Max Planck Institute for Chemical Physics of Solids, Noethnitzer Straße 40, 01187 Dresden, Germany.
J Phys Condens Matter. 2012 Jul 25;24(29):294201. doi: 10.1088/0953-8984/24/29/294201.
Both CeCu2Si2 and YbRh2Si2 crystallize in the tetragonal ThCr2Si2 crystal structure. Recent neutron-scattering results on normal-state CeCu2Si2 reveal a slowing down of the quasielastic response which complies with the scaling expected for a quantum critical point (QCP) of itinerant, i.e., three-dimensional spin-density-wave (SDW), type. This interpretation is in full agreement with the non-Fermi-liquid behavior observed in transport and thermodynamic measurements. The momentum dependence of the magnetic excitation spectrum reveals two branches of an overdamped dispersive mode whose coupling to the heavy charge carriers is strongly retarded. These overdamped spin fluctuations are considered to be the driving force for superconductivity in CeCu2Si2 (Tc = 600 mK). The weak antiferromagnet YbRh2Si2 (TN = 70 mK) exhibits a magnetic-field-induced QCP at BN = 0.06 T (B⊥c). There is no indication of superconductivity down to T = 10 mK. The magnetic QCP appears to concur with a breakdown of the Kondo effect. Doping-induced variations of the average unit-cell volume result in a detachment of the magnetic and electronic instabilities. A comparison of the properties of these isostructural compounds suggests that 3D SDW QCPs are favorable for unconventional superconductivity. The question whether a Kondo-breakdown QCP may also give rise to superconductivity, however, remains to be clarified.
CeCu2Si2 和 YbRh2Si2 均结晶为四方 ThCr2Si2 晶体结构。最近对正常态 CeCu2Si2 的中子散射结果表明，准弹性响应的减缓符合巡游量子临界点（QCP）的标度预期，即三维自旋密度波（SDW）类型。这种解释与在输运和热力学测量中观察到的非费米液体行为完全一致。磁激发谱的动量依赖性揭示了两个过阻尼弥散模式的分支，其与重载流子的耦合被强烈延迟。这些过阻尼的自旋涨落被认为是 CeCu2Si2 超导性的驱动力（Tc = 600 mK）。弱反铁磁 YbRh2Si2（TN = 70 mK）在 BN = 0.06 T（B⊥c）的磁场下表现出 QCP。在 T = 10 mK 以下没有超导性的迹象。磁 QCP 似乎与 Kondo 效应的破裂同时发生。平均单元体积的掺杂诱导变化导致磁和电子不稳定性的分离。对这些同构化合物性质的比较表明，3D SDW QCP 有利于非常规超导性。然而，Kondo 破坏 QCP 是否也可能导致超导性，这一问题仍有待澄清。
