14 research outputs found
Manifestations of fine features of the density of states in the transport properties of KOs2O6
We performed high-pressure transport measurements on high-quality single
crystals of KOs2O6, a beta-pyrochlore superconductor. While the resistivity at
high temperatures might approach saturation, there is no sign of saturation at
low temperatures, down to the superconducting phase. The anomalous resistivity
is accompanied by a nonmetallic behavior in the thermoelectric power (TEP) up
to temperatures of at least 700 K, which also exhibits a broad hump with a
maximum at 60 K. The pressure influences mostly the low-energy electronic
excitations. A simple band model based on enhanced density of states in a
narrow window around the Fermi energy (EF) explains the main features of this
unconventional behavior in the transport coefficients and its evolution under
pressure
Exact solution of electronic transport in semiconductors dominated by scattering on polaronic impurities
The scattering of electrons on impurities with internal degrees of freedom is
bound to produce the signatures of the scatterer's own dynamics and results in
nontrivial electronic transport properties. Previous studies of polaronic
impurities in low-dimensional structures, like molecular junctions and
one-dimensional nanowire models, have shown that perturbative treatments cannot
account for a complex energy dependence of the scattering cross section in such
systems. Here we derive the exact solution of polaronic impurities shaping the
electronic transport in bulk (3D) systems. In the model with a short-ranged
electron-phonon interaction, we solve for and sum over all elastic and
inelastic partial cross sections, abundant in resonant features. The
temperature dependence of the charge mobility shows the power-law dependence,
, with being highly sensitive to impurity
parameters. The latter may explain nonuniversal power-law exponents observed
experimentally, e.g. in high-quality organic molecular semiconductors.Comment: 5 pages, 6 figure
Diffusion of triplet excitons in an operational Organic Light Emitting Diode
Measurements of the diffusion length L for triplet excitons in small
molecular-weight organic semiconductors are commonly carried out using a
technique in which a phosphorescent-doped probe layer is set in the vicinity of
a supposed exciton generation zone. However, analyses commonly used to retrieve
ignore microcavity effects that may induce a strong modulation of the
emitted light as the position of the exciton probe is shifted. The present
paper investigates in detail how this technique may be improved to obtain more
accurate results for L. The example of 4,4'-bis(carbazol-9-yl)1,1'-biphenyl
(CBP) is taken, for which a triplet diffusion length of L=16 +/- 4 nm (at 3
mA/cm2) is inferred from experiments. The influence of triplet-triplet
annihilation, responsible for an apparent decrease of L at high current
densities, is theoretically investigated, as well as the 'invasiveness' of the
thin probe layer on the exciton distribution. The interplay of microcavity
effects and direct recombinations is demonstrated experimentally with the
archetypal trilayer structure
[N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)]-4,4'-diaminobiphenyl (NPB)/CBP/
2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (named bathocuproine, BCP). It is
shown that in this device holes do cross the NPB/CBP junction, without the
assistance of electrons and despite the high energetic barrier imposed by the
shift between the HOMO levels. The use of the variable-thickness doped layer
technique in this case is then discussed. Finally, some guidelines are given
for improving the measure of the diffusion length of triplet excitons in
operational OLEDs, applicable to virtually any small molecular-weight material.Comment: Accepted for publication in Physical Review
Preferential out-of-plane conduction and quasi-one-dimensional electronic states in layered 1T-TaS 2
Layered transition metal dichalcogenides (TMDs) are commonly classified as quasi-two-dimensional materials, meaning that their electronic structure closely resembles that of an individual layer, which results in resistivity anisotropies reaching thousands. Here, we show that this rule does not hold for 1T-TaS2—a compound with the richest phase diagram among TMDs. Although the onset of charge density wave order makes the in-plane conduction non-metallic, we reveal that the out-of-plane charge transport is metallic and the resistivity anisotropy is close to one. We support our findings with ab initio calculations predicting a pronounced quasi-one-dimensional character of the electronic structure. Consequently, we interpret the highly debated metal-insulator transition in 1T-TaS2 as a quasi-one-dimensional instability, contrary to the long- standing Mott localisation picture. In a broader context, these findings are relevant for the newly born field of van der Waals heterostructures, where tuning interlayer interactions (e.g., by twist, strain, intercalation, etc.) leads to new emergent phenomena