Insulator-to-metal transition in NbO2 on ultrafast time scales

• Applicant: Dr. Marc Herzog
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 524986497

The transition-metal oxide Niobium dioxide (NbO2) is very closely related to Vanadium dioxide (VO2), which is intensively investigated since decades due to its insulator-metal transition (IMT). In coexistence with a structural phase transition, NbO2 also exhibits an IMT, however, at a much higher phase transition temperature of T_C=1080 K, which is advantageous for certain technological applications. In analogy to VO2, the physical mechanism responsible for these effects in NbO2 is highly debated. Electron correlations (Mott mechanism) or the specific lattice structure (Peierls mechanism) are particularly relevant in stabilizing the insulating phase. In the past, time-resolved experiments after laser pulse excitation have proven to be excellent means in order to address this kind of scientific questions. The proposed project aims at revealing the physical nature of the IMT in NbO2 by means of various ultrafast techniques. In order to be sensitive to the different subsystems, the experimental methods are divided into two categories. The laser-induced changes of the electronic band structure during the IMT will be investigated for the first time using optical pump-probe spectroscopy with broadband visible laser pulses as well as pulses in the near- and mid-infrared spectral range. Due to a photon energy smaller than the insulator band gap, the latter is particularly sensitive to the collapse of the band gap during the ultrafast laser-induced IMT. The optical spectroscopy measurements will be complemented by time-resolved x-ray diffraction. Such experiments have never been reported on NbO2. They will shed light on the dynamical processes regarding the structural phase transition in NbO2, which occurs concomitantly at the transition temperature T_C of the IMT indicating a strong coupling between them. Specifically, the investigation of the structural component of the phase transition is realized by time-resolved observation of the crystal symmetry, the crystallographic unit cell as well as acoustic and optical phonons. In addition, time-resolved measurements of the expansion dynamics yield information about the thermal conductivity of NbO2 across its IMT, which is crucial to infer whether NbO2 may be a suitable material to be used for functional and switchable heat transport. Moreover, the project will investigate if and how the negative thermal expansion (NTE) of NbO2 found at thermal equilibrium manifests on ultrafast time scales (picoseconds). Such an ultrafast contraction would offer completely new means of generating unconventional hypersonic wavepackets, which can serve as elastic stimulus in magnetically ordered or nonlinear materials.

• DFG Programme: Research Grants