Yang Chia-Jung, Kliemt Kristin, Krellner Cornelius, Kroha Johann, Fiebig Manfred, Pal Shovon
Department of Materials, ETH Zurich, Zurich, Switzerland.
Physikalisches Institut, Goethe-Universität Frankfurt, Frankfurt, Germany.
Nat Phys. 2023;19(11):1605-1610. doi: 10.1038/s41567-023-02156-7. Epub 2023 Jul 31.
When a system close to a continuous phase transition is subjected to perturbations, it takes an exceptionally long time to return to equilibrium. This critical slowing down is observed universally in the dynamics of bosonic excitations, such as order-parameter collective modes, but it is not generally expected to occur for fermionic excitations. Here using terahertz time-domain spectroscopy, we find evidence for fermionic critical slowing down in YbRhSi close to a quantum phase transition between an antiferromagnetic phase and a heavy Fermi liquid. In the latter phase, the relevant quasiparticles are a quantum superposition of itinerant and localized electronic states with a strongly enhanced effective mass. As the temperature is lowered on the heavy-Fermi-liquid side of the transition, the heavy-fermion spectral weight builds up until the Kondo temperature ≈ 25 K, then decays towards the quantum phase transition and is, thereafter, followed by a logarithmic rise of the quasiparticle excitation rate below 10 K. A two-band heavy-Fermi-liquid theory shows that this is indicative of the fermionic critical slowing down associated with heavy-fermion breakdown near the quantum phase transition. The critical exponent of this breakdown could be used to classify this system among a wider family of fermionic quantum phase transitions that is yet to be fully explored.
当一个接近连续相变的系统受到微扰时，它需要极长的时间才能恢复到平衡状态。这种临界慢化现象在玻色子激发的动力学过程中普遍存在，比如序参量集体模式，但一般认为在费米子激发过程中不会出现。在此，我们利用太赫兹时域光谱技术，发现在YbRhSi中靠近反铁磁相和重费米液体之间的量子相变处存在费米子临界慢化的证据。在后一个相中，相关的准粒子是巡游电子态和局域电子态的量子叠加，其有效质量大幅增强。当温度在相变的重费米液体一侧降低时，重费米子光谱权重逐渐增加，直至近藤温度≈25 K，然后朝着量子相变方向衰减，此后，在低于10 K时准粒子激发率呈对数上升。双带重费米液体理论表明，这意味着在量子相变附近与重费米子崩溃相关的费米子临界慢化。这种崩溃的临界指数可用于在尚未充分探索的更广泛的费米子量子相变家族中对该系统进行分类。
