铜氧化物高温超导体中纳米尺度的电子态对理解这类材料的复杂物理特性和非常规超导机理具有重要的意义。本文采用数值精确对角化方法研究了纳米尺度四方哈伯德团簇中对角跃迁t'和近邻库仑相互作用V对电子对束缚能、自旋和电荷能隙及其对应跨越温度的物理影响。研究结果表明对角跃迁t'有效地扩展了电子配对、自旋和电荷配对的哈伯德U参数区间，升高了它们对应的跨越温度；与此相反，近邻库仑相互作用V显著地缩小了电子配对、自旋和电荷配对的哈伯德U参数区间，并降低了它们对应的跨越温度。当V与哈伯德U产生的有效电子吸引相互作用能量尺度相当时，在整个U值区间不能形成稳定的电子配对态。我们的研究结果说明t'有利于形成电子配对的超导态，而V将抑制超导态的形成。 In the high-Tc cuprates, the nanoscale electronic state is important for understanding their com-plex physical properties and unconventional superconducting mechanism. In this work, numerical exact diagonalization method has been used to study the effect of diagonal hopping t' and nearest-neighbor Coulomb interaction V on the electron pair binding energy, spin and charge gaps and corresponding crossover temperatures in the square Hubbard clusters. The calculated results show that t' extends the regime of U in which electron, spin and charge pairings are formed, and corresponding crossover temperature increased. On the contrary, V exhibits opposite effects com- pared to t'. When V approaches to the energy scale of effective attraction induced by U, electron pairing is completely destroyed. Our findings demonstrate that t' can help the formation of superconducting state, while V will suppress the superconductivity.
铜氧化物高温超导体中纳米尺度的电子态对理解这类材料的复杂物理特性和非常规超导机理具有重要的意义。本文采用数值精确对角化方法研究了纳米尺度四方哈伯德团簇中对角跃迁t'和近邻库仑相互作用V对电子对束缚能、自旋和电荷能隙及其对应跨越温度的物理影响。研究结果表明对角跃迁t'有效地扩展了电子配对、自旋和电荷配对的哈伯德U参数区间，升高了它们对应的跨越温度；与此相反，近邻库仑相互作用V显著地缩小了电子配对、自旋和电荷配对的哈伯德U参数区间，并降低了它们对应的跨越温度。当V与哈伯德U产生的有效电子吸引相互作用能量尺度相当时，在整个U值区间不能形成稳定的电子配对态。我们的研究结果说明t'有利于形成电子配对的超导态，而V将抑制超导态的形成。 In the high-Tc cuprates, the nanoscale electronic state is important for understanding their com-plex physical properties and unconventional superconducting mechanism. In this work, numerical exact diagonalization method has been used to study the effect of diagonal hopping t' and nearest-neighbor Coulomb interaction V on the electron pair binding energy, spin and charge gaps and corresponding crossover temperatures in the square Hubbard clusters. The calculated results show that t' extends the regime of U in which electron, spin and charge pairings are formed, and corresponding crossover temperature increased. On the contrary, V exhibits opposite effects com- pared to t'. When V approaches to the energy scale of effective attraction induced by U, electron pairing is completely destroyed. Our findings demonstrate that t' can help the formation of superconducting state, while V will suppress the superconductivity.
In the high-Tc cuprates, the nanoscale electronic state is important for understanding their com- plex physical properties and unconventional superconducting mechanism. In this work, numerical exact diagonalization method has been used to study the effect of diagonal hopping t' and near- est-neighbor Coulomb interaction V on the electron pair binding energy, spin and charge gaps and corresponding crossover temperatures in the square Hubbard clusters. The calculated results show that t' extends the regime of U in which electron, spin and charge pairings are formed, and corresponding crossover temperature increased. On the contrary, V exhibits opposite effects com- pared to t'. When V approaches to the energy scale of effective attraction induced by U, electron pairing is completely destroyed. Our findings demonstrate that t' can help the formation of super- conducting state, while V will suppress the superconductivity.
The emergence of superconductivity (SC) in lattice models, such as the attractive Hubbard one, has renewed interest since the realization of cold-atom experiments. However, reducing the temperature in these experiments is a bottleneck; therefore, investigating how to increase the energy scale for SC is crucial to cold atoms. In view of this, we examine the effects of next-nearest-neighbor hoppings (t′) on the pairing properties of the attractive Hubbard model in a square lattice. To this end, we analyze the model through unbiased Quantum Monte Carlo simulations for fixed density n=0.87, and perform finite-size scaling analysis to the thermodynamic limit. As our main result, we notice that the existence of further hopping channels leads to an enhancement of the pairing correlations, which, in turn, increases the ground-state order parameter Δ. Finally, at finite temperatures, for t′/t≠0, this enhancement of pairing correlations leads to an increase in the critical temperature Tc. That is, the fine-tuning of second-neighbor hoppings increases the energy scales for SC, and may be a route by which cold-atom experiments can achieve such a phase and to help us further understand the nature of this phenomenon.
