Superconductivity in the Hubbard model on a square lattice near half filling is studied using an optimization (or correlated) variational Monte Carlo method. Second-order processes of the strong-coupling expansion are considered in the wave functions beyond the Gutzwiller factor. Superconductivity of d x 2 −y 2 -wave is widely stable, and exhibits a crossover around U = Uco ∼ 12t from a BCS type to a new type. For U > ∼ Uco (U < ∼ Uco), the energy gain in the superconducting state is derived from the kinetic (potential) energy. Condensation energy is large and ∝ exp(−t/J) [tiny] on the strong [weak] coupling side of Uco. Cuprates belong to the strong-coupling regime.PACS numbers: 74.20. Mn, 71.10.Fd, 71.30.+h In a superconducting (SC) transition, conventional BCS superconductors follow a low-frequency sum rule of optical conductivity σ 1 (ω) [1]. However, recent experiments have shown that cuprate superconductors violate this sum rule [2]. This violation implies a gain in kinetic energy (K) in the transition, because the sum of σ 1 (ω) is proportional to −K [3]. Such kinetic-energy-driven superconductivity [4] sharply contrasts with that of the conventional BCS superconductors, where the transition is induced by the lowering of potential energy [5].The Hubbard model on a square lattice,is often used as a simple model which probably seizes the essence of cuprates [6]. In spite of its importance, reliable knowledge is limited particularly in the intermediate and strong coupling regimes. It is still controversial whether superconductivity is realized in this model. In the strong coupling region, the Hubbard model is mapped to t-J-type models, where the d x 2 −y 2 -wave superconductivity is concluded by exact diagonalization [7] and variation methods [8,9]. On the other hand, in the weakcoupling region, many quantum Monte Carlo (QMC) studies [10] came to negative conclusions for U/t = 2-4. Unfortunately, QMC is ineffective in larger-U regimes due to the negative sign problem. In contrast, RPA calculations [11], fluctuation exchange approximations [12], renormalization-group [13,14] and perturbative [15,16] studies concluded d x 2 −y 2 -wave superconductivity. Besides, variational Monte Carlo (VMC) studies using the Gutzwiller projection argued that an antiferromagnetic (AF) order prevails widely near half filling, and narrows the SC region [17, 18].The purpose of this letter is to resolve the above discrepancy, and to explore the possibility of the kineticenergy-driven superconductivity in the two-dimensional Hubbard model. By carefully studying the wave functions with vital improvement on the Gutzwiller projection, it is found that the d-wave SC state is stabilized even in the weak coupling region, but its energy gain is too small to be observed in QMC. Furthermore, we find a crossover at U co ∼ 12t, over which the SC transition is induced by the lowering of kinetic energy. This indicates that the high-T c superconductivity should be understood in the context of strong correlation.A VMC method [19] is useful to...

With high-Tc cuprates in mind, the properties of correlated d x 2 −y 2 -wave superconducting (SC) and antiferromagnetic (AF) states are studied for the Hubbard (t-t ′ -U ) model on square lattices using a variational Monte Carlo method. We employ simple trial wave functions including only crucial parameters, such as a doublon-holon binding factor indispensable for describing correlated SC and normal states as doped Mott insulators. The U/t, t ′ /t, and δ (doping rate) dependences of relevant quantities are systematically calculated. As U/t increases, a sharp crossover of SC properties occurs at Uco/t ∼ 10 from a conventional BCS type to a kinetic-energy-driven type for any t ′ /t. As δ decreases, Uco/t is smoothly connected to the Mott transition point at half filling. For U/t < ∼ 5, steady superconductivity corresponding to the cuprates is not found, whereas the d-wave SC correlation function P ∞ d rapidly increases for U/t > ∼ 6 and becomes maximum at U = Uco. Comparing the δ dependence of P ∞ d with an experimentally observed dome-shaped Tc and condensation energy, we find that the effective value of U for cuprates should be larger than the bandwidth, for which the t-J model is valid. Analyzing the kinetic energy, we reveal that, for U > Uco, only doped holes (electrons) become charge carriers, which will make a small Fermi surface (hole pocket), but for U < Uco all the electrons (holes) contribute to conduction and will make an ordinary large Fermi surface, which is contradictory to the feature of cuprates. By introducing an appropriate negative (positive) t ′ /t, the SC (AF) state is stabilized. In the underdoped regime, the strength of SC for U > Uco is determined by two factors, i.e., the AF spin correlation, which creates singlet pairs (pseudogap), and the charge mobility dominated by Mott physics. In this connection, we argue that the electrons near the antinodal points in the momentum space play a leading role in stabilizing the d-wave state, in contrast to the dichotomy of electronic roles in the momentum space proposed for the two-gap problem. We also show the instability of the hole-doped AF state against phase separation.

