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Growth of infinite-layer (Sr, Nd)CuO2 films by MBE
Physica C 471 (2011) 185–187
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Growth of infinite-layer (Sr, Nd)CuO2 films by MBE Y. Krockenberger ⇑, H. Yamamoto NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi, Kanagawa 243-0198, Japan
a r t i c l e
i n f o
Article history: Received 8 December 2010 Received in revised form 20 December 2010 Accepted 22 December 2010 Available online 25 December 2010 Keywords: High-Tc superconductivity Phase control Molecular beam epitaxy Infinite layer Cuprate Reduction
a b s t r a c t Single phase, c-axis oriented infinite layer (IL) Sr1xNdxCuO2 thin films were epitaxially grown on (1 1 0) DyScO3 substrates by molecular beam epitaxy (MBE). Electron impact emission spectroscopy (EIES) is the tool of choice for a stringent stoichiometry control which is essential for Sr1xNdxCuO2 thin film preparation. As-grown films contain excess oxygen and therefore a reduction process is necessary to induce superconductivity. In the present study special attention was paid to the underdoped region since the optimization of the reduction process becomes more difficult for lower doping levels x. Sr0.95Nd0.05CuO2 films (underdoped) became superconducting with T onset = 30 K and T zero = 11.4 K after an annealing c c procedure. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction The electron-doped infinite layer (IL) compound Sr1xRExCuO2 (RE = La, Nd, Sm, Eu, Gd and Pr; Tc = 43 K) [1,2] has the simplest crystallographic structure among all of the superconducting cuprates. Stacked CuO2 planes, composed of four-fold coordinated Cu2+ ions, with RE and Sr atoms as inter-plane separators form the crystal. In the ideal structure [3] no apical oxygen is present and there is no charge reservoir layer. The parent compound of this phase is SrCuO2 (x = 0). Prepared at ambient pressure, SrCuO2 has an orthorhombic structure and contains double chains Cu2O2 of square planar groups sharing edges joined through rock-salt type SrO layer [4,5]. In order to stabilize the IL-SrCuO2, high pressure synthesis is required, and hence, no bulk single crystal has been reported so far. However, high quality single crystalline films can be prepared by molecular beam epitaxy (MBE) or pulsed laser deposition (PLD) technique by appropriate choices of substrates and growth conditions. Karimoto et al. [6] have deposited superconducting Sr1xLaxCuO2 thin films by MBE. The Sr1xLaxCuO2 thin films are not only superconducting but even show metallic resistivity behavior. The IL system belongs to the class of electron-doped cuprates. Another representative of this class is the so-called T0 -phase. Both
⇑ Corresponding author. Address: NTT Basic Research Labs., 3-1 MorinosatoWakamiya, Atsugi-shi, Kanagawa 243-0198, Japan. Tel.: +81 46 240 4458; fax: +81 46 240 4729. E-mail address: [email protected] (Y. Krockenberger). 0921-4534/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2010.12.013
are layered copper oxides where copper is square planar coordinated. Typically, the electronic phase diagram is plotted as a function of the carrier concentration. On the electron doped side of the phase diagram the magnetic- and superconducting properties of T0 Nd2xCexCuO4 are shown while on the hole doped side of La2xSrxCuO4 [7]. The phase diagram was originally prepared by Luke et al. [8] for T0 -Nd2xCexCuO4, where the superconducting region is small. Later, a modified phase diagram was published by Brinkmann et al. [9] where an optimized annealing procedure was performed on bulk samples. In that phase diagram the superconducting region is significantly expanded and superconductivity was observed for doping levels as low as x = 0.04. In a recent report by Matsumoto et al. [10] the authors show that, superconductivity appears even in T0 -RE2CuO4 (x = 0.00). Moreover, the superconducting transition temperature increases with decreasing the doping concentration [11]. Obviously, the material properties significantly depend on its annealing history since T0 -Nd2xCexCuO4 is not an insulating antiferromagnet and a superconductor, simultaneously. It is known that the as-grown specimens of the cuprates with square-planar-coordinated copper contain a fair amount of interstitial oxygen. Evacuation of interstitial sites during a post-reduction process – while simultaneously preserving regular oxygen sites occupied – is an exigency to obtain superconductivity. Since this complicated oxygen diffusion process has not been considered in previously prepared bulk samples, a revision of the electronic phase diagram for T0 -RE2-xCexCuO4, especially in the underdoped regime becomes feasible. A similar behavior is expected for the Sr1xRExCuO2 system and therefore the primary focus within the present investigation is the underdoped region of IL-Sr1xNdxCuO2.
