Unconventional Electrodynamic Response of the Quasi-One-Dimensional Organic Conductor (TMTSF)2ClO4

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Unconventional Electrodynamic Response of the Quasi-One-Dimensional Organic Conductor

(TMTSF)2ClO4

N. Cao, T. Timusk, K. Bechgaard

To cite this version:

N. Cao, T. Timusk, K. Bechgaard. Unconventional Electrodynamic Response of the Quasi-One-

Dimensional Organic Conductor (TMTSF)2ClO4. Journal de Physique I, EDP Sciences, 1996, 6 (12),

pp.1719-1726. �10.1051/jp1:1996184�. �jpa-00247277�

Unconventional Electrodynamic Response of the

Quasi.One.Dimensional Organic Conductor (TMTSF)2Cl04

N. Cao

(~),

T. Timusk

(~,*)

^{and K.}

Bechgaard (~)

(~)

Department

of

Physics

and

Astronomy,

Mcmaster University, Hamilton, Ontario,

Canada LBS 4Ml

(~)

Department

of Solid State

Physics,

Ris» National

Laboratory,

4000

Roskilde,

Denmark

(Received

18 June 1996, revised 1

August

1996,

accepted

18

August 1996)

PACS.78.30.-j

Infrared and Raman spectra PACS.75.30.Fv

Spin-density

_waves

Abstract. The polarized optical reflectance of the quasi-one-dimensional organic ^conductor

(TMTSF)2Cl04

has been measured

along

the chain _axis from the far-infrared (+~8

mev)

to the visible (+~1

eV)

at temperatures between 10 and 300 K. A self-consistent description of the far infrared reflectance and the

high

metallic

conductivity

of

(TMTSF)iCl04 implies

that _{a narrow} mode at _zero

frequency

_carnes the transport current, ^{and there} is no Drude

peak corresponding

to

single partiale

motion. As the temperature _is lowered below 100

K,

the

spectral weight

of

the _narrow mode grows in

parallel

with several bands _in the far infrared: _a broad band with

a

gap-like

onset at

(2A

_m 170

cm~~)

and several low

lying phonons.

These observations _are consistent with

a process of collective

charge

transport

by

_a

sliding charge density

_wave.

1. Introduction

It has been 24 years since

Igor Schegolev

introduced the orgamc

charge

transfer salts [1j ^{to the}

physicists,

and the passage of time has not

brought

forth _a

completely satisfactory explanation

of their

properties.

In

particular,

their

remarkably large

electrical

conductivity

has been the

primary subject

of interest and

controversy

^{since the earhest work of Coleman et ai.} [2j on

TTF-TCNQ.

Models of collective

[2,3]

and

single particle [4,5j transport

have been

proposed

to

explain

this

phenomenon.

In the collective

description,

1D

fluctuating

Frôlich [6j

sliding charge density

_waves

(CDW'S)

_carry the current. In the

single particle

model the

organic

materials _are considered to be _a

quasi-one-dimensional

metal

behaving

like _a Fermi

hqmd

with

extraordinarily large scattering

times.

The earliest evidence in

support

^{of the collective}

picture

_was the observation I?i of a

discrep-

ancy between the

high

de

conductivity

and the low far mfrared

conductivity

of

TTF-TCNQ

at low

temperatures.

^{Instead of the}

strong

^Drude

peak

that _one would

expect

^for a

good metal,

the mfrared

conductivity

remains

low,

close to the 300 K

value,

while the de

conductivity

mcreases

by

_more than _an order of

magnitude

_[4j

just

before the

phase

transition into the _insu-

lating

CDW _state. While there _{is some} shift of

spectral weight

towards low

frequencies,

which suggests

improved

metallic

conductivity,

a

pseudogap develops

_m the _ai

(uJ)

spectrum

[8j,

^thus

(*)

Author for

correspondence (e-mail: timuskslmcmaster.ca)

©

Les

Éditions

_de

Physique

1996

1720 JOURNAL DE

PHYSIQUE

I N°12

eifectively reducing

the far infrared

conductivity

to a very low value. The

imagmary part of, a(uJ)

shows that there _is considerable

spectral weight

at very low

frequencies

below _m 10

cm~~,

and this _can be attributed to a collective mode

[9,10].

In addition to

TTF-TCNQ,

these

phe-

nomena are also manifest in members of the

family IX

=

Cl04, PF6, AsF6

_18,

9,11-15].

It is

important

to

recognize

the

optical signature

^of this collective

mode,

which is

superim- posed

_{upon a} low

conductivity single particle background.

