Thermal denaturation of DNA studied with neutron scattering

The melting transition of DNA, whereby the strands of the double-helix structure completely separate at a certain temperature, has been characterized using neutron scattering. A Bragg peak from B-form fiber DNA has been measured as a function of temperature, and its widths and integrated intensities have been interpreted using the Peyrard-Bishop-Dauxois model with only one free parameter. The experiment is unique, as it gives spatial correlation along the molecule through the melting transition where other techniques cannot.     

Photoelectron angular distributions from O K shell of oriented CO molecules: A critical comparison between theory and experiment

The dynamical information (ten dipole matrix elements and eight phase differences) has been deduced from the measured angular distributions of photoelectrons from O K shell of oriented CO molecules near the ionization threshold in the region of a sigma(*) shape resonance. Light polarization parallel and perpendicular to the molecular axis has been used. An important contribution of six lsigma partial waves with 0相似文献   

Nonlinear dynamics of a convection loop: A quantitative comparison of experiment with theory

The flow in a loop of fluid, heated at constant flux over the bottom half and cooled at constant temperature over the top half, is shown to be qualitatively and quantitatively described by the nonlinear dynamics of the Lorenz model over a wide range of parameters encompassing three different chaotic flow regimes. The boundaries of the five flow regimes are determined experimentally and compared with a scaling relationship which relates the physical parameters of the loop and working fluid to the parameters of the Lorenz model. The characteristics of the three chaotic flow regimes are also discussed.     

Leak rate of seals: Effective-medium theory and comparison with experiment

Seals are extremely useful devices to prevent fluid leakage. We present an effective-medium theory of the leak rate of rubber seals, which is based on a recently developed contact mechanics theory. We compare the theory with experimental results for seals consisting of silicon rubber in contact with sandpaper and sand-blasted PMMA surfaces.     

Thermal denaturation of fluctuating DNA driven by bending entropy

A statistical model of homopolymer DNA, coupling internal base-pair states (unbroken or broken) and external thermal chain fluctuations, is exactly solved using transfer kernel techniques. The dependence on temperature and DNA length of the fraction of denaturation bubbles and their correlation length is deduced. The thermal denaturation transition emerges naturally when the chain fluctuations are integrated out and is driven by the difference in bending (entropy dominated) free energy between broken and unbroken segments. Conformational properties of DNA, such as persistence length and mean-square-radius, are also explicitly calculated, leading, e.g., to a coherent explanation for the experimentally observed thermal viscosity transition.     

Shape resonances of oriented molecules: ab initio theory and experiment on hydrocarbon molecules

We report ab initio calculations of the x-ray absorption cross section for the near edge x-ray absorption fine structure of C2H6, and C2H4, and C2H2 at the C K-edge, based on a full multiple scattering formalism. The angular dependence of the electric dipole transition in the calculations as well as the angular dependent experiments for the oriented molecules give a good opportunity to compare both. The resonance can be assigned to a sigma(*) shape resonance. The multiple scattering formalism and the experiment agree well and thereby support the existence of such features in the spectra.     

DNA单分子弹性理论

随着单分子操纵技术的发明与发展，人们已经可以对单个生物大分子施以力或力矩，并测量它们的物理性质，DNA单分子的力学实验表明，在分子尺度上理解生物大分子的生化过程，力与能量是同等重要的结构与功能参数。一个梯子模型被用来描述双链DNA的外力拉伸曲线，在这个模型中，DNA是由许多碱基对(梯子的横杆，横杆之间存在吸引势)连接两条聚核苷酸虫链(梯子的两侧)形成的高分子，利用路径积分法得出的理论曲线与实验曲线吻合得很好，对于单链DNA，用分立的杂化高分子链统计理论的母函数方法来计算其弹性行为，得出与实验相符合的外力引起的解链相变结果。此外，对于抑瘤蛋白p53识别序列DNA微环弹性进行分析，发现其弹性模量只是通常随机序列的三分之一。     

