Non-equilibrium structural Dynamics of incommensurate Charge-Density Waves

Diffractive Probing with a micron-scale ultrafast Electron Gun

by Gero Storeck
Doctoral thesis
Date of Examination:2020-06-12
Date of issue:2020-07-01
Advisor:Prof. Dr. Claus Ropers
Referee:Prof. Dr. Claus Ropers
Referee:Prof. Dr. Stefan Mathias
Persistent Address: http://dx.doi.org/10.53846/goediss-8062

 

 

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Abstract

English

In recent years, charge-density wave (CDW) systems have been studied extensively, as they provide a diverse testing field for basic concepts in electron-phonon coupling, electron correlation, and structural phase transitions. In particular, time-resolved techniques have participated in that process, disentangling the dynamics of the various degrees of freedoms in such complex materials. As a recently developed pump-probe technique, ultrafast low-energy electron diffraction provides complementary insight into the CDW-coupled structural dynamics at the surface. This cumulative thesis covers the investigation of the incommensurate CDWs phases in layered tantalum disulfide, employing a new miniaturized electron gun in the ULEED setup. In a first study, the design and fabrication process of the miniaturized electron gun are described. Finite element modeling supports the design process and provides helpful insight into the performance of the device and estimates for voltages as well as pulse duration. Photolithography and focused-ion-beam etching were used for building a contact support and the gun assembly, including the nanotip emitter, lens electrodes and the shielding. The pulse duration and transverse beam quality were extracted using the transient electric field effect at a copper grid and static diffraction patterns, respectively. In a second study, the structural dynamics in the incommensurate and nearly commensurate CDW phase of tantalum disulfide were investigated employing 1 ps temporal resolution. The diffraction intensities of main lattice spots and CDW satellites, as well as the diffuse background, indicate a multi-step relaxation process. The comparison of different groups of diffraction spots allowed to correct for the phonon-related reductions, yielding the CDW-associated periodic lattice distortion (PLD). The persistent reduction of the PLD amplitude and fluence-dependent relaxation cycles reveal a structural non-equilibrium situation exhibiting time constants exceeding typical phonon equilibration times. This is discussed in the context of hot populations of CDW excitation modes. Satellite spot broadening at the highest fluence points to the creation of CDW dislocation defects.
Keywords: ultrafast low-energy electron diffraction; ULEED; structural dynamics; transition metal dichalcogenide; charge-density wave; incommensurateness; 1T-TaS2; miniaturized electron gun
 
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