720 research outputs found

    Two-Staged Magnetoresistance Driven by Ising-like Spin Sublattice in SrCo6O11

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    A two-staged, uniaxial magnetoresistive effect has been discovered in SrCo6O11 having a layered hexagonal structure. Conduction electrons and localized Ising spins are in different sublattices but their interpenetration makes the conduction electrons sensitively pick up the stepwise field-dependence of magnetization. The stepwise field-dependence suggests two competitive interlayer interactions between ferromagnetic Ising-spin layers, i.e., a ferromagnetic nearest-layer interaction and an antiferromagnetic next-nearest-layer interaction. This oxide offers a unique opportunity to study nontrivial interplay between conduction electrons and Ising spins, the coupling of which can be finely controlled by a magnetic field of a few Tesla.Comment: 14 pages, 4 figures, accepted for publication in Phys. Rev. Let

    Universal behavior at discontinuous quantum phase transitions

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    Discontinuous quantum phase transitions besides their general interest are clearly relevant to the study of heavy fermions and magnetic transition metal compounds. Recent results show that in many systems belonging to these classes of materials, the magnetic transition changes from second order to first order as they approach the quantum critical point (QCP). We investigate here some mechanisms that may be responsible for this change. Specifically the coupling of the order parameter to soft modes and the competition between different types of order near the QCP. For weak first order quantum phase transitions general results are obtained. In particular we describe the thermodynamic behavior at this transition when it is approached from finite temperatures. This is the discontinuous equivalent of the non-Fermi liquid trajectory close to a conventional QCP in a heavy fermion material.Comment: 7 pages, 3 figure

    Theory of magnetic excitons in the heavy-fermion superconductor UPd2Al3UPd_{2}Al_{3}

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    We analyze the influence of unconventional superconductivity on the magnetic excitations in the heavy fermion compound UPd2_2Al3_3. We show that it leads to the formation of a bound state at energies well below 2Δ0\Delta_0 at the antiferromagnetic wave vector {\textbf Q}=(0,0,π/c)(0,0,\pi/c). Its signature is a resonance peak in the spectrum of magnetic excitations in good agreement with results from inelastic neutron scattering. Furthermore we investigate the influence of antiferromagnetic order on the formation of the resonance peak. We find that its intensity is enhanced due to intraband transitions induced by the reconstruction of Fermi surface sheets. We determine the dispersion of the resonance peak near {\textbf Q} and show that it is dominated by the magnetic exciton dispersion associated with local moments. We demonstrate by a microscopic calculation that UPd2_2Al3_3 is another example in which the unconventional nature of the superconducting order parameter can be probed by means of inelastic neutron scattering and determined unambiguously.Comment: 6 pages, 4 figure

    Magnetic and superconducting instabilities in the periodic Anderson model: an RPA stud

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    We study the magnetic and superconducting instabilities of the periodic Anderson model with infinite Coulomb repulsion U in the random phase approximation. The Neel temperature and the superconducting critical temperature are obtained as functions of electronic density (chemical pressure) and hybridization V (pressure). It is found that close to the region where the system exhibits magnetic order the critical temperature T_c is much smaller than the Neel temperature, in qualitative agreement with some T_N/T_c ratios found for some heavy-fermion materials. In our study, all the magnetic and superconducting physical behaviour of the system has its origin in the fluctuating boson fields implementing the infinite on-site Coulomb repulsion among the f-electrons.Comment: 9 pages, 2 figure

    Pressure-induced anomalous magnetism and unconventional superconductivity in CeRhIn5 : 115In-NQR Study under Pressure

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    We report 115^{115}In nuclear-quadrupole-resonance (NQR) measurements of the pressure(PP)-induced superconductor CeRhIn5_5 in the antiferromagnetic (AF) and superconducting (SC) states. In the AF region, the internal field HintH_{int} at the In site is substantially reduced from Hint=1.75H_{int}=1.75 kOe at P=0 to 0.39 kOe at P=1.23P=1.23 GPa, while the N\'eel temperature slightly changes with increasing PP. This suggests that either the size in the ordered moment MQ(P)M_{Q}(P) or the angle θ(P)\theta (P) between the direction of MQ(P)M_{Q}(P) and the tetragonal cc axis is extrapolated to zero at P∗=1.6±0.1P^*=1.6 \pm 0.1 GPa at which a bulk SC transition is no longer emergent. In the SC state at P=2.1P=2.1 GPa, the nuclear spin-lattice relaxation rate 115(1/T1)^{115}(1/T_1) has revealed a T3T^3 dependence without the coherence peak just below TcT_c, giving evidence for the unconventional superconductivity. The dimensionality of the magnetic flutuations in the normal state are also discussed.Comment: 8pages,4figures,submitted to Phys. Rev. B. Rapid

