48 research outputs found
Quantum critical behavior induced by Mn impurity in CuGeO3
Results of high frequency (60-315 GHz) studies of ESR in CuGeO3 single
crystals containing 0.9% of Mn impurity are reported. Quantitative EPR line
shape analysis allowed concluding that low temperature magnetic susceptibility
for T <40 K diverges following power law with the critical exponent 0.81 and
therefore manifests onset of a quantum critical (QC) regime. We argue that
transition into Griffiths phase occurs at TG~40 K and disorder produced by Mn
impurity in quantum spin chains of CuGeO3 may lead to co-existence of the QC
regime and spin-Peierls dimerisation.Comment: 2 pages, submitted to SCES05 proceeding
Renormalization of the asymptotically expanded Yang-Mills spectral action
We study renormalizability aspects of the spectral action for the Yang-Mills
system on a flat 4-dimensional background manifold, focusing on its asymptotic
expansion. Interpreting the latter as a higher-derivative gauge theory, a
power-counting argument shows that it is superrenormalizable. We determine the
counterterms at one-loop using zeta function regularization in a background
field gauge and establish their gauge invariance. Consequently, the
corresponding field theory can be renormalized by a simple shift of the
spectral function appearing in the spectral action.
This manuscript provides more details than the shorter companion paper, where
we have used a (formal) quantum action principle to arrive at gauge invariance
of the counterterms. Here, we give in addition an explicit expression for the
gauge propagator and compare to recent results in the literature.Comment: 28 pages; revised version. To appear in CMP. arXiv admin note:
substantial text overlap with arXiv:1101.480
Transition Spectra for a BCS Superconductor with Multiple Gaps: Model Calculations for MgB_2
We analyze the qualitative features in the transition spectra of a model
superconductor with multiple energy gaps, using a simple extension of the
Mattis-Bardeen expression for probes with case I and case II coherence factors.
At temperature T = 0, the far infrared absorption edge is, as expected,
determined by the smallest gap. However, the large thermal background may mask
this edge at finite temperatures and instead the secondary absorption edges
found at Delta_i+Delta_j may become most prominent. At finite T, if certain
interband matrix elements are large, there may also be absorption peaks at the
gap difference frequencies | Delta_i-Delta_j | . We discuss the effect of
sample quality on the measured spectra and the possible relation of these
predictions to the recent infrared absorption measurement on MgB_2
Disorder driven quantum critical behavior in CuGeO3 doped with magnetic impurity
For the CuGeO3 doped with 1% of Fe the quantum critical behavior in a wide
temperature range 1-40 K is reported. The critical exponents for susceptibility
along different crystallographic axes are determined: a=0.34 (B//a and B//c)
and a=0.31 (B//b). New effect of the frequency dependence of the critical
exponent is discussed.Comment: Submitted to SCES0
Multiband model for penetration depth in MgB2
The results of first principles calculations of the electronic structure and
the electron-phonon interaction in MgB2 are used to study theoretically the
temperature dependence and anisotropy of the magnetic field penetration depth.
The effects of impurity scattering are essential for a proper description of
the experimental results. We compare our results with experimental data and we
argue that the two-band model describes the data rather well.Comment: submitted to Phys. Rev.
Factorization of integrals, defining the beta-function, into integrals of total derivatives in N=1 SQED, regularized by higher derivatives
Some calculations in supersymmetric theories, made with the higher derivative
regularization, show that the beta-function is given by integrals of total
derivatives. This is qualitatively explained for the N=1 supersymmetric
electrodynamics in all orders.Comment: 14 page