Coexistence of antiferromagnetism and unconventional superconductivity in a quasi-one-dimensional flat-band system:Creutz lattice

We study the coexistence of antiferromagnetism and unconventional superconductivity on the Creutz lattice which shows strictly flat bands in the noninteracting regime.The famous renormalized mean-field theory is used to deal with strong electron-electron repulsive Hubbard interaction in the effective low-energy t-J model,the superfluid weight of the unconventional superconducting state has been calculated via the linear response theory.An unconventional superconducting state with both spin-singlet and staggered spin-triplet pairs emerges beyond a critical antiferromagnetic coupling interaction,while antiferromagnetism accompanies this state.The superconducting state with only spin-singlet pairs is dominant with paramagnetic phase.The A phase is analogous to the pseudogap phase,which shows that electrons go to form pairs but do not cause a supercurrent.We also show the superfluid behavior of the unconventional superconducting state and its critical temperature.It is proven directly that the flat band can effectively raise the critical temperature of superconductivity.It is implementable to simulate and control strongly-correlated electrons'behavior on the Creutz lattice in the ultracold atoms experiment or other artificial structures.Our results may help the understanding of the interplay between unconventional superconductivity and magnetism.

Non-perturbative dynamics of flat-band systems with correlated disorder

We develop a numerical method for the time evolution of Gaussian wave packets on flat-band lattices in the presence of correlated disorder.To achieve this,we introduce a method to generate random on-site energies with prescribed correlations.We verify this method with a one-dimensional(1D)cross-stitch model,and find good agreement with analytical results obtained from the disorder-dressed evolution equations.This allows us to reproduce previous findings,that disorder can mobilize 1D flat-band states which would otherwise remain localized.As explained by the corresponding disorder-dressed evolution equations,such mobilization requires an asymmetric disorder-induced coupling to dispersive bands,a condition that is generically not fulfilled when the flat-band is resonant with the dispersive bands at a Dirac point-like crossing.We exemplify this with the 1D Lieb lattice.While analytical expressions are not available for the two-dimensional(2D)system due to its complexity,we extend the numerical method to the 2D a–T3 model,and find that the initial flat-band wave packet preserves its localization when a=0,regardless of disorder and intersections.However,when a̸=0,the wave packet shifts in real space.We interpret this as a Berry phase controlled,disorder-induced wave-packet mobilization.In addition,we present density functional theory calculations of candidate materials,specifically Hg1−xCdxTe.The flat-band emerges near the G point(α=0)in the Brillouin zone.

Recent progress on fabrication and flat-band physics in 2D transition metal dichalcogenides moiré superlattices

Moiré superlattices are formed when overlaying two materials with a slight mismatch in twist angle or lattice constant. They provide a novel platform for the study of strong electronic correlations and non-trivial band topology, where emergent phenomena such as correlated insulating states, unconventional superconductivity, and quantum anomalous Hall effect are discovered. In this review, we focus on the semiconducting transition metal dichalcogenides(TMDs) based moiré systems that host intriguing flat-band physics. We first review the exfoliation methods of two-dimensional materials and the fabrication technique of their moiré structures. Secondly, we overview the progress of the optically excited moiré excitons, which render the main discovery in the early experiments on TMD moiré systems. We then introduce the formation mechanism of flat bands and their potential in the quantum simulation of the Hubbard model with tunable doping, degeneracies, and correlation strength. Finally, we briefly discuss the challenges and future perspectives of this field.

Temperature and doping dependent flat-band superconductivity on the Lieb-lattice

We consider the superconducting properties of Lieb lattice, which produces a flat-band energy spectrum in the normal state under the strong electron–electron correlation. Firstly, we show the hole-doping dependent superconducting order amplitude with various electron–electron interaction strengths in the zero-temperature limit. Secondly, we obtain the superfluid weight and Berezinskii–Kosterlitz–Thouless(BKT) transition temperature with a lightly doping level. The large ratio between the gap-opening temperature and BKT transition temperature shows similar behavior to the pseudogap state in high-T_(c) superconductors. The BKT transition temperature versus doping level exhibits a dome-like shape in resemblance to the superconducting dome observed in the high-T_(c) superconductors. However, unlike the exponential dependence of T_(c) on the electron–electron interaction strength in the conventional high-T_(c) superconductors, the BKT transition temperature for a flat band system depends linearly on the electron–electron interaction strength. We also show the doping-dependent superconductivity on a lattice with the staggered hoping parameter in the end. Our predictions are amenable to verification in the ultracold atoms experiment and promote the understanding of the anomalous behavior of the superfluid weight in the high-T_(c) superconductors.

