119,870 research outputs found
Theory optical excitation spectra and depolarization dynamics in bilayer WS from viewpoint of excimers
We investigate the optical excitation spectra and the photoluminescence
depolarization dynamics in bilayer WS. A different understanding of the
optical excitation spectra in the recent photoluminescence experimentby Zhu
{\em et al.} [arXiv:1403.6224] in bilayer WS is proposed. In the
experiment, four excitations (1.68, 1.93, 1.99 and 2.37 eV) are observed and
identified to be indirect exciton for the valley, trion, A exciton and
B exciton excitations, respectively, with the redshift for the A exciton energy
measured to be 3050 meV when the sample synthesized from monolayer to
bilayer. According to our study, by considering there exist both the
intra-layer and charge-transfer excitons in the bilayer WS, with
inter-layer hopping of the hole, there exists excimer state composed by the
superposition of the intra-layer and charge-transfer exciton states.
Accordingly, we show that the four optical excitations in the bilayer WS
are the A charge-transfer exciton, excimer, excimer and B
intra-layer exciton states, respectively, with the calculated resonance
energies showing good agreement with the experiment. In our picture, the
speculated indirect exciton, which involves a high-order phonon
absorption/emission process, is not necessary. Furthermore, the binding energy
for the excimer state is calculated to be 40 meV, providing reasonable
explanation for the experimentally observed energy redshift of the A exciton.
Based on the excimer states, we further derive the exchange interaction
Hamiltonian. Then the photoluminescence depolarization dynamics due to the
electron-hole exchange interaction is studied in the pump-probe setup by the
kinetic spin Bloch equations. We find that ......Comment: 14 pages, 2 figure
Gauge-invariant theory of optical response to THz pulses in s-wave and (+)-wave superconducting semiconductor quantum wells
We investigate the optical response to the THz pulses in the s-wave and
(+)-wave superconducting semiconductor quantum wells by using the
gauge-invariant optical Bloch equations, in which the gauge structure in the
superconductivity is explicitly retained. By using the gauge transformation,
not only can the microscopic description for the quasiparticle dynamics be
realized, but also the dynamics of the condensate is included, with the
superfluid velocity and the effective chemical potential naturally
incorporated. We reveal that the superfluid velocity itself can contribute to
the pump of quasiparticles (pump effect), with its rate of change acting as the
drive field to drive the quasiparticles (drive effect). Specifically, the drive
effect can contribute to the formation of the blocking region for the
quasiparticle, which directly suppresses the anomalous correlation of the
Cooper pairs. We find that both the pump and drive effects contribute to the
oscillations of the Higgs mode with twice the frequency of the optical field.
However, it is shown that the contribution from the drive effect to the
excitation of Higgs mode is dominant as long as the driven superconducting
momentum is less than the Fermi momentum. This is in contrast to the conclusion
from the Liouville or Bloch equations in the literature, in which the drive
effect on the anomalous correlation is overlooked with only the pump effect
considered.Furthermore, in the gauge-invariant optical Bloch equations, the
charge neutrality condition is {\em consistently} considered based on the
two-component model for the charge, in which the charge imbalance of
quasiparticles can cause the fluctuation of the effective chemical potential.
......Comment: 33 pages, 16 figure
Gapped triplet -wave superconductivity in strong spin-orbit-coupled semiconductor quantum wells in proximity to -wave superconductor
We show that the {\it gapped} triplet superconductivity, i.e., a triplet
superconductor with triplet order parameter, can be realized in strong
spin-orbit-coupled quantum wells in proximity to -wave superconductor. It is
revealed that with the singlet order parameter induced from the superconducting
proximity effect, in quantum wells, not only can the triplet pairings arise due
to the spin-orbit coupling, but also the triplet order parameter can be induced
due to the repulsive effective electron-electron interaction, including the
electron-electron Coulomb and electron-phonon interactions. This is a natural
extension of the work of de Gennes, in which the repulsive-interaction-induced
singlet order parameter arises in the normal metal in proximity to -wave
superconductor [Rev. Mod. Phys. {\bf 36}, 225 (1964)]. Specifically, we derive
the effective Bogoliubov-de Gennes equation, in which the self-energies due to
the effective electron-electron interactions contribute to the singlet and
triplet order parameters. It is further shown that for the singlet order
parameter, it is efficiently suppressed due to this self-energy
renormalization; whereas for the triplet order parameter, it is the -wave
() one with the -vector parallel to the effective
magnetic field due to the spin-orbit coupling. Finally, we perform the
numerical calculation in InSb (100) quantum wells. Specifically, we reveal that
the Coulomb interaction is much more important than the electron-phonon
interaction at low temperature. Moreover, it shows that with proper electron
density, the minimum of the renormalized singlet and the maximum of the induced
triplet order parameters are comparable, and hence can be experimentally
distinguished.Comment: 15 pages, 8 figures, PRB in pres
Anomalous D'yakonov-Perel' spin relaxation in InAs (110) quantum wells under strong magnetic field: role of Hartree-Fock self-energy
We investigate the influence of the Hartree-Fock self-energy, acting as an
effective magnetic field, on the anomalous D'yakonov-Perel' spin relaxation in
InAs (110) quantum wells when the magnetic field in the Voigt configuration is
much stronger than the spin-orbit-coupled field. The transverse and
longitudinal spin relaxations are discussed both analytically and numerically.
