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Superconductivity in the misfit layer compounds (BiSe)1.10(NbSe2) and (BiS)1.11(NbS2)
Solid State Communications,Vol. 101, No. 3, pp. 149-153, 1997 Copyright0 1996 Elsevier Science Ud Printedin Great Britain. All tights reserved 0038-1098/97 $17.00+.00
Pergamon
PII: soo38-1098(%)00561-3
SUPERCONDUCTIVITY
IN THE MISFIT LAYER COMPOUNDS
(BiSe)l,lo(lVbSez) AND (BiS)I,II(NbS2)
A. Nader,‘,” A. Briggs’ and Y. Gotoh ‘Centre de Recherches sur les T&s Basses Temptrature, laboratoire associC 5 1’UniversitCJoseph Fourier, C.N.R.S., BP 166, 38042 Grenoble-CCdex 9, France 2Atomic Energy Commission of Syria, Physics Department, BP 6091, Damascus, Syria 3National Institute of Materials and Chemical Research, Highashi, Tsukuba, Ibaraki, 305 Japan (Received
8 July 1996 by P. Burlet)
We report on the temperature dependence of the critical magnetic fields of the misfit layer compounds (BiSe)l,lo(NbSe2) and (BiS),,,,(NbS2) in the directions parallel and perpendicular to the layer structure. They both behave as anisotropic 3D superconductors. Copyright 0 1996 Elsevier Science Ltd Keywords: A. superconductors, D. electronic transport.
1. INTRODUCTION Many of the so-called misfit layer compounds have now been synthesized and studied by crystallographers [l-3]. Their general formula is (MX),(TX,), where M: Pb, Sn, Bi, Rare Earth, T: Ta, Nb, X: S, Se, n = 1.08-1.23, m = 1,2. The average structure consists of an alternating sequence of one lMX[ layer and m 1‘IX21 slabs stacked along their c direction. The 1MX 1layer is two atoms thick with M in a distorted pyramidal coordination; giving a quasi NaCl-type structure JTX2/ sandwiches are three atoms thick with the T atom in slightly distorted trigonal prism of the chalcogenide atoms, the quasi hexagonal structure of these slabs resembles that of the transition metal dichalcogenides. Each of the two subsystems is characterised by a unit cell and a space group which match perfectly along the b and c directions, but a mismatch occurs in the a direction and seems to be in the range of fi. Some of these misfit layer compounds are known to become superconducting, but few of their properties in the superconducting state have been investigated [4-71, though comparison to transition metal dichalcogenides make them of great interest and invokes stimulating questions about interaction between layers. We report below the determination of the upper critical magnetic field Hc2 on (Bis)l,ll(NbSz) and (BiSe)l,lo(NbSez) single crystals for the directions parallel and perpendicular to the layer structure down
to 0.04 K by magnetoresistance measurements. Our results are compared to those obtained on NbSe* and other misfit layer compounds.
