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634_ition in this electrondoped sample. A clear peak of C_T shows up at Tc = 36 K for the hole-doped Ba0.5 K0.5 Fe2 As2 superconducting sample

By Beatrice Torres,2014-10-14 20:30
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634_ition in this electrondoped sample. A clear peak of C_T shows up at Tc = 36 K for the hole-doped Ba0.5 K0.5 Fe2 As2 superconducting sample

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     Thermodynamic properties of Ba1?x Mx Fe2 As2 (M = La and K)

     J. K. Dong,1 L. Ding,1 H. Wang,1 X. F. Wang,2 T. Wu,2 X. H. Chen,2 and S. Y. Li1,?

     1 Department of Physics, Surface Physics Laboratory (National Key Laboratory), and Advanced Materials Laboratory, Fudan University, Shanghai 200433, P. R. China 2 Hefei National Laboratory for Physical Science at Microscale and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China (Dated: June 30, 2008)

     arXiv:0806.3573v2 [cond-mat.supr-con] 30 Jun 2008

     The speci?c heat C(T ) of BaFe2 As2 single crystal, electron-doped Ba0.7 La0.3 Fe2 As2 and holedoped Ba0.5 K0.5 Fe2 As2 polycrystals were measured. For undoped BaFe2 As2 single crystal, a very sharp speci?c heat peak was observed at 136 K. This is attributed to the structural and antiferromagnetic transitions occurring at the same temperature. C(T ) of the electron-doped nonsuperconducting Ba0.7 La0.3 Fe2 As2 also shows a small peak at 120 K, indicating a similar but weaker structural/antiferromagnetic transition. For the hole-doped superconducting Ba0.5 K0.5 Fe2 As2 , a clear peak of C/T was observed at Tc = 36 K, which is the highest peak seen at superconducting transition for iron-based high-Tc superconductors so far. The electronic speci?c heat coe?cient ?Ã and Debye temperature ??D of these compounds were obtained from the low temperature data.

     PACS numbers: 74.25.Bt, 74.25.Ha

     The recent discovery of superconductivity with Tc as high as 55 K in iron-based LnO1?x Fx FeAs (Ln = La, Sm, Ce, Nd, Pr, Gd, Tb, Dy) has attracted great attention.1,2,3,4,5,6,7,8,9 It is believed that the FeAs layers are responsible for the high-Tc superconductivity and the LnO layers provide electron carriers through ?uorine

    doping,1,2,3,4,5,6,7,8,9 by simply introducing oxygen vacancies,10 or by substituting Ln3+ with Th4+ .11 This is very similar to the high-Tc cuprate superconductors. Since increasing the number of CuO2 layers in one unit cell has been a promising way to elevate Tc for cuprates, many e?orts have been put into searching superconductivity in iron-based compounds with multiple FeAs layers. The ternary iron arsenide BaFe2 As2 with the tetragonal ThCr2 Si2 -type structure was ?rst suggested by Rotter et al.12 that it can serve as a parent compound for oxygen-free iron arsenide superconductors. Very soon, superconductivity with Tc = 38 K was indeed discovered in Ba1?x Kx Fe2 As2 .13 In contrast to electrondoped LnO1?x Fx FeAs, the carriers in

    Ba1?x Kx Fe2 As2 are holes introduced by potassium doping, which has been veri?ed by Hall coe?cient and thermoelectric power measurements.14 Later, superconductivity was also found in Sr1?x Mx Fe2 As2 (M = K and Cs) and Ca1?x Nax Fe2 As2 with Tc ?? 38 and 20 K, respectively.15,16,17 Interestingly, the electron-doped Ba1?x Lax Fe2 As2 shows no sign of superconductivity.14 For the parent compound BaFe2 As2 , resistivity, speci?c heat, and susceptibility show clear anomaly near 140 K.12,18 Similar anomalies were also observed in SrFe2 As2 and EuFe2 As2 at relatively higher temperature near 200 K.15,19,20,21 Neutron scattering experiment has demonstrated that in BaFe2 As2 there is a phase transition to a long-ranged antiferromagnetic state at 142 K where a ?rst-order structural transition from tetragonal to orthorhombic symmetry also occurs.22 This is analogous to that in LaOFeAs compound, where structural and spin-

