Grüneisen Ratio Divergence at the Quantum Critical Point inCeCu6−xAg

Abstract

The heavy fermion system CeCu 6−x Ag x is studied at its antiferromagnetic quantum critical point, x c = 0.2, by low temperature (T ≥ 50 mK) specific heat, C(T ), and volume thermal expansion, β(T ), measurements. Whereas C/T ∝ log(T 0 /T ) would be compatible with the predictions of the itinerant spin-density-wave (SDW) theory for two-dimensional critical spin fluctuations, β(T )/T and the Grüneisen ratio, Γ(T ) ∝ β/C, diverge much weaker than expected, in strong contrast to this model. Both C and β, plotted as a function of the reduced temperature t = T /T 0 with T 0 = 4.6 K are similar to what was observed for YbRh 2 (Si 0.95 Ge 0.05 ) 2 (T 0 = 23.3 K) indicating a striking discrepancy to the SDW prediction in both systems.

Thermodynamics of the coupled spin-dimer system close to a quantum phase transition

“…In particular, a divergence of the so-called Grüneisen parameter Γ = α/C is expected, when a pressure-dependent QCP is approached [17]. Experimentally, a diverging Γ(T ) has indeed been observed for different heavy-fermion compounds [18,19]. Since the phase transition of TlCuCl 3 is extremely sensitive to pressure [20,21,22,23] and the control parameter may be easily tuned by a variation of the field, this compound is ideally suited to study such generic properties of a QCP.…”

Separation of Energy Scales in Undoped YbRh2Si2Under Hydrostatic Pressure

“…[2][3][4] This results, e.g., in a divergence of the Grüneisen ratio, À / =C, or magnetic Grüneisen parameter À mag / ðÀdM=dTÞ=C, (: volume thermal expansion, C: electronic specific heat, M: magnetization) for QCPs tuned by pressure and magnetic field, respectively. 5) Experiments on the HF metals CeCu 6Àx M x (M ¼ Au, 6) Ag 7) ) and YbRh 2 are incompatible with the standard picture of an antiferromagnetic (AF) QCP, which describes a spin-densitywave transition. Unconventional quantum criticality, [11][12][13][14] which qualitatively differs from the predictions of the standard theory, may arise due to a destruction of Kondo screening, leading to a decomposition of the heavy quasiparticles into conduction electrons and local magnetic moments.…”

Critical elasticity at zero and finite temperature

“…Moreover, Γ also changes sign close to a quantum critical point, and the sign changes reveal the locations of entropy acummulation [3]. Measurements of Γ [4,5,6,7] and its magnetic counterpart, the magnetocaloric effect [8,9,10,11,12,13], have proven useful in the investigation of various quantum critical systems, see Refs. [14,15,16] for reviews.…”