Thesis High-Resolution Photoemission Study of Kondo Insulators ...

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26 Chapter 2. Photoemission Spectroscopy contamination appears at higher temperatures under the same sample condition. Impurities through the light capillary can be an origin of the contamination. Such contamination in the photoemission spectra develops in proportion to the time of exposing the sample to the light although it does not grow while the sample is kept away from the He lamp. These impurities accompanying the He flow are difficult to detect since the local pressure at the sample position, which may be considerably different from that at the gauge tube, is responsible for contamination. Possible sources of the impurities are (i) the impurities originally included in the gas, (ii) degasing from the tube, where sufficient baking is difficult, (iii) contamination at the first stage pumping, and (iv) contamination at the second stage pumping. We discuss all of the possibilities below. The impurities originally included in the He gas [(i)] is rather small amount. We used He gas of the highest purity (99.9999%) that is commercially available. The impurities at room temperature are: Molecule Amount H2O 0.5 ppm N2 < 0.1 ppm O2 < 0.05 ppm CO < 0.02 ppm CO2 < 0.02 ppm CH4 < 0.01 ppm Since the local pressure at the sample position is at least higher than that at the second stage (3 × 10−6 Torr), the contribution of the 1 ppm impurities, originally included in the He gas, is 3 × 10−12 Torr at most. Nevertheless, in order to remove H2O, which is the main component of the impurities in the gas, we pass the He gas into a temporary storage tank through a molecular sieve trap7 cooled with liquid nitrogen. The degasing from the tube [(ii)] might be crucial. We have installed a turbo pump for the gas-introducing tube to bake it out. Pumping only through the He lamp, whose conductance is very low, from the measurement chamber would not be sufficient on 7 We did not provide a trap with an auto-release valve, with which a commercial molecular sieve trap is equipped, because its seal is not compatible with an ultrahigh vacuum. Thus we had to pump the trap whenever we finished passing the gas. Note that the high-pressure limit of a standard ultrahigh vacuum chamber is about 5 atm.
2.2. Experimental 27 baking out the tube. Entering impurities at the differentially pumping stages [(iii) and (iv)] could be the most probable origins of the contamination. In the original setup, we pumped the second stage and the rotary feedthrough of the manipulator by one turbo pump simultaneously and found that the impurities entering from the rotary feedthrough was leaked into the chamber. As for the first-stage pumping, it is equipped with a rotary pump. We have tried both a turbo pump and a rotary pump in the other system, which is equipped with the same VG lamp and the VSW CLASS-150 analyzer, and have found no impurities entering the chamber through the light capillary. 2.2.2 Background Correction Intensity (arb. units) raw spectrum He I spectrum background LuB 12 -200 -100 0 Energy relative to E F (meV) Figure 2.6: Raw photoemission data of LuB12, the linear background due to the satellite lines, and the background-corrected He I photoemission spectrum. Before dividing the He I (He I α, 21.22 eV) photoemission spectrum by the FD function, one should remember that photoemission signals excited by the He II (40.8 eV) and He I β (23.08 eV) lines, the energy of which are higher than that of He I α, always overlap the He I spectrum as a background. Although the intensity ratio of the satellite lines to the He I α line is much less than one, the ratio is fixed to be
Page 1: Thesis High-Resolution Photoemissio
Page 4 and 5: ii CONTENTS 5.2 Experimental . . .
Page 6 and 7: 2 Chapter 1. Introduction Figure 1.
Page 8 and 9: 4 Chapter 1. Introduction Figure 1.
Page 10 and 11: 6 Chapter 1. Introduction peaks, wh
Page 12 and 13: 8 Chapter 1. Introduction only the
Page 14 and 15: 10 Chapter 1. Introduction directio
Page 16 and 17: 12 Chapter 1. Introduction activati
Page 18 and 19: 14 References [1.14] F. Iga, M. Kas
Page 21 and 22: Chapter 2 Photoemission Spectroscop
Page 23 and 24: 2.1. General Principles 19 Photoele
Page 25 and 26: 2.1. General Principles 21 where R(
Page 27 and 28: 2.2. Experimental 23 DOS Intensity
Page 29: 2.2. Experimental 25 Figure 2.5: Il
Page 33: References [2.1] O. Gunnarsson and
Page 36 and 37: 32 Chapter 3. Photoemission Study o
Page 48 and 49: 44 References the Kondo peak positi
Page 50 and 51: 46 Chapter 4. Evolution of Electron
Page 63 and 64: References [4.1] G. Aeppli and Z. F
Page 65: References 61 [4.25] P. Weibel,M. G
Page 68 and 69: 64 Chapter 5. Temperature Dependenc
Page 78 and 79: 74 References [5.10] N. Shimizu, F.
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Chapter 6 Substitution and Temperat
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6.1. Introduction 79 Figure 6.2: Ma
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6.3. Results and Discussion 81 6.3
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6.3. Results and Discussion 83 DOS
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6.3. Results and Discussion 85 dist
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6.3. Results and Discussion 87 Apar
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6.3. Results and Discussion 89 of E
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6.3. Results and Discussion 91 the
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6.4. Summary 93 and consistent anal
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96 References [6.11] M. J. Rozenber
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98 Chapter 7. Temperature and Co-Su
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100 Chapter 7. Temperature and Co-S
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References [7.1] G. Aeppli and Z. F
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Chapter 8 Temperature and Al-Substi
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8.2. Experimental 113 and Fisk [8.2
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8.3. Results and Discussion 115 8.3
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8.3. Results and Discussion 117 Int
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8.3. Results and Discussion 119 dep
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8.3. Results and Discussion 121 σ
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8.4. Summary 123 temperature magnet
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126 References [8.14] T. Tonogai, A
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128 Chapter 9. Conclusion has shown
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130 Chapter 9. Conclusion (a) FeSi
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132 Chapter 9. Conclusion developed
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