2.0
close
open
rate (Å/s)
1.5
0.5
Nd 0.0
0
400
800
(330)DyScO3 (003)
I (arb. units)
3
10
2
20
40
o
60
2θ ( )
80
100
Fig. 2. X-ray diffraction pattern of a Sr0.95Nd0.05CuO2 film grown on (1 1 0) DyScO3 substrate. The film is single-phase and all peaks can be indexed as (0 0 l) reflections of IL phase.
Fig. 3. RHEED pattern of a Sr0.95Nd0.05CuO2 film grown on (1 1 0) DyScO3 substrate along [0 1] direction.
570
25400 20000
o
560
5000
550
100
540 530 520 510
Sr Cu
1.0
4
10
10
Growth temperature ( C)
Epitaxial c-axis oriented thin films of Sr1xNdxCuO2, 900–950 Å thick, were grown in a custom designed ultra-high-vacuum chamber from metal sources using RF-activated atomic oxygen or ozone. Details of our MBE growth technique are described elsewhere [12]. In this study (1 1 0) DyScO3 (DSO) substrates have been also used, only. In a previous study by Karimoto et al. [6] (1 1 0) KTaO3 substrates have been also used for the growth of IL Sr1xLaxCuO2 thin films. However, the surface morphology of (1 1 0) KTaO3 is unstable due to the migration of K2O segregations though the lattice constant is larger (a = 0.3989 nm). The pseudo-cubic lattice constant of DSO is a = 3.944 Å while SrCuO2 has an in-plane lattice constant of a = 3.95 Å. The growth rate was approx. 1.6 Å/s. The present investigation has been carried out by full electron impact emission spectroscopy (EIES) control of the individual metal sources. Nd was used instead of La simply due to its stronger emission lines allowing better control by EIES. A stringent stoichiometry control is inevitable stabilizing the IL phase since, beside the IL phase, Sr2CuO3 and Sr14Cu24O41 phases exist on the copper-poor and –rich side, respectively. In Fig. 1 the flux-rates of Sr, Nd and Cu are plotted as a function of time. A proper choice of evaporant-crucible combinations together with a careful tuning of the electron beam parameters allows a constant co-evaporation throughout the growth. In Fig. 2, a X-ray diffraction pattern of a Sr0.95Nd0.05CuO2 film grown on (1 1 0) DSO is shown. The film is c-axis oriented and its c-axis length is 3.424 Å. Reflection high energy electron diffraction (RHEED) indicates that the film is grown epitaxially (Fig. 3). The diffraction pattern shown in Fig. 2 is a result of an optimization process performed for various growth temperatures (510–570 °C). In Fig. 4, the intensity of the (0 0 2) diffraction peak is plotted as a function of the doping concentration x (0.04 < x < 0.09) and the growth temperature. In total, 16 samples are summarized in Fig. 4 (black dots) where the annealing condition has not been altered (500 °C, 10 min). For low doping concentrations x the optimal growth temperature is lower compared to higher doping levels. At zero doping (SrCuO2), 500 °C was found to be an optimal growth temperature. It should be mentioned, that the phrase ‘‘optimal’’ refers to crystallographic qualities, not superconducting ones. As-grown IL films are subsequently annealed in situ under ultrahigh vacuum conditions (oxygen partial pressure <1010 Torr). The annealing process is a diffusion-driven process where time and temperature are the key parameters. In the IL system, the anneal-
(002)
5
10
(220)DyScO3
2. Experimental, results, and discussion
(001)
Y. Krockenberger, H. Yamamoto / Physica C 471 (2011) 185–187
(110)DyScO3
186
0.04
0.05
0.06
0.07
0.08
0.09
Sr1-xNdxCuO2 Fig. 4. Intensities of the (0 0 2) diffraction peak are plotted as a function of growth temperature and doping concentration. High intensities are shown in red color while low ones are blue. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
1200
time (s) Fig. 1. Flux-rates of individual metal sources controlled by EIES are plotted as a function of time. At the starting point (time = 0 s) the electron-beam power is continuously increased until the source melts (Cu, Nd) or sublimes (Sr). ‘‘Open’’ and ‘‘close’’ indicate the opening and closing of the shutter in front of the substrate. Here, the growth time is 600 s.
