Order-by-disorder near criticality in XY pyrochlore magnets

Article

关键词: QUANTUM FLUCTUATIONS;    SPIN-GLASS;    ANTIFERROMAGNET;    LATTICE;    SYSTEM;    MODEL;   

DOI  :  10.1103/PhysRevB.91.174424

来源: SCIE

【 摘 要 】

We consider a system of spins on the sites of a three-dimensional pyrochlore lattice of corner-sharing tetrahedra interacting with a predominant effective xy exchange. In particular, we investigate the selection of a long-range ordered state with broken discrete symmetry induced by thermal fluctuations near the critical region. At the standard mean-field theory (s-MFT) level, in a region of the parameter space of this Hamiltonian that we refer to as Gamma(5) region, the ordered state possesses an accidental U(1) degeneracy. In this paper, we show that fluctuations beyond s-MFT lift this degeneracy by selecting one of two states (so-called psi(2) and psi(3)) from the degenerate manifold, thus exposing a certain form of order-by-disorder (ObD). We analytically explore this selection at the microscopic level and close to criticality by elaborating upon and using an extension of the so-called TAP method, originally developed by Thouless, Anderson, and Palmer to study the effect of fluctuations in spin glasses. We also use a single-tetrahedron cluster-mean-field theory (c-MFT) to explore over what minimal length scale fluctuations can lift the degeneracy. We find the phase diagrams obtained by these two methods to be somewhat different since c-MFT only includes the shortest-range fluctuations. General symmetry arguments used to construct a Ginzburg-Landau theory to lowest order in the order parameters predict that a weak magnetic moment m(z) along the local < 111 > ((z) over cap) direction is generically induced for a system ordering into a psi(2) state, but not so for psi(3) ordering. Both E-TAP and c-MFT calculations confirm this weak fluctuation-induced m(z) moment. Using a Ginzburg-Landau theory, we discuss the phenomenology of multiple phase transitions below the paramagnetic phase transition and within the Gamma(5) long-range ordered phase.

【 授权许可】

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