Non-equilibrium dynamics of quantum-impurity systems close to quantum critical points
Final Report Abstract
The project on quantum impurity systems close to quantum critical points had three major foci: (i) The scanning tunnelling spectra have been calculated for an Au-PTCDA monomer complex on a gold surface. For the proper modeling of this system, we have used ab-initio results from density functional theory (LDA) as the input parameter for our numerical renormalization group calculation. Our ab-initio LDA+NRG approach predicts a Kondo temperature of 37K which excellently agrees with the experimental measured TK = (38 ± 8)K. We extended our investigation to dimers formed by two neighbouring Au-PTCDA monomers. We could prove that this system is close to a quantum phase transition between a local singlet phase and an S = 1 Kondo regime, where the control parameter is the strength of the chemical bond between two magnetic moment carrying π-orbitals of the monomers induced by the additional Au-atom. The chemical properties of the π-orbitals are determined by the relative positions of the two Au-atom of the two PTCDA molecules. We found quantitative agreement with the experiments, proving the predictive power of our ab-initio many-body approach. (ii) We investigated the spatial and temporal propagation of spin-spin correlation functions in the Kondo model with ferromagnetic and antiferromagnetic coupling out of equilibrium. Using the time-dependent numerical renormalization group (TD-NRG) approach, we have shown that spatial spin correlations build up even outside of the light cone. These results are confirmed by an analytic perturbation theory, which becomes exact at short times, and are connected to intrinsic spin correlations of the non-interacting Fermi sea. (iii) We calculated the real-time dynamics in a pseudo-gap Anderson model out of equilibrium after a strong quench using the TD-NRG. In such a system, quantum phase transitions between a local moment and a strong coupling phase have been established in equilibrium. We found thermalization in the long-time limit of expectation values for quenches within the strong coupling regime. For quenches into the local moment phase, all local observables approach a steady-state that deviates from its thermodynamics expectation value. We could link the deviations between the steady-state and the equilibrium values to the distance of the control parameter from the critical coupling. The inverse of the crossover energy scale can be viewed as a characteristic length scale for the spatial extension of the local moment: the better the localization of the local moment, the larger the deviation between steady-state and the equilibrium values.
Publications
- Spectral properties of a molecular wire in the Kondo regime. physica status solidi (b) (2013)
A. Greuling, R. Temirov, B. Lechtenberg, F. B. Anders, M. Rohlfing, F. S. Tautz
- Real-time dynamics induced by quenches across the quantum critical points in gapless Fermi systems with a magnetic impurity. Phys. Rev. B 90, 235145 (2014)
Christian Kleine, Julian Mußhoff, Frithjof B. Anders
(See online at https://doi.org/10.1103/PhysRevB.90.235145) - Spatial and temporal propagation of Kondo correlations. Phys. Rev. B 90, 045117 (2014)
Benedikt Lechtenberg and Frithjof B. Anders
(See online at https://doi.org/10.1103/PhysRevB.90.045117) - Equilibrium and nonequilibrium dynamics close to impurity quantum phase transitions. Dissertation, Fakultät Physik an der
Technischen Universität Dortmund, Dez 2015, VIII, 185 S.
Benedikt Lechtenberg
- Transfering spin into an extended π-orbital of a large molecule. Phys. Rev. B 91, 144415 (2015)
Taner Esat, Thorsten Deilmann, Benedikt Lechtenberg, Christian Wagner, Peter Krüger, Ruslan Temirov, Frithjof B. Anders, Michael Rohlfing, F. Stefan Tautz
(See online at https://doi.org/10.1103/PhysRevB.91.144415) - A chemically driven quantum phase transition in a two-molecule Kondo system. Nature Physics
Taner Esat, Benedikt Lechtenberg, Thorsten Deilmann, Christian Wagner, Peter Krüger, Ruslan Temirov, Michael Rohlfing, Frithjof B. Anders, and F. Stefan Tautz
(See online at https://doi.org/10.1038/NPHYS3737)
