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Electric Pulsed Induced Changes in Oxides 2012

K. Shibuya

Phase control of bistable metal-insulator states in

vanadium dioxide

AIST


Where is RIKEN and AIST?

RIKEN :the institute of physical and chemical research

Narita airport

City center

2

60km


Background of vanadium dioxide

10-4

10-3

10-2

10-1

100

101

102

Res

istiv

ity (

cm

)

4003002001000

Temperature (K)

V0.91W0.09O2

VO2

Room temperature

Metal

Insulator

TMI above room temperature

Chemical substitution

Metal InsulatorTMI = 340 K

Rutile P42/mnm Monoclinic P21/C

~ 0.5 eV

3


4

Sneak current TiO2

VO2

TiO2/VO2


Background of vanadium dioxide

Insulator

Metal

Smart window

Tran

smitt

ance

Temperature

TMI

M. Soltani et al, J. Vac. Sci. Thechnol. A25, 971 (2005).

After AIST press lease (2005)

Photo switch

Thermochromic property

SapphireVO2

λ=1.55 µm

Thermal control Electric-field, photo, pressure control

Metal

PhotonElectric field

Winter Summer

Pressure

5


Outline

1. Phase control by chemical substitution2. TiO2/VO2 superlattices3. VO2/Nb:TiO2 junctions4. Electric-field-induced phase transition5. X-ray photo-induced phase transition6. High-pressure-induced phase transition

6


Overall supervisionRIKEN & U. Tokyo: M. Kawasaki & Y. Tokura

X-ray Diffraction RIKEN: D. Okuyama, Y. Taguchi, & T. ArimaKEK: Y. Yamasaki, K. Kobayashi, H. Nakao, R. Kumai, & Y. Murakami

Photoemission spectroscopyU. Tokyo: E. Sakai, K. Yoshimatsu, H. Kumigashira, & M. Oshima

Optical studyGIST: J. S. Lee

Electric Double Layer Transistor (ELDT)RIKEN: M. Nakano, T. Hatano, & Y. Iwasa

High pressure measurementRIKEN: C. Terakura

Acknowledgements

7


1. Phase control by chemical substitution

8


400

300

200

100

0

Tem

pera

ture

(K)

0.200.150.100.050.00

x in V1-xWxO2

-4

-2

0

2

Log(Resistivity/

cm)

Metal

Insulator Insulator

Electron-doping in VO2 thin films

K. Shibuya et al, APL 96, 022102 (2010)

Spin-singlet formation vs. kinetic energyThe dimerization is destabilized with electron-dopingV1-xWxO2 /TiO2(001)

Electron-doping can control the transition temperature and the metallic state is stabilized.

(Mott)

9


Optical study for V1-xWxO2 thin films

Metal

Metal

Insulator

Insulator

Lower doping (x=0.05) Charge and lattice dynamics

J. S. Lee, K. Shibuya et al, PRB 85, 155110 (2012)

10


Photoemission study for V1-xWxO2 thin films

E. Sakai, K. Shibuya et al, PRB 84, 195132 (2011)

Spin-Peierls

Mott insulator

Lower doping

Higher doping

Electron

V4+

V3+

Electron correlation

MetalInsulator

Electronic structure

W5d+O2p

11


2. TiO2/VO2 superlattices

12


TiO2/VO2 superlattices

3d1/3d0 interface

TiO2(001)VO2

TiO2

VO2

TiO2

VO2

TiO2

nmnmnm

Motivation:Interface properties in rutile

A superlattice of rutile-type layers (m=5)

PRL 102, 166803 (2009) Prediction:VO2 metallic ground state

13


TiO2/VO2 superlattices

Structural change

Robust V-V dimerization

(TiO2)m/(VO2)n

Neither metallic nor ferromagnetic

Low temp

High temp

14


Both W:VO2 and TiO2 layers are rutile.

TiO2/V0.92W0.08O2 superlattices

The revival of the insulating ground state was observed.

The inherently metallic electron-doped VO2 layer was alternatively utilized.

K. Shibuya et al, PPB 82, 205118 (2010).

15


The bulk-like metallic phase is stabilized when the film thickness exceeds 6 ML.

SrVO3/SrTiO3

Photoemission study:K. Yoshimatsu et al., PRL 104, 147601 (2010).

In Perovskite

single layer

single layer

TiO2/VO2

TiO2/W:VO2

SrVO3/SrTiO3

Discussion

Spin-singlet state is favored at the interfaces due to strong electron-lattice interaction.

D. H. Kim et al., Solid State Commun. 114, 473 (2000).

16


3. VO2/Nb:TiO2 junctions

17


V1-xCrxO2 thin films400

350

300

250

200

150

Tem

pera

ture

(K)

0.300.250.200.150.100.050.00y in V1-yCryO2

-3

-2

-1

0

1

2

Log (Resistivity/ cm

)

Insulator

MetalThin film

Bulk

18Phys. Rev. B 8, 1323 (1973)


1 2 1 1

21

′1

1

VO2/Nb:TiO2 junction

19

TiO2 VO2

No indication of depletion layer in VO2

Hole-dopingElectron-doping

Nb:0.05wt%


VO2/Nb:TiO2 junction

20

VO2 work function∼ 5 eV

A. Sawa et al, APL 90, 252102 (2007)

Perovskite


4. Electric-field-induced phase transition

21


22

Electric-field induced phase transition

VO2

M. Nakano, K. Shibuya et al, Nature 487, 459 (2012)

a,b, A schematic (a) and an optical micrograph (b) of an EDLT with a coplanar gate electrode. A small droplet of an ionic liquid, DEME-TFSI, covers both a channel and a gate electrode so that ions can freely move between these areas back and forth depending on a gate voltage, enabling electrical switching.


