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2+δ single crystals
Physiea C 185-18~, (1991) 1235-1236 North-Holland
Growth and anisotropic properties of superconducting La2.xCal+xCU206.xt2+s single crystals Takao Watanabe, Takao Ishii*, Kyoichi Kinoshita, and Azusa Matsuda NTI" Basic Research Laboratories, Midori-cho, Musashino-shi, Tokyo 180, Japan Single crystals of La2.xCat+xCU206.x~+S have been grown by the CuO flux method. After annealing under a high partial pressure of 0 2, these crystals showed zero resistance at 41 K and a perfect shielding effect (3% Meissner signal). These crystals also showed metallic behaviour parallel (a,b) and perpendicular (c) to the basal plane, with an anisotropy of about 30 at room temperature. T13e Ginzburg-Landau coherence lengths were estimated from He2 measurements: ~ab=33 A and ~c--12 A. The anisotropy factor of 3 is fairly small for a high-To superconductor.
Superconductivity has recently been achieved in La2_x(Cal_ySry)l+xCU206_xtZ+8 and in La2.x Cal~vCugOr._..r,.~xcompounds (2-1-2-6) synthesized . - ~ .~ v- . . . . 1,2,3 under high O 2 pressure . The sunple 2-1-2-6 4 crystal structure contains only a pyramidal Cu-O plane, whereas Ba2YCu307 has both pyramids and a chain. The electrical properties of 2-1-2-6 single crystals will therefore provide useful information about the transport mechanism in typical high-Tc superconductors. This paper reports the growth o f L a 2 . x C a l + x C U 2 0 6 . x / 2 + 5 single crystals and the anisotropic superconducting properties of these crystals. Single crystals of La2.xCal+xCU206_x/2+8 were grown in a Pt crucible by the CuO flux method, with starting compositions La2_xCal+xCU2Oy:CUO in the ratio of 1:2. These mixtures were heated in air to 1200 °C for 2 h, cooled at 4 °C/h to 900 °C, and then furnace-cooled to room temperature. Crystals found in the cavity at the bottom of the crucible were square plates with an average size of lxlx0.3 ram. No Pt contamination was detected by electron-probe microanalysis (EPMA). The La:Ca ratio of the crystals was 1.87:1.13 and it varied little for various starting values of x.
These crystals were superconductive after annealing at 1080 °C for 100 h under a 0 2 partial pressure of 300-atm: Resistance was zero at 41 K. The temperature dependence of the magnetic susceptibility is shown in the inset of Fig. 1. A perfect shielding effect, which indicates bulk superconductivity, was observed. On the other hand, the Meissner signal was about 3 % of the ideal value, which suggests strong flux pinning exists. Figure 1 shows for both as-grown and annealed crystals, the temperature dependence of electrical resistivity (measured by Montgomery's method) parallel and perpendicular to the basal plane. For both directions, resistivity of the as-grown crystal is almost constant from room temperature to 50 K, where it begins to increase with decreasing temperature. This suggests that carder transport at low temperatures is dominated by localization. Tne pe/p~ anisotropy of fne as-grown crystal is estimated to be about 300. The annealed crysta~ shows metallic behaviour in both directions, with a pc/Pab anisotropy that is about 30 at room temperature and that increases slightly with decreasing temperature. This anisotropy is similar to or even smaller than that of Ba2YCu3075.
*Present address: NTr Applied Electronics Laboratories, 162 Tokai, Ibaraki 319-11, Japan 0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V.
All rights reserved.
1236
Z Watanabeet aL / SuperconductingLa:.xCaz+xCu20~R+s single oystals
10 2 1.00
.,,.pGCas_grown] 101
E u,
H.t¢
0.2,5 -.50 -.75
o
10o
\(3- 0{75 (3. 0.50
~ \ ~
I0-t
,
,
20 30 60 50 60 Tempera.ture (K) \ pato(a.s-grown}
0
10
._~ . I¢1 m
I :
i °r
~/li l lor~/~hr~ :i .: ! :
'~ 0.25
,
;t :
5r ~ J
-1,00 t
S :
2T
4T
~.'i/11 ~. i 70 .
0.: 0 "17 ~N 1.00 ~tc
r"--
n" 10-z
\ p¢(3OO-~,trn O2
anneated)
0 :7
0.75 .. 0.50
I0 -3
'ST ~
pan(300-atm 02 annea.ted~.._.......~ 0.25 n
10-4 0
I
t
100
Temperature
I
. . . . . .." j . . , 0
300
10
,..
