Raman Spectroscopy Study of the Pyrochlore Superconductors KOs 2 O 6 and RbOs 2 O 6 and Joachim Schoenes Institut für Physik der Kondensierten Materie,TU Braunschweig in collaboration with Klaus Doll 1, Janusz Karpinski 2, Zbigniew Bukowski 2 1 Max-Planck-Institut für Festkörperforschung, Stuttgart, Germany 2 Laboratorium für Festkörperphysik, ETH Zürich, Switzerland
Outline Introduction Raman effect Raman measurements for different geometries Conclusions and future prospects 2
Pyrochlore Superconductors S. Yonezawa, Y. Muraoka, Y. Matsushita, Z. Hiroi, J. Phys.: Condens. Matter 16, L9 (2004). G. Schuck, S. M. Kazakov, K. Rogacki, N. D. Zhigadlo, J. Karpinski, Phys. Rev. B 73, 144506 (2006). Z. Hiroi, S. Yonezawa, Y. Nagao, J. Yamaura, Phys. Rev B 76, 014523 (2007). -pyrochlore: A 2 B 2 O 6 O -pyrochlore: a 2 B 2 O 6 A Cd 2 Re 2 O 7 : T c = 1 K KOs 2 O 6 : T c = 9.6 K RbOs 2 O 6 : T c = 6.3 K CsOs 2 O 6 : T c = 3.3 K 3
Proposed Crystal Structures for KOs 2 O 6 3 F43m Fd m Os O K (Rb) (111) oriented crystals from J. Karpinski and Z. Bukowski 4 1 mm
K (Rb) in a Cage of OsO 6 Hiroi et al., Phys. Rev. B 76, 014523 (2007). 5
Superconductivity in KOs2O6 Resistivity Z. Hiroi et al., Phys. Rev. B 76, 014523 (2007). Magnetic Properties G. Schuck et al., Phys. Rev. B 73, 144506 (2006). 6
Specific Heat in KOs2O6 TP Tc Hiroi et al., Phys. Rev. B 76, 014523 (2007). Einstein Temperatures: E1 = 22 K, E2 = 61 K 7
Superconducting Parameters in KOs2O6 2 0 / kbtc = 5 (BCS = 3.5) C / Tc = 2.87 (BCS = 1.43) Tc2 / Hc2 = 0.128 (BCS = 0.168) ep = 2.38 Strong Electron-Phonon Coupling in KOs2O6 8
Motivation for Raman Experiments Raman spectroscopy is powerful to clarify the crystal structure Exact determination of the phonon frequencies Temperature dependence of the Raman spectra give information about phonon relaxation processes like phonon-phonon or electronphonon coupling 9
Raman Effect Stokes Anti-Stokes Example: Raman spectrum of Si Inelastic light scattering by phonons E1 Stokes elastic scattered light v=1 v=0-800 -600 Anti-Stokes Antistokes Stokes Intensity (arb. units) h -400-200 0 200 400 600 E0 800-1 Raman shift (cm ) Normal Raman Resonance Raman 10
Raman Spectrometer LN2 Cryostat Mirror Soleil-BabinetCompensator Interference filter Ar - Laser, 514.5 nm CCD Polarisation filter Sample Objectivlens Projection lens LHe-Cryostat Doublemonochromator Computer Signal processor Singlemonochromator CCD Electronics 11
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Factor Group Analysis for K(Rb)Os2O6 Space group Os K (Rb) O Raman active modes IR active modes Fd3m 16c 8b 48f A1g+Eg+4T2g 5T1u F43m 16e 3A1+3E+9T2 9T2 4c, 4b 24f, 24g 13
Polarization and Angular Dependence of the Raman Intensity Ag parallel Eg parallel T2g parallel 90 90 90 45 45 45 0 0 0 T2g perpendicular Eg perpendicular 90 90 Ag perpendicular 90 45 45 0 0 45 0 14
Micro-Raman Spectra at Room Temperature KOs2O6 T2g (241.6) parallel perpendicular K rattling mode Ag (488.6) Eg (267.2) T2g (696.3) T2g (74.8) T2g (413.4) 100 200 300 400 500 600-1 Raman shift (cm ) 700 800 Intensity (arb. units) Intensity (arb. units) T2g (238.1) RbOs2O6 parallel perpendicular Rb rattling mode Ag (492.9) T2g (60.3) Eg (267.8) T2g (694.6) T2g (416.9) 100 200 300 400 500 600-1 Raman shift (cm ) 700 800 J. Schoenes, A.-M. Racu, K. Doll, Z. Bukowski, J. Karpinski, Phys. Rev. B 77, 134515 (2008). 15
Angular Dependence of the Intensity 90 120 = 45 60 T2g 240 cm parallel -1 90 120 30 150 180 150 0 210 0 330 210 300 240 270 60 150 Eg 260 cm parallel 90-1 120 30 180 300 270 Eg 260 cm perpendicular 30 180 330 210-1 60 150 0 240 300 270 90 120-1 30 180 330 240 T2g 415 cm parallel 60 0 330 210 240 300 270 16
Theory and Experiment KOs2O6 experiment Fd3m theory F43m theory RbOs2O6 experiment Fd3m theory 74,87 71 70 60,3 60 143 228 800 RbOs2O6 274 275 241,6 265 267,2 285 278 267,8 278-1 theory (cm ) 600 238,1 301 400 319 KOs2O6 326 200 442 0 0 100 200 300 400 500 600 700 413,4 477 476 416,9 466 488,6 530 531 492,9 531 694,6 765-1 543 experiment (cm ) 698 696,3 768 769 17
Conclusions KOs2O6 and RbOs2O6 belong to the Fd3m space group The Raman active rattling modes have been identified at 74.8 cm-1 (K) and 60.3 cm-1 (Rb) Future Work Electron-phonon coupling from temperature dependent Raman spectra 18