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Tunneling spectroscopy of layered nitride superconductors MNCl0.7 (M = Hf and Zr)
Physica C 445–448 (2006) 77–79 www.elsevier.com/locate/physc
Tunneling spectroscopy of layered nitride superconductors MNCl0.7 (M = Hf and Zr) T. Takasaki a, T. Ekino b
b,*
, S. Yamanaka
c
a Graduate School of Advanced Sciences of Matter, Hiroshima University, Higashi-Hiroshima 739-8530, Japan Faculty of Integrated Arts and Sciences, Hiroshima University, 1-7-1 Kagamiyama, Higashi-Hiroshima 739-8521, Japan c Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan
Available online 15 May 2006
Abstract We have carried out tunneling measurements for MNCl0.7 (M = Hf and Zr) with Tc 24 K and 14 K, respectively, using breakjunction technique. Observed gap structures can be reproduced by the superconductor–insulator–superconductor (SIS) conductance with the BCS density of states. The estimated gap ratio 2D/kBTc P 5 for both compounds is turned out to be larger than that of the conventional superconductors. Ó 2006 Elsevier B.V. All rights reserved. PACS: 74.50.+r; 74.70. b; 74.70.Dd; 74.25.Jb Keywords: Tunneling spectroscopy; Superconducting gap; HfNCl; ZrNCl
1. Introduction The electron-doped layered nitride compound b-HfNCl (HfNCl0.7) is known to exhibit superconductivity with its highest Tc = 24 K [1]. This value is nearly the same as Tc = 25.5 K of Li0.48(THF)yHfNCl [2], and higher than that of the intermetallic compounds except for MgB2 [3]. Photoemission [4], X-ray absorption [4], lSR [5], and NMR studies [6] clarified that the electronic conduction of electron-doped b-HfNCl is strongly two dimensional and coming from metal nitride double honeycomb layers [6], which is consistent with the band calculation [7]. For Li0.48(THF)yHfNCl, the superconducting properties have been investigated by several groups [6,8,9]. The large gap ratio 2D/kBTc P 5 [8], quite small isotope effect [6] and a large electron–phonon coupling constant k > 3 [9] in terms of conventional McMillan’s prediction were reported. Here, we report on the tunneling measurement of MNCl0.7
*
Corresponding author. Tel.: +81 82 424 6552; fax: +81 82 424 0757. E-mail address: [email protected] (T. Ekino).
0921-4534/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2006.03.082
(M = Hf and Zr) to clarify the nature of superconducting gap. Polycrystalline samples showing Tc 24 K (M = Hf) and 14 K (Zr) were used with break-junction technique [1,10]. In this technique, the superconductor–insulator– superconductor (SIS) junction is formed, and the peakto-peak separation ( Vp–p) corresponds to 4D/e (2D: superconducting gap). 2. Results and discussion Fig. 1 shows the tunneling conductance of HfNCl0.7 at 4.2 K. From the measurements of 15 junctions, the gap values can be classified into those shown in Fig. 1(a) and (b). In Fig. 1(a), the sharp gap-edge structures are observed at ±8 mV concomitant with Josephson peak at zero bias and small in-gap peak structures at ±2–3 mV. The spectrum can be described by the SIS tunneling conductance with the broadened BCS density of states [11,12]. The gap value is estimated to be D = 3.7–3.9 meV with quite small broadening parameter c = 0.1–0.2 meV. This gap size is slightly smaller than the typical gap value of Li0.48(THF)yHfNCl with Tc = 25.5 K [8]. The small peaks around ±2–3 mV
78
T. Takasaki et al. / Physica C 445–448 (2006) 77–79
2.0
Δ = 3.7 meV γ = 0.2 meV
1.0
