Electron Tunneling Studies in the Charge Density Wave State of the Quasi Bidimensional Metal η-Mo4O11

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Electron Tunneling Studies in the Charge Density Wave State of the Quasi Bidimensional Metal η-Mo4O11

J. Sorbier, H. Tortel, C. Schlenker, H. Guyot

To cite this version:

J. Sorbier, H. Tortel, C. Schlenker, H. Guyot. Electron Tunneling Studies in the Charge Density Wave

State of the Quasi Bidimensional Metal η-Mo4O11. Journal de Physique I, EDP Sciences, 1995, 5 (2),

pp.221-233. �10.1051/jp1:1995124�. �jpa-00247053�

Classification

Physics Abstracts

73.40G 71.45L

Electron Tunneling Studies in the Charge Density Wave State of the Quasi Bidimensional Metal i~-Mo4Oii

J-P-

Sorbier(~),

H.

Tortel(~),

C.

Schlenker(~)

and H.

Guyot(~)

(~) Laboratoire d'Electronique des Milieux Condensés, Université de Provence, Centre de St

Jérôme, 13397 Marseille Cedex 20, France

(~) Laboratoire d'Etudes des Propriétés Electroniques des Solides, CNRS, B-P. 166, 38042 Grenc- ble Cedex 9, France

(Received 30 May 1994, accepted in final form 27 October

1g94)

Abstract. Electron tunneling studies bave been perforrned _on junctions made with the

quasi bidimensional oxide ~-Mo4Oii and _a lead film separated by _an insulating oxide loyer.

The density of states in the vicinity of the Fermi level has been obtained along two directions

(parallel

and perpendicular to the loyers) in the ternperature _range 1 K-90 K. The data frorn trie differential conductance _{as a} function of de bras voltage _are consistent with gap openings taking place at Tpi _" loo K and Tp2 ₌ 30 K. Bath electron instabilities _are attributed to charge density _waves and the anisotropy of trie density of states _is related to trie anisotropy of the Fermi surface. For tunneling current in the plane of the loyers, weak oscillations _are found in trie density of states at low temperatures. They _are possibly due to trie existence of several types of small electron and hale pockets induced _on trie Fermi surface by trie charge density wave gap

opemngs.

l. Introduction

Molybdenum

bronzes and oxides have been

extensively

studied in the last decade in relation with their low-dimensional electronic

properties

and the onset of Peierls' instabilities

leading

to

charge density

_wave

(CDW)

states [1].

Among

these

compounds,

the Mo4Oii oxides _are quasi bidimensional

(2D)

due to _a

crystal

structure built with infinite

layers

of

Mo06

octahedra

separated by Mo04

tetrahedra [2]. ^Since the

hybridized d-p

band is

partially

filled and _since the conduction electrons _are confined into the Mo06

layers,

these

compounds

_are metallic with

quasi-cylindrical

Fermi surfaces. The two

phases, q-Mo4Oii

and

~f-Mo4Oii

have

neighbounng crystal

structures and show _a Peierls

instability

towards _a metallic CDW state around loo K.

Only

the monoclinic

q-phase

shows _a second

instability

in the

vicinity

of 30 K.

Up

_{to now,} the nature of this

instability

_was not dear [3]. ^We report ^{in this article electron}

tunneling

studies

performed

_on

junctions

made with

single crystals

of

i~-Mo4Oii

and _a lead counterelectrode

separated by

an

msulating layer.

The results establish that the low temperature

instability

is

© Les Editions de Physique 1995

due _{to a} gap

opening, possibly

of the CDW type. ^{We also show that the residual}

density

of states

(DOS)

is

anisotropic.

The transport

properties

of

i~-Mo4Oii

have been studied in detail

by Guyot

et ai.

[4,5],

^who

established that the electrical

resistivity

is

strongly

anisotropic, ^the

resistivity being

two orders of

magnitude larger along

the a* axis

perpendicular

to the

layers

than in the

layer plane.

