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Fast oscillations in (TMTSF)2X
ELSEVIER
$nthatic Metals 85 (1997) 1531-1532
Fast Oscillations in (TMTSF)zX M. J. Naughton, a,bZc** J.P. Ulmetb, I.J. Lee*, J.M. Fabred “Departments ofphysics and Chemistry, State University of New York, Buffalo, NY 14260 USA bService National des Champs Magnetiques Pulses, 31077 Toulouse, France ‘National High Magnetic Field Laboratory Tallahassee, Fl32310 USA dLaboratoira de Chimie Organique Structurale, 34095 Montpellier, France Abstract We report on the temperature dependence of the “fast oscillation” phenomenon in quasi-one dimensional molecular conductors. Magnetoresistance and magnetization measurements to 30T in the ambient pressure spin density wave state have been taken for the X = N03, PF6 and AsF6 members of the (TMTSF)2X family. Complementary data on X=AsF6 are presented by Ulmet et al. in these proceedings. All three compounds exhibit a peak in the transverse magnetoresistances pm (B//c *, I//a) and psr (B//c*, Iilb) and their oscillatory components around 3K, with the oscillations vanishing at higher and lower temperatures. No magnetic moment oscillations are detected in PF6 or AsF6, while only extremely weak oscillations (6m-10-10 emu) are seen in the NO3 salt above 25T. Also, no osciilations were observed in longitudinal magnetoresistance pz (BIiI//c*) measurements performed on AsF6 and PF6. Possible scenarios which have been used to explain the data include imperfect nesting at Tsow leaving behind quantizable closed orbits, and quantum interference effects arising from magnetic breakdown orbits. Another may involve a new phase transition inside the SDW state. Keywords: organrcsuperconductors, magnetotransport A major unsolved mystery in the Bechgaard salts (TMTSF)2X is the origin of the “fast oscillations,” SdH-like resistance oscillations permeating all conducting phases of these materials. They are periodic with inverse field, but have a unique temperature dependence which has the oscillations vanishing at low temperature. However, corresponding oscillations in the magnetization (the dHvA effect) have only been detected in one salt, X=C104, and then only in the field-induced SDW state.[l,2] According to baud structure determinations, the normal state Fermi surface contains only one pair of open sheets, such that no closed orbits should be present in the high temperature metallic phase.[3] Nonetheless, clear SdH-like oscillations have been observed in this metallic phase in the Re04 [4] and C104 [2] salts at fields below the threshold for FISDW formation. In addition, similar resistance oscillations have been detected in the ambient pressure SDW states in the PF6 [5] AsF6 [6,7] and NO3 [8,9] salts, and, of course, in the FISDW state in C104 [lO,ll] and Re0,,[4] all with T and B-independent frequencies A(l/B) near 250T. Recent theoretical effort [12,13,14] has focused on the possibility of magnetic breakdown across an anion ordering gap (in those salts which have anion ordering) and quantum interference effects. It is therefore useful to have thermodynamic information on the oscillations, as interference orbits should occur exclusively in transport (i.e. no magnetic moment). In order to test this hypothesis, originally proposed by Yan et al.[12] to explain SdH-like effects in Clod, we set out to determine to what extent these oscillations appear in magnetization in the nonFISDW spin density wave state. We present here results from this simultaneous magnetization and resistance study of the X = N03, PF6 and AsF6 members of the TMTSF system. *This researchwas supported by the National ScienceFoundation, under grant numbers DMR-9258579 and DMR-93 11739. 0379-67791971S17.00 0 1997 Elsevier ScienceS.A All rights reserved PII 90379-6779(96)4467-0
For PF6 and AsF6, we find no evidence of oscillatory behavior in the magnetic moment up to 30T, in the range 0.5K to IOK (not shown). On the other hand, we confirm the result of Ulmet, et al.[7] on AsF6 with regard to the T-dependence of the magnetoresistance and the fast oscillations, for both materials. We show the magnetoresistance R, of PF6 below in Fig. 1. A relationship can be seen between the magnitudes of the magnetoresistance and its oscillatory component, with both decreasing at low T. This, of course, is quite unexpected and difficult to explain using the Lifshitz-Kosevich model used to describe quantized orbits, which predicts saturation of the oscillation amplitude as T-+0. This is shown in more detail for PF6 and AsF6 in Fig. 2, where we plot the oscillatory resistance only.