10
dI/dV (μS)
dI/dV (mS)
12
HfNCl0.7 T = 4.2 K
1.5
(a)
14
(a)
ZrNCl0.7 T = 4.2 K
8 6 4
0.5
Δ = 4.0 meV γ = 0.3 meV
2 0
0 120
HfNCl0.7 T = 4.2 K
0.2
dI/dV (mS)
100
dI/dV (μS)
(b)
(b)
80 60
ZrNCl0.7 T = 4.2 K
0.1
40 Δ = 5.0 meV γ = 0.5 meV
20 0 -20
-10
0
10
Δ = 2.3 meV γ = 0.1 meV
0 -20
20
-10
Voltage (mV)
could be primarily due to n-particle tunneling, but they deserve further investigation [12]. In Fig. 1(b), the gap edge peaks at ±10–11 mV occur with moderate strength. The broadness of this gap is quite similar to that of the typical gap structures of Li0.48(THF)yHfNCl [8]. This gap also has the zero-bias peak and a pair of small peaks in the low bias region like the spectrum in (a). The gap parameter can be estimated to be D = 5 meV, which is larger than that of Fig. 1(a). Since electron doping process could cause the carrier inhomogeneity [1], the superconducting gap might be distributed like in this case, which is similar to that of high-Tc cuprates [13]. We also have carried out tunneling measurements on ZrNCl0.7 with Tc 14 K to clarify how the decrease in Tc affects the gap size in MNCl0.7 [14]. As shown in Fig. 2, two kinds of gap structures with Vp–p = 16 mV and 7 mV are observed in 20 junctions. The Vp–p = 16 mV of ZrNCl0.7 in Fig. 2(a) is comparable to that of HfNCl0.7 in Fig. 1 in spite of the different Tc between two compounds. Fig. 3 shows the temperature variations of the tunnel conductance for HfNCl0.7. The value of Vp–p at 4.2 K is estimated to be 16 mV. We can see the inner peak structures at ±2–3 mV, which will be discussed in the subsequent report after more detailed investigation. The conductance slightly changes at 12 K. With further increas-
10
20
Fig. 2. Tunneling conductance of two different break junctions for ZrNCl0.7 at 4.2 K. (a) the large gap, (b) the small gap [14]. The broken curve is the calculated SIS conductance with BCS density of states.
23.5 18.4
dI/dV (mS)
Fig. 1. Tunneling conductance of two different break junctions (a) and (b) for HfNCl0.7 at 4.2 K. The broken curve is the calculated SIS conductance with BCS density of states.
0
Voltage (mV)
12.3
7.9
0.4 4.2
0.2
T (K)
HfNCl0.7
0.0 -20
-10
0
10
20
Voltage (mV) Fig. 3. Temperature variations of the tunneling conductance for HfNCl0.7.
ing the temperature above 12 K, the gap structure becomes gradually broad, and it disappears around Tc. Fig. 4 shows the temperature dependence of the gap value for HfNCl0.7 taken from Fig. 3. The error bar at 4.2 K corresponds to the gap distribution from 15 junctions. The gap value seems to become zero at around Tc. The temperature dependence of gap value from the BCS
T. Takasaki et al. / Physica C 445–448 (2006) 77–79
According to the band calculation [7], it is suggested that the different gap ratio 2D/kBTc between HfNCl and ZrNCl0.7 could be connected with the different electronic structures, namely, the former consists of only one Hf–N band, while the latter includes Zr–N and Zr–Zr bands [7]. In such case, the different carrier concentration would occur between Hf–N and Zr–N bands even at the same x in MNCl1 x.
HfNCl0.7
20
15
Vp-p (mV)
79
BCS
10
3. Conclusion 5
0 0
5
10 15 20 Temperature (K)
25
30
Fig. 4. Temperature dependence of Vp–p (= 4D/e) for the HfNCl0.7 break junction as shown in Fig. 3. Open square at 4.2 K is the average gap value from 15 junctions, and the error bar corresponds to the gap distribution. The broken line represents the BCS prediction.
