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At least 469 records · Page 26Linked to original sources
PubMed2000

Dynamical properties near the metal-insulator transition: evidence for electron-assisted variable range hopping

We report independent measurements (between 20 and 200 mK) of the electronic specific heat C(e), the electron-phonon coupling G(e-ph), and the electron-phonon relaxation time tau(e-ph) (from 10(-2) to 10(-5) s) for NbxSi1-x Anderson insulator thin films. We show that the usual equation tau(e-ph) = C(e)/G(e-ph) holds only if the resistance is solely related to the electron temperature. We conclude that at sufficiently low temperatures variable range hopping transport is assisted by electron-electron interactions alone and is independent of the phonon distribution.

Journal Article↗
PubMed2001

Universal crossover from band to hopping conduction in molecular organic semiconductors.

The charge transport in a variety of herringbone-stacked organic molecular semiconductors is investigated in the temperature range from 10 to 550 K. A crossover from coherent bandlike charge transport with mobilities up to several thousand cm (2)/V s at low temperature to an incoherent hopping motion at high temperatures is observed. This is attributed to the localization of the charge carrier due to increased electron-phonon interaction and, finally, the formation of a lattice polaron.

Journal Article↗
PubMed2001

Variable range hopping and electrical conductivity along the DNA double helix.

We present a model to describe electrical conductivity along the DNA double helix. In this model, DNA is considered as a one-dimensional disordered system, and electrons are transported via variable range hopping between localized states. Thermal structural fluctuations in DNA further localize electronic wave functions, giving rise to a temperature-dependent localization length. The model quantitatively explains the temperature dependence of the conductivity observed in the lambda phage DNA (lambda-DNA).

Bacteriophage lambda↗
PubMed2001

Hopping transport of positrons in hydrogenated amorphous silicon.

Positron beam timing spectroscopy has been used to investigate positron diffusion in hydrogenated amorphous silicon between 85 and 350 K. The diffusivity is determined from both the contribution of the surface, where the positrons have a different characteristic annihilation rate, and its effect on the shape of the bulk annihilation rate distribution. We observe a single positron state with a temperature independent lifetime of 322 ps. From the temperature dependence of the positron diffusion we show that this is a localized state and present the first ever direct observation of hopping diffusion of positrons in solids. The migration enthalpy for positrons in this state is found to be 17.7(3) meV.

Journal Article↗
PubMed2002

Hopping conductivity in the quantum Hall effect: revival of universal scaling.

We have measured the temperature dependence of the conductivity sigma(xx) of a two-dimensional electron system deep into the localized regime of the quantum Hall plateau transition. Using variable-range hopping theory we extract directly the localization length xi from this experiment. We use our results to study the scaling behavior of xi as a function of the filling factor distance /deltanu/ to the critical point of the transition. We find for all samples a power-law behavior xi equivalent to /deltanu/(-gamma) in agreement with the theoretically proposed universal exponent gamma = 2.35.

Journal Article↗
PubMed2002

Variable range hopping in two-dimensional systems of interacting electrons.

Computer modeling of the variable-range-hopping (VRH) conductivity in two-dimensional systems has been done by a kinetic Monte Carlo method, which includes some new elements. Study of the temperature dependence of the conductivity, testing of the different scaling relations, and study of the size effect show the detailed validity of the Efros-Shklovskii theory of the VRH in the system of interacting electrons. It has also been shown that simultaneous transitions of many electrons are not important. The reasons for disagreement with previous computational works are thoroughly analyzed.

Journal Article↗
PubMed2003

Anderson transition in low-dimensional disordered systems driven by long-range nonrandom hopping.

The single-parameter scaling hypothesis predicts the absence of delocalized states for noninteracting quasiparticles in low-dimensional disordered systems. We show analytically, using a supersymmetric method combined with a renormalization group analysis, as well as numerically that extended states may occur in the one- and two-dimensional Anderson model with a nonrandom hopping falling off as some power of the distance between sites. The different size scaling of the bare level spacing and the renormalized magnitude of the disorder seen by the quasiparticles finally results in the delocalization of states at one of the band edges of the quasiparticle energy spectrum.

