Hall Effect Anomalies and Phase Transitions in the Organic Superconductors, κ(BEDT-TTF)2Cu(NCS)2 and β-(BEDT-TTF)2I3

HAL Id: jpa-00247287

https://hal.archives-ouvertes.fr/jpa-00247287

Submitted on 1 Jan 1996

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Hall Effect Anomalies and Phase Transitions in the Organic Superconductors, κ(BEDT-TTF)2Cu(NCS)2

and β-(BEDT-TTF)2I3

Keizo Murata

To cite this version:

Keizo Murata. Hall Effect Anomalies and Phase Transitions in the Organic Superconductors,

κ(BEDT-TTF)2Cu(NCS)2 and β-(BEDT-TTF)2I3. Journal de Physique I, EDP Sciences, 1996, 6

(12), pp.1865-1873. �10.1051/jp1:1996195�. �jpa-00247287�

Hall Elllect Anomalies and Phase Transitions in trie Organic Superconductors, ~t-(BEDT-TTF)2Cu(NCS)2

and fl-(BEDT-TTF)213

Keizo Murata

(*)

Electrotechnical

Laboratory,

1-1-4, Umezono, Tsukuba lbaraki, 305

Japan

(Received

13

May1996,

revised 22

August, accepted

_.5

September1996)

PACS.74.70K

Organic superconductors

PACS.72.15Lh Relaxation limes and _mean free

patins

PACS.72.15Eb Electrical and thermal conduction in

crystalline

menais and

alloys

Abstract. Trie Hall coefficient, Ru

(T)

for

organic

conductors _is

strongly

temperature

depen-

dent _even if they _are metallic. The temperature

dependence

of Ru

(T)

_is found tu exhibit _une-

to-one

correspondence

with trie structure of the Fermi surface, hence _con be called "Standard"

for each surface. Ii turned Dut that the

simple temperature-independent

Ru

(T)

is

only

realized in _a material where trie Fermi surface _is round and

simple

for these low dimensional systems.

The "Standard"

Ru(T)

for each Fermi surface is obtained

empirically

by _comparing

RH(Tl's

of

relatively high

_pressure, where low temperature navet stores _or excitations

are suppressed.

In _{some cases,}

Ru(T)

deviates

abruptly

from trie standard

Ru(T)-behavior,

which suggests _a

phase transition _or lis precursor. We show the

examples

of

~-(BEDT-TTF)2Cu(NCS)2

and of

fl-(BEDT-TTF)213

in which the anomaly _in

RH(T) corresponds

tu _a

peak (r-

GO

K)

of antifer-

romagnetic

fluctuation, and tu 20

K-phase

transition which is a~sociated with low-Tc _supercon-

ductivity, respectively.

As _{seen in} these two

exarnples,

trie Hall eflect is

strongly

influenced by trie electronic state, and therefore, _can be _~ useful trot for

locating phase

transitions.

1. Introduction

The Hall effect is

generally

known to show carrier

density

in metallic and

semiconducting systems by

trie

relation, RH(T)

₌

1/~e,

where _{n is} trie carrier

density

and _e, trie electron

charge.

A

simple

consequence of this relation _is that metallic materials show _a

temperature tndependent RH (T),

_at least wheu trie Fermi energy is much

higher

thon 300 K. In trie

past

several years, we bave examiued trie Hall effect of

orgauic

couductors aud

supercouductors

from

quasi-oue-

to

quasi-two-dimeusioual (QID, Q2D)

couductors aud fouud almost

always

_a

temperature

dependent RH (T). Although

simulations of

RH(T)

aud its

proof

_{are very} dilficult due to restricted

experimeutal

information aud

evideuce,

at least for trie

Q2D _system,

_we fouud

empirically

oue-to-oue

correspondence

^with trie

temperature depeudeuce

of

resistivity, p(T),

trie temperature

dependence

of Hall

effect, RH(T),

and trie structure of trie Fermi

surface,

~N"hich _are shown

schematically

in

Figure

1.

