Dr. Jan Ravnik

Jan Ravnik
Paul Scherrer Institut
Forschungsstrasse 111
5232 Villigen PSI

Jan Ravnik is a postdoctoral fellow on a Marie Curie cofound fellowship. He was born in Ljubljana, Slovenia. He did his B.Sc, M.Sc and his PhD in physics at the Faculty of Mathematics and Physics at the University of Ljubljana. As a masters and a PhD student he was doing his research work in the group of prof. dr. Dragan D. Mihailovic at the Jozef Stefan Institute in Ljubljana. His research started by doing ultrafast optical experiments, where he measured the phonon footprints during an insulator to metal transition in 1T-TaS2. Later, he also built a mathematical model to describe the observed phase transition. During his PhD, he got experience in using a four probe scanning tunneling microscope, coupled with an optical laser system and with a self-built THz pulse system to examine photoinduced phase transitions in correlated materials.

The research work of Jan Ravnik is focused on production and testing of nanoscale electronic switching elements and superconducting circuits. He is using the knowledge of ultrafast phase transitions in correlated materials, to make electronic cryogenic memory devices, based on an insulator to metal phase transition in 1T-TaS2. The material can be switched to the metallic stat optically and electrically, which is very convenient to perform measurements on different timescales. Jan Ravnik is also working with superconducting thin films, nanowires and three-terminal superconducting devices. He is using the available X-ray facilities to characterize the phase transitions and get a deeper insight into the fundamental physical mechanisms that drive them.

A time-domain phase diagram of metastable states in a charge ordered quantum material. RAVNIK, Jan, DIEGO, Michele, GERASIMENKO, Yaroslav, VASKIVSKYI, Yevhenii, VASKIVSKYI, Igor, MERTELJ, Tomaž, VODEB, Jaka, MIHAILOVIĆ, Dragan. Nature Communications, 2021, 12, doi: 10.1038/s41467-021-22646-7.
A number of metastable electronically ordered quantum states were recently reported in the prototypical layered dichalcogenide 1T-TaS2, created by either current injection in devices or by optical excitation. This paper presents a temporally-evolving phase diagram of the metastable states reached by photoexcitation at different laser fluences and different temperatures. Phase diagrams in general are considered to represent the phases in equilibrium. Presenting the phase diagram for metastable states introduces a time axis, which requires measurements on different timescales. This paper presents such a ‘dynamical’ phase diagram spanning between 10-12 s and 103 s, defined by the timescales of the measurement techniques, namely femtosecond spectroscopy and scanning tunnelling microscopy respectively. Beside the conceptual novelty, the results show how the phase boundaries evolve with time, and how the excitation conditions influence long-term metastability, thus opening the way to the development of new metastable quantum materials.

Quantum jamming transition to a correlated electron glass in 1T-TaS2. GERASIMENKO, Yaroslav, VASKIVSKYI, Igor, LITSKEVICH, Maksim, RAVNIK, Jan, VODEB, Jaka, DIEGO, Michele, KABANOV, Viktor V., MIHAILOVIĆ, Dragan. Nature materials, 2019, 11, doi: 10.1038/s41563-019-0423-3.
Controlled optical or electromagnetic perturbations can lead to an amorphous metastable state of strongly correlated electrons in a quasi-two-dimensional dichalcogenide. Scanning tunnelling microscopy reveals a hyperuniform pattern of localized charges, whereas multitip surface nanoscale conductivity measurements and tunneling spectroscopy show a conducting state that is different from conventional Coulomb glasses and manybody localized systems. The state is stable up to room temperature and shows no signs of either local charge order or phase separation. The mechanism for its formation is attributed to a localization of electrons through mutual interactions. Theoretical calculations confirm the correlations between localized charges to be crucial for the state’s unusual stability.

Real-time observation of the coherent transition to a metastable emergent state in 1T-TaS2. RAVNIK, Jan, VASKIVSKYI, Igor, MERTELJ, Tomaž, MIHAILOVIĆ, Dragan.Physical review. B, 2018, vol. 97, no. 7, 075304-1-075304-7, doi: 10.1103/PhysRevB.97.075304.
Coherent three-pulse time-resolved femtosecond spectroscopy for the first time shows the transition to a hidden metastable state in 1T -TaS2 in real time. Relying on spectral differences between phonon modes in the equilibrium states and in themetastable state, and temperature-tuning themetastable state lifetime, we perform stroboscopicmeasurements of the electronic response and switching of coherent oscillation frequency through the transition. Very fast coherent switching of the collective-mode frequency is observed (within one phonon oscillation period 400 fs), comparable to the electronic time scale300 fs. A slower, 4.7-ps process is attributed to lattice relaxation. The observations are described well by a fast electronic band structure transformation into the metastable state, consistent with a topological transition.

Strain-induced metastable topological networks in laser-fabricated TaS2 polytype heterostructures for nanoscale devices. RAVNIK, Jan, VASKIVSKYI, Igor, GERASIMENKO, Yaroslav, DIEGO, Michele, KABANOV, Viktor V., MIHAILOVIĆ, Dragan.  ACS applied nano materials, 2019, vol. 2, no. 6, 3743-3751, doi:10.1021/acsanm.9b00644.
The stacking of layered materials into heterostructures offers diverse possibilities for generating deformed Moiré states arising from their mutual interaction. Here we report two-dimensional nanoscale strain networks formed within a single prismatic (H) polytype monolayer of TaS2 created in-situ on the surface of an orthorhombic 1T-TaS2 single crystal by a low-temperature laser-induced polytype transformation. The networks revealed by scanning tunneling microscopy (STM) take on diverse configurations at different temperatures, including extensive double stripes and a twisted 3-gonal mesh of connected 6-pronged vertices.

Configurational electronic states in layered transition metal dichalcogenides. VODEB, Jaka, KABANOV, Viktor V., GERASIMENKO, Yaroslav, VENTURINI, Rok, RAVNIK, Jan, VAN MIDDEN, Marion, ZUPANIČ, Erik, ŠUTAR, Petra, MIHAILOVIĆ, Dragan.  New journal of physics, 2019, vol. 21, 083001-1-083001-15, doi:10.1088/1367-2630/ab3057.

Molybdenum carbide nanowires : facile synthesis, a new hybrid phase and their use as transparent electrodes. VENGUST, Damjan, RAVNIK, Jan, MRZEL, Aleš, VILFAN, Mojca. RSC advances, 2016, vol. 6, no. 93, 90806-90812, doi: 10.1039/C6RA18650E.

Surface decoration of MoSI nanowires and MoS2 multi-wall nanotubes and platinum nanoparticle encapsulation. KOVIČ, Andrej, MRZEL, Aleš, RAVNIK, Jan, ŠTURM, Sašo, VILFAN, Mojca. Materials letters, 2015, vol. 159, 333-336, doi: 10.1016/j.matlet.2015.07.021.