Low-Energy Excitations and the Electronic Specific Heat of YbBiPt

Abstract

Neutron scattering data from the cubic heavy-fermion compound YbBiPt are presented, along with an interpretation in terms of strongly broadened crystal-field levels. To first order, the ground state is sixfold degenerate, consisting of a doublet (17) and quartet (I' s). Integration over these levels reproduces the observed linear specific-heat coefficient y of 8 Jmol ' K to within~20%. At the

High-resolution x-ray diffraction study of the heavy-fermion compound YbBiPt

“…20 On the other hand, inelastic neutron scattering experiments found a scattering peak at E = 5.7 meV, which was identified as a CEF level transition to an excited doublet, and two inelastic peaks centered at ≈ 1 and 2 meV, which become resolved at T 10 K and lie on top of two broad quasielastic signals. 7 Experiments have not yet detected any dispersion associated with the peaks at ≈ 1 and 2 meV, which suggests that they may correspond to CEF level transitions. As for the broad quasielastic signals, they are also seen above 10 K, and it has been proposed that they arise from transitions occurring within the Γ 7 doublet and Γ 8 quartet.…”

“…For example, several previous studies have suggested that a lattice distortion occurs at T ≈ 6 K which results in a lowering of the cubic symmetry. 7,11,12 Specifically, inelastic neutron scattering data suggest that the J = 7/2 groundstate magnetic multiplet of the Yb is split by the CEF further than allowed for by cubic point symmetry, 7 and specific heat data show a feature at 6 K that cannot be described only by a simple Schottky-term. 12 In addition, electron spin resonance experiments on Er-doped YbBiPt suggest that slight distortions away from cubic symmetry occur in the vicinity of the rare-earth sites, 11 and measurements of the thermal expansion have shown that the thermal expansion coefficient changes sign close to 6 K. 5 In this paper, we present results from high-resolution, highenergy x-ray diffraction experiments on single-crystal samples of YbBiPt.…”

“…As for the broad quasielastic signals, they are also seen above 10 K, and it has been proposed that they arise from transitions occurring within the Γ 7 doublet and Γ 8 quartet. 7 Broad quasielastic scattering in other heavy-fermion compounds has been associated with the hybridization between the localized 4f electrons and itinerant charges and has been related to the value of T K . 21,22 If the two inelastic peaks at E ≈ 1 and 2 meV correspond to CEF level transitions, then the point symmetry of the Yb 3+ sites would be lower than cubic, since for the cubic CEF level scheme only the Γ 7 ↔ Γ 8 and Γ 8 ↔ Γ 6 transitions have nonzero transition probabilities.…”

“…19 The solid blue line corresponds to the CEF level scheme given in Ref. 7, which is appropriate for a CEF with lower than cubic symmetry acting on the Yb 3+ ions. It consists of 4 doublets located at 0, 1, 2, and 6 meV.…”

“…YbBiPt is a heavy-fermion compound which manifests an extraordinary Sommerfeld coefficient of ≈ 8 J/mol-K 2 and spin-density-wave type antiferromagnetic (AFM) order below a Néel temperature of T N = 0.4 K. [1][2][3][4][5][6] The low value of T N reflects the fact that the dominant magnetic energy scales are all small and comparable -the Kondo temperature T K ≈ 1 K, 2 the Weiss temperature θ W ≈ −2 K, 5 and the crystallineelectric-field (CEF) splitting is on the order of 1 -10 K. 7 As a consequence, the compound's electronic states are very responsive to small applied magnetic fields and pressures, 5,8 and recent neutron diffraction measurements have shown that the AFM order is quite fragile. 6 The magnetic phase diagram indicates that, as T → 0 K, AFM order persists up to a critical applied magnetic field of µ 0 H c ≈ 0.4 T, followed by a region of non-Fermi liquid behavior up to ≈ 0.8 T, above which Fermi-liquid behavior occurs up to at least 6 T. 5 It has been proposed that a magnetic-field-induced quantum-critical point occurs at µ 0 H c .…”