Pairing state in multicomponent superconductors

We use the microscopic weak coupling theory to predict the pairing state in superconductors of cubic, hexagonal, or tetragonal symmetry, where the order parameter is multicomponent, i.e., transforms according to either a 2-dimensional or a 3-dimensional representation of the crystal point group. We show that the superconducting phase usually breaks the time-reversal symmetry for singlet multicomponent superconductors. The superconducting order parameter for triplet superconductors in most cases turns out to be non-magnetic.Comment: 7 page

Effects of Magnetic Order on the Upper Critical Field of UPt3_3

I present a Ginzburg-Landau theory for hexagonal oscillations of the upper critical field of UPt$_3$ near $T_c$. The model is based on a $2D$ representation for the superconducting order parameter, $\vec{\eta}=(\eta_1,\eta_2)$, coupled to an in-plane AFM order parameter, $\vec{m}_s$. Hexagonal anisotropy of $H_{c2}$ arises from the weak in-plane anisotropy energy of the AFM state and the coupling of the superconducting order parameter to the staggered field. The model explains the important features of the observed hexagonal anisotropy [N. Keller, {\it et al.}, Phys. Rev. Lett. {\bf 73}, 2364 (1994).] including: (i) the small magnitude, (ii) persistence of the oscillations for $T\rightarrow T_c$, and (iii) the change in sign of the oscillations for $T> T^{*}$ and $T< T^{*}$ (the temperature at the tetracritical point). I also show that there is a low-field crossover (observable only very near $T_c$) below which the oscillations should vanish.Comment: 9 pages in a RevTex (3.0) file plus 2 postscript figures (uuencoded). Submitted to Physical Review B (December 20, 1994)

Reduction of Pauli paramagnetic pair-breaking effect in antiferromagnetic superconductors

Antiferromagnetic superconductors in a magnetic field are studied. We examine a mechanism which significantly reduces the Pauli paramagnetic pair-breaking effect. The mechanism is realized even in the presence of the orbital pair-breaking effect. We illustrate it using a three-dimensional model with an intercalated magnetic subsystem. The upper critical field is calculated for various parameters. It is shown that the upper critical field can reach several times the pure Pauli paramagnetic limit. The possible relevance to the large upper critical field observed in the heavy fermion antiferromagnetic superconductor CePt_3Si discovered recently is briefly discussed. We try to understand the large upper critical field in the compound CePt_3Si and field-induced superconductivity in the compound CePb_3 within a unified framework.Comment: 5 pages, 2 figures, revtex4, minor correction