Raman Spectroscopy Study of the Pyrochlore Superconductors KOs 2 O 6 and RbOs 2 O 6

Transcription

1 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

2 Outline Introduction Raman effect Raman measurements for different geometries Conclusions and future prospects 2

3 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, (2006). Z. Hiroi, S. Yonezawa, Y. Nagao, J. Yamaura, Phys. Rev B 76, (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

4 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

5 K (Rb) in a Cage of OsO 6 Hiroi et al., Phys. Rev. B 76, (2007). 5

6 Superconductivity in KOs2O6 Resistivity Z. Hiroi et al., Phys. Rev. B 76, (2007). Magnetic Properties G. Schuck et al., Phys. Rev. B 73, (2006). 6

7 Specific Heat in KOs2O6 TP Tc Hiroi et al., Phys. Rev. B 76, (2007). Einstein Temperatures: E1 = 22 K, E2 = 61 K 7

8 Superconducting Parameters in KOs2O6 2 0 / kbtc = 5 (BCS = 3.5) C / Tc = 2.87 (BCS = 1.43) Tc2 / Hc2 = (BCS = 0.168) ep = 2.38 Strong Electron-Phonon Coupling in KOs2O6 8

9 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

10 Raman Effect Stokes Anti-Stokes Example: Raman spectrum of Si Inelastic light scattering by phonons E1 Stokes elastic scattered light v=1 v= Anti-Stokes Antistokes Stokes Intensity (arb. units) h E Raman shift (cm ) Normal Raman Resonance Raman 10

11 Raman Spectrometer LN2 Cryostat Mirror Soleil-BabinetCompensator Interference filter Ar - Laser, nm CCD Polarisation filter Sample Objectivlens Projection lens LHe-Cryostat Doublemonochromator Computer Signal processor Singlemonochromator CCD Electronics 11

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13 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

14 Polarization and Angular Dependence of the Raman Intensity Ag parallel Eg parallel T2g parallel T2g perpendicular Eg perpendicular Ag perpendicular

15 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) Raman shift (cm ) 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) Raman shift (cm ) J. Schoenes, A.-M. Racu, K. Doll, Z. Bukowski, J. Karpinski, Phys. Rev. B 77, (2008). 15

16 Angular Dependence of the Intensity = T2g 240 cm parallel Eg 260 cm parallel Eg 260 cm perpendicular T2g 415 cm parallel

17 Theory and Experiment KOs2O6 experiment Fd3m theory F43m theory RbOs2O6 experiment Fd3m theory 74, , RbOs2O , , , theory (cm ) , KOs2O , , , , , experiment (cm ) ,

18 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