2. EXPERIMENTAL The growth of the misfit layer compounds and (BiSe)1.10(NbSe2) has been (BiS)l.ll(N%) described elsewhere [8-lo]. They appear as thin platelets with a diameter of approximately 2mm and a thickness of some tens of microns. The in-plane magnetoresistance (perpendicular to c direction) was measured by the four probe method using a low frequency bridge, contacts were made with silver paint. The electrical current was always in the plane of the crystal, perpendicular to the magnetic field and much lower than the critical current such that the resistance was current-independent. Measurements between 0.04 and 0.6K were performed using a top loading dilution refrigerator and an 8T superconducting magnet, for temperatures up to 4.2 K we preferred to use a 3He cryostat. We define the upper critical field as the field for which the measured resistance is equal to a pre-defined fraction of normal state resistance RN (which unless otherwise stated is 0.5RN). For measurements of critical field parallel to the layer structure, the sample position was adjusted parallel to the magnetic field with an accuracy of about 3”. 149
MISFIT LAYER COMPOUNDS (BiSe),.ie(NbSez) AND (BiS)i.ii(Nl&)
150
Table 1. Lattice parameters of the subsystems BiX and NbXz (X: S, Se) of the misfit layer compounds (BiS)1.1i(NbS2) and (BiSe)i.&NbSez). For (BiS),,i1(NbS2) the mismatch direction was taken on the b direction and the parameter b is six times larger than in other misfit layer compounds
c (4 NbSe2
BiSe (BiS)i.li(NbSz) as2
BiS
3.437 6.255
5.983 5.983
24.203 24.203
5.750 5.752
3.330 36.15
23.000 23.000
Table 2. Summarized results obtained on samples S 1 and $2 of (BiSe)1,io(NbSe2)
TcWI AT, (K) (wRlldT)rc (m,z$dT)n &300
K&4
(TK-‘) (TK-‘) K)
p (4.2 K) (Ohm m)
Sl
s2
2.36 0.12 -0.237 -0.305 4 5 x 1o-7
2.37 0.18 -0.268 3 8 x 1O-7
Vol. 101, No. 3
3. RESULTS AND DISCUSSION The crystallographic structure of (BiS)i.ii(Nb&) and (BiSe)i,io(NbSez) has been developed in Refs [8, 9 and lo]. They both have a rather complex structure with short Bi-Bi bonds and non-bonded S-S (Se-Se) pairs in the BiS (BiSe) layer. Their lattice parameters are shown in Table 1. 3.1. (BiSe)l.lo(A%Se2) Measurements were performed on two samples. We obtained comparable results for both, which are summarized in Table 2. Figure 1 shows the resistivity change between room temperature and 4.2 K of our sample S 1, this variation is linear down to 2OK, where the residual resistivity is reached and R@,sx)/&2 x) - 4, the superconducting transition vs temperature is shown in the inset, the critical temperature taken for 50% of the normal state resistivity was found to be 2.36K and the transition width (AT,) between 0.1 RN and 0.9 RN is 0.12 K. The upper critical magnetic field Hc2 is shown as a function of temperature in Fig. 2; it was taken as the field for which the magnetoresistance is equal to 0.5 RN, because of the transition width H,,(T) has been also sketched for 0.1 RN, the anisotropy and the general shape are not changed, only absolute values are intluenced. The upper critical magnetic field perpendicular to the
2 m6
1.5 lW6
1 1P
5 lo-’
Fig. 1. Resistivity change between room temperature and 4.2K of (BiSe)i,io(NbSe2)-Sl, superconducting transition.
the inset shows the
Vol. 101, No. 3
MISFIT LAYER COMPOUNDS
151
(BiSe)1.10(NbSe2) AND (BiS),,tt(NbS,)
0.7 0.6 1
x 'X X
0.5
3
;;; g % X"
hr.. :
0.4
-
0.3 -
yIA
X
x
A YX .X
A A
0.2 -
A 4%
0.1 -
A
or....'.""..."""""= 0 0.5 1
1.5
2
2.5
-UK) Fig. 2. Critical magnetic field vs temperature of (BiSe)t,r@bSe&Sl perpendicular to the layer structure. is linear near T, and (dHCz,ldT)rC = -0.237 TK-l, the slope changes at lower temperature as predicted from conventional models. The parallel critical magnetic field HCzi, varies linearly with temperature and no sign of saturation is observed down to the lowest temperature reached, its slope near T, is -0.305TK-‘. Anisotropy near T, is 1.29 and increases to 1.4 when temperature is decreased, we also measured the anisotropy at 0.320 K using a rotating system whose accuracy is better than 0.1 degree and we obtained comparable results. These values should be compared to those obtained on NbSez single crystals [ll] whose anisotropy near T, is 2.4 and increases to 3.2 in lower temperature and that of NbSez intercalated with TCNQ molecules
layer structure
[12] which is 20 at 0.85 T,. By using the simple effective mass model we estimate room zero temperature coherence lengths for both
principal directions as r/l(O)- 24OA and IL(O) LI186i(, as expected from the crystal structure lL(0) is smaller than {l,(O),but much higher than that of NbSe2 (24A) [ll]. As a result, this compound behaves as a 3D anisotropic superconductor and cannot be regarded as an intercalate of NbSez because coupling between layers is too strong to be explained by a Josephson coupling model suggested for highly layered superconducting systems.
