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Scanning tunneling spectroscopy on La2−xSrxCuO4 under magnetic field
Physica C 412–414 (2004) 250–253 www.elsevier.com/locate/physc
Scanning tunneling spectroscopy on La2xSrxCuO4 under magnetic field T. Kato *, S. Okitsu, M. Murakoso, M. Yokoi, R. Saitou, T. Maruyama, H. Sakata Department of Physics, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan Received 29 October 2003; accepted 8 December 2003 Available online 6 May 2004
Abstract Low temperature scanning tunneling microscopy and spectroscopy measurements have been performed on the optimally doped La1:85 Sr0:15 CuO4 (LSCO) single crystal at 4.2 K in a magnetic field of 6 T applied along the c axis. The spatial distribution of the local density of sates (LDOS) has been successfully obtained on the ab plane. Although the weak distribution of the zero-bias conductance has been observed, the significant change in LDOS originating from the vortex core states has not been observed within our resolution. This result suggests that the vortex core state is rather small in LSCO. 2004 Elsevier B.V. All rights reserved. PACS: 74.50.+r; 74.72.)h; 74.72.Dn Keywords: Scanning tunneling microscopy; Scanning tunneling spectroscopy; High temperature superconductor; La2x Srx CuO4
1. Introduction In addition to topographic imaging with the atomic resolution, low temperature scanning tunneling microscopy and spectroscopy (LT-STM/ STS) can provide direct mapping of the local density of states (LDOS) on a sample surface in real space. A large number of LT-STM/STS measurements have been performed on the high-Tc superconductors (HTSC), mainly on Bi- and Ybased cuprates such as Bi2 Sr2 CaCu2 O8þd (Bi2212)
*
Corresponding auhor. Tel.: +81-3-5228-8219; fax: +81-35261-8475. E-mail address: [email protected] (T. Kato).
and YBa2 Cu3 O7d (YBCO) [1–4]. As regards La2x Srx CuO4 (LSCO) family, on the other hand, LT-STM/STS experiments have not been carried out extensively because of the difficulty in preparing the clean surface and its high surface sensitivity [5–7]. Recently, we have succeeded in observing the topographic images on the ab plane of LSCO single crystals for the first time [8], and verified that LT-STM/STS technique is applicable to LSCO as well as Bi2212 and YBCO. Under the magnetic fields, the superconducting vortices were successfully imaged in YBCO and Bi2212 [9–11] by mapping the low energy additional states in the core. Disordered or oblique square lattice were observed and the LDOS inside the core has similar peaks at ±5.5 meV and ±7
0921-4534/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2003.12.037
T. Kato et al. / Physica C 412–414 (2004) 250–253
meV attributed to the core states in YBCO and Bi2212, respectively. Nevertheless, the interpretation of the core state is different from each other, and there have been several disagreements with the theoretical predictions of the extended quasiparticle states in the d-wave vortices [12]. Consequently, the investigation of the vortices in LSCO by STM/STS is significant for understanding the electronic structure of the vortex core in HTSC. In this paper, we report the LT-STM/STS study on the optimally doped LSCO single crystal at 4.2 K under the magnetic field.
2. Experimental An optimally doped LSCO single crystal (x ¼ 0:15) was grown by a traveling-solvent floating-zone technique [13], cut into 1 · 1 · 3 mm3 rectangles along the c axis, and annealed in air at 900 C for 50 h and 500 C for 50 h. The superconducting transition temperature Tc was determined to be 38 K from dc-SQUID magnetization measurements. The STM/STS measurements were performed using a laboratory build LT-STM. The sample mounted in the LT-STM was cooled down to 4.2 K in pure helium gas and then, in order to obtain a clean and flesh surface for the measurements, cracked perpendicular to the c axis prior to the experiments. The prepared surface was highly flat and almost parallel to the ab plane. Electrochemically etched Au wire was used as a STM tip, which was perpendicular to the exposed ab plane. In this configuration, the tunneling differential conductance dI=dV primarily yields an angular average of the density of states over the ab plane. The bias voltage V was applied to the sample, namely negative and positive bias correspond to occupied and unoccupied states, respectively. After the preparation of the sample surface, a magnetic field of 6 T was applied along the c axis (zero-field cooling), and then surface images and the LDOS maps were obtained by STM/STS. The STM measurements were performed in the constant current mode. Typical tunneling parameters were tunneling current I ¼ 0:03 nA and sample bias voltage V ¼ 300 mV giving a tunneling conductance G ¼ 0:1 nS. We took a large tip-surface
251
spacing in order to avoid the surface degradation by the scanning tip. In the STS measurements, the LDOS maps were acquired with 64 · 64 pixels. Each dI=dV curve was obtained by numerical derivative of the measured current–voltage (I–V ) curve. The typical tunneling condition was I ¼ 0:06 nA and V ¼ 30 mV, and hence G ¼ 2:0 nS.
