Research
A New Scheme to Determine the Spin Hall Coefficient
The recent interest in the spin Hall
effect has incited a large number of theoretical studies.
Experimentally, it is still very difficult to determine the spin Hall
coefficient. We proposed a new scheme to determine the spin Hall
coefficient of a semiconductor sample. The scheme is
simple, accessible to most of experimental groups in the field.
Our scheme employs a continuous circularly-polarized laser light beam that induces an inhomogeneous spin density in the sample. We then measure the induced charge current along the direction transverse to the gradient of the spin density. By applying the Einstein relation and Onsager relation derived from the general principles of non-equilibrium thermodynamics, we can determine the spin Hall coefficient from the measurement.
The scheme was applied in a recent experimental study on InGaAs quantum well. The spin Hall coefficient was estimated to be ~e/8p. The value is surprisingly close to the theoretical prediction of the intrinsic spin Hall effect.
See the preprint for details.
Our scheme employs a continuous circularly-polarized laser light beam that induces an inhomogeneous spin density in the sample. We then measure the induced charge current along the direction transverse to the gradient of the spin density. By applying the Einstein relation and Onsager relation derived from the general principles of non-equilibrium thermodynamics, we can determine the spin Hall coefficient from the measurement.
The scheme was applied in a recent experimental study on InGaAs quantum well. The spin Hall coefficient was estimated to be ~e/8p. The value is surprisingly close to the theoretical prediction of the intrinsic spin Hall effect.
See the preprint for details.
Proper Definition of Spin Current in Spin-Orbit Coupled Systems
The
study of spin-Hall effect and
spin transport has attracted much attention recently. One of
the
fundamental issues is the definition of spin current. An
intuitive choice of the spin
current is to replace the
electron charge -e
in the
usual charge current with the spin operator. Unfortunately,
this
approach neglects one of the
most important aspects of spins: unlike the charge, spins are
not conserved in general. This has caused some apparent
difficulties already observed by a number of authors.
Our work surveyed this issue in depth. We found that the conventional spin current includes the extraneous contribution from the local spin torque that must be deducted before the resulting conserved spin current can be considered as the true transport current. It turns out that the proper definition of the spin current should be the total time derative of the spin displacement operator rsz. The spin Hall coefficients of Rashba models, both linear and cubed k, were recently re-calculated using the proper definition. It was found that in the presence of the non-magnetic impurities, there is no intrinsic spin Hall effect. This contradicts with the most of the earlier conclusions, and suggests that it may be necessary to re-survey the whole subject thoroughly.
See this PRL paper and these slides for the theory.
Our work surveyed this issue in depth. We found that the conventional spin current includes the extraneous contribution from the local spin torque that must be deducted before the resulting conserved spin current can be considered as the true transport current. It turns out that the proper definition of the spin current should be the total time derative of the spin displacement operator rsz. The spin Hall coefficients of Rashba models, both linear and cubed k, were recently re-calculated using the proper definition. It was found that in the presence of the non-magnetic impurities, there is no intrinsic spin Hall effect. This contradicts with the most of the earlier conclusions, and suggests that it may be necessary to re-survey the whole subject thoroughly.
See this PRL paper and these slides for the theory.
Electron Dynamics in Exotic Materials
For
certain class of solids such as
ferromagnetic metals, it had been found that the (Bloch) electron
follows different dynamics from the one found in textbooks.
Most
notably, there exists an extra "magnetic field" in reciprocal (k-)
space that could drive the
motion of electrons. In theory, all solids that lack
either time-reversal or spatial inversion symmetry could possess such a
field. So, it is foreseeable that a whole class of
new
materials could be discovered/synthesized in laboratory in the near
future.
Our research try to laid down the theoretical ground for this new class of exotic materials. Based on the previous works on the effective dynamics of Bloch electron, we are able to determine the complete form of the new quantum mechanics of Bloch electron in these exotic materials. One of its most notable features is that the phase space of the electron acquires a measure which is non-homogeneous in general. This will have profound effects in equilibrium and transport properties. Its could also have important implications to some fundamental aspects of the condensed matter physics, such as the Fermi liquid theory.
See this PRL paper and these slides for a detailed survey of the theory.
Our research try to laid down the theoretical ground for this new class of exotic materials. Based on the previous works on the effective dynamics of Bloch electron, we are able to determine the complete form of the new quantum mechanics of Bloch electron in these exotic materials. One of its most notable features is that the phase space of the electron acquires a measure which is non-homogeneous in general. This will have profound effects in equilibrium and transport properties. Its could also have important implications to some fundamental aspects of the condensed matter physics, such as the Fermi liquid theory.
See this PRL paper and these slides for a detailed survey of the theory.
Direct Extraction of Eliashberg Function From Photoemission
It turns out
the photoemission
technique can reveal more in-depth information about electron
correlation in solids than that we had expected! The development of a
systematic
technique to
extract the Eliashberg function directly from the high resolution angle
resolved photoemission
data made this possible.
The technique had been successfully applied to Be(1010)
surface.
Recently this technique was also applied to high-Tc
materials and the
fine structures of the electron-boson coupling were identified in the LSCO
compound for the first time. A
number of experimental groups have employed the technique to study the
properties of other
materials.
Look at these slides to have a rough idea of the technique. The computer codes doing the analysis can be downloaded in the Files section.
Surface Structural Phase Transitions Induced by Electron-Mediated Adatom-Adatom Interaction
The electron-phonon coupling is enhanced on many surfaces. In metallic adlayers, it may lead to very strong adatom-adatom interaction mediated by electrons. This is responsible for the surface structural phase transition observed in Sn/Ge(111). Our paper in Physical Review Letters explained many puzzling behaviors of the transition based on this model. The prediction of the paper, i.e., the system may enter into a glassy phase at low temperature, has been confirmed by a recent experiment in iso-electronic Pb/Ge(111) system.Look at these slides for a brief introduction.
"Zero Resistance States" in Microwave Radiated 2DEG
Recent experimental discoveries of "zero resistance state" in a microwave radiated 2DEG revealed that intriguing and unexpected transport behaviors may arise in a far-from-equilibrium system. Our paper published in Physical Review Letters provided a simple theoretical rationale for the occurrence of the "zero resistance state". From the physical view, this is the typical behavior of a far-from-equilibrium driven system, albeit in a quantum system. It is still not clear whether or not the quantum nature can play essential role in the far-from-equilibrium transport.See these slides for a brief introduction.