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Two gap superconductivity in Ba0.55K0.45Fe2As2 single crystals studied by the directional point-contact Andreev reflection spectroscopy
ARTICLE IN PRESS Physica B 404 (2009) 3220–3222
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Two gap superconductivity in Ba0:55 K0:45 Fe2 As2 single crystals studied by the directional point-contact Andreev reflection spectroscopy P. Szabo´ a, Z. Pribulova a, G. Prista sˇ b, S.L. Bud’ko b, P.C. Canfield b, P. Samuely a, a b
ˇ a rik University, Watsonova 47, SK-04001 Koˇsice, Slovakia Centre of Low Temperature Physics, IEP Slovak Academy of Sciences & P.J.Saf Ames Laboratory and Iowa State University, Ames, IA 50011, USA
a r t i c l e in f o
PACS: 74.50.þr 74.7o.Dd
a b s t r a c t First directional point-contact Andreev reflection spectroscopy on the Ba0:55 K0:45 Fe2 As2 single crystals is presented. The spectra show significant differences when measured in the ab plane in comparison with those measured in the c direction. In the latter case no traces of superconducting energy gap could be found, just a reduced point-contact conductance persisting up to about 100 K and indicating reduced density of states. On the other hand within the ab plane two nodeless superconducting energy gaps DS 2–5 meV and DL 9–11 meV are detected. & 2009 Elsevier B.V. All rights reserved.
1. Introduction Extremely interesting physics has come with the discovery of high-Tc superconductivity in the iron pnictides [1]. REFeAsO(F) systems with various rare earth (RE) elements bring a new class of layered high-Tc materials with numerous similarities with high-Tc cuprates, from antiferromagnetism in parent compounds (albeit metallic), through electron and hole doping as a route leading to superconductivity, to a possible unconventional pairing mechanism. Beside REFeAsO(F), another three groups within those superconductors have been identified. Namely, AFe2 As2 with A ¼ Ba, Sr and Ca made superconducting by chemical substitution [2] or chemical pressure [3], Lix FeAs [4] and a-FeSe [5] are intensively studied. One of the most fundamental issues for unveiling the superconducting mechanism of these multiband systems concerns the symmetry of the superconducting order parameter(s). There are already lot of theoretical predictions on this topic but also a body of experimental studies is emerging. Band structure calculations have shown disconnected sheets of Fermi surface with possibly different superconducting energy gaps. A minimal model has to include two bands: the hole band around the G point and the electron one around the M point [6]. In contrast to the multiband but conventional s-wave scenario in MgB2 [7], here, the extended s-wave pairing with a sign reversal of the order parameter between different Fermi surface sheets has been proposed by Mazin et al. [8]. Experimental results are providing controversial conclusions. Some of them suggest the gap with nodes, as the Hc1
Corresponding author.
E-mail address: [email protected] (P. Samuely). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.07.108
magnetization measurements on LaFeAsO(F) [9] showing a clear linear temperature behavior at low temperatures. Matano et al. [10] in their NMR studies on PrFeAsO(F) found two superconducting energy gaps but in contrast to the case of MgB2 here with nodes. On the other hand the ARPES measurements [11] has found two nodeless and nearly isotropic gaps in Ba0:6 K0:4 Fe2 As2 and the penetration depth studies have provided the nodeless superconducting energy gap in NdFeAsO0:9 F0:1 with remarkably small coupling 2D=kB Tc 2 [12]. The point-contact Andreev reflection (PCAR) spectroscopy has been very powerful technique in investigating the superconducting order parameter even in case of multiple gaps like in MgB2 [7]. The PCAR spectroscopy data known on iron pnictides so far have also brought conflicting results. Wang et al. [13] found a nodal superconductivity with multiple gaps in SmFeAsO(F). Chen et al. [14] have presented a surprisingly conventional superconducting energy gap with a medium coupling equal to 2D=kB Tc 3:7. The recent data of Yates et al. [15] obtained on the 45 K NdFeAsO0:85 show also an indication of the superconducting energy gap with 2D=kB Tc ¼ 3:6. In our previous PCAR studies on the NdFeAsO(F) polycrystals indications for two