367 research outputs found
Theory of Mott insulator/band insulator heterostructure
A theory of heterostructures comprised of LaTiO (a Mott insulator) and
SrTiO (a band insulator) is presented. The band structure of the Ti %
-electrons is treated with a nearest neighbor tight-binding approximation; the
electric fields arising from the La/Sr charge difference and the
carriers are treated within a Hartree approximation; and the on-site
interactions are treated by unrestricted Hartree-Fock. The phase diagram as a
function of interaction strength and layer number is determined and predictions
are made for optical conductivity experiments. A note worthy finding is that
the edges of the heterostructure are generally metallic.Comment: 11 pages, 9 figure
Digital compensation of the side-band-rejection ratio in a fully analog 2SB sub-millimeter receiver
In observational radio astronomy, sideband-separating receivers are
preferred, particularly under high atmospheric noise, which is usually the case
in the sub-millimeter range. However, obtaining a good rejection ratio between
the two sidebands is difficult since, unavoidably, imbalances in the different
analog components appear. We describe a method to correct these imbalances
without making any change in the analog part of the sideband-separating
receiver, specifically, keeping the intermediate-frequency hybrid in place.
This opens the possibility of implementing the method in any existing receiver.
We have built hardware to demonstrate the validity of the method and tested it
on a fully analog receiver operating between 600 and 720GHz. We have tested the
stability of calibration and performance vs time and after full resets of the
receiver. We have performed an error analysis to compare the digital
compensation in two configurations of analog receivers, with and without
intermediate frequency (IF) hybrid. An average compensated sideband rejection
ratio of 46dB is obtained. Degradation of the compensated sideband rejection
ratio on time and after several resets of the receiver is minimal. A receiver
with an IF hybrid is more robust to systematic errors. Moreover, we have shown
that the intrinsic random errors in calibration have the same impact for
configuration without IF hybrid and for a configuration with IF hybrid with
analog rejection ratio better than 10dB. Compensated rejection ratios above
40dB are obtained even in the presence of high analog rejection. The method is
robust allowing its use under normal operational conditions at any telescope.
We also demonstrate that a full analog receiver is more robust against
systematic errors. Finally, the error bars associated to the compensated
rejection ratio are almost independent of whether IF hybrid is present or not
Ultra-pure digital sideband separation at sub-millimeter wavelengths
Deep spectral-line surveys in the mm and sub-mm range can detect thousands of
lines per band uncovering the rich chemistry of molecular clouds, star forming
regions and circumstellar envelopes, among others objects. The ability to study
the faintest features of spectroscopic observation is, nevertheless, limited by
a number of factors. The most important are the source complexity (line
density), limited spectral resolution and insufficient sideband (image)
rejection (SRR). Dual Sideband (2SB) millimeter receivers separate upper and
lower sideband rejecting the unwanted image by about 15 dB, but they are
difficult to build and, until now, only feasible up to about 500 GHz
(equivalent to ALMA Band 8). For example ALMA Bands 9 (602-720 GHz) and 10
(787-950 GHz) are currently DSB receivers. Aims: This article reports the
implementation of an ALMA Band 9 2SB prototype receiver that makes use of a new
technique called calibrated digital sideband separation. The new method
promises to ease the manufacturing of 2SB receivers, dramatically increase
sideband rejection and allow 2SB instruments at the high frequencies currently
covered only by Double Sideband (DSB) or bolometric detectors. Methods: We made
use of a Field Programmable Gate Array (FPGA) and fast Analog to Digital
Converters (ADCs) to measure and calibrate the receiver's front end phase and
amplitude imbalances to achieve sideband separation beyond the possibilities of
purely analog receivers. The technique could in principle allow the operation
of 2SB receivers even when only imbalanced front ends can be built,
particularly at very high frequencies. Results: This digital 2SB receiver shows
an average sideband rejection of 45.9 dB while small portions of the band drop
below 40 dB. The performance is 27 dB (a factor of 500) better than the average
performance of the proof-of-concept Band 9 purely-analog 2SB prototype
receiver.Comment: 5 page
ALMA Band 9 upgrade: a feasibility study
We present the results of a study on the feasibility of upgrading the
existing ALMA Band 9 receivers (602-720 GHz). In the current configuration,
each receiver is a dual channel heterodyne system capable of detecting
orthogonally polarized signals through the use of a wire grid and a compact
arrangement of mirrors. The main goals of the study are the upgrade of the
mixer architecture from Double-Sideband (DSB) to Sideband-separating (2SB), the
extension of the IF and RF bandwidth, and the analysis of the possibilities of
improving the polarimetric performance. We demonstrate the performance of 2SB
mixers both in the lab and on-sky with the SEPIA660 receiver at APEX, which
shows image rejection ratios exceeding 20 dB and can perform successful
observations of several spectral lines close to the band edges. The same
architecture in ALMA Band 9 would lead to an increase in the effective spectral
sensitivity and a gain of a factor two in observation time. We set up also an
electromagnetic model of the optics to simulate the polarization performance of
the receivers, which is currently limited by the cross-polar level and the beam
squint, i.e. pointing mismatch between the two polarizations. We present the
results of the simulations compared to the measurements and we conclude that
the use of a polarizing grid is the main responsible of the limitations.Comment: to appear in Proc. of the mm Universe 2023 conference, Grenoble
(France), June 2023, published by F. Mayet et al. (Eds), EPJ Web of
conferences, EDP Science
Non conventional screening of the Coulomb interaction in low dimensional and finite size system
We study the screening of the Coulomb interaction in non polar systems by
polarizable atoms. We show that in low dimensions and small finite size systems
