Anomalous Phonon Redshift In K Doped Bafe 2as 2 Iron Pnictides-PDF Free Download

Anomalous phonon redshift in K doped BaFe 2As 2 iron pnictides
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PHYSICAL REVIEW B 91 104510 2015, Anomalous phonon redshift in K doped BaFe2 As2 iron pnictides. B Xu 1 2 Y M Dai 3 B Shen 2 H Xiao 2 Z R Ye 4 A Forget 5 D Colson 5 D L Feng 4 H H Wen 6 C C Homes 3. X G Qiu 2 and R P S M Lobo1 7 8, LPEM ESPCI ParisTech PSL Research University 10 rue Vauquelin F 75231 Paris Cedex 5 France. Beijing National Laboratory for Condensed Matter Physics Institute of Physics Chinese Academy of Sciences P O Box 603. Beijing 100190 China, Condensed Matter Physics and Materials Science Department Brookhaven National Laboratory Upton New York 11973 USA. State Key Laboratory of Surface Physics Department of Physics and Advanced Materials Laboratory Fudan University. Shanghai 200433 China, IRAMIS SPEC CEA 91191 Gif sur Yvette France. National Laboratory of Solid State Microstructures and Department of Physics Nanjing University Nanjing 210093 China. CNRS UMR 8213 F 75005 Paris France, Sorbonne Universite s UPMC Paris 6 F 75005 Paris France.
Received 26 January 2015 revised manuscript received 16 February 2015 published 12 March 2015. The effect of K Co and P dopings on the lattice dynamics in the BaFe2 As2 system is studied by infrared. spectroscopy We focus on the phonon at 253 cm 1 the highest energy in plane infrared active Fe As mode in. BaFe2 As2 Our studies show that Co and P dopings lead to a blueshift of this phonon in frequency which can be. simply interpreted by the change in lattice parameters induced by doping In sharp contrast an unusual redshift. of the same mode was observed in the K doped compound at odds with the above explanation This anomalous. behavior in K doped BaFe2 As2 is more likely associated with the coupling between lattice vibrations and other. channels such as charge or spin This coupling scenario is also supported by the asymmetric line shape and. intensity growth of the phonon in the K doped compound. DOI 10 1103 PhysRevB 91 104510 PACS number s 74 25 Gz 78 30 j 74 25 Kc. Despite extensive studies on high Tc superconductivity Infrared spectroscopy is a standard tool to investigate. in iron pnictides since its discovery 1 the question of lattice dynamics providing information on the coupling. what plays an important role in the pairing mechanism of between lattice vibrations and electrons or spins 11 14. this class of superconductors remains enigmatic Theoretical Although optical investigations into the lattice dynamics. calculations have demonstrated that unlike the traditional of iron pnictides have been conducted by many groups. BCS superconductors an electron phonon interaction is not 14 17 a comparison study of different iron pnictides in. sufficient to account for such a high Tc in iron pnictides particular different substitution types in the same family is still. 2 Therefore the s pairing with a sign reversal in the gap absent. function that is mediated by spin fluctuations was proposed We fill this gap by comparing the behavior of the. by Mazin et al 3 However the above scenario seems far 253 cm 1 in plane infrared active Fe As mode in BaFe2 As2. from being thoroughly established The s pairing state is parent compound of the Ba122 family and three different. expected to be sensitive to impurity scattering 4 Indeed doping types K Co and P dopings We observe a blueshift. on one hand in and out of plane dopings affect Tc and the of this mode in the Co and P doped BaFe2 As2 as well as. unpaired quasiparticle density in the BaFe2 As2 family 5 an anomalous redshift of the same mode in the K doped. but on the other hand superconductivity is robust against compound The latter cannot be explained by the change in. impurities in 1111 materials 6 7 Furthermore a large iron lattice parameters induced by doping A close inspection of. isotope effect has been reported in SmFeAsO1 x Fex and the phonon line shape and intensity leads us to the conclusion. Ba1 x Kx Fe2 As2 8 indicating that electron phonon coupling that the coupling between lattice vibrations and other channels. plays some role in the pairing mechanism By taking these is stronger in K doped BaFe2 As2 and thus responsible for the. facts into account Kontani and Onari proposed that electron anomalous redshift of the phonon. phonon coupling arising from the Fe ion oscillation can induce High quality single crystals of BaFe2 As2 BFA. orbital fluctuations mediating the s pairing without sign TN 138 K Ba0 6 K0 4 Fe2 As2 K40 Tc 39 K and. reversal 9 A recent quantitative convergent beam electron Ba Fe0 92 Co0 