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Magnetic Field (T) Fie. Magnetoresistanceof PFs in the ambient pressure SDW phase (Tsow=12K). Below 3K, both Ap/po and the fast oscillations amplitudes are suppressed. Inset: R,(T) at zero field, reaching 16M at 1.3K.
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AsF6, PF6 and N03, for data in the range 25-30T. All 3 show a characteristic peak between 2 and 4K. Moreover, the AsF6 FO amplitude, defined either by -(total)--(ambient) or via FFT’s, tracks remarkably with the ambient in Fig. 3(a). We note also that oscillations in AsF6 occur with R(ambient) nearly 1MSI. Finally, we show magnetization data on N03, where a very small oscillatory component is observed. The magnetic torque was measured at several temperatures. Atop a generally smooth variation, we find weak oscillations. We show this below, along with the oscillatory magnetoresistance, measured simultaneously. There appear oscillations in the magnetization above 25T, with a maximum amplitude of 5x10”’ emu. Due to this minute signal, we will not claim with certainty that the features in Fig. 4 arise from the dHvA effect. Additional experiments are underway at higher fields (>35T), which may help clarify the situation.
m t! 4 P s 2 * s 0
85 (1997) 1531-1532
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_ &I. z 2.OK- 4 -4 2.2K- ; _ 02
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SdH-like oscliiations in (TMTSF)lPFa and (TMTSF)?AsFh.
The amplitude of the oscillatory magnetoresistance grows with decreasing temperature until -3K, below which it falls rapidly to zero. We see that PF6 and AsFb have characteristically identical behavior. No oscillations were detected between 0.4K and 1.5K; nor were anv observed in R, measurements, at any T. The temperature dependence of the ambient magnetoresistance, defined by a low-order polynomial fit to the raw data to disregard the oscillations, also shows a rapid falloff at low temperature, as shown below in Fig. 3(a) for AsF6. Fig. 3(b) shows summary data on the amplitudes of the fast oscillations, for
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B CT) u Oscillatory components of the magnetoresistance(-) and the magnetization (0) in (TMTSF)zNOj. There is a correlation between the two quantities, which we tentatively ascribe to the WvA effect. Both amplitudes decreaseabove and below 2K. In summary, we have shown that the TMTSF fast oscillations in transverse magnetoresistance pm vanish at low temperature in the X=PF6, AsF6 and NOI salts. This coincides with a reduction in the ambient magnetoresistance. We find no evidence for oscillations in pZ or magnetization in AsF6 and PF6, and weak evidence for the latter in NO,, likely due to closed orbits at T< TAO. The vanishing of the FO at low T may be due to the opening of the SDW gap, reducing the probability for quantum interference orbits,[l4] or perhaps to a coexisting CDW-SDW state. References [l] JS. Brooks, et al., J. Mag. Mag. Mat, 54-57.637 (1986); P.M. Chaikin, et al., Physica 1438, 383 (1986); R.V. Chamberlin, et al., [2] [3] [4] [5]
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(a) ambient magnetoresistancein AsF6,showing peak near 2.5K,
(b) fast osclllatlon amplitude vs. T for all three TMTSF compounds
Jap. J. Appl. Phys. 2& 575 (1987). X. Yan, et al., Phys. Rev. Bx, 1799(1987).
P.M. Grant, J. Phys. France44, C3-847 (1983). H. Shweti, et al., Phys. Rev. Lett. & 667 (1986). J.P. Ulmet, et al., J. Phys. (Paris) Lett, 46, L-5354 (1985). [6] A. Audouard, J.P.Ulmet,J.M.Fabre,Synth. Met. a 751 (1995). [7] J.P.Ulmet, et al. (preprint), and theseproceedings. [8] W. Kang, et al., P.M. Chaikin, Phys. Rev. Lett, a,2812 (1990). [9] A. Audouxd, et al., Europhys. Lett. 25, 363 (1994). [lo] P.M. Chaikin, et al., Phys. Rev. Lett. 2, 2333 (1983). [I I] J.P. Ulmet, et al. , Sol. State Commun.a 753 (1986). [12] X. Yan, et al., Synth. Met. 27, B145 (1988). [13] L.P. Gor’kov and A.G. Lebed’, Phys. Rev. Bu, 1362(1995). [14] K. Machida, et al., Phys. Rev, Bfi, 8946 (1995); B53, 5461 (1996).
