954 research outputs found
Ground-state Wave Function Of Shallow Donors In Uniaxially Stressed Silicon: Piezohyperfine Constants Determined By Electron-nuclear Double Resonance
Stress-induced changes in the ENDOR spectra of a shallow donor electron interacting with various Si29 nuclei neighboring the donor have been experimentally and theoretically investigated. For each of the three measured donors - As, P, and Sb - the compressional, uniaxial stress was applied along the [001] axis and its magnitude corresponded to strains up to 10-3. To describe the observed linear and quadratic shifts and splitting\u27s of the lines in an ENDOR shell, we have defined a set of piezo hyperfine constants. One piezo hyperfine constant was measured for each axis-class shell; three independent piezo hyperfine constants were measured for each shell of the other shell symmetry classes. Piezo hyperfine constants are reported for more than 15 measured lattice shells about each donor. Analysis of the results shows that the constants for any one shell can be attributed primarily to changes in the Fermi contact hyperfine constants at the various lattice sites within that shell. Consequently, the stress-induced changes are directly related to wave-function density changes at specific points in the lattice. Calculations of these wave-function density changes have been performed using a model based on the valley-repopulation effect and on an effect due to the redistribution of the radial envelope function (RREF effect). The calculations and experimental results are qualitatively in good agreement. The quantitative theoretical accuracy is not sufficient to match all the experimental shells to the actual lattice shells, but a new match of shell Q and the (1,1,5) shell has been determined and other matchings are suggested. The theoretical and experimental results provide information on two intrinsic lattice parameters: the deformation-potential constant Îu, and the location of the conduction-band minimum k0. Difficulties with assigning a value to Îu because of the RREF effect are discussed. A revised spin-resonance value for Îu was found to be 10 ± 1 eV. Applying the above model to the previously matched shells [A and (0,0,4), B and (4,4,0), K and (0,0,8)], one finds an average k0=(0.86±0.02)kmax. © 1970 The American Physical Society
Signature of Electronic Correlations in the Optical Conductivity of the Doped Semiconductor Si:P
Electronic transport in highly doped but still insulating silicon at low
temperatures is dominated by hopping between localized states; it serves as a
model system of a disordered solid for which the electronic interaction can be
investigated. We have studied the frequency-dependent conductivity of
phosphorus-doped silicon in the THz frequency range (30 GHz to 3 THz) at low
temperatures K. The crossover in the optical conductivity from a
linear to a quadratic frequency dependence as predicted by Efros and Shklovskii
is observed qualitatively; however, the simple model does not lead to a
quantitative agreement. Covering a large range of donor concentration, our
temperature- and frequency-dependent investigations reveal that electronic
correlation effects between the localized states play an important and complex
role at low temperatures. In particular we find a super-linear frequency
dependence of the conductivity that highlights the influence of the density of
states, i.e. the Coulomb gap, on the optical conductivity. When approaching the
metal-to-insulator transition by increasing doping concentration, the
dielectric constant and the localization length exhibit critical behavior.Comment: 9 pages, 8 figures, 1 tabl
Decoherence of electron spin qubits in Si-based quantum computers
Direct phonon spin-lattice relaxation of an electron qubit bound by a donor
impurity or quantum dot in SiGe heterostructures is investigated. The aim is to
evaluate the importance of decoherence from this mechanism in several important
solid-state quantum computer designs operating at low temperatures. We
calculate the relaxation rate as a function of [100] uniaxial strain,
temperature, magnetic field, and silicon/germanium content for Si:P bound
electrons. The quantum dot potential is much smoother, leading to smaller
splittings of the valley degeneracies. We have estimated these splittings in
order to obtain upper bounds for the relaxation rate. In general, we find that
the relaxation rate is strongly decreased by uniaxial compressive strain in a
SiGe-Si-SiGe quantum well, making this strain an important positive design
feature. Ge in high concentrations (particularly over 85%) increases the rate,
making Si-rich materials preferable. We conclude that SiGe bound electron
qubits must meet certain conditions to minimize decoherence but that
spin-phonon relaxation does not rule out the solid-state implementation of
error-tolerant quantum computing.Comment: 8 figures. To appear in PRB-July 2002. Revisions include: some
references added/corrected, several typos fixed, a few things clarified.
