Spontaneous Symmetry Breaking for Scalar QED with Non-minimal Chern-Simons Coupling

We investigate the two-loop effective potential for both minimally and non-minimally coupled Maxwell-Chern-Simons theories. The non-minimal gauge interaction represents the magnetic moment interaction between a charged scalar and the electromagnetic field. In a previous paper we have shown that the two loop effective potential for this model is renormalizable with an appropriate choice of the non-minimal coupling constant. We carry out a detailed analysis of the spontaneous symmetry breaking induced by radiative corrections. As long as the renormalization point for all couplings is chosen to be the true minimum of the effective potential, both models predict the presence of spontaneous symmetry breaking. Two loop corrections are small compared to the one loop result, and thus the symmetry breaking is perturbatively stable.Comment: Revtex 25 pages, 9 figure

Exponential Temperature Dependence of Penetration Depth in Single Crystal MgB_2

The temperature dependence of the London penetration depth,lambda(T), was measured in both single crystal and polycrystalline MgB_2 samples by a high-resolution, radio frequency technique. A clear exponential temperature dependence of lambda(T) was observed at low temperature, indicating s-wave pairing. A BCS fit to the lowest temperature data gives an in-plane energy gap Delta of 2.6\pm0.2 meV (2\Delta/T_c=1.5\pm0.1), which is significantly smaller than the standard BCS weak coupling value of 3.5. We find that the data are best described by a two-gap model.Comment: Minor corrections added, as per published articl

Destruction of chain-superconductivity in YBa_2Cu_4O_8 in a weak magnetic field

We report measurements of the temperature dependent components of the magnetic penetration depth {\lambda}(T) in single crystal samples of YBa_2Cu_4O_8 using a radio frequency tunnel diode oscillator technique. We observe a downturn in {\lambda}(T) at low temperatures for currents flowing along the b and c axes but not along the a axis. The downturn in {\lambda}_b is suppressed by a small dc field of ~0.25 T. This and the zero field anisotropy of {\lambda}(T) likely result from proximity induced superconducting on the CuO chains, however we also discuss the possibility that a significant part of the anisotropy might originate from the CuO2 planes.Comment: 5 page

Temperature dependent anisotropy of the penetration depth and coherence length in MgB$_2

We report measurements of the temperature dependent anisotropies ($\gamma_\lambda$ and $\gamma_\xi$) of both the London penetration depth $\lambda$ and the upper critical field of MgB$_2$. Data for $\gamma_\lambda=\lambda_c/\lambda_a$ was obtained from measurements of $\lambda_{a}$ and $\lambda_c$ on a single crystal sample using a tunnel diode oscillator technique. $\gamma_\xi=H_{c2}^{\parallel c}/H_{c2}^{\bot c}$ was deduced from field dependent specific heat measurements on the same sample. $\gamma_\lambda$ and $\gamma_\xi$ have opposite temperature dependencies, but close to $T_c$ tend to a common value ($\gamma_\lambda\simeq \gamma_\xi=1.75\pm0.05$). These results are in good agreement with theories accounting for the two gap nature of MgB$_2$Comment: 4 pages with figures (New version

Non-minimal Maxwell-Chern-Simons theory and the composite Fermion model

The magnetic field redefinition in Jain's composite fermion model for the fractional quantum Hall effect is shown to be effectively described by a mean-field approximation of a model containing a Maxwell-Chern-Simons gauge field non-minimally coupled to matter. Also an explicit non-relativistic limit of the non-minimal (2+1)D Dirac equation is derived.Comment: 10 pages, no figure