- Email: [email protected]
Infrared reflectivity and conductivity of YBa2Cu3O7−δ thin films
Physica C 162-164 (1989) 1085-1086
North-Holland
INFRARED REFLECTIVITY AND CONDUCTIVITY OF YBa2Cu307-t THIN FILMS K.F. RENK, J. SCHI]TZMANN, W. OSE, M. FRANZ, J. KELLER, U. HOFMANN, H. ESCHRIG a, B. ROAS *+, B. HENSEL +, and G. SAEMANN-ISCHENKO + Fakultgt Physik, Universit~t Regensburg, 8400 Regensburg, W.Germany * Siemens AG, Forschungslabor, 8520 Erlangen, W.Germany + Physikalisches Institut, Universitgt Erlangen, 8520 Erlangen, W.Germany We report an infrared reflectivity study of epitaxial YBa2Cu307-6 thin films. Results for samples with different values of To, obtained by Fe doping or ion bombardment, give evidence for a gap 2A(0) ~ 7 kBTc. For ion bombarded films we find a soft mode that may correspond to a giant-amplitude oxygen vibration. The dynamical conductivity in the normal state can be described by a strongly frequency dependent conductivity in the Cu02 planes and a weakly frequency dependent conductivity in the Cu03 chains.
We report on a study of the infrared reflectivity of a high quality epitaxial YBa2Cu307_6 thin film and of perturbed films; for a survey on earlier infrared studies see Ref.[1]. The film (thickness 350 nm), prepared on a SrTiO3 substrate by laser ablation [2], had a d.c. resistivity (60 pf~cm at 100 K) increasing above Tc ( ~ 91 K) proportional to temperature and a critical current density of 2.2 • 10s A/cm 2 at 77 K. The c-axis was perpendicular to the film plane. We produced films with lower Tc either by doping with F e (3% before laser ablation) or by irradiating with 25 MeV zeO ions [3]. All films showed sharp (resistive) superconducting transitions. We measured the reflectivity for perpendicularly incident radiation and probed therefore the dynamical conductivity in the (a,b)-plane. Experimental results are shown in Fig. 1. For the unperturbed film the reflectivity at low temperature (8 K) is nearly 1 up to a frequency of 100 cm -z, decreases then 1.0 ~
~"~I~K
,,,%
slightly and shows a strong decrease above 400 cm -1. The 8 K curve then decreases at 600 cm -1 below the normal state curve and reaches the normal state curve again above 1000 cm -1. The reflectivity decreases towards higher temperatures (Fig. 1) and also towards higher frequencies [4]. The F e doped film shows also a strong temperature dependence of the reflectivity (Fig. 1). Structure is caused by absorption in the substrate; the F e doped film was thinner than the other films. The irradiated samples have reduced reflectivities with smaller influence of superconductivity. In Fig. 2 we have drawn the reflectivity enhancement (Rs - / ~ ) / R ~ where R, is the reflectivity for T < < Tc and P~ for T ~ To. The ranges of enhancement coincide with the ranges predicted by the Mattis-Bardeen theory (dotted) for 2A(0)/kBTc ~ 7. Deviations from theory may he due to partially normalconducting behavior or, especially for the unperturbed film, due to a gap structure [5].
IRRAD~T~
o.9
UN~m'me~
~_1,0
'
'
'
K'
0
'
o:=
ff a9 W
,:
o:
0~
0
bOO
0
200 v (cm -1 )
400
02 0 ~ , ~
/.,
o.z.
T¢f30K 200
0.4
'
K
600
FIGURE 1 Far-infrared reflectivity of YBa2Cu307-6 films. a on leave from ZFW Dresden, GDR 0921-4534/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
Q2.~
.
\
v (cm -11
FIGURE 2 Reflectivity enhancement in the superconducting state.
