Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS 3 Tomislav Ivek,...

Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3

Tomislav Ivek, Tomislav Vuletić, Silvia TomićInstitut za fiziku, Zagreb, Croatia

Ana Akrap, Helmuth Berger, László ForróEcole Polytechnique Fédérale, Lausanne, Switzerland

T. Ivek et al., Phys. Rev. B 78, 035110 (2008).

29 August 2008T. Ivek: Collective Charge Excitations below the Metal-to-Insulator

Transition in BaVS3

BaVS3 Consists of VS3 chains separated by

Ba atoms Neighboring VS6 octahedra share a

face, stack along c-axis

Room Temperature: primitive hexagonal unit

2 formula units per primitive cell

At ~240 K: transition to orthorhombic structure

At ~70 K: monoclinic structure Internal distortion of VS6 octahedra Tetramerization of V4+ chains

Ba

V

S

Lechermann et al.,PRB 76, 085101 (2007)


Transition in BaVS3

BaVS3 2 electrons in:

a wide A1g band (dz2) narrow Eg1, Eg2 bands (et2g)S2

S2

S2

S2

S1

S1

Filling of bands governed by Coulomb repulsion, local Hund’s rule coupling

A1g, Eg1 close to half-filling

Metal-to-insulator phase transition at TMI≈70 K Diffuse x-ray scattering: Fagot et al., PRL 90,

196401 (2003) pretransition fluctuations up to 170 K qc ≈ 2kF (A1g) superstructure characteristic for a Peierls transition and

Charge Density Wave ground state No charge disproportionation in anomalous x-ray

scattering! - Fagot et al., PRB 73, 033102 (2006) Magnetic transition at Tχ≈30 K: incommensurate

magnetic ordering (Nakamura et al., J. Phys. Soc. Jpn. 69, 2763 (2000), Mihály et al., PRB 61, R7831 (2000))

• Nature of MI transition?• Ground state?

Lechermann et al., PRB 76, 085101 (2007)LDA + DMFT


Transition in BaVS3

Samples

Needle-like single crystals grown along c-axis, hexagonal cross-section

3 x 0.25 x 0.25 mm3

Important quality check: suppression of insulating phase at 20 kbar

Contacts: evaporated 50 nm chrome evaporated 50 nm gold DuPont silver paint 6838

cured at 350°C for 10 min in vacuum


Transition in BaVS3

0.01 Hz – 10 MHz Complex conductivity ->

Complex dielectric function Insulating phase

single symmetrically widened overdamped loss peak

reminiscent of a Charge Density Wave phason response (Littlewood, PRB 36, 3108 (1987))

Low-Frequency Dielectric Spectroscopy

2D Graph 1

Frequency (Hz)

10-1 100 101 102 103 104 105 106 107

(1

04 )

0

2

4

6

8

10

12

14

16

'-HF ''20 K35 K50 K

red.jnb/0206

BaVS3

What is the connection of this relaxation with the MI transition?

Ivek et al., PRB 78, 035110 (2008)


Transition in BaVS3

Metal-InsulatorPhase Transition TMI ≈ 67K: peak in dc

resistivity derivation dc gap 2Δ≈500 K

corresponds to the optical gap (Kézsmárki et al., PRL 96, 186402 (2006))

Peak in Δε at the same T!

Screening by free charge carriers

1/T (1000/K)

20 40 60 80 100

(

cm)

10-3

100

103

106

d ln

/ d

(1/T

) (K

)

0

500

1000

1500

2000

2500

Temperature (K)

300 67 30 20 15 10

TMI

103

104

105

106

107

1/T (1/1000 K-1)

20 40 60 80 100

(

cm)

10-3

100

103

106

0 (s

)

10-9

10-6

10-3

100

b)

d ln

/ d

(1/T

)

0

500

1000

1500

2000

2500

Temperature (K)

300 67 30 20 15 10

a)

c)

TMI

TMI

T


Transition in BaVS3

CDW Phasons? Do we have a long-

wavelength, phason response?

