Transcript Kolektivna pobuđenja naboja ispod prijelaza metal

Collective Charge Excitations below the
Metal-to-Insulator Transition in BaVS3
Tomislav Ivek, Tomislav Vuletić, Silvia Tomić
Institut za fiziku, Zagreb, Croatia
Ana Akrap, Helmuth Berger, László Forró
Ecole Polytechnique Fédérale, Lausanne, Switzerland
T. Ivek et al., Phys. Rev. B 78, 035110 (2008).
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
Ba
S
V
 At ~240 K: transition to orthorhombic
structure
 At ~70 K: monoclinic structure
 Internal distortion of VS6 octahedra
 Tetramerization of V4+ chains
29 August 2008
Lechermann et al.,
PRB 76, 085101 (2007)
T. 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
S1
S1
S2
 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))
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Lechermann et al., PRB 76, 085101 (2007)
LDA + DMFT
• Nature of MI transition?
• Ground state?
T. 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
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T. Ivek: Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3
Low-Frequency Dielectric Spectroscopy
Ivek et al., PRB 78, 035110 (2008)


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))
16
BaVS3
14
12
(104)

10
'-HF ''
20 K
35 K
50 K
8
6
4
2

What is the connection of this
relaxation with the MI transition?
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0
10-1 100 101 102 103 104 105 106 107
Frequency (Hz)
T. Ivek: Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3

15
10
a)
106
TMI
103
500
100
0
500 7
10
0 106
105
10-3
TMI
20
40
T
60
80
100
1/T (1000/K)
104
103
b)
100
c)
106
10-3
103
10-6
100
10-9
10-3
20
40
60
80
100
-1
1/T (1/1000 K )
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 (cm)
d ln / d(1/T)
1000
Peak in Δε at the same T!
Screening by free charge
carriers
20
15001000
0 (s)

T30
MI
10
T. Ivek: Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3
(cm)

TMI ≈ 67K: peak in dc
resistivity derivation
dc gap 2Δ≈500 K
corresponds to the optical
gap (Kézsmárki et al., PRL
96, 186402 (2006))
20001500


300 67 Temperature
30
20 (K)15
2500
300 67
25002000
d ln / d(1/T) (K)
Metal-Insulator
Phase Transition
Temperature (K)
BaVS3 BLACK V-I
Do we have a longwavelength, phason
response?
300 67
2500
TMI
2000
a)
1500
0.020
1000
0.030
500
0.025

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
0.020
0
107
0.015
0.010
TMI
0.010
0.005
106
105
0
0.005
10
104
20
T
30
0.000
40
50
E (V/cm)
103
b)
100
c)
0.000
0 (s)

 (0)
(-
(0))/
0.015
BaVS3, 20K
-0.005
10-3
0.01
0.1
10-6
1
10
106
100 103
E (V/cm)
100
10-9
10-3
20
40
60
80
100
-1
1/T (1/1000 K )
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T. Ivek: Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3
(cm)

Temperature
(K)
INDIGO nonlinearity.jnb/0114
30
20
15
10
(-(0))/(0)
CDW Phasons?
d ln / d(1/T)
BaVS3 BLACK V-I
Hopping conduction?
Energy (eV)
0.01

Cross-over frequency far
above the observed
dielectric response
0.02
300K
85K
60K


Optical conductivity not
enhanced compared to
dc values
10K
•.
Kézsmárki et al.,
PRL 96, 186402 (2006)
Not a candidate
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T. Ivek: Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3
10
6

BaVS3 c-axis
030417b (RED)
12
10 /
Ferroelectric nature
of the MI transition?
14
8
TC=67K
Below TMI: noncentrosymmetric
6
structure with aBaVS
polar axis in the
4
reflection plane of VS3 chains
2
High polarizability of electron
0
system coupled to V4+ displacements
30
40
50
60
70
80
90
could induce high Δε
Temperature (K)
Curie
Law
BVS (Fagot et al., Solid State Sci. 7, 718 (2005)):
some charge
=C/|T-T |
disproportionation at low T
T =67K
4+ environment,
But, overestimated due to a nonsymmetric V
C(T<T )=2.5 10 (full line, fit)
thermal contraction, imprecise atomic coordinates
(FouryC(T>T )=5 10 (dotted line, prediction from theory)
Leylekian (2007))
Charge redistribution not larger than 0.01e (Fagot et al., PRB 73,
033102 (2006))
FE cannot explain our dielectric results
3