Our previous mean-field study concerning superconducting pairings in the extended Hubbard model with on-site repulsive and intersite attractive interaction and arbitrary electron density is extended to include the effects of nearest-neighbor exchange (J) and correlated hopping (K) interaction. Detailed numerical analysis of s-, p-, and d-wave pairing solutions is performed for the two-dimensional square lattice with nearest- (t) and next-nearest- (t2) neighbor hopping, and the resulting mutual stability phase diagrams are given as a function of band filling and the interaction parameters. Antiferromagnetic exchange can enhance both the d-wave and the extended s-wave pairing. If it prevails over the repulsive intersite density-density interaction, such a term can be a leading mechanism for superconductivity. However, its effects strongly depend on the band filling and the tt2 ratio, giving in particular for t2=0s-wave (d-wave) pairing as most favorable in the low (high) density limit, but increasing t2 can even reverse this tendency. The correlated hopping term which breaks the electron-hole symmetry affects s-wave pairing mostly. A condensation transition (phase separation), possible in the presence of the attractive density-density interaction, is also analyzed within the random-phase approximation, and stability conditions of such an electron droplet phase with respect to other types of ordering are determined as a function of the band filling for the two-dimensional square lattice.
Many theoretical approaches find d-wave superconductivity in the prototypical one-band Hubbard model for high-temperature superconductors. At strong coupling ($U\ensuremath{\ge}W$, where U is the on-site repulsion and W=8t the bandwidth) pairing is controlled by the exchange energy J=4t2/U. One may then surmise, ignoring retardation effects, that near-neighbor Coulomb repulsion V will destroy superconductivity when it becomes larger than J, a condition that is easily satisfied in cuprates, for example. Using cellular dynamical mean-field theory with an exact diagonalization solver for the extended Hubbard model, we show that pairing at strong coupling is preserved, even when $V\ensuremath{\gg}J$, as long as $V\ensuremath{\lesssim}U/2$. While at weak coupling V always reduces the spin fluctuations and hence d-wave pairing, at strong coupling, in the underdoped regime, the increase of $J=4{t}^{2}/(U\ensuremath{-}V)$ caused by V increases binding at low frequency while the pair-breaking effect of V is pushed to high frequency. These two effects compensate in the underdoped regime, in the presence of a pseudogap. While the pseudogap competes with superconductivity, the proximity to the Mott transition that leads to the pseudogap, and retardation effects, protect d-wave superconductivity from V.
In the present work the Roth s two-pole approximation (Phys. Rev. 184, 451 (1969)) has been used to investigate the role of d− p hybridization in the normal and superconducting states of an extended d− p Hubbard model. Superconductivity with singlet dx2−y2 -wave pairing is treated by following Beenen and Edwards formalism (Phys. Rev. B 52, 13636 (1995)). In the first part of this work, the effects of the hybridization on the superconductivity, in the hole-doped regime, have been studied treating Roth s band shift within two different approximations. In the first one, the band shift has been calculated in the limit U →¥ (U is the Coulomb interaction), with zero temperature and without consider the superconducting effects. These regards, are restrict to the band shift. In the other parts of the problem, U, the temperature and the superconducting effects have been considered finites. In the second approximation, the Coulomb interaction, the temperature and the superconductivity have been considered in the calculation of some relevant correlation functions present in the Roth s band shift. The obtained results show that the hybridization acts in the sense of to suppresses the superconductivity. Also, it has been verified that the first approximation overstimates the effects of the hybridization on the superconductivity. In the second part of these work, hoppings to second-nearest-neighbors have been included in the model with the purpose of reproduces adequately the asymetries (mainly those related with the Fermi surface, band structure and phase diagram) between the hole- and electron-doped systems. Particularly, it is shown that the crossover from hole-like to electronlike Fermi surface is deeply affected by the d − p hybridization in the hole doping case. It has been verified that the effect of the hybridization is most pronounced around the saddle-points, where the superconducting gap is maximum in the particular case of dx2−y2 -wave symmetry. As a consequence, the critical temperature Tc is directly affected by the hibridization. Moreover, the obtained results suggest that in the hole doped regime, the hybridization may act on the transport properties of the system due to the sign changes of the Hall coefficient when the crossover of the Fermi surface occurs. In the electron doped case, the crossover in the Fermi surface is not verified. Nevertheless, as the hybridization suppresses the density of states near the Fermi level, the superconductivity is affected. The topology of the Fermi surface in the hole and electron doping regime agree with recent experimental ARPES results for La2−xSrxCuO4 (hole doping) and Nd2−xCexCuO4 (electron doping).
The Hubbard model with local on-site repulsion is generally thought to possess a superconducting ground state for appropriate parameters, but the effects of more realistic long-range Coulomb interactions have not been studied extensively. We study the influence of these interactions on superconductivity by including nearest- and next-nearest-neighbor extended Hubbard interactions in addition to the usual on-site terms. Utilizing numerical exact diagonalization, we analyze the signatures of superconductivity in the ground states through the fidelity metric of quantum information theory. We find that nearest and next-nearest neighbor interactions have thresholds above which they destabilize superconductivity regardless of whether they are attractive or repulsive, seemingly due to competing charge fluctuations.