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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)
29 August 2008T. Ivek: Collective Charge Excitations below the Metal-to-Insulator
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
29 August 2008T. Ivek: Collective Charge Excitations below the Metal-to-Insulator
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
29 August 2008T. Ivek: Collective Charge Excitations below the Metal-to-Insulator
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)
29 August 2008T. Ivek: Collective Charge Excitations below the Metal-to-Insulator
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
29 August 2008T. Ivek: Collective Charge Excitations below the Metal-to-Insulator
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
29 August 2008T. Ivek: Collective Charge Excitations below the Metal-to-Insulator
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)
29 August 2008T. Ivek: Collective Charge Excitations below the Metal-to-Insulator
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
29 August 2008T. Ivek: Collective Charge Excitations below the Metal-to-Insulator
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)
29 August 2008T. Ivek: Collective Charge Excitations below the Metal-to-Insulator
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
29 August 2008T. Ivek: Collective Charge Excitations below the Metal-to-Insulator
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
29 August 2008T. Ivek: Collective Charge Excitations below the Metal-to-Insulator
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
29 August 2008T. Ivek: Collective Charge Excitations below the Metal-to-Insulator
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
29 August 2008T. Ivek: Collective Charge Excitations below the Metal-to-Insulator
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
29 August 2008T. Ivek: Collective Charge Excitations below the Metal-to-Insulator
Transition in BaVS3
Contacts
29 August 2008T. Ivek: Collective Charge Excitations below the Metal-to-Insulator
Transition in BaVS3
Low-Frequency Dielectric Spectroscopy
Complex conductivity as a function of frequency
29 August 2008T. Ivek: Collective Charge Excitations below the Metal-to-Insulator
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
29 August 2008T. Ivek: Collective Charge Excitations below the Metal-to-Insulator
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
29 August 2008T. Ivek: Collective Charge Excitations below the Metal-to-Insulator
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
29 August 2008T. Ivek: Collective Charge Excitations below the Metal-to-Insulator
Transition in BaVS3
= (0)-(): dielectric strength
0: mean relaxation time
(1-): relaxation time distribution width
Havriliak-Negami model dielectric function
G B
29 August 2008T. Ivek: Collective Charge Excitations below the Metal-to-Insulator
Transition in BaVS3