共查询到20条相似文献,搜索用时 15 毫秒
1.
We present a method for determining the effective driving potential for a molecular motor from measurements of its stochastic
position versus time. In developing the method we can make precise the previously vague notions of ‘Brownian ratchet’ and
‘power stroke’, and suggest means to experimentally distinguish between the two. In particular, we distinguish between two
kinds of ratchets: ratchets that rectify large fluctuations and ratchets that bias small fluctuations.
Received: 24 October 2001 / Accepted: 11 February 2002 / Published online: 22 April 2002 相似文献
2.
We perform Monte Carlo simulations of an existing electrophoretic microchannel device used for the size separation of large
DNA fragments. This device is normally operated with a constant (dc) driving field. In contrast, we consider the case of a
varying (ac) driving field, in the zero-frequency limit. We find that a time-asymmetric pulse can yield interesting migration
regimes, in particular bidirectional transport for different molecular sizes. We also study a spatially asymmetric version
of the device and show that it can rectify unbiased but non-equilibrium molecular motion, in agreement with previous predictions
for entropic ratchets. Finally, at finite frequency we uncover a resonance for the molecular velocity in the channel which
could lead to improved performance.
Received: 16 November 2001 / Accepted: 11 February 2002 / Published online: 22 April 2002 相似文献
3.
Superconducting quantum interference devices (SQUIDs) are very well suited for experimental investigations of ratchet effects.
This is due to the periodicity of the Josephson coupling energy with respect to the phase difference δ of the superconducting
macroscopic wave function across a Josephson junction. We show first that, within the resistively and capacitively shunted
junction model, the equation of motion for δ is equivalent to the motion of a particle in the so-called tilted washboard potential,
and we derive the conditions which have to be satisfied to build a ratchet potential based on asymmetric dc SQUIDs. We then
present results from numerical simulations and experimental investigations of dc SQUID ratchets with critical-current asymmetry
under harmonic excitation (periodically rocking ratchets). We discuss the impact of important properties like damping or thermal
noise on the operation of SQUID ratchets in various regimes, such as adiabatically slow or fast nonadiabatic excitation.
Received: 22 November 2001 / Accepted: 14 January 2002 / Published online: 22 April 2002 相似文献
4.
H. Linke T.E. Humphrey P.E. Lindelof A. Löfgren R. Newbury P. Omling A.O. Sushkov R.P. Taylor H. Xu 《Applied Physics A: Materials Science & Processing》2002,75(2):237-246
Quantum ratchets are Brownian motors in which the quantum dynamics of particles induces qualitatively new behavior. We review
a series of experiments in which asymmetric semiconductor devices of sub-micron dimensions are used to study quantum ratchets
for electrons. In rocked quantum-dot ratchets electron-wave interference is used to create a non-linear voltage response, leading to a ratchet effect. The direction of
the net ratchet current in this type of device can be sensitively controlled by changing one of the following experimental
variables: a small external magnetic field, the amplitude of the rocking force, or the Fermi energy. We also describe a tunneling
ratchet in which the current direction depends on temperature. In our discussion of the tunneling ratchet we distinguish between
three contributions to the non-linear current–voltage characteristics that lead to the ratchet effect: thermal excitation
over energy barriers, tunneling through barriers, and wave reflection from barriers. Finally, we discuss the operation of
adiabatically rocked tunneling ratchets as heat pumps.
Received: 8 February 2002 / Accepted: 11 February 2002 / Published online: 22 April 2002 相似文献
5.
Csaba László Sajti Svea Petersen Ana Menéndez-Manjón Stephan Barcikowski 《Applied Physics A: Materials Science & Processing》2010,101(2):259-264
In-situ functionalization of gold nanoparticles with fluorophore-tagged oligonucleotides is studied by comparing femtosecond
laser ablation in stationary liquid and in biomolecule flow. Femtosecond laser pulses induce significant degradation to sensitive
biomolecules when ablating gold in a stationary solution of oligonucleotides. Contrary, in-situ conjugation of nanoparticles
in biomolecule flow considerably reduces the degree of degradation studied by gel electrophoresis and UV–Vis spectrometry.
