Low temperature properties of Yb1.2-xEuxMo6S 8 under pressure

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Low temperature properties of Yb1.2-xEuxMo6S 8 under pressure

J. Beille, B. Cheaito, M.S. Torikachvili, M.B. Maple

To cite this version:

J. Beille, B. Cheaito, M.S. Torikachvili, M.B. Maple. Low temperature properties of Yb1.2-xEuxMo6S 8 under pressure. Journal de Physique, 1988, 49 (3), pp.481-484. �10.1051/jphys:01988004903048100�.

�jpa-00210719�

Low temperature properties ^of Yb1.2-xEuxMo6S8 ^{under pressure}

J. Beille

(1),

B. Cheaito

(1),

M. S. Torikachvili

(2, *)

^{and M. B.} Maple

(2)

(1) Laboratoire Louis Néel, CNRS, 38042 Grenoble Cedex, ^France

(2) Institute for Pure and Applied Physical ^Sciences, University ^of California, ^San Diego, ^La Jolla, ^CA 92093, U.S.A.

(Reçu ^{le 22} juillet 1987, accept6 le 17 novembre 1987)

Résumé. ²⁰¹⁴ Les propriétés ^des composés pseudoternaires Yb1.2-xEuxMo6S8 ^{ont été} étudiées à l’aide des

mesures de résistivité électrique ^{et de} susceptibility magnétique ^{ac sous} pression hydrostatique jusqu’à ^{20 kbar}

et quasi hydrostatique jusqu’à 100 kbar. Un diagramme ^de phase ^de température-pression-concentration ^d’Eu

montrant les phases cristallographiques, ^{normales et} superconductrices ^{de ces} composés ^a été construit.

Abstract. ²⁰¹⁴ The properties ^of Yb1.2-xEuxMo6S8 compounds ^{have been} investigated by ^{means of electrical} resistivity ^{and ac} magnetic susceptibility measurements under hydrostatic pressure up ^to 20 kbar and quasi- hydrostatic pressure up ^to 100 kbar. A temperature-pressure-Eu concentration phase diagram showing ^the crystallographic, normal and superconducting phases ^{of these} compounds ^was constructed.

Classification

Physics ^Abstracts

72.15 ^- 74.10 ^- 74.70

1. Introduction.

Most ternary rare-earth R

molybdenum

^chal- cogenides with nominal

composition R.,M06X8 (where x

= 1.0-1.2 ; ^{X =} S,

Se)

^are

superconducting

and their

superconducting

transition temperatures

Tc’s

^{show a} systematic variation with R

[1]

^{that can}

be accounted for in terms of the theory of Abrikosov and Gor’kov

(AG) [2].

Exceptions ^{to this} systematics

include

Cel.2MO6X8

^and

Eul.2MO6X8 (X

^{= S and}

Se)

which do not exhibit

superconductivity,

^{and also}

Ybl.2MO6S8,

which has a

Tc

^much higher ^than expected

[1].

^Recent

crystal

^{structure refinement}

studies on single

crystals

^of

RxM06S8 (R

^{= Ce, Eu,} Ho, ^and

Yb)

^{indicate an} upper limit for the R concentration x = 1.0,

[3, 4]

^which suggests ^{that the} sintered specimens ^{with x} > 1.0 utilized in the early

work as well as on this one contained

impurity

phases. Baillif et al.

[5]

found that a structural

phase

transition at - 109 K prevents

superconductivity

from occurring ⁱⁿ

EuM06S8

^{at ambient} pressure.

This structural transformation can be

suppressed by

hydrostatic pressure

[6],

^and pressure-induced super-

conductivity ⁱⁿ

EuM06S8

^{occurs at a}

T, [7-9]

^{which is}

(*) ^{Present address :} Department ^of Physics, ^San Diego

State University, ^San Diego, ^CA 92182, ^U.S.A.

also much higher ^{than what one would} expect ^from the AG theory, ^{as is the case for}

Ybl.2MO6S8

^{at zero}

pressure. While ^most R ions in the ternary

R.,M06X8

compounds ^are trivalent, ^Eu

[10]

^{and Yb}

[4, 11-12]

are in nearly ^{divalent states,} and it is

possible

^that

the high Tc values of - 7.3 K for

Ybl.2Mo6S8 (sintered chunk)

and - 14 K for

Eul.2MO6S8 (extrapolated

value to zero pressure ^{on a} melted

sample) [8]

^are

associated with the large unit cell volume due to the divalent character of the Yb and Eu ions, ^as suggested by Sergent et ^al.

