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Strain effects on electronic structure of Fe0.75Ru0.25Te
Materials Chemistry and Physics xxx (2016) 1e6
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Strain effects on electronic structure of Fe0.75Ru0.25Te M.J. Winiarski a, *, M. Samsel-Czekała a, A. Ciechan b a b
lna 2, 50-422, Wrocław, Poland Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Oko w 32/46, 02-668, Warsaw, Poland Institute of Physics, Polish Academy of Sciences, al. Lotniko
h i g h l i g h t s Ru-doped FeTe systems are investigated by density-functional theory methods. Structural and electronic properties of Fe0.75Ru0.25Te and parent FeTe are studied. The double-stripe antiferromagnetic ground state is predicted for both systems. The single-stripe antiferromagnetic phase may be induced by tensile strain. Tensile strained Fe0.75Ru0.25Te is a candidate for a new Fe-based superconductor.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 September 2016 Received in revised form 3 November 2016 Accepted 13 November 2016 Available online xxx
Structural and electronic properties of a hypothetical Fe0.75Ru0.25Te alloy and the parent FeTe compound have been investigated from first principles within the density functional theory (DFT). For both systems the double-stripe antiferromagnetic ground state is predicted at ambient pressure. The incorporation of Ru atoms into FeTe in the nonmagnetic phase leads to a deep valley of density of states in the vicinity of the Fermi level and the DOS at the Fermi level is significantly diminished in the considered solid solution. The single-stripe antiferromagnetic phase in Fe0.75Ru0.25Te may be induced by tensile strain. These findings suggest that strained thin films of Fe1xRuxTe are good candidates for new superconducting Febased materials. © 2016 Elsevier B.V. All rights reserved.
Keywords: superconductors Electronic structure Crystal structure Deformation
1. Introduction Despite the relatively low superconducting critical temperature (Tc) of 8 K [1] exhibited by bulk FeSe in equilibrium conditions, a rapid increase of Tc up to 37 K [2e5] was revealed in this system under hydrostatic pressure. Superconductivity (SC) in iron chalcogenides was also reported for solid solutions FeSe1xTex (with the maximum Tc ¼ 15 K for x ¼ 0.5 [6e10]) and FeTe1xSx (Tc ¼ 10 K for x ¼ 0.2 [11e16]). Because the SC phenomenon in this family of compounds is somehow related to the tetrahedral coordination of Fe atoms [3,5,17], some further modifications of Tc were obtained due to strain. Namely, the compressive biaxial (ab-plane) or uniaxial (c-axis) strain lead to the increase of Tc's in superconducting Fe(Se,Te) systems [18e21]. The tensile strain suppresses superconductivity of FeSe [22], whereas for FeTe such strain allows a manifestation of SC in its thin films [17].
* Corresponding author. E-mail address: [email protected] (M.J. Winiarski).
The electronic structure of superconducting iron chalcogenides has been investigated both experimentally [23e29] and theoretically [30e45]. The multi-gap SC in FeSe-based systems originates probably from the interband interactions between the holelike and electron like Fermi surface (FS) sheets, e.g. by means of antiferromagnetic (AFM) spin fluctuations [46,47], which are connected with the imperfect nesting q z (0.5,0.5) (2p/a), spanning these FS sheets in the iron chalcogenides [30,33,36e40]. Because such fluctuations in iron chalcogenides are closely related to the singlestripe AFM order, compounds with the double-stripe AFM order do not exhibit SC [48]. Furthermore, the magnetic ordering in Fe-based chalcogenides is connected with the chalcogen atom position in the unit cell (u.c.) [49] and can be tuned by specific strain [41]. Despite the fact that the doping with the Ru atoms arises the SC in iron pnictides [50e54], there are no experimental reports on Rudoped iron chalcogenides. Our recent first principles study [55] has suggested that such systems may be good candidates for novel superconducting Fe-based materials. Namely, they exhibit the same topology of FS with desirable nesting features. The magnetic
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Please cite this article in press as: M.J. Winiarski, et al., Strain effects on electronic structure of Fe0.75Ru0.25Te, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.11.024
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ground state of Fe1xRuxSe alloys is rather the single-stripe type, whereas in Fe1xRuxTe systems the double-stripe phase is preferred. Interestingly, the calculated values of stabilization energy for both orders in Fe0.75Ru0.25Te are close to each other, whereas only the double-stripe phase was predicted as energetically favorable for systems with higher Ru contents. It is a well-known fact that the density functional theory (DFT) calculations poorly reproduce structural parameters for strongly anisotropic systems with a layered structure. This issue has been extensively discussed for ferropnictides [56]. Interestingly, the careful examination of numerous DFT-based results for FeSe and FeTe [55] led to the conclusion that the pseudopotentials in the standard LDA approach yields reasonable structural results when compared to experimental data and results of calculations beyond the standard parameterizations of exchange-correlation energy, i.e. with the van der Waals interaction corrections included [57,58]. In this work, structural and electronic modifications by the substitution of Fe with Ru atoms in the parent FeTe compound are discussed based on our DFT results for Fe0.75Ru0.25Te. Because the previous theoretical predictions for RuTe [55] have suggested that this hypothetical compound exhibits the same FS topology as those of FeSe or FeTe, the transition between the single- and doublestripe AFM orders in the Fe0.75Ru0.25Te solid solution may arise spin-fluctuation mediated SC in this system, analogously to the case of superconducting FeTe [41]. Some effects connected with the isotropic and ab-plane strain on the magnetic ground state of Fe0.75Ru0.25Te are carefully investigated. 2. Computational methods Band structure calculations for Fe1xRuxTe systems have been carried out in the framework of DFT, i.e. with the Abinit package [59], using the norm-conserving pseudopotentials, generated with APE software [60]. The local density approximation (LDA) [61] of the exchange-correlation potential was employed. The 3d4s4p states for Fe and the 4d5s5p states for the Ru and Te pseudoatoms were selected as a valence-band basis. The 65 Ha energy cutoff for plane wave basis and the 6 6 6 k-point grids (shifted by (0.5,0.5,0.5)) were chosen. The total energy convergence of 1.0 108 Ha and the tolerance of the maximal force of 5.0 105 Ha were used. The nonmagnetic (NM) phase of Fe0.75Ru0.25Te and FeTe was simulated with the tetragonal supercells (8-atoms in 2a 2a c multiplication of the primitive u.c.) of the PbO-type whereas in spin-polarized calculations (LSDA) the (2a 2a c) and (2a a c) supercells were employed for single- (AFM1) and double-stripe (AFM2) antiferromagnetic orders. Equilibrium geometries of studied systems were obtained with the full optimization of lattice parameters and the free zTe position in the u.c. due to Hellmann-Feynman forces. Such a relaxation leads to the orthorhombic (AFM1) and monoclinic (AFM2) distortions of the u.c. The isotropic strain (hydrostatic pressure) was simulated with the full optimization of particular geometry of the u.c. for selected volume. The strain in ab-plane was simulated for an arbitrary set of lattice parameters a via the optimization of the free zTe position and lattice parameter c. Because several experimental studies for thin films of FeSexTe1x deposited on various substrates revealed a very small variation of the lattice parameter c in such systems [17e19,21,22], the tensile strain was also considered for the constant lattice parameter c, arbitrarily selected from the equilibrium of the tetragonal PbO-type u.c. The calculated values of the magnetic stabilization energy are related to the nonmagnetic phase. 3. Results and discussion The lattice parameters (a, b) and the 2c/(a þ b) ratio as well as
the average chalcogen atom height (〈hTe〉) calculated for the tetragonal NM and monoclinic AFM2 phases of Fe0.75Ru0.25Te and FeTe, compared to the available experimental data for FeTe, are gathered in Table 1. The agreement between the obtained structural parameters and the experimental data for the parent FeTe compound is satisfactory, particularly in the AFM2 phase in which the 2c/(a þ b) ratio and 〈hTe〉 are only slightly overestimated by the LDA pseudopotential approach used here. As one can expect, the substitution of Fe with Ru atoms in the parent FeTe compound leads to an increase of the u.c. volume, however, the lattice parameters a and b predicted for Fe0.75Ru0.25Te are closer to each other when compared to those of the former compound. Therefore, the anisotropy of u.c. in AFM2 phase in Ru-doped system is somehow reduced. It is worth noting that an incorporation of 25 atomic % of Ru into FeTe host system only slightly changed its 2c/(a þ b) and 〈hTe〉, being important for stabilization of the AFM2 ground state [41,49]. The AFM2 phase predicted for Fe0.75Ru0.25Te exhibits the magnetic stabilization energy of 97 meV, being lower than that of the parent FeTe compound (120 meV). The calculated DOSs of Fe0.75Ru0.25Te and FeTe in the NM, AFM1, and AFM2 phase are presented in Fig. 1. The overall shapes of the total DOSs below the Fermi level (EF) in the NM phase are similar in both studied systems, being dominated by the Fe 3d states in the energy range down to 2 eV where the contributions of the Te 5p states become important on equal footing. As seen in Fig. 1 (b), the presented here DOS plots for FeTe are in very good agreement with some earlier theoretical results [30,41,55]. Interestingly, in Fe0.75Ru0.25Te the Ru 4d states form relatively flat maxima in a wide range of binding energy. A specific shape of DOS was revealed in this system in the vicinity of EF, since the EF is located in the valley that is anomalously deep when compared with other iron chalcogenides in the NM phase [39,42], including RuSe and RuTe [55]. Therefore, the solid solution Fe0.75Ru0.25Te, containing 25 atomic % of Ru, may exhibit essentially different electronic structure than the parent FeTe system. Such a feature has not been revealed in DOS plots for the AFM1 and AFM2 phases, depicted in Fig. 1 (c,e), and may reflect an unstable character of the NM phase of Fe0.75Ru0.25Te. Furthermore, a little shift of EF to a relatively higher value of energy, e.g. caused by strain, may restore the shape of DOS similar to that of FeTe. Because the main aim of this work is a study of a magnetic ground state of Fe0.75Ru0.25Te for a wide range of structural parameters, the issue of the valley in DOS plot for the NM phase of this system will not be further discussed. As presented in Fig. 1 (e) and (f) the overall shapes of the total DOSs in the AFM2 phase for both studied systems are substantially different from those of the NM phase. Interestingly, in Fe0.75Ru0.25Te the contributions of the Fe 3d states, which are shifted to the higher binding energies, form less distinctive peaks. In turn, in FeTe the DOS in the vicinity of EF in the AFM2 phase, as depicted in Fig. 1 (f), remains sharp. It is worth noting that in this system the Fermi level is located at the local maximum of DOS, which was also
Table 1 Lattice parameters a and b, 2c/(a þ b) ratio, and average tellurium anion height, 〈hTe〉, in fully optimized u.c. of the nonmagnetic PbO-type (tetragonal) and double-stripe (AFM2, monoclinic) phases calculated for Fe0.75Ru0.25Te and FeTe.
