Anomalous superfluid density in quantum critical superconductors

Abstract

When a second-order magnetic phase transition is tuned to zero temperature by a non-thermal parameter, quantum fluctuations are critically enhanced, often leading to the emergence of unconventional superconductivity. In these `quantum critical' superconductors it has been widely reported that the normal-state properties above the superconducting transition temperature $T_c$ often exhibit anomalous non-Fermi liquid behaviors and enhanced electron correlations. However, the effect of these strong critical fluctuations on the superconducting condensate below $T_c$ is less well established. Here we report measurements of the magnetic penetration depth in heavy-fermion, iron-pnictide, and organic superconductors located close to antiferromagnetic quantum critical points showing that the superfluid density in these nodal superconductors universally exhibit, unlike the expected $T$-linear dependence, an anomalous 3/2 power-law temperature dependence over a wide temperature range. We propose that this non-integer power-law can be explained if a strong renormalization of effective Fermi velocity due to quantum fluctuations occurs only for momenta $\mathbf{k}$ close to the nodes in the superconducting energy gap $\Delta (\mathbf{k})$. We suggest that such `nodal criticality' may have an impact on low-energy properties of quantum critical superconductors.

Main text (5 pages, 3 figures) + Supporting Information (3 pages, 4 figures). Published in PNAS Early Edition on February 12, 2013

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• From Kondo Lattices to Kondo Superlattices
• How many quantum phase transitions exist inside the superconducting dome of the iron pnictides?
• Zero-field Quantum Critical Point in CeCoIn${}_5$
• Evolution of the superconducting energy gap structure concomitant with Fermi surface reconstruction in the heavy-fermion superconductor CeCoIn5
• Effect of magnetic criticality and Fermi-surface topology on the magnetic penetration depth
• Anomalous Scaling of the Penetration Depth in Nodal Superconductors
• Quasi-two-dimensional Fermi surfaces of the heavy-fermion superconductor Ce${}_2$PdIn${}_8$
• Evolution of quasiparticle excitations with critical mass enhancement in superconducting AFe2As2 (A = K, Rb, and Cs)
• Impact of ordering competition on the global phase diagram of iron pnictides
• Observation of the Non-linear Meissner Effect
• Strange superconductivity near an antiferromagnetic heavy fermion quantum critical point
• Renormalizations in unconventional superconducting states of Ce${}_{1-x}$Yb${}_x$CoIn${}_5$
• Fermionology in the Kondo-Heisenberg model: the case of CeCoIn${}_5$
• Thermodynamic properties of nodal superconductors close to a magnetic quantum critical point
• Zero-Field Quantum Critical Point in Ce${}_{0.91}$Yb${}_{0.09}$CoIn${}_5$
• Superfluid response in heavy fermion superconductors
• Microscopic Insights into London Penetration Depth: Application to CeCoIn${}_5^{}$
• Magnetic Quantum Criticality inside the Superconducting State Revealed by Penetration Depth Scaling with Local $T_{\mathrm{c}}$