Competition between Pauli and orbital effects in a charge-density wave system

We present angular dependent magneto-transport and magnetization measurements on α-(ET)2MHg(SCN)4 compounds at high magnetic fields and low temperatures. We find that the low temperature ground state undergoes two subsequent field-induced density-wave type phase transitions above a critical angle of the magnetic field with respect to the crystallographic axes. This new phase diagram may be qualitatively described assuming a charge density wave ground state which undergoes field-induced transitions due to the interplay of Pauli and orbital effects. Typeset using REVTEX 1 Low dimensional electronic systems characterized by a quasi-one-dimensional (Q1D) Fermi surface tend to form either a charge-density wave (CDW) or a spin-density wave (SDW) ground state at low temperatures as a consequence of one-dimensional instabilities [1,2]. High magnetic fields have proved to be useful to investigate, and even manipulate these ground states, since the effects are quite different for the CDW and the SDW cases. The Zeeman (Pauli) energy is expected to suppress a CDW state because a CDW couples only bands with the same spin. In a magnetic field it is not possible to have the same nesting wave vector Q for both spin-up and spin-down bands (see [3]). In analogy with the Pauli effect in superconductors [4], the Zeeman energy, μ B ρ(EF)B , (where ρ(EF) is the density of states at the Fermi level) competes with the CDW condensation energy, −ρ(EF)∆(0). The transition temperature is expected to decrease with increasing field, and above a certain threshold field (≃ ∆(0)/μB) a uniform CDW is no longer energetically favorable. Consequently, a CDW may be suppressed by high magnetic fields. In contrast, for a SDW system, the nesting property is not affected by the Zeeman term because a SDW couples spin-up with spin-down states. The nesting condition is actually improved by high magnetic fields due to the magnetic field induced one-dimensionalization of the Q1D electronic orbits. Thus for an imperfectly nested Fermi surface, the SDW transition temperature can actually increase with increasing magnetic field [5,6]. The role of orbital effects on SDW systems has been well established in the Q1D organic Bechgaard salts [7]. By using a simple BCS relation, we can obtain a rough estimate for the critical field necessary to suppress a uniform CDW: Bc = 1.765kB/μBTc, where kB is the Boltzmann constant, μB is the Bohr magneton, and Tc is the transition temperature to the DW state. However, the relatively high transition temperatures (≥ 30 K) of most CDW systems, like for example, the molybdenum bronzes [2] implies the need for very high magnetic fields, of the order of 100 tesla or more, in order to suppress the CDW ground state via the Zeeman energy. This limitation has prevented the observation of this field-induced suppression. In this work, we argue that the α-(ET)2MHg(SCN)4 (where M = K, Tl and Rb) organic conductors may be the first compounds whose ground state is driven towards new DW states