30 research outputs found
Quadrupolar spectra of nuclear spins in strained InGaAs quantum dots
Self-assembled quantum dots (QDs) are born out of lattice mismatched
ingredients where strain plays an indispensable role. Through the electric
quadrupolar coupling, strain affects the magnetic environment as seen by the
nuclear spins. To guide prospective single-QD nuclear magnetic resonance (NMR)
as well as dynamic nuclear spin polarization experiments, an atomistic insight
to the strain and quadrupolar field distributions is presented. A number of
implications of the structural and compositional profile of the QD have been
identified. A high aspect ratio of the QD geometry enhances the quadrupolar
interaction. The inclined interfaces introduce biaxiality and the tilting of
the major quadrupolar principal axis away from the growth axis; the alloy
mixing of gallium into the QD enhances both of these features while reducing
the quadrupolar energy. Regarding the NMR spectra, both Faraday and Voigt
geometries are investigated, unraveling in the first place the extend of
inhomogeneous broadening and the appearance of the normally-forbidden
transitions. Moreover, it is shown that from the main extend of the NMR spectra
the alloy mole fraction of a single QD can be inferred. By means of the
element-resolved NMR intensities it is found that In nuclei has a factor of
five dominance over those of As. In the presence of an external magnetic field,
the borderlines between the quadrupolar and Zeeman regimes are extracted as 1.5
T for In and 1.1 T for As nuclei. At these values the nuclear spin
depolarization rates of the respective nuclei get maximized due to the
noncollinear secular hyperfine interaction with a resident electron in the QD.Comment: Published version, 13 pages, 9 figure
Nuclear spin squeezing via electric quadrupole interaction
Control over nuclear spin fluctuations is essential for processes that rely
on preserving the quantum state of an embedded system. For this purpose,
squeezing is a viable alternative, so far that has not been properly exploited
for the nuclear spins. Of particular relevance in solids is the electric
quadrupole interaction (QI), which operates on nuclei having spin higher than
1/2. In its general form, QI involves an electric field gradient (EFG)
biaxiality term. Here, we show that as this EFG biaxiality increases, it
enables continuous tuning of single-particle squeezing from the one-axis
twisting to the two-axis countertwisting limits. A detailed analysis of QI
squeezing is provided, exhibiting the intricate consequences of EFG biaxiality.
The initial states over the Bloch sphere are mapped out to identify those
favorable for fast initial squeezing, or for prolonged squeezings. Furthermore,
the evolution of squeezing in the presence of a phase-damping channel and an
external magnetic field are investigated. We observe that dephasing drives
toward an anti-squeezed terminal state, the degree of which increases with the
spin angular momentum. Finally, QI squeezing in the limiting case of a
two-dimensional EFG with a perpendicular magnetic field is discussed, which is
of importance for two-dimensional materials, and the associated beat patterns
in squeezing are revealed.Comment: Published version in contents, 10 pages, 9 figure
Disorder-free localization around the conduction band edge of crossing and kinked silicon nanowires
We explore ballistic regime quantum transport characteristics of
oxide-embedded crossing and kinked silicon nanowires (NWs) within a large-scale
empirical pseudopotential electronic structure framework, coupled to the
Kubo-Greenwood transport analysis. A real-space wave function study is
undertaken and the outcomes are interpreted together with the findings of
ballistic transport calculations. This reveals that ballistic transport edge
lies tens to hundreds of millielectron volts above the lowest unoccupied
molecular orbital, with a substantial number of localized states appearing in
between, as well as above the former. We show that these localized states are
not due to the oxide interface, but rather core silicon-derived. They manifest
the wave nature of electrons brought to foreground by the reflections
originating from NW junctions and bends. Hence, we show that the crossings and
kinks of even ultraclean Si NWs possess a conduction band tail without a
recourse to atomistic disorder.Comment: Published version, 7 pages, 9 figure
Stark effect, polarizability and electroabsorption in silicon nanocrystals
Demonstrating the quantum-confined Stark effect (QCSE) in silicon
