Carriers In Momentum Space Research Articles

Ab initio real-time quantum dynamics of charge carriers in momentum space.

Application of the non-adiabatic molecular dynamics (NAMD) approach is limited to studying carrier dynamics in the momentum space, as a supercell is required to sample the phonon excitation and electron-phonon (e-ph) interaction at different momenta in a molecular dynamics simulation. Here we develop an ab initio approach for the real-time charge carrier quantum dynamics in the momentum space (NAMD_k) by directly introducing e-ph coupling into the Hamiltonian based on the harmonic approximation. The NAMD_k approach maintains the zero-point energy and includes memory effects of carrier dynamics. The application of NAMD_k to the hot carrier dynamics in graphene reveals the phonon-specific relaxation mechanism. An energy threshold of 0.2 eV-defined by two optical phonon modes-separates the hot electron relaxation into fast and slow regions with lifetimes of pico- and nanoseconds, respectively. The NAMD_k approach provides an effective tool to understand real-time carrier dynamics in the momentum space for different materials.

Circular Photogalvanic Effect in the Weyl Semimetal TaAs**Supported by the National Basic Research Program of China under Grant Nos 2015CB921300, the National Key Research and Development Program of China under Grant Nos 2016YFA0300300, 2017YFA0504703 and 2017YFA0302900, the National Natural Science Foundation of China under Grant Nos 11604372, 11474323 and 11774391, the Strategic Priority Research Program (B) of the Chinese Academy

Weyl semimetal (WSM) is expected to be an ideal spintronic material owing to its spin currents carried by the bulk and surface states with spin-momentum locking. The generation of a sizable photocurrent is predicted in non-centrosymmetric WSM arising from the broken inversion symmetry and the linear energy dispersion that is unique to Weyl systems. In our recent measurements, the circular photogalvanic effect (CPGE) is discovered in the TaAs WSM. The CPGE voltage is proportional to the helicity of the incident light, reversing direction if the radiation helicity changes handedness, a periodical oscillation therefore appears following the alteration of light polarization. We herein attribute the CPGE to the asymmetric optical excitation of the Weyl cone, which could result in an asymmetric distribution of photoexcited carriers in momentum space according to an optical spin-related selection rule.

Anisotropy of Excitation and Relaxation of Photogenerated Charge Carriers in Graphene

We present a pump-probe experiment on graphene, which reveals a pronounced dependence of the pump-induced transmission on the angle between pump and probe polarization. It reflects a strong anisotropy of the pump-induced occupation of photogenerated carriers in momentum space. Within 150 fs after excitation, an isotropic carrier distribution is established. The experiments are well described by microscopic modeling, which identifies carrier-phonon scattering to be the main relaxation mechanism giving rise to an isotropic carrier distribution.

Synchrotron infrared transmission spectroscopy of a quantum cascade laser correlated to gain models

We measure the broadband optical gain and absorption spectra between 0.1 and 0.7 eV in a two-phonon resonance design quantum cascade laser, with the infrared beam of a synchrotron light source, and correlate them to established simulation models, based on the density matrix formalism, and on the non-equilibrium Green's function theory. We show that accounting for the distribution of carriers in momentum space improves the description of the high-energy absorption from the excited states located close to the active wells and results in an accurate prediction of the gain.

A multi-component Fermi surface in the vortex state of an underdoped high-Tc superconductor

To understand the origin of superconductivity, it is crucial to ascertain the nature and origin of the primary carriers available to participate in pairing. Recent quantum oscillation experiments on high-transition-temperature (high-T(c)) copper oxide superconductors have revealed the existence of a Fermi surface akin to that in normal metals, comprising fermionic carriers that undergo orbital quantization. The unexpectedly small size of the observed carrier pocket, however, leaves open a variety of possibilities for the existence or form of any underlying magnetic order, and its relation to d-wave superconductivity. Here we report experiments on quantum oscillations in the magnetization (the de Haas-van Alphen effect) in superconducting YBa(2)Cu(3)O(6.51) that reveal more than one carrier pocket. In particular, we find evidence for the existence of a much larger pocket of heavier mass carriers playing a thermodynamically dominant role in this hole-doped superconductor. Importantly, characteristics of the multiple pockets within this more complete Fermi surface impose constraints on the wavevector of any underlying order and the location of the carriers in momentum space. These constraints enable us to construct a possible density-wave model with spiral or related modulated magnetic order, consistent with experimental observations.

