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Anomalous superconductivity in black phosphorus under high pressures
0038-1098/84 $3.00 + .OO Pergamon Press Ltd.
Solid State Communications,Vo1.49,No.9, pp.879-881, 1984 Printed in Great Britain.
ANOMALOUS SUPERCONDUCTIVITYIN BLACK PHOSPHORUS UNDER HIGH PREXXRKS Haruki Kawamura, Ichimin Shirotani* and Kyoji Tachikawa National Research Institute for Metals, Sakura, Niihari, Ibaraki, 305 Japan *Applied Science for Energy, Muroran Institute of Technology, Mizumoto, Muroran, 050 Japan (Received 30 November 1983 by W. Sasaki)
Pressure induced superconductivityin single crystals of black phosphorus has been studied. Maximum onset T, was near 13 K. The anomalous superconductivitymay be explained in terms of excitonic mechanism.
carried out along two different paths. In path (A), the pressure up to 15 GPa was applied at room temperature, so black phosphorus was completely transformed intothesimple cubic phase, and then the pressure cell was cooled down to liquid helium temperature. Therefore, in path (A), the dependenceof the superconducting transition temperatureonthe pressure forthesimple cubic phase is determined. This experiment was performed in order to compare with the superconductivity along path (B). The transition temperature of the simple cubic phase is about 6 K and it is slightly increased by the pressure. This result is in fairly good agreement with other results (10). In path(B), the sample was suddenly cooled down to 4.5 K without applying pressure prior
In recent years, large single crystals of black phosphorus are obtained by the crystal growth under high pressures and at high temperatures (1). Many physical properties have been clarified (2). At atmospheric pressure and room temperature black phosphorus is a semiconductor and its crystal structure is orthorhombic. It is transferred into rhombohedral structure in the pressure range from 4 to 8 GPa (3). This phase transition is very sluggish even at room temperature and these two phases are mixed in the wide pressure range. The black phosphorus under still higher pressures above 10 GPa is particularly interesting,for it is crystallized inasimple cubic structure and shows a metallic conduction. Ontheotherhand,new mechanisms for superconductivity have been proposed theoretically after BCS theory. The excitonic superconductivity proposed by Little is most famous of them (4). Stimulated by his proppsal,organicsuperconductors have been synthesized (5). However, the appearance of superconductivity on these compounds has been considered to be based on BCS mechanism. Recently, it is discussed whether excitonic superconductivityis realized or not in Nb-Ge multi phase system (6), but so far it has not been clarified. Multi phase systems like this are nevertheless useful to investigate the excitonic superconductivity. Since black phosphorus has polymorphism under high pressures, multi phase system of black phosphorus should be realized by making use of this polymorphism. In this article, anomalous superconductivityin single crystals of black phosphorus at high pressures is described. The high pressure experiment at low temperature is performed by the use of our newlydeveloped apparatus. This system consists of a diamond anvil pressure cell mounted on booster which is actuated by helium gas. Therefore, we can change pressures continuously in the cryostat. Details of this system is described in other paper (7). The measurements of resistance are performed by the ordinary four probe method similar to Sakaiss one (8). Figure 1 shows the schematic phase diagram ofblackphosphorus (9). Resistance measurements of black phosphorus under high pressures were
Semi-
Semi-Mete1
Metal
Conductor
52 Y
\
0,
I
I
4.2
5 Pressure
10 (GPa)
Fig. 1. Schematic phase diagram of black phosphorus. Two different paths along whichthepresent experiment was done are also illustrated. 879
880
ANOMALOUS
SUPERCONDUCTIVITY
to cooling. Then, the pressure was increased up to about 30 GPa. Figure 2 shows the dependence of the resistance on the pressure at 4.5 K. The scale which is illustrated in the upper side of the figure showstheapproximate pressure applied to the sample. The resistivity estimated from the dimension of the sample is given at the righthand side of the figure. The resistance is insensitive to pressure uptoabout2GPa. Though the sample is mounted at room temperature and the pressure cell is clamped at nearly atmospheric pressure, the pressure is increasedupto 2 GPa because of the thermal shrinkage of the pressure cell during the course of cooling. The resistance is monotonically decreased with increasing pressure and abruptly decreased around 12 GPa. It represents the appearance of superconductivityat4.5 K. Triangular marks show the residual resistance of the normal state just above the superconducting transition temperature.
Vol. 49, No. 9
IN BLACK PHOSPHORUS
transition from the normal to superconducting state. The reproducibility of the experimental results is very good and this phenomenon along path (B) is considered intrinsic.