    Analysis of Superconductivity in d-p Model on Basis of Perturbation Theory

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    We investigate the mass enhancement factor and the superconducting transition temperature in the d-p model for the high-\Tc cuprates. We solve the \'Eliashberg equation using the third-order perturbation theory with respect to the on-site Coulomb repulsion UU. We find that when the energy difference between d-level and p-level is large, the mass enhancement factor becomes large and \Tc tends to be suppressed owing to the difference of the density of state for d-electron at the Fermi level. From another view point, when the energy difference is large, the d-hole number approaches to unity and the electron correlation becomes strong and enhances the effective mass. This behavior for the electron number is the same as that of the f-electron number in the heavy fermion systems. The mass enhancement factor plays an essential role in understanding the difference of \Tc between the LSCO and YBCO systems.Comment: 4pages, 9figures, to be published in J. Phys. Soc. Jp

    Possibility of valence-fluctuation mediated superconductivity in Cd-doped CeIrIn5_5 probed by In-NQR

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    We report on a pressure-induced evolution of exotic superconductivity and spin correlations in CeIr(In1−x_{1-x}Cdx_{x})5_5 by means of In-Nuclear-Quadrupole-Resonance (NQR) studies. Measurements of an NQR spectrum and nuclear-spin-lattice-relaxation rate 1/T11/T_1 have revealed that antiferromagnetism induced by the Cd-doping emerges locally around Cd dopants, but superconductivity is suddenly induced at TcT_c = 0.7 and 0.9 K at 2.34 and 2.75 GPa, respectively. The unique superconducting characteristics with a large fraction of the residual density of state at the Fermi level that increases with TcT_c differ from those for anisotropic superconductivity mediated by antiferromagnetic correlations. By incorporating the pressure dependence of the NQR frequency pointing to the valence change of Ce, we suggest that unconventional superconductivity in the CeIr(In1−x_{1-x}Cdx_{x})5_5 system may be mediated by valence fluctuations.Comment: Accepted for publication in Physical Review Letter

    Exotic superconductivity in the coexistent phase of antiferromagnetism and superconductivity in CeCu2(Si0.98Ge0.02)2: A Cu-NQR study under hydrostatic pressure

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    We report a pressure (PP) effect on CeCu2_2(Si0.98_{0.98}Ge0.02_{0.02})2_2 where an antiferromagnetic (AFM) order at TN∼T_N \sim 0.75 K coexists with superconductivity below Tc∼T_c \sim 0.4 K\@. At pressures exceeding P=0.19P = 0.19 GPa, the AFM order is suppressed, which demonstrates that the sudden emergence of AFM order due to the Ge doping is ascribed to the intrinsic lattice expansion. The exotic superconductivity at P=0P = 0 GPa is found to evolve into a typical heavy-fermion one with a line-node gap above P=0.91P = 0.91 GPa\@. We highlight that the anomalous enhancement in nuclear spin-lattice relaxation rate 1/T11/T_1 that follows a T1TT_1T = const. behavior well below TcT_c at PP = 0 GPa is characterized by the persistence of low-lying magnetic excitations, which may be inherent to the coexistent state of antiferromagnetism and superconductivity.Comment: 5 pages with 4 figures embedded in the text. To be published in J. Phys. Soc. Jp

    Evidence for ferromagnetic spin-pairing superconductivity in UGe2_2: A 73^{73}Ge-NQR study under pressure

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    We report that a novel type of superconducting order parameter has been realized in the ferromagnetic states in UGe2_2 via 73^{73}Ge nuclear-quadrupole-resonance (NQR) experiments performed under pressure (PP). Measurements of the nuclear spin-lattice relaxation rate (1/T1)(1/T_1) have revealed an unconventional nature of superconductivity such that the up-spin band is gapped with line nodes, but the down-spin band remains gapless at the Fermi level. This result is consistent with that of a ferromagnetic spin-pairing model in which Cooper pairs are formed among ferromagnetically polarized electrons. The present experiment has shed new light on a possible origin of ferromagnetic superconductivity, which is mediated by ferromagnetic spin-density fluctuations relevant to the first-order transition inside the ferromagnetic states.Comment: 5 pages, 5 figure
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