in the directions (X) parallel and (A)
superconducting transition between 0.1 RN and 0.9 RN is less than 0.03 K, the resistivity variation between room temperature and 4.2K is linear down to 25 K when
residual resistivity is reached, with a small change in slope occurring at 150 K. The critical magnetic field parallel and perpendicular to the layer structure as a function of temperature is shown in Fig. 4, Hczl and Hc211 vary linearly with temperature in the temperature range under the consideration, the anisotropy calculated from our data is 2.4, lower than that of (PbS)t,r,(NbS,) which is 8.8 [4], the slope of Hc2 and Hczll near T, is respectively -O.O39TK-’ and -0.093 TK-‘, so the coherence length at zero temperature can be estimated in the two principal directions to be r,,(O) w 143OA and r,(O) m 600 A which is much higher than the distance between two superconducting NbS2 slabs, as a result this compound behaves as a 3D anisotropic superconductor. The critical temperature of this compound is much lower than that of other similar compounds of the same family like (SnS)1,1,(NbS2) (T, = 2.75 K) [5,6] or (PbS),+,(Nb.&) (T, = 2.475 K) [4]. Table 3. Summarized results obtained on samples d 1 and d2 of (BiS)I.trW%) dl Tc W
The resistivity measurements were performed on two samples whose results are summarized in Table 3. The critical temperature of our sample d 1 (see Fig. 3) corresponding to 0.5 RN is 0.415 K and the width of the
AT, (K) (~,zlldT), (TK-‘) (HczllldT)rc (TK-‘) R,,oo,'R,4, &4.2K) (Ohm
m)
0.415 0.03 -0.039 -0.093 7 1 x 1o-6
d2
0.345 0.03 10 4 x lo-’
152
MISFIT LAYER COMPOUNDS
(BiSe)r,r,(NbSe2)
AND (BiS)r.rr(NbSJ
Vol. 101, No. 3
7 UP 6 UT6 5 UP 4 1o-6 3 UP 2 1ti6 1 1r6
0
100
50
T
Fig. 3. Resistivity variation superconducting transition.
between
room temperature
Ettema and Haas [13] suggest a valency close to three for Bi in (BiS)I.11(NbS2); probably a high level of Nb substitution into the pseudo quadratic BiS layer occurs, as a result, Nb substituting for Bi would have the Nb3+ oxidation state and equivalent transformation of Nb4+ into Nb3+ in the quasi hexagonal NbS2 layer would occur, and an equal number of Bi ions will take the 3+ valence. A similar model has been suggested by Moe10 et al. in Ref. [14] for [(Pb,Sn)S],[(Ti,Nb)S2],, these
200
150
250
300
W)
and 4.2K
of (BiS)t,tt(Nb&dl,
the inset shows the
authors propose that the critical temperature for (MS),(NbS&,, misfit la er compounds is decreased as the proportion of Nb L is increased in the NbS2 layer. This substitution would occur in a lower proportion in (BiSe),,,a(NbSe2) because NbSez slabs are less favourable to Nb3+. The anisotropy of both compounds is lower than expected from the layered structure, this point needs further investigation.
X
X
l
X
. A
.
Fig. 4. Critical magnetic field vs temperature perpendicular to the layer structure.
of (BiSe),.,,(IW&)-dl
in the directions
(X) parallel
and (A)
Vol. 101, No. 3
MISFIT LAYER COMPOUNDS (BiSe)t,tO(NbSez) AND (BiS),.t1(NbS2) 4. CONCLUSION
We have determined the critical temperature and the temperature dependence of the upper critical magnetic field, in the direction parallel and perpendicular to the layer structure of the misfit layer compounds (BiSe)t,&%Se2) and (BiS)I.11(NbS2) down to 0.04 K, they show both a very weak anisotropy. The critical temperature of (BiS)t,,,(Nb!$) is much lower than that of other similar compounds of the same family.
