5,152 research outputs found
Decay rate and other properties of the positronium negative ion
A new method for detecting the positronium minus ion is described, and the possibility of a long positronium mean free path in a solid is discussed
Strongly Enhanced Hole-Phonon Coupling in the Metallic State of the Dilute Two-Dimensional Hole Gas
We have studied the temperature dependent phonon emission rate () of a
strongly interacting (22) dilute 2D GaAs hole system using a standard
carrier heating technique. In the still poorly understood metallic state, we
observe that () changes from () to ()
above 100mK, indicating a crossover from screened piezoelectric(PZ) coupling to
screened deformation potential(DP) coupling for hole-phonon scattering.
Quantitative comparison with theory shows that the long range PZ coupling
between holes and phonons has the expected magnitude; however, in the metallic
state, the short range DP coupling between holes and phonons is {\it almost
twenty times stronger} than expected from theory. The density dependence of
() shows that it is {\it easier} to cool low density 2D holes in GaAs
than higher density 2D hole systems.Comment: To appear in Phys. Rev. Let
Suppression of weak localization effects in low-density metallic 2D holes
We have measured the conductivity in a gated high-mobility GaAs two
dimensional hole sample with densities in the range (7-17)x10^9 cm^-2 and at
hole temperatures down to 5x10^-3 E_F. We measure the weak localization
corrections to the conductivity g=G/(e^2/h) as a function of magnetic field
(Delta g=0.019 +/- 0.006 at g=1.5 and T=9 mK) and temperature (d ln g/dT<0.0058
and 0.0084 at g=1.56 and 2.8). These values are less than a few percent of the
value 1/pi predicted by standard weak localization theory for a disordered 2D
Fermi liqui
Positronium reflection and positronium beams
Specular reflection of positronium, Ps was observed and that there is adequate intensity at higher energies to make further study worthwhile was established. The scattering appears to be restricted to the outermost surface with a mean free path of (0.75 + or - 0.15)A for Ps in LiF(100). With a greater intensity Ps beam one should see higher order diffraction beams as the result of the periodicity of the surface. Ps diffraction thus offers the possibility of being a novel and valuable probe to study the outermost surface and to study adsorbants on it. Two methods for producing Ps beams are described
The metallic resistance of a dilute two-dimensional hole gas in a GaAs quantum well: two-phase separation at finite temperature?
We have studied the magnetotransport properties of a high mobility
two-dimensional hole gas (2DHG) system in a 10nm GaAs quantum well (QW) with
densities in range of 0.7-1.6*10^10 cm^-2 on the metallic side of the
zero-field 'metal-insulator transition' (MIT). In a parallel field well above
B_c that suppresses the metallic conductivity, the 2DHG exhibits a conductivity
g(T)~0.3(e^2/h)lnT reminiscent of weak localization. The experiments are
consistent with the coexistence of two phases in our system: a metallic phase
and a weakly insulating Fermi liquid phase having a percolation threshold close
to B_c
The "recoil" correction of order to hyperfine splitting of positronium ground state
The "recoil" correction of order to the hyperfine splitting of
positronium ground state was found. The formalism employed is based on the
noncovariant perturbation theory in QED. Equation for two-particle component of
full (many-body) wave function is used, in which effective Hamiltonian depends
on the energy of a system. The effective Hamiltonian is not restricted to the
nonrelativistic region, so there is no need in any regularization. To evaluate
integrals over loop momenta, they are divided into "hard" and "soft" parts,
coming from large and small momenta respectively. Soft contributions were found
analytically, and hard ones are evaluated by numerical integration. Some soft
terms due to the retardation cancel each other. To calculate the "hard"
contributions, a great number of noncovariant graphs is replaced by only a few
covariant ones. The hard contribution was found in two ways. The first way is
to evaluate contributions of separate graphs, using the Coulomb gauge. The
second one is to calculate full hard contribution as a whole using the Feynman
gauge. The final result for the "recoil" correction is 0.381(6) m\al^6 and
agrees with those of previous papers. Diagram-to-diagram comparison with the
revised results of Adkins&Sapirstein was done. All the results agree, so the
"recoil" correction is now firmly established. This means a considerable
disagreement with the experimental data.Comment: 28 pages, latex including latex figure
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