http://www.recercat.cat:80/handle/2072/48815 Thu, 23 May 2013 02:16:56 GMT 2013-05-23T02:16:56Z http://www.recercat.cat:80/bitstream/id/34146/ http://www.recercat.cat:80/handle/2072/48815 http://www.recercat.cat:80/handle/2072/208356 Rotational Doppler effect in magnetic resonance Lendínez Escudero, Sergi; Chudnovsky, Eugene M., 1948-; Tejada Palacios, Javier We compute the shift in the frequency of the spin resonance in a solid that rotates in the field of a circularly polarized electromagnetic wave. Electron-spin resonance, nuclear magnetic resonance, and ferromagnetic resonance are considered. We show that contrary to the case of the rotating LC circuit, the shift in the frequency of the spin resonance has strong dependence on the symmetry of the receiver. The shift due to rotation occurs only when rotational symmetry is broken by the anisotropy of the gyromagnetic tensor, by the shape of the body or by magnetocrystalline anisotropy. General expressions for the resonance frequency and power absorption are derived and implications for experiment are discussed. http://www.recercat.cat:80/handle/2072/208356 http://www.recercat.cat:80/handle/2072/207942 Renewal equations for option pricing Montero Torralbo, Miquel In this paper we will develop a methodology for obtaining pricing expressions for financial instruments whose underlying asset can be described through a simple continuous-time random walk (CTRW) market model. Our approach is very natural to the issue because it is based in the use of renewal equations, and therefore it enhances the potential use of CTRW techniques in finance. We solve these equations for typical contract specifications, in a particular but exemplifying case. We also show how a formal general solution can be found for more exotic derivatives, and we compare prices for alternative models of the underlying. Finally, we recover the celebrated results for the Wiener process under certain limits. http://www.recercat.cat:80/handle/2072/207942 http://www.recercat.cat:80/handle/2072/207939 Monotonic continuous-time random walks with drift and stochastic reset events Montero Torralbo, Miquel; Villarroel, Javier In this paper we consider a stochastic process that may experience random reset events which suddenly bring the system to the starting value and analyze the relevant statistical magnitudes. We focus our attention on monotonic continuous-time random walks with a constant drift: The process increases between the reset events, either by the effect of the random jumps, or by the action of the deterministic drift. As a result of all these combined factors interesting properties emerge, like the existence (for any drift strength) of a stationary transition probability density function, or the faculty of the model to reproduce power-law-like behavior. General formulas for two extreme statistics, the survival probability, and the mean exit time, are also derived. To corroborate in an independent way the results of the paper, Monte Carlo methods were used. These numerical estimations are in full agreement with the analytical predictions. http://www.recercat.cat:80/handle/2072/207939 http://www.recercat.cat:80/handle/2072/207941 Exit times in non-Markovian drifting continuous-time random-walk processes Montero Torralbo, Miquel; Villarroel, Javier By appealing to renewal theory we determine the equations that the mean exit time of a continuous-time random walk with drift satisfies both when the present coincides with a jump instant or when it does not. Particular attention is paid to the corrections ensuing from the non-Markovian nature of the process. We show that when drift and jumps have the same sign the relevant integral equations can be solved in closed form. The case when holding times have the classical Erlang distribution is considered in detail. http://www.recercat.cat:80/handle/2072/207941 http://www.recercat.cat:80/handle/2072/207940 Parrondo-like behavior in continuous-time random walks with memory Montero Torralbo, Miquel The continuous-time random walk (CTRW) formalism can be adapted to encompass stochastic processes with memory. In this paper we will show how the random combination of two different unbiased CTRWs can give rise to a process with clear drift, if one of them is a CTRW with memory. If one identifies the other one as noise, the effect can be thought of as a kind of