Owen Moulding and Sven Friedemann, University of Bristol.
TiSe2 features a charge density wave (CDW) driven by condensation of excitons, i.e., pairs of electrons and holes, alongside electronphonon coupling. The CDW transition at 202 K gaps out most of the Fermi surface. Quantum-oscillation measurements at the HFML Nijmegen provide a clear view of the newly formed Fermi surface inside the CDW state. Above the CDW transition, the Fermi surface consists of a hole-like cylindrical pocket and an electron-like distorted and tilted ellipsoidal pocket. The formation of electron-hole pairs gaps out the complete hole pocket but leaves a small part of the electron states. Indeed, our quantum-oscillation measurements show that the low-temperature state contains a single electron-like ellipsoid without tilt.
Identifying the size and shape of the Fermi surface was done with angular-dependent measurements in magnetic fields up to 35 T and using a rotator probe inside a 3-He cryostat at the HFML, Nijmegen. The increase of the quantum-oscillation frequency shows that the Fermi surface is approximately ellipsoid, and the semi-axes have been extracted from the data (Figure). Together with the effective mass determined from the temperature dependence of the quantumoscillation amplitude, this allows detailed comparison with specific heat, ARPES, and our 2-band transport analysis which shows that this is the only pocket. This knowledge of the Fermi surface of TiSe2 enabled a study across the Fermi-surface reconstruction at the CDW transition. The team of Dr. Friedemann analyzed magnetoresistance and Hall resistivity measurements performed at the University of Bristol to trace the elimination of the hole pocket and the shrinkage of the electron pocket. They found that electron scattering is maximum right at the CDW transition and, thus, probably driven by the CDW fluctuations and highlights that the CDW is dominating the electronic properties of TiSe2.
Figure: Highest resolution in the quantum oscillations measurements were realized with samples optimized for large resistance signal and using low-noise transformers. (a) The oscillations show a single frequency in TiSe2 at lowest temperatures. (b) From the temperature dependence of the amplitude the effective electron mass was extracted. (c) The angular dependence of the quantumoscillation frequency is fitted by an ellipsoid model (red line).
Fermi Surface Reconstruction and Electron Dynamics at the Charge-Density-Wave Transition in TiSe2, P. Knowles, B. Yang, T. Muramatsu,
O. Moulding, J. Buhot, C. J. Sayers, E. Da Como, and S. Friedemann, Phys. Rev. Lett. 124, 167602 (2020).