Electronic Properties of α-NaV2O2

Abstract

1 General properties of α ′ −NaV 2 O 5 and motivation for a spectroscopic study Introduction -α ′ −NaV 2 O 5 is one of the several phases in the class of Na x V 2 O 5 systems [1] and until now it is by far the most studied of them. Since 1996 this compound (denoted in the following simply by NaV 2 O 5 ) has received considerable attention because it was thought to be the second realization, after CuGeO 3 , of a quasi-one dimensional (1D) inorganic material displaying a spin-Peierls (SP) transition. The interest was justified given the scarcity of inorganic materials having this property, which is quite interesting especially for the scientific community working in the field of low dimensional quantum spin systems. However, it turned out that the physics of NaV 2 O 5 is more complicated and intriguing than that and the degrees of freedom involved are not only the ones describing the spins and the lattice.What is a SP transition? We discussed in Chapter [2] the general properties of a Peierls distortion which is a transition to charge density wave state. This means that below some temperature T P the crystal gets distorted and the electronic density acquires a periodic spatial modulation, a process during which the loss in elastic energy is compensated by the gain in the kinetic energy of the electrons. A crucial role is played by the nesting properties of the Fermi surface (i.e. the property that enables one to connect points of the Fermi surface by wavevector characteristic of other excitations, in this case phonons) which makes low dimensional systems especially susceptible to such an instability. A pure SP transition is one in which the lattice distortion is caused by the magneto-elastic coupling, the gain in energy in this case being related to the spin degrees of freedom [3]. In other words, it is a lattice instability driven by the magnetic interactions. This phenomenon leads to the formation of a spin-singlet (S = 0) ground state and the opening of a spin-gap in the magnetic excitation spectrum, i.e. a finite energy is required to excite the system from its ground state to lowest triplet (S = 1) state. A signature of a SP state is thus an isotropic activated temperature dependence in the uniform magnetic susceptibility below the transition at T SP . In addition, as opposed to an usual Peierls transition, the direct participation of the spins leads to specific predictions for the dependence of T SP on external magnetic field H [4]. This has to do with the fact that in the spin case the filling factor of the electronic band and accordingly the magnitude of the nesting wavevectors can be varied continuously by a magnetic field. This statement should not be taken ad litteram, but in the sense that the magnetic problem can be mapped onto a fermion like system by using a transformation of the spin operators, the magnetic field playing the role of the chemical potential, see Refs. [3,4] for more details.So what is the difference between CuGeO 3 and NaV 2 O 5 ? In the former, the SP nature of the transition observe...

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