Scanning-tunneling microscopy/spectroscopy and break-junction tunneling spectroscopy of FeSe₁–xTex

The iron-chalcogenide superconductor FeSe₁–xTex (0.5 < x < 1) was investigated by scanning-tunneling microscopy/ spectroscopy (STM/STS) and break-junction techniques. In the STM topography of the samples, randomly distributed Te and Se surface atomic structure patterns correlate well with th...

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Datum:	2013
Hauptverfasser:	Ekino, T., Sugimoto, A., Gabovich, A.M.
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Sprache:	English
Veröffentlicht:	Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України 2013
Schriftenreihe:	Физика низких температур
Schlagworte:	К 75-летию со дня рождения И. К. Янсона
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spelling	irk-123456789-1182282017-05-30T03:02:48Z Scanning-tunneling microscopy/spectroscopy and break-junction tunneling spectroscopy of FeSe₁–xTex Ekino, T. Sugimoto, A. Gabovich, A.M. К 75-летию со дня рождения И. К. Янсона The iron-chalcogenide superconductor FeSe₁–xTex (0.5 < x < 1) was investigated by scanning-tunneling microscopy/ spectroscopy (STM/STS) and break-junction techniques. In the STM topography of the samples, randomly distributed Te and Se surface atomic structure patterns correlate well with the bulk composition, demonstrating that nanoscale surface features directly reflect bulk properties. The high-bias STS measurements clarified the gap-like structure at ≈ 100–300 meV, which is consistent with the break-junction data. These highenergy structures were also found in sulfur substituted FeS₀.₁Te₀.₉. Possible origin of such spectral peculiarities is discussed. The superconducting gap 2Δ ≈ 3.4 ± 0.2 meV at temperature T = 4.2 K was found in the break junction of FeSe₁–xTex with the critical temperature Tc ≈ 10 K. The corresponding characteristic gap to Tc ratio 2Δ/kBTc ≈ 4 ± 0.2 indicates moderate superconducting coupling (kB is the Boltzmann constant). 2013 Article Scanning-tunneling microscopy/spectroscopy and break-junction tunneling spectroscopy of FeSe₁–xTex / T. Ekino, A. Sugimoto, A.M. Gabovich // Физика низких температур. — 2013. — Т. 39, № 3. — С. 343–353. — Бібліогр.: 50 назв. — англ. 0132-6414 PACS: 74.50.+r, 74.55.+v, 74.70.–b, 74.70.Xa http://dspace.nbuv.gov.ua/handle/123456789/118228 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
institution	Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection	DSpace DC
language	English
topic	К 75-летию со дня рождения И. К. Янсона К 75-летию со дня рождения И. К. Янсона
spellingShingle	К 75-летию со дня рождения И. К. Янсона К 75-летию со дня рождения И. К. Янсона Ekino, T. Sugimoto, A. Gabovich, A.M. Scanning-tunneling microscopy/spectroscopy and break-junction tunneling spectroscopy of FeSe₁–xTex Физика низких температур
description	The iron-chalcogenide superconductor FeSe₁–xTex (0.5 < x < 1) was investigated by scanning-tunneling microscopy/ spectroscopy (STM/STS) and break-junction techniques. In the STM topography of the samples, randomly distributed Te and Se surface atomic structure patterns correlate well with the bulk composition, demonstrating that nanoscale surface features directly reflect bulk properties. The high-bias STS measurements clarified the gap-like structure at ≈ 100–300 meV, which is consistent with the break-junction data. These highenergy structures were also found in sulfur substituted FeS₀.₁Te₀.₉. Possible origin of such spectral peculiarities is discussed. The superconducting gap 2Δ ≈ 3.4 ± 0.2 meV at temperature T = 4.2 K was found in the break junction of FeSe₁–xTex with the critical temperature Tc ≈ 10 K. The corresponding characteristic gap to Tc ratio 2Δ/kBTc ≈ 4 ± 0.2 indicates moderate superconducting coupling (kB is the Boltzmann constant).
format	Article
author	Ekino, T. Sugimoto, A. Gabovich, A.M.
author_facet	Ekino, T. Sugimoto, A. Gabovich, A.M.
author_sort	Ekino, T.
