title: Signature of a continuous quantum phase transition in non-equilibrium energy absorption: Footprints of criticality on higher excited states.
abstract: Understanding phase transitions in quantum matters constitutes a significant part of present day condensed matter physics. Quantum phase transitions concern ground state properties of many-body systems, and hence their signatures are expected to be pronounced in low-energy states. Here we report signature of a quantum critical point manifested in strongly out-of-equilibrium states with finite energy density with respect to the ground state and extensive (subsystem) entanglement entropy, generated by an external pulse. These non-equilibrium states are evidently completely disordered (e.g., paramagnetic in case of a magnetic ordering transition). The pulse is applied by switching a coupling of the Hamiltonian from an initial value (λI) to a final value (λF) for sufficiently long time and back again. The signature appears as non-analyticities (kinks) in the energy absorbed by the system from the pulse as a function of λF at critical-points (i.e., at values of λF corresponding to static critical-points of the system). As one excites higher and higher eigenstates of the final Hamiltonian H(λF) by increasing the pulse height (|λF - λI|), the non-analyticity grows stronger monotonically with it. This implies adding contributions from higher eigenstates help magnifying the non-analyticity, indicating strong imprint of the critical-point on them. Our findings are grounded on exact analytical results derived for Ising and XY chains in transverse field.
authors.affiliation: R.R.R. Mahavidyalaya, Radhanagar, Hooghly 712406, India.
authors.forename: Sirshendu
authors.initials: S
authors.lastname: Bhattacharyya
authors.affiliation: Department of Physics, University of Calcutta, 92 Acharya Prafulla Chandra Road, Kolkata 700009, India.
authors.forename: Subinay
authors.initials: S
authors.lastname: Dasgupta
authors.affiliation: Theoretical Physics Department, Indian Association for the Cultivation of Science, 2A &2B Raja S. C. Mullick Road, Kolkata - 700032, India.
authors.forename: Arnab
authors.initials: A
authors.lastname: Das