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Pseudogap features of intrinsic tunneling in Bi2212 single crystals
Physica C 362 (2001) 286±289
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Pseudogap features of intrinsic tunneling in Bi2212 single crystals A. Yurgens a,*, D. Winkler b, T. Claeson a, Seong-Ju Hwang c, Jin-Ho Choy c a
Department of Microelectronics and Nanoscience, Chalmers University of Technology, SE-41296 Gothenburg, Sweden b Imego ± Institute of Microelectronics in Gothenburg, SE-41133 Gothenburg, Sweden c Department of Chemistry, Seoul National University, Seoul 151-742, South Korea Received 23 August 2000
Abstract The c-axis intrinsic tunneling properties of Bi2212 and HgBr2 ±Bi2212 single crystals have been measured in the temperature range 4.2±250 K. The 7±15-unit-cell-high mesa structures on the surfaces of these single crystals were investigated. Clear superconductor±insulator±superconductor like tunneling curves were observed for current applied in the c-axis direction. The dynamic conductance shows both sharp peaks at the superconducting energy gap (SG), followed by dips and wide maxima at larger voltages. The maxima together with depressed conductance at zero voltage persist in the whole temperature range, illustrating the presence of the pseudogap in the quasiparticle excitation spectra, while the SG peaks decrease in voltage and lose their distinctiveness at T ! Tc . The intercalation of Bi2212 with HgBr2 molecules results in a drastic increase of the c-axis lattice constant and resistivity qc . Despite this, the overall shape of the qc T -dependence and the characteristic temperature T at which an upturn in the qc T -dependence sets in upon cooling do not change. This implies that T is not aected by coupling between the CuO2 bilayers. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 74.72.Hs; 74.50.r Keywords: Pseudogap; Intrinsic tunneling; Bi2212
1. Introduction The pseudogap (PG) in electronic excitation spectra is one of the most important features of high-Tc superconductors (HTS) [1]. Such a gap was reported to exist in both underdoped and overdoped samples, and was observed to transform
* Corresponding author. Tel.: +46-31-772-3474; fax: +46-31772-3471. E-mail address: [email protected] (A. Yurgens).
into the superconducting energy gap (SG) upon cooling in a number of experiments [1,2]. The tunneling spectroscopy is particularly sensitive to the density of states (DOS) at the Fermi level and can therefore be used to study any gap in the quasiparticle excitation spectrum. The most common experimental methods, STM and pointcontact techniques are surface probes, and are therefore aected by surface deterioration. In our experiments, we study the tunneling properties of Bi2212 using the intrinsic spectroscopy [3], for which such problems are unimportant. The atomic perfection of the naturally
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 6 8 9 - X
A. Yurgens et al. / Physica C 362 (2001) 286±289
287
occurring tunnel junctions in this material provides a reliable basis for the tunneling spectroscopy inside the single crystal giving a high degree of homogeneity and reproducibility.
2. Experiment The experiments are similar to earlier intrinsictunneling experiments [4,5]. We used lithographically patterned 7±15-unit-cell-high mesa structures on the surface of Bi2212 single crystals to study the c-axis transport properties [6]. In Bi2212, with rather poor thermal and electrical conductivity, the Joule heating at high bias current may become essential, and a short-pulse technique may be needed to overcome such a problem [5]. To decrease heating we have used intercalation by inert HgBr2 molecules to lengthen the c-axis of the host Bi2212 single crystals [7]. Correspondingly, the intrinsic tunneling barriers are becoming wider which results in a drastic decrease of the c-axis critical current density. In turn, the heat released during the current sweep to reach the normal-state tunneling parts of the current±voltage (I±V ) characteristics also drops o [8].
Fig. 1. Typical current±voltage characteristics of a stack of IJJs in a HgBr2 ±Bi2212 single crystal at T 4:2 K. The inset shows the I±V curve at large bias. Note that the I±V branches are evenly spaced in voltage and there is no back-bending at the superconducting gap voltage, con®rming the absence of overheating.