[9],

and it is therefore not

surprising that these two compounds ^{do not follow}

Abrikosov-Gor’kov’s

systematics

for trivalent R in

RM06S8 compounds.

It has been

suggested

^{that the}

crystallographic

transformation in

EuM06S8

^{at -109} K may be

accompanied by ^{the formation of a}

charge density

wave

(CDW)

^{which removes a}

portion

^{of the Fermi}

surface

[13].

High pressure

experiments

^{on sintered}

Snl.2-,,EU,,MO6S8

samples

[13]

^{revealed a} pressure

dependence ^of Tc ^that

suggested

^{that the}

portion

^of

the Fermi surface removed by ^{the CDW decreases}

with pressure, resulting ^{in an} increase in the

density

of states at the Fermi level and a

corresponding

increase in Tc. However, measurements of electrical

resistivity

^vs. temperature ^under hydrostatic pressure

on a

high quality

^melted sample ^of

EuM06S8

^indi-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01988004903048100

482

cated that

superconductivity

^{sets in}

abruptly

^with Tc - ^{12 K as soon as the} pressure ^is

sufficiently high

to suppress the

crystallographic

transformation com-

pletely

(Per --

^13.2

kbar ) [6].

^This suggests ^{that a} CDW which _may occur in the triclinic phase ^{does not}

coexist with

superconductivity.

A recent

study

^of Yb 1.2 -

xEuxMo6Sg pseudotemary

compounds revealed that the upper ^critical magnetic

field Hc2 ^of

Ybl.2Mo6S8

^is

substantially

^enhanced by partial substitution of Eu for Yb

[14].

^{As in}

Snl,2 - XEuxMo6S8 [15], Pbl - xEUxMo6S8 [15],

^and

Lal,2 -xEuxMo6S8 [16],

the enhancement of

Hc2

ⁱⁿ

Ybl.2 - xEUxMo6S8 [14]

^was attributed to the com-

pensation ^{of the}

applied

magnetic ^field

by

^a

negative exchange

^{field due to an}

antiferromagnetic exchange

interaction between the conduction electron

spins

and the Eu 2+

magnetic

^{moments, via a mechanism}

first suggested

by

^{Jaccarino and Peter}

(JP) [17].

^The

data and a theoretical calculation of

Hc2(T)

suggest that

Ybl.2-xEuxMo6Sg compounds

^{with x - 0.5 are} good candidates for

magnetic

field induced _supercon-

ductivity,

provided

high

quality homogeneous

^sam- ples ^{could be}

prepared [14].

Reported ^{herein are} the results of an investigation

that we carried out on Yb-Eu

pseudoternary

molyb-

denum sulfide

compounds

with nominal

composition Yb1.2 - xEuxM06Sg

by ^{means of} measurements of electrical

resistivity

p and ac magnetic

susceptibility

Xac under hydrostatic pressure up ^to 20 kbar and

quasi-hydrostatic

pressure up ^to 100 kbar. Prelimi- nary results ^were discussed in reference

[18].

2. Experimental ^details.

The

Ybl.2 - xEUxM06S8

compounds ^were prepared by sintering according ^{to the} description in reference

[14].

^Low

frequency

^ac electrical

resistivity

^{and ac}

magnetic susceptibility

measurements under

hydros-

tatic pressure ^were carried out at the

University

^of California,

San Diego (UCSD)

and Louis Neel

Laboratory (LNL)

ⁱⁿ

self-clamped

^Be-Cu pressure cells utilizing ^{a 1:1}

isoamyl

^alcohol: n-pentane

mixture as a pressure

transmitting

^{medium. A}

Bridgman

^{anvil cell}

capable

^of

achieving quasi-hyd-

rostatic pressures up ^{to -} 140 kbar

[19]

^{was used at}

LNL for the

resistivity

measurements under

quasi- hydrostatic

pressure. ^{Since the} electrical resistance of samples ^{extracted from the sintered}

pellets

^was

strongly

sample

dependent,

^no attempt ^{was made to}

determine absolute values of

resistivity.

