513 research outputs found
Controlling the Spin Polarization of the Electron Current in a Semimagnetic Resonant-Tunneling Diode
The spin filtering effect of the electron current in a double-barrier
resonant-tunneling diode (RTD) consisting of ZnMnSe semimagnetic layers has
been studied theoretically. The influence of the distribution of the magnesium
ions on the coefficient of the spin polarization of the electron current has
been investigated. The dependence of the spin filtering degree of the electron
current on the external magnetic field and the bias voltage has been obtained.
The effect of the total spin polarization of the electron current has been
predicted. This effect is characterized by total suppression of the spin-up
component of electron current, that takes place when the Fermi level coincides
with the lowest Landau level for spin-up electrons in the RTD semimagnetic
emitter
Pathways to Meteoritic Glycine and Methylamine
Glycine and methylamine are meteoritic water-soluble organic compounds that provide insights into the processes that occurred before, during, and after the formation of the Solar System. Both glycine and methylamine and many of their potential synthetic precursors have been studied in astrophysical environments via observations, laboratory experiments, and modeling. Despite these studies, the synthetic mechanisms for their formation leading to their occurrence in meteorites remain poorly understood. Typical 13C-isotopic values (13C) of meteoritic glycine and methylamine are 13C-enriched relative to their terrestrial counterparts; thus, analyses of their stable carbon isotopic compositions (13C/12C) may be used not only to assess terrestrial contamination in meteorites but also to provide information about their synthetic routes inside the parent body. Here, we examine potential synthetic routes of glycine and methylamine from a common set of precursors present in carbonaceous chondrite meteorites, using data from laboratory analyses of the well-studied CM2 Murchison meteorite. Several synthetic mechanisms for the origins of glycine and methylamine found in carbonaceous chondrites may be possible, and the prevalence of these mechanisms will largely depend on (a) the molecular abundance of the precursor molecules and (b) the levels of processing (aqueous and thermal) that occurred inside the parent body. In this work, we also aim to contextualize the current knowledge about gas-phase reactions and irradiated ice grain chemistry for the synthesis of these species through parent body processes. Our evaluation of various mechanisms for the origins of meteoritic glycine and methylamine from simple species shows what work is still needed to evaluate both the abundances and isotopic compositions of simpler precursor molecules from carbonaceous chondrites as well as the effects of parent body processes on those abundances and isotopic compositions. The analyses presented here combined with the indicated measurements will aid a better interpretation of quantitative analysis of reaction rates, molecular stability, and distribution of organic products from laboratory simulations of interstellar ices, astronomical observations, and theoretical modeling
Resonance-like piezoelectric electron-phonon interaction in layered structures
We show that mismatch of the piezoelectric parameters between layers of
multiple-quantum well structures leads to modification of the electron-phonon
interaction. In particular, short-wavelength phonons propagating perpendicular
to the layers with wavevector close to , where is the period of
the structure, induce a strong smoothly-varying component of the
piezo-potential. As a result, they interact efficiently with 2D electrons. It
is shown, that this property leads to emission of collimated
quasi-monochromatic beams of high-frequency acoustic phonons from hot electrons
in multiple-quantum well structures. We argue that this effect is responsible
for the recently reported monochromatic transverse phonon emission from
optically excited GaAs/AlAs superlattices, and provide additional experimental
evidences of this.Comment: 6 pages, 7 figure
Spin relaxation of two-dimensional holes in strained asymmetric SiGe quantum wells
We analyze spin splitting of the two-dimensional hole spectrum in strained
asymmetric SiGe quantum wells (QWs). Based on the Luttinger Hamiltonian, we
obtain expressions for the spin-splitting parameters up to the third order in
the in-plane hole wavevector. The biaxial strain of SiGe QWs is found to be a
key parameter that controls spin splitting. Application to SiGe field-effect
transistor structures indicates that typical spin splitting at room temperature
varies from a few tenth of meV in the case of Si QW channels to several meV for
the Ge counterparts, and can be modified efficiently by gate-controlled
variation of the perpendicular confining electric field. The analysis also
shows that for sufficiently asymmetric QWs, spin relaxation is due mainly to
the spin-splitting related D'yakonov-Perel' mechanism. In strained Si QWs, our
estimation shows that the hole spin relaxation time can be on the order of a
hundred picoseconds at room temperature, suggesting that such structures are
suitable for p-type spin transistor applications as well
Controlled lasing from active optomechanical resonators
Planar microcavities with distributed Bragg reflectors (DBRs) host, besides
confined optical modes, also mechanical resonances due to stop bands in the
phonon dispersion relation of the DBRs. These resonances have frequencies in
the sub-terahertz (10E10-10E11 Hz) range with quality factors exceeding 1000.
The interaction of photons and phonons in such optomechanical systems can be
drastically enhanced, opening a new route toward manipulation of light. Here we
implemented active semiconducting layers into the microcavity to obtain a
vertical-cavity surface-emitting laser (VCSEL). Thereby three resonant
excitations -photons, phonons, and electrons- can interact strongly with each
other providing control of the VCSEL laser emission: a picosecond strain pulse
injected into the VCSEL excites long-living mechanical resonances therein. As a
result, modulation of the lasing intensity at frequencies up to 40 GHz is
observed. From these findings prospective applications such as THz laser
control and stimulated phonon emission may emerge
Resonance-like electrical control of electron spin for microwave measurement
We demonstrate that the spin-polarized electron current can interact with a
microwave electric field in a resonant manner. The spin-orbit interaction gives
rise to an effective magnetic field proportional to the electric current. In
the presence of both dc and ac electric field components, electron spin
resonance occurs if the ac frequency matches with the spin precession frequency
that is controlled by the dc field. In a device consisting of two
spin-polarized contacts connected by a two-dimensional channel, this mechanism
allows electrically tuned detection of the ac signal frequency and amplitude.
For GaAs, such detection is effective in the frequency domain around tens of
gigahertz.Comment: 10 pages, 2 figure
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