(*)

Present address:

Dept.

of Mat. Sci., Fac. of

Sci.,

Osaka

City

Univ., 3-3-138,

Sugimoto, Sumiyoshi-

ku, Osaka 558, Japan

e-mail:

muratak©soi.oeaka-cu.ac.jp

@

Les

Éditions

_de

_Physique

₁₉₉₆

1866 JOURNAL DE

PHYSIQUE

I N°12

J-tYPe ~ ~ '"'~~°~

^~~

^~

>o

~

K-type ~

^'"'

'°~ /

< o

~

T-type ~

lin

'°g

tl-(BEDT-TTF~I

~~

i~ j

4

~ og

~

20kbar ² bar

Fermi

surface RH ^Vs

Temp

_{p vs}

Temp

Fig.

1. Schematic sketch of trie Fermi Surface, trie Hall coefficient _~ers~s temperature ^and resistivity

~ersus temperature for fl-, ~- and T-types of

quasi-two-dimensional

_organic conductors.

It is

interesting

to recall that Fermi surfaces of trie

fl-,

_{~-, o-} and r-types can be

developed

from _a

siugle

round Fermi surface _as showu _lu

Figure

2.

By

_companug

Figures

1 and

2,

it is

worth

noting

that trie Fermi surface of trie

fi-type

is trie most

simple

_amoug

ail,

1.e. _a round Fermi surface _is located

just

inside trie first Brillouin _zone. When trie Fermi surface folds

due _{to contact} with trie Brillouin _zone

boundary,

there appears a

nearly singular

curvature

m trie Fermi surface. Arouud the

siugular

curvature lu the Fermi

surface,

_vF varies

strougly

with

k,

aud

consequently

trie _r

scattering

lifetime becomes

sensitively k-dependeut.

This is trie _{pnmary reasou} for _trie temperature

depeudeuce

of

RH(T).

Marked

examples

of these

siugular

curvatures _are trie

edges

of trie leus orbit of trie

~-type,

and trie

edges

of trie star-

shaped

Fermi surface of trie r-type.

(In

_r-type

couductors,

there must be another _reason for temperature

dependeuce

_m

RH (T).

The

system

con uot be treated withiu trie framework used for

degeuerate

menais due to trie Fermi energy which is in trie _same order of

magnitude

_as trie

room

temperature.)

For the ideal 1D

system

^with

k-independent

_{r, m} the relaxation time

approximation

_lu the semiclassical treatmeut,

symmetric (in k-space)

relaù~ation time does not

produce

Hall

voltage,

_{smce a~y,} which is

proportional

to trie Hall

voltage,

vamshes

according

to trie

integral,

a~y o~

f

dl _x £, where £ = vfr

[ii. Therefore,

trie Hall

voltage,

when it is detected in trie actual

QID system,

it is due _to the imbalance from _the symmetry. ^In other

words, RH (T)

_{is very} much

>-type

0

-

~

K-type

~-type

_-

~ ~

a-type

~ ~

example:

tx-(BEDT-TIF)~MHg(SCN)4

Fig.

2.

Typical

two-dimensional Fermi surfaces reconstructed from trie

original

round surfaces.

dominated

by

trie

dispersion (or diffusion) along

trie second and third conductive _axes. Then trie Hall

signal

con either be

positive

_or

negative irrespective

of trie value _or

polarity expected

from trie

charge

^transfer. So it is _more dilficult in lD thon in 2D to draw ont trie "standard"

RH(T) correspondiug

to trie Fermi surfaces. Schematic

(= rougir) image

of trie Fermi surface

is less sulficieut to defiue

RH (T)

iii

QID.

Trie

importance

of trie

transport properties aloug

trie less couductive _{axes are}

poiuted

ont to

iuterpret

trie

temperature depeudeut RH(T)

iii TTF-

TCNQ _[2].

Auother

interpretation

for trie temperature

depeudent RR(T)

is, for

instance,

in

terms of _a precursor of trie

phase

transition _or of collective

excitation,

such _as CDW in TTF-

TCNQ

_[2] and SD~V in

(DMET)2Au(CN)2 [3,4].

