Nonequilibrium Quantum Magnetism
Quantum Magnetophononics
Quantum spin systems are one of the best theoretical platforms for investigating many-body states with highly nontrivial entanglement properties. Experiments in quantum materials offer the cleanest realisations of these models on macroscopic lengthscales. With the advent of powerful laser sources able to access all the energy scales of interest in condensed matter, nonequilibrium many-body physics has become an experimental science. However, applying these sources to insulating quantum magnets requires a suitable coupling channel: the magnetic field of the light is very weak, direct photon-to-spin excitations are also weak (they depend on anisotropies in the spin Hamiltonian) and Raman-type processes are difficult to control. We have chosen to focus on exciting the spin sector through the lattice excitations -- magnetophononics -- which is the most versatile and frequency-selective mechanism available.
Driven Dissipative Systems
Most theoretical studies to date have focused on the intrinsic dynamics in the presence of the light field. Even when this provides a constant driving, as in the case of Floquet physics, little attention has been paid to the damping, and thus these studies are overlooking a steady build-up of energy and increase in the system temperature. In order to describe these aspects appropriately, we use the Lindblad formalism to model the dissipative terms acting on the system and hence to access the energy flow at each stage of the driving and damping process. In this framework one has quantum master equations describing the time-evolution of the expectation values of the quantum operators.
Most theoretical studies to date have focused on the intrinsic dynamics in the presence of the light field. Even when this provides a constant driving, as in the case of Floquet physics, little attention has been paid to the damping, and thus these studies are overlooking a steady build-up of energy and increase in the system temperature. In order to describe these aspects appropriately, we use the Lindblad formalism to model the dissipative terms acting on the system and hence to access the energy flow at each stage of the driving and damping process. In this framework one has quantum master equations describing the time-evolution of the expectation values of the quantum operators.
Nonequilibrium Steady States (NESS)
We investigated the NESS of the spin system established by a continuous driving, delineating parameter regimes in driving frequency, damping and spin-phonon coupling for the establishment of physically meaningful NESS and their related non-trivial properties. Focusing on the regime of generic weak spin-phonon coupling, we characterised the NESS by their frequency and wave-vector content, explored their transient and relaxation behavior, and discussed the energy flow, the system temperature, and the critical role of the type of bath adopted [1]. This was our foundation study to set the framework for more complicated and realistic modelling, including of different types of spin system, different types of bath, strong spin-phonon coupling, pulsed driving and ultimately the quantitative modelling of experiments currently being designed to control coherent many-body spin states in quantum magnetic materials.
We investigated the NESS of the spin system established by a continuous driving, delineating parameter regimes in driving frequency, damping and spin-phonon coupling for the establishment of physically meaningful NESS and their related non-trivial properties. Focusing on the regime of generic weak spin-phonon coupling, we characterised the NESS by their frequency and wave-vector content, explored their transient and relaxation behavior, and discussed the energy flow, the system temperature, and the critical role of the type of bath adopted [1]. This was our foundation study to set the framework for more complicated and realistic modelling, including of different types of spin system, different types of bath, strong spin-phonon coupling, pulsed driving and ultimately the quantitative modelling of experiments currently being designed to control coherent many-body spin states in quantum magnetic materials.
Nonlinear Quantum Magnetophononics
We extended the magnetophononics concept to control a collective quantum spin state by using coherent terahertz (THz) pulses to drive infrared (IR)-active phonons in the quantum antiferromagnet SrCu2(BO3)2. We demonstrated experimentally how the coherent lattice displacements caused by two IR-active phonons at 3.78 and 4.60 THz create a non-equilibrium population of the S = 0 component of the characteristic two-triplon bound-state in this material [2]. We established the theoretical framework for modulation of the magnetic interactions by nonlinear mixing of harmonic phonons (here a difference frequency) and the consequent breaking of perfect frustration in the driven lattice structure that allows two-triplon creation, which we verified by density functional theory (DFT) calculations.
We extended the magnetophononics concept to control a collective quantum spin state by using coherent terahertz (THz) pulses to drive infrared (IR)-active phonons in the quantum antiferromagnet SrCu2(BO3)2. We demonstrated experimentally how the coherent lattice displacements caused by two IR-active phonons at 3.78 and 4.60 THz create a non-equilibrium population of the S = 0 component of the characteristic two-triplon bound-state in this material [2]. We established the theoretical framework for modulation of the magnetic interactions by nonlinear mixing of harmonic phonons (here a difference frequency) and the consequent breaking of perfect frustration in the driven lattice structure that allows two-triplon creation, which we verified by density functional theory (DFT) calculations.
[1] Dynamical Properties of a Driven Dissipative Dimerized S = 1/2 Chain,
M. Yarmohammadi, C. Meyer, B. Fauseweh, B. Normand and G. S. Uhrig,
unpublished (arXiv:2009.14805).
[2] Nonlinear Quantum Magnetophononics in SrCu2(BO3)2,
F. Giorgianni, B. Wehinger, S. Allenspach, N. Colonna, C. Vicario, E. Pomjakushina, P. Puphal,
B. Normand and Ch. Rüegg,
unpublished.
M. Yarmohammadi, C. Meyer, B. Fauseweh, B. Normand and G. S. Uhrig,
unpublished (arXiv:2009.14805).
[2] Nonlinear Quantum Magnetophononics in SrCu2(BO3)2,
F. Giorgianni, B. Wehinger, S. Allenspach, N. Colonna, C. Vicario, E. Pomjakushina, P. Puphal,
B. Normand and Ch. Rüegg,
unpublished.