Following photoexcitation and initial relaxation phenomena on fs and ps timescales, the quasi-equilibrium is achieved, where the temperature is higher than the initial temperature, and its recovery is governed by heat diffusion. The increase in the sample temperature can be estimated based on the published values for the specific heat [Suz85] and the absorbed energy density, determined from the published optical properties of 1T-TaS2 [Bea75]. From the value of reflectivity at a wavelength of λ = 387 nm, R ∼20 %, and the optical pene-tration depth of ∼90 nm [Bea75] it follows that the absorbed energy density in our 30 nm thin film is∼ 170 (340) J/cm3 at a excitation fluence of F = 2.4 (4.8) mJ/cm2. With the specific heat,Cp∼1.85 J/cm3K, which is nearly temperature independent above 200 K, the resulting temperature increase is ∆T ≈90(180) K.
Appendix B.
Zusammenfassung
Die vorliegende Diplomarbeit beschäftigt sich mit der strukturellen Dynamik von Charge-Density-Wave (CDW) Systemen. Aufgrund ihrer quasi niedrigen Dimensionalität sind CDWs ideale Modellsysteme um die Wechselwirkungen von unterschiedlichen Freiheits-graden wie Spins, Elektronen, Gitter, etc. – welche bei makroskopischen Quantephänome-nen wie Hochtemperatursupraleitung und Riesenmagnetowiderstand auftreten – zu er-forschen. In dieser Hinsicht sind Femtosekunden (fs) zeitaufgelöste Techniken ideale Werk-zeuge um ultraschnelle Prozesse in besagten Materialen anzuregen und gleichzeitig die ver-schiedenen Relaxationswege und Wechselwirkungen zwischen verver-schiedenen Subsystemen nachzuverfolgen [Ave01, Oga05, Kus08]. Besonders 1T-TaS2 beherbergt eine Vielzahl von sog. korrelierten Phänomenen, angefangen vomMott-Isolator-Zustand [Tos76, Faz79] über Supraleitung unter Druck [Sip08, Liu09] bis hin zur Ausbildung unterschiedlich kommen-surabler CDWs [Wil75, Spi97].
Im Folgenden werden photoinduzierte transiente Änderungen der Reflektion und Trans-mission von 1T-TaS2 bei verschiedenen CDW Phasen präsentiert. Im Experiment wur-den unter anderem eine kohärente CDW Amplituwur-denmode und zwei Relaxationszeitskalen τfast ∼200 fs undτslow ∼4 ps beobachtet, welche typisch für CDW Systeme sind [Dem99].
Weiterhin wird die Frequenzverschiebung der Amplitudenmode und die Änderung der Relaxationszeitskalen beim Übergang von der kommensurablen zur fast-kommensurablen Phase diskutiert.
Eine Grundvoraussetzung für die direkte Untersuchung der strukturellen Dynamik durch fs Elektronenbeugung ist die Verfügbarkeit von Proben mit einer Dicke von einigen zehn Nanometern, gleichzeitig aber mit lateralen Abmessungen im Bereich von mehreren hun-dert Mikrometern. Die Herstellung von 1T-TaS2Proben mit einer Abmessung von 30 nm× 100µm×100µm wird ebenso vorgestellt wie deren Charakterisierung durch temperaturab-hängige Transmissionselektronenmikroskopie (TEM), Rasterelektronenmikroskopie (REM) und energiedispersive Röntgenspektroskopie (EDS).
Die Erzeugung von fs Elektronpulsen kann durch das kompakte Design einer Elektro-nenkanone ermöglicht werden [Siw04, Dwy05]. In dieser Arbeit wird die Entwicklung einer kompakten, rückseitig beleuchteten 30 kV Elektronenkanone beschrieben. Obwohl die voll-ständige Charakterisierung noch nicht abgeschlossen ist, werden die Haupteigenschaften der Konstruktion erläutert.
Schließlich werden direkte Messungen der Strukturdynamik von 1T-TaS2 durch fs Elek-tronenbeugung vorgestellt. Es zeigt sich, dass die periodische Gittermodulierung der CDW innerhalb von τmelt ≈ 170±40 fs kollabiert. Dies ist schneller ist als die Hälfte der ko-rrespondierenden Amplitudenmode und deutet daher auf einen elektronisch getriebenen Prozess hin. Der Energietransfer zu optischen Phononen findet innerhalb von τe-ph ≈ 350±50 fs statt. Die Relaxation der CDW passiert auf einer Zeitskala von τrec ∼ 4 ps, welche identisch zum rein optisch gemessenen Wert τslow ist. Die Relaxation des
Ord-nungsparamters in CDW Systemen läuft daher auf der Picosekunden Zeitskala ab. Bei Anregeflüssen, die vergleichbar sind mit der Energie um 1T-TaS2 in den kommensurablen CDW Zustand zu heizen, wird beobachtet, dass der Phasenübergang auf der sub ps Zeitskala stattfindet. Dieses Experiment zeigt, dass komplementäre Erkenntnisse zur Erforschung von komplexen Phänomenen – wie das Schmelzen und Relaxieren des Ordnungsparamters in CDW Materialen – durch fs Elektronenbeugung gewonnen werden können.
58
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