REFERENCES 1. 2. 3. 4.
Wiegers, G.A., Meetsma, A., Haange, R.J. and de Boer, J.L., Mat. Res. Bull., 23, 1988, 1551. Wiegers, G.A. and Meetsma, A., J. All. & Comp., 178, 1992, 351. Rouxel, J., Meerschaut, A. and Wiegers, G.A., J. All. & Comp., 229, 1995, 144. Smontara, A., Monceau, P., Guemas, L., Meerschaut, A., Rabu, P. and Rouxel, J., Fizika, 21, 1989, 201.
5.
153
Reefman, D., Baak, J., Brom, H.B. and Wiegers, G.A., Solid State Commun., 75, 1990, 47. 6. Reefman, D., Koorvaar, P., Brom, H.B. and Wiegers, G.A., Synthetic Metals, 41-43, 1991, 3775. 7. Monceau, P., Chen, J., Laborde, O., Briggs, A., Auriel, C., Roesky, R., Meerschaut, A. and Rouxel, J., Physica, B194-196, 1994, 2361. 8. Gotoh, Y., Akimoto, J., Goto, M., Oosawa, Y. and Onoda, M., J. Solid State Chem., 116, 1995, 61. 9. Oosawa, Y., Gotoh, Y. and Onoda, M., Chemistry Letters, 1989, 1563. 10. Zhou, W.Y., Meetsma, A., de Boer, J.L. and Wiegers, G.A., Mat. Res. Bull., 27, 1992, 563. 11. Toyota, N., Nakattsuji, H., Noto, K., Hoshi, A., Kobayashi, N., Muto, Y. and Onodera, Y., J. Low Temp. Phys., 25, 1976, 485. 12. Obolenskii, M.A., Chashka, Kh.B., Beletskii, V.I., Balla, D.D. and Stradub, V.A., Sov. J. Low Temp. Phys., 8, 1992, 86. 13. Ettema, A.R.H.F. and Haas, C., J. Phys. Cond. Mat., 5, 1993, 3817. 14. Moelo, Y., Meerschaut, A., Rouxel, J. and Auriel, C., Chem. Mater,, 7, 1995, 1759.
Pergamon
PII: soo38-1098(%)00561-3
SUPERCONDUCTIVITY
IN THE MISFIT LAYER COMPOUNDS
(BiSe)l,lo(lVbSez) AND (BiS)I,II(NbS2)
A. Nader,‘,” A. Briggs’ and Y. Gotoh ‘Centre de Recherches sur les T&s Basses Temptrature, laboratoire associC 5 1’UniversitCJoseph Fourier, C.N.R.S., BP 166, 38042 Grenoble-CCdex 9, France 2Atomic Energy Commission of Syria, Physics Department, BP 6091, Damascus, Syria 3National Institute of Materials and Chemical Research, Highashi, Tsukuba, Ibaraki, 305 Japan (Received
8 July 1996 by P. Burlet)
We report on the temperature dependence of the critical magnetic fields of the misfit layer compounds (BiSe)l,lo(NbSe2) and (BiS),,,,(NbS2) in the directions parallel and perpendicular to the layer structure. They both behave as anisotropic 3D superconductors. Copyright 0 1996 Elsevier Science Ltd Keywords: A. superconductors, D. electronic transport.
1. INTRODUCTION Many of the so-called misfit layer compounds have now been synthesized and studied by crystallographers [l-3]. Their general formula is (MX),(TX,), where M: Pb, Sn, Bi, Rare Earth, T: Ta, Nb, X: S, Se, n = 1.08-1.23, m = 1,2. The average structure consists of an alternating sequence of one lMX[ layer and m 1‘IX21 slabs stacked along their c direction. The 1MX 1layer is two atoms thick with M in a distorted pyramidal coordination; giving a quasi NaCl-type structure JTX2/ sandwiches are three atoms thick with the T atom in slightly distorted trigonal prism of the chalcogenide atoms, the quasi hexagonal structure of these slabs resembles that of the transition metal dichalcogenides. Each of the two subsystems is characterised by a unit cell and a space group which match perfectly along the b and c directions, but a mismatch occurs in the a direction and seems to be in the range of fi. Some of these misfit layer compounds are known to become superconducting, but few of their properties in the superconducting state have been investigated [4-71, though comparison to transition metal dichalcogenides make them of great interest and invokes stimulating questions about interaction between layers. We report below the determination of the upper critical magnetic field Hc2 on (Bis)l,ll(NbSz) and (BiSe)l,lo(NbSez) single crystals for the directions parallel and perpendicular to the layer structure down
to 0.04 K by magnetoresistance measurements. Our results are compared to those obtained on NbSe* and other misfit layer compounds.
