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Laser induced photovoltages in B-riched MgB2 thin films
Optik 122 (2011) 2234–2236
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Laser induced photovoltages in B-riched MgB2 thin films Yang Wang a,b , Songqing Zhao b , Kun Zhao a,b,c,∗ a
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China Laboratory of Optic Sensing and Detecting Technology, College of Science, China University of Petroleum, Beijing 102249, China c International Center for Materials Physics, Chinese Academy of Science, Shenyang 110016, China b
a r t i c l e
i n f o
Article history: Received 5 October 2010 Accepted 7 February 2011
Keywords: MgB2 thin film Ultrafast photovoltaic effect Dember effect Position-sensitive photodetectors
a b s t r a c t Ultrafast photoelectric effects have been observed in MgB2 thin films fabricated by chemical vapor deposition on MgO (1 1 1) substrates. The rise time and full width at half-maximum of the photoresponse pulse signals were about 2.4 and 4 ns under the irradiation of a 248 nm laser pulse of 20 ns in duration through the MgO substrate at ambient temperature without any bias. Furthermore, the signal polarity is directly bound up with the laser illumination positions, while no photovoltage was observed when the MgO (1 1 1) single crystal was irradiated. The inner origin mechanism of the present positions-dependent photovoltaic response was discussed. © 2011 Elsevier GmbH. All rights reserved.
1. Introduction The discovery of the binary metallic MgB2 superconductor with a spectacularly high transition temperature of 39 K has attracted great scientific interest [1]. Recently, MgB2 has been fabricated in various forms: bulk, single crystals, thin films, tapes and nanowires by some effective technique [2–6]. Various scientific and technological milestones are now being reached, such as dynamics studies of MgB2 [7], investigations on critical current densities and critical fields [8], measurements of the specific heat, magnetic susceptibility and resistance [9–11]. Since the discovery of superconductivity in MgB2 [1], there have been several theoretical studies to search for the potential high-Tc binary and ternary borides in isoelectronic systems, such as BeB2 , CaB2 , hole-doped systems Mg1−x Lix B2 , Mg1−x Nax B2 , Mg1−x Cux B2 and related compounds [12–15]. In the paper, we utilized chemical vapor deposition (CVD) method to synthesize MgB2 thin films on MgO substrates, and focused on the laser-induced photovoltage at room temperature without any applied bias. We have observed the open-circuit photovoltaic pulse with a rise time of about 2.4 ns and full width at half maximum (FWHM) of about 4 ns when the MgB2 thin films were irradiated through MgO substrate by a laser pulse of 20 ns duration and 248 nm wavelength. On the other hand, our results show that the change of signal polarity with laser spot movement along the MgO substrate side. Meanwhile, the inner origin mechanism of the
∗ Corresponding author at: Laboratory of Optic Sensing and Detecting Technology, College of Science, China University of Petroleum, Beijing 102249, China. Tel.: +86 10 89731037. E-mail address: [email protected] (K. Zhao). 0030-4026/$ – see front matter © 2011 Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2011.02.016
movements of nonequilibrium charge carries (electrons and holes) is discussed below.
2. Experimental MgB2 thin films were prepared on mirror double-polished MgO (1 1 1) substrate by CVD technique. The size of the MgO substrate is 5 mm × 10 mm with the thickness of 0.5 mm. The CVD setup consists of a water cooled stainless steel tube and a susceptor, which is inductively heated. During the deposition, the carrier gas is purified N2 with a flow rate the order of 300 sccm to 1.5 slm at a pressure of about 1 × 104 Pa. Bulk pure Mg (purity 99.99%) is used as the Mg source. When the susceptor is heated to 650–700 ◦ C, which is high enough for epitaxial growth of MgB2 film, pure Mg bulks are also heated, which generates a high Mg vapor pressure next to the substrate. 1000 ppm Diborane (B2 H6 ) was diluted in H2 (1 vol%) as the B source. B2 H6 was admitted directly onto the substrates, whereas the gas in the reactor was continuously evacuated by a mechanical pump. MgB2 film started to grow when the boron precursor gas is introduced into the reactor. The parameter of typical conditions is described below: the flow rate of B2 H6 /H2 mixture of 50 sccm, the substrate temperature of 650–700 ◦ C and the total pressure of chamber of 2 × 104 Pa. After holding at the temperature for 20 min, the film was quenched to room temperature in 30 min. The film growth stops when the boron precursor gas is switched off. The structure of MgB2 films was characterized by X-ray diffraction (XRD) using Ni-filtered Cu K␣ radiation. For the photovoltaic measurement, two Ag electrodes (width of 1 mm) separated by 8 mm were prepared on the film surface, and the irradiated area
Y. Wang et al. / Optik 122 (2011) 2234–2236
Fig. 1. X-ray diffraction pattern for the CVD-prepared MgB2 thin film on MgO (1 1 1) substrate.