superconducting energy gaps have been found [16]. In the following we present the first PCAR directional studies on iron pnictides, namely on the Ba0:55 K0:45 Fe2 As2 single crystals. The spectra have shown significant differences when measured in the ab plane and in the c direction. In the latter case no traces of superconducting energy gap could be found but a reduced pointcontact conductance persisting up to about 100 K. On the other hand within the ab plane two nodeless superconducting energy gaps DS 2–5 meV and DL 9–11 meV are detected. There also spectra with reduced conductance background together with broadened Andreev-type enhancement of the conductance could
ARTICLE IN PRESS P. Szab´ o et al. / Physica B 404 (2009) 3220–3222
3221
Fig. 1 shows the PCAR spectra on Ba0:55 K0:45 Fe2 As2 single crystal measured with the point-contact current preferably within the ab plane of the single crystal. The inset of the figure presents the raw data taken at 4.5 and 27 K. The spectral background obviously reveals asymmetry, being higher at positive bias voltage, i.e. when the electrons are flowing from the superconductor into the tip. This feature revealed also above Tc , is
opposite to the one revealed with many previous PCAR measurements on iron pnictides, as those of Chen et al. [14], our previous measurements on the polycrystalline NdFeAsO(F) [16], etc. Spectra measured in the superconducting state are normalized to the normal state spectrum (here at 27 K). The main figure then represents the resulting normalized data. The remarkable feature at the lowest temperature is a double enhanced point-contact conductance. The first enhancement of the point-contact conductance starts at about 720 mV while the second, superimposed on the first one, is symmetrically located between 76 mV. The form of the spectrum obviously reminds the two gap spectrum of MgB2 for a highly transparent junction. There the both conductance enhancements are due to the Andreev reflection of quasiparticles on the small and large superconducting energy gaps. Indeed the spectrum from Fig. 1 taken at the lowest temperature could be easily fit by the two gap BTK formula resulting to the two superconducting energy gaps: the small DS 3 meV and the large one DL 9 meV. With the increasing temperature both gap structures are smoothly smeared out and disappear between 23 and 25 K. The details of the fitting procedure showing a common transition temperature of the both gaps and more statistics of the similar data are given elsewhere [18]. From the data obtained on many junctions we observe that the small gap is scattered between 2 and 5 meV while the large one between 9 and 11 meV. Completely different picture yields the measurements with the point-contact current perpendicular to the platelette of the single crystals, i.e. along the c axis crystallographic direction. In a huge statistics we have collected, very reproducible spectra are found without any enhanced point-contact conductance and coherence peaks due to the Andreev reflection on the superconducting energy gap. On the contrary the spectra display always a reduced conductance around the zero bias. We remark that this kind of a ‘‘tunneling-like’’ conductance spectra could not be modified to more Andreev reflection-like ones, even if the junction resistance was decreased significantly by applying much higher pressure on the normal tip. However this may originate simply from surface reconstruction or contamination. Moreover as could be seen from measurements at higher temperature and from ZBC vs. temperature (inset of Fig. 2) the effect of the reduced conductance persists well above the transition temperature Tc 27 K. In the forthcoming paper also the spectra measured at the ab plane showing beside the two Andreev-like enhanced conductances also the reduced conductance around zero bias at higher temperatures
Fig. 1. Normalized PCAR spectra of the junction with the current mostly within the ab plane of the Ba0:55 K0:45 Fe2 As2 single crystal showing two superconducting gaps. Inset—the raw data taken at 4.5 and 27 K.
Fig. 2. Spectrum showing a reduced conductance even above Tc . Inset—Zero-bias conductance as a function of temperature.
be found. Some of the ab plane spectra reveal also the zero-bias peak which could be a finger print of the order parameter sign reversal between the different Fermi sheets.
