41 research outputs found
History-dependent relaxation and the energy scale of correlation in the Electron-Glass
We present an experimental study of the energy-relaxation in
Anderson-insulating indium-oxide films excited far from equilibrium. In
particular, we focus on the effects of history on the relaxation of the excess
conductance dG. The natural relaxation law of dG is logarithmic, namely
dG=-log(t). This may be observed over more than five decades following, for
example, cool-quenching the sample from high temperatures. On the other hand,
when the system is excited from a state S_{o} in which it has not fully reached
equilibrium to a state S_{n}, the ensuing relaxation law is logarithmic only
over time t shorter than the time t_{w} it spent in S_{o}. For times t>t_{w}
dG(t) show systematic deviation from the logarithmic dependence. It was
previously shown that when the energy imparted to the system in the excitation
process is small, this leads to dG=P(t/t_{w}) (simple-aging). Here we test the
conjecture that `simple-aging' is related to a symmetry in the relaxation
dynamics in S_{o} and S_{n}. This is done by using a new experimental procedure
that is more sensitive to deviations in the relaxation dynamics. It is shown
that simple-aging may still be obeyed (albeit with a modified P(t/t_{w})) even
when the symmetry of relaxation in S_{o} and S_{n} is perturbed by a certain
degree. The implications of these findings to the question of aging, and the
energy scale associated with correlations are discussed
Anomalous Hopping Exponents of Ultrathin Films of Metals
The temperature dependence of the resistance R(T) of ultrathin
quench-condensed films of Ag, Bi, Pb and Pd has been investigated. In the most
resistive films, R(T)=Roexp(To/T)^x, where x=0.75. Surprisingly, the exponent x
was found to be constant for a wide range of Ro and To in all four materials,
possibly implying a consistent underlying conduction mechanism. The results are
discussed in terms of several different models of hopping conduction.Comment: 6 pages, 5 figure
Time Dependent Development of the Coulomb Gap
We show that the time development of the Coulomb gap in a Coulomb glass can
involve very long relaxation times due to electron rearrangement and hopping.
We find that an applied magnetic field reduces the rate of electron hopping
and, hence, Coulomb gap formation. These results are consistent with recent
conductance experiments on thin semiconducting and metallic films.Comment: 4 pages, Latex, 3 encapsulated postscript figure
Electrical transport studies of quench condensed Bi films at the initial stage of film growth: Structural transition and the possible formation of electron droplets
The electrical transport properties of amorphous Bi films prepared by
sequential quench deposition have been studied in situ. A
superconductor-insulator (S-I) transition was observed as the film was made
increasingly thicker, consistent with previous studies. Unexpected behavior was
found at the initial stage of film growth, a regime not explored in detail
prior to the present work. As the temperature was lowered, a positive
temperature coefficient of resistance (dR/dT > 0) emerged, with the resistance
reaching a minimum before the dR/dT became negative again. This behavior was
accompanied by a non-linear and asymmetric I-V characteristic. As the film
became thicker, conventional variable-range hopping (VRH) was recovered. We
attribute the observed crossover in the electrical transport properties to an
amorphous to granular structural transition. The positive dR/dT found in the
amorphous phase of Bi formed at the initial stage of film growth was
qualitatively explained by the formation of metallic droplets within the
electron glass.Comment: 7 pages, 6 figure
Expression, Purification and Refolding of a Human Na V 1.7 Voltage Sensing Domain with Native-like Toxin Binding Properties
The voltage-gated sodium channel NaV1.7 is an important target for drug development due to its role in pain perception. Recombinant expression of full-length channels and their use for biophysical characterization of interactions with potential drug candidates is challenging due to the protein size and complexity. To overcome this issue, we developed a protocol for the recombinant expression in E. coli and refolding into lipids of the isolated voltage sensing domain (VSD) of repeat II of NaV1.7, obtaining yields of about 2 mg of refolded VSD from 1 L bacterial cell culture. This VSD is known to be involved in the binding of a number of gating-modifier toxins, including the tarantula toxins ProTx-II and GpTx-I. Binding studies using microscale thermophoresis showed that recombinant refolded VSD binds both of these toxins with dissociation constants in the high nM range, and their relative binding affinities reflect the relative IC50 values of these toxins for full-channel inhibition. Additionally, we expressed mutant VSDs incorporating single amino acid substitutions that had previously been shown to affect the activity of ProTx-II on full channel. We found decreases in GpTx-I binding affinity for these mutants, consistent with a similar binding mechanism for GpTx-I as compared to that of ProTx-II. Therefore, this recombinant VSD captures many of the native interactions between NaV1.7 and tarantula gating-modifier toxins and represents a valuable tool for elucidating details of toxin binding and specificity that could help in the design of non-addictive pain medication acting through NaV1.7 inhibition
PRE-TREATMENT AND ENZYMATIC HYDROLYSIS OF BANANA (Musa acuminata x balbisiana) PSEUDOSTEM FOR ETHANOL PRODUCTION