ing temperature is typically lower compared to the growth temperature. Annealing temperatures higher than the growth temperatures result in decomposition or phase transformation of the IL phase. Fig. 5 shows the resistivity as a function of temperature plotted for a Sr0.95Nd0.05CuO2 film grown on (1 1 0) DSO. The
Y. Krockenberger, H. Yamamoto / Physica C 471 (2011) 185–187
are simultaneously evacuated becomes a difficult task for lower doping levels x, though our present results indicate that a reassessment of the under- and zero-doped region is highly encouraging in the IL cuprates.
0.4
0.3
ρ (mΩ cm)
187
3. Conclusion
0.2
0.1
0.0
0
50
100
150
200
250
300
T (K) Fig. 5. Plot of resistivity vs. temperature of a 900 Å thick Sr0.95Nd0.05CuO2 film. The superconducting transition temperature is 11.4 K.
film was annealed for 10 min at 500 °C. The resistivity behavior is metallic between room temperature and 170 K, whereas semiconducting behavior is observed between 170 K and the onset of superconductivity around 30 K. A superconducting transition temperature of 11.4 K has been measured. This transition temperature is higher than that observed in previous studies on Sr0.95La0.05CuO2 [6], most likely due to an improved annealing process. Note that, in the underdoped region, the appearance of a minima in the resistivity curve usually reflects the fact that the reduction process has been insufficient to evacuate interstitial oxygen sites completely. Several key-parameters are required to induce superconductivity in IL Sr1xRExCuO2 (underdoped range) by MBE. At first, a singlephase IL structure (high degree of crystallinity) is necessary. For single-phase IL films, (a) precise rate control of constituent cations, (b) stringent control of growth temperature, (c) appropriate choice of substrate [6], are required. Secondly, removal of excess apical oxygen without removing oxygen from the CuO2 planes is a challenge. For this purpose, an (d) appropriate usage of active oxygen during growth as well as a (e) well-designed post-reduction process is required. Especially, the second criterion is a difficult task. The formation of oxygen vacancies in CuO2 layers, resulting in the so-called long-c phase (c = 3.62 Å) [13] is one of the origins responsible for decreased or suppressed superconducting properties. Preservation of regular oxygen sites while interstitial sites
Single-phase IL Sr1xNdxCuO2 thin films have been grown epitaxially on (1 1 0) DSO substrate by MBE. The growth conditions have been optimized in the underdoped regime in order to obtain high quality IL films. Vacuum annealing is required to induce superconductivity. Sr0.95Nd0.05CuO2 films became superconducting after an annealing treatment at 500 °C for 10 min. Further investigations are required in order to optimize the annealing conditions at low doping levels. Acknowledgements The authors acknowledge fruitful discussions with S. Karimoto and lucrative communication with M. Naito. References [1] M.G. Smith, A. Manthiram, J. Zhou, J.B. Goodenough, J.T. Markert, Nature (London) 351 (1991) 549. [2] G. Er, Y. Miyamoto, F. Kanamaru, S. Kikkawa, Physica C 181 (1991) 206. [3] J.D. Jorgensen, P.G. Radaelli, D.G. Hinks, J.L. Wagner, S. Kikkawa, G. Er, F. Kanamaru, Phys. Rev. B 