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Charge-density-wave paraconductivity
ELSEVIER
Synthetic
Metals
71 (1995) 1893-1894
Charge-Dexity-Wave
Pa.raconductivity
M. Dressela*, A. Schwartz”, A. Blank”, 7‘. Csiba”, G. Griiner”, B.P. Gorshunovb, A.A. Volkovb, G.V. Kozlovb, and L. Degiorgi’ of California, Los Angeles, 405 Hilgard Ave., Los Angeles, CA 90024-1547’ a Department of Physics, University b Institute of General Physics, Russian Academy of Sciences, Vavilov str. 38, Moscow 117942, R.ussia+ ’ Laboratorium FestkGrperphysik, Eidgenijssische Technische Hochschule, CH-8093 Ziirich, Switzerlandt Abstract The dynamic (above
conductivity
using a combination in the spectra develops ture
and permittivity
180 K and 260 K, respectively) of different
below
50 cm-’
at low T. We associate
and give a collective
a usual single-particle
of Ii0 3MoO 3 a.nd (TaSer)zI
these
Russian
behavior
met,allic beha.vior in the fluctuating
phase
lo5 cm-‘,
pronounced excitations are discovered along which the charge density wave
region,
below TMF but above TCDW,
a substantial anisotropy connected with the one-dimensional nature of the charge transport. The excitation spectra parallel to the chains, however, show two overall features: decreasing conductivity with decreasing frequency in the far infrared range [v < 1000 cm-‘], and an enhanced conductivity at low frequencies. As we can see from the K,,sMo03 spectra, lowering the temperature enha.nces both features, clearly indicating the development of a pseudo-gap of the same order of magnitude as the CDW gap below the transition. For (TaSeq)zI the drop in conductivity below the pseudo-gap is even more developed since Tc~w = 260 I<. At low frequencies we find a pronounced enha.ncement of 011 below 50 cm-‘. This implies an additional contribution Q* to the conductivity for l? parallel to the chains with characteristic frequencies v 2 10 cmiu the case of K0,sMo03, and v z 20 cm-’ for (TaSe_,)zI, which we believe is a result of the fluctuating CDW response. The interaction of the fluctuating CDW segments with impurities may result in a pinning like behavior as has been observed for the CDW ground state. In conclusion, the experiments on both Ii0.3M003 and (TaSed)zI reported here give clear evidence for important deviations from conventional metallic bellavior in the fluctuating region, below TMF but above ‘&L)~J, the temperture where long range order develops. Our results on the anisotropy clearly establish the collective mode aspect of
von Humboldt-Foundation Research
the U.S.-R ussian collaboration is supported by NSF Grant No. 9216500 Foundation Grant No. 93-02-16110 +Supported by the Swiss National Science Foundation
0379-6779/95/$09.50 Q 1995 Elsevier Science S.A. All rights reserved SSDI 0379-6779(94)03095-N
1 -
with charge-density-wave fluctuations which exist even at room temperaFor t.he transverse polarization, l? perpendicular to the chains, The experiments give clear of t,he conductivity and permitt,ivity spectra is observed.
by NSF Grant No. 9218745; by the
range
to the conduct,ivity.
It is expected that quasi one-dimensional systems show fluctuation effects above the three-dimensional ordering transition, and that deviations from Fermi liquid behavior occur [J]. Clear evidence for fluctuation effects has been observed in several inorganic linear chain compounds, such as Ko.~MOOS, (TaSeq)zI, and Ta& which develop a charge-densitywave ground state at low temperatures [TCDW = 180 K for K~.JMoO~ and 260 K for (TaSed)zI]. The magnetic susceptibility strongly decreases with decreasing temperature [2], suggestive of an opening of a pseudo-gap. Photoemission experiments [3] indicate the absence of a sharp Fermi edge, and this is interpreted as evidence for non-Fermi liquid behavior due to one-dimensional fluctuations. We have employed various techniques to obtain the frequency dependent conductivty 6 = (T + i(tm - c)vc/2 over an extremely wide frequency range [4]. The experimental results for K0.3M003 and (TaSed)zI are presented in Figs. 1 and 2, respectively, together with the results of dispersion analysis. In the upper panels the absorptivity A in both orientations is displayed as a function of’frequency. The lower panels show the frequency dependent conductivity. Instead of a Kramers-Kronig analysis, we used a minimum set of harmonic oscillators and Drude terms necessary to describe A(u) as well a.s the conductivity and dielectric constant. For both compounds K0.3MoOs and (TaSea)sI, the electrodynamic response is dominated by *Fellow of the Alexander
in the conducting
frequency
excitations
evidence for important deviations from conventional the temperature where long range order develops.
‘Supported +Supported
single crystals
for t.he firs+ time, over a broad
spectroscopic techniques. In bot.h compounds clearly for the electric field parallel t.he cha.ins, the direction
contribution
Drude
spectra
have been measured,
M. Dressel et al. I Synthetic Metals 71 (1995) 1893-1894
1894
Ko.3Mo%
100
T = 300 K
Elb
T=300K
.,*
,’
,;r,’ ,’ ,’
,’T ,’
n
Cavity Perturbation ?? Reflectivity Reflectivity --- Fit
2
104
??
