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Superconductivity and transport properties in LaRu4Sb12 single crystals probed by radiation-induced disordering
Physica C 460–462 (2007) 691–693 www.elsevier.com/locate/physc
Superconductivity and transport properties in LaRu4Sb12 single crystals probed by radiation-induced disordering B. Goshchitskii *, S. Naumov, N. Kostromitina, A. Karkin Institute of Metal Physics, S. Kovalevskoi Street, 18, Ekaterinburg 620219, Russia Available online 27 March 2007
Abstract Resistivity q(T) and Hall coefficient RH(T) in magnetic fields H up to 14 T were studied in superconducting (Tc = 3.3 K) LaRu4Sb12 single crystals disordered by fast neutron irradiation. Atomic disordering leads to increase in residual resistivity q0, decrease of Hall number and suppression of superconductivity. The upper critical field slope dHc2/dT increases approximately linear with q0. The irradiation effects are almost recovered after annealing at 500 C. The observed radiation-induced effects in LaRu4Sb12 are compared with those in PrOs4Sb12 in terms of unconventional mechanisms of superconductivity. 2007 Elsevier B.V. All rights reserved. Keywords: Superconductivity; Neutron irradiation; Transport properties
A number of filled skutterudite compounds RET4Pn12 (RE = rare earth; T = transition metal; Pn = pnictogen) show heavy fermion behaviour coexisting with the state of unconventional superconductivity (SC) [1,2]. To understand the SC features of this class of materials, we studied the response, in the SC and normal states, of the electron system of LaRu4Sb12 to disordering on the atomic scale induced by fast neutron irradiation. Single crystals of LaRu4Sb12 were prepared by the molten-metal-flux growth method with Sb flux [2]. Lanthanum, ruthenium and antimony as the initial components were placed in a graphite container and a quartz ampoule; the ampoule was slowly (20 h) heated to 950 C, held for 3– 5 h at this temperature and then cooled to 640–700 C at a rate of 2–5 C/h. Samples 1.0 · 0.7 · 0.2 mm3 in size were exposed to irradiation with fast neutron fluence of 5 · 1018 cm2 at Tirr = (330 ± 10) K and annealed isochronally within 0.5 h to Tann = 500 C. Neutron irradiation leads to increase in residual resistivity q0 (from 3.5 to 200 lX cm) and some increase in resistivity slope dq/dT, while the form of curves q(T)
remains approximately the same (Fig. 1). In the initial state, the Hall number nH = 1/(RHe) slightly increases with the rise of temperature (from 2.5 · 1021 at 4.2 K to 2.8 · 1021 at 300 K). After irradiation, nH (300 K) drops to 1.7 · 1021 cm3 (which corresponds to the observed increase of dq/dT), and nH (4.2 K) remains almost unvaried. Thus, following irradiation, nH slightly decreases with the rise of temperature. The SC state gets quickly suppressed by radiationinduced disordering, and recovers at Tann = 300 C (the SC temperature Tc > 1.4 K at q0 < 120 lX cm). The properties of the samples, both in the SC and the normal states, get almost completely recovered after annealing at 500 C. A very similar behaviour was observed in another filled skutterudite compound, PrOs4Sb12, with lower Tc 1.8 K [3]. In the SC region (q0 < 120 lX cm), the upper critical field slope (dHc2/dT) increases substantially and displays almost linear dependence on q0 (Fig. 2). The estimate of coherence length n and mean free path length l, using the expressions n2 ¼ U0 =2pð0:69dH c2 =dT ÞT c ;
*
Corresponding author. Tel.: +7 343 3744494; fax: +7 343 3740003. E-mail address: [email protected] (B. Goshchitskii).