3. Results and discussion Remarkably enough, the intercalation does not change the quality of the I±V 's [8]. These usually consist of evenly voltage-spaced hysteretic quasiparticle branches, see Fig. 1. The total number of branches seen in an I±V plot tells how many intrinsic Josephson junctions (IJJ) are enclosed in the mesa [6]. Fig. 2 shows the c-axis dynamic conductance r dI=dV (V ) for HgBr2 ±Bi2212 at dierent temperatures. It was obtained by dierentiating the last branch corresponding to all 14 IJJ being in the quasiparticle state. Clear superconductor±insulator±superconductor-like tunneling curves are seen for current applied in the c-axis direction. The dynamic conductance shows sharp peaks at the SG, followed
Fig. 2. The average c-axis dynamic conductance r of HgBr2 ± Bi2212 at dierent T. V is the voltage per junction, Tc 70 K.
288
A. Yurgens et al. / Physica C 362 (2001) 286±289
by dips and wide maxima at larger voltages. The maxima together with a depressed conductance at zero voltage persist in the whole temperature range, illustrating the presence of a PG in the quasiparticle excitation spectra, while the SGpeaks decrease in voltage and loose their distinctiveness at T ! Tc . Both SG- and PG-features are clearly resolved at T < Tc . Moreover, the position of the PGfeature is independent of T at T P Tc , while the SG-voltage decreases with T. Therefore, the two phenomena coexist below Tc [9,10]. The wide maxima seem to slightly decrease in voltage with temperature at T 6 Tc , which, in fact, is explained by the presence of temperature-dependent dips at 3D T . Note also that, if the area of junctions is re2 , which may be presumed scaled down to 5 5 A for STM, r is even smaller than typical STM [2] values r 10 12 X 1 . This proves the high quality of intrinsic tunnel junctions. Fig. 3 shows the normalized resistance R for both the pristine Bi2212-, and HgBr2 ±Bi2212mesas of the same areas and heights. It is seen, that the overall shape of the R T -dependence and the characteristic temperature T at which an upturn in R T sets in upon cooling do not change. This
Fig. 3. Superconducting transition curves for Bi2212-, and HgBr2 ±Bi2212-samples. The shape is the same despite the 20fold dierence in absolute values of resistance.
implies that T is not aected by the coupling between the CuO2 bilayers. The PG-feature in Bi2212 is thus derived from the properties of the bilayers alone. PG appears to be a general property of all copper oxides. Numerous models have been developed to explain the main experimental facts, like precursor-superconductivity, spin±charge-separation, or interlayer-exchange models, see Ref. [1] for a review. However, none of the available theoretical models provide a single consistent view of the phenomenon. There is a fundamental common feature of all HTS compounds, the long-known van Hove singularity (VHS) in the density of electronic states which is expected to lie closely below the Fermi energy. It can be derived from tight-band structure calculations (see, for instance Ref. [11]), according to the bare dispersion nk 2t cos kx a cos ky a 4t0 cos kx a cos ky a, including nextnearest-neighbor hopping. The VHS was pointed out as a potential candidate to explain the observed PG-features, 1 and it is the main acting mechanism in the microscopic theory of PG by Onufrieva et al. [13,14]. Although our data alone are not sucient to conclusively distinguish the right model of PG, we ®nd our experiments to be consistent with the VHS-scenario. The wide maxima at V 0:1 eV (see Fig. 2) can in fact be a signature of VHS. Indeed, for t 0:25 eV, and t0 0:4t, normally used to model the Fermi surface of Bi2212, VHS is lying 0.3 eV below the Fermi level. Calculations involving effective band structure parameters extracted from ARPES data [15] give about 35 meV for the VHS position [16]. This means that V 0:1 eV is quite reasonable for a VHS peak in tunneling spectra of Bi2212 [11]. In general, a two-dimensional VHS leads to a tendency for structural or superconducting instability, and there should be a large electron±phonon coupling involved in the VHS-scenario. The dips just above the SG-peaks can possibly be associated
1
See Ref. [12] for an extensive review of VHS-scenario.