K.F. Renk et al. I Infrared reflectivity and conductivity of YBa:Cu~Oz_6 thin films
1086
~"15 E u
4O
: ~ 10 20
-'lOOK
>-
FILH
p-
CHAIN 100
T [K)
200
0
300
0
FIGURE 3 Soft mode in irradiated YBa2Cu3OT_6 films. Most striking is the appearance of a low frequency soft mode in the perturbed films (Fig. 1). The frequency of this mode (Fig. 3), lying in the acoustic-phonon region, and also the damping decrease strongly with decreasing temperature. According to an extremely large oscillator strength (Fig. 1), the soft mode has giant vibrational amplitude. We suggest that the mode corresponds to a vibration of oxygen ions along the (a,b)plane. It may be present also in unperturbed films but not directly observable by reasons of symmetry; perturbations leading to local symmetry reduction may give rise to infrared activity. The mode may be involved in superconducting pairing of charge carriers by processes discussed in various theoretical studies [6,7]. At present, we cannot completely exclude that the mode has to do with the substrate. By a Kramers-Kronig transformation of reflectivity curves we obtain for the unperturbed film (above To) a conductivity (Fig. 4a) that decreases strongly with increasing frequency. Because of twinning the film has no preferential a- and b-axis orientation. We describe therefore the conductivity as a sum apt + ½ach where avl is the plane conductivity and ach the chain conductivity which we characterize by a Drude term trch = j~eZ/rh (a[ _ iw)-I where N, rh and ~ are concentration, effective mass and scattering rate of charge carriers in the chains. We then obtain, with N/r~ = 4. 1021 cm-3]mo (too = free-electron mass) and "~ = 4.4.103 cm-~according to results for untwinned crystals [8] for ach the dotted curve of Fig. 4b and for ach the solid curves. The plane conductivity decreases at high frequencies (hw > kT) approximately as w -!. This result gives evidence for hopping conductivity in the planes [9]. According to the large value of ~ hopping conductivity occurs also in the chains. For a further analysis we describe the plane conductivity by art = Ne2 /m(oo) [7(w) - iwm(w)/m(oo)] -1 where N, re(w) and 7(w) are concentration, mass and scattering rate of the holes in the planes. We find N/m(oo) = 8-1021 cm-Z/mo (for 100 K) and m(w) and 7(w) of Fig. 5. According to a strong frequency dependence we estimate for the plane mass an interaction parameter ), ~ 1 for low energy excitations; these can be phonons (e.g. the soft mode of Fig. 3) or spin states.
,
2
I
0 2 v (103cm-1)
4
FIGURE 4 Dynamical conductivities. For the scattering rate we find h7 ~ 2 kT for hw < < kT and h 7 ~ ½hw for hw > > kT. We attribute the chain conductivity to almost localized electron states responsible for intrinsic doping of the planes [10] and find with N = N a ratio rh = 2m(oo); i.e., the chain band may be slightly narrower than the plane band. Assuming N = N = 8.1021 cm -3 (0.7 holes per Cu02) we have m(eo) = too, m(O) .~ 4 mo (at 100 K) and ~
2m
o .
M
4
"I00K
~2 0
i
|
i
i
1
v 1103cm"11
FIGURE 5 Mass and damping of holes in CuO planes. The work was supported by the Bundesministerium ffir Forschung und Technologie. [ 1] T. Timusk and D. Tanner, Infrared Properties of High-To Superconductors, eel. by D.M. Ginsberg (World Scientific Publishing Company), 1989. li] B. Roas et al., Appl. Phys. Lett. 53 (1988)1557. B. Roas et al., Appl. Phys. Lett. 54 (1989) 1051. J. Schfitzmann et al., Europhys. Lett. 8 (1989) 679. U. Hofmann et al., Solid State Comm. 70 (1989) 325. [ 6] J.R. Hardy and J.W. Flocken, Phys. Rev. Lett. 60 (1988) 2191. [7] H. Eschrig and G. Seifert, Physica Scripta T 25 (1989) 88. [8 t B. Koch et al., preprint. T.M. Rice and F.C. Zhang, Phys. Rev. B. 39 (1989) 815. [I0] H. Eschrig, Physica C, in print.