A

strong anisotropy _was also found between the b* and c* axis in this

plane. X-ray

diffraction and electron diffraction studies established that the

anomaly

which takes

place

at loo K is due to _a CDW transition, _as shown

by

^{satellites found} at qi "

(o,

o.23 b*,

o)

_[4,GI. Furthermore

Môssbauer studies

supporting

the existence of _an

anomaly

in the

Debye-Waller

factor around loo K corroborated this model [7]. ^Thermal

conductivity

_as well _as

optical reflectivity changes

below loo K _were also found to be consistent with _a CDW

instability [8,9].

Electrical

resistivity

studies under

hydrostatic

_pressure showed that both transition temperatures

Tpi

" loo K and

Tp2 = 30 K _{are pressure}

dependent

_[10]. But while

Tpi

^{is found} to increase with _pressure, Tp2

is

decreasing.

Band structure calculations

performed by

the

tight-binding

method _{in a two-} dimensional

approximation

_are consistent with

nesting properties

of the Fermi surface

along

the b* axis and the _qi vector found

experimentally

_[11]. Further

analysis

of the band struéture lead to the model of the sc-called "hidden

nesting"

_or "hidden

one-dimensionality",

related _to the

presence of

quasi

one-dimensional chains of Mo06 octahedra

along

the three

crystallographic

directions b and b ₊

/

_c

[12,13].

Due to _a weak

hybridation

between these three

chains,

the Fermi surface _may be considered in _a first

approximation

_as the

superposition

of three quasi one-dimensional Fermi surfaces. The

high

temperature CDW transition at

Tpi

is related to gap

openings along

b*, as shown

by X-ray

results and electron diffraction [4].

Preliminary X-ray

results mdicate that the low temperature transition may be related to CDW satellites found at q2 =

(?,

o.42 b*, o.28

c*)

^therefore

corresponding

to gap

openings along

other parts of the Fermi surface [13].

High magnetic

^field transport studies at low temperatures had shown the existence of Shubnikov-de Haas oscillations for

magnetic

fields

perpendicular

to the

layers

_[5,GI. De Haas

van

Alphen

oscillations _were also found _on the

magnetization

[5]. ^{These results} were consis- tent with the existence _on the low temperature ^{state Fermi surface}

(T

_{< Tp2} of small electron and hole

pockets

with _{a size} of the order of lo~~ _the

high

temperature bidimensional

(b*, c*)

Brillouin _zone. More recent magnetoresistance and Hall effect data in fields _up to 26 T also show

hysteresis phenomena

between _up and

down-sweeps

of the

magnetic

field [14]. ^{This is} attributed to _an irreversible effect of the

magnetic

field _on the CDW state. Studies of

pulsed

laser induced _transient thermoelectric effect have also been

performed recently

between 4.2 K and 300 K [15]. ^{The existence of several} relaxation times is attributed to the

complexity

of the

Fermi surface in the different CDW states. A model for the Fermi surface at low temperatures, with several types ^{of electron} and hole

pockets

is also

proposed

_{[15], see} also Ref.

[13],

_p.

167).

Electron

tunneling

^studies _can m

principle

_give information _on the

change

of the DOS _near the Fermi level due to CDW _gap

openings.

^{Such results} have been obtained

previously

_on NbSe3 Î16]. ^The temperature

dependence

of the Peierls gap can also be obtained

by

this

technique.

In the _case of

i~-Mo4Oii,

it had been found earlier that the CDW satellite

amplitude

and therefore the Peierls _gap show _a BCS type ^law _as a function of temperature [4]. ^In a quasi one-dimensional system such _a law is

expected

to

obey

the mean-field

theory

of the Peierls transition. In this context it _was relevant to

perform

electron

tunnelling

studies _on

q-Mo4Oii

in order to study ^the two mstabilities at

Tpi

and Tp~ ^{and the} temperature

dependence

of the DOS.

0

FERMI

Ea3

_LEVEL

$

_Eai Ea2

~ £~

ù

~

ai

~

E~

Fig. l. Bond scheme structure _{in a} metal-insulating-sernimetal junction.

2.