T1.0
-
e a 2 .o-
I :13.0GPa 2 : 15.0 3 : 16.9 4 : 16.6 5 : 20.7 6: 22.5 7 : 24.4 6:273 9: 29.0
5
a
aJo. "
-
5 ;i; .:
a
100
Fig. 3. Superconducting each fixed pressure.
transition
curves at
10 Figure 4 shows the dependence of superconducting transition temperature on the pressure along both (A) and (B)pathway. Transition temperatures are defined at the midpoint of the transition. Inpath (B), T, is increased steeply with the increasing pressure, and this behavior of T, greatly differs from that of the simple This anomalous increase of T, in cubic phase. an element without d electrons would not be explicable within the scope of BCS mechanism. Since the phase transition from orthorhombit to rhombohedral structure is very sluggish even at room temperature, orthorhombic phase of
5
11
1.0 20 3.0 He Gas Pressure (MPd
4x1
10 9
Fig. 2. Dependence of resistance on the pressure at 4.5 K along path (B). The scale illustrated in the upper side shows the pressure given to the sample. Approximate resistivity is also scaled on the righthand side.
8 2 ;7 6
Figure 3 shows the superconducting transition curves at each fixed pressure. These transtate sitions from normal to superconducting around the are commenced at the resistivity The remnant resistivity order of 10v5 ohm-cm. at the temperature below the termination point of the transition is order of 10s7 ohm-cm. This remnant resistivity is attributed to the contact resistance and the resistivity of the sample is So these transiconsidered to be nearly zero. to the electronic tions should be attributed
5
A’
15
25 Press%
(GPa)
Fig. 4. Dependence of T, on the pressure. T, along path (A) is indicated by circles and triangular marks show T, along path (B).
30
Vol.
ANOMALOUS
49, No. 9
SUPERCONDUCTIVITY
black phosphorus should be still retained at very low temperature even ifthepressure, under which the transition from orthorhombic to rhombohedral phase is expected, is applied. However, therearemany dislocations in these pressed crystals. As atoms surrounding dislocations are relatively mobile, phase transition will take place only around dislocations and orthorhombic phase wouldbetransferred into the simple cubic phase through the rhombohedral phase under enough high pressure. it is Therefore, considered that black phosphorus is converted into a mixed system consisting of metallic fine threads dispersed in the semiconducting matrix. The appearance of superconductivity in the system-like this has been suggested by Fukuyama
881
IN BLACK PHOSPHORUS
(11). Itisplausible that the superconductivity along path (B) is caused by the excitonic mechanism. Naturally it is difficult to rule out the possibility of the transforming intoother forms from the lack of detailed study of theP-Tphase diagram at low temperature region. The x-ray analysis under high pressure and at low temperature may be useful to clarify the anomalous superconductivity on black phosphorus.
Acknowledgement - This study was partly supported by Special Co-ordination Funds for Promating Science and Technology of the Science and Technology Agency of the Japanese Government.
References
1.
I. Shiratani, R. Maniwa, H. Sato, A. Fukizawa, N. Sato, Y. Maruyama, H. Inokuchi and S. Akimoto, Nippon Kagakukaishi, (1981) 1604. I. Shirotani, Mol. Cryst. Liq. Cryst. 86 (1982) 1943.
2.
T. Takahashi, I. Shirotani, S. Suzuki and T. Sagawa, Solid State Commun. 45 (1983) 945. M. Ikezawa, Y. Kondo and I. Shirotani, .I. Phys. Sot. Japan, 52 (1983) 1518.
3.
J. C. Jamieson,
4.
L. A. Little, Phys. Rev. Al34 (1964) 1416. V. L. Ginzburg, Comtemp. Phys. 9 (1968) 355. D. Allender, J. Bray and J. Bardeen, Phys. Rev. B7 (1973) 1020.
5.
D. Jerome,
6.
C. M. Knoedler and D. H. Douglass, J. Low Temp. Phys. 37 (1979) 189. A. K. Ghosh and D. H. Douglass, Phys. Rev. Lett. 37 (1976) 32.
7.
H. Kawamura,
8.
N. Sakai, T. Kajiwara,
9.
T. Kikegawa
Science,
A. Mazaud,
139 (1963) 1291.
M. Ribaut and K. Bechgaard,
0. Shimomura
T. Kajiwara,
and K. Tachikawa,
Rev. Sci. Instrum.
K. Tsuji and S. Minomura,
and H. Iwasaki,
J. Physique
J. Appl. Crystallogr.
to be published.
Rev. Sci. Instrum. to be published.
10.
I. V. Berman and N. B. Brandt, ZhETF Pis'ma, 7 (1968) 412. J. Wittig and B. T. Matthias, Science, 160 (1968) 994.
11.
H. Fukuyama,
J. Phys. Sot. Japan, 51 (1982) 1709.
Lett. 41 (1980) 95.
