IR Pump-Probe Study of Phase Separated Hole-Doped Manganite La1/4Pr3/8Ca3/8MnO3

Jaewook AhnKAIST - physics

Ultrafast Dynamics in Strongly-Correlated Materials.

IRMMM-THz, September 23rd, 2009, Pusan

Coauthors Kyeong-Jin Jang and Jongseok Lim (KAIST) Jihee Kim and Ki-Ju Yee (Chungnam Nat. Univ.) Jai Seok Ahn (Pusan Nat. University)

Fundings

Charge Lattice

Orbital Spin

Correlated

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-re-solved 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.K

AIS

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ics

Palo Alto, CA 1872

Ultrafast Phenomena in 1872

Leland Stanford Eadweard Muybridge

Time Resolution:1/60th of a second

Bet: Do all four hooves of a galloping horse ever simultaneously leave the ground?

Courtesy of R. Trebino

Ultrafast phenomena in condensed matter

Materials with correlated electrons exhibit some of the most intrigu-ing 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

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Current Days Ultrafast Phenomena

M. Uehara et al., Nature 399, 560(1999)

Motivation : La5/8-yPryCa3/8MnO3

IIIIII

Structural phase transi-tions

III : paramagnetic insula-tor

II : short-range ferromagnetic metal short-range charge-ordering phase

I : long-range FM and CO phases

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 technolo-gies such as read/write heads for high-capacity magnetic storage and spintronics.

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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 EarthCaMnO3 :Alkaline Earth

Perovskite structure ABO3 A in an 8 BO6 octahedra

A : 12-fold oxygen coordination

B : 6-fold oxygen coordination

33/83/81/4 MnOCaPrLa

3xx-1 MnOAERE

B

AO

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M. Uehara et al., Nature 399, 560(1999)

Motivation : La5/8-yPryCa3/8MnO3

IIIII

I

Structural phase transi-tions

III : paramagnetic insula-tor

II : short-range ferromagnetic metal short-range charge-ordering phase

I : long-range FM and CO phases

17K (I) 120K(II)

In TC≤T≤TCO region (II),two phases (FM and CO) co-exist.

M. Uehara et al., Nature 399, 560(1999)

20K (I)

Charge-disordered domain(ferromagnetic metal-lic) Charge-ordered do-

main

Mixed Phase in La5/8-yPryCa3/8MnO3

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17K (I) 120K(II)

20K (I)

Charge-disordered domain(ferromagnetic metal-lic) Charge-ordered do-

main

Mixed Phase in La5/8-yPryCa3/8MnO3

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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.

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 sam-pling/sec

Pulse power=10mW, energy=20nJ

Refer-ence

Pump

ProbeBS

BS

LensSam-ple

PhotoDiode

Shaker

Laser

Our Measurements

Fast oscillation Slow oscillation

Jang, Lim, Ahn, et al., (2009)

Reflectance change in ~ps

dR/R

2,13,2,1

)cos(i

ti

iii

ti

ii eBteADR

R

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.

Displacive Excitation of Coherent Phonon

ttee tt sin

'cos

DECP Model Fit :

DECP (Displacive Excitation of Coherent Phonon)

Coherent phonon generation

where n(t) is the electron densityin excitation band

( )( ) ( )

dn tP t n t

dt

0

22

2[ ( )] 2

Q QQ n t

t t

0( ) ( )pumpn t g t e d

20

2 2 00

( )

( ){ (cos sin )}2

pump

Q t

g t e e d

Coherent phonon generation

0( ) ( )pumpn t g t e d

20

2 2 00

( ) ( ){ (cos sin )}2

pumpQ t g t e e d

where ,

2 20

( ) 1( ) ( )

R t R Rn t Q t

R R n Q

20

2 20

c s i2

o)

s n( t t tR t

Ae B e eR

t t

DECP (Displacive Excitation of Coherent Phonon)

Two Optical Phonons

5.15 THz2.43 THz

Non-oscillatory relaxationsiA

Is this anharmonic behavior ? - X (not enough fluence) - 10,000 times smaller than Lindemann cri-

teria 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 ?

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.

Strain propagation in bulk

1. The strained layer which is generated by pump pulse at surface moves through sample at velocity of Cs.

2. The interference of reflected probe pulse at the surface and at z shows an oscillatory behavior.

strained layer

R. Liu et al. PRB 72 (2005)

Strain propagation in bulk

( , ) sin 2sin( )R

t n kt

Td

R

20 1 2 ( )r r r r O n 2

1i k zr e n 2 ( )

2i k z dr e n

2 2

0 02

0

( , ) sin( ) sin(2 ) cos(2 )r r rR

t k d n k z k d k z k dR r

where 2

probe

s s

cT

n C nC

n n i

KAIST

Coherent Phonon Amplitudes

Reflectance change in ~ps

dR/R

2,13,2,1

)cos(i

ti

iii

ti

ii eBteADR

R

Scenario: Charge-Ordering Gap

]/)(exp[)(

21

]2/)(/[)(3

TkTT

Tka

TkTITB

BCOCO

B

BCO

BCS-like Gap Explains the Coherent Phonon Bahavior.

Crossover of phonon amplitudes may be also coupled with the relaxation ?

Relaxation I

Fast

ele

c-

tronic re

-

laxatio

n

coherentphonondephas-ing

In a few ps, the dR/R is related to a change phonon temper-ature. - the excited electrons relax through electron-phonon coupling.

dR/R

Spin-lat-tice relax-ation

Metallic behav-

ior:

Acoustic Phonon

Relaxation II

Spin-lattice relaxation

On a longer time scale,spin reorientation may occur.

dR/R

#

# M. Uehara et al., Nature 399, 560(1999)

Strain Pulse Propagation

Summary

Fast

ele

c-

tron

ic re-

laxatio

n

coherentphonondephas-ing

Spin-lat-tice relax-ation

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 re-laxation

dR/R

IR Pump-Probe Study of Phase Separated Hole-Doped Manganite La1/4Pr3/8Ca3/8MnO3

Jaewook AhnKAIST - physics

Ultrafast Dynamics in Strongly-Correlated Materials.

IRMMM-THz, September 23rd, 2009, Pusan

Coauthors Kyeong-Jin Jang and Jongseok Lim (KAIST) Jihee Kim and Ki-Ju Yee (Chungnam Nat. Univ.) Jai Seok Ahn (Pusan Nat. University)

Fundings

Charge Lattice

Orbital Spin

Correlated

fs study of CO phase?

Coherent phonons in La1-xCaxMnO3 : Lim et al,. PRB 71, 134403 (2005).

These oscillations are related to charge-ordering phase transition.

~2.2 THz oscillation below TCO

~55 GHz oscillation above TCO