Fe0.75Ru0.25Te LDA (NM) LSDA (AFM2) FeTe LDA (NM) LSDA (AFM2) Ref. [62] exp.
a (Å)
b (Å)
2c/(a þ b)
〈hTe〉 (Å)
3.788 3.766
3.788 3.710
1.568 1.630
1.660 1.804
3.701 3.730 3.831
3.701 3.617 3.783
1.599 1.660 1.645
1.666 1.775 1.747
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The presence of Ru atoms in Fe0.75Ru0.25Te cause a slight decrease of the chalcogen atom height when compared with that of the FeTe parent compound. Therefore, the structural modifications related to the substitution of Fe with Ru atoms in FeTe are different than those connected with the substitution with the Te atom into the Se site in FeSe [64e66]. Meantime, the substitution with S atoms into Te sites in FeTe leads to a significant decrease of 〈hTe〉 and as a result to magnetic transition from AFM2 to AFM1 phases [42]. These effects are connected with the differences between ionic radii of particular atoms and strong anisotropy of the crystal structure of iron chalcogenides, i.e. rather weak bonding between Fe-Te layers. The dependence of structural parameters for the tetragonal NM, orthorhombic AFM1, and monoclinic AFM2 phases of Fe0.75Ru0.25Te as a function of the u.c. volume is presented in Fig. 2. In such a layered structure an increase of the u.c. volume leads to a nonlinear increase of the ab-plane lattice parameters and an enhancement of anisotropy expressed in the 2c/(a þ b) ratio. Interestingly, these simultaneous effects influence only slightly the chalcogen atom height. It is worth noting that the predicted 〈hTe〉 for Fe0.75Ru0.25Te in AFM1 and AFM2 phases are close to each other whereas that of NM one is significantly lower. As presented in Fig. 3, in Fe0.75Ru0.25Te the AFM2 phase is more favorable due to slightly higher stabilization energy. Therefore, the possible AFM1 and AFM2 ground states are almost degenerate. The difference in stabilization energy between AFM2 and AFM1 phases depends somehow on the volume of u.c., decreasing almost linearly under compressive strain. One can consider that the u.c. volume of 78 Å3 corresponds to the hydrostatic pressure of about 6.5 GPa. Because the relatively low hydrostatic pressure of 2 GPa leads to a ferromagnetic phase of FeTe [67], the ferromagnetic ordering could also be expected in compressively strained Fe0.75Ru0.25Te systems,
Fig. 1. The total and orbital projected electronic DOS (LDA) for (a,c,e) Fe0.75Ru0.25Te and (b,d,f) FeTe in nonmagnetic (NM), single- (AMF1), and double-stripe (AFM2) phases, respectively.
reported in the previous DFT-based investigations with the GGA approach [32]. The values of DOS at the Fermi level N(EF) ¼ 2.19 and 2.17 states/ eV/f.u. obtained for FeTe in the NM and AFM2 phases, respectively, are slightly higher than N(EF) ¼ 2.14 states/eV/f.u [55]. calculated with the full-potential code for the same structural parameters. In Fe0.75Ru0.25Te the lower values of predicted N(EF), 0.89 and 0.91 states/eV/f.u. for both studied phases, suggest a weaker metallic character than that of FeTe. A diminishing of N(EF) in Ru-based based compounds when compared to that of Fe-based systems is exhibited by chalcogenides [55] and other intermetallics, e.g. Lu2 Ru3 Si5 [63]. In the hypothetical AFM1 phase, that is not a ground state of these systems in equilibrium conditions, the DOS in the vicinity of EF in Fe0.75Ru0.25Te is higher than that of FeTe (see Fig. 1 (c,d)). The incorporation of Ru atoms into the FeTe host system might lead to an increase of N(EF) (from 2.87 to 3.16 states/eV/f.u.), which is an opposite effect to that revealed in the AFM2 phase.
Fig. 2. Structural parameters (a) a*, (b) c/a*, (c) 〈hTe〉 calculated for unit cell of Fe0.75Ru0.25Te as a function of volume of nonmagnetic (NM), single- (AFM1) and double- (AFM2) stripe AFM phases. Note, that for AFM2 a* denotes (aþb)/2 while c/a* denotes 2c/(aþb). The equilibrium structural parameters for NM and AFM2 phases are marked with solid symbols.
Please cite this article in press as: M.J. Winiarski, et al., Strain effects on electronic structure of Fe0.75Ru0.25Te, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.11.024
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Fig. 3. Energy gain per formula unit (f.u.) for transition between the double- (AFM2) and single- (AFM1) stripe antiferromagnetic orders calculated for Fe0.75Ru0.25Te as a function of the unit cell volume. The equilibrium volume for AFM2 phase is marked with a solid vertical line.
