Transport and anisotropy in single-crystalline SrFe{}_{2}As{}_{2} and A_{0.6}K{}_{0.4}Fe{}_{2}As{}_{2} (A = Sr, Ba) superconductors

Transport and anisotropy in single-crystalline SrFeAs and KFeAs ( = Sr, Ba) superconductors

G. F. Chen    Z. Li    J. Dong    G. Li    W. Z. Hu    X. D. Zhang    X. H. Song    P. Zheng    N. L. Wang    J. L. Luo Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
Abstract

We have successfully grown high quality single crystals of SrFeAs and AKFeAs(A=Sr, Ba) using flux method. The resistivity, specific heat and Hall coefficient have been measured. For parent compound SrFeAs, an anisotropic resistivity with / as large as 130 is obtained at low temperatures. A sharp drop in both in-plane and out-plane resistivity due to the SDW instability is observed below 200 K. The angular dependence of in-plane magnetoresistance shows 2-fold symmetry with field rotating within ab plane below SDW transition temperature. This is consistent with a stripe-type spin ordering in SDW state. In K doped AKFeAs(A=Sr. Ba), the SDW instability is suppressed and the superconductivity appears with T above 35 K. The rather low anisotropy in upper critical field between Hab and Hc indicates inter-plane coupling play an important role in hole doped Fe-based superconductors.

pacs:
74.70.-b, 74.62.Bf, 74.25.Gz

The recent discovery of superconductivity with transition temperature T26 K in LaFeAsOF has generated tremendous interest in scientific communityKamihara08 (). Shortly after this discovery, the T was raised to 41-55 K by replacing La by rare-earth Ce, Sm, Pr, Nd, etc, making those systems with T exceeding 50 K.Chen1 (); XHChen (); Ren2 (); CWang () The undoped quaternary compounds crystallize in a tetragonal ZrCusiAs-type structure, which consists of alternate stacking of edge-sharing FeAs tetrahedral layers and LaO tetrahedral layers along c-axis. Very recently, superconductivity with T up to 38 K was discovered in AFeAs(A=Ba, Sr, Ca) upon K or Na-doping.Rotter2 (); Chen (); Wu (); Sasmal (); Wu2 () AFeAs compounds crystallize in a tetragonal ThCrSi-type structure with identical FeAs tetrahedral layers as in LaFeAsO, but separated by single elemental A-layers. These compounds contain no oxygen in A layers. The simpler structure of AFeAs system makes it more suitable for research of intrinsic physical properties of Fe-based compounds.

Except for a relatively high transition temperature, the system displays many interesting properties. The existence of a spin-density-wave (SDW) instability in parent LaFeAsODong () was indicated by specific heat, optical measurements and first principle calculations, and subsequently confirmed by neutron scattering,Cruz () NMR,Ishida () sR,Klauss () and MössbaureKitao () spectroscopic measurements. The superconductivity only appears when SDW instability was suppressed by doping carriers or applying pressure. The competition between superconductivity and SDW instability was identified in other rare-earth substituted systems Chen1 (); Ding (); Chen2 (). Besides the SDW instability, a structural distortion from tetragonal to monoclinic were also observed for both ReFeAsO(Re=rare earth) and AFeAs (A=Ba, Sr, Ca)Rotter1 (); Bao (); Ni (); Ni3 (); Ni2 (); Tegel (). The structural transition temperature were found to occur at slightly higher than SDW transition temperature in LaFeAsOCruz (), but the two transitions occur simultaneously in AFeAs(A=Ba, Sr, Ca).Rotter1 (); Bao (); Zhao () The band structure calculation and neutron scattering experiments indicated a stripe-type antiforremagnetic structure of Fe moments in SDW state in LaFeAsO.Dong (); Cruz () In such a spin structure, a 4-fold spin rotation symmetry is broken and reduced to 2-fold. It is interesting to see if such a 2-fold symmetry could be observed in angular-dependent magneto-resistance measurements.