Infrared and

microwaùe

reflectance

experiments

show that the

spectrum

^is characterized

by

_a low

frequency region

of _very

high

reflectance R >

99%

followed

by

_a

plasma edge

where the reflectance

drops rapidly

to the lower far infrared value of R _m

90To.

In

(TMTSF)2PF6

this

edge

is at m 15

cm~~ [10,14j,

which

implies

that the

plasma frequency

of the _narrow mode should be of the order of1ooo

cm~~.

In

(TMTSF)2Cl04

the

plasma frequency

of the _narrow mode is _{even more} diflicult to

quantify

since the

position

of the

edge,

which has _never been observed

directly,

is less than 5

cm~~.

This is the iower limit of the accurate measurements

provided by Ng

et ai.

Taking

this _as

the _upper limit of the

edge

_we

get

an even lower

plasma frequency

for the collective mode

m 650

cm~~ [16j.

In terms of the

partial

_sum rule for the

optical conductivity,

the

plasma frequency

of the

narrow mode determines the _area under the

conductivity

_curve from _zero

frequency

to the

plasma edge, provided

it is well defined and the

plasmons

_are not

overdamped (r

_<

_uJp).

To estimate the

damping

constant r of the _narrow mode _{we can,} for

example

_assume that the mode has _a Drude line

shape,

_a Lorenzian centered _{on zero}

frequency,

and then find the width

given by

r

=

uJ( /47ra

where _a is the de

conductivity.

That this

simple

model is at least

approximately

correct is shown

by

the microwave data of Donov

fi4j

^{which shows that the} reflectance

below the

plasma edge

follows the

predicted

1- _uJ

/27ra(0),

where

a(o)

_m 35 x

10~ Q~~ cm~~

is close to the measured de

conductivity.

(TMTSF)2Cl04

is

unique

in

that,

in its relaxed state, ^obtained

by coohng

the

sample slowly through

its structural transition at 24

K,

it remains metallic down to 1.2 K. At this

point

it reaches _a very

high conductivity

before

undergoing

a three-dimensional

superconducting

transition

[1î]. Again, using

a Drude picture for the _narrow

mode,

its width _can be estimated from the

high

de

conductivity

_to be _m o.ors

cm~~ (150 MHz) _[11].

Another

anomaly

of the

organic charge

transfer salts is the

temperature dependence

of the de

resistivity,

which is

approximately quadratic

_m T. In

ordinary metals,

where the electron-

phonon

interaction

dominates,

the

resistivity

varies

linearly

with respect ^{to T. Also between} 1 and 20

K,

where _m conventional metals the

electron-phonon

interaction is frozen eut, ^{the de}

resistivity

of

(TMTSF)2Cl04

shows _a

strong

iinear

temperature dependence [18j.

This

points

to a

non-phonon scattering

mechanism similar _to what is _{seen m} the

high Tc

cuprates. For these

materials,

the

lmearity

_can be observed to very low temperatures ^{and trie}

resistivity

_curve

goes

through

_zero, instead of _m

fl/4

_{as one} would expect for _a low

lying

boson

spectrum

^with

a characteristic

frequency

fl. A

large anomaly

_m the NMR relaxation rate for T _< 25 K

[19j

and

suppression

^of trie thermal

conductivity

below 60 K

[20, 21]

^also

suggest

that _a

pseudogap

is

developing

in trie

density

of states and that collective eifects _may be important.

The

single partiale picture,

_on the other

hand,

is

supported by

_a number of

experiments

in

magnetic

^fields. For

example,

the observation of

oscillatory phenomena

in

high magnetic

fields [22] ^bave

generally

been

interpreted

in terms of

single particles moving

in orbits

[23].

Trie _very

large magnetoresistance

^normal to trie chains in

(TMTSF)~Cl04

_is in accord with Kohler's rule [24] which _assumes that

magnetic

field

transport

eflects _can be characterized

by

àJcT. Sl1lCe trie dC

reslstlvlty

_p lS

prOpOrtlO1laÎ

tO

1/T, pÎOtS

Of

mag1letOreSiStanCe

_{aS a} fUnction of

H/p (Kohler plots)

should all fall _on the _{same curve.} However _one finds that Kohler's ruk is violated for

transport along

the

chains, precisely

the direction where there _is

anomalously

high

de

conductivity [25].

We _now review the

existing optical

data. The far-mfrared reflectance of relaxed

(TMTSF)2 Cl04 along

the chain axis, trie

highly conducting direction,

bas been measured between 2 and 60 K for

frequencies

_up to

+~ 200

cm~l by Ng

et ai.

