Thermal Activation between Landau Levels in the Organic Superconductorβ′′−(BEDT−TTF)2

Abstract

We show that Shubnikov-de Haas oscillations in the interlayer resistivity of the organic superconductor β ′′ -(BEDT-TTF)2SF5 CH2CF2SO3 become very pronounced in magnetic fields ∼ 60 T. The conductivity minima exhibit thermally-activated behaviour that can be explained simply by the presence of a Landau gap, with the quasi-one-dimensional Fermi surface sheets contributing negligibly to the conductivity. This observation, together with complete suppression of chemical potential oscillations, is consistent with an incommensurate nesting instability of the quasi-one-dimensional sheets.PACS numbers: 74.70. Kn, 78.20.Ls, 71.20.Rv The quantizing effect of a magnetic field on a chargecarrier system is well known [1]. In metals, this leads to oscillations of the free energy and quasiparticle density of states as Landau levels cross the Fermi energy [2]. The effect is very pronounced in quasi-two-dimensional (Q2D) metals containing Fermi surfaces (FSs) that are approximately cylindrical [3]. Recently there has also been interest in analogous effects in quasi-one-dimensional (Q1D) FS sections, which can lead to magnetic-field-induced quantization [4] and localization [5].In this paper we describe the magnetoresistance of the organic superconductor. Bandstructure calculations suggest that this material possesses a FS comprising a Q2D cylinder and a pair of Q1D sheets [6]. However, Shubnikov-de Haas (SdH) and de Haas-van Alphen (dHvA) measurements reveal that the Q2D cylinder has only one third the expected cross-section [7,8,9]. Angle-dependent magnetoresistance oscillation (AMRO) [8,10] and millimetre-wave magnetoconductivity experiments [11] show that the cross-section of this Q2D pocket resembles an elongated diamond. The same experimental techniques find no evidence for the presence of Q1D Fermi sheets at low temperatures [8,10,11], unlike the situation in other Q2D organic metals [3]. By contrast, in order to explain the observation of a fixed chemical potential µ in the dHvA effect [7], Wosnitza et al. proposed Q1D states which have an enormous density of states, exceeding the estimates from bandstructure calculations by at least an order of magnitude [7]. Moreover, using a simple formula for the background magnetoresistance, Wosnitza et al. suggested that the Q1D sheets become localised in a magnetic field [12]. In the present paper, we show that magnetoresistance data suggest a much simpler explanation. The thermally activated behaviour of the data at integer Landau level filling factors is explained entirely in terms of a Landau gap. Moreover, the failure of the Q1D sheets to contribute to the conductivity together with their ability to fix µ is explained by their nesting to form an incommensurate density-wave ground state. This mechanism is supported by the temperature dependence of the resistivity at B = 0.Single crystals (∼ 1.0 × 0.5 × 0.2 mm 3 ) of β ′′ -(BEDT-TTF) 2 SF 5 CH 2 CF 2 SO 3 were prepared using standard electrochemical techniques [6]. Contacts were applied using 12.5 µm Pt wires and graphite pain...

Quasi-two-dimensional organic superconductors: A review

“…[66]), although intensive searches continue for for departures from this comforting behaviour. However, much work remains to be done on the question of the interactions involved in the superconductivity.…”

“…In α-(BEDT-TTF) 2 NH 4 Hg(SCN) 4 , which is believed to have a simple Fermi surface consisting of a pair of sheets and a closed pocket, a cyclotron resonance corresponding to a mass of 1.9m e was measured [64,65]; this may be compared with an effective mass from magnetic quantum oscillations of 2.5m e [67]. β ′′ -(BEDT-TTF) 2 SF 5 CH 2 CF 2 SO 3 , again with a Fermi surface believed to consist of a single pocket and a pair of sheets (but see Reference [66]), exhibited a cyclotron resonance corresponding to a mass of 2.2m e [31] (see Figure 9) the effective mass from Shubnikovde Haas oscillations is 1.9m e [68].…”

Robust superconducting state in the low-quasiparticle-density organic metalsβ″−(BEDT−TTF)4[(Bangura1, Coldea2, Singleton3 et al. 2005Phys. Rev. BSelf Cite

“…Analogous Fermisurface reconstructions have been suggested for other ␤Љ salts, including ␤Љ-͑BEDT-TTF͒ 2 AuBr 2 ͑where a plethora of small Fermi surface pockets results͒, 38 the ␤Љ-͑BEDT-TTF͒ 4 ͓͑H 3 O͒M͑C 2 O 4 ͒ 3 ͔ ·C 5 H 5 N salts, 14 and ␤Љ-͑BEDT-TTF͒ 2 SF 5 CH 2 CF 2 SO 3 ; in the latter case it appears that the Fermi-surface nesting is more efficient. 39 As in the case of the ␤Љ-͑BEDT-TTF͒ 4 ͓͑H 3 O͒M͑C 2 O 4 ͒ 3 ͔ ·C 5 H 5 N salts, 14 the appearance of the Shubnikov-de Haas oscillations is affected by M, with the lower-frequency series being much more visible for M =Fe ͓see Figs. 3͑b͒ and 3͑c͔͒.…”

“…͑5͒ will give an apparent orbital anisotropy that is orders of magnitude too small. 39,43 Moreover, the dissipative mechanisms giving rise to the peak in R zz are unlikely to be the same for in-plane and out-of-plane fields, 39 as evidenced by the distinct temperature dependences of the resistive critical fields ͑Fig. 9͒ and the rather different appearance of the peak at 90% ͑see …”

The Landau band effects in the quantum magnetic oscillations and the deviations from the quasiclassical Lifshitz–Kosevich theory in quasi-two-dimensional conductors

“…They are caused by the chemical potential oscillations [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22], magnetic breakdown [4][5][6][7][8][23][24][25][26][27][28][29], dispersive and incoherent transport of electrons across the layers [4][5][6][7][8][9][10][30][31][32][33][34][35][36], spin-(SDW) and charge-density-waves (CDW) [4][5][6][7]. The first observation of the QMO in layered superconductors in 1976 [37] was a surprise since magnetic field penetrates into the bulk of a superconductor in a form of the Ab...…”