prediction (2D(0 K)/kBTc = 3.53, Tc = 23.5 K) is given by the broken curve. Although observed gap values are larger than the BCS one, overall gap decreasing feature is similar to the BCS curve. By taking the gap disappearing temperature as Tc, the observed gap values lead to the gap ratio 2D(0 K)/kBTc = 4–5, which seems to exceed the conventional strong-coupling one [12]. From our measurements for 40 junctions of HfNCl0.7 and Li0.48(THF)yHfNCl, the ratio 2D(0 K)/kBTc 5 is commonly obtained, confirming the strong-coupling superconductivity for both the HfNCl compounds. For ZrNCl0.7, both larger and smaller gaps shown in Fig. 2 disappear at bulk Tc [14]. Then, the gap ratios are estimated to be 2D(0 K)/kBTc = 6–8 and 3–4, respectively. The former value is comparable or even larger than the our results of HfNCl0.7 and Li0.48(THF)yHfNCl [8], while the latter one is similar to the BCS ratio [12]. It should be noted that the gap ratio 2D/kBTc P 5 obtained for both MNCl0.7 (M = Hf and Zr) exceeds that of the conventional superconductors. The gap distribution and the large gap structures in these layered nitride superconductors can be connected with several origins. Since the electrical conductivity is produced by the electron doping from the semiconducting states, mesoscopic inhomogeneity causing the locally different superconducting properties may be easily induced. Actually, in high-Tc cuprates, granular superconducting domains are formed by oxygen annealing process, which results in observed scattered gap values [13]. For polycrystalline samples, each grain should influence neighboring regions through the proximity effect, and it may cause the characteristic dip-hump structures for the proximityinduced gaps [15]. Expected strong gap anisotropy caused by the highly two dimensional electronic structures is also a possible origin of the gap distribution [4–7].
We have carried out tunneling measurements of layered nitride superconductors MNCl0.7 (M = Hf and Zr). The gap values of Vp–p = 20 mV and 16 mV for M = Hf and Zr, respectively, lead to the extremely strong-coupling gap ratio 2D/kBTc P 5, which indeed exceeds that of the conventional superconductors. The observed distribution of the gap may be related to either the intrinsic inhomogeneity of local carrier density or the anisotropic gap originating from the layered structure. The more detailed experiments are now in progress to clarify the meaning of this strong-coupling ratio. Acknowledgements This work was supported in part by the Grant-in-Aid for COE research (No. 13CE2002) and for Scientific Research (No. 15540346) of the Ministry of Education, Culture, Sports, Science and Technology of Japan. References [1] L. Zhu, S. Yamanaka, Chem. Mater. 15 (2003) 1897. [2] S. Yamanaka, K. Hotehama, H. Kawaji, Nature 392 (1998) 580. [3] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, J. Akimitsu, Nature 410 (2001) 63. [4] T. Yokoya, Y. Ishiwata, S. Shin, S. Shamoto, K. Iizawa, T. Kajitani, I. Hase, T. Takahashi, Phys. Rev. B 64 (2001) 153107. [5] T. Ito, Y. Fudamoto, A. Fukaya, I.M. Gat-Malureanu, M.I. Larkin, P.L. Russo, A. Savici, Y.J. Uemura, K. Groves, R. Breslow, K. Hotehama, S. Yamanaka, P. Kyriakou, M. Rovers, G.M. Luke, K.M. Kojima, Phys. Rev. B 69 (2004) 134522. [6] H. Tou, Y. Maniwa, S. Yamanaka, Phys. Rev. B 67 (2003) 100509(R). [7] C. Felser, R. Seshadri, J. Matter. Chem 9 (1999) 459. [8] T. Ekino, T. Takasaki, H. Fujii, S. Yamanaka, Physica C 388–389 (2003) 573. [9] Y. Taguchi, M. Hisakabe, Y. Ohishi, S. Yamanaka, Y. Iwasa, Phys. Rev. B 70 (2004) 104506. [10] T. Ekino, T. Takabatake, H. Tanaka, H. Fujii, Phys. Rev. Lett. 75 (1995) 4262. [11] R.C. Dynes, V. Narayanamurti, J.P. Garno, Phys. Rev. Lett. 41 (1978) 1509. [12] E.L. Wolf, in: R.J. Elliot, J.A. Krumhansl, W. Marshall, D.H. Wilkinson (Eds.), Principles of Electron Tunneling Spectroscopy, Oxford University Press, New York, 1985, pp. 132–136, p. 524–530. [13] K.M. Lang, V. Madhavan, J.E. Hoffman, E.W. Hudson, H. Eisaki, S. Uchida, J.C. Davis, Nature 415 (2002) 412. [14] T. Takasaki, T. Ekino, H. Fujii, S. Yamanaka, J. Phys. Soc. Jpn. 74 (2005) 2586. [15] T. Ekino, T. Takasaki, T. Muranaka, J. Akimitsu, H. Fujii, Phys. Rev. B 67 (2003) 094504.
