Electronic specific heat of Ba1−xKxFe2As

Abstract

Using a high-resolution differential technique we have determined the electronic specific heat coefficient γ (T ) = C el /T of Ba 1−x K x Fe 2 As 2 with x = 0 to 1.0, at temperatures (T ) from 2 K to 380 K and in magnetic fields H = 0 to 13 T. In the normal state γ n (x,T ) increases strongly with x at low temperature, compatible with a mass renormalization ∼12 at x = 1, and decreases weakly with x at high temperature. A superconducting transition is seen in all samples from x = 0.2 to 1, with transition temperatures and condensation energies peaking sharply at x = 0.4. Superconducting coherence lengths ξ ab ∼ 20Å and ξ c ∼ 3Å are estimated from an analysis of Gaussian fluctuations. For many dopings we see features in the H and T dependencies of γ s (T ,H ) in the superconducting state that suggest superconducting gaps in three distinct bands. A broad "knee" and a sharp mean-field-like peak are typical of two coupled gaps. However, several samples show a shoulder above the sharp peak with an abrupt onset at T c,s and a T dependence γ s (T ) ∝ 1 − T /T c,s . We provide strong evidence that the shoulder is not due to doping inhomogeneity and suggest it is a distinct gap with an unconventional T dependenceWe estimate band fractions and T = 0 gaps from three-band α-model fits to our data and compare the x dependencies of the band fractions with spectroscopic studies of the Fermi surface.The electronic specific heat γ measured to room temperature contains a wealth of quantitative information about the electronic spectrum of metallic systems over an energy region ±100 meV about the Fermi level, crucial for understanding high-temperature superconductivity. Measurements of the electronic specific heat have played an important role in revealing key properties of the copper-oxide-based "cuprate" high-temperature superconductors (HTSCs). Some examples include the normal-state "pseudogap," 1-4 the bulk sample inhomogeneity length scale, 5 and more recently evidence that the superconducting transition temperature is suppressed due to superconducting fluctuations. 6 In this work we extend such measurements to the iron-arsenide-based "pnictide" HTSCs. Here we present results obtained for polycrystalline samples of Ba 1−x K x Fe 2 As 2 (x = 0 to 1.0) using a high-resolution differential technique. 7 With this technique we directly measure the difference in the specific-heat capacities of a doped sample and an undoped reference sample (BaFe 2 As 2 ). This eliminates most of the large phonon term from the raw data and yields a curve dominated by the difference in electronic terms. Features of the electronic specific heat that would otherwise be masked by the large phonon background are then clearly visible in the raw data over the entire temperature range. Central to the success of this technique are measurements on a series of samples at closely spaced doping intervals, so that systematic trends in the the relatively small difference in phonon terms between sample and reference can be identified and appropriate corrections made. ...

Superconducting gap evolution in overdopedBaFe2(As1−xPx)2single crystals through nanocal

“…The behavior is different in comparison with Co-doped samples 24,26 , where the two gaps have fairly constant weights throughout the entire doping range. It is instead in agreement with K-doped samples 23 , where the weight of the second gap is increasing with doping, as in the present case.…”

“…Values of γ r /γ n in the range of 10−20% have been reported for Co-doped samples close to optimal doping 24,25 (γ r /γ n ∼ 5% at optimal doping), much higher than for K-doped samples, where γ r /γ n accounts to only a few percent 22,23 . This is in agreement with a fully gapped state detected in K-doped samples over a wide range of the phase diagram 20 , while nodal behavior is detected from thermal conductivity measurements in Co-doped samples as soon as x moves away from optimal doping 8 .…”

“…A nodeless s-wave order parameter is expected to give a linear dependence of γ r as a function of field 41 , while a d-wave order parameter a square root dependence with field 42 . An intermediate behavior is generally interpreted in iron-based superconductors as due to the presence of gaps with different amplitudes 23,26 . The gap imbalance becomes more pronounced as γ r (H) deviates further from a linear relation.…”

Electric field gradient wave (EFGW) in iron-based superconductor Ba0.6K0.4Fe2As2 studied by Mössbauer spectroscopy

“…Generally suitable substitution of any element in the '122' parent compounds leads to suppression of SDW and eventual appearance of the superconductivity [26,27]. The difference in total molar specific heat coefficients tot p tot s    between superconductor (s) and parent compound (p) versus temperature is also shown together with the electronic specific heat coefficient el s  of the superconductor versus temperature [18].…”

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

“…Our analysis depends on the validity of the fitting expression, Eq. (12), and the fit to C(H). Unlike the conventional analyses, it does not involve a pre-determined quantitative expression for C lat .…”