Symmetry and Temperature dependence of the Order parameter in MgB2 from point contact measurements

We have performed differential conductance versus voltage measurements of Au/MgB2 point contacts. We find that the dominant component in the conductance is due to Andreev reflection. The results are fitted to the theoretical model of BTK for an s-wave symmetry from which we extract the value of the order parameter (Delta) and its temperature dependence. From our results we also obtain a lower experimental bound on the Fermi velocity in MgB2.Comment: 7 pages (Including figure captions) and 4 figure

Local and macroscopic tunneling spectroscopy of Y(1-x)CaxBa2Cu3O(7-d) films: evidence for a doping dependent is or idxy component in the order parameter

Tunneling spectroscopy of epitaxial (110) Y1-xCaxBa2Cu3O7-d films reveals a doping dependent transition from pure d(x2-y2) to d(x2-y2)+is or d(x2-y2)+idxy order parameter. The subdominant (is or idxy) component manifests itself in a splitting of the zero bias conductance peak and the appearance of subgap structures. The splitting is seen in the overdoped samples, increases systematically with doping, and is found to be an inherent property of the overdoped films. It was observed in both local tunnel junctions, using scanning tunneling microscopy (STM), and in macroscopic planar junctions, for films prepared by either RF sputtering or laser ablation. The STM measurements exhibit fairly uniform splitting size in [110] oriented areas on the order of 10 nm2 but vary from area to area, indicating some doping inhomogeneity. U and V-shaped gaps were also observed, with good correspondence to the local faceting, a manifestation of the dominant d-wave order parameter

A self-consistent quantum master equation approach to molecular transport

We propose a self-consistent generalized quantum master equation (GQME) to describe electron transport through molecular junctions. In a previous study [M.Esposito and M.Galperin. Phys. Rev. B 79, 205303 (2009)], we derived a time-nonlocal GQME to cure the lack of broadening effects in Redfield theory. To do so, the free evolution used in the Born-Markov approximation to close the Redfield equation was replaced by a standard Redfield evolution. In the present paper, we propose a backward Redfield evolution leading to a time-local GQME which allows for a self-consistent procedure of the GQME generator. This approach is approximate but properly reproduces the nonequilibrium steady state density matrix and the currents of an exactly solvable model. The approach is less accurate for higher moments such as the noise.Comment: 9 pages, 4 figure

Quantum Transition State Theory for proton transfer reactions in enzymes

We consider the role of quantum effects in the transfer of hyrogen-like species in enzyme-catalysed reactions. This study is stimulated by claims that the observed magnitude and temperature dependence of kinetic isotope effects imply that quantum tunneling below the energy barrier associated with the transition state significantly enhances the reaction rate in many enzymes. We use a path integral approach which provides a general framework to understand tunneling in a quantum system which interacts with an environment at non-zero temperature. Here the quantum system is the active site of the enzyme and the environment is the surrounding protein and water. Tunneling well below the barrier only occurs for temperatures less than a temperature $T_0$ which is determined by the curvature of potential energy surface near the top of the barrier. We argue that for most enzymes this temperature is less than room temperature. For physically reasonable parameters quantum transition state theory gives a quantitative description of the temperature dependence and magnitude of kinetic isotope effects for two classes of enzymes which have been claimed to exhibit signatures of quantum tunneling. The only quantum effects are those associated with the transition state, both reflection at the barrier top and tunneling just below the barrier. We establish that the friction due to the environment is weak and only slightly modifies the reaction rate. Furthermore, at room temperature and for typical energy barriers environmental degrees of freedom with frequencies much less than 1000 cm$^{-1}$ do not have a significant effect on quantum corrections to the reaction rate.Comment: Aspects of the article are discussed at condensedconcepts.blogspot.co

Perception about the importance of feeding in a group of hematologic cancer patients

