Divergence of the Grüneisen Ratio at Quantum Critical Points in Heavy Fermion Metals
R. KüchlerN. OeschlerP. GegenwartT. CichorekK. NeumaierO. TegusC. GeibelJ. A. MydoshF. SteglichLijun ZhuQimiao Si
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We present low-temperature volume thermal expansion, beta, and specific heat, C, measurements on high-quality single crystals of CeNi2Ge2 and YbRh2(Si0.95Ge0.05)(2) which are located very near to quantum critical points. For both systems, beta shows a more singular temperature dependence than C, and thus the Grüneisen ratio Gamma proportional to beta/C diverges as T-->0. For CeNi2Ge2, our results are in accordance with the spin-density wave (SDW) scenario for three-dimensional critical spin fluctuations. By contrast, the observed singularity in YbRh2(Si0.95Ge0.05)(2) cannot be explained by the itinerant SDW theory but is qualitatively consistent with a locally quantum critical picture.Keywords:
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The first theoretical description of the itinerant antiferromagnetic state in heavy-fermion systems is presented in detail. We analyze the phase diagram, the stability of the phases, the magnetic susceptibility, and the specific heat. For the case in which the gap vanishes in points on the Fermi surface, the deduced results are in good agreement with the experimental data.
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This letter deals with the suppressive effect of the nonmagnetic impurity, doped into the parent compound of the high-Tc superconductor, on the spin density wave (SDW) state. Taking into account the van Hove singularity in the Fermi surface of the two-dimensional square lattice, our results indicate that the nonmagnetic impurity can lead to sharp reduction of the SDW order, which is in qualitative agreement with the experimental reports.
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We study the instability of the metallic state towards the formation of a new ground state in graphene doped near the van Hove singularity. The system is described by the Hubbard model and a field theoretical approach is used to calculate the charge and spin susceptibility. We find that for repulsive interactions, within the random phase approximation, there is a competition between ferromagnetism and spin-density wave (SDW). It turns out that a SDW with a triangular geometry is more favorable when the Hubbard parameter is above the critical value U_c(T), which depends on the temperature T, even if there are small variations in the doping. Our results can be verified by ARPES or neutron scattering experiments in highly doped graphene.
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Abstract The phonon response of a spin density wave superconductor (SDW-SC) is calculated. The phonon self energy due to electron-phonon interaction, which involves the electronic density response function, is evaluated explicitly for the coexistent SDW-SC state. It has a square-root singularity at a frequency corresponding to the superconducting gap 2Δ. As a consequence, the spectral density function of the SDW-phonon will show a peak at 2Δ The possibility of observing the 2Δ-peak by Raman scattering is discussed. The relevance of the results to high-temperature superconductors is pointed out. The available Raman data for YBa2Cu3O7-δare compared with the results of the calculation. Key Words: SuperconductivitySpin density wave (SDW)Phonon spectral functionRaman ScatteringHigh temperature superconductors.
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An interplay between the superconductivity and spin density wave (SDW) is studied from the perturbative point of view based on the two dimensional Hubbard model. It is shown that the d-pairing superconductivity is induced by spin fluctuations near the SDW boundary. The superconducting transition temperature ( T c ) is found to be remarkably suppressed by the renormalization through the electron self-energy, and to be sensitive to the band parameters due to nesting of the Fermi surface and the presence of the van Hove singularity as well as to the Fermi energy and the on-site interaction. As a consequence T c is enhanced by the next-nearest-neighbour hopping integral. The relation between the present result and experiments of high T c superconductors is discussed.
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Two-dimensional Hubbard model with both nearest- and next-nearest-neighbor transfers is studied. A singularity in the charge susceptibility at the Mott transition point δ→0 is observed, leading to the anomaly of the charge mass m c * in the form m c * ∝|δ| -1 , where δ is the doping concentration. The singularity is similar to that in the case without the next-nearest-neighbor transfer. This indicates that the singularity in the charge mass is a universal nature of this model, irrespective of its band structure.
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