7
Cavity Pert. Transmission Reflectivity Fit _
??
--
f I
I
(a)
10-3
,f
~lliil i,n DCValues Cavity Pert. (300 K) Fabry-Perot (300 K
Ellb ----q
-i E lo3 0
*
DCValue
0 Cavity Pert.
‘T
??
C
~
b
L
102
j:
Transmissiorl Fit
:
Elc
10’
lo-’
(b) .
_
10”
2 &ic $! ^
10’ IO‘ 1oj Frequency (cm-‘)
0- ’
lo4
Figure 1: a) Frequency dependence of the room temperature absorptivity of I
the conductivity, while the frequency dependences in the direction parallel to the chains give evidence for novel features such as a finite conductivity and pinning-like behavior. These together with photoemission experiments reported earlier [3] are essential features of the one-dimensional systems when correlations are important. Whether these experiments reflect simply the opening of a pseudo-gap, or give evidence for non-Fermi liquid beha.vior remains to be seen. Experiments on alloys, currently being carried out, could further clarify the roles played by fluctuations and impurities in these highly anisotropic materials.
;
:b)
‘.
too
10’
102
Frequency
? 2 A,, ,
i&
104
uu105
(cn-‘)
Figure 2: a) Frequency dependence of the room temperature absorptivity of (TaSed)zI in both orientations ,!? 11c and E _L c. The squares were obtained from the surface resistance, the circles calculated from quasi-optical transmission measurements. The solid lines show the optical reflectivity results. The dashed lines show the results of the dispersion analysis of the data. b) Frequency dependence of the conductivity in both directions. The solid circles show the result of a direct measurement of the conductivity by transmission experiments, while the lines represent the fit. The open squares are calculated from the surface impedance measurements. The open arrow indicates the single particle gap.
References PI
D.J.
Scalapino,
Y. Imry, and P. Pincus, Phys. Rev. B Int. J. Afor/. Phys. 5 (1991)
11 (1975) 2042; H. Schultz.
57
PI
D.C. Johnston, Phys. Rev. Letl. 52 (1984) 2049; D.C. Johnston, M. Maki, and G. Griine~, Solid Stnte Commun. 53 (1985) 5
131 B. Dardel,
M. Malterre, M. Grioni, P. Weibel. Y. Baer, and F. Levy Phys. Rev. Lett. 67 (1!391) 3144
PI
B.P. Gorshunov et al., Phys. Rev. Lett and references therein.
73 (1994) 3OS,
Synthetic
Metals
71 (1995) 1893-1894
Charge-Dexity-Wave
Pa.raconductivity
M. Dressela*, A. Schwartz”, A. Blank”, 7‘. Csiba”, G. Griiner”, B.P. Gorshunovb, A.A. Volkovb, G.V. Kozlovb, and L. Degiorgi’ of California, Los Angeles, 405 Hilgard Ave., Los Angeles, CA 90024-1547’ a Department of Physics, University b Institute of General Physics, Russian Academy of Sciences, Vavilov str. 38, Moscow 117942, R.ussia+ ’ Laboratorium FestkGrperphysik, Eidgenijssische Technische Hochschule, CH-8093 Ziirich, Switzerlandt Abstract The dynamic (above
conductivity
using a combination in the spectra develops ture
and permittivity
180 K and 260 K, respectively) of different
below
50 cm-’
at low T. We associate
and give a collective
a usual single-particle
of Ii0 3MoO 3 a.nd (TaSer)zI
these
Russian
behavior
met,allic beha.vior in the fluctuating
phase
lo5 cm-‘,
pronounced excitations are discovered along which the charge density wave
region,
below TMF but above TCDW,
a substantial anisotropy connected with the one-dimensional nature of the charge transport. The excitation spectra parallel to the chains, however, show two overall features: decreasing conductivity with decreasing frequency in the far infrared range [v < 1000 cm-‘], and an enhanced conductivity at low frequencies. As we can see from the K,,sMo03 spectra, lowering the temperature enha.nces both features, clearly indicating the development of a pseudo-gap of the same order of magnitude as the CDW gap below the transition. For (TaSeq)zI the drop in conductivity below the pseudo-gap is even more developed since Tc~w = 260 I<. At low frequencies we find a pronounced enha.ncement of 011 below 50 cm-‘. This implies an additional contribution Q* to the conductivity for l? parallel to the chains with characteristic frequencies v 2 10 cmiu the case of K0,sMo03, and v z 20 cm-’ for (TaSe_,)zI, which we believe is a result of the fluctuating CDW response. The interaction of the fluctuating CDW segments with impurities may result in a pinning like behavior as has been observed for the CDW ground state. In conclusion, the experiments on both Ii0.3M003 and (TaSed)zI reported here give clear evidence for important deviations from conventional metallic bellavior in the fluctuating region, below TMF but above ‘&L)~J, the temperture where long range order develops. Our results on the anisotropy clearly establish the collective mode aspect of
von Humboldt-Foundation Research
the U.S.-R ussian collaboration is supported by NSF Grant No. 9216500 Foundation Grant No. 93-02-16110 +Supported by the Swiss National Science Foundation
0379-6779/95/$09.50 Q 1995 Elsevier Science S.A. All rights reserved SSDI 0379-6779(94)03095-N
1 -
with charge-density-wave fluctuations which exist even at room temperaFor t.he transverse polarization, l? perpendicular to the chains, The experiments give clear of t,he conductivity and permitt,ivity spectra is observed.
by NSF Grant No. 9218745; by the
range
to the conduct,ivity.
