903 research outputs found

    Statistical Transfer Matrix Study of the ±J\pm J Multileg Ising Ladders and Tubes

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    Finite temperature properties of symmetric ±J\pm J multileg Ising ladders and tubes are investigated using the statistical transfer matrix method. The temperature dependences of the specific heat and entropy are calculated. In the case of tubes, it is found that the ground state entropy shows an even-odd oscillation with respect to the number of legs. The same type of oscillation is also found in the ground state energy. On the contrary, these oscillations do not take place in ladders. From the temperature-dependence of the specific heat, it is found that the lowest excitation energy is 4J for even-leg ladders while it is 2J otherwise, The physical origin of these behaviors is discussed based on the structure of excitations.Comment: 6 pages, 9 figure

    Model for the low-temperature magnetic phases observed in doped YBa_2Cu_3O_{6+x}

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    A classical statistical model for the antiferromagnetic (AFM) ordering of the Cu-spins in the CuO_2 planes of reduced YBa_2Cu_3O_{6+x} type materials is presented. The magnetic phases considered are the experimentally observed high-temperature AFI phase with ordering vector Q_I=(1/2,1/2,0), and the low-temperature phases: AFII with Q_II=(1/2,1/2,1/2) and intermediate TA (Turn Angle) phases TAI, TAII and TAIII with components of both ordering vectors. It is shown that the AFII and TA phases result from an effective ferromagnetic (FM) type coupling mediated by free spins in the CuO_x basal plane. Good agreement with experimental data is obtained for realistic model parameters.Comment: 11 pages, 2 Postscript figures, Submitted to Phys.Rev.Let

    Ordered phase and scaling in ZnZ_n models and the three-state antiferromagnetic Potts model in three dimensions

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    Based on a Renormalization-Group picture of ZnZ_n symmetric models in three dimensions, we derive a scaling law for the ZnZ_n order parameter in the ordered phase. An existing Monte Carlo calculation on the three-state antiferromagnetic Potts model, which has the effective Z6Z_6 symmetry, is shown to be consistent with the proposed scaling law. It strongly supports the Renormalization-Group picture that there is a single massive ordered phase, although an apparently rotationally symmetric region in the intermediate temperature was observed numerically.Comment: 5 pages in REVTEX, 2 PostScript figure

    Tilted vortex lattice in irradiate Bi2Sr2CaCu2O8+δ single crystals

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    In order to enlighten the structure of vortex matter in irradiated layered Bi 2 Sr 2 CaCu 2 O 8+δ single crystals, the interaction of Josephson vortices and pancake vortices in was investigated by means of the local ac-magnetic permeability measurements by using the miniature local coils, while vortex matter in pristine crystals was studied by in-plane resistivity measurements. The transition anomaly, separating the strong pinning phase and the weak pinning vortex phase was found by both techniques deep in the vortex solid phase solid near ab-plane, indicating crossover from the vortex chains + lattice phase to tilted vortex chains phase. While the columnar defects affect strongly the first-order vortex-lattice melting transition, the magnetic permeability anomaly, associated with the crossover from vortex chains + lattice phase to tilted lattice, is surprisingly still clear, deep in the vortex solid phase. However, the stronger columnar defects eventually affect the crossover anomaly that it disappears too

    Magnetic-Field Induced Quantum Critical Point in YbRh2_2Si2_2

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    We report low-temperature calorimetric, magnetic and resistivity measurements on the antiferromagnetic (AF) heavy-fermion metal YbRh2_2Si2_2 (TN={T_N =} 70 mK) as a function of magnetic field BB. While for fields exceeding the critical value Bc0{B_{c0}} at which TN0{T_N\to0} the low temperature resistivity shows an AT2{AT^2} dependence, a 1/(BBc0){1/(B-B_{c0})} divergence of A(B){A(B)} upon reducing BB to Bc0{B_{c0}} suggests singular scattering at the whole Fermi surface and a divergence of the heavy quasiparticle mass. The observations are interpreted in terms of a new type of quantum critical point separating a weakly AF ordered from a weakly polarized heavy Landau-Fermi liquid state.Comment: accepted for publication in Phys. Rev. Let

    Theory of Quasi-Universal Ratio of Seebeck Coefficient to Specific Heat in Zero-Temperature Limit in Correlated Metals