A. Yurgens et al. / Physica C 362 (2001) 286±289
with strong electron±phonon coupling eects, as suggested earlier [17].
4. Conclusions The sharp peaks at the SG, and the dips and wide maxima at larger voltages are concluded to be common features of intrinsic tunneling spectra of Bi2212 and HgBr2 ±Bi2212. The latter maxima, together with depressed conductance at zero voltage, demonstrate the presence of a PG in the DOS. The SG-peaks decrease in voltage and loose their distinctiveness at T ! Tc , while the PG-features are temperature independent, implying independence of the two phenomena. The overall shape of the qc T -dependence and the characteristic temperature T are fully governed by properties of CuO2 bilayers alone and are not aected by the coupling between them. Features of intrinsic tunneling in Bi2212 and HgBr2 ±Bi2212, including the PG, may be explained by the presence of a VHS close to the Fermi level.
Acknowledgements Support by the Swedish Superconductivity Consortium and the Swedish Research Council for Engineering Sciences (TFR) is greatly appreciated.
289
References [1] T. Timusk, B. Statt, Rep. Prog. Phys. 62 (1999) 61. [2] Ch. Renner, B. Revaz, J.-Y. Genoud, K. Kadowaki, é. Fisher, Phys. Rev. Lett. 80 (1998) 149. [3] K. Schlenga, R. Kleiner, G. Hecht®scher, M. M oûle, S. Schmitt, P. M uller, C. Helm, C. Preis, F. Forsthofer, J. Keller, H.L. Johnson, M. Veith, E. Steinbeiû, Phys. Rev. B 57 (1998) 14518. [4] I.F.G. Parker, C.E. Gough, M. Endres, P.J. Thomas, G. Yang, A. Yurgens, in: I. Bozovic, D. Pavuna (Eds.), Conference on Superlattices II: Native and Arti®cial, SPIE, Bellingham, WA, 1998, SPIE Proc. 3480 (1998) 11. [5] M. Suzuki, T. Watanabe, A. Matsuda, Phys. Rev. Lett. 82 (1999) 5361. [6] A. Yurgens, D. Winkler, N.V. Zavaritsky, T. Claeson, Appl. Phys. Lett. 70 (1997) 1760. [7] J.-H. Choy, S.-J. Hwang, N.-G. Park, J. Am. Chem. Soc. 119 (1997) 1624. [8] A. Yurgens, D. Winkler, T. Claeson, S.-J. Hwang, J.-H. Choy, Int. J. Mod. Phys. B 13 (1999) 3758. [9] V.M. Krasnov, A. Yurgens, D. Winkler, P. Delsing, T. Claeson, Phys. Rev. Lett. 84 (2000) 5860. [10] J. Demsar, B. Podobnik, V.V. Kabanov, Th. Wolf, D. Mihailovic, Phys. Rev. Lett. 82 (1999) 4918. [11] A.J. Fedro, D.D. Koelling, Phys. Rev. B 47 (1993) 14342. [12] R.S. Markiewicz, J. Phys. Chem. Solids 58 (1997) 1179. [13] F. Onufrieva, P. Pfeuty, M. Kiselev, Phys. Rev. Lett. 82 (1999) 2370. [14] F. Onufrieva, P. Pfeuty, Phys. Rev. Lett. 82 (1999) 3136. [15] M.R. Norman, M. Randeria, H. Ding, J.C. Campuzano, Phys. Rev. B 52 (1995) 615. [16] Z. Yusof, J.F. Zasadzinski, L. Coey, N. Miyakawa, Phys.Rev. B 58 (1998) 514. [17] Y. DeWilde, N. Miyakawa, P. Guptasarma, M. Iavarone, L. Ozyuzer, J.F. Zasadzinski, P. Romano, D.G. Hinks, C. Kendziora, G.W. Crabtree, K.E. Gray, Phys. Rev. Lett. 80 (1998) 153.