stochastic resonance. The ultimate origin of this phenomenon is the same as that of the Parrondo paradox in game theory. http://www.recercat.cat:80/handle/2072/207940 http://www.recercat.cat:80/handle/2072/207938 A New Mechanism of Quark Energy Loss Casalderrey-Solana, Jorge; Fernández Moreno, Daniel; Mateos, David (Mateos Solé) We show that a heavy quark moving sufficiently fast through a quark-gluon plasma may lose energy by Cherenkov-radiating mesons. We demonstrate that this takes place in all strongly coupled, large-Nc plasmas with a gravity dual. The energy loss is exactly calculable in these models despite being an O(1/Nc)-effect. We discuss phenomenological implications for heavy-ion collision experiments. http://www.recercat.cat:80/handle/2072/207938 http://www.recercat.cat:80/handle/2072/202652 Magnetic properties of amorphous Fe-Si compositionally modulated thin films Martínez Perea, Benjamin; Moreu, M. A.; Labarta, Amílcar; Obradors, Xavier; Tejada Palacios, Javier The present paper reports on a magnetometric study of Fe‐Si compositionally modulated thin films. The low‐temperature dependence of the magnetization exhibit Bloch's T3/2 dependence with a minor T5/2 correction term. The spin‐wave stiffness constant deduced from the temperature coefficient depends on the characteristic modulation length and its values are much lower that in glassy alloys. http://www.recercat.cat:80/handle/2072/202652 http://www.recercat.cat:80/handle/2072/202653 Spin-glass behaviour in mixed metal oxides with a rutile-type structure Labarta, Amílcar; Obradors, Xavier; Tejada Palacios, Javier; Berry, Frank J., 1947-; Sarson, M. We report here on the magnetic properties of compounds of composition Fe1−xCrxSbO4 and Fe1−xGaxSbO4. The introduction of paramagnetic Cr3+ and diamagnetic Ga3+ into the rutile‐related iron antimonate lattice does not destroy the antisite atomic ordering which exists in iron antimonate of composition FeSbO4. The initial slope of the Curie temperature dependence on x is similar in both series, indicating that Fe3+‐Cr3+ interactions are very small. The magnetic susceptibility measurements recorded from the compounds of composition Fe1−xCrxSbO4, x&0.4, and Fe0.9Ga0.1SbO4 show them to behave as spin glasses at low temperatures. The inhibition of compounds of the type Fe1−xCrxSbO4, x&0.4, and Fe1−xGaxSbO4, x&0.1 to undergo a spin‐glass transition above 4.2 K is associated with a dilution effect. http://www.recercat.cat:80/handle/2072/202653 http://www.recercat.cat:80/handle/2072/202651 Magnetic properties of an Fe/Cu granular multilayer Rubistein, Mark; Tejada Palacios, Javier; Zhang, Xixiang A Cu/Fe granular film, formed from a multilayer film and composed of particles of Fe imbedded in Cu, has had several of its important properties investigated. Measurements include ferromagentic resonance, magnetoresistance, Mössbauer effect, magnetic viscosity, and magnetization. The two‐phase behavior of these immiscible alloy systems, and the effect of multilayering on the shape of the magnetic precipitates, can explain some of these properties. An explanation of the ferromagnetic resonance line shape is proffered. An extraordinary macroscopic quantum tunneling effect is found to govern the magnetic relaxation at the lowest temperatures. http://www.recercat.cat:80/handle/2072/202651 http://www.recercat.cat:80/handle/2072/202649 Cation distribution and magnetization of BaFe12-2xCoxSnxO19 (x=0.9,1.28) single crystals Sandiumenge Ortiz, Felip; Martínez Perea, Benjamin; Batlle Gelabert, Xavier; Galí Medina, Salvador, 1949-; Obradors, Xavier The distribution of Sn4+ cations within the five crystallographic sites of the magnetoplumbite (M) ‐like compound BaFe12−2xCoxSnxO19 has been analyzed using single‐crystal x‐ray‐diffraction data. The species Fe3+ and Co2+ cannot be distinguished using x rays because of their very similar atomic numbers; however, the calculation of the apparent valencies for the different sites allows an insight into the Co2+ cation segregation. The use of previous data from neutron powder diffraction allows a precise picture of the cation distribution, which indicates a pronounced site selectivity for both Sn4+ and Co2+ cations. The Sn4+ cations prefer the 4f2 sites and to a much lower extent the 12k sites, while they do not enter the octahedral 2a sites at all. Co2+ cations are distributed among