is 2 mm × 5 mm, as shown in Fig. 1. The sample was irradiated by a KrF ultraviolet laser pulse with a wavelength of 248 nm and a duration of 20 ns at a 2 Hz repetition rate, and laser energy density was kept at 0.35 mJ/mm2 . The sample was irradiated at three different positions by pulse laser through substrate at ambient temperature without any bias. The voltage signals were recorded with a digital oscilloscope terminated into 50 . 3. Results and discussions The XRD –2 pattern for the CVD-prepared MgB2 thin film on MgO (1 1 1) substrate is shown in Fig. 1. In this scan, the diffraction (0 0 l) peaks of MgB2 film indicate that the film deposited on MgO substrate is aligned with the c axis. An envelope observed in the pattern indicates the presence of amorphous boron. Fig. 2 shows a typical transient photovoltage induced by 248 nm pulsed laser irradiation from the substrate side with three different positions. The 10–90% rise time, FWHM and the peak of amplitudes are about 1.2 s, 40 s and 3.8 mV and 1.2 s, 90 s and −2.9 mV for position (a) and position (c), respectively. We have not observed any photovoltaic signal when the MgB2 thin films were irradiated through the substrate at the center position (position (b)) between the two contacts. As the photon energy of 248 nm wavelength (∼5 eV) is much smaller than the band gap of MgO (∼7.8 eV), the nonequilibrium charge carries are created only in the MgB2 thin film when the MgB2 thin film is irradiated through the substrate. Our further measurement confirmed this result that no photovoltaic signal was observed when the MgO substrate (1 1 1) single crystal was irradiated under the same experimental condition mentioned above. This inversion of the signal when laser spot was moving form position (a) to position (c) can be explained below: nonequilibrium
Fig. 2. Photovoltaic response of MgB2 thin film to 248 nm laser pulse in duration of 20 ns when the film was irradiated through the MgO substrate (back-side) with different positions. The inset shows the schematic circuit of the sample measurement; here, a, b and c denote the different illuminated positions.
2235
Fig. 3. The zoom of the sharp rise of the open-circuit photovoltage for variation with time after excitation with a 248 nm laser pulse on the MgO substrate. The panorama of the variation is shown in the inset.
charge carriers (electrons and holes) are generated by laser irradiation, and the unequal diffusion velocity of electron and hole leads to the phenomenon. The reason causing the photovoltage is the mobility difference between holes and electrons. The much larger mobility of electrons than that of holes makes the separation of electron–hole pairs, which results in a transient distribution of electrons far away from the laser spot and the holes staying closer to the spot. This transient distribution definitely causes a higher electric potential in the region closer to the spot, which is in agree with the Dember effect [16]. Based on this effect, MgB2 is expected to make a new type of candidate for position-sensitive photodetectors. To determine the photovoltaic behaviors of the rise time of the signal, we changed the range of oscilloscope to show the details of the rise time for whole scence. The realistic rise time is about 2.4 ns and the FWHM is about 4 ns under the same experimental condition mentioned above (Fig. 3). It should be noted that there is a sharp rise of the pulse at the very beginning, and the maximum of the absolute value of the photovoltage was obtained at 14.3 mV, then the photovoltage signal sharply decreased to 4 mV in 300 ns and maintained amplitude unchanged more than 2000 ns. It also should be noted that this amplitude come in line with the maximum of the absolute value of the photovoltage of Fig. 2(a). It means that we observed the realistic rise time of the signal when the MgB2 thin films were irradiated through the substrate. The decay portion and slight oscillation of the pulse response resulted from an impedance mismatching in the circuit.