2. Experiment The Ba0:55 K0:45 Fe2 As2 single crystals were grown out of a Sn flux as described in details in Ref. [17]. To avoid losses of potassium several precautions were made including an excess of potassium content in an initial stoichiometry. Typical dimensions of the resulting crystals are 1–2 1–2 0:05–0:1 mm3 . The crystallographic c axis is perpendicular to the plane of the platelike crystals. Before the point-contact (PC) experiment a fresh surface of the crystal was made via cleavage by a scotch tape. The point-contact measurements have been realized via the standard lock-in technique in a special point-contact approaching system with lateral and vertical movements of the PC tip by a differential screw mechanism. The microconstrictions were prepared in situ by pressing different metallic tips (copper and platinum, formed either mechanically or by electrochemical etching) on different parts of the fresh surface of the superconductor. For the measurements with the point-contact current in the ab plane we used sometimes the single crystal as a tip pressed on the massive, chemically etched copper. In order to focus on the data in the proper spectroscopic regime, the precautions were made to avoid the junctions with heating effects. In the following, only the junctions without the conductance dips and irreversibilities in voltage dependences are presented. The local superconducting transition temperature Tc was determined from the temperature dependence of the pointcontact spectra. The transition temperatures found in this way varied between 23 and 27 K.
3. Results and discussion
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P. Szab´ o et al. / Physica B 404 (2009) 3220–3222
probably reveal the sign reversal of the order parameter which would explain occasional observations of the ZBP effect in the PCAR spectra [13–16] below Tc . Very recently Gonnelli et al. [22] have also obtained pronounced two-gap spectra on the LaFeAsO1x Fx polycrystals as well as they have shown an existence of the reduced DOS in the normal state up to about 140 K, close the Neel temperature of the undoped system. Their size of the large and small gaps are remarkably similar to those found in the present work.
4. Conclusions
Fig. 3. PCAR spectra of the point-contact junction with the current mostly within the ab plane of Ba0:45 K0:45 Fe2 As2 single crystal showing single gap and the ZBP effect.
will be shown. Thus we can conclude that this effect is a spectral feature related to a reduced DOS in the normal state. In some cases of the ab plane spectra only a single gap could be observed. Fig. 3 brings one of these. The spectrum at 4.5 K reveals the gap-like peaks at 72 mV on the overall enhanced pointcontact conductance due to the Andreev reflection. The spectrum shows a very peculiar development with increased temperature. While the position of the gap-like peaks is rapidly shrinking with increasing temperature, surprisingly the intensity of the conductance spectrum near the zero-bias voltage increases. At 10 K the gap-like peaks are completely gone and the only visible feature of the spectrum is the zero-bias peak (ZBP) which at even higher temperatures looses its intensity and vanishes at the local Tc , here between 20 and 27 K. This effect was very regularly observed in our previous PCAR measurements on the NdFeAsO(F) polycrystalline samples [16]. There the effect was ascribed to a possibility of summing over several channels within one pointcontact junction. Recently Choi and Bang [19] have elaborated a model of ‘‘sp’’ pairing (s-wave order parameter with sign reversal) for iron pnictides in analogy with the behavior of the superconducting/ ferromagnet (S/F) bilayers where the order parameter sign change can also happen. By their calculations the sp state can produce the zero-bias peak in the local density of states at the superconducting/normal interface. This can be detected in the tunneling or PCAR spectroscopy measured on the normal metal/superconductor junctions and explain the ZBP observed in Fig. 3. Mazin and Johannes [20] proposed recently fluctuating magnetic domains as important clue for explanation of superconducting and normalstate properties in iron pnictides. Goko et al. [21] have even brought evidence for coexistence of superconductivity and strong static magnetic order in a partial volume fraction of ðBa; KÞFe2 As2 and similar systems. They have shown a strong magnetism existing below about 70 K in ðBa; KÞFe2 As2 . Then, the point-contact junctions made on or nearby such ‘‘magnetic’’ spots could
The first directional PCAR studies have been performed on the single crystalline Ba0:55 K0:45 Fe2 As2 . Significant differences in the spectra are observed when measured within the ab planes and the c direction. Within the planes two superconducting gaps with DS 2–5 meV and DL 9–11 meV are detected. For the PCAR measurements perpendicular to the planes no traces of superconducting energy gap(s) could be found, just a conductance reduced around the zero bias is observed. This effect persisting up to about 100 K and indicating reduced DOS even in the normal state, is possibly connected with the magnetism existing in a substantial volume fraction even in the superconducting state.
