The Study of Coherent Optical and Acoustic Phonons

거대자기저항 효과 물질과 다강체 물질에서의 결맞음 포논에 관한 연구 The Study of Coherent Optical and Acoustic Phonons in Correlated Electron Materials (CMR, Multiferroic) 박사학위 청구논문 심사 2010년 5월 7일 장경진 KAIST 한국과학기술원 물리학과 초고속양자광학연구실 Ultrafast Quantum Optics Lab.

Content Ultrafast dynamics of lattice motion depend on phases ! Ultrafast dynamics? - experimental tools Lattice motion? - coherent optical phonon - coherent acoustic phonon Phases? - hole-doped manganite (La 2/8 Pr 3/8 Ca 3/8 Mn. O 3) KAIST - hexagonal manganites (Lu. Mn. O 3, YMn. O 3) Ultrafast Quantum Optics Lab.

Pump-probe method v Need two short pulses - powerful pump pulse and weaker probe pulse v Two optical path lengths are different in order to make time delay. KAIST - two types of delay generation (shaker and mechanical delay) v The photo-induced changes in reflectivity or transmission are measured. Ultrafast Quantum Optics Lab.

Experimental setup KAIST Realization of pump-probe method 1. 20 -fs pulse duration : enough to measure sub-picosecond dynamics 2. 400 k. Hz repetition rate : higher energy per pulse (tens n. J order) (for 80 MHz repetition rate, a few n. J order) 3. Raster scanning shaker : eliminate high frequency parts so data is clean 4. Cryostat : from 10 K to 300 K filled with liquid He Ultrafast Quantum Optics Lab.

What is observed? Jang, Lim, Ahn, et al. , PRB, under review KAIST Coherent oscillations in differential reflectance are measured as function of probe delay. Ultrafast Quantum Optics Lab.

Coherent optical phonon generation Impulsive Stimulated Raman Scattering - sin-like - transparent KAIST - optic phonon in Si Riffe et al. , PRB 76, 085207(2007) - coherent Eg phonons of Bi Ishioka et al. , J. Appl. Phys. 100, 093501 (2006) Displacive Excitation of Coherent Phonon - cos-like - opaque - A 1 or A 1 g modes in Bi, Sb, Te, Ti 2 O 3 Zeiger et al. , PRB 45, 768 (1992) - A 1 g mode of Bi, associated with the Peierls distortion Zijlstra et al. , PRB 74, 220301 (2006) Optical phonons in results are generated by DECP ! Ultrafast Quantum Optics Lab.

Coherent acoustic phonon generation Strained Pulse propagation - sin-like - propagating coherent acoustic phonon in Inx. Ga 1 -x. N/Ga. N heterostructure : Liu et al. , PRB 72, 195335 (2005) KAIST Acoustic phonons in results are generated by strained pulse propagation ! Ultrafast Quantum Optics Lab.

Optical phonon : DECP (Displacive Excitation of Coherent Phonon) n(t) : the electron density in excitation band P(t) : laser power density β : electronic decay constant n(t ) KAIST ω0 : mode freq, γ : damping constant of mode and Ultrafast Quantum Optics Lab.

KAIST Acoustic Phonon : Strained pulse propagation 1. The strained layer which is generated by pump pulse at surface moves through sample at velocity of vs. 2. The interference of reflected probe pulse at the surface and at z shows an oscillatory behavior. Liu et al. , PRB 72, 195335 (2005) Ultrafast Quantum Optics Lab.

Ultrafast dynamics of lattice motion depend on phases ! Ultrafast dynamics? - experimental tools Lattice motion? - coherent optical phonon - coherent acoustic phonon Phases? - hole-doped manganite (La 2/8 Pr 3/8 Ca 3/8 Mn. O 3) KAIST - hexagonal manganites (Lu. Mn. O 3, YMn. O 3) Ultrafast Quantum Optics Lab.

Hole doped manganite : La 1 -x. Cax. Mn. O 3 KAIST v Parent material, La. Mn. O 3, is antiferromagnetic charge transfer insulator v With Ca doping, d-orbital configuration is changed. v various phases as functions of doping rate and temperature Hole doped manganite meets the temperature-dependent phase study ! Ultrafast Quantum Optics Lab.

