Fermionic Ising Glasses with BCS Pairing Interaction. Tricritical Behaviour

We have examined the role of the BCS pairing mechanism in the formation of the magnetic moment and henceforth a spin glass (SG) phase by studying a fermionic Sherrington-Kirkpatrick model with a local BCS coupling between the fermions. This model is obtained by using perturbation theory to trace out the conduction electrons degrees of freedom in conventional superconducting alloys. The model is formulated in the path integral formalism where the spin operators are represented by bilinear combinations of Grassmann fields and it reduces to a single site problem that can be solved within the static approximation with a replica symmetric Ansatz. We argue that this is a valid procedure for values of temperature above the de Almeida-Thouless instability line. The phase diagram in the T-g plane, where g is the strength of the pairing interaction, for fixed variance J^2/N of the random couplings J_{ij}, exhibits three regions: a normal paramagnetic (NP) phase, a spin glass (SG) phase and a pairing (PAIR) phase where there is formation of local pairs.The NP and PAIR phases are separated by a second order transition line g=g_{c}(T) that ends at a tricritical point T_{3}=0.9807J, g_{3}=5,8843J, from where it becomes a first order transition line that meets the line of second order transitions at T_{c}=0.9570J that separates the NP and the SG phases. For T<T_{c} the SG phase is separated from the PAIR phase by a line of first order transitions. These results agree qualitatively with experimental data in Gd_{x}Th_{1-x}RU_{2}.Comment: 26 pages, 5 figures, to appear in The European Physical Journal

Charged Particles and the Electro-Magnetic Field in Non-Inertial Frames of Minkowski Spacetime: I. Admissible 3+1 Splittings of Minkowski Spacetime and the Non-Inertial Rest Frames

By using the 3+1 point of view and parametrized Minkowski theories we develop the theory of {\it non-inertial} frames in Minkowski space-time. The transition from a non-inertial frame to another one is a gauge transformation connecting the respective notions of instantaneous 3-space (clock synchronization convention) and of the 3-coordinates inside them. As a particular case we get the extension of the inertial rest-frame instant form of dynamics to the non-inertial rest-frame one. We show that every isolated system can be described as an external decoupled non-covariant canonical center of mass (described by frozen Jacobi data) carrying a pole-dipole structure: the invariant mass and an effective spin. Moreover we identify the constraints eliminating the internal 3-center of mass inside the instantaneous 3-spaces. In the case of the isolated system of positive-energy scalar particles with Grassmann-valued electric charges plus the electro-magnetic field we obtain both Maxwell equations and their Hamiltonian description in non-inertial frames. Then by means of a non-covariant decomposition we define the non-inertial radiation gauge and we find the form of the non-covariant Coulomb potential. We identify the coordinate-dependent relativistic inertial potentials and we show that they have the correct Newtonian limit. In the second paper we will study properties of Maxwell equations in non-inertial frames like the wrap-up effect and the Faraday rotation in astrophysics. Also the 3+1 description without coordinate-singularities of the rotating disk and the Sagnac effect will be given, with added comments on pulsar magnetosphere and on a relativistic extension of the Earth-fixed coordinate system.Comment: This paper and the second one are an adaptation of arXiv 0812.3057 for publication on Int.J.Geom. Methods in Modern Phys. 77

Spin glass freezing in Kondo lattice compounds

It is presented a theory that describes a spin glass phase at finite temperatures in Kondo lattice systems with an additional RKKY interaction represented by long range, random couplings among localized spins like in the Sherrington- Kirkpatrick (SK) spin glass model. The problem is studied within the functional integral formalism where the spin operators are represented by bilinear combinations of fermionic (anticommuting) Grassmann variables. The Kondo and spin glass transitions are both described with the mean field like static ansatz that reproduces good results in the two well known limits. At high temperatures and low values of the Kondo coupling there is a paramagnetic (disordered) phase with vanishing Kondo and spin glass order parameters. By lowering the temperature a second order transition line is found at Tsg to a spin glass phase. For larger values of the Kondo coupling there is a second order transition line at roughly Tk to a Kondo ordered state. For T<Tsg the transition between the Kondo and spin glass phases becomes first order.Comment: 21 pages, 1 figure, to appear on Phys. Rev.

Efficient and Accurate Modeling of Conformational Transitions in Proteins: The Case of c-Src Kinase

The theoretical computational modeling of large conformational transitions occurring in biomolecules still represents a challenge. Here, we present an accurate "in silico" description of the activation and deactivation mechanisms of human c-Src kinases, a fundamental process regulating several crucial cell functions. Our results clearly show that by applying an efficient and automated algorithm able to drive the molecular dynamics (MD) sampling along the pathway between the two c-Src conformational states - the active state and the inactive state - it is possible to accurately describe, at reduced computational costs, the molecular mechanism underlying these large conformational rearrangements. This procedure, combining the MD simulations with the sampling along the well-defined principal motions connecting the two conformational states, allows to provide a description well beyond the present computational limits, and it is easily applicable to different systems where the structures of both the initial and final states are known

