Contribution Of Thermal Fluctuations Research Articles

Energy shift of a uniformly moving two-level atom through a thermal reservoir

We investigate the implications of an atomic constant velocity in the energy shift of a two-level atom inside the thermal bath of a quantum scalar field, which is described by the Bose-Einstein distribution. The use of DDC formalism shows that the contribution of thermal fluctuations on the atomic level shifts depends on the atomic velocity and the temperature of the heat reservoir but the contribution of radiation reaction is totally insusceptible. The resulting energy shifts are analyzed and examined in detail under different circumstances. The atomic uniform linear motion always broadens the atomic level spacing in the limit of low temperature but narrows down it in the limit of high temperature. Our work clearly indicates that the moving heat reservoir shifts the atomic levels in a way quite different from that of the static one.

Settling of a vesicle in the limit of quasispherical shapes

AbstractVesicles are drops of radius of a few tens of micrometres bounded by an impermeable lipid membrane of approximately 4 nm thickness in a viscous fluid. The salient characteristics of such a deformable object are a membrane rigidity governed by flexion due to curvature energy and a two-dimensional membrane fluidity characterized by a local membrane incompressibility. This provides unique properties with strong constraints on the internal volume and membrane area. Yet, when subjected to external stresses, vesicles exhibit a large deformability. The deformation of a settling vesicle in an infinite flow is studied theoretically, assuming a quasispherical shape and expanding all variables of the problem onto spherical harmonics. The contribution of thermal fluctuations is neglected in this analysis. A system of equations describing the temporal evolution of the shape is derived with this formalism. The final shape and the settling velocity are then determined and depend on two dimensionless parameters: the Bond number and the excess area. This simultaneous study leads to three stationary shapes, an egg-like shape already observed in an analogous experimental configuration in the limit of weak flow magnitude (Chatkaew, Georgelin, Jaeger & Leonetti, Phys. Rev. Lett, 2009, vol. 103(24), 248103), a parachute-like shape and a non-trivial non-axisymmetrical shape. The final shape depends on the initial conditions: prolate or oblate vesicle and orientation compared with gravity. The analytical solution in the small deformation regime is compared with numerical results obtained with a three-dimensional code. A very good agreement between numerical and theoretical results is found.

Contribution of the intrinsic curvature to measured DNA persistence length

The persistence length of DNA, a, depends both on the intrinsic curvature of the double helix and on the thermal fluctuations of the angles between adjacent base-pairs. We have evaluated two contributions to the value of a by comparing measured values of a for DNA containing a generic sequence and for an “intrinsically straight” DNA. In each 10 bp segment of the intrinsically straight DNA an initial sequence of five bases is repeated in the sequence of the second five bases, so any bends in the first half of the segment are compensated by bends in the opposite direction in the second half. The value of a for the latter DNA depends, to a good approximation, on thermal fluctuations only; there is no intrinsic curvature. The values of a were obtained from measurements of the cyclization efficiency for short DNA fragments, about 200 bp in length. This method determines the persistence length of DNA with exceptional accuracy, due to the very strong dependence of the cyclization efficiency of short fragments on the value of a. We find that the values of a for the two types of DNA fragment are very close and conclude that the contribution of the intrinsic curvature to a is at least 20 times smaller than the contribution of thermal fluctuations. The relationship between this result and the angles between adjacent base-pairs, which specify the intrinsic curvature, is analyzed.

RDS and IRDS filters for high-speed CCD video sensors

In this brief, we present two filters called "reflection delayed noise suppression" (RDS) and "integration reflection delayed noise suppression" (IRDS) which are useful to increase dynamics of video charge-coupled device (CCD) cameras. The RDS is a band-pass filter built with a parallel short-circuited line. It significantly lowers the most important noise, called reset noise. Unfortunately, this signal processing unit increases the contribution of thermal fluctuations by at least 3 dB. To overcome this disadvantage, a reset low-pass filter can be added to the RDS cell, which leads to the IRDS method. For both methods, we will describe the principle and compare their effects in a theoretical way, and by experimentation on sensing-node amplifier and reset noise in the CCD sensor.

RDS and IRDS filters for fast CCD video sensors

In this paper we present two filters called Reflection Delayed noise Suppression RDS and Integration Reflection Delayed noise Suppression IRDS which are useful to increase the dynamics of video CCD cameras. The RDS is a band-pass filter built with a parallel short-circuited line. It lowers significantly the most important noise which is called reset noise. Unfortunately, this signal processing unit increases the contribution of thermal fluctuations by about 3 dB. To overcome this disadvantage, a triggered low-pass filter can be added to the RDS which leads to the IRDS cell. First, we will describe both systems, and then we will compare their effects, in a theoretical way and by experimentation, on the unsampled noises ( i.e. the thermal and flicker noises) generated in the CCD sensor.

Nuclear level densities including thermal and quantal fluctuations.

The nuclear level density for solvable model Hamiltonians is calculated as a function of the excitation energy, taking into account both large-amplitude thermal and small-amplitude quantal fluctuations. The formalism, which is applied to simple systems displaying a phase transition (deformed to spherical, superfluid to normal) as a function of the temperature, accurately reproduces the exact solutions. The contribution of thermal fluctuations changes the level density by an order of magnitude. Quantal fluctuations are essential to reproduce the absolute energies of the system and thus to relate the excitation energy to temperature.

Contribution of thermal fluctuations to the scattering and the gauge effect of longevity

The distribution function of longevity of a sample under load due to thermal fluctuations is calculated. It reveals a size effect. Fair agreement is observed between theoretical results and available experimental data for a series containing about 102 samples. This allows us to suppose that the usually observed scattering and the size effect of longevity are predominantly due to the thermally activated nature of the fracture process.