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Superconductivity in Y2PdGe3
Physica B 312–313 (2002) 152–154
Superconductivity in Y2PdGe3 E.V. Sampathkumarana,*, Subham Majumdara, W. Schneiderb, S.L. Molodtsovb, C. Laubschatb a
Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400005, India b Institut fur und Mikrostrukturephysik, Technishe Universitat . Oberflachen . . Dresden, D-01062 Dresden, Germany
Abstract The results of electrical resistance, magnetization and heat-capacity measurements in Y2 PdGe3 ; crystallizing in an AlB2 -derived hexagonal structure, establish that this compound is superconducting below 3 K: This is the first superconductor among the ternary members derived from the hexagonal AlB2 structure. The band structure calculations reveal that the Fermi level ðEF Þ is dominated by Y 4d density of states, and therefore these states are presumably responsible for superconductivity; this situation is somewhat different from that in recently discovered high-temperature superconductor MgB2 (isostructural to Y2 PdGe3 Þ; in which superconductivity is attributed to B. r 2002 Elsevier Science B.V. All rights reserved. PACS: 74.70.Ad; 71.20.Eh Keywords: Superconductivity; Y2 PdGe3
Recent observation [1] of superconductivity with Tc close to 40 K in MgB2 has caused considerable excitement among condensed matter physicists. This compound crystallizes in the hexagonal AlB2 -type structure, which is made up of alternating hexagonal layers of Al and graphite-like honeycomb layers of B atoms. This structure is generally unfavorable for superconductivity among rare-earth (including Y) compounds. Thus, for instance, among the rare-earth disilicides and germanides having the same electron concentration ðYSi2 ; YGe2 ; LaSi2 and LaGe2 Þ; only those with the a-ThSi2 structure ðYGe2 ; LaSi2 and LaGe2 ; structurally slightly different from AlB2 Þ have been found to be superconducting; even for ThSi2 ; the atetragonal form exhibits a higher Tc ð3:16 KÞ than the bform which is of AlB2 -type ð2:4 KÞ [2]. Therefore, it is of interest to search for superconductors among rare-earthbased compounds crystallizing in the AlB2 -type struc*Corresponding author. Tel.: +91-22-215-2971; fax: +9122-215-2110. E-mail address: [email protected] (E.V. Sampathkumaran).
ture. It is also of interest to look for such superconductors not containing B, in view of the proposals that B plays a vital role to favor superconductivity in this structure in MgB2 [3]. Here, we report the results of electrical resistivity ðrÞ; magnetic susceptibility ðwÞ and heat-capacity ðCÞ measurements as well as the results of electronic structure calculations in a ternary compound, Y2 PdGe3 ; forming in the AlB2 -derived structure [4], establishing superconductivity below ðTc ¼ 3 KÞ; presumably arising from Y 4d band unlike the situation in MgB2 : The samples were prepared by arc melting and ( and c ¼ characterized by X-ray diffraction ða ¼ 4:192 A ( [4]. The absence of superstructure lines in the 4:000 AÞ X-ray diffraction pattern indicates that Pd and Ge ions are not crystallographically ordered in the hexagonal network and these ions therefore occupy B site randomly in the MgB2 structure. Thus, otherwise, there is no difference in the crystal structure between these two compounds. The r (1.4–300 KÞ; the field-cooled (FC) and the zero field-cooled (ZFC) w ðH ¼ 25 OeÞ below 10 K; and the C data (2.5–30 KÞ were obtained as discussed in Ref. [4].