although the pressure required for the occurrence of such a spin order is predicted here to be relatively higher. Meantime, the recent DFT-based investigations suggested quite complex magnetic and structural phase diagrams for the parent FeTe system [68]. This issue for Fe0.75Ru0.25Te requires some further investigations, which are beyond the scope of the present work. The values of chalcogen atom height calculated for Fe0.75Ru0.25Te in NM, AFM1 and AFM2 phases and magnetic stabilization energies (AFM1 and AFM2, vs. NM) as a function of the lattice parameters of tetragonal u.c., arbitrarily selected to simulate the abplane strain, are presented in Fig. 4. As depicted in Fig. 4 (a), the increase of the lattice parameter a causes the approximately linear decrease of 〈hTe〉, similarly to the case of the parent FeTe compound [41]. The obtained values of 〈hTe〉 in AFM2 phase are higher than those of the AFM1 state. This difference is even enhanced with the increase of tensile strain. However, a careful examination of particular atomic positions in the u.c. of Fe0.75Ru0.25Te, presented in Fig. 4 (b), indicates that in the AFM2 phase the chalcogen atoms in the vicinity of Ru atoms adopt more distant positions from the host Fe-Te layer than those bounded with Fe atoms. As a result of the chemical pressure related to the incorporation of Ru atoms, in the Fe-Te region of the u.c. in this phase the hTe is lower than that of the AFM1 phase. Despite that such an effect does not affect the magnetic ground state of Fe0.75Ru0.25Te, being AFM2, it may explain why the values of stabilization energy calculated for both orders (AFM1,AFM2) in this system are so close to each other. Furthermore, as presented in Fig. 4 (c), the AFM1 order is more preferable for lattice parameters bigger than z 4 Å (hTe < 1.62 Å). A similar effect was reported for FeTe for the relatively lower lattice parameters, a > 3.87 Å and hTe < 1.72 Å [41]. Interestingly, as presented in Fig. 5, the energy gain owing to a switch between AFM1 and AFM2 orders is very low for the whole range of studied strain in the ab-plane, both of compressive and tensile types. Furthermore, the lattice parameter a required for the transition between AFM2 and AFM1 orders is almost independent of the lattice parameter c, i.e., the simulation of tensile strain with an arbitrarily chosen constant c leads to the same conclusions as calculations employing the optimization of c. Only the particular values of stabilization energy are relatively lower in the former case due to the lack of the full relaxation of forces in the u.c. It is worth mentioning that the enhanced electron-electron correlations between the Fe 3d states in iron chalcogenides are driven by Hund's rule coupling [69,70]. Because in FeSe the electron doping restores the Fermi-liquid behavior, whereas hole doping
Fig. 4. Calculated (a) 〈hTe〉 for nonmagnetic (NM), single- (AFM1) and double- (AFM2) stripe antiferromagnetic orders for Fe0.75Ru0.25Te, (b) selected maximal and minimal (in the vicinity of Ru and Fe atom) hTe for AFM2 phase compared with the corresponding hTe of AFM1 phase, and (c) magnetic stabilization energy (vs. nonmagnetic) for various values of a for the PbO-type u.c., simulating the ab-plane strain in thin films deposited on tetragonal or cubic substrates. The equilibrium cell parameter a is marked with a solid vertical line.
Fig. 5. Energy gain per f.u. for transition between the double- (AFM2) and single(AFM1) stripe antiferromagnetic orders calculated for Fe0.75Ru0.25Te for fully relaxed (triangles) and constant equilibrium for the PbO-type unit cell (circles) cell parameter c as a function of lattice parameter a, simulating the ab-plane strain in thin films deposited on tetragonal or cubic substrates. The equilibrium cell parameter a is marked with a solid vertical line.
enhances strange-metallic properties [71], one can expect that the substitution of Fe atoms with Ru atoms in iron chalcogenides should lead to the decrease of Hund's coupling and the diminishing of magnetic interactions. This fact explains the relatively lower magnetic stabilization energies for Fe1xRuxTe alloys than those of the parent FeTe compound as well as the relatively lower hTe
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Acknowledgments This work was supported by the National Science Center of Poland under grants No. 2013/08/M/ST3/00927. Calculations were partially performed on ICM supercomputers of Warsaw University (Grant No. G46-13) and in Wroclaw Center for Networking and Supercomputing (Project No. 158). References
Fig. 6. The total and orbital projected electronic DOS (LDA) for Fe0.75Ru0.25Te in nonmagnetic phase for (a) equilibrium and (b) tensile strained cell parameters.
required for the stabilization of the AFM1 phase. Furthermore, as depicted in Fig. 6, tensile strain that should induce the transition between AFM2 and AFM1 spin arrangements (related to lattice parameters a > 4 Å) changes substantially the overall shape of DOS for Fe1xRuxTe. In comparison with the case of the equilibrium structural parameters in the NM phase, the contributions of the Fe 3d states in the tensile-strained system are more localized, forming one broad peak with higher intensity. Interestingly, the discussed earlier valley of the total DOS at EF remains unchanged, leading to almost equal N(EF) ¼ 0.87 states/eV/ f.u. to that predicted for the unstrained case of Fe1xRuxTe. Such a low value of N(EF) indicates that SC in Fe1xRuxTe systems may be significantly diminished when compared to that of FeTe. However, it is worth noting that any clear correlation between N(EF) and Tc's of FeSexTe1x superconductors was not revealed from previous studies of their electronic structure [35,37,38,40].
4. Conclusions Structural, magnetic, and electronic properties of Fe0.75Ru0.25Te and FeTe have been studied by ab initio calculations. Because the incorporation of Ru atoms into FeTe host system modifies insignificantly the chalcogen atom height, both studied systems exhibit the double-stripe antiferromagnetic ground state. However, the calculated (LDA) magnetic stabilization energy of the single-stripe phase for Fe0.75Ru0.25Te is close to that of the above ground state. The single-stripe phase in this alloy may be induced by tensile strain, e.g. in thin films on tetragonal or cubic substrates with abplane lattice parameters exceeding 4 Å, analogously to the strained thin films of FeTe [17]. The thin films of various iron chalcogenides are extensively investigated [72], therefore the Ru-doped FeTe systems may be good candidates for new superconducting Febased materials.