23

Electric-field induced phase transitionNon-volatile action

Insulator

MetalThe temperature dependence of the sheet resistivity under different VG. Once a positive gate bias is applied, the metal-insulator transition temperature decreases drastically. Above VG = 0.8 V, the sample shows almost metallic behavior.


￥


M. Nakano, K. Shibuya et al, Nature 487, 459 (2012).

Gate voltage

Carrier injection by E

24


5. X-ray photo-induced phase transition

25

Electric Pulsed Induced Changes in Oxides 2012PRL 87, 237401 (2001)

Optical phonon

Rutile

Monoclinic

VO2 single crystal at 300 K

Photo-induced phase transition at room temperatureBut not persistent

26


X-ray induced phase transitionV0.935W0.065O2

X-ray

X-ray inducedt = 80 nm

hν = 9.8 keV

K. Shibuya et al, PRB 84, 165108 (2011)

X-ray

27


A negligible relaxation process was seen without X-ray irradiation. The persistent phase transition was confirmed.

X-ray induced phase transition

XRD patterns during the phase transition

metal

insulator0 s

2000 s

Volu

me

frac

tion

of

the

met

al p

hase

Con

duct

ivity

（S/

cm）

Irradiation time (s)

T = 9 K

X-ray photon MetalInsulator

8.4 × 1011 photon/cm2 s

30nm

28


Flux dependence and scaling

Photon flux1012101310141015 Percolation

processfor t

he in

sula

ting

phas

e

Converted from

diffraction intensity data

scVV 0

29

The insulator-metal transition is well-scaled with the time-integrated photon density, independent of flux density.


6. High-pressure-induced phase transition

30


[100]M2 // [001]R

more insulating

Adv. Mater. 22, 5134 (2010).

31


V0.94W0.06O2/TiO2(001)

High-pressure effect

Pressure cell

Cubic anvil systemHydrostatic pressure

t = 100 nm

0GPa

10GPa

Hydrostatic pressure induces an insulator‐metal transition.

32


With use of DAC and synchrotron X-ray

X-ray diffraction under high pressure

V0.94W0.06O2/TiO2(001)

(101)Insulator

Metal

TMI

TMI

33


High pressure induces a correlated metal.

Pressure

Pressure

Hall measurements for V0.933W0.067O2 under hydrostatic pressures

With use of a piston-type pressure cell

34


3D phase diagram

MetalIns.

Pressure-induced insulator-metal transition was seen at the phase boundary.35


Summary

MetalVO2

Structure change

Electron-doping

(External) Pressure

X-ray(Light)

YesYes

Electric field Structurestrain

Insulator

YesYes YesYes

No, so farNo, so far

The metallic state is stabilized by chemical substitution. The collective dimerization is robust even at the interfaces. Electronic phases of VO2 can be controlled by external stimuli such as electric field, X‐ray, and high‐pressure. The electronic state is closely interrelated with the lattice‐structural change. The electric‐field control is the most powerful perturbation for device application.

YesYes

36

Electron correlation&

Electron lattice coupling

Electron correlation&

Electron lattice coupling


Back-up

37


Y. Muraoka et al., APL 80, 583 (2002).

VO2

TiO2Lattice mismatch (%)

a (nm) c (nm)

0.45520.4593

0.28460.2959

‐0.89 ‐3.82

RutileRutile

Crystal

VO2(001)/TiO2(001)

-4

-2

0

2

c-ax

is la

ttice

mis

mat

ch (%

)

-4 -2 0 2 4

a-axis mismatch lattice (%)

VO2

V0.93W0.07O2

V0.80W0.20O2

TiO2

V0.67W0.33O2

All thin films were grown on TiO2(001) surfaces by PLD.38


X-ray diffractionIn

tens

ity (a

rb. u

nits

)

6866646260

2theta (degrees)

x=0.20

x=0.13

x=0.09

x=0.07

x=0.04

x=0

TiO2(002)

x=0.33

V1-xWxO2

105

103

101

0.298

0.296

0.294

0.292

0.290

0.288

0.286

0.284

0.282

c-ax

is la

ttice

con

stan

t (nm

)

0.200.150.100.050.00

x in V1-xWxO2

Powder (Israelsson et al.) Thin films (this work)

TiO2

monoclinic

rutile

The out‐of‐plane lattice constants change linearly against W‐doping concentration.

thickness: 30 – 40 nm

Tetragonal

monoclinic

Tetragonal

monoclinic

Vegard’s law

39


Optical study for V1-xWxO2

Larger mass of W

V-O stretching

The V-V dimerization is systematically weaken with W concentration.