These

apparently

different _causes

resulting

in trie

temperature dependent RH (T)

_are

actually

uot

iudepeudeut,

siuce trie

dispersion along

trie less couductive _axes

strougly

influences trie variation of trie Hall effect _as well _as trie

phase

transitions. It is a matter of _course that materials with lower

dimeusiouality

are more

likely

to show low

temperature

iustabilities. lu other

words,

materials

properties

at lower dimension _{are more} sensitive to trie 2ud aud 3rd _axes trausfer _{euergy or} to trie

warpiug

of trie Fermi surface. This situation makes it _more dilficult to draw out

typical

aud standard

RH(T)

for lower dimeusioual materials. Eveu _lu trie 2D materials, with

iucreasiug

_pressure,

low temperature iustabilities _{are more}

suppressed,

aud

thus,

"standard"

RH (T)

for each Fermi

surface,

_eveu

though

it is

schematic,

_is

relatively easily

obtaiued.

lu this _{paper, we} revisited

RH (T)

of trie

~-type

and

fl-type

couductors and show that

RH (T)

ceases

chaugiug

with pressure a

beyoud

certain pressure, where _a

typical

aud "Standard

RH (T)

for each Fermi surface _are realized. Wheu suddeu deviatious from those

RH(T)

_are

fouud,

it

1868 JOURNAL DE

PHYSIQUE

I N°12

~

5

~ .9

)

~

Î~

~ ~

É

#

'

~

i

# 1

à

~

'

0 50 100 150 200 250 300

Temperature (K)

Fig.

3.

Temperature dependence

of Hall coefficient of

K-(BEDT-TTF)2Cu(NCS)2

ai ambient and other pressures.

Magnetic

field is

perpendicular

tu trie

plane.

is an indication of _a

phase

transition _{or some}

peaking

in electronic excitation. Thus trie Hall effect is _a useful and sensitive tool for

observiug

_a

phase

transition.

2.

~z-(BEDT-TTF)2Cu(NCS)2

This material shows _a

hump

in the

resistivity

_versus

temperature

^{around loo K and under-} goes a

superconducting

transition at 10 K. The

resistivity hump

had been controversial until

a series of materais from trie

insulating

to _more metallic _ones

appeared systematically.

It is understood _since that trie materials _con be

categorized

from insulator _to metal in trie _or-

der, N-(BEDT-TTF)2Cu[N(CN)2)Cl, N-(d~, BEDT-TTF)2Cu[N(CN)2)Br, ~-(h8, BEDT-TTF)2 Cu[N(CN)2)Br,

and

N-(BEDT-TTF)2Cu(NCS)2.

This

insulating

state is driven

by

electron

correlation,

like _a Mott insulator which bas been discussed in correction with NMR

experi-

ments

[à-7j

_as well _as

theoretically _[8j.

Trie

hump

at 100 K in

N-(BEDT-TTF)2Cu(NCS)2

con then be understood _{as a crossover} from trie Mott insulator to a metal

by lowering

trie

temperature.

In ail of trie

N-compounds, (TIT)~~

of

~~C-NMR

_exhibits

a

peak.

In

~-(BEDT-TTF) 2Cu(N CS)2

and in

~-(BEDT-TTF)2Cu[N(CN)2)Br,

this

peak

is located around 55 60 K

indicating

a maximum in trie

antiferromagnetic

fluctuation but _net trie real transition because trie fine

shape

of NMR is still visible below 55 60 K. Previous to these NMR

studies,

there bave been several reports that indicate anomalous

change

around 60 70 K, which bave been referred _{to m} reference

[9j.

^{Trie Hall}

coefficient, RH(T),

which is

positive, gradually

increases _on

decreasmg

trie

RH(T) temperature.

^{Below 60} 70

K,

trie

slope

of

(dRH(T)/dT( suddenly

increases and shows _a

peak

around 10 K

[9,10j.

No

appreciable change

_is observed around 100 K

similarly

to

NMR-(TIT)~~

and to static

susceptibility _[5, 6j.

This sudden increase _m

(dRR(T)/dT(

below 60 70 K coincides

exactly

with trie

peak

in

(TIT)~~

of

~~C-NMR.

Now trie 60 70 K

transition,

_{we con compare} trie

RH(T)

studied under

higher

_pressures.

If _we

overlap

trie

RH(T)

of P ₌ 0 with other

RH (T)

of

higher

_pressures, it is quite evident that trie 60 70 K is

really

_an anomalous temperature as shown _m

Figure

3. We therefore could bave _a

strong implication

of _a transition around 60 70 K net from trie

slope change

m

jdRH(T)/dT(

but rallier from trie _contrast between trie ambient and

higher

_{pressures m} trie Hall effect. We exammed trie

specific

heat _across this

temperature

_range, but could trot detect

any

singular signal il ii.

