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IR Pump-Probe Study of Phase Separated Hole-Doped Manganite La 1/4 Pr 3/8 Ca 3/8 MnO 3

IR Pump-Probe Study of Phase Separated Hole-Doped Manganite La 1/4 Pr 3/8 Ca 3/8 MnO 3. Lattice. Charge. Ultrafast Dynamics in Strongly-Correlated Materials. Jaewook Ahn KAIST - physics. Correlated. Orbital. Spin. Coauthors Kyeong -Jin Jang and Jongseok Lim (KAIST)

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IR Pump-Probe Study of Phase Separated Hole-Doped Manganite La 1/4 Pr 3/8 Ca 3/8 MnO 3

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  1. IR Pump-Probe Study of Phase Separated Hole-Doped ManganiteLa1/4Pr3/8Ca3/8MnO3 Lattice Charge Ultrafast Dynamics in Strongly-Correlated Materials. Jaewook Ahn KAIST - physics Correlated Orbital Spin Coauthors Kyeong-Jin Jang and Jongseok Lim (KAIST) Jihee Kim and Ki-Ju Yee (Chungnam Nat. Univ.) Jai Seok Ahn (Pusan Nat. University) • Fundings IRMMM-THz, September 23rd, 2009, Pusan

  2. Ultrafast Spectroscopy Lab

  3. Summary (1) We report the generation of coherent optical and acoustic phonons in a mixed valence manganite LPCMO using femtosecond infrared pump-probe spectroscopy. (2) Temperature-dependent measurements of the time-resolved optical reflectance, obtained over a range of 5-300~K, revealed that the energy of the photoexcited electrons dissipated, during relaxation, to acoustic phonons in the high-temperature paramagnetic phase and to optical phonons in the low-temperature charge ordering phase. (3) We suggest a phenomenological charge ordering gap opening mechanism to explain the crossover behavior observed during electron-lattice relaxation in the vicinity of the charge ordering phase transition. KAIST - Physics

  4. Time Resolution: 1/60th of a second Ultrafast Phenomena in 1872 Bet: Do all four hooves of a galloping horse ever simultaneously leave the ground? Leland Stanford Eadweard Muybridge Palo Alto, CA 1872 Courtesy of R. Trebino

  5. Ultrafast phenomena in condensed matter Current Days Ultrafast Phenomena KAIST - Physics Materials with correlated electrons exhibit some of the most intriguing phenomena in condensed matter physics. A new experimental tools is now allowing researchers to probe the electronic structure of these materials, which can exist in a rich variety of phases. Photo-induced phase transitions Ultrafast dynamics in superconductors Correlated dynamics in semiconductors Dynamics of coherent excitations Spin dynamics

  6. II III I Motivation : La5/8-yPryCa3/8MnO3 Structural phase transitions III : paramagnetic insulator II : short-range ferromagnetic metal short-range charge-ordering phase I : long-range FM and CO phases M. Ueharaet al., Nature 399, 560(1999)

  7. Colossal Magneto-Resistance Colossal magnetoresistance (CMR) is a property of some materials, mostly manganese-based perovskiteoxides, that enables them to dramatically change their electrical resistance in the presence of a magnetic field. Initially discovered in 1993 by von Helmolt et al., this property is not explained by any current physical theories, including conventional magnetoresistance or the double-exchange mechanism. The understanding and application of CMR offers tremendous opportunties for the development of new technologies such as read/write heads for high-capacity magnetic storage and spintronics. KAIST - Physics

  8. Trivalent rare earth: La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Tb, Lu (Lanthanide 57-71) Alkaline earth: Be, Mg, Ca, Sr, Ba, Ra (Group 2) LaMnO3 :Rare Earth CaMnO3 :Alkaline Earth Perovskite structure ABO3 A in an 8 BO6 octahedra A : 12-fold oxygen coordination B : 6-fold oxygen coordination A O B KAIST - Physics

  9. II III I Motivation : La5/8-yPryCa3/8MnO3 Structural phase transitions III : paramagnetic insulator II : short-range ferromagnetic metal short-range charge-ordering phase I : long-range FM and CO phases M. Ueharaet al., Nature 399, 560(1999)

  10. 20K (I) Charge-disordered domain(ferromagnetic metallic) Charge-ordered domain 17K (I) 120K(II) Mixed Phase in La5/8-yPryCa3/8MnO3 In TC≤T≤TCO region (II),two phases (FM and CO) coexist. KAIST - Physics M. Ueharaet al., Nature 399, 560(1999)

  11. 20K (I) Charge-disordered domain(ferromagnetic metallic) Charge-ordered domain 17K (I) 120K(II) Mixed Phase in La5/8-yPryCa3/8MnO3 In LPCMO, there coexist ferromagnetic metallic (FM) phase and charge-ordering insulating (CO) phase. Fs study of phase-separated manganite, there is a strong opportunity to optically control the competing ground states between metallic and insulating phases. KAIST - Physics