tetrahedral and octahedral sites displaying a clear preference for the tetrahedral 4f1 sites. Magnetic measurements indicate that the compound still exhibits uniaxial anisotropy with the easy direction parallel to the c axis. Nevertheless, the magnetic structure shows a considerable degree of noncolinearity. A strong reduction of the magnetic anisotropy regarding that of the undoped compound is also detected. http://www.recercat.cat:80/handle/2072/202649 http://www.recercat.cat:80/handle/2072/202650 Structural and magnetic characterization of the lithiated iron oxide LixFe3O4 Fontcuberta i Griñó, Josep; Rodríguez, J.; Pernet, Michel; Longworth, Gary; Goodenough, John B. The Rietveld profile‐analysis method is used to investigate the x‐ray diffraction pattern of lithiated Fe3O4. It is shown that, after exposure to air, pure magnetite coexists with a lithium‐inserted LixFe3O4 phase. The Mössbauer spectra at 300 and 4.2 K have been used to estimate the lithium content of the sample, the pure magnetite concentration, and the iron distribution over the available 16c and 16d sites of the spinel structure. Magnetization measurements from 4.2 to 120 K with an external magnetic field up to 150 kOe have been used to obtain the saturation magnetic moment, the magnetic anisotropy constants, and the susceptibility. It is concluded that a noncollinear spin structure should be present in Li0.5Fe3O4. These results indicate that there is no room‐temperature extrusion of iron even for x→2.0, but that on exposure to air LixFe3O4 samples with x&0.5 are oxidized at room temperature by delithiation. http://www.recercat.cat:80/handle/2072/202650 http://www.recercat.cat:80/handle/2072/195797 Contribuciones de Einstein a la teoría cuántica (1905-1925) Navarro Veguillas, Luis El mito Einstein se ha formado básicamente en torno a sus teorías relativistas que, entre otras implicaciones, conducían a una original concepción del espacio-tiempo e incluso a una nueva forma de descripción del mundo físico. No obstante, sus contribuciones a la teoría cuántica -aunque no tan divulgadas- son de tal envergadura que por sí solas constituyen un aporte más que suficiente para que su autor ocupara un destacadísimo lugar entre los grandes creadores científicos de todos los tiempos. En el presente trabajo nos proponemos justificar tan categórica afirmación, empleando los medios que la historiografía moderna ofrece e incluyendo también resultados de algunas investigaciones propias. http://www.recercat.cat:80/handle/2072/195797 http://www.recercat.cat:80/handle/2072/184066 A high-sensitivity differential scanning calorimeter with magnetic field for magnetostructural transitions Marcos, Jordi; Casanova i Fernàndez, Fèlix; Batlle Gelabert, Xavier; Labarta, Amílcar; Planes Vila, Antoni; Mañosa, Lluís We have developed a differential scanning calorimeter capable of working under applied magnetic fields of up to 5 T. The calorimeter is highly sensitive and operates over the temperature range 10¿300 K. It is shown that, after a proper calibration, the system enables determination of the latent heat and entropy changes in first-order solid¿solid phase transitions. The system is particularly useful for investigating materials that exhibit the giant magnetocaloric effect arising from a magnetostructural phase transition. Data for Gd5(Si0.1Ge0.9)4 are presented. http://www.recercat.cat:80/handle/2072/184066 http://www.recercat.cat:80/handle/2072/183978 Dynamics of driven three-dimensional thin films: from hdrophilic to superhydrophobic substrates Ledesma Aguilar, Rodrigo Andrés; Hernández Machado, Aurora; Pagonabarraga Mora, Ignacio We study the forced displacement of a thin film of fluid in contact with vertical and inclined substrates of different wetting properties, that range from hydrophilic to hydrophobic, using the lattice-Boltzmann method. We study the stability and pattern formation of the contact line in the hydrophilic and superhydrophobic regimes, which correspond to wedge-shaped and nose-shaped fronts, respectively. We find that contact lines are considerably more stable for hydrophilic substrates and small inclination angles. The qualitative behavior of the front in the linear regime remains independent of the wetting properties of the substrate as a single dispersion relation describes the stability of both wedges and noses. Nonlinear patterns show a