This is reasonable if trie 60 Ii

anomaly

is of

purely

electronic

origin.

In such _{a case, an}

anomaly

_m

specific heat,

_even if _one

exists,

_can be buried in trie lattice contribution to trie

specific

heat.

Moreover,

this is consistent with trie fact that this is trie

peak

in the

antiferromagnetic

fluctuation in

~-(BEDT-TTF)2Cu(NCS)2

but trot trie transition obtained from observation of the

fine-shape

of NMR

[12].

The suppression

by

_pressure of the 60 70 K "transition" observed

by

Hall effect

study

under pressure in

~-(BEDT-TTF)2Cu(NCS)2

_may

actually

_occur. In _a similar

material,

~-(BEDT-TTF)2Cu[N(CN)2)Br,

the

suppression

of the

magnetic

fluctuation

peak by

_pressure

is observed

by (TIT)~~

of NMR

[6].

About the "standard"

RH(T),

Sushko et ai. studied in

~-(BEDT-TTF)2Cu[N(CN)2)Cl

in trie _{pressure range} of 4.5 10

kbar,

and found almost _no

change

_m

RH(T)

with pressure

[13].

It is

important

to note that this is trie pressure region where

already superconductivity

is

suppressed similarly

_as m

~-(BEDT-TTF)2Cu(NCS)2.

3.

fl-(BEDT-TTF)213

As _seen

Figure 4, RH (T)

of this material varies

extremely

little with temperature down to 20 K.

Although

this result is

quite simple,

and looks _more like standard

metal,

tt is worth

remarktng,

since flot

RH (T)

is

exceptional

_among the

Q2D organic

couductors.

By comparing

^with

RH (T)

aud the Fermi surface of other

types,

it is noticed that this standard behavior of

RH(T)

is caused

by

the fact that the Fermi surface does not touch the first Brillouin _zone

boundary, leaving

_no

singular

_area in the Fermi surface. In other

words,

_no

particularly

anomalous

r-(mean

free

time)

_area in

kF-space

_is found.

Another

interesting

^feature in this

region

is that _even

though p(T)

and

RH (T)

exhibit metallic

behavior,

the _mean free

path, t,

calculated

by

_a

=

ne~r/m

is smaller than trie lattice constant around 300

K;

t

= 0.13

À

_{for 300 K and 130 at}

temperature just

above

superconducting

T~

[14, là]. (The

lattice constants are 6.6

À,

9.1 in the 2D

plane

and 15.3

perpendicular

to the

plane.)

The mobilities _{are p}

=

RH

_{x a} is 0.2

cm~ IV

_s at _room

temperature

^and 80

cm~

_V

s at 20 K. From such

high

to low

temperatures

ail

through, RH (T)

is constant _as if it is

contiuuously

_a coherent metal. It is net well understood

yet why

metallic behavior is

exhibited with such

parameters

ⁱⁿ

high (= room) temperature.

Turning

back to the relation between the Hall effect aud the

phase transitions,

_{we compare} the

RH(Tl's

studied under _{vanous pressures.} From

Figure 4, RH(Tl's

_are identical above 7

kbar,

and _{we can} thus obtain trie "standard"

RH(T)

for this material.

By comparing

^with

this,

anomalies _m

RH(T)

_are

quite

dear at 17à K

[14]

^{and 20 K}

[14-16].

This material bas two

supercouductiug

transition

temperatures,

^{8 K and 1-à K}

[17,18].

Trie

superconductivity

of1.à K is associated with trie incommensurate

superstructure

^fouud

by Emge

et ai.

[19]

^with

a wave vector of

(0.08,

0.27,

0.20à)

that appear ^at 17à K. Trie

superconductivity

at 1.à K is

a metastable state

[20, 21]

^which can be converted to 8 K state

through

2 K

by annealing

trie

sample

at around 103 K in trie time scale of 100 hours

[22, 23].

