Quantum Hall Effect and Rapid Oscillations in (Tmtsf) 2 Pf 6 under Pressure
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QUANTUM HALL EFFECT AND RAPID OSCILLATIONS IN (TMTSF) 2 PF 6UNDER PRESSURE S. T. HANNAHSa, J. S. BROOKS', W. KANG', P. M. CHAIKINb,c, L. Y. CHIANGc, AND R. UPASANIb,c 'Department of Physics, Boston University, Boston MA 02215 bDepartment of Physics, Princeton University, Princeton NJ 08544 cExxon Research And Engineering Co. Rt. 22E, Annandale, NJ 08801 ABSTRACT We present magnetotransport data and the phase diagram derived from them for (TMTSF) 2 PF 6 under sufficient pressure that the zero field Spin Density Wave (SDW) is suppressed and the material is superconducting. Application of a large magnetic field perpendicular to the conducting plane then leads to the cascade of Field Induced Spin Density Wave (FISDW) transitions. The transitions are in good agreement with the Standard model for these transitions and in contrast to the more complicated behavior seen in the ClO 4 salt. In addition Hall and longitudinal resistivity indicates a behavior much closer to that observed in conventional Quantum Hall devices than in the C10 4 salt or previous studies of PF 6 . We do observe the "rapid" Schubnikov de Haas like oscillations in magnetoresistance at high field similar to those seen in C10 4, even though in the present case there is no evidence for anion ordering as some theories would require. The integer and fractional quantum Hall effects (QHE) have been previously reported only in quantum wells formed in semiconductor heterostructures [1] and in a few weakly coupled layers. [2] The only bulk materials which have shown QHE-like behavior are the Bechgaard salts (TMTSF) 2 X [31 where X is the anion C10 4 , ReO 4 , or PF 6 . Theoretical work has shown that there should be a QHE associated with the field induced spin density wave (FISDW) states [4] and experimental observation of the QHE [5-6] in these salts has been suggested. However, the interpretation has been complicated by anomalies and additional transitions which question the basic theoretical understanding of these materials. We have made measurements of the transport properties of PF 6 under pressure. These are in almost complete agreement with expectations for a system undergoing a series of FISDW transitions to states with filled Landau bands exhibiting QHE. [7] Previous studies on (TMTSF) 2 C10 4have found instead of the regular progression of Hall steps n, n - 1, .... 1,0 there were intervening phases where the Hall resistance (pry) changed sign, [8] there was not a transition from n = 1 to 0 but rather apparently [61 from n = 1 to n = 1/3, there was no evidence for the n = 0 state anywhere in the phase diagram, and there was a reentrance of the metallic state at high field. [9] The material (TMTSF) 2 PF 6 differs from the other (TMTSF) 2salts, C104 and ReO 4 The anions are centrosymmetric and thus there can be no anion ordering transition and below 12 K at ambient pressure there is the formation of a SDW phase. This SDW phase can be suppressed by application of hydrostatic pressure exceeding a critical pressure P,. This results in a metallic (and below 1
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