

I

1

6

8

I

IO T (K)

I2

I

14

16

18

20

Rg. 2 Resistance vs temperature for BaMo& for several apphed magnetic fields; sample under 4.5 GPa quaslhydrostatlc pressure.

30-

BaMo6Ss

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0

2

4

\

\

\

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6 T (Kl

8

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12

Rg. 3. Upper cntlcal magnetic field, HCz( T) vs temperature for superconductmg BaMo& at two quaslhydrostatic pressures. Open circles represent onset transltlon temperature; for closed symbol HC2( T) cntenon, see mset of fig. 1

values are commensurate with other divalent Chevrel phases, including EuMo,&, where again T, is strongly affected by pressure. However, unlike Eu, Ba carries no magnetic moment and we do not expect the non-monotonic HC2( T-C P) curves of EuMo,& which are caused by negative localized moment-conduction electron exchange effects [ 12 1.

Meul [ 131 has reviewed extensively the normal properties of the divalent Chevrel (M2+Mo&) compounds. He identities two normal state electronic transitions m these materials. The onset of the higher temperature transition is manifested by a weak minimum m the temperature dependence of the electrical resistivity, p ( T). The lower temperature transition is associated with the actual rhombohedral-to-triclimc structural transition. The temperature band between these two transitions is called an “intermediate” region. (Sn- and PbMo&, which are ambient pressure superconductors, are seen as being in the “intermediate” phase at T,; neutron scattering measurements [ 141 identify slight lattice distortions below about 130 K and 110 K, respectively [ 141, but no transition to the triclinic phase.) It is suggested that the increase in p(T) for T-C T(pml,) (the onset of the higher temperature transition) for the Chevrel phases is the result of partial gapping of the Fermi surface caused by very small distortions of the rhombohedral lattice [ 51. This was observed directly for Sn- and PbMo& using neutron scattering [ 141. Hall effect measurements [ 131 in the rhombohedral phase suggest the number of charge carriers changes rapidly with decreasing temperature due to this gapping. The energy gap in most of the M2+Mo& compounds should be small compared to thermal energies for T> 100 K, except for BaMo&, where it is of the same order of magnitude [ 13 1. Of all the (M’+Mo&) compounds, the resistive and Hall effect anomalies at ambient pressure in BaMo& in the “intermediate” region are by far the largest. For BaMo& the upper transition occurs below about 250 K, whereas the structural transition, which appears to be first order, takes place at about 120 K ( 174 K in single crystals [ 15 ] ). The electrical resistivity of sintered samples increases by about a factor of 16 from room temperature to 120 K, then decreases and reaches a minimum at about 50 K, and finally p( T) increases for T-C 50 K, possibly due to localization effects [ 16 1. (Suggestions of non-linearity in the I- Vcharacteristics of BaMo& implicate charge density wave excitations in the transport property changes around 120 K [ 161. ) It is important to note that BaMo& has the largest unit cell of

250

Yao Yu Shu et al / Upper cntrcal)eld

of the pressure Induced superconductor BaMo&

all the M*+Mo& compounds, 292.7 A3 vs 219.3 8, for PbMo& [ 13 1. This means the distance between the MO& clusters is the largest of any of the divalent Chevrel phases and that the overlap between the 4d MO bands is probably the least, i.e. electrons tend to remain localized around the MO octahedra. This seems to correlate with the strength of the transport anomalies which signal the lattice distortions for the divalent Chevrel phases. Pressure, however, is seen to reduce the intercluster distance and thus increase hybridization between the cluster MO 4d bands, eventually forcing the system into the metallic state even down to the lowest temperatures, where conditions then become favorable for the onset of superconductivity. Thus pressure suppresses both the weak “upper” partial gapping transition as well as the “lower” structural transition. (The latter may be first order in BaMo& at ambient pressure.) The results presented in this paper show that at sufficiently high pressures, enough to produce superconductivity, p( T) is metallic-like for T, -e Tc 300 K, suggesting a pressure induced suppression of both “transitions” alluded to above. The Hall coefficient of sintered samples of BaMo& increases by more than two orders of magnitude between room temperature and 150 K [ 131, far larger changes than in any other Chevrel phase. It would be interesting to carry out Hall effect measurements at high pressures in order to see if the carrier concentration remains relatively constant between room temperature and T,. In general, measurements carried out at extreme conditions such as those presented in this paper can reveal features of the band structure and can be correlated with the results of Chevrel phase band structure calculations [ 17 1.

Acknowledgements

We are pleased to acknowledge many helpful discussions with Dr. Simon Foner of the Francis Bitter National Magnet Laboratory, which is supported by the National Science Foundation, The Tufts-based work was supported by NSF DMR 8502077, and the Argonne National Laboratory-based work was

supported by the Department of Energy, Basic Energy Sciences under contract W-31-109-ENG-38.

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