0921-4534/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.03.131
l ¼ ð3p2 Þ1=3 h=ðq0 e2 n2=3 Þ;
692
B. Goshchitskii et al. / Physica C 460–462 (2007) 691–693
10
As it has been shown, in the compounds with electron– phonon pairing, such as MgB2 [4], the decrease of Tc under atomic disordering is due to the decrease of N(EF), which, however, remains finite in a strongly disordered system, therefore Tc > 0. Fast SC suppression in LaRu4Sb12 under disordering does not correspond to the electron–phonon scenario. The first probable explanation to the observed radiation effects in LaRu4Sb12 consists in that decrease of Tc takes place due to the ‘‘pair-breaking’’ effects of the d-wave order parameter described by the Abrikkosov–Gor’kov (AG) formula
1
lnðT c0 =T c Þ ¼ wða þ 1=2Þ wð1=2Þ;
2 3 4
400
5 6 7
300
ρ, μΩcm
8 9
200
where w is the digamma function, the ‘‘pair-breaking’’ parameter is a = h/(2pkBTcs). This equation gives Tc = 0 at s 6 sc = h/(0.88kBTc0), or at q0 P q0c = (0.88kBTc0m*)/(hne2). Estimating the effective electron mass m* 10me, from the expression
100
c ¼ ð3p2 Þ
0 0
50
100
150
200
250
300
350
T, K Fig. 1. Resistivities q as a function of temperature T for initial (1), irradiated (2) and annealed at Tann = 100 C (3), 200 C (4), 250 C (5), 300 C (6), 350 C (7), 400 C (8), 450 C (9), and 500 C (10) single crystal LaRu4Sb12.
-dHc2/dT, T/K
0.8
Hc2,T
0.3
0.2
9
0.6 0.4 0.2
8
0.0
0.1
7
10
6
0.0
0
50 100 ρ0, μΩcm
1
1.5
2.0
2.5
3.0
3.5
T, K Fig. 2. Hc2 vs. T for LaRu4Sb12, curve numbers are the same as in Fig. 1. Inset shows dHc2/dT as a function of q0.
˚ , l 2000 A ˚ for the initial (non-irradiated) gives n 300 A ˚ ˚ for the sample irradiated sample, and n 200 A, l 60 A and annealed at 300 C, if we use the low-T value of nH 2.5 · 1021 cm3 as the concentration of charge carriers n. Hence, the initial sample is related to the ‘‘clean’’ limit l n, while the sample irradiated and annealed at 300 C is related to the ‘‘dirty’’ limit l n. Since the coherence has the form 1/n2 1/n0(1/n0 + 1/l), the linear dependence of dHc2/dT on q0 corresponds to constancy of ‘‘band’’ parameters, such as, e.g., density of electron states N(EF).
1=3
k B Rn1=3 V mol m =ð3h2 Þ;
˚ 3 [5], we obtain with c = 36 mJ/(mol K2) and Vmol = 403 A q0c 5 lX cm. This value is significantly smaller than q0 100 lX cm, where, in our case, SC still exists. Thus, the AG theory overestimates the decrease of Tc in LaRu4Sb12. There is another qualitative explanation of Tc suppression under disordering in the case of exotic (non-phonon) mechanism of SC, associated with irradiation-induced suppression of respective quasi-particles, which are responsible for exotic pairing. As distinct from phonon states, which vary but slightly under atomic disordering, these delicate quasi-particle states, connected, in all probability, with the heavy fermion phenomenon, may strongly depend on disordering and disappear when atomic displacements become more or less significant. Suppression of the heavy fermion states under radiation-induced disordering has been also observed in a number of systems, such as PrOs4Sb12 [3], CeCu6 [6] and CeT2X2 (T = Ni, Cu, Pd; X = Si, Ge) [7]. In conclusion, we have shown that neutron irradiation leads to suppression of superconductivity in LaRu4Sb12. The linear dependence of dHc2/dT on q0 is related to the constancy of N(EF); the latter points to an exotic (non-phonon) mechanism of superconductivity in this compound. Acknowledgments Work was carried out with the financial support of the Federal Agency for Science and Innovations (State Contract No. 02.452.11.7004), the Presidium of RAS Programs of Fundamental Research: ‘‘Quantum Macrophysics’’ (Project No. 4 of UB RAS) and ‘‘Effect atomic-crystalline and electron structure on properties of condensed matter’’ (Project No. 9 of UB RAS), RFBR (Project No. 04-0216053).
B. Goshchitskii et al. / Physica C 460–462 (2007) 691–693
References [1] N. Takeda, M. Ishikawa, J. Phys. Soc. Jpn. 69 (2000) 868. [2] B.C. Sales et al., Phys. Rev. B 56 (1997) 15081. [3] A. Karkin et al., Physica B 359–361 (2005) 913.
[4] [5] [6] [7]
A. Karkin et al., Pis’ma v ZhETF 73 (2001) 640. E.D. Bauer et al., J. Phys.: Condens. Mat. 13 (2001) 4495. A.S. Krivoshchekov et al., Physica B 359–361 (2005) 178. A.E. Karkin et al., Physica B 359–361 (2005) 160.
