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M Cardona

Publications and source records attributed to M Cardona.

At least 19 recordsLinked to original sources
PubMed2006

Optical detection and ionization of donors in specific electronic and nuclear spin States.

We resolve the remarkably sharp bound exciton transitions of highly enriched 28Si using a single-frequency laser and photoluminescence excitation spectroscopy, as well as photocurrent spectroscopy. Well-resolved doublets in the spectrum of the 31P donor reflect the hyperfine coupling of the electronic and nuclear donor spins. The optical detection of the nuclear spin state, and selective pumping and ionization of donors in specific electronic and nuclear spin states, suggests a number of new possibilities which could be useful for the realization of silicon-based quantum computers.

Journal Article↗
PubMed2006

Measurement of the surface strain induced by reconstructed surfaces of GaAs (001) using photoreflectance and reflectance-difference spectroscopies.

We report photoreflectance-difference and reflectance-difference measurements on reconstructed GaAs (001) surfaces. From these data the linear and quadratic electro-optic coefficient spectra are determined in the important 2.8-3.4 eV spectral region. The surface strain and fields induced by the surface reconstruction are also determined. We show experimentally that between c(4x4) and (2x4) surfaces, there is an inversion of the surface electric field which we attribute to a direct piezo-electric effect related to the surface strain induced by reconstruction.

Journal Article↗
PubMed2005

Markov chains: computing limit existence and approximations with DNA.

We present two algorithms to perform computations over Markov chains. The first one determines whether the sequence of powers of the transition matrix of a Markov chain converges or not to a limit matrix. If it does converge, the second algorithm enables us to estimate this limit. The combination of these algorithms allows the computation of a limit using DNA computing. In this sense, we have encoded the states and the transition probabilities using strands of DNA for generating paths of the Markov chain.

Algorithms↗
PubMed2003

Impurity absorption spectroscopy in 28Si: the importance of inhomogeneous isotope broadening.

We report high-resolution infrared absorption spectra of the neutral donors phosphorus and lithium, and the neutral acceptor boron, in isotopically pure 28Si crystals. Surprisingly, many of the transitions are much sharper than previously reported in natural Si. In particular, the 2p(0) line of phosphorus in 28Si has a full width at half maximum of only 4.2 microeV, about 5 times less than the narrowest 2p(0) line previously reported for natural Si, making it the narrowest shallow impurity transition yet observed. The widely held assumptions that the impurity transitions previously reported in high quality samples of natural Si revealed the true, homogeneous linewidths, are thus shown to be incorrect. The sharper transitions in 28Si also reveal new substructures in the boron and lithium spectra.

Journal Article↗
PubMed2003

Raman-active c-axis plasma modes in multilayer high-Tc cuprate superconductors.

The spectacular changes of the A(1g)-polarized Raman spectra of trilayer and four-layer high-T(c) cuprate superconductors below T(c) observed in recent experiments can be explained in terms of a new type of excitation that occurs in these systems, a Raman active c-axis plasmon. Its frequency, intensity, resonance properties, polarization properties, and coupling to phonon modes involving vibrations of the planar oxygens are estimated and shown to agree with those of the observed A(1g) superconductivity-induced electronic peak.

Journal Article↗
PubMed2003

Dispersive phonon linewidths: the E2 phonons of ZnO.

Phonon linewidths can exhibit a large variation when either pressure or isotopic masses are changed. These effects yield detailed information about the mechanisms responsible for linewidths and lifetimes, e.g., anharmonicity or isotopic disorder. We report Raman measurements of the linewidth of the upper E2 phonons of ZnO crystals with several isotopic compositions and their dependence on pressure. Changes by a factor of 12 are observed at a given temperature. Comparison with calculated densities of one-phonon states, responsible for isotope scattering, and of two-phonon states, responsible for anharmonic decay, yields a consistent picture of these phenomena. Isotopic disorder broadening by 7 cm(-1) is found in samples with mixed 16O-18O content, whereas the anharmonic processes involve decay into sums and differences of two phonons.

Journal Article↗
PubMed2003

Origin of the residual acceptor ground-state splitting in silicon.

