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.
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.
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.
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.
"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.
It had been hoped that a series of experiments could have provided
evidences that a metallic phase may exist in two-dimensions. This
contradicts with the well known weak localization theorem. Our
Studies
suggested that the observed transport anomaly could be well understood
in a semi-classical picture in which the incoherent transport
dominates. In general, the transition is the Mott-Hubbard
type,
and the weak localization theorem, upon which many earlier arguments
were based, is not relevant.