20
/ i ./'. J , 30
Tempera.ture
(K)
Figure 1 Temperature dependence of electrical resistivity parallel (P,b) and perpendicular (po) to the basal plane for as-grown and annealed La~ .Teat ..Cu.O single crystals. (Inset shows the temperature dependence of the magnetic susceptibility of an annealed crystal.)
OT
/i/
u
200
1T
/
I
40
50
(K)
Figure 2 Temperature dependence of resistivity plotted as a function of magnetic fields parallel and perpendicular to the c-axis of an annealed LaI s~Cat ..Cu.O single crystal. Normahzed resistivity P/Po is the ratio of p(T,H) to p(T=50 K, H=0 T). References
Figure 2 shows the temperature dependence of resistivity plotted as a function of magnetic fields parallel and perpendicular to the c-axis. The temperature dependence of the upper critical magnetic field, He2(T), was determined from the midpoint of the transition. According to the Werthamaro Helfand-Hohenberg theory for type II superconductors, Hc2(0) may be estimated as Hc2(0) =-0.69Tc(dHc2/dT)T=Tc. Thus, He2//(0) and Hc2_L(0) are estimated to be 85.4 T and 30.2 T. Using these values of Hc2(0), the coherence length (~) along the c-axis is 12 ,~ and the coherence length in the basal plane is 33 A. The anisotropy factor of 3 is fairly small for a high-Tc superconductor.
1. R. J. Cava, B. Batlogg, R. B. van Dover, J. J. Krajewski, J. V. Waszczak, R. M. Fleming, W. F. Peck Jr., L. W. Rupp Jr., P. Marsh, A. C. W. P. James, and L. F. Schneemeyer, Nature 345 (1990) 602. 2. K. Kinoshita, H. Shibata, and T. Yamada, Jpn. J. Appl. Phys. 29 (1990) L1632. J.
IL~.• I X J U t
lalUta,
II. OlUUa.i~
o.l.iu
r 6111~.Ud,
FllySll,;ii
C 171 (19~3) 523. 4. N. Nguyen, L. Er-Rakho, C. Michel, J. Choisnet, and B. Raveau, Mater.Res. Bull. 15 (1980) 891• 5. T. A. Friedrnann, M. W. Rabin, J. Giapinzakis, J. P. Rice, and D. M. Ginsberg, Phys. Rev. B 42 (1990) 6217.
Growth and anisotropic properties of superconducting La2.xCal+xCU206.xt2+s single crystals Takao Watanabe, Takao Ishii*, Kyoichi Kinoshita, and Azusa Matsuda NTI" Basic Research Laboratories, Midori-cho, Musashino-shi, Tokyo 180, Japan Single crystals of La2.xCat+xCU206.x~+S have been grown by the CuO flux method. After annealing under a high partial pressure of 0 2, these crystals showed zero resistance at 41 K and a perfect shielding effect (3% Meissner signal). These crystals also showed metallic behaviour parallel (a,b) and perpendicular (c) to the basal plane, with an anisotropy of about 30 at room temperature. T13e Ginzburg-Landau coherence lengths were estimated from He2 measurements: ~ab=33 A and ~c--12 A. The anisotropy factor of 3 is fairly small for a high-To superconductor.
Superconductivity has recently been achieved in La2_x(Cal_ySry)l+xCU206_xtZ+8 and in La2.x Cal~vCugOr._..r,.~xcompounds (2-1-2-6) synthesized . - ~ .~ v- . . . . 1,2,3 under high O 2 pressure . The sunple 2-1-2-6 4 crystal structure contains only a pyramidal Cu-O plane, whereas Ba2YCu307 has both pyramids and a chain. The electrical properties of 2-1-2-6 single crystals will therefore provide useful information about the transport mechanism in typical high-Tc superconductors. This paper reports the growth o f L a 2 . x C a l + x C U 2 0 6 . x / 2 + 5 single crystals and the anisotropic superconducting properties of these crystals. Single crystals of La2.xCal+xCU206_x/2+8 were grown in a Pt crucible by the CuO flux method, with starting compositions La2_xCal+xCU2Oy:CUO in the ratio of 1:2. These mixtures were heated in air to 1200 °C for 2 h, cooled at 4 °C/h to 900 °C, and then furnace-cooled to room temperature. Crystals found in the cavity at the bottom of the crucible were square plates with an average size of lxlx0.3 ram. No Pt contamination was detected by electron-probe microanalysis (EPMA). The La:Ca ratio of the crystals was 1.87:1.13 and it varied little for various starting values of x.
