Pressure effects on strained FeSe0.5Te0.5 thin films
aa r X i v : . [ c ond - m a t . s up r- c on ] A ug Pressure effects on strained FeSe . Te . thin films M. Gooch, a) B. Lorenz, S. X. Huang, C. L. Chien, and C. W. Chu T CSUH and Department of Physics, University of Houston, Houston, Texas 77204-5002,USA Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218,USA T CSUH and Department of Physics, University of Houston, Houston, Texas 77204-5002,USA and Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720,USA (Dated: 12 November 2018)
The pressure effect on the resistivity and superconducting T c of prestrained thin films of the iron chalcogenidesuperconductor FeSe . Te . is studied. Films with different anion heights above the Fe layer showing differentvalues of ambient pressure T c ’s are compressed up to a pressure of 1.7 GPa. All films exhibit a significantincrease of T c with pressure. The results cannot solely be explained by a pressure-induced decrease of theanion height but other parameters have to be considered to explain the data for all films. I. INTRODUCTION
The discovery of superconductivity in iron pnictidecompounds has revived superconductivity research withthe goal to search for new superconducting compoundswith high critical temperatures as well as novel physi-cal phenomena and mechanisms of superconductivity.
With the theoretical proposal of multiband superconduc-tivity with s ± pairing symmetry and the experimentalevidence for nodeless two-gap superconductivity in theoptimally doped pnictides, it became obvious verysoon that a new class of high-temperature superconduc-tors different from the known high- T c cuprates had beenfound. Of particular interest is the close proximity of thesuperconducting state to magnetic order in form of a spindensity wave (SDW) state that extends from the under-doped toward to the optimally doped region, in the phasediagram of most iron pnictides. This has raised ques-tions about the role of antiferromagnetic fluctuations inthe pairing mechanism of FeAs superconductors.The new class of superconducting materials is struc-turally very flexible and many different compounds havebeen studied (see for example the review of Ref. [15]).The common feature of all is the Fe As layer as the ac-tive layer for superconductivity. Unlike the CuO planesin the cuprates, the Fe As layer is not flat but it formsa puckered slab with the planar Fe layer sandwiched be-tween two As sheets. The Fe As layers are separatedby different blocks of rare earth and oxygen (LaOFeAs),alkaline earth ions (SrFe As ), or more complex assem-blies of ions. The Fe-ions are located in the center ofa distorted As tetrahedron, the shape and dimension ofwhich is considered to be essential for high superconduct-ing temperatures. In an attempt to clarify the relationship between thetetrahedral dimensions and high- T c superconductivity,Huang et al. have recently investigated a series of thin a) Electronic mail: [email protected] films of the iron chalcogenide FeSe . Te . . This com-pound has the simplest structure and it consists exclu-sively of Fe (Se,Te) layers isostructural to the Fe As layers, stacked along the c -axis of the tetragonal struc-ture. It was demonstrated that high quality c -axis ori-ented films can be deposited on MgO with different sub-strate temperatures, T s , which determines the biaxialstrain in the FeSe . Te . films. The decrease of T s re-sults in an increase of the c -axis and a decrease of the a -axis. Most importantly, no chemical substitution orcharge doping is necessary and the shape (angle) of theFe-Se/Te tetrahedron remained approximately constant.The parameter controlling the superconducting T c wasidentified as the anion hight above the Fe-layer, h Se/T e ,that decreases with decreasing substrate temperature.Upon decreasing h Se/T e ( T s ), superconductivity arises be-low a critical value with T c rapidly increasing toward itsmaximum that is close to the superconducting T c of bulkFeSe . Te . . The anion height of the films with T c,max isclose to the value of h Se/T e for the bulk material. Withfurther decreasing h Se/T e the superconducting T c falls offquickly and vanishes below another critical value. The effect of hydrostatic pressure on T c of iron pnic-tides and chalcogenides has been