arXiv subjects

Norman Kretzschmar

Publications and source records attributed to Norman Kretzschmar.

4 recordsLinked to original sources

arXiv2020

Collapse of layer dimerization in the photo-induced hidden state of 1T-TaS2

Photo-induced switching between collective quantum states of matter is a fascinating rising field with exciting opportunities for novel technologies. Presently very intensively studied examples in this regard are nanometer-thick single crystals of the layered material 1T-TaS2 , where picosecond laser pulses can trigger a fully reversible insulator-to-metal transition (IMT). This IMT is believed to be connected to the switching between metastable collective quantum states, but the microscopic nature of this so-called hidden quantum state remained largely elusive up to now. Here we determine the latter by means of state-of-the-art x-ray diffraction and show that the laser-driven IMT involves a marked rearrangement of the charge and orbital order in the direction perpendicular to the TaS2-layers. More specifically, we identify the collapse of inter-layer molecular orbital dimers, which are a characteristic feature of the insulating phase, as a key mechanism for the non-thermal IMT in 1T-TaS2, which indeed involves a collective transition between two truly long-range ordered electronic crystals.

cond-mat.str-el↗

arXiv2016

Analog Simulation of Weyl Particles with Cold Atoms

We study theoretically, numerically, and experimentally the relaxation of a collisionless gas in a quadrupole trap after a momentum kick. The non-separability of the potential enables a quasi thermalization of the single particle distribution function even in the absence of interactions. Suprinsingly, the dynamics features an effective decoupling between the strong trapping axis and the weak trapping plane. The energy delivered during the kick is redistributed according to the symmetries of the system and satisfies the Virial theorem, allowing for the prediction of the final temperatures. We show that this behaviour is formally equivalent to the relaxation of massless relativistic Weyl fermions after a sudden displacement from the center of a harmonic trap.

cond-mat.quant-gas↗

arXiv2015

Simultaneous sub-Doppler laser cooling of fermionic $^6$Li and $^{40}$K on the D$_1$ line: Theory and Experiment

We report on simultaneous sub-Doppler laser cooling of fermionic $^6$Li and $^{40}$K using the D$_1$ optical transitions. We compare experimental results to a numerical simulation of the cooling process applying a semi-classical Monte Carlo wavefunction method. The simulation takes into account the three dimensional optical molasses setup and the dipole interaction between atoms and the bichromatic light field driving the D$_1$ transitions. We discuss the physical mechanisms at play, we identify the important role of coherences between the ground state hyperfine levels and compare D$_1$ and D$_2$ sub-Doppler cooling. In 5 ms, the D$_1$ molasses phase largely reduces the temperature for both $^6$Li and $^{40}$K at the same time, with a final temperature of 44 $μ$K and 11 $μ$K, respectively. For both species this leads to a phase-space density close to $10^{-4}$. These conditions are well suited to directly load an optical or magnetic trap for efficient evaporative cooling to quantum degeneracy.

physics.atom-ph↗

arXiv2012

Sub-Doppler laser cooling of fermionic 40K atoms in three-dimensional gray optical molasses

We demonstrate sub-Doppler cooling of 40K on the D_1 atomic transition. Using a gray molasses scheme, we efficiently cool a compressed cloud of 6.5x10^8 atoms from ~ 4\mK to 20uK in 8 ms. After transfer in a quadrupole magnetic trap, we measure a phase space density of ~10^-5. This technique offers a promising route for fast evaporation of fermionic 40K.

cond-mat.quant-gas↗