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    It is shown that the quasi-universal ratio q=limT0eS/C±1q=\lim_{T\to0}eS/C\sim\pm1 of the Seebeck coefficient to the specific heat in the limit of T=0 observed in a series of strongly correlated metals can be understood on the basis of the Fermi liquid theory description. In deriving this result, it is crucial that a relevant scattering arises from impurities, but not from the mutual scattering of quasiparticles. The systematics of the sign of qq is shown to reflect the sign of the logarithmic derivative of the density of states and the inverse mass tensor of the quasiparticles, explaining the systematics of experiments. In particular, the positive sign of qq for Ce-based and f3f^{3}-based heavy fermions, and the negative sign for Yb-based and f2f^{2}-based heavy fermions, are explained. The case of non-Fermi liquid near the quantum critical point (QCP) is briefly mentioned, showing that the ratio qq decreases considerably toward antiferromagnetic QCP while it remains essentially unchanged for the ferromagnetic QCP or QCP due to a local criticality.Comment: 12 pages, 1 figur

    Spin Dynamics at Very Low Temperature in Spin Ice Dy2_2Ti2_2O7_7

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    We have performed AC susceptibility and DC magnetic relaxation measurements on the spin ice system Dy2_2Ti2_2O7_7 down to 0.08 K. The relaxation time of the magnetization has been estimated below 2 K down to 0.08 K. The spin dynamics of Dy2_2Ti2_2O7_7 is well described by using two relaxation times (τS\tau_{\rm S} (short time) and τL\tau_{\rm L} (long time)). Both τS\tau_{\rm S} and τL\tau_{\rm L} increase on cooling. Assuming the Arrhenius law in the temperature range 0.5-1 K, we obtained an energy barrier of 9 K. Below 0.5 K, both τS\tau_{\rm S} and τL\tau_{\rm L} show a clear deviation from the thermal activated dynamics toward temperature independent relaxation, suggesting a quantum dynamics.Comment: 4 page

    First-Principles Study for the Anisotropy of Iron-based Superconductors toward Power and Device Applications

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    Performing the first-principles calculations, we investigate the anisotropy in the superconducting state of iron-based superconductors to gain an insight into their potential applications. The anisotropy ratio γλ\gamma_\lambda of the c-axis penetration depth to the ab-plane one is relatively small in BaFe2As2 and LiFeAs, i.e., γλ3\gamma_\lambda \sim 3, indicating that the transport applications are promising in these superconductors. On the other hand, in those having perovskite type blocking layers such as Sr2ScFePO3 we find a very large value, γλ200\gamma_\lambda \sim 200, comparable to that in strongly anisotropic high-Tc cuprate Bi2Sr2CaCu2O{8-\delta}. Thus, the intrinsic Josephson junction stacks are expected to be formed along the c-axis, and novel Josephson effects due to the multi-gap nature are also suggested in these superconductors.Comment: 5 pages, 4 figure

    The London theory of the crossing-vortex lattice in highly anisotropic layered superconductors

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    A novel description of Josephson vortices (JVs) crossed by the pancake vortices (PVs) is proposed on the basis of the anisotropic London theory. The field distribution of a JV and its energy have been calculated for both dense (aλJa\lambda_J) PV lattices with distance aa between PVs, and the nonlinear JV core size λJ\lambda_J. It is shown that the ``shifted'' PV lattice (PVs displaced mainly along JVs in the crossing vortex lattice structure), formed in high out-of-plane magnetic fields transforms into the PV lattice ``trapped'' by the JV sublattice at a certain field, lower than Φ0/γ2s2\Phi_0/\gamma^2s^2, where Φ0\Phi_0 is the flux quantum, γ\gamma is the anisotropy parameter and ss is the distance between CuO2_2 planes. With further decreasing BzB_z, the free energy of the crossing vortex lattice structure (PV and JV sublattices coexist separately) can exceed the free energy of the tilted lattice (common PV-JV vortex structure) in the case of γs<λab\gamma s<\lambda_{ab} with the in-plane penetration depth λab\lambda_{ab} if the low (Bx<γΦ0/λab2B_x<\gamma\Phi_0/\lambda_{ab}^2) or high (BxΦ0/γs2B_x\gtrsim \Phi_0/\gamma s^2) in-plane magnetic field is applied. It means that the crossing vortex structure is realized in the intermediate field orientations, while the tilted vortex lattice can exist if the magnetic field is aligned near the cc-axis and the abab-plane as well. In the intermediate in-plane fields γΦ0/λab2BxΦ0/γs2\gamma\Phi_0/\lambda_{ab}^2\lesssim B_x \lesssim \Phi_0/\gamma s^2, the crossing vortex structure with the ``trapped'' PV sublattice seems to settle in until the lock-in transition occurs since this structure has the lower energy with respect to the tilted vortex structure in the magnetic field H{\vec H} oriented near the abab-plane.Comment: 15 pages, 6 figures, accepted for publication in PR
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