Growth and characterization of A{}_{1-x}K{}_{x}Fe{}_{2}As{}_{2} (A = Ba, Sr) single crystals with x=0 \sim 0.4

Growth and characterization of AKFeAs (A = Ba, Sr) single crystals with =0 0.4

Huiqian Luo, Zhaosheng Wang, Huan Yang, Peng Cheng, Xiyu Zhu and Hai-Hu Wen hhwen@aphy.iphy.ac.cn National Laboratory for Superconductivity, Institute of Physics and
National Laboratory for Condensed Matter Physics, P. O. Box 603 Beijing, 100190, P. R. China
Abstract

Single crystals of AKFeAs (A=Ba, Sr) with high quality have been grown successfully by FeAs self-flux method. The samples have sizes up to 4 mm with flat and shiny surfaces. The X-ray diffraction patterns suggest that they have high crystalline quality and -axis orientation. The non-superconducting crystals show a spin-density-wave (SDW) instability at about 173 K and 135 K for Sr-based and Ba-based compound, respectively. After doping K as the hole dopant into the BaFeAs system, the SDW transition is smeared, and superconducting samples with the compound of BaKFeAs (0 0.4) are obtained. The superconductors characterized by AC susceptibility and resistivity measurements exhibit very sharp superconducting transition at about 36 K, 32 K, 27 K and 23 K, respectively.

pacs:
74.25.Fy, 74.62.Bf, 74.70.Ad, 61.50.-f

1 Introduction

The newly discovered superconductivity in iron-oxypnictides superconductors stimulates intensive researches on high-temperature superconductivity beside the cuprate system. Just in several months, the superconducting transition temperature () was promoted to 55 K in the electron doped system [1, 2, 3, 4, 5, 6], as well as 25 K in hole doped LaSrOFeAs compound [7]. Because of the layered structure, the doping behavior and many other properties of the iron-based system are very similar to those of the copper-oxides, it has been thus expected that higher may be found in multi-layer systems. Soon after, single crystals of LnFeAs(OF) (Ln=Pr, Nd, Sm) were grown successfully by NaCl/KCl-flux method [8, 9, 10], while the sub-millimeter sizes limit the experimental study on them [11, 12]. Therefore, FeAs-based single crystals with high crystalline quality, homogeneity and large sizes are highly desired for precise measurements of the properties.

Very recently, the BaFeAs compound in a tetragonal ThCrSi-type structure with infinite Fe-As layers was reported [13]. By replacing the alkaline earth elements ( Ba and Sr ) with alkali elements ( Na, K and Cs ), superconductivity up to 38 K was discovered both in hole-doped and electron doped samples [14, 15, 16, 17]. The varies from 2.7 K in CsFeAs to 38 K in AKFeAs (A=Ba, Sr) [14, 18]. Meanwhile, the superconductivity also could be induced in the parent phase by high pressure [19, 20] or replacing partial Fe by Co [21, 22]. More excitingly, large single crystals could be obtained by Sn-flux method in this family for the rather low melting temperature and the intermetallic characteristics [23, 24, 25]. However, single crystals with high homogeneity and less contamination are still hard to be obtained by this method [26]. To avoid these problems, the FeAs self-flux method may be more appropriate.

Here we report the successful growth of AKFeAs (A=Ba,Sr) single crystals by self-flux method using FeAs as the flux, both non-superconducting parent phase AFeAs (A=Ba,Sr) and superconducting BaKFeAs crystals are obtained. The measurements of X-ray diffraction (XRD) indicate high crystalline quality on both kinds of samples, and the results of AC susceptibility show sharp superconducting transitions on the superconducting ones. The resistivity measurements on the non-superconducting phase suggest a clear resistivity anomaly which is induced by the formation of the spin-density-wave (SDW) and structure transition [13, 27, 28, 29]. The doping dependence of -axis lattice constant and are very consistent with the reported results from polycrystalline samples [30, 31].

2 Experiments

The AKFeAs (A=Ba,Sr) single crystals were grown by FeAs self-flux method. The FeAs precursor was synthesized by the reaction of Fe powders (Alfa Aesar, 99.99% in purity) and As chips (99.999%) at 500 C for 10 hours and then 700 C for 20 hours in a sealed silica tube. The starting materials of FeAs, and high purity Ba or Sr (Alfa Aesar, 99.2% in purity) were mixed in 4:(1-), then a soft bulk of K with proper amount was added to cover the powder. The whole procedure was performed in a glove box with a protective argon atmosphere where both concentrations of O and HO were less than 1 ppm. The mixture was placed in an alumina oxide crucible with 15 mm 35 mm in dimension and sealed under vacuum in a silica tube with 18 mm 90 mm in dimension. Because the silica tube would be broken due to the gas pressure of potassium at the temperature around 1000 C, the superconducting samples could only be obtained by using an limited amount of potassium and a thick enough silica tube ( t 3 mm ). For example, if the total mass of staring material was supposed to be 2.0 g with the ratio of Ba : K : Fe : As = 0.6 : 0.4 : 4.0 : 4.0, the mass for each materials was m(Ba)= 0.265 g, m(K)= 0.050 g and m(FeAs)= 1.684 g, respectively. Considering the losing of K during the growth (about 0.12g in most cases), the total amount of K should be 0.170 g. It should be noted that the safe amount of K is less than 0.25g in our conditions for preventing the explosion of silica tube. The actual contents of K in the as-grown crystals were determined by quantitative analysis in the later measurements. The sealed silica tube was placed in a muffle furnace and heated up to a high temperature at about 1000 1150 C for melting completely. Then it was cooled down to a temperature below 800 C at a very slow speed which is less than 10 C/hour. The melting temperature and cooling down speed depend on the ratio of Ba:K in the staring material. Finally, the muffle furnace was powered off. After it was cooled down to room temperature, the tube was fetched out and broken. The crystals were obtained by cleaving the as-grown bulks. Then they were selected and shaped under a microscope.

