- Email: [email protected]
Application of a mm-wave free electron laser to the study of high-Tc superconductors
cm
Nuclear Instruments
and Methods in Physics Research A 358 (1995) AES 9%AES 10
. __ !s
!!d ELSEVIER
NUCLEAR INSTRUMENTS 8 METHODS IN PHYSICS RESEARCH Sectlon A
Application of a mm-wave free electron laser to the study of high-T, superconductors E. Giovenale
*,
a,
V. Boffa b, P. Dore ‘, A. Doria a, G.P. Gallerano a, M.F. Kimmitt d, R. Trippetti ‘, I. Spassovsky a~1
aENEA, INN-FIS, P.O. Box 65, 00044 Frascati, Italy b ENEA, ERG-FUS, P.O. Box 65, 00044 Frascati, Italy ’ Physics Department, lJniversit& di Roma “La Sapienza”, 00185 Rome, Italy d Physics Department, University of Essex, Colchester, CO4 3SQ, UK
The discovery of high-T, superconductors has led to much research on the optical properties of these materials [l]. Reflection and transmission measurements carried out below the critical temperature T, in the mm-wave and far infrared region, in particular, can show evidence of the superconducting energy gap, as shown in the pioneering works of Glover and Tinkham [2] on conventional superconductors. In general, information on the frequency-dependent complex conductivity can be derived from spectroscopic measurements in the infrared region. In studying the optical properties of high-T, superconductors, optical conductivity a(v) (i.e. the real part of the complex conductivity) is usually obtained from KramersKronig (K-K) trasformations of reflectance spectra R(v). In the case of thin films deposited on transparent substrates only reflectance measurements are possible. At low frequencies, U(V) can thus be strongly affected [3] by the extrapolations employed to extend the measured spectra down to zero frequency. Therefore, an accurate determination of the optical conductivity V(V) at low frequencies (Y < 100 cm-‘) is still an open problem. In studying the low frequency behaviour of C(Y), we extended far infrared measurements down to frequencies as low as possible. Reflectance R(v) and transmittance T(v) spectra of thin YrBa.&uaO,_, (YBCO) films deposited on MgO have been measured down to 15 cm-’ by using a conventional Michelson interferometer equipped with a silicon bolometer [4]. We remark that low-frequency transmittance measurements gave more reliable results than those obtained through reflectance measurements. However, accurate transmittance measurements require a transparent substrate and a powerful source; indeed, the trans-
* Corresponding author. Tel. + 39 6 94001, 5334, e-mail [email protected]. ’ ENEA-ICTP Fellow.
fax + 39 6 9400
0168-9002/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDf 0168-9002(94)01299-7
mission of a high-quality film, even if of a thickness of the order of 50 nm, is very low. We have in program mm-wave and far infrared transmission and reflection measurements on thin YBCO films deposited on LaAlO,. This substrate allows epitaxial film growth and is transparent up to about 100 cm-‘, as shown by recent preliminary results (Fig. 11. High-quality films will be prepared by laser ablation at ENEA (ERG-FUS) [5]. Far infrared measurements will be made down to 15 cm-’ at the Physics Department of the Rome University, by using the above mentioned apparatus. For mm-wave measurements, the ENEA Compact FEL facility will be employed as radiation source, owing to its high peak power and continuous tunability features [6]. Preliminary measurements have been carried out on the substrate in order to choose the better experimental configuration and to confirm the data obtained with a conventional equip-
0.4 T(v)
0.2
0.1
v (cmmL)
Fig. 1. LaAlO, transmittance for T = 295, 150, 100, 60 and 20 K. Sample thickness = 0.5 mm.
EXTENDED
SYNOPSES
ABS 10
E. Giocenale ei al./ Nucl. lnstr. and Meth. in Phys. Res. A 358 119951 ABS 9-ABS BEAM SPLll-UNG SYSTEM
10
PYROELECTRIC DETECTOR MOLECTRON P4-32
RADIATION
PYROELECTRIC DETECTOR MOLECTRON P4-32
‘, ,’
i
BEAM SPLITTING SYSTEM Fig. 2. Proposed experimental
The main difficulties of the measurement at low frequency arise from the fact that the sample size is comparable with the wavelength of the radiation. It is thus necessary to design an experimental apparatus in order to avoid diffraction and interference problems. Moreover the refraction index of the substrate has been measured to be about 4.5. This value, together with a sample thickness of 0.5 mm, can cause Fabry-Perot interference effects in the sample at wavelengths between 2 and 3 mm. Preliminary measurements have been carried out in this wavelength range (3-5 cm-‘), and interference effects in the sample have been identified as the most annoying source of systematic errors. New measurements have been planned using coherent spontaneous emission radiation [6,7]. This broad band emission will be used, together with a FabryPerot spectrometer, to obtain a continuous spectrum for the sample transmission, and the interference fringes will be eliminated with a proper procedure. On the basis of the above results, the experimental setup shown in Fig. 2 has been designed. A waveguide system, using a directional coupler to measure simultaneously transmittance and reflectance, can also be considered for this experiment. The use of the compact FEL facility, which will extend operation up to 400 pm (25 cm-‘) in 1995, together with a conventional far infrared apparatus, will allow the user to
layout for mm-wave
measurements.
