In some metals containing a sub-lattice of rare earth or actinide ions, free local $f$ spins at high temperatures dissolve into the sea of quantum conduction electrons at low temperatures, where they become mobile excitations. Once mobile, the spins acquire charge, forming electrons of heavy mass, known as heavy fermions. In turn, the incorporation of heavy charges into the conduction sea leads to an increase in the volume of the Fermi surface. This process, called Kondo scattering, is accompanied by a dramatic, temperature dependent transformation of the electronic interactions and masses. Since the Kondo phenomena is controlled by quantum fluctuations, here we ask, at which point does the Fermi surface change character? A priori, the answer is not clear, since near its onset, the Kondo effect cannot be described as a simple hybridization of electronic eigenstates. Conventional descriptions of this Kondo scattering process consider that hybridization, Fermi volume change, and $f$-electron mobility occur simultaneously. However, using angle resolved photoemission spectroscopy to measure the evolution of excitations, we find that the changes of the Fermi surface emerge at temperatures an order of magnitude higher than the opening of the hybridization gap, and two orders of magnitude higher than the onset of the coherent character of the $f$-electrons. We suggest that the large changes in Fermi volume, driven by electronic fluctuations, occur at temperatures where the various $Γ_x \to Γ_y$ crystal field-split $f$ levels become accessible to conduction states of the corresponding symmetries. The separation of these energy scales significantly modifies the conventional description of the Kondo lattice effect, which still lacks a full theoretical description.
Imbalanced thee-component Fermi gas with attractive interactions: Multiple FFLO-pairing, Bose-Fermi and Fermi-Fermi mixtures versus collapse and phase separation
Time-resolved collapse and revival of the Kondo state near a quantum phase transition
多体物理学最成功的范例之一是准粒子的概念：强相互作用物质中的激发行为就像自由空间中的弱相互作用粒子。金属中的准粒子是非常坚固的物体。然而，当一个系统的基态在量子临界点（QCP）发生质变时，准粒子可能会分裂并让位于一种奇异的量子流体态。这种崩溃的性质是激烈的争论，因为涌现的量子流体在高温下控制着材料的性质，甚至可能与某些化合物中超导性的发生有关。在这里，我们跟踪CeCu $_{6-x}$ Au $_{x}$中的重费米子准粒子的动力学，并在多体计算的支持下，在时间分辨实验中监测它们向QCP的演化。太赫兹脉冲破坏了多体重费米子态。在延迟的相位相干太赫兹反射的发射下，重费米子状态恢复，相干时间比通常与相关金属相关的时间长100倍。准粒子的重量崩溃的QCP，但其形成温度保持不变-现象被认为是相互排斥的。在同一个实验中的共存要求我们对量子临界性的看法进行修正。
