We report on the observation of magnetic quantum ratchet effect in metal-oxide-semiconductor field-effect-transistors on silicon surface (Si-MOSFETs). We show that the excitation of an unbiased transistor by ac electric field of terahertz radiation a
t normal incidence leads to a direct electric current between the source and drain contacts if the transistor is subjected to an in-plane magnetic field. The current rises linearly with the magnetic field strength and quadratically with the ac electric field amplitude. It depends on the polarization state of the ac field and can be induced by both linearly and circularly polarized radiation. We present the quasi-classical and quantum theories of the observed effect and show that the current originates from the Lorentz force acting upon carriers in asymmetric inversion channels of the transistors.
The performance of the solid deuterium ultra-cold neutron source at the pulsed reactor TRIGA Mainz with a maximum peak energy of 10 MJ is described. The solid deuterium converter with a volume of V=160 cm3 (8 mol), which is exposed to a thermal neutr
on fluence of 4.5x10^13 n/cm2, delivers up to 550 000 UCN per pulse outside of the biological shield at the experimental area. UCN densities of ~ 10/cm3 are obtained in stainless steel bottles of V ~ 10 L resulting in a storage efficiency of ~20%. The measured UCN yields compare well with the predictions from a Monte Carlo simulation developed to model the source and to optimize its performance for the upcoming upgrade of the TRIGA Mainz into a user facility for UCN physics.
We observe photocurrents induced in single layer graphene samples by illumination of the graphene edges with circularly polarized terahertz radiation at normal incidence. The photocurrent flows along the sample edges and forms a vortex. Its winding d
irection reverses by switching the light helicity from left- to right-handed. We demonstrate that the photocurrent stems from the sample edges, which reduce the spatial symmetry and result in an asymmetric scattering of carriers driven by the radiation electric field. The developed theory is in a good agreement with the experiment. We show that the edge photocurrents can be applied for determination of the conductivity type and the momentum scattering time of the charge carriers in the graphene edge vicinity.
We study terahertz radiation induced ratchet currents in low dimensional semiconductor structures with a superimposed one-dimensional lateral periodic potential. The periodic potential is produced by etching a grating into the sample surface or depos
iting metal stripes periodically on the sample top. Microscopically, the photocurrent generation is based on the combined action of the lateral periodic potential, verified by transport measurements, and the in-plane modulated pumping caused by the lateral superlattice. We show that a substantial part of the total current is caused by the polarization-independent Seebeck ratchet effect. In addition, polarization-dependent photocurrents occur, which we interpret in terms of their underlying microscopical mechanisms. As a result, the class of ratchet systems needs to be extended by linear and circular ratchets, sensitive to linear and circular polarizations of the driving electro-magnetic force.
We demonstrate the injection of pure valley-orbit currents in multi-valley semiconductors and present the theory of this effect. We studied photo-induced transport in $n$-doped (111)-oriented silicon metal-oxide-semiconductor field effect transistors
at room temperature. By shining circularly polarized light on exact oriented structures with six equivalent valleys, non-zero electron fluxes within each valley are generated, which compensate each other and do not yield a net electric current. By disturbing the balance between the valley fluxes, in this work by applying linearly polarized radiation as well as by introducing a nonequivalence of the valleys by disorientation, we approve that the pure valley currents can be converted into a measurable electric current.
We report the observation of the circular ac Hall effect where the current is solely driven by the crossed ac electric and magnetic fields of circularly polarized radiation. Illuminating an unbiased monolayer sheet of graphene with circularly polariz
ed terahertz radiation at room temperature generates - under oblique incidence - an electric current perpendicular to the plane of incidence, whose sign is reversed by switching the radiation helicity. Alike the classical dc Hall effect, the voltage is caused by crossed electric and magnetic fields which are however rotating with the lights frequency.
We report on the observation of photon helicity driven currents in graphene. The directed net electric current is generated in single layer graphene by circularly polarized terahertz laser radiation at normal as well as at oblique incidence and chang
es its sign upon reversing the radiation helicity. The phenomenological and microscopic theories of the observed photocurrents are developed. We demonstrate that under oblique incidence the current is caused by the circular photon drag effect in the interior of graphene sheet. By contrast, the effect at normal incidence stems from the sample edges, which reduce the symmetry and result in an asymmetric scattering of carriers driven by the radiation field. Besides a photon helicity dependent current we also observe photocurrents in response to linearly polarized radiation. The microscopic mechanisms governing this effect are discussed.
We report on the circular and linear photogalvanic effects caused by free-carrier absorption of terahertz radiation in electron channels on (001)-oriented and miscut silicon surfaces. The photocurrent behavior upon variation of the radiation polariza
tion state, wavelength, gate voltage and temperature is studied. We present the microscopical and phenomenological theory of the photogalvanic effects, which describes well the experimental results. In particular, it is demonstrated that the circular (photon-helicity sensitive) photocurrent in silicon-based structures is of pure orbital nature originating from the quantum interference of different pathways contributing to the absorption of monochromatic radiation.
