Thermopower of doped and damaged NbSe3

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iiSolid State Communications, Voi.39, pp.553-557. Pergamon Press Ltd. 1981. Printed in Great Britain.

TBERMOPOWER

OF D(IPED M~D DAmaGED NbSe 3

P. M. Cbaikin, Department

0038-]098/81/280553-05502.00/0

W. W. Fuller and R. Lacoe

of Physics, Los Angeles,

University of California CA 9~024 U.S.A.

J. F. Kwak and R. L. Greene IBM Research Labs San Jose,

Department

and J. C. Eckert and N. P. Ong of Physics, University of Southern Californla Los Angeles, CA 9007 U.S.A.

(Received

3 February

1981 by H. Suhl)

We have measured the thermoelectric power of pure NbSe 3 as well as samples which have been substitutionally doped with isoelectronic Ta and the charged impurity Ti and separate samples which have been radiation damaged by 2.5 MeV protons. We find that 5% Ta doping supresses the lower temperature charge density wave transition. In contrast, the radiation damaged samples and 0.1% Ti samples with larger residual resistivities than the Ta doped samples retain the CDW transitions. A discussion is given of the difference between doping and radiation damage.

Niobium trise!enide (NbSe 3) is quite remarkable material. It undergoes two charge density wave (CDW) transitions at 145K and 59K but remains metallic at low temperature, i-3 Below the CDW transitions the conductivity has been found to be highly electric field and frequency dependent, a behavior attributed to depinning of the CDW. 4-6 At low temperatures there have been conflicting reports of superconductivity in pure NbSe 3 at ambient pressure. 7-9 However, when the CDW transitions are suppressed by pressure application above 6 kbar the material becomes a superconductor at 2.5K. I0 Bulk superconductivity has also been seen in Ta and Ti doped samples. 9,11 Since the nonlinear conductivity and its frequency dependence depend critically on the strength and number of sites which pin the charge density wave, there have been several studies of the influence ~ ~vbstitutional impurities and of damage. ~-~-4 The purpose of this paper is to investigate the differences between these imperfections in light of the differences observed in the conductivity measurements and the presence of low temperature superconductivity for the doped samples and no superconductivity for irradiated samples. The thermopower is a particularly useful and sensitive way of probing CDW transitions especially when carriers of two different signs are present and the transition affects only one type of carrier. The pure and doped samples were prepared at the University of Southern California by mixing stoichiometric quantities of the previously prepared transition metal alloy and

selenium. The mixture was then heated to 720°C for 3 weeks. The samples which resulted were bundles of flat thin fibers. Single fibers were removed and were in ideal geometry for thermopower measurements. For damage studies single fibers were reproved from a bundle of pure NbSe3, mounted on a quartz slide by indium soldering the fiber ends and placed in the Van de Graff accelerator at the California State University in Los Angeles. The samples were then irradiated with 2.5 MeV protons. The penetration depth of these protons is ~ 20D which is larger than the thickness of the samples so that damage occurs uniformly. The amount of damage is calculated from the beam current, exposure time, scattering cross section and number of eroding displacements. 13,15 From known values a beam current of 10NA/mm 2 produces 10 -4 defects/atom in a three minute exposure. However, this number is only good to a factor of 2 in absolute value due to uncertainties in the order of the cascade with two atoms, Nb and Se, involved in the compound. Relative values of damage are of course linear in radiation time. The thermopower was measured in a conventional apparatus described previously using a slow AC technique. 16 The absolute thermopower for pure NbSe 3 and Ta doped samples is shown in figure i. Our results for pure NbSe 3 are in agreement with the published data of reference 17. The two charge density wave transitions are easily observable as sharp changes in slope at 145K and 59K. The usual interpretation of this data is that the 145K CDW involves the formation of 553

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Vol. 39, No. 4

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a gap on the part of the Fermi surface with hole-like carriers. The negative carriers remaining are less compensated and the thermopower becomes more negative. Similarly the 59K CDW produces a gap on an electron like region and the thermopower becomes more positive (less negative). ~ i l e it is clear that gaps are being formed at the CDW transitions the assignment of carrier sign (electron or hole) is somewhat clouded by comparison with the Hall coefficient% 8 The 145K transition appears similarly in the two measurements. However at 59K the Hall constant first increases negatively and then turns around and crosses zero at ~ 15K. The two transport coefficients measure different averages of the Fermi surface parameter. (The Hall effect measures ~xy whereas the thermopower probes the derivative 9_~ ). So the oE sign change in the slope at 59K is not contradictory. It merely indicates a topologically complicated Fermi surface that is not simply describable by an ellipsoidal effective mass. When 0.5% Ta is substituted the thermopower remains largely unchanged. Both CDW transitions are clearly observed although the transition temperatures have been shifted downward and some smearing of the transition has occured. This is in general agreement with resistivity studies. However, the loss in conductivity at the lower transition is a smaller fraction of the total conductivity than in pure samples (1/2 for pure, 1/4 for 0.5% Ta).12,13 One would thus expect that the fraction of the Fermi surface affected by the CDW is decreased for the doped sample. On the other hand the thermopower indicates that the CDW gap is largely unchanged for this dopant level. This would tend to imply that the Ta impurities affect the scattering of the nested region of the Fermi surface more than the unnested regions, at temperatures above the lower transition. With 5% Ta doping the thermopower indicates the upper CDW transition as a gradual negative

