1,760 research outputs found
Direct link between Coulomb blockade and shot noise in a quantum coherent structure
We analyze the current-voltage characteristic of a quantum conduction channel
coupled to an electromagnetic environment of arbitrary frequency-dependent
impedance. In the weak blockade regime the correction to the ohmic behavior is
directly related to the channel current fluctuations vanishing at perfect
transmission in the same way as shot noise. This relation can be generalized to
describe the environmental Coulomb blockade in a generic mesoscopic conductor
coupled to an external impedance, as the response of the latter to the current
fluctuations in the former.Comment: 12 pages, 2 figures, submitted to Phys. Rev. Let
Evidence for saturation of channel transmission from conductance fluctuations in atomic-size point contacts
The conductance of atomic size contacts has a small, random, voltage
dependent component analogous to conductance fluctuations observed in diffusive
wires (UCF). A new effect is observed in gold contacts, consisting of a marked
suppression of these fluctuations when the conductance of the contact is close
to integer multiples of the conductance quantum. Using a model based on the
Landauer-Buettiker formalism we interpret this effect as evidence that the
conductance tends to be built up from fully transmitted (i.e., saturated)
channels plus a single, which is partially transmitted.Comment: An error in Eq.(2) was corrected, where a square root was added to
the factor (1-cos(gamma)). This results in a revised estimate for the mean
free path of 5 nm, which is now fully consistent with the estimates from the
series resistance and the thermopowe
Quantum Conductance in Semimetallic Bismuth Nanocontacts
Electronic transport properties of bismuth nanocontacts are analyzed by means
of a low temperature scanning tunneling microscope. The subquantum steps
observed in the conductance versus elongation curves give evidence of atomic
rearrangements in the contact. The underlying quantum nature of the conductance
reveals itself through peaks in the conductance histograms. The shape of the
conductance curves at 77 K is well described by a simple gliding mechanism for
the contact evolution during elongation. The strikingly different behaviour at
4 K suggests a charge carrier transition from light to heavy ones as the
contact cross section becomes sufficiently small.Comment: 5 pages including 4 figures. Accepted for publication in Phys. Rev.
Let
Identificação de métodos para superação da dormência em sementes de cornichão.
O cornichão é uma leguminosa forrageira amplamente utilizada no Rio Grande do Sul que se destaca por sua versatilidade, tolerância à acidez e à baixa fertilidade, além de excelente valor nutritivo, conferido por elevados teores de proteína e digestibilidade. Entretanto, um dos problemas da espécie é seu estabelecimento lento, devido à dormência tegumentar, exigindo o emprego de tratamentos pré-germinativos que permitam a superação da dormência das sementes e expressão da máxima germinação do lote. O objetivo do presente trabalho foi identificar métodos eficientes, rápidos, seguros e de fácil padronização para a superação da dormência de sementes de cornichão. O trabalho foi conduzido no Laboratório de Análise de Sementes da Embrapa Clima Temperado e foram utilizadas sementes de cornichão de uma linhagem avançada de melhoramento. As sementes foram submetidas a doze tratamentos visando à superação da dormência: pré-esfriamento (10 ºC) por 7 dias, imersão em água a temperatura ambiente por 8 e 16 horas, escarificação mecânica com lixa por 20, 40, 60, 80 e 100 segundos, a 1750 rpm, e escarificação mecânica com revestimento das pás do cilindro do escarificador com borracha de 0,7 cm de espessura, por 5, 10, 15 e 20 segundos. Após cada tratamento, as sementes foram submetidas ao teste de germinação a 20-30 ºC, por 12 dias, sendo avaliadas quanto à percentagem de germinação. O tratamento de pré-esfriamento, atualmente recomendado para superação da dormência das sementes de cornichão pelas Regras para Análise de Sementes, não foi eficiente para promover a germinação das mesmas. A escarificação mecânica com revestimento das pás do cilindro do escarificador com borracha, por 5 segundos, é eficiente para superação da dormência de sementes de cornichão.Título em inglês: Identification of methods to overcome seed dormancy in lotus
Connective neck evolution and conductance steps in hot point contacts
Dynamic evolution of the connective neck in Al and Pb mechanically
controllable break junctions was studied during continuous approach of
electrodes at bias voltages V_b up to a few hundred mV. A high level of power
dissipation (10^-4 - 10^-3 W) and high current density (j > 10^10 A/cm^2) in
the constriction lead to overheating of the contact area, electromigration and