As for trie

properties

and rotes of trie Hall

effect, first,

ii is

interesting

^from trie

viewpoint

of trie

transport property

^{itself that}

already

at 17à

K,

where _no

siguificaut change

in trie Fermi

surface relative to trie Brillouin _zone

boundary

_is

expected,

trie

RH (T)

is influenced

(reduced) by

8Slo. Next, _{a new} transition _was

strongly implied

at 20 K

by observing

_a

sharp

kink there

and _> 40Yo

drop

_m

RH (Ti

below 20 K

[14]

^which was then confirmed _{as a} transition but not

trie _crossover of trie

scattenng

mechanism

by specific

heat measurement

[24].

^{Since this 20}

K transition _was obvions _m

RH(T)

and in

speofic heat,

it is noted that trie

RH(T) study

_is

thus _a

po~v"erful

tool for

locating phase

transitions.

Looking

^around at other works _concernmg trie 20 K

anomaly,

_{we are} remmded of trie NMR

Korringa relation,

which does net continue

187o JOURNAL DE

PHYSIQUE

I N°12

5

Ù w+

~

~ OOOO'OOO°OaO

f

~ ,,,

Îf

~

Î__

#

g

à1

~Î~"~

_h

'~>

~ '

a)

_{50 100 150 200} 250 300

Temperature (K)

~'%

~

~ _l '

~

~

~ °

' Q

c©

j O

£ -

>

O >

~ fl

~

>

'

perature (K)

Fig.

4.

Temperature dependence

of Hall coefficient of

fl-(BEDT-TTF)213

ai anibient and orner pressures for T

= o 300 K

la)

and ils _expansion for low temperature

16). Magnetic

field is perpen- dicular to trie

plane.

The values of pressure in _a

clamp

cell

correspond

to low temperature. In

our

pressure medium and equipment, room temperature is

always

1.5 kb

independent

of

room temperature values. Arrow

in

(b)

shows _a

example

of _an

anomaly

_m RH

(T)

for 5 kb. Data for 3 -10 kb

are for

trie _same

sample.

TO fit with o kb data of _a dilferent sample, trie value ai 100 K is adjusted taking

into account trie data from reference [28].

across this

temperature [2à,26].

Trie

~H-NMR

_{relaxation behavior becomes}

non-siugle

aud

that situation _is

comphcated _[27].

We _come back to this discussion later.

One of trie

advantages

of trie

RH(T) study

_is that _{we can}

easily

extend trie work under pressure. Since anomalies _are

always

_seen in

RH (T),

_as shown

by

_{an arrow} in

Figure

4b for à kb for

example,

_{we cau}

plot

those

temperatures agamst

_pressure

together

with

T~'s

of trie

superconductivity,

as shown in

Figure

à. It is _seen that trie

point

at 20 K at P

= 0

cannot be couuected to other points associated with trie

high T~'s. By annealing

at 103 K for

à0 -loo

hours,

8 K

supercouductivity

is observed

by specific heat,

aud 20 K

anomaly

in

RH (T) disappears _[28].

What is known from

Figure

5 is that 1.5 K

superconductivity

_is related with lîà K

through

20 K

anomaly,

while 8 K

superconductivity

_is related

through

la K

anomaly.

With this

figure,

_as

N-(BEDT-TTF)2Cu(NCS)2 superconductivity

is

suppressed

where

RR(T)

ceases

changiug

with _pressure, it is

suggestive

that trie

superconductivity

_is assisted with _non-

conventional electromc excitation.

Although

trie _reason to reahze 1-à K

superconductivity

_m trie basic

background

of 8 K bas uot beeu solved

yet,

_we

proposed previously

that trie

deusity

of _states for the 1-à K state _is

25

J'-(BEDT-TIF)~l~

Çi

R~(T)

Î ₁₅

~ é

Î

_la Low Temp

f

^Metallic region

~ ~

O 2 4 6 8 la 12

Pressure (kbar)

Fig.

5. Trie temperature-pressure

diagram

of

fl-(BEDT-TTF)213 plotted

with Tc

(superconductiv- ity)

and THaii

(Hall

elfect

anomaly).

Ii is _seen that ai Tuait ai 20 K for low Tc, trie store does non continue to trie

points

associated with trie

high

Tc state. The metallic region ^below trie temperature of trie Hall elfect

anomaly

_is shown _as "low temperature metallic

region".

