Ultrafast, coherent time evolution of terahertz-pumped heavy-fermion systems

The characterisation of new quantum phases of matter has recently been intensified by the application of terahertz (THz) spectroscopy in the time domain to heavy-fermion (HF) systems [1-3]. It was experimentally shown that a single-cycle THz pulse disrupts the strongly correlated Kondo ground state in HF compounds such as CeCu_6−xAu_x, and that it recovers after a characteristic delay time, the Kondo coherence time τ*_K= ħ / (k_BT_K), accompanied by the emission of a temporally confined THz echo pulse. In this way, time-domain THz spectroscopy provides direct, background-free access to both the quasi-particle spectral weight and the characteristic time/energy scales, across a HF quantum phase transition [1-2].

In the present work we develop the theoretical description of the HF non-equilibrium dynamics. The matter fields are captured by an Anderson model described by a time-dependent version of the non-equilibrium Non-Crossing Approximaton. The THz photons are treated as a quantum field with its own dynamics and are coupled to the HF-system by a dipole interaction. In this way, not only can incident THz pulses with arbitrary pulse shape be considered but also the photon quantum dynamics allow for re-emission of radiation and, thereby, the necessary release of energy during the relaxation dynamics to the HF ground state. These coupled dynamics are solved by a newly developed adaptive two-time stepping algorithm [4] which captures the exponentially long memory time inherent to the system, given by τ*_K. We also discuss the thermalisation to ambient temperature in terms of a Lindblad-like coupling to the electromagnetic environment as a bath.