Abstract-Time-resolved collapse and revival of the Kondo state near a quantum phase transition

C. Wetli, S. Pal, J. Kroha, K. Kliemt, C. Krellner, O. Stockert, H. v. Löhneysen,  M. Fiebig,

https://www.nature.com/articles/s41567-018-0228-3

One of the most successful paradigms of many-body physics is the concept of quasiparticles: excitations in strongly interacting matter behaving like weakly interacting particles in free space. Quasiparticles in metals are very robust objects. Nevertheless, when a system’s ground state undergoes a qualitative change at a quantum critical point (QCP)1, the quasiparticles may disintegrate and give way to an exotic quantum-fluid state of matter. The nature of this breakdown is intensely debated2,3,4,5, because the emergent quantum fluid dominates material properties up to high temperatures and might even be related to the occurrence of superconductivity in some compounds6. Here we trace the dynamics of heavy-fermion quasiparticles in CeCu6−xAux and monitor their evolution towards the QCP in time-resolved experiments, supported by many-body calculations. A terahertz pulse disrupts the many-body heavy-fermion state. Under emission of a delayed, phase-coherent terahertz reflex the heavy-fermion state recovers, with a coherence time 100 times longer than typically associated with correlated metals7,8. The quasiparticle weight collapses towards the QCP, yet its formation temperature remains constant—phenomena believed to be mutually exclusive. Coexistence in the same experiment calls for revisions in our view on quantum criticality.

Abstract-Gauge-invariant theory of quasiparticle and condensate dynamics in response to terahertz optical pulses in superconducting semiconductor quantum wells. II. ( s + p )-wave superconductivity in the strong spin-orbit coupling limit

T. Yu and M. W. Wu

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.96.155312

We investigate the quasiparticle and condensate dynamics in response to the terahertz (THz) optical pulses in the strong spin-orbit-coupled ($s+p$)-wave superconducting semiconductor quantum wells by using the gauge-invariant optical Bloch equations in the quasiparticle approximation. Both the dynamics of triplet and singlet superconductivity are studied in response to the THz optical pulses. Specifically, for the triplet superconductivity, we predict that in the ($s+p$)-wave superconducting (100) quantum wells, with the vector potential parallel to the quantum wells, the optical field can cause the total spin polarization of Cooper pairs, oscillating with the frequency of the optical field. The direction of the total Cooper-pair spin polarization is shown to be parallel to the vector potential. For the singlet superconductivity, we show that due to the large spin-orbit coupling in InSb (100) quantum wells, there exist two Fermi surfaces including the inner and outer ones. In this specific configuration, the superconducting momentum can be tuned to be larger than the inner Fermi momentum but smaller than the outer one. We find that in this regime, the dynamics of the Higgs mode and charge imbalance shows different features in comparison with the conventional $s$-wave case.

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Abstract-Time-resolved collapse and revival of the Kondo state near a quantum phase transition

Ch. Wetli (1), J. Kroha (2,3), K. Kliemt (4), C. Krellner (4), O. Stockert (5), H. von Loehneysen (6), M. Fiebig (1) ((1) ETH Zurich, Switzerland, (2) Univ Bonn, Germany, (3) CCM Zhejiang Univ, Hangzhou, China, (4) Univ Frankfurt, Germany (5) MPI-CPfS Dresden, Germany (6) KIT Karlsruhe, Germany)

(Submitted on 13 Mar 2017)

https://arxiv.org/abs/1703.04443

One of the most successful paradigms of many-body physics is the concept of quasiparticles: excitations in strongly interacting matter behaving like weakly interacting particles in free space. Quasiparticles in metals are very robust objects. Yet, when a system's ground state undergoes a qualitative change at a quantum critical point (QCP), these quasiparticles can disintegrate and give way to an exotic quantum-fluid state of matter where the very notion of particles comprising the system breaks down. The nature of this breakdown is intensely debated, because the emergent quantum fluid dominates the material properties up to high temperature and might even be related to the occurence of superconductivity in some compounds. Here we control the resurgence of heavy-fermion quasiparticles out of a photoexcited nonequilibrium state and monitor their dynamics towards the QCP in a time-resolved experiment, supported by many-body calculations. A terahertz pulse transforms heavy fermions in CeCu5.9Au0.1 into light electrons. Under emission of a delayed, phase-coherent terahertz reflex the heavy-fermion state recovers, with a memory time 100 times longer than the coherence time typically associated with metals. The quasiparticle weight collapses towards the QCP, yet its formation temperature remains almost constant. This suggests a revised view of quantum criticality in between disintegration and preservation of the quasiparticle picture.

Abstract-Higgs mode excitation in superconductors by intense terahertz pulse

Ryusuke Matsunaga, Ryo Shimano

The Univ. of Tokyo (Japan)

Proc. SPIE 9835, Ultrafast Bandgap Photonics, 98351G (May 13, 2016); doi:10.1117/12.2223010

http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=2523983

Recent development of intense terahertz (THz) pulse generation technique has offered novel opportunities to reveal ultrafast phenomena in a variety of materials on tabletop experiments and provided a new pathway toward ultrafast control of quantum phases. Here we present our recent study of nonequilibrium dynamics in metallic superconductors NbN excited by intense THz pulse. Since the superconducting gap energy is located in the THz frequency range, the intense THz pulse excitation makes it possible to instantaneously excite high-density quasiparticles at the gap edge without injecting excess energies. It has also become possible to coherently drive the superconducting ground state without exciting incoherent quasiparticles by tuning the pump frequency below the gap energy. The ultrafast dynamics of the order parameter induced by such an intense low energy excitation is directly probed, and the nature of a collective excitation, namely the Higgs amplitude mode, is revealed. Efficient THz higher-harmonic generation from a superconductor is discovered, manifesting the nonlinear coupling between the THz wave and the Higgs mode. We also report the experimental results in a multi-gap superconductor MgB2. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.