North-Holland
INFRARED REFLECTIVITY AND CONDUCTIVITY OF YBa2Cu307-t THIN FILMS K.F. RENK, J. SCHI]TZMANN, W. OSE, M. FRANZ, J. KELLER, U. HOFMANN, H. ESCHRIG a, B. ROAS *+, B. HENSEL +, and G. SAEMANN-ISCHENKO + Fakultgt Physik, Universit~t Regensburg, 8400 Regensburg, W.Germany * Siemens AG, Forschungslabor, 8520 Erlangen, W.Germany + Physikalisches Institut, Universitgt Erlangen, 8520 Erlangen, W.Germany We report an infrared reflectivity study of epitaxial YBa2Cu307-6 thin films. Results for samples with different values of To, obtained by Fe doping or ion bombardment, give evidence for a gap 2A(0) ~ 7 kBTc. For ion bombarded films we find a soft mode that may correspond to a giant-amplitude oxygen vibration. The dynamical conductivity in the normal state can be described by a strongly frequency dependent conductivity in the Cu02 planes and a weakly frequency dependent conductivity in the Cu03 chains.
We report on a study of the infrared reflectivity of a high quality epitaxial YBa2Cu307_6 thin film and of perturbed films; for a survey on earlier infrared studies see Ref.[1]. The film (thickness 350 nm), prepared on a SrTiO3 substrate by laser ablation [2], had a d.c. resistivity (60 pf~cm at 100 K) increasing above Tc ( ~ 91 K) proportional to temperature and a critical current density of 2.2 • 10s A/cm 2 at 77 K. The c-axis was perpendicular to the film plane. We produced films with lower Tc either by doping with F e (3% before laser ablation) or by irradiating with 25 MeV zeO ions [3]. All films showed sharp (resistive) superconducting transitions. We measured the reflectivity for perpendicularly incident radiation and probed therefore the dynamical conductivity in the (a,b)-plane. Experimental results are shown in Fig. 1. For the unperturbed film the reflectivity at low temperature (8 K) is nearly 1 up to a frequency of 100 cm -z, decreases then 1.0 ~
~"~I~K
,,,%
slightly and shows a strong decrease above 400 cm -1. The 8 K curve then decreases at 600 cm -1 below the normal state curve and reaches the normal state curve again above 1000 cm -1. The reflectivity decreases towards higher temperatures (Fig. 1) and also towards higher frequencies [4]. The F e doped film shows also a strong temperature dependence of the reflectivity (Fig. 1). Structure is caused by absorption in the substrate; the F e doped film was thinner than the other films. The irradiated samples have reduced reflectivities with smaller influence of superconductivity. In Fig. 2 we have drawn the reflectivity enhancement (Rs - / ~ ) / R ~ where R, is the reflectivity for T < < Tc and P~ for T ~ To. The ranges of enhancement coincide with the ranges predicted by the Mattis-Bardeen theory (dotted) for 2A(0)/kBTc ~ 7. Deviations from theory may he due to partially normalconducting behavior or, especially for the unperturbed film, due to a gap structure [5].
IRRAD~T~
o.9
UN~m'me~
~_1,0
'
'
'
K'
0
'
o:=
ff a9 W
,:
o:
0~
0
bOO
0
200 v (cm -1 )
400
02 0 ~ , ~
/.,
o.z.
T¢f30K 200
0.4
'
K
600
FIGURE 1 Far-infrared reflectivity of YBa2Cu307-6 films. a on leave from ZFW Dresden, GDR 0921-4534/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
Q2.~
.
\
v (cm -11
FIGURE 2 Reflectivity enhancement in the superconducting state.
