330 research outputs found
Rigorous derivation of coherent resonant tunneling time and velocity in finite periodic systems
The velocity of resonant tunneling electrons in finite periodic
structures is analytically calculated in two ways. The first method is based on
the fact that a transmission of unity leads to a coincidence of all still
competing tunneling time definitions. Thus, having an indisputable resonant
tunneling time we apply the natural definition
to calculate the velocity. For the second method we
combine Bloch's theorem with the transfer matrix approach to decompose the wave
function into two Bloch waves. Then the expectation value of the velocity is
calculated. Both different approaches lead to the same result, showing their
physical equivalence. The obtained resonant tunneling velocity is
smaller or equal to the group velocity times the magnitude of the complex
transmission amplitude of the unit cell. Only at energies where the unit cell
of the periodic structure has a transmission of unity equals the
group velocity. Numerical calculations for a GaAs/AlGaAs superlattice are
performed. For typical parameters the resonant velocity is below one third of
the group velocity.Comment: 12 pages, 3 figures, LaTe
An All-Cryogenic THz Transmission Spectrometer
This paper describes a THz transmission spectrometer for the spectral range
of 2-65 cm^-1 (100 GHz to 2 THz) with a spectral resolution of at least 1.8
cm^-1 (50 GHz) where the source, sample, and detector are all fully contained
in a cryogenic environment. Cyclotron emission from a two-dimensional electron
gas heated with an electrical current serves as a magnetic field tunable
source. The spectrometer is demonstrated at 4.2 K by measuring the resonant
cyclotron absorption of a second two dimensional electron gas. Unique aspects
of the spectrometer are that 1) an ultra-broadband detector is used and 2) the
emitter is run quasi-continuously with a chopping frequency of only 1 Hz. Since
optical coupling to room temperature components is not necessary, this
technique is compatible with ultra-low temperature (sub 100 mK) operation.Comment: 7 pages, 5 figures. Author affiliation and funding acknowledgements
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Infrared emission spectrum and potentials of and states of Xe excimers produced by electron impact
We present an investigation of the Xe excimer emission spectrum
observed in the near infrared range about 7800 cm in pure Xe gas and in
an Ar (90%) --Xe (10%) mixture and obtained by exciting the gas with energetic
electrons. The Franck--Condon simulation of the spectrum shape suggests that
emission stems from a bound--free molecular transition never studied before.
The states involved are assigned as the bound state with atomic limit and the dissociative state with limit. Comparison with the spectrum simulated by using theoretical
potentials shows that the dissociative one does not reproduce correctly the
spectrum features.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
The intrinsic features of the specific heat at half-filled Landau levels of two-dimensional electron systems
The specific heat capacity of a two-dimensional electron gas is derived for
two types of the density of states, namely, the Dirac delta function spectrum
and that based on a Gaussian function. For the first time, a closed form
expression of the specific heat for each case is obtained at half-filling. When
the chemical potential is temperature-independent, the temperature is
calculated at which the specific heat is a maximum. Here the effects of the
broadening of the Landau levels are distinguished from those of the different
filling factors. In general, the results derived herein hold for any
thermodynamic system having similar resonant states.Comment: 11 pages, 1 figure, to appear in J Low Temp Phys (2010
Generation of coherent terahertz pulses in Ruby at room temperature
We have shown that a coherently driven solid state medium can potentially
produce strong controllable short pulses of THz radiation. The high efficiency
of the technique is based on excitation of maximal THz coherence by applying
resonant optical pulses to the medium. The excited coherence in the medium is
connected to macroscopic polarization coupled to THz radiation. We have
performed detailed simulations by solving the coupled density matrix and
Maxwell equations. By using a simple -type energy scheme for ruby, we have
demonstrated that the energy of generated THz pulses ranges from hundreds of
pico-Joules to nano-Joules at room temperature and micro-Joules at liquid
helium temperature, with pulse durations from picoseconds to tens of
nanoseconds. We have also suggested a coherent ruby source that lases on two
optical wavelengths and simultaneously generates THz radiation. We discussed
also possibilities of extension of the technique to different solid-state
materials
Scanning Capacitance Microscopy Investigations of Focused Ion Beam Damage in Silicon
In this article, we explore the application of Scanning Capacitance Microscopy (SCM) for studying focused ion beam (FIB) induced damage in silicon. We qualitatively determine the technologically important beam shape by measuring the SCM image of FIB processed implantation spots and by comparison of topographical and SCM data. Further, we investigate the question how deep impinging ions generate measurable damage below the silicon surface. For this purpose, trenches were manufactured using FIB and analyzed by SCM in cross sectional geometry
Persistent spin splitting of a two-dimensional electron gas in tilted magnetic fields
By varying the orientation of the applied magnetic field with respect to the
normal of a two-dimensional electron gas, the chemical potential and the
specific heat reveal persistent spin splitting in all field ranges. The
corresponding shape of the thermodynamic quantities distinguishes whether the
Rashba spin-orbit interaction RSOI, the Zeeman term or both dominate the
splitting. The interplay of the tilting of the magnetic field and RSOI resulted
to an amplified splitting in weak fields. The effects of changing the RSOI
strength and the Landau level broadening are also investigated.Comment: 10 pages, 5 figure
Plasma instability and amplification of electromagnetic waves in low-dimensional electron systems
A general electrodynamic theory of a grating coupled two dimensional electron
system (2DES) is developed. The 2DES is treated quantum mechanically, the
grating is considered as a periodic system of thin metal strips or as an array
of quantum wires, and the interaction of collective (plasma) excitations in the
system with electromagnetic field is treated within the classical
electrodynamics. It is assumed that a dc current flows in the 2DES. We consider
a propagation of an electromagnetic wave through the structure, and obtain
analytic dependencies of the transmission, reflection, absorption and emission
coefficients on the frequency of light, drift velocity of 2D electrons, and
other physical and geometrical parameters of the system. If the drift velocity
of 2D electrons exceeds a threshold value, a current-driven plasma instability
is developed in the system, and an incident far infrared radiation is
amplified. We show that in the structure with a quantum wire grating the
threshold velocity of the amplification can be essentially reduced, as compared
to the commonly employed metal grating, down to experimentally achievable
values. Physically this is due to a considerable enhancement of the grating
coupler efficiency because of the resonant interaction of plasma modes in the
2DES and in the grating. We show that tunable far infrared emitters, amplifiers
and generators can thus be created at realistic parameters of modern
semiconductor heterostructures.Comment: 28 pages, 15 figures, submitted to Phys. Rev.
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