47 (1993) 14654. [4] Chr. L. Teske, Hk. Muller-Buschbaum, Z. Anorg. Allg. Chem. 379 (1970) 234. [5] M.T. Gambardella, B. Domengès, B. Raveau, Mater. Res. Bull. 27 (1992) 629. [6] S. Karimoto, K. Ueda, M. Naito, T. Imai, Appl. Phys. Lett. 79 (2001) 2767; S. Karimoto, K. Ueda, M. Naito, T. Imai, Appl. Phys. Lett. 84 (2004) 2136; S. Karimoto, K. Ueda, M. Naito, T. Imai, Physica C 378–381 (2002) 127. [7] M.B. Maple, Mater. Res. Soc. Bull. 15 (1990) 60. [8] G.M. Luke, L.P. Le, B.J. Sternlieb, Y.J. Uemura, J.H. Brewer, R. Kadono, R.F. Kiefl, S.R. Kreitzman, T.M. Riseman, C.E. Stronach, M.R. Davis, S. Uchida, H. Takagi, Y. Tokura, Y. Hidaka, T. Murakami, J. Gopalakrishnan, A.W. Sleight, M.A. Subramanian, E.A. Early, J.T. Markert, M.B. Maple, C.L. Seaman, Phys. Rev. B 42 (1990) 7981. [9] M. Brinkmann, T. Rex, H. Bach, K. Westerholt, Phys. Rev. Lett. 74 (1995) 4927– 4930; M. Brinkmann, T. Rex, H. Bach, K. Westerholt, Physica C 269 (1996) 76. [10] O. Matsumoto, A. Utsuki, A. Tsukada, H. Yamamoto, T. Manabe, M. Naito, Phys. Rev. B 79 (2009) 100508. R. [11] O. Matsumoto, A. Utsuki, A. Tsukada, H. Yamamoto, T. Manabe, M. Naito, Physica C 469 (2009) 924. [12] M. Naito, H. Sato, H. Yamamoto, Physica C 293 (1997) 36. [13] B. Mercey, A. Gupta, M. Hervieu, B. Raveau, J. Solid State Chem. 116 (1995) 37.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Growth of infinite-layer (Sr, Nd)CuO2 films by MBE Y. Krockenberger ⇑, H. Yamamoto NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi, Kanagawa 243-0198, Japan
a r t i c l e
i n f o
Article history: Received 8 December 2010 Received in revised form 20 December 2010 Accepted 22 December 2010 Available online 25 December 2010 Keywords: High-Tc superconductivity Phase control Molecular beam epitaxy Infinite layer Cuprate Reduction
a b s t r a c t Single phase, c-axis oriented infinite layer (IL) Sr1xNdxCuO2 thin films were epitaxially grown on (1 1 0) DyScO3 substrates by molecular beam epitaxy (MBE). Electron impact emission spectroscopy (EIES) is the tool of choice for a stringent stoichiometry control which is essential for Sr1xNdxCuO2 thin film preparation. As-grown films contain excess oxygen and therefore a reduction process is necessary to induce superconductivity. In the present study special attention was paid to the underdoped region since the optimization of the reduction process becomes more difficult for lower doping levels x. Sr0.95Nd0.05CuO2 films (underdoped) became superconducting with T onset = 30 K and T zero = 11.4 K after an annealing c c procedure. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction The electron-doped infinite layer (IL) compound Sr1xRExCuO2 (RE = La, Nd, Sm, Eu, Gd and Pr; Tc = 43 K) [1,2] has the simplest crystallographic structure among all of the superconducting cuprates. Stacked CuO2 planes, composed of four-fold coordinated Cu2+ ions, with RE and Sr atoms as inter-plane separators form the crystal. In the ideal structure [3] no apical oxygen is present and there is no charge reservoir layer. The parent compound of this phase is SrCuO2 (x = 0). Prepared at ambient pressure, SrCuO2 has an orthorhombic structure and contains double chains Cu2O2 of square planar groups sharing edges joined through