clear dependence on wetting properties and substrate inclination angle. The effect is quantified in terms of the pattern growth rate, which vanishes for the sawtooth pattern and is finite for the finger pattern. Sawtooth shaped patterns are observed for hydrophilic substrates and low inclination angles, while finger-shaped patterns arise for hydrophobic substrates and large inclination angles. Finger dynamics show a transient in which neighboring fingers interact, followed by a steady state where each finger grows independently. http://www.recercat.cat:80/handle/2072/183978 http://www.recercat.cat:80/handle/2072/183976 Three-dimensional aspects of fluid flows in channels. I. Meniscus and thin film regimes Ledesma Aguilar, Rodrigo Andrés; Hernández Machado, Aurora; Pagonabarraga Mora, Ignacio We study the forced displacement of a fluid-fluid interface in a three-dimensional channel formed by two parallel solid plates. Using a lattice-Boltzmann method, we study situations in which a slip velocity arises from diffusion effects near the contact line. The difference between the slip and channel velocities determines whether the interface advances as a meniscus or a thin film of fluid is left adhered to the plates. We find that this effect is controlled by the capillary and Péclet numbers. We estimate the crossover from a meniscus to a thin film and find good agreement with numerical results. The penetration regime is examined in the steady state. We find that the occupation fraction of the advancing finger relative to the channel thickness is controlled by the capillary number and the viscosity contrast between the fluids. For high viscosity contrast, lattice-Boltzmann results agree with previous results. For zero viscosity contrast, we observe remarkably narrow fingers. The shape of the finger is found to be universal. http://www.recercat.cat:80/handle/2072/183976 http://www.recercat.cat:80/handle/2072/183977 The Nonequilibrium thermodynamics of small systems Bustamante, Carlos; Liphardt, Jan; Ritort Farran, Fèlix The interactions of tiny objects with their environments are dominated by thermal fluctuations. Guided by theory and assisted by new micromanipulation tools, scientists have begun to study such interactions in detail. http://www.recercat.cat:80/handle/2072/183977 http://www.recercat.cat:80/handle/2072/183975 Three-dimensional aspects of fluid flows in channels. II. Effects of meniscus and thin film regimes on viscous fingers Ledesma Aguilar, Rodrigo Andrés; Pagonabarraga Mora, Ignacio; Hernández Machado, Aurora We perform a three-dimensional study of steady state viscous fingers that develop in linear channels. By means of a three-dimensional lattice-Boltzmann scheme that mimics the full macroscopic equations of motion of the fluid momentum and order parameter, we study the effect of the thickness of the channel in two cases. First, for total displacement of the fluids in the channel thickness direction, we find that the steady state finger is effectively two-dimensional and that previous two-dimensional results can be recovered by taking into account the effect of a curved meniscus across the channel thickness as a contribution to surface stresses. Second, when a thin film develops in the channel thickness direction, the finger narrows with increasing channel aspect ratio in agreement with experimental results. The effect of the thin film renders the problem three-dimensional and results deviate from the two-dimensional prediction. http://www.recercat.cat:80/handle/2072/183975 http://www.recercat.cat:80/handle/2072/183946 Effects of the epitaxial layer thickness on the noise properties of Schottky barrier diodes Gomila Lluch, Gabriel; Bulashenko, O. M. An analytical theory to describe the combined effects of the epitaxial layer thickness and the ohmic contact on the noise properties of Schottky barrier diodes is presented. The theory, which provides information on both the local and the global noise properties, takes into account the finite size of the epitaxial layer and the effects of the back ohmic contact, and applies to the whole range of applied bias. It is shown that by scaling down the epitaxial layer thickness, the current regime in which the noise temperature displays a shot-noise-like behavior increases at the cost of reducing the current range in which the thermal-noise-like behavior