Microscopic force driving the photoinduced ultrafast phase transition: Time-dependent density functional theory simulations of IrTe2

Abstract

Photoinduced phase transitions can have complex and intriguing behaviors more than material ground-state dynamics. Understanding the underlying mechanism can help us to design new ways to manipulate the materials. A variety of mechanisms has been proposed to explain the photoinduced phase transitions of IrTe 2 ,but a consensus has yet to be reached. Here, we study the photo-induced phase transitions of IrTe 2 by performing the real-time time-dependent density functional theory (rt-TDDFT) simulations in combination with the occupation constrained DFT method. We reveal that the microscopic driving force for the photo-induced phase transitions arises from the tendency to lower the energy levels occupied by the excited carriers, through the increase or decrease of the associated atomic pair distances, depending on whether the newly occupied states are antibonding or bonding states, respectively. The geometric constraints between different bonds represented by the Poisson ratio can bring together different tendencies from different atomic pairs, thus forming a complex intriguing dynamic picture depending on the intensity of the excitation. We also find that phonons don't play a primary role, but can assist the phase transition. These findings imply that one can control the structural phase transitions by selectively exciting photocarriers into designated atomic states using appropriate photon sources.

Break of symmetry at the surface of IrTe$_2$ upon phase transition measured by X-ray photoelectron diffraction

“…This displacement leads to an apparent local disordered environment for the dimerised Ir atoms, resulting in XPD diffractograms with broad intensities. In addition, our XPD measurements confirm that Te atoms also undergo large displacements relative to the Ir emitter, suggesting their participation in the stabilisation of the Ir dimers, as also established using DFT calculations [21,35] and experimental investigations [19,35]. The simulated diffractograms permit to estimate these displacements with a accuracy of about 5% of the lattice parameters.…”

The critical role of hot carrier cooling in optically excited structural transitions

“…5,7,9,25 An alternative, yet related, the picture is that the transient change in the PESs results in a non-thermal excitation of soft phonon modes, which leads to a critically damped nuclear motion following these soft phonon modes to the end phase of the PIPT. 5,25 Our previous work 26 has also pointed out that the atomic forces for driving the PIPT in IrTe2 arising from occupation of the Ir-Ir dimer antibonding (bonding) states by optically excited electrons (holes). 26 All these proposed explanations consider the photoexcited carriers as the cause for the change of PESs or the generation of additional atomic driving forces but disregard completely the phenomena of their relaxation to lower energy levels that occurred within the first hundred femtoseconds after photoexcitation.…”

“…5,25 Our previous work 26 has also pointed out that the atomic forces for driving the PIPT in IrTe2 arising from occupation of the Ir-Ir dimer antibonding (bonding) states by optically excited electrons (holes). 26 All these proposed explanations consider the photoexcited carriers as the cause for the change of PESs or the generation of additional atomic driving forces but disregard completely the phenomena of their relaxation to lower energy levels that occurred within the first hundred femtoseconds after photoexcitation.…”

“…8 We have also demonstrated that one can control the structural phase transitions by selectively exciting photocarriers into designated excited electronic states. 26 The hot carrier cooling will undoubtedly alter the carrier's occupation to the excited states, thus changing the atomic driving forces. Besides, there are many possible effects in a PIPT, including electronelectron and electron-phonon interactions and thermal fluctuations.…”

“…An improvement has been made in some recent rt-TDDFT simulations using a Gaussianenvelope laser pulse to represent the actual laser light. 24,26,29 But the use of Ehrenfest dynamics in the rt-TDDFT is unlikely to describe the carrier cooling correctly since it lacks the detailed balance. As a matter of fact, the Ehrenfest dynamics tend to overheat the electronic subsystem.…”