3. Results and discussion Fig. 1 shows an area of 40 · 40 nm2 topographic image at 4.2 K under the magnetic field of 6 T. Many steps and narrow terraces reflecting the layered structure along the c axis can be observed. This is a typical feature of the ab plane of LSCO prepared by cracking at 4.2 K [8]. Fig. 2 shows 44 spectra chosen randomly from 64 · 64 points on the same area as Fig. 1. Note that these spectra were raw data and not normalized. Most of the obtained spectra exhibit the following characteristics; a large amount of zero-bias conductance (ZBC), small magnitude of the gap-edge peaks, and the asymmetric V-shaped background conductance with higher value at negative bias. While the amounts of ZBC varied spatially from 0.6 to 1.8 nS, the superconducting gap D defined as the half of the energy between the shoulders at about ±10 meV was almost constant. There is no significant change in the tunneling spectra in the magnetic field compared to those in zero magnetic field [8,14].
Fig. 1. A STM image (40 · 40 nm2 ) of the ab plane prepared by cracking at 4.2 K. Many narrow terraces can be seen.
252
T. Kato et al. / Physica C 412–414 (2004) 250–253
dI/dV (nS)
6
4
2
0
-40 -30 -20 -10
0
10
20
30
40
Bias voltage (mV) Fig. 2. Representative dI=dV spectra obtained on ab plane at 4.2 K in the magnetic field of 6 T applied along the c axis.
In order to visualize the spatial distribution of the LDOS, the ZBC was mapped in a gray scale. As shown in Fig. 3(a), the value of the ZBC was varied weakly on the length scale of 10–20 nm and no significant change was observed. The variation in ZBC is approximately 40% of the conductance at about 30 meV. There are no correlations between the ZBC map and the topographic image shown in Fig. 1. Fig. 3(b) shows the spectra in the black (A) and white (B) region (corresponding to low and high ZBC, respectively) depicted in Fig. 3(a). In the white region, the ZBC rises up slightly and the gap feature tends to be smeared compared to the black region. Because D of LSCO is much smaller than that of YBCO and Bi2212, the core states are expected to appear at much lower energies in LSCO, namely, close to Fermi energy. Therefore, the observed change in ZBC may be the signature of the vortices. However, this is suspected by the following reasons. One is that the nearly same degree of the spatial distribution of the ZBC was observed even in the absence of the magnetic field. The origin of this weak change in ZBC is not clear. Possible explanations are the inhomogeneity in superconducting properties reported in Bi2212 [3], inhomogeneous carrier distribution or existence of the excess oxygen [14]. Another is that there is no spatial configuration in the increase in the ZBC corresponding to the
Fig. 3. (a) A ZBC map, measured on the same area as Fig. 1. White and black correspond to high and low ZBC, respectively. (b) Almost 150 superposed dI=dV spectra correspond to the area A and B depicted in Fig. 3(a).
square vortex lattice, which is observed by small angle neutron scattering [15]. The failure in observing the vortices in LSCO in this experiment suggests that the core state in LSCO is rather small as in the case of Bi2212 [11]. This is in contrast to the large core states in YBCO [9] and the origin of this difference is an interesting problem. The further experiments with higher resolution and stability are needed for the imaging of the vortices in LSCO and the resolving the problem of the core states. In summary, we have performed LT-STM/STS measurements on optimally doped LSCO single crystal at 4.2 K in the magnetic field of 6 T applied along the c axis. The spatial distribution of the LDOS has been successfully obtained on the ab plane. Although the weak distribution of the ZBC has been observed, the significant signature of the vortex core states has not been observed within our resolution. This result suggests that the vortex core state in LSCO is rather small in contrast to YBCO.
T. Kato et al. / Physica C 412–414 (2004) 250–253
Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research of the Ministry of Education, Culture, Sports, Science and Technology of Japan.