     density-wave transitions were observed, but occurring at di?erent temperatures, 155 K and 137 K respectively.23 In both LnOFeAs and AFe2 As2 (A = Ba, Sr, and Ca), antiferromagnetism gives way to superconductivity as electrons or holes are doped, indicating antiferromagnetic spin ?uctuations may play an important role in these systems. While the speci?c heat peak near 140 K was observed in BaFe2 As2 polycrystalline sample,12 it was absent from the C(T ) curve of the ?rst reported BaFe2 As2 single crystal.24 The resistivity behavior of their single crystal24 was also di?erent from previous polycrystalline sample.13 These unusual resistivity and speci?c heat behaviors may result from the contamination of Sn ?ux in the crystal.24 Therefore it is interesting to check the intrinsic speci?c heat behavior of BaFe2 As2 single crystal with high purity. There are also no speci?c heat data for superconducting A1?x Mx Fe2 As2 (A = Ba, Sr, and Ca; M = K, Cs, and Na) compounds so far. In this Brief Report, we present the ?rst speci?c heat results of high quality BaFe2 As2 single crystal, electron-doped Ba0.7 La0.3 Fe2 As2 and hole-doped Ba0.5 K0.5 Fe2 As2 polycrystals. Very sharp speci?c heat peak at 136 K was observed for our BaFe2 As2 single crystal, 3 times higher than that in previous polycrystalline sample. A much smaller peak near 120 K was also observed for Ba0.7 La0.3 Fe2 As2 , indicating a weak

    structural/antiferromagnetic transition in this electrondoped sample. A clear peak of C/T shows up at Tc = 36 K for the hole-doped Ba0.5 K0.5 Fe2 As2 superconducting sample. By ?tting the low temperature data, the electronic speci?c heat coe?cient ?Ã and Debye temperature ??D of these compounds were obtained. The single crystals of BaFe2 As2 and polycrystalline samples with nominal composition Ba0.7 La0.3 Fe2 As2 and Ba0.5 K0.5 Fe2 As2 were prepared as in Ref. 14 and 18. By employing self-?ux method (FeAs as ?ux), our BaFe2 As2 single crystals are free of contamination from other ele-

     2

     0.9 Ba0.5K0.5Fe2As2 Ba0.7La0.3Fe2As2 BaFe2As2 0.6

     250 BaFe2As2 Ba0.7La0.3Fe2As2 Ba0.5K0.5Fe2As2

     250

     Ts

     200

     x 0.5 Ts

     C (J / mol K)

     200

     ?Ñ (m? cm)

     150

     200 150

     100

     100 130 132 134 136 138 140

     0.3

     50

     0.0

     0

     50

     100

     150

     0

     0

     50

     100

     150

     200

     T (K)

     FIG. 1: (Color online) Resistivity of BaFe2 As2 single crystal, electron-doped Ba0.7 La0.3 Fe2 As2 and hole-doped Ba0.5 K0.5 Fe2 As2 polycrystals. Arrows denote the structural and antiferromagnetic transitions occurring at the same temperature Ts .

