A unique insight into the dynamics of symmetry breaking (SB) can be obtained from real-time experiments on “freely evolving” phase transitions in condensed matter systems. Such experiments offer analogies with other time-evolving systems in cosmology and elementary particle collisions, and even in collective social phenomena. Charge ordered materials and superconductors are two examples where different symmetries are broken on femtosecond timescales which can be investigated with femtosecond laser spectroscopy techniques. The trajectories through the transition can be investigated via the response of single particle (fermionic) and collective (bosonic) excitations with very high temporal resolution. This enables us to observe new phenomena, such as coherent oscillations of the order parameter through the transition and dispersive Higgs-like particle emission resulting from domain wall annihilation. Weakly interacting massive particles (phonons) which interact with the order parameter offer analogies with dark matter existing prior to the transition. Beyond standard theory, non-equilibrium trajectories to hidden states of matter are also observed under conditions where the fermionic and bosonic excitations are far out of equilibrium, reaching states which are unreachable under adiabatic conditions.
B.04 Guest lecture
COBISS.SI-ID: 27822631The aim of the conference entitled “Photoinduced phase transitions and cooperative phenomena - 5” (PIPT5) was to bring together leading scientists in the field to present their most recent results, and through the exchange of ideas consider new challenges and develop new ideas in this rapidly developing field. Since the first PIPT meeting, the field has grown steadily and some very rapid developments have taken place in the last few years. As time-resolved spectroscopy has become mainstream and has been accepted and understood by the wider community, new opportunities arise. The recent rapid expansion of new experimental techniques on one hand, and new theory developments of nonequilibrium systems on the other, together with the discovery of new systems offering cooperative phenomena, the meeting was an exciting one. The main topics covered by the conference were: New phenomena in nonequilibrium collective behavior, New materials exhibiting photoinduced phase transitions and cooperative phenomena, New time-resolved experimental technologies for studies of nonequilibrium phenomena, Theoretical developments in nonequilibrium phase transitions.
B.02 Presiding over the programming board of a conference
COBISS.SI-ID: 27772967The recent discovery of a photo-induced, metastable state in TaS2 promises a deeper understanding of non-equilibrium phase changes in complex materials. Notably, this state can only be induced with a sub-ps laser pulse, which reversibly switches from the insulating equilibrium state to the conductive metastable state. Here, we employ angle-resolved photoemission spectroscopy (ARPES), femtosecond time-resolved ARPES and low-energy electron diffraction (LEED) to investigate this photo-induced state. ARPES evidences that the metastable state is characterized by an ungapped Fermi surface near the gamma-point. trARPES suggests that the transition is driven by a transient imbalance of electron and hole populations while LEED does not resolve dramatic structural changes. We discussed similarities and differences to known phases of TaS2.
B.04 Guest lecture
COBISS.SI-ID: 27774247