693
Superconductivity and transport properties in LaRu4Sb12 single crystals probed by radiation-induced disordering B. Goshchitskii *, S. Naumov, N. Kostromitina, A. Karkin Institute of Metal Physics, S. Kovalevskoi Street, 18, Ekaterinburg 620219, Russia Available online 27 March 2007
Abstract Resistivity q(T) and Hall coefficient RH(T) in magnetic fields H up to 14 T were studied in superconducting (Tc = 3.3 K) LaRu4Sb12 single crystals disordered by fast neutron irradiation. Atomic disordering leads to increase in residual resistivity q0, decrease of Hall number and suppression of superconductivity. The upper critical field slope dHc2/dT increases approximately linear with q0. The irradiation effects are almost recovered after annealing at 500 C. The observed radiation-induced effects in LaRu4Sb12 are compared with those in PrOs4Sb12 in terms of unconventional mechanisms of superconductivity. 2007 Elsevier B.V. All rights reserved. Keywords: Superconductivity; Neutron irradiation; Transport properties
A number of filled skutterudite compounds RET4Pn12 (RE = rare earth; T = transition metal; Pn = pnictogen) show heavy fermion behaviour coexisting with the state of unconventional superconductivity (SC) [1,2]. To understand the SC features of this class of materials, we studied the response, in the SC and normal states, of the electron system of LaRu4Sb12 to disordering on the atomic scale induced by fast neutron irradiation. Single crystals of LaRu4Sb12 were prepared by the molten-metal-flux growth method with Sb flux [2]. Lanthanum, ruthenium and antimony as the initial components were placed in a graphite container and a quartz ampoule; the ampoule was slowly (20 h) heated to 950 C, held for 3– 5 h at this temperature and then cooled to 640–700 C at a rate of 2–5 C/h. Samples 1.0 · 0.7 · 0.2 mm3 in size were exposed to irradiation with fast neutron fluence of 5 · 1018 cm2 at Tirr = (330 ± 10) K and annealed isochronally within 0.5 h to Tann = 500 C. Neutron irradiation leads to increase in residual resistivity q0 (from 3.5 to 200 lX cm) and some increase in resistivity slope dq/dT, while the form of curves q(T)
remains approximately the same (Fig. 1). In the initial state, the Hall number nH = 1/(RHe) slightly increases with the rise of temperature (from 2.5 · 1021 at 4.2 K to 2.8 · 1021 at 300 K). After irradiation, nH (300 K) drops to 1.7 · 1021 cm3 (which corresponds to the observed increase of dq/dT), and nH (4.2 K) remains almost unvaried. Thus, following irradiation, nH slightly decreases with the rise of temperature. The SC state gets quickly suppressed by radiationinduced disordering, and recovers at Tann = 300 C (the SC temperature Tc > 1.4 K at q0 < 120 lX cm). The properties of the samples, both in the SC and the normal states, get almost completely recovered after annealing at 500 C. A very similar behaviour was observed in another filled skutterudite compound, PrOs4Sb12, with lower Tc 1.8 K [3]. In the SC region (q0 < 120 lX cm), the upper critical field slope (dHc2/dT) increases substantially and displays almost linear dependence on q0 (Fig. 2). The estimate of coherence length n and mean free path length l, using the expressions n2 ¼ U0 =2pð0:69dH c2 =dT ÞT c ;
*
Corresponding author. Tel.: +7 343 3744494; fax: +7 343 3740003. E-mail address: [email protected] (B. Goshchitskii).