$nthatic Metals 85 (1997) 1531-1532
Fast Oscillations in (TMTSF)zX M. J. Naughton, a,bZc** J.P. Ulmetb, I.J. Lee*, J.M. Fabred “Departments ofphysics and Chemistry, State University of New York, Buffalo, NY 14260 USA bService National des Champs Magnetiques Pulses, 31077 Toulouse, France ‘National High Magnetic Field Laboratory Tallahassee, Fl32310 USA dLaboratoira de Chimie Organique Structurale, 34095 Montpellier, France Abstract We report on the temperature dependence of the “fast oscillation” phenomenon in quasi-one dimensional molecular conductors. Magnetoresistance and magnetization measurements to 30T in the ambient pressure spin density wave state have been taken for the X = N03, PF6 and AsF6 members of the (TMTSF)2X family. Complementary data on X=AsF6 are presented by Ulmet et al. in these proceedings. All three compounds exhibit a peak in the transverse magnetoresistances pm (B//c *, I//a) and psr (B//c*, Iilb) and their oscillatory components around 3K, with the oscillations vanishing at higher and lower temperatures. No magnetic moment oscillations are detected in PF6 or AsF6, while only extremely weak oscillations (6m-10-10 emu) are seen in the NO3 salt above 25T. Also, no osciilations were observed in longitudinal magnetoresistance pz (BIiI//c*) measurements performed on AsF6 and PF6. Possible scenarios which have been used to explain the data include imperfect nesting at Tsow leaving behind quantizable closed orbits, and quantum interference effects arising from magnetic breakdown orbits. Another may involve a new phase transition inside the SDW state. Keywords: organrcsuperconductors, magnetotransport A major unsolved mystery in the Bechgaard salts (TMTSF)2X is the origin of the “fast oscillations,” SdH-like resistance oscillations permeating all conducting phases of these materials. They are periodic with inverse field, but have a unique temperature dependence which has the oscillations vanishing at low temperature. However, corresponding oscillations in the magnetization (the dHvA effect) have only been detected in one salt, X=C104, and then only in the field-induced SDW state.[l,2] According to baud structure determinations, the normal state Fermi surface contains only one pair of open sheets, such that no closed orbits should be present in the high temperature metallic phase.[3] Nonetheless, clear SdH-like oscillations have been observed in this metallic phase in the Re04 [4] and C104 [2] salts at fields below the threshold for FISDW formation. In addition, similar resistance oscillations have been detected in the ambient pressure SDW states in the PF6 [5] AsF6 [6,7] and NO3 [8,9] salts, and, of course, in the FISDW state in C104 [lO,ll] and Re0,,[4] all with T and B-independent frequencies A(l/B) near 250T. Recent theoretical effort [12,13,14] has focused on the possibility of magnetic breakdown across an anion ordering gap (in those salts which have anion ordering) and quantum interference effects. It is therefore useful to have thermodynamic information on the oscillations, as interference orbits should occur exclusively in transport (i.e. no magnetic moment). In order to test this hypothesis, originally proposed by Yan et al.[12] to explain SdH-like effects in Clod, we set out to determine to what extent these oscillations appear in magnetization in the nonFISDW spin density wave state. We present here results from this simultaneous magnetization and resistance study of the X = N03, PF6 and AsF6 members of the TMTSF system. *This researchwas supported by the National ScienceFoundation, under grant numbers DMR-9258579 and DMR-93 11739. 0379-67791971S17.00 0 1997 Elsevier ScienceS.A All rights reserved PII 90379-6779(96)4467-0
For PF6 and AsF6, we find no evidence of oscillatory behavior in the magnetic moment up to 30T, in the range 0.5K to IOK (not shown). On the other hand, we confirm the result of Ulmet, et al.[7] on AsF6 with regard to the T-dependence of the magnetoresistance and the fast oscillations, for both materials. We show the magnetoresistance R, of PF6 below in Fig. 1. A relationship can be seen between the magnitudes of the magnetoresistance and its oscillatory component, with both decreasing at low T. This, of course, is quite unexpected and difficult to explain using the Lifshitz-Kosevich model used to describe quantized orbits, which predicts saturation of the oscillation amplitude as T-+0. This is shown in more detail for PF6 and AsF6 in Fig. 2, where we plot the oscillatory resistance only.
