FORUM DQMP

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Day / Time
Date(s) - 24/01/2017
13 h 00 min - 16 h 00 min

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Tuesday, January 24th 2017 – 13h00
Auditoire Stückelberg
Ecole de physique

Coffee and tea will be available from
12h50 at the entrance of the Auditoire

New insight on the CDW phase of 1T-CuxTiSe2
Marcello Spera (group of Prof. Renner)

The origin of the charge density wave (CDW) phase in 1T-TiSe2 has been matter of debate since its discovery in 1976. Despite the large amount of data available, it is still unclear whether the origin of this phase is purely electronic, elastic mechanisms or a combination thereof. The appearance of superconductivity upon Cu intercalation (2006) and with high pressure (2009) triggered further interest in understanding the microscopic mechanisms governing the CDW ground state of this material, seemingly in competition with superconductivity.
We study the impact of Cu intercalation on the CDW in 1T-CuxTiSe2 by scanning tunneling microscopy and spectroscopy. Cu atoms, identified through density functional theory modeling, are found to intercalate randomly on the octahedral site in the van der Waals gap and to dope delocalized electrons near the Fermi level. While the CDW modulation period does not depend on Cu content, we observe the
formation of charge stripe domains at low Cu content (x<0.02) and a breaking up of the commensurate order into 2×2 domains at higher Cu content. The latter shrink with increasing Cu concentration and tend to be phase shifted. These findings invalidate a nesting scenario and a proposed excitonic pairing as the primary CDW formation mechanism in this material. New STM imaging shows a very strong energy
dependence of the CDW pattern that can be understood in terms of a local chemical potential shift due to electrons doped by Cu. This data questions scenarios of competing CDW and superconductivity.

Disentangling the surface and bulk contributions of massless carriers in Cd3As2
Ana Akrap (group of Prof. van der Marel)

Cadmium arsenide has recently been identified as a 3D Dirac semimetal, with two separated Dirac nodes. While the available ARPES data shows that these cones extend over several hundreds of meV, our magneto-optical spectra up to 33 T imply that the energy range of Dirac cones is an order of magnitude smaller. We clearly observe massless carriers, but these are not Dirac-like.
We study the infrared properties of several single crystals with different doping levels and crystal orientations. Our spectra imply that two contributions are present in their optical conductivity, both in zero and finite magnetic fields. Contrary to a recent report showing Fermi arcs in Cd3As2, we propose that these two contributions originate from the bulk of the crystal, as well as a thin depletion layer on the surface.
Forum Committee : C.Lichtensteiger, A.Tamai, N.Ubrig (14.11.2016)

Réalisation : Sur Mesure concept