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Effects of doping on Eu1.5Ce0.5RuSr2Cu2O10: an STM study
Physica B 284}288 (2000) 647}648
E!ects of doping on Eu Ce RuSr Cu O : an STM study Y. Levi, I. Felner, U. Asaf, O. Millo* Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel
Abstract Scanning tunneling spectroscopy (STS) is used to study changes in the spatial distribution of the density of states in high-temperature superconductors as a function of oxygen and hydrogen doping. The STS data correlate well with macroscopic transport measurements. Upon oxygen doping, STS exhibits an overall increase in the superconducting gap, in agreement with the raising of ¹ . Small hydrogen doping lowers ¹ until the samples become insulating at high ! ! concentrations. At the same time, STS images show a development of insulating regions which coalesce as the doping is increased. This suggests that the superconductor to insulator transition takes place in a percolative fashion, even though the samples are nominally single phase. 2000 Elsevier Science B.V. All rights reserved. Keywords: Density of states; Spatial distribution; STM/STS
The Eu Ce RuSr Cu O compound is the "rst known Cu}O-based material in which superconductivity (SC) and weak ferromagnetism coexist, where the SC transition temperature is signi"cantly smaller than that corresponding to the magnetic transition [1,2]. It is also known for other cuprate systems that both SC and magnetic properties are sensitive to O concentration and H doping. In this paper we focus on the e!ects of O and H doping at the microscopic level, using cryogenic scanning tunneling spectroscopy. The `as-prepareda samples (APS) [1,2] showed ¹ of ! 32 K [Fig. 1(a)]. Upon annealing at ¹"9003C for 24 h under various O pressures, up to 150 atm., ¹ in! creased monotonically up to 49 K [Fig. 1(a), inset]. At an intermediate pressure of 75 atm the resistivity versus ¹ plot exhibits two in#ection points [see Fig. 1(b) and inset], indicating the coexistence of two SC phases. The spatial distributions of the SC gaps, for the APS and the annealed sample are exhibited by the histograms in Fig. 2. In the APS, a small fraction of the surface is normal, i.e., the I}< curves show a gapless structure. The rest of the surface manifests gaps between 3 and 5.5 meV. After annealing, two peaks are observed, con"rming the
* Corresponding author. Tel.:#972 2 658 5670; fax.: 972 2 658 4437 E-mail address: [email protected] (O. Millo)
existence of two SC phases (a trace of the higher ¹ phase is present also in the APS). The ratio between ! the large- and small-gap values is in agreement with that of the two transition temperatures extracted from Fig. 1(b), namely 32 and 45 K.
Fig. 1. (a) Resistivity of the APS as a function of temperature. Inset: ¹ as a function of O pressure in the annealing process. (b) ! Same as (a) but for the sample annealed under 75 atm. Inset: The derivative of the curve, highlighting the two critical temperatures.
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 2 3 0 1 - 7
648
Y. Levi et al. / Physica B 284}288 (2000) 647}648
The e!ect of H doping is to degrade SC. ¹ is lowered ! with increasing H concentration until the system becomes insulating. In order to see how the transition occurs at the microscopic level, we acquired current images of the surface at a bias where the density of states (DOS) of the di!erent phases is considerably di!erent. Even at very low H dopings, isolated insulating regions embedded in the SC matrix start to appear. The signature of a region that becomes insulating is a broadening of the gap in the DOS, above the SC gap, D, resulting in small currents in the current images taken at a bias close to D+5 meV (see Fig. 3). As more H is driven into the material, the insulating regions grow and coalesce, suggesting that the superconductor to insulator transition takes place in a percolative fashion at the microscopic scale, even though the samples are nominally homogeneous.
References Fig. 2. Histograms showing the spatial distribution of the SC energy gaps for the APS and the sample annealed under 75 atm oxygen. Inset: Tunneling dI/d< versus < curves, one taken on a region of small gaps (dotted), the other on a large-gap region (solid).
Fig. 3. Current image taken at a bias of 5 mV on a lightly H doped sample. The bright regions (large current) are SC while the dark ones (small current) are insulating.
[1] I. Felner, U. Asaf, Y. Levi, O. Millo, Phys. Rev. B 55 (1997) R3374. [2] I. Felner, U. Asaf, S.D. Goren, C. Korn, Phys. Rev. B 57 (1998) 550.