Screening by free charge carriers: Littlewood

Unexpected Δε behavior CDW: Δε(T)~const.≈107

Lack of a significant non-linear dc conductivity – no sliding

Another DW phason fingerprint: a narrow microwave pinned mode no experimental results

BaVS3 BLACK V-I

E (V/cm)

0.01 0.1 1 10 100

(-

(0))

/(0

)

-0.005

0.000

0.005

0.010

0.015

0.020

BaVS3, 20K

INDIGO nonlinearity.jnb/0114BaVS

3 BLACK V-I

E (V/cm)

0 10 20 30 40 50

(-

(0))

/(0

)

0.000

0.005

0.010

0.015

0.020

0.025

0.030

26 K

103

104

105

106

107

1/T (1/1000 K-1)

20 40 60 80 100

(

cm)

10-3

100

103

106

0 (s

)

10-9

10-6

10-3

100

b)

d ln

/ d

(1/T

)

0

500

1000

1500

2000

2500

Temperature (K)

300 67 30 20 15 10

a)

c)

TMI

TMI

T


Transition in BaVS3

Hopping conduction?

Cross-over frequency far above the observed dielectric response

Optical conductivity not enhanced compared to dc values

Not a candidate

300K

85K

60K

10K

0.01 0.02

Energy (eV)

•.Kézsmárki et al.,PRL 96, 186402 (2006)


Transition in BaVS3

Ferroelectric nature of the MI transition? Below TMI: noncentrosymmetric

structure with a polar axis in thereflection plane of VS3 chains

High polarizability of electronsystem coupled to V4+ displacementscould induce high Δε

BVS (Fagot et al., Solid State Sci. 7, 718 (2005)): some charge disproportionation at low T

But, overestimated due to a nonsymmetric V4+ environment, thermal contraction, imprecise atomic coordinates (Foury-Leylekian (2007))

Charge redistribution not larger than 0.01e (Fagot et al., PRB 73, 033102 (2006))

FE cannot explain our dielectric results

Temperature (K)3040506070809002468101214

106 /

red.jnb/0208

TC=67K

BaVS3 c-axis

030417b (RED)

Curie Law=C/|T-TC|

Tc=67K

C(T<Tc)=2.5.106 (full line, fit)

C(T>Tc)=5.106 (dotted line, prediction from theory)

BaVS3


Transition in BaVS3

Orbital ordering? No charge modulation in the

insulating phase Fagot et al., Lechermann et al.:

modulation of orbital occupancy

51V NMR and NQR measurements suggest an orbital ordering below TMI that is fully developed only at Tx (Nakamura et al., PRL 79, 3779 (1997))

Magnetic susceptibility (Mihály et al., PRB 61, R7831 (2000)): lack of magnetic long-range order between TMI and Tχ

Magnetic anisotropy (M. Miljak, unpublished): AF domain structure below Tχ

Fagot et al., PRB 73, 033102 (2006)


Transition in BaVS3

Interpretation in the context ofOrbital Order

Δε ~ collective excitation density, i.e. number of domain walls

Domains consolidate: number of domain walls diminishes with cooling

Δε decreases only down to Tχ Below that a long-range spin ordering

is established and Δε stays constant

Temperature (K)3006730201510103104105106107

1/T (1/1000 K-1)20 40 60 80 100

18th

RU

N

dre

ssel

sam

ple

2 S

r 14C

u24

O41

20th

RU

N

dre

ssel

sam

ple

2 S

r 11C

a 3Cu

24O

41

2 co

ntac

ts

S

ep.

2001

. -

- J

un.