title	Scanning-tunneling microscopy/spectroscopy and break-junction tunneling spectroscopy of FeSe₁–xTex
title_short	Scanning-tunneling microscopy/spectroscopy and break-junction tunneling spectroscopy of FeSe₁–xTex
title_full	Scanning-tunneling microscopy/spectroscopy and break-junction tunneling spectroscopy of FeSe₁–xTex
title_fullStr	Scanning-tunneling microscopy/spectroscopy and break-junction tunneling spectroscopy of FeSe₁–xTex
title_full_unstemmed	Scanning-tunneling microscopy/spectroscopy and break-junction tunneling spectroscopy of FeSe₁–xTex
title_sort	scanning-tunneling microscopy/spectroscopy and break-junction tunneling spectroscopy of fese₁–xtex
publisher	Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
publishDate	2013
topic_facet	К 75-летию со дня рождения И. К. Янсона
url	http://dspace.nbuv.gov.ua/handle/123456789/118228
citation_txt	Scanning-tunneling microscopy/spectroscopy and break-junction tunneling spectroscopy of FeSe₁–xTex / T. Ekino, A. Sugimoto, A.M. Gabovich // Физика низких температур. — 2013. — Т. 39, № 3. — С. 343–353. — Бібліогр.: 50 назв. — англ.
series	Физика низких температур
work_keys_str_mv	AT ekinot scanningtunnelingmicroscopyspectroscopyandbreakjunctiontunnelingspectroscopyoffese1xtex AT sugimotoa scanningtunnelingmicroscopyspectroscopyandbreakjunctiontunnelingspectroscopyoffese1xtex AT gabovicham scanningtunnelingmicroscopyspectroscopyandbreakjunctiontunnelingspectroscopyoffese1xtex
first_indexed	2025-07-08T13:35:32Z
last_indexed	2025-07-08T13:35:32Z
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fulltext	© T. Ekino, A. Sugimoto, and A.M. Gabovich, 2013 Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 3, pp. 343–353 Scanning-tunneling microscopy/spectroscopy and break-junction tunneling spectroscopy of FeSe1–xTex T. Ekino and A. Sugimoto Hiroshima University, Graduate School of Integrated Arts and Sciences, Higashi-Hiroshima 739-8521, Japan E-mail: ekino@hiroshima-u.ac.jp A.M. Gabovich Institute of Physics, National Academy of Sciences of Ukraine, 46 Nauka Ave., Kyiv 03028, Ukraine Received November 12, 2012 The iron-chalcogenide superconductor FeSe1–xTex (0.5 < x < 1) was investigated by scanning-tunneling mi- croscopy/spectroscopy (STM/STS) and break-junction techniques. In the STM topography of the samples, ran- domly distributed Te and Se surface atomic structure patterns correlate well with the bulk composition, demon- strating that nanoscale surface features directly reflect bulk properties. The high-bias STS measurements clarified the gap-like structure at ≈ 100–300 meV, which is consistent with the break-junction data. These high- energy structures were also found in sulfur substituted FeS0.1Te0.9. Possible origin of such spectral peculiarities is discussed. The superconducting gap 2Δ ≈ 3.4 ± 0.2 meV at temperature T = 4.2 K was found in the break junc- tion of FeSe1–xTex with the critical temperature Tc ≈ 10 K. The corresponding characteristic gap to Tc ratio 2Δ/kBTc ≈ 4 ± 0.2 indicates moderate superconducting coupling (kB is the Boltzmann constant). PACS: 74.50.+r Tunneling phenomena; Josephson effects; 74.55.+v Tunneling phenomena: single particle tunneling and STM; 74.70.–b Superconducting materials other than cuprates; 74.70.Xa Pnictides and chalcogenides. Keywords: tunneling spectroscopy, break junction, scanning-tunneling microscopy/spectroscopy, energy gap, iron-based superconductors, FeSe1–xTex. 1. Introduction The discovery of iron-arsenide superconductor LaFeAsO1–xFx exhibiting Tc = 26 K in 2008 [1] stimu- lated the subsequent synthesis of novel iron-based super- conductors, e.g., LiFeAs [2], BaFe2As2 [3], and Fe(Se,Te) [4]. In particular, SmFeAsO1–xFx compound has the high- est Tc 55 K [5–8], which is a record among non-cuprate superconductors. As for the microscopic Cooper pairing mechanism supporting such high Tc’s, several possible candidates were proposed [9,10] and until now it is unclear which of them is a true one. Among iron-based superconductors there is a PbO type β-FeSe with the simplest crystal structure. Its specific fea- ture, especially beneficial for surface studies, is a good cleavability over the ab-plane. The sample surface is be- lieved to be so stable that it is not expected to be recon- structed. The superconductivity of FeSe maintains against substitution of the chalcogenide ions such as Fe(Se,M) (M = Si, Sb, S, Te) [11,12], and Fe(Te,S) [13]. For in- stance, Tc of Te substituted compound Fe(Se,Te) can be easily manipulated by changing the Se/Te composition ratio. In particular, Tc of FeSe is enhanced up to ~15 K by replacing Se by Te, which has the larger ion radius, while Tc of FeSe is only 8 K. Furthermore, Tc rises up to 37 K under the high pressure of 8.9 GPa [14], which is especial- ly remarkable because such a high critical temperature occurs in a pure binary compound system. Therefore, it seems very important to investigate gradual composition changes and their influence on superconductivity micro- scopically, e.g., as a function of the Se/Te ratio. In this paper, single crystals of Fe1.01Se1–xTex (x = 0.5–1) were