Femtosecond pump-probe study of carrier- relaxation dynamics in the Metal-Insulator transition compound NdNiO

Femtosecond pump-probe study of carrier- relaxation dynamics in the Metal-Insulator

transition compound NdNiO

P.Ruello

, B.Perrin

, T.Pézeril

, V.Gusev

, P.Laffez

.

1/ Laboratoire de Physique de l’Etat Condensé, UMR 6087 CNRS-Université du Maine

Av. O.Messiaen 72085 Le Mans, France

2/ Laboratoire des Milieux Désordonnés et Hétérogènes, UMR 7603 CNRS- Université Pierre et Marie Curie, 4 Pl. Jussieu, 75252 Paris, France

Metal-insulator transition in Thin NdNiO ₃ film

I U

1 10 100 1000 10000 100000

50 100 150 200 250 300 350

T(K)

resistivity (ohm.cm)

NdNiO

/Si(100)

Literature survey

Structural point of view

•Structural refinements (neutron diffraction) (Garcia-Muñoz, PRB 1992 )

•Raman spectroscopy and TEM (Zaghrioui et al, PRB 2001 ) The origin of the MIT in the RNiO

compound

(R= Sm, Pr, Nd)

MIT = first order phase transition T<T

T>T

P2

/n Pbnm

•Decrease of the Ni-O bounds

•Tilt of the super- exchange angle

O-Ni-O

The ZSA Model (Zaanen et al, )

E_F

Ni(3d)

O(2p)

∆

W

T<T

T>T

Ni Ni

O θ Ni O Ni

Ni

O Nd

Satisfactory for NdNiO

, SmNiO

, PrNiO

,

Dynamics point of view

IR Spectroscopy

•At T<T

self-trapping of electron in a polaronic medium is evidenced

•Isotopic exchange (

O /

O)

T

(

O) - T

(

O)=+10.3K

Electron-phonon interaction is

therefore a driving mechanism for

MIT

Experimental set-up

Reflectivity measurement

probe

λ/4 λ/4

pump H

H Detector V

Si Diode

Pump beam stop

cryostat

λ/4

polarisator

AOM Acousto-Optic Modulator Lock-In Amplifier

probe

λ/4 λ/4

pump H

H Detector V

Si Diode

Pump beam stop

cryostat

λ/4

polarisator

AOM Acousto-Optic Modulator Lock-In Amplifier

λ =770nm (h ν =1,6 eV) Pump energy = 180mW Probe energy = 13mW Flux ~ 0,6mJ.cm

(1 photon for 42 unit cells)

Sample = 120nm film

ξ =60nm (absorption length)

NdNiO ₃ : Charge transfer Metal-insulator

10eV

O(2p)→Ni(5d4f)

5eV

CT excitation range

Ni (3d) O (2p)

Ni (3d) LH Ni (3d) UH

Zaanen et al, PRL 1985, Katsufuji et al, PRB 1995

Ni(5d4f)

Mott transition ?

Mott transition

In our study :

Laser irradiation creates 0,02 e

/Ni

Recent study

: If number of photoexcited

electron is >0,1e

/Ni [Ni(chxn)

Br]Br

Probably no Mott

transition induced

by the Laser

Results

Transient reflectivity versus temperature

R R

∆

270K

124K T

C B

A

C

Characteristics of optoacoustical spectra

•A-No change of the first echo shape but an evanescent echo with decreasing temperature

•B-A large increase of the magnitude of the fast component : new electronic dynamics when

T<T

•C- strong thermal signal below the MIT

Thermal variation of the first acoustic echo

•The magnitude decreases when T decreases

•The shape remains the same

No variation of

⇒ n, κ , dn/d η , d κ /d η

270K 222K 200K 182K 164K

146K

Consistent with optical conductivity measurement on bulk

(Katsufuji et al, PRB 1998 )

The shape remains the same No variation of

⇒ n, κ , dn/d η , d κ /d η are constant

E=1,6eV

Probe energy

The echo shape

Fast component versus temperature

The optical parameters, at

thermodynamic equilibrium, remain the same for E=hv=1,6eV :

⇒ The sharp increase is then related

to intrinsic dynamics

Fitting of the leaving echo at the short time scale

Classical model (Maris et al,

Thomsen et al) : - thermal stress is the only parameter - no hot electron diffusion

200K

124K

164K

Renormalized fast component versus T

Relaxation time

270K ⇒ 124K

τ ~ 0.5ps ⇒ τ ~2.5ps

Thermal signal

At τ > few ps, the thermodynamical equilibrium is reached

Heat conductivity κ

>>> κ

The heat transport is then less efficient in the insulating state.