This demonstrates trie

ambiguity

_m trie definition of trie anomalous temperature.

smaller thon that of trie 8 K at trie 20 K transition

through possible nesting

of trie Fermi surface

[14, 24].

This

deusity

of state _{scenario can} not be consistent with trie

~H-NMR results,

which tells about

3%/kbar/kbar

decrease in trie

deusity

of states in

spite

of _some relaxation

anomaly

at P

= o

through

around 20 K

[2à].

Very recently,

Kanoda et ai. bave

reported

_a detailed static

susceptibility

and

~~C-NMR

concemiug trie 20 K transition

[29]

^{which renewed trie}

previous reports

_ou

susceptibility _[30].

Trie static

susceptibility

decreases with

temperature

^{aud levels off below 20 K.}

By auuealiug

at 103

K,

trie level-off

susceptibility

iucreased. This is consistent with _our

previous deusity

of state

picture implied

_ou trie Hall effect measurement. Further. incommensurate structure _are seeu to be

suppressed by

trie NMR fine

shape by aunealing

in trie time scale of 700 tirs.

4. Conclusion

The Hall effect of

orgauic

metaiiic couductors _are

generally

_very

temperature depeudent.

This is due to trie low dimensional nature of these inaterials. If trie materials _are lower in

dimension, RH (T)

_as well _as trie low dimensional instabilities _{are more}

sensitively

^influenced

by

trie transfer energy

along

trie less conductive _axes. This situation

might

be trie _reason that _{we cari} obtain

typical

"standard"

RH(T)

for each

typical

Fermi surface

especially

for 2D materials under

relatively higher

_pressures, where delicate transfer _energy structure is less

important.

Among these, fl-(BEDT-TTF)213

is trie

special

_case, which shows almost temperature ^Inde-

pendent RH (T).

It is

explained

that trie Fermi surface _is trie _most

simple

without _any

singular point

in

k-space

lu terms of

scatteriug

lifetime.

We note that for ail trie _cases

~-(BEDT-TTF)2Cu(NCS)2, ~-(BEDT-TTF)2Cu[N(CN)2)Cl

and

fl-(BEDT-TTF)213, RH(T)

is

already

constant with _pressure where

superconductivity

is

suppressed.

This

might

indicate that trie

superconductivity

is realized in trie environment of novel low dimensional instabilities.

1872 JOiÎRNAL DE

PHYSIQUE

I N°12

Finally regarding

trie standard

RH(T),

trie anomalies in

RH(T)

which _are

usually

related to

phase

transitions _are

clearly

found. Thus the Hall effect is _a useful tool for

locating phase

transitions.

References

iii Ong N.P., Phys.

Reu. B 43

(1991)

193.

[2]

Cooper

J.

R., Miljak M., Delplanque G.,

Jérome

D., Weger

M., Fabre J. M. and Giral

L.,

J.

Phys.

France 38

(1977)

1097.

[3] Murata K., Kikuchi

K.,

Honda

Y.,

Komazaki

T.,

Saito

K., Kobayashi

Ii. and Ikemoto

I.,

Trie

Physics

and

Chemistry

^of

Organic Superconductors (Proc.

of trie ISSP Int.

Symp., Tokyo, 1989) Springer

Proc.

Phys. 51,

234.

[4] Honda

Y.,

Murata

K.,

Kikuchi

K.,

Saito

K.,

Ikemoto I. and

Kobayashi K.,

Soitd State Commun. 71

(1989)1087.

[5] ^Kawamoto

A., Miyagawa A.,

Nakazawa Y. and Kanoda

K., Phys.

Reu. Lett. 74

(1995)

3455.

[6]

Mayaffre H.,

Wzietek

P.,

Lenoir

C.,

Jérome D. and Batail

P., Europhys.

Lett. 28

(1994)

205.

[7]

Miyagawa A.,

Kawamoto A., Nakazawa Y. and Kanoda

K., Phys.

Reu. Lett. 75

(1995

l174.

[8] H. Kino and H.

Fukuyama,

J.

Phys.

Soc.

Jpn

64

(1995)

2726.

[9] Murata

K.,

Ishibashi

M.,

Honda

Y.,

Fortune N.