Upconversion of partition noise in semiconductors operating under periodic large-signal conditions

By means of Monte Carlo simulations of bulk semiconductors operating in the periodic large-signal regime, we show the existence of upconversion of hot-carrier partition noise associated with the fluctuations between different groups of carriers in momentum space, characterized by different dynamical properties. The signature of the upconversion phenomenon is predicted by an analytical model and confirmed by the spectral analysis of the instantaneous spectral density of velocity fluctuations. As applications, we investigate the cases in which the two groups of carriers pertain to a single band in the presence of strong low-temperature optical-phonon emission and to lowest- and upper-valley populations in compound semiconductors.

DC field response of hot carriers under circular polarized intense microwave fields in semiconductors limited to two-dimension

Hot carrier dynamics under intense microwave fields is investigated theoretically for the case that the dominant scattering process is optical phonon emission, and the carrier motion limited to two-dimension is intrinsic. When the microwave amplitude is appropriately large, an accumulated distribution of carriers in momentum space appears. The system of this motion is found to cause various peculiar DC fields response, e.g. strong non-linearity, negative differential conductivity, and negative response, under realistic physical conditions [J. Phys. Soc. Jpn 68 (1995) 2994]. In the proper strength of microwave and DC electric fields, especially in the case of circular polarized microwave fields, the carrier motions are converged to some trajectories in momentum space [Proceeding of 25th International Conference on the Physics of Semiconductors, 2001]. Resultantly a new type of accumulated distribution of carriers in momentum space appears. This situation is a sort of population inversion of carriers and causes various phenomena including a negative differential conductivity and/or a negative response appeared in the drift velocity vs. DC field relation not found in the case of linear polarized microwave fields. The carrier motion restricted in two-dimensional band may be essential and effective for these properties.

Carrier Accumulation in Momentum Space and Related Phenomena under Intense Microwave Fields in Semiconductors. II. Quasi-One-Dimensional Systems

When the dominant scattering mechanism is optical phonon emission, the application of appropriately intense microwave fields is expected to cause an accumulated distribution of carriers in momentum space and related peculiar phenomena, as described in a previous paper which treated usual bulk semiconductors. It is shown here that the carrier accumulation effect is more pronounced in one-dimensional systems in the direction of the microwave fields, because of the absence of transverse degrees of freedom. Computations are made for electrons in GaAs crystals confined by strong magnetic fields of 12∼24 T. The results show that peculiar dc field responses including absolute negative resistance are expected under realistic experimental conditions, e.g. in samples with more than 10 14 cm -3 charged impurity concentration.

Theory of femtosecond ellipsometry in Ge at 1.5 eV

Intense photoexcitation of a semiconductor using femtosecond laser pulses affects its optical properties. We consider the transient photoinduced changes Δε to the dielectric function ε of bulk Ge after excitation with 100-fs laser pulses at 1.5 eV creating a carrier density of 4×10 18 cm −3 due to three mechanisms: (i) diffusion of carriers from the surface into the bulk, (ii) relaxation of carriers in momentum space, and (iii) many-body effects, including band gap renormalization, collisional broadening, screening of the excitonic Coulomb enhancement, and band filling.

Carrier Accumulation in Momentum Space and Related Phenomena under Intense Microwave Fields in Semiconductors*

Hot carrier dynamics under intense microwave fields is investigated theoretically for the case that the dominant scattering process is optical phonon emission. When the microwave amplitude is appropriately large, an accumulated distribution of carriers in momentum space appears. This distribution is a new type of dynamic population inversion and is found to cause various peculiar dc field responses, e.g. strong nonlinearity, negative differential conductivity, and often negative response, under realistic physical conditions.

Bunching of Carriers in Momentum Space under Crossed Magnetic and Intense Microwave Fields in Semiconductors

The motion of a carrier in momentum space is examined in detail for the case that an intense microwave electric field and a static magnetic field perpendicular to it are applied to semiconductors in which the predominant scattering process is the optical phonon emission. The motion changes the type in a systematic way depending on the strengths of the fields. Particularly in intermediate magnetic field range, the motion rapidly converges to a terminal motion independent of the initial condition, leading to a bunching of the carriers in the momentum space. The convergence becomes slow as the magnetic field is decreased. In strong magnetic fields, the motion becomes of long period or chaotic. The type of the motion changes periodically with the electric field. The calculation is carried out for two waveforms, square and sinusoidal. The results are very similar to each other as for the bunching, implying its generic property.