dominates. This improvement in noise temperature is limited by the effects of the ohmic contact, which appear for large currents. The theory is formulated on general trends, allowing its application to the noise analysis of other semiconductor devices operating under strongly inhomogeneous distributions of the electric field and charge concentrations. http://www.recercat.cat:80/handle/2072/183946 http://www.recercat.cat:80/handle/2072/183724 Classical predictive electrodynamics of two charges with radiation: Energy and 3. momentum balance and scattering crossections-II Lapiedra, Ramon, 1940-; Marqués Truyol, Francisco; Molina, Alfred We deal with a classical predictive mechanical system of two spinless charges where radiation is considered and there are no external fields. The terms (2,2)Paa of the expansion in the charges of the HamiltonJacobi momenta are calculated. Using these, together with known previous results, we can obtain the paa up to the fourth order. Then we have calculated the radiated energy and the 3-momentum in a scattering process as functions of the impact parameter and the incident energy for the former and 3-momentum for the latter. Scattering cross-sections are also calculated. Good agreement with well known results, including those of quantum electrodynamics, has been found. http://www.recercat.cat:80/handle/2072/183724 http://www.recercat.cat:80/handle/2072/183723 Classical predictive electrodynamics of two charges with radiation: General framework. I. Lapiedra, Ramon, 1940-; Molina, Alfred Outgoing radiation is introduced in the framework of the classical predictive electrodynamics using LorentzDiracs equation as a subsidiary condition. In a perturbative scheme in the charges the first radiative self-terms of the accelerations, momentum and angular momentum of a two charge system without external field are calculated. http://www.recercat.cat:80/handle/2072/183723 http://www.recercat.cat:80/handle/2072/183722 Front form and point form formulation in P.R.M. noninteractions theorems Jan, Xavier; Molina, Alfred; Iranzo Fernández, Vicente The front form and the point form of dynamics are studied in the framework of predictive relativistic mechanics. The non-interaction theorem is proved when a Poincar-invariant Hamiltonian formulation with canonical position coordinates is required. http://www.recercat.cat:80/handle/2072/183722 http://www.recercat.cat:80/handle/2072/183721 Exact temporal evolution for some non-linear diffusion process Garrido, L. (Luis), 1930-; Masoliver, Jaume, 1951- Exact solutions to FokkerPlanck equations with nonlinear drift are considered. Applications of these exact solutions for concrete models are studied. We arrive at the conclusion that for certain drifts we obtain divergent moments (and infinite relaxation time) if the diffusion process can be extended without any obstacle to the whole space. But if we introduce a potential barrier that limits the diffusion process, moments converge with a finite relaxation time. http://www.recercat.cat:80/handle/2072/183721 http://www.recercat.cat:80/handle/2072/183720 On a class of exact solutions to the Fokker-Planck equations Garrido, L. (Luis), 1930-; Masoliver, Jaume, 1951- In this paper we study under which circumstances there exists a general change of gross variables that transforms any FokkerPlanck equation into another of the OrnsteinUhlenbeck class that, therefore, has an exact solution. We find that any FokkerPlanck equation will be exactly solvable by means of a change of gross variables if and only if the curvature tensor and the torsion tensor associated with the diffusion is zero and the transformed drift is linear. We apply our criteria to the Kubo and Gompertz models. http://www.recercat.cat:80/handle/2072/183720 http://www.recercat.cat:80/handle/2072/183719 General Interaction Picture from Action Principle for Mechanics Garrido, L. (Luis), 1930- In this paper we consider a general action principle for mechanics written by means of the elements of a Lie algebra. We study the physical reasons why we have to choose precisely a Lie algebra to write the action principle. By means of such an action principle we work out the equations of motion and a technique to evaluate perturbations in a general mechanics that is equivalent to a general interaction picture. Classical or quantum mechanics come out as particular cases