Spin-thermopower in interacting quantum dots

TL;DR

This paper investigates the spin-thermopower in quantum dots under magnetic fields using analytical and numerical methods, revealing large spin-Seebeck coefficients near the Kondo temperature and providing simplified models.

Contribution

It introduces a detailed analysis of spin-thermopower in quantum dots, including a mapping to negative-U systems and analytical expressions for various regimes.

Findings

Large spin-Seebeck coefficient near the Kondo temperature.

Pure spin current can be driven without charge current in symmetric cases.

Behavior remains similar under particle-hole symmetry breaking with appropriate voltage.

Abstract

Using analytical arguments and the numerical renormalization group method we investigate the spin-thermopower of a quantum dot in a magnetic field. In the particle-hole symmetric situation the temperature difference applied across the dot drives a pure spin current without accompanying charge current. For temperatures and fields at or above the Kondo temperature, but of the same order of magnitude, the spin-Seebeck coefficient is large, of the order of k_B/e. Via a mapping, we relate the spin-Seebeck coefficient to the charge-Seebeck coefficient of a negative-U quantum dot where the corresponding result was recently reported by Andergassen et al. in Phys. Rev. B 84, 241107 (2011). For several regimes we provide simplified analytical expressions. In the Kondo regime, the dependence of the spin-Seebeck coefficient on the temperature and the magnetic field is explained in terms of the shift of the Kondo resonance due to the field and its broadening with the temperature and the field. We also consider the influence of breaking the particle-hole symmetry and show that a pure spin current can still be realized provided a suitable electric voltage is applied across the dot. Then, except for large asymmetries, the behavior of the spin-Seebeck coefficient remains similar to that found in the particle-hole symmetric point.