The past decade has witnessed rapid advances in the development of "semiconductor spintronics," an area of research that broadly aims to exploit electron spin for qualitatively new semiconductor device functionality of both semi-classical and quantum character. This collaborative project is aimed at harnessing recent fundamental discoveries in semiconductor spintronics (such as the spin Hall effect and the enhancement of spin coherence in micro-resonators) for the systematic control of coherent spin phenomena in micro-patterned semiconductor devices. We will develop static and dynamical electrical measurements of the spin Hall effect as well seek pathways for enhancing the magnitude of this phenomenon. We also intend to pursue investigations that explore the entanglement and coherent manipulation of spins in coupled optical microcavities, with the ultimate goal of coherently controlling a single spin. Finally, we will conduct experiments that exploit the exchange interaction across interfaces for coherent spin control in both paramagnetic and ferromagnetic semiconductor heterostructures. Methods of investigation include spatially-resolved femtosecond optical spectroscopies, variable-temperature magnetotransport, direct magnetization, scanning probe microscopies, molecular beam epitaxial growth, and submicron fabrication techniques. The project is an integrated effort between the two principal investigators, emphasizing fundamental discovery in condensed matter physics, but with a clear eye on phenomena that could be of potential importance for future information technologies. The project combines sophisticated measurement techniques with advanced materials engineering, thus providing cutting edge training in both fundamental physics and materials science for undergraduate and graduate students.
Non-technical Contemporary information technology relies on the charge of electrons for computation (logic) and the magnetic properties called spin of electrons for permanent storage. The past decade has witnessed rapid advances in the development of "semiconductor spintronics," an area of research that broadly aims to integrate these traditionally separate functionalities. This proposal is aimed at the fundamental frontiers of semiconductor spintronics, where we seek to control the behavior of electron spin in microscopically patterned semiconductor chips. By exploiting the consequences of special relativity in solid state crystals, we will develop electrical means of probing and harnessing the "spin Hall effect," a contemporary spin analog of the classical Hall effect discovered over a century ago. Enhancing our fundamental understanding of the spin Hall effect may allow us to envision new classes of spintronic devices that exploit the non-intuitive laws of quantum physics for new types of logic without the need for magnetic fields or magnetic materials. We also intend to develop experiments that explore the quantum control of spins in finely tuned "micro-resonators," micron sized "boxes" that trap light and thus enhance its interaction with the spin of electrons. Our ultimate goal is the quantum mechanical control of a single electron spin in such boxes, enabling both computation and optical communication in a single device. Finally, we will develop experiments that exploit the interaction between magnetic ions and itinerant electrons across exquisitely designed interfaces. The project is an integrated effort between the two principal investigators, emphasizing fundamental discovery but with a clear eye on phenomena that could be of potential importance for future information technologies. The project combines sophisticated measurement techniques with advanced materials engineering, thus providing cutting edge training in both fundamental physics and materials science for undergraduate and graduate students.
When a quantum particle such as an electron or proton is placed in a magnetic field, quantum mechanics tells us that its behavior is governed by a physical property known as its "spin." One of the forefront areas of condensed matter physics – quantum spintronics -- focuses on developing a fundamental understanding and complete quantum mechanical control over the behavior of spin in materials such as semiconductors, metals and insulators, with the aim of building the foundations of future platforms for information technology. This project was broadly aimed at understanding the response of electronic, ionic and nuclear spins in solid state crystals, principally semiconductors, to various external and internal "knobs" such as magnetic fields, electric fields, incident light, strain and physical geometry. The project was a collaboration between two PIs – Professor Nitin Samarth (Penn State) and Professor David Awschalom (UC-Santa Barbara, Univ. Chicago -- who used an experimental approach that combined sophisticated spatio-temporally resolved spectroscopic measurements with the design and synthesis of novel spin-engineered model systems via molecular beam epitaxy. The collaboration mapped out the phenomenological behavior of both ensembles and small numbers of spins in a range of novel semiconductor geometries, from three-dimensional (bulk) crystals to lower dimensional structures such as two-dimensional quantum wells and thin films and quasi-one-dimensional nanowires. The principal scientific advances of the project centered on four major outcomes. First, the PIs discovered how to probe and control the elastic behavior of boundaries between magnetic domains (known as "domain walls") using simple-to-measure electrical signals (voltages). The technique allowed the first probe of magnetic domain wall dynamics in a previously inaccessible regime where the flexing of a domain wall is measured at nanometer length scales. Second, the PIs discovered how to coherently manipulate the spin of electrons in a current using the imprinting of nuclear spins in a lattice by a vicinal magnetic layer. This technique provides a potentially powerful protocol for future developments of "quantum gate" – an essential element of a quantum computer -- in a standard semiconductor material (GaAs) that is commonly used in contemporary optoelectronic devices. Third, the PIs showed how to make coupled micro-resonators from GaAs quantum structures, demonstrating an all-optical method of controlling the light emission of these evanescently coupled GaAs "photonic molecules" by the polarization state of the incident excitation. This optical control is an important functionality that, if extended to a large array, could lead to the creation of optical flip-flop and gated logic devices with multi-node capabilities. Finally, the PIs used the time-resolved imaging of the dynamics of a phenomenon known as the "spin Hall effect" to understand the all electrical generation and manipulation of spin currents in semiconductors. Finally, the materials synthesis carried out for this project also led to many other side projects with other collaborators, also resulting in diverse scientific advances. All these results were published in the highest quality scientific journals. The broader scientific impact of the work in this collaborative project is manifold. The PIs and their personnel disseminated their results to broad audiences, beyond the immediate subfield of interest, through many invited talks and plenary lectures at major international and U.S. scientific conferences, such as the 2009, 2010, 2011 and 2012 March meetings of the American Physical Society, the conference on Magnetism and Magnetic Materials, the International Conference on Magnetism, Spintech V & VI, EP2DS18/MSS14, CLEO/QELS, and PASPS VI & VII. During the funding period, the PIs also presented over 120 colloquia and seminars on this work at prominent universities, government, and industrial and research laboratories, in addition to national and international meetings and summer schools. The broader scientific impact in condensed matter physics is evidenced by the PIs receiving the following awards: Dan Tsui Lecturer, Chinese Academy of Sciences (Awschalom, 2008), Penn State Faculty Scholar Medal in the Physical Sciences (Samarth, 2008), Outstanding Physics Alumnus Award, Purdue University (Samarth, 2008); National Academy of Engineering (Awschalom, 2011); Huang Kun Colloquium Lecturer in Semiconductor Science and Technology, Chinese Academy of Sciences (Samarth, 2011), Miegunyah Distinguished Fellow, University of Melbourne (Awschalom, 2012), Fellow of the AAAS (Samarth, 2013). In addition, graduate students working on this project were also recognized with awards: Peter Eklund Lectureship, Penn State University and the European Physical Society Thesis Prize. The diverse group of students who worked on this project, including 3 women at Penn State and 3 women at UCSB, have all proceeded to positions in academia (through postdocs at national labs, followed by faculty positions) as well as positions in industry, thus contributing in a critical manner to the development of the science and engineering work force within the US.