[1] F.C. Hsu, J.Y. Luo, K.W. Yeh, T.K. Chen, T.W. Huang, P.M. Wu, Y.C. Lee, Y.L. Huang, Y.Y. Chu, D.C. Yan, M.K. Wu, Proc. Natl. Acad. Sci. U. S. A. 105 (2008) 14262e14264. [2] Y. Mizuguchi, F. Tomioka, S. Tsuda, T. Yamaguchi, Y. Takano, Appl. Phys. Lett. 93 (2008) 152505. [3] S. Medvedev, T.M. McQueen, I. Trojan, T. Palasyuk, M.I. Eremets, R.J. Cava, S. Naghavi, F. Casper, V. Ksenofontov, G. Wortmann, C. Felser, Nat. Mater. 8 (2009) 630e633. [4] S. Margadonna, Y. Takabayashi, Y. Ohishi, Y. Mizuguchi, Y. Takano, T. Kagayama, T. Nakagawa, M. Takata, K. Prassides, Phys. Rev. B 80 (2009) 064506. [5] H. Okabe, N. Takeshita, K. Horigane, T. Muranaka, J. Akimitsu, Phys. Rev. B 81 (2010) 205119. [6] K.W. Yeh, T.W. Huang, Y.L. Huang, T.K. Chen, F.C. Hsu, P.M. Wu, Y.C. Lee, Y.Y. Chu, C.L. Chen, J.Y. Luo, D.C. Yan, M.K. Wu, Europhys. Lett. 84 (2008) 37002. [7] M.H. Fang, H.M. Pham, B. Qian, T.J. Liu, E.K. Vehstedt, Y. Liu, L. Spinu, Z.Q. Mao, Phys. Rev. B 78 (2008) 224503. [8] D.J. Gawryluk, J. Fink-Finowicki, A. Wisniewski, R. Pu zniak, V. Domukhovski, R. Diduszko, M. Kozłowski, M. Berkowski, Supercond. Sci. Technol. 24 (2011) 065011. [9] V. Fedorchenko, G.E. Grechnev, V.A. Desnenko, A.S. Panfilov, S.L. Gnatchenko, V.V. Tsurkan, J. Deisenhofer, H.-A. Krug von Nidda, A. Loidl, D.A. Chareev, O.S. Volkova, A.N. Vasiliev, Low. Temp. Phys. 37 (2011) 83e89. [10] S. Panfilov, V.A. Pashchenko, G.E. Grechnev, V.A. Desnenko, A.V. Fedorchenko, A.N. Bludov, S.L. Gnatchenko, D.A. Chareev, E.S. Mitrofanov, A.N. Vasiliev, Low. Temp. Phys. 40 (2014) 615e620. [11] Y. Mizuguchi, F. Tomioka, S. Tsuda, T. Yamaguchi, Y. Takano, Appl. Phys. Lett. 94 (2009) 012503. [12] Y. Mizuguchi, Y. Hara, K. Deguchi, S. Tsuda, T. Yamaguchi, K. Takeda, H. Kotegawa, H. Tou, Y. Takano, Supercond. Sci. Technol. 23 (2010) 054013. [13] R. Hu, E.S. Bozin, J.B. Warren, C. Petrovic, Phys. Rev. B 80 (2009) 214514. [14] P. Zajdel, P.Y. Hsieh, E.E. Rodriguez, N.P. Butch, J.D. Magill, J. Paglione, P. Zavalij, M.R. Suchomel, M.A. Green, J. Am. Chem. Soc. 132 (2010) 13000e13007. [15] V.P.S. Awana, A. Pal, A. Vajpayee, B. Gahtori, H. Kishan, Phys. C 471 (2011) 77e82. [16] C. Dong, H. Wang, Q. Mao, R. Khan, X. Zhou, C. Li, J. Yang, B. Chen, M. Fang, J. Phys. Condens. Matter 25 (2013) 385701. [17] Y. Han, W.Y. Li, L.X. Cao, X.Y. Wang, B. Xu, B.R. Zhao, Y.Q. Guo, J.L. Yang, Phys. Rev. Lett. 104 (2010) 017003. [18] S.X. Huang, C.L. Chien, V. Thampy, C. Broholm, Phys. Rev. Lett. 104 (2010) 217002. , M.R. Cimberle, [19] E. Bellingeri, I. Pallecchi, R. Buzio, A. Gerbi, D. Marre M. Tropeano, M. Putti, A. Palenzona, C. Ferdeghini, Appl. Phys. Lett. 96 (2010) 102512. [20] E. Bellingeri, S. Kawale, V. Braccini, R. Buzio, A. Gerbi, A. Martinelli, M. Putti, I. Pallecchi, G. Balestrino, A. Tebano, C. Ferdeghini, Supercond. Sci. Technol. 25 (2012) 084022. [21] W.Z. Si, Z.W. Lin, Q. Jie, W.G. Yin, J. Zhou, G. Gu, P.D. Johnson, Q. Li, Appl. Phys. Lett. 95 (2009) 052504. [22] Y.F. Nie, E. Brahimi, J.I. Budnick, W.A. Hines, M. Jain, B. Wells, Appl. Phys. Lett. 94 (2009) 242505. [23] F. Chen, et al., Phys. Rev. B 81 (2010) 014526. [24] K. Nakayama, et al., Phys. Rev. Lett. 105 (2010) 197001. [25] A. Tamai, et al., Phys. Rev. Lett. 104 (2010) 097002. [26] H. Miao, et al., Phys. Rev. B 85 (2012) 094506. [27] I. Perez, J.A. McLeod, R.J. Green, R. Escamilla, V. Ortiz, A. Moewes, Phys. Rev. B 90 (2014) 014510. [28] I. Perez, J.A. McLeod, R.J. Green, R. Escamilla, V. Ortiz, A. Moewes, J. Phys. Chem. C 118 (2014) 25150e25157. [29] J. Maletz, et al., Phys. Rev. B 89 (2014) 220506(R). [30] A. Subedi, L. Zhang, D.J. Singh, M.H. Du, Phys. Rev. B 78 (2008) 134514. [31] Y. Ding, Y. Wang, J. Ni, Solid State Commun. 149 (2009) 505e509. [32] F. Ma, W. Ji, J. Hu, Z.Y. Lu, T. Xiang, Phys. Rev. Lett. 102 (2009) 177003. [33] P.P. Singh, J. Phys. Condens. Matter 22 (2010) 135501. [34] S. Chadov, D. Scharf, G.H. Fecher, C. Felser, L. Zhang, D.J. Singh, Phys. Rev. B 81 (2010) 104523. [35] H. Shi, Z.B. Huang, J.S. Tse, H.Q. Lin, J. Appl. Phys. 110 (2011) 043917. [36] A. Ciechan, M.J. Winiarski, M. Samsel-Czekała, Acta Phys. Pol. A 121 (2012) 820e823. [37] M.J. Winiarski, M. Samsel-Czekała, A. Ciechan, Europhys. Lett. 100 (2012)
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47005. [38] M.J. Winiarski, M. Samsel-Czekała, A. Ciechan, J. Alloys Compd. 566 (2013) 187e190. [39] A. Ciechan, M.J. Winiarski, M. Samsel-Czekała, Intermetallics 41 (2013) 44e50. [40] M.J. Winiarski, M. Samsel-Czekała, A. Ciechan, Intermetallics 52 (2014) 97e100. [41] A. Ciechan, M.J. Winiarski, M. Samsel-Czekała, J. Phys. Condens. Matter 26 (2014) 025702. [42] A. Ciechan, M.J. Winiarski, M. Samsel-Czekała, J. Alloys Compd. 630 (2015) 100e105. [43] S. Kumar, P.P. Singh, Solid State Commun. 220 (2015) 49e56. [44] S. Kumar, P.P. Singh, Mater. Res. Express 3 (2016) 056001. [45] S. Kumar, P.P. Singh, J. Alloys Compd. 658 (2016) 1041e1059. [46] T. Imai, K. Ahilan, F.L. Ning, T.M. McQueen, R.J. Cava, Phys. Rev. Lett. 102 (2009) 177005. [47] Y. Qiu, et al., Phys. Rev. Lett. 103 (2009) 067008. [48] T.J. Liu, et al., Nat. Mater. 9 (2010) 716e720. [49] C.Y. Moon, H.J. Choi, Phys. Rev. Lett. 104 (2010) 057003. [50] S. Sharma, A. Bharathi, S. Chandra, V.R. Reddy, S. Paulraj, A.T. Satya, V.S. Sastry, A. Gupta, C.S. Sundar, Phys. Rev. B 81 (2010) 174512. [51] V. Brouet, et al., Phys. Rev. Lett. 105 (2010) 087001. [52] R.S. Dhaka, et al., Phys. Rev. Lett. 107 (2011) 267002. [53] N. Xu, et al., Phys. Rev. B 86 (2012) 064505. [54] L. Ma, G.F. Ji, J. Dai, X.R. Lu, M.J. Eom, J.S. Kim, B. Normand, W. Yu, Phys. Rev. Lett. 109 (2012) 197002. [55] M.J. Winiarski, M. Samsel-Czekała, A. Ciechan, J. Appl. Phys. 116 (2014) 223903.
[56] I.I. Mazin, M.D. Johannes, L. Boeri, K. Koepernik, D.J. Singh, Phys. Rev. B 78 (2008) 085104. [57] F. Ricci, G. Profeta, Phys. Rev. B 87 (2013) 184105. [58] Q.Q. Ye, K. Liu, Z.Y. Lu, Phys. Rev. B 88 (2013) 205130. [59] X. Gonze, et al., Comput. Phys. Commun. 180 (2009) 2582e2615. [60] M. Oliveira, F. Nogueira, Comput. Phys. Commun. 178 (2008) 524e534. [61] J.P. Perdew, Y. Wang, Phys. Rev. B 45 (1992) 13244. [62] A. Martinelli, A. Palenzona, M. Tropeano, C. Ferdeghini, M. Putti, M.R. Cimberle, T.D. Nguyen, M. Affronte, C. Ritter, Phys. Rev. B 81 (2010) 094115. [63] M. Samsel-Czekała, M.J. Winiarski, Intermetallics 31 (2012) 186e190. [64] D. Louca, K. Horigane, A. Llobet, R. Arita, S. Ji, N. Katayama, S. Konbu, K. Nakamura, T.Y. Koo, P. Tong, K. Yamada, Phys. Rev. B 81 (2010) 134524. [65] B. Joseph, A. Iadecola, A. Puri, L. Simonelli, Y. Mizuguchi, Y. Takano, N.L. Saini, Phys. Rev. B 82 (2010) 020502(R). [66] F. Caglieris, F. Ricci, G. Lamura, A. Martinelli, A. Palenzona, I. Pallecchi, A. Sala, G. Profeta, M. Putti, Sci. Technol. Adv. Mater. 13 (2012) 054402. [67] M. Bendele, A. Maisuradze, B. Roessli, S.N. Gvasaliya, E. Pomjakushina, S. Weyeneth, K. Conder, H. Keller, R. Khasanov, Phys. Rev. B 87 (2013) 060409(R). [68] M. Monni, F. Bernardini, G. Profeta, S. Massidda, Phys. Rev. B 87 (2013) 094516. [69] M.D. Johannes, I.I. Mazin, Phys. Rev. B 79 (2009) 220510(R). [70] M. Aichhorn, S. Biermann, T. Miyake, A. Georges, M. Imada, Phys. Rev. B 82 (2010) 064504. [71] A. Liebsch, H. Ishida, Phys. Rev. B 82 (2010) 155106. [72] P. Mele, Sci. Technol. Adv. Mater. 13 (2012) 054301.