investigated by means of the scanning tunneling mi- croscopy (STM) and spectroscopy (STS), and the results of the nanoscale surface measurements within the large Te range were compared and discussed. The tunneling spec- troscopy measurements of the superconducting gapping were also carried out by the break-junction (BJ) method which is extremely sensitive to the electronic spectrum variations in the superconducting state. T. Ekino, A. Sugimoto, and A.M. Gabovich 344 Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 3 2. Experimental Fe1.01Se1–xTex single crystals were synthesized by a standard process [15]. The mixed powder of Fe, Se, and Te pressed into pellet was double-sealed in an evacuated quartz tube, which was held at 1000 °C for 36 hours to cool down to 400 °C at a rate of –3 °C/h, followed by fur- nace cooling to room temperature, T. The pristine samples thus obtained were annealed at 400 °C for 100 hours. The electron probe micro analyzer (EPMA) was employed to determine the actual composition of the crystal. The resistivity measurements were done by a standard dc four probe method. The STM apparatus used in this experiment was an Omicron LT-UHV-STM system, which has been modified to further reduce external disturbance of the sample [16,17]. The sample was cleaved along the layer in situ at T = 77 K in an ultra-high vacuum (UHV) sample prepara- tion chamber of ~10–8 Pa to avoid any contamination or migration of atoms on the crystal surface. The Pt/Ir wire was used as the tunneling tip, which was cleaned by a high-voltage field emission process with Au single crystal target before the scanning operation. The STM measure- ments were carried out at T = 4.9–77 K under the UHV condition of ~ 10–8 Pa evacuated by the ion pump. A con- stant current mode was adopted to obtain the STM images. The BJ tunneling spectra were measured using an ac mod- ulation technique with lock-in amplifier. By this method, fresh and clean superconductor–insulator–superconductor (SIS) junction interface can be obtained along the crack of the thin platelet single crystal at T = 4.2 K [18,19]. 3. Results and discussion Prior to the STM/STS measurements, we determined the composition ratio x from EPMA in Fe1.01Se1–xTex with 0.5 < x < 1 (hereafter, we denote it as FeSe1–xTex). The results showed that the analyzed compositions of Te and Se were in a good agreement with nominal x. The found Te content exceeded x in the range 0.2 < x < 0.5. The analyzed Fe content 1.02 was slightly larger than the nominal value of 1.01. Single crystals of nominal x < 0.2 were not obtained in the present synthesis procedures. From the temperature dependence of resistivity, the maxi- mum Tc was determined: Tc (onset) = 15.9 K, Tc(0) = 14.5 K for x = 0.5. The critical temperature decreases with either increasing or decreasing x from 0.5. The lowest onset Tc was found as ~ 11 K for x = 0.9. In the T-dependence of resistivity for the annealed FeTe sample (x = 1), a noticea- ble drop in the resistivity, which is related to the tetragon- al-to-orthorhombic structural phase transition, was ob- served at around 72 K [20–23]. As the Te content x decreases, the resistivity-drop temperature also goes down accompanying the increase of resistivity. Such a feature was not observed for x = 0.9 before the annealing process. Before the STM measurements, we confirmed the sur- face electronic cleanliness by measuring the dependence of tunneling current I on the tip-sample distance z, I(z). Figure 1 shows that I(z) curve is exponential. From the perfect linearity of the logI-z dependence, the local work function, φ = 5.2 eV, is determined. The spatial resolution of the tip was ascertained by monitoring the atomic struc- tures of gold single crystal during the tip preparation process. For the STM measurements, the sample bias voltage V = ± 0.01–0.8 V and the tunneling current It = = 0.3–0.4 nA were adopted as the feedback conditions, but the results did not substantially depend on these parame- ters. The STM images did not depend on the annealing process. Clear surface spots of atomic arrangements form- ing the square-lattice structures were always observed. The period of lattice spot structures is ~ 0.37–0.38 nm for all compositions x, thus no distinguishable differences were found among them within the resolution. The STM images for FeSe1–xTex with various x meas- ured at 4.9 K are shown in Fig. 2. Since the crystal struc- ture consists of the stacking of edge-sharing FeSe4 tetrahe- dral layers, the cleaved surface is always the same. This is very advantageous for the investigation of the ab-plane properties in this compound. They exhibit the coexistence of the bright and dark spots for all the topographies. In Fig. 2(a), bright and dark spots are shown to be randomly distributed over the wide area of the atomic image. Such coexistence is realized only for the limited range of com- positions x, where bright and dark spots correspond to Te and Se atoms, respectively, as was discussed earlier [15]. Similar STM images were also reported in Refs. 24–27. Fig. 1. Dependence of the tunneling current I on the tip-sample distance z between PtIr tip and FeSe1–xTex single crystal. The upper-right inset shows the optical micrograph of the single crys- tal, while the lower left inset displays a histogram of the local work function (the barrier height). 