Acknowledgments This work has been supported by the Slovak Research and Development Agency under the Contract nos. VVCE-0058-07, APVV-0346-07 and LPP-0101-06 and by the EC Framework Programme MTKD-CT-2005-030002. Centre of Low Temperature Physics is operated as the Centre of Excellence of the Slovak Academy of Sciences. The liquid nitrogen for the experiment has been sponsored by the U.S. Steel Koˇsice, s.r.o. Work at the Ames Laboratory was supported by the Department of Energy—Basic Energy Sciences under Contract no. DE-AC02-07CH11358. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
Y. Kamihara, et al., J. Am. Chem. Soc. 130 (2008) 3296. M. Rotter, et al., Phys. Rev. Lett. 101 (2008) 107006. M.S. Torikachvili, et al., Phys. Rev. Lett. 101 (2008) 057006. X.C. Wang, et al., Solid State Commun. 148 (2008) 538. F.C. Hsu, et al., Proc. Natl. Acad. Sci. USA 105 (2008) 14262. S. Raghu, et al., Phys. Rev. B 77 (2008) 220503(R). ´ et al., Phys. Rev. Lett. 87 (2001) 137005. P. Szabo, I.I. Mazin, et al., Phys. Rev. Lett. 101 (2008) 057003. C. Ren, et al., Chinese Phys. Lett. 25 (2008) 2215. K. Matano, et al., Europhys. Lett. 83 (2008) 57001. H. Ding, et al., Europhys. Lett. 83 (2008) 47001. C. Martin, et al., arXiv:0807.0876, 2008. Y.-L. Wang, et al., Supercond. Sci. Technol. 22 (2009) 015018. T.Y. Chen, et al., Nature 453 (2008) 1224. K.A. Yates, et al., Supercond. Sci. Technol. 21 (2008) 092003. P. Samuely, et al., Supercond. Sci. Technol. 22 (2009) 014003. N. Ni, et al., Phys. Rev. B 78 (2008) 014507. P. Szabo, et al., Phys. Rev. B 79 (2009) 012503. H.-Y. Choi, Y. Bang, arXiv:0807.4604, 2008. I.I. Mazin, et al., arXiv:0807.3737, 2008. T. Goko, et al., arXiv:0808.1425, 2008. R. Gonnelli, et al., Phys. Rev. B 79 (2009) 184526.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Two gap superconductivity in Ba0:55 K0:45 Fe2 As2 single crystals studied by the directional point-contact Andreev reflection spectroscopy P. Szabo´ a, Z. Pribulova a, G. Prista sˇ b, S.L. Bud’ko b, P.C. Canfield b, P. Samuely a, a b
ˇ a rik University, Watsonova 47, SK-04001 Koˇsice, Slovakia Centre of Low Temperature Physics, IEP Slovak Academy of Sciences & P.J.Saf Ames Laboratory and Iowa State University, Ames, IA 50011, USA
a r t i c l e in f o
PACS: 74.50.þr 74.7o.Dd
a b s t r a c t First directional point-contact Andreev reflection spectroscopy on the Ba0:55 K0:45 Fe2 As2 single crystals is presented. The spectra show significant differences when measured in the ab plane in comparison with those measured in the c direction. In the latter case no traces of superconducting energy gap could be found, just a reduced point-contact conductance persisting up to about 100 K and indicating reduced density of states. On the other hand within the ab plane two nodeless superconducting energy gaps DS 2–5 meV and DL 9–11 meV are detected. & 2009 Elsevier B.V. All rights reserved.
1. Introduction Extremely interesting physics has come with the discovery of high-Tc superconductivity in the iron pnictides [1]. REFeAsO(F) systems with various rare earth (RE) elements bring a new class of layered high-Tc materials with numerous similarities with high-Tc cuprates, from antiferromagnetism in parent compounds (albeit metallic), through electron and hole doping as a route leading to superconductivity, to a possible unconventional pairing mechanism. Beside REFeAsO(F), another three groups within those superconductors have been identified. Namely, AFe2 As2 with A ¼ Ba, Sr and Ca made superconducting by chemical substitution [2] or chemical pressure [3], Lix FeAs [4] and a-FeSe [5] are intensively studied. One of the most fundamental issues for unveiling the superconducting mechanism of these multiband systems concerns the symmetry of the superconducting order parameter(s). There are already lot of theoretical predictions on this topic but also a body of experimental studies is emerging. Band structure calculations have shown disconnected sheets of Fermi surface with possibly different superconducting energy gaps. A minimal model has to include two bands: the hole band around the G point and the electron one around the M point [6]. In contrast to the multiband but conventional s-wave scenario in MgB2 [7], here, the extended s-wave pairing with a sign reversal of the order parameter between different Fermi surface sheets has been proposed by Mazin et al. [8]. Experimental results are providing controversial conclusions. Some of them suggest the gap with nodes, as the Hc1
Corresponding author.
