Journal of the Physical Society of Japan
Vol. 69, No. 10, October, 2000, pp. 3470-3471

Short Notes

Pressure-Induced Magnetoresistancein NbSe3
Syuma Yasuzuka, Yoshitoshi Okajima,
Satoshi Tanda, Nao Takeshita1 , Nobuo Môri1
and Kazuhiko Yamaya
Department of Applied Physics, Hokkaido University,
Kita-ku, Sapporo 060-8628
1 The Institute for Solid State Physics, The University of Tokyo,
Kashiwanoha, Kashiwa-shi, Chiba 277-8581(Received June 15, 2000)
KEYWORDS: NbSe3 , charge-density-wave, magnetoresistance,magnetic field, pressure, Fermi surface

Currently, a great deal of attention is being paid tothe magnetotransport accompanying the charge- or spindensity-wave (CDW or SDW) states in low-dimensional
electron systems as related to the topology of the Fermi
surface (FS).1) This occurs via a variety of physical phenomena, such as rapid oscillations, magic angles, andShubnikov-de Haas oscillations.1, 2) Applying pressure
generally leads to a drastic change in the topology of theFS in these systems.2, 3) Therefore it is of great interest
to study the effect of pressure on magnetotransport inthese systems.1-3)
NbSe3 , which undergoes CDW transitions at T1 =142 K and T2 = 58 K,4) exhibits large magnetoresistance(LMR)5) just below T2 in a magnetic field perpendicular
to the high-conductivity axis (b-axis). There is a longstanding controversy over whether the origin of the LMR
can be explained in terms of the magnetic-field-induced
enhancement of the CDW gap or not.6) The present authors7) discovered recently that the LMR immediately
vanished in a high strength magnetic field of H = 12 T
when the T2 -CDW was suppressed by pressure. Basedon these observations as well as many previous transport8-10) and X-ray studies,11) the LMR has been explained simply by considering the response of normal
carriers in small pockets created by an imperfect nesting of the T2 -CDW, showing that it is negative for themagnetic-field-induced enhancement of the CDW gap.7)
By contrast, the magnetoresistance (MR) in the T1 CDW phase (T ≥ T2 ) of NbSe3 is negligibly small atambient pressure.5) This is probably due to nearly perfect nesting in the T1 -CDW transition.12) It is known
that applying pressure causes the collapse of nearly perfect nesting, that is, imperfect nesting induced by highpressure. Therefore, MR due to imperfect nesting underhigh pressure may appear even in the T1 -CDW phase.
We will focus our investigation on whether a new MR is
induced by pressure in the T1 -CDW phase or not.
NbSe3 crystals were grown by vapor transport insealed evacuated quartz tubes using Nb wire and Se
shots heated at 740◦ C for two weeks. The resistance wasmeasured using the conventional four-probe DC method,where current is parallel to the b axis. A modified-Bridgman anvil device13) was employed in order to generate high pressure with H parallel to the a∗ -axis.
Figure 1 shows the temperature dependence of resistance (left-hand scale) for H = 0 and 12 T and the MRfor H = 12 T (right-hand scale) between 0 and 80 K at
P = 0 kbar. The magnetic field enhances the resistanceanomaly due to the T2 -CDW formation (T2 = 55 K at
H = 0 T). A large ∆ρ/ρ0 appears immediately upon theformation of the the T2 -CDW phase; a LMR is observed
below T2 . However, above T2 ∆ρ/ρ0 is negligibly small.
T2 is independent of the magnetic fields within our experimental error of ±0.5 K. These results agree well withprevious results.5)
Figure 2 shows the temperature dependence of resistance (left-hand scale) for H = 0 and 12 T and the MR
for H = 12 T (right-hand scale) between 0 and 40 K
at P = 7.2 kbar. The T2 -CDW transition temperature(T2 = 23 K) and the resistance anomaly associated with
T2 -CDW formation are suppressed by increased pressure.
Despite the considerable suppression of the T2 -CDW
phase however, a large hump in resistance can clearly beobserved below T2 for H = 12 T. Again, ∆ρ/ρ0 increases
below T2 , and a LMR is observed even under pressure.
These results agree with our previous results measuredfor H parallel to the c-axis.7) By contrast, ∆ρ/ρ0 is small
above T2 , with a magnitude of about 0.1 at 30 K. However, this value is about two orders of magnitude larger
than that at ambient pressure, which is about 3×10−3 at
70 K. Even if the temperature factor is considered in thetemperature dependence of ∆ρ/ρ0 ∼ (ωc τ )2 , where ωcis the cyclotron frequency and τ is the relaxation time,the ∆ρ/ρ0 above T2 obtained at 7.2 kbar is significantly
large. This indicates the new appearance of the MR