E!ects of doping on Eu Ce RuSr Cu O : an STM study Y. Levi, I. Felner, U. Asaf, O. Millo* Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel
Abstract Scanning tunneling spectroscopy (STS) is used to study changes in the spatial distribution of the density of states in high-temperature superconductors as a function of oxygen and hydrogen doping. The STS data correlate well with macroscopic transport measurements. Upon oxygen doping, STS exhibits an overall increase in the superconducting gap, in agreement with the raising of ¹ . Small hydrogen doping lowers ¹ until the samples become insulating at high ! ! concentrations. At the same time, STS images show a development of insulating regions which coalesce as the doping is increased. This suggests that the superconductor to insulator transition takes place in a percolative fashion, even though the samples are nominally single phase. 2000 Elsevier Science B.V. All rights reserved. Keywords: Density of states; Spatial distribution; STM/STS
The Eu Ce RuSr Cu O compound is the "rst known Cu}O-based material in which superconductivity (SC) and weak ferromagnetism coexist, where the SC transition temperature is signi"cantly smaller than that corresponding to the magnetic transition [1,2]. It is also known for other cuprate systems that both SC and magnetic properties are sensitive to O concentration and H doping. In this paper we focus on the e!ects of O and H doping at the microscopic level, using cryogenic scanning tunneling spectroscopy. The `as-prepareda samples (APS) [1,2] showed ¹ of ! 32 K [Fig. 1(a)]. Upon annealing at ¹"9003C for 24 h under various O pressures, up to 150 atm., ¹ in! creased monotonically up to 49 K [Fig. 1(a), inset]. At an intermediate pressure of 75 atm the resistivity versus ¹ plot exhibits two in#ection points [see Fig. 1(b) and inset], indicating the coexistence of two SC phases. The spatial distributions of the SC gaps, for the APS and the annealed sample are exhibited by the histograms in Fig. 2. In the APS, a small fraction of the surface is normal, i.e., the I}< curves show a gapless structure. The rest of the surface manifests gaps between 3 and 5.5 meV. After annealing, two peaks are observed, con"rming the
* Corresponding author. Tel.:#972 2 658 5670; fax.: 972 2 658 4437 E-mail address: [email protected] (O. Millo)
existence of two SC phases (a trace of the higher ¹ phase is present also in the APS). The ratio between ! the large- and small-gap values is in agreement with that of the two transition temperatures extracted from Fig. 1(b), namely 32 and 45 K.
Fig. 1. (a) Resistivity of the APS as a function of temperature. Inset: ¹ as a function of O pressure in the annealing process. (b) ! Same as (a) but for the sample annealed under 75 atm. Inset: The derivative of the curve, highlighting the two critical temperatures.
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 2 3 0 1 - 7
648
Y. Levi et al. / Physica B 284}288 (2000) 647}648
The e!ect of H doping is to degrade SC. ¹ is lowered ! with increasing H concentration until the system becomes insulating. In order to see how the transition occurs at the microscopic level, we acquired current images of the surface at a bias where the density of states (DOS) of the di!erent phases is considerably di!erent. Even at very low H dopings, isolated insulating regions embedded in the SC matrix start to appear. The signature of a region that becomes insulating is a broadening of the gap in the DOS, above the SC gap, D, resulting in small currents in the current images taken at a bias close to D+5 meV (see Fig. 3). As more H is driven into the material, the insulating regions grow and coalesce, suggesting that the superconductor to insulator transition takes place in a percolative fashion at the microscopic scale, even though the samples are nominally homogeneous.
References Fig. 2. Histograms showing the spatial distribution of the SC energy gaps for the APS and the sample annealed under 75 atm oxygen. Inset: Tunneling dI/d< versus < curves, one taken on a region of small gaps (dotted), the other on a large-gap region (solid).
Fig. 3. Current image taken at a bias of 5 mV on a lightly H doped sample. The bright regions (large current) are SC while the dark ones (small current) are insulating.
[1] I. Felner, U. Asaf, Y. Levi, O. Millo, Phys. Rev. B 55 (1997) R3374. [2] I. Felner, U. Asaf, S.D. Goren, C. Korn, Phys. Rev. B 57 (1998) 550.