1 mm 0 2 4 6 8 Local work function, eV F re q u en cy 4 2 0,1 8 6 4 2 0,1 8 6 4 0.1 0.2 z, nm I, n A Scanning-tunneling microscopy/spectroscopy and break-junction tunneling spectroscopy of FeSe1–xTex Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 3 345 The dynamic range of the topographic contrast slightly depends on the frames. The bright spots are located just on the atomic grid points of the square lattice structure for x = 0.5–0.9 (Figs. 2(a)–(d)), while the bright shining spots shown for x = 1, Fig. 2(e), seem to be slightly away from the formal atomic positions and extend to a few atoms. Therefore, the origin of these spots in Fig. 2(e) is probably different from that of the bright spots in Figs. 2(a)–(d). By assuming these spots in Fig. 2(e) as excess Fe atoms, the clusters are counted in the larger area of topography, and the ratio of ~ 0.05 was obtained. This is consistent with the data from EPMA. The atomic spots in Fig. 2(e) except the discussed Fe spots are homogeneous, which are naturally attributed to Te atoms. Interestingly, Te atoms as the brigh- ter spots in Figs. 2(a)–(d) turn into the darker spots in Fig. 2(e), but it is understood if we consider the difference in the atomic sizes (Te atom having larger size than Se one) for Figs. 2(a)–(d) or the height of the excess Fe atoms on the surface for Fig. 2(e), respectively, which results in relative change in the contrast. A direct evidence of distinguishing atoms in line with the discussion is given in Fig. 3, where the STM topogra- phy and its cross section along the indicated line is pre- sented for x = 0.7 and x = 0.9. The ripple patterns reflect- ing the sample-tip distance being kept constant are readily seen in the cross sectional profiles. The positions of spiky dips among the regular shallow ripples in the height pro- files in Figs. 3(a) and (b) correspond to the darker spots in the topography. The depth of the spikes is ~ 0.02–0.03 nm for both profiles. Since the ionic radii of Te and Se are 0.221 and 0.198 nm, respectively, the difference in the height z reflects just the difference of ionic radii 0.023 nm be- tween Te and Se. Therefore, the difference in ion sizes be- tween them is now visualized. Figure 3(c) shows the dark- spot ratio in the STM images versus the Se content (1–x) determined from EPMA. The dark-spot ratio was defined by counting the numbers of dark nd and bright nb spots within the area of typically 30×30 nm and then normalized as nd/(nb + nd). The bright shinning spots discussed above were sometimes observed in the whole x ranges, but they were omitted from the counting. Almost perfect linear rela- tionship was obtained between the dark-spot ratio on the STM image and the Se content taken from EPMA. From these results, we identify the origin of the bright and dark spots in the whole investigated range of 0.5 < x <0.9. The results testify that the bulk characteristics directly correlate with the nanoscale local features of the freshly cleaved crystal surface. These convincing topographic studies with atomic resolution are ensured by the high quality of the local tunnel barriers manifested by the constancy of the measured work function. In spite of such clear observations of the surface atomic arrangements of FeSe1-xTex with precise identification of Te and Se atoms on the crystal surface, we observed by STS no clear local superconducting gaps. The electronic surface inhomogeneity might be a possible primary origin Fig. 2. STM topography of FeSe1–xTex for 0.5 < x < 1. T = 4.9 K, I = 0.3 nA, V = –0.4 V. T. Ekino, A. Sugimoto, and A.M. Gabovich 346 Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 3 of the obscure superconducting gap structures. To confirm such a nanoscale electronic irregularity we built STS con- ductance mappings. Figure 4 demonstrates the STM topography and the mapping of the differential conductance dI/dV at zero bias and V = 30 mV for FeSe0.3Te0.7 in the superconducting state at 4.9 K. The patch-like patterns in the conductance map of 5×5 nm roughly resemble those of STM topogra- phy in the same region, which is due to Te/Se distributions as indicated in Fig. 2. However, a closer look indicates slight discrepancies between the contrast patterns of the topography and conductance mapping. This can be ex- plained by the manifestation of the local density of states integrated over the energy revealed in the topography while density of states itself determines the conductance mapping. To clarify this point further, the line profiles of the conductance are shown for the line cuts from three dif- ferent locations. The conductance magnitude becomes re- markably inhomogeneous when the bias voltage exceeds +20–30 mV while it is fairly homogeneous for the negative bias. This indicates the existence of impurity states in the empty band. Since the energy scale