Biomedical subjects

J J Sáenz

Publications and source records attributed to J J Sáenz.

12 recordsLinked to original sources

PubMed2006

Conductance oscillations in squashed carbon nanotubes.

We report measurements on the radial electromechanical properties of single walled carbon nanotubes. By measuring the conductance of the nanotube, we show that a gap is opened while squashing the nanotubes and that during the deformation stages we observe at least two open-close cycles of the gap. We employ a novel experimental setup where an atomic force microscope tip is used both as an electrode and to induce radial deformations. In contrast with prior experiments reported, this technique allows direct probing of the local electronic structure of carbon nanotubes as they are radially deformed.

Journal Article↗

PubMed2005

Electromagnetic surface modes in structured perfect-conductor surfaces.

Surface-bound modes in metamaterials forged by drilling periodic hole arrays in perfect-conductor surfaces are investigated by means of both analytical techniques and rigorous numerical solution of Maxwell's equations. It is shown that these metamaterials cannot be described in general by local, frequency-dependent permitivities and permeabilities for small periods compared to the wavelength, except in certain limiting cases that are discussed in detail. New related metamaterials are shown to exhibit exciting optical properties that are elucidated in the light of our simple analytical approach.

Journal Article↗

PubMed2005

Sensing dipole fields at atomic steps with combined scanning tunneling and force microscopy.

The electric field of dipoles localized at the atomic steps of metal surfaces due to the Smoluchowski effect were measured from the electrostatic force exerted on the biased tip of a scanning tunneling microscope. By varying the tip-sample bias the contribution of the step dipole was separated from changes in the force due to van der Waals and polarization forces. Combined with electrostatic calculations, the method was used to determine the local dipole moment in steps of different heights on Au(111) and on the twofold surface of an Al-Ni-Co decagonal quasicrystal.

Journal Article↗

PubMed2005

Full transmission through perfect-conductor subwavelength hole arrays.

Light transmission through 2D subwavelength hole arrays in perfect-conductor films is shown to be complete (100%) at some resonant wavelengths even for arbitrarily narrow holes. Conversely, the reflection on a 2D planar array of nonabsorbing scatterers is shown to be complete at some wavelengths regardless how small the scatterers are. These results are proven analytically and corroborated by rigorous numerical solution of Maxwell's equations. This work supports the central role played by dynamical diffraction during light transmission through subwavelength hole arrays and it provides a systematics to analyze more complex geometries and many of the features observed in connection with transmission through hole arrays.

Journal Article↗

PubMed2004

Photonic properties of strongly correlated colloidal liquids.

The optical and structural properties of dense colloidal suspensions in the presence of long-range electrostatic repulsion are determined from both light and small-angle neutron scattering experiments. Short-range structural order induces an enhancement of the scattering strength while at the same time the total transmission shows strong wavelength dependence, reminiscent of a photonic crystal. Interestingly, the interplay between diffusive scattering and local order leads to negative values of the scattering anisotropy parameter. The tunable optical properties of these liquids furthermore suggest potential applications such as transparency switches or filters.

Journal Article↗

PubMed2002

Conductance distributions in quasi-one-dimensional disordered wires.

A detailed analysis of the distribution of conductances P(g) of quasi-one-dimensional disordered wires in the metal-insulator crossover is presented. P(g) obtained from a Monte Carlo solution of the Dorokhov, Mello, Pereyra, and Kumar (DMPK) scaling equation is in full agreement with "tight-binding" numerical calculations of bulk disordered wires. Perturbation theory is shown to be valid even for mean dimensionless conductances of the order of 1. In the crossover regime <, similar 1, P(g) presents a sharp feature at g=1 which is different from that observed in surface disordered wires.

Journal Article↗

PubMed2002

Quantum conductance in semimetallic bismuth nanocontacts.

Electronic transport properties of bismuth nanocontacts are analyzed using a low temperature scanning tunneling microscope. The subquantum steps observed in the conductance versus elongation curves give evidence of atomic rearrangements in the contact. The quantum nature of the conductance reveals itself through peaks in the conductance histograms. The shape of the curves at 77 K is described by a simple gliding mechanism for the contact evolution during elongation. The different behavior at 4 K suggests a transition from light to heavy charge carriers as the contact cross section is decreased.