Nothing dramati
Analysis of the Surface Density and Reactivity of Perfluorophenylazide and the Impact on Ligand Immobilization
Perfluorophenylazide (PFPA) chemistry is a novel method for tailoring the surface properties of solid surfaces and nanoparticles. It is general and versatile, and has proven to be an efficient way to immobilize graphene, proteins, carbohydrates, and synthetic polymers. The main thrust of this work is to provide a detailed investigation on the chemical composition and surface density of the PFPA tailored surface. Specifically, gold surfaces were treated with PFPA-derivatized (11-mercaptoundecyl) tetra(ethylene glycol) (PFPA-MUTEG) mixed with 2-[2-(2-mercaptoethoxy)ethoxy]ethanol (MDEG) at varying solution mole ratios. Complementary analytical techniques were employed to characterize the resulting films including Fourier transform infrared spectroscopy to detect fingerprints of the PFPA group, x-ray photoelectron spectroscopy and ellipsometry to study the homogeneity and uniformity of the films, and near edge x-ray absorption fine structures to study the electronic and chemical structure of the PFPA groups. Results from these studies show that the films prepared from 90:10 and 80:20 PFPA-MUTEG/MDEG mixed solutions exhibited the highest surface density of PFPA and the most homogeneous coverage on the surface. A functional assay using surface plasmon resonance with carbohydrates covalently immobilized onto the PFPAmodified surfaces showed the highest binding affinity for lectin on the PFPA-MUTEG/MDEG film prepared from a 90:10 solution
Assembly and structure of α-helical peptide films on hydrophobic fluorocarbon surfaces
The structure, orientation and formation of amphiphilic α-helix model peptide films on fluorocarbon surfaces has been monitored with sum frequency generation (SFG) vibrational spectroscopy, near edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). The α-helix peptide is a 14-mer of hydrophilic lysine and hydrophobic leucine residues with a hydrophobic periodicity of 3.5. This periodicity yields a rigid amphiphilic peptide with leucine and lysine side chains located on opposite sides. XPS composition analysis confirms the formation of a peptide film that covers about 75% of the surface. NEXAFS data are consistent with chemically intact adsorption of the peptides. A weak linear dichroism of the amide Ï* is likely due to the broad distribution of amide bond orientations inherent to the α-helical secondary structure. SFG spectra exhibit strong peaks near 2865 cm(â1) and 2935 cm(â1) related to aligned leucine side chains interacting with the hydrophobic surface. Water modes near 3200 cm(â1) and 3400 cm(â1) indicate ordering of water molecules in the adsorbed--peptide fluorocarbon surface interfacial region. Amide I peaks observed near 1655 cm(â1) confirm that the secondary structure is preserved in the adsorbed peptide. A kinetic study of the film formation process using XPS and SFG showed rapid adsorption of the peptides followed by a longer assembly process. Peptide SFG spectra taken at the airâbuffer interface showed features related to well ordered peptide films. Moving samples through the buffer surface led to the transfer of ordered peptide films onto the substrates
Electrical properties of isotopically enriched neutron-transmutation-doped ^{70} Ge:Ga near the metal-insulator transition
We report the low temperature carrier transport properties of a series of
nominally uncompensated neutron-transmutation doped (NTD) ^{70} Ge:Ga samples
very close to the critical concentration N_c for the metal-insulator
transition. The concentration of the sample closest to N_c is 1.0004N_c and it
is unambiguously shown that the critical conductivity exponent is 0.5.
Properties of insulating samples are discussed in the context of Efros and
Shklovskii's variable range hopping conduction.Comment: 8 pages using REVTeX, 8 figures, published versio
X-Band ESR Determination of Dzyaloshinsky-Moriya Interaction in 2D SrCu(BO) System
X-band ESR measurements on a single crystal of SrCu(BO) system in
a temperature range between 10 K and 580 K are presented. The temperature and
angular dependence of unusually broad ESR spectra can be explained by the
inclusion of antisymmetric Dzyaloshinsky-Moriya (DM) interaction, which yields
by far the largest contribution to the linewidth. However, the well-accepted
picture of only out-of-plane interdimer DM vectors is not sufficient for
explanation of the observed angular dependence. In order to account for the
experimental linewidth anisotropy we had to include sizable in-plane components
of interdimer as well as intradimer DM interaction in addition to the
out-of-plane interdimer one. The nearest-neighbor DM vectors lie perpendicular
to crystal anisotropy c-axis due to crystal symmetry. We also emphasize that
above the structural phase transition occurring at 395 K dynamical mechanism
should be present allowing for instantaneous DM interactions. Moreover, the
linewidth at an arbitrary temperature can be divided into two contributions;
namely, the first part arising from spin dynamics governed by the spin
Hamiltonian of the system and the second part due to significant spin-phonon
coupling. The nature of the latter mechanism is attributed to phonon-modulation
of the antisymmetric interaction, which is responsible for the observed linear
increase of the linewidth at high temperatures.Comment: 17 pages, 4 figures, submitted to PR
Individual charge traps in silicon nanowires: Measurements of location, spin and occupation number by Coulomb blockade spectroscopy
We study anomalies in the Coulomb blockade spectrum of a quantum dot formed
in a silicon nanowire. These anomalies are attributed to electrostatic
interaction with charge traps in the device. A simple model reproduces these
anomalies accurately and we show how the capacitance matrices of the traps can
be obtained from the shape of the anomalies. From these capacitance matrices we
deduce that the traps are located near or inside the wire. Based on the
occurrence of the anomalies in wires with different doping levels we infer that
most of the traps are arsenic dopant states. In some cases the anomalies are
accompanied by a random telegraph signal which allows time resolved monitoring
of the occupation of the trap. The spin of the trap states is determined via
the Zeeman shift.Comment: 9 pages, 8 figures, v2: section on RTS measurements added, many
improvement
Materials with Colossal Dielectric Constant: Do They Exist?
Experimental evidence is provided that colossal dielectric constants, epsilon
>= 1000, sometimes reported to exist in a broad temperature range, can often be
explained by Maxwell-Wagner type contributions of depletion layers at the
interface between sample and contacts, or at grain boundaries. We demonstrate
this on a variety of different materials. We speculate that the largest
intrinsic dielectric constant observed so far in non-ferroelectric materials is
of order 100.Comment: 3 figure
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