[Paper Review] Specific heat of the iron-based high-$T_c$ superconductor SmO$_{1-x}$F$_x$FeAs

L. Ding, C. He+5 more|ArXiv.org|Apr 23, 2008

Iron-based superconductors research3 citations

TL;DR

This study investigates the specific heat of SmO1-xFxFeAs, revealing a 130 K structural/SDW transition in the undoped parent compound, suppression of this transition with F doping, and a clear superconducting specific heat anomaly near Tc=54 K in doped samples—unlike in La-based analogs. A sharp 4.6 K specific heat peak in the parent compound is attributed to antiferromagnetic ordering of Sm3+ ions, confirmed by magnetic entropy analysis and comparison to Sm2-xCexCuO4-δ.

ABSTRACT

The specific heat $C(T)$ of new iron-based high-$T_c$ superconductor SmO$_{1-x}$F$_x$FeAs ($0 \leq x \leq 0.2$) was systematically studied. For undoped $x$ = 0 sample, a specific heat jump was observed at 130 K. This is attributed to the structural or spin-density-wave (SDW) transition, which also manifests on resistivity as a rapid drop. However, this jump disappears with slight F doping in $x$ = 0.05 sample, although the resistivity drop still exists. The specific heat $C/T$ shows clear anomaly near $T_c$ for $x$ = 0.15 and 0.20 superconducting samples. Such anomaly has been absent in LaO$_{1-x}$F$_x$FeAs. For the parent compound SmOFeAs, $C(T)$ shows a sharp peak at 4.6 K, and with electron doping in $x$ = 0.15 sample, this peak shifts to 3.7 K. It is interpreted that such a sharp peak results from the antiferromagnetic ordering of Sm$^{3+}$ ions in this system, which mimics the electron-doped high-$T_c$ cuprate Sm$_{2-x}$Ce$_x$CuO$_{4-δ}$.

Motivation & Objective

• To systematically measure the specific heat of SmO1-xFxFeAs across a range of electron doping levels (x = 0 to 0.2) to map its thermodynamic phase transitions.
• To clarify the nature of the 130 K anomaly in the parent compound and determine whether it arises from structural, SDW, or combined transitions.
• To investigate the presence and characteristics of superconducting specific heat anomalies near Tc in Sm-based iron pnictides, contrasting with the absence of such anomalies in La-based analogs.
• To identify and characterize low-temperature specific heat features linked to magnetic ordering of Sm3+ ions in the parent and doped compounds.

Proposed method

• Specific heat was measured via the relaxation method in a Quantum Design PPMS across 2–300 K for polycrystalline SmO1-xFxFeAs samples (x = 0, 0.05, 0.15, 0.20).
• Magnetic field-dependent specific heat was measured at 8 T for superconducting samples (x = 0.15 and 0.20) to probe superconducting gap symmetry.
• Electronic and magnetic contributions to specific heat were separated using the formula C = γT + βT³ + Cm, with Cm representing magnetic specific heat.
• The magnetic entropy S was calculated from Cm(T) to confirm the ground state degeneracy of Sm3+ ions, with saturation to R ln2 indicating a doublet ground state.
• Data were fitted in the range 14–20 K to extract the electronic specific heat coefficient γ and phonon coefficient β, though caution was applied due to limited temperature range and potential magnetic correlations.

Experimental results

Research questions

• RQ1 Does the 130 K specific heat jump in SmOFeAs correspond to a structural transition, an SDW transition, or both?
• RQ2 How does electron doping via F substitution affect the suppression of the 130 K transition and the emergence of superconductivity?
• RQ3 Why is a clear superconducting specific heat anomaly observed in SmO1-xFxFeAs but not in LaO1-xFxFeAs, despite similar Tc values?
• RQ4 What causes the sharp 4.6 K specific heat peak in the parent compound, and how does it evolve with electron doping?
• RQ5 Is the low-temperature specific heat anomaly in SmO1-xFxFeAs due to magnetic ordering of Sm3+ ions, and how does this compare to electron-doped cuprates?

Key findings

• A specific heat jump at 130 K in the x = 0 sample indicates a structural or spin-density-wave (SDW) transition, which is suppressed upon F doping (x = 0.05), despite a persistent resistivity drop.
• Clear specific heat anomalies near Tc = 54 K are observed for x = 0.15 and x = 0.20 samples, indicating a higher superfluid density compared to La-based analogs where such anomalies were absent.
• A sharp specific heat peak at 4.6 K in the x = 0 sample is attributed to antiferromagnetic ordering of Sm3+ ions, with the peak shifting to 3.7 K upon electron doping (x = 0.15).
• The magnetic entropy associated with the transition saturates to R ln2, confirming a doublet ground state for Sm3+ ions in the crystal field.
• The electronic specific heat coefficient γ is exceptionally high (119.4 mJ/mol·K²) for the x = 0 sample, reflecting strong magnetic correlations, though fitting over a limited temperature range (14–20 K) raises concerns about reliability.
• The γ value increases only slightly (from 81.0 to 83.7 mJ/mol·K²) under 8 T, indicating weak field dependence, unlike the strong field dependence observed in La-based systems.