As a step towards resolving the spatially-resolved electronic structure of the layered transition metal dichalcogenide 1T-TaS2 – and eventually electronically switched cryomemory devices – we have performed angle-resolved photoemission spectroscopy (ARPES). Using a photon energy of 72 eV and the micrometer spot size available at the spectromicroscopy beamline of the Elettra synchrotron, we measured the band structure and high-statistics Fermi surface of 11, 500 and 600 nm thick flakes. Also, the first-order phase transition from the nearly-commensurate to the commensurate CDW state leads to a prominent splitting of the Ta 4f core levels which we have mapped spatially. In addition, using 400 eV soft X-ray ARPES at the ADRESS beamline of the Swiss Light Source synchrotron, we established the kz dependence of the band structure of a 110-nm thick flake, which reveals a two dimensionality of the electronic structure.
It is an outstanding goal to unveil the fingerprints of universal quantum dynamics at eigenstate transitions. Focusing on quadratic fermionic Hamiltonians, we identify physical observables that exhibit critical behavior at the transition. Our result is based on two ingredients: (a) A relationship between the observable time evolution in a many-body state and the transition probabilities in single-particle states, and (b) a scale invariance of transition probabilities, which generalizes the recent result for survival probabilities. We then show that these properties give rise to a critical behavior in the quantum quench dynamics of observables, which share the common eigenbasis with the Hamiltonian before the quench. We numerically demonstrate this phenomenon at the localization transition in the three-dimensional Anderson model, for which the critical bahavior can be detected in experimentally relevant observables such as site occupations and particle imbalance.
Laser-induced melting plays a crucial role in advancing manufacturing technology and ultrafast science; however, its atomic processes and microscopic mechanisms remain elusive due to complex interplays between many degrees of freedom within a timescale of ~100 femtoseconds. We employ MgO as an example to investigate the nonequilibrium mechanisms of laser-driven ultrafast nonthermal melting of wide-gap materials. Our study is based on real-time time-dependent density functional theory (rt-TDDFT) molecular dynamics (MD) simulations. We report here that laser melting is greatly accelerated by tunnel ionization processes. The tunneling processes generate a large number of photocarriers and results in intense energy absorption, instantaneously changing the potential energy surface of lattice configuration. The strong electron-phonon couplings and fast carrier relaxation enable the efficient energy transfer between electrons and the lattice. The modulation of melting thresholds and phase boundary demonstrate the possibility of manipulating phase transition on demand. A shock wave curve is also obtained at moderate conditions (P < 50 GPa), extending Hugoniot curve to new regimes. These results account well for the latest ultrafast melting experiments, and provide atomistic details and nonequilibrium mechanism of photoinduced ultrafast phase transitions in wide-gap materials.
