Doping effect on spin-Peierls instability.

Abstract

We study the effects of doping on spin-Peierls (SP) systems using the unimodular mean-field theory. The impurity spins affect the singlet valence bond field and renormalize the magnetic excitations. The SP transition temperature and the energy gap of magnetic excitations are reduced by factors ~ n« and ~ nf, respectively, with rii as the impurity density. At certain value of rij, a gapless SP phase occurs, and the interaction between impurities becomes RKKY-like. The recently observed reduction of SP transition temperature upon doping and occurrence of a spin glass phase is interpreted using the proposed theory. MIRAMARE TRIESTE January 1994 'Present address: International School for Advanced Studies (SISSA), Via Beirut No.24, 34013 Trieste, Italy. The theoretically predicted spin-Peierls (SP) state with alternating bond length has been observed, so far only in a few quasi-one-dimensional (ID) organic compounds with antiferromagnetic (AF) interactions [1]. Very recently, it was found in an inorganic compound CuGeO:, [2]. Moreover, a drastic reduction of SP transition temperature Tr was observed upon Zn-doping and a spin glass (SG) phue appeared in the doping range 0.02 < n, < 0.08 [3]. In this Letter we propose a theory of SP transition in doped systems explaining these findings and making further predictions on a gapleat SP state to be checked by experiments. The SP transition is driven by the interaction between the (1-D) spin-1/2 chains and the three-dimensional (3-D) lattice,which makes a mean-field (MF) approach available due to its suppression of fluctuations. Below the SP transition an uniform AF chain is deformed into an alternating AF chain with a singlet ground state and a magnetic gap [4]. Up to now there are two successful theories of SP transition, i.e. that of Pytte [S] and of Cross and Fisher [6]. In these theories, a fermion representation via the Jordan-Wigner transformation (JWT) is used to describe the spin-1/2 chain, and the fermion-phonon interactions are taken into account in the random phase approximation. However, it is difficult to investigate effects of impurity doping upon the SP system) within this approach due to nonlocal features of JWT. P.W. Anderson [7] has proposed the Resonant-Valence-Bond model to describe twodimensional spin-1/2 AF systems. Later, Arovas and Girvin [8] put forward an unimodulr as the impurity density, whereas the decrease of the energy gap is proportional to nf and it collapses at some value of n,, but the system is still in the SP phase, i.e. the spin-lattice dimeriz&tion remains. Thus we predict the existence of a gnpiess SP state to be checked by direct experiments. This is rather similar to superconductors doped with paramagnetic impurities which reduce the energy gap and eventually give rise to gapless superconductivity [13]. The Hamiltonian describing an impurity-doped SP system is [12,14] = #„ #1 = Zi H = E An, = ff(a + 1)5, • S,+, + AnI • Si, 5TM J(a,l)Sn, Ei 2A-u, (1)