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Pseudogap features of intrinsic tunneling in Bi2212 single crystals A. Yurgens a,*, D. Winkler b, T. Claeson a, Seong-Ju Hwang c, Jin-Ho Choy c a
Department of Microelectronics and Nanoscience, Chalmers University of Technology, SE-41296 Gothenburg, Sweden b Imego ± Institute of Microelectronics in Gothenburg, SE-41133 Gothenburg, Sweden c Department of Chemistry, Seoul National University, Seoul 151-742, South Korea Received 23 August 2000
Abstract The c-axis intrinsic tunneling properties of Bi2212 and HgBr2 ±Bi2212 single crystals have been measured in the temperature range 4.2±250 K. The 7±15-unit-cell-high mesa structures on the surfaces of these single crystals were investigated. Clear superconductor±insulator±superconductor like tunneling curves were observed for current applied in the c-axis direction. The dynamic conductance shows both sharp peaks at the superconducting energy gap (SG), followed by dips and wide maxima at larger voltages. The maxima together with depressed conductance at zero voltage persist in the whole temperature range, illustrating the presence of the pseudogap in the quasiparticle excitation spectra, while the SG peaks decrease in voltage and lose their distinctiveness at T ! Tc . The intercalation of Bi2212 with HgBr2 molecules results in a drastic increase of the c-axis lattice constant and resistivity qc . Despite this, the overall shape of the qc T -dependence and the characteristic temperature T at which an upturn in the qc T -dependence sets in upon cooling do not change. This implies that T is not aected by coupling between the CuO2 bilayers. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 74.72.Hs; 74.50.r Keywords: Pseudogap; Intrinsic tunneling; Bi2212
1. Introduction The pseudogap (PG) in electronic excitation spectra is one of the most important features of high-Tc superconductors (HTS) [1]. Such a gap was reported to exist in both underdoped and overdoped samples, and was observed to transform
* Corresponding author. Tel.: +46-31-772-3474; fax: +46-31772-3471. E-mail address: [email protected] (A. Yurgens).
into the superconducting energy gap (SG) upon cooling in a number of experiments [1,2]. The tunneling spectroscopy is particularly sensitive to the density of states (DOS) at the Fermi level and can therefore be used to study any gap in the quasiparticle excitation spectrum. The most common experimental methods, STM and pointcontact techniques are surface probes, and are therefore aected by surface deterioration. In our experiments, we study the tunneling properties of Bi2212 using the intrinsic spectroscopy [3], for which such problems are unimportant. The atomic perfection of the naturally
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 6 8 9 - X
A. Yurgens et al. / Physica C 362 (2001) 286±289
287
occurring tunnel junctions in this material provides a reliable basis for the tunneling spectroscopy inside the single crystal giving a high degree of homogeneity and reproducibility.
2. Experiment The experiments are similar to earlier intrinsictunneling experiments [4,5]. We used lithographically patterned 7±15-unit-cell-high mesa structures on the surface of Bi2212 single crystals to study the c-axis transport properties [6]. In Bi2212, with rather poor thermal and electrical conductivity, the Joule heating at high bias current may become essential, and a short-pulse technique may be needed to overcome such a problem [5]. To decrease heating we have used intercalation by inert HgBr2 molecules to lengthen the c-axis of the host Bi2212 single crystals [7]. Correspondingly, the intrinsic tunneling barriers are becoming wider which results in a drastic decrease of the c-axis critical current density. In turn, the heat released during the current sweep to reach the normal-state tunneling parts of the current±voltage (I±V ) characteristics also drops o [8].
Fig. 1. Typical current±voltage characteristics of a stack of IJJs in a HgBr2 ±Bi2212 single crystal at T 4:2 K. The inset shows the I±V curve at large bias. Note that the I±V branches are evenly spaced in voltage and there is no back-bending at the superconducting gap voltage, con®rming the absence of overheating.
