Interplay of exchange field and Kondo effect in quantum impurity systems out of equilibrium

Equilibrium studies of how magnetic leads affect the transport through a quantum dot system in the Kondo regime has been well established over the last two decades. Non-equilibrium investigations into such systems have been limited by the lack of a reliable theoretical framework applicable for accurate calculations of transport in far-from-equilibrium conditions. This gap has been recently filled by a hybrid quench approach combining the Numerical Renormalization Group (NRG) and time-dependent Density Matrix Renormalization Group (tDMRG) methods in the matrix product state framework [1]. This method has the ability to treat the correlations exactly while keeping the system out of equilibrium. Thus, in this work, we investigate the non-equilibrium spin-resolved transport properties of quantum dots (molecules) coupled to ferromagnetic leads using the hybrid NRG-tDMRG thermofield quench approach. In particular, we study the behavior of the zero-bias conductance peak, a characteristic feature of the Kondo effect, under various system parameters. In the absence of an exchange field, we observe that the lead spin polarization reduces the Kondo energy scale in the applied bias voltage, showing qualitative agreement to the analytical predictions [2]. However, when there is an exchange field in the system, the Kondo resonance gets suppressed and the zero-bias conductance peak becomes split with respect to the value of the exchange field. These split-Kondo peaks are further studied under the influence of various external parameters, such as applied magnetic field and temperature, to provide new insights into the behavior of the system out of equilibrium. Our work provides quantitatively accurate results for nonequilibrium behavior of quantum dot spin valves that may serve as a benchmark for future theoretical and experimental research.
This work was supported by the Polish National Science Centre from funds awarded through the decision No.2017/27/B/ST3/00621.

[1] F. Schwarz et. al., Phys. Rev. Lett., 121, 137702 (2018).
[2] J. Martinek et al., Phys. Rev. Lett., 91, 127203 (2003).

E-mail for corresponding author: [email protected]