223 research outputs found

    Quantum critical point in the spin glass-antiferromagnetism competition in Kondo-lattice systems

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    A theory is proposed to describe the competition among antiferromagnetism (AF), spin glass (SG) and Kondo effect. The model describes two Kondo sublattices with an intrasite Kondo interaction strength JKJ_{K} and an interlattice quantum Ising interaction in the presence of a transverse field Γ\Gamma. The interlattice coupling is a random Gaussian distributed variable (with average −2J0/N-2J_0/N and variance 32J2/N32 J^{2}/N) while the Γ\Gamma field is introduced as a quantum mechanism to produce spin flipping. The path integral formalism is used to study this fermionic problem where the spin operators are represented by bilinear combinations of Grassmann fields. The disorder is treated within the framework of the replica trick. The free energy and the order parameters of the problem are obtained by using the static ansatz and by choosing both J0/JJ_0/J and Γ/J≈(Jk/J)2\Gamma/J \approx (J_k/J)^2 to allow, as previously, a better comparison with the experimental findings. The results indicate the presence of a SG solution at low JK/JJ_K/J and for temperature T<TfT<T_{f} (TfT_{f} is the freezing temperature). When JK/JJ_K/J is increased, a mixed phase AF+SG appears, then an AF solution and finally a Kondo state is obtained for high values of JK/JJ_{K}/J. Moreover, the behaviors of the freezing and Neel temperatures are also affected by the relationship between JKJ_{K} and the transverse field Γ\Gamma. The first one presents a slight decrease while the second one decreases towards a Quantum Critical Point (QCP). The obtained phase diagram has the same sequence as the experimental one for Ce2Au1−xCoxSi3Ce_{2}Au_{1-x}Co_{x}Si_{3}, if JKJ_{K} is assumed to increase with xx, and in addition, it also shows a qualitative agreement concerning the behavior of the freezing and the Neel temperatures.Comment: 11 pages, 3 figures, accepted for publication in J. Phys.

    One-step replica symmetry breaking solution for a highly asymmetric two-sublattice fermionic Ising spin glass model in a transverse field

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    The one-step replica symmetry breaking (RSB) is used to study a two-sublattice fermionic infinite-range Ising spin glass (SG) model in a transverse field Γ\Gamma. The problem is formulated in a Grassmann path integral formalism within the static approximation. In this model, a parallel magnetic field HH breaks the symmetry of the sublattices. It destroys the antiferromagnetic (AF) order, but it can favor the nonergodic mixed phase (SG+AF) characterizing an asymmetric RSB region. In this region, intra-sublattice disordered interactions VV increase the difference between the RSB solutions of each sublattice. The freezing temperature shows a higher increase with HH when VV enhances. A discontinue phase transition from the replica symmetry (RS) solution to the RSB solution can appear with the presence of an intra-sublattice ferromagnetic average coupling. The Γ\Gamma field introduces a quantum spin flip mechanism that suppresses the magnetic orders leading them to quantum critical points. Results suggest that the quantum effects are not able to restore the RS solution. However, in the asymmetric RSB region, Γ\Gamma can produce a stable RS solution at any finite temperature for a particular sublattice while the other sublattice still presents RSB solution for the special case in which only the intra-sublattice spins couple with disordered interactions.Comment: 11 pages, 8 figures, accepted for publication in Phys. Rev.

    Clustering in disordered ferromagnets: The Curie temperature in diluted magnetic semiconductors

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    We theoretically investigate impurity correlation and magnetic clustering effects on the long-range ferromagnetic ordering in diluted magnetic semiconductors, such as Ga1−xMnxAs\textrm{Ga}_{1-x}\textrm{Mn}_{x}\textrm{As}, using analytical arguments and direct Monte Carlo simulations. We obtain an analytic formula for the ferromagnetic transition temperature TcT_{c} which becomes asymptotically exact in the strongly disordered, highly dilute (i.e. small xx) regime. We establish that impurity correlations have only small effects on TcT_{c} with the neutrally correlated random disorder producing the nominally highest TcT_{c}. We find that the ferromagnetic order is approached from the high temperature paramagnetic side through a random magnetic clustering phenomenon consistent with the percolation transition scenario.Comment: 5 pages, 4 figure

    Polaron percolation in diluted magnetic semiconductors

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    We theoretically study the development of spontaneous magnetization in diluted magnetic semiconductors as arising from a percolation of bound magnetic polarons. Within the framework of a generalized percolation theory we derive analytic expressions for the Curie temperature and the magnetization, obtaining excellent quantitative agreement with Monte Carlo simulation results and good qualitative agreement with experimental results.Comment: 5 page

    Tetragonal tungsten bronze compounds: relaxor vs mixed ferroelectric - dipole glass behavior

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    We demonstrate that recent experimental data (E. Castel et al J.Phys. Cond. Mat. {\bf 21} (2009), 452201) on tungsten bronze compound (TBC) Ba2_2Prx_xNd1−x_{1-x}FeNb4_4O15_{15} can be well explained in our model predicting a crossover from ferroelectric (x=0x=0) to orientational (dipole) glass (x=1x=1), rather then relaxor, behavior. We show, that since a "classical" perovskite relaxor like Pb(Mn1/3_{1/3} Nb2/3_{2/3})O3_3 is never a ferroelectric, the presence of ferroelectric hysteresis loops in TBC shows that this substance actually transits from ferroelectric to orientational glass phase with xx growth. To describe the above crossover theoretically, we use the simple replica-symmetric solution for disordered Ising model.Comment: 5 two-column pages, 4 figure

    Thermodynamic Properties and Phase Transitions in a Mean-Field Ising Spin Glass on Lattice Gas: the Random Blume-Emery-Griffiths-Capel Model

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    The study of the mean-field static solution of the Random Blume-Emery-Griffiths-Capel model, an Ising-spin lattice gas with quenched random magnetic interaction, is performed. The model exhibits a paramagnetic phase, described by a stable Replica Symmetric solution. When the temperature is decreased or the density increases, the system undergoes a phase transition to a Full Replica Symmetry Breaking spin-glass phase. The nature of the transition can be either of the second order (like in the Sherrington-Kirkpatrick model) or, at temperature below a given critical value, of the first order in the Ehrenfest sense, with a discontinuous jump of the order parameter and accompanied by a latent heat. In this last case coexistence of phases takes place. The thermodynamics is worked out in the Full Replica Symmetry Breaking scheme, and the relative Parisi equations are solved using a pseudo-spectral method down to zero temperature.Comment: 24 pages, 12 figure

    Suppression of carrier induced ferromagnetism by composition and spin fluctuations in diluted magnetic semiconductors

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    We suggest an approach to account for spatial (composition) and thermal fluctuations in "disordered" magnetic models (e.g. Heisenberg, Ising) with given spatial dependence of magnetic spin-spin interaction. Our approach is based on introduction of fluctuating molecular field (rather than mean field) acting between the spins. The distribution function of the above field is derived self-consistently. In general case this function is not Gaussian, latter asymptotics occurs only at sufficiently large spins (magnetic ions) concentrations nin_i. Our approach permits to derive the equation for a critical temperature TcT_c of ferromagnetic phase transition with respect to the above fluctuations. We apply our theory to the analysis of influence of composition fluctuations on TcT_c in diluted magnetic semiconductors (DMS) with RKKY indirect spin-spin interaction.Comment: 6 pages, 2 figure
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