Various techniques were used to characterize our samples. The crystal surface morphology and composition were examined by scanning electron microscopy (SEM, Hitachi S-4200) and the energy dispersive X-ray (EDX, Oxford-6566, installed in the S-4200 apparatus) analysis. While the X-ray diffraction of the crystals was carried out by a Mac- Science MXP18A-HF equipment with scan to examine the crystalline quality of the samples. radiation of Cu target was used, and the continuous scanning range of 2 is from 10 to 80. The raw data of XRD was analyzed by PowderX software where the zero-shift and -elimination and other factors were taken into account [32]. The AC susceptibility was measured on an Oxford cryogenic system Maglab-EXA-12. An alternating magnetic field (H=1 Oe) was applied perpendicular to the -plane at a frequency =333 Hz when the AC susceptibility measurement was undertaken. The was derived from AC susceptibility curve by the point where the real part of the susceptibility becomes flat. The resistivity measurements were carried out on a Quantum Design Physical Property Measurement System (PPMS) by a standard four-probe method with a low contact resistance ( ).

3 Results and discussion

By varying the content of potassium in the starting material, we obtained non-superconducting crystals in both Ba-based and Sr-based compound and superconducting samples with the composition of BaKFeAs. Figure 1 shows the photograph of some crystals cleaved from the as-grown bulks. They all have very shiny plate-like cleaved surfaces. The sizes of the largest one are about 2.5 mm4.0 mm0.2 mm, and others have sizes up to 2 mm. Because the real contents of each element always deviate from the starting material in the flux method. The composition of our single crystals was determined by the energy dispersive X-ray (EDX) analysis. 3 5 pieces of as-grown single crystal were selected out carefully from each batch. Then they were cleaved under microscope and taken EDX measurement immediately before the surface degenerate in air. A typical EDX spectrum is shown in Figure 2. The inset is the SEM photograph of this crystal, which shows a very flat surface morphology and the layered structure. A brief summary of the properties of BaKFeAs single crystals is given in Table 1. We successfully obtained four categories superconducting samples with K doping level in = 0.40, 0.28, 0.25 and 0.23. The non-superconducting crystals also contain a little K which is less than 10% for both Ba-based and Sr-based compound.

The crystal structure of non-superconducting samples was examined by X-ray diffraction measurement with incident X-ray along the -axis. The typical diffraction patterns are shown in Figure 3. Only sharp peaks along () could be observed, and full-width-at-half-maximum (FWHM) of each peak is around 0.10. These indicate high -axis orientation and crystalline quality in our samples. The raw data of XRD was analyzed by PowderX software with the zero-shift and -elimination and other factors taken into account [32]. The -axis parameters were calculated and also presented in Table 1. For the non-superconducting samples, the -aixs lattice constant is about 13.07 Å for Ba-based compound and 12.58 Å for Sr-based compound, respectively. The magnitudes of -axis are very close to those results in the polycrystalline parent phase without K doing, where they were reported to be 13.02 Å [13] for Ba-based compound and 12.40 Å [16] for Sr-based compound. In addition, it should be noted that the EDX has a uncertainty around 10%, and it will be larger especially for light elements. It is possible that the real contents of K in non-superconducting samples are less than the magnitude shown in Table 1. Thus we just mark them using the chemical formula as parent compounds BaFeAs and SrFeAs. Figure 3(b) also displays the XRD pattern for the sample with = 0.40. A clear shift for each peak shows up, which indicates that the lattice has a little variation after doping K into the parent phase.

The AC susceptibility measurement was used to character the superconducting BaKFeAs single crystals. Figure 4 shows three typical groups of the susceptibility curves. The was determined as the onset point of , and the transition width was defined as =(90%) - (10%). There are a flat diamagnetic signal in low temperature region and a very sharp superconducting transition around , where the demagnetizing factor is not taken into account in these measurements. The increases gradually as more and more K were doped into the samples. The superconducting transition is almost the same among the single crystals cleaved from the same batch, which indicates that our samples are very homogeneous.

