Laboratory for Solid State Physics, ETH Zurich, 8093 Zurich, Switzerland.
Institute for Quantum Electronics, ETH Zurich, 8093 Zurich, Switzerland.
Phys Rev Lett. 2018 Aug 24;121(8):087206. doi: 10.1103/PhysRevLett.121.087206.
Prior to the development of pulsed lasers, one assigned a single local temperature to the lattice, the electron gas, and the spins. With the availability of ultrafast laser sources, one can now drive the temperature of these reservoirs out of equilibrium. Thus, the solid shows new internal degrees of freedom characterized by individual temperatures of the electron gas T_{e}, the lattice T_{l} and the spins T_{s}. We demonstrate an analogous behavior in the spin polarization of a ferromagnet in an ultrafast demagnetization experiment: At the Fermi energy, the polarization is reduced faster than at deeper in the valence band. Therefore, on the femtosecond time scale, the magnetization as a macroscopic quantity does not provide the full picture of the spin dynamics: The spin polarization separates into different parts similar to how the single temperature paradigm changed with the development of ultrafast lasers.
在脉冲激光器发展之前，人们为晶格、电子气和自旋赋予了单一的局部温度。随着超快激光源的出现，人们现在可以使这些储层的温度达到非平衡状态。因此，固体表现出具有电子气 T_e、晶格 T_l 和自旋 T_s 各自温度的新的内部自由度。我们在超快退磁实验中展示了铁磁体中自旋极化的类似行为：在费米能处，极化的衰减速度比在价带深处更快。因此，在飞秒时间尺度上，作为宏观量的磁化并不能提供自旋动力学的全貌：自旋极化分为不同的部分，类似于超快激光发展带来的单一温度范例的变化。