rock-salt type SrO layer [4,5]. In order to stabilize the IL-SrCuO2, high pressure synthesis is required, and hence, no bulk single crystal has been reported so far. However, high quality single crystalline films can be prepared by molecular beam epitaxy (MBE) or pulsed laser deposition (PLD) technique by appropriate choices of substrates and growth conditions. Karimoto et al. [6] have deposited superconducting Sr1xLaxCuO2 thin films by MBE. The Sr1xLaxCuO2 thin films are not only superconducting but even show metallic resistivity behavior. The IL system belongs to the class of electron-doped cuprates. Another representative of this class is the so-called T0 -phase. Both
⇑ Corresponding author. Address: NTT Basic Research Labs., 3-1 MorinosatoWakamiya, Atsugi-shi, Kanagawa 243-0198, Japan. Tel.: +81 46 240 4458; fax: +81 46 240 4729. E-mail address: [email protected] (Y. Krockenberger). 0921-4534/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2010.12.013
are layered copper oxides where copper is square planar coordinated. Typically, the electronic phase diagram is plotted as a function of the carrier concentration. On the electron doped side of the phase diagram the magnetic- and superconducting properties of T0 Nd2xCexCuO4 are shown while on the hole doped side of La2xSrxCuO4 [7]. The phase diagram was originally prepared by Luke et al. [8] for T0 -Nd2xCexCuO4, where the superconducting region is small. Later, a modified phase diagram was published by Brinkmann et al. [9] where an optimized annealing procedure was performed on bulk samples. In that phase diagram the superconducting region is significantly expanded and superconductivity was observed for doping levels as low as x = 0.04. In a recent report by Matsumoto et al. [10] the authors show that, superconductivity appears even in T0 -RE2CuO4 (x = 0.00). Moreover, the superconducting transition temperature increases with decreasing the doping concentration [11]. Obviously, the material properties significantly depend on its annealing history since T0 -Nd2xCexCuO4 is not an insulating antiferromagnet and a superconductor, simultaneously. It is known that the as-grown specimens of the cuprates with square-planar-coordinated copper contain a fair amount of interstitial oxygen. Evacuation of interstitial sites during a post-reduction process – while simultaneously preserving regular oxygen sites occupied – is an exigency to obtain superconductivity. Since this complicated oxygen diffusion process has not been considered in previously prepared bulk samples, a revision of the electronic phase diagram for T0 -RE2-xCexCuO4, especially in the underdoped regime becomes feasible. A similar behavior is expected for the Sr1xRExCuO2 system and therefore the primary focus within the present investigation is the underdoped region of IL-Sr1xNdxCuO2.
2.0
close
open
rate (Å/s)
1.5
0.5
Nd 0.0
0
400
800
(330)DyScO3 (003)
I (arb. units)
3
10
2
20
40
o
60
2θ ( )
80
100
Fig. 2. X-ray diffraction pattern of a Sr0.95Nd0.05CuO2 film grown on (1 1 0) DyScO3 substrate. The film is single-phase and all peaks can be indexed as (0 0 l) reflections of IL phase.
Fig. 3. RHEED pattern of a Sr0.95Nd0.05CuO2 film grown on (1 1 0) DyScO3 substrate along [0 1] direction.