Tunneling

Effect:

Special Aspects

2,1. DIRECTIVITY _{oF THE} TUNNELING EFFECT. Giaever first

proved experimentally

the

possibilities given by

the

tunneling

eflect

Il?].

He demonstrated that _a

simple conductivity

measurement at the terminais of _a N-I-S sandwich

(N being

_a normal

metal,

I _an

insulating

layer

and S _a

superconductor) provides

the DOS of the

superconductor

around the Fermi level.

The measurements established the existence of the BCS _gap and _a strong

electron-phonon coupling

inside lead.

An important factor in the calculation of the

tunneling

current is the evaluation of the transmission factor D which is defined _as

being

the

probability

current associated with the fraction of the incident _wave which is transmitted. This calculation _was

largely developed

in the literature [18]. ^{If is the}

angle

between the incident _{wave vector} and the normal to the

junction plane,

in the trame of the WKB

approximation,

the transmission factor _may be

expressed

in the

following

_way:

D _c~

Do

_exp

(-oÙ~)

where

Do

is the transmission factor in the _case where the _wave vector is normal _to the

junction plane.

The transmission factor is

multiplied by 1le

for

changes

of _a few

degrees.

Therefore

only

the electrons which have their _wave vector normal to the

junction plane

_{may cross} the

insulating layer.

2.2. CASE _oF MULTI-VALLEY CoNDucTioN BANDS. Esaki

[19-21]

^{considered the} case of

a M-I-SM

junction,

where SM is _a semimetal,

(as

is shown _m

Fig. l)

and demonstrated that if the band

edges

^{of the} semimetal _are less

deep

than the conduction bond

edge

^{of the} metallic

counterelectrode,

the

tunneling

conductance reflects the DOS of the semimetal.

It follows that the

tunneling

spectroscopy ^{in the} case of _a metal-insulator- semimetal

provides

the semimetal DOS

along

the

crystallographic

direction normal _to the

junction plane.

This

density

will further be noted "DOS".

3.

Experimental Technique

In order to

verify

the

quality

of the

tunneling

spectra, we have used _a lead counterelectrode in which the

superconductive

DOS is well known. The

superconducting phase

is

only

used _{as a}

quality

test and is then

suppressed by applying

a weak

magnetic

field. A

typical

characteristic of the

tunneling

conductance _is shown in

Figure

2.

Lead dots of 3000

À

_{thickness are}

evaporated through mylar

masks into _{a vacuum envi-} ronment

(10~6 Torr).

The

samples

_were first deaved _or washed in decinormal arnmonia. We have studied four

crystallographic

directions _a, b, c, and b _{+ c.} The b _{+ c} direction has _no

geometrical relationship

with the

nesting

_wave vector q2 but it _was chosen because it is _an easy

deavage

direction. The

quantity

and the rate of lead

evaporation

is measured

by using

-j ^>

~ ~

@

)

3

1.5

o ,)

m 1

Ô l.2 Bm0

@

, [

É ".

C ,

é

~ i

fi ,

E

Z

o.5

-io -s o s io

Appfled Voltage (mV)

Fig. 2. Dilferential conductance in Pb

linsulator /~-Mo4Oii

junction rneasured with current parallel

to b _{+ c} axis. The dashed fine represents the superconducting lead DOS at 1 K. Trie elfects of the

superconducting energy gap and of the phonons _{are seen.} The fuit bne represents the "DOS" of

~-Mo4Oii along the b + _c direction at 1 K. A rnagnetic field of o.3 T is applied.

a quartz balance. We believe that _a natural

insulating interface, probably PbO,

is created.

Ohmic contacts are then realized _onto the

sample

and the lead dots with silver poste. The collection of the

tunneling

resistance data dV

Idi

_uers~ls

V,

for _a temperature ^T and _a bias

voltage

V, is made

by

_a lock-in detection system ^{which allows} an

analogic

measurement of the differential resistance.

4.

Experimental

Results

4.1. DIFFERENTIAL CONDUCTANCE _{ALONG THE a} Axis.

Figure

3 shows the _curves of the

normalized differential conductance

N(l~ T),

_at different temperatures.

