We have studied the antiferromagnetic Ising chain in a transverse magnetic field hx and uniform longitudinal field hz. Using the density matrix renormalization group calculation combined with a finite-size scaling the ground state phase diagram in (hx, hz) plane is determined. It is shown that there is an order-disordered transition line in this plane and the critical properties belong to the universality class of the two-dimensional Ising model. Based on the perturbation theory in hz the scaling behavior of the mass gap in the vicinity of the critical point (hx = 1/2, hz = 0) is established. It is found that the form of the transition line near the classical multicritical point (hx = 0, hz = 1) is linear. The connection of the considered quantum model with the quasi-one-dimensional classical Ising model in the magnetic field is discussed.

We report magnetic-susceptibility, specific-heat, and Raman-scattering investigations of ␣-TeVO 4 containing V-O edge-sharing chains. These chains promote a system of ferromagnetic/antiferromagnetic spin-1/2 Heisenberg alternating exchange chains with pronounced spin frustration. The magnetic susceptibility and Raman scattering evidence a crossover at T ‫ء‬ = 85 K with different slopes of the reciprocal susceptibility and a magnetic phase transition into a long-range-ordered state at T c = 16 K. From Raman-scattering data a strong mutual coupling between lattice and magnetic degrees of freedom is deduced. A comparison to model calculations and prior Raman scattering on other chain systems yields a plausible interpretation of the microscopic mechanism for the crossover behavior.

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