nanocrystals (NCs) embedded in oxide has been rather elusive, unlike the other
materials. Here, the recent experimental data from ion-implanted Si NCs is
unambiguously explained within the context of QCSE using an atomistic
pseudopotential theory. This further reveals that the majority of the Stark
shift comes from the valence states which undergo a level crossing that leads
to a nonmonotonic radiative recombination behavior with respect to the applied
field. The polarizability of embedded Si NCs including the excitonic effects is
extracted over a diameter range of 2.5--6.5 nm, which displays a cubic scaling,
, with C/(Vm), where is the NC
diameter. Finally, based on intraband electroabsorption analysis, it is
predicted that p-doped Si NCs will show substantial voltage tunability, whereas
n-doped samples should be almost insensitive. Given the fact that bulk silicon
lacks the linear electro-optic effect as being a centrosymmetric crystal, this
may offer a viable alternative for electrical modulation using p-doped Si NCs.Comment: Published version, 7 pages, 7 figure
Carrier-induced refractive index change and optical absorption in wurtzite InN and GaN: Fullband approach
Based on the full band electronic structure calculations, first we consider
the effect of n-type doping on the optical absorption and the refractive index
in wurtzite InN and GaN. We identify quite different dielectric response in
either case; while InN shows a significant shift in the absorption edge due to
n-type doping, this is masked for GaN due to efficient cancellation of the
Burstein-Moss effect by the band gap renormalization. For high doping levels
the intraband absorption becomes significant in InN. Furthermore, we observe
that the free-carrier plasma contribution to refractive index change becomes
more important than both band filling and the band gap renormalization for
electron densities above 10~cm in GaN, and 10~cm in
InN. As a result of the two different characteristics mentioned above, the
overall change in the refractive index due to n-type doping is much higher in
InN compared to GaN, which in the former exceeds 4\% for a doping of
10~cm at 1.55~m wavelength. Finally, we consider intrinsic
InN under strong photoexcitation which introduces equal density of electron and
holes thermalized to their respective band edges. The change in the refractive
index at 1.55~m is observed to be similar to the n-doped case up to a
carrier density of 10~cm. However, in the photoexcited case this
is now accompanied by a strong absorption in this wavelength region due to
intravalence band transition. Our findings suggest
that the alloy composition of InGaN can be optimized in the
indium-rich region so as to benefit from high carrier-induced refractive index
change while operating in the transparency region to minimize the losses. These
can have direct implications for InN-containing optical phase modulators and
lasers.Comment: Revised with an appendix, two additional figures, and more
discussions; 10 pages, 14 figures; published versio
Nuclear magnetic resonance inverse spectra of InGaAs quantum dots: Atomistic level structural information
A wealth of atomistic information is contained within a self-assembled
quantum dot (QD), associated with its chemical composition and the growth
history. In the presence of quadrupolar nuclei, as in InGaAs QDs, much of this
is inherited to nuclear spins via the coupling between the strain within the
polar lattice and the electric quadrupole moments of the nuclei. Here, we
present a computational study of the recently introduced inverse spectra
nuclear magnetic resonance technique to assess its suitability for extracting
such structural information. We observe marked spectral differences between the
compound InAs and alloy InGaAs QDs. These are linked to the local biaxial and
shear strains, and the local bonding configurations. The cation-alloying plays
a crucial role especially for the arsenic nuclei. The isotopic line profiles
also largely differ among nuclear species: While the central transition of the
gallium isotopes have a narrow linewidth, those of arsenic and indium are much
broader and oppositely skewed with respect to each other. The statistical
distributions of electric field gradient (EFG) parameters of the nuclei within
the QD are analyzed. The consequences of various EFG axial orientation
characteristics are discussed. Finally, the possibility of suppressing the
first-order quadrupolar shifts is demonstrated by simply tilting the sample
with respect to the static magnetic field.Comment: Published version, 17 pages, 18 figure