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 1 5 5 9 - 9
E.V. Sampathkumaran et al. / Physica B 312–313 (2002) 152–154
With respect to the high-temperature r behavior, there is a normal metallic behavior of r with the variation of T: However, at 3 K; there is a sharp drop of r (Fig. 1) to zero as T is lowered, as if this compound is superconducting below 3 K: To support this finding, the w has also been measured at low fields ð25 OeÞ and we find that there is an onset of strong diamagnetism below 3 K and the divergence of ZFC and FC w is typical of that expected for type-II superconductors. A comparison of the magnitude of the value of ZFC-w at 2 K with that of other known standard superconductors like Pb establishes bulk nature of superconductivity. The plot of magnetization (M) versus H (see Fig. 1, inset) at 1:7 K is typical of that of type-II superconductors with a value of lower critical field of about 400 Oe: In order to render further support to the bulk nature of superconductivity, C was also measured down to 2:5 K and the upturn observed in the data obtained below 3 K (see Fig. 1, bottom inset) is sufficient to establish bulk superconductivity; the broadened signal around the transition could possibly be due to inhomogeneities. The values of the Debye temperature and the electronic term ðgÞ inferred from the data in the range 5–15 K turn to be about 130 K and 2:5 mJ=mol K2 ; respectively. If one
0 T = 1.7 K
0.4
Y2PdGe3 -4
25 Oe ZFC
-8
M (emu / g)
χ (emu / mol)
FC
0.0
-0.4 0
4 H ( kOe)
8
0.2
Y-4d Pd-4d Ge-4p
Density of states
C (mJ/mol K)
80
ρ (mΩ cm)
employs this value of g; the value of DC=gTc turns to be close to 3, which is far away from the weak-coupling value of 1.35. Also, the band structure calculations were performed using a full potential nonorthogonal local orbital minimum-basis band structure scheme (FPLO) [5] in its scalar relativistic version employing the Perdew and Zunger exchange–correlation potential [6]. The set of atomic orbitals included the Y 5s, 5p, and 4d states, the Pd 5s, 5p, and 4d states as well as the Ge 4s and 4p states. The orbitals are solutions of an atomic-like . Schrodinger equation with an additional confining potential ðr=r0 Þ4 : The compression radii, r0 ; of the orbitals were optimized in a way, minimizing the total energy. The k sums were carried out using the linear tetrahedron method at a mesh of 133 k-points in the irreducible part of the Brillouin zone. The results of such calculations are shown in Fig. 2, which reveals that the Fermi level is clearly dominated by Y 4d density of states, and not by Ge 4p states (at least about 5 times less); therefore, we believe that the superconductivity in this compound arises from Y 4d electrons. To conclude, Y2 PdGe3 serves as the first superconductor among ternary compounds crystallizing in the hexagonal AlB2 structure. Apparently, the band that is responsible for superconductivity is mainly from Al site. This situation is different from that in MgB2 ; in which B site determines superconductivity, as B 2p states are the ones, which dominate the Fermi level [3] and direct B–B bond is rather strong causing metallicity. At this point, it may be remarked that the lattice constants ( and c ¼ of MgB2 are much smaller ða ¼ 3:086 A ( than that of the Y compound. The relatively 3:504 AÞ large unit-cell and the Pd ions intervening the Ge atoms (in the planar nets of the unit cell) apparently weaken direct Ge–Ge bonding in the latter. Naturally, the
T (K)
0.4
153
60
40
Y2PdGe3
20 3
4
T (K)
0.0 0
2
4
6
8
10
T (K) Fig. 1. Electrical resistivity and magnetic susceptibility ðH ¼ 25 OeÞ behavior at low temperatures for Y2 PdGe3 : The top inset shows the field dependence of M at 1:7 K: The bottom inset shows the plot of heat capacity versus temperature. The line through the r data serves as a guide to the eyes.
6
4
EF 2 Binding energy (eV)
-2
-4
Fig. 2. Partial density of states for Y-4d, Pd-4d and Ge-4p as a function of binding energy for Y2 PdGe3 : The data for Ge-4s, Pd-5p, Pd-5s, Y-5p and Y-5s are not shown in the figure, as their contributions are negligibly small.
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overlap of the spatially extended Y 4d orbital dominates conductivity, thereby explaining the differences in the band structure. References [1] J. Nagamatsu, et al., Nature 410 (2001) 63.
[2] B. Chevalier, et al., Solid State Commun. 49 (1984) 753. [3] G. Satta, et al., Cond-mat/0102358; N.I. Medveda, et al, Cond-mat/0103157; A. Reyes-Serrato, D.H. Galvan, Cond-mat/0103477. [4] S. Majumdar, E.V. Sampathkumaran, Phys. Rev. B 63 (2001) 172407. [5] K. Koepernik, H. Eschrig, Phys. Rev. B 59 (1999) 1743. [6] J.P. Perdew, A. Zunger, Phys. Rev. B 23 (1981) 5048.
