Abstract
We consider structural instabilities exhibited by the one-dimensional (1D) (arene)2X family of organic conductors in relation with their electronic and magnetic properties. With a charge transfer of one electron to each anion X, these salts exhibit a quarter-filled (hole) conduction band located on donor stacks. Compounds built with donors such as fluorenthene, perylene derivatives and anions X such as PF6 or AsF6 exhibit a high temperature (TP ~ 170 K) conventional Peierls transition that is preceded by a sizeable regime of 1D 2kF charge density wave fluctuations (kF is the Fermi wave vector of the 1D electron gas located on Per stack). Surprisingly, and probably because of the presence of a multi-sheet warped Fermi surface, the critical temperature of the Peierls transition is considerably reduced in the perylene series α-(Per)2[M(mnt)2] where X is the dithiolate molecule with M = Au, Cu, Co and Fe. Special attention will be devoted to physical properties of α-(Per)2[M(mnt)2] salts with M = Pt, Pd and Ni which incorporate segregated S = 1/2 1D antiferromagnetic (AF) dithiolate stacks coexisting with 1D metallic Per stacks. We analyze conjointly the structural and magnetic properties of these salts in relation with the 1D spin-Peierls (SP) instability located on the dithiolate stacks. We show that the SP instability of Pd and Ni derivatives occurs in the classical (adiabatic) limit while the SP instability of the Pt derivative occurs in the quantum (anti-adiabatic) limit. Furthermore, we show that in Pd and Ni derivatives 1st neighbor direct and frustrated 2nd neighbor indirect (through a fine tuning with the mediated 2kF RKKY coupling interaction on Per stacks) AF interactions add their contribution to the SP instability to stabilize a singlet-triplet gap. Our analysis of the data show unambiguously that magnetic α-(Per)2[M(mnt)2] salts exhibit the physics expected for a two chain Kondo lattice.
Highlights
Since the discovery of the so-called Peierls transition in the Krogmann salt K2Pt(CN)4Br0.3-3H2O (KCP) [1] in 1973, nearly than 45 years ago, many investigations have shown that most one-dimensional (1D) conductors are subject to a coupled electronic-structural instability transition at the 2kF critical wave vector
Due to the electron–phonon coupling the Peierls transition consists in a 2kF modulated wave of bond distances, forming a so-called bond ordered wave (BOW), accompanied by a 2kF modulation of the electronic density, forming a so-called charge density wave (CDW); these two waves being in quadrature
Since 4 × 2kDF = b*, the 2kF BOW/CDW modulation wave length is in fourth-fold commensurate relation with the chain periodicity b, so that the phase of the CDW modulation should be pinned in the structure by the four-fold lattice potential
Summary
Since the discovery of the so-called Peierls transition in the Krogmann salt K2Pt(CN)4Br0.3-3H2O (KCP) [1] in 1973, nearly than 45 years ago, many investigations have shown that most one-dimensional (1D) conductors are subject to a coupled electronic-structural instability transition at the 2kF critical wave vector (kF being the Fermi wave vector of the 1D electron gas). Between TPMF and TP, local fluctuations in direct space open a partial gap (i.e., a pseudo-gap) in the electronic structure More explanations concerning these distinctive features can be found in a recent review [3]. Because of the relative importance of electron repulsions U and V with respect to t//, organic conductors develop another type of CDW instability at the critical 4kF wave vector consisting in a Wigner (or Mott-Hubbard) type of charge localization [2]. In 2:1 organic salts, intensively studied in recent years [5], a spin-charge decoupling accompanies the charge localization phenomenon When such a decoupling is achieved, the localized S = 1/2 degrees of freedom remain available to order in anti-ferromagnetic (AF) or non-magnetic singlet paired ground states. AB band where the hole wave function is basically localized in the anti-bonding state of the dimer
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