Journal Article↗
PubMed2003

Variable-range hopping and quantum creep in one dimension.

We study the quantum nonlinear response to an applied electric field E of a one-dimensional pinned charge-density wave or Luttinger liquid in the presence of disorder. From an explicit construction of low-lying metastable states and of bounce instanton solutions between them, we demonstrate quantum creep v=e(-c/E(1/2)) as well as a sharp crossover at E=E(*) towards a linear response form consistent with variable-range hopping arguments, but dependent only on electronic degrees of freedom.

Journal Article↗
PubMed2003

Hopping between localized Floquet states in periodically driven quantum dots.

The dynamic localization in energy space, suppression of the absorption of energy from an external microwave field due to quantum interference, was analyzed recently for a closed quantum dot in the absence of electron-electron interactions. Here a weak interaction is shown to lead to a finite absorption and heating, which may be viewed as hopping between localized Floquet states. The heating rate grows together with the electronic temperature, eventually destroying the localization.

Journal Article↗
PubMed2004

Variable range hopping conduction in semiconductor nanocrystal solids.

The temperature and electrical field dependent conductivity of n-type CdSe nanocrystal thin films is investigated. In the low electrical field regime, the conductivity follows sigma approximately exp([-(T(*)/T)(1/2)] in the temperature range 10<T<120 K. At high electrical field, the conductivity is strongly field dependent. At 4 K, the conductance increases by 8 orders of magnitude over one decade of bias. At a very high field, conductivity is temperature independent with sigma approximately exp([-(E(*)/E)(1/2)]. The complete behavior is very well described by variable range hopping with a Coulomb gap.

Journal Article↗
PubMed2005

Orbital ac spin-Hall effect in the hopping regime.

The Rashba and Dresselhaus spin-orbit interactions are both shown to yield the low temperature spin-Hall effect for strongly localized electrons coupled to phonons. A frequency-dependent electric field E(omega) generates a spin-polarization current, normal to E, due to interference of hopping paths. At zero temperature the corresponding spin-Hall conductivity is real and is proportional to omega2. At nonzero temperatures the coupling to the phonons yields an imaginary term proportional to omega. The interference also yields persistent spin currents at thermal equilibrium, at E=0. The contributions from the Dresselhaus and Rashba interactions to the interference oppose each other.

Journal Article↗
PubMed2005

Efficient sampling of protein structures by model hopping.

We introduce a novel simulation method, model hopping, that enhances sampling of low-energy configurations in complex systems. The approach is illustrated for a protein-folding problem. Thermodynamic quantities of proteins with up to 46 residues are evaluated from all-atom simulations with this method.

Algorithms↗
PubMed2005

Coulomb blockade and hopping conduction in PbSe quantum dots.

We report for the first time on rich and tunable transport phenomena in closed-packed arrays of PbSe colloidal nanocrystals (NCs) in the form of thin films. As the interdot coupling is increased, the system evolves from an insulating regime dominated by Coulomb blockade to a semiconducting regime, where hopping conduction is the dominant transport mechanism. The observed phenomena can be interpreted using the framework established mainly in the context of transport measurements in metallic quantum dots and disordered semiconductors.

Journal Article↗
PubMed2005

Trajectory surface hopping in the time-dependent Kohn-Sham approach for electron-nuclear dynamics.

The mean-field treatment of electron-nuclear interaction results in many qualitative breakdowns in the time-dependent Kohn-Sham (TDKS) density functional theory. Examples include current-induced heating in nanoelectronics, charge dynamics in quantum dots and carbon nanotubes, and relaxation of biological chromophores. The problem is resolved by the trajectory surface-hopping TDKS approach, which is illustrated by the photoinduced electron injection from a molecular chromophore into TiO2, and the excited-state relaxation of the green fluorescent protein chromophore.