Ablating gold with 100 μJ femtosecond laser pulses DNA sequence does not degrade, while the degree of fluorophore tag degradation
was 84% in stationary solution compared to 5% for 1 mL/min liquid flow. It is concluded that femtosecond laser-induced degradation
of biomolecules is triggered by absorption of nanoparticle conjugates suspended in the colloid and not by ablation of the
target. Quenching of nanoparticle size appears from 0.5 μM biomolecule concentration for 0.3 μg/s nanoparticle productivity
indicating the successful surface functionalization. Finally, increasing the liquid flow rate from stationary to 450 mL/min
enhances nanoparticle productivity from 0.2 μg/s to 1.5 μg/s, as increasing liquid flow allows removal of light absorbing
nanoparticles from the ablation zone, avoiding attenuation of subsequent laser photons. 相似文献
6.
J.M.R. Parrondo B.J. de Cisneros 《Applied Physics A: Materials Science & Processing》2002,75(2):179-191
We review the literature on the energetics of Brownian motors, distinguishing between forced ratchets, chemical motors – driven
out of equilibrium by differences of chemical potential, and thermal motors – driven by temperature differences. The discussion
is focused on the definition of efficiency and the compatibility between the models and the laws of thermodynamics.
Received: 13 November 2001 / Accepted: 10 January 2002 / Published online: 22 April 2002 相似文献
7.
W. Lieber X.S. Yi N. Nontasut D. Curticapean 《Applied physics. B, Lasers and optics》2002,75(4-5):487-491
The bandwidth behavior of graded-index multimode fibers (GI-MMFs) for different launching conditions is investigated to understand
and characterize the effect of differential mode delay. In order to reduce the launch-power distribution the near field of
a single-mode fiber is used to produce a controlled restricted launch. The baseband response is measured by observing the
broadening of a narrow input pulse (time-domain measurement). The paper verifies the degradation in bandwidth due to profile
distortion by scanning the spot of the single-mode fiber with a transversal offset from the center of the test sample. In
addition, the impact of the launch-power distribution tuned by different spot-size diameters is demonstrated. Measurements
were taken on ‘older’ 50-μm and 62.5-μm GI-MMFs as well as on laser-performance-optimized fibers more recently developed.
Received: 12 November 2001 / Final version: 26 June 2002 / Published online: 25 September 2002
RID="*"
ID="*"Corresponding author. Fax: +49-781/205-242, E-mail: opto@fh-offenburg.de 相似文献
8.
Anthony J.C. Ladd 《Molecular physics》2018,116(21-22):3121-3133
ABSTRACTA shear flow breaks the spherical symmetry of a flexible polymer, which has some interesting consequences for the electrophoresis of polyelectrolytes. In addition to introducing a chain-length dependence of the electrophoretic velocity, there is also the possibility of migration of the molecule perpendicular to the direction of coaxial gradients in pressure and electric potential. This has been shown to produce a rapid and highly localised concentration of DNA within a microfluidic capillary, with a number of potential applications to on-chip preparation and analysis of genomic DNA. In this paper, dedicated to Prof. Daan Frenkel, I will describe a calculation of the electrophoretic motion of a long polyelectrolyte under a coaxial flow and electric field. 相似文献
9.
The capillary array electrophoretic NMR (CA-ENMR) was developed to study protein mixtures in biological buffer solutions of high ionic strength. By enhancing the strength of the effective electric field across the sample, the technique permits the detection of the electrophoretic motion of 1 mM lysozyme in 50 mM NaH(2)PO(4) aqueous solution, which was previously not achievable using conventional ENMR. Heat-induced convection was dramatically reduced by blocking convective current loops and by improving the efficiency of heat exchange. Thus, the capability of ENMR study of electrolyte solutions was extended from low to high ionic strength. In addition, capillary walls reduced rf-induced electrical eddy current, thereby maintaining good probe Q factors. Because of its parallel configuration to the static magnetic field, the capillary array chamber produced no susceptibility distortions of the ENMR signal. The technique offers great potential in characterizing multiple protein conformations and protein interactions in solution. 相似文献
10.
D.C. Cabra G.L. Rossini 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,29(4):529-532
We study the breaking of parity symmetry in the 2+1 Gross-Neveu model at finite temperature with chemical potential μ, in
the presence of an external magnetic field. We find that the requirement of gauge invariance, which is considered mandatory
in the presence of gauge fields, breaks parity at any finite temperature and provides for dynamical mass generation, preventing
symmetry restoration for any non-vanishing μ. The dynamical mass becomes negligibly small as temperature is raised.
Received 4 April 2002 / Received in final form 12 July 2002 Published online 15 October 2002
RID="a"
ID="a"e-mail: cabra@venus.fisica.unlp.edu.ar 相似文献
11.
