IR Pump-Probe Study of Phase Separated Hole-Doped Manganite La1/4Pr3/8Ca3/8MnO3 Jaewook Ahn KAIST - physics Ultrafast Dynamics in Strongly-Correlated Materials. Charge Lattice Orbital Spin Correlated 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

Ultrafast Spectroscopy Lab

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

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 Time Resolution: 1/60th of a second Courtesy of R. Trebino

Ultrafast phenomena in condensed matter Current Days Ultrafast Phenomena Ultrafast phenomena in condensed matter 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 KAIST - Physics

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

Colossal Magneto-Resistance Colossal magnetoresistance (CMR) is a property of some materials, mostly manganese-based perovskite oxides, 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

Alkaline earth: Be, Mg, Ca, Sr, Ba, Ra (Group 2) LaMnO3 :Rare Earth 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

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

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

Mixed Phase in La5/8-yPryCa3/8MnO3 20K (I) Charge-disordered domain (ferromagnetic metallic) Charge-ordered domain 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. 17K (I) 120K(II) KAIST - Physics

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

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 Reference Pump Probe BS Lens Sample Photo Diode Shaker Laser

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

Reflectance change in ~ps dR/R

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.) Anharmonic behavior may appear at high fluence exp.

Coherent phonon generation DECP (Displacive Excitation of Coherent Phonon) where n(t) is the electron density in excitation band

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

Two Optical Phonons 2.43 THz 5.15 THz Non-oscillatory relaxations

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

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

What about the slow oscillations ?

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

Strain propagation in bulk 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)

Strain propagation in bulk where

Coherent Phonon Amplitudes KAIST

Reflectance change in ~ps dR/R

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

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 coherent phonon dephasing Metallic behavior:

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

Summary Fs study of CO-FM mixed phase. Fast electronic relaxation coherent phonon dephasing Spin-lattice relaxation 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

IR Pump-Probe Study of Phase Separated Hole-Doped Manganite La1/4Pr3/8Ca3/8MnO3 Jaewook Ahn KAIST - physics Ultrafast Dynamics in Strongly-Correlated Materials. Charge Lattice Orbital Spin Correlated 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

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.