$(V, T) N(V, T)

= ~~

~(V,loo)

dI/dV(V, loo),

the differential conductance at _a temperature above

Tpi, (loo K)

is taken _{as a} base hne and inform _us about the barrier deformations with _a bios

voltage.

N(V,T)

_is in fact the convolution

product

of the "DOS" with the first derivative of the Fermi function. However _we will _assume that at low temperatures

N(V,T) gives

the

"DOS",

the derivative of the Fermi function

being

considered _{as a} Dirac function.

Figure

4 shows the dilferential

tunneling

conductance _{as a} function of temperature at nuit bias

voltage.

This

curve represents the condensation rate at the Fermi level. It _varies between 50$lo and 80$lo from

sample

to

sample.

A steeper variation _appears at temperatures ^{below 30 K.} This

phenomenon

may be related to the second CDW transition. One _{cari note} that these results _are similar to those observed in NbSe3 [16].

4.2. DIFFERENTIAL CONDUCTANCE ALONG THE b + c Axis. In this direction, _as is shown

in

Figure

2, the "DOS" _seems to be temperature

mdependent

_as if it _{were a} normal metal.

~ /

'

~

£

~+

#~

y~

~>

Ô

~ ~ l' /

, >

> /

~ +Î~

%fÉ Î

~~ ° 50 ioo

5° ° 50 Temperature (K)

Applied Voltage (mV)

Fig. 4

Fig. 3

Fig. 3. Norrnahzed differential tunnehng conductance _versus applied voltage along the _a aXis at djlferent temperatures.

Fig. 4. Temperature dependence of the dilferential conductance for _zero bias voltage, for current

along the _a aXis.

à -j>

~ ~

w

31

~

)

é n

É

#

a 0,3T

iT

io -s o s io

Applled Voltage (mV)

Fig. 5. Dilferential conductance enlarged _versus apphed voltage at dilferent ternperatures along the _{b +}

c direction with _an applied magnetic ^field of o.3 T. The dashed fine represents the "DOS" _at 1 K with _an apphed rnagnetic field of1 T.

However,

Figure

5 shows that the

enlarged

curves

(xlo

_or

x20)

exhibit below 30 K weak oscillations _m the "DOS". The

magnitude

of these oscillations is maximal at the lowest _tem- perature

reached,

1.2 K, and decreases

quickly

with

increasing

temperatures. The

energies

corresponding

to these minima _are -9, -4.5, -2, o, 2, 4.5, and 9 mev. The "DOS" which _was minimal at the Fermi level at 1 K becomes maximal at 4 K. A

magnetic

^{field of1 T directed}

along

the _a axis has _no influence _on the "DOS".

El _{30 K}

w

3 1

~ g

é 'C

É

# o

-io 5 o 5 io

Applfed Voltage (mV)

Fig. 6. Dilferential conductance _versus applied voltage at dilferent temperatures along the b direc- tion for _a magnetic field of o.3 T.

~

~~

d _~

@

3

1

~ ',

)

,_..

é n

$

#

à

io s o s io

Applled Voltage (mV)

Fig. 7. Evolution for various temperatures ^of the amplified "DOS" along the _c direction for _a o.3 T magnetic field.

4.3. DiFFERENTIAL CONDUCTANCE _{ALONG THE} b _{AND c} Axis. The temperature

changes

of the "DOS" _are

comparatively

weak

along

the b and _c directions _as shown in

Figures

6 and 7. For both directions there _{is a} relative maximum of the "DOS" at the Fermi level. Weak

oscillations _are also found at the _sonne

energies

_as

along

the _{+ c} direction.

They

_{are more} visible

along

^the c axis than

along

the b _axis.

One should note that

along

the _c direction the "DOS" _is influenced

by

the

application

of _a

magnetic field,

_as shown in

Figure

8. This effect _appears for fields above _a threshold value of

approximately

o.5 T.

Therefore,

it is clear that the "DOS" is

anisotropic

since the

properties

_are different

along

the

(b, c) plane

^and

along

^the a axis. The temperature

dependence

of the "DOS" is the strongest

along

the _a axis. An increase of the condensation _rate takes

place

below 30 K. In ail the

explored

directions of the

(b, c) plane, symmetrical

^minima are found at energies of -9, -4.5, -2, 2,

4.5,

^and 9 mev.