This study focuses on the potential of permanent magnets as thermoelectric converters. It is found that a SmCo5-type magnet exhibits the large anomalous Ettingshausen effect (AEE) at room temperature and that its charge-to-heat current conversion coefficient is more than one order of magnitude greater than that of typical ferromagnetic metals. The large AEE is an exclusive feature of the SmCo5-type magnet among various permanent magnets in practical use, which is independent of the conventional performance of magnets based on static magnetic properties. The experimental results show that the large AEE originates from the intrinsic transverse thermoelectric conductivity of SmCo5. This finding makes a connection between permanent magnets and thermal energy engineering, providing the basis for creating "thermoelectric permanent magnets.

First-principles calculation of D022 Mn3Ge shows a fully spin-polarized Δ1 band at the Fermi level, low saturation magnetization MS=180 emu/cm3, high uniaxial magnetic anisotropy Ku=23 Merg/cm3, and low Gilbert damping α=9×10-4. We also experimentally investigate D022 Mn3+xGe epitaxial films grown on MgO(100) substrates with different compositions x. The films exhibit a coercivity of about 20 kOe, MS of about 130 emu/cm3, and Ku of about 10 Merg/cm3 at x=0.55 (78 at.% Mn). These indicate that D022 Mn3Ge is a good candidate for spin-transfer-torque random access memory.

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Local magnetic moments around a nonmagnetic impurity in the two-dimensional t-J model are studied within Gutzwiller approximations and the Bogoliubov-de Gennes theory. We show that the resonance peak in the local density of states expected around the impurity splits into two when local magnetic moments are induced near the impurity. Therefore, in the recent scanning tunneling spectroscopy data on Bi 2 Sr 2 Ca(Cu 1Ϫx Zn x ) 2 O 8ϩ␦ ͓Pan et al., Nature ͑London͒ 403, 746 ͑2000͔͒ which show the sharp resonance peak, there is no evidence of the existence of local moments around the Zn impurities. It is shown that the absence of local moments is caused by the tendency of electrons to avoid the impurity.

A magnetic tunnel junction with a perpendicular magnetic easy-axis (p-MTJ) is a key device for spintronic non-volatile magnetoresistive random access memory (MRAM). Co-Fe-B alloy-based p-MTJs are being developed, although they have a large magnetisation and medium perpendicular magnetic anisotropy (PMA), which make it difficult to apply them to a future dense MRAM. Here, we demonstrate a p-MTJ with an epitaxially strained MnGa nanolayer grown on a unique CoGa buffer material, which exhibits a large PMA of more than 5 Merg/cm3 and magnetisation below 500 emu/cm3; these properties are sufficient for application to advanced MRAM. Although the experimental tunnel magnetoresistance (TMR) ratio is still low, first principles calculations confirm that the strain-induced crystal lattice distortion modifies the band dispersion along the tetragonal c-axis into the fully spin-polarised state; thus, a huge TMR effect can be generated in this p-MTJ.

The superconducting state around a well-isolated nonmagnetic impurity in the high-Tc superconductors is studied using the two-dimensional t-J model. The spatial dependence of the order parameter and the local density of states are obtained from the numerical diagonalization of the Bogoliubov-de Gennes equation derived using the Gutzwiller approximation. We find a zero-energy peak in the local density of states around the impurity. Different from the previous results on a vortex or surfaces in the t-J model, the splitting of the zero-energy peak is not found. The zero-energy states corresponding to the zero-energy peak is approximately localized around the impurity.