3. Results and discussion Remarkably enough, the intercalation does not change the quality of the I±V 's [8]. These usually consist of evenly voltage-spaced hysteretic quasiparticle branches, see Fig. 1. The total number of branches seen in an I±V plot tells how many intrinsic Josephson junctions (IJJ) are enclosed in the mesa [6]. Fig. 2 shows the c-axis dynamic conductance r dI=dV (V ) for HgBr2 ±Bi2212 at dierent temperatures. It was obtained by dierentiating the last branch corresponding to all 14 IJJ being in the quasiparticle state. Clear superconductor±insulator±superconductor-like tunneling curves are seen for current applied in the c-axis direction. The dynamic conductance shows sharp peaks at the SG, followed
Fig. 2. The average c-axis dynamic conductance r of HgBr2 ± Bi2212 at dierent T. V is the voltage per junction, Tc 70 K.
288
A. Yurgens et al. / Physica C 362 (2001) 286±289
by dips and wide maxima at larger voltages. The maxima together with a depressed conductance at zero voltage persist in the whole temperature range, illustrating the presence of a PG in the quasiparticle excitation spectra, while the SGpeaks decrease in voltage and loose their distinctiveness at T ! Tc . Both SG- and PG-features are clearly resolved at T < Tc . Moreover, the position of the PGfeature is independent of T at T P Tc , while the SG-voltage decreases with T. Therefore, the two phenomena coexist below Tc [9,10]. The wide maxima seem to slightly decrease in voltage with temperature at T 6 Tc , which, in fact, is explained by the presence of temperature-dependent dips at 3D T . Note also that, if the area of junctions is re2 , which may be presumed scaled down to 5 5 A for STM, r is even smaller than typical STM [2] values r 10 12 X 1 . This proves the high quality of intrinsic tunnel junctions. Fig. 3 shows the normalized resistance R for both the pristine Bi2212-, and HgBr2 ±Bi2212mesas of the same areas and heights. It is seen, that the overall shape of the R T -dependence and the characteristic temperature T at which an upturn in R T sets in upon cooling do not change. This
Fig. 3. Superconducting transition curves for Bi2212-, and HgBr2 ±Bi2212-samples. The shape is the same despite the 20fold dierence in absolute values of resistance.
implies that T is not aected by the coupling between the CuO2 bilayers. The PG-feature in Bi2212 is thus derived from the properties of the bilayers alone. PG appears to be a general property of all copper oxides. Numerous models have been developed to explain the main experimental facts, like precursor-superconductivity, spin±charge-separation, or interlayer-exchange models, see Ref. [1] for a review. However, none of the available theoretical models provide a single consistent view of the phenomenon. There is a fundamental common feature of all HTS compounds, the long-known van Hove singularity (VHS) in the density of electronic states which is expected to lie closely below the Fermi energy. It can be derived from tight-band structure calculations (see, for instance Ref. [11]), according to the bare dispersion nk 2t cos kx a cos ky a 4t0 cos kx a cos ky a, including nextnearest-neighbor hopping. The VHS was pointed out as a potential candidate to explain the observed PG-features, 1 and it is the main acting mechanism in the microscopic theory of PG by Onufrieva et al. [13,14]. Although our data alone are not sucient to conclusively distinguish the right model of PG, we ®nd our experiments to be consistent with the VHS-scenario. The wide maxima at V 0:1 eV (see Fig. 2) can in fact be a signature of VHS. Indeed, for t 0:25 eV, and t0 0:4t, normally used to model the Fermi surface of Bi2212, VHS is lying 0.3 eV below the Fermi level. Calculations involving effective band structure parameters extracted from ARPES data [15] give about 35 meV for the VHS position [16]. This means that V 0:1 eV is quite reasonable for a VHS peak in tunneling spectra of Bi2212 [11]. In general, a two-dimensional VHS leads to a tendency for structural or superconducting instability, and there should be a large electron±phonon coupling involved in the VHS-scenario. The dips just above the SG-peaks can possibly be associated
1
See Ref. [12] for an extensive review of VHS-scenario.