References [1] Ch. Renner, Ø. Fischer, Phys. Rev. B 51 (1995) 9208. [2] S. Kaneko, N. Nishida, K. Mochiku, K. Kadowaki, Physica C 298 (1998) 105. [3] K.M. Lang, V. Madhavan, J.E. Hoffman, E.W. Hudson, H. Eisaki, S. Uchida, J.C. Davis, Nature 415 (2002) 412. [4] H.L. Edwards, J.T. Markert, A.L. de Lozanne, Phys. Rev. Lett. 69 (1992) 2967. [5] J.R. Kirtley, C.C. Tsuei, S.I. Park, C.C. Chi, J. Rozen, M.W. Shafer, Phys. Rev. B 35 (1987) 7216.
253
[6] S. Pan, K.W. Ng, A.L. de Lozanne, J.M. Tarascon, L.H. Greene, Phys. Rev. B 35 (1987) 7220. [7] M. Oda, T. Matsuzaki, N. Momono, M. Ido, Physica C 341–348 (2000) 847. [8] T. Kato, H. Morimoto, A. Katagiri, S. Okitsu, H. Sakata, Physica C 392–396 (2003) 221. [9] I. Maggio-Aprile, Ch. Renner, A. Erb, E. Walker, Ø. Fischer, Phys. Rev. Lett. 75 (1995) 2754. [10] Ch. Renner, B. Revaz, K. Kadowaki, I. Maggio-Aprile, Ø. Fischer, Phys. Rev. Lett. 80 (1998) 3606. [11] S.H. Pan, E.W. Hudson, A.K. Gupta, K.-W. Ng, H. Eisaki, S. Uchida, J.C. Davis, Phys. Rev. Lett. 85 (2000) 1536. [12] M. Franz, Z. Tesanovic, Phys. Rev. Lett. 80 (1998) 4763. [13] I. Tanaka, K. Yamane, H. Kojima, J. Crystal Growth 96 (1989) 711. [14] T. Kato, S. Okitsu, H. Sakata (in preparation). [15] R. Gilardi, J. Mesot, A. Drew, U. Divakar, S.L. Lee, E.M. Forgan, O. Zaharko, K. Conder, V.K. Aswal, C.D. Dewhurst, R. Cubitt, N. Momono, M. Oda, Phys. Rev. Lett. 88 (2002) 217003.
Scanning tunneling spectroscopy on La2xSrxCuO4 under magnetic field T. Kato *, S. Okitsu, M. Murakoso, M. Yokoi, R. Saitou, T. Maruyama, H. Sakata Department of Physics, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan Received 29 October 2003; accepted 8 December 2003 Available online 6 May 2004
Abstract Low temperature scanning tunneling microscopy and spectroscopy measurements have been performed on the optimally doped La1:85 Sr0:15 CuO4 (LSCO) single crystal at 4.2 K in a magnetic field of 6 T applied along the c axis. The spatial distribution of the local density of sates (LDOS) has been successfully obtained on the ab plane. Although the weak distribution of the zero-bias conductance has been observed, the significant change in LDOS originating from the vortex core states has not been observed within our resolution. This result suggests that the vortex core state is rather small in LSCO. 2004 Elsevier B.V. All rights reserved. PACS: 74.50.+r; 74.72.)h; 74.72.Dn Keywords: Scanning tunneling microscopy; Scanning tunneling spectroscopy; High temperature superconductor; La2x Srx CuO4
1. Introduction In addition to topographic imaging with the atomic resolution, low temperature scanning tunneling microscopy and spectroscopy (LT-STM/ STS) can provide direct mapping of the local density of states (LDOS) on a sample surface in real space. A large number of LT-STM/STS measurements have been performed on the high-Tc superconductors (HTSC), mainly on Bi- and Ybased cuprates such as Bi2 Sr2 CaCu2 O8þd (Bi2212)
*
Corresponding auhor. Tel.: +81-3-5228-8219; fax: +81-35261-8475. E-mail address: [email protected] (T. Kato).