2. EXPERIMENTAL The growth of the misfit layer compounds and (BiSe)1.10(NbSe2) has been (BiS)l.ll(N%) described elsewhere [8-lo]. They appear as thin platelets with a diameter of approximately 2mm and a thickness of some tens of microns. The in-plane magnetoresistance (perpendicular to c direction) was measured by the four probe method using a low frequency bridge, contacts were made with silver paint. The electrical current was always in the plane of the crystal, perpendicular to the magnetic field and much lower than the critical current such that the resistance was current-independent. Measurements between 0.04 and 0.6K were performed using a top loading dilution refrigerator and an 8T superconducting magnet, for temperatures up to 4.2 K we preferred to use a 3He cryostat. We define the upper critical field as the field for which the measured resistance is equal to a pre-defined fraction of normal state resistance RN (which unless otherwise stated is 0.5RN). For measurements of critical field parallel to the layer structure, the sample position was adjusted parallel to the magnetic field with an accuracy of about 3”. 149
MISFIT LAYER COMPOUNDS (BiSe),.ie(NbSez) AND (BiS)i.ii(Nl&)
150
Table 1. Lattice parameters of the subsystems BiX and NbXz (X: S, Se) of the misfit layer compounds (BiS)1.1i(NbS2) and (BiSe)i.&NbSez). For (BiS),,i1(NbS2) the mismatch direction was taken on the b direction and the parameter b is six times larger than in other misfit layer compounds
c (4 NbSe2
BiSe (BiS)i.li(NbSz) as2
BiS
3.437 6.255
5.983 5.983
24.203 24.203
5.750 5.752
3.330 36.15
23.000 23.000
Table 2. Summarized results obtained on samples S 1 and $2 of (BiSe)1,io(NbSe2)
TcWI AT, (K) (wRlldT)rc (m,z$dT)n &300
K&4
(TK-‘) (TK-‘) K)
p (4.2 K) (Ohm m)
Sl
s2
2.36 0.12 -0.237 -0.305 4 5 x 1o-7
2.37 0.18 -0.268 3 8 x 1O-7
Vol. 101, No. 3
3. RESULTS AND DISCUSSION The crystallographic structure of (BiS)i.ii(Nb&) and (BiSe)i,io(NbSez) has been developed in Refs [8, 9 and lo]. They both have a rather complex structure with short Bi-Bi bonds and non-bonded S-S (Se-Se) pairs in the BiS (BiSe) layer. Their lattice parameters are shown in Table 1. 3.1. (BiSe)l.lo(A%Se2) Measurements were performed on two samples. We obtained comparable results for both, which are summarized in Table 2. Figure 1 shows the resistivity change between room temperature and 4.2 K of our sample S 1, this variation is linear down to 2OK, where the residual resistivity is reached and R@,sx)/&2 x) - 4, the superconducting transition vs temperature is shown in the inset, the critical temperature taken for 50% of the normal state resistivity was found to be 2.36K and the transition width (AT,) between 0.1 RN and 0.9 RN is 0.12 K. The upper critical magnetic field Hc2 is shown as a function of temperature in Fig. 2; it was taken as the field for which the magnetoresistance is equal to 0.5 RN, because of the transition width H,,(T) has been also sketched for 0.1 RN, the anisotropy and the general shape are not changed, only absolute values are intluenced. The upper critical magnetic field perpendicular to the
2 m6
1.5 lW6
1 1P
5 lo-’
Fig. 1. Resistivity change between room temperature and 4.2K of (BiSe)i,io(NbSe2)-Sl, superconducting transition.
