Low temperature specific heat of the hole-dopedBa0.6K0.4Fe2As2

Abstract

We report the specific heat (SH) measurements on single crystals of hole doped FeAs-based superconductor Ba 0.6 K 0.4 Fe 2 As 2 . It is found that the electronic SH coefficient γ e (T ) is not temperature dependent and increases almost linearly with the magnetic field in low temperature region. These point to a fully gapped superconducting state. Surprisingly the sharp SH anomaly ∆C/T | Tc reaches a value of 98 mJ/molK 2 suggesting a very high normal state quasiparticle density of states (γ n ≈ 63mJ/molK 2 ). A detailed analysis reveals that the γ e (T ) cannot be fitted with a single gap of s-wave symmetry due to the presence of a hump in the middle temperature region. However, our data indicate that the dominant part of the superconducting condensate is induced by an s-wave gap with the magnitude of about 6 meV. PACS numbers: 74.20.Rp, 74.25.Bt, 65.40.Ba, 74.70.Dd The discovery of high temperature superconductivity in the FeAs-based system has stimulated enormous interests in the field of condensed matter physics and material sciences [1]. The superconductivity has not only been discovered in the electron doped samples, but also in the hole-doped ones [2,3]. The central issues concerning the superconductivity mechanism are about the symmetry and the magnitude of the superconducting gap. The experimental results obtained so far are, however, highly controversial. The low temperature specific heat (SH) measurements in the F-doped LaFeAsO samples revealed a nonlinear magnetic field dependence of the SH coefficient γ e , which was attributed to the presence of a nodal gap [4]. This was later supported by many other measurements based on µSR [5,6,7], NMR[8, 9, 10], magnetic penetration [11] and point contact Andreev spectrum (PCAS) [12]. On the other hand, the PCAS on the Fdoped SmFeAsO indicated a feature of s-wave gap [13], some measurements [14,15,16,17] also gave support to this conclusion. It is important to note that most of the conclusions drawn for a nodal gap were obtained on the electron doped LnFeAsO samples (abbreviated as FeAs-1111, Ln stands for the rare earth elements) which are characterized by a low charge carrier density and thus low superfluid density [18]. For the FeAs-1111 phase, it is very difficult to grow crystals with large sizes, therefore most of the measurements on the pairing symmetry so far were made on polycrystalline samples. This is much improved in the (Ba, S r) 1−x K x Fe 2 As 2 (denoted as FeAs-122) system since sizable crystals can be achieved [19,20]. Preliminary data by angle resolved photoemission spectroscopy (ARPES) on these crystals show two groups of superconducting gaps (∆ 1 ≈ 12 meV, ∆ 2 ≈ 6 meV) all with s-wave symmetry [21,22,23]. It is known that the surface of this type of single crystals decay or reconstruct very quickly, this may give obstacles to get repeatable data when using the surface sensitive tools. Thus solid conclusions about the gap symmetry and magnitude from bulk measurements are highly desired.Specific heat (SH) is one of the powerful tools to meas...

Specific Heat of Ba0.59K0.41Fe2As2, an Fe-Pnictide Superconductor with Tc = 36.9 K, and a New Method for Identifying the Electron Contribution

“…Ba 0.6 K 0.4 Fe 2 As 2 : Welp et al 29 ;Mu et al 38 The specific heat of a sample with T c = 35.8 K was measured from 2 to 50 K. The low-T data gave B 3 and a γ v (H) that was approximately linear in H. Extrapolation of γ v (H) to H c2 , which was taken to be 100 T, gave γ n = 71 mJ K -2 mol -1 , which included a residual γ r = 7.7 mJ K -2 mol -1 . C lat , represented by five terms in Eq (3) with B 3 fixed at the value derived in the low-T fit, was obtained by fitting the normal-state data in the narrow interval above T c with γ n fixed at 71 mJ K -2 mol -1 .…”

Evidence for multiple gaps in the specific heat of LiFeAs crystals

“…7 In addition, some of the reports of electronic specific heat, C p,e / T, below T c have been so different that d-wave and s-wave pairings were proposed for LaFeAs͑O 0.9 F 0.1 ͒ and ͑Ba 0.6 K 0.4 ͒Fe 2 As 2 , respectively. 6,10 The situation is actually even more complicated. Different gap features have been suggested for the same ͑Ba 0.6 K 0.4 ͒Fe 2 As 2 crystals from two different bulk probes, specific heat, and the lower critical field H c1 .…”

“…Although a more-or-less universal trend is expected based on the common layered structure and the comparable superconducting transition temperatures T c within the FeAs family, the properties observed ͑even the basic thermodynamic parameters͒ are rather divergent. The doping dependencies of T c , the Sommerfeld coefficient ␥ ͑a measure of the density of states͒, 4,6 the specific-heat anomaly ⌬C p / T at T c ͑a parameter representing the pair-coupling strength͒, the temperature dependence of the superconducting gap below T c ͑i.e., the wave-function symmetry͒ ͑Refs. 7-10͒ and the residual C p / T at the zero-temperature limit ͑an indicator of the possible phase separation͒ all vary significantly from one member to another.…”

“…6 A much lower ␥ Ϸ 6-8 mJ/ mole K 2 in LaFeAs͑O 0.9 F 0.1 ͒ was then used to accommodate the difference. However, an apparent ␥ = 121 mJ/ mole K 2 , but a ⌬C p / T c as small as that of LaFeAs͑O 0.9 F 0.1 ͒, is observed in SmFeAs͑O 0.9 F 0.1 ͒.…”

Observation of ordered vortices with Andreev bound states in Ba0.6K0.4Fe2As2

“…They all exhibit a very clean x-ray diffraction (XRD) pattern, extremely low residual specific heat coefficient in the zero temperature limit (γ 0 = 1.24 mJ/mol-K 2 , comparing to the normal state value γ n >50 mJ/mol-K 2 ) 30 , and a sharp superconducting transition.…”