and YBa2 Cu3 O7d (YBCO) [1–4]. As regards La2x Srx CuO4 (LSCO) family, on the other hand, LT-STM/STS experiments have not been carried out extensively because of the difficulty in preparing the clean surface and its high surface sensitivity [5–7]. Recently, we have succeeded in observing the topographic images on the ab plane of LSCO single crystals for the first time [8], and verified that LT-STM/STS technique is applicable to LSCO as well as Bi2212 and YBCO. Under the magnetic fields, the superconducting vortices were successfully imaged in YBCO and Bi2212 [9–11] by mapping the low energy additional states in the core. Disordered or oblique square lattice were observed and the LDOS inside the core has similar peaks at ±5.5 meV and ±7
0921-4534/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2003.12.037
T. Kato et al. / Physica C 412–414 (2004) 250–253
meV attributed to the core states in YBCO and Bi2212, respectively. Nevertheless, the interpretation of the core state is different from each other, and there have been several disagreements with the theoretical predictions of the extended quasiparticle states in the d-wave vortices [12]. Consequently, the investigation of the vortices in LSCO by STM/STS is significant for understanding the electronic structure of the vortex core in HTSC. In this paper, we report the LT-STM/STS study on the optimally doped LSCO single crystal at 4.2 K under the magnetic field.
2. Experimental An optimally doped LSCO single crystal (x ¼ 0:15) was grown by a traveling-solvent floating-zone technique [13], cut into 1 · 1 · 3 mm3 rectangles along the c axis, and annealed in air at 900 C for 50 h and 500 C for 50 h. The superconducting transition temperature Tc was determined to be 38 K from dc-SQUID magnetization measurements. The STM/STS measurements were performed using a laboratory build LT-STM. The sample mounted in the LT-STM was cooled down to 4.2 K in pure helium gas and then, in order to obtain a clean and flesh surface for the measurements, cracked perpendicular to the c axis prior to the experiments. The prepared surface was highly flat and almost parallel to the ab plane. Electrochemically etched Au wire was used as a STM tip, which was perpendicular to the exposed ab plane. In this configuration, the tunneling differential conductance dI=dV primarily yields an angular average of the density of states over the ab plane. The bias voltage V was applied to the sample, namely negative and positive bias correspond to occupied and unoccupied states, respectively. After the preparation of the sample surface, a magnetic field of 6 T was applied along the c axis (zero-field cooling), and then surface images and the LDOS maps were obtained by STM/STS. The STM measurements were performed in the constant current mode. Typical tunneling parameters were tunneling current I ¼ 0:03 nA and sample bias voltage V ¼ 300 mV giving a tunneling conductance G ¼ 0:1 nS. We took a large tip-surface
251
spacing in order to avoid the surface degradation by the scanning tip. In the STS measurements, the LDOS maps were acquired with 64 · 64 pixels. Each dI=dV curve was obtained by numerical derivative of the measured current–voltage (I–V ) curve. The typical tunneling condition was I ¼ 0:06 nA and V ¼ 30 mV, and hence G ¼ 2:0 nS.
3. Results and discussion Fig. 1 shows an area of 40 · 40 nm2 topographic image at 4.2 K under the magnetic field of 6 T. Many steps and narrow terraces reflecting the layered structure along the c axis can be observed. This is a typical feature of the ab plane of LSCO prepared by cracking at 4.2 K [8]. Fig. 2 shows 44 spectra chosen randomly from 64 · 64 points on the same area as Fig. 1. Note that these spectra were raw data and not normalized. Most of the obtained spectra exhibit the following characteristics; a large amount of zero-bias conductance (ZBC), small magnitude of the gap-edge peaks, and the asymmetric V-shaped background conductance with higher value at negative bias. While the amounts of ZBC varied spatially from 0.6 to 1.8 nS, the superconducting gap D defined as the half of the energy between the shoulders at about ±10 meV was almost constant. There is no significant change in the tunneling spectra in the magnetic field compared to those in zero magnetic field [8,14].
Fig. 1. A STM image (40 · 40 nm2 ) of the ab plane prepared by cracking at 4.2 K. Many narrow terraces can be seen.
252
T. Kato et al. / Physica C 412–414 (2004) 250–253
dI/dV (nS)
6
4
2
0
-40 -30 -20 -10
0
10
20
30
40
Bias voltage (mV) Fig. 2. Representative dI=dV spectra obtained on ab plane at 4.2 K in the magnetic field of 6 T applied along the c axis.