4. Conclusions In conclusion, transient laser-induced voltages have been observed in MgB2 thin films epitaxially grown on MgO (1 1 1) at room temperature without any applied bias. The maximum of the absolute value of the photovoltage is obtained at 14.3 mV, and the rise time was ∼2.4 ns and the full width at half-maximum was ∼4 ns when the film was irradiated through the substrate. Furthermore, the signal polarity is reversed when the films are irradiated through the substrate from position a to position c. We have not observed any photovoltaic signal when the MgO substrate (1 1 1) single crystal were irradiated under the same experimental condition mentioned above. The inner origin mechanism of the movements of nonequilibrium charge carries (electrons and holes) is discussed. The experimental results have potential applications for photoelectric detector based on Dember effect. Further investigation, both experimental and theoretical, on the mechanism of the multifunctional properties of electricity and optics in such systems are undergoing.
2236
Y. Wang et al. / Optik 122 (2011) 2234–2236
Acknowledgements This work has been supported by New Century Excellent Talents in University, National Natural Science Foundation of China, and Research Fund for the Doctoral Program of Higher Education. References [1] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, J. Akimitsu, Superconductivity at 39 K in magnesium diboride, Nature 410 (2001) 63–64. [2] X.H. Zeng, A.V. Pogrebnyakov, A. Kotcharov, J.E. Jones, X.X. Xi, E.M. Lysczek, J.M. Redwing, S.Y. Xu, Q. Li, J. Lettieri, D.G. Schlom, W. Tian, X.Q. Pan, Z.K. Liu, In situ epitaxial MgB2 thin films for superconducting electronics, Nat. Mater. 1 (2002) 35–38. [3] Y. Wu, B. Messer, P. Yang, Superconducting MgB2 nanowires, Adv. Mater. 13 (2001) 1487–1489. [4] S. Lee, H. Mori, T. Masui, Yu. Eltsev, A. Yamamoto, S. Tajima, Growth, structure analysis and anisotropic superconducting properties of MgB2 single crystals, J. Phys. Soc. Jpn. 70 (2001) 2255–2258. [5] X.X. Xi, A.V. Pogrebnyakov, S.Y. Xu, K. Chen, Y. Cui, E.C. Maertz, C.G. Zhuang, Q. Li, D.R. Lamborn, J.M. Redwing, Z.K. Liu, A. Soukiassian, D.G. Schlom, X.J. Weng, E.C. Dickey, Y.B. Chen, W. Tian, X.Q. Pan, S.A. Cybart, R.C. Dynes, MgB2 thin films by hybrid physical-chemical vapor deposition, Phys. C 456 (2007) 22–37. [6] A. Brinkman, D. Mijatovic, G. Rijnders, V. Leca, H.J.H. Smilde, I. Oomen, A.A. Golubov, F. Roesthuis, S. Harkema, H. Hilgenkamp, D.H.A. Blank, H. Rogalla, Superconducting thin films of MgB2 on Si by pulsed laser deposition, Phys. C 353 (2001) 1–4. [7] Y. Bugoslavsky, G.K. Perkins, X. Qi, L.F. Cohen, A.D. Caplin, Vortex dynamics in superconducting MgB2 and prospects for applications, Nature 410 (2001) 563–565.