For the transverse configuration, it is found that the spin relaxation is very
sensitive to the Hartree-Fock effective magnetic field, which is very different
from the conventional D'yakonov-Perel' spin relaxation. Even an extremely small
spin polarization () can significantly influence the behavior of the
spin relaxation. It is further revealed that this comes from the {\em unique}
form of the effective inhomogeneous broadening, originated from the mutually
perpendicular spin-orbit-coupled field and strong magnetic field. It is shown
that this effective inhomogeneous broadening is very small and hence very
sensitive to the Hartree-Fock field. Moreover, we further find that in the spin
polarization dependence, the transverse spin relaxation time decreases with the
increase of the spin polarization in the intermediate spin polarization regime,
which is also very different from the conventional situation, where the spin
relaxation is always suppressed by the Hartree-Fock field. It is revealed that
this {\em opposite} trends come from the additional spin relaxation channel
induced by the HF field. For the longitudinal configuration, we find that the
spin relaxation can be either suppressed or enhanced by the Hartree-Fock field
if the spin polarization is parallel or antiparallel to the magnetic field.Comment: 10 pages, 2 figure
Hot-electron effect in spin relaxation of electrically injected electrons in intrinsic Germanium
The hot-electron effect in the spin relaxation of electrically injected
electrons in intrinsic Germanium is investigated by the kinetic spin Bloch
equations both analytically and numerically. It is shown that in the
weak-electric-field regime with ~kV/cm, our calculations has
reasonable agreement with the recent transport experiment in the spin-injection
configuration [Phys. Rev. Lett. {\bf 111}, 257204 (2013)]. We reveal that the
spin relaxation is significantly enhanced at low temperature in the presence of
weak electric field ~V/cm, which originates from the obvious
center-of-mass drift effect due to the weak electron-phonon interaction,
whereas the hot-electron effect is demonstrated to be less important. This can
explain the discrepancy between the experimental observation and the previous
theoretical calculation [Phys. Rev. B {\bf 86}, 085202 (2012)], which deviates
from the experimental results by about two orders of magnitude at low
temperature. It is further shown that in the strong-electric-field regime with
~kV/cm, the spin relaxation is enhanced due to the
hot-electron effect, whereas the drift effect is demonstrated to be marginal.
Finally, we find that when ~kV/cm which lies in the
strong-electric-field regime, a small fraction of electrons ()
can be driven from the L to valley, and the spin relaxation rates are
the same for the and L valleys in the intrinsic sample without
impurity. With the negligible influence of the spin dynamics in the
valley to the whole system, the spin dynamics in the L valley can be measured
from the valley by the standard direct optical transition method.Comment: 10 pages, 3 figures, to be published in JPC
Spin diffusion in ultracold spin-orbit coupled K gas
We investigate the steady-state spin diffusion for ultracold spin-orbit
coupled K gas by the kinetic spin Bloch equation approach both
analytically and numerically. Four configurations, i.e., the spin diffusions
along two specific directions with the spin polarization perpendicular
(transverse configuration) and parallel (longitudinal configuration) to the
effective Zeeman field are studied. It is found that the behaviors of the
steady-state spin diffusion for the four configurations are very different,
which are determined by three characteristic lengths: the mean free path
, the Zeeman oscillation length and the spin-orbit
coupling oscillation length . It is analytically revealed and
numerically confirmed that by tuning the scattering strength, the system can be
divided into {\it five} regimes: I, weak scattering regime (); II, Zeeman field-dominated moderate scattering regime
(); III, spin-orbit coupling-dominated
moderate scattering regime (); IV,
relatively strong scattering regime (); V, strong scattering regime (), with representing the crossover length
between the relatively strong and strong scattering regimes. In different
regimes, the behaviors of the spacial evolution of the steady-state spin
polarization are very rich, showing different dependencies on the scattering
strength, Zeeman field and spin-orbit coupling strength. The rich behaviors of
the spin diffusions in different regimes are hard to be understood in the
framework of the simple drift-diffusion model or the direct inhomogeneous
broadening picture in the literature. ...Comment: 19 pages, 5 figure
Valley depolarization due to inter- and intra-valley electron-hole exchange interactions in monolayer MoS
We investigate the valley depolarization due to the electron-hole exchange
interaction in monolayer MoS. Both the long- and short-range parts of the
intra- and inter-valley electron-hole exchange interactions are calculated. We
find that both the long- and short-range exchange interactions can cause the
inter- and intra-valley bright exciton transitions. With the intra-valley
bright exciton transition channel nearly forbidden due to the large splitting
of the valence bands, the inter-valley channel due to the exchange interaction
can cause the valley depolarization efficiently by the Maialle-Silva-Sham
mechanism [Phys. Rev. B {\bf 47}, 15776 (1993)]. With only the long-range
exchange interaction, the calculations show good agreement with the recent