Phases of La 5/8 -y. Pry. Ca 3/8 Mn. O 3 20 K (III) Charge-disordered domain (ferromagnetic metallic) Charge-ordered domain III II I M. Uehara et al. , Nature 399, 560 (1999) 17 K (III) I : paramagnetic insulator II : short-range ferromagnetic metal short-range charge-ordering phase KAIST III : long-range FM and CO phases 120 K (II) Different two phases coexist below TCO ! Ultrafast Quantum Optics Lab.

Two coherent optical phonons Oscillation amplitudes by FT of oscillating part KAIST v A 5. 15 -THz component is about 10 times smaller than a 2. 43 -THz component. v From the low temperature Raman study on charge-ordering Bi 1 -x. Cax. Mn. O 3, this fast mode (5. 15 THz) is inferred the Mn vibration. Optical phonons (inferred the Mn ion vibration) are generated at low temperature ! Ultrafast Quantum Optics Lab.

CO phase relation The relation of 2. 43 THz optical phonon and CO phase I : PI II : s-r CO and FM III : l-r FM and CO Jang et al. , PRB, under review KAIST v The optical mode requires a CO phase (unequal Mn-O distance). v The metallic phase(equal Mn-O distance) doesn’t have optical mode generation. Optical phonon is related to the charge-ordering phase ! Ultrafast Quantum Optics Lab.

Coherent acoustic phonon KAIST v The temperature dependence of acoustic phonon is opposite to that of optical phonon. v Below TCO, the amplitude of acoustic mode decreases. Acoustic phonon is not generated as the charge ordering is formed ! Ultrafast Quantum Optics Lab.

§ Lattice motion related charge-ordering phase in La 2/8 Pr 3/8 Ca 3/8 Mn. O 3. § Spin-lattice coupling in Lu. Mn. O 3. KAIST § Phonon related iso-structural transition in hexagonal manganites. La 2/8 Pr 3/8 Ca 3/8 Mn. O 3 Lu. Mn. O 3 YMn. O 3 Optical phonon (DECP) 2. 4 THz & 5. 1 THz 3. 6 THz X Acoustic phonon (strained pulse propagation) 34 GHz 47 GHz 31 GHz phonon-CO phase coupling Ultrafast Quantum Optics Lab.

Why multiferroic material? Multiferroic is good material to study of coupling between spin and lattice by optical measurement ! KAIST Eerenstein et al. , Nature 442, 759 (2003) Multiferroic - materials that possess two or all three of the so-called ‘ferroic’ properties (ferroelectricity, ferromagnetism, ferroelasticity) Ultrafast Quantum Optics Lab.

Hexagonal manganites : (Y or Lu)Mn. O 3 S. Lee et al. , Nature 451 (2008) KAIST v v Hexagonal structure Ferroelectric ordering at high temperature ~ 900 K along c-axis Antiferromagnetic transition near 80 or 90 K. Mn ions consist ideal triangle structure in the ab-plane above TN, while Mn ions move toward or far away O 3 ion below TN. No structural phase transition at TN. Isostructural transition through magnetic ordering ! Ultrafast Quantum Optics Lab.

Our essential insight vs Do we obtain remarkable result near TN? KAIST D. Lim et al. , APL 83, 4800 (2003) Our experiment Previous work Our experiment No optical phonon ? Acoustic phonons at all temperatures Change near TN ? Ultrafast Quantum Optics Lab.

Results of Lu. Mn. O 3 Two coherent phonons are observed and disappeared in magnetic ordered phase ! KAIST Jang et al. , NJP 12, 023017 (2010) v Coherent optical phonon ~ 280 fs period v Coherent acoustic phonon ~ 21 ps period Ultrafast Quantum Optics Lab.

Coherent optical phonon in Lu. Mn. O 3 3. 63 THz optical phonon at room temperature Lu 1 Lu 2 S. -T. Lou et al. , PRB 79, 214301 (2009) KAIST v 3. 63 THz optical phonon is result of an A 1 -symmetry LO phonon (3. 61 THz). v Lu 1 and Lu 2 atoms are distinguished by ferroelectric transition. Coherent optical phonon involves motions of Lu atoms along the c-axis in opposite direction ! Ultrafast Quantum Optics Lab.

Coherent acoustic phonon in Lu. Mn. O 3 KAIST v The mode hardens as temperature is lowered to ~160 K. v Weakly softening behavior below ~160 K down to 90 K. The acoustic phonon is strongly coupled to spin fluctuations above and below TN ! Ultrafast Quantum Optics Lab.