cover the complete low-frequency range, thus obtaining reliable information on the low-frequency conductivity of high-T, materials.
References
[II
D.B. Tanner and T. Timusk, Optical properties of high-T, superconductors, in: Physical Properties of High Temperature Superconductors, ed. D.M. Ginsberg (World Scientific, Singapore, 1992). 121R.E. Glover and M. Tinkham, Phys. Rev. 104 (1956) 844. 131D. Miller and P.L. Richards, Phys. Rev. B 47 (1993) 12308. [41 S. Cunsolo, P. Dore, S. Lupi, R. Trippetti, C.P. Varsamis and A. Sherman, Physica C 211 (1993) 22; P. Dore, A. Paolone, S. Tatoni, C.P. Varsamis, V. Boffa. C. Romeo, S. Barbanera and F. Murtas, Int. J. of IR&MM waves, in press. bl C. Romeo, V. Boffa, S. Bollanti, C. Alvani, M. Penna, S. Barbanera, P. Castrucci, R. Leoni and F. Murtas, Physica C 180 (1991) 77. [d F. Ciocci, R. Bartolini, A. Doria, G.P. Gallerano, E. Giovenale. M.F. Kimmitt, G. Messina and A. Renieri, Phys. Rev. Lett. 70 (1993) 928. 171A. Doria, R. Bartolini, J. Feinstein, G.P. Gallerano and R.H. Pantell. IEEE J. Quantum Eletron. QE-29 (1993) 1428.
Nuclear Instruments
and Methods in Physics Research A 358 (1995) AES 9%AES 10
. __ !s
!!d ELSEVIER
NUCLEAR INSTRUMENTS 8 METHODS IN PHYSICS RESEARCH Sectlon A
Application of a mm-wave free electron laser to the study of high-T, superconductors E. Giovenale
*,
a,
V. Boffa b, P. Dore ‘, A. Doria a, G.P. Gallerano a, M.F. Kimmitt d, R. Trippetti ‘, I. Spassovsky a~1
aENEA, INN-FIS, P.O. Box 65, 00044 Frascati, Italy b ENEA, ERG-FUS, P.O. Box 65, 00044 Frascati, Italy ’ Physics Department, lJniversit& di Roma “La Sapienza”, 00185 Rome, Italy d Physics Department, University of Essex, Colchester, CO4 3SQ, UK
The discovery of high-T, superconductors has led to much research on the optical properties of these materials [l]. Reflection and transmission measurements carried out below the critical temperature T, in the mm-wave and far infrared region, in particular, can show evidence of the superconducting energy gap, as shown in the pioneering works of Glover and Tinkham [2] on conventional superconductors. In general, information on the frequency-dependent complex conductivity can be derived from spectroscopic measurements in the infrared region. In studying the optical properties of high-T, superconductors, optical conductivity a(v) (i.e. the real part of the complex conductivity) is usually obtained from KramersKronig (K-K) trasformations of reflectance spectra R(v). In the case of thin films deposited on transparent substrates only reflectance measurements are possible. At low frequencies, U(V) can thus be strongly affected [3] by the extrapolations employed to extend the measured spectra down to zero frequency. Therefore, an accurate determination of the optical conductivity V(V) at low frequencies (Y < 100 cm-‘) is still an open problem. In studying the low frequency behaviour of C(Y), we extended far infrared measurements down to frequencies as low as possible. Reflectance R(v) and transmittance T(v) spectra of thin YrBa.&uaO,_, (YBCO) films deposited on MgO have been measured down to 15 cm-’ by using a conventional Michelson interferometer equipped with a silicon bolometer [4]. We remark that low-frequency transmittance measurements gave more reliable results than those obtained through reflectance measurements. However, accurate transmittance measurements require a transparent substrate and a powerful source; indeed, the trans-
* Corresponding author. Tel. + 39 6 94001, 5334, e-mail [email protected]. ’ ENEA-ICTP Fellow.