E-mail address: [email protected] (P. Samuely). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.07.108
magnetization measurements on LaFeAsO(F) [9] showing a clear linear temperature behavior at low temperatures. Matano et al. [10] in their NMR studies on PrFeAsO(F) found two superconducting energy gaps but in contrast to the case of MgB2 here with nodes. On the other hand the ARPES measurements [11] has found two nodeless and nearly isotropic gaps in Ba0:6 K0:4 Fe2 As2 and the penetration depth studies have provided the nodeless superconducting energy gap in NdFeAsO0:9 F0:1 with remarkably small coupling 2D=kB Tc 2 [12]. The point-contact Andreev reflection (PCAR) spectroscopy has been very powerful technique in investigating the superconducting order parameter even in case of multiple gaps like in MgB2 [7]. The PCAR spectroscopy data known on iron pnictides so far have also brought conflicting results. Wang et al. [13] found a nodal superconductivity with multiple gaps in SmFeAsO(F). Chen et al. [14] have presented a surprisingly conventional superconducting energy gap with a medium coupling equal to 2D=kB Tc 3:7. The recent data of Yates et al. [15] obtained on the 45 K NdFeAsO0:85 show also an indication of the superconducting energy gap with 2D=kB Tc ¼ 3:6. In our previous PCAR studies on the NdFeAsO(F) polycrystals indications for two superconducting energy gaps have been found [16]. In the following we present the first PCAR directional studies on iron pnictides, namely on the Ba0:55 K0:45 Fe2 As2 single crystals. The spectra have shown significant differences when measured in the ab plane and in the c direction. In the latter case no traces of superconducting energy gap could be found but a reduced pointcontact conductance persisting up to about 100 K. On the other hand within the ab plane two nodeless superconducting energy gaps DS 2–5 meV and DL 9–11 meV are detected. There also spectra with reduced conductance background together with broadened Andreev-type enhancement of the conductance could
ARTICLE IN PRESS P. Szab´ o et al. / Physica B 404 (2009) 3220–3222
3221
Fig. 1 shows the PCAR spectra on Ba0:55 K0:45 Fe2 As2 single crystal measured with the point-contact current preferably within the ab plane of the single crystal. The inset of the figure presents the raw data taken at 4.5 and 27 K. The spectral background obviously reveals asymmetry, being higher at positive bias voltage, i.e. when the electrons are flowing from the superconductor into the tip. This feature revealed also above Tc , is
opposite to the one revealed with many previous PCAR measurements on iron pnictides, as those of Chen et al. [14], our previous measurements on the polycrystalline NdFeAsO(F) [16], etc. Spectra measured in the superconducting state are normalized to the normal state spectrum (here at 27 K). The main figure then represents the resulting normalized data. The remarkable feature at the lowest temperature is a double enhanced point-contact conductance. The first enhancement of the point-contact conductance starts at about 720 mV while the second, superimposed on the first one, is symmetrically located between 76 mV. The form of the spectrum obviously reminds the two gap spectrum of MgB2 for a highly transparent junction. There the both conductance enhancements are due to the Andreev reflection of quasiparticles on the small and large superconducting energy gaps. Indeed the spectrum from Fig. 1 taken at the lowest temperature could be easily fit by the two gap BTK formula resulting to the two superconducting energy gaps: the small DS 3 meV and the large one DL 9 meV. With the increasing temperature both gap structures are smoothly smeared out and disappear between 23 and 25 K. The details of the fitting procedure showing a common transition temperature of the both gaps and more statistics of the similar data are given elsewhere [18]. From the data obtained on many junctions we observe that the small gap is scattered between 2 and 5 meV while the large one between 9 and 11 meV. Completely different picture yields the measurements with the point-contact current perpendicular to the platelette of the single crystals, i.e. along the c axis crystallographic direction. In a huge statistics we have collected, very reproducible spectra are found without any enhanced point-contact conductance and coherence peaks due to the Andreev reflection on the superconducting energy gap. On the contrary the spectra display always a reduced conductance around the zero bias. We remark that this kind of a ‘‘tunneling-like’’ conductance spectra could not be modified to more Andreev reflection-like ones, even if the junction resistance was decreased significantly by applying much higher pressure on the normal tip. However this may originate simply from surface reconstruction or contamination. Moreover as could be seen from measurements at higher temperature and from ZBC vs. temperature (inset of Fig. 2) the effect of the reduced conductance persists well above the transition temperature Tc 27 K. In the forthcoming paper also the spectra measured at the ab plane showing beside the two Andreev-like enhanced conductances also the reduced conductance around zero bias at higher temperatures
Fig. 1. Normalized PCAR spectra of the junction with the current mostly within the ab plane of the Ba0:55 K0:45 Fe2 As2 single crystal showing two superconducting gaps. Inset—the raw data taken at 4.5 and 27 K.