The residual ground-state splitting of acceptors in high-quality silicon has been studied intensely by different experimental techniques for several decades. Recently, photoluminescence studies of isotopically pure silicon revealed the ground-state splitting to result from the random distribution of isotopes in natural silicon. Here we present a new model that explains these surprising experimental results, and discuss the implications for acceptor ground-state splittings observed in other isotopically mixed semiconductors, as well as for the acceptor ground state in semiconductor alloys.

Journal Article↗
PubMed2002

"Intrinsic" acceptor ground state splitting in silicon: an isotopic effect.

One of the oldest open questions in semiconductor physics is the origin of the small splittings of the neutral acceptor ground state in silicon which lead to a distribution of doublet splittings rather than the fourfold-degenerate state of Gamma(8) symmetry expected in the absence of perturbations. Here we show that these acceptor ground state splittings are absent in the photoluminescence spectra of acceptor bound excitons in isotopically purified 28Si, demonstrating conclusively the surprising result that the splittings previously observed in natural Si result from the randomness of the Si isotopic composition.

Journal Article↗
PubMed2001

Photoluminescence of isotopically purified silicon: how sharp are bound exciton transitions?

We report the first high resolution photoluminescence studies of isotopically pure Si (99.896% (28)Si). New information is obtained on isotopic effects on the indirect band gap energy, phonon energies, and phonon broadenings, which is in good agreement with calculations and previous results obtained in Ge and diamond. Remarkably, the linewidths of the no-phonon boron and phosphorus bound exciton transitions in the (28)Si sample are much narrower than in natural Si and are not well resolved at our maximum instrumental resolution of approximately 0.014 cm(-1). The removal of the dominant broadening resulting from isotopic randomness in natural Si reveals new fine structure in the boron bound exciton luminescence.

Journal Article↗
PubMed2001

X-ray standing wave analysis of the effect of isotopic composition on the lattice constants of Si and Ge.

The x-ray standing wave (XSW) technique is used to measure the isotopic mass dependence of the lattice constants of Si and Ge. Backreflection allows substrates of moderate crystallinity to be used while high order reflection yields high accuracy. The XSW, generated by the substrate, serves as a reference for the lattice planes of an epilayer of different isotopic composition. Employing XSW and photoemission, the position of the surface planes is determined from which the lattice constant difference Deltaa is calculated. Scaled to DeltaM = 1 amu we find (Deltaa/a) of -0.36x10(-5) and -0.88x10(-5) for Ge and -1.8x10(-5) and -3.0x10(-5) for Si at 300 and 30 K, respectively.

Journal Article↗
PubMed2001

Phonon dispersion curves in wurtzite-structure GaN determined by inelastic x-ray scattering.

We have investigated the lattice dynamics of a wurtzite GaN single crystal by inelastic x-ray scattering. Several dispersion branches and phonons at high-symmetry points have been measured, including the two zone-center Raman- and infrared-inactive silent modes. The experiments have been complemented by ab initio calculations. They are in very good agreement with our measurements, not only for phonon energies, but also for scattering intensities, thus validating the correctness of the eigenvectors. Other phenomenological and ab initio theories exhibit significant differences.

Journal Article↗
PubMed2000

Leishmania (Viannia) guyanensis: a new minicircle class exclusive to this specie isolated from a DNA cosmid library useful for taxonomic purposes.

A new minicircle class exclusive to this specie isolated from a DNA cosmid library useful for taxonomic purposes. Experimental Parasitology 94, 143-149. In this paper we describe a new minicircle class exclusive to Leishmania (Viannia) guyanensis. The minicircle class was obtained with the aid of a total DNA cosmid library. The library was screened with an EcoRI fragment isolated from L. (V.) guyanensis (M4147). After screening seven clones were selected which showed strong hybridisation. Clones were digested and hybridised with the same probe. After hybridisation only one clone containing the desired fragment was positive. The fragment sized around 1000 bp was subcloned into pBluescript for sequencing. Sequence analysis using the GCG programme showed no substantial homology with any sequences previously reported, apart from the expected homology with the conserved region of Leishmania kDNA sequences. The probe hybridised strongly only to L. (V.) guyanensis kDNA after medium stringency washing.

Animals↗
PubMed1999

Subjective versus statistical model assessment of mortality risk in open heart surgical procedures.