570
25400 20000
o
560
5000
550
100
540 530 520 510
Sr Cu
1.0
4
10
10
Growth temperature ( C)
Epitaxial c-axis oriented thin films of Sr1xNdxCuO2, 900–950 Å thick, were grown in a custom designed ultra-high-vacuum chamber from metal sources using RF-activated atomic oxygen or ozone. Details of our MBE growth technique are described elsewhere [12]. In this study (1 1 0) DyScO3 (DSO) substrates have been also used, only. In a previous study by Karimoto et al. [6] (1 1 0) KTaO3 substrates have been also used for the growth of IL Sr1xLaxCuO2 thin films. However, the surface morphology of (1 1 0) KTaO3 is unstable due to the migration of K2O segregations though the lattice constant is larger (a = 0.3989 nm). The pseudo-cubic lattice constant of DSO is a = 3.944 Å while SrCuO2 has an in-plane lattice constant of a = 3.95 Å. The growth rate was approx. 1.6 Å/s. The present investigation has been carried out by full electron impact emission spectroscopy (EIES) control of the individual metal sources. Nd was used instead of La simply due to its stronger emission lines allowing better control by EIES. A stringent stoichiometry control is inevitable stabilizing the IL phase since, beside the IL phase, Sr2CuO3 and Sr14Cu24O41 phases exist on the copper-poor and –rich side, respectively. In Fig. 1 the flux-rates of Sr, Nd and Cu are plotted as a function of time. A proper choice of evaporant-crucible combinations together with a careful tuning of the electron beam parameters allows a constant co-evaporation throughout the growth. In Fig. 2, a X-ray diffraction pattern of a Sr0.95Nd0.05CuO2 film grown on (1 1 0) DSO is shown. The film is c-axis oriented and its c-axis length is 3.424 Å. Reflection high energy electron diffraction (RHEED) indicates that the film is grown epitaxially (Fig. 3). The diffraction pattern shown in Fig. 2 is a result of an optimization process performed for various growth temperatures (510–570 °C). In Fig. 4, the intensity of the (0 0 2) diffraction peak is plotted as a function of the doping concentration x (0.04 < x < 0.09) and the growth temperature. In total, 16 samples are summarized in Fig. 4 (black dots) where the annealing condition has not been altered (500 °C, 10 min). For low doping concentrations x the optimal growth temperature is lower compared to higher doping levels. At zero doping (SrCuO2), 500 °C was found to be an optimal growth temperature. It should be mentioned, that the phrase ‘‘optimal’’ refers to crystallographic qualities, not superconducting ones. As-grown IL films are subsequently annealed in situ under ultrahigh vacuum conditions (oxygen partial pressure <1010 Torr). The annealing process is a diffusion-driven process where time and temperature are the key parameters. In the IL system, the anneal-
(002)
5
10
(220)DyScO3
2. Experimental, results, and discussion
(001)
Y. Krockenberger, H. Yamamoto / Physica C 471 (2011) 185–187
(110)DyScO3
186
0.04
0.05
0.06
0.07
0.08
0.09
Sr1-xNdxCuO2 Fig. 4. Intensities of the (0 0 2) diffraction peak are plotted as a function of growth temperature and doping concentration. High intensities are shown in red color while low ones are blue. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
1200
time (s) Fig. 1. Flux-rates of individual metal sources controlled by EIES are plotted as a function of time. At the starting point (time = 0 s) the electron-beam power is continuously increased until the source melts (Cu, Nd) or sublimes (Sr). ‘‘Open’’ and ‘‘close’’ indicate the opening and closing of the shutter in front of the substrate. Here, the growth time is 600 s.
ing temperature is typically lower compared to the growth temperature. Annealing temperatures higher than the growth temperatures result in decomposition or phase transformation of the IL phase. Fig. 5 shows the resistivity as a function of temperature plotted for a Sr0.95Nd0.05CuO2 film grown on (1 1 0) DSO. The
Y. Krockenberger, H. Yamamoto / Physica C 471 (2011) 185–187
are simultaneously evacuated becomes a difficult task for lower doping levels x, though our present results indicate that a reassessment of the under- and zero-doped region is highly encouraging in the IL cuprates.
0.4
0.3
ρ (mΩ cm)
187
3. Conclusion
0.2
0.1
0.0
0
50
100
150
200
250
300
T (K) Fig. 5. Plot of resistivity vs. temperature of a 900 Å thick Sr0.95Nd0.05CuO2 film. The superconducting transition temperature is 11.4 K.