53 (1982) 499.
Solid State Communications,Vo1.49,No.9, pp.879-881, 1984 Printed in Great Britain.
ANOMALOUS SUPERCONDUCTIVITYIN BLACK PHOSPHORUS UNDER HIGH PREXXRKS Haruki Kawamura, Ichimin Shirotani* and Kyoji Tachikawa National Research Institute for Metals, Sakura, Niihari, Ibaraki, 305 Japan *Applied Science for Energy, Muroran Institute of Technology, Mizumoto, Muroran, 050 Japan (Received 30 November 1983 by W. Sasaki)
Pressure induced superconductivityin single crystals of black phosphorus has been studied. Maximum onset T, was near 13 K. The anomalous superconductivitymay be explained in terms of excitonic mechanism.
carried out along two different paths. In path (A), the pressure up to 15 GPa was applied at room temperature, so black phosphorus was completely transformed intothesimple cubic phase, and then the pressure cell was cooled down to liquid helium temperature. Therefore, in path (A), the dependenceof the superconducting transition temperatureonthe pressure forthesimple cubic phase is determined. This experiment was performed in order to compare with the superconductivity along path (B). The transition temperature of the simple cubic phase is about 6 K and it is slightly increased by the pressure. This result is in fairly good agreement with other results (10). In path(B), the sample was suddenly cooled down to 4.5 K without applying pressure prior
In recent years, large single crystals of black phosphorus are obtained by the crystal growth under high pressures and at high temperatures (1). Many physical properties have been clarified (2). At atmospheric pressure and room temperature black phosphorus is a semiconductor and its crystal structure is orthorhombic. It is transferred into rhombohedral structure in the pressure range from 4 to 8 GPa (3). This phase transition is very sluggish even at room temperature and these two phases are mixed in the wide pressure range. The black phosphorus under still higher pressures above 10 GPa is particularly interesting,for it is crystallized inasimple cubic structure and shows a metallic conduction. Ontheotherhand,new mechanisms for superconductivity have been proposed theoretically after BCS theory. The excitonic superconductivity proposed by Little is most famous of them (4). Stimulated by his proppsal,organicsuperconductors have been synthesized (5). However, the appearance of superconductivity on these compounds has been considered to be based on BCS mechanism. Recently, it is discussed whether excitonic superconductivityis realized or not in Nb-Ge multi phase system (6), but so far it has not been clarified. Multi phase systems like this are nevertheless useful to investigate the excitonic superconductivity. Since black phosphorus has polymorphism under high pressures, multi phase system of black phosphorus should be realized by making use of this polymorphism. In this article, anomalous superconductivityin single crystals of black phosphorus at high pressures is described. The high pressure experiment at low temperature is performed by the use of our newlydeveloped apparatus. This system consists of a diamond anvil pressure cell mounted on booster which is actuated by helium gas. Therefore, we can change pressures continuously in the cryostat. Details of this system is described in other paper (7). The measurements of resistance are performed by the ordinary four probe method similar to Sakaiss one (8). Figure 1 shows the schematic phase diagram ofblackphosphorus (9). Resistance measurements of black phosphorus under high pressures were
Semi-
Semi-Mete1
Metal
Conductor
52 Y
\
0,
I
I
4.2
5 Pressure
10 (GPa)
Fig. 1. Schematic phase diagram of black phosphorus. Two different paths along whichthepresent experiment was done are also illustrated. 879
880
ANOMALOUS
SUPERCONDUCTIVITY
to cooling. Then, the pressure was increased up to about 30 GPa. Figure 2 shows the dependence of the resistance on the pressure at 4.5 K. The scale which is illustrated in the upper side of the figure showstheapproximate pressure applied to the sample. The resistivity estimated from the dimension of the sample is given at the righthand side of the figure. The resistance is insensitive to pressure uptoabout2GPa. Though the sample is mounted at room temperature and the pressure cell is clamped at nearly atmospheric pressure, the pressure is increasedupto 2 GPa because of the thermal shrinkage of the pressure cell during the course of cooling. The resistance is monotonically decreased with increasing pressure and abruptly decreased around 12 GPa. It represents the appearance of superconductivityat4.5 K. Triangular marks show the residual resistance of the normal state just above the superconducting transition temperature.
Vol. 49, No. 9
IN BLACK PHOSPHORUS
transition from the normal to superconducting state. The reproducibility of the experimental results is very good and this phenomenon along path (B) is considered intrinsic.