Temperature dependence of phonons Jang et al. , NJP 12, 023017 (2010) § Indicate the coupling of coherent phonons with the AFM ordering transition. § Similar disappearance of phonon is found at the structural transitions in other materials. § By neutron scattering, isostructural phase transition (Mn ions move) at TN. KAIST Spin-phonon coupling with iso-structural transition ! Ultrafast Quantum Optics Lab.

Our experimental achievement Jang et al. , NJP 12, 023017 (2010) KAIST Do we obtain remarkable result near TN? Previous work Our experiment System Amplifier system Cavity dumper system Optical phonon No Observed Temperature dependence No change Exist only above TN Ultrafast Quantum Optics Lab.

§ Lattice motion related charge-ordering phase in La 2/8 Pr 3/8 Ca 3/8 Mn. O 3. § Spin-lattice coupling in Lu. Mn. O 3. KAIST § Phonon related iso-structural transition in hexagonal manganites. La 2/8 Pr 3/8 Ca 3/8 Mn. O 3 Lu. Mn. O 3 YMn. O 3 Optical phonon (DECP) 2. 4 THz & 5. 1 THz 3. 6 THz X Acoustic phonon (strained pulse propagation) phonon-CO phase coupling spin-lattice coupling 34 GHz 47 GHz 31 GHz Ultrafast Quantum Optics Lab.

General behavior of hexagonal manganites? S. Lee et al. , Nature 451 (2008) KAIST For YMn. O 3, Mn ions move opposite to that of Lu. Mn. O 3 as magnetic ordering occurs. Spin-phonon coupling is general behavior of hexagonal manganites ? Ultrafast Quantum Optics Lab.

Result of YMn. O 3 1. Coherent acoustic phonon (period ~32 ps) above TN KAIST 2. Below TN, other frequency mode is observed in the magnetic ordering phase. Similar behavior in other hexagonal manganite ! Ultrafast Quantum Optics Lab.

Comparison between hexagonal manganites Same behavior in Lu. Mn. O 3 and YMn. O 3 ! KAIST Jang et al. , NJP 12, 023017 (2010) Jang et al. , in preparation v Used pump pulse energy is near resonant d-d transition. v Acoustic phonons are strongly coupled to spin fluctuation above and below TN. Ultrafast Quantum Optics Lab.

§ Lattice motion related charge-ordering phase in La 2/8 Pr 3/8 Ca 3/8 Mn. O 3. § Spin-lattice coupling in Lu. Mn. O 3. KAIST § Phonon related iso-structural transition in hexagonal manganites. La 2/8 Pr 3/8 Ca 3/8 Mn. O 3 Lu. Mn. O 3 YMn. O 3 Optical phonon (DECP) 2. 4 THz & 5. 1 THz 3. 6 THz X Acoustic phonon (strained pulse propagation) phonon-CO phase coupling spin-lattice coupling 34 GHz 47 GHz 31 GHz iso-structural transition Ultrafast Quantum Optics Lab.

Conclusion § Lattice motion related charge-ordering phase in La 2/8 Pr 3/8 Ca 3/8 Mn. O 3. § Spin-lattice coupling in Lu. Mn. O 3. KAIST § Phonon related iso-structural transition in hexagonal manganites. La 2/8 Pr 3/8 Ca 3/8 Mn. O 3 Lu. Mn. O 3 YMn. O 3 Optical phonon (DECP) 2. 4 THz & 5. 1 THz 3. 6 THz X Acoustic phonon (strained pulse propagation) phonon-CO phase coupling spin-lattice coupling 34 GHz 47 GHz 31 GHz iso-structural transition Ultrafast Quantum Optics Lab.

KAIST Ultrafast Quantum Optics Lab.

KAIST Ultrafast Quantum Optics Lab.

Time-scale of phonon KAIST Coherent phonons (solid state lattice vibration) Time scale < 10 -12 sec Ultrafast Quantum Optics Lab.

Phonons in solid 10 -15 Energy transfer to the electrons 10 -13 Optical phonons 10 -12 10 -11 Acoustic phonons time (s) Equilibrium poisitions of atoms KAIST Acoustic vibration: The two atoms on the unit cell vibrate along the same direction Optical vibration: The two atoms on the unit cell vibrate in opposing motion Ultrafast Quantum Optics Lab.

