Anisotropic phase diagram and superconducting fluctuations of single - crystalline

We report on the specific-heat determination of the anisotropic phase diagram of single crystals of optimally doped SmFeAsO1-xFx. In zero field, we find a clear cusplike anomaly in C/T with ΔC/Tc=24 mJ/mol K2 at Tc=49.5 K. In magnetic fields along the c axis, pronounced superconducting fluctuations induce broadening and suppression of the specific-heat anomaly which can be described using threedimensional lowest-Landau-level scaling with an upper critical field slope of –3.5 T/K and an anisotropy of Γ =8. The small value of ΔC/Tc yields a Sommerfeld coefficient � � 8 mJ/mol K2, indicating that SmFeAsO1-xFx is characterized by a modest density of states and strong coupling. DOI: https://doi.org/10.1103/PhysRevB.83.100513 Posted at the Zurich Open Repository and Archive, University of Zurich ZORA URL: https://doi.org/10.5167/uzh-50926 Accepted Version Originally published at: Welp, U; Chaparro, C; Koshelev, A; Kwok, W; Rydh, A; Zhigadlo, N; Karpinski, J; Weyeneth, S (2011). Anisotropic phase diagram and superconducting fluctuations of single-crystalline SmFeAsO0.85F0.15.PhysicalReview.B, 83(10) : 100513. DOI: https://doi.org/10.1103/PhysRevB.83.100513 Anisotropic Phase Diagram and Superconducting Fluctuations in SmFeAsO0.85F0.15 U. Welp, C. Chaparro, A. E. Koshelev, W. K. Kwok, A. Rydh, N. D. Zhigadlo, J. Karpinski, S. Weyeneth Materials Science Division, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA Department of Physics, Stockholm University, SE-10691 Stockholm, Sweden Laboratory for Solid State Physics, ETH Zurich, Schafmattstr. 16, CH-8093 Zurich, Switzerland Physik-Institut der Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland We report on the specific heat determination of the anisotropic phase diagram of single crystals of optimally doped SmFeAsO1-xFx. In zero-field, the optimally doped compound displays a clear cusp-like anomaly in C/T with ΔC/Tc = 24 mJ/molK at Tc = 49.5 K. In magnetic fields applied along the c-axis, we find pronounced superconducting fluctuations induced broadening and suppression of the specific heat anomaly which can be described using three-dimensional lowest-Landau-level scaling with an upper critical field slope of -3.5 T/K and an anisotropy of Γ = 8. The small value of ΔC/Tc yields a Sommerfeld coefficient γ ~ 8 mJ/molK indicating that SmFeAsO1-xFx is characterized by a modest density of states and strong coupling. Following the initial discovery [1] of superconductivity at temperatures up to 26 K in LaFeAsO1-xFx, superconductivity has been found in a large number of materials whose common structural motif is the presence of FeAs (or FeSe,Te) planes [2, 3]. Various families of FeAs-superconductors can be distinguished, most notably the (Rare Earth)1111 materials derived from the original LaFeAsO1-xFx, and the 122-family derived from Ba1-xKxFe2As2 [4]. Superconductivity arises upon electron or hole doping or due to the application of pressure from an antiferromagnetic parent compound. The highest values of Tc of ~56 K (resistive onset) were achieved in Smand Gd-based 1111-materials [5]. The high values of Tc, and the prospect of unconventional s-symmetry of the superconducting order parameter, pairing mediated by spin fluctuations and multi-band superconductivity have generated tremendous interest in these new superconductors. The FeAs-superconductors have distinguishing macroscopic properties such as an enormous upper critical field combined with a small superconducting anisotropy. The upper critical field, Hc2, its anisotropy and the specific heat anomaly associated with the superconducting transition are fundamental bulk characteristics that shed additional light on the microscopic length scales, the Fermi surface topology and electronic structure of the superconductor. Here we present the first single crystal specific heat measurements of SmFeAsO0.85F0.15 to determine the anisotropic phase diagram and the effect of superconducting fluctuations in this material. A clear cusp-like anomaly is observed at the superconducting transition with height of ! "C /T c # 24 mJ/molK which is substantially smaller than the prediction based on the scaling ! "C /T c #T c 2 reported for various Ba-122 based materials [6]. The shape of the zero-field transition and its evolution in applied magnetic fields reveal pronounced superconducting fluctuation effects which can be consistently described in the framework of 3D lowest Landau level (LLL) scaling yielding an upper critical field slope of -3.5 T/K for H || c and a coherence length anisotropy Γ = 8. The strong superconducting fluctuations are manifested in the very large value of the Ginzburg number Gi ~ 1.6 10. Entropy conservation and the low value of the specific heat anomaly imply that the Sommerfeld coefficient of the electronic specific heat, γ ~ 8 mJ/molK, is lower than previously anticipated, identifying SmFeAsO0.85F0.15 as a superconductor with modest density of states and strong coupling. Calorimetric measurements were conducted using a membrane-based steady-state acmicro-calorimeter [7] with a thermocouple composed of Au-1.7%Co and Cu films deposited onto a 150 nm thick Si2N4-membrane as thermometer. This technique enables high precision measurement of the specific heat of sub-micro gram samples. The absolute accuracy of our specific heat data was checked against gold samples of similar size as our pnictide crystals. SmFeAsO0.85F0.15 crystals with approximate sizes of 108x95x7 μm (sample I) and 130x79x13 μm (sample II) were grown in a high-pressure synthesis procedure using NaCl/KCl flux [8]. The samples were mounted onto the thermocouple using Apiezon N grease. An ac-heater current at 23 Hz was adjusted to induce 50 to 200 mK oscillations of the sample temperature. Figure 1 shows the low field magnetization at the superconducting transition of both crystals. The temperature independent magnetization at low temperatures and a transition width of ~ 1.5 K underline the high quality of the crystals. The inset of Fig. 2a displays the specific heat anomaly near Tc ~ 49.5 K of sample I in zero-field. The specific heat is essentially linear in temperature above Tc up to 60 K, the highest temperature measured. We use the linear extrapolation of the normal state specific heat Cn plus a small correction described in detail below as background to analyze the specific heat of SmFeAsO0.85F0.15 in the temperature range close to Tc(H). At lower temperatures the background specific heat will deviate from linear as the Debye function approaches the characterstic T-dependence, and the superconducting contribution will be overestimated. The main panels of Fig. 2 shows the superconducting specific heat Cs/T of sample I in various fields applied along the c-axis and ab-plane, respectively. Similar data were obtained for sample II. In zero-field a clear almost cusp-like anomaly is observed with a height of ~ 24 mJ/molK, about twice the value reported on a polycrystalline sample [9] and close to the value of 19 mJ/molK obtained on a polycrystalline sample of oxygen deficient F-free SmFeAsO1-x with Tc = 54.6 K [10]. However, our value for ΔC/Tc is almost an order of magnitude smaller than what would be expected on the basis of the