studied extensively forbulk compounds. The T c of bulk FeSe . Te . , as de-fined by the onset of the resistivity drop, was raised from14 K at ambient pressure to 19 K at about 3.6 GPa; how-ever, with a significant broadening of the superconduct-ing transition. With further increasing pressure, T c did decline quickly with an extrapolated disappearanceof superconductivity above 10 GPa. Since the major ef-fect of pressure is the compression of the lattice, it can beanticipated that pressure will also deform the Fe-Se/Tetetrahedra of thin films and have a significant effect onthe crucial parameters like the tetrahedral angle or theanion height and consequently modify the superconduct-ing T c . We have therefore studied a series of strainedthin films of FeSe . Te . of varying ambient-pressure an-ion height, deposited at different temperatures on MgOsubstrates. II. EXPERIMENTAL
Films of thickness 400( ±
50) nm have been prepared bypulsed laser deposition (PLD) using the same target ofcomposition FeSe . Te . , as described in Ref. [20], andfour different substrate temperatures have been selected.All films have exclusively c-axis orientation. The com-position of the films is close to the target compositionsince the PLD results in a congruent film deposition andthe estimated vapor pressure of FeSe . Te . is smallerthan 10 − Torr at 500 o C, well above the chosen sub-strate temperatures. According to the deposition tem-peratures the films are labeled as follows: A ( T s =240 o C),B ( T s =290 o C), C ( T s =340 o C), and D ( T s =400 o C). Anadditional thin layer of chromium was deposited on topof the films to protect the FeSe . Te . films and to makesure that they are not affected by exposure to air ormoisture during the preparation for high-pressure exper-iments and by the pressure transmitting medium.Pressure up to 1.7 GPa was generated in a piston-cylinder clamp cell and a 1:1 mixture of FluorinertFC70:FC77 was used as a liquid pressure medium. Theresistivity of the four samples was measured in standardfour lead configuration in the plane of the films employingthe ac resistance bridge, LR700 (Linear Research). Thepressure was measured in situ at low temperature by fol-lowing the superconducting transition of a lead manome-ter. III. RESULTS AND DISCUSSION
The ambient pressure resistivity data are shown in Fig.1. The films A, B, C are superconducting and the fourthsample (D) exhibits a semiconducting temperature de-pendence of the resistivity above 2 K. The superconduct-ing T c ’s of samples A, B, and C, as determined by themidpoint of the resistivity drop, are 7 K, 10 K, and 4 K,respectively. This trend is consistent with data for sim-ilar films reported before and it reflects the systematicchange of T c with the increase of h Se/T e . Upon application of hydrostatic pressure the resistivitydrop of samples A, B, C shifts roughly in parallel towardhigher temperature, i.e. T c increases for all three sam-ples. The results of the pressure dependent resistivitymeasurements are shown in Fig. 2 (a to d). While pres-sure does increase T c of all superconducting films, therate of increase is very different. Although the ambientpressure T c ’s of samples A, B, and C differ by more than100 %, their T c ’s at the highest pressure of this investiga-tion are relatively close at 13 K, 16 K, and 12.5 K, respec-tively. The largest (relative) change is observed in film C( T s =340 o C) with an increase of T c by a factor of three.The width of the superconducting transition, as definedby the temperature difference between the 10 % and 90% resistivity drop, is of the order of 1.5 K to 2.5 K, in rea-sonable agreement with ambient pressure data obtainedfor thin films and high-quality single crystals. T S = 240 C o T S = 290 C o T S = 340 C o T S = 400 C o R ( T ) / R ( ) T (K)