[8] Y. Bugoslavsky, L.F. Cohen, G.K. Perkins, M. Polichetti, T.J. Tate, R. Gwilliam, A.D. Caplin, Enhancement of the high-field critical current density of superconducting MgB2 by proton irradiation, Nature 411 (2001) 561–563. [9] F. Bouquet, R.A. Fisher, N.E. Phillips, D.G. Hinks, J.D. Jorgensen, Specific heat of Mg11 B2 : evidence for a second energy gap, Phys. Rev. Lett. 87 (2001) 047001–047005. [10] Y. Wang, T. Plackowski, A. Junod, Specific heat in the superconducting and normal state (2–300 K, 0–16T), and magnetic susceptibility of the 38 K superconductor MgB2 : evidence for a multicomponent gap, Phys. C 355 (2001) 179–193. [11] S. Patnaik, L.D. Cooley, A. Gurevich, A.A. Polyanskii, J. Jiang, X.Y. Cai, A.A. Squitieri, M.T. Naus, M.K. Lee, J.H. Choi, L. Belenky, S.D. Bu, J. Letteri, X. Song, D.G. Schlom, S.E. Babcock, C.B. Eom, E.E. Hellstrom, D.C. Larbalestier, Electronic anisotropy, magnetic field-temperature phase diagram and their dependence on resistivity in c-axis oriented MgB2 thin films, Supercond. Sci. Technol. 14 (2001) 315–319. [12] N.I. Medvedeva, J.E. Medvedeva, A.L. Ivanovskii, V.G. Zubkov, A.J. Freeman, Band structure of superconductingMgB2 compound and modeling of related ternary systems, JETP Lett. 73 (2001) 336–340. [13] N.I. Medvedeva, A.L. Ivanovskii, J.E. Medvedeva, A.J. Freeman, Electronic structure of superconducting MgB2 and related binary and ternary borides, Phys. Rev. B 64 (2001) 020502–020514. [14] P. Ravindran, P. Vajeeston, R. Vidya, A. Kjekshus, H. Fjellvig, Detailed electronic structure studies on superconducting MgB2 and related compounds, Phys. Rev. B 64 (2001) 224509–224524. [15] M.J. Mehl, D.A. Papaconstantopoulos, D.J. Singh, Effects of C, Cu and Be substitutions in superconducting MgB2 , Phys. Rev. B 64 (2001) 140509–140513. [16] K.J. Jin, K. Zhao, H.B. Lu, L. Liao, G.Z. Yang, Dember effect induced photovoltage in pervoskite p-n heterojunctions, Appl. Phys. Lett. 91 (2007) 081906.1–081906.3.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Laser induced photovoltages in B-riched MgB2 thin films Yang Wang a,b , Songqing Zhao b , Kun Zhao a,b,c,∗ a
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China Laboratory of Optic Sensing and Detecting Technology, College of Science, China University of Petroleum, Beijing 102249, China c International Center for Materials Physics, Chinese Academy of Science, Shenyang 110016, China b
a r t i c l e
i n f o
Article history: Received 5 October 2010 Accepted 7 February 2011
Keywords: MgB2 thin film Ultrafast photovoltaic effect Dember effect Position-sensitive photodetectors
a b s t r a c t Ultrafast photoelectric effects have been observed in MgB2 thin films fabricated by chemical vapor deposition on MgO (1 1 1) substrates. The rise time and full width at half-maximum of the photoresponse pulse signals were about 2.4 and 4 ns under the irradiation of a 248 nm laser pulse of 20 ns in duration through the MgO substrate at ambient temperature without any bias. Furthermore, the signal polarity is directly bound up with the laser illumination positions, while no photovoltage was observed when the MgO (1 1 1) single crystal was irradiated. The inner origin mechanism of the present positions-dependent photovoltaic response was discussed. © 2011 Elsevier GmbH. All rights reserved.
