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Register a new userSpin fluctuations influence on quasiparticle spectrum of realistic p-d model
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In the present work the multiband p-d model for $CuO_2$-layer is treated. It was shown that for the realistic set of parameters besides Zhang-Rice two-particle singlet there is non-negligible contribution of two-particle triplet state to the top of the valence band. Also shown, that to gain quantitative agreement with experimental data the minimal approximation should include the spin fluctuations beyond the Hubbard-I scheme. Quasiparticle spectrum, obtained in this approximation, is in fairly good agreement with ARPES data on Bi2212 High-$T_c$ compound.
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Polarized- and unpolarized-neutron scattering measurements of the spin excitation spectrum in the stripe-ordered phase of La2NiO4+d (d = 0.11) are presented. At low energies, the magnetic spectral weight is found to shift anomalously towards the two-dimensional antiferromagnetic wave vector, similar to the low-energy dispersions observed in cuprate superconductors. While spin-wave spectra in stripe phases can exhibit an apparent inward dispersion, we find that the peak shifts measured here cannot be accounted for by this effect. Possible extensions of the model are discussed.
In cuprate superconductors, superconductivity appears below the CDW transition temperature $T_{CDW}$. However, many-body electronic states under the CDW order are still far from understood. Here, we study the development of the spin fluctuations under the presence of $d$-wave bond order (BO) with wavevector $q=(pi/2,0),(0,pi/2)$, which is derived from the paramagnon interference mechanism in recent theoretical studies. Based on the $4 times 1$ and $4 times 4$ cluster Hubbard models, the feedback effects between spin susceptibility and self-energy are calculated self-consistently by using the fluctuation-exchange (FLEX) approximation. It is found that the $d$-wave BO leads to a sizable suppression of the nuclear magnetic relaxation rate $1/T_1$. In contrast, the reduction in $T_c$ is small, since the static susceptibility $chi^s(Q_s)$ is affected by the BO just slightly. It is verified that the $d$-wave BO scenario is consistent with the experimental electronic properties below $T_{CDW}$.
The rich phenomenology of twisted bilayer graphene (TBG) near the magic angle is believed to arise from electron correlations in topological flat bands. An unbiased approach to this problem is highly desirable, but also particularly challenging, given the multiple electron flavors, the topological obstruction to defining tight binding models and the long-ranged Coulomb interactions. While numerical simulations of realistic models have thus far been confined to zero temperature, typically excluding some spin or valley species, analytic progress has relied on fixed point models away from the realistic limit. Here we present for the first time unbiased Monte Carlo simulations of realistic models of magic angle TBG at charge-neutrality. We establish the absence of a sign problem for this model in a momentum space approach, and describe a computationally tractable formulation that applies even on breaking chiral symmetry and including band dispersion. Our results include (i) the emergence of an insulating Kramers inter-valley coherent ground state in competition with a correlated semi-metal phase, (ii) detailed temperature evolution of order parameters and electronic spectral functions which reveal a `pseudogap regime, in which gap features are established at a higher temperature than the onset of order and (iii) predictions for electronic tunneling spectra and their evolution with temperature. Our results pave the way towards uncovering the physics of magic angle graphene through exact simulations of over a hundred electrons across a wide temperature range.
Spin-triplet superconductors are of extensive current interest because they can host topological state and Majorana ferimons important for quantum computation. The uranium based heavyfermion superconductor UTe$_2$ has been argued as a spin-triplet superconductor similar to UGe$_2$, URhGe, and UCoGe, where the superconducting phase is near (or coexists with) a ferromagnetic (FM) instability and spin-triplet electron pairing is driven by FM spin fluctuations. Here we use neutron scattering to show that although UTe$_2$ exhibits no static magnetic order down to 0.3 K, its magnetism is dominated by incommensurate spin fluctuations near antiferromagnetic (AF) ordering wave vector and extends to at least 2.6 meV. We are able to understand the dominant incommensurate spin fluctuations of UTe$_2$ in terms of its electronic structure calculated using a combined density functional and dynamic mean field theory.
Neutron scattering can provide detailed information about the energy and momentum dependence of the magnetic dynamics of materials provided sufficiently large single crystals are available. This requirement has limited the number of rare earth high temperature superconducting materials that have been studied in any detail. However, improvements in crystal growth in recent years has resulted in considerable progress in our understanding of the behaviour of the magnetism of the CuO planes in both the superconducting and normal state. This review will focus primarily on the spin fluctuations in La_{2-x}Sr_{x}CuO_{4} and YBa_{2}Cu_{3}O_{7-x} since these are the two systems for which the most detailed results are available. Although gaps in our understanding remain, there is now a consistent picture of on the spin fluctuation spectra in both systems as well as the changes induced by the superconducting transition. For both La_{2-x}Sr_{x}CuO_{4} and underdoped YBa_{2}Cu_{3}O_{7-x} the normal state response is characterised by incommensurate magnetic fluctuations. The low energy excitations are suppressed by the superconducting transition with a corresponding enhancement in the response at higher energies. For YBa_{2}Cu_{3}O_{7-x} the superconducting state is accompanied by the rapid development of a commensurate resonant response whose energy varies with T_{c}. In underdoped samples this resonance persists above T_{c}.
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