film was annealed for 10 min at 500 °C. The resistivity behavior is metallic between room temperature and 170 K, whereas semiconducting behavior is observed between 170 K and the onset of superconductivity around 30 K. A superconducting transition temperature of 11.4 K has been measured. This transition temperature is higher than that observed in previous studies on Sr0.95La0.05CuO2 [6], most likely due to an improved annealing process. Note that, in the underdoped region, the appearance of a minima in the resistivity curve usually reflects the fact that the reduction process has been insufficient to evacuate interstitial oxygen sites completely. Several key-parameters are required to induce superconductivity in IL Sr1xRExCuO2 (underdoped range) by MBE. At first, a singlephase IL structure (high degree of crystallinity) is necessary. For single-phase IL films, (a) precise rate control of constituent cations, (b) stringent control of growth temperature, (c) appropriate choice of substrate [6], are required. Secondly, removal of excess apical oxygen without removing oxygen from the CuO2 planes is a challenge. For this purpose, an (d) appropriate usage of active oxygen during growth as well as a (e) well-designed post-reduction process is required. Especially, the second criterion is a difficult task. The formation of oxygen vacancies in CuO2 layers, resulting in the so-called long-c phase (c = 3.62 Å) [13] is one of the origins responsible for decreased or suppressed superconducting properties. Preservation of regular oxygen sites while interstitial sites
Single-phase IL Sr1xNdxCuO2 thin films have been grown epitaxially on (1 1 0) DSO substrate by MBE. The growth conditions have been optimized in the underdoped regime in order to obtain high quality IL films. Vacuum annealing is required to induce superconductivity. Sr0.95Nd0.05CuO2 films became superconducting after an annealing treatment at 500 °C for 10 min. Further investigations are required in order to optimize the annealing conditions at low doping levels. Acknowledgements The authors acknowledge fruitful discussions with S. Karimoto and lucrative communication with M. Naito. References [1] M.G. Smith, A. Manthiram, J. Zhou, J.B. Goodenough, J.T. Markert, Nature (London) 351 (1991) 549. [2] G. Er, Y. Miyamoto, F. Kanamaru, S. Kikkawa, Physica C 181 (1991) 206. [3] J.D. Jorgensen, P.G. Radaelli, D.G. Hinks, J.L. Wagner, S. Kikkawa, G. Er, F. Kanamaru, Phys. Rev. B 47 (1993) 14654. [4] Chr. L. Teske, Hk. Muller-Buschbaum, Z. Anorg. Allg. Chem. 379 (1970) 234. [5] M.T. Gambardella, B. Domengès, B. Raveau, Mater. Res. Bull. 27 (1992) 629. [6] S. Karimoto, K. Ueda, M. Naito, T. Imai, Appl. Phys. Lett. 79 (2001) 2767; S. Karimoto, K. Ueda, M. Naito, T. Imai, Appl. Phys. Lett. 84 (2004) 2136; S. Karimoto, K. Ueda, M. Naito, T. Imai, Physica C 378–381 (2002) 127. [7] M.B. Maple, Mater. Res. Soc. Bull. 15 (1990) 60. [8] G.M. Luke, L.P. Le, B.J. Sternlieb, Y.J. Uemura, J.H. Brewer, R. Kadono, R.F. Kiefl, S.R. Kreitzman, T.M. Riseman, C.E. Stronach, M.R. Davis, S. Uchida, H. Takagi, Y. Tokura, Y. Hidaka, T. Murakami, J. Gopalakrishnan, A.W. Sleight, M.A. Subramanian, E.A. Early, J.T. Markert, M.B. Maple, C.L. Seaman, Phys. Rev. B 42 (1990) 7981. [9] M. Brinkmann, T. Rex, H. Bach, K. Westerholt, Phys. Rev. Lett. 74 (1995) 4927– 4930; M. Brinkmann, T. Rex, H. Bach, K. Westerholt, Physica C 269 (1996) 76. [10] O. Matsumoto, A. Utsuki, A. Tsukada, H. Yamamoto, T. Manabe, M. Naito, Phys. Rev. B 79 (2009) 100508. R. [11] O. Matsumoto, A. Utsuki, A. Tsukada, H. Yamamoto, T. Manabe, M. Naito, Physica C 469 (2009) 924. [12] M. Naito, H. Sato, H. Yamamoto, Physica C 293 (1997) 36. [13] B. Mercey, A. Gupta, M. Hervieu, B. Raveau, J. Solid State Chem. 116 (1995) 37.