[16],

at 6 K to looo

cm~~ by Eldridge

and Bates

[26],

^and at 2 K

by

Challener et ai.

[2î].

Kikuchi et ai. bave measured trie _near mfrared chain-axis reflectance above sooo

cm~~ [28]. Recently,

room-temperature chain axis data bave also been

published by

Pedron _et ai. from

+~ 10 to _+~ loooo

cm~~ [29].

^To

clarify

trie

issues of

single partiale

_vs. collective

transport

^{it is}

obviously important

to make

systematic

far

infrared measurements _{as a} function of

magnetic

field and

temperature.

While _some data exist

m

magnetic

fields

they

_are limited in

temperature

and

frequency

_coverage,

focussing mainly

on trie

temperature dependence

of low

lying

features

[16, 27,30-32].

There is considerable variation in trie available

data,

both in _terms of trie

magnitude

of trie reflectance and in trie _presence and absence of

phonon

features. Some of this variation _can be attributed to thermal

cycling

of

samples,

which tends to reduce the

strength

of

phonons,

and incorrect

Kramers-Kronig extrapolations

to

high frequencies.

Irradiation

by

defects has also been

reported

to reduce the

magnitude

of

phonon peaks.

Most of the measurements _are

supposed

to be in the relaxed state, but there is _a

possibility

that _some of the

samples

were not

fully

annealed.

With

improved

reflectance

techniques,

infrared groups are now able to obtain trie

optical properties

of diflicult

samples

to _a

high degree

of

reproducibility _[33, 34].

^Thus we are now

ready

to

complete

trie measurements of trie chain-axis reflectance of

(TMTSF)2Cl04

in the

missmg frequency

_range from

+~ 60 to _+~ 8000

cm~~

_over

a

larger

_range of

temperatures.

In

this _{paper we} report on some

preliminary

measurements.

2.

Experimental

Results

Single crystals

3.o x o-à _x o.2

mm3

_{in the}

a, b, ^and c directions

respectively,

_were assembled mto _a mosaic of rive

crystals

with the ab

plane facing

the incident radiation. The

sample

_was cooled

slowly

below 40 K (+~1 ^K

/min)

to reach the relaxed state of the anion order-disorder

phase

transition at 24 K

[35].

^{An in situ}

evaporation technique

_was used _{to correct any errors} due to the

roughness

of the surface

[33].

Figure

1 shows the chain axis reflectance at

300,

200, loo, and 10 K. Dur room-temperature data _are

quite

similar to those

given by

Pedron et ai.

except

for _a few

percent magnitude

diflerence

[29],

^and the

high frequency

data _are in

agreement

^with Kikuchi et ai.

[28].

^Above 150

cm~~,

_our data match well with trie reflectance

reported by

Challener et

ai.,

but these authors show _a

clip

m reflectance clown to

80%

below this

frequency;

a

clip

net seen

by

other

investigators.

In

contrast, Eldridge

and Bates

[26]

^show a reflectance that

roughly

_agrees

with _ours in the _same

region

but

drops

to

70%

above 150

cm~~

_The

magnitude

of the 10 K reflectance in the

present study

matches the data

point

of

Ng

et ai. at 60

cm~~ [16].

^{It should} be noted that the measurements _were clone

by focussing

the radiation _{on a}

sample

with toroidal

mirrors whereas

Ng

et ai.

employed

_an immersion

cryostat

with

light

_pipe

optics.

In both _cases

an

overcoating technique

_was used to correct for diffraction eflects from the

irregular

surface.

The variation _m the reflectance of

(TMTSF)2Cl04

_as measured

by

diiferent _{groups is}

larger

than _can be attributed to variations _m

experimental techniques.

The variation cannot be due to the _use of small

samples

and _{mosaics smce} the metallic

overcoating technique eifectively

elimmates such _errors, for it

gives

reflectance values that _are accurate to better than _one

percent [33j.

^{This has been checked in}

simple systems

^{where the} reflectance _can be calculated from de

conductivity

and Drude

theory.

Stainless steel _{is a}

good system

^with an absolute

reflectance similar to trie

samples

under

study

here.

1722 JOURNAL DE

PHYSIQUE

I N°12

E(e'i~f~

0.01 O.lo 1.00

(TMTSF)~clo~

~ ~ Ella

l~lv>il(&j,,j_{1~%~~_}

>l '1 ^{' '} à _{' '}

' 100 K

~

~ i

g

~ o

©

',

200 K

~",,

, oe

100 1000 10000

Frequency

(cm"~)

Fig.

l. Frequency

dependent polarized

reflectance of

(TMTSF)2Cl04 along

the chain axis.