2002

.

a)

Run 68 large heater=1100 K

=19meV

red.jnb/1001

Run 69, 4 applied contsRun 68

Run 18

180K

Run 73 SII

Run 37

R2c

ont/ R

4con

t 100101

GREY

1st mode

2nd mode

V+V-

I+I-

TC=66K

TMI

T

Primary order parameter for the MI phase transition:1D Charge Density Wave instability

Orbital ordering transition happens at TMI, driven via structural changes, tetramerization

Domains of OO gradually develop in size with lowering temperature OO coupled with spin degrees of freedom, drives the spin-ordering into an AF-

like ground state below 30K; domains persist! Short-wavelength excitations of domain walls


Transition in BaVS3

Conclusion

BaVS3 – system with orbital degeneracy

Metal-Insulator transition at TMI~67 K

Magnetic transition at Tχ=30 K Low-Frequency Dielectric Spectroscopy: the

observed mode cannot be assigned to phason excitations

Density of excitations decreases from TMI with decreasing T, becomes constant under Tχ

Short-wavelength excitations <-> Orbital Ordering


Transition in BaVS3

Hopping

b) frequency marking the onset of ac conduction cross is

roughly proportional to the dc conductivity: Barton-Nakajima-Namikawa relationconnects dc and dielectric loss peak frequency

-1: dc -1

Dyre and Schroeder, Rev.Modern Physics 72, 873 (2000)

- BaVS at low T: dc 10-5 – 10-6 -1cm-1

→ cross expected at > 1 MHz

- For BaVS simple calculation yields: cross (25 K) = 360 MHz and cross (50 K) = 3.8 GHz

1/T (1000/K)

20 40 60 80 100

(

-1cm

-1)

10-6

10-3

100

103

Temperature (K)

300 67 30 20 15 10

red.jnb/0106

TC=66K

TMI

TMI

BaVS3

T.Vuletic et al., Physics Reports 428, 169 (2006).

c) 00 1ns is too long to be attributed to quasi-particles


Transition in BaVS3

Temperature (K)020406080100 ''

(10

6 )

0.000.020.040.060.08

10 kHz100 kHz1 MHz '

(106 )

0.00.20.40.60.81.01.2

Col 1001 vs Col 1002 Col 1001 vs Col 1004 Col 1001 vs Col 1006

BaVS3

red.jnb /0255


Transition in BaVS3

red.jnb/0254BaVS3 c-axis

030417b (RED)

Temperature (K)20406080100101102103104105

106 /' -

H

F)

TMI=67 K

Curie Law=C/|T-TMI|

TMI=67K

C(T<TMI)=2.5.106 (full line, fit)

C(T>TMI)=5.106 (dotted line, prediction from theory)

BaVS3

1 MHz

10 kHz

100 Hz

Temperature (K)3040506070809002468101214

106 /

red.jnb/0208

TC=67K

BaVS3 c-axis

030417b (RED)

Curie Law=C/|T-TC|

Tc=67K

C(T<Tc)=2.5.106 (full line, fit)

C(T>Tc)=5.106 (dotted line, prediction from theory)

BaVS3


Transition in BaVS3

Contacts


Transition in BaVS3

Low-Frequency Dielectric Spectroscopy

Complex conductivity as a function of frequency


Transition in BaVS3

ac

lock-in

strujno pretpojačalo

V VoutVin~I

I

Low frequencies, high impedances

Lock-in + current preamplifier

Voltage output Measuring the current 10 mHz – 3 kHz Resistances up to 1 TΩ

sample


Transition in BaVS3

Autobalancing bridge

~10 Hz up to ~100 MHz Resistances up to ~1

GΩ Virtual ground avoids

capacitive coupling to ground

Lc is kept at 0 potential by a feedback loop


Transition in BaVS3

Dana analysis

We measure complex admittance Y=G+iB as a function of frequency

After subtracting the background, complex dielectric function is given by


Transition in BaVS3

= (0)-(): dielectric strength

0: mean relaxation time

(1-): relaxation time distribution width

Havriliak-Negami model dielectric function

G B


Transition in BaVS3

Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS 3 Tomislav Ivek,...

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