Kübler, Carl

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Kübler
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Carl
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Ultrafast insulator-metal phase transition in VO2 studied by multiterahertz spectroscopy

2011, Pashkin, Alexej, Kübler, Carl, Ehrke, Henri, Lopez, Ricardo, Halabica, Andrej, Haglund, Richard, Huber, Rupert, Leitenstorfer, Alfred

The ultrafast photoinduced insulator-metal transition in VO2 is studied at different temperatures and excitation fluences using multi-THz probe pulses. The spectrally resolved midinfrared response allows us to trace separately the dynamics of lattice and electronic degrees of freedom with a time resolution of 40 fs. The critical fluence of the optical pump pulse, which drives the system into a long-lived metallic state, is found to increase with decreasing temperature. Under all measurement conditions, we observe a modulation of the eigenfrequencies of the optical phonon modes induced by their anharmonic coupling to the coherent wave-packet motion of V-V dimers at 6.1 THz. Furthermore, we find a weak quadratic coupling of the electronic response to the coherent dimer oscillation resulting in a modulation of the electronic conductivity at twice the frequency of the wave-packet motion. The findings are discussed in the framework of a qualitative model based on an approximation of local photoexcitation of the vanadium dimers from the insulating state.

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Coherent structural dynamics and electronic correlations during an ultrafast insulator-to-metal phase transition in VO2

2007, Kübler, Carl, Ehrke, Henri, Lopez, Rene, Halabica, Andrej, Haglund, Richard F., Leitenstorfer, Alfred

We directly trace the multi-THz conductivity of VO2 during an insulator-metal transition triggered by a 12-fs light pulse. The femtosecond dynamics of lattice and electronic degrees of freedom are spectrally discriminated. A coherent wave packet motion of V-V dimers at 6 THz modulates the lattice polarizability for approximately 1 ps. In contrast, the electronic conductivity settles to a constant value already after one V-V oscillation cycle. Based on our findings, we propose a qualitative model for the nonthermal phase transition.

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THz Slow Motion of an Ultrafast Insulator-Metal Transition in VO2 : Coherent Structural Dynamics and Electronic Correlations

2009, Huber, Rupert, Kübler, Carl, Ehrke, Henri, Lopez, Ricardo, Halabica, Andrej, Haglund, Richard F., Leitenstorfer, Alfred

The multi-THz conductivity of VO2 recorded during a photoinduced insulator-metal transition directly reveals the femtosecond dynamics of V-V stretching modes and electronic correlations. We suggest a novel qualitative model for the nonthermal phase transition.

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Femtosecond terahertz studies of many-body correlations : from ultrafast phonon-plasmon dynamics to an insulator- metal transition

2007, Kübler, Carl

Ziel der vorliegenden Arbeit war die experimentelle Untersuchung von Vielteilchen-Korrelationen auf ultrakurzen Zeitskalen. Folgende beispielhafte und repräsentative Szenarien wurden mittels zeitaufgelöster ultrabreitbandiger THz-Spektroskopie studiert: - Im polaren Halbleitermaterial Indiumphosphid (InP), einem idealen Laborsystem dessen Grundzustand sehr gut verstanden ist, wurde der Aufbau von Korrelationen zwischen elektronischen und Gitterfreiheitsgraden in Echtzeit verfolgt. - In dem stark korrelierten Materialsystem Vanadiumdioxid (VO_{2}) wurde das Wechselspiel zwischen Gitterschwingungen und elektronischen Korrelationen während eines ultraschnellen photoinduzierten Isolator-Metall Übergangs beobachtet, in einem Regime jenseits der Gültigkeit der Born-Oppenheimer Näherung.

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Ultrafast insulator-metal transition in VO2 : interplay between coherent lattice motion and electronic correlations

2009, Leitenstorfer, Alfred, Kübler, Carl, Lopez, Rene, Halabica, Andrej, Haglund, Richard F., Huber, Rupert

The multi-THz conductivity of VO2 recorded during a photoinduced insulator-metal transition directly reveals the femtosecond dynamics of V-V stretching modes and electronic correlations. The strongly correlated character of the electronic system leads to a transient behaviour of the conductivity that cannot be understood in a Born-Oppenheimer picture. Based on our measurements with a time resolution of 12 fs and recent theoretical simulations for the electronic structure in steady state, we suggest a novel qualitative model for the non-thermal phase transition. The extremely high speed for the photoinduced phase transition is elegantly explained via a directed wave packet motion in the electronically excited state of the dimer that transfers the crystal structure of the insulator into the configuration of the metallic phase.