valley polarization experiments, including the time-resolved valley
polarization measurement, the pump-probe experiment and the steady-state PL
polarization measurement. We further show that for the A-exciton with large
(small) center-of-mass momentum, the long-range exchange interaction can cause
the {\em fast} ({\em slow}) inter-valley exciton transition.Comment: 8 pages, 2 figur
Novel valley depolarization dynamics and valley Hall effect of exciton in mono- and bilayer MoS
We investigate the valley depolarization dynamics and valley Hall effect of
exciton due to the electron-hole exchange interaction in mono- and bilayer
MoS by solving the kinetic spin Bloch equations. The effect of the exciton
energy spectra by the electron-hole exchange interaction is explicitly
considered. For the valley depolarization dynamics, in the monolayer MoS,
it is found that in the strong scattering regime, the conventional motional
narrowing picture is no longer valid, and a novel valley depolarization channel
is opened. For the valley Hall effect of exciton, in both the mono- and bilayer
MoS, with the exciton equally pumped in the K and K' valleys, the system
can evolve into the equilibrium state where the valley polarization is parallel
to the effective magnetic field due to the exchange interaction. With the drift
of this equilibrium state by applied uniaxial strain, the exchange interaction
can induce the {\it momentum-dependent} valley/photoluminesence polarization,
which leads to the valley/photoluminesence Hall current. Specifically, the
disorder strength dependence of the valley Hall conductivity is revealed. In
the strong scattering regime, the valley Hall conductivity decreases with the
increase of the disorder strength; whereas in the weak scattering regime, it
saturates to a constant, which can be much larger than the one in Fermi system
due to the absence of the Pauli blocking.Comment: 14 pages, 7 figure
Spin relaxation in ultracold spin-orbit coupled K gas
We report the anomalous Dyakonov-Perel' spin relaxation in ultracold
spin-orbit coupled K gas when the coupling between and
states (atcing as the effective Zeeman magnetic field) is much
stronger than the spin-orbit coupled field. Both the transverse and
longitudinal spin relaxations are investigated with small and large spin
polarizations. It is found that with small spin polarization, the transverse
(longitudinal) spin relaxation is divided into four (two) regimes: the normal
weak scattering regime, the anomalous Dyakonov-Perel'-like regime, the
anomalous Elliott-Yafet-like regime and the normal strong scattering regime
(the anomalous Elliott-Yafet-like regime and the normal strong scattering
regime), with only the normal weak scattering regime being in the weak
scattering limit. This is very different from the conventional situation under
the weak magnetic field, which is divided into the weak and strong scattering
regimes according to the weak/strong scattering limit. With large spin
polarization, we find that the Hartree-Fock self-energy, which acts as an
effective magnetic field, can markedly suppress the transverse spin relaxation
in both weak and strong scattering limits. Moreover, by noting that as both the
momentum relaxation time and the Hartree-Fock effective magnetic field vary
with the scattering length in cold atoms, the anomalous Dyakonov-Perel'-like
regime is suppressed and the transverse spin relaxation is hence divided into
three regimes in the scattering length dependence: the normal weak scattering
regime, the anomalous Elliott-Yafet-like regime and the strong scattering
regime. On the other hand, the longitudinal spin relaxation is again divided
into the anomalous EY-like and normal strong scattering regimes. ...Comment: 12 pages, 3 figure
Anomalous Hall effect in semiconductor quantum wells in proximity to chiral p-wave superconductors
By using the gauge-invariant optical Bloch equation, we perform a microscopic
kinetic investigation on the anomalous Hall effect in chiral p-wave
superconducting states. Specifically, the intrinsic anomalous Hall conductivity
in the absence of the magnetic field is zero as a consequence of Galilean
invariance in our description. As for the extrinsic channel, a finite anomalous
Hall current is obtained from the impurity scattering with the optically
excited normal quasiparticle current even at zero temperature. From our kinetic
description, it can be clearly seen that the excited normal quasiparticle
current is due to an induced center-of-mass momentum of Cooper pairs through
the acceleration driven by ac electric field. For the induced anomalous Hall
current, we show that the conventional skew-scattering channel in the linear
response makes the dominant contribution in the strong impurity interaction. In
this case, our kinetic description as a supplementary viewpoint mostly confirms
the results of Kubo formalism in the literature. Nevertheless, in the weak
impurity interaction, this skew-scattering channel becomes marginal and we
reveal that a novel induction channel from the Born contribution dominates the
anomalous Hall current. This novel channel, which has long been overlooked in
the literature, is due to the particle-hole asymmetry by nonlinear optical
excitation. Finally, we study the case in the chiral p-wave superconducting
state with a transverse conical magnetization, which breaks the Galilean
invariance. In this situation, the intrinsic anomalous Hall conductivity is no
longer zero. Comparison of this intrinsic channel with the extrinsic one from
impurity scattering is addressed.Comment: 17 pages, 8 figure
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