Seminars

17 Apr 2024
  • Bernd Braunecker (University of St. Andrews)

    17 Apr 2024  4:00 pm - 5:00 pm
    Turing building, Lovell Seminar room.

    Coherent backaction between spins and an electronic bath: Non-Markovian dynamics, strong correlations and non-thermodynamic quantum electron cooling

    I will present a systematic analytical approach for calculating the dynamics of a spin in contact with a bath of fermions [1,2] and show how strong correlations modify the dynamics [3,4]. I will focus in particular on the free induction decay. Although this is the basic tool of magnetic resonance measurements and thus has been studied since decades, its theoretical foundations date back to the 1950s and do not take into account quantum coherent memory effects that affect the short time behaviour and produce a non-Markovian dynamics. I will show that it is possible to derive analytic expressions that cover the full time range, showing how the quantum correlation dominated dynamics at short times evolves smoothly into the conventional exponential thermal decay at long times. The expressions can be obtained at any temperature through a systematic study of the pole structure of the memory kernel of the density matrix, based on the Nakajima-Zwanzig master equation in the weak coupling limit.
    This gives not only access to the non-Markovian short-time behaviour but provides also a clear formula for the standard exponential decay times, and how the non-Markovian contribution produces a fast initial decay of the spin amplitude, known as the initial slip.

    I will show results for Fermi liquids and for Luttinger liquids as prime examples of strongly correlated systems. For the long-time behaviour I can confirm how strong correlations modify the Korringa relation, which expresses the temperature dependence of the exponential decay times. For the short-time behaviour I show how the non-Markovian decay is systematically tuned as a function of interaction strength, which may be connected to the modification of the statistics of the elementary excitations. I will conclude with the proposal to employ the fact that the non-Markovian initial decay to transport heat out of the electron system. As this decay is quantum and not temperature fluctuation based it is largely temperature independent and thus effective even at very low temperatures which are a bottleneck for standard magnetic refrigeration. I will argue that a repeated re-initialisation of a cluster of spins could offer thus a viable route to further cool electrons at very low temperatures, in a non-thermodynamic way as it is quantum driven and out of equilibrium.

    [1] S. Matern, D. Loss, J. Klinovaja and B. Braunecker, Phys. Rev. B 100, 134308 (2019).
    [2] S. Matern, PhD Thesis, St. Andrews (2020).
    [3] T. Boorman, PhD Thesis, St. Andrews, submitted (2024).
    [4] T. Boorman and B. Braunecker, papers in preparation.

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8 May 2024
  • Frank A Schindler (Imperial college)

    8 May 2024  4:00 pm - 5:00 pm

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