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**1 - 9**of**9**### Engineering Acknowledgments

, 1996

"... Personal thanks is given to my advisors, Prof. T.E. Schlesinger and Prof. D.D. Stancil for their knowledge, advice, encouragement, and guidance. Special thanks goes to Yi Chiu, Anna Chernakova, Venkat Gopalan, Matthew Kawas, and Jun Li for the invaluable discussion, suggestion and assistance. I woul ..."

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Personal thanks is given to my advisors, Prof. T.E. Schlesinger and Prof. D.D. Stancil for their knowledge, advice, encouragement, and guidance. Special thanks goes to Yi Chiu, Anna Chernakova, Venkat Gopalan, Matthew Kawas, and Jun Li for the invaluable discussion, suggestion and assistance. I would like to thank Prof. S. Tan for his assistance and suggestions regarding lithium tantalate deposition. Special thanks is given to Mark Mescher and Takeshi Abe for their assistance in using the sputtering machine. I would also like to thank many graduate students and postdoctors who provided technical assistance, suggestions, and help, especially Wei Yang and Jing Zhang. Finally, I would like to thank Betty, Anne, Meg, Elaine, Lynn and all the others for the tremendous help and kindness which they have given.

### Excitonic Condensation in Bilayer Systems Committee:

"... I would like to express my gratitude to my advisor, Allan MacDonald. He is a great physicist and great advisor who always shows patience and kindness. He has taught me physics and the different ways to approach physics. He is truly a role model to his students. I would like to thank Rafi Bistritzer ..."

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I would like to express my gratitude to my advisor, Allan MacDonald. He is a great physicist and great advisor who always shows patience and kindness. He has taught me physics and the different ways to approach physics. He is truly a role model to his students. I would like to thank Rafi Bistritzer and Hongki Min on the graphene project. I would also like to Wei-Cheng for the many very helpful discussions and information. Thanks to Tami on our new project and for the suggestions for the future. Also, thanks to Maria, Ion, Joseph, Yafis and Dagim for your help from time to time. Special thanks to Becky Drake; her very kindly assistance has always been touching. Finally, I would like to thank my family for all their encouragement and support.

### Acknowledgements

, 2004

"... This thesis is a product of the training, guidance, and overall direction provided by ..."

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This thesis is a product of the training, guidance, and overall direction provided by

### FACULTÉ DES SCIENCES

"... Excitations avec texture de spin et de pseudospin dans le graphène ..."

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### Director

, 2009

"... Composition du jury Le, le jury a accepte ́ le mémoire de M. Luo dans sa version finale. ..."

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Composition du jury Le, le jury a accepte ́ le mémoire de M. Luo dans sa version finale.

### Prepared for submission to JHEP Holographic D3-probe-D5 Model of a Double Layer Dirac Semimetal

"... Abstract: The possibility of inter-layer exciton condensation in a holographic D3-probe-D5 brane model of a strongly coupled double monolayer Dirac semi-metal in a magnetic field is studied in detail. It is found that, when the charge densities on the layers are exactly balanced so that, at weak cou ..."

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Abstract: The possibility of inter-layer exciton condensation in a holographic D3-probe-D5 brane model of a strongly coupled double monolayer Dirac semi-metal in a magnetic field is studied in detail. It is found that, when the charge densities on the layers are exactly balanced so that, at weak coupling, the Fermi surfaces of electrons in one monolayer and holes in the other monolayer would be perfectly nested, inter-layer condensates can form for any separation of the layers. The case where both monolayers are charge neutral is special. There, the inter-layer condensate occurs only for small separations and is replaced by an intra-layer exciton condensate at larger separations. The phase diagram for charge balanced monolayers for a range layer separations and chemical potentials is found. We also show that, in semi-metals with multiple species of massless fermions, the balance of charges required for Fermi surface nesting can occur spontaneously by breaking some of the internal symmetry of the monolayers. This could have important consequences for experimental attempts to find inter-layer condensates. ar X iv

### A Holographic Quantum Hall Ferromagnet

"... A detailed numerical study of a recent proposal for exotic states of the D3-probe D5 brane system with charge density and an external magnetic field is presented. The state has a large number of coincident D5 branes blowing up to a D7 brane in the presence of the worldvolume electric and magnetic fi ..."

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A detailed numerical study of a recent proposal for exotic states of the D3-probe D5 brane system with charge density and an external magnetic field is presented. The state has a large number of coincident D5 branes blowing up to a D7 brane in the presence of the worldvolume electric and magnetic fields which are necessary to construct the holographic state. Numerical solutions have shown that these states can compete with the the previously known chi-ral symmetry breaking and maximally symmetric phases of the D3-D5 system. Moreover, at integer filling fractions, they are incompressible with integer quan-tized Hall conductivities. In the dual superconformal defect field theory, these solutions correspond to states which break the chiral and global flavor symme-tries spontaneously. The region of the temperature-density plane where the D7 brane has lower energy than the other known D5 brane solutions is identified. A hypothesis for the structure of states with filling fraction and Hall conductivity greater than one is made and tested by numerical computation. A parallel with the quantum Hall ferromagnetism or magnetic catalysis phenomenon which is observed in graphene is drawn. As well as demonstrating that the phenomenon can exist in a strongly coupled system, this work makes a number of predictions of symmetry breaking patterns and phase transitions for such systems. 1 ar

### PSEUDOSPIN TRANSFER TORQUES AND THE EFFECT OF DISORDER IN SEMICONDUCTOR ELECTRON BILAYERS BY

"... We use self-consistent quantum transport theory to investigate the influ-ence of electron-electron interactions on interlayer transport in semiconduc-tor electron bilayers in the absence of an external magnetic field. We con-clude that, even though spontaneous pseudospin order does not occur at zero ..."

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We use self-consistent quantum transport theory to investigate the influ-ence of electron-electron interactions on interlayer transport in semiconduc-tor electron bilayers in the absence of an external magnetic field. We con-clude that, even though spontaneous pseudospin order does not occur at zero field, interaction-enhanced quasiparticle tunneling amplitudes and pseu-dospin transfer torques do alter tunneling I-V characteristics, and can lead to time-dependent response to a dc bias voltage. In addition, we study the effects of disorder on the interlayer transport properties of disordered semiconductor bilayers. We find that the addition of material disorder to the system affects interlayer interactions leading to significant deviations in the interlayer transfer characteristics. In particular, we find that disorder decreases and broadens the tunneling peak, effectively reducing the interacting system to a non-interacting system. Our results suggest that the experimental observation of interaction-enhanced interlayer