642 research outputs found
Electrical properties of isotopically enriched neutron-transmutation-doped ^{70} Ge:Ga near the metal-insulator transition
We report the low temperature carrier transport properties of a series of
nominally uncompensated neutron-transmutation doped (NTD) ^{70} Ge:Ga samples
very close to the critical concentration N_c for the metal-insulator
transition. The concentration of the sample closest to N_c is 1.0004N_c and it
is unambiguously shown that the critical conductivity exponent is 0.5.
Properties of insulating samples are discussed in the context of Efros and
Shklovskii's variable range hopping conduction.Comment: 8 pages using REVTeX, 8 figures, published versio
Dopant-induced crossover from 1D to 3D charge transport in conjugated polymers
The interplay between inter- and intra-chain charge transport in bulk
polythiophene in the hopping regime has been clarified by studying the
conductivity as a function of frequency (up to 3 THz), temperature and doping
level. We present a model which quantitatively explains the observed crossover
from quasi-one-dimensional transport to three-dimensional hopping conduction
with increasing doping level. At high frequencies the conductivity is dominated
by charge transport on one-dimensional conducting chains.Comment: 4 pages, 2 figure
Materials with Colossal Dielectric Constant: Do They Exist?
Experimental evidence is provided that colossal dielectric constants, epsilon
>= 1000, sometimes reported to exist in a broad temperature range, can often be
explained by Maxwell-Wagner type contributions of depletion layers at the
interface between sample and contacts, or at grain boundaries. We demonstrate
this on a variety of different materials. We speculate that the largest
intrinsic dielectric constant observed so far in non-ferroelectric materials is
of order 100.Comment: 3 figure
Crystal Field and Dzyaloshinsky-Moriya Interaction in orbitally ordered La_{0.95}Sr_{0.05}MnO_3: An ESR Study
We present a comprehensive analysis of Dzyaloshinsky-Moriya interaction and
crystal-field parameters using the angular dependence of the paramagnetic
resonance shift and linewidth in single crystals of La_{0.95}Sr_{0.05}MnO_3
within the orthorhombic Jahn-Teller distorted phase. The Dzyaloshinsky-Moriya
interaction (~ 1K) results from the tilting of the MnO_6 octahedra against each
other. The crystal-field parameters D and E are found to be of comparable
magnitude (~ 1K) with D ~= -E. This indicates a strong mixing of the |3z^2-r^2>
and |x^2-y^2> states for the real orbital configuration.Comment: 12 pages, 6 figure
Stability of sub-surface oxygen at Rh(111)
Using density-functional theory (DFT) we investigate the incorporation of
oxygen directly below the Rh(111) surface. We show that oxygen incorporation
will only commence after nearly completion of a dense O adlayer (\theta_tot =
1.0 monolayer) with O in the fcc on-surface sites. The experimentally suggested
octahedral sub-surface site occupancy, inducing a site-switch of the on-surface
species from fcc to hcp sites, is indeed found to be a rather low energy
structure. Our results indicate that at even higher coverages oxygen
incorporation is followed by oxygen agglomeration in two-dimensional
sub-surface islands directly below the first metal layer. Inside these islands,
the metastable hcp/octahedral (on-surface/sub-surface) site combination will
undergo a barrierless displacement, introducing a stacking fault of the first
metal layer with respect to the underlying substrate and leading to a stable
fcc/tetrahedral site occupation. We suggest that these elementary steps,
namely, oxygen incorporation, aggregation into sub-surface islands and
destabilization of the metal surface may be more general and precede the
formation of a surface oxide at close-packed late transition metal surfaces.Comment: 9 pages including 9 figure files. Submitted to Phys. Rev. B. Related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
First-principles calculations of the self-trapped exciton in crystalline NaCl
The atomic and electronic structure of the lowest triplet state of the
off-center (C2v symmetry) self-trapped exciton (STE) in crystalline NaCl is
calculated using the local-spin-density (LSDA) approximation. In addition, the
Franck-Condon broadening of the luminescence peak and the a1g -> b3u absorption
peak are calculated and compared to experiment. LSDA accurately predicts
transition energies if the initial and final states are both localized or
delocalized, but 1 eV discrepancies with experiment occur if one state is
localized and the other is delocalized.Comment: 4 pages with 4 embeddded figure
Ultrafast structure and dynamics in ionic liquids: 2D-IR spectroscopy probes the molecular origin of viscosity
The viscosity of imidazolium ionic liquids increases dramatically when the strongest hydrogen bonding location is methylated. In this work, ultrafast two-dimensional vibrational spectroscopy of dilute thiocyanate ion ([SCN] -) in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4C1im][NTf2]) and 1-butyl-2,3- dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([C4C 1C12im][NTf2]) shows that the structural reorganization occurs on a 26 ± 3 ps time scale and on a 47 ± 15 ps time scale, respectively. The results suggest that the breakup of local ion-cages is the fundamental event that activates molecular diffusion and determines the viscosity of the fluids. © 2014 American Chemical Society
Water Dynamics at Protein Interfaces: Ultrafast Optical Kerr Effect Study
The behavior of water molecules surrounding a protein can have an important bearing on its structure and function. Consequently, a great deal of attention has been focused on changes in the relaxation dynamics of water when it is located at the protein surface. Here we use the ultrafast optical Kerr effect to study the H-bond structure and dynamics of aqueous solutions of proteins. Measurements are made for three proteins as a function of concentration. We find that the water dynamics in the first solvation layer of the proteins are slowed by up to a factor of 8 in comparison to those in bulk water. The most marked slowdown was observed for the most hydrophilic protein studied, bovine serum albumin, whereas the most hydrophobic protein, trypsin, had a slightly smaller effect. The terahertz Raman spectra of these protein solutions resemble those of pure water up to 5 wt % of protein, above which a new feature appears at 80 cm–1, which is assigned to a bending of the protein amide chain
Colossal dielectric constants in transition-metal oxides
Many transition-metal oxides show very large ("colossal") magnitudes of the
dielectric constant and thus have immense potential for applications in modern
microelectronics and for the development of new capacitance-based
energy-storage devices. In the present work, we thoroughly discuss the
mechanisms that can lead to colossal values of the dielectric constant,
especially emphasising effects generated by external and internal interfaces,
including electronic phase separation. In addition, we provide a detailed
overview and discussion of the dielectric properties of CaCu3Ti4O12 and related
systems, which is today's most investigated material with colossal dielectric
constant. Also a variety of further transition-metal oxides with large
dielectric constants are treated in detail, among them the system La2-xSrxNiO4
where electronic phase separation may play a role in the generation of a
colossal dielectric constant.Comment: 31 pages, 18 figures, submitted to Eur. Phys. J. for publication in
the Special Topics volume "Cooperative Phenomena in Solids: Metal-Insulator
Transitions and Ordering of Microscopic Degrees of Freedom
- …