08 2 As2 Co08 Tc 23 K were grown with. diffraction study by Ma et al 10 has revealed strong coupling a self flux method 18 19 whereas the BaFe2 As0 85 P0 15 2. between Fe orbital fluctuations and anion dipole polarizations P15 TN 90 K single crystals were grown without flux. in Ba Fe1 x Cox 2 As2 They suggest that a full understanding 20 The ab plane reflectivity R was measured at a. of the pairing mechanism in iron pnictides can only be reached near normal angle of incidence on Bruker IFS113v and IFS66v. by considering the charge spin orbital lattice and anion spectrometers An in situ gold overfilling technique 21 was. polarizations all together in a consistent theory used to obtain the absolute reflectivity of the samples Data. from 30 to 15 000 cm 1 were collected at different tempera. tures on freshly cleaved surfaces for each sample and then we. ymdai lanl gov extended the reflectivity to 40 000 cm 1 at room temperature. lobo espci fr with an AvaSpec 2048 14 optical fiber spectrometer. 1098 0121 2015 91 10 104510 5 104510 1 2015 American Physical Society. B XU et al PHYSICAL REVIEW B 91 104510 2015, FIG 1 Color online The reflectivity in the far infrared region. FIG 2 Color online The low frequency optical conductivity. for BFA K40 Co08 and P15 at a 300 K and b 150 K The red. 1 for BFA K40 Co08 and P15 at a 300 K and b 150 K, arrows indicate the ab plane infrared active Ba mode at 94 cm 1 and. Panels c and d show the enlarged view of panels a and b. the Fe As mode at 253 cm 1 in BFA, respectively focusing on the phonon at 253 cm 1 with offset The. thin black lines through the data are the Lorentzian fitting results The. Figure 1 a shows the far infrared reflectivity at room vertical dashed lines indicate the phonon frequency for BFA. temperature 300 K for all four compounds Metallic behavior. can be realized in all these materials by their relatively high. at 300 and 150 K respectively The low frequency 1, reflectivity that approaches unity at zero frequency In addition. exhibits a prominent Drude like metallic behavior for all. two sharp features indicated by the arrows representing. materials at both 300 and 150 K consistent with the reflectivity. the symmetry allowed ab plane infrared active Eu modes. were observed at 94 and 253 cm 1 in BFA consistent with. The optical conductivity can be conveniently parametrized. previous works 15 16 The 253 cm 1 mode is also clearly. by a Drude Lorentz model, observed on the room temperature R in the doped BFA.
compounds Figure 1 b displays R for the same materials 2 p j. at 150 K where similar features are revealed 1 2 2. Here we would like to point out that BFA exhibits structural Z0 k. 2 j 1j k 0 k 2 k2 2, and magnetic phase transitions at 140 K 22 with a consequent 1. renormalization of the infrared phonon spectra 15 17. whereas such transitions are either suppressed or absent in where Z0 is the vacuum impedance The first term describes a. the doped compounds meaning that below 140 K these sum of free carrier Drude responses each characterized by a. compounds are in different phases In order to avoid effects plasma frequency p j and a scattering rate 1 j The second. related to these phase transitions the temperature window in term is a sum of Lorentz oscillators each having a resonance. our study is constrained between 150 and 300 K frequency 0 k a line width k and an oscillator strength k. In the following we concentrate on the 253 cm 1 mode The optical response of Fe based superconductors FeSCs. which involves the displacements of Fe and As atoms can be modeled reasonably well by the superposition of two. 15 16 To investigate the doping effect on this mode in a Drude components and a series of Lorentz terms over a wide. straightforward way we calculated the optical conductivity via frequency range which has been discussed in detail in previous. Kramers Kronig analysis of the reflectivity At low frequency works 23 26. we employed a Hagen Rubens R 1 A extrapolation In addition to the gross features the 253 cm 1 mode man. Above 40 000 cm 1 the highest measured frequency we ifests itself as a sharp peak in the optical conductivity Figure. utilized a constant reflectivity up to 12 5 eV followed by a 2 c highlights the region around the 253 cm 1 phonon for all. free electron 4 response compounds The dashed line denotes the phonon peak position. Figures 2 a and 2 b show the real part of the optical of BFA It can be immediately noticed that the phonon in the. conductivity 1 in the far infrared region for all compounds K doped Ba122 compound shifts to a lower frequency redshift. ANOMALOUS PHONON RED SHIFT IN K DOPED BaFe PHYSICAL REVIEW B 91 104510 2015. TABLE