Journal Article↗
PubMed2006

Anomalous Hall effect in La0.7Sr0.3MnO3/SrTiO3 superlattices: hopping transport and a probe of dimensionality.

The anomalous Hall effect (AHE) in La0.7Sr0.3MnO3/SrTiO3 (LSMO/STO) superlattices was analyzed using theories of the Berry phase effect. The temperature-dependent anomalous Hall coefficient Rs is found to be expressed by -Rs = Rs0 exp[(T/T0)1.5], where Rs0 is a prefactor and T0 is in units of kelvin, and is explained by the carrier-hopping mechanism. In addition, a scaling analysis for the Hall resistivity as a function of magnetization supports the scenario of the Berry phase effects. Furthermore, a shifting extremum of Hall resistivity in LSMO/STO superlattices is observed and regarded as a probe of dimensionality in quasi-two-dimensional systems.

Journal Article↗
PubMed1992

Factors affecting antibacterial activity of hop compounds and their derivatives.

The antibacterial effect of weak acids derived from the hop plant (Humulus lupulus L.) increased with decreasing pH. Analysis of the minimum inhibitory concentration of such compounds against Lactobacillus brevis IFO 3960 over pH 4-7 suggests that undissociated molecules were mainly responsible for inhibition of bacterial growth. The antibacterial activity of trans-isohumulone was ca 20 times greater than that of humulone, 11 times greater than that of colupulone and nine times greater than that of trans-humulinic acid when the degree of ionization was taken into account. Monovalent cations (K+, Na+, NH4+, Rb+, Li+) stimulated antibacterial activity of trans-isohumulone but the effect was smaller than that observed with H+. The response to divalent cations varied: Ca2+ had little effect on antibacterial activity, whereas Mg2+ reduced activity. Lipid materials and beta-cyclodextrin also antagonized the antibacterial action of trans-isohumulone.

Anti-Bacterial Agents↗
PubMed2006

An overview of fungal community diversity in diseased hop plantations.

Samples were taken from several hop fields presenting various symptoms. Fifty-nine pure filamentous fungal strains were isolated and identified through genomic DNA preparations, PCR amplification of the ribosomal DNA internal transcribed spacer region and database interrogations. The most frequent genera were Alternaria (16 isolates) and Epicoccum (14 isolates). The ecosystem was shown to be very diverse, since as many as 27 species belonging to 17 genera were recovered. Furthermore, many of the isolated fungi are known to be involved in phytopathogenesis.

Alternaria↗
PubMed2000

Local hopping of IS3 elements into the A+T-rich part of the high-pathogenicity island in Yersinia enterocolitica 1B, O:8.

The high-pathogenicity island (Yen HPI) of Yersinia enterocolitica biogroup (BG) 1B strains is associated with mouse virulence. Three repeated sequences are clustered on the A+T-rich part of the Yen HPI downstream of the fyuA yersiniabactin receptor gene in Y. enterocolitica O:8 strains WA-314 and 8081. In addition to IS1328 and IS1400, the RS3 repeated sequence consists of a novel insertion sequence, IS1329, inserted into the remnants of IS1222. This partial IS retains both 44-bp inverted terminal repeats (ITRs) of IS1222 but has suffered deletions of different sizes in strains WA-314 and 8081. IS1329 is 1243-bp long, carries 25-bp imperfect ITRs and two consecutive orfs capable to encode 110-amino acid (aa) and 249-aa proteins, respectively. IS1329 is present only in BG 1B Y. enterocolitica strains. Similarly to IS1400, IS1329 and IS1222 belong to the IS3 group of mobile elements and seem to have preference for the 'local hopping' into the A+T-rich part of the Yen HPI. These insertion sequences may be responsible for the imprecise deletions of the Yen HPI in strain WA-314.

AT Rich Sequence↗