     T (K)

     FIG. 2: (Color online) Speci?c heat of BaFe2 As2 , Ba0.7 La0.3 Fe2 As2 and Ba0.5 K0.5 Fe2 As2 samples. The parent compound BaFe2 As2 single crystal shows a very sharp peak at 136 K, enlarged in the insert. A small peak is also visible at 120 K for Ba0.7 La0.3 Fe2 As2 .

     ments. Resistivity were measured by the standard fourprobe method. Speci?c heat measurements were performed in a Quantum Design physical property measurement system (PPMS) via the relaxation method. Magnetic ?eld H = 8 T was applied for the Ba0.5 K0.5 Fe2 As2 superconducting sample. Fig. 1 shows the resistivity of BaFe2 As2 single crystal, Ba0.7 La0.3 Fe2 As2 and Ba0.5 K0.5 Fe2 As2 polycrystals. Abrupt resistivity drop can be seen at Ts = 134 K for BaFe2 As2 single crystal,

    consistent with previous reports.12,14 Similar resistivity drop was also observed for Ba0.7 La0.3 Fe2 As2 sample, but less abrupt and shifting to lower temperature Ts = 122 K. No superconducting transition was observed at low temperature down to 5 K for the electron-doped Ba0.7 La0.3 Fe2 As2 , similar to previous Ba0.85 La0.15 Fe2 As2 sample.14 The hole-doped Ba0.5 K0.5 Fe2 As2 sample shows a sharp superconducting transition starting at 38 K and reaching zero resistivity at 34 K. We use the middle point of transition 36 K as its Tc . The speci?c heat C(T ) of BaFe2 As2 , Ba0.7 La0.3 Fe2 As2 and Ba0.5 K0.5 Fe2 As2 samples are shown in Fig. 2. For the BaFe2 As2 single crystal, one can see a very sharp peak (enlarged in the inset) with ?C ?Ö 130 J / mol K, which is di?erent from the ?rst reported BaFe2 As2 single crystal by Ni et al.,24 and consistent with previous result on polycrystalline sample.12 The height of speci?c heat peak in the insert of Fig. 2 is also 3 times higher than that in polycrystalline sample where ?C ?Ö 35 J / mol K.12 This proves the high quality of our BaFe2 As2 single crystal. It is believed that the use of self-?ux method of growth gives the high purity of our samples, thus the intrinsic speci?c heat behavior of BaFe2 As2 single crystal.

     As found in SrFe2 As2 ,19 the abrupt resistivity drop and sharp speci?c heat peak in Fig. 1 and 2 should be compatible with a ?rst-order phase transition in BaFe2 As2 . Neutron scattering experiment has demonstrated such ?rst-order structural phase transition, accompanied by a long-range antiferromagnetic transition.22 The entropy connected with the transition ?S ?? 1 J / mol K is estimated from the area under the C(T ) peak in the insert of Fig. 2. This small value of ?S is almost the same as that in SrFe2 As2 .19 In Fig. 2, C(T ) of the electron-doped Ba0.7 La0.3 Fe2 As2 sample also manifests a small peak at 120 K. This is consistent with the resistivity drop at 122 K in Fig. 1, indicating a similar but weaker structural/antiferromagnetic transition. No anomaly of C(T ) was observed above Tc for the hole-doped Ba0.5 K0.5 Fe2 As2 sample. These results show that hole doping suppresses structural/antiferromagnetic transition more e?ciently than electron doping in Ba1?x Mx Fe2 As2 system. Fig. 3 plots C/T vs T for the superconducting Ba0.5 K0.5 Fe2 As2 sample from 30 to 40 K in zero and H = 8 T magnetic ?elds. A clear peak shows up at Tc = 36 K and it is suppressed to lower temperature by 8 T ?eld. Rough estimation gives the speci?c heat jump ?C/T ?Ö 15 mJ / mol K2 at Tc in zero ?eld. Previously, there was no visible anomaly on C/T near Tc for LaO1?x Fx FeAs,25,26 and only a small kink was observed at Tc for SmO1?x Fx FeAs.27 To our knowledge, the C/T peak in Fig. 3 is the highest peak seen at the superconducting transition in iron-based high-Tc superconductors so far. This may be attributed to the better quality, or higher super?uid density of the Ba0.5 K0.5 Fe2 As2 sample.