this screening deviates strongly from that conventionally assumed. In fact in
one dimension the short range interaction is strongly screened and the long
range interaction is anti-screened thereby strongly reducing the gradient of
the Coulomb interaction and therefore the correlation effects. We argue that
this effect explains the success of mean field single particle theories for
large molecules.Comment: 4 pages, 5 figure
An SIS-based sideband-separating heterodyne mixer optimized for the 600 to 720 GHz band
The Atacama Large Millimeter Array (ALMA) is the largest radio astronomical enterprise ever proposed. When completed, each of its 64 constituting radio-telescopes will be able to hold 10 heterodyne receivers covering the spectroscopic windows allowed by the atmospheric transmission at the construction site, the altiplanos of the northern Chilean Andes. In contrast to the sideband-separating (2SB) receivers being developed at low frequencies, double-side-band (DSB) receivers are being developed for the highest two spectroscopic windows (bands 9 and 10). Despite of the well known advantages of 2SB mixers over their DSB counterparts, they have not been implemented at the highest-frequency bands as the involved dimensions for some of the radio frequency components are prohibitory small. However, the current state-of-the-art micromachining technology has proved that the structures necessary for this development are attainable. Here we report the design, modeling, realization, and characterization of a 2SB mixer for band 9 of ALMA (600 to 720 GHz). At the heart of the mixer, two superconductor-insulator-superconductor (SIS) junctions are used as mixing elements. The constructed instrument presents an excellent performance as shown by two important figures of merit: noise temperature of the system and side band ratio, both of them within ALMA specifications
Study of the Calibration Channel Width for a Digital Sideband Separating System for SIS 2SB Receiver
A Digital Sideband Separating (DSS) system has been recently applied to a full 2SB receiver, i.e., one with the analog IF hybrid still in place. This concept allows reaching IRR level around 45 dB and it presents additional advantages in calibration stability compared to the case when no IF hybrid is present. If implemented in multipixel cameras, the DSS system relaxes the requirements for the IRR level of the analog receiver substantially enabling to reach at least an IRR of 30 dB with relatively simple hardware. It would be ideal for spectral line surveys since it practically eliminates the line confusion in addition to rejecting the atmospheric noise in the image band. Therefore, the DSS system is a potential option for a future ALMA upgrade. Here we present our study on an important practical question: how wide should the calibration-channel width in order to reach a desired IRR level? This parameter determines, for a large part, the calibration speed of the DSS system and influences the back-end architecture. We estimate that for currently installed ALMA bands (B3-B8), the channel width of the DSS system can be at least 45 MHz to reach a 30db IRR level in entire band
ALMA Band 9 upgrade: A feasibility study
We present the results of a study on the feasibility of upgrading the existing ALMA Band 9 receivers (602-720 GHz). In the current configuration, each receiver is a dual channel heterodyne system capable of detecting orthogonally polarized signals through the use of a wire grid and a compact arrangement of mirrors. The main goals of the study are the upgrade of the mixer architecture from Double-Sideband (DSB) to Sideband-separating (2SB), the extension of the IF and RF bandwidth, and the analysis of the possibilities of improving the polarimetric performance. We demonstrate the performance of 2SB mixers both in the lab and on-sky with the SEPIA660 receiver at APEX, which shows image rejection ratios exceeding 20 dB and can perform successful observations of several spectral lines close to the band edges. The same architecture in ALMA Band 9 would lead to an increase in the effective spectral sensitivity and a gain of a factor two in observation time. We set up also an electromagnetic model of the optics to simulate the polarization performance of the receivers, which is currently limited by the cross-polar level and the beam squint, i.e. pointing mismatch between the two polarizations. We present the results of the simulations compared to the measurements and we conclude that the use of a polarizing grid is the main responsible of the limitations
Low-Noise Sis Receivers for New Radio-Astronomy Projects
We have developed, manufactured, and tested a waveguide mixer in the range 211-275 GHz on the basis of the superconductor-insulator-superconductor (SIS) tunnel structures. The methods of manufacturing high-quality tunnel structures on quartz substrates have been worked out. To extend the receiver band, the Nb/AlOx/Nb and Nb/AlN/NbN tunnel junctions with a high current density of up to 20 kA/cm2 are employed. The dependence of the characteristics of the receiving elements on the signal frequency is simulated for the intermediate-frequency band 4-12 GHz. The measurements demonstrate a good agreement of the input band of the receiving structures with the calculated results. The uncorrected noise temperature of the receiver amounts to 24 K at a frequency of 265 GHz, which is only two times higher than the quantum limit. The receivers under development are intended for a number of newly-built ground-based radio telescopes ("Suffa" and LLAMA), as well as for the "Millimetron" space program
Single domain transport measurements of C60 films
Thin films of potassium doped C60, an organic semiconductor, have been grown
on silicon. The films were grown in ultra-high vacuum by thermal evaporation of
C60 onto oxide-terminated silicon as well as reconstructed Si(111). The
substrate termination had a drastic influence on the C60 growth mode which is
directly reflected in the electrical properties of the films. Measured on the
single domain length scale, these films revealed resistivities comparable to
bulk single crystals. In situ electrical transport properties were correlated
to the morphology of the film determined by scanning tunneling microscopy. The
observed excess conductivity above the superconducting transition can be
attributed to two-dimensional fluctuations.Comment: 4 pages, 4 figure
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