316 research outputs found

### Two-Nucleon L·S Potential in Pseudoscalar Meson Theory

Nonstatic corrections to the two-nucleon potential of Brueckner and Watson and of Gartenhaus are computed within the framework of the γ5 theory. These terms appear as spin-orbit corrections of order μ/M to the static potentials.
The S matrix is calculated in second and fourth order for a reduced form of the relativistic theory. The potential is then chosen so as to duplicate this S matrix to the required order in the coupling constant and μ/M. We consider to what extent our reduction of the γ5 theory changes its character.
The resulting potentials are given in analytic form for no cutoff in momentum space and in numerical form for the Gaussian cutoff employed by Gartenhaus. We give also some additional static corrections to previous potentials. A qualitative comparison is made with the experimental observations in nucleon-nucleon scattering, the fine structure in the splitting of the He5 nucleus, and the contribution of the nonstatic potential to the magnetic moment of the deuteron

### A new electromagnetic mode in graphene

A new, weakly damped, {\em transverse} electromagnetic mode is predicted in
graphene. The mode frequency $\omega$ lies in the window
$1.667<\hbar\omega/\mu<2$, where $\mu$ is the chemical potential, and can be
tuned from radiowaves to the infrared by changing the density of charge
carriers through a gate voltage.Comment: 5 pages, 4 figure

### Artificial trapping of a stable high-density dipolar exciton fluid

We present compelling experimental evidence for a successful electrostatic
trapping of two-dimensional dipolar excitons that results in stable formation
of a well confined, high-density and spatially uniform dipolar exciton fluid.
We show that, for at least half a microsecond, the exciton fluid sustains a
density higher than the critical density for degeneracy if the exciton fluid
temperature reaches the lattice temperature within that time. This method
should allow for the study of strongly interacting bosons in two dimensions at
low temperatures, and possibly lead towards the observation of quantum phase
transitions of 2D interacting excitons, such as superfluidity and
crystallization.Comment: 11 pages 4 figure

### Imaging density disturbances in water with 41.3 attosecond time resolution

We show that the momentum flexibility of inelastic x-ray scattering may be
exploited to invert its loss function, alowing real time imaging of density
disturbances in a medium. We show the disturbance arising from a point source
in liquid water, with a resolution of 41.3 attoseconds ($4.13 \times 10^{-17}$
sec) and 1.27 $\AA$ ($1.27 \times 10^{-8}$ cm). This result is used to
determine the structure of the electron cloud around a photoexcited molecule in
solution, as well as the wake generated in water by a 9 MeV gold ion. We draw
an analogy with pump-probe techniques and suggest that energy-loss scattering
may be applied more generally to the study of attosecond phenomena.Comment: 4 pages, 4 color figure

### Magnetic Field Induced Insulating Phases at Large $r_s$

Exploring a backgated low density two-dimensional hole sample in the large
$r_s$ regime we found a surprisingly rich phase diagram. At the highest
densities, beside the $\nu=1/3$, 2/3, and 2/5 fractional quantum Hall states,
we observe both of the previously reported high field insulating and reentrant
insulating phases. As the density is lowered, the reentrant insulating phase
initially strengthens, then it unexpectedly starts weakening until it
completely dissapears. At the lowest densities the terminal quantum Hall state
moves from $\nu=1/3$ to $\nu=1$. The intricate behavior of the insulating
phases can be explained by a non-monotonic melting line in the $\nu$-$r_s$
phase space

### Mobility of slow electrons in a polar crystal

We have obtained an approximate expression for the impedance function at all frequencies, temperatures, and coupling strengths of an electron coupled to a polar lattice (a system commonly called a polaron). The starting point for the calculation is the quantum mechanical expression for the expected current. The phonon coordinates are eliminated from this expression by well-known field-theory techniques. The resulting exact "influence functional" is then approximated by a corresponding quadratic "influence functional" which, it is hoped, imitates the real polaron. Correction terms are computed to account for the difference between the approximate impedance and the exact polaron impedance in a manner closely analogous to Feynman's treatment of the polaron self-energy. In fact, the analytic evaluation of the expression for the impedance obtained here is carried out using the approximate "influence functional" that was successfully employed in minimizing the binding (and free) energy of the polaron in earlier calculations. However, the accuracy obtained using this approximation, for the impedance calculation, is less satisfactory and its limitations are discussed. Nevertheless, beginning at intermediate coupling strengths, the approximate impedance produces a level structure of increasing complexity and narrowing resonances as the coupling strengthens. This suggests that further refinements may be fruitful. Methods for finding a better quadratic influence functional for use in our impedance expression as well as ways of improving the expression further are suggested. A comparison of our results with those of the Boltzmann equation points up interesting differences which arise from reversing the order of taking limits of zero frequency and coupling

### Nuclear structure correction to the hyperfine structure in hydrogen

In previous papers, corrections to the hyperfine structure (hfs) in hydrogen of relative order αm / M have been calculated by treating the proton as a point particle with an anomalous magnetic moment in addition to its Dirac moment. In this paper the proton is treated as a particle with structure by making use of the high-energy electron-proton-scattering data. Corrections of the previous work in which only a point particle was considered are calculated by using the Feynman formulation of quantum electrodynamics. It is also shown that exactly the same terms may be obtained by using the covariant Bethe-Salpeter equation.
The calculated shift of -35 parts per million (including the "recoil corrections") is not in agreement with the combined results of several experiments (-1.4±18 parts per million). A possible source of this difference is meson corrections to a two-photon form-factor which is taken here as the product of two (Hofstadter) single-photon form-factors

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