~

~/g

d

@ 3

1

)

~

é n

0.3T "'

~

ÎÎ~

-io -s o s io

Applled Voltage (mV)

Fig. 8. Idem along the _c axis for dilferent rnagnetic fields at several temperatures. In this

case the

rnagnetic field is applied along trie _a aXis.

We _can remark that the oscillations of t>he "DOS" _are stronger

along

+ c direction than

along

b _{or c.} This is due to the fact that

along

this direction

junctions

_are realized _onto

freshly

deaved

crystals

_givmg better

tunneling junctions.

A minimum _at the Fermi level is found in addition to other minima

along

the b _{+ c} direction.

5. Discussion

Tunneling

spectroscopy

provides

the "DOS" in the valence and in the conduction bonds _neon the Fermi level. Our data show that there _{is no} true metal-semiconductor transition. This is due _to the _quasi bidimensional character of this

compound

and _{to an}

imperfect nesting

of the Fermi surface. We will _now correlate the

experimental

_curves to

simple

band structure models.

S-1. EVOLUTION OF THE "DOS" WITH TEMPERATURE

5.1.1. Evolution

alo~lg

trie _a Axis.

Along

^this

direction,

the condensation _rate is the

highest.

The

tunneling

conductance varies

monotonously

with the bios

voltage

and there exists _a

unique

extremum situated at the Fermi level. We _{can assume} a

simple

band scheme at 1 K in which

the "DOS" _near the Fermi level is

parabolic,

_as shown _m the mset of

Figure

9a. In this

approximation N(E)

varies _as

E~/~

_{both in the valence and conduction bonds.}

Figure

9 represents ^the

tunneling

conductance at

K,

50

K,

and 90 K calculated

by

_means of the

following

relation

(22)

~~ ^~~

(E

^eV

-(V)

_c~

/ _N(E)

^{~ kT}

'~~ kT _{~ ~} ^~ dE

1

+ exp

~

kT

(ai

ç d

~j g

~ 4

)

o

g

~ ³ _{CO ndoebon}

g Ba nd

3 K

(

2

E

~ ^'

~

Valence 1

.30 .20 -10 0 10 20 30

Applled Voltage (mv~

(bi

~ ⁵

d

+iÉ

g 4 90K

)

K

g K

~ ₃ K

K

# Énà~~

_K

j

₂ ^~

o E 10K

~ ' lK

me

~'~

0~

-30 .20 .10 0 10 20 30

ApplIe0 Voltage (mv~

Fig. 9.

a)

Numerical simulation of the differential conductance at K, 50 K, and 90 K for rigid

parabolic bond scheme. The two bond extrema _are located at Ef. b) Same _as previously with

no rigid

bonds changmg with temperature _up to T

= loo K according to _a mean-field law.

N(E)

is the

projection

of the DOS _near the band

edges along

the

crystallographic

direction

investigated.

The

computed

_curves show that there is _a

crossing

of the _curves which does not appear

experimentally (Fig. 9a).

We have therefore assumed that there _{is a} band

splitting

which

mcreases below

Tp.

^{In this model the} extrema of the two bands follow _a mean-field law:

lT

^1/2

~ÎÎÎ~

~0

(l fl)