current-enhanced diffusion of atoms out of the "hot spot". At a low electrode
approach rate (10 - 50 pm/s) the transverse dimension of the neck and the
conductance of the junction depend on V_b and remain nearly constant over the
approach distance of 10 - 30 nm. For V_b > 300 mV the connective neck consists
of a few atoms only and the quantum nature of conductance manifests itself in
abrupt steps and reversible jumps between two or more levels. These features
are related to an ever changing number of individual conductance channels due
to the continuous rearrangement in atomic configuration of the neck, the
recurring motion of atoms between metastable states, the formation and breaking
of isolated one-atom contacts and the switching between energetically
preferable neck geometries.Comment: 21 pages 10 figure
Subharmonic gap structure in d-wave superconductors
We present a self-consistent theory of current-voltage characteristics of
d-wave/d-wave contacts at arbitrary transparency. In particular, we address the
open problem of the observation of subharmonic gap structure (SGS) in cuprate
junctions. Our analysis shows that: (i) the SGS is possible in d-wave
superconductors, (ii) the existence of bound states within the gap results in
an even-odd effect in the SGS, (iii) elastic scattering mechanisms, like
impurities or surface roughness, may suppress the SGS, and (iv) in the presence
of a magnetic field the Doppler shift of the Andreev bound states leads to a
very peculiar splitting of the SGS, which is an unambiguous fingerprint of
d-wave superconductivity.Comment: Revtex4, 4 pages, 5 figure
The Formal Dynamism of Categories: Stops vs. Fricatives, Primitivity vs. Simplicity
Minimalist Phonology (MP; Pöchtrager 2006) constructs its theory based on the phonological epistemological principle (Kaye 2001) and exposes the arbitrary nature of standard Government Phonology (sGP) and strict-CV (sCV), particularly with reference to their confusion of melody and structure.
For Pöchtrager, these are crucially different, concluding that place of articulation is melodic (expressed with elements), while manner of articulation is structural. In this model, the heads (xN and xO) can license and incorporate the length of the other into their own interpretation, that is xN influences xO projections as well as its own and vice versa. This dynamism is an aspect of the whole framework and this paper in particular will show that stops and fricatives evidence a plasticity of category and that, although fricatives are simpler in structure, stops are the more primitive of the two.
This will be achieved phonologically through simply unifying the environment of application of the licensing forces within Pöchtrager's otherwise sound onset structure. In doing so, we automatically make several predictions about language acquisition and typology and show how lenition in Qiang (Sino-Tibetan) can be more elegantly explained
From favorable atomic configurations to supershell structures: a new interpretation of conductance histograms
Title: From favorable atomic configurations to supershell structures: a new
interpretation of conductance histograms Authors: A. Hasmy (IVIC), E. Medina
(IVIC), P.A. Serena (CSIC,IVIC) Comments: 7 pages, 3 figures,
cond-mat.anwar.10825 Subj-class: Soft Condensed MatterComment: 7 pages, 3 figuresSubject: fput HMS.tex HMS-FIG1.ps HMS-FIG2.ps
HMS-FIG3.p
Transitions between "" and "0" states in superconductor -- ferromagnet -- superconductor junctions
Experimental and theoretical study of superconductor (S) -- ferromagnet (F)
-- superconductor junctions showed that in certain range of parameters (e.g.,
the length of the ferromagnet , the exchange field, ) the
ground state of a SFS junction corresponds to superconducting phase difference
or 0. The phase diagram of a SFS junction with the respect to and 0
states is investigated in this letter in space. It is
shown that the phase diagram is very sensitive to the geometry of the system,
in particular, to the amount of disorder.Comment: Minor changes. References adde
Subharmonic Shapiro steps and assisted tunneling in superconducting point contacts
We analyze the current in a superconducting point contact of arbitrary
transmission in the presence of a microwave radiation. The interplay between
the ac Josephson current and the microwave signal gives rise to Shapiro steps
at voltages V = (m/n) \hbar \omega_r/2e, where n,m are integer numbers and
\omega_r is the radiation frequency. The subharmonic steps (n different from 1)
are a consequence of the ocurrence of multiple Andreev reflections (MAR) and
provide an unambiguous signature of the peculiar ac Josephson effect at high
transmission. Moreover, the dc current exhibits a rich subgap structure due to
photon-assisted MARs.Comment: Revtex, 4 pages, 4 figure
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