arXiv subjects

M. J. Graf

Publications and source records attributed to M. J. Graf.

78 records · Page 5Linked to original sources

arXiv1998

Suppression of Superconductivity in UPt_3 Single Crystals

High quality single crystals of UPt_3 have been prepared by vertical float-zone refining in ultra-high vacuum. We find that the superconducting transition temperature can be varied systematically by annealing, revealing that the transition temperature intrinsic to UPt_3 is 563 +/- 5 mK. The suppression of the superconducting transition from defects is consistent with a modified Abrikosov-Gor'kov formula that includes anisotropic pairing, Fermi surface anisotropy, and anisotropic scattering by defects.

cond-mat.supr-con↗

arXiv1996

Universality in Transport Processes of Unconventional Superconductors

We show that some of the low temperature transport coefficients (e.g., electrical and thermal conductivities, viscosity and sound attenuation) are {\it universal}, i.e., independent of the impurity concentration and phase shift for specific classes of unconventional superconductors. The existence of a universal limit depends on the symmetry of the order parameter and is achieved at low temperatures $k_B T \ll γ\ll Δ_0$, where $γ$ is the bandwidth of the impurity induced Andreev bound states. The density of states is finite at zero energy and leads to the re-appearance of the Wiedemann-Franz law deep in the superconducting phase for $k_B T \ll γ$. Our findings also show that impurity concentration studies at low temperatures can distinguish between different order parameter symmetries.

cond-mat↗

arXiv1995

Thermal Conductivity of Superconducting UPt$_3$ at Low Temperatures

We study the thermal conductivity within the E$_{1g}$ and E$_{2u}$ models for superconductivity in UPt$_3$ and compare the theoretical results for electronic heat transport with recently measured results reported by Lussier, Ellman and Taillefer. The existing data down to $T/T_c\approx 0.1$ provides convincing evidence for the presence of both line and point nodes in the gap, but the data can be accounted for either by an E$_{1g}$ or E$_{2u}$ order parameter. We discuss the features of the pairing symmetry, Fermi surface, and excitation spectrum that are reflected in the thermal conductivity at very low temperatures. Significant differences between the E$_{1g}$ and E$_{2u}$ models are predicted to develop at excitation energies below the bandwidth of the impurity-induced Andreev bound states. The zero-temperature limit of the $\bf \hat c$ axis thermal conductivity, $\lim_{T\to 0} κ_c/T$, is \underline{universal} for the E$_{2u}$ model, but non-universal for the E$_{1g}$ model. Thus, impurity concentration studies at very low temperatures should differentiate between the nodal structures of the E$_{2u}$ and E$_{1g}$ models.

cond-mat↗

arXiv1995

Infrared Conductivity in Layered $d$-wave Superconductors

We calculate the infrared conductivity of a stack of coupled, two-dimensional superconducting planes within the Fermi liquid theory of superconductivity. We include the effects of random scattering processes and show that the presence of even a small concentration of resonant impurities, in a $d$-wave superconductor, has an important effect on both the in-plane and $c$-axis transport properties, which could serve as signatures for $d$-wave pairing.

cond-mat↗

arXiv1995

Electronic thermal conductivity and the Wiedemann-Franz law for unconventional superconductors

We use the quasiclassical theory of superconductivity to calculate the electronic contribution to the thermal conductivity. The theory is formulated for low temperatures when heat transport is limited by electron scattering from random defects and for superconductors with nodes in the order parameter. We show that certain eigenvalues of the thermal conductivity tensor are universal at low temperature, $k_B T\ll γ$, where $γ$ is the bandwidth of impurity bound states in the superconducting phase. The components of the electrical and thermal conductivity also obey a Wiedemann-Franz law with the Lorenz ratio, $L(T)=κ/σT$, given by the Sommerfeld value of $L_{\!S}=({π^2}/{3})(k_B/e)^2$ for $k_BT\llγ$. For intermediate temperatures the Lorenz ratio deviates significantly from $L_{\!S}$, and is strongly dependent on the scattering cross section, and qualitatively different for resonant vs.\ nonresonant scattering. We include comparisons with other theoretical calculations and the thermal conductivity data for the high $T_c$ cuprate and heavy fermion superconductors.

cond-mat↗

arXiv1995

Electromagnetic Absorption in an Anisotropic Layered Superconductor

We calculate the infrared conductivity tensor of a layered superconductor considering two different order parameter symmetries: strongly anisotropic $s$-wave with line nodes, and pure ($d_{x^2-y^2}$) $d$-wave. The calculations are performed within the quasiclassical theory of superconductivity and include the effects of non-magnetic scattering processes. We discuss to what extent measurements of the optical absorption can be relied upon to distinguish between these two order parameter symmetries.

cond-mat↗