of the inhomogeneity starting at 20–30 meV is much higher than Tc ~ 10 K (~ 0.9 meV) of FeSe0.3Te0.7, it does not seem to be directly related to the superconducting properties. The Bardeen- Cooper-Schrieffer (BCS) pairing energy is about 2 meV, but no trace of a superconducting gap is seen in any of local conductance profiles in the low bias regions below 10 mV. It stems from the figure that there are no serious local na- noscale inhomogeneities at zero bias which would be able to smear superconducting-like spectra with their small energy gaps. (This is in contrast to the pattern at V > 20–30 meV where the substantial irregularity does exist.) The gap- related features might have been washed out by surface variations of the electronic properties emerging despite low-T and UHV cleaving conditions. Nevertheless, it is impossible now to indicate any sound reason of such varia- tions. One might also consider the absence of gapping be- ing due to bad resolution of the STS chamber. However, measurements with the same STM/STS apparatus demon- strated conspicuous atomic arrangements and the super- conducting gap features in the other layered superconduc- tor β-ZrNCl having a similar Tc = 13–15 K [28]. The existence of the superconducting gap could be found by examination of the zero-bias conductance in the superconducting state. Corresponding attempts were made and the results obtained are shown in Fig. 4, in which the dark-contrast region near zero-bias corresponding to the depression of the electronic density of states might be due to the manifestation of the superconductivity gap, because such deep dark contrast becomes generally weak upon warming. Anyway, there are no traces of the apparent gap structure in the observed line profiles. Hence, another effective method of BJ tunneling spec- troscopy (BJTS) [18] was applied to reveal superconduct- ing gaps in the materials concerned. In this technique, the symmetric geometry of electrodes is realized so that the overwhelming majority of junctions are of the SIS nature. Since the crack section is perpendicular to the plate the tunneling current passes mainly through the ab crystal plane. Quite a number of iron-based superconductors are be- lieved to manifest multiple or anisotropic gaps [29] as a consequence of the multiple electronic band structure [10]. Fig. 3. Line profiles of the STM topography for FeSe0.3Te0.7 (a) and FeSe0.1Te0.9 (b). Dark spot ratio from STM versus Se content (1–x) from the electron-probe micro analyzer (EPMA) (c). Scanning-tunneling microscopy/spectroscopy and break-junction tunneling spectroscopy of FeSe1–xTex Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 3 347 In particular, two-gap behavior was demonstrated in FeSe1–x [30]. We sought for this phenomenon in our sam- ples of FeSe1–xTex but succeeded in the observation of the maximum-gap features only. This major gap, which is of the order of Tc if the gaps are relatively weakly coupled, is a parameter that at least approximately reflects the strength of the pairing interaction [31]. Figure 5(a) shows three representative BJ tunneling con- ductances measured at T = 4.2 K for FeSe0.5Te0.5, which possesses the highest bulk Tc = 15 K among the series FeSe1–xTex. The top curve shows a conspicuous depletion of the electron density of states as well as inexpressive conduc- tance peaks. The pattern can be associated with the presence of the superconducting gap. The gap smearing correlates with a large observed zero-bias leakage conductance as compared to the standard BCS density of states. At the same time, the middle and bottom curves in Fig. 5(a) reveal more subtle gap-like features demonstrat- ing hints of multigapness. The inner coherent peaks of the double-gap structure in the middle curve correspond to the gap peaks of the top curve positioned at ± 2–3 mV, while the outer ones correlate with the rather strong peaks of the bottom curve positioned at ± 5–6 mV. Since the latter bias voltages are twice as much as the former ones, a plausible conclusion may be made that the outer and inner peaks are generated by tunneling through SIS (the bottom curve) and SIN (the top curve, N stands for a normal metal) junctions, respectively rather than by random scatter of multiple gap values due to the sample inhomogeneity. Indeed, SIN junc- tions are often formed instead of SIS counterparts even in symmetric break junctions. This can be understood as the consequence of the crack occurrence at a crystal defect or grain boundary [32]. Fig. 4. The STM topography, the dI/dV mapping, and the dI/dV line profiles for FeSe0.3Te0.7 at 4.9 K. T. Ekino, A. Sugimoto, and A.M. Gabovich 348 Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 3 The above arguments were checked by the conductance fitting using the broadened BCS density of states with the account of thermal broadening. In the fitting, the well- known approximation for the density of states, N(E, Γ) = = \|Re{(E – iΓ)/[(E – iΓ)2 – Δ2]1/2}\|, was used, where Γ and Δ are the phenomenological broadening and gap parame- ters, respectively [33]. The fitted results are depicted