when we make realizations of the Lie algebra by derivations into the algebra of products of functions or operators, respectively. Later on we develop in particular the applications of the action principle to classical and quantum mechanics, seeing that in this last case it agrees with Schwinger's action principle. The main contribution of this paper is to introduce a perturbation theory and an interaction picture of classical mechanics on the same footing as in quantum mechanics. http://www.recercat.cat:80/handle/2072/183719 http://www.recercat.cat:80/handle/2072/183718 Generalized adiabatic invariance Garrido, L. (Luis), 1930- In this paper we find the quantities that are adiabatic invariants of any desired order for a general slowly time-dependent Hamiltonian. In a preceding paper, we chose a quantity that was initially an adiabatic invariant to first order, and sought the conditions to be imposed upon the Hamiltonian so that the quantum mechanical adiabatic theorem would be valid to mth order. [We found that this occurs when the first (m - 1) time derivatives of the Hamiltonian at the initial and final time instants are equal to zero.] Here we look for a quantity that is an adiabatic invariant to mth order for any Hamiltonian that changes slowly in time, and that does not fulfill any special condition (its first time derivatives are not zero initially and finally). http://www.recercat.cat:80/handle/2072/183718 http://www.recercat.cat:80/handle/2072/183717 Strong coupling solution to Schrödinger Equation: the mixing of states Garrido, L. (Luis), 1930-; Navarro Veguillas, Luis In this paper we give some ideas that can be useful to solve Schrödinger equations in the case when the Hamiltonian contains a large term. We obtain an expansion of the solution in reciprocal powers of the large coupling constant. The procedure followed consists in considering that the small part of the Hamiltonian engenders a motion adiabatic to the motion generated by the large part of the same. http://www.recercat.cat:80/handle/2072/183717 http://www.recercat.cat:80/handle/2072/183716 Realizations of Poincar group induced by second order differential systems. Non interaction theorem Jan, Xavier; Llosa, Josep; Marqués Truyol, Francisco; Montoto i Gayete, Amadeu A generalization of the predictive relativistic mechanics is studied where the initial conditions are taken on a general hypersurface of M4. The induced realizations of the Poincar group are obtained. The same procedure is used for the Galileo group. Noninteraction theorems are derived for both groups. http://www.recercat.cat:80/handle/2072/183716 http://www.recercat.cat:80/handle/2072/183715 On the spinning axis representation Azcárraga, José A. de; Navarro Veguillas, Luis A new spinning axis representation is introduced. It allows us to calculate the evolution operator of a system with slowly varying time dependent Hamiltonian with the desired degree of approximation in the parameter used for describing its dynamical evolution. The procedure is compared with a previously existing one and applied to a simple example. http://www.recercat.cat:80/handle/2072/183715 http://www.recercat.cat:80/handle/2072/183714 Hamiltonian formalism for nonlocal Lagrangians Llosa, Josep; Vives, J. A Hamiltonian formalism is set up for nonlocal Lagrangian systems. The method is based on obtaining an equivalent singular first order Lagrangian, which is processed according to the standard Legendre transformation and then, the resulting Hamiltonian formalism is pulled back onto the phase space defined by the corresponding constraints. Finally, the standard results for local Lagrangians of any order are obtained as a particular case. http://www.recercat.cat:80/handle/2072/183714 http://www.recercat.cat:80/handle/2072/183713 A family of inhomogeneous cosmological Einstein - Rosen metrics Oliver, Gabriel; Verdaguer Oms, Enric, 1950- Some generalized soliton solutions of the cosmological EinsteinRosen type defined in the space-time region t2=z2 in terms of canonical coordinates are considered. Vacuum solutions are studied and interpreted as cosmological models. Fluid solutions are also considered and are seen to represent inhomogeneous cosmological models that become homogeneous at t?8. A subset of them evolve toward isotropic FriedmannRobertsonWalker metrics. http://www.recercat.cat:80/handle/2072/183713