K.F. Renk et al. I Infrared reflectivity and conductivity of YBa:Cu~Oz_6 thin films
1086
~"15 E u
4O
: ~ 10 20
-'lOOK
>-
FILH
p-
CHAIN 100
T [K)
200
0
300
0
FIGURE 3 Soft mode in irradiated YBa2Cu3OT_6 films. Most striking is the appearance of a low frequency soft mode in the perturbed films (Fig. 1). The frequency of this mode (Fig. 3), lying in the acoustic-phonon region, and also the damping decrease strongly with decreasing temperature. According to an extremely large oscillator strength (Fig. 1), the soft mode has giant vibrational amplitude. We suggest that the mode corresponds to a vibration of oxygen ions along the (a,b)plane. It may be present also in unperturbed films but not directly observable by reasons of symmetry; perturbations leading to local symmetry reduction may give rise to infrared activity. The mode may be involved in superconducting pairing of charge carriers by processes discussed in various theoretical studies [6,7]. At present, we cannot completely exclude that the mode has to do with the substrate. By a Kramers-Kronig transformation of reflectivity curves we obtain for the unperturbed film (above To) a conductivity (Fig. 4a) that decreases strongly with increasing frequency. Because of twinning the film has no preferential a- and b-axis orientation. We describe therefore the conductivity as a sum apt + ½ach where avl is the plane conductivity and ach the chain conductivity which we characterize by a Drude term trch = j~eZ/rh (a[ _ iw)-I where N, rh and ~ are concentration, effective mass and scattering rate of charge carriers in the chains. We then obtain, with N/r~ = 4. 1021 cm-3]mo (too = free-electron mass) and "~ = 4.4.103 cm-~according to results for untwinned crystals [8] for ach the dotted curve of Fig. 4b and for ach the solid curves. The plane conductivity decreases at high frequencies (hw > kT) approximately as w -!. This result gives evidence for hopping conductivity in the planes [9]. According to the large value of ~ hopping conductivity occurs also in the chains. For a further analysis we describe the plane conductivity by art = Ne2 /m(oo) [7(w) - iwm(w)/m(oo)] -1 where N, re(w) and 7(w) are concentration, mass and scattering rate of the holes in the planes. We find N/m(oo) = 8-1021 cm-Z/mo (for 100 K) and m(w) and 7(w) of Fig. 5. According to a strong frequency dependence we estimate for the plane mass an interaction parameter ), ~ 1 for low energy excitations; these can be phonons (e.g. the soft mode of Fig. 3) or spin states.
,
2
I
0 2 v (103cm-1)
4
FIGURE 4 Dynamical conductivities. For the scattering rate we find h7 ~ 2 kT for hw < < kT and h 7 ~ ½hw for hw > > kT. We attribute the chain conductivity to almost localized electron states responsible for intrinsic doping of the planes [10] and find with N = N a ratio rh = 2m(oo); i.e., the chain band may be slightly narrower than the plane band. Assuming N = N = 8.1021 cm -3 (0.7 holes per Cu02) we have m(eo) = too, m(O) .~ 4 mo (at 100 K) and ~
2m
o .
M
4
"I00K
~2 0
i
|
i
i
1
v 1103cm"11
FIGURE 5 Mass and damping of holes in CuO planes. The work was supported by the Bundesministerium ffir Forschung und Technologie. [ 1] T. Timusk and D. Tanner, Infrared Properties of High-To Superconductors, eel. by D.M. Ginsberg (World Scientific Publishing Company), 1989. li] B. Roas et al., Appl. Phys. Lett. 53 (1988)1557. B. Roas et al., Appl. Phys. Lett. 54 (1989) 1051. J. Schfitzmann et al., Europhys. Lett. 8 (1989) 679. U. Hofmann et al., Solid State Comm. 70 (1989) 325. [ 6] J.R. Hardy and J.W. Flocken, Phys. Rev. Lett. 60 (1988) 2191. [7] H. Eschrig and G. Seifert, Physica Scripta T 25 (1989) 88. [8 t B. Koch et al., preprint. T.M. Rice and F.C. Zhang, Phys. Rev. B. 39 (1989) 815. [I0] H. Eschrig, Physica C, in print.