4,372 research outputs found
Signatures of High-Intensity Compton Scattering
We review known and discuss new signatures of high-intensity Compton
scattering assuming a scenario where a high-power laser is brought into
collision with an electron beam. At high intensities one expects to see a
substantial red-shift of the usual kinematic Compton edge of the photon
spectrum caused by the large, intensity dependent, effective mass of the
electrons within the laser beam. Emission rates acquire their global maximum at
this edge while neighbouring smaller peaks signal higher harmonics. In
addition, we find that the notion of the centre-of-mass frame for a given
harmonic becomes intensity dependent. Tuning the intensity then effectively
amounts to changing the frame of reference, going continuously from inverse to
ordinary Compton scattering with the centre-of-mass kinematics defining the
transition point between the two.Comment: 25 pages, 16 .eps figure
The role of electron-electron interactions in two-dimensional Dirac fermions
The role of electron-electron interactions on two-dimensional Dirac fermions
remains enigmatic. Using a combination of nonperturbative numerical and
analytical techniques that incorporate both the contact and long-range parts of
the Coulomb interaction, we identify the two previously discussed regimes: a
Gross-Neveu transition to a strongly correlated Mott insulator, and a
semi-metallic state with a logarithmically diverging Fermi velocity accurately
described by the random phase approximation. Most interestingly, experimental
realizations of Dirac fermions span the crossover between these two regimes
providing the physical mechanism that masks this velocity divergence. We
explain several long-standing mysteries including why the observed Fermi
velocity in graphene is consistently about 20 percent larger than the best
values calculated using ab initio and why graphene on different substrates show
different behavior.Comment: 11 pages, 4 figure
Coarse-graining microscopic strains in a harmonic, two-dimensional solid and its implications for elasticity: non-local susceptibilities and non-affine noise
In soft matter systems the local displacement field can be accessed directly
by video microscopy enabling one to compute local strain fields and hence the
elastic moduli using a coarse-graining procedure. We study this process for a
simple triangular lattice of particles connected by harmonic springs in
two-dimensions. Coarse-graining local strains obtained from particle
configurations in a Monte Carlo simulation generates non-trivial, non-local
strain correlations (susceptibilities), which may be understood within a
generalized, Landau type elastic Hamiltonian containing up to quartic terms in
strain gradients (K. Franzrahe et al., Phys. Rev. E 78, 026106 (2008)). In
order to demonstrate the versatility of the analysis of these correlations and
to make our calculations directly relevant for experiments on colloidal solids,
we systematically study various parameters such as the choice of statistical
ensemble, presence of external pressure and boundary conditions. We show that
special care needs to be taken for an accurate application of our results to
actual experiments, where the analyzed area is embedded within a larger system,
to which it is mechanically coupled. Apart from the smooth, affine strain
fields, the coarse-graining procedure also gives rise to a noise field made up
of non-affine displacements. Several properties of this noise field may be
rationalized for the harmonic solid using a simple "cell model" calculation.
Furthermore the scaling behavior of the probability distribution of the noise
field is studied and a master curve is obtained.Comment: 16 pages, 12 figure
Geometric Frustration and Dimensional Reduction at a Quantum Critical Point
We show that the spatial dimensionality of the quantum critical point
associated with Bose--Einstein condensation at T=0 is reduced when the
underlying lattice comprises a set of layers coupled by a frustrating
interaction. Our theoretical predictions for the critical temperature as a
function of the chemical potential correspond very well with recent
measurements in BaCuSiO [S. E. Sebastian \textit{et al}, Nature
\textbf{411}, 617 (2006)].Comment: 5 pages, 2 figure
Overscreened multi-channel SU(N) Kondo model : large-N solution and Conformal Field Theory
The multichannel Kondo model with SU(N) spin symmetry and SU(K) channel
symmetry is considered. The impurity spin is chosen to transform as an
antisymmetric representation of SU(N), corresponding to a fixed number of
Abrikosov fermions . For more
than one channel (K>1), and all values of N and Q, the model displays non-Fermi
behaviour associated with the overscreening of the impurity spin. Universal
low-temperature thermodynamic and transport properties of this non-Fermi liquid
state are computed using conformal field theory methods. A large-N limit of the
model is then considered, in which K/N and Q/N are held fixed. Spectral
densities satisfy coupled integral equations in this limit, corresponding to a
(time-dependent) saddle-point. A low frequency, low-temperature analysis of
these equations reveals universal scaling properties in the variable
, also predicted from conformal invariance. The universal scaling
form is obtained analytically and used to compute the low-temperature universal
properties of the model in the large-N limit, such as the T=0 residual entropy
and residual resistivity, and the critical exponents associated with the
specific heat and susceptibility. The connections with the ``non-crossing
approximation'' and the previous work of Cox and Ruckenstein are discussed.Comment: 39 pages, RevTeX, including 5 figures in encapsulated postscript
forma
The intensity dependent mass shift: existence, universality and detection
The electron mass shift in a laser field has long remained an elusive
concept. We show that the mass shift can exist in pulses but that it is neither
unique nor universal: it can be reduced by pulse shaping. We show also that the
detection of mass shift effects in laser-particle scattering experiments is
feasible with current technology, even allowing for the transverse structure of
realistic beams.Comment: 5 pages, 4 figures. V2: references added, introduction expande
Fractional ac Josephson effect in unconventional superconductors
For certain orientations of Josephson junctions between two p_x-wave or two
d-wave superconductors, the subgap Andreev bound states produce a 4pi-periodic
relation between the Josephson current I and the phase difference phi: I ~
sin(phi/2). Consequently, the ac Josephson current has the fractional frequency
eV/h, where V is the dc voltage. In the tunneling limit, the Josephson current
is proportional to the first power (not square) of the electron tunneling
amplitude. Thus, the Josephson current between unconventional superconductors
is carried by single electrons, rather than by Cooper pairs. The fractional ac
Josephson effect can be observed experimentally by measuring frequency spectrum
of microwave radiation from the junction.Comment: 8 pages, 3 figures, RevTEX 4; v2. - minor typos corrected in proof
Solution of the two impurity, two channel Kondo Model
We solve the two-impurity two-channel Kondo model using a combination of
conformal invariance and bosonisation techniques. The odd-even symmetric case
is analysed in detail. The RKKY interaction turns out to be exactly marginal,
resulting in a line of non-Fermi liquid fixed points. Explicit formulae are
given for the critical exponents and for the finite-size spectrum, which depend
continuously on a single parameter. The marginal line spans a range of values
of the RKKY coupling which goes from the infinitely strong ferromagnetic
point (associated with a 4-channel spin-1 Kondo model) to a finite
antiferromagnetic critical value beyond which a Fermi liquid is
recovered. We also find that, when the odd-even symmetry is broken, the
marginal line is unstable for ferromagnetic , while for antiferromagnetic
it extends into a manifold of fixed points.Comment: 9 pages, preprint LPTENS 94/1
Bosonization in the two-channel Kondo model
The bosonization of the anisotropic two-channel Kondo model is shown
to yield two equivalent representations of the original problem. In a straight
forward extension of the Emery-Kivelson approach, the interacting resonant
level model previously derived by the Anderson-Yuval technique is obtained. In
addition, however, a ``(,)'' description is also found. The
strong coupling fixed point of the (,) model was originally
postulated to be related to the intermediate coupling fixed point of the
two-channel Kondo model. The equivalence of the , model to the
two-channel Kondo model is formally established. A summary of what one may
learn from a simple study of these different representations is also given.Comment: 5 pages, latex (uses revtex and epsf macros) with 1 postscript figur
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