Please cite this article in press as: M.J. Winiarski, et al., Strain effects on electronic structure of Fe0.75Ru0.25Te, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.11.024
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Strain effects on electronic structure of Fe0.75Ru0.25Te M.J. Winiarski a, *, M. Samsel-Czekała a, A. Ciechan b a b
lna 2, 50-422, Wrocław, Poland Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Oko w 32/46, 02-668, Warsaw, Poland Institute of Physics, Polish Academy of Sciences, al. Lotniko
h i g h l i g h t s Ru-doped FeTe systems are investigated by density-functional theory methods. Structural and electronic properties of Fe0.75Ru0.25Te and parent FeTe are studied. The double-stripe antiferromagnetic ground state is predicted for both systems. The single-stripe antiferromagnetic phase may be induced by tensile strain. Tensile strained Fe0.75Ru0.25Te is a candidate for a new Fe-based superconductor.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 September 2016 Received in revised form 3 November 2016 Accepted 13 November 2016 Available online xxx
Structural and electronic properties of a hypothetical Fe0.75Ru0.25Te alloy and the parent FeTe compound have been investigated from first principles within the density functional theory (DFT). For both systems the double-stripe antiferromagnetic ground state is predicted at ambient pressure. The incorporation of Ru atoms into FeTe in the nonmagnetic phase leads to a deep valley of density of states in the vicinity of the Fermi level and the DOS at the Fermi level is significantly diminished in the considered solid solution. The single-stripe antiferromagnetic phase in Fe0.75Ru0.25Te may be induced by tensile strain. These findings suggest that strained thin films of Fe1xRuxTe are good candidates for new superconducting Febased materials. © 2016 Elsevier B.V. All rights reserved.
Keywords: superconductors Electronic structure Crystal structure Deformation
1. Introduction Despite the relatively low superconducting critical temperature (Tc) of 8 K [1] exhibited by bulk FeSe in equilibrium conditions, a rapid increase of Tc up to 37 K [2e5] was revealed in this system under hydrostatic pressure. Superconductivity (SC) in iron chalcogenides was also reported for solid solutions FeSe1xTex (with the maximum Tc ¼ 15 K for x ¼ 0.5 [6e10]) and FeTe1xSx (Tc ¼ 10 K for x ¼ 0.2 [11e16]). Because the SC phenomenon in this family of compounds is somehow related to the tetrahedral coordination of Fe atoms [3,5,17], some further modifications of Tc were obtained due to strain. Namely, the compressive biaxial (ab-plane) or uniaxial (c-axis) strain lead to the increase of Tc's in superconducting Fe(Se,Te) systems [18e21]. The tensile strain suppresses superconductivity of FeSe [22], whereas for FeTe such strain allows a manifestation of SC in its thin films [17].
* Corresponding author. E-mail address: [email protected] (M.J. Winiarski).
The electronic structure of superconducting iron chalcogenides has been investigated both experimentally [23e29] and theoretically [30e45]. The multi-gap SC in FeSe-based systems originates probably from the interband interactions between the holelike and electron like Fermi surface (FS) sheets, e.g. by means of antiferromagnetic (AFM) spin fluctuations [46,47], which are connected with the imperfect nesting q z (0.5,0.5) (2p/a), spanning these FS sheets in the iron chalcogenides [30,33,36e40]. Because such fluctuations in iron chalcogenides are closely related to the singlestripe AFM order, compounds with the double-stripe AFM order do not exhibit SC [48]. Furthermore, the magnetic ordering in Fe-based chalcogenides is connected with the chalcogen atom position in the unit cell (u.c.) [49] and can be tuned by specific strain [41]. Despite the fact that the doping with the Ru atoms arises the SC in iron pnictides [50e54], there are no experimental reports on Rudoped iron chalcogenides. Our recent first principles study [55] has suggested that such systems may be good candidates for novel superconducting Fe-based materials. Namely, they exhibit the same topology of FS with desirable nesting features. The magnetic
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Please cite this article in press as: M.J. Winiarski, et al., Strain effects on electronic structure of Fe0.75Ru0.25Te, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.11.024
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ground state of Fe1xRuxSe alloys is rather the single-stripe type, whereas in Fe1xRuxTe systems the double-stripe phase is preferred. Interestingly, the calculated values of stabilization energy for both orders in Fe0.75Ru0.25Te are close to each other, whereas only the double-stripe phase was predicted as energetically favorable for systems with higher Ru contents. It is a well-known fact that the density functional theory (DFT) calculations poorly reproduce structural parameters for strongly anisotropic systems with a layered structure. This issue has been extensively discussed for ferropnictides [56]. Interestingly, the careful examination of numerous DFT-based results for FeSe and FeTe [55] led to the conclusion that the pseudopotentials in the standard LDA approach yields reasonable structural results when compared to experimental data and results of calculations beyond the standard parameterizations of exchange-correlation energy, i.e. with the van der Waals interaction corrections included [57,58]. In this work, structural and electronic modifications by the substitution of Fe with Ru atoms in the parent FeTe compound are discussed based on our DFT results for Fe0.75Ru0.25Te. Because the previous theoretical predictions for RuTe [55] have suggested that this hypothetical compound exhibits the same FS topology as those of FeSe or FeTe, the transition between the single- and doublestripe AFM orders in the Fe0.75Ru0.25Te solid solution may arise spin-fluctuation mediated SC in this system, analogously to the case of superconducting FeTe [41]. Some effects connected with the isotropic and ab-plane strain on the magnetic ground state of Fe0.75Ru0.25Te are carefully investigated. 2. Computational methods Band structure calculations for Fe1xRuxTe systems have been carried out in the framework of DFT, i.e. with the Abinit package [59], using the norm-conserving pseudopotentials, generated with APE software [60]. The local density approximation (LDA) [61] of the exchange-correlation potential was employed. The 3d4s4p states for Fe and the 4d5s5p states for the Ru and Te pseudoatoms were selected as a valence-band basis. The 65 Ha energy cutoff for plane wave basis and the 6 6 6 k-point grids (shifted by (0.5,0.5,0.5)) were chosen. The total energy convergence of 1.0 108 Ha and the tolerance of the maximal force of 5.0 105 Ha were used. The nonmagnetic (NM) phase of Fe0.75Ru0.25Te and FeTe was simulated with the tetragonal supercells (8-atoms in 2a 2a c multiplication of the primitive u.c.) of the PbO-type whereas in spin-polarized calculations (LSDA) the (2a 2a c) and (2a a c) supercells were employed for single- (AFM1) and double-stripe (AFM2) antiferromagnetic orders. Equilibrium geometries of studied systems were obtained with the full optimization of lattice parameters and the free zTe position in the u.c. due to Hellmann-Feynman forces. Such a relaxation leads to the orthorhombic (AFM1) and monoclinic (AFM2) distortions of the u.c. The isotropic strain (hydrostatic pressure) was simulated with the full optimization of particular geometry of the u.c. for selected volume. The strain in ab-plane was simulated for an arbitrary set of lattice parameters a via the optimization of the free zTe position and lattice parameter c. Because several experimental studies for thin films of FeSexTe1x deposited on various substrates revealed a very small variation of the lattice parameter c in such systems [17e19,21,22], the tensile strain was also considered for the constant lattice parameter c, arbitrarily selected from the equilibrium of the tetragonal PbO-type u.c. The calculated values of the magnetic stabilization energy are related to the nonmagnetic phase. 3. Results and discussion The lattice parameters (a, b) and the 2c/(a þ b) ratio as well as
the average chalcogen atom height (〈hTe〉) calculated for the tetragonal NM and monoclinic AFM2 phases of Fe0.75Ru0.25Te and FeTe, compared to the available experimental data for FeTe, are gathered in Table 1. The agreement between the obtained structural parameters and the experimental data for the parent FeTe compound is satisfactory, particularly in the AFM2 phase in which the 2c/(a þ b) ratio and 〈hTe〉 are only slightly overestimated by the LDA pseudopotential approach used here. As one can expect, the substitution of Fe with Ru atoms in the parent FeTe compound leads to an increase of the u.c. volume, however, the lattice parameters a and b predicted for Fe0.75Ru0.25Te are closer to each other when compared to those of the former compound. Therefore, the anisotropy of u.c. in AFM2 phase in Ru-doped system is somehow reduced. It is worth noting that an incorporation of 25 atomic % of Ru into FeTe host system only slightly changed its 2c/(a þ b) and 〈hTe〉, being important for stabilization of the AFM2 ground state [41,49]. The AFM2 phase predicted for Fe0.75Ru0.25Te exhibits the magnetic stabilization energy of 97 meV, being lower than that of the parent FeTe compound (120 meV). The calculated DOSs of Fe0.75Ru0.25Te and FeTe in the NM, AFM1, and AFM2 phase are presented in Fig. 1. The overall shapes of the total DOSs below the Fermi level (EF) in the NM phase are similar in both studied systems, being dominated by the Fe 3d states in the energy range down to 2 eV where the contributions of the Te 5p states become important on equal footing. As seen in Fig. 1 (b), the presented here DOS plots for FeTe are in very good agreement with some earlier theoretical results [30,41,55]. Interestingly, in Fe0.75Ru0.25Te the Ru 4d states form relatively flat maxima in a wide range of binding energy. A specific shape of DOS was revealed in this system in the vicinity of EF, since the EF is located in the valley that is anomalously deep when compared with other iron chalcogenides in the NM phase [39,42], including RuSe and RuTe [55]. Therefore, the solid solution Fe0.75Ru0.25Te, containing 25 atomic % of Ru, may exhibit essentially different electronic structure than the parent FeTe system. Such a feature has not been revealed in DOS plots for the AFM1 and AFM2 phases, depicted in Fig. 1 (c,e), and may reflect an unstable character of the NM phase of Fe0.75Ru0.25Te. Furthermore, a little shift of EF to a relatively higher value of energy, e.g. caused by strain, may restore the shape of DOS similar to that of FeTe. Because the main aim of this work is a study of a magnetic ground state of Fe0.75Ru0.25Te for a wide range of structural parameters, the issue of the valley in DOS plot for the NM phase of this system will not be further discussed. As presented in Fig. 1 (e) and (f) the overall shapes of the total DOSs in the AFM2 phase for both studied systems are substantially different from those of the NM phase. Interestingly, in Fe0.75Ru0.25Te the contributions of the Fe 3d states, which are shifted to the higher binding energies, form less distinctive peaks. In turn, in FeTe the DOS in the vicinity of EF in the AFM2 phase, as depicted in Fig. 1 (f), remains sharp. It is worth noting that in this system the Fermi level is located at the local maximum of DOS, which was also
Table 1 Lattice parameters a and b, 2c/(a þ b) ratio, and average tellurium anion height, 〈hTe〉, in fully optimized u.c. of the nonmagnetic PbO-type (tetragonal) and double-stripe (AFM2, monoclinic) phases calculated for Fe0.75Ru0.25Te and FeTe.