+ àcross

P

jn 1/2

jçmin _{~$ ~$}

con j~ ^cross

P

Z M A ~

~ _j

q

' ~

C°

i

_Y°

~~

lai

~~~

Fig, lo.

a)

Schematic diagram showmg the high temperature Fermi surface from Canadell _et ai.

Shaded _areas correspond to electron pockets. b) Proposed electron and noie remaining pockets at low temperatures [11, 12]. Trie _arrows indicate trie nesting _wave vector associated with the first and the second CDW transition.

Ao is trie value of trie

pseudc-gap

defined

by

trie mean-field

theory,

and A~ra~~ is the

overlap

energy of the bands at T

= o K. The evolution with temperature of the different _curves is shown in

Figure

9b with

Ao

" 15 mev, Tp = loo K, and A~r~~~ = o mev. The calculated

curves are more similar to the

experimental

_ones.

This

simple

band model therefore supports ^{the fact} that

along

the _a

direction,

_a band

splitting

takes

place

when T is

lowered,

the temperature

dependence following

_a mean-field

theory.

5.1.2. Evolution

along

trie b,_c, and trie b+c A~is. Canadell and

Whangbo il

_2] have calculated

m a 2D

approximation

_a band _structure in the (b,

c) plane

m the normal state above the first Peierls transition.

They

also obtained the Fermi surface which indudes three sheets, shown in

Figure

loa.

They

bave noted that the Fermi surface _con be obtained

by

the

superposition

of three

quasi-lD

Fermi surfaces. The

nesting

vector of the first CDW transition, o.23 b*,

would relate sheets of this Fermi surface. One _may

speculate,

_as in reference [15] ^{that below} Tp

ellipsoidal

electron and hole

pockets

_are left

by

the CDW _gap

openings (Fig. lob).

The

major

_axis ^of these

pockets

would be

along

the c* axis.

The oscillations that _we have observed _are not due to _a

phonon

spectrum since

they change

with _an

applied magnetic

^field.

Along

the

explored

directions the "DOS" _varies little with temperature and the oscillations

disappear

above 25 K. A dassical "DOS" is then observed.

These oscillations may therefore be due to the existence of these small pockets.

Let _{us assume a}

simple

model of two

parabolic

bands

overlapping

at the Fermi level at

1 K. The extremum of these bands would follow _a mean-field law towards temperature as the

expression

given

m Section 5.1.1. The calculated "DOS" is shown _m

Figure

11, with

Ao

_"

4.5 mev and A~ra~~ ₌ 2 mev. Two minima which

correspond

to the extremum of each bands appear ^{at 1} K. It _can be _seen that there is _no

overlappmg

of the differential conductance in the

vicinity

of the Fermi

level,

this is due to _a too

large splitting

of these bands towards tem-

perature. The parameter

Ao

which

gives computed

_curves

corresponding

to the

expenmental

data obtained

along

b _{or c} direction is lower than the _mean field

prediction

and takes the value 1 mev.

On the basis of _our

experimental

results it _seems that there _are at least three

couples

of electron and hole bands in the (b,

c) plane.

The extrema of these bands _are

symmetrical

and situated at 2, 4.5 and 9 mev _on both sides of the Fermi level.

The minimum of the "DOS" at the Fermi level found in the b + c direction at 1 K could be

explained

in the

following

_way:

along

this axis the faces of the

crystals

_are

possibly

not

perfectly

parallel

to the _a axis. Therefore the

junction plane

_may contain _{an a} axis comportent. ^The

T=1.2K 12K<T<30K é~

~nwx

Et

When T >ncreases

b

ç dÎ>

~ ~

g

1 ₆

il

é

° 5

«

$à

#

° K

3

30

Applled ^oltage (mV)

Fig. Il. -

scheme ^{epresent olution} of _thesebonds towards temperature, b) _imulation of the

"DOS" corresponding to trie _baud model proposed above with _AD " 4.5

~neV^mean-field

Across = 2 mev. The value of trie _extre~na of _trie ilferential conductance

are

orresponding to values

of he bond edges weighted by trie derivative of the Fermi function.

simulation of the tunneling onductance, at _ifferent

temperatures

which _takes into account

two erlapping bands

and

two ointive bands at ^{the Fermi} level at

Figure

12.

The verlapping bands follow ^a ean-field law up to _{30 but}

with

Ao

"

while

the _jointive

bonds

follow also _a mean-field law _up to loti K. This last set _of ^uds is

weighted by

a