1. Introduction The discovery of the binary metallic MgB2 superconductor with a spectacularly high transition temperature of 39 K has attracted great scientific interest [1]. Recently, MgB2 has been fabricated in various forms: bulk, single crystals, thin films, tapes and nanowires by some effective technique [2–6]. Various scientific and technological milestones are now being reached, such as dynamics studies of MgB2 [7], investigations on critical current densities and critical fields [8], measurements of the specific heat, magnetic susceptibility and resistance [9–11]. Since the discovery of superconductivity in MgB2 [1], there have been several theoretical studies to search for the potential high-Tc binary and ternary borides in isoelectronic systems, such as BeB2 , CaB2 , hole-doped systems Mg1−x Lix B2 , Mg1−x Nax B2 , Mg1−x Cux B2 and related compounds [12–15]. In the paper, we utilized chemical vapor deposition (CVD) method to synthesize MgB2 thin films on MgO substrates, and focused on the laser-induced photovoltage at room temperature without any applied bias. We have observed the open-circuit photovoltaic pulse with a rise time of about 2.4 ns and full width at half maximum (FWHM) of about 4 ns when the MgB2 thin films were irradiated through MgO substrate by a laser pulse of 20 ns duration and 248 nm wavelength. On the other hand, our results show that the change of signal polarity with laser spot movement along the MgO substrate side. Meanwhile, the inner origin mechanism of the
∗ Corresponding author at: Laboratory of Optic Sensing and Detecting Technology, College of Science, China University of Petroleum, Beijing 102249, China. Tel.: +86 10 89731037. E-mail address: [email protected] (K. Zhao). 0030-4026/$ – see front matter © 2011 Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2011.02.016
movements of nonequilibrium charge carries (electrons and holes) is discussed below.
2. Experimental MgB2 thin films were prepared on mirror double-polished MgO (1 1 1) substrate by CVD technique. The size of the MgO substrate is 5 mm × 10 mm with the thickness of 0.5 mm. The CVD setup consists of a water cooled stainless steel tube and a susceptor, which is inductively heated. During the deposition, the carrier gas is purified N2 with a flow rate the order of 300 sccm to 1.5 slm at a pressure of about 1 × 104 Pa. Bulk pure Mg (purity 99.99%) is used as the Mg source. When the susceptor is heated to 650–700 ◦ C, which is high enough for epitaxial growth of MgB2 film, pure Mg bulks are also heated, which generates a high Mg vapor pressure next to the substrate. 1000 ppm Diborane (B2 H6 ) was diluted in H2 (1 vol%) as the B source. B2 H6 was admitted directly onto the substrates, whereas the gas in the reactor was continuously evacuated by a mechanical pump. MgB2 film started to grow when the boron precursor gas is introduced into the reactor. The parameter of typical conditions is described below: the flow rate of B2 H6 /H2 mixture of 50 sccm, the substrate temperature of 650–700 ◦ C and the total pressure of chamber of 2 × 104 Pa. After holding at the temperature for 20 min, the film was quenched to room temperature in 30 min. The film growth stops when the boron precursor gas is switched off. The structure of MgB2 films was characterized by X-ray diffraction (XRD) using Ni-filtered Cu K␣ radiation. For the photovoltaic measurement, two Ag electrodes (width of 1 mm) separated by 8 mm were prepared on the film surface, and the irradiated area
Y. Wang et al. / Optik 122 (2011) 2234–2236
Fig. 1. X-ray diffraction pattern for the CVD-prepared MgB2 thin film on MgO (1 1 1) substrate.
is 2 mm × 5 mm, as shown in Fig. 1. The sample was irradiated by a KrF ultraviolet laser pulse with a wavelength of 248 nm and a duration of 20 ns at a 2 Hz repetition rate, and laser energy density was kept at 0.35 mJ/mm2 . The sample was irradiated at three different positions by pulse laser through substrate at ambient temperature without any bias. The voltage signals were recorded with a digital oscilloscope terminated into 50 . 3. Results and discussions The XRD –2 pattern for the CVD-prepared MgB2 thin film on MgO (1 1 1) substrate is shown in Fig. 1. In this scan, the diffraction (0 0 l) peaks of MgB2 film indicate that the film deposited on MgO substrate is aligned with the c axis. An envelope observed in the pattern indicates the presence of amorphous boron. Fig. 2 shows a typical transient photovoltage induced by 248 nm pulsed laser irradiation from the substrate side with three different positions. The 10–90% rise time, FWHM and the peak of amplitudes are about 1.2 s, 40 s and 3.8 mV and 1.2 s, 90 s and −2.9 mV for position (a) and position (c), respectively. We have not observed any photovoltaic signal when the MgB2 thin films were irradiated through the substrate at the center position (position (b)) between the two contacts. As the photon energy of 248 nm wavelength (∼5 eV) is much smaller than the band gap of MgO (∼7.8 eV), the nonequilibrium charge carries are created only in the MgB2 thin film when the MgB2 thin film is irradiated through the substrate. Our further measurement confirmed this result that no photovoltaic signal was observed when the MgO substrate (1 1 1) single crystal was irradiated under the same experimental condition mentioned above. This inversion of the signal when laser spot was moving form position (a) to position (c) can be explained below: nonequilibrium
Fig. 2. Photovoltaic response of MgB2 thin film to 248 nm laser pulse in duration of 20 ns when the film was irradiated through the MgO substrate (back-side) with different positions. The inset shows the schematic circuit of the sample measurement; here, a, b and c denote the different illuminated positions.