Fig. 1. Temperature dependence of resistance (left-hand scale)for H = 0 and 12 T and magnetoresistance for H = 12 T (righthand scale) between 0 and 80 K at P = 0 kbar.

Fig. 2. Temperature dependence of resistance (left-hand scale)for H = 0 and 12 T and magnetoresistance for H = 12 T (righthand scale) between 0 and 40 K at P = 7.2 kbar.

3470

Short NotesFig. 3. Temperature dependence of magnetoresistance ∆ρ/ρ0 for
H = 12 T under three pressures. Inset is resistance between 0and 140 K under three pressures for H = 0 T.

above T2 under pressure. We call this pressure-induced
magnetoresistance (PIMR).
Temperature dependence of the MR is shown in Fig. 3,where ∆ρ/ρ0 is plotted for H = 12 T under pressures including those above P1 , where the T1 -CDW phase disappears completely. The superconducting transitions takeplace near 4.0 K at H = 0 T. As seen in the inset in Fig. 3,small humps corresponding to the T1 -CDW phase transition are observed at around T1 = 90 K at 17 kbar and84 K at 19 kbar, whereas at P = 24 kbar, the T1 -CDW
phase is completely suppressed. P1 can be estimated tobe below 24 kbar. This is quite a bit smaller than thatfound in the previous study (P1 = 35 kbar),14) which
may be due to the difference in the pressure transmitting medium; in the previous work, a solid medium wasused, whereas in the present work a fluid medium (3MFluorinert) was used.
When the T1 -CDW phase is present, the PIMR increases with decreasing temperature below T1 for P =17 kbar and 19 kbar, and then rises sharply below about
40 K. The values of ∆ρ/ρ0 at 4.2 K are about 1. The
PIMR for P = 24 kbar increases gradually even below
40 K, and the value at 4.2 K is one order smaller thanthose for 17 kbar and 19 kbar. The PIMR increases withpressure up to P1 , above which the PIMR decreases
rapidly. We find that the PIMR effectively vanishes assoon as the T1 -CDW phase is completely suppressed bypressure, exhibiting the same behavior as the case ofLMR appearing in the T2 -CDW phase.
The most reasonable explanation for the pressure dependence of the PIMR is that the number of normal

3471

carriers in the pocket, as caused by a collapse of nearly
perfect nesting, increases with increasing pressure. However, if the T1 -CDW phase vanishes under higher pressure, the pockets will disappear at the same time, resulting in the strong decrease of PIMR. Namely, PIMR
substantially disappears as the T1 -CDW is completelysuppressed. Thus, we can explain consistently the appearance and disappearance of PIMR by assuming theexistence of normal carriers generated at ungapped FS
as induced by pressure.
By comparing the pressure dependence of LMR withthat of PIMR, we find that the existence of the T2 - andT1 -CDW phase are essential to the occurrence of LMR
and PIMR, respectively. This means that tiny residualpieces of FS created by imperfect nesting due to CDWformation can generally lead to MR even at relatively
high temperature.
In conclusion, we discovered that MR newly appears in
the T1 -CDW phase at elevated pressures. The results canbe explained by pressure-induced changes in the topologyof the FS as a result of the nesting effect in the formationof T1 -CDW in NbSe3 , indicating that the application ofpressure is a powerful tool in changing the topology ofthe FS in CDW and SDW systems.
We thank Dr. T. Nakanishi (Nihon University, Tokyo)
for his help with high-pressure measurements using themodified-Bridgman anvil device.
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