The residual entropy of Shastry-Sutherland lattice of rare-earth tetraborides

Abstract

The experimental temperature dependence of thulium tetraboride specific heat Cp(T) and other heat capacity data for other RE-tetraborides were investigated in the 2–300 K temperature interval. Anomalies of Cp(T) dependence of TmB4 at TN1 = 9.6 K and TN2 = 11.4 K due to the antiferromagnetic ordering have been revealed. The ground state of the ion Tm3+ was confirmed as a doublet. The diffuse anomaly of Cp(T) is attributed to the Schottky contribution to the specific heat of the tetraboride, which is caused by the influence of the crystal electric field (CEF). The presence of the residual (zero-point) entropy S0 of the magnetic moments system of Tm3+ ions, due to frustration of the Shastry-Sutherland lattice, is detected. As a measure of the frustration of the system, a new characteristic of frustrated systems, the entropy frustration figure of merit, the value of which depends on the zero-point entropy of the system, is introduced: fS = S0/ΔSm max, where ΔSm max is the maximum value of entropy change of the boride magnetic subsystem. For TmB4 and tetraborides of Gd, Tb, Dy, Ho, and Er, fs values determined from thermal measurements, and f = Өc-w/TN (Өc-w denotes the Curie-Weiss temperature; TN is Neel temperature), calculated according to the magnetic measurement data, are practically identical.