Superconductivity in Y2PdGe3 E.V. Sampathkumarana,*, Subham Majumdara, W. Schneiderb, S.L. Molodtsovb, C. Laubschatb a
Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400005, India b Institut fur und Mikrostrukturephysik, Technishe Universitat . Oberflachen . . Dresden, D-01062 Dresden, Germany
Abstract The results of electrical resistance, magnetization and heat-capacity measurements in Y2 PdGe3 ; crystallizing in an AlB2 -derived hexagonal structure, establish that this compound is superconducting below 3 K: This is the first superconductor among the ternary members derived from the hexagonal AlB2 structure. The band structure calculations reveal that the Fermi level ðEF Þ is dominated by Y 4d density of states, and therefore these states are presumably responsible for superconductivity; this situation is somewhat different from that in recently discovered high-temperature superconductor MgB2 (isostructural to Y2 PdGe3 Þ; in which superconductivity is attributed to B. r 2002 Elsevier Science B.V. All rights reserved. PACS: 74.70.Ad; 71.20.Eh Keywords: Superconductivity; Y2 PdGe3
Recent observation [1] of superconductivity with Tc close to 40 K in MgB2 has caused considerable excitement among condensed matter physicists. This compound crystallizes in the hexagonal AlB2 -type structure, which is made up of alternating hexagonal layers of Al and graphite-like honeycomb layers of B atoms. This structure is generally unfavorable for superconductivity among rare-earth (including Y) compounds. Thus, for instance, among the rare-earth disilicides and germanides having the same electron concentration ðYSi2 ; YGe2 ; LaSi2 and LaGe2 Þ; only those with the a-ThSi2 structure ðYGe2 ; LaSi2 and LaGe2 ; structurally slightly different from AlB2 Þ have been found to be superconducting; even for ThSi2 ; the atetragonal form exhibits a higher Tc ð3:16 KÞ than the bform which is of AlB2 -type ð2:4 KÞ [2]. Therefore, it is of interest to search for superconductors among rare-earthbased compounds crystallizing in the AlB2 -type struc*Corresponding author. Tel.: +91-22-215-2971; fax: +9122-215-2110. E-mail address: [email protected] (E.V. Sampathkumaran).
ture. It is also of interest to look for such superconductors not containing B, in view of the proposals that B plays a vital role to favor superconductivity in this structure in MgB2 [3]. Here, we report the results of electrical resistivity ðrÞ; magnetic susceptibility ðwÞ and heat-capacity ðCÞ measurements as well as the results of electronic structure calculations in a ternary compound, Y2 PdGe3 ; forming in the AlB2 -derived structure [4], establishing superconductivity below ðTc ¼ 3 KÞ; presumably arising from Y 4d band unlike the situation in MgB2 : The samples were prepared by arc melting and ( and c ¼ characterized by X-ray diffraction ða ¼ 4:192 A ( [4]. The absence of superstructure lines in the 4:000 AÞ X-ray diffraction pattern indicates that Pd and Ge ions are not crystallographically ordered in the hexagonal network and these ions therefore occupy B site randomly in the MgB2 structure. Thus, otherwise, there is no difference in the crystal structure between these two compounds. The r (1.4–300 KÞ; the field-cooled (FC) and the zero field-cooled (ZFC) w ðH ¼ 25 OeÞ below 10 K; and the C data (2.5–30 KÞ were obtained as discussed in Ref. [4].