Fig. 2. Spectrum showing a reduced conductance even above Tc . Inset—Zero-bias conductance as a function of temperature.
be found. Some of the ab plane spectra reveal also the zero-bias peak which could be a finger print of the order parameter sign reversal between the different Fermi sheets.
2. Experiment The Ba0:55 K0:45 Fe2 As2 single crystals were grown out of a Sn flux as described in details in Ref. [17]. To avoid losses of potassium several precautions were made including an excess of potassium content in an initial stoichiometry. Typical dimensions of the resulting crystals are 1–2 1–2 0:05–0:1 mm3 . The crystallographic c axis is perpendicular to the plane of the platelike crystals. Before the point-contact (PC) experiment a fresh surface of the crystal was made via cleavage by a scotch tape. The point-contact measurements have been realized via the standard lock-in technique in a special point-contact approaching system with lateral and vertical movements of the PC tip by a differential screw mechanism. The microconstrictions were prepared in situ by pressing different metallic tips (copper and platinum, formed either mechanically or by electrochemical etching) on different parts of the fresh surface of the superconductor. For the measurements with the point-contact current in the ab plane we used sometimes the single crystal as a tip pressed on the massive, chemically etched copper. In order to focus on the data in the proper spectroscopic regime, the precautions were made to avoid the junctions with heating effects. In the following, only the junctions without the conductance dips and irreversibilities in voltage dependences are presented. The local superconducting transition temperature Tc was determined from the temperature dependence of the pointcontact spectra. The transition temperatures found in this way varied between 23 and 27 K.
3. Results and discussion
ARTICLE IN PRESS 3222
P. Szab´ o et al. / Physica B 404 (2009) 3220–3222
probably reveal the sign reversal of the order parameter which would explain occasional observations of the ZBP effect in the PCAR spectra [13–16] below Tc . Very recently Gonnelli et al. [22] have also obtained pronounced two-gap spectra on the LaFeAsO1x Fx polycrystals as well as they have shown an existence of the reduced DOS in the normal state up to about 140 K, close the Neel temperature of the undoped system. Their size of the large and small gaps are remarkably similar to those found in the present work.
4. Conclusions
Fig. 3. PCAR spectra of the point-contact junction with the current mostly within the ab plane of Ba0:45 K0:45 Fe2 As2 single crystal showing single gap and the ZBP effect.