2235
Fig. 3. The zoom of the sharp rise of the open-circuit photovoltage for variation with time after excitation with a 248 nm laser pulse on the MgO substrate. The panorama of the variation is shown in the inset.
charge carriers (electrons and holes) are generated by laser irradiation, and the unequal diffusion velocity of electron and hole leads to the phenomenon. The reason causing the photovoltage is the mobility difference between holes and electrons. The much larger mobility of electrons than that of holes makes the separation of electron–hole pairs, which results in a transient distribution of electrons far away from the laser spot and the holes staying closer to the spot. This transient distribution definitely causes a higher electric potential in the region closer to the spot, which is in agree with the Dember effect [16]. Based on this effect, MgB2 is expected to make a new type of candidate for position-sensitive photodetectors. To determine the photovoltaic behaviors of the rise time of the signal, we changed the range of oscilloscope to show the details of the rise time for whole scence. The realistic rise time is about 2.4 ns and the FWHM is about 4 ns under the same experimental condition mentioned above (Fig. 3). It should be noted that there is a sharp rise of the pulse at the very beginning, and the maximum of the absolute value of the photovoltage was obtained at 14.3 mV, then the photovoltage signal sharply decreased to 4 mV in 300 ns and maintained amplitude unchanged more than 2000 ns. It also should be noted that this amplitude come in line with the maximum of the absolute value of the photovoltage of Fig. 2(a). It means that we observed the realistic rise time of the signal when the MgB2 thin films were irradiated through the substrate. The decay portion and slight oscillation of the pulse response resulted from an impedance mismatching in the circuit.
4. Conclusions In conclusion, transient laser-induced voltages have been observed in MgB2 thin films epitaxially grown on MgO (1 1 1) at room temperature without any applied bias. The maximum of the absolute value of the photovoltage is obtained at 14.3 mV, and the rise time was ∼2.4 ns and the full width at half-maximum was ∼4 ns when the film was irradiated through the substrate. Furthermore, the signal polarity is reversed when the films are irradiated through the substrate from position a to position c. We have not observed any photovoltaic signal when the MgO substrate (1 1 1) single crystal were irradiated under the same experimental condition mentioned above. The inner origin mechanism of the movements of nonequilibrium charge carries (electrons and holes) is discussed. The experimental results have potential applications for photoelectric detector based on Dember effect. Further investigation, both experimental and theoretical, on the mechanism of the multifunctional properties of electricity and optics in such systems are undergoing.
