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

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46 Chapter 4. Evolution of Electronic States in the Kondo Alloy ... Yb 4f state and B sp valence bands have not been clarified. By studying how the gap collapses with temperature, alloying and magnetic field, one may be able to answer such questions. In this Chapter, we present a photoemission spectroscopy (PES) study of the alloy system Yb1−xLuxB12, in which the gap closes as Lu is substituted for Yb. We have observed both the 4f- and valence-band electronic structures using various photon energies and discussed how the 4f and conduction electrons interact with each other and evolve with Lu-substitution. Figure 4.1: Arrhenius plot for the electrical resistivity of Yb1−xLuxB12 [4.11]. Yb1−xLuxB12 has a UB12-type crystal structure in the whole composition range 0 ≤ x ≤ 1. Figure 4.1 shows that the semiconducting behavior of YbB12 persists up to x ∼ 0.5 in the electrical resistivity [4.11]. The magnetic susceptibility rapidly decreases below ∼ 60 K in the Yb-rich region while it saturates to a constant value in the Lu-rich region [4.11] as shown in Fig. 4.2. It shows a broad maximum at about 75 K and follows a Curie-Weiss law above ∼ 150 K. Magnetic contributions to the low temperature specific heat of Yb1−xLuxB12 are presented in Fig. 4.3. They show a T - linear behavior in Lu-rich (x ≥ 0.5) samples while in x ∼ 0.25 samples they show a clear Schottky-type behavior similar to pure YbB12 with its maximum at ∼ 40 K [4.6], again
4.2. Experimental 47 Figure 4.2: Magnetic susceptibility per mole rare earth (Yb1−xLux) ofYb1−xLuxB12 [4.11]. showing that the gap disappears only for a large amount of Lu-substitution. The Lu 4f level forms the closed-shell 4f 14 configuration and is located well below EF [4.12]. Thus in the first approximation, the extra electrons added by Lu-substitution are trapped in the Lu 4f level and do not contribute to the transport properties. Nevertheless the Lu-substitution leads to the doping of a small number of conduction electrons because the valence of Lu (3+) is somewhat larger than that of Yb (∼ 2.86+) [4.4]. 4.2 Experimental Polycrystalline samples of Yb0.5Lu0.5B12 and LuB12 were prepared by reducing Yb2O3 and Lu2O3 at 2200 ◦ C. The LuB12 samples contained a trace amount (∼3%) of Yb. PES measurements were performed using He resonance lines (He I: hν = 21.2 eV and He II: 40.8 eV) and synchrotron radiation (hν = 125 eV). The latter measurements were made at beam line BL-3B of the Photon Factory, National Laboratory for High Energy Physics. The Fermi edge of Au film evaporated on the sample surface after each series of measurements was used to determine the Fermi level (EF ) position and the instrumental resolution. The resolution was 21-25, ∼28 and ∼55 meV for He I, He II and hν = 125 eV, respectively. All the measurements were done at ∼ 30 K. The base pressure of the spectrometer was ∼ 5 × 10−11 Torr for the He I and He II
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 and 30: 2.2. Experimental 25 Figure 2.5: Il
Page 31 and 32: 2.2. Experimental 27 baking out the
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 52 and 53: 48 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.
Page 81 and 82: Chapter 6 Substitution and Temperat
Page 83 and 84: 6.1. Introduction 79 Figure 6.2: Ma
Page 85 and 86: 6.3. Results and Discussion 81 6.3
Page 87 and 88: 6.3. Results and Discussion 83 DOS
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Page 97: 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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