Magnetic field dependent behavior of the CDW ground state in Per2M(mnt)2 (M = Au, Pt)
Introduction
The α phases of Per2M(mnt)2 where Per = perylene, mnt = maleonitriledithiolate and M = Au, Pt, (or other transition metals) have been the subject of many studies [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12] due to the nature of conducting and magnetic instabilities that may arise in the two chains (perylene or M(mnt)2). Here a charge density wave (CDW) instability arises in the perylene chains; when M has a localized magnetic moment (e.g. Pt), a spin-Peierls transition may occur. Previous work to 18 T has shown that the CDW transition temperature was decreasing according to mean field predictions [13]. More recently, Graf and co-workers [14], [15] have shown that the low temperature, high resistance CDW phases in Per2M(mnt)2 (for M = Au, Pt) are suppressed in high magnetic fields of order of the Pauli field BP. A sketch of the magnetic field dependent phase diagrams based in Refs. [14], [15] and the present work is given in Fig. 1.
For fields above BP, the resistance rises again for both the Pt and Au compounds (except for B ∥ b for M = Au). We have described this second, high resistance phase as a field induced charge density wave (FICDW) phase, based on comparisons with current theoretical work in this area [16], [17]. At even higher fields, the FICDW is re-entrant to a second low resistance state for M = Pt. An intervening low resistance state in the vicinity of BP appears between the CDW and FICDW phases, which for some samples and field orientations appears to be activated. However, in the case of B ∥ c for M = Pt, metallic behavior in the temperature dependent resistance has been was observed below 1.8 K in the vicinity of BP.
In reference to Fig. 1, we emphasize that the phase boundaries depicted are based on both temperature dependent resistance measurements at constant field, and magnetoresistance (MR) measurements at constant temperatures. Hence implicit in the MR data is the notion that as the field destroys or stabilizes a field dependent gap Δ(B), the MR at constant T will fall or rise with the gap, and from BCS, we assume that Tc(B) ∼ Δ(B). Hence the MR signal vs. B on a log scale will resemble Tc(B) vs. B to a rough approximation. Further extensive R vs. T for increments in B will be needed to unambiguously determine the shape of Tc(B).
The purpose of this report is to summarize our results to date on these systems, to present some more recent, albeit, preliminary data, and to put our findings in the context of current theoretical work.
Section snippets
Theoretical background
Fujita et al. have shown that in the absence of interchain bandwidth, where no orbital effects enter, a commensurate CDW can undergo a transition to an incommensurate ICDW in the presence of a high magnetic field [18]. Here a soliton-like structure can arise in the lattice and charge density, and the ICDW gap will generally decrease with increasing magnetic field. An increase in the magnetization at the CDW-ICDW transition is also predicted [18]. Zanchi, Bjelis, and Montambaux [16] treated a
Experimental results
The results presented here are made with conventional 4-terminal transport, pressure, and thermopower methods. We note that great care is needed in cooling down the samples, and that 18 h or more from room temperature to 4.2 K are necessary to insure that the sample will be truly metallic before entering the CDW ground state. Measurements were carried out in 18 T, 33 T, and 45 T superconducting, resistive, and hybrid magnets respectively at the NHMFL and in 60 T at the LANL pulsed field facility.
Discussion
Although Per2M(mnt)2 is thought to be a nearly perfect one-dimensional conductor along the perylene chains, our angular dependent magnetotransport data indicate the possible influence of orbital effects. To more carefully examine this issue, the electronic structure of the Per2M(mnt)2 system has recently been re-examined by Canadell et al. [25]. Computations with single-ζ integrals showed a large ratio between the intrachain (∼150 meV for the b-axis) to interchain (∼2 meV for the a-axis; ∼0 meV for
Acknowledgments
Supported by NSF-DMR 02-03532 and a NSF GK-12 Fellowship (DG). NHMFL is supported by the NSF and the State of Florida. Work in Portugal was supported by FCT under contract POCT/FAT/39115/2001.
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