In this work, we present resistivity, specific heat and Hall coefficient measurements on single crystals SrFeAs and AKFeAs (A=Sr, Ba). A large anisotropic resistivity with / 130 is observed for SrFeAs. Below 200K, a sharp drop in both in-plane and out-plane resistivity due to the SDW instability is observed, similar to previously results on polycrystal samplesChen (). Moreover, the angular-dependence of in-plane and out-plane magnetoresistance shows 2-fold symmetry with field rotating within ab plane below SDW transition temperature. The breaking of 4-fold rotation symmetry to 2-fold provides transport evidence for the formation of a stripe-type antiforremagnetic structure of Fe moments in SDW state. Moreover, the anisotropy of upper critical field for high quality single crystal SrKFeAs is investigated. The inter-layer coupling is relatively strong in superconducting samples.

The parent single crystals SrFeAs were prepared by the high temperature solution method using Sn as flux, similar to the procedure described in refNi3 (). The superconducting crystals AKFeAs (A=Sr, Ba) were prepared by the high temperature solution method using FeAs as flux. The starting materials of Sr or Ba, K, and FeAs in a ratio of 0.5:1:4 were put into an alumina crucible and sealed in welded Ta crucible under 1.6 atmosphere of argon. The Ta crucible were then sealed in an evacuated quartz ampoule and heated at 1150 for 5 hours and cooled slowly to 800 over 50 hours. The plate-like crystals with sizes up to 10mm5m0.5mm could be obtained after breaking the alumina crucible. Both scanning electron microscopy/energy dispersive x-ray (SEM/EDX) and induction-coupled plasma (ICP) analysis revealed that the elemental composition of the crystal is AKFeAs (A=Sr, Ba) with . Figure 1(a) shows X-ray diffraction pattern of parent SrFeAs with the 00 reflections. The lattice constant c = 0.1239 nm was calculated from the higher order peaks, comparable to that of polycrystalline sampleChen (). Figure 1(b) shows the EDX analysis of BaKFeAs crystal.

The in-plane resistivity was measured by the standard 4-probe method. The out-plane resistivity was measured using a typical method for layered materials as shown in inset of Fig. 2(b). The silver paste was used to cover almost all area of upper and lower side of the measured sample for two current leads, and leaves two small holes in the center of both sides for the voltage leads. Assuming that the current density is uniformly distributed throughout the cross section, the resistivity can then be measured with: = R S /d, where R is the resistance, S is area of cross section and d is the thickness of the sample. The error comes mostly from the measurement of geometry factors. The ac magnetic susceptibility was measured with a modulation field in the amplitude of 10 Oe and a frequency of 333 Hz. The Hall coefficient measurement was done using a five-probe technique. The specific heat measurement was carried out using a thermal relaxation calorimeter. All these measurements were preformed down to 1.8K in a Physical Property Measurement System(PPMS) of Quantum Design company.

Figure 1: (Color online) (a) Single crystal X-ray diffraction pattern for SrFeAs. The inset shows the photograph of a SrFeAs single crystal (length scale 1 mm). (b) EDX analysis of BaKFeAs crystal.

Figure 2(a) shows the temperature dependence of in-plane resistivity in zero field and 8 T, and Fig. 2(b) shows the out-plane resistivity for SrFeAs in zero field. They have similar T-dependent behavior and exhibit a strong anomaly at about 200 K: the resistivity drops steeply below this temperature. This is a characteristic feature related to SDW instability and a structural distortion in parent compounds of Fe-based high T superconductors.Dong (); Cruz () The transition temperature is slightly lower than that observed in poly-crystalline SrFeAs ( 205 K). The anisotropic resistivity / is found to be 13065 at 25 K. It is comparable with that of layered cuprates YBaCuO.Friedmann () From Fig.2(a), we find that the SDW transition temperature is insensitive to the applied field, however a large positive magnetoresistance is observed at low temperatures. At 10 K, the magnetoresistance [-]/ reaches as high as 25. The behavior is similar to that observed in polycrystal LaFeAsO.Dong (). The large positive magnetoresistance can be understood from the suppression of SDW order by applied field and thus the spin scattering is enhanced.

Figure 2: (Color online) (a) The in-plane resistivity for SrFeAs in zero field and 8 T, respectively. (b) The out-plane resistivity in zero field. The inset shows a sketch for how to measure .