M.W. Todd R.A. Provencal T.G. Owano B.A. Paldus A. Kachanov K.L. Vodopyanov M. Hunter S.L. Coy J.I. Steinfeld J.T. Arnold 《Applied physics. B, Lasers and optics》2002,75(2-3):367-376
A novel instrument, based on cavity-ringdown spectroscopy (CRDS), has been developed for trace gas detection. The new instrument
utilizes a widely tunable optical parametric oscillator (OPO), which incorporates a zinc–germanium–phosphide (ZGP) crystal
that is pumped at 2.8 μm by a 25-Hz Er,Cr:YSGG laser. The resultant mid-IR beam profile is nearly Gaussian, with energies
exceeding 200 μJ/pulse between 6 and 8 μm, corresponding to a quantum conversion efficiency of approximately 35%. Vapor-phase
mid-infrared spectra of common explosives (TNT, TATP, RDX, PETN and Tetryl) were acquired using the CRDS technique. Parts-per-billion
concentration levels were readily detected with no sample preconcentration. A collection/flash-heating sequence was implemented
in order to enhance detection limits for ambient air sampling. Detection limits as low as 75 ppt for TNT are expected, with
similar concentration levels for the other explosives.
Received: 1 April 2002 / Revised version: 13 June 2002 / Published online: 12 September 2002
RID="*"
ID="*"Corresponding author. Fax: +1-408/524-0551, E-mail: mtodd@picarro.com 相似文献
12.
Field emission from single-walled carbon nanotubes (SWNTs) aligned on a patterned gold surface is reported. The SWNT emitters
were prepared at room temperature by a self-assembly monolayer technique. SWNTs were cut into sub-micron lengths by sonication
in an acidic solution. Cut SWNTs were attached to the gold surface by the reaction between the thiol groups and the gold surface.
The field-emission measurements showed that the turn-on field was 4.8 V/μm at an emission current density of 10 μA/cm2. The current density was 0.5 mA/cm2 at 6.6 V/μm. This approach provides a novel route for fabricating CNT-based field-emission displays.
Received: 3 May 2002 / Accepted: 6 May 2002 / Published online: 4 December 2002
RID="*"
ID="*"Corresponding author. Fax: +82-54/279-8298, E-mail: ce20047@postech.ac.kr 相似文献
13.
A. Crunteanu M. Pollnau G. Jänchen C. Hibert P. Hoffmann R.P. Salathé R.W. Eason C. Grivas D.P. Shepherd 《Applied physics. B, Lasers and optics》2002,75(1):15-17
We report, to our knowledge, the first active channel waveguide in Ti:sapphire. We have created ∼1.4-μm high ribs in a ∼10-μm
thick Ti:sapphire planar waveguide by reactive ion etching. Following excitation by an Ar-ion laser, the rib structure showed
channel-waveguide fluorescence emission. The mode profiles and the beam-parameter values (M2) were measured. The coupling efficiency of fluorescence emission into a single-mode fiber was an order of magnitude higher
than for fluorescence from unstructured planar regions of the waveguide. Such devices are of interest as low-threshold tunable
lasers and as broadband light sources in low-coherence interferometry.
Received: 22 December 2002 / Revised version: 30 March 2002 / Published online: 8 August 2002 相似文献
14.
A. Martin P. Odier J.-F. Pinton S. Fauve 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,18(2):337-341
Magnetohydrodynamics studies in laboratory experiments have long been restricted to low magnetic Reynolds number flows, mainly
as a result of the very high magnetic diffusivity λ = 1/μσ of common conducting fluids (μ is the fluid's magnetic permeability
and σ its electrical conductivity). The best conductivities are found in liquid metals which have a unit magnetic permeability,
relative to vacuum. We show experimentally that a suspension of solid particles with a high magnetic permeability in a liquid
metal yields an effective medium that has a high electrical conductivity and an enhanced magnetic permeability. The dispersion
of the beads results from the turbulent fluid motion. The range of accessible magnetic Reynolds number can be increased by
a factor of as much as 4 in our experimental setup.
Received 6 March 2000 and Received in final form 13 July 2000 相似文献
15.