A.,

Tokumoto

M.,

Konoshita N. and

Anzai

H.,

Soiid State Commun. 76

(1990)

377.

[10]

^Murata

K.,

Ishibashi M., Fortune

N-A-,

Tokumoto

M.,

Konoshita

N.,

Anzai

H.,

Kikuchi K., Saito

K.,

Ikemoto I. and Takahashi

T., Phystca

C185-189

(1991)

2685.

[iii

Fortune N-A- and àfurata

K., unpublished.

[12]

^Kanoda

K., private

communication.

[13j

^{Sushko Yu.}

V.,

Shirakawa

N.,

Murata

K.,

Kubo

Y.,

Kushch N-D- and

Yagubskii E-B-,

Proc. Int. Conf. _on Sci. and Tech. of Snth.

Metals.,'96.

[14]

^Murata

K.,

Ishibashi

M.,

Honda

Y.,

Tokumoto

M.,

Kinoshita N. and Anzai

H.,

^J.

Phys.

Soc.

Jpn

58

(1989)

3469.

[15]

^Murata

K.,

Ishibashi

M.,

Fortune N. A.. Komazaki

T.,

Honda

Y.,

Tokumoto

M.,

Kinoshita N, and Anzai

H., Synth.

Met. 41-43

(1991)

2163.

[16j

Konn-Hamzic

B.,

Ferra L. and

Cooper J-R-, Phys.

Reu. B 41

(1990)

11646. Trie

temper-

ature

they pointed

eut was

actually

40 K instead of 20 K.

Ii?i

Murata

K.,

Tokumoto

M.,

Anzai

H.,

Banda

H.,

Saito

G., Kajimura

K. and

Ishiguro T,

J.

Phys.

Soc.

Jpn

54

(1985) 1236,

2084.

[18]

^Laukhin

V.N.,

Kostuchenko

E-E-,

Sushko Yu. V. aud

Schegolev I.F.,

Pis'ma Zh.

Eksp.

Teor. Fiz. 41

(1985)

68.

[19] Emge

T.

J.,. Leuug

P. C.

W,

Beno M.

A.,

Schultz A.

J., Wang

^H.

H.,

Sowa L.M. and Williams

I.hI., Phys.

Reu. B 30

(1984)

6780.

[20j Ravy

S.,

Pouget

J.

P.,

Moret R. and Lenoir

C., Phys.

Reu. B 37

(1988)

5113.

[21j Ravy S.,

Moret R. and

Pouget J-P-, Phys.

Reu. B 38

(1988)

4469-4480.

[22j

^Kanoda

K.,

Akiba

K.,

Takahashi T. and Saito

G., Phys.

Reu. B 42

(1990)

6700.

[23] Kagoshima S.,

Hasumi

M., Nogami Y.,

Kinoshita N. Anzai

H.,

Tokumoto M. and Saito

G.,

Soiid state Commun. 71

(1989)

843.

[24j

^{Fortune N.}

A.,

Murata

K.,

Ikeda K. and Takahashi

T., Phys.

Reu. Lett. 68

(1992)

2933.

[25j

^Mamwa

Y.,

Takahashi

T., Takigawa M.,

Yasuoka

H.,

Saito

G.,

Murata

K.,

Tokumoto M.

and Anzai

H., Jpn.

J.

Appi. Phys. Suppi.

26-3

(1987)

1361.

[26j

^Creuzet

F.,

Bourbonnais

C.,

Jérome D.. Schweitzer D. and Keller

H-J-, Europhys.

Lett. 1

(1986)

467.

[27j

^Kato

M., Kobayashi Y.,

Nakamura T. and Takahashi

T., Phystca

8194-196

(1994)

1993.

[28]

^Fortune

N-A-,

Murata

K.,

Ikeda

K.,

Nakamura T. and Takahashi

T., Synth.

Met. 70

(1995)

903.

[29]

^{Kanoda K. et ai., Proc. lut. Conf.} on Sci. and Tech. of Snth.

Metals.,'96.

[30]

^Rothaemel

B.,

Forro

L., Ùooper

J.

R., Schilling

J.

S., Weger M., Bele, P.,

Brunner

H.,

Schweitzer D. and Keller

H-J-, Phys.

Reu. B 34

(1986)

704.