FIG. 1. (Color online) Resistivity vs. temperature of strainedFeSe . Te . thin films deposited at different substrate tem-peratures T s .FIG. 2. (Color online) Pressure effect on the resistivity ofthe FeSe . Te . films. (a) T s =240 o C, (b) T s =290 o C, (c) T s =340 o C, and (d) T s =400 o C. In contrast, film D maintains its semiconducting tem-perature characteristics although the slope of the resis-tivity ρ ( T ) clearly decreases with pressure (Fig. 2d). Adrop of the resistivity develops at pressures above 0.8GPa indicating the possible onset of superconductivity inthis film. However, the resistance drop at 4.2 K reachesonly 30 % of the value at T c and the transition appearsto be very broad at the maximum pressure of this study.This indicates that the superconducting state may be fil-amental and a percolating superconducting path is notachieved throughout the film. It is possible that bulk su-perconductivity in sample D requires much higher pres-sure values.The pressure dependence of T c for the three supercon-ducting films A, B, and C is shown in Fig. 3a. Up tothe maximum pressure of this study, T c increases contin-uously with external pressure. The T c of film B, start-ing with the highest ambient value of 10.5 K, rises to16.5 K at 1.7 GPa. This value is comparable with the T c of bulk FeSe . Te . at about the same pressure, although the ambient pressure T c of this film is clearlylower than that of the bulk compound (13 K). The criti-cal temperatures of films A and C increase with pressurebut remain lower at 1.7 GPa since the ambient pressurevalues are smaller. However, plotting the relative change, T c ( p ) /T c (0), Figure 3b shows a significantly larger pres-sure effect on the normalized T c of sample C. The criticaltemperature increases by a factor of three upon compres-sion of the film. The relative change of T c is approxi-mately the same for films A and B.The results of the high pressure study of the three su-perconducting films raise the question about the most rel-evant parameters affected by pressure. The anion height h Se/T e is apparently an important control parameter for T c at ambient conditions, as shown for the current films and many other iron pnictides. For bulk samples ofFeSe, detailed structural studies at high pressure haverevealed that the anion height h Se decreases with pres-sures up to 2 GPa (the pressure range of the currentexperiment). A strict correlation of T c and h Se wasdemonstrated and discussed in high-pressure experimentson FeSe. The effect of hydrostatic pressure on h Se/T e offilms attached to a substrate, however, is more complexand needs a detailed discussion.While pressure is applied uniformly to the samplethrough the liquid pressure medium, the structural dis-tortion of the FeSe . Te . thin films will be affected bythe MgO substrate, different from the compression ofbulk samples. A qualitative picture can be derived bycomparing the compressibilities of the film and substratematerials. The compressibility of MgO is κ MgO =0.0154GPa − . Detailed structural data at high pressure arenot available for FeSe . Te . but it is justified to usecompressibility data for FeSe instead. The compressibil-ity (below 2 GPa) for FeSe is about two times larger( κ F eSe =0.032 GPa − ) than that of MgO. The at-tachment of the FeSe . Te . films to the less compress-ible MgO substrate results in a relatively larger uniaxialstrain effect along the c-axis (perpendicular to the sub-strate) of the film as compared to bulk samples under thesame pressure. It appears conceivable to conclude thatthe reduction of the anion height of the films on MgOat a given pressure is even amplified with respect to thesimilar effect in bulk single crystals.It seems therefore natural to consider the compres-sion of the tetrahedra and the decrease of h Se/T e withpressure as a driving mechanism to the observed raise of T c . This effect is certainly essential to understand thehuge increase of T c ( p ) /T c (0) of film C (Fig. 3b). Thisfilm starts with the largest h Se/T e of the superconductingsamples at ambient pressure, significantly larger than theoptimal anion height. Therefore, the reduction of h Se/T e under pressure moves the anion closer to the optimal dis-tance from the Fe plane and results in a large positive T C ( K ) P (GPa) o C 290 o C 340 o C (a) T C ( p ) / T C ( ) P (GPa) o C 290 o C 340 o C (b) FIG. 3. (Color online) Pressure dependence of (a) the criticaltemperatures T c and (b) the relative changes T c ( p ) /T c (0) forthe three superconducting films. In (a) the width of the tran-sition as defined between the 10 % and 90 % drop of resistivityis indicated by vertical dotted bars. pressure effect on T c .This argument, however, cannot solely account for the T c increase of sample A for which