Time resolved electrical injection of coherent spin packets through a Schottky barrier [Elektronische Ressource] / vorgelegt von Lars Reiner Schreiber

Sujets

• Physik

Informations

• Publié par: rheinisch-westfalischen_technischen_hochschule_-rwth-_aachen
• Publié le: 01 janvier 2008
• Nombre de lectures: 7
• Langue: English
• Poids de l'ouvrage: 51 Mo

Extrait

Time-resolved electrical injection of coherent spin packets
through a Schottky barrier
Von der Fakultät für Mathematik, Informatik und Naturwissenschaften der
Rheinisch-Westfälischen Technischen Hochschule Aachen zur Erlangung des
akademischen Grades eines Doktors der Naturwissenschaften genehmigte Dissertation
vorgelegt von
Diplom-Physiker (Univ.) Lars Reiner Schreiber (M.Phil. Univ. of Cambridge)
aus Hilden
Universitätsprofessor Dr. sc. nat. Gernot Güntherodt
Berichter:
Univr Dr. rer. nat. Hans Lüth
Tag der mündlichen Prüfung: 7. März 2008
Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online verfügbar.Contents
Contents i
List of Figures v
List of Tables ix
1 Introduction 1
2 Theory 5
2.1 Coherent spin precession . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.1 Single electron spin precession . . . . . . . . . . . . . . . . . . . . . . 5
2.1.2 Precession of spin ensembles . . . . . . . . . . . . . . . . . . . . . . . 7
2.1.3 Summary of spin precession . . . . . . . . . . . . . . . . . . . . . . . 13
2.2 III-V heterostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.3 Spin injection and spin pumping . . . . . . . . . . . . . . . . . . . . . . . . 16
2.3.1 Electrical spin injection . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.3.2 Electrical spin i and coherent spin precession . . . . . . . . . 20
2.3.3 Optical spin pumping . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.4 Spin relaxation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.5 Dynamic nuclear polarization . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3 Experimental set-up 31
3.1 Set-up of time-resolved electrical spin injection . . . . . . . . . . . . . . . . 31
3.1.1 The probe laser pulses . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.1.2 Voltage pulse generation and phase coupling . . . . . . . . . . . . . . 34
3.1.3 The magneto-optical cryostat . . . . . . . . . . . . . . . . . . . . . . 36
3.1.4 The Faraday rotation angle . . . . . . . . . . . . . . . . . . . . . . . 38
3.1.5 The detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.1.6 Dual lock-in technique . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.2 Set-up of the all-optical pump-probe experiment . . . . . . . . . . . . . . . 40
3.2.1 One-color and two-color experiment . . . . . . . . . . . . . . . . . . 40
3.2.2 Modifications to the dual lock-in technique . . . . . . . . . . . . . . 43
3.3 Automation of the experiment . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4 Sample properties 45
4.1 Layer structure of the samples under investigation . . . . . . . . . . . . . . 46
4.1.1 Electrical injection samples Rx and CA139 . . . . . . . . . . . . . . 46
4.1.2 Sample GaAs2E16 and the (In,Ga)As/GaAs QWs A, B, C and D . . 48
4.2 Spin injection, transport and detection . . . . . . . . . . . . . . . . . . . . . 50
4.2.1 Magnetic anisotropy of the Fe injector . . . . . . . . . . . . . . . . . 52
4.2.2 Spin dephasing time in the semiconductor . . . . . . . . . . . . . . . 54
iContents
4.2.3 Influence of hot carrier relaxation on the spin . . . . . . . . . . . . . 58
4.2.4 Spin injection efficiency . . . . . . . . . . . . . . . . . . . . . . . . . 61
4.3 Electrical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
4.3.1 Basics of high frequency electronics . . . . . . . . . . . . . . . . . . . 64
4.3.2 Coplanar waveguides and high frequency connection . . . . . . . . . 67
4.3.3 I-V characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
4.3.4 Vector network analysis . . . . . . . . . . . . . . . . . . . . . . . . . 71
4.3.5 Time-domain reflectometry . . . . . . . . . . . . . . . . . . . . . . . 73
4.4 Summary of the sample properties and outlook . . . . . . . . . . . . . . . . 79
5 Time-resolved electrical spin injection 81
5.1 Proof of time-resolved electrical spin injection . . . . . . . . . . . . . . . . . 83
5.1.1 Flipping the injector magnetization . . . . . . . . . . . . . . . . . . . 84
5.1.2 Pulsed injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
5.1.3 Peculiarities of pulsed electrical injection . . . . . . . . . . . . . . . . 90
5.1.4 Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . 92
5.2 Continuous electrical and optical spin injection . . . . . . . . . . . . . . . . 92
5.2.1 Hanlé depolarization as a function of bias . . . . . . . . . . . . . . 93