will be shown. Thus we can conclude that this effect is a spectral feature related to a reduced DOS in the normal state. In some cases of the ab plane spectra only a single gap could be observed. Fig. 3 brings one of these. The spectrum at 4.5 K reveals the gap-like peaks at 72 mV on the overall enhanced pointcontact conductance due to the Andreev reflection. The spectrum shows a very peculiar development with increased temperature. While the position of the gap-like peaks is rapidly shrinking with increasing temperature, surprisingly the intensity of the conductance spectrum near the zero-bias voltage increases. At 10 K the gap-like peaks are completely gone and the only visible feature of the spectrum is the zero-bias peak (ZBP) which at even higher temperatures looses its intensity and vanishes at the local Tc , here between 20 and 27 K. This effect was very regularly observed in our previous PCAR measurements on the NdFeAsO(F) polycrystalline samples [16]. There the effect was ascribed to a possibility of summing over several channels within one pointcontact junction. Recently Choi and Bang [19] have elaborated a model of ‘‘sp’’ pairing (s-wave order parameter with sign reversal) for iron pnictides in analogy with the behavior of the superconducting/ ferromagnet (S/F) bilayers where the order parameter sign change can also happen. By their calculations the sp state can produce the zero-bias peak in the local density of states at the superconducting/normal interface. This can be detected in the tunneling or PCAR spectroscopy measured on the normal metal/superconductor junctions and explain the ZBP observed in Fig. 3. Mazin and Johannes [20] proposed recently fluctuating magnetic domains as important clue for explanation of superconducting and normalstate properties in iron pnictides. Goko et al. [21] have even brought evidence for coexistence of superconductivity and strong static magnetic order in a partial volume fraction of ðBa; KÞFe2 As2 and similar systems. They have shown a strong magnetism existing below about 70 K in ðBa; KÞFe2 As2 . Then, the point-contact junctions made on or nearby such ‘‘magnetic’’ spots could
The first directional PCAR studies have been performed on the single crystalline Ba0:55 K0:45 Fe2 As2 . Significant differences in the spectra are observed when measured within the ab planes and the c direction. Within the planes two superconducting gaps with DS 2–5 meV and DL 9–11 meV are detected. For the PCAR measurements perpendicular to the planes no traces of superconducting energy gap(s) could be found, just a conductance reduced around the zero bias is observed. This effect persisting up to about 100 K and indicating reduced DOS even in the normal state, is possibly connected with the magnetism existing in a substantial volume fraction even in the superconducting state.
Acknowledgments This work has been supported by the Slovak Research and Development Agency under the Contract nos. VVCE-0058-07, APVV-0346-07 and LPP-0101-06 and by the EC Framework Programme MTKD-CT-2005-030002. Centre of Low Temperature Physics is operated as the Centre of Excellence of the Slovak Academy of Sciences. The liquid nitrogen for the experiment has been sponsored by the U.S. Steel Koˇsice, s.r.o. Work at the Ames Laboratory was supported by the Department of Energy—Basic Energy Sciences under Contract no. DE-AC02-07CH11358. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
Y. Kamihara, et al., J. Am. Chem. Soc. 130 (2008) 3296. M. Rotter, et al., Phys. Rev. Lett. 101 (2008) 107006. M.S. Torikachvili, et al., Phys. Rev. Lett. 101 (2008) 057006. X.C. Wang, et al., Solid State Commun. 148 (2008) 538. F.C. Hsu, et al., Proc. Natl. Acad. Sci. USA 105 (2008) 14262. S. Raghu, et al., Phys. Rev. B 77 (2008) 220503(R). ´ et al., Phys. Rev. Lett. 87 (2001) 137005. P. Szabo, I.I. Mazin, et al., Phys. Rev. Lett. 101 (2008) 057003. C. Ren, et al., Chinese Phys. Lett. 25 (2008) 2215. K. Matano, et al., Europhys. Lett. 83 (2008) 57001. H. Ding, et al., Europhys. Lett. 83 (2008) 47001. C. Martin, et al., arXiv:0807.0876, 2008. Y.-L. Wang, et al., Supercond. Sci. Technol. 22 (2009) 015018. T.Y. Chen, et al., Nature 453 (2008) 1224. K.A. Yates, et al., Supercond. Sci. Technol. 21 (2008) 092003. P. Samuely, et al., Supercond. Sci. Technol. 22 (2009) 014003. N. Ni, et al., Phys. Rev. B 78 (2008) 014507. P. Szabo, et al., Phys. Rev. B 79 (2009) 012503. H.-Y. Choi, Y. Bang, arXiv:0807.4604, 2008. I.I. Mazin, et al., arXiv:0807.3737, 2008. T. Goko, et al., arXiv:0808.1425, 2008. R. Gonnelli, et al., Phys. Rev. B 79 (2009) 184526.