h Se/T e actually isalready lower than the optimal value at ambient pres-sure. A further compression should be counterproductivein raising the superconducting T c . Therefore, externalpressure must also change other parameters favoring thesuperconducting state and a higher T c . Changes of theband structure and the Fermi surface may play an im-portant role in understanding the pressure effects in ironpnictides and chalcogenides. For example, the density ofstates, the position of the Fermi energy, the topology ofthe Fermi surface and the possible nesting feature can allbe affected by external pressure. A charge transfer fromthe chalcogenide to the iron layer could also be inducedby pressure. We conclude that the effect of pressure on T c of iron chalcogenides cannot be reduced to only one pa-rameter ( h Se/T e ), as suggested from high pressure studiesof bulk samples, and further studies are needed for abetter understanding.The pressure data for the resistivity of the semicon-ducting film D (Fig. 2d) indicate the possible onset of su-perconductivity with the drop of the resistivity at higherpressures. While the temperature dependence of the re-sistivity is still semiconducting above this temperature,the decrease of the resistivity by 50 % is not small. Thechange of the temperature dependence of the resistivitywith pressure suggests that a metallic state can possiblyachieved at higher values of pressure. It can be expectedthat this will also result in a zero resistance state andbulk superconductivity at low temperature. IV. SUMMARY AND CONCLUSIONS
The study of the effect of pressure on prestrained thinfilms of FeSe . Te . reveals a complex behavior of the su-perconducting critical temperature. While the T c of allfilms increases from their ambient pressure values, therelative increase of T c ( p ) /T c (0) is significantly larger forthe film C with the largest anion height above the Feplane at ambient pressure. The results suggest that theeffect of pressure on reducing the anion height is essen-tial to understand the behavior of film C. However, thiseffect cannot solely explain the T c increase with pressureof films B and A with an initial anion height closer toor even below the optimal value. Therefore, the pressureeffects on additional parameters related to the electronicstructure and a possible charge transfer and the corre-sponding change of T c have to be considered. ACKNOWLEDGMENTS
This work is supported in part by the T.L.L. TempleFoundation, the John J. and Rebecca Moores Endow-ment, the State of Texas through TCSUH, the U.S. AirForce Office of Scientific Research, and the the U.S. De-partment of Energy. Y. Kamihara, T. Watanabe, M. Hirano, and H. Hosono, J. Am.Chem. Soc. , 3296 (2008). H. Takahashi, K. Igawa, K. Arii, Y. Kamihara, M. Hirano, andH. Hosono, Nature , 376 (2008). X. H. Chen, T. Wu, G. Wu, R. H. Liu, H. Chen, and D. F. Fang,Nature , 761 (2008). G. F. Chen, Z. Li, D. Wu, G. Li, W. Z. Hu, J. Dong, P. Zheng,J. L. Luo, and N. L. Wang, Phys. Rev. Lett. , 247002 (2008). Z. A. Ren, J. Yang, W. Lu, W. Yi, G. C. Che, X. L. Dong, L. L.Sun, and Z. X. Zhao, Mater. Sci. Innov. , 105 (2008). I. I. Mazin, D. J. Singh, M. D. Johannes, and M. H. Du, Phys.Rev. Lett. , 057003 (2008). A. V. Chubukov, D. E. Efremov, and I. Eremin, Phys. Rev. B , 134512 (2008). V. Cvetkovic and Z. Tesanovic, EPL , 37002 (2009). F. Wang, H. Zhai, Y. Ran, A. Vishwanat, and D.-H. Lee, Phys.Rev. Lett. , 047005 (2009). T. Y. Chen, Z. Tesanovic, R. H. Liu, X. H. Chen, and C. L.Chien, Nature (London) , 1224 (2008). H. Ding, P. Richard, K. Nakayama, T. Sugawara, T. Arakane,Y. Sekiba, A. Takayama, S. Souma, T. Sato, T. Takahashi,Z. Wang, X. Dai, Z. Fang, G. F. Chen, J. L. Luo, and N. L.Wang, EPL , 47001 (2008). K. Matano, Z. A. Ren, X. L. Dong, L. L. Sun, Z. X. Zhao, andG.-Q. Zheng, EPL , 57001 (2008). K. Hashimoto, T. Shibauchi, T. Kato, K. Ikada, R. Okazaki,H. Shishido, M. Ishikado, H. Kito, A. Iyo, H. Eisaki, S. Shamoto,and Y. Matsuda, Phys. Rev. Lett. , 017002 (2009). G. Mu, H. Luo, Z. Wang, L. Shan, C. Ren, and H.-H. Wen, Phys.Rev. B , 174501 (2009). J. Paglione and R. L. Greene, Nature Physics , 645 (2010). C.-H. Lee, A. Iyo, H. Eisaki, H. Kito, M. T. Fernandez-Diaz,T. Kumai, K. Miyazawa, K. Kihou, H. Matsuhata, M. Braden,and K. Yamada, J. Phys. Soc. Jpn.