In a system with strong coupling of charge carrier and background magnetism, angular-dependent-magnetoresistance(AMR) is a useful tool to detect the magnetic structure of background magnetism. AMR measurements have been successfully used in understanding the spin structure of slightly doped high T cuprates and charge ordering NaCoO systems.XHChen3 (); Hu () For this purpose, we measured the in-plane resistivity with magnetic field of 10 T rotating within the ab plane for SrFeAs. Figure 3 shows AMR () at 10, 30, 50, 100, 150 and 200 K, respectively. shows a clear 2-fold oscillation below 200K. However, the 2-fold oscillation disappears at 200 K, a temperature just above T. Above T, the spin orientations of Fe moments are random so there is no angular dependent magnetoresistance arising from spin scattering. However below T, a large MR is observed at low temperature and it originates from the enhanced spin scattering while SDW order is suppressed in external magnetic field. Therefore the symmetry of the AMR oscillation reflect directly the symmetry of spin structure. The observation of 2-fold symmetry oscillations in SrFeAs in low temperatures indicates the stripe-type spin structure of Fe moments in ground state. The recent neutron diffraction measurements on SrFeAs single crystals Zhao () and polycrystalline Kaneko () indicate that the SDW transition is indeed presented in SrFeAs system below a transition temperature of 200-220 K. The stripe-type antiferromagnetic spin structure is found in the SDW state. Moreover, the angular-dependent magnetization measurement on BaFeAs crystal with field rotating within the ab plane also show a 2-fold symmetry of magnetization below SDW transition temperatureWang (), consistent with the AMR measurements on SrFeAs crystals. Therefore AMR can act as a probe for SDW order in these systems without having to wait for neutron beam time. Similar 2-fold symmetry oscillations of AMR has been observed in NaCoO which was attributed to ordering of charge stripes.Hu ()

Figure 3: (Color online) Angular dependence of in-plane magnetoresistance() at different temperatures in external magnetic field of 10T rotating within ab plane for SrFeAs

To get more information about the SDW and structural phase transition, we preformed specific heat and Hall resistivity measurement for SrFeAs. Figure 4(a) shows the temperature dependence of specific heat C from 2 to 230 K. We can see clearly a sharp -function shape peak at about 200 K with C 185 J/molK. This is a characteristic feature of a first-order phase transition. The transition temperature agrees well with that observed in resistivity measurement. The latent heat of the transition is estimated being 46350 J/mol by integral the area of the specific heat data around the transition peak after subtracting the background. The background is estimated using a linear fit of the specific heat data above and below the transition. It worth noting that only one peak at around 200 K is observed in SrFeAs, different from that of LaFeAsO where two subsequent peaks at 155 K and 143 K were observed in specific heat data corresponding to structural and SDW transition, respectively.Jin () This suggests the structural transition and SDW transition occur at same temperature in SrFeAs, similar to that observed in BaFeAs.Rotter1 (); Bao () At low temperatures, a good linear T dependence of C/T is observed indicates that the specific heat C is mainly contributed by electrons and phonons [see inset Fig. 4(a)]. The fit yields the electronic coefficient =6.5 mJ/mol.K and the Debye temperature = 245 K. Note that the electronic coefficient is significantly smaller than the values obtained from the band structure calculations for non-magnetic state.Xiang () This can be explained that a partial energy gap is opened below SDW transition, the smaller experimental value here could be naturally accounted for by the gap formation which removes parts of the density of states below the phase transition. Compared with band calculation, similar smaller electronic specific heat coefficient was also observed in LaOFeAs due to gap opening originated from SDW instability.Dong () In comparison, we also measured specific heat of superconducting crystal SrKFeAs prepared using FeAs as flux. It is found that no structural and SDW transition can be observed in specific heat data. Instead, a specific heat anomaly at a superconducting transition temperature with T35.6 K is observed. The specific heat jump C/T is found to be 48 mJ/mol.K.

Figure 4: (Color online) (a) Temperature dependence of specific heat C for SrFeAs. Inset: T dependence of C/T in low temperatures. (b)Temperature dependence of specific heat C for SrKFeAs. Inset: C/T around superconducting transition.