fax + 39 6 9400
0168-9002/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDf 0168-9002(94)01299-7
mission of a high-quality film, even if of a thickness of the order of 50 nm, is very low. We have in program mm-wave and far infrared transmission and reflection measurements on thin YBCO films deposited on LaAlO,. This substrate allows epitaxial film growth and is transparent up to about 100 cm-‘, as shown by recent preliminary results (Fig. 11. High-quality films will be prepared by laser ablation at ENEA (ERG-FUS) [5]. Far infrared measurements will be made down to 15 cm-’ at the Physics Department of the Rome University, by using the above mentioned apparatus. For mm-wave measurements, the ENEA Compact FEL facility will be employed as radiation source, owing to its high peak power and continuous tunability features [6]. Preliminary measurements have been carried out on the substrate in order to choose the better experimental configuration and to confirm the data obtained with a conventional equip-
0.4 T(v)
0.2
0.1
v (cmmL)
Fig. 1. LaAlO, transmittance for T = 295, 150, 100, 60 and 20 K. Sample thickness = 0.5 mm.
EXTENDED
SYNOPSES
ABS 10
E. Giocenale ei al./ Nucl. lnstr. and Meth. in Phys. Res. A 358 119951 ABS 9-ABS BEAM SPLll-UNG SYSTEM
10
PYROELECTRIC DETECTOR MOLECTRON P4-32
RADIATION
PYROELECTRIC DETECTOR MOLECTRON P4-32
‘, ,’
i
BEAM SPLITTING SYSTEM Fig. 2. Proposed experimental
The main difficulties of the measurement at low frequency arise from the fact that the sample size is comparable with the wavelength of the radiation. It is thus necessary to design an experimental apparatus in order to avoid diffraction and interference problems. Moreover the refraction index of the substrate has been measured to be about 4.5. This value, together with a sample thickness of 0.5 mm, can cause Fabry-Perot interference effects in the sample at wavelengths between 2 and 3 mm. Preliminary measurements have been carried out in this wavelength range (3-5 cm-‘), and interference effects in the sample have been identified as the most annoying source of systematic errors. New measurements have been planned using coherent spontaneous emission radiation [6,7]. This broad band emission will be used, together with a FabryPerot spectrometer, to obtain a continuous spectrum for the sample transmission, and the interference fringes will be eliminated with a proper procedure. On the basis of the above results, the experimental setup shown in Fig. 2 has been designed. A waveguide system, using a directional coupler to measure simultaneously transmittance and reflectance, can also be considered for this experiment. The use of the compact FEL facility, which will extend operation up to 400 pm (25 cm-‘) in 1995, together with a conventional far infrared apparatus, will allow the user to
layout for mm-wave
measurements.
cover the complete low-frequency range, thus obtaining reliable information on the low-frequency conductivity of high-T, materials.
References
[II
D.B. Tanner and T. Timusk, Optical properties of high-T, superconductors, in: Physical Properties of High Temperature Superconductors, ed. D.M. Ginsberg (World Scientific, Singapore, 1992). 121R.E. Glover and M. Tinkham, Phys. Rev. 104 (1956) 844. 131D. Miller and P.L. Richards, Phys. Rev. B 47 (1993) 12308. [41 S. Cunsolo, P. Dore, S. Lupi, R. Trippetti, C.P. Varsamis and A. Sherman, Physica C 211 (1993) 22; P. Dore, A. Paolone, S. Tatoni, C.P. Varsamis, V. Boffa. C. Romeo, S. Barbanera and F. Murtas, Int. J. of IR&MM waves, in press. bl C. Romeo, V. Boffa, S. Bollanti, C. Alvani, M. Penna, S. Barbanera, P. Castrucci, R. Leoni and F. Murtas, Physica C 180 (1991) 77. [d F. Ciocci, R. Bartolini, A. Doria, G.P. Gallerano, E. Giovenale. M.F. Kimmitt, G. Messina and A. Renieri, Phys. Rev. Lett. 70 (1993) 928. 171A. Doria, R. Bartolini, J. Feinstein, G.P. Gallerano and R.H. Pantell. IEEE J. Quantum Eletron. QE-29 (1993) 1428.