Another

plausible explanation

for the variation is the presence of _a surface

layer

of _a diflerent

composition

than that of the

sample.

Trie variation in trie thickness of the

layer

coula then be

a variable that affects trie measured reflectance.

However,

it is diflicult to see what material would

produce

trie observed eflects. A

simple

dielectric _on

top

of _a metal bas little influence

m the far infrared. This _can be

justified by examining

the eflects of

ice,

which often shows _up

m

systems

^with

inadequate

_vacuum

trappmg,

as a band around 3200

cm~~, provided

that the

ice is

present

in thicknesses

greater

than _a few hundred

angstroms.

On _a metallic surface the far infrared ice lines _are not

visible,

_even if the thickness is

enough

to

bring

trie reflectance at 3200

cm~~

_down

by 50$i.

Simulations with trie known

optical

constants of ice agree with these

observations

[36j.

To extract trie real and

imaginary parts

^of

a(uJ), by Kramers-Kronig analysis,

trie data must be

extrapolated

outside trie range of measured reflectance. At

high frequency,

we extended

our data between 8000 and 25000

by using

trie

room-temperature

^{data of Kikuchi} et ai.

[28j,

a

constant value between 25000 and

lo~ cm~~,

and free electron behaviour

(R(uJ)

_c~

uJ~~) beyond

lo~ cm~~.

_{Since trie low}

frequency

limit of _our measurements _was 60

cm~~

_{we used trie data} of

Ng

et ai.

[16j

on trie

low-frequency

side down to _+~ 5

cm~~.

To do trie lowest

frequency extrapolation,

trie

parameters

of trie _narrow mode must first be estimated. Trie

followmg procedure

_was used: We assumed that trie mode had _a Drude

shape,

and fit the measured

reflectance to _{a narrow}

peak

with _a

plasma frequency

_uJpn and width _~n using trie known de

conductivity.

Trie 10 K values found from trie fit _were 634 and o.034

cm~~, respectively.

A series of Lorentz oscillators _was used to represent trie rest of trie infrared

conductivity. Figure

2 shows trie fitted reflectance _as trie solid _curve. We then used this fitted reflectance _in the 0 to 5

cm~~ region

_as the lowest

frequency

extension in the Kramers

Kronig analysis

of

R(uJ).

Other

assumptions

for the

plasma frequency

of the _narrow mode do net affect the overall

a(uJ)

in the

region

of

actually

measured data _{uJ >} 5

cm~~

_but the

amplitude

of the low

frequency phonon

line _is

strongly

aflected

by

the

strength

of the assumed low

frequency

mode. This is

illustrated in

Figure

3 where the other choices for the

plasma frequency

shown _in

Figure

2 _are used in the KK

extrapolation

to calculate the

conductivity.

oo

j'j

0.98

(

"

fl (TMTSF)~CiO~

(

_T=10K

~

Extrapolation Extrapolation2 Extrapolation 3

0 4 8 12 16 20

Wavenumber (cm'~

Fig.

2. The measured low

frequency

reflectance

(the

solid

hne),

and the fitted low

frequency

extensions

(ail

the data below 5 cm~~ _The

sharp plasma edges

below 4 cm~~

are caused by _{a narrow} mode at low

frequency.

ioooo

(TMTSF)~CIO~

[

--- Extrapolation 1

z~

5

-.

à

c~

% à

Î

0

cm~~)

Fig.

3. The calculated low

frequency conductivity

with different extensions to low

frequency

_as

shown _in

Figure

2. The sohd hne _{uses a} low

frequency

extension from _a least squared fit to the

measured

reflectivity.

The real

part

of

a(uJ)

for

(TMTSF)~Cl04 along

the chain _{axis m} the far-infrared is shown

m

Figure

4. The

corresponding

values of trie de

conductivity _[17j

are also shown. In accord with ail previous data _on this

system,

^the far infrared

a(uJ)

is qmte ^{small in} contrast with trie much

larger

de

conductivity [17j.