2236
Y. Wang et al. / Optik 122 (2011) 2234–2236
Acknowledgements This work has been supported by New Century Excellent Talents in University, National Natural Science Foundation of China, and Research Fund for the Doctoral Program of Higher Education. References [1] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, J. Akimitsu, Superconductivity at 39 K in magnesium diboride, Nature 410 (2001) 63–64. [2] X.H. Zeng, A.V. Pogrebnyakov, A. Kotcharov, J.E. Jones, X.X. Xi, E.M. Lysczek, J.M. Redwing, S.Y. Xu, Q. Li, J. Lettieri, D.G. Schlom, W. Tian, X.Q. Pan, Z.K. Liu, In situ epitaxial MgB2 thin films for superconducting electronics, Nat. Mater. 1 (2002) 35–38. [3] Y. Wu, B. Messer, P. Yang, Superconducting MgB2 nanowires, Adv. Mater. 13 (2001) 1487–1489. [4] S. Lee, H. Mori, T. Masui, Yu. Eltsev, A. Yamamoto, S. Tajima, Growth, structure analysis and anisotropic superconducting properties of MgB2 single crystals, J. Phys. Soc. Jpn. 70 (2001) 2255–2258. [5] X.X. Xi, A.V. Pogrebnyakov, S.Y. Xu, K. Chen, Y. Cui, E.C. Maertz, C.G. Zhuang, Q. Li, D.R. Lamborn, J.M. Redwing, Z.K. Liu, A. Soukiassian, D.G. Schlom, X.J. Weng, E.C. Dickey, Y.B. Chen, W. Tian, X.Q. Pan, S.A. Cybart, R.C. Dynes, MgB2 thin films by hybrid physical-chemical vapor deposition, Phys. C 456 (2007) 22–37. [6] A. Brinkman, D. Mijatovic, G. Rijnders, V. Leca, H.J.H. Smilde, I. Oomen, A.A. Golubov, F. Roesthuis, S. Harkema, H. Hilgenkamp, D.H.A. Blank, H. Rogalla, Superconducting thin films of MgB2 on Si by pulsed laser deposition, Phys. C 353 (2001) 1–4. [7] Y. Bugoslavsky, G.K. Perkins, X. Qi, L.F. Cohen, A.D. Caplin, Vortex dynamics in superconducting MgB2 and prospects for applications, Nature 410 (2001) 563–565.
[8] Y. Bugoslavsky, L.F. Cohen, G.K. Perkins, M. Polichetti, T.J. Tate, R. Gwilliam, A.D. Caplin, Enhancement of the high-field critical current density of superconducting MgB2 by proton irradiation, Nature 411 (2001) 561–563. [9] F. Bouquet, R.A. Fisher, N.E. Phillips, D.G. Hinks, J.D. Jorgensen, Specific heat of Mg11 B2 : evidence for a second energy gap, Phys. Rev. Lett. 87 (2001) 047001–047005. [10] Y. Wang, T. Plackowski, A. Junod, Specific heat in the superconducting and normal state (2–300 K, 0–16T), and magnetic susceptibility of the 38 K superconductor MgB2 : evidence for a multicomponent gap, Phys. C 355 (2001) 179–193. [11] S. Patnaik, L.D. Cooley, A. Gurevich, A.A. Polyanskii, J. Jiang, X.Y. Cai, A.A. Squitieri, M.T. Naus, M.K. Lee, J.H. Choi, L. Belenky, S.D. Bu, J. Letteri, X. Song, D.G. Schlom, S.E. Babcock, C.B. Eom, E.E. Hellstrom, D.C. Larbalestier, Electronic anisotropy, magnetic field-temperature phase diagram and their dependence on resistivity in c-axis oriented MgB2 thin films, Supercond. Sci. Technol. 14 (2001) 315–319. [12] N.I. Medvedeva, J.E. Medvedeva, A.L. Ivanovskii, V.G. Zubkov, A.J. Freeman, Band structure of superconductingMgB2 compound and modeling of related ternary systems, JETP Lett. 73 (2001) 336–340. [13] N.I. Medvedeva, A.L. Ivanovskii, J.E. Medvedeva, A.J. Freeman, Electronic structure of superconducting MgB2 and related binary and ternary borides, Phys. Rev. B 64 (2001) 020502–020514. [14] P. Ravindran, P. Vajeeston, R. Vidya, A. Kjekshus, H. Fjellvig, Detailed electronic structure studies on superconducting MgB2 and related compounds, Phys. Rev. B 64 (2001) 224509–224524. [15] M.J. Mehl, D.A. Papaconstantopoulos, D.J. Singh, Effects of C, Cu and Be substitutions in superconducting MgB2 , Phys. Rev. B 64 (2001) 140509–140513. [16] K.J. Jin, K. Zhao, H.B. Lu, L. Liao, G.Z. Yang, Dember effect induced photovoltage in pervoskite p-n heterojunctions, Appl. Phys. Lett. 91 (2007) 081906.1–081906.3.