Fe0.75Ru0.25Te LDA (NM) LSDA (AFM2) FeTe LDA (NM) LSDA (AFM2) Ref. [62] exp.
a (Å)
b (Å)
2c/(a þ b)
〈hTe〉 (Å)
3.788 3.766
3.788 3.710
1.568 1.630
1.660 1.804
3.701 3.730 3.831
3.701 3.617 3.783
1.599 1.660 1.645
1.666 1.775 1.747
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The presence of Ru atoms in Fe0.75Ru0.25Te cause a slight decrease of the chalcogen atom height when compared with that of the FeTe parent compound. Therefore, the structural modifications related to the substitution of Fe with Ru atoms in FeTe are different than those connected with the substitution with the Te atom into the Se site in FeSe [64e66]. Meantime, the substitution with S atoms into Te sites in FeTe leads to a significant decrease of 〈hTe〉 and as a result to magnetic transition from AFM2 to AFM1 phases [42]. These effects are connected with the differences between ionic radii of particular atoms and strong anisotropy of the crystal structure of iron chalcogenides, i.e. rather weak bonding between Fe-Te layers. The dependence of structural parameters for the tetragonal NM, orthorhombic AFM1, and monoclinic AFM2 phases of Fe0.75Ru0.25Te as a function of the u.c. volume is presented in Fig. 2. In such a layered structure an increase of the u.c. volume leads to a nonlinear increase of the ab-plane lattice parameters and an enhancement of anisotropy expressed in the 2c/(a þ b) ratio. Interestingly, these simultaneous effects influence only slightly the chalcogen atom height. It is worth noting that the predicted 〈hTe〉 for Fe0.75Ru0.25Te in AFM1 and AFM2 phases are close to each other whereas that of NM one is significantly lower. As presented in Fig. 3, in Fe0.75Ru0.25Te the AFM2 phase is more favorable due to slightly higher stabilization energy. Therefore, the possible AFM1 and AFM2 ground states are almost degenerate. The difference in stabilization energy between AFM2 and AFM1 phases depends somehow on the volume of u.c., decreasing almost linearly under compressive strain. One can consider that the u.c. volume of 78 Å3 corresponds to the hydrostatic pressure of about 6.5 GPa. Because the relatively low hydrostatic pressure of 2 GPa leads to a ferromagnetic phase of FeTe [67], the ferromagnetic ordering could also be expected in compressively strained Fe0.75Ru0.25Te systems,
Fig. 1. The total and orbital projected electronic DOS (LDA) for (a,c,e) Fe0.75Ru0.25Te and (b,d,f) FeTe in nonmagnetic (NM), single- (AMF1), and double-stripe (AFM2) phases, respectively.
reported in the previous DFT-based investigations with the GGA approach [32]. The values of DOS at the Fermi level N(EF) ¼ 2.19 and 2.17 states/ eV/f.u. obtained for FeTe in the NM and AFM2 phases, respectively, are slightly higher than N(EF) ¼ 2.14 states/eV/f.u [55]. calculated with the full-potential code for the same structural parameters. In Fe0.75Ru0.25Te the lower values of predicted N(EF), 0.89 and 0.91 states/eV/f.u. for both studied phases, suggest a weaker metallic character than that of FeTe. A diminishing of N(EF) in Ru-based based compounds when compared to that of Fe-based systems is exhibited by chalcogenides [55] and other intermetallics, e.g. Lu2 Ru3 Si5 [63]. In the hypothetical AFM1 phase, that is not a ground state of these systems in equilibrium conditions, the DOS in the vicinity of EF in Fe0.75Ru0.25Te is higher than that of FeTe (see Fig. 1 (c,d)). The incorporation of Ru atoms into the FeTe host system might lead to an increase of N(EF) (from 2.87 to 3.16 states/eV/f.u.), which is an opposite effect to that revealed in the AFM2 phase.
Fig. 2. Structural parameters (a) a*, (b) c/a*, (c) 〈hTe〉 calculated for unit cell of Fe0.75Ru0.25Te as a function of volume of nonmagnetic (NM), single- (AFM1) and double- (AFM2) stripe AFM phases. Note, that for AFM2 a* denotes (aþb)/2 while c/a* denotes 2c/(aþb). The equilibrium structural parameters for NM and AFM2 phases are marked with solid symbols.
Please cite this article in press as: M.J. Winiarski, et al., Strain effects on electronic structure of Fe0.75Ru0.25Te, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.11.024
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Fig. 3. Energy gain per formula unit (f.u.) for transition between the double- (AFM2) and single- (AFM1) stripe antiferromagnetic orders calculated for Fe0.75Ru0.25Te as a function of the unit cell volume. The equilibrium volume for AFM2 phase is marked with a solid vertical line.