A. K. Murugesh A. Uthayanan C. Lekakou 《Applied Physics A: Materials Science & Processing》2010,100(1):135-144
This paper contains an in-depth analysis of the electrophoresis of multi-wall carbon nanotubes (MWNTs) in liquid epoxy where
electrophoresis experiments under DC and AC fields were carried out for five different types of multi-wall carbon nanotubes
(MWNTs). DC electrophoresis and particle image velocimetry were used to determine the electrophoretic particle mobility and
zeta potential, where the MWNTs with the largest outer diameter and length led to the highest mobility values. The orientation
and agglomeration of MWNTs into “striation” lines under AC electrophoresis were investigated by analysing the hue, saturation
and intensity of the transmitted polarised light under microscope, following a schedule of step-wise applied voltage in the
range of 0 to 100 V. Plots of hue and saturation as a function of the applied voltage were used to assess the degree of orientation
and density of orientated MWNT structures, respectively, and to determine an optimum AC electric field value for the orientation
of a specific MWNT type by electrophoresis. 相似文献
16.
A. K. Vershovskii M. V. Balabas A. É. Ivanov V. N. Kulyasov A. S. Pazgalev E. B. Aleksandrov 《Technical Physics》2006,51(1):112-117
A new fast three-component variometer-magnetometer based on a cesium sensor is developed and tested. The device is intended
for measuring the longitudinal component of the geomagnetic field in the range from 20 to 65 μT and two transverse components
in the range ±1 μT. The reproducibility of measurements is ±0.15 nT, and the noise-limited sensitivity is 0.01 nT (in terms
of the rms deviation) or 0.04˝ for a measurement time of 0.1 s. 相似文献
17.
M. Dogterom M.E. Janson C. Faivre-Moskalenko A. van der Horst J.W.J. Kerssemakers C. Tanase B.M. Mulder 《Applied Physics A: Materials Science & Processing》2002,75(2):331-336
Polymerization ratchets formed by the assembly of actin filaments and microtubules are possibly the simplest realizations
of biological thermal ratchets. A variety of experimental evidence exists that significant forces are generated by these processes,
but quantitative studies lag far behind similar studies for molecular motors such as kinesin and myosin. Here we present a
discussion of the theory of polymerization ratchets as well as experimental techniques used in our laboratory for the study
of forces generated by single growing microtubules. Data obtained with these techniques provide us with valuable information
that may eventually allow us to distinguish between different models for the growth of microtubules.
Received: 15 January 2002 / Accepted: 11 February 2002 / Published online: 22 April 2002 相似文献
18.
Thin TiO2 films obtained by cathodic electrosynthesis from an acidic aqueous bath containing TiOSO4, H2O2 and KNO3 on conductive glass indium tin oxide have been physically and electrochemically characterised. Secondary ion mass spectroscopy
profile of the crystallised gel after heat treatment at 400 °C shows the presence of TiO2 with traces of TiO and oxygen. X-ray patterns confirm the presence of anatase nanocrystallites for the annealed film and
an amorphous structure for the non-annealed gel. Scattering electron microscopy surface micrographies reveal an opened porous
nanostructure of the deposits. Cyclic voltammetry and impedance spectroscopic measurements reveal the different behaviour
of the films obtained before and after the annealing, showing an important electrical activity of the non-annealed films.
The dependence of capacitance values with potential in the anodic domain of depletion is obviously remarked from impedance
plots for both gel and crystal forms of the film, which confirmed the fact that films obtained in this way have n-type properties.
The potential of flat band equals −0.6 V/Ag/AgCl in pH range of 6.5 has been estimated according to Mott–Shottky curves for
the crystallised oxide; meanwhile, the Mott–Shottky curve for the hydrated gel was nonlinear. 相似文献
19.
J. Békési S. Szatmári P. Simon G. Marowsky 《Applied physics. B, Lasers and optics》2002,75(4-5):521-524
Applying the combination of a solid-state Ti:Sa laser system and a newly developed wide-aperture, discharge-pumped KrF amplifier,
output pulses with over 9 W average power at 300 Hz have been achieved in a single output beam. The frequency-tripled seed
pulses of the Ti:Sa system – delivering approximately 10 μJ energy at 248 nm – were amplified to over 30 mJ using a 3-pass
off-axis amplification scheme. The optical set-up has been fitted to the amplifier’s parameters, and stored-energy measurements
were carried out with different parameters in order to optimize the operational conditions of the device for the highest average
power.
Received: 10 June 2002 / Published online: 25 September 2002
RID="*"
ID="*"Corresponding author. Fax: +49-551/503-599, E-mail: jbekesi@llg.gwdg.de 相似文献
20.
We study the cosmic time evolution of an effective quantum field theory energy-momentum tensor T
μν
and show that, as a consequence of the effective nature of the theory, T
μν
is such that the vacuum energy decreases with time. We find that the zero point energy at present time is washed out by the
cosmological evolution. The implications of this finding for the cosmological constant problem are investigated. 相似文献