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| Spin Hall effect | |
|---|---|
| Name | Spin Hall effect |
| Field | Condensed matter physics |
| Discovered | 1971; experimentally observed 2004 |
| Discoverer | M. I. Dyakonov; first optical detection by Y. K. Kato and colleagues |
Spin Hall effect. The Spin Hall effect is a transport phenomenon in which an electric current in a conductor or semiconductor gives rise to a transverse spin current and spin accumulation at sample boundaries. It links charge transport phenomena in systems studied by researchers at institutions such as Bell Labs, IBM Research, and MIT with spintronics platforms developed at Stanford University, Tohoku University, and University of California, Berkeley. The effect connects theoretical frameworks from groups led by figures like Mikhail Dyakonov, Jairo Sinova, and Shoucheng Zhang to experimental techniques pioneered by teams including Yasuhiro Kato and David Awschalom.
The Spin Hall effect was predicted in 1971 by Mikhail Dyakonov and Vladimir Perel and gained renewed attention following experimental demonstrations in the early 2000s by groups at University of California, Santa Barbara and University of Minnesota. It manifests in nonmagnetic conductors and magnetic heterostructures and complements phenomena such as the Hall effect, anomalous Hall effect, and quantum Hall effect. Spin Hall physics underpins technologies pursued by companies like Intel and Samsung and informs initiatives funded by agencies including the National Science Foundation and the European Research Council.
Mechanisms responsible for transverse spin currents include intrinsic processes tied to band structure and extrinsic scattering processes. Intrinsic contributions arise from spin–orbit coupling in crystalline solids described in systems investigated by groups at IBM Research and observed in materials like platinum studied by teams at Max Planck Institute for Chemical Physics of Solids. Extrinsic mechanisms include skew scattering and side-jump scattering off impurities characterized in experiments at Argonne National Laboratory and theory from researchers at University of Cambridge. The interplay between spin accumulation at boundaries and spin diffusion lengths is analyzed in the context of spin relaxation channels reported by investigators at Riken and Oak Ridge National Laboratory.
The intrinsic Spin Hall conductivity is often derived from Berry curvature of Bloch bands in multiband models developed by theorists at Princeton University and Harvard University. Semiclassical Boltzmann approaches with spin-dependent scattering terms were advanced by groups at University of Geneva and University of California, San Diego. Kubo linear-response formalisms used by authors at Leiden University and University of Oxford connect to topological classifications introduced in work associated with Kane and Mele and Charles Kane's collaborators. Model Hamiltonians include Rashba and Dresselhaus spin–orbit couplings studied at University of Texas at Austin and Northwestern University, while numerical methods such as density functional theory calculations have been applied by teams at Lawrence Berkeley National Laboratory and Los Alamos National Laboratory.
Optical detection using Kerr rotation microscopy pioneered by groups at University of California, Santa Barbara and University of Minnesota provided early direct observation. Electrical detection schemes using nonlocal spin valves and inverse Spin Hall voltages were implemented in experiments at Korea Advanced Institute of Science and Technology and Tohoku University. Spin pumping via ferromagnetic resonance in heterostructures combining materials from Japan Fine Ceramics Center and Argonne National Laboratory has been exploited, while spin-torque ferromagnetic resonance techniques used by teams at University of Groningen and University of Leeds probe dynamics. Cryogenic facilities at National High Magnetic Field Laboratory enabled low-temperature studies that complement room-temperature work at industrial labs like Hitachi.
Materials with large spin–orbit coupling such as platinum, tantalum, and tungsten have been central to studies at Max Planck Society laboratories and Riken collaborations. Semiconductors including gallium arsenide and indium gallium arsenide were investigated in projects at Nanjing University and University of Tokyo. Topological insulators produced by groups at Korea Institute of Science and Technology and Yale University show strong spin–momentum locking relevant to Spin Hall phenomena. Applications include spin–orbit torque magnetic random-access memory developed by companies like Samsung Electronics and Western Digital, spin-based logic proposals from research at Intel Labs, and nanoscale oscillators explored at IMEC and TSMC.
Optical Kerr and Faraday rotation used by laboratories at University of California, Santa Barbara enable spatially resolved spin accumulation mapping. Electrical detection via inverse Spin Hall effect voltages is standard in setups replicated at Argonne National Laboratory and University of Wisconsin–Madison. Spin-polarized scanning tunneling microscopy techniques from teams at University of Hamburg and University of Basel offer local probes, while X-ray magnetic circular dichroism experiments at synchrotrons like SLAC National Accelerator Laboratory and European Synchrotron Radiation Facility provide element-specific detection. Time-resolved pump–probe spectroscopy executed at Stanford Linear Accelerator Center assesses ultrafast spin dynamics.
Outstanding issues include quantitative reconciliation between intrinsic and extrinsic contributions championed by theorists at University of California, Los Angeles and University of Florida, control of spin currents in two-dimensional materials researched at Columbia University and University of Manchester, and integration of Spin Hall effects with superconducting platforms investigated at Argonne National Laboratory and University of Copenhagen. Prospects involve device scaling efforts at TSMC, quantum information interfacing pursued by groups at Google and Microsoft Research, and materials discovery enabled by high-throughput computations at Lawrence Livermore National Laboratory and Sandia National Laboratories.