Flat-band voltage shift in metal-gate/high-k/Si stacks

In metal-gate/high-k stacks adopted by the 45 nm technology node, the fiat-band voltage （Vfb） shift remains one of the most critical challenges, particularly the flat-band voltage roll-off （Vfb roll-off） phenomenon in p-channel metal- oxide-semiconductor （pMOS） devices with an ultrathin oxide layer. In this paper, recent progress on the investigation of the Vfb shift and the origin of the Vfb roll-off in the metal-gate/high-k pMOS stacks are reviewed. Methods that can alleviate the Vfb shift phenomenon are summarized and the future research trend is described.

A two-dimensional threshold voltage analytical model for metal-gate/high-k/SiO_2 /Si stacked MOSFETs

In this paper the influences of the metal-gate and high-k/SiO 2 /Si stacked structure on the metal-oxide-semiconductor field-effect transistor（MOSFET） are investigated.The flat-band voltage is revised by considering the influences of stacked structure and metal-semiconductor work function fluctuation.The two-dimensional Poisson＇s equation of potential distribution is presented.A threshold voltage analytical model for metal-gate/high-k/SiO 2 /Si stacked MOSFETs is developed by solving these Poisson＇s equations using the boundary conditions.The model is verified by a two-dimensional device simulator,which provides the basic design guidance for metal-gate/high-k/SiO 2 /Si stacked MOSFETs.

Realistic flat-band model based on degenerate p-orbitals in two-dimensional ionic materials

Though several theoretical models have been proposed to design electronic flat-bands, the definite experimental realization in two-dimensional atomic crystal is still lacking. Here we propose a novel and realistic flat-band model based on threefold degenerate p-orbitals in two-dimensional ionic materials. Our theoretical analysis and first-principles calculations show that the proposed flat-band can be realized in 1 T layered materials of alkali-metal chalogenides and metal-carbon group compounds. Some of the former are theoretically predicted to be stable as layered materials(e.g., K2 S), and some of the latter have been experimentally fabricated in previous works(e.g., Gd2 CCl2). More interestingly, the flat-band is partially filled in the heterostructure of a K2 S monolayer and graphene layers. The spin polarized nearly flatband can be realized in the ferromagnetic state of a Gd2 CCl2 monolayer, which has been fabricated in experiments. Our theoretical model together with the material predictions provide a realistic platform for the study of flat-bands and related exotic quantum phases.

Flat-band ferromagnetism of SU(N) Hubbard model on Tasaki lattices

We investigate the para-ferro magnetic transition of the repulsive SU(N) Hubbard model on a type of oneand two-dimensional decorated cubic lattices, referred as Tasaki lattices, which feature massive singleparticle ground state degeneracy. Under certain restrictions for constructing localized many-particle ground states of flat-band ferromagnetism, the quantum model of strongly correlated electrons is mapped to a classical statistical geometric site-percolation problem, where the nontrivial weights of different configurations must be considered. We prove rigorously the existence of para-ferro transition for the SU(N) Hubbard model on one-dimensional Tasaki lattice and determine the critical density by the transfer-matrix method. In two dimensions, we numerically investigate the phase transition of SU(3),SU(4) and SU(10) Hubbard models by Metropolis Monte Carlo simulation. We find that the critical density exceeds that of standard percolation, and increases with spin degrees of freedom, implying that the effective repulsive interaction becomes stronger for larger N. We further rigorously prove the existence of flat-band ferromagnetism of the SUeNT Hubbard model when the number of particles equals to the degeneracy of the lowest band in the single-particle energy spectrum.

High temperature property studies of the 6H-SiC MOS capacitor

N-type and p-type 6H-SiC metal oxide semiconductor (MOS) capacitor samples are fabricated with a typical method,and the high frequency capacitor voltage (C-V) curves of these samples are measured at temperatures ranging from 293 to 533 K.There exists huge difference between the n-type and p-type samples.Flat-band voltage shift of the n-type sample becomes larger with temperature rising,but that of the p-type sample have very little change.This may be caused by the residual Al in the p-type oxide.Both types of the SiC samples follow the same rule of flat-band voltage changing with temperature.But their mechanisms are different as temperature is above 453 K.Of both types the p-type SiC is more suitable for high temperature applications.