C

c
6
.
c
.
6
c


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T. Ivek: Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3
Orbital ordering?
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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
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Fagot et al., PRB 73, 033102 (2006)
T. Ivek: Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3
Interpretation in the context of
Orbital Order
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Primary order parameter for the MI phase transition:
1D Charge Density Wave instability
TC=66K
Orbital ordering transition happens at TMI, driven via structural changes,
tetramerization
red.jnb/10
Domains of OO gradually develop in size with lowering temperature
OO coupled with spin degrees of freedom, drives the spin-ordering into an AFlike ground state below 30K; domains persist!
Short-wavelength excitations of domain walls
Temperature (K)
30
20
15
300 67



Δε ~ collective excitation density, i.e.
number of180K
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
107
TMI
106


10
T
105
104
103
a)
20
40
60
80
100
nt
1/T (1/1000 K-1)
101
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T. Ivek: Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3
Conclusion
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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
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T. Ivek: Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3
Hopping
TC=66K
Dyre and Schroeder, Rev.Modern Physics 72, 873 (2000)
red.jn

b) frequency marking the onset of ac conduction ncross is
roughly proportional to the dc conductivity: Barton-Nakajima-Namikawa relation
connects dc and dielectric
loss peak frequency 0-1: dc  0-1
T
MI
–
10-6
→ ncross expected at > 1 MHz
Temperature (K)
-1cm-1
300 67
30
20
15
10
103
- For BaVS simple calculation yields:
ncross (25 K) = 360 MHz and ncross (50 K) = 3.8 GHz
T.Vuletic et al., Physics Reports 428, 169 (2006).
BaVS3
100
 (-1cm-1)
- BaVS at low T: dc 
10-5
TMI
10-3
10-6
c) 00  1ns is too long to be attributed to quasi-particles
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20
40
60
1/T (1000/K)
80
T. Ivek: Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3
100
red.jnb /0255
1.2
BaVS3
' (106)
1.0
Col 1001 vs Col 1002
Col 1001 vs Col 1004
Col 1001 vs Col 1006
0.8
10 kHz
100 kHz
1 MHz
0.6
0.4
0.2
'' (106)
0.0
0.08
0.06
0.04
0.02
0.00
0
20
40
60
80
100
Temperature (K)
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T. Ivek: Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3
red.jnb/0254
14
BaVS3
105
12
10 /
10
6
1 MHz
103
6
10 /' - HF)
104
102
BaVS3 c-axis
030417b (RED)
10 kHz
8
TC=67K
6
100 Hz
BaVS3
TMI=67 K
4
101
2
100
20
40
60
80
0
30
40
50
60
70
80
90
Temperature (K)
Temperature (K)
Curie Law
=C/|T-TC|
Curie Law
=C/|T-TMI|
Tc=67K
TMI=67K
C(T<Tc)=2.5.106 (full line, fit)
C(T<TMI)=2.5.106 (full line, fit)
C(T>Tc)=5.106 (dotted line, prediction from theory)
6
C(T>TMI)=5.10 (dotted line, prediction from theory)
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T. Ivek: Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3
Contacts
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T. Ivek: Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3
Low-Frequency Dielectric Spectroscopy

Complex conductivity as a
function of frequency
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T. Ivek: Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3
Low frequencies, high impedances





Lock-in + current
preamplifier
Voltage output
Measuring the current
10 mHz – 3 kHz
Resistances up to 1 TΩ
lock-in
Vin~I
V
ac
sample
Vout
I
strujno
pretpojačalo
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T. 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
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T. 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
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T. Ivek: Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3
Havriliak-Negami model dielectric function
 = (0)-(): dielectric strength

0: mean relaxation time

(1-): relaxation time distribution width
G
B

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T. Ivek: Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3
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T. Ivek: Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3
29 August 2008
T. Ivek: Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3