Dr Moritz Hoesch - Giant Kohn Anomaly and the phase transition in charge-density-wave ZrTe3
09 Mar 2010
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Diamond Light Source

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The Charge Density Wave (CDW) in ZrTe3 is driven by a Fermi-surface nesting of the quasi 1-dimensional Fermi surface sheet of the Te p_x orbitals. The material is made of prismatic ZrTe_3 chains that are laterally linked through Te p-bonding. The CDW develops below T_CDW = 63 K with a modulation vector q_CDW in the a*-c*-plane perpendicular to the prismatic chains.

In this seminar I will present lattice dynamics data from inelastic x-ray scattering (IXS) that show the soft mode mechanism of the charge density wave and reveal the thermodynamics of the phase transition. This first observation of the phonon softening in disagreement to mean field theory is complemented by diffuse x-ray scattering data that show the phase transition to be compatible with an order parameter of dimension n = 2 [1].

Cleavage of the crystal exposes well-ordered surfaces in the a-b plane. The opening of a gap in the electronic structure was previously observed by angle resolved photoemission (ARPES) and the full temperature dependence was discussed to be compatible with a BCS model [2]. Our new high-resolution ARPES data show all bands split in two. A comparison with first principles scalar-relativistic band structure calculations shows that the observed electronic structure is not fully compatible with the expectation from bulk ZrTe_3 [3]. Rather, the surface relaxation gives rise to a subtly modified behaviour in the top layer. The splitting arises in the bi-layer of the topmost (ZrTe3)_3 chains. This leads to the interesting conclusion that controlled modifications of the CDW system are possible at the surface and their properties can be studied by surface sensitive techniques.

[1] M. Hoesch et al. Phys. Rev. Lett. 102 (2009) 086402.

[2] T. Yokoya et al., Phys. Rev. B 71 (2005) 140504(R).

[3] M. Hoesch et al., Phys. Rev. B 80, 075423, 2009.

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