Transient Lamb waves: comparison between theory and experiment

This paper presents an experimental investigation of the generation of transient Lamb waves inside an immersed plate by a time limited signal. The transient Lamb waves are the solutions of the characteristic equation of the plate for complex frequency and real slowness, in contrast to the leaky Lamb waves, for which the slowness is complex and the frequency is real. By using a large transducer in agreement with the real slowness assumption, the excitation of these transient Lamb waves is observed experimentally both in reflection and transmission for a tone burst excitation. Particular attention is paid to the A0 mode for which the amplitude increases exponentially in time. As a result, the total reflected waveform contains in this case a transient contribution, coming from the generation of the A0 mode, that precedes the forced regime. It is shown that there is a very good agreement between the theoretical and the experimental time exponential amplitude variations of the transient Lamb waves. For the A0 mode, the transient signal is qualitatively explained by a group velocity higher than the phase velocity.     

Far-infrared electrodynamics of superconducting Nb: comparison of theory and experiment

Complex conductivity spectra of superconducting Nb are calculated from the first principles in the frequency region around the energy gap and compared with the experimental results. The raw experimental data obtained on thin films can be precisely described by these calculations.     

Directed motion for delta-kicked atoms with broken symmetries: comparison between theory and experiment

We report an experimental investigation of momentum diffusion in the delta-function kicked rotor where time symmetry is broken by a two-period kicking cycle and spatial symmetry by an alternating linear potential. We exploit this, and a technique involving a moving optical potential, to create an asymmetry in the momentum diffusion that is due to the classical chaotic diffusion. This represents a realization of a type of Hamiltonian quantum ratchet.     

The semiclassical theory of a three-mirror laser and its comparison with experiment

Semiclassical laser theory is used to analyse the operation of a single-mode three-mirror laser system. Experimental results, using a precise and highly flexible on-line computer system, show excellent agreement with theory.     

A comparison of experiment and theory for sound propagation in variable area ducts

An experimental and analytical program has been carried out to evaluate sound suppression techniques in ducts that produce refraction effects due to axial velocity gradients. The analytical program employs a computer code based on the method of multiple scales to calculate the influence of axial variations due to slow changes in the cross-sectional area as well as transverse gradients due to the wall boundary layers. Detailed comparisons between the analytical predictions and the experimental measurements have been made. The circumferential variations of pressure amplitudes and phases at several axial positions have been examined in straight and variable area ducts, with hard walls and lined sections, and with and without a mean flow. Reasonable agreement between the theoretical and experimental results has been found.     

Polariton luminescence and additional boundary conditions: comparison between theory and experiment

Polariton luminescence spectra in semiconductors have been calculated as a function of temperature and impurity trapping rates using a two branch polariton model. Typically, the theoretical spectra show two structures: a peak near the transverse exciton energy and a shoulder near the longitudinal exciton energy. While the lower energy peak is relatively insensitive to the choice of additional boundary conditions (ABC), the higher energy shoulder strongly depends on ABC. The theoretical curves obtained with Pekar's ABC are in quantitative agreement with experimental spectra reported in CdS, CdSe and CuCl. However, significant discrepancies between theory and experiment are found in GaAs and ZnTe.     

Longitudinal parametric oscillation in photorefractive sillenites: comparison between theory and experiment

By using an alternative setup for photorefractive parametric oscillation in which wave mixing between the recording beams is avoided it has become possible to make more detailed comparisons with the space-charge wave theory. In the present paper we compare the experimental features of longitudinal parametric oscillation observed in a crystal of Bi₁₂SiO₂₀ with the theoretical predictions. Received: 24 November 1998 / Revised version: 13 January 1999 / Published online: 7 April 1999     

Critical temperature of a trapped Bose gas: comparison of theory and experiment

We apply the projected Gross-Pitaevskii equation (PGPE) formalism to the experimental problem of the shift in critical temperature Tc of a harmonically confined Bose gas as reported in Gerbier et al., Phys. Rev. Lett. 92, 030405 (2004). The PGPE method includes critical fluctuations and we find the results differ from various mean-field theories, and are in best agreement with experimental data. To unequivocally observe beyond mean-field effects, however, the experimental precision must either improve by an order of magnitude, or consider more strongly interacting systems. This is the first application of a classical field method to make quantitative comparison with experiment.