There _{is no sign} of Drude

absorption,

mstead the

optical

1724 JOURNAL DE

PHYSIQUE

I N°12

o ~~

E(mev)

~° BO 80 i~~ ~~~

25000

i~~ùù (TMTSF)~CIO_~

~

8000 ^~~~~

~°°°°

6000 10 K

~~~~~

~

4000

~~2000

10000

°o

5 10 15 20

Frequency

(cm~

5000

~E

_Î

^°

j

26 100 K

~ 5000

0

200 K 2500

0

300 K

°0

_{200 400}

600 800 1000

Frequency (cm'~)

Fig.

4. Real part of

a(w)

of

(TMTSF)2Cl04 along

the chain _axis. The fuit circles

on the left

ordinate _are the values of the de

conductivity

from reference [12]. ^{The 10 K de value} _{(+~ 2} x 10~

Q~~cm)

is toc

high

ta be shown

on the scale shown. A gap with the value 2A _ci 170 cm~~ (~- 21

mev)

is evident in

a(w)

at 10 K. The inset is

a(w)

at 10 K between 5 and 20 cm~~. _{The dashed fine}

is the result of

Kramers-Kronig

analysis in the extrapolated _region

(the

details of this extrapolation

are described in the

text).

conductivity

is dominated

by

_{a very} broad

band,

with

a(uJ) increasing

with

frequency.

^As trie

temperature

^{is lowered below 100}

K,

the

spectral weight

^{of the}

phonon

modes _grows and there is _an overall shift of

spectral weight

to lower

frequencies.

A gap

develops

in the broad band below 1î0

cm~~ (2A

_ci 21

mev).

The reflectance

fitting procedure,

described above shows that the

spectrai weight

of trie _narrow mode _as well _as that of the

phonons

_{grow as} the

temperature

is lowered.

3. Discussion

In

discussing

the data it is

imperative

to address the

question

of the existence of the _narrow

mode _{since we} do not obtain it

directly

from the real part of

a(uJ).

It has been

suggested

that trie anomalous reflectance of trie

organic

conductors _can be accounted for

by

a "broken strand"

model,

where trie

conducting

chains _are broken into

segments

^{of finite}

length by

defects and cracks

[37j. ^However,

experiments

performed

_on

(TMTSF)2PF6 _{[13, 38]}

have revealed that it has

high

reflectance _{at microwave}

frequencies

which is associated with trie _narrow mode

component

^{of trie}

conductivity.

As _a surface with defects and cracks should bave lower

reflectance _at lower

frequencies,

trie above mentioned observations would

negate

_any

suggestion

that

imperfections

are

responsible

for trie anomalous reflectance.

The _presence of the _narrow mode is also evident in the reflectance

edge

_seen

by

Jacobsen et ai. in the far mfrared reflectance of

(TMTSF)2PF6 _{(Ref. [10], Fig. 16).}

_{Jacobsen et ai.}

estimate,

from trie

spectral weight

of the _narrow

mode,

_a

plasma frequency

of 900

cm~~,

whereas

analysis

of the recent microwave data of Dressel et ai. [15]

gives

_a

plasma frequency

of 1100

cm~~

_It

can be _seen from the

good agreement

of these data that trie _so called _gap between trie microwave and trie infrared bas been

bridgea

and both _sets of measurements

support

trie

concept ^of a narrow mode of low

spectral weight

_as trie

charge

carrier in

(TMTSF)2PF6.

To

conclude,

_we note that trie absence of Drude

absorption

and _a

spectral weight

consistent with trie carrier concentration in trie _a

direction, (and

in trie b direction [16] shows that

single partiale transport

is diflusive in aii directions and at ail temperatures. Trie _reason for trie

high conductivity,

_m trie chain direction _{is a narrow mode} of

large

_mass

accompanied by

a gap of the order of17o

cm~~

_in

a

relatively

low

conductivity single partiale

band. These

observations, along

with the observation of the

growth

of

phonon lines,

_are in accord with

predictions

for

the

conductivity

_{spectrum m} the _presence of

sliding charge density

_wave

transport [39].

We

finally

return,

briefly,

to the

question

of

reconciling

these observations with the _over-

whelmmg

evidence in support ^{of band} transport

by single partiales,

_{as seen} in _a

variety

of

magnetotransport experiments

_m

high

^fields. It is clear that the _answer lies in careful _re- flectance measurements in

high

fields measurements that would map ^out the

phase boundary

between the CDW _zero field

region

and field

region

where the CDW is

destroyed

and

single partiale

transport becomes effective.

Acknowledgments

We would like to

acknowledge

C.

Bourbonnais,

V.

Emery,

D. Jérome and J. Musfeldt for valuable

discussions,

R. A. Duncan and G. Hewitson for technical support. This work _was funded

by

the Natural Science and

Engineering

Research Council of Canada

(NSERC)

and

by

trie Canadian Institute for Advanced

Research, (CIAR).

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