اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدAnisotropy of excitation and relaxation of photogenerated Dirac electrons in graphene
128
0
0.0
(
0
)
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
We investigate the polarization dependence of the carrier excitation and relaxation in epitaxial multilayer graphene. Degenerate pump-probe experiments with a temporal resolution of 30 fs are performed for different rotation angles of the pump-pulse polarization with respect to the polarization of the probe pulse. A pronounced dependence of the pump-induced transmission on this angle is found. It reflects a strong anisotropy of the pump-induced occupation of photogenerated carriers in momentum space even though the band structure is isotropic. Within 150 fs after excitation an isotropic carrier distribution is established. Our observations imply the predominant role of collinear scattering preserving the initially optically generated anisotropy in the carrier distribution. The experiments are well described by microscopic time-, momentum, and angle-resolved modelling, which allows us to unambiguously identify non-collinear carrier-phonon scattering to be the main relaxation mechanism giving rise to an isotropic distribution in the first hundred fs after optical excitation.
قيم البحث
اقرأ أيضاً
The relativistic nature of Dirac electrons and holes in graphene profoundly affects the way they interact with impurities. Signatures of the relativistic behavior have been observed recently in scanning tunneling measurements on individual impurities
, but the conductance measurements in this regime are typically dominated by electron and hole puddles. Here we present measurements of quantum interference noise and magnetoresistance in graphene pn junctions. Unlike the conductance, the quantum interference noise can provide access to the scattering at the Dirac point:it is sensitive to the motion of a single impurity, it depends strongly on the fundamental symmetries that describe the system and it is determined by the phase-coherent phenomena which are not necessarily obscured by the puddles. The temperature and the carrier density dependence of resistance fluctuations and magnetoresistance in graphene p-n junctions at low temperatures suggest that the noise is dominated by the quantum interference due to scattering on impurities and that the noise minimum could be used to determine the point where the average carrier density is zero. At larger carrier densities, the amplitude of the noise depends strongly on the sign of the impurity charge, reflecting the fact that the electrons and the holes are scattered by the impurity potential in an asymmetric manner.
We have investigated a new feature of impurity cyclotron resonances common to various localized potentials of graphene. A localized potential can interact with a magnetic field in an unexpected way in graphene. It can lead to formation of anomalous b
oundstates that have a sharp peak with a width $R$ in the probability density inside the potential and a broad peak of size magnetic length $ell$ outside the potential. We investigate optical matrix elements of anomalous states, and find that they are unusually small and depend sensitively on magnetic field. The effect of many-body interactions on their optical conductivity is investigated using a self-consistent time-dependent Hartree-Fock approach (TDHFA). For a completely filled Landau level we find that an excited electron-hole pair, originating from the optical transition between two anomalous impurity states, is nearly uncorrelated with other electron-hole pairs, although it displays a substantial exchange self-energy effects. This absence of correlation is a consequence of a small vertex correction in comparison to the difference between renormalized transition energies computed within the one electron-hole pair approximation. However, an excited electron-hole pair originating from the optical transition between a normal and an anomalous impurity states can be substantially correlated with other electron-hole states with a significant optical strength.
We demonstrate the interaction between surface acoustic waves and Dirac electrons in monolayer graphene at low temperatures and high magnetic fields. A metallic interdigitated transducer launches surface waves that propagate through a conventional pi
ezoelectric GaAs substrate and couple to large-scale monolayer CVD graphene films resting on its surface. Based on the induced acousto-electric current, we characterize the frequency domains of the transducer from its first to the third harmonic. We find an oscillatory attenuation of the SAW velocity depending on the conductivity of the graphene layer. The acousto-electric current reveals additional fine structure that is absent in pure magnetotransport. In addition we find a shift between the acousto-electric longitudinal voltage and the velocity change of the SAW. We attribute this shift to the periodic strain field from the propagating SAW that slightly modifies the Dirac cone.
The relativistic nature of charge carriers in graphene is expected to lead to an angle- dependent transmission through a potential barrier, where Klein tunneling involves annihilation of an electron and a hole at the edges of the barrier. The signatu
res of Klein tunneling have been observed in gated graphene devices, but the angle dependence of the transmission probability has not been directly observed. Here we show measurements of the angle-dependent transmission through quasi-ballistic graphene heterojunctions with straight and angled leads, in which the barrier height is controlled by a shared gate electrode. Using a balanced differential measurement technique, we isolate the angle-dependent contribution to the resistance from other angle-insensitive, gate-dependent and device-dependent effects. We find large oscillations in the transmission as a function of the barrier height in the case of Klein tunneling at a 45 deg angle, as compared to normal incidence. Our results are consistent with the model that predicts oscillations of the transmission probability due to interference of chiral carriers in a ballistic barrier. The observed angle dependence is the key element behind focusing of electrons and the realization of a Veselago lens in graphene.
Dirac carriers in graphene are commonly characterized by a pseudospin degree of freedom, arising from the degeneracy of the two inequivalent sublattices. The inherent chirality of the quasiparticles leads to a topologically non-trivial band structure
, where the in-plane component of sublattice spin and momentum are intertwined. Equivalently, sublattice imbalance is intimately connected with angular momentum, inducing a torque of opposite sign at each Dirac point. In this work we develop an intuitive picture that associates sublattice spin and winding number with angular momentum. We develop a microscopic perturbative model to obtain the finite angular momentum contributions along the main crystallographic directions. Our results can be employed to determine the angular dependence of the g-factor and of light absorption in honeycomb bipartite structures.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
