- Email: [email protected]
Quasiparticle excitations in newly discovered antiperovskite superconductor ZnNNi3
Physica C 470 (2010) S705–S706
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Quasiparticle excitations in newly discovered antiperovskite superconductor ZnNNi3 Kazuki Ohishi a,*, Takashi U. Ito b, Wataru Higemoto b, Takahiro Yamazaki c, Akira Uehara c, Katsuya Kozawa c, Yoshihide Kimishima c, Masatomo Uehara c a b c
Advanced Meson Science Laboratory, Nishina Center for Accelerator-Based Science, RIKEN, Wako 351-0198, Japan Advanced Science Research Center, Japan Atomic Energy Agency, Ibaraki 319-1195, Japan Department of Physics, Faculty of Engineering, Yokohama National University, Yokohama 240-8501, Japan
a r t i c l e
i n f o
Article history: Accepted 24 October 2009 Available online 28 October 2009 Keywords: ZnNNi3 Superconductivity Penetration depth lSR
a b s t r a c t We report on transverse field muon spin rotation measurements of the superconducting penetration depth k in newly discovered antiperovskite superconductor ZnNNi3 in order to investigate the symmetry of order parameter. The penetration depth at T = 0 K is estimated to be k(0) = 362(2) nm. Temperature dependence of muon spin relaxation rate rv shows T2 dependence, suggesting unconventional superconductor. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of superconductivity in MgCNi3 with Tc 8 K by He et al. [1], antiperovskite superconductors has attracted much interest because it is considered that the ferromagnetic correlation is associated with the superconductivity in MgCNi3. Band structure calculations point out the presence of a large peak in the density of states, located just below the Fermi surface, leading to a predictions of a quantum phase transition to a ferromagnetic ground state with hole doping [2,3]. Actually, the emergence of ferromagnetism has been observed in carbon-deficient (Mg, Zn)CyNi3 (y < 0.7) [4]. It is considered that the magnetic interactions in superconducting systems have often manifested themselves in anisotropic pairing, i.e., high-Tc cuprates and heavy fermion systems. Experimental efforts to reveal the superconducting-gap symmetry have been carried out [5–9], but a consensus on the gap symmetry and the origin of superconductivity has not been obtained. Recently, Uehara et al. have succeeded to discover a new antiperovskite-type superconductor ZnNyNi3 which is the first antiperovskite nitride superconductor [10]. It is reported that ZnNyNi3 falls into the bulk superconducting state below Tc ’ 3 K, as confirmed by a jump of specific heat as well as large diamagnetism due to the Meissner effect associated with the occurrence of zero resistivity. The DC magnetization curve indicates that the super-
* Corresponding author. E-mail address: [email protected] (K. Ohishi). 0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2009.10.084
conductivity is of type-II with the lower critical field Hc1 = 6.9 mT and the upper critical field Hc2 0.96 T. In this paper, we report on the quasiparticle excitations in the flux-line lattice (FLL) state of ZnNyNi3 studied by muon spin rotation (lSR). The magnetic penetration depth k, which reflects the population of quasiparticles in the superconducting state, is determined microscopically by measuring the muon spin relaxation due to the spacial inhomogeneity of magnetic induction in the FLL. In our preliminary analysis, the temperature dependence of muon spin relaxation rate rv / 1/k2 deviates from that based on the BCS superconductors with an isotropic gap. 2. Experimental The polycrystalline samples of ZnNNi3 used in this experiment were synthesized in NH3 gas atmosphere. Details of synthesis method are given in Ref. [10]. The inset of Fig. 1 shows the temperature dependence of susceptibility measured at H = 2 mT after zero filed cooling and field cooling for the present sample. The superconducting transition temperature Tc was estimated to be ’2.6 K. The lSR experiments down to 20 mK were performed at the M15 beamline at TRIUMF, Vancouver, Canada. The lSR spectra were described by a two-component function, which corresponds to muons stopping in the sample and silver sample holder, respectively:
2 2 b ¼ A exp r t exp ðixt þ /Þ þ AAg exp ixAg t þ / ; A0 PðtÞ 2
ð1Þ
S706
K. Ohishi et al. / Physica C 470 (2010) S705–S706
rv ðTÞ ¼ rv ð0Þ½1 ðT=T c Þ4 :
0.8 ZnNNi3
0.7
The fitting analysis by the same formula with an arbitrary power,
TF- SR H = 50 mT
n=4
n = 2.2(1)
0.6
ð3Þ
rv ðTÞ ¼ rv ð0Þ½1 ðT=T c Þn ;
ð4Þ 1
-1
[ s ]
0.5
v
0.4 0
-1
:w
0.2
FC [emu/mol]
0.3
-2
0.1
ZFC H = 2 mT
-3 1.5
2.0
0.0 0.0
0.5
2.5 T [K]
1.0
3.0
3.5
1.5 T [K]
2.0
2.5
3.0
Fig. 1. Temperature dependence of muon spin relaxation rate originated from the flux-line lattice state in ZnNNi3 at H = 50 mT. The solid curve shows a result of fitting by a relation rv / 1/k2 / 1 (T/Tc)n with n and Tc being free parameters. Dotted curve are obtained when n = 4. Inset shows the temperature dependence of magnetic susceptibility at H = 2 mT.