J. S. Lee, K. Shibuya et al, PRB 85, 155110 (2012)40


V4+ V3+V3.5+V3.8+ V3.3+

A. Perucchi et al.J. Phys.: Condens. Matter 21, 323202 (2009).

Comparing with V magnéli phases: VnO2n-1

Vanadium magnéli phases VO2 V2O3

A similar picture can be applied to W:VO2?

400

300

200

100

0

Tran

sitio

n te

mpe

ratu

re (K

)

0.300.250.200.150.100.050.00

x in V1-xWxO2

Metal

Insulator

n=3

n=2

n=4

n=5

n=6

V2n-1On TMI TN

PI

AFI

W:VO2

41


S. Kachi, AIP Conf. Proc. 10, 714 (1973).42


The magnetic inflection point corresponds to the metal‐insulator transition temperature.

101

102

103

104

105

106

Res

istiv

ity (O

hm c

m)

4003002001000

Temperature (K)

TMI

0.006

0.005

0.004

0.003

0.002

0.001

0.000

M/H

(em

u/m

ol)

3000 Oe

Antiferromagnetic?

43

V0.85W0.15O2 polycrystal


TiO2/V1-xWxO2 (x = 0 or 0.08) superlattices

3d1/3d0 superlattices in Perovskite

- LaTiO3/SrTiO3 (metallic interface)A. Ohtomo and H. Y. Hwang, Nature 427, 423 (2004).

- SrVO3/SrTiO3 (metal-insulator transition)D. H. Kim et al., Solid State Commun. 114, 473 (2000).

3d1/3d0 interface

TiO2(001)VO2

TiO2

VO2

TiO2

VO2

TiO2

nmnmnm

Motivation:Interface properties in rutile

A superlattice of rutile-type layers(m=5)

Strong e-lattice coupling?44


Robust V-V dimerization

Neither metallic nor ferromagnetic K. Shibuya et al, PPB 82, 205118 (2010)

(TiO2)m/(W:VO2)n

(TiO2)m/(VO2)n

TiO2/V1-xWxO2 (x = 0 or 0.08) superlattices

45


46



47



48



(1)

(2)

(3)

(4)

XRD patterns

The X-ray induced phase transition is accompanied with the revival of metallic conduction. The conductivity is strongly coupled with lattice.

X-ray was irradiated at 9 K,and off during heating.

rutile

monoclinic

Lattice constant and resistivityOn

Off

5.4 × 1014 photon/cm2 s

49


Insulating phase

metal fraction

The metal volume fraction was estimated, assuming that two phases exist which form a nanoscale phase separation.

The threshold difference between conductivity and metal volume fraction was observed, indicating a percolative conduction.

80 nm

Threshold photon density

3DPercolation model

V

σ

50


Possible mechanism

X-ray

Photo-excitation

Core-level excitation

The spin-singlet dimmer is deformed

Transiently d electron is removed.

The d electron comes back.

Or coupling with optical phonon?

Electronic change?Structural change?

51


Previous studies on pressure effect on VO2

10 kbar=1 GPa

C. N. Berglund and A. Jayaraman, Phys. Rev. 185, 1034 (1969).

E. Arcangeletti et al., PRL 98, 196406 (2007).

VO2 single crystal

52


10-4

10-3

10-2

10-1

100

101

Res

isiti

vity

( c

m)

320300280260240220200

Temperature (K)

VO2 0GPa 2GPa 4GPa 6GPa 8GPa

8GPa

2GPa

0 GPa

10-3

10-2

10-1

100

101

Res

istiv

ity (

cm

)

300250200150100

Temperature (K)

V0.98W0.02O2 0GPa 1GPa 2GPa 4GPa 6GPa 10GPa

Composition dependence

320

300

280

260

240

220

200

180

Tem

pera

ture

(K)

86420Pressure (GPa)

(a) x = 0

-3

-2

-1

0

1

Log(Resistivity/

cm)

300

250

200

150

100

Tem

pera

ture

(K)

1086420Pressure (GPa)

-3

-2

-1

0

1

Log(Resistivity/

cm)

(b) x = 0.02

The insulating phase remains in x = 0 and 0.02.

VO2/TiO2(001) V0.98W0.02O2/TiO2(001)

53


MatteoMitrano∗,1 BeatriceMaroni,2 CarloMarini,3 MichaelHanfand,3 BobyJoseph,1 PaoloPostorino,1,† and Lorenzo Malavasi2

1 Dipartimento di Fisica, Universita di Roma ”LaSapienza”, Piazzale Aldo Moro 2, 00185 Roma, Italy2 Dipartimento di Chimica, Sezione Chimica Fisica, INSTM, Universita di Pavia, Viale Taramelli 16, 27100 Pavia, Italy

3 European Synchrotron Radiation Facility, 6 Rue Jules Horowitz, BP220, 38043 Grenoble Cedex, France

M1

M3

M2

M1

M1

M1

Mx

Mx

Mx

M1: P21/cMx: P21/c

M. Mitrano et al., Phys. Rev. B 85, 184108 (2012).54

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