3,596 research outputs found
On Epsilon Expansions of Four-loop Non-planar Massless Propagator Diagrams
We evaluate three typical four-loop non-planar massless propagator diagrams
in a Taylor expansion in dimensional regularization parameter
up to transcendentality weight twelve, using a recently
developed method of one of the present coauthors (R.L.). We observe only
multiple zeta values in our results.Comment: 3 pages, 1 figure, results unchanged, discussion improved, to appear
in European Physical Journal
Role of interference in MM-wave driven DC transport in two dimensional electron gas
In this paper we point out that in addition to the density of states effect
proposed in Ref.\cite{durst,anderson} one should consider the effect of
constructive interference between the multi-MM-wave-photon processes shown in
Fig.2. This process enhances the dark value of the conductivity. When the
sample is very pure, i.e., when the transport life time is very long, this
interference effect quickly diminishes as the MM-wave frequency deviates from
the cyclotron frequency. In this paper we also present the linear response
theory in the presence of strong harmonic time-dependent perturbation
The effect of interstitial clusters and vacancies on the STM image of graphite
Making use of the tight-binding Green's function technique, we have
calculated the STM images of graphite with surface and sub-surface defects,
while taking into account the relaxation of the lattice due to defects. We have
demonstrated that two different physical mechanisms may result in the formation
of hillocks in the STM images: buckling of the graphite surface due to
interstitials between the uppermost graphite layers and the enhancement of the
electron density of states close to the Fermi energy on the carbon atoms in the
vicinity of vacancies. Our results indicate that small hillocks may originate
both from the interstitial clusters and from the vacancies. By contrast,
however, large hillocks in excess of 10 \AA~ in diameter can be caused only by
interstitial clusters.Comment: Submitted to Surface Scienc
Nucleation at finite temperature beyond the superminispace model
The transition from the quantum to the classical regime of the nucleation of
the closed Robertson-Walker Universe with spacially homogeneous matter fields
is investigated with a perturbation expansion around the sphaleron
configuration. A criterion is derived for the occurrence of a first-order type
transition, and the related phase diagram for scalar and vector fields is
obtained. For scalar fields both the first and second order transitions can
occur depending on the shape of the potential barrier. For a vector field, here
that of an O(3) nonlinear -model, the transition is seen to be only of
the first order.Comment: 15 pages, 3 figure
Higher Derivative Operators from Transmission of Supersymmetry Breaking on S_1/Z_2
We discuss the role that higher derivative operators play in field theory
orbifold compactifications on S_1/Z_2 with local and non-local (Scherk-Schwarz)
breaking of supersymmetry. Integrating out the bulk fields generates
brane-localised higher derivative counterterms to the mass of the brane (or
zero-mode of the bulk) scalar field, identified with the Higgs field in many
realistic models. Both Yukawa and gauge interactions are considered and the
one-loop results found can be used to study the ``running'' of the scalar field
mass with respect to the momentum scale in 5D orbifolds. In particular this
allows the study of the behaviour of the mass under UV scaling of the momentum.
The relation between supersymmetry breaking and the presence of higher
derivative counterterms to the mass of the scalar field is investigated. This
shows that, regardless of the breaking mechanism, (initial) supersymmetry
cannot, in general, prevent the emergence of such operators. Some implications
for phenomenology of the higher derivative operators are also presented.Comment: 29 pages, LaTeX. Added Section 4 ("Phenomenological implications:
living with ghosts?") and Appendix
Vector Leptoquarks Could Be Rather Light?
Some low-energy manifestations of a minimal extension of the Standard Model
based on the quark-lepton symmetry of the
Pati-Salam type are analysed. Given this symmetry a new type of mixing in the
quark-lepton interactions is shown to be required. An additional arbitrariness
of the mixing parameters could allow to decrease noticeably the lower bound on
the leptoquark mass originated from the and decays and the conversion. The only mixing independent bound emerging from the cosmological
limit on the decay width is .Comment: 10 pages, LaTeX, Yaroslavl, Yaroslavl State University preprint
YARU-HE-94/0
On the Origin of Peak-dip-hump Structure in the In-plane Optical Conductivity of the High Cuprates; Role of Antiferromagnetic Spin Fluctuations of Short Range Order
An improved U(1) slave-boson approach is applied to study the optical
conductivity of the two dimensional systems of antiferromagnetically correlated
electrons over a wide range of hole doping and temperature. Interplay between
the spin and charge degrees of freedom is discussed to explain the origin of
the peak-dip-hump structure in the in-plane conductivity of high
cuprates. The role of spin fluctuations of short range order(spin singlet pair)
is investigated. It is shown that the spin fluctuations of the short range
order can cause the mid-infrared hump, by exhibiting a linear increase of the
hump frequency with the antiferromagnetic Heisenberg coupling strength
M-Theory Giant Gravitons with C field
We find giant graviton configurations of an M5-brane probe in the D=11
supergravity background generated by a stack of non-threshold (M2,M5) bound
states. The M5-brane probe shares three directions with the background and
wraps a two-sphere transverse to the bound states. For a particular value of
the worldvolume gauge field of the PST formalism, there exist solutions of the
equations of motion for which the M5-brane probe behaves as a wave propagating
in the (M2,M5) background. We have checked that the probe breaks the
supersymmetry of the background exactly as a massless particle moving along the
trajectory of its center of mass.Comment: 11 pages, LaTeX; v2: references adde
Differential equations and high-energy expansion of two--loop diagrams in D dimensions
New method of calculation of master integrals using differential equations
and asymptotical expansion is presented. This method leads to the results exact
in space-time dimension having the form of the convergent power series. As
an application of this method, we calculate the two--loop master integral for
"crossed--triangle" topology which was previously known only up to O(\ep)
order. The case when a topology contains several master integrals is also
considered. We present an algorithm of the term-by-term calculation of the
asymptotical expansion in this case and analyze in detail the "crossed--box"
topology with three master integrals.Comment: 13 pages,8 figures, uses elsart.cls. Misprints correcte
Proximity to a Nearly Superconducting Quantum Critical Liquid
The coupling between superconductors and a quantum critical liquid that is
nearly superconducting provides natural interpretation for the Josephson effect
over unexpectedly long junctions, and the remarkable stripe-spacing dependence
of the critical temperature in LSCO and YBCO superconductors.Comment: four two-column pages, no figure
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