although the pressure required for the occurrence of such a spin order is predicted here to be relatively higher. Meantime, the recent DFT-based investigations suggested quite complex magnetic and structural phase diagrams for the parent FeTe system [68]. This issue for Fe0.75Ru0.25Te requires some further investigations, which are beyond the scope of the present work. The values of chalcogen atom height calculated for Fe0.75Ru0.25Te in NM, AFM1 and AFM2 phases and magnetic stabilization energies (AFM1 and AFM2, vs. NM) as a function of the lattice parameters of tetragonal u.c., arbitrarily selected to simulate the abplane strain, are presented in Fig. 4. As depicted in Fig. 4 (a), the increase of the lattice parameter a causes the approximately linear decrease of 〈hTe〉, similarly to the case of the parent FeTe compound [41]. The obtained values of 〈hTe〉 in AFM2 phase are higher than those of the AFM1 state. This difference is even enhanced with the increase of tensile strain. However, a careful examination of particular atomic positions in the u.c. of Fe0.75Ru0.25Te, presented in Fig. 4 (b), indicates that in the AFM2 phase the chalcogen atoms in the vicinity of Ru atoms adopt more distant positions from the host Fe-Te layer than those bounded with Fe atoms. As a result of the chemical pressure related to the incorporation of Ru atoms, in the Fe-Te region of the u.c. in this phase the hTe is lower than that of the AFM1 phase. Despite that such an effect does not affect the magnetic ground state of Fe0.75Ru0.25Te, being AFM2, it may explain why the values of stabilization energy calculated for both orders (AFM1,AFM2) in this system are so close to each other. Furthermore, as presented in Fig. 4 (c), the AFM1 order is more preferable for lattice parameters bigger than z 4 Å (hTe < 1.62 Å). A similar effect was reported for FeTe for the relatively lower lattice parameters, a > 3.87 Å and hTe < 1.72 Å [41]. Interestingly, as presented in Fig. 5, the energy gain owing to a switch between AFM1 and AFM2 orders is very low for the whole range of studied strain in the ab-plane, both of compressive and tensile types. Furthermore, the lattice parameter a required for the transition between AFM2 and AFM1 orders is almost independent of the lattice parameter c, i.e., the simulation of tensile strain with an arbitrarily chosen constant c leads to the same conclusions as calculations employing the optimization of c. Only the particular values of stabilization energy are relatively lower in the former case due to the lack of the full relaxation of forces in the u.c. It is worth mentioning that the enhanced electron-electron correlations between the Fe 3d states in iron chalcogenides are driven by Hund's rule coupling [69,70]. Because in FeSe the electron doping restores the Fermi-liquid behavior, whereas hole doping
Fig. 4. Calculated (a) 〈hTe〉 for nonmagnetic (NM), single- (AFM1) and double- (AFM2) stripe antiferromagnetic orders for Fe0.75Ru0.25Te, (b) selected maximal and minimal (in the vicinity of Ru and Fe atom) hTe for AFM2 phase compared with the corresponding hTe of AFM1 phase, and (c) magnetic stabilization energy (vs. nonmagnetic) for various values of a for the PbO-type u.c., simulating the ab-plane strain in thin films deposited on tetragonal or cubic substrates. The equilibrium cell parameter a is marked with a solid vertical line.
Fig. 5. Energy gain per f.u. for transition between the double- (AFM2) and single(AFM1) stripe antiferromagnetic orders calculated for Fe0.75Ru0.25Te for fully relaxed (triangles) and constant equilibrium for the PbO-type unit cell (circles) cell parameter c as a function of lattice parameter a, simulating the ab-plane strain in thin films deposited on tetragonal or cubic substrates. The equilibrium cell parameter a is marked with a solid vertical line.
enhances strange-metallic properties [71], one can expect that the substitution of Fe atoms with Ru atoms in iron chalcogenides should lead to the decrease of Hund's coupling and the diminishing of magnetic interactions. This fact explains the relatively lower magnetic stabilization energies for Fe1xRuxTe alloys than those of the parent FeTe compound as well as the relatively lower hTe
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Acknowledgments This work was supported by the National Science Center of Poland under grants No. 2013/08/M/ST3/00927. Calculations were partially performed on ICM supercomputers of Warsaw University (Grant No. G46-13) and in Wroclaw Center for Networking and Supercomputing (Project No. 158). References
Fig. 6. The total and orbital projected electronic DOS (LDA) for Fe0.75Ru0.25Te in nonmagnetic phase for (a) equilibrium and (b) tensile strained cell parameters.
required for the stabilization of the AFM1 phase. Furthermore, as depicted in Fig. 6, tensile strain that should induce the transition between AFM2 and AFM1 spin arrangements (related to lattice parameters a > 4 Å) changes substantially the overall shape of DOS for Fe1xRuxTe. In comparison with the case of the equilibrium structural parameters in the NM phase, the contributions of the Fe 3d states in the tensile-strained system are more localized, forming one broad peak with higher intensity. Interestingly, the discussed earlier valley of the total DOS at EF remains unchanged, leading to almost equal N(EF) ¼ 0.87 states/eV/ f.u. to that predicted for the unstrained case of Fe1xRuxTe. Such a low value of N(EF) indicates that SC in Fe1xRuxTe systems may be significantly diminished when compared to that of FeTe. However, it is worth noting that any clear correlation between N(EF) and Tc's of FeSexTe1x superconductors was not revealed from previous studies of their electronic structure [35,37,38,40].
4. Conclusions Structural, magnetic, and electronic properties of Fe0.75Ru0.25Te and FeTe have been studied by ab initio calculations. Because the incorporation of Ru atoms into FeTe host system modifies insignificantly the chalcogen atom height, both studied systems exhibit the double-stripe antiferromagnetic ground state. However, the calculated (LDA) magnetic stabilization energy of the single-stripe phase for Fe0.75Ru0.25Te is close to that of the above ground state. The single-stripe phase in this alloy may be induced by tensile strain, e.g. in thin films on tetragonal or cubic substrates with abplane lattice parameters exceeding 4 Å, analogously to the strained thin films of FeTe [17]. The thin films of various iron chalcogenides are extensively investigated [72], therefore the Ru-doped FeTe systems may be good candidates for new superconducting Febased materials.