0921-4534/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.03.131
l ¼ ð3p2 Þ1=3 h=ðq0 e2 n2=3 Þ;
692
B. Goshchitskii et al. / Physica C 460–462 (2007) 691–693
10
As it has been shown, in the compounds with electron– phonon pairing, such as MgB2 [4], the decrease of Tc under atomic disordering is due to the decrease of N(EF), which, however, remains finite in a strongly disordered system, therefore Tc > 0. Fast SC suppression in LaRu4Sb12 under disordering does not correspond to the electron–phonon scenario. The first probable explanation to the observed radiation effects in LaRu4Sb12 consists in that decrease of Tc takes place due to the ‘‘pair-breaking’’ effects of the d-wave order parameter described by the Abrikkosov–Gor’kov (AG) formula
1
lnðT c0 =T c Þ ¼ wða þ 1=2Þ wð1=2Þ;
2 3 4
400
5 6 7
300
ρ, μΩcm
8 9
200
where w is the digamma function, the ‘‘pair-breaking’’ parameter is a = h/(2pkBTcs). This equation gives Tc = 0 at s 6 sc = h/(0.88kBTc0), or at q0 P q0c = (0.88kBTc0m*)/(hne2). Estimating the effective electron mass m* 10me, from the expression
100
c ¼ ð3p2 Þ
0 0
50
100
150
200
250
300
350
T, K Fig. 1. Resistivities q as a function of temperature T for initial (1), irradiated (2) and annealed at Tann = 100 C (3), 200 C (4), 250 C (5), 300 C (6), 350 C (7), 400 C (8), 450 C (9), and 500 C (10) single crystal LaRu4Sb12.
-dHc2/dT, T/K
0.8
Hc2,T
0.3
0.2
9
0.6 0.4 0.2
8
0.0
0.1
7
10
6
0.0
0
50 100 ρ0, μΩcm
1
1.5
2.0
2.5
3.0
3.5
T, K Fig. 2. Hc2 vs. T for LaRu4Sb12, curve numbers are the same as in Fig. 1. Inset shows dHc2/dT as a function of q0.
˚ , l 2000 A ˚ for the initial (non-irradiated) gives n 300 A ˚ ˚ for the sample irradiated sample, and n 200 A, l 60 A and annealed at 300 C, if we use the low-T value of nH 2.5 · 1021 cm3 as the concentration of charge carriers n. Hence, the initial sample is related to the ‘‘clean’’ limit l n, while the sample irradiated and annealed at 300 C is related to the ‘‘dirty’’ limit l n. Since the coherence has the form 1/n2 1/n0(1/n0 + 1/l), the linear dependence of dHc2/dT on q0 corresponds to constancy of ‘‘band’’ parameters, such as, e.g., density of electron states N(EF).
1=3
k B Rn1=3 V mol m =ð3h2 Þ;
˚ 3 [5], we obtain with c = 36 mJ/(mol K2) and Vmol = 403 A q0c 5 lX cm. This value is significantly smaller than q0 100 lX cm, where, in our case, SC still exists. Thus, the AG theory overestimates the decrease of Tc in LaRu4Sb12. There is another qualitative explanation of Tc suppression under disordering in the case of exotic (non-phonon) mechanism of SC, associated with irradiation-induced suppression of respective quasi-particles, which are responsible for exotic pairing. As distinct from phonon states, which vary but slightly under atomic disordering, these delicate quasi-particle states, connected, in all probability, with the heavy fermion phenomenon, may strongly depend on disordering and disappear when atomic displacements become more or less significant. Suppression of the heavy fermion states under radiation-induced disordering has been also observed in a number of systems, such as PrOs4Sb12 [3], CeCu6 [6] and CeT2X2 (T = Ni, Cu, Pd; X = Si, Ge) [7]. In conclusion, we have shown that neutron irradiation leads to suppression of superconductivity in LaRu4Sb12. The linear dependence of dHc2/dT on q0 is related to the constancy of N(EF); the latter points to an exotic (non-phonon) mechanism of superconductivity in this compound. Acknowledgments Work was carried out with the financial support of the Federal Agency for Science and Innovations (State Contract No. 02.452.11.7004), the Presidium of RAS Programs of Fundamental Research: ‘‘Quantum Macrophysics’’ (Project No. 4 of UB RAS) and ‘‘Effect atomic-crystalline and electron structure on properties of condensed matter’’ (Project No. 9 of UB RAS), RFBR (Project No. 04-0216053).
B. Goshchitskii et al. / Physica C 460–462 (2007) 691–693
References [1] N. Takeda, M. Ishikawa, J. Phys. Soc. Jpn. 69 (2000) 868. [2] B.C. Sales et al., Phys. Rev. B 56 (1997) 15081. [3] A. Karkin et al., Physica B 359–361 (2005) 913.
[4] [5] [6] [7]
A. Karkin et al., Pis’ma v ZhETF 73 (2001) 640. E.D. Bauer et al., J. Phys.: Condens. Mat. 13 (2001) 4495. A.S. Krivoshchekov et al., Physica B 359–361 (2005) 178. A.E. Karkin et al., Physica B 359–361 (2005) 160.
693