Spin-wave contribution to specific heat of superconducting antiferromagnets RNi 2B 2C (R=Er, Dy)

Abstract

RNi 2B 2C (R=Tm, Er, Ho, Dy) superconducts at, respectively, T c=11, 11, 8, 6 K. Below, respectively, T N≃1.5, 6, 5, 10.6 K, each (For D Y below T c) shows a coexistence of superconductivity and antiferromagnetism. Since their magnetic structures consist of weakly coupled magnetic RC that are separated by superconducting Ni 2B 2, it is of interest to investigate the character and dimensionality of the involved spin-wave SW excitations. For that purpose, magnetic specific heat C M( T) of well characterized RNi 2B 2C (R=Er, Dy) were measured in the coexistence regime. For R=Ho, SW contribution was reported as 0.29 T 3exp(−5.3/ T) J/mol K, indicative of magnon excitation in a 3d antiferromagnetic (AF) state with an energy gap Δ≈ k B T N. For R=Tm, a reported linear-in- T contribution implies a gapless magnon excitation in a 2d FM sheet. The SW contribution of R=Er below 2 K is 0.27( Δ 2+4 TΔ+6 T 2)exp(− Δ/ T) J/mol K where Δ=5.9±0.1 K: a gapped dispersion relation of a 2d FM sheet. The contribution of R=Dy below 2.5 K is 0.05 T J/mol K: a 2d FM gapless dispersion relation. Assuming an anisotropic Neel AF ground structure and using standard SW theory we are able to obtain an explicit expression for the energy gap in terms of anisotropy field and interplanar coupling. Such an expression is used qualitatively to rationalize the observed differences in the magnon contributions of these superconducting AFs.