I The vibrational parameters for oscillator fits to the Increasing the carrier concentration will increase the electronic. infrared active Fe As mode observed in different doped compounds background potentially leading to a growth in the screening. at 300 and 150 K where 0 and are the oscillator frequency effect However the screening effect only affects the intensity. strength and linewidth respectively All units are in cm 1 of the phonon but has no influence on the frequency 27. In addition the screening effect on the 253 cm 1 mode in the. 300 K 150 K Ba122 system has been demonstrated to be very weak or absent. Doping 0 0 15 Therefore we can safely rule out this possibility Disorder. in the Fe As layers may reduce the intensity of this mode but. BFA 253 10 211 85 3 77 258 00 217 13 3 43 virtually has no effect on the phonon frequency either Finally. K40 251 30 321 50 5 51 255 88 322 74 4 51 the change in lattice parameters due to chemical pressure. Co08 254 94 208 26 6 31 259 75 208 86 5 93, may play an important role in shifting the phonon frequency. P15 257 34 175 10 5 92 261 78 181 78 5 88, Generally the frequency shift of a phonon is closely related. to the change in the bond length l of the associated atoms. or softening whereas P and Co dopings lead to a shift of this The phonon shifts toward a higher frequency if l shrinks. mode to a higher frequency blueshift or hardening Exactly The expected frequency shift can be estimated via a simple. the same behavior is observed at 150 K as shown in Fig 2 d formula 0 0 l l 3 2 28 By comparison with BFA the. In order to quantitatively analyze the behavior of the Fe As bond length l is 0 3 shorter in K40 22 29 30 0 5. phonon upon doping we fit it to a Lorentz oscillator with shorter in Co08 31 and 0 8 shorter in P15 32 Based. a linear background in a narrow frequency range centered at on these parameters the doping induced frequency shift of. the phonon resonance frequency for all the materials at all the 253 cm 1 mode in BFA can be easily estimated in all. measured temperatures The fitting results at 300 and 150 K three compounds The open diamonds in Fig 4 a denote the. are shown as thin solid lines through the corresponding data calculated phonon frequency by considering the Fe As bond. in Figs 2 c and 2 d respectively The fitting parameters are length for each material which we compare to the values. summarized in Table I determined from the experiment solid squares Note that. Figure 3 shows the phonon resonance frequency 0 deter the measured phonon frequency agrees very well with the. mined from the fit for all the compounds at seven measured calculation in both Co08 and P15 suggesting that the phonon. temperatures between 150 and 300 K For each material 0. increases upon cooling following a quadratic T dependence. indicated by the black dashed lines which is expected. in the absence of structural or magnetic transitions The T. dependence of this phonon is in good agreement with previous. works 15 For all the measured temperatures with respect to. BFA red solid squares 0 is smaller in K doped compounds. blue solid circles but larger in the Co green solid diamonds. and P doped pink solid triangles materials, We now trace the origin of the phonon frequency shift. induced by different doping types Doping usually has three. main effects on materials i changing the carrier concen. tration by adding electrons electron doping or holes hole. doping ii introducing disorder and iii application of. chemical pressure that can change the lattice parameters. FIG 4 Color online a The black solid squares denote the. resonance frequency 0 of the Fe As mode at 150 K for BFA. K40 Co08 and P15 The red open diamonds are the calculated. frequency by considering the Fe As bond length change only for. each compound b The green solid circles and blue solid triangles. portray the Fano parameter 1 q 2 and phonon intensity 2 determined. FIG 3 Color online The resonance frequency 0 of the Fe As from the Fano fit at 150 K respectively Inset the phonon line shape. mode as a function of temperature for all the compounds The black without the electronic background for all the compounds The solid. dashed lines represent a quadratic temperature dependence lines through the data are the Fano fitting results. simply interpreted by the change in lattice parameters induced by doping In sharp contrast an unusual redshift of the same mode was observed in the K doped compound at odds with the above explanation This anomalous behavior in K doped BaFe 2As is more likely associated with the coupling between lattice vibrations and other

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