[1] F.C. Hsu, J.Y. Luo, K.W. Yeh, T.K. Chen, T.W. Huang, P.M. Wu, Y.C. Lee, Y.L. Huang, Y.Y. Chu, D.C. Yan, M.K. Wu, Proc. Natl. Acad. Sci. U. S. A. 105 (2008) 14262e14264. [2] Y. Mizuguchi, F. Tomioka, S. Tsuda, T. Yamaguchi, Y. Takano, Appl. Phys. Lett. 93 (2008) 152505. [3] S. Medvedev, T.M. McQueen, I. Trojan, T. Palasyuk, M.I. Eremets, R.J. Cava, S. Naghavi, F. Casper, V. Ksenofontov, G. Wortmann, C. Felser, Nat. Mater. 8 (2009) 630e633. [4] S. Margadonna, Y. Takabayashi, Y. Ohishi, Y. Mizuguchi, Y. Takano, T. Kagayama, T. Nakagawa, M. Takata, K. Prassides, Phys. Rev. B 80 (2009) 064506. [5] H. Okabe, N. Takeshita, K. Horigane, T. Muranaka, J. Akimitsu, Phys. Rev. B 81 (2010) 205119. [6] K.W. Yeh, T.W. Huang, Y.L. Huang, T.K. Chen, F.C. Hsu, P.M. Wu, Y.C. Lee, Y.Y. Chu, C.L. Chen, J.Y. Luo, D.C. Yan, M.K. Wu, Europhys. Lett. 84 (2008) 37002. [7] M.H. Fang, H.M. Pham, B. Qian, T.J. Liu, E.K. Vehstedt, Y. Liu, L. Spinu, Z.Q. Mao, Phys. Rev. B 78 (2008) 224503. [8] D.J. Gawryluk, J. Fink-Finowicki, A. Wisniewski, R. Pu zniak, V. Domukhovski, R. Diduszko, M. Kozłowski, M. Berkowski, Supercond. Sci. Technol. 24 (2011) 065011. [9] V. Fedorchenko, G.E. Grechnev, V.A. Desnenko, A.S. Panfilov, S.L. Gnatchenko, V.V. Tsurkan, J. Deisenhofer, H.-A. Krug von Nidda, A. Loidl, D.A. Chareev, O.S. Volkova, A.N. Vasiliev, Low. Temp. Phys. 37 (2011) 83e89. [10] S. Panfilov, V.A. Pashchenko, G.E. Grechnev, V.A. Desnenko, A.V. Fedorchenko, A.N. Bludov, S.L. Gnatchenko, D.A. Chareev, E.S. Mitrofanov, A.N. Vasiliev, Low. Temp. Phys. 40 (2014) 615e620. [11] Y. Mizuguchi, F. Tomioka, S. Tsuda, T. Yamaguchi, Y. Takano, Appl. Phys. Lett. 94 (2009) 012503. [12] Y. Mizuguchi, Y. Hara, K. Deguchi, S. Tsuda, T. Yamaguchi, K. Takeda, H. Kotegawa, H. Tou, Y. Takano, Supercond. Sci. Technol. 23 (2010) 054013. [13] R. Hu, E.S. Bozin, J.B. Warren, C. Petrovic, Phys. Rev. B 80 (2009) 214514. [14] P. Zajdel, P.Y. Hsieh, E.E. Rodriguez, N.P. Butch, J.D. Magill, J. Paglione, P. Zavalij, M.R. Suchomel, M.A. Green, J. Am. Chem. Soc. 132 (2010) 13000e13007. [15] V.P.S. Awana, A. Pal, A. Vajpayee, B. Gahtori, H. Kishan, Phys. C 471 (2011) 77e82. [16] C. Dong, H. Wang, Q. Mao, R. Khan, X. Zhou, C. Li, J. Yang, B. Chen, M. Fang, J. Phys. Condens. Matter 25 (2013) 385701. [17] Y. Han, W.Y. Li, L.X. Cao, X.Y. Wang, B. Xu, B.R. Zhao, Y.Q. Guo, J.L. Yang, Phys. Rev. Lett. 104 (2010) 017003. [18] S.X. Huang, C.L. Chien, V. Thampy, C. Broholm, Phys. Rev. Lett. 104 (2010) 217002. , M.R. Cimberle, [19] E. Bellingeri, I. Pallecchi, R. Buzio, A. Gerbi, D. Marre M. Tropeano, M. Putti, A. Palenzona, C. Ferdeghini, Appl. Phys. Lett. 96 (2010) 102512. [20] E. Bellingeri, S. Kawale, V. Braccini, R. Buzio, A. Gerbi, A. Martinelli, M. Putti, I. Pallecchi, G. Balestrino, A. Tebano, C. Ferdeghini, Supercond. Sci. Technol. 25 (2012) 084022. [21] W.Z. Si, Z.W. Lin, Q. Jie, W.G. Yin, J. Zhou, G. Gu, P.D. Johnson, Q. Li, Appl. Phys. Lett. 95 (2009) 052504. [22] Y.F. Nie, E. Brahimi, J.I. Budnick, W.A. Hines, M. Jain, B. Wells, Appl. Phys. Lett. 94 (2009) 242505. [23] F. Chen, et al., Phys. Rev. B 81 (2010) 014526. [24] K. Nakayama, et al., Phys. Rev. Lett. 105 (2010) 197001. [25] A. Tamai, et al., Phys. Rev. Lett. 104 (2010) 097002. [26] H. Miao, et al., Phys. Rev. B 85 (2012) 094506. [27] I. Perez, J.A. McLeod, R.J. Green, R. Escamilla, V. Ortiz, A. Moewes, Phys. Rev. B 90 (2014) 014510. [28] I. Perez, J.A. McLeod, R.J. Green, R. Escamilla, V. Ortiz, A. Moewes, J. Phys. Chem. C 118 (2014) 25150e25157. [29] J. Maletz, et al., Phys. Rev. B 89 (2014) 220506(R). [30] A. Subedi, L. Zhang, D.J. Singh, M.H. Du, Phys. Rev. B 78 (2008) 134514. [31] Y. Ding, Y. Wang, J. Ni, Solid State Commun. 149 (2009) 505e509. [32] F. Ma, W. Ji, J. Hu, Z.Y. Lu, T. Xiang, Phys. Rev. Lett. 102 (2009) 177003. [33] P.P. Singh, J. Phys. Condens. Matter 22 (2010) 135501. [34] S. Chadov, D. Scharf, G.H. Fecher, C. Felser, L. Zhang, D.J. Singh, Phys. Rev. B 81 (2010) 104523. [35] H. Shi, Z.B. Huang, J.S. Tse, H.Q. Lin, J. Appl. Phys. 110 (2011) 043917. [36] A. Ciechan, M.J. Winiarski, M. Samsel-Czekała, Acta Phys. Pol. A 121 (2012) 820e823. [37] M.J. Winiarski, M. Samsel-Czekała, A. Ciechan, Europhys. Lett. 100 (2012)
Please cite this article in press as: M.J. Winiarski, et al., Strain effects on electronic structure of Fe0.75Ru0.25Te, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.11.024
6
M.J. Winiarski et al. / Materials Chemistry and Physics xxx (2016) 1e6
47005. [38] M.J. Winiarski, M. Samsel-Czekała, A. Ciechan, J. Alloys Compd. 566 (2013) 187e190. [39] A. Ciechan, M.J. Winiarski, M. Samsel-Czekała, Intermetallics 41 (2013) 44e50. [40] M.J. Winiarski, M. Samsel-Czekała, A. Ciechan, Intermetallics 52 (2014) 97e100. [41] A. Ciechan, M.J. Winiarski, M. Samsel-Czekała, J. Phys. Condens. Matter 26 (2014) 025702. [42] A. Ciechan, M.J. Winiarski, M. Samsel-Czekała, J. Alloys Compd. 630 (2015) 100e105. [43] S. Kumar, P.P. Singh, Solid State Commun. 220 (2015) 49e56. [44] S. Kumar, P.P. Singh, Mater. Res. Express 3 (2016) 056001. [45] S. Kumar, P.P. Singh, J. Alloys Compd. 658 (2016) 1041e1059. [46] T. Imai, K. Ahilan, F.L. Ning, T.M. McQueen, R.J. Cava, Phys. Rev. Lett. 102 (2009) 177005. [47] Y. Qiu, et al., Phys. Rev. Lett. 103 (2009) 067008. [48] T.J. Liu, et al., Nat. Mater. 9 (2010) 716e720. [49] C.Y. Moon, H.J. Choi, Phys. Rev. Lett. 104 (2010) 057003. [50] S. Sharma, A. Bharathi, S. Chandra, V.R. Reddy, S. Paulraj, A.T. Satya, V.S. Sastry, A. Gupta, C.S. Sundar, Phys. Rev. B 81 (2010) 174512. [51] V. Brouet, et al., Phys. Rev. Lett. 105 (2010) 087001. [52] R.S. Dhaka, et al., Phys. Rev. Lett. 107 (2011) 267002. [53] N. Xu, et al., Phys. Rev. B 86 (2012) 064505. [54] L. Ma, G.F. Ji, J. Dai, X.R. Lu, M.J. Eom, J.S. Kim, B. Normand, W. Yu, Phys. Rev. Lett. 109 (2012) 197002. [55] M.J. Winiarski, M. Samsel-Czekała, A. Ciechan, J. Appl. Phys. 116 (2014) 223903.
[56] I.I. Mazin, M.D. Johannes, L. Boeri, K. Koepernik, D.J. Singh, Phys. Rev. B 78 (2008) 085104. [57] F. Ricci, G. Profeta, Phys. Rev. B 87 (2013) 184105. [58] Q.Q. Ye, K. Liu, Z.Y. Lu, Phys. Rev. B 88 (2013) 205130. [59] X. Gonze, et al., Comput. Phys. Commun. 180 (2009) 2582e2615. [60] M. Oliveira, F. Nogueira, Comput. Phys. Commun. 178 (2008) 524e534. [61] J.P. Perdew, Y. Wang, Phys. Rev. B 45 (1992) 13244. [62] A. Martinelli, A. Palenzona, M. Tropeano, C. Ferdeghini, M. Putti, M.R. Cimberle, T.D. Nguyen, M. Affronte, C. Ritter, Phys. Rev. B 81 (2010) 094115. [63] M. Samsel-Czekała, M.J. Winiarski, Intermetallics 31 (2012) 186e190. [64] D. Louca, K. Horigane, A. Llobet, R. Arita, S. Ji, N. Katayama, S. Konbu, K. Nakamura, T.Y. Koo, P. Tong, K. Yamada, Phys. Rev. B 81 (2010) 134524. [65] B. Joseph, A. Iadecola, A. Puri, L. Simonelli, Y. Mizuguchi, Y. Takano, N.L. Saini, Phys. Rev. B 82 (2010) 020502(R). [66] F. Caglieris, F. Ricci, G. Lamura, A. Martinelli, A. Palenzona, I. Pallecchi, A. Sala, G. Profeta, M. Putti, Sci. Technol. Adv. Mater. 13 (2012) 054402. [67] M. Bendele, A. Maisuradze, B. Roessli, S.N. Gvasaliya, E. Pomjakushina, S. Weyeneth, K. Conder, H. Keller, R. Khasanov, Phys. Rev. B 87 (2013) 060409(R). [68] M. Monni, F. Bernardini, G. Profeta, S. Massidda, Phys. Rev. B 87 (2013) 094516. [69] M.D. Johannes, I.I. Mazin, Phys. Rev. B 79 (2009) 220510(R). [70] M. Aichhorn, S. Biermann, T. Miyake, A. Georges, M. Imada, Phys. Rev. B 82 (2010) 064504. [71] A. Liebsch, H. Ishida, Phys. Rev. B 82 (2010) 155106. [72] P. Mele, Sci. Technol. Adv. Mater. 13 (2012) 054301.
Please cite this article in press as: M.J. Winiarski, et al., Strain effects on electronic structure of Fe0.75Ru0.25Te, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.11.024