In order to visualize the spatial distribution of the LDOS, the ZBC was mapped in a gray scale. As shown in Fig. 3(a), the value of the ZBC was varied weakly on the length scale of 10–20 nm and no significant change was observed. The variation in ZBC is approximately 40% of the conductance at about 30 meV. There are no correlations between the ZBC map and the topographic image shown in Fig. 1. Fig. 3(b) shows the spectra in the black (A) and white (B) region (corresponding to low and high ZBC, respectively) depicted in Fig. 3(a). In the white region, the ZBC rises up slightly and the gap feature tends to be smeared compared to the black region. Because D of LSCO is much smaller than that of YBCO and Bi2212, the core states are expected to appear at much lower energies in LSCO, namely, close to Fermi energy. Therefore, the observed change in ZBC may be the signature of the vortices. However, this is suspected by the following reasons. One is that the nearly same degree of the spatial distribution of the ZBC was observed even in the absence of the magnetic field. The origin of this weak change in ZBC is not clear. Possible explanations are the inhomogeneity in superconducting properties reported in Bi2212 [3], inhomogeneous carrier distribution or existence of the excess oxygen [14]. Another is that there is no spatial configuration in the increase in the ZBC corresponding to the
Fig. 3. (a) A ZBC map, measured on the same area as Fig. 1. White and black correspond to high and low ZBC, respectively. (b) Almost 150 superposed dI=dV spectra correspond to the area A and B depicted in Fig. 3(a).
square vortex lattice, which is observed by small angle neutron scattering [15]. The failure in observing the vortices in LSCO in this experiment suggests that the core state in LSCO is rather small as in the case of Bi2212 [11]. This is in contrast to the large core states in YBCO [9] and the origin of this difference is an interesting problem. The further experiments with higher resolution and stability are needed for the imaging of the vortices in LSCO and the resolving the problem of the core states. In summary, we have performed LT-STM/STS measurements on optimally doped LSCO single crystal at 4.2 K in the magnetic field of 6 T applied along the c axis. The spatial distribution of the LDOS has been successfully obtained on the ab plane. Although the weak distribution of the ZBC has been observed, the significant signature of the vortex core states has not been observed within our resolution. This result suggests that the vortex core state in LSCO is rather small in contrast to YBCO.
T. Kato et al. / Physica C 412–414 (2004) 250–253
Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research of the Ministry of Education, Culture, Sports, Science and Technology of Japan.
References [1] Ch. Renner, Ø. Fischer, Phys. Rev. B 51 (1995) 9208. [2] S. Kaneko, N. Nishida, K. Mochiku, K. Kadowaki, Physica C 298 (1998) 105. [3] K.M. Lang, V. Madhavan, J.E. Hoffman, E.W. Hudson, H. Eisaki, S. Uchida, J.C. Davis, Nature 415 (2002) 412. [4] H.L. Edwards, J.T. Markert, A.L. de Lozanne, Phys. Rev. Lett. 69 (1992) 2967. [5] J.R. Kirtley, C.C. Tsuei, S.I. Park, C.C. Chi, J. Rozen, M.W. Shafer, Phys. Rev. B 35 (1987) 7216.
253
[6] S. Pan, K.W. Ng, A.L. de Lozanne, J.M. Tarascon, L.H. Greene, Phys. Rev. B 35 (1987) 7220. [7] M. Oda, T. Matsuzaki, N. Momono, M. Ido, Physica C 341–348 (2000) 847. [8] T. Kato, H. Morimoto, A. Katagiri, S. Okitsu, H. Sakata, Physica C 392–396 (2003) 221. [9] I. Maggio-Aprile, Ch. Renner, A. Erb, E. Walker, Ø. Fischer, Phys. Rev. Lett. 75 (1995) 2754. [10] Ch. Renner, B. Revaz, K. Kadowaki, I. Maggio-Aprile, Ø. Fischer, Phys. Rev. Lett. 80 (1998) 3606. [11] S.H. Pan, E.W. Hudson, A.K. Gupta, K.-W. Ng, H. Eisaki, S. Uchida, J.C. Davis, Phys. Rev. Lett. 85 (2000) 1536. [12] M. Franz, Z. Tesanovic, Phys. Rev. Lett. 80 (1998) 4763. [13] I. Tanaka, K. Yamane, H. Kojima, J. Crystal Growth 96 (1989) 711. [14] T. Kato, S. Okitsu, H. Sakata (in preparation). [15] R. Gilardi, J. Mesot, A. Drew, U. Divakar, S.L. Lee, E.M. Forgan, O. Zaharko, K. Conder, V.K. Aswal, C.D. Dewhurst, R. Cubitt, N. Momono, M. Oda, Phys. Rev. Lett. 88 (2002) 217003.