Resonant Nernst Effect in the Metallic and Field-Induced Spin Density Wave States of(TMTSF)2ClO4

Abstract

We examine an unusual phenomenon where, in tilted magnetic fields near magic angles parallel to crystallographic planes, a "giant" resonant Nernst signal has been observed by Wu et al.[Phys. Rev. Lett. 91 56601(2003)] in the metallic state of an organic conducting Bechgaard salt. We show that this effect appears to be a general feature of these materials, and is also present in the field induced spin density wave phase with even larger amplitude. Our results place new restrictions on models that treat the metallic state as an unconventional density wave or as a state with finite Cooper pairing.PACS Numbers (72.15.Jf, 72.15.Gd, 75.30.Fv) 2 The Bechgaard salts, (TMTSF) 2 X (X=PF 6 , ClO 4 , AsF 6 , ReO 4 ,…) continue to attract attention due to the proximity of novel ground states including antiferromagnetic spin density waves (SDW), unconventional superconducting (SC) and metallic states, and magnetic field induced spin density wave (FISDW) states [1,2]. Recently, arguments have been made for the coexistence of the SDW, SC, and metallic states along phase boundaries [3][4][5], for the possibility that the metallic state is an unconventional SDW phase [6], and even for the survival of p-wave Cooper pairing in the metallic state [7].Most studied are (TMTSF) 2 PF 6 (under pressure to remove the ambient pressure SDW phase) and (TMTSF) 2 ClO 4 . These materials are quasi-one-dimensional (Q1D) organic conductors with transfer integrals of order 250, 25, and 0.25 meV for the a, b, and c axis directions respectively. In the SC state, evidence for p-wave pairing exists since the critical field H c2 exceeds the Pauli limit when the field is aligned parallel to the molecular planes [8,9] [12,13]. Here the field is aligned along crystal planes composed of the a-axis and integer combinations of the b and c unit cell parameters. Above a threshold field H th , the FISDW state (with a cascade of subphases) is stabilized due to the quantized nesting of the Q1D Fermi surface [1,15]. Unlike the tan(θ) dependence, the threshold and subphases in the FISDW phase are driven by the field component perpendicular to the conducting ab planes, i.e. the "cosine law" H ⊥ =H cos(θ).In this Letter we examine the longitudinal (Seebeck or TEP) and transverse (Nernst) thermoelectric effects in the metallic and FISDW phases in (TMTSF) 2 ClO 4 (hereafter ClO 4 ). The Nernst effect, an analogue of the Hall effect, is the transverse voltage induced by a temperature gradient in a magnetic field. It has been studied in cuprates due to its sensitivity to the vortex state, where large Nernst signal can be induced by phase slip 3 [16,17]. On the other hand, quasi particles in normal Fermi liquids with one type of charge carriers are known to generate very small Nernst effect because of so called the "Sondheimer cancellation" [18,19]. When more than two types of carriers are involved, the cancellation is not perfect and the Nernst effect can be finite with an order of (e/k B )(T/T F )(ω c τ) where k B T F is the Fermi energy, ω c is the cyclotron...

Observation of Orbital Resonance Hall Effect in(TMTSF)2ClO4

“…The signal is consistent with 2D conducting planes originating from an orbital resonance of motion along the field direction at the MA distinct from the molecular ab plane. This measurement not only sheds new light on previously unresolved Lebed resonance behavior [13,14] but also establishes the emergence of an orbital resonance-driven Hall effect and an emergent low-dimensional system ripe for further exploration.Most commonly, the MA effect is observed as a dip structure in the interlayer resistance R zz as a function of angle θ at the MA condition defined aswhere b and c are lattice constants in the y and z direction, p and q are integers, and θ is measured from the z axis. A microscopic and macroscopic depiction of the crystal are shown inset in Fig.…”

“…The signal is consistent with 2D conducting planes originating from an orbital resonance of motion along the field direction at the MA distinct from the molecular ab plane. This measurement not only sheds new light on previously unresolved Lebed resonance behavior [13,14] but also establishes the emergence of an orbital resonance-driven Hall effect and an emergent low-dimensional system ripe for further exploration.…”

Numerical investigation of Nernst effect in quasi-one-dimensional systems

“…The giant Nernst resonances and giant Nernst effect have also been observed in the sister compound ͑TMTSF͒ 2 ClO 4 . 23,24 Both the sign change at the magic angles and the large magnitude of the signal are not yet explained, but the effect appears generic for these materials. Any theoretical description of the magic angle effect should explain both resistivity dips and the resonantlike Nernst effect.…”

“…[21][22][23] It has been shown that the Boltzmann transport fails to explain the resonantlike Nernst effect at magic angles and their giant magnitude, when there is no magnetic field component along the a axis ͓B x = 0 curve in Fig. 2͑a͔͒.…”

Non-Fermi-liquid magic angle effects in high magnetic fields

“…On the other hand, in many cases, the LMA phenomena are experimentally claimed [9-13] to be of a nonFermi-liquid nature. This was claimed by Chaikin's group for the Nernst effect in (TMTSF) 2 PF 6 [9,10,12], by Brooks' group for the Nernst effect in (TMTSF) 2 ClO 4 [11], and recently by Uji's group [13] for the Hall effect in (TMTSF) 2 ClO 4 . In our opinion, some non-Fermi-liquid effects were also observed in the LMA resistive experiments in the Q1D compound (Per) 2 Au(mnt) 2 by Graf et al [14].…”