806 research outputs found
Cyclotron motion and magnetic focusing in semiconductor quantum wells with spin-orbit coupling
We investigate the ballistic motion of electrons in III-V semiconductor
quantum wells with Rashba spin-orbit coupling in a perpendicular magnetic
field. Taking into account the full quantum dynamics of the problem, we explore
the modifications of classical cyclotron orbits due to spin-orbit interaction.
As a result, for electron energies comparable with the cyclotron energy the
dynamics are particularly rich and not adequately described by semiclassical
approximations. Our study is complementary to previous semiclassical approaches
concentrating on the regime of weaker fields.Comment: 14 pages, 8 figures included, version to appear in Phys. Rev.
Thin-film quantum dot photodiode for monolithic infrared image sensors
Imaging in the infrared wavelength range has been fundamental in scientific, military and surveillance applications. Currently, it is a crucial enabler of new industries such as autonomous mobility (for obstacle detection), augmented reality (for eye tracking) and biometrics. Ubiquitous deployment of infrared cameras (on a scale similar to visible cameras) is however prevented by high manufacturing cost and low resolution related to the need of using image sensors based on flip-chip hybridization. One way to enable monolithic integration is by replacing expensive, small-scale III-V-based detector chips with narrow bandgap thin-films compatible with 8- and 12-inch full-wafer processing. This work describes a CMOS-compatible pixel stack based on lead sulfide quantum dots (PbS QD) with tunable absorption peak. Photodiode with a 150-nm thick absorber in an inverted architecture shows dark current of 10(-6) A/cm(2) at 2 V reverse bias and EQE above 20% at 1440 nm wavelength. Optical modeling for top illumination architecture can improve the contact transparency to 70%. Additional cooling (193 K) can improve the sensitivity to 60 dB. This stack can be integrated on a CMOS ROIC, enabling order-of-magnitude cost reduction for infrared sensors
Atomic layer deposition of titanium nitride for quantum circuits
Superconducting thin films with high intrinsic kinetic inductance are of
great importance for photon detectors, achieving strong coupling in hybrid
systems, and protected qubits. We report on the performance of titanium nitride
resonators, patterned on thin films (9-110 nm) grown by atomic layer
deposition, with sheet inductances of up to 234 pH/square. For films thicker
than 14 nm, quality factors measured in the quantum regime range from 0.4 to
1.0 million and are likely limited by dielectric two-level systems.
Additionally, we show characteristic impedances up to 28 kOhm, with no
significant degradation of the internal quality factor as the impedance
increases. These high impedances correspond to an increased single photon
coupling strength of 24 times compared to a 50 Ohm resonator, transformative
for hybrid quantum systems and quantum sensing.Comment: 10 pages, 8 figures including supplemental material
Giant Magnetothermal Conductivity Switching in Semimetallic WSi Single Crystals
Materials able to rapidly switch between thermally conductive states by
external stimuli such as electric or magnetic fields can be used as
all-solid-state thermal switches and open a myriad of applications in heat
management, power generation and cooling. Here, we show that the large
magnetoresistance that occurs in the highly conducting semimetal
-WSi single crystals leads to dramatically large changes in
thermal conductivity at temperatures <100 K. At temperatures <20 K, where
electron-phonon scattering is minimized, the thermal conductivity switching
ratio between zero field and a 9T applied field can be >7. We extract the
electronic and lattice components of the from the thermal conductivity
measurements and show that the Lorenz number for this material approximates the
theoretical value of L. From the heat capacity and thermal diffusivity,
the speed of thermal conductivity switching is estimated to range from 1 x
10 seconds at 5 K to 0.2 seconds at 100 K for a 5-mm long sample. This
work shows that WSi, a highly conducting multi-carrier semimetal, is a
promising thermal switch component for low-temperature applications such
cyclical adiabatic demagnetization cooling, a technique that would enable
replacing He-based refrigerators.Comment: 20 pages, 6 figure
Photoluminescence spectra of point defects in semiconductors: validation of first principles calculations
Optically and magnetically active point defects in semiconductors are
interesting platforms for the development of solid-state quantum technologies.
Their optical properties are usually probed by measuring photoluminescence
spectra, which provide information on excitation energies and on the
interaction of electrons with lattice vibrations. We present a combined
computational and experimental study of photoluminescence spectra of defects in
diamond and SiC, aimed at assessing the validity of theoretical and numerical
approximations used in first principles calculations, including the use of the
Franck-Condon principle and the displaced harmonic oscillator approximation. We
focus on prototypical examples of solid-state qubits, the divacancy centers in
SiC and the nitrogen-vacancy in diamond, and we report computed
photoluminescence spectra as a function of temperature that are in very good
agreement with the measured ones. As expected we find that the use of hybrid
functionals leads to more accurate results than semilocal functionals.
Interestingly our calculations show that constrained density functional theory
(CDFT) and time-dependent hybrid DFT perform equally well in describing the
excited state potential energy surface of triplet states; our findings indicate
that CDFT, a relatively cheap computational approach, is sufficiently accurate
for the calculations of photoluminescence spectra of the defects studied here.
Finally, we find that only by correcting for finite-size effects and
extrapolating to the dilute limit, one can obtain a good agreement between
theory and experiment. Our results provide a detailed validation protocol of
first principles calculations of photoluminescence spectra, necessary both for
the interpretation of experiments and for robust predictions of the electronic
properties of point defects in semiconductors
In-plane magnetic field-induced spin polarization and transition to insulating behavior in two-dimensional hole systems
Using a novel technique, we make quantitative measurements of the spin
polarization of dilute (3.4 to 6.8*10^{10} cm^{-2}) GaAs (311)A two-dimensional
holes as a function of an in-plane magnetic field. As the field is increased
the system gradually becomes spin polarized, with the degree of spin
polarization depending on the orientation of the field relative to the crystal
axes. Moreover, the behavior of the system turns from metallic to insulating
\textit{before} it is fully spin polarized. The minority-spin population at the
transition is ~8*10^{9} cm^{-2}, close to the density below which the system
makes a transition to an insulating state in the absence of a magnetic field.Comment: 4 pages with figure
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