3. Results and discussion.

Curves of electrical

resistivity

^versus temperature ^for the five

Ybl,2 - xEuxMo6S8

compounds ^{with x =} 0, 0.4, 0.6, 0.8 and 1.2 at

atmospheric

pressure ^are

displayed

ⁱⁿ figure ^{1. The behaviour of} p

(T)

^evolves

from metallic-like in

Ybl.2MO6S8

^{to a} behaviour in which the

resistivity

^{shows a broad minimum as Eu is}

Fig. ^{1. -} Electrical resistivity ^vs. temperature ^for Ybl.2 _ xEuxMo6S8 compounds ^{with x =} 0, 0.4, 0.6, ^{0.8 and} 1.2 at atmospheric pressure. The solid lines ^are guides ^to

the eye.

substituted for Yb,

presumably

^{due to the onset of}

the triclinic distortion. The

resistivity

of the Eu-rich

Ybl.2 -

,Eu.,Mo6S8

compounds ^{at low} temperatures ^is

about two orders of magnitude higher ^{that at room} temperature.

Defining

^the temperature ^{at which the} triclinic distortion occurs as the temperature Ts at

which the derivative

dp /dT

^{first exhibits a discon-} tinuity upon

cooling,

^a phase

diagram

^{can be con-} structed, in which the

boundary

between the rhombohedral and the triclinic

phases

^{can be}

display-

ed as a function of Eu concentration, ^{as shown in} figure ^{2a. The}

dependence

^of Tc ^{on x determined}

from the

midpoint

of the transitions in X ac and p is

also

displayed

ⁱⁿ

figure

^{2a. The} detailed behaviour of

Tc (x)

is shown in figure 2b, ^{where the vertical}

bars represent ^the temperature ^{interval between} 10 % and 90 % of the transition from the normal to the

superconducting

^state.

Fig. ^{2. -} (a) Crystallographic ^and superconducting ^tran-

sition temperatures T,, ^and Tc, respectively, ^{versus x for} Ybl.2-.,EU.,Mo6S8 compounds. ^The values of T, ^were

inferred from discontinuities in ap /aT upon cooling ^and

the values of Tc ^were determined from the midpoints ^of

transitions in _p (A) ^and X ac (N) ; (b) superconducting

transition temperature Tc ^{vs. Eu} concentration x for sintered pellets ^of Yb1.2 - xEuxMo6Sg compounds.

The behaviour of _p

(T )

^for

YbO.2Eul.OM06S8

^under

various

hydrostatic

pressures between 0 and 20 kbar

can be seen in figure ^{3. The effect of} pressure ^{is to}

depress

T, very

rapidly

^{and to} suppress the upturn ^of the resistivity ^{at low} temperatures. ^{At 7 kbar}

p

(T) displays

^a broad minimum ant - 67 K followed

by

sharp drop

^{at - 5 K due to the onset of} supercon-

ductivity, ^{as confirmed}

by

X ac measurements. The pressure

dependence

^of Tc determined from resistivi- ty measurements is

displayed

^{in the inset of}

figure

^3.

Superconductivity ^is

quickly

established in a narrow

pressure range and Tc ^{achieves a maximum near}

10 kbar. At higher pressures,

T,

decreases almost

linearly

with pressure ^{at a rate} of -0.13K/kbar,

which is close to the rate found for

EuM06S8-

Curves _p

(T)

^for

Ybo,6Euo.6Mo6Sg

under various

quasi-hydrostatic

pressures between 4.4 kbar and 103.5 kbar are

displayed

ⁱⁿ figure 4. At 4.4 kbar,

which is the lowest pressure ^at which electrical contact between the sample and the leads could be

Fig. ^{3. -} Electrical resistivity ^vs. temperature ^for Ybo.2Eu1.0Mo6SS ^at hydrostatic pressures between 0 and 20 kbar. The pressure dependence ^of T, extracted from

resistivity measurements ( ) ^is displayed in the inset.

Fig. ^{4. -} Curves of electrical resistivity ^vs. temperature for Ybo,6Euo.6Mo6Sg ^at quasi-hydrostatic pressures between 4.4 kbar and 103.5 kbar.

established, ^the

resistivity

^{exhibits a broad minimum}

at - 115 K, ^followed by ^an increase in _p as the temperature is lowered and a

drop

^{at -} 8 K that is associated with the onset of

superconductivity.