77, Suppl. C , 44 (2008). J. Zhao, Q. Huang, C. de la Cruz, S. Li, J. W. Lynn, Y. Chen,M. A. Green, G. F. Chen, G. Li, Z. Li, J. L. Luo, N. L. Wang,and P. Dai, Nature Materials , 953 (2008). Y. Mizuguchi, Y. Hara, K. Deguchi, S. Tsuda, T. Yamaguchi,T. K, H. Kotegawa, H. Tou, and Y. Takano, Supercond. Sci.Technol. , 054013 (2010). H. Okabe, N. Takeshita, K. Horigane, T. Muranaka, and J. Akim-itsu, Phys. Rev. B , 205119 (2010). S. X. Huang, C. L. Chien, V. Thampy, and C. Broholm, Phys.Rev. Lett. , 217002 (2010). C. W. Chu and B. Lorenz, Physica C , 385 (2009). K. Horigane, N. Takeshita, C.-H. Lee, H. Hiraka, and K. Yamada,J. Phys. Soc. Jpn. , 063705 (2009). C.-L. Huang, C.-C. Chou, K.-F. Tseng, Y.-L. Huang, F.-C. Hsu,K.-W. Yeh, M.-K. Wu, and H.-D. Yang, J. Phys. Soc. Jpn. ,084710 (2009). G. Tsoi, A. K. Stemshorn, Y. K. Vohra, P. M. Wu, F. C. Hsu,Y. L. Huang, M. K. Wu, K. W. Yeh, and S. T. Weir, J. Phys.:Condens. Matter , 232201 (2009). I. Tsukada, M. Hanawa, S. Komiya, T. Akiike, R. Tanaka,Y. Imai, and A. Maeda, Phys. Rev. B , 054515 (2010). B. C. Sales, A. S. Sefat, M. A. McGuire, R. Y. Jin, D. Mandrus,and Y. Mozharivskyj, Phys. Rev. B , 094521 (2009). T. Taen, Y. Tsuchiya, Y. Nakajima, and T. Tamegai, Phys. Rev.B , 092502 (2009). K. W. Yeh, C. T. Ke, T. W. Huang, T. K. Chen, Y. L. Huang,P. M. Wu, and M. K. Wu, Cryst. Growth Des. , 4847 (2009). S. Margadonna, Y. Takabayashi, Y. Ohishi, Y. Mizuguchi,Y. Takano, T. Kagayama, T. Nakagawa, M. Takata, and K. Pras-sides, Phys. Rev. B , 064506 (2009). R. Y. Goble and S. D. Scott, Can. J. Mineral. , 273 (1085). D. Braithwaite, B. Salce, G. Lapertot, F. Bourdarot, C. Martin,D. Aoki, and M. Hanfland, J. Phys.: Condens. Matter , 232202(2009). G. Garbarino, A. Sow, P. Lejay, A. Sulpice, P. Toulemonde,W. Crichton, M. Mezouar, and M. Nunez-Regueiro, EPL86