It is expected that quasi one-dimensional systems show fluctuation effects above the three-dimensional ordering transition, and that deviations from Fermi liquid behavior occur [J]. Clear evidence for fluctuation effects has been observed in several inorganic linear chain compounds, such as Ko.~MOOS, (TaSeq)zI, and Ta& which develop a charge-densitywave ground state at low temperatures [TCDW = 180 K for K~.JMoO~ and 260 K for (TaSed)zI]. The magnetic susceptibility strongly decreases with decreasing temperature [2], suggestive of an opening of a pseudo-gap. Photoemission experiments [3] indicate the absence of a sharp Fermi edge, and this is interpreted as evidence for non-Fermi liquid behavior due to one-dimensional fluctuations. We have employed various techniques to obtain the frequency dependent conductivty 6 = (T + i(tm - c)vc/2 over an extremely wide frequency range [4]. The experimental results for K0.3M003 and (TaSed)zI are presented in Figs. 1 and 2, respectively, together with the results of dispersion analysis. In the upper panels the absorptivity A in both orientations is displayed as a function of’frequency. The lower panels show the frequency dependent conductivity. Instead of a Kramers-Kronig analysis, we used a minimum set of harmonic oscillators and Drude terms necessary to describe A(u) as well a.s the conductivity and dielectric constant. For both compounds K0.3MoOs and (TaSea)sI, the electrodynamic response is dominated by *Fellow of the Alexander
in the conducting
frequency
excitations
evidence for important deviations from conventional the temperature where long range order develops.
‘Supported +Supported
single crystals
for t.he firs+ time, over a broad
spectroscopic techniques. In bot.h compounds clearly for the electric field parallel t.he cha.ins, the direction
contribution
Drude
spectra
have been measured,
M. Dressel et al. I Synthetic Metals 71 (1995) 1893-1894
1894
Ko.3Mo%
100
T = 300 K
Elb
T=300K
.,*
,’
,;r,’ ,’ ,’
,’T ,’
n
Cavity Perturbation ?? Reflectivity Reflectivity --- Fit
2
104
??
7
Cavity Pert. Transmission Reflectivity Fit _
??
--
f I
I
(a)
10-3
,f
~lliil i,n DCValues Cavity Pert. (300 K) Fabry-Perot (300 K
Ellb ----q
-i E lo3 0
*
DCValue
0 Cavity Pert.
‘T
??
C
~
b
L
102
j:
Transmissiorl Fit
:
Elc
10’
lo-’
(b) .
_
10”
2 &ic $! ^
10’ IO‘ 1oj Frequency (cm-‘)
0- ’
lo4
Figure 1: a) Frequency dependence of the room temperature absorptivity of I
the conductivity, while the frequency dependences in the direction parallel to the chains give evidence for novel features such as a finite conductivity and pinning-like behavior. These together with photoemission experiments reported earlier [3] are essential features of the one-dimensional systems when correlations are important. Whether these experiments reflect simply the opening of a pseudo-gap, or give evidence for non-Fermi liquid beha.vior remains to be seen. Experiments on alloys, currently being carried out, could further clarify the roles played by fluctuations and impurities in these highly anisotropic materials.
;
:b)
‘.
too
10’
102
Frequency
? 2 A,, ,
i&
104
uu105
(cn-‘)
Figure 2: a) Frequency dependence of the room temperature absorptivity of (TaSed)zI in both orientations ,!? 11c and E _L c. The squares were obtained from the surface resistance, the circles calculated from quasi-optical transmission measurements. The solid lines show the optical reflectivity results. The dashed lines show the results of the dispersion analysis of the data. b) Frequency dependence of the conductivity in both directions. The solid circles show the result of a direct measurement of the conductivity by transmission experiments, while the lines represent the fit. The open squares are calculated from the surface impedance measurements. The open arrow indicates the single particle gap.
References PI
D.J.
Scalapino,
Y. Imry, and P. Pincus, Phys. Rev. B Int. J. Afor/. Phys. 5 (1991)
11 (1975) 2042; H. Schultz.
57
PI
D.C. Johnston, Phys. Rev. Letl. 52 (1984) 2049; D.C. Johnston, M. Maki, and G. Griine~, Solid Stnte Commun. 53 (1985) 5
131 B. Dardel,
M. Malterre, M. Grioni, P. Weibel. Y. Baer, and F. Levy Phys. Rev. Lett. 67 (1!391) 3144
PI
B.P. Gorshunov et al., Phys. Rev. Lett and references therein.
73 (1994) 3OS,