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Magnetic Field (T) Fie. Magnetoresistanceof PFs in the ambient pressure SDW phase (Tsow=12K). Below 3K, both Ap/po and the fast oscillations amplitudes are suppressed. Inset: R,(T) at zero field, reaching 16M at 1.3K.
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ALJ. Naughtun etal. /SyntheticMetals
AsF6, PF6 and N03, for data in the range 25-30T. All 3 show a characteristic peak between 2 and 4K. Moreover, the AsF6 FO amplitude, defined either by -(total)--(ambient) or via FFT’s, tracks remarkably with the ambient in Fig. 3(a). We note also that oscillations in AsF6 occur with R(ambient) nearly 1MSI. Finally, we show magnetization data on N03, where a very small oscillatory component is observed. The magnetic torque was measured at several temperatures. Atop a generally smooth variation, we find weak oscillations. We show this below, along with the oscillatory magnetoresistance, measured simultaneously. There appear oscillations in the magnetization above 25T, with a maximum amplitude of 5x10”’ emu. Due to this minute signal, we will not claim with certainty that the features in Fig. 4 arise from the dHvA effect. Additional experiments are underway at higher fields (>35T), which may help clarify the situation.
m t! 4 P s 2 * s 0
85 (1997) 1531-1532
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_ &I. z 2.OK- 4 -4 2.2K- ; _ 02
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B CT) w
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SdH-like oscliiations in (TMTSF)lPFa and (TMTSF)?AsFh.
The amplitude of the oscillatory magnetoresistance grows with decreasing temperature until -3K, below which it falls rapidly to zero. We see that PF6 and AsFb have characteristically identical behavior. No oscillations were detected between 0.4K and 1.5K; nor were anv observed in R, measurements, at any T. The temperature dependence of the ambient magnetoresistance, defined by a low-order polynomial fit to the raw data to disregard the oscillations, also shows a rapid falloff at low temperature, as shown below in Fig. 3(a) for AsF6. Fig. 3(b) shows summary data on the amplitudes of the fast oscillations, for
t
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B CT) u Oscillatory components of the magnetoresistance(-) and the magnetization (0) in (TMTSF)zNOj. There is a correlation between the two quantities, which we tentatively ascribe to the WvA effect. Both amplitudes decreaseabove and below 2K. In summary, we have shown that the TMTSF fast oscillations in transverse magnetoresistance pm vanish at low temperature in the X=PF6, AsF6 and NOI salts. This coincides with a reduction in the ambient magnetoresistance. We find no evidence for oscillations in pZ or magnetization in AsF6 and PF6, and weak evidence for the latter in NO,, likely due to closed orbits at T< TAO. The vanishing of the FO at low T may be due to the opening of the SDW gap, reducing the probability for quantum interference orbits,[l4] or perhaps to a coexisting CDW-SDW state. References [l] JS. Brooks, et al., J. Mag. Mag. Mat, 54-57.637 (1986); P.M. Chaikin, et al., Physica 1438, 383 (1986); R.V. Chamberlin, et al., [2] [3] [4] [5]
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8 4 6 Temperature (K)
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(a) ambient magnetoresistancein AsF6,showing peak near 2.5K,
(b) fast osclllatlon amplitude vs. T for all three TMTSF compounds
Jap. J. Appl. Phys. 2& 575 (1987). X. Yan, et al., Phys. Rev. Bx, 1799(1987).
P.M. Grant, J. Phys. France44, C3-847 (1983). H. Shweti, et al., Phys. Rev. Lett. & 667 (1986). J.P. Ulmet, et al., J. Phys. (Paris) Lett, 46, L-5354 (1985). [6] A. Audouard, J.P.Ulmet,J.M.Fabre,Synth. Met. a 751 (1995). [7] J.P.Ulmet, et al. (preprint), and theseproceedings. [8] W. Kang, et al., P.M. Chaikin, Phys. Rev. Lett, a,2812 (1990). [9] A. Audouxd, et al., Europhys. Lett. 25, 363 (1994). [lo] P.M. Chaikin, et al., Phys. Rev. Lett. 2, 2333 (1983). [I I] J.P. Ulmet, et al. , Sol. State Commun.a 753 (1986). [12] X. Yan, et al., Synth. Met. 27, B145 (1988). [13] L.P. Gor’kov and A.G. Lebed’, Phys. Rev. Bu, 1362(1995). [14] K. Machida, et al., Phys. Rev, Bfi, 8946 (1995); B53, 5461 (1996).