increase beginning at the temperature where this transition occurs with pure samples. The lower temperature transition is not apparent at all. The fact that the thermopower reaches a maximum and then tends toward zero is probably more indicative of the thermodynamic constraint that for a system with finite conductivity at T = 0 the thermopower must be zero. The thermopower then leads to the conclusion that 5% Ta doped samples have a smeared upper CDW transition but essentially no lower transition. This result is in agreement with resistivity measurements which however show a slightly increasing resistance as temperature is lowered below ~ 40 K. 12 Moreover, the absence of the lower transition leaves more states available at the, Fermi surface for Cooper pairing and helps explain why 5% Ta doping produces bulk superconductivity in NbSe 3 whereas pure samples are not superconducting• II It is also interesting to note that the thermopower has the same type of rounded negative maximum with 5% Ta doping as it does when measuring in pure samples in a high electric field. 19 Samples doped with 0.1% Ti show virtually the same dependence in thermopower as the 5% Ta doped samples. However, resistivity measurements (Fig. 2) show that the two CDW phase

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transitions survive the Ti doping at this level. The lightly doped (0.01%) sample is qualitatively similar to the pure case. However, the two samples of nominal 0.1% Ti concentration show different behavior at low temperatures. In the sample with filled circles the restivity continues to rise monotonically below 30K while in the sample with open circles the resistivity resumes metallic behavior at low temperatures• This crossover behavior in two samples obtained from the same growth batch shows the marked sensitivity of the low-temperature resistivity to (non-isoelectronic) impurity concentration at this dopant level. For proton irradiated

THERMOPOWER OF DOPED AND DAMAGED NbSe 3

Vol. 39, No. 4

samples 14 a similar crossover behavior is observed in going from 0.05% to 0.08% defect concentrations. Measurements of the nonOhmicity reveal that the Ti doped samples 20 and irradiated samples (the same defect concentration) have similar minimum threshold fields (ET (~ 3 v/cm for a residual resistivity ratio RRR of ~ 4~. The threshold field also scales as (RRR) -~ in both cases as opposed to a (PURR)-2 dependence in Ta doped samples. 12 However, bulk superconductivity is observed 9 in the 0.1% Ti sample at 2.2K (ambient pressure) whereas no bulk superconductivity has been observed in the irradiated samples aside from vestigial traces of the filamentary superconductivity seen in pure samples. The thermopower for radiation damaged samples is shown in Fig. 3. Both CDW transi-

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tions are evident for all of the dosages although the case of the most highly damaged sample can be argued. It is to be noted that the 0.04% damaged sample has a much higher residual resistivity than the Ta doped sample of comparable concentration. However it is comparable to the spread observed in the 0.1% Ti doped samples. Aside from the CDW transitions there are two additional points of interest in Fig. 3. First, the room temperature thermopower value decreases with increasing radiation damage. Secondly, for high dosage samples the thermopower is much more positive below the lower transition than for the pure samples.

