IR absorption due photo-generIR absorption due photo-generated carriers in quantum paraated carriers in quantum para

electric SrTiOelectric SrTiO33

H. Okamura, M. Matsubara, T. Nanba – Kobe Univ.

K. Tanaka – Kyoto Univ.

SrTiOSrTiO33: “Quantum Paraelectric”: “Quantum Paraelectric”

~ 35 K, but paraelectric down to 0.3 K. Quantum (zero-point) fluctuations of Ti

~ 20000

35 KMüller et al, PRB 19 3593 (’79).

Photo-induced enhancement of Photo-induced enhancement of dielectric constantdielectric constant

enhanced by UV photo-excitation Photocarriers vs lattice

Hasegawa et al., JPSJ 72, 41 (‘03) Takesada et al., JPSJ 72, 37 (‘03)

Optical properties (EOptical properties (Eg g = 3.2 eV)= 3.2 eV)

Luminescence

IL(t) localized quasiparticles Polarons, self-trapped excitons, etc.

Absorption

Grabner, Phys. Rev. 177, 1315 (1969).

Stokes shift ~ 0.7 eV

Large energy relaxation before recombination

Photoconductivity of SrTiOPhotoconductivity of SrTiO33

Mobile quasiparticles are present.

Katsu et al., Jpn. J. Appl. Phys. 39 (2000) 2657.

Eg

High conductivity when h > Eg

This work: This work: IR absorption due to photocarriersIR absorption due to photocarriers

c.b. Ti 3d

v.b. O 2p

hin

Information about photocarriers and in-gap states

hIR

IR absorption

hPL

・ Stokes shift (0.7 eV)

・ Relaxation processes

・ In-gap states

ExperimentalExperimental

STO single crystals

Near-normal incidence, R() and T().

Frequency-doubled Ti:Sapphire laser.

IR beam line BL43IR at SPring-8.

0

0.5

1.0

0.01 0.1 1 100

2

4

6

Photon Energy (eV)

Re

flect

ivity

Co

nd

uct

ivity

(1

03

-1cm

-1)

RR(() and ) and ) : 4 meV - 35 eV) : 4 meV - 35 eV

SrTiO3

295 K

Transparent region

Eg

Transmission with UV laser on / offTransmission with UV laser on / off

Two broad absorption bands (in-gap states). Stronger absorption at lower T. 　　　　　

　　　 (similar to luminescence)

0

0.5

1.0

0 0.5 1.0 1.5

SrTiO3laser = 365 nm (h = 3.4 eV)Plaser = 0.22 W/cm-2

8 K

80 K

Photon Energy (eV)

- T

/ T

(

10

-2)

(Photo-induced decrease in the IR transmission)

Excitation Power DependenceExcitation Power Dependence

0

1

2

0 0.2 0.4 0.6 0.8 1.0 1.2

SrTiO3 T = 8 Klaser = 365 nm (3.40 eV)

Laser power [W/cm2]

1.00.64

0.32

0.16

0.08

0.03

Photon Energy (eV)

-T

/ T

[1

0-2

]

Reflection with UV laser on / offReflection with UV laser on / off

Absorption also observed in the reflection. Not simple Drude response Photo-induced mid-IR absorption band

0

0.005

0.010

0.015

0.020

0 0.2 0.4 0.6 0.8 1.0

Transmission

Reflection

(laser) = 365 nmPower =0.9 W / cm2

T = 8 K

Photon Energy (eV)

Ph

oto

-ind

uce

d a

bso

rptio

n

Model for photo-enhancement oModel for photo-enhancement of f based on polarons based on polarons

Strong e-ph coupling Soft mode (Ti) Breathing mode (O)

K. Nasu, Phys. Rev. B 67, 174111 (2003).

t2g

eg

(3d)1

t2g

eg

“Large polarons” and “small polarons” Large polarons (extended over many sites): conduc

tion, metastable Small polarons (at one site) : no conduction, stable

Analogy with strongly correlated Analogy with strongly correlated “dirty metal” oxides“dirty metal” oxides

Conduction due to hopping and/or tunneling Chemically- doped Mott insulators. “Incoherent peak” in optical spectra

Coherent peak (usual Drude peak)

Alternative view : binding energy of polarons Microscopic models needed.

Incoherent peak (mid-IR peak)

0≠0

SummarySummaryIR absorption in SrTiO3 under photo-excit

ations above Eg

– Two broad absorption bands (mIR-visible)

– Incoherent carrier dynamics (“generalized Drude response”) and/or real trapping states.

– Microscopic origin ? large / small polarons ?

– Similar to those observed for n- and p-type SrTiO3 (IR, photoemission, and XAS)

Kramers-Kronig analysisKramers-Kronig analysis

3 phonon modes.

-500

0

500

1000

200 400 600

Im]

Re[]

x10

0

500

1000

1500

0 200 400 600

Wavenumber (cm-1)

[

-1cm

-1] SrTiO3

295 K

Soft phonons vs temperatureSoft phonons vs temperature

Detailed temperature dependence of the softening

0.85

0.95

20 40 60 80 100 120

8 K40 K80 K120 K160 K200 K240 K295 K

80 K

8 K

295 K

Wavenumber (cm-1)

Re

flect

ivity

0

10

20

0 100 200 300

Temperature (K)P

ea

k W

idth

(cm

-1)

540

542

544

546

0 50 100 150 200 250 300

172

173

174

175

0 50 100 150 200 250 300

Pe

ak

po

sitio

n (

cm-1

)

020406080

0 50 100 150 200 250 300

Temperature (K)

Temperature dependenceTemperature dependence

W !

?

Lower-frequency data needed !! (in progress).

?

?

?