TY  - JOUR
A1  - Anders, Janet
A1  - Sait, Connor R. J.
A1  - Horsley, Simon A. R.
T1  - Quantum Brownian motion for magnets
JF  - New journal of physics : the open-access journal for physics
N2  - Spin precession in magnetic materials is commonly modelled with the classical phenomenological Landau-Lifshitz-Gilbert (LLG) equation. Based on a quantized three-dimensional spin + environment Hamiltonian, we here derive a spin operator equation of motion that describes precession and includes a general form of damping that consistently accounts for memory, coloured noise and quantum statistics. The LLG equation is recovered as its classical, Ohmic approximation. We further introduce resonant Lorentzian system-reservoir couplings that allow a systematic comparison of dynamics between Ohmic and non-Ohmic regimes. Finally, we simulate the full non-Markovian dynamics of a spin in the semi-classical limit. At low temperatures, our numerical results demonstrate a characteristic reduction and flattening of the steady state spin alignment with an external field, caused by the quantum statistics of the environment. The results provide a powerful framework to explore general three-dimensional dissipation in quantum thermodynamics.
KW  - open quantum systems
KW  - coloured and quantum noise
KW  - memory effects
KW  - spin
KW  - dynamics
KW  - LLG equation
KW  - magnetisation
KW  - quantum thermodynamics
Y1  - 2022
U6  - https://doi.org/10.1088/1367-2630/ac4ef2
SN  - 1367-2630
VL  - 24
IS  - 3
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Koc, A.
A1  - Reinhardt, M.
A1  - Reppert, Alexander von
A1  - Rössle, Matthias
A1  - Leitenberger, Wolfram
A1  - Gleich, M.
A1  - Weinelt, M.
A1  - Zamponi, Flavio
A1  - Bargheer, Matias
T1  - Grueneisen-approach for the experimental determination of transient spin and phonon energies from ultrafast x-ray diffraction data: gadolinium
JF  - Journal of physics : Condensed matter
N2  - We study gadolinium thin films as a model system for ferromagnets with negative thermal expansion. Ultrashort laser pulses heat up the electronic subsystem and we follow the transient strain via ultrafast x-ray diffraction. In terms of a simple Grueneisen approach, the strain is decomposed into two contributions proportional to the thermal energy of spin and phonon subsystems. Our analysis reveals that upon femtosecond laser excitation, phonons and spins can be driven out of thermal equilibrium for several nanoseconds.
KW  - ultrafast
KW  - x-ray diffraction
KW  - magnetostriction
KW  - nonequilibrium
KW  - spin
KW  - phonon
KW  - rare earth
Y1  - 2017
U6  - https://doi.org/10.1088/1361-648X/aa7187
SN  - 0953-8984
SN  - 1361-648X
VL  - 29
SP  - 5884
EP  - 5891
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  -