Tunneling spectroscopy of layered nitride superconductors MNCl0.7 (M = Hf and Zr) T. Takasaki a, T. Ekino b
b,*
, S. Yamanaka
c
a Graduate School of Advanced Sciences of Matter, Hiroshima University, Higashi-Hiroshima 739-8530, Japan Faculty of Integrated Arts and Sciences, Hiroshima University, 1-7-1 Kagamiyama, Higashi-Hiroshima 739-8521, Japan c Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan
Available online 15 May 2006
Abstract We have carried out tunneling measurements for MNCl0.7 (M = Hf and Zr) with Tc 24 K and 14 K, respectively, using breakjunction technique. Observed gap structures can be reproduced by the superconductor–insulator–superconductor (SIS) conductance with the BCS density of states. The estimated gap ratio 2D/kBTc P 5 for both compounds is turned out to be larger than that of the conventional superconductors. Ó 2006 Elsevier B.V. All rights reserved. PACS: 74.50.+r; 74.70. b; 74.70.Dd; 74.25.Jb Keywords: Tunneling spectroscopy; Superconducting gap; HfNCl; ZrNCl
1. Introduction The electron-doped layered nitride compound b-HfNCl (HfNCl0.7) is known to exhibit superconductivity with its highest Tc = 24 K [1]. This value is nearly the same as Tc = 25.5 K of Li0.48(THF)yHfNCl [2], and higher than that of the intermetallic compounds except for MgB2 [3]. Photoemission [4], X-ray absorption [4], lSR [5], and NMR studies [6] clarified that the electronic conduction of electron-doped b-HfNCl is strongly two dimensional and coming from metal nitride double honeycomb layers [6], which is consistent with the band calculation [7]. For Li0.48(THF)yHfNCl, the superconducting properties have been investigated by several groups [6,8,9]. The large gap ratio 2D/kBTc P 5 [8], quite small isotope effect [6] and a large electron–phonon coupling constant k > 3 [9] in terms of conventional McMillan’s prediction were reported. Here, we report on the tunneling measurement of MNCl0.7
*
Corresponding author. Tel.: +81 82 424 6552; fax: +81 82 424 0757. E-mail address: [email protected] (T. Ekino).
0921-4534/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2006.03.082
(M = Hf and Zr) to clarify the nature of superconducting gap. Polycrystalline samples showing Tc 24 K (M = Hf) and 14 K (Zr) were used with break-junction technique [1,10]. In this technique, the superconductor–insulator– superconductor (SIS) junction is formed, and the peakto-peak separation ( Vp–p) corresponds to 4D/e (2D: superconducting gap). 2. Results and discussion Fig. 1 shows the tunneling conductance of HfNCl0.7 at 4.2 K. From the measurements of 15 junctions, the gap values can be classified into those shown in Fig. 1(a) and (b). In Fig. 1(a), the sharp gap-edge structures are observed at ±8 mV concomitant with Josephson peak at zero bias and small in-gap peak structures at ±2–3 mV. The spectrum can be described by the SIS tunneling conductance with the broadened BCS density of states [11,12]. The gap value is estimated to be D = 3.7–3.9 meV with quite small broadening parameter c = 0.1–0.2 meV. This gap size is slightly smaller than the typical gap value of Li0.48(THF)yHfNCl with Tc = 25.5 K [8]. The small peaks around ±2–3 mV
78
T. Takasaki et al. / Physica C 445–448 (2006) 77–79
2.0
Δ = 3.7 meV γ = 0.2 meV
1.0
10
dI/dV (μS)
dI/dV (mS)
12
HfNCl0.7 T = 4.2 K
1.5
(a)
14
(a)
ZrNCl0.7 T = 4.2 K
8 6 4
0.5
