arXiv subjects

R. Ramirez

Publications and source records attributed to R. Ramirez.

21 records · Page 2Linked to original sources

arXiv2007

Strong covalent bonding between two graphene layers

We show that two graphene layers stacked directly on top of each other (AA stacking) form strong chemical bonds when the distance between planes is 0.156 nm. Simultaneously, C-C in-plane bonds are considerably weakened from partial double-bond (0.141 nm) to single bond (0.154 nm). This polymorphic form of graphene bilayer is meta-stable w.r.t. the one bound by van der Waals forces at a larger separation (0.335 nm) with an activation energy of 0.16 eV/cell. Similarly to the structure found in hexaprismane, C forms four single bonds in a geometry mixing 90^{0} and 120^{0} angles. Intermediate separations between layers can be stabilized under external anisotropic stresses showing a rich electronic structure changing from semimetal at van der Waals distance, to metal when compressed, to wide gap semiconductor at the meta-stable minimum.

cond-mat.mtrl-sci↗

arXiv2002

p-Type semiconducting properties in lithium-doped MgO single crystals

The phenomenally large enhancement in conductivity observed when Li-doped MgO crystals are oxidized at elevated temperatures was investigated by dc and ac electrical measurements in the temperature interval 250-673 K. The concentration of ([Li]^{0}) centers (Li^{+} ions each with a trapped hole) resulting from oxidation was monitored by optical absorption measurements. Both dc and ac experiments provide consistent values for the bulk resistance. The electricalconductivity of oxidized MgO:Li crystals increases linearly with the concentration of ([Li]^{0}) centers. The conductivity is thermally activated with an activation energy of (0.70 +/- 0.01) eV, which is independent of the ([Li]^{0}) content. The \textit{standard semiconducting} mechanism satisfactorily explains these results. Free holes are the main contribution to band conduction as they are trapped at or released from the ([Li]^{0})-acceptor centers. In as-grown MgO:Li crystals, electrical current increases dramatically with time due to the formation of ([Li]^{0}) centers. The activation energy values between 1.3 and 0.7 eV are likely a combination of the activation energy for the creation of ([Li]^{0}) centers and the activation energy of ionization of these centers. Destruction of ([Li]^{0}) centers can be induced in oxidized crystals by application of an electric field due to Joule heating up to temperatures at which ([Li]^{0}) centers are not stable.

cond-mat.mtrl-sci↗

arXiv2000

Semiconducting Characteristics of Magnesium-Doped Al2O3 Single Crystals

DC and AC electrical measurements were performed to investigate the electrical conductivity of alpha-Al2O3:Mg samples with different concentrations of [Mg]^0 centers .The concentration of [Mg]^0 centers was monitored by the opticalabsorption peak at 2.56 eV. At low electrical fields, DC measurements reveal blocking contacts. Steady electroluminescence is emitted at the negative electrodeindicating that the majority of carriers are holes. Low voltage AC measurements show that the equivalent circuit for the sample is the bulk resistance in series with the junction capacitance connected in parallel with a capacitance, which represents the dielectric constant of the sample. The values determined for the bulk resistance in both DC and AC experiments are in good agreement. The electrical conductivity of Al2O3:Mg crystals increases linearly with the concentration of [Mg]^0 centers, regardless of the amount of other impurities also present in the crystals, and is four times higher in the $c_{\perpendicular}$ than in the $c_{\parallel}$ direction. The conductivity is thermally activated with an activation energy of 0.68 eV, which is independent of: 1) the [Mg]^0 content, 2) the crystallographic orientation, and 3) the concentration of other impurities. These results favor the {\it small-polaron-motion} mechanism.

cond-mat.mtrl-sci↗