0 0,2 0,4 0,6 0,8 1 1,2

90 140 190 240

T(K)

Thermal gap at 43ps

D=K/C

K/ σ =(3/2)*(k/e)²T

Simplistic model

(Wiedeman-Franz law)

D

Why a change of the dynamics at MIT?

Change of the electron

lattice interaction evidenced by photoinduced absorption in IR range

(Mroginski et al, PRB 1999)

Polaron peaks at : - 0,1eV

- 0,28eV

(Massa et al, PRB 1997)

Strong similitude with HTSC and CDW SC

• Large increase of the fast component

• Increase of the relaxation time

⇒ Hot phonon bottleneck

(Kabanov et al, PRB 1999, Demsar et al, PRL 1999)

Single particule relaxation

mechanism in a schematic parabolic band

T<T

T>T

Phonon emission

holes

E

E

Phonon

emission

Photo-induced absorption

Argon Laser irradiation (488 nm) IR

spectroscopy

- ∆ T/T=(T

-T)/T

Polaron states are formed and detected

ν

The femto-second experiment

time resolved photo-induced reflectivity

⇒ The polaron relaxation might contribute to the overall relaxation

process.

Consistency between relaxation time and electron-lattice interaction strength

• According to this point of view the relaxation process would be driven by e-lattice interaction (trapping) only

-Is this approach can explain the sharp

increase of the fast

component of ∆ R/R ?

0 2 4 6 8 10 12 14 16

50 100 150 200 250 300 350 400 450

T(K)

SmNiO₃

TMI

Mroginski et al, PRB 1999

Polaron binding energy E

~h ϖη /2

NdNiO

Kabanov et al, PRB 1999

YBACUO

Similitude with HTSC and CDW SC

Magnitude

of the fast

component

Single particule relaxation mechanism in a schematic parabolic band

T<T

T>T

Phonon emission

holes

Phonon emission

E

E

High frequency phonon

emission

Fast component increase

• laser pulse (0,6mJ.cm

) ⇒ 0,025 e-/Ni site

⇒ Frequency plasma of the photoexcited electron accumulated in the bottom of the CB

⇒ ω

~ (h ω

~0,8eV)

E

> E

⇒ Fermi’s golden rule OK

∆ R/R ~- δ ( n

 <H

>  ² ρ

(E))

~ - δ ( n

)  <H

>  ² ρ

(E)

n_pe : photo-excited carrier

E

Transient reflectivity of the fast component

E

E

E

E

E

E

Bottleneck effect

• Accumulation of relaxed electron in the bottom of the CB. A near steady state equilibrium between high frequency phonon

and photo-excited carrier Phonon with E(=hw

) > E

only

are concerned

In NdNiO

Only LO phonon

fulfill this condition

Optical phonon in NdNiO ₃

w(LO) (cm-1) hv(LO) eV 185,1 0,02295 321,8 0,03990 391,5 0,04855 428,8 0,05317 435,8 0,05404 542,1 0,06722 567,1 0,07032 1108,5 0,13745

E

~ 0,1eV

May are concerned in

hot phonon bottleneck

effect

Summary

• Evidence of a change of the electronic relaxation mechanism above and below MIT

• Both processes might contribute to this relaxation process :

* polaron self-trapping of photo-excited carriers

* bottleneck effect due to LO phonon

Quantitative calculations and modelling are required to

assess the above idea

Unanswered question

• Why the acoustic echo magnitude is vanishing when t decreases ?

* problem of detection ?

* intrinsic property ?

Perspectives

* Bottleneck effect exists :

⇔ LO phonon lifetime τ

> τ

(Raman studies of LO phonon linewidth)

* New studies on MIT compounds NdNiO

with various electronic structure

Ni(3d)

O(2p)

∆

150nm

17nm 70nm

E_g

W

Laffez et al, Eur.

Phys. J. (2003)