Specific Heat of Disordered Superconductors Induced by Negative Centers

We show that superconductors with inhomogeneous order parameters can show similar features as anisotropic ones. In this paper we study the low temperature specific heat dependence in such a system and we show that the disorder associated with randomly distributed attractive centers convert the BCS temperature exponential behaviour into a power law formula. To describe superconductivity we used a random version of the negative U Hubbard model, while the disorder was treated by means of the Coherent Potential Approximation (CPA).Comment: 2 pages, 3 figures, to apear in Journal of Magnetism and Magnetic Material

Superconducting Pairing Amplitude and Local Density of States in Presence of Repulsive Centers

We study the properties of superconductor in presence of a finite concentration of repulsive centers. The superconductor is described by the negative $U$ Hubbard model while repulsive centers are treated as randomly distributed impurities with repulsive interaction. Analyzing the paring potential and local density of states at impurity sites we find a wide range of the system parameters where the $\pi$ - like state could possibly be realized. Comparison of our results to the single repulsive center case is also given.Comment: 2 pages, 2 figures, Physica B in pres

Transition to chaos and escape phenomenon in two degrees of freedom oscillator with a kinematic excitation

We study the dynamics of a two-degrees-of-freedom (two DOF) nonlinear oscillator representing a quartercar model excited by a road roughness profile. Modelling the road profile by means of a harmonic function we derive the Melnikov criterion for a system transition to chaos or escape. The analytically obtained estimations are confirmed by numerical simulations. To analyze the transient vibrations we used recurrences.Comment: 13 pages, 16 figures, in pres

Charge and Phase Fluctuations in Attractive Hubbard Model

Using the negative U Hubbard model we analyze normal state properties of a superconductor. In this model there exists a characteristic pairing temperature T_P above a superconducting critical temperature. Below T_P electrons start to form incoherent pairs. The fluctuations in charge and phase are precursors of charge density wave and superconductivity phases depending on band filling. They lead naturally to pseudogap opening in the density of states.Comment: 4 pages, 2 figures, Physica C (accepted

Intermittent behaviour of a Cracked Rotor in the resonance region

Vibrations of the Jeffcott rotor are modelled by a three degree of freedom system including coupling between lateral and torsional modes. The crack in a rotating shaft of the rotor is introduced via time dependent stiffness with off diagonal couplings. Applying the external torque to the system allows to observe the effect of crack "breathing" and gain insight into the system. It is manifested in the complex dynamic behaviour of the rotor in the region of internal resonance, showing a quasi--periodic motion or even non-periodic behaviour. In the present paper report, we show the system response to the external torque excitation using nonlinear analysis tools such as bifurcation diagram, phase portraits, Poincare maps and wavelet power spectrum. In the region of resonance we study intermittent motions based on laminar phases interrupted by a series nonlinear beats.Comment: 12 pages, 6 figure

Dynamics of a Gear System with Faults in Meshing Stiffness

Gear box dynamics is characterised by a periodically changing stiffness. In real gear systems, a backlash also exists that can lead to a loss in contact between the teeth. Due to this loss of contact the gear has piecewise linear stiffness characteristics, and the gears can vibrate regularly and chaotically. In this paper we examine the effect of tooth shape imperfections and defects. Using standard methods for nonlinear systems we examine the dynamics of gear systems with various faults in meshing stiffness.Comment: 10 pages, 8 figure

Properties of the π\pi state induced by impurities in a d-wave superconductor

We study the properties of a quantum impurity embedded in a superconducting host of a d-wave symmetry. The superconductor is described by the extended negative $U$ Hubbard model while the impurity introduces a repulsive interaction to the system around the central site. We discuss the influence of this repulsion on the local properties (density of states, electron pairing and spontaneous current around the impurity) of the superconductor. We show the condition of $\pi$ - like behaviour, defined as two subsystems having a phase difference of $\pi$, in the system by using a proper combination of attractive pairing bond interaction and repulsive one between electrons located at the impurity and nearest neighbour sites.Comment: 2 pages, 1 figur