P. McHale | Searcharxiv

arXiv2004

Strong-coupling theory of magnetic-exciton-mediated superconductivity in UPd$_2$Al$_3$

There is compelling evidence from inelastic-neutron-scattering and tunneling experiments that the heavy-fermion superconductor UPd$_2$Al$_3$ can be understood as a dual system consisting of magnetic excitons, arising from crystal-field-split U$^{4+}$ levels, coupled to delocalised f-electrons. We have computed the superconducting transition temperature and the mass renormalisation arising from a dual model with maximal spin anisotropy using a strong-coupling approach. We find an instability to two possible opposite-spin-pairing states with even- or odd-parity gap functions. Each has a line node perpendicular to the c-direction, in agreement with NMR relaxation-rate, specific-heat and thermal-conductivity measurements. In addition, both have total spin component $S_z$=0, compatible with the observation of a pronounced Knight shift and $H_{c2}$ Pauli limiting. For parameter values appropriate to UPd$_2$Al$_3$, we determine the dependence of the superconducting transition temperature $T_c$ on a phenomenological coupling constant $g$ and we investigate the associated mass enhancement and its anisotropy.

cond-mat.str-el↗

arXiv2002

Sensitivity of the Superconducting Transition Temperature to Changes in the Spin-Fluctuation Spectral Weight

In the simplest model of magnetic pairing, the transition temperature to the superconducting state depends on the dynamical susceptibility $χ({\bf q},ω)$. We discuss how $T_c$ is affected by different momentum and frequency parts of $χ({\bf q},ω)$ for nearly antiferromagnetic and nearly ferromagnetic metals in two dimensions. While in the case of phonon-mediated superconductivity any addition of spectral weight to $α^2F(ω)$ at $ω>0$ leads to an increase in $T_c$, we find that adding magnetic spectral weight at any momentum ${\bf q}$ and low frequencies ($[0:3T_c]$ and $[0:(5-9)T_c]$ for nearly antiferromagnetic and ferromagnetic metals respectively) leads to a suppression of $T_c$. The most effective frequency and momentum range consists of large momenta ${\bf q} \sim (π,π)$ and frequencies around $10T_c$ for nearly antiferromagnetic metals and small momenta ${\bf q} \sim 0$ and frequencies of approximately $(13-22)T_c$ for nearly ferromagnetic metals.

cond-mat.str-el↗