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Acta Crystallogr B Struct Sci Cryst Eng Mater ; 78(Pt 4): 627-636, 2022 Aug 01.
Artigo em Inglês | MEDLINE | ID: mdl-35975829


Bond valence analysis has been applied to various copper chalcogenides with copper valence excess, i.e. where the formal valence of copper exceeds 1. This approach always reveals a copper bond valence excess relative to the unit value, correlated to an equivalent ligand bond valence deficit. In stoichiometric chalcogenides, this corresponds to one ligand electron in excess per formula unit relative to the valence equilibrium considering only CuI. This ligand electron in excess is 50/50 shared between all or part of the Cu-atom positions, and all or part of the ligand-atom positions. In Cu3Se2, only one of the two Cu positions is involved in this sharing. It would indicate a special type of multicentre bonding (`one-electron co-operative bonding'). Calculated and ideal structural formulae according to this bond valence distribution are presented. At the crystal structure scale, Cu-ligand bonds implying the single electron in excess form one-, two- or three-dimensional subnetworks. Bond valence distribution according to two two-dimensional subnets is detailed in covellite, CuS. This bond valence description is a formal crystal-chemical representation of the metallic conductivity of holes (mixing between Cu 3d bands and ligand p bands), according to published electronic band structures. Bond valence analysis is a useful and very simple prospective approach in the search for new compounds with targeted specific physical properties.

Cobre , Elétrons , Cobre/química , Ligantes
Nano Lett ; 13(8): 3648-53, 2013 Aug 14.
Artigo em Inglês | MEDLINE | ID: mdl-23826620


We study the Mott insulator compound GaTa4Se8 in which we previously discovered an electric-field-induced resistive transition. We show that the resistive switching is associated to the appearance of metallic and super-insulating nanodomains by means of scanning tunneling microscopy/spectroscopy (STM/STS). Moreover, we show that local electronic transitions can be controlled at the nanoscale at room temperature using the electric field of the STM tip. This opens the way for possible applications in resistive random access memories (RRAM) devices.

Phys Rev Lett ; 102(12): 127005, 2009 Mar 27.
Artigo em Inglês | MEDLINE | ID: mdl-19392315


We report a detailed scanning tunneling microscopy study of a superconductor in a strong vortex confinement regime. This is achieved in a thin nanoisland of Pb having a size d about 3 times the coherence length, and a thickness h such that h<

Adv Mater ; 20(14): 2760-5, 2008 Jul 17.
Artigo em Inglês | MEDLINE | ID: mdl-25213903


Experimental evidence of a nonvolatile electric-pulse-induced insulator-to-metal transition and possible superconductivity in the Mott insulator GaTa4 Se8 is reported. Scanning tunneling microscopy experiments show that this unconventional response of the system to short electric pulses arises from a nanometer-scale electronic phase separation generated in the bulk material.