Introducción: Los pacientes oncohematológicos presentan habitualmente déficits nutricionales derivados de su enfermedad, sus tratamientos y efectos secundarios, etc. Sin embargo, apenas existen datos descriptivos del empleo de recomendaciones dietéticas, y de la auto-percepción del paciente de su utilidad en su enfermedad. Objetivos: Evaluar la percepción de la importancia de la alimentación en un grupo de pacientes con cáncer hematológico. Materiales y métodos: 111 pacientes oncohematológicos auto-registraron un cuestionario con preguntas sociosanitarias y de percepción personal de la importancia de la alimentación en su enfermedad. Resultados: La edad media fue 40,0 ± 12,8 años (64,86% mujeres). El diagnóstico y el tratamiento más frecuente fue linfoma (83,78%) y quimioterapia (92,80%). Un 75% consideró “muy importante” la relación entre enfermedad oncohematológica y estado nutricional. Sólo un 54,1% recibió consejo dietético. Y un 53,2% y 50,5% consideró que el consumo de suplementos nutricionales mejoraría su calidad de vida y/o enfermedad respectivamente. Conclusiones: Un porcentaje importante de pacientes presentaron estados avanzados de su enfermedad. Sin embargo, la mitad del colectivo no había recibido pautas dietéticas a pesar del elevado interés por recibir atención nutricional. Por tanto, la valoración nutricional del paciente oncohematológico así como la implementación de un soporte nutricional personalizado deberían formar parte de la práctica clínica habitualIntroduction: Oncohematologic patients usually present nutritional deficits associated with the disease, the treatments and side effects, etc. However, there are hardly any descriptive data about the dietary recommendations used and the patient’s self-perception of its usefulness in their disease. Aim: To assess the self-perception of the nutritional importance in a group of oncohematologic patients. Materials and methods: 111 oncohematologic patients self-reported a questionnaire containing items about socio-sanitary data and self-perception of the nutritional importance in their disease. Results: The mean age was 40.0 ± 12.8 years (64.86% women). The most frequent diagnosis and treatment was lymphoma (83.78%) and chemoteraphy (92.80%). 75% considered “very important” relationship between oncohematologic disease and nutritional status. Only 54.1% received dietary advice. And 53.2% and 50.5% considered that the use of nutritional supplements improve quality of life and/or disease respectively. Conclusions: A significant percentage of patients had advanced stages of their disease. However, half the group had not received dietary advice despite high interest in receiving nutritional care. Therefore, the nutritional assessment and the personalized nutritional support implementation of the patient should be included in routine clinical practic

Master-equations for the study of decoherence

Different structures of master-equation used for the description of decoherence of a microsystem interacting through collisions with a surrounding environment are considered and compared. These results are connected to the general expression of the generator of a quantum dynamical semigroup in presence of translation invariance recently found by Holevo.Comment: 10 pages, latex, no figures, to appear in Int. J. Theor. Phy

Test Particle in a Quantum Gas

A master equation with a Lindblad structure is derived, which describes the interaction of a test particle with a macroscopic system and is expressed in terms of the operator valued dynamic structure factor of the system. In the case of a free Fermi or Bose gas the result is evaluated in the Brownian limit, thus obtaining a single generator master equation for the description of quantum Brownian motion in which the correction due to quantum statistics is explicitly calculated. The friction coefficients for Boltzmann and Bose or Fermi statistics are compared.Comment: 9 pages, revtex, no figure

Scanning tunneling spectroscopy of high-temperature superconductors

Tunneling spectroscopy played a central role in the experimental verification of the microscopic theory of superconductivity in the classical superconductors. Initial attempts to apply the same approach to high-temperature superconductors were hampered by various problems related to the complexity of these materials. The use of scanning tunneling microscopy/spectroscopy (STM/STS) on these compounds allowed to overcome the main difficulties. This success motivated a rapidly growing scientific community to apply this technique to high-temperature superconductors. This paper reviews the experimental highlights obtained over the last decade. We first recall the crucial efforts to gain control over the technique and to obtain reproducible results. We then discuss how the STM/STS technique has contributed to the study of some of the most unusual and remarkable properties of high-temperature superconductors: the unusual large gap values and the absence of scaling with the critical temperature; the pseudogap and its relation to superconductivity; the unprecedented small size of the vortex cores and its influence on vortex matter; the unexpected electronic properties of the vortex cores; the combination of atomic resolution and spectroscopy leading to the observation of periodic local density of states modulations in the superconducting and pseudogap states, and in the vortex cores.Comment: To appear in RMP; 65 pages, 62 figure