     3

     0.52 Ba0.5K0.5Fe2As2 0.50 H=0T H=8T

     0.48

     0.46

     Ba0.7 La0.3 Fe2 As2 and Ba0.5 K0.5 Fe2 As2 samples at low temperature. For BaFe2 As2 single crystal, the data between 2 and 6 K can be linearly ?tted to C/T = ?à + ?ÂT 2 , which gives the electronic speci?c heat coe?cient ?à = 6.1 ?À 0.3 mJ / mol K2 and ? = 1.51 ?À 0.01 mJ / mol K4 . This value of ?à is smaller than that obtained in polycrystal sample (16 mJ / mol K2 ),12 and much smaller than that of the ?rst reported BaFe2 As2 single crystal (37 mJ / mol K2 ).24 Using the equation ? = (12?Ð 4 nkB )/(5??3 ), D where n is the number of atoms per formula unit, we estimate the Debye temperature ??D = 186 K for BaFe2 As2 single crystal. For Ba0.7 La0.3 Fe2 As2 and Ba0.5 K0.5 Fe2 As2 samples, the data between 2 and 8 K can also be ?tted as above, which gives ?à = 15.2 ?À 0.1 mJ / mol K2 , ? = 0.586 ?À 0.003 mJ / mol K4 , and ?à = 9.1 ?À 0.2 mJ / mol K2 , ? = 0.653 ?À 0.005 mJ / mol K4 , respectively. Comparing with the parent compound, electron doping seems to increase ?à in the non-superconducting Ba0.7 La0.3 Fe2 As2 . The residual ?à = 9.1 mJ / mol K2 of the superconducting Ba0.5 K0.5 Fe2 As2 sample suggests nodes in the superconducting gap. However, since these two samples are polycrystals, their values of ?à may be not completely intrinsic. The Debye temperature of these two samples are estimated to be ??D = 254 and 246 K, respectively. In summary, we have studied the speci?c heat of high quality BaFe2 As2 single crystal, electron-doped Ba0.7 La0.3 Fe2 As2 and hole-doped Ba0.5 K0.5 Fe2 As2 polycrystals. For BaFe2 As2 single crystal, a very sharp speci?c heat peak at 136 K was observed, consistent with the polycrystal result. A small peak near 120 K was also observed for electron-doped Ba0.7 La0.3 Fe2 As2 , indicating a weak structural/antiferromagnetic transition. A clear peak of C/T can be seen at Tc = 36 K for the hole-doped Ba0.5 K0.5 Fe2 As2 sample, which is the highest one among all iron-based high-Tc superconductors so far. By ?tting the low temperature data, we obtain the electronic speci?c heat coe?cient ?à and Debye temperature ??D of these compounds. This work is supported by the Natural Science Foundation of China, the Ministry of Science and Technology of China (973 project No: 2006CB601001 and National Basic Research Program No:2006CB922005), and STCSM of China.

     C/T (J / mol K )

     2

     0.44 30

     32

     34

     36

     38

     40

     T (K)

     FIG. 3: (Color online) C/T vs crystalline sample near Tc = 36 netic ?elds. Rough estimation ?C/T ?Ö 15 mJ / mol K2 at Tc

     0.12 BaFe2As2 Ba0.7La0.3Fe2As2 Ba0.5K0.5Fe2As2 0.09

     T for Ba0.5 K0.5 Fe2 As2 polyK in zero and H = 8 T maggives the speci?c heat jump in zero ?eld.

     C/T (J / mol K )

     2

     0.06

     0.03

     0.00

     0

     20

     2

     40

     60

     2

     80

     T (K )

     FIG. 4: (Color online) C/T vs T 2 for BaFe2 As2 , Ba0.7 La0.3 Fe2 As2 and Ba0.5 K0.5 Fe2 As2 samples. The lines are linear ?ts at low temperature.

     In Fig.

     4, C/T vs T 2 is plotted for BaFe2 As2 ,

     Electronic address: shiyan li@fudan.edu.cn

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