as dashed lines in Figs. 5(b) and (c). The overall features in- cluding the broadened gap peaks and especially the sub- stantial leakage conductance at zero bias are fairly well reproduced. On the other hand, the calculated conductance peaks are much shaper than the experimental data. The best fitting parameters at T = 4.2 K are Δ = 1.6–1.8 meV and Γ = 0.2–0.3 meV, respectively. As for the microscopic origin of Γ, there exist several possible factors such as gap anisotropy, inhomogeneity, etc., although there are no quantitative estimations. On the other hand, relatively large but rather common value obtained here implies the extrin- sic interface influence. Such a large Γ value probably re- flects the depression of the superconducting gap features in STS. From the fitting results in Figs. 5(b) and (c), in which the correspondence between experimental data and calcu- lated curves is rather good in the zero bias region, the fea- ture of sub-gap conductance seems to have nothing in common with any non-conventional anisotropic pairing. Figure 6 shows the T-dependence of the conductance in the gap region corresponding to Fig. 5(b). The gap struc- ture already broadened at 4.2 K is further broadened upon warming and smeared out at a local Tc = 10 K in the junc- tion. This is much lower than the bulk Tc = 15 K from the resistivity measurements. The figure also indicates the SIN junction formation in BJ as is indicated in Fig. 5(b), where the gap structure is gradually smeared out but the gap-edge peaks do not shift to zero at T → Tc, which is in contrast to the behavior of SIS junctions [32]. The low-T gap value 2Δ = 3.2–3.6 meV and the junction local Tc = 10 K give the characteristic gap to Tc ratio 2Δ/kBTc = 3.7–4.2. This is slightly larger than the BCS value ≈ 3.5, indicating a mod- erate or strong-coupling. Similar gap values were reported in the earlier STS studies [24,25,27]. Fig. 6. Temperature evolution of dI/dV for FeSe0.5Te0.5. The conductance is shifted up for the clarity. Fig. 5. The dI/dV curves of FeSe0.5Te0.5 (a) at 4.2 K from break- junction tunneling spectroscopy (BJTS). The dI/dV fittings using the broadened BCS density of states (dotted curves) (b) and (c). Scanning-tunneling microscopy/spectroscopy and break-junction tunneling spectroscopy of FeSe1–xTex Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 3 349 In the BJ measurements, substantial variation of both gaps was found. Most probably, it is because BJTS is sen- sitive to local changes in stoichiometry as described in our previous measurements [34]. These BJ studies reveal an upper limit of the superconducting gap value. The solid curve in Fig. 7(a) shows the maximum gap size observed in this series of measurements, where the peak-to-peak separation of Vp–p ≈ 13–14 mV was obtained in the gap structure with moderately pronounced peaks. This gap may be attributed to the maximum Tc = 15 K in this com- pound series. The representative outer gap structure in Fig. 5(a) is also shown for the comparison. We can recog- nize that the larger gap size possesses the larger conduc- tance leakage (probably reflecting the low superconduct- ing volume fraction). This is consistent with the fact that we often obtained immature gap size and Tc, as shown in Figs. 5 and 6. This set of thermodynamic parameters cor- relates well with Vp–p ≈ 40 mV and Tc ≈ 48 K of the su- perconducting NdFeAsO0.9F0.1 found in our BJTS mea- surements [35], thereby supporting the idea of a common mechanism of superconductivity among the iron-based compounds [10,36]. In fact, one-unit-cell film of FeSe fabricated by molecu- lar beam epitaxy was found to exhibit superconductivity above 50 K and reveals the gap values of ≈ 20 mV and ≈ 40 mV in STS investigations [37,38]. Notwithstanding the difference in Tc, the coherent peak positions in Ref. 37 correlate with our results of Vp–p ≈ 40 mV for the symme- tric BJ junction. In Fig. 7(b), superconducting zero-T gap versus Tc plots for several iron-based superconductors are shown together with those of some copper oxides as well as of previously investigated by us layered nitrochloride superconductors [28,39]. All the measurements were done by the fixed design of BJTS. The plots demonstrate a re- markable difference in the slope reflecting the difference in the coupling strength and, possibly, in the underlying me- chanism of Cooper pairing. Namely, the slope is about the s-wave BCS weak-coupling value in iron-based supercon- ductors, while it is more than twice that in the copper- oxide and nitrochloride superconductors. Therefore, it is clear that cuprates and nitrochlorides possess unusually large energy scale of the pairing interaction, the origin of which remains to be clarified [39,40]. As for cuprates, it might be a consequence of the charge-density-wave (CDW) influence on superconductivity [41]. We have extended the bias voltage range to investigate the background normal-state electronic states. Figure 8(a) shows representative tunneling conductances obtained in both STS (A and B) and BJTS (C and D) measurements. The STS conductance is