[Paper Review] Mass enhancement in multiple bands approaching optimal doping in a high-temperature superconductor

Camilla M. Moir, Scott Riggs+8 more|arXiv (Cornell University)|Aug 26, 2016

Iron-based superconductors research33 references3 citations

TL;DR

This study uses high-magnetic-field heat capacity measurements in BaFe2(As1−xPx)2 to directly probe the electronic quasiparticle mass enhancement near optimal doping. By suppressing superconductivity with magnetic fields, the researchers extract the normal-state electronic density of states and find a linear extrapolation of inverse mass to zero at a critical doping of x ≈ 0.28 ± 0.015, indicating a quantum critical point driving mass enhancement in this iron-based superconductor.

ABSTRACT

Pnictides provide an opportunity to study the effects of quantum criticality in a multi-band high temperature superconductor. Quasiparticle mass divergence near optimal doping, observed in two major classes of high-temperature superconductors, pnictides and cuprates, is a direct experimental indicator of enhanced electronic interactions that accompany quantum criticality. Whether quasiparticles on all Fermi surface pockets in BaFe2(As1-xPx)2 are affected by quantum criticality is an open question, which specific heat measurements at high magnetic fields can directly address. Here we report specific heat measurements up to 35T in BaFe2(As1-xPx)2 over a broad doping range, 0.44 <= x <= 0.6. We observe saturation of C/T in the normal state at all dopings where superconductivity is fully suppressed. Our measurements demonstrate that quasiparticle mass increases towards optimal doping in multiple pockets, some of which exhibit even stronger mass enhancement than previously reported from quantum oscillations of a single pocket.

Motivation & Objective

• To directly measure the quasiparticle mass in the normal state of a high-temperature superconductor near optimal doping, avoiding model-dependent assumptions from superconducting transition jumps.
• To test the hypothesis of quantum criticality in iron-based superconductors by probing the electronic density of states in the normal state.
• To resolve discrepancies in interpreting quantum oscillation data by providing a thermodynamic measurement of mass enhancement.
• To determine whether the observed mass divergence near optimal doping is consistent with a quantum critical point, as seen in cuprates.

Proposed method

• High-magnetic-field heat capacity measurements were performed on BaFe2(As1−xPx)2 samples across a broad doping range, with fields applied along the c-axis.
• The saturation of specific heat C/T at high fields (H > H_sat) was used to extract the normal-state electronic density of states, (C/T)_sat.
• Low-field deviations from √H behavior were analyzed to identify nodal superconducting gaps, consistent with line-node pairing symmetry.
• The zero-field extrapolated C/T value, (C/T)_extrap, was subtracted from (C/T)_sat to isolate the quasiparticle contribution to the density of states.
• The inverse of the extracted quasiparticle mass was determined from the field-independent component (C/T)_H, which scales with the total quasiparticle mass on superconducting Fermi pockets.
• Linear extrapolation of the inverse mass to zero was used to estimate the critical doping for mass divergence.

Experimental results

Research questions

• RQ1 Does the quasiparticle mass in BaFe2(As1−xPx)2 exhibit a divergence near optimal doping, as suggested by quantum oscillation data?
• RQ2 Can the normal-state electronic density of states be directly measured in high-temperature superconductors using high magnetic fields to suppress superconductivity?
• RQ3 Is the observed mass enhancement consistent with a quantum critical point, as seen in cuprate superconductors?
• RQ4 What is the origin of the background C/T component (γ_bg) observed at low fields, and does it affect the interpretation of mass enhancement?
• RQ5 How does the field-independent electronic entropy in the normal state reconcile with theories of planckian dissipation and linear-in-field resistivity?

Key findings

• The high-field saturation of C/T at H > H_sat provides a direct measurement of the normal-state electronic density of states in BaFe2(As1−xPx)2, enabling a model-independent determination of quasiparticle mass.
• The inverse quasiparticle mass extrapolates linearly to zero at a critical doping of x ≈ 0.28 ± 0.015, indicating a divergent effective mass near optimal doping.
• The observed mass divergence is consistent with a quantum critical point, supporting a common quantum critical origin of superconductivity in both iron-based and cuprate high-temperature superconductors.
• The background C/T component (γ_bg) increases with doping but is unlikely due to pair-breaking, as low-field deviations from √H behavior diminish with increasing x, contradicting pair-breaking expectations.
• The normal-state electronic entropy is nearly independent of magnetic field above H_sat, contradicting predictions of planckian dissipation and suggesting a need to revise current theoretical models of quantum criticality in these materials.
• The measured mass enhancement is strongly correlated with the superconducting Fermi pockets, confirming that the enhanced correlations are intrinsic to the superconducting state near optimal doping.