the inset shows the
Vol. 101, No. 3
MISFIT LAYER COMPOUNDS
151
(BiSe)1.10(NbSe2) AND (BiS),,tt(NbS,)
0.7 0.6 1
x 'X X
0.5
3
;;; g % X"
hr.. :
0.4
-
0.3 -
yIA
X
x
A YX .X
A A
0.2 -
A 4%
0.1 -
A
or....'.""..."""""= 0 0.5 1
1.5
2
2.5
-UK) Fig. 2. Critical magnetic field vs temperature of (BiSe)t,r@bSe&Sl perpendicular to the layer structure. is linear near T, and (dHCz,ldT)rC = -0.237 TK-l, the slope changes at lower temperature as predicted from conventional models. The parallel critical magnetic field HCzi, varies linearly with temperature and no sign of saturation is observed down to the lowest temperature reached, its slope near T, is -0.305TK-‘. Anisotropy near T, is 1.29 and increases to 1.4 when temperature is decreased, we also measured the anisotropy at 0.320 K using a rotating system whose accuracy is better than 0.1 degree and we obtained comparable results. These values should be compared to those obtained on NbSez single crystals [ll] whose anisotropy near T, is 2.4 and increases to 3.2 in lower temperature and that of NbSez intercalated with TCNQ molecules
layer structure
[12] which is 20 at 0.85 T,. By using the simple effective mass model we estimate room zero temperature coherence lengths for both
principal directions as r/l(O)- 24OA and IL(O) LI186i(, as expected from the crystal structure lL(0) is smaller than {l,(O),but much higher than that of NbSe2 (24A) [ll]. As a result, this compound behaves as a 3D anisotropic superconductor and cannot be regarded as an intercalate of NbSez because coupling between layers is too strong to be explained by a Josephson coupling model suggested for highly layered superconducting systems.
in the directions (X) parallel and (A)
superconducting transition between 0.1 RN and 0.9 RN is less than 0.03 K, the resistivity variation between room temperature and 4.2K is linear down to 25 K when
residual resistivity is reached, with a small change in slope occurring at 150 K. The critical magnetic field parallel and perpendicular to the layer structure as a function of temperature is shown in Fig. 4, Hczl and Hc211 vary linearly with temperature in the temperature range under the consideration, the anisotropy calculated from our data is 2.4, lower than that of (PbS)t,r,(NbS,) which is 8.8 [4], the slope of Hc2 and Hczll near T, is respectively -O.O39TK-’ and -0.093 TK-‘, so the coherence length at zero temperature can be estimated in the two principal directions to be r,,(O) w 143OA and r,(O) m 600 A which is much higher than the distance between two superconducting NbS2 slabs, as a result this compound behaves as a 3D anisotropic superconductor. The critical temperature of this compound is much lower than that of other similar compounds of the same family like (SnS)1,1,(NbS2) (T, = 2.75 K) [5,6] or (PbS),+,(Nb.&) (T, = 2.475 K) [4]. Table 3. Summarized results obtained on samples d 1 and d2 of (BiS)I.trW%) dl Tc W
The resistivity measurements were performed on two samples whose results are summarized in Table 3. The critical temperature of our sample d 1 (see Fig. 3) corresponding to 0.5 RN is 0.415 K and the width of the
AT, (K) (~,zlldT), (TK-‘) (HczllldT)rc (TK-‘) R,,oo,'R,4, &4.2K) (Ohm
m)
0.415 0.03 -0.039 -0.093 7 1 x 1o-6
d2
0.345 0.03 10 4 x lo-’
152
MISFIT LAYER COMPOUNDS
(BiSe)r,r,(NbSe2)
AND (BiS)r.rr(NbSJ
Vol. 101, No. 3
7 UP 6 UT6 5 UP 4 1o-6 3 UP 2 1ti6 1 1r6
0
100
50
T
Fig. 3. Resistivity variation superconducting transition.