Δ = 4.0 meV γ = 0.3 meV
2 0
0 120
HfNCl0.7 T = 4.2 K
0.2
dI/dV (mS)
100
dI/dV (μS)
(b)
(b)
80 60
ZrNCl0.7 T = 4.2 K
0.1
40 Δ = 5.0 meV γ = 0.5 meV
20 0 -20
-10
0
10
Δ = 2.3 meV γ = 0.1 meV
0 -20
20
-10
Voltage (mV)
could be primarily due to n-particle tunneling, but they deserve further investigation [12]. In Fig. 1(b), the gap edge peaks at ±10–11 mV occur with moderate strength. The broadness of this gap is quite similar to that of the typical gap structures of Li0.48(THF)yHfNCl [8]. This gap also has the zero-bias peak and a pair of small peaks in the low bias region like the spectrum in (a). The gap parameter can be estimated to be D = 5 meV, which is larger than that of Fig. 1(a). Since electron doping process could cause the carrier inhomogeneity [1], the superconducting gap might be distributed like in this case, which is similar to that of high-Tc cuprates [13]. We also have carried out tunneling measurements on ZrNCl0.7 with Tc 14 K to clarify how the decrease in Tc affects the gap size in MNCl0.7 [14]. As shown in Fig. 2, two kinds of gap structures with Vp–p = 16 mV and 7 mV are observed in 20 junctions. The Vp–p = 16 mV of ZrNCl0.7 in Fig. 2(a) is comparable to that of HfNCl0.7 in Fig. 1 in spite of the different Tc between two compounds. Fig. 3 shows the temperature variations of the tunnel conductance for HfNCl0.7. The value of Vp–p at 4.2 K is estimated to be 16 mV. We can see the inner peak structures at ±2–3 mV, which will be discussed in the subsequent report after more detailed investigation. The conductance slightly changes at 12 K. With further increas-
10
20
Fig. 2. Tunneling conductance of two different break junctions for ZrNCl0.7 at 4.2 K. (a) the large gap, (b) the small gap [14]. The broken curve is the calculated SIS conductance with BCS density of states.
23.5 18.4
dI/dV (mS)
Fig. 1. Tunneling conductance of two different break junctions (a) and (b) for HfNCl0.7 at 4.2 K. The broken curve is the calculated SIS conductance with BCS density of states.
0
Voltage (mV)
12.3
7.9
0.4 4.2
0.2
T (K)
HfNCl0.7
0.0 -20
-10
0
10
20
Voltage (mV) Fig. 3. Temperature variations of the tunneling conductance for HfNCl0.7.
ing the temperature above 12 K, the gap structure becomes gradually broad, and it disappears around Tc. Fig. 4 shows the temperature dependence of the gap value for HfNCl0.7 taken from Fig. 3. The error bar at 4.2 K corresponds to the gap distribution from 15 junctions. The gap value seems to become zero at around Tc. The temperature dependence of gap value from the BCS
T. Takasaki et al. / Physica C 445–448 (2006) 77–79
According to the band calculation [7], it is suggested that the different gap ratio 2D/kBTc between HfNCl and ZrNCl0.7 could be connected with the different electronic structures, namely, the former consists of only one Hf–N band, while the latter includes Zr–N and Zr–Zr bands [7]. In such case, the different carrier concentration would occur between Hf–N and Zr–N bands even at the same x in MNCl1 x.
HfNCl0.7
20
15
Vp-p (mV)
79
BCS
10
3. Conclusion 5
0 0
5
10 15 20 Temperature (K)
25
30
Fig. 4. Temperature dependence of Vp–p (= 4D/e) for the HfNCl0.7 break junction as shown in Fig. 3. Open square at 4.2 K is the average gap value from 15 junctions, and the error bar corresponds to the gap distribution. The broken line represents the BCS prediction.