Optical phonon : ISRS • ISRS (Impulsive Stimulated Raman Scattering) (not used in our experiment) where polarizability KAIST driving force : Ultrafast Quantum Optics Lab.

Complement KAIST where , Ultrafast Quantum Optics Lab.

Optical phonon : DECP KAIST v β is large - oscillatory term is depressed due to - rapid return of the quasi-equilibrium displacement to zero - sin-like oscillation is not negligible (phase shift from cosine) Ultrafast Quantum Optics Lab.

Fit by DECP (LPCMO) KAIST 예심 Sb 0. 009 Bi -0. 0174 Te 0. 0351 Ti 2 O 3 0. 0425 Ultrafast Quantum Optics Lab.

Complement KAIST where Ultrafast Quantum Optics Lab.

Complement KAIST Ferromagnetic metal - Colossal Magnetoresistance Charge-ordering insulator - Anti-ferromagnetic insulator Ultrafast Quantum Optics Lab.

Complement 20 K (I) In LPCMO, there coexist ferromagnetic metallic (FM) phase and charge-ordering insulating (CO) phase. Charge-disordered domain (ferromagnetic metallic) Charge-ordered domain KAIST 17 K (I) Fs study of phase-separated manganite, there is a strong opportunity to optically control the competing ground states between metallic and insulating phases. 120 K(II) Ultrafast Quantum Optics Lab.

Coherent optical phonon KAIST lose coherence get random in 1~10 ps (dephasing) The dephasing is caused by coupling with electron and phonon, and by scattering by crystalline impurities and defects. Ultrafast Quantum Optics Lab.

KAIST Electronic decay of DECP Ultrafast Quantum Optics Lab.

KAIST Phonon decay of DECP Ultrafast Quantum Optics Lab.

KAIST Cosine-like DECP Ultrafast Quantum Optics Lab.

Damping amplitudes of phonons KAIST In the CO phase, the different charges on the Mn 3+ and Mn 4+ sites cause out-of-phase Mn vibrations have a nonzero net charge fluctuation that couples to the polarizability. (Mn 3+-O-Mn 4+) Ultrafast Quantum Optics Lab.

Complement KAIST Non-oscillatory relaxations Ultrafast Quantum Optics Lab.

Complement KAIST • Two absorption bands in s(w) in Bi 1 -x. Cax. Mn. O 3 : Liu et al. , PRL 81, 4684 (1998). • Two IR bands in s(w) in La 1/8 Sr 7/8 Mn. O 3 : Jung et al. , PRB 59, 3793 (1999). • Electron Microscopy of Co domains La 5/8 -y. Pry. Ca 3/8 Mn. O 3 : Uehara et al, . Nature 399, 560 (1999). • Two absorption bands in s(w) in La 5/8 -y. Pry. Ca 3/8 Mn. O 3 : Lee et al. , PRB 65, 115118 (2002). • Coherent phonons in La 1 -x. Cax. Mn. O 3 : Lim et al, . PRB 71, 134403 (2005). • Raman scattering study of Lax. Pry. Ca 1 -x-y. Mn. O 3 : Kim et al. , PRB 77, 134411 (2008). Dearth of femtosecond study Ultrafast Quantum Optics Lab.

KAIST Complement Amelitchev et al. , PRB 63, 104430 (2001) Ultrafast Quantum Optics Lab.

KAIST Fast oscillation v 3. 63 THz coherent optical phonon v optical phonon maintain till to 10 ps v below 90 K, no optical phonon Slow oscillation v 47 GHz coherent acoustic phonon v below 90 K, acoustic phonon is disappeared. Ultrafast Quantum Optics Lab.

Optical phonon KAIST v Similar with the lowest optical phonon mode (3. 57 THz) reported by Souchkov et al. * *Souchkov (2003) et al. , PRL 91, 027203 Acoustic phonon v Weakly softening behavior below ~160 K down to 90 K. v The acoustic phonon is strongly coupled to spin fluctuations above and below TN. Ultrafast Quantum Optics Lab.

KAIST Complement Ultrafast Quantum Optics Lab.

KAIST Fast & slow oscillations Ultrafast Quantum Optics Lab.

Dephasing and relaxation times KAIST § The excited electrons relax faster than nuclei. § For acoustic dephasing time, the skin depth is considered. § Another time scale is the absorption time constant, Ultrafast Quantum Optics Lab.

KAIST Complement Ultrafast Quantum Optics Lab.