^How-

ever, the

superconducting

transition was not com-

plete

^{down to 1.7} K, ^{the lower} temperature ^{limit of} the apparatus. ^{As the}

quasi-hydrostatic

pressure ^was further increased, the minimum in _p

(T)

^was progres-

sively suppressed.

Although

^a

drop

ⁱⁿ p ^{ant -} 8 K suggests ^{the onset of}

superconductivity

^{at 9.7} kbar,

the

superconducting

transition was not complete

down to 1.7 K either. The

incompleteness

^{of the}

superconducting

transitions is

probably

^associated

with pressure

inhomogeneities.

^No

superconductivity

was observed above 20 kbar. A temperature-hydros-

tatic pressure

phase diagram

^for

Ybo.6Euo.6Mo6Sg

was constructed and is

displayed

ⁱⁿ

figure

5a, ^and

the pressure

dependence

^of Tc ^is displayed ⁱⁿ figure ^5b.

Fig. ^{5. -} (a) Hydrostatic pressure dependence ^of T, ^and Tc ^for Ybo,6Euo.6Mo6Sg, showing the distinction between the rhombohedral and the triclinic phases ; (b) pressure dependence ^of T,,. ^Data points ^denoted by triangles (A) ^are from electrical resistivity measurements, and by squares (0) ^{from ac} magnetic susceptibility

measurements.

4. Conclusions.

The Yb-Eu pseudoternary

molybdenum

^{sulfide com-} pounds with nominal

composition Ybl,2-xEuxM06S8

exhibit remarkable

crystallographic

and supercon- ductive properties ^{that are} very sensitive to pressure.

The compound

Ybl.2Mo6Sg

^is

superconducting

^at

zero pressure with 7c - ^{7.3 K.} Upon partial ^substitu-

tion of Eu for Yb, Tc decreases with Eu concen-

tration until superconductivity ^is

completely

sup-

pressed ^{at x --} 0.7. The effect of pressure ^on Yb-rich

Ybl _ xEuxMo6Sg compounds

^{is to} depress

Tc

^{at a rate}

similar to

Ybl.2MO6S8.

^At intermediate compositions Tc is first enhanced

by

pressure, reaches ^a maximum and then starts to decrease. The Eu-rich compounds,

which are not

superconducting

^{at ambient} pressure, become

superconducting

^{in a narrow} pressure range, above which Tc ^is

depressed by

pressure. The

temperature TS at ^{which the} rhombohedral-triclinic

crystallographic

distortion that prevents supercon-

484

Fig. ^{6. -} Three dimensional phase diagram ^of Ybl.2-,,EU.,MO6S8 compounds displaying the pressure and concentration dependences ^of T, ^and T,. ^Data points

denoted by triangles (A) ^are from electrical resistivity

measurements, and by squares (0) ^{from ac} magnetic susceptibility measurements.

ductivity ^from occurring ^{at ambient} pressure takes

place

ⁱⁿ

EUM06S8,

^{and which} determines the onset

of the non-metallic behaviour, ^is suppressed by

pressure and by

substituting

Yb for Eu. A three- dimensional phase diagram ^{of the}

Ybl,2-xEuxM06Sg pseudoternary compounds

^{in which} Ts ^and

Tc

^are displayed ^{as a} function of pressure and concentration is shown in

figure

^6.

In light ^{of the recent}

availability

^of

single

crystals

of

EuM06S8

^and

YbM06S8 [3-4],

^an attempt ^to grow

Yb1 - xEuxM06Sg single

crystals ^{and a} study ^{of the}

effect of pressure ^on these compounds ^{at low} temperatures ^{as well a} search for

magnetic

^field

induced

superconductivity

^at intermediate _compo- sitions is in order.

Acknowledgments.

Research at UCSD was supported

by

^{the US} Depart-

ment of Energy ^{under Grant} No. DE-FG03- 86ER45230. Research at Louis Neel Laboratory ^was supported by the DRET under Grant No. 84/109.

The authors would like to thank Chris Seaman for

preparing ^the figures.

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