A. Yurgens et al. / Physica C 362 (2001) 286±289
with strong electron±phonon coupling eects, as suggested earlier [17].
4. Conclusions The sharp peaks at the SG, and the dips and wide maxima at larger voltages are concluded to be common features of intrinsic tunneling spectra of Bi2212 and HgBr2 ±Bi2212. The latter maxima, together with depressed conductance at zero voltage, demonstrate the presence of a PG in the DOS. The SG-peaks decrease in voltage and loose their distinctiveness at T ! Tc , while the PG-features are temperature independent, implying independence of the two phenomena. The overall shape of the qc T -dependence and the characteristic temperature T are fully governed by properties of CuO2 bilayers alone and are not aected by the coupling between them. Features of intrinsic tunneling in Bi2212 and HgBr2 ±Bi2212, including the PG, may be explained by the presence of a VHS close to the Fermi level.
Acknowledgements Support by the Swedish Superconductivity Consortium and the Swedish Research Council for Engineering Sciences (TFR) is greatly appreciated.
289
References [1] T. Timusk, B. Statt, Rep. Prog. Phys. 62 (1999) 61. [2] Ch. Renner, B. Revaz, J.-Y. Genoud, K. Kadowaki, é. Fisher, Phys. Rev. Lett. 80 (1998) 149. [3] K. Schlenga, R. Kleiner, G. Hecht®scher, M. M oûle, S. Schmitt, P. M uller, C. Helm, C. Preis, F. Forsthofer, J. Keller, H.L. Johnson, M. Veith, E. Steinbeiû, Phys. Rev. B 57 (1998) 14518. [4] I.F.G. Parker, C.E. Gough, M. Endres, P.J. Thomas, G. Yang, A. Yurgens, in: I. Bozovic, D. Pavuna (Eds.), Conference on Superlattices II: Native and Arti®cial, SPIE, Bellingham, WA, 1998, SPIE Proc. 3480 (1998) 11. [5] M. Suzuki, T. Watanabe, A. Matsuda, Phys. Rev. Lett. 82 (1999) 5361. [6] A. Yurgens, D. Winkler, N.V. Zavaritsky, T. Claeson, Appl. Phys. Lett. 70 (1997) 1760. [7] J.-H. Choy, S.-J. Hwang, N.-G. Park, J. Am. Chem. Soc. 119 (1997) 1624. [8] A. Yurgens, D. Winkler, T. Claeson, S.-J. Hwang, J.-H. Choy, Int. J. Mod. Phys. B 13 (1999) 3758. [9] V.M. Krasnov, A. Yurgens, D. Winkler, P. Delsing, T. Claeson, Phys. Rev. Lett. 84 (2000) 5860. [10] J. Demsar, B. Podobnik, V.V. Kabanov, Th. Wolf, D. Mihailovic, Phys. Rev. Lett. 82 (1999) 4918. [11] A.J. Fedro, D.D. Koelling, Phys. Rev. B 47 (1993) 14342. [12] R.S. Markiewicz, J. Phys. Chem. Solids 58 (1997) 1179. [13] F. Onufrieva, P. Pfeuty, M. Kiselev, Phys. Rev. Lett. 82 (1999) 2370. [14] F. Onufrieva, P. Pfeuty, Phys. Rev. Lett. 82 (1999) 3136. [15] M.R. Norman, M. Randeria, H. Ding, J.C. Campuzano, Phys. Rev. B 52 (1995) 615. [16] Z. Yusof, J.F. Zasadzinski, L. Coey, N. Miyakawa, Phys.Rev. B 58 (1998) 514. [17] Y. DeWilde, N. Miyakawa, P. Guptasarma, M. Iavarone, L. Ozyuzer, J.F. Zasadzinski, P. Romano, D.G. Hinks, C. Kendziora, G.W. Crabtree, K.E. Gray, Phys. Rev. Lett. 80 (1998) 153.