factor 1/2. The seem to _be in good _greement with the xperimental

data.

Our

are three

electrons and

holes

ockets. The _bonds

all

cross the ermi

level.

The

alues of the

extrema of the hole bonds

are

2.2 and 7.3 _mev and for the _electron bonds are o.4

We cannote that if _the

energies are not identical to _those

proposed by Sasaki, _they are of

>

~~

(

~

(r

É# o

-io

Fig,

12. -

with temperature up

bonds ^with at Ef at 1 K changing with e~nperature up to 100 to

law, with _Ai # 15

mev.

the same order.

The ternperature seems to nfluence all

are _vanishing when

related to _the econd transition ecause above this temperature _there are no cillations. The

change

in

the

rate along the a _axis may also be

related

to a

corresponding to a second CDW transition.

5.2.

OF A

MAGNETIC FIELD: OSSIBILITY OF A

MAGNETIC^REAKDOWN.

-

The

effect of a magnetic field

in the (b, c) plane was investigated. _The

field

has modified

the c

irection.

We have ^sumed

that

the

"DOS"

in

a field of o.3 T (this

field

is _used

in

the ead superconductivity)

that the effect of B is gnificant only

above 0.5 T. erefore

this

effect is not elated to _the

uantization m

Landau

_levels, since the _Landau levels eing periodic in energy, the

position of

the _relative minima of the DOS"

would vary

linearly with the magnetic field.

One

_notes that

the

extrema in trie "DOS" appearapproximately at trie ^same

positions,

_what-

ever the value of the field (Fig. _8).

A

possible xplanation

of

the

be the presence of a agneticreakdown between _different

ellipsoïdal

ockets at _the

Fermi

surface. Indeed, Blount _[23] has _proposed

that in

_the

presence of

a

magnetic field electrons

or holes may tunnel"_cross

a

gap A separating

the pockets.

_The ^ransition probability

written

as:

~~

~ ~~~ _~ B _~

with

A~m*

~°

o

tion low

threshold

magnetic

field of the order of o.5 T, _we obtain _a distance between

pockets

in the

reciprocal

_space of o.3

À~~.

At _a temperature ^{of1 K the effect of} a

magnetic

field is _more

significant

_on the "DOS" _at

an energy of 4.5 mev than for the other

energies.

At _a temperature ^{of 4}

K,

the "DOS" at the Fermi level does not

change

until B ₌ 1 T. On the other hand the minimum of the "DOS" for o.3 T which is at

roughly

6 mev is subdivided

into two minima _at 4.5 mev and 9 mev, ^the absolute minimum

being

at 4.5 mev.

Above 10 K the influence of the

rnagnetic

field is

progressively

weakened and

disappears completely

above 18 K. The characteristic

energies

_are the _{same as} those

concerning

the elec-

trons and holes

pockets. Therefore,

these results _may be due to a

magnetic

breakdown related

to the presence of electrons and holes

pockets

at the Fermi surface.

The different effects of the

magnetic

field for different bios

voltages

would be

explained by

a

slight

variation of the effective _mass around _{a mean} effective _mass o.l _mû this

causing

different threshold fields between the

pockets.

In the present case the

pocket having

the lowest effective

mass would be at 4.5 mev from the Fermi level.

6. Conclusion

To _our

knowledge,

it is the first time that _one observes

by tunneling

^studies the existence

of electron and hole

pockets

in _a

charge density

_wave

compound.

We have been able _to

separate three

symmetrical couples

of electron and conduction bands with

edges

at 2,

4.5,

and 9 mev from the Fermi level in the low temperature CDW state of

i~-Mo4Oii.

These bands

are induced

by

the second low temperature ^CDW transition. Furthermore there

might

be a

magnetic

breakdown

along

the _c axis, the threshold field

being

at

roughly

o.5 T.

A band scheme simulation

describing

the behaviour of the

tunneling

conductance

along

the _a axis corroborates that the Peierls transition dots on

overlappmg

valence and conduction bands and tends to

split

these bands

according

to _a mean-field law.

Acknowledgments

We _are

grateful

to the members of Laboratoire d'

Electronique

des Milieux Condensés who

helped

us

continuously, especially

to Dr A. Fournel for

interesting

discussions and to J-B- Faure for technical assistance. Thanks _are also due to G. Fourcaudot for the

crystal growth.

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