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Submitted on 1 Jan 1978
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ELECTRICAL RESISTIVITY ANOMALIES IN THE
LINEAR COMPOUND NbSe3 DOWN TO 7 mK
P. Haen, Jean Mignot, P. Monceau, M. Núñez Regueiro
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, suppl&menf au no 8, Tome 39, aolit 1978, page C6-703
ELECTRICAL RESISTIVITY ANOMALIES
I N THE LINEAR
COMPOUND NbSe3 DOWN T O 7 mKP. Haen, J.M. Mignot, P. Monceau and M. Niiiiez Regueiro
Centre de Recherches sur Zes -2s Basses Tempgratures, C. N. R. S., BP 166 X, 38041 Gren~bZe Ceder, France.
RBsum6.- Nous d6crivons des mesures de r6sistivit6 du compos6 lin6aire NbSe3 jusqu'ii des templratu- res de 7 mK. Deux chiites de la r6sistivit6 se produisent 1 2 , 2 K and 0.4 K, mettant en dvidence deux nouvelles transitions. Nous observons d'importants effets non lindaires en dessous de 2,2 K. Nous
sugggrons que ces deux transitions sont associbes aux deux ondes de densitb de charge qui prennent naissance dans ce compoc6 B 145K et 59 K.
Abstract.- We report on detailed resistivity measurements of the linear compound NbSe down to 7 mK. 3
Two drops in resistivity occur at 2.2 K and 0.4 K indicating two new transitions. Strongly non-li- near effects are observed below 2.2 K. We suggest to associate these transitions with the CDW's which appear in this compound at 145 K and 59 K.
The 1 D transition-metal trichalcogenide NbSe3 has very recently arisen a large interest on account
0,' its remarkable properties. .Two phase transitions occur at T 1 = 145 K and T2 = 59 K where the resisti- vity shows a sharp increase /1,2/. These anomalies were ascribed to the formation of charge density waves (cDW'S). Direct evidences for the formation of CDW's by electron diffraction and X-ray scattering measurements indicate that two independent inconmen-
surate CDW's coexist in NbSe below 50 K /3,5/. The 3
resistive anomalies associated with the CDW's are strongly non-linear with the applied current /6/ and drastically suppressed by a microwave field /7/. These results have been interpreted by Bardeen/8/ as the evidence of the ~rShlich conductivity from moving CDW's. Magnetization measurements down to
50 mK have shown no trace of superconductivity but NbSe becomes a bulk superconductor under pressure
3
with a variation of the critical temperature pressure of 0.6 K/kbar /g/. We also reported that the resistivity-of NbSe3 drops below 2.2 K by 30 to 75
X
of its residual value at 4.2 K.and reaches a plateau near near 1.5 K /10/. The resistivity below 2.2.K was shown to be strongly non-linear for cur- rent densities higher than l ~ Afnm - ~.
The measure- ments reported here were performed in the mixingchamber of a dilution refrigerator down to 7 mK using a four wire low frequency a.c.method.9igure
1 shows the temperature variation of the resistivi- ty for several current densities of a NbSe3 sample labelled A, a bundle of threads with a resistance ratio of 80 between 300 K and 4.2 K, whose resisti- nity temperature variation between 4.2.K and 1.2 K
was published previously (figure 4 of reference /IO/)
Fig.1 : Resistance of NbSe3(sample A =bundle of threads, R /R = 80) vs. temperature for d'f-
P
ferent cur?% idisities. From 1 to 8, j(A/mm ) = 0.52 ; 0.26; 0.13 ; 0.052; 0.026; 0.013; 0.0052; 0.0013.Curve number 8 of figure 1 represents the low electric field limit, obtained for. current densities
2
ties lower than I O - ~ A/=
.
Below 2.2.K this resis- tivity drops by 75 % of the residual value at 4.2 K and reaches a plateau. Below 0.4 it decreases line- arly down to 7 mK. It has been checked that the mea- surements performed between 1.2 K and 4.2 K in li-quid or gaseous helium give identical result.