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 1 5 5 9 - 9
E.V. Sampathkumaran et al. / Physica B 312–313 (2002) 152–154
With respect to the high-temperature r behavior, there is a normal metallic behavior of r with the variation of T: However, at 3 K; there is a sharp drop of r (Fig. 1) to zero as T is lowered, as if this compound is superconducting below 3 K: To support this finding, the w has also been measured at low fields ð25 OeÞ and we find that there is an onset of strong diamagnetism below 3 K and the divergence of ZFC and FC w is typical of that expected for type-II superconductors. A comparison of the magnitude of the value of ZFC-w at 2 K with that of other known standard superconductors like Pb establishes bulk nature of superconductivity. The plot of magnetization (M) versus H (see Fig. 1, inset) at 1:7 K is typical of that of type-II superconductors with a value of lower critical field of about 400 Oe: In order to render further support to the bulk nature of superconductivity, C was also measured down to 2:5 K and the upturn observed in the data obtained below 3 K (see Fig. 1, bottom inset) is sufficient to establish bulk superconductivity; the broadened signal around the transition could possibly be due to inhomogeneities. The values of the Debye temperature and the electronic term ðgÞ inferred from the data in the range 5–15 K turn to be about 130 K and 2:5 mJ=mol K2 ; respectively. If one
0 T = 1.7 K
0.4
Y2PdGe3 -4
25 Oe ZFC
-8
M (emu / g)
χ (emu / mol)
FC
0.0
-0.4 0
4 H ( kOe)
8
0.2
Y-4d Pd-4d Ge-4p
Density of states
C (mJ/mol K)
80
ρ (mΩ cm)
employs this value of g; the value of DC=gTc turns to be close to 3, which is far away from the weak-coupling value of 1.35. Also, the band structure calculations were performed using a full potential nonorthogonal local orbital minimum-basis band structure scheme (FPLO) [5] in its scalar relativistic version employing the Perdew and Zunger exchange–correlation potential [6]. The set of atomic orbitals included the Y 5s, 5p, and 4d states, the Pd 5s, 5p, and 4d states as well as the Ge 4s and 4p states. The orbitals are solutions of an atomic-like . Schrodinger equation with an additional confining potential ðr=r0 Þ4 : The compression radii, r0 ; of the orbitals were optimized in a way, minimizing the total energy. The k sums were carried out using the linear tetrahedron method at a mesh of 133 k-points in the irreducible part of the Brillouin zone. The results of such calculations are shown in Fig. 2, which reveals that the Fermi level is clearly dominated by Y 4d density of states, and not by Ge 4p states (at least about 5 times less); therefore, we believe that the superconductivity in this compound arises from Y 4d electrons. To conclude, Y2 PdGe3 serves as the first superconductor among ternary compounds crystallizing in the hexagonal AlB2 structure. Apparently, the band that is responsible for superconductivity is mainly from Al site. This situation is different from that in MgB2 ; in which B site determines superconductivity, as B 2p states are the ones, which dominate the Fermi level [3] and direct B–B bond is rather strong causing metallicity. At this point, it may be remarked that the lattice constants ( and c ¼ of MgB2 are much smaller ða ¼ 3:086 A ( than that of the Y compound. The relatively 3:504 AÞ large unit-cell and the Pd ions intervening the Ge atoms (in the planar nets of the unit cell) apparently weaken direct Ge–Ge bonding in the latter. Naturally, the
T (K)
0.4
153
60
40
Y2PdGe3
20 3
4
T (K)
0.0 0
2
4
6
8
10
T (K) Fig. 1. Electrical resistivity and magnetic susceptibility ðH ¼ 25 OeÞ behavior at low temperatures for Y2 PdGe3 : The top inset shows the field dependence of M at 1:7 K: The bottom inset shows the plot of heat capacity versus temperature. The line through the r data serves as a guide to the eyes.
6
4
EF 2 Binding energy (eV)
-2
-4
Fig. 2. Partial density of states for Y-4d, Pd-4d and Ge-4p as a function of binding energy for Y2 PdGe3 : The data for Ge-4s, Pd-5p, Pd-5s, Y-5p and Y-5s are not shown in the figure, as their contributions are negligibly small.
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E.V. Sampathkumaran et al. / Physica B 312–313 (2002) 152–154
overlap of the spatially extended Y 4d orbital dominates conductivity, thereby explaining the differences in the band structure. References [1] J. Nagamatsu, et al., Nature 410 (2001) 63.
[2] B. Chevalier, et al., Solid State Commun. 49 (1984) 753. [3] G. Satta, et al., Cond-mat/0102358; N.I. Medveda, et al, Cond-mat/0103157; A. Reyes-Serrato, D.H. Galvan, Cond-mat/0103477. [4] S. Majumdar, E.V. Sampathkumaran, Phys. Rev. B 63 (2001) 172407. [5] K. Koepernik, H. Eschrig, Phys. Rev. B 59 (1999) 1743. [6] J.P. Perdew, A. Zunger, Phys. Rev. B 23 (1981) 5048.