T1.0
-
e a 2 .o-
I :13.0GPa 2 : 15.0 3 : 16.9 4 : 16.6 5 : 20.7 6: 22.5 7 : 24.4 6:273 9: 29.0
5
a
aJo. "
-
5 ;i; .:
a
100
Fig. 3. Superconducting each fixed pressure.
transition
curves at
10 Figure 4 shows the dependence of superconducting transition temperature on the pressure along both (A) and (B)pathway. Transition temperatures are defined at the midpoint of the transition. Inpath (B), T, is increased steeply with the increasing pressure, and this behavior of T, greatly differs from that of the simple This anomalous increase of T, in cubic phase. an element without d electrons would not be explicable within the scope of BCS mechanism. Since the phase transition from orthorhombit to rhombohedral structure is very sluggish even at room temperature, orthorhombic phase of
5
11
1.0 20 3.0 He Gas Pressure (MPd
4x1
10 9
Fig. 2. Dependence of resistance on the pressure at 4.5 K along path (B). The scale illustrated in the upper side shows the pressure given to the sample. Approximate resistivity is also scaled on the righthand side.
8 2 ;7 6
Figure 3 shows the superconducting transition curves at each fixed pressure. These transtate sitions from normal to superconducting around the are commenced at the resistivity The remnant resistivity order of 10v5 ohm-cm. at the temperature below the termination point of the transition is order of 10s7 ohm-cm. This remnant resistivity is attributed to the contact resistance and the resistivity of the sample is So these transiconsidered to be nearly zero. to the electronic tions should be attributed
5
A’
15
25 Press%
(GPa)
Fig. 4. Dependence of T, on the pressure. T, along path (A) is indicated by circles and triangular marks show T, along path (B).
30
Vol.
ANOMALOUS
49, No. 9
SUPERCONDUCTIVITY
black phosphorus should be still retained at very low temperature even ifthepressure, under which the transition from orthorhombic to rhombohedral phase is expected, is applied. However, therearemany dislocations in these pressed crystals. As atoms surrounding dislocations are relatively mobile, phase transition will take place only around dislocations and orthorhombic phase wouldbetransferred into the simple cubic phase through the rhombohedral phase under enough high pressure. it is Therefore, considered that black phosphorus is converted into a mixed system consisting of metallic fine threads dispersed in the semiconducting matrix. The appearance of superconductivity in the system-like this has been suggested by Fukuyama
881
IN BLACK PHOSPHORUS
(11). Itisplausible that the superconductivity along path (B) is caused by the excitonic mechanism. Naturally it is difficult to rule out the possibility of the transforming intoother forms from the lack of detailed study of theP-Tphase diagram at low temperature region. The x-ray analysis under high pressure and at low temperature may be useful to clarify the anomalous superconductivity on black phosphorus.
Acknowledgement - This study was partly supported by Special Co-ordination Funds for Promating Science and Technology of the Science and Technology Agency of the Japanese Government.
References
1.
I. Shiratani, R. Maniwa, H. Sato, A. Fukizawa, N. Sato, Y. Maruyama, H. Inokuchi and S. Akimoto, Nippon Kagakukaishi, (1981) 1604. I. Shirotani, Mol. Cryst. Liq. Cryst. 86 (1982) 1943.
2.
T. Takahashi, I. Shirotani, S. Suzuki and T. Sagawa, Solid State Commun. 45 (1983) 945. M. Ikezawa, Y. Kondo and I. Shirotani, .I. Phys. Sot. Japan, 52 (1983) 1518.
3.
J. C. Jamieson,
4.
L. A. Little, Phys. Rev. Al34 (1964) 1416. V. L. Ginzburg, Comtemp. Phys. 9 (1968) 355. D. Allender, J. Bray and J. Bardeen, Phys. Rev. B7 (1973) 1020.
5.
D. Jerome,
6.
C. M. Knoedler and D. H. Douglass, J. Low Temp. Phys. 37 (1979) 189. A. K. Ghosh and D. H. Douglass, Phys. Rev. Lett. 37 (1976) 32.
7.
H. Kawamura,
8.
N. Sakai, T. Kajiwara,
9.
T. Kikegawa
Science,
A. Mazaud,
139 (1963) 1291.
M. Ribaut and K. Bechgaard,
0. Shimomura
T. Kajiwara,
and K. Tachikawa,
Rev. Sci. Instrum.
K. Tsuji and S. Minomura,
and H. Iwasaki,
J. Physique
J. Appl. Crystallogr.
to be published.
Rev. Sci. Instrum. to be published.
10.
I. V. Berman and N. B. Brandt, ZhETF Pis'ma, 7 (1968) 412. J. Wittig and B. T. Matthias, Science, 160 (1968) 994.
11.
H. Fukuyama,
J. Phys. Sot. Japan, 51 (1982) 1709.
Lett. 41 (1980) 95.
53 (1982) 499.