Journal Article↗

PubMed2002

Finite-size effects on intensity correlations in random media.

The correlations between waves transmitted through random media are analyzed by use of a random-matrix approach and numerical simulations of rough waveguides. Although the intensity and conductance fluctuations are practically independent of the sample length, the correlations present a strong dependence on the length of the disordered region. In waveguide geometries the long-range correlations C((2)) and C((3)), usually associated to intensity and conductance fluctuations, respectively, become negative as the length of the system decreases. Our results provide a new interpretation of recent optical experiments on disordered slab geometries.

Journal Article↗

PubMed2001

Universal conductance distributions in the crossover between diffusive and localization regimes.

The full distribution of the conductance P(G) in quasi-one-dimensional wires with rough surfaces is analyzed from the diffusive to the localization regime. In the crossover region, where the statistics is dominated by only one or two eigenchannels, the numerically obtained P(G) is found to be independent of the details of the system with the average conductance as the only scaling parameter. For < or =2e(2)/h, the shape of P(G) remarkably agrees with those predicted by random matrix theory for two fluctuating transmission eigenchannels.

Journal Article↗

PubMed2001

Resonant radiation pressure on neutral particles in a waveguide.

A theoretical analysis of electromagnetic forces on neutral particles in a hollow waveguide is presented. We show that the effective scattering cross section of a very small (Rayleigh) particle can be strongly modified inside a waveguide. The coupling of the scattered dipolar field with the waveguide modes induces a resonant enhanced backscattering state of the scatterer-guide system close to the onset of new modes. The particle effective cross section can then be as large as the wavelength even far from any transition resonance. As we will show, a small particle can be strongly accelerated along the guide axis while being highly confined in a narrow zone of the cross section of the guide.

Journal Article↗

PubMed2001

Early prognosis in severe sepsis via analyzing the monocyte immunophenotype.

OBJECTIVE: To analyze the early discriminative predictive information regarding the immunophenotype components of patients with sepsis, and its potential use as a prognosis tool. DESIGN: Observational prospective clinical study. SETTING: Intensive care unit (ICU) in a University Hospital. PATIENTS: Thirty-five patients admitted with severe sepsis. MEASUREMENTS: Analysis of peripheral blood on admission and 48 h later of the absolute white cell count and the immunophenotype of lymphocyte (CD3, CD3-HLADR, CD4, CD8, CD4/CD8 ratio, CD19, and CD25) and monocyte (CD13, CD13-HLADR, CD14, CD14-HLADR, CD13-CD14, and CD4) subpopulations. RESULTS: Due to its high correlation, the immunophenotypic profile studied at admission and 48 h later showed the same prognosis power regardless of the time of performance. The univariate analysis between groups (survival versus death) confirmed the prognostic significance of the total monocyte count and its subpopulations; significant differences were observed from the beginning only in the CD19 lymphocyte subpopulation. Multivariate analysis was performed using logistic regression with survival as the dependent variable. The final model comprised monocytes beta = 0.002 (P = 0.025) and CD13-HLADR beta = 0.016 (P = 0.029). The monocytes receiver operating characteristic (ROC) area obtained was 0.819 (confidence interval 0.663-0.976 at 95 %), the CD13-HLADR ROC area was 0.810 (confidence interval 0.658-0.963), and the monocytes + CD13-HLADR ROC area was 0.918 (confidence interval 0.807-1.000). CONCLUSIONS: A single blood sample test obtaining the absolute monocyte and CD13-HLADR subpopulation count in the first days of admission could contribute to simplifying the classification of patients with severe sepsis into high- and low-mortality risk.

APACHE↗

PubMed1992

Is it possible to observe biological macromolecules by electrostatic force microscopy?

Biological macromolecules such as proteins are charged species in solution. The exact charge depends upon the pH, temperature and ionic strength of the medium in general. This suggests the possibility of employing electrostatic force microscopy as a useful tool for the observation of these macromolecules. Our calculations show that electrostatic forces could be used for this purpose, thus helping to remove ambiguities in the location and identification of proteins. The long-range nature of electrostatic forces could make them useful to locate these macromolecules on the appropriate substrate while, at the same time, overcoming important disturbing effects such as lateral displacement of the macromolecules or destructive contact with the sample.

Electrochemistry↗