obtained by averaging 4096 spec- tra in a 10×10 nm region, while the BJTS conductance is a single trace from a junction. The revealed broadened gap- like structure in STS spectra appears to be asymmetric with respect to zero bias, but the bias polarity of the enhanced peak in the asymmetric conductance depends on the mea- surement run as shown in two characteristic curves of Fig. 8(a). The asymmetric gap structure of curve A pos- sesses a moderately enhanced peak appearing at the negative bias of ~ –300 mV, while a subtle peak at the positive bias ≈ 100 mV becomes distinct only after normalizing by the background value. Note that the bias polarity is defined with respect to the sample. The high-energy gap structures found in STS persist even at 77 K, showing the peaks at –300 mV and +100mV, as displayed in Fig. 8(b). The sub-gap conductance depends linearly on the bias showing a very weak bend at –100–150 mV, which becomes visible after the normalization. This bias is symmetric with respect to the smaller peak occurring at +100 mV in the normalized conductance, which is distinctly manifested in the raw conductance data of B only in the positive bias. Curves C and D in Fig. 8(a) describe the high-energy structure of the BJTS conductance similar to that observed by STS. The conductance is reasonably symmetric in ac- cordance with the SIS nature of the junction. The well- distinguished gap-edge structures are seen at ≈ ±300 mV for C with small bends at ±100mV, accompanying an in- Fig. 7. The maximum gap structure observed in the measure- ments of FeSe1–xTex (a). Peak-to-peak value of the superconduct- ing gap 2Δp–p versus Tc(K) for several superconductors [35,38,39] (b). FeSe Te1–x x La Sr CuO1.85 0.15 4 Nd Ce CuO1.85 0.15 4 Bi Sr CuO2 2 6 �-HfNCl �-ZrNCl (a) BJTS = 4.2 KT 8 7 6 5 0.17 0.16 0.15 –10 –5 0 5 10 Voltage, mV d I/ d V , m S 15–15 (b) NdFeAsO F0.9 0.1 40 30 20 10 0 10 20 30 40 50 Tc, K 2 , m eV � p – p T. Ekino, A. Sugimoto, and A.M. Gabovich 350 Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 3 tensive broad zero-bias peak due to a superconducting weak-link behavior of the junction. The broadened double- peak structure is seen in D at ±400 mV and ±150 mV, pos- sessing the slightly larger energy than in C. These gap fea- tures are in agreement with the STS spectra. The lack of superconductivity manifestations in spec- trum D is probably due to break of the sample in the non- superconducting region or the gap averaging out by inho- mogeneity. This explanation correlates with relatively large conductance leakage. It might also happen that the absence of superconductivity is due to the insufficient resolution of the ac modulation voltage (> 1 mV) in this case. The line profiles of the conductance mapping are shown in Figs. 9(a) and (b) from the STS measurements at 4.9 K within the length scale of ≈ 10 nm. The broadened peaks at ≈ – 300 mV (Fig. 9(a)) and +100 mV (Fig. 9(b)) are evi- dent anywhere in the spatial region, which demonstrate that the conductance below \|V\| = 400 mV is fairly homoge- neous at least in this range with small accidental variations. The broad peak structure at –300 mV is consistent with the binding energy of 300 meV in the density of states ori- ginating from iron d-state as observed in the valence-band photoemission spectroscopy [42]. On the other hand, the conductance peak positions which change with the bias polarity (–300 mV and +100 mV) could be due to the tunneling from different sections of the Fermi surface. In fact, the angle-resolved photoemission spectroscopy along the high-symmetry line of the Brillouin zone showed a broad peak feature at ≈ 300 meV around the Γ point and a nondispersive band at ≈ 100 meV around the M point, res- pectivey [43]. The energies of asymmetric peaks with strongly asymmetric conductance background observed in STS could be explained by the difference in the tunneling probability along the different directions in the Brillouin zone. Since the negative bias corresponds to the electron tunneling from the sample into the tip, the observed peak at –300 mV might reflect the bottom of the electron band, while the +100 mV peak reflects the empty states of the top of the hole like band, both of which are located at the Γ point. The conductance features like this exist in the broad composition range, showing similar nanoscale homogeneity. The remarkable segmental dependence of the conduc- tance background occurring intensively in the particular surface regions, that is, stronger positive or negative bias dependences in Figs.9(a) or (b), respectively, could hide the peak in the opposite bias. Such features could happen when the shape of the potential barrier plays an important role. In the vacuum tunneling using an STM tip, local en- hancement of the electric field modifying the barrier yields a strong bias dependence. The density-wave formation in the layered compound may be another plausible driving force of the conductance asymmetry as predicted by the theory [44]. As for the gap size observed here, it is typical to those of the charge den- sity wave compounds [45,46]. Specifically, the asymmetry in the gap structure could be a manifestation of the influ- ence of the CDW order parameter phase. This is a possible reason of the pseudogap (dip-hump) peak existence on one bias polarity branch only [47]. The existence of the asymme- tric gap feature in the STS conductance could be a smoking gun of the CDW appearance in FeSe1–xTex, which might exist in iron-based compounds along with spin density waves (SDWs) [36]. At the same time, SDW can also pro- mote asymmetry of the tunneling conductance [44]. Assuming the energy scale of the conventional CDW compounds, the present gap might disappear at ≈ 100–250 K. This is in accordance with Fig. 8(b), where the gap-like structures are observed even at 77 K. In fact, the structural phase transition near 90 K has been reported in FeSe [48], and confirmed in our own resistivity measurements [49]. These characteristic features can be associated with high bias gap-like structures observed here. The high-energy gap like structure is reproduced in the sulfur-substituted compound. Figure 10 shows the STS conductance profile for Fe1.02S0.1Te0.9 where composi- tions were determined by EPMA. Rather homogeneous line profile in the spatial range of 10 nm is similar to that in the Se-substituted compound. The pronounced peak at Fig. 8. The dI/dV curves for FeSe0.5Te0.5 obtained from STS (A, B) and BJTS (C, D) at T = 4–5 K (a), and STS at 77 K (b). –1000 0 1000 Voltage, mV 1 0 B JT S , m S S T S , n S d I/ d V 2 0 FeSe Te0.5 0.5(a) C (BJTS) D (BJTS) A (STS) B (STS) 4.2 K (BJTS) – 4.9 K (STS) –500 0 500 (b) 1 0 –500 0 500 STS = 77 KT d I/ d V , n S Voltage, mV Scanning-tunneling microscopy/spectroscopy and break-junction tunneling spectroscopy of FeSe1–xTex Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 3 351 ≈ –400 mV and a weak structure at +100–200 mV are slightly larger than in the Se compound, but essentially in the same energy range. The difference is probably due to modifications of electronic states upon the change of the ion sizes, since the S ion is 7–9% smaller than the Se one. In order to study the high-energy feature in connection with the magnetic phase transition at 72 K [20–23], the STS measurements have been carried out for the non- superconducting Fe1.01Te. The results showing the shallow zero-bias depression of conductance and no apparent high- energy gap structure are in contrast to Fig. 8, although both conductance spectra display noticeable asymmetry with respect to the bias voltage. This difference of the observed electronic structures in a normal metal FeTe and a super- conductor FeSe1–xTex is consistent with calculations [50]. 4. Summary We have synthesized and measured the surface and electronic properties of Fe1.01Se1–xTex (0.5 < x < 1) single crystals by means of STM/STS and BJ tunneling spectros- copy. The STM topographies distinguish Se and Te atoms in the whole x range, indicating that surface local events directly reflect the bulk properties. The STS and break junction measurements clarified the asymmetric gap-like structure at ≈ ±100 mV and –300 mV. These features seem to be consistent with the photoemission spectra, while the distinct asymmetric gap-like feature can be explained by a possible CDW formation. Such structures are also ob- served in sulfur substituted Fe1.02S0.1Te0.9, but not in Fe1.01Te. The superconducting gap structure is probed by BJ tunneling. A smeared BCS density of states with 2Δ (4 K) ≈ 3.4 ± 0.2 meV for Tc ~ 10 K was observed. The characteristic gap ratio 2Δ(0)/kBTc = 3.7–4.2 indicates an intermediate superconducting coupling strength. Acknowledgements We thank the Natural Science Center for Basic Research and Development, Hiroshima University for EPMA analysis and supplying liquid helium. This work was supported by a Fig. 9. Line profiles of dI/dV for FeSe0.5Te0.5 measured by STS at 4.9 K, showing the peak at –300 mV (a) and +100 mV (b). The pro- files of (a) and (b) were taken from different samples. (a) (b) x, n m T = 4.9 K T = 4.9 K 4 2 0 –2 –4 –600–400–200 0 200 400 600 4 2 0 –400 –200 0 200 400 4 2 0 2 1 0 –1 –2 x, n m Voltage, mV Voltage, mV d I/ d V , n S d I/ d V , n S Fig. 10. Line profile of dI/dV from STS for FeS0.1Te0.9 at 4.9 K. T. Ekino, A. Sugimoto, and A.M. Gabovich 352 Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 3 Grand-in-Aid for Scientific Research (245403770) of the Ministry of Education, Culture, Sports, Science, and Tech- nology (MEXT) of Japan. The work was partially supported by the Project N 8 of the 2012-2014 Scientific Cooperation Agreement between Poland and Ukraine. 1. Y. Kamihara, T. Watanabe, M. Hirono, and H. Hosono, J. Am. Chem. Soc. 130, 3296 (2008). 2. X.C. Wang, Q.Q. Liu, Y.X. Lv, W.B. Gao, L.X. Yang, R.C. Yu, F.Y. Li, and C.Q. Jin, Solid State Commun. 148, 538 (2008). 3. M. Rotter, M. Tegel, and D. Johrendt, Phys. Rev. Lett. 101, 107006 (2008). 4. F.C. 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Scanning-tunneling microscopy/spectroscopy and break-junction tunneling spectroscopy of FeSe₁–xTex

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