Local Tunneling Study of Three-Dimensional Order Parameter in the π\pi-band of Al-doped MgB2_2 Single Crystals

We have performed local tunneling spectroscopy on high quality Mg$_{1-x}$Al$_x$B$_2$ single crystals by means of Variable Temperature Scanning Tunneling Spectroscopy (STS) in magnetic field up to 3 Tesla. Single gap conductance spectra due to c-axis tunneling were extensively measured, probing different amplitudes of the three-dimensional $\Delta_\pi$ as a function of Al content. Temperature and magnetic field dependences of the conductance spectra were studied in S-I-N configuration: the effect of the doping resulted in a monotonous reduction of the locally measured $T_C$ down to 24K for x=0.2. On the other hand, we have found that the gap amplitude shows a maximum value $\Delta_\pi= 2.3$ meV for x=0.1, while the $\Delta_\pi / T_C$ ratio increases monotonously with doping. The locally measured upper critical field was found to be strongly related to the gap amplitude, showing the maximum value $H_{c2}\simeq3T$ for x=0.1 substituted samples. For this Al concentration the data revealed some spatial inhomogeneity in the distribution of $\Delta_\pi$ on nanometer scale.Comment: 4 pages, 3 figure

Exact analytical evaluation of time dependent transmission coefficient from the method of reactive flux for an inverted parabolic barrier

In this paper we derive a general expression for the transmission coefficient using the method of reactive flux for a particle coupled to a harmonic bath surmounting a one dimensional inverted parabolic barrier. Unlike Kohen and Tannor [J. Chem. Phys. 103, 6013 (1995)] we use a normal mode analysis where the unstable and the other modes have a complete physical meaning. Importantly our approach results a very general expression for the time dependent transmission coefficient not restricted to overdamped limit. Once the spectral density for the problem is know one can use our formula to evaluate the time dependent transmission coefficient. We have done the calculations with time dependent friction used by Xie [Phys. Rev. Lett 93, 180603 (2004)] and also the one used by Kohen and Tannor [J. Chem. Phys. 103, 6013 (1995)]. Like the formula of Kohen and Tannor our formula also reproduces the results of transition state theory as well as the Kramers theory in the limits t->0 and t->infinity respectively

Probing the superfluid velocity with a superconducting tip: the Doppler shift effect

We address the question of probing the supercurrents in superconducting (SC) samples on a local scale by performing Scanning Tunneling Spectroscopy (STS) experiments with a SC tip. In this configuration, we show that the tunneling conductance is highly sensitive to the Doppler shift term in the SC quasiparticle spectrum of the sample, thus allowing the local study of the superfluid velocity. Intrinsic screening currents, such as those surrounding the vortex cores in a type II SC in a magnetic field, are directly probed. With Nb tips, the STS mapping of the vortices, in single crystal 2H-NbSe_2, reveals both the vortex cores, on the scale of the SC coherence length $\xi$, and the supercurrents, on the scale of the London penetration length $\lambda$. A subtle interplay between the SC pair potential and the supercurrents at the vortex edge is observed. Our results open interesting prospects for the study of screening currents in any superconductor.Comment: 4 pages, 5 figure

Reply to Comment on "Completely positive quantum dissipation"

This is the reply to a Comment by R. F. O'Connell (Phys. Rev. Lett. 87 (2001) 028901) on a paper written by the author (B. Vacchini, ``Completely positive quantum dissipation'', Phys.Rev.Lett. 84 (2000) 1374, arXiv:quant-ph/0002094).Comment: 2 pages, revtex, no figure

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

Exact quantum states of a general time-dependent quadratic system from classical action

A generalization of driven harmonic oscillator with time-dependent mass and frequency, by adding total time-derivative terms to the Lagrangian, is considered. The generalization which gives a general quadratic Hamiltonian system does not change the classical equation of motion. Based on the observation by Feynman and Hibbs, the propagators (kernels) of the systems are calculated from the classical action, in terms of solutions of the classical equation of motion: two homogeneous and one particular solutions. The kernels are then used to find wave functions which satisfy the Schr\"{o}dinger equation. One of the wave functions is shown to be that of a Gaussian pure state. In every case considered, we prove that the kernel does not depend on the way of choosing the classical solutions, while the wave functions depend on the choice. The generalization which gives a rather complicated quadratic Hamiltonian is simply interpreted as acting an unitary transformation to the driven harmonic oscillator system in the Hamiltonian formulation.Comment: Submitted to Phys. Rev.

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

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