prediction (2D(0 K)/kBTc = 3.53, Tc = 23.5 K) is given by the broken curve. Although observed gap values are larger than the BCS one, overall gap decreasing feature is similar to the BCS curve. By taking the gap disappearing temperature as Tc, the observed gap values lead to the gap ratio 2D(0 K)/kBTc = 4–5, which seems to exceed the conventional strong-coupling one [12]. From our measurements for 40 junctions of HfNCl0.7 and Li0.48(THF)yHfNCl, the ratio 2D(0 K)/kBTc 5 is commonly obtained, confirming the strong-coupling superconductivity for both the HfNCl compounds. For ZrNCl0.7, both larger and smaller gaps shown in Fig. 2 disappear at bulk Tc [14]. Then, the gap ratios are estimated to be 2D(0 K)/kBTc = 6–8 and 3–4, respectively. The former value is comparable or even larger than the our results of HfNCl0.7 and Li0.48(THF)yHfNCl [8], while the latter one is similar to the BCS ratio [12]. It should be noted that the gap ratio 2D/kBTc P 5 obtained for both MNCl0.7 (M = Hf and Zr) exceeds that of the conventional superconductors. The gap distribution and the large gap structures in these layered nitride superconductors can be connected with several origins. Since the electrical conductivity is produced by the electron doping from the semiconducting states, mesoscopic inhomogeneity causing the locally different superconducting properties may be easily induced. Actually, in high-Tc cuprates, granular superconducting domains are formed by oxygen annealing process, which results in observed scattered gap values [13]. For polycrystalline samples, each grain should influence neighboring regions through the proximity effect, and it may cause the characteristic dip-hump structures for the proximityinduced gaps [15]. Expected strong gap anisotropy caused by the highly two dimensional electronic structures is also a possible origin of the gap distribution [4–7].
We have carried out tunneling measurements of layered nitride superconductors MNCl0.7 (M = Hf and Zr). The gap values of Vp–p = 20 mV and 16 mV for M = Hf and Zr, respectively, lead to the extremely strong-coupling gap ratio 2D/kBTc P 5, which indeed exceeds that of the conventional superconductors. The observed distribution of the gap may be related to either the intrinsic inhomogeneity of local carrier density or the anisotropic gap originating from the layered structure. The more detailed experiments are now in progress to clarify the meaning of this strong-coupling ratio. Acknowledgements This work was supported in part by the Grant-in-Aid for COE research (No. 13CE2002) and for Scientific Research (No. 15540346) of the Ministry of Education, Culture, Sports, Science and Technology of Japan. References [1] L. Zhu, S. Yamanaka, Chem. Mater. 15 (2003) 1897. [2] S. Yamanaka, K. Hotehama, H. Kawaji, Nature 392 (1998) 580. [3] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, J. Akimitsu, Nature 410 (2001) 63. [4] T. Yokoya, Y. Ishiwata, S. Shin, S. Shamoto, K. Iizawa, T. Kajitani, I. Hase, T. Takahashi, Phys. Rev. B 64 (2001) 153107. [5] T. Ito, Y. Fudamoto, A. Fukaya, I.M. Gat-Malureanu, M.I. Larkin, P.L. Russo, A. Savici, Y.J. Uemura, K. Groves, R. Breslow, K. Hotehama, S. Yamanaka, P. Kyriakou, M. Rovers, G.M. Luke, K.M. Kojima, Phys. Rev. B 69 (2004) 134522. [6] H. Tou, Y. Maniwa, S. Yamanaka, Phys. Rev. B 67 (2003) 100509(R). [7] C. Felser, R. Seshadri, J. Matter. Chem 9 (1999) 459. [8] T. Ekino, T. Takasaki, H. Fujii, S. Yamanaka, Physica C 388–389 (2003) 573. [9] Y. Taguchi, M. Hisakabe, Y. Ohishi, S. Yamanaka, Y. Iwasa, Phys. Rev. B 70 (2004) 104506. [10] T. Ekino, T. Takabatake, H. Tanaka, H. Fujii, Phys. Rev. Lett. 75 (1995) 4262. [11] R.C. Dynes, V. Narayanamurti, J.P. Garno, Phys. Rev. Lett. 41 (1978) 1509. [12] E.L. Wolf, in: R.J. Elliot, J.A. Krumhansl, W. Marshall, D.H. Wilkinson (Eds.), Principles of Electron Tunneling Spectroscopy, Oxford University Press, New York, 1985, pp. 132–136, p. 524–530. [13] K.M. Lang, V. Madhavan, J.E. Hoffman, E.W. Hudson, H. Eisaki, S. Uchida, J.C. Davis, Nature 415 (2002) 412. [14] T. Takasaki, T. Ekino, H. Fujii, S. Yamanaka, J. Phys. Soc. Jpn. 74 (2005) 2586. [15] T. Ekino, T. Takasaki, T. Muranaka, J. Akimitsu, H. Fujii, Phys. Rev. B 67 (2003) 094504.