3,956 research outputs found
Theory of Bubble Nucleation and Cooperativity in DNA Melting
The onset of intermediate states (denaturation bubbles) and their role during
the melting transition of DNA are studied using the Peyrard-Bishop-Daxuois
model by Monte Carlo simulations with no adjustable parameters. Comparison is
made with previously published experimental results finding excellent
agreement. Melting curves, critical DNA segment length for stability of bubbles
and the possibility of a two states transition are studied.Comment: 4 figures. Accepted for publication in Physical Review Letter
Density of states of colloidal glasses
Glasses are structurally liquid-like, but mechanically solid-like. Most
attempts to understand glasses start from liquid state theory. Here we take the
opposite point of view, and use concepts from solid state physics. We determine
the vibrational modes of a colloidal glass experimentally, and find soft
low-frequency modes that are very different in nature from the usual acoustic
vibrations of ordinary solids. These modes extend over surprisingly large
length scales
Crystal growth and ambient and high pressure study of the reentrant superconductor Tm_2Fe_3Si_5
We report single crystal growth of the reentrant superconductor Tm_2Fe_3Si_5,
and measurements of the anisotropic static magnetic susceptibility \chi(T) and
isothermal magnetization M(H), ac susceptibility \chi_ac(T), electrical
resistivity \rho(T) and heat capacity C(T) at ambient pressure and \chi_ac(T)
at high pressure. The magnetic susceptibility along the c-axis \chi_c(T) shows
a small maximum around 250 K and does not follow the Curie-Weiss behavior while
the magnetic susceptibility along the a-axis \chi_a(T) follows a Curie-Weiss
behavior between 130 K and 300 K with a Weiss temperature \theta and an
effective magnetic moment \mu_eff which depend on the temperature range of the
fit. The easy axis of magnetization is perpendicular to the c-axis and
\chi_a/\chi_c = 3.2 at 1.8 K. The ambient pressure \chi_ac(T) and C(T)
measurements confirm bulk antiferromagnetic ordering at T_N = 1.1 K. The sharp
drop in \chi_ac below T_N is suggestive of the existence of a spin-gap. We
observe superconductivity only under applied pressures P\geq 2 kbar. The
temperature-pressure phase diagram showing the non-monotonic dependence of the
superconducting transition temperature T_c on pressure P is presented.Comment: 7 pages, 8 figure
Improved superlensing in two-dimensional photonic crystals with a basis
We study propagation of light in square and hexagonal two-dimensional
photonic crystals. We show, that slabs of these crystals focus light with
subwavelength resolution. We propose a systematic way to increase this
resolution, at an essentially fixed frequency, by employing a hierarchy of
crystals of the same structure, and the same lattice constant, but with an
increasingly complex basis.Comment: 16 Pages, 5 Figure
Electron spin relaxation via flexural phonon modes in semiconducting carbon nanotubes
This work considers the g-tensor anisotropy induced by the flexural thermal
vibrations in one-dimensional structures and its role in electron spin
relaxation. In particular, the mechanism of spin-lattice relaxation via
flexural modes is studied theoretically for localized and delocalized
electronic states in semiconducting carbon nanotubes in the presence of
magnetic field. The calculation of one-phonon spin-flip process predicts
distinctive dependencies of the relaxation rate on temperature, magnetic field
and nanotube diameter. Comparison with the spin relaxation caused by the
hyperfine interaction clearly suggests the relative efficiency of the proposed
mechanism at sufficiently high temperatures. Specifically, the longitudinal
spin relaxation time in the semiconducting carbon nanotubes is estimated to be
as short as 30 microseconds at room temperature.Comment: 18 pages, 7 figure
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