555

The actual value of the thermopower is quite difficult to calculate for simple systems and extraordinarily difficult for a complicated Fermi surface. Thus we can only guess at an explanation. One possibility is that the electron like part of the Fermi surface involves wavefunctions that are predominantly on the Nb atoms. These atoms are slightly more susceptible to radiation damage than the Se atoms because of their cross section. The relative conductivity of this protion of the Fermi surface would be reduced and so would its contribution to the thermopower. The additional positive thermopower below the CDW transition is also puzzling. In resistivity studies we have found that the defect scattering of conduction electrons is enhanced by the presence of the CDW's. This results from their pinning the CDW in a non-periodic fashion so that the CDW itself can contribute to scattering of electrons in the unnested, ungapped regions of the Fermi surface, If this additional scattering is mostly on regions adjoining the nested region their mobility would decrease. Since the nested region involved negative carriers so would the adjoining regions. Thus the total decrease in negative carrier contribution to the thermopower would be even more reduced and the thermopower more positlve. It is clear that chemical doping and radiation damage have a strong effect on the CDW transitions and an even more pronounced effect on the dynamics, namely depinning of the condensate. For clarity we discuss these two effects separately. First, the introduction of impurities and defects tend to smear both CDW transitions in a continuous way, with the lower transition being more susceptible. Isoelectronic (Ta) doping causes less damage than either non isoelectronic (Ti) doping or proton radiation. From the thermopower alone one might infer that Ti doping at the 0.1% level has the same approximate effect on the lower transition as 5% Ta doping, and is more drastic than a 0.1% defect level in the irradiated samples (Fig. 3 solid triangles). Furthermore the appearance of bulk superconductivity 9 in the 0.1% Ti samples at 2.2K in contrast to only vestigial traces of filamentary superconductivity in the irradiated samples indicates that more of the Fermi surface survives the CDW transitions in the Ti samples. Pressure studies by Monceau et a120 on the CDW transitions have shown that the lower transition is almost totally suppressed at 6 kbar, indicating an unusually high sensitivity of the nesting condition to changes in the band-width. The inference is that Ti is more disruptive to the CDW transition than defects because it shifts the Fermi level. However, one should treat these interpretations with qualification. The resistivity vs. temperature profile of the radiation damaged samples 15 are strikingly similar to that of the 0.01% and 0.1% Ti doped samples~ The lower CDW transition is certainly not substantially suppressed by a 0.1% level of Ti atoms (see Fig. 2), Assuming a rigid band model we estimate that the shift in Fermi level with a 0.1% doping of Ti equals 10-3 ~E F where

THERMOPOWER OF DOPED AND DAMAGED NbSe 3

556

n is of the order of one. If E F is taken to be several electron volts the induced shift equals the thermal broadening at 60K. It would appear that a doping level of several times the I000 ppm used is required to effect the nesting condition significantly. On the other hand because of the high sensitivity of the nesting condition (as indicated by the thermopower, pressure and superconductivity studies) once this level is exceeded the lower CDW transition is expected to be greatly affected by the Ti concentration. Such is not the case with the irradiated samples. The second aspect is the effect of impurities and defects on the motion of the CDW condensate. Non-Ohmic measurementsl2, 21 show that both E o and E T at their minimum valses scale as (RR) -2 when Ta is introduced. Recent work21,14 shows that E T scales as (RRR) -I for both Ti irradiated samples in the i00 ppm range. The curves of E T vs. (RRR) -I lie on the same line within the data scatter for both kinds of defects. Therefore it may be concluded that Ta acts as a weak impurity while Ti and radiation-induced defects act as strong pinning sites. The power law behavior in all three cases is consistent with the theory of Lee and Rice. 22 Finally we comment on the cross over behavior of the resistivity in Fig. 2. In all highly doped or damaged samples (exceeding 0.1% Ti or 0.08% defect concentration) the low temperature resistivity rises monotonically with decreasing temperature instead of resuming metallic behavior. We offer two explanations for ~his observation. In the first, the CDW gap fails to saturate to its maximum value, as has been shown by Zittartz 23 to occur in the excitonic insulator when a large concentration of pair-breaking impurities is present. The continual removal of free carriers leads to a slow

Vol. 39, No. 4

rise in the resistivity if the mobility is impurity dominated in this temperature range. In the second model the presence of a large number of strong pinning sites dictates that the CDW loses its phase and amplitude coherence as it strives to optimize their value at each pinning site. In such a situation--more appropriately called a CDW glass-the disorder may lead to localization of the surviving free carriers in very large diameter orbits. The slow quasi logarithmic rise in resistivity at temperatures below 4K is consistent with this picture although the rapid rise in the 10-40K range is still a puzzle. In conclusion we have studied the thermopower of Ta and Ti doped samples of NbSe 3 as well as irradiated samples. Previous results have shown that Ta acts as a weak pinning site ET ~ C 2) whereas both Ti and radiation induced defects act as strong pinning sites (ET ~ C). What we find from the thermopower results is that the lower CDW transition is suppressed by doping with 5% Ta. Radiation damage samples however show both CDW transitions even for residual scattering rates exceeding that of the 5% Ta doping. This suggests that the Ta doped samples may have important changes to the nesting condition carried by Fermi level shift~ Such shifts may also be present in Ti samples for concentrations several times the 0.1% level with precursor effects at lower levels. We would llke ,, to acknowledge useful discussions with G. Gruner, T, Holstein, P. Pincus and Jim Savage. Research at UCLA supported by NSF under grant DMR 79-08560 (P. M. Chaikin) and ONR under grant N00014-76-C-1079 (W. W. Fuller) and at U.S.C. under grant NSF DMR 79-05418. P. M. Chaikin is an A. P. Sloan Foundation Fellow, W. W. Fuller is an IBM Predoctoral Fellow.

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THERMOPOWER OF DOPED AND DAMAGED NbSe 3

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