where A0 (=A + AAg) is the total positron decay asymmetry, A and AAg, x and xAg are the partial asymmetries and central frequencies for ZnNNi3 and the Ag sample holder, respectively. r is the muon spin depolarization rate and / is the initial phase. A Gaussian relaxation function gives a good description of the lineshape for polycrystalline samples. We define rn to reflect the spin relaxation in the normal state where it is dominated by the nuclear magnetic moments, and in the superconducting state r2 ¼ r2n þ r2v with rv being due to the formation of FLL. The parameter rn was evaluated by fitting the time spectra above Tc, yielding the temperature-averaged rn = 0.134(2) ls1 at H = 50 mT. rv is related to magnetic penetration depth k as follows [11]: 4
2 1=2
2
rv ¼ 4:83 10 ð1 hÞ½1 þ 3:9ð1 hÞ =k ;
with Tc as a free parameter yields rv(0) = 0.74(1) ls , Tc = 2.3(1) K and n = 2.2(1). The result is shown by solid curve in Fig. 1. Following the relation of Eq. (2), we estimate the penetration depth at 0 K to be k(0) = 362(2) nm, which is consistent with that estimated from Hc1 [10]. As shown in Fig. 1, it is apparent that the temperature dependence of rv is different from that of conventional BCS superconductors (n = 4, dotted curve). Temperature dependence with n = 2 is expected in the superconductors having line nodes with impurities scattering or nonlocal effect. Considering the strongly suppressed specific heat jump and the deviation from exponential temperature dependence of electronic specific heat [10], it suggests that ZnNNi3 is unconventional superconductor. On the other hand, it is noted that the superconducting transition in this sample is broad DTc1K as shown in the inset of Fig. 1. The power of n in this model depends on the gradient of jdrv/dTj just below Tc, i.e., the gradient for n = 4 is larger than that for n = 2.2 as seen in the figure. Therefore, we cannot exclude the possibility that such a broad transition reduces the change in rv, resulting in seemingly-reduction of jdrv/dTj. 4. Conclusions In conclusion, we have performed TF-lSR experiments as a function of temperature on a polycrystalline sample of ZnNNi3. Muon spin relaxation rate rv due to FLL state give k(0) = 362(2) nm. Empirical fit suggests T2 dependence of penetration depth. Acknowledgement We would like to thank the staff of TRIUMF for their technical support during the experiments.
