Phase transitions in transition metal dichalcogenides studied by femtosecond electron diffraction

[摘要] ENGLISH ABSTRACT: Low-dimensional materials are known to undergo phase transitions to differently or-dered states, when cooled to lower temperatures. These phases often show a periodicmodulation of the charge density (called a charge density wave – CDW) coupled witha periodic perturbation of the crystal lattice (called a periodic lattice distortion – PLD).Although many experiments have been performed and much has been learnt aboutCDW phases in low-dimensional materials, the reasons for their existence are stillnot fully understood yet. Many processes, involving either strong electron–electron orelectron–lattice coupling, have been observed which all might play a role in explainingthe formation of different phases under different conditions.With the availability of femtosecond lasers it has become possible to study materialsunder highly nonequilibrium conditions. By suddenly introducing a known amountof energy into the system, the equilibrium state is disturbed and the subsequent relax-ation processes are then observed on timescales of structural and electronic responses.These experiments can deliver valuable information about the complex interactionsbetween the different constituents of condensed matter, which would be inaccessibleunder equilibrium conditions.We use time resolved electron diffraction to investigate the behaviour of a CDWsystem perturbed by a short laser pulse. From the observed changes in the diffractionpatterns we can directly deduce changes in the lattice structure of our sample.A femtosecond electron diffraction setup was developed at the Laser Research In-stitute in Stellenbosch, South Africa. Short laser pulses produce photo electrons whichare accelerated to an energy of 30 keV. Despite space charge broadening effects, elec-tron pulses shorter than 500 fs at sample position can be achieved. Technical detailsof this system and its characterisation as well as sample preparation techniques andanalysis methods are described in detail in this work.Measurements on two members of the quasi-two-dimensional transition metal di-chalcogenides, namely 4Hb-TaSe2 and 1T-TaS2, are shown and discussed. Both showfast (subpicosecond) changes due to the suppression of the PLD and a rapid heatingof the lattice. When the induced temperature rise heats the sample above a phase tran-sition temperature, a complete transformation into the new phase was observed. For4Hb-TaSe2 we found that the recovery to the original state is significantly slower ifthe PLD was completely suppressed compared to only disturbing it. On 1T-TaS2 wecould not only study the suppression of the original phase but also the formation ofthe higher energetic CDW phase. Long (100 ps) time constants were found for the tran-sition between the two phases. These suggest the presence of an energy barrier whichhas to be overcome in order to change the CDW phase. Pinning of the CDW by de-fects in the crystal structure result in such an energy barrier and consequently lead toa phase of domain growth which is considerably slower than pure electron or latticedynamics.

[发布机构] Stellenbosch University