Figure 5 shows the temperature dependence of resistivity under zero field. The applied current is 5 mA, and it flows in the -plane during the measurements. For BaFeAs and SrFeAs, a strong anomaly shows up at = 135 K and 173 K, respectively ( Figure 5(a) ). The resistivity has a nearly -linear dependence above this temperature and sharply drops down below this temperature. This resistivity anomaly could be attributed to the SDW instability and structure transition which was also observed in other systems [16, 27, 28, 29]. However, the characteristic temperatures found in our samples are lower than those in other reports both for BaFeAs (= 140 K) [13, 29] and SrFeAs (= 195 K) [16]. It may be induced by doping a little bit of potassium into the parent compounds. Increasing hole-doping further will suppress the SDW transition, and superconductivity eventually emerges [30]. Therefore, the superconducting BaKFeAs samples were obtained by adding more K into the starting material. Figure 5(b) shows the temperature dependence of resistivity for the superconducting single crystals. The SDW anomaly is smeared in the normal state, and a superconducting transition emerges at lower temperatures. The (conset)s for different doping are about 36.6 K, 31.4 K, 28.7 K and 24.5 K. The resistivity data also indicates sharp transition in our samples with = 0.44, 0.49, 0.71 and 0.40 K ( 90% - 10% of normal state resistivity ). Furthermore, if we extrapolate the data just above the superconducting transition by a straight line, it could be roughly estimated that the residual resistivity is almost close to zero for the sample with = 36 K. This indicates that our samples are rather clean.

When preparing this paper, we became aware that two papers working on the BaKFeAs polycrystalline samples with series doping levels were posted on arXiv [30, 31]. Thus we made a comparison on the doping dependence of -axis and between the polycrystalline samples and our single crystals. The result is shown in Figure 6. Our data is very consistent with the data in Reference [31], while a little deviation is found for the data in Reference [30]. However, the general behaviors are almost the same between the polycrystalline samples and single crystals. The -axis expands almost linearly as increasing the content of K. While the increases quickly as a little K was doped into the parent compound, then it grows slowly between = 0.3 0.4. It seems that the maximal of is achieved between = 0.4 0.5. Therefore, our samples with = 0 0.4 reside in the ”underdoped” regime. Growth of the ”overdoped” single crystals is underway.

4 Summary

In summary, we have successfully grown the single crystals of AKFeAs (A=Ba, Sr) with high quality by using FeAs as the self-flux. By varying the K content during the growth, we obtained non-superconducting Ba(Sr)FeAs single crystals and superconducting BaKFeAs single crystals with = 0.23, 0.25, 0.28 and 0.40. The samples have sizes up to 4 mm with flat and shiny cleaved surfaces. The X-ray diffraction patterns with only (00) peaks suggest that they have high crystalline quality. The superconductivity is characterized by AC susceptibility and resistivity measurements which exhibit very sharp superconducting transitions. While the temperature dependence of resistivity on the non-superconducting crystals shows that the SDW instability and structure transition occur at about 173 K and 135 K for Sr-based and Ba-based compound, respectively. The doping dependence of -axis parameters and are consistent with the previous data from polycrystalline samples [30, 31], which indicates the effects of different potassium doping.

Acknowledgement

This work was financially supported by the Natural Science Foundation of China, the Ministry of Science and Technology of China (973 Projects Nos. 2006CB601000, 2006CB921802 and 2006CB921300), and Chinese Academy ofSciences (Project ITSNEM). The authors acknowledge the helps from Lihong Yang and Hong Chen for the XRD measurements, and the helpful discussions with Lei Fang, Lei Shan and Cong Ren at IOP, CAS.

References

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  • 00 no. 0Ba0:0K (Å) (K) typical sizes (mm)
    0-1 0.600:00.40 13.344 36.3
    0-2 0.720:00.28 13.226 31.8
    0-3 0.750:00.25 13.196 27.5
    0-4 0.770:00.23 13.189 23.4
    0-5 0.920:00.08 13.077 00
    0-6 0.940:00.06 13.065 00
Table 1: A brief summary of the properties of BaKFeAs single crystals: The actual cationic compositions (Ba:K) determined by EDX, -axis parameters, critical temperature, and typical sizes of crystals.

Figure 1: Photograph of the AKFeAs (A = Ba,Sr) crystals cleaved from the as-grown bulk. The crystals have rather shiny surfaces with sizes up to 4 mm.

Figure 2: A typical EDX spectrum for one single crystal. The inset is the SEM photograph of this crystal, which shows a very flat surface morphology and the layered structure.

Figure 3: Typical XRD patterns for cleaved crystals. The FWHM of each peak is around 0.10. A clear shift was observed after doping 40% amount of K in the BaKFeAs system.

Figure 4: Temperature dependence of AC susceptibility for superconducting BaKFeAs single crystals. The was derived by the point where the real part of the susceptibility becomes flat, and the transition width was defined as =(90%) - (10%).

Figure 5: (a).Temperature dependence of resistivity for BaFeAs and SrFeAs crystals. The SDW anomaly happens at 135 K and 173 K, respectively. (b).Temperature dependence of resistivity for the superconducting single crystals. The inset is the zooming in graph around the superconducting transition.

Figure 6: Doping dependence of the -axis and for our single crystals (blue points). The open black squares and green triangles are polycrystalline data from ref. 30 and ref. 31, respectively.
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