ð2Þ
where h = H/Hc2. 3. Results and discussion Temperature dependence of rv at H = 50 mT is shown in Fig. 1. rv increases with decreasing temperature below 2.3 K due to the formation of the FLL. This temperature is slightly lower than that observed by susceptibility measurements. According to the empirical two-fluid model approximately valid for conventional BCS superconductors, we have
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
T. He et al., Nature 411 (2001) 54. J.H. Shin et al., Phys. Rev. B 64 (2001) 180510. R. H. Rosner et al., Phys. Rev. Lett. 88 (2002) 027001. M. Uehara et al., J. Phys. Chem. Solids 68 (2007) 2178. P.M. Singer et al., Phys. Rev. Lett. 87 (2001) 257601. R. Prozorov et al., Phys. Rev. B 68 (2003) 180502. Z.Q. Mao et al., Phys. Rev. B 67 (2003) 094502. J.-Y. Lin et al., Phys. Rev. B 67 (2003) 052501. G.J. MacDougall et al., Physica B 374–375 (2006) 263. M. Uehara et al., J. Phys. Soc. Jpn. 78 (2009) 033702. E.H. Brandt, Phys. Rev. B 37 (1988) 2349.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Quasiparticle excitations in newly discovered antiperovskite superconductor ZnNNi3 Kazuki Ohishi a,*, Takashi U. Ito b, Wataru Higemoto b, Takahiro Yamazaki c, Akira Uehara c, Katsuya Kozawa c, Yoshihide Kimishima c, Masatomo Uehara c a b c
Advanced Meson Science Laboratory, Nishina Center for Accelerator-Based Science, RIKEN, Wako 351-0198, Japan Advanced Science Research Center, Japan Atomic Energy Agency, Ibaraki 319-1195, Japan Department of Physics, Faculty of Engineering, Yokohama National University, Yokohama 240-8501, Japan
a r t i c l e
i n f o
Article history: Accepted 24 October 2009 Available online 28 October 2009 Keywords: ZnNNi3 Superconductivity Penetration depth lSR
a b s t r a c t We report on transverse field muon spin rotation measurements of the superconducting penetration depth k in newly discovered antiperovskite superconductor ZnNNi3 in order to investigate the symmetry of order parameter. The penetration depth at T = 0 K is estimated to be k(0) = 362(2) nm. Temperature dependence of muon spin relaxation rate rv shows T2 dependence, suggesting unconventional superconductor. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of superconductivity in MgCNi3 with Tc 8 K by He et al. [1], antiperovskite superconductors has attracted much interest because it is considered that the ferromagnetic correlation is associated with the superconductivity in MgCNi3. Band structure calculations point out the presence of a large peak in the density of states, located just below the Fermi surface, leading to a predictions of a quantum phase transition to a ferromagnetic ground state with hole doping [2,3]. Actually, the emergence of ferromagnetism has been observed in carbon-deficient (Mg, Zn)CyNi3 (y < 0.7) [4]. It is considered that the magnetic interactions in superconducting systems have often manifested themselves in anisotropic pairing, i.e., high-Tc cuprates and heavy fermion systems. Experimental efforts to reveal the superconducting-gap symmetry have been carried out [5–9], but a consensus on the gap symmetry and the origin of superconductivity has not been obtained. Recently, Uehara et al. have succeeded to discover a new antiperovskite-type superconductor ZnNyNi3 which is the first antiperovskite nitride superconductor [10]. It is reported that ZnNyNi3 falls into the bulk superconducting state below Tc ’ 3 K, as confirmed by a jump of specific heat as well as large diamagnetism due to the Meissner effect associated with the occurrence of zero resistivity. The DC magnetization curve indicates that the super-
* Corresponding author. E-mail address: [email protected] (K. Ohishi). 0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2009.10.084
conductivity is of type-II with the lower critical field Hc1 = 6.9 mT and the upper critical field Hc2 0.96 T. In this paper, we report on the quasiparticle excitations in the flux-line lattice (FLL) state of ZnNyNi3 studied by muon spin rotation (lSR). The magnetic penetration depth k, which reflects the population of quasiparticles in the superconducting state, is determined microscopically by measuring the muon spin relaxation due to the spacial inhomogeneity of magnetic induction in the FLL. In our preliminary analysis, the temperature dependence of muon spin relaxation rate rv / 1/k2 deviates from that based on the BCS superconductors with an isotropic gap. 2. Experimental The polycrystalline samples of ZnNNi3 used in this experiment were synthesized in NH3 gas atmosphere. Details of synthesis method are given in Ref. [10]. The inset of Fig. 1 shows the temperature dependence of susceptibility measured at H = 2 mT after zero filed cooling and field cooling for the present sample. The superconducting transition temperature Tc was estimated to be ’2.6 K. The lSR experiments down to 20 mK were performed at the M15 beamline at TRIUMF, Vancouver, Canada. The lSR spectra were described by a two-component function, which corresponds to muons stopping in the sample and silver sample holder, respectively:
2 2 b ¼ A exp r t exp ðixt þ /Þ þ AAg exp ixAg t þ / ; A0 PðtÞ 2
ð1Þ
S706
K. Ohishi et al. / Physica C 470 (2010) S705–S706
rv ðTÞ ¼ rv ð0Þ½1 ðT=T c Þ4 :
0.8 ZnNNi3
0.7
The fitting analysis by the same formula with an arbitrary power,
TF- SR H = 50 mT
n=4
n = 2.2(1)
0.6
ð3Þ
rv ðTÞ ¼ rv ð0Þ½1 ðT=T c Þn ;
ð4Þ 1
-1
[ s ]
0.5
v
0.4 0
-1
:w
0.2
FC [emu/mol]
0.3
-2
0.1
ZFC H = 2 mT
-3 1.5
2.0
0.0 0.0
0.5
2.5 T [K]
1.0
3.0
3.5
1.5 T [K]
2.0
2.5
3.0
Fig. 1. Temperature dependence of muon spin relaxation rate originated from the flux-line lattice state in ZnNNi3 at H = 50 mT. The solid curve shows a result of fitting by a relation rv / 1/k2 / 1 (T/Tc)n with n and Tc being free parameters. Dotted curve are obtained when n = 4. Inset shows the temperature dependence of magnetic susceptibility at H = 2 mT.