These crystals were superconductive after annealing at 1080 °C for 100 h under a 0 2 partial pressure of 300-atm: Resistance was zero at 41 K. The temperature dependence of the magnetic susceptibility is shown in the inset of Fig. 1. A perfect shielding effect, which indicates bulk superconductivity, was observed. On the other hand, the Meissner signal was about 3 % of the ideal value, which suggests strong flux pinning exists. Figure 1 shows for both as-grown and annealed crystals, the temperature dependence of electrical resistivity (measured by Montgomery's method) parallel and perpendicular to the basal plane. For both directions, resistivity of the as-grown crystal is almost constant from room temperature to 50 K, where it begins to increase with decreasing temperature. This suggests that carder transport at low temperatures is dominated by localization. Tne pe/p~ anisotropy of fne as-grown crystal is estimated to be about 300. The annealed crysta~ shows metallic behaviour in both directions, with a pc/Pab anisotropy that is about 30 at room temperature and that increases slightly with decreasing temperature. This anisotropy is similar to or even smaller than that of Ba2YCu3075.
*Present address: NTr Applied Electronics Laboratories, 162 Tokai, Ibaraki 319-11, Japan 0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V.
All rights reserved.
1236
Z Watanabeet aL / SuperconductingLa:.xCaz+xCu20~R+s single oystals
10 2 1.00
.,,.pGCas_grown] 101
E u,
H.t¢
0.2,5 -.50 -.75
o
10o
\(3- 0{75 (3. 0.50
~ \ ~
I0-t
,
,
20 30 60 50 60 Tempera.ture (K) \ pato(a.s-grown}
0
10
._~ . I¢1 m
I :
i °r
~/li l lor~/~hr~ :i .: ! :
'~ 0.25
,
;t :
5r ~ J
-1,00 t
S :
2T
4T
~.'i/11 ~. i 70 .
0.: 0 "17 ~N 1.00 ~tc
r"--
n" 10-z
\ p¢(3OO-~,trn O2
anneated)
0 :7
0.75 .. 0.50
I0 -3
'ST ~
pan(300-atm 02 annea.ted~.._.......~ 0.25 n
10-4 0
I
t
100
Temperature
I
. . . . . .." j . . , 0
300
10
,..
20
/ i ./'. J , 30
Tempera.ture
(K)
Figure 1 Temperature dependence of electrical resistivity parallel (P,b) and perpendicular (po) to the basal plane for as-grown and annealed La~ .Teat ..Cu.O single crystals. (Inset shows the temperature dependence of the magnetic susceptibility of an annealed crystal.)
OT
/i/
u
200
1T
/
I
40
50
(K)
Figure 2 Temperature dependence of resistivity plotted as a function of magnetic fields parallel and perpendicular to the c-axis of an annealed LaI s~Cat ..Cu.O single crystal. Normahzed resistivity P/Po is the ratio of p(T,H) to p(T=50 K, H=0 T). References
Figure 2 shows the temperature dependence of resistivity plotted as a function of magnetic fields parallel and perpendicular to the c-axis. The temperature dependence of the upper critical magnetic field, He2(T), was determined from the midpoint of the transition. According to the Werthamaro Helfand-Hohenberg theory for type II superconductors, Hc2(0) may be estimated as Hc2(0) =-0.69Tc(dHc2/dT)T=Tc. Thus, He2//(0) and Hc2_L(0) are estimated to be 85.4 T and 30.2 T. Using these values of Hc2(0), the coherence length (~) along the c-axis is 12 ,~ and the coherence length in the basal plane is 33 A. The anisotropy factor of 3 is fairly small for a high-Tc superconductor.
1. R. J. Cava, B. Batlogg, R. B. van Dover, J. J. Krajewski, J. V. Waszczak, R. M. Fleming, W. F. Peck Jr., L. W. Rupp Jr., P. Marsh, A. C. W. P. James, and L. F. Schneemeyer, Nature 345 (1990) 602. 2. K. Kinoshita, H. Shibata, and T. Yamada, Jpn. J. Appl. Phys. 29 (1990) L1632. J.
IL~.• I X J U t
lalUta,
II. OlUUa.i~
o.l.iu
r 6111~.Ud,
FllySll,;ii
C 171 (19~3) 523. 4. N. Nguyen, L. Er-Rakho, C. Michel, J. Choisnet, and B. Raveau, Mater.Res. Bull. 15 (1980) 891• 5. T. A. Friedrnann, M. W. Rabin, J. Giapinzakis, J. P. Rice, and D. M. Ginsberg, Phys. Rev. B 42 (1990) 6217.