between
room temperature
Ettema and Haas [13] suggest a valency close to three for Bi in (BiS)I.11(NbS2); probably a high level of Nb substitution into the pseudo quadratic BiS layer occurs, as a result, Nb substituting for Bi would have the Nb3+ oxidation state and equivalent transformation of Nb4+ into Nb3+ in the quasi hexagonal NbS2 layer would occur, and an equal number of Bi ions will take the 3+ valence. A similar model has been suggested by Moe10 et al. in Ref. [14] for [(Pb,Sn)S],[(Ti,Nb)S2],, these
200
150
250
300
W)
and 4.2K
of (BiS)t,tt(Nb&dl,
the inset shows the
authors propose that the critical temperature for (MS),(NbS&,, misfit la er compounds is decreased as the proportion of Nb L is increased in the NbS2 layer. This substitution would occur in a lower proportion in (BiSe),,,a(NbSe2) because NbSez slabs are less favourable to Nb3+. The anisotropy of both compounds is lower than expected from the layered structure, this point needs further investigation.
X
X
l
X
. A
.
Fig. 4. Critical magnetic field vs temperature perpendicular to the layer structure.
of (BiSe),.,,(IW&)-dl
in the directions
(X) parallel
and (A)
Vol. 101, No. 3
MISFIT LAYER COMPOUNDS (BiSe)t,tO(NbSez) AND (BiS),.t1(NbS2) 4. CONCLUSION
We have determined the critical temperature and the temperature dependence of the upper critical magnetic field, in the direction parallel and perpendicular to the layer structure of the misfit layer compounds (BiSe)t,&%Se2) and (BiS)I.11(NbS2) down to 0.04 K, they show both a very weak anisotropy. The critical temperature of (BiS)t,,,(Nb!$) is much lower than that of other similar compounds of the same family.
REFERENCES 1. 2. 3. 4.
Wiegers, G.A., Meetsma, A., Haange, R.J. and de Boer, J.L., Mat. Res. Bull., 23, 1988, 1551. Wiegers, G.A. and Meetsma, A., J. All. & Comp., 178, 1992, 351. Rouxel, J., Meerschaut, A. and Wiegers, G.A., J. All. & Comp., 229, 1995, 144. Smontara, A., Monceau, P., Guemas, L., Meerschaut, A., Rabu, P. and Rouxel, J., Fizika, 21, 1989, 201.
5.
153
Reefman, D., Baak, J., Brom, H.B. and Wiegers, G.A., Solid State Commun., 75, 1990, 47. 6. Reefman, D., Koorvaar, P., Brom, H.B. and Wiegers, G.A., Synthetic Metals, 41-43, 1991, 3775. 7. Monceau, P., Chen, J., Laborde, O., Briggs, A., Auriel, C., Roesky, R., Meerschaut, A. and Rouxel, J., Physica, B194-196, 1994, 2361. 8. Gotoh, Y., Akimoto, J., Goto, M., Oosawa, Y. and Onoda, M., J. Solid State Chem., 116, 1995, 61. 9. Oosawa, Y., Gotoh, Y. and Onoda, M., Chemistry Letters, 1989, 1563. 10. Zhou, W.Y., Meetsma, A., de Boer, J.L. and Wiegers, G.A., Mat. Res. Bull., 27, 1992, 563. 11. Toyota, N., Nakattsuji, H., Noto, K., Hoshi, A., Kobayashi, N., Muto, Y. and Onodera, Y., J. Low Temp. Phys., 25, 1976, 485. 12. Obolenskii, M.A., Chashka, Kh.B., Beletskii, V.I., Balla, D.D. and Stradub, V.A., Sov. J. Low Temp. Phys., 8, 1992, 86. 13. Ettema, A.R.H.F. and Haas, C., J. Phys. Cond. Mat., 5, 1993, 3817. 14. Moelo, Y., Meerschaut, A., Rouxel, J. and Auriel, C., Chem. Mater,, 7, 1995, 1759.