where A0 (=A + AAg) is the total positron decay asymmetry, A and AAg, x and xAg are the partial asymmetries and central frequencies for ZnNNi3 and the Ag sample holder, respectively. r is the muon spin depolarization rate and / is the initial phase. A Gaussian relaxation function gives a good description of the lineshape for polycrystalline samples. We define rn to reflect the spin relaxation in the normal state where it is dominated by the nuclear magnetic moments, and in the superconducting state r2 ¼ r2n þ r2v with rv being due to the formation of FLL. The parameter rn was evaluated by fitting the time spectra above Tc, yielding the temperature-averaged rn = 0.134(2) ls1 at H = 50 mT. rv is related to magnetic penetration depth k as follows [11]: 4
2 1=2
2
rv ¼ 4:83 10 ð1 hÞ½1 þ 3:9ð1 hÞ =k ;
with Tc as a free parameter yields rv(0) = 0.74(1) ls , Tc = 2.3(1) K and n = 2.2(1). The result is shown by solid curve in Fig. 1. Following the relation of Eq. (2), we estimate the penetration depth at 0 K to be k(0) = 362(2) nm, which is consistent with that estimated from Hc1 [10]. As shown in Fig. 1, it is apparent that the temperature dependence of rv is different from that of conventional BCS superconductors (n = 4, dotted curve). Temperature dependence with n = 2 is expected in the superconductors having line nodes with impurities scattering or nonlocal effect. Considering the strongly suppressed specific heat jump and the deviation from exponential temperature dependence of electronic specific heat [10], it suggests that ZnNNi3 is unconventional superconductor. On the other hand, it is noted that the superconducting transition in this sample is broad DTc1K as shown in the inset of Fig. 1. The power of n in this model depends on the gradient of jdrv/dTj just below Tc, i.e., the gradient for n = 4 is larger than that for n = 2.2 as seen in the figure. Therefore, we cannot exclude the possibility that such a broad transition reduces the change in rv, resulting in seemingly-reduction of jdrv/dTj. 4. Conclusions In conclusion, we have performed TF-lSR experiments as a function of temperature on a polycrystalline sample of ZnNNi3. Muon spin relaxation rate rv due to FLL state give k(0) = 362(2) nm. Empirical fit suggests T2 dependence of penetration depth. Acknowledgement We would like to thank the staff of TRIUMF for their technical support during the experiments.
ð2Þ
where h = H/Hc2. 3. Results and discussion Temperature dependence of rv at H = 50 mT is shown in Fig. 1. rv increases with decreasing temperature below 2.3 K due to the formation of the FLL. This temperature is slightly lower than that observed by susceptibility measurements. According to the empirical two-fluid model approximately valid for conventional BCS superconductors, we have
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
T. He et al., Nature 411 (2001) 54. J.H. Shin et al., Phys. Rev. B 64 (2001) 180510. R. H. Rosner et al., Phys. Rev. Lett. 88 (2002) 027001. M. Uehara et al., J. Phys. Chem. Solids 68 (2007) 2178. P.M. Singer et al., Phys. Rev. Lett. 87 (2001) 257601. R. Prozorov et al., Phys. Rev. B 68 (2003) 180502. Z.Q. Mao et al., Phys. Rev. B 67 (2003) 094502. J.-Y. Lin et al., Phys. Rev. B 67 (2003) 052501. G.J. MacDougall et al., Physica B 374–375 (2006) 263. M. Uehara et al., J. Phys. Soc. Jpn. 78 (2009) 033702. E.H. Brandt, Phys. Rev. B 37 (1988) 2349.