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Science ; 358(6368): 1314-1317, 2017 12 08.
Artigo em Inglês | MEDLINE | ID: mdl-29217574


Bose condensation has shaped our understanding of macroscopic quantum phenomena, having been realized in superconductors, atomic gases, and liquid helium. Excitons are bosons that have been predicted to condense into either a superfluid or an insulating electronic crystal. Using the recently developed technique of momentum-resolved electron energy-loss spectroscopy (M-EELS), we studied electronic collective modes in the transition metal dichalcogenide semimetal 1T-TiSe2 Near the phase-transition temperature (190 kelvin), the energy of the electronic mode fell to zero at nonzero momentum, indicating dynamical slowing of plasma fluctuations and crystallization of the valence electrons into an exciton condensate. Our study provides compelling evidence for exciton condensation in a three-dimensional solid and establishes M-EELS as a versatile technique sensitive to valence band excitations in quantum materials.

J Chem Phys ; 130(16): 164706, 2009 Apr 28.
Artigo em Inglês | MEDLINE | ID: mdl-19405615


Scanning tunneling microscopy reveals a free radical-induced surface chain reaction in the chlorination of nanoscale patterns on an otherwise H-passivated (masked) Si(100). While scanning probe methods can be used to pattern active surface regions with single-bond precision, follow-up selective chemical vapor deposition with polyatomic molecules can produce various filling characteristics. On active surface regions, molecular Cl(2) undergoes an atom abstraction reaction in which a Si dangling bond abstracts one atom of the incident Cl(2) molecule while the complementary Cl atom is scattered away from the initial abstraction site either back into the vacuum or to be captured by a second dangling bond and adsorbed there, or to react with a nearby adsorbed H atom to form volatile HCl. In contrast, I(2) undergoes only dissociative adsorption on two immediately neighboring dangling bonds, whereby two I-Si bonds are formed simultaneously upon cleavage of the I(2) bond. The different chemisorption processes of the two model diatomic molecular gases place intrinsic limitations on atomic-scale lithography and processing: Adsorption of Cl(2) results in spillage over the prepatterned regions of active bonds. In contrast, adsorption of I(2) is a pair process and results in under-filling.

Science ; 301(5636): 1078-80, 2003 Aug 22.
Artigo em Inglês | MEDLINE | ID: mdl-12934002


We report an experimental determination of the phonon dispersion curves in a face-centered cubic (fcc) delta-plutonium-0.6 weight % gallium alloy. Several unusual features, including a large elastic anisotropy, a small-shear elastic modulus C', a Kohn-like anomaly in the T1[011] branch, and a pronounced softening of the [111] transverse modes, are found. These features can be related to the phase transitions of plutonium and to strong coupling between the lattice structure and the 5f valence instabilities. Our results also provide a critical test for theoretical treatments of highly correlated 5f electron systems as exemplified by recent dynamical mean field theory calculations for delta-plutonium.