BACKGROUND: The aim of this study was to compare the predictive accuracy for open heart surgical mortality between a statistical model based on collection of clinical data and surgeons' subjective risk assessment. METHODS: Predictive discrimination of both risk assessments (surgeons' and model) was compared through the area under the receiver operating characteristic curve. Logistic regression analysis was used to assess the relation between surgeons' and model predictions to actual outcomes. Calibration of the subjective estimates was evaluated with a chi2 test. RESULTS: Overall, the area under the receiver operating characteristic curve was 0.76 for the statistical model and 0.70 for the subjective assessment. Logistic regression analysis showed that the statistical model remained significant after accounting for the subjective assessment. Calibration of subjective mortality predictions was poor. CONCLUSIONS: Surgeons' risk assessment tends to cluster in the middle ranges of risk. Subjective assessment seems accurate in identifying the two extremes of risk but is inaccurate for intermediate risk levels. A multivariate statistical model improves the accuracy of subjective predictions.

Aged↗
PubMed1999

Randomized double-blind trial of beta-carotene and vitamin C in women with minor cervical abnormalities.

A double-blind, placebo-controlled, randomized, factorial study using a daily oral administration of 30 mg beta-carotene and/or 500 mg vitamin C was conducted in 141 women with colposcopically and histologically confirmed minor squamous atypia or cervical intra-epithelial neoplasia (CIN) I. Over approximately 2 years of follow-up, 43 lesions regressed to normal and 13 progressed to CIN II. The regression rate was slightly higher, but not significantly so, in those randomized to beta-carotene compared to no beta-carotene (hazard ratio = 1.58, 95% CI: 0.86-2.93, P = 0.14) and slightly lower, but not statistically significant, for those randomized to vitamin C compared to no vitamin C (hazard ratio = 0.65, 95% CI: 0.35-1.21, P = 0.17). In a model with no interaction, the progression rate was slightly higher in those randomized to beta-carotene (hazard ratio = 1.75, 95% CI: 0.57-5.36, P = 0.32) and also in those randomized to vitamin C (hazard ratio = 2.40, 95% CI: 0.74-7.80, P = 0.13). Neither of these were statistically significant. However, there was some evidence of an interaction effect of the two compounds on the progression rate (P = 0.052), with seven of the progressed lesions occurring in those randomized to both vitamins compared to a total of six in the three other groups. The currently available evidence from this and other trials suggests that high doses of these compounds are unlikely to increase the regression or decrease the progression of minor atypia and CIN I.

Adult↗
PubMed1999

Transmission of tuberculosis from a seven-year-old child in a Sydney school.

OBJECTIVES: To determine whether a 7-year-old child with extrapulmonary and pulmonary tuberculosis (TB) and direct smear positive sputum for acid-fast bacilli was infectious to home and school contacts, and to ascertain potential adult sources of infection for these contacts. METHODS: Contact tracing by Mantoux testing was conducted on 220 children at a primary school and after-school care facility, and 59 selected adults considered potential sources of infection. RESULTS: The participation rate for the children was 98% and 92% for the adults. Mantoux positivity (induration >/= 10 mm, or >/= 15 mm with previous BCG) among children was 13% at the school (anticipated rate 2-3%), 26% among school staff, and 7% among children at the after-school care centre where the index case attended. One exposed adult hospital staff member converted from Mantoux negative to positive. No other cases of TB disease were detected among children or adults tested. CONCLUSION: Although spread of TB from children to others is rare, the findings of this investigation indicate that transmission of TB from a young child to other children and an adult may have occurred, and that sputum testing and contact tracing for sputum smear positive children should be considered.

Adult↗
PubMed1998

Isotopic mass and lattice constant: X-ray standing wave measurements

The molecular volume of crystals depends on their isotopic masses. This influence originates from the zero-point motion and the resulting small differences in lattice constants. This effect was measured with high precision by using an x-ray standing wave. The standing wave is generated during Bragg reflection and thus is in phase with the planes of the substrate crystal, which is covered with a homoepitaxial film that has a different isotopic composition than the substrate. The positions of the surface planes of the film with respect to the substrate planes are revealed by the photoelectrons excited by the maxima of the standing wave. For germanium-76 on natural germanium(111), a difference in lattice constant of -1.1 x 10(-5) and -2.5 x 10(-5) at 300 and 54 kelvin, respectively, was found. The results are in good agreement with theoretical predictions.

Journal Article↗