  12. Study of Phase Separation • Two absorption bands in s(w) in Bi1-xCaxMnO3 : Liu et al., PRL 81, 4684 (1998). • Two IR bands in s(w) in La1/8Sr7/8MnO3 :Jung et al., PRB 59, 3793 (1999). • Electron Microscopy of Co domains La5/8-yPryCa3/8MnO3 : Uehara et al,. Nature 399, 560 (1999). • Two absorption bands in s(w) in La5/8-yPryCa3/8MnO3 : Lee et al., PRB 65, 115118 (2002). • Coherent phonons in La1-xCaxMnO3 : Lim et al,. PRB 71, 134403 (2005). • Raman scattering study of LaxPryCa1-x-yMnO3 : Kim et al., PRB 77, 134411 (2008). Dearth of femtosecond study

  13. Shaker Pump Reference Probe BS Laser BS PhotoDiode Lens Sample Pump-probe method : mechanical delay • Strong pump pulse+ weaker probe pulse • Two optical path lengths are different in order to make time delay.- two types of delay generation (shaker and mechanical delay) • The photoinduced changes in reflectivity or transmission are measured • Shaker:10 Hz, Laser pulses:400 kHz 20,000 sampling/sec • Pulse power=10mW, energy=20nJ

  14. Our Measurements Fast oscillation Slow oscillation Jang, Lim, Ahn, et al., (2009)

  15. Reflectance change in ~ps dR/R

  16. Displacive Excitation of Coherent Phonon DECP Model Fit : • Pumped by a laser pulse, first electronically excited system rapidly comes to quasi-equilibrium state within nuclear response time, then, nuclear A1g coordinate is displaced with no change in lattice symmetry. • Only A1g symmetry • ~Cos(wt) dependence ( note: ISRS ~sin(wt) etc.) • Anharmonicbehavior may appear at high fluence exp.

  17. Coherent phonon generation DECP (Displacive Excitation of Coherent Phonon) where n(t) is the electron densityin excitation band

  18. Coherent phonon generation DECP (Displacive Excitation of Coherent Phonon) where ,

  19. Two Optical Phonons 2.43 THz 5.15 THz Non-oscillatory relaxations

  20. Is this anharmonic behavior ? - X (not enough fluence) - 10,000 times smaller than Lindemann criteria for melting • Raman forbidden mode ? - O (CO phase mode, newly found) - BiCaMnO3 (2000) - no direct evidence for LPCMO

  21. Amelitchev et al., PRB 63, 104430 (2001).

  22. What about the slow oscillations ?

  23. I (PI) Change of reflectance in long time • Coherent acoustic phonons with freqeuncy of 50 GHz exist above TCO. • Note: cos(wt) behavior : DECP • It is explained by propagating strain pulse mechanism.

  24. Strain propagation inbulk strained layer The strained layer which is generated by pump pulse at surface moves through sample at velocity of Cs. The interference of reflected probe pulse at the surface and at z shows an oscillatory behavior. R. Liu et al. PRB 72 (2005)

  25. Strain propagation inbulk where

  26. Coherent Phonon Amplitudes KAIST

  27. Reflectance change in ~ps dR/R

  28. Scenario: Charge-Ordering Gap Crossover of phonon amplitudes may be also coupled with the relaxation ? BCS-like Gap Explains the Coherent Phonon Bahavior.

  29. Relaxation I dR/R In a few ps, the dR/R is related to a change phonon temperature. Acoustic Phonon Fast electronic relaxation - the excited electrons relax through electron-phonon coupling. Spin-lattice relaxation coherentphonondephasing Metallic behavior:

  30. Spin-lattice relaxation Relaxation II dR/R Strain Pulse Propagation # On a longer time scale,spin reorientation may occur. # M. Uehara et al., Nature 399, 560(1999)

  31. Fast electronic relaxation coherentphonondephasing Spin-lattice relaxation Summary dR/R • Fs study of CO-FM mixed phase. • CO phase: three relaxations (2 coherent phonons + spin-lattice ) • T<Tc : weaker coherent phonon generation + metallic behavior • Coherent phonons: 2.5, 5.1 THz (A1g Raman modes), 50 GHz(acoustic) 2 fast relaxation(0.16, 0.52 ps) + spin-lattice relaxation

  32. IR Pump-Probe Study of Phase Separated Hole-Doped ManganiteLa1/4Pr3/8Ca3/8MnO3 Lattice Charge Ultrafast Dynamics in Strongly-Correlated Materials. Jaewook Ahn KAIST - physics Correlated Orbital Spin Coauthors Kyeong-Jin Jang and Jongseok Lim (KAIST) Jihee Kim and Ki-Ju Yee (Chungnam Nat. Univ.) Jai Seok Ahn (Pusan Nat. University) • Fundings IRMMM-THz, September 23rd, 2009, Pusan

  33. fs study of CO phase? • Coherent phonons in La1-xCaxMnO3 : Lim et al,. PRB 71, 134403 (2005). ~2.2 THz oscillation below TCO ~55 GHz oscillation above TCO These oscillations are related to charge-ordering phase transition.

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