Hall coefficient R as a function of temperature between 20 and 300 K for SrFeAs is shown in Fig. 5. For comparison, R(T) for LaFeAsO is also shown in inset of Fig. 5. Above 200 K, the Hall coefficient is negative and nearly temperature independent for SrFeAs, indicating conduction carriers are dominated by electrons. The carrier density is estimated being n = 1.510cm at 300 K if one-band model is simply adopted. It is nearly an order higher than that of LaFeAsO with n 1.810cm at 300 K obtained by same method. Optical measurement also indicates a quite large plasma frequency, 1.5 eV.WZHu () The large carrier number for SrFeAs indicate that it is a good metal. Below 200 K, the Hall coefficient increases slightly to a positive value, and then drops dramatically to a very large negative value. The absolute value of R at 2 K is about 35 times larger than that at 300 K. The dramatic change reflects the reconstruction of Fermi surface after SDW transition. The band calculation show that there are 3 hole pockets around point and 2 electron pockets around M pointDong (). The experiments seem to indicate that, upon cooling below the SDW transition temperature, hole pockets are almost fully gapped while the electron pockets are partially gapped. Therefore, at low temperatures, the R reflect mainly the the un-gapped electron density around M point, which is significantly small in comparison with its initial value above 200K.

Figure 5: (Color online) Temperature dependence of Hall coefficient for SrFeAs. For comparison, the temperature dependence of Hall coefficient of LaFeAsO is shown in the inset.

In addition to parent SrFeAs, the anisotropy of resistivity and upper critical field for single crystals AKFeAs (A=Sr, Ba) is also investigated. Figure 6(a) shows the temperature dependence of the in-plane resistivity in zero field for SrKFeAs and BaKFeAs crystals. decreases with decreasing temperature and shows a downward curvature for SrKFeAs, consistent with the polycrystal sample. With further decreasing temperature, an extremely sharp superconducting transition at 35.5 K with transition width about 0.3 K is observed, indicating high homogeneity of the sample. A sharp superconducting transition for BaKFeAs is also observed with T=38 K, slightly higher than that of SrKFeAs. The anisotropic resistivity / in normal state for SrKFeAs is found to be 21, much lower than that of parent SrFeAs.

Figure 6: (Color online) (a) Temperature dependence of the in-plane electrical resistivity for SrKFeAs and BaKFeAs in zero field, respectively. (b) and (c) Temperature dependence of the in-plane electrical resistivity for SrKFeAs at fixed fields up to 14T for H c and H ab, respectively. (d)H(T) plot for H c(closed square) and H ab(open circle) , respectively.

Figure 6(b) and figure 6(c) show (T) for SrKFeAs in external magnetic fields up to 14 T along c-axis and within ab plane, respectively. We can see the superconducting transition is broadened slightly in magnetic fields up to 14 T. The behavior is different from polycrystalline LaFeAsO where the superconducting transition is broadened strongly in magnetic fields.Chen0 () Figure 6(d) shows H-T curves for both Hab and Hc, respectively, where T is adopted by a criterion of 90 of normal state resistivity. The curves H(T) are very steep with slopes -dH /dT =7.57 T/K for Hab and -dH/dT=3.80 T/K for Hc. This indicates the upper critical fields is extremely high for SrKFeAs. Using the Werthamer-Helfand-Hohenberg formulaWHH () H(0)=-0.69(dH/dt)T and taking T=35.5K, the upper critical fields are: H=185.4 T and H=93.1 T, respectively. The anisotropy ratio =H/H2.0.Ni () The value of is close to that of BaKFeAs with between 2.5 to 3.5. It is lower than that of F-doped NdFeAsOWen () with 4.3-4.9, and much lower than high T cuprates, for example 7-10 for YBCO.Nanda () The lower values of / and indicate that the inter-plane coupling in SrKFeAs is relative strong.

To summarize, SrFeAs and AKFeAs(A=Sr, Ba) single crystals were prepared by flux method. The angular dependence of in-plane resistivity was measured for SrFeAs. A 2-fold symmetry of oscillations in AMR is observed at low temperatures which possible indicate a stripe-type spin structure below SDW temperature. In K doped AKFeAs(A=Sr. Ba), the SDW instability is suppressed and instead superconductivity appears with T above 35 K. The upper critical field is rather high with H=185.4 T, while its anisotropy is rather low. The inter-plane coupling may play an important role in this material.

Acknowledgements.
We acknowledge Z. D. Wang, T. Xiang and Z. Fang for helpful discussions, and Q. M. Meng in experiments. This work is supported by the National Science Foundation of China, the Knowledge Innovation Project of the Chinese Academy of Sciences, and the 973 project of the Ministry of Science and Technology of China.

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