Fig.2 : Semi-log plot of the resistance of sample A vs. the current density, at different tempefatu- res. From 1 to 9, T(K) = 2.09; 1.99; 1.90; 1.81;
1.60; 1.245. 0.697. 0.206; 0.042.
Figure 2 shows the resistivity as a function of the current density j for different temperatures.
-
Fig.3 : Resistance of two NbSe monocrystals vs.T 3
for the low current density limit (j < 1 0 - ~ ~/mm2) Lower curve:sample B (R 00/~4.2. = 30). Upper curve: sample C (R300/R4.2 = 77.
In figure 3, we present the resistivity varia- tion for two other samples, B and C in the low cur-
-
3rent density limit (j < 10 ~Imm2). These samples
were monocrystals and their typical dimensions were 5 X 0.05 X 0.01 mm3. Both ends of the samples were
glued with silver paint on sintered silver to impro- ve the thermal coupling between the sample and the helium bath. The lawer part of figure 3 is the va- riation of sample B, which has a resistance ratio of 30. The drop below 2.2 K is only 35
Z
but we ob- serve a second drop below 0.4 K as for sample A. However the residual resistance at 7 m!is very different from zero. The upper part of figure 3 is for sample C which has a resistance ratio of 7. The drop below 2.2 K is very broad and the second drop is wiped out.The non-linear properties below 2.2 K and the absence of increase of the diamagnetic (less than 10-3 of the volume) down to 50 mK in fields as low as 1 Oe preclude any description of NbSe3 as a bulk superconductor. We previously discussed that super- conducting filaments could explain the results /10/. Filaments in NbSe may have a morphological origin.
3
On the other hand we have observed large Shubnikov- de Haas oscillations in relatively low fields /11/ which indicate that NbSe3 is a rather homogeneous material. However, the two low temperature transi- tions may be associated with t?.e high temperature ones at T 1 = 145 K and T2 = 59 K, where CDW's deve- lop in the crystals. Electron diffraction patterns reveal superlattice spots below the T 1 transition which indicate a 3D ordering between the CDW's in- duced on each chain /3,4/. Filaments can be seen as domains where the CDW's havea.coherenceofphase. By some mechanism at low temperature the CDW'S ineach filament may carry a current (Frijhlich mode), which explains the drop to a lower resistivity at 2.2 K and 0.4 K
.
The diameter of these filaments must be small compared to the London penetration depth X to give no Meissner effect. Under pressure the transverse coherence length increases, thus increa- sing the size of the filaments and bulk superconduc-1
tivity is observed (susceptibility of
-
-V when47T
the pressure is higher than 2 or 3 kbars /g/).. Several difficulties obviously exist in the framework of this interpretation which should only be considered a putative one. Nevertheless, any further disccssion on the properties of NbSe3 must take into account the occurence of the two low temperature transitions that we have observed. ACKNOWLEDGEMENTS.- We would like to thank A.Meer-
References
/l/ Haen,P., Monceau,P., Tissier,B., Waysand,G., Meerschaut,A., Molinie,P., Rouxel,J., Proceedings Fourteenth International Con- ference on Low Temperature Physics (Otaniami,Finland,l975) vo1.5, p. 445
/ 2 / Chaussy,J., Haen,P.,Lasjaunias.J.C., Monceau,P., Waysand,G., Waintal,A., Meerschaut,A., Molinie,P., Rouxel,J., Solid State Commun.
20
(1976) 759./ 3 / Tsutsumi,K., Takagaki,T., Yamamoto,M., Shiozaki,Y., Ido,M., Sambongi ,L., Yartaya,K. ,Abe,Y., Phys.Rev.Letters
2
(1977) 1675./ 4 / Hodeau,J.L., Marezio,M., Roucau,C., Ayroles,R., Meerschaut,A., Rouxel, J., Monceau,P., to be published.
/ 5 / Fleming,R.M., Moncton,D.E., McWhan, D.B.,to be published.
/7/ Ong.N.P.,Monceau,P., Phys.rev.
B
(1977) 3 4 4 3 ./a/
Bardeen, J. to be published./g/ Monceau,P., Peyrard,J., Richard,J., Molinie,P.,Phys.Rev. Letters
2
(1977) 160/ 101 Ha=n,P.
