Ž .Ž . The possibility of superconductivity SC in the ground state of the two-dimensional 2D Hubbard model was investigated by means of the variational Monte Carlo method. The energy gain of the d-wave SC state, obtained as the difference of the minimum energy with a finite gap and that with zero gap, was examined with respect to dependences on U, electron density r and next nearest neighbor transfer t X mainly on the 10 = 10 lattice. It was found to be maximized around Ž .X U s 8 the energy unit is nearest neighbor transfer t . It was shown to sharply increase for negative values of t and have a broad peak for t X ; y0.10. For these value of t X the energy gain was a smooth increasing function of r almost independent of the shell structure in the region starting from ; 0.76 up to the upper bound of investigation 0.92. This clearly indicates that the result is already close to the value in the bulk limit. For t X s 0, the energy gain depended on the electronic shell state. This suggests the 10 = 10 lattice is not sufficiently large for this case, although it is highly plausible that the bulk limit value is finite. Competition between the SC and the commensurate SDW states was also investigated. When t X s 0, the ground state is SDW in the range of r G; 0.84. The SC region slightly extends up to ; 0.87 for t X ; y0.10. Consequently the present results strongly support an assertion that the 2D Hubbard model with t X ; y0.1 drives SC by itself in the r region from ; 0.76 to ; 0.87. The above features are in a fair agreement with the phase diagram of the optimally and overly hole-doped cuprates. The energy gain in the SC state with suitable parameters is found to be in reasonable agreement with the condensation energy in the SC state of YBa Cu O . The corresponding t-J model proves to give an 2 3 7order-of-magnitude larger energy gain, which questions its validity. q 1998 Elsevier Science B.V. All rights reserved.

We propose a Monte Carlo method, which is a hybrid method of the quantum Monte Carlo method and variational Monte Carlo theory, to study the Hubbard model. The theory is based on the off-diagonal and the Gutzwiller type correlation factors which are taken into account by a Monte Carlo algorithm. In the 4×4 system our method is able to reproduce the exact results obtained by the diagonalization. An application is given to investigate the half-filled band case of two-dimensional square lattice. The energy is favorably compared with quantum Monte Carlo data.

The ground state of the two-dimensional three-band Hubbard model in the oxide superconductors is investigated by using the variational Monte Carlo method. The Gutzwiller-projected BCS and SDW wave functions are employed in search for a possible ground state with respect to dependences on electron density. Antiferromagnetic correlations are considerably strong near half-filling. It is shown that the d-wave state may exist away from half-filling for both the hole and electron doping cases. Overall structure of the phase diagram obtained by our calculations qualitatively agrees with experimental indications. The superconducting condensation energy is in reasonable agreement with the experimental value obtained from specific heat and critical magnetic field measurements for optimally doped samples. The inhomogeneous SDW state is also examined near 1/8 doping. Incommensurate magnetic structures become stable due to hole doping in the underdoped region, where the transfer tpp between oxygen orbitals plays an important role in determining a stable stripe structure.

We report a uniaxial pressure-dependence of magnetism in layered perovskite strontium ruthenate Sr 3 Ru 2 O 7 . By applying a relatively small uniaxial pressure, greater than 0.1 GPa normal to the RuO 2 layer, ferromagnetic ordering manifests below 80 K from the enhancedparamagnet. Magnetization at 1 kOe and 2 K becomes 100 times larger than that under ambient condition. Uniaxial pressure dependence of Curie temperature T C suggests the first order magnetic transition. Origin of this uniaxial-pressure induced ferromagnetism is discussed in terms of the rotation of RuO 6 octahedra within the RuO 2 plane.KEYWORDS: Sr3Ru2O7, Sr2RuO4, ferromagnetism, uniaxial pressure, nearly ferromagnetic metal, magnetization, structural distortion §1. IntroductionTuning the magnetism in solid state compounds via tiny perturbation is one of the central issues for strongly-correlated electron systems such as heavy-fermion (HF) intermetallic compounds.Since those materials tend to reveal the high susceptibility of the electronic properties to relatively small external pressures (≤ GPa). For instance, antiferromagnetic HF compounds (CeCu 2 Ge 2 , 1)3) ) and a ferromagnetic HF compound (UGe 2 4) ) reveal superconductivity under several GPa hydrostatic pressures, which reduce the magnetic ordering temperature towards absolute zero. Around the diminishing region, the superconductivity appears. Several GPa pressures, which is necessary to induce the superconductivity in above HF compounds, are relatively small *

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