Potassium chromium jarosite, KCr3(OH)6(SO4)2 (Cr-jarosite), is considered a promising candidate to display spin liquid behavior due to the strong magnetic frustration imposed by the crystal structure. However, the ground state...
Abstract:We investigated the antiferromagnetic phase transition in the frustrated and multiferroic hexagonal manganites h-YMnO 3 (YMO) and h-(Y 0.98 Eu 0.02 )MnO 3 (YEMO). Elastic neutron scattering was used to study, in detail, the phase transition in YMO and YEMO under zero pressure and in YMO under a hydrostatic pressure of 1.5 GPa. Under conditions of zero pressure, we found critical temperatures of T N = 71.3(1) K and 72.11(5) K and the critical exponent 0.22(2) and β = 0.206(3), for YMO and YEMO, respectively. This is in agreement with earlier work by Roessli et al. Under an applied hydrostatic pressure of 1.5 GPa, the ordering temperature increased to T N = 75.2(5) K, in agreement with earlier reports, while β was unchanged. Inelastic neutron scattering was used to determine the size of the anisotropy spin wave gap close to the phase transition. From spin wave theory, the gap is expected to close with a critical exponent, β , identical to the order parameter β. Our results indicate that the gap in YEMO indeed closes at T N = 72.4(3) K with β = 0.24(2), while the in-pressure gap in YMO closes at 75.2(5) K with an exponent of β = 0.19(3). In addition, the low temperature anisotropy gap was found to have a slightly higher absolute value under pressure. The consistent values obtained for β in the two systems support the likelihood of a new universality class for triangular, frustrated antiferromagnets.
We have used exact numerical diagonalization to study the excitation spectrum and the dynamic spin correlations in the s = 1/2 next-next-nearest neighbor Heisenberg antiferromagnet on the square lattice, with additional 4-spin ring exchange from higher order terms in the Hubbard expansion. We have varied the ratio between Hubbard model parameters, t/U , to obtain different relative strengths of the exchange parameters, while keeping electrons localized. The Hubbard model parameters have been parametrized via an effective ring exchange coupling, Jr, which have been varied between 0 J and 1.5 J. We find that ring exchange induces a quantum phase transition from the (π, π) ordered Neèl state to a (π/2, π/2) ordered state. This quantum critical point is reduced by quantum fluctuations from its mean field value of Jr/J = 2 to a value of ∼ 1.1. At the quantum critical point, the dynamical correlation function shows a pseudo-continuum at q-values between the two competing ordering vectors. arXiv:1812.04277v1 [cond-mat.str-el] 11 Dec 2018 A a b d c e f B i j k l J J J J r
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