Spin Glass and antiferromagnetism in Kondo lattice disordered systems

The competition between spin glass (SG), antiferromagnetism (AF) and Kondo effect is studied here in a model which consists of two Kondo sublattices with a gaussian random interaction between spins in differents sublattices with an antiferromagnetic mean Jo and standard deviation J. In the present approach there is no hopping of the conduction electrons between the sublattices and only spins in different sublattices can interact. The problem is formulated in the path integral formalism where the spin operators are expressed as bilinear combinations of Grassmann fields which can be solved at mean field level within the static approximation and the replica symmetry ansatz. The obtained phase diagram shows the sequence of phases SG, AF and Kondo state for increasing Kondo coupling. This sequence agrees qualitatively with experimental data of the Ce_{2} Au_{1-x} Co_{x} Si_{3} compound.Comment: 7 pages, 1 figure, submitted to EPJ

Wage Adjustment Practices and the Link between Price and Wages: Survey Evidence from Colombian Firms

The aim of this paper is to explore firms’ wage adjustment practices in the Colombian formal labor market; specifically, the timing and frequency of wage increases, as well as the link between wage and price changes. To this end, we use an ad hoc survey of 1,305 small, medium and large firms belonging to all economic sectors, except the public sector. The results show most of the firms adjust base wages annually, mainly during the first quarter, which suggests wage changes in Colombia are time-dependent. Also, wage increases were concentrated around observed inflation and none of the firms cut wages. Moreover, factors associated with the performance of firms and workers alike are the main determinants of wage adjustments. Regarding the link between wages and price changes, econometric results indicate this relationship is stronger in sectors where labor costs represent a higher share of total costs and in firms operating in sectors with higher labor productivity.Wage increases, labor market, survey evidence, logit models, Colombia. Classification JEL: D22, J30, C25.

Massive Scaling Limit of beta-Deformed Matrix Model of Selberg Type

We consider a series of massive scaling limits m_1 -> infty, q -> 0, lim m_1 q = Lambda_{3} followed by m_4 -> infty, Lambda_{3} -> 0, lim m_4 Lambda_{3} = (Lambda_2)^2 of the beta-deformed matrix model of Selberg type (N_c=2, N_f=4) which reduce the number of flavours to N_f=3 and subsequently to N_f=2. This keeps the other parameters of the model finite, which include n=N_L and N=n+N_R, namely, the size of the matrix and the "filling fraction". Exploiting the method developed before, we generate instanton expansion with finite g_s, epsilon_{1,2} to check the Nekrasov coefficients (N_f =3,2 cases) to the lowest order. The limiting expressions provide integral representation of irregular conformal blocks which contains a 2d operator lim frac{1}{C(q)} : e^{(1/2) \alpha_1 \phi(0)}: (int_0^q dz : e^{b_E phi(z)}:)^n : e^{(1/2) alpha_2 phi(q)}: and is subsequently analytically continued.Comment: LaTeX, 21 pages; v2: a reference adde

Full Aging in Spin Glasses

The discovery of memory effects in the magnetization decays of spin glasses in 1983 began a large effort to determine the exact nature of the decay. While qualitative arguments have suggested that the decay functions should scale as $t_{w}$, the only time scale in the system, this type of scaling has not yet been observed. In this letter we report strong evidence for the scaling of the TRM magnetization decays as a function of $t_{w}$. By varying the rate and the profile that the sample is cooled through its transition temperature to the measuring temperature, we find that the cooling plays a major role in determining scaling. As the effective cooling time decreases, $\frac {t}{t_{w}}$scaling improves and for $t_{c}^{eff}<20s$ we find almost perfect $\frac{t}{t_{w}}$ scaling. We also find that subtraction of a stationary term from the magnetization decay has a small effect on the scaling but changes the form of the magnetization decay and improves overlap between curves produced with different $t_{w}$.Comment: 4 pages, 3 figure

Downward Wage Rigidities and Other Firms’ Responses to an Economic Slowdown: Evidence from a Survey of Colombian Firms

This paper uses a wage setting survey of 1,305 Colombian firms to explore the nature and sources of wage rigidities. This is the first study of a non-European emerging economy that uses evidence from a survey of firms to analyse this topic. The survey was carried out during the first half of 2009, when the Colombian economy was showing signs of a slowdown in economic activity and increasing unemployment. The sample is fully representative of the population under study. The results provide evidence of nominal and real downward wage rigidities in the country. The most important factor in not reducing base wages during an economic slowdown is to avoid the loss of more experienced and productive workers, which is related to the efficiency wage theory in its adverse selection version. In addition, ordered logit regressions were used to determine what factors are related to wage rigidities. The findings indicate that, in general, permanent contracts, workforce composition, labour intensity and the presence of collective agreements play an important role in explaining wage rigidities in the country.Wage rigidities, survey evidence, efficiency wages, Colombia, labour market, ordered logit. Classification JEL: C25, J30, J50