No abstract

Zero-field susceptibility and specific heat of Pb[Cu(SO 4 )(OH) 2 ] (linarite) single crystal were measured. In order to verify that linarite is a quasi-one-dimensional system with competing nearest-neighbour and nextnearest-neighbour in-chain exchange interaction (J 1 ≈ -30 K, J 2 ≈ 15 K), theoretical results based on electronic structure calculations within the LDA and a phenomenological modelling using the finitetemperature transfer-matrix method are taken into account. The possibility of various ground states is discussed in terms of the screened onsite repulsion U s : commensurate Neel or spin-Peierls phases versus incommensurate spiral states with acute or obtuse pitch angles.

Monte Carlo simulations in the three-dimensional (3D) Ashkin-Teller model on a cubic lattice are performed in the regions of the two-parameter space diagram where Ising-type phase transitions are expected. The scaling behavior of the Binder cumulant Q and the magnetic susceptibility in the critical region are exploited. In simulations the periodic boundary conditions and the Metropolis algorithm are used. Starting from Ising critical exponents and applying the finite-size-scaling analysis of the cumulant Q with nonlinear corrections, the accurate positions of the critical couplings on the continuous phase transition lines are calculated. For these couplings the critical exponent y(h) is calculated analyzing the magnetic susceptibility in the framework of the finite-size scaling with corrections. The values of y(h) agree with the 3D Ising model value along two lines determined by the order parameter [ssigma] .