Banana (M. acuminata x balbisiana) is an abundant lignocellulosic waste material in large plantations all over the Philippines, especially in Mindanao, which can be utilized as substrate in producing high-value products like ethanol. To compensate for the low yield based on total weight of substrate due to the high moisture content of banana pseudostem, there is the primary challenge to make the conversion of this lignocellulosic biomass into monomeric sugar and then into ethanol more efficiently in order to achieve yields that would make it cost-competitive. Hence, this study evaluated the effects of solid loading, incubation time and amount of enzyme on yield of reducing sugars in the enzymatic hydrolysis process and attempted to optimize the significant factors by Response Surface Methodology (RSM), specifically using Box-Behnken design. There was significant improvement on the reducing sugar yield of the pretreated banana pseudostem at 20 h incubation time, 15 g solid loading and 0.55 % enzyme concentration. Ethanol production was observed to be higher in the detoxified substrate although biomass was higher for the non-detoxified substrate. As to our knowledge, the present study is the first attempt to produce second generation ethanol using banana pseudostem waste as feedstock in the Philippines
Evidence of Vortices on the Insulating Side of the Superconductor-Insulator Transition
The magnetoresistance of ultrathin insulating films of Bi has been studied
with magnetic fields applied parallel and perpendicular to the plane of the
sample. Deep in the strongly localized regime, the magnetoresistance is
negative and independent of field orientation. As film thicknesses increase,
the magnetoresistance becomes positive, and a difference between values
measured in perpendicular and parallel fields appears, which is a linear
function of the magnetic field and is positive. This is not consistent with the
quantum interference picture. We suggest that it is due to vortices present on
the insulating side of the superconductor-insulator transition.Comment: 4 pages, 3 figure
Electron-Assisted Hopping in Two Dimensions
We have studied the non-ohmic effects in the conductivity of a
two-dimensional system which undergoes the crossover from weak to strong
localization with decreasing electron concentration. When the electrons are
removed from equilibrium with phonons, the hopping conductivity depends only on
the electron temperature. This indicates that the hopping transport in a system
with a large localization length is assisted by electron-electron interactions
rather than by the phonons.Comment: 5 pages, 4 figure
Electron Glass in Ultrathin Granular Al Films at Low Temperatures
Quench-condensed granular Al films, with normal-state sheet resistance close
to 10 k, display strong hysteresis and ultraslow, non-exponential
relaxation in the resistance when temperature is varied below 300 mK. The
hysteresis is nonlinear and can be suppressed by a dc bias voltage. The
relaxation time does not obey the Arrhenius form, indicating the existence of a
broad distribution of low energy barriers. Furthermore, large resistance
fluctuations, having a 1/f-type power spectrum with a low-frequency cut-off,
are observed at low temperatures. With decreasing temperature, the amplitude of
the fluctuation increases and the cut-off frequency decreases. These
observations combine to provide a coherent picture that there exists a new
glassy electron state in ultrathin granular Al films, with a growing
correlation length at low temperatures.Comment: RevTeX 3.1, 4 pages, 4 figures (EPS files) (Minor Additions
The butterflies of Barro Colorado Island, Panama: local extinction since the 1930s
Few data are available about the regional or local extinction of tropical butterfly species.When confirmed, local extinction was often due to the loss of host-plant species. We usedpublished lists and recent monitoring programs to evaluate changes in butterfly compositionon Barro Colorado Island (BCI, Panama) between an old (1923–1943) and a recent (1993–2013) period. Although 601 butterfly species have been recorded from BCI during the1923–2013 period, we estimate that 390 species are currently breeding on the island,including 34 cryptic species, currently only known by their DNA Barcode Index Number.Twenty-three butterfly species that were considered abundant during the old period couldnot be collected during the recent period, despite a much higher sampling effort in recenttimes. We consider these species locally extinct from BCI and they conservatively represent6% of the estimated local pool of resident species. Extinct species represent distant phylo-genetic branches and several families. The butterfly traits most likely to influence the proba-bility of extinction were host growth form, wing size and host specificity, independently ofthe phylogenetic relationships among butterfly species. On BCI, most likely candidates forextinction were small hesperiids feeding on herbs (35% of extinct species). However, con-trary to our working hypothesis, extinction of these species on BCI cannot be attributed toloss of host plants. In most cases these host plants remain extant, but they probably subsistat lower or more fragmented densities. Coupled with low dispersal power, this reducedavailability of host plants has probably caused the local extinction of some butterfly species.Many more bird than butterfly species have been lost from BCI recently, confirming thatsmall preserves may be far more effective at conserving invertebrates than vertebrates and,therefore, should not necessarily be neglected from a conservation viewpoin