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Ground State and the Metal-insulator Transition in (Pr1−yYy)1−xCaxCoO3 (0.45 ≤ x ≤ 0.55) Cobaltites
A. J. Bar´on-Gonz´alez,J. L. Garc´ıa-Mu˜noz,J. Herrero-Mart´ın,C. Frontera,G. Sub´ıas,J. Blasco 한국물리학회 2013 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.63 No.3
Pr0.5Ca0.5CoO3 exhibits a non-conventional metal-insulator transition (MIT) in which the insulatingstate is due toa volume contraction (2%) and electron transfer from Pr to Co sites. Pr1−xCaxCoO3cobaltites with Ca doping around x 0.5 (0.45 ≤ x ≤ 0.55) have been investigated. Different ground states and phase transitions are reported in samples at both sides of the half-dopedcomposition. The relevance of the Pr/Ca ratio around half-doping and the amplitude of the changesobserved at the MIT are analyzed. Besides the expected variation in the transition temperature, wereport an enhanced Pr3+/Pr4+ transformation at the MIT in Pr0.45Y0.05Ca0.50CoO3[0.25(5) e−/Pr ion and TMI = 120 K] with respect to Pr0.5Ca0.5CoO3[0.15(5) e−/ Pr ion and TMI = 75 K].
Control of Thermal and Electronic Transport in Defect-Engineered Graphene Nanoribbons
Haskins, Justin,Kınacı, Alper,Sevik, Cem,Sevinç,li, Hâ,ldun,Cuniberti, Gianaurelio,Ç,ağ,ın, Tahir American Chemical Society 2011 ACS NANO Vol.5 No.5
<P>The influence of the structural detail and defects on the thermal and electronic transport properties of graphene nanoribbons (GNRs) is explored by molecular dynamics and non-equilibrium Green’s function methods. A variety of randomly oriented and distributed defects, single and double vacancies, Stone−Wales defects, as well as two types of edge form (armchair and zigzag) and different edge roughnesses are studied for model systems similar in sizes to experiments (>100 nm long and >15 nm wide). We observe substantial reduction in thermal conductivity due to all forms of defects, whereas electrical conductance reveals a peculiar defect-type-dependent response. We find that a 0.1% single vacancy concentration and a 0.23% double vacancy or Stone−Wales concentration lead to a drastic reduction in thermal conductivity of GNRs, namely, an 80% reduction from the pristine one of the same width. Edge roughness with an rms value of 7.28 Å leads to a similar reduction in thermal conductivity. Randomly distributed bulk vacancies are also found to strongly suppress the ballistic nature of electrons and reduce the conductance by 2 orders of magnitude. However, we have identified that defects close to the edges and relatively small values of edge roughness preserve the quasi-ballistic nature of electronic transport. This presents a route of independently controlling electrical and thermal transport by judicious engineering of the defect distribution; we discuss the implications of this for thermoelectric performance.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2011/ancac3.2011.5.issue-5/nn200114p/production/images/medium/nn-2011-00114p_0005.gif'></P>