5.2.2 Spin injection rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
5.2.3 Spin dephasing time . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
5.2.4 Injected spin density . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
5.2.5 Probe laser energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
5.2.6 Summary of continuous injection results . . . . . . . . . . . . . . . . 100
5.3 Spin injection and precession in the time-domain . . . . . . . . . . . . . . . 100
5.3.1 Experimental observations . . . . . . . . . . . . . . . . . . . . . . . . 101
5.3.2 A model for pulsed spin injection . . . . . . . . . . . . . . . . . . . . 105
5.3.3 Data analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
5.3.4 Summary of spin injection and precession in the time-domain . . . . 123
5.4 Time-resolved spin injection and resonant spin amplification . . . . . . . . . 124
5.4.1 The extended model for resonant spin amplification . . . . . . . . . . 125
5.4.2 Experiment vs. simulation . . . . . . . . . . . . . . . . . . . . . . . . 125
5.4.3 Sophisticated pulse patterns and outlook . . . . . . . . . . . . . . . . 132
5.4.4 Reverse bias pulse and outlook . . . . . . . . . . . . . . . . . . . . . 134
5.4.5 Summary of time-resolved spin injection and resonant spin amplifi-
cation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
6 Dynamic nuclear polarization by electric spin injection 137
6.1 Superposition of pulsed and direct current . . . . . . . . . . . . . . . . . . . 137
6.1.1 Experimental observations and geometrical considerations . . . . . . 138
6.1.2 Time constant of dynamic nuclear polarization . . . . . . . . . . . . 142
6.1.3 Nuclear magnetic resonance . . . . . . . . . . . . . . . . . . . . . . . 145
6.1.4 Summary of the superposition of pulsed and direct current . . . . . . 148
6.2 All-optical dynamic nuclear polarization . . . . . . . . . . . . . . . . . . . . 150
6.2.1 The role of the pump laser energy . . . . . . . . . . . . . . . . . . . 152
6.2.2 Time constant of nuclear polarization . . . . . . . . . . . . . . . . . 154
6.2.3 Nuclear polarization as a function of the external magnetic field . . . 158
6.2.4 Summary of all-optical dynamic nuclear polarization . . . . . . . . . 162
6.3 Bias-controlled dynamic nuclear polarization . . . . . . . . . . . . . . . . . . 163
iiContents
6.3.1 Switching the direct-current and nuclear polarization time . . . . . . 164
6.3.2 Nuclear magnetic field dependence on the magnitude of the external
bias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
6.3.3 Nuclear magnetic field separated for the nuclei species . . . . . . . . 167
6.3.4 Summary of bias-controlled dynamic nuclear polarization . . . . . . 169
7 Anisotropic spin dephasing 171
7.1 Samples under investigation and measurement geometry . . . . . . . . . . . 172
7.2 Experimental observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174
7.3 Amodelfortheanomalousspinprecessionfordeterminationoftheanisotropy176
7.4 Calculation of the spin dephasing anisotropy . . . . . . . . . . . . . . . . . . 181
7.5 Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
8 Conclusion 185
A Mathematical proofs I
A.1 General ansatz for the evolution of the spin-induced net magnetization . . . I
A.1.1 Net magnetization of a time-dependent repetitive spin injection rate I
A.1.2 Repetitive and abrupt spin injection . . . . . . . . . . . . . . . . . . III
A.1.3 Repetitive spin injection regarding a pulse width Δw . . . . . . . . III
A.1.4 Repetitive spin ing the charging and discharging of
the Schottky diode . . . . . . . . . . . . . . . . . . . . . . . . . . . V
Acronyms and symbols IX
Acknowledgements XI
Bibliography XIV
iiiList of Figures
1.1 Schematic of the basic idea of the experiment . . . . . . . . . . . . . . . . . 2
2.1 Coordinates on a Bloch sphere . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 Larmor precession of a spin ensemble . . . . . . . . . . . . . . . . . . . . . 9
2.3 Hanlé depolarization for continuous injection . . . . . . . . . . . . . . . . . 10
2.4 Resonant spin amplification for repetitive pulsed injection . . . . . . . . . . 12
2.5 Energy gap and lattice constant for III-V compounds . . . . . . . . . . . . . 14
2.6 Band structure of GaAs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.7 Spindephasingtimeinn-bulkGaAs asafunctionofthedopingconcentration 16
2.8 Band diagram of an unbiased In Ga As/GaAs spin LED . . . . . . . . . 180.1 0.9
2.9 Geometries for electrical spin injection . . . . . . . . . . . . . . . . . . . . . 20
2.10 Electroluminescence polarization hysteresis loops . . . . . . . . . . . . . . . 21
2.11 Spin current flowing