Observation of flat-band localized state in a one-dimensional diamond momentum lattice of ultracold atoms

We investigated the one-dimensional diamond ladder in the momentum lattice platform. By inducing multiple twoand four-photon Bragg scatterings among specific momentum states, we achieved a flat band system based on the diamond model, precisely controlling the coupling strength and phase between individual lattice sites. Utilizing two lattice sites couplings, we generated a compact localized state associated with the flat band, which remained localized throughout the entire time evolution. We successfully realized the continuous shift of flat bands by adjusting the corresponding nearest neighbor hopping strength, enabling us to observe the complete localization process. This opens avenues for further exploration of more complex properties within flat-band systems, including investigating the robustness of flat-band localized states in disordered flat-band systems and exploring many-body localization in interacting flat-band systems.

Layered kagome compound Na_(2)Ni_(3)S_(4)with topological flat band

We report structural and electronic properties of Na_(2)Ni_(3)S_(4),a quasi-two-dimensional compound composed of alternating layers of[Ni_(3)S_(4)]^(2-)and Na^(+).The compound features a remarkable Ni-based kagome lattice with a square planar configuration of four surrounding S atoms for each Ni atom.Magnetization and electrical measurements reveal a weak paramagnetic insulator with a gap of about 0.5 eV.Our band structure calculation highlights a set of topological flat bands of the kagome lattice derived from the rotated dxz-orbital with C_(3)+T symmetry in the presence of crystal-field splitting.

基于色散调控氟碲酸盐玻璃光纤的O-U波段平坦光频梳产生

光学频率梳(简称光频梳)作为一种优秀的多波长光源在通信领域具有巨大的应用潜力。通过将光频梳光源与波分复用技术(WDM)、空分复用技术结合(SDM),通信系统可以具有百Tbit/s量级的传输速率,在5G/6G通信、物联网、自动驾驶等方面具有重要的应用价值。目前实现光频梳的方法主要有以下四种,分别为基于锁模激光器产生飞秒光频梳、基于光学微腔产生光频梳、基于电光调制器产生光频梳以及基于行波四波混频产生光频梳。它们各具特点,但均很难同时实现宽光谱、高信噪比、高平坦度、高的单梳齿功率以及梳齿频率间隔大范围可调,这在一定程度上影响了光频梳在光通信领域的应用。本文提出基于受激布里渊激光腔产生种子梳,采用腔外负色散光纤进行脉冲压缩,进一步利用色散调控的高非线性氟碲酸盐光纤进行扩谱,从而实现光频梳产生。数值计算结果表明,通过该系统,我们可以得到重复频率大范围可调、光谱覆盖整个O-U波段且在O-U波段梳齿强度标准差小于5 dB的平坦光频梳,证明了通过基于布里渊激光腔与色散调控高非线性氟碲酸盐光纤的光学系统产生可用于光通信的平坦光频梳的可行性。

Machine learning of theΓ-point gap and flat bands of twisted bilayer graphene at arbitrary angles

The novel electronic properties of bilayer graphene can be fine-tuned via twisting,which may induce flat bands around the Fermi level with nontrivial topology.In general,the band structure of such twisted bilayer graphene(TBG)can be theoretically obtained by using first-principles calculations,tight-binding method,or continuum model,which are either computationally demanding or parameters dependent.In this work,by using the sure independence screening sparsifying operator method,we propose a physically interpretable three-dimensional(3D)descriptor which can be utilized to readily obtain theΓ-point gap of TBG at arbitrary twist angles and different interlayer spacings.The strong predictive power of the descriptor is demonstrated by a high Pearson coefficient of 99%for both the training and testing data.To go further,we adopt the neural network algorithm to accurately probe the flat bands of TBG at various twist angles,which can accelerate the study of strong correlation physics associated with such a fundamental characteristic,especially for those systems with a larger number of atoms in the unit cell.

Improvement of memory characteristics by employing a charge trapping layer with combining bent and flat energy bands

Designed ZrxSi1-xO2 films with combining bent and flat energy bands are employed as a charge trapping layer for memory capacitors.Compared to a single bent energy band,the bandgap structure with combining bent and flat energy bands exhibits larger memory window,faster program/erase speed,lower charge loss even at 200℃ for 104s,and wider temperature insensitive regions.The tunneling thickness together with electron recaptured efficiency in the trapping layer,and the balance of two competing electron loss mechanisms in the bent and flat energy band regions collectively contribute to the improved memory characteristics.Therefore,the proposed ZrxSi1-xO2 with combining bent and flat energy bands should be a promising candidate for future nonvolatile memory applications,taking into consideration of the trade-off between the operation speed and retention characteristics.

Surface Majorana flat bands in j=3/2 superconductors with singlet-quintet mixing

Recent experiments[Science Advances 4 eaao4513(2018)]have revealed the evidence of nodal-line superconductivity in half-Heusler superconductors,e.g.,YPt Bi.Theories have suggested the topological nature of such nodal-line superconductivity and proposed the existence of surface Majorana flat bands on the(111)surface of half-Heusler superconductors.Due to the divergent density of states of the surface Majorana flat bands,the surface order parameter and the surface impurity play essential roles in determining the surface properties.We study the effect of the surface order parameter and the surface impurity on the surface Majorana flat bands of half-Heusler superconductors based on the Luttinger model.To be specific,we consider the topological nodal-line superconducting phase induced by the singlet-quintet pairing mixing,classify all the possible translationally invariant order parameters for the surface states according to irreducible representations of C3vpoint group,and demonstrate that any energetically favorable order parameter needs to break the time-reversal symmetry.We further discuss the energy splitting in the energy spectrum of surface Majorana flat bands induced by different order parameters and non-magnetic or magnetic impurities.We propose that the splitting in the energy spectrum can serve as the fingerprint of the pairing symmetry and mean-field order parameters.Our theoretical prediction can be examined in the future scanning tunneling microscopy experiments.

Tunable bandgaps and flat bands in twisted bilayer biphenylene carbon

Owing to the interaction between the layers,the twisted bilayer two-dimensional(2 D)materials exhibit numerous unique optical and electronic properties different from the monolayer counterpart,and have attracted tremendous interests in current physical research community.By means of first-principles and tight-binding model calculations,the electronic properties of twisted bilayer biphenylene carbon(BPC)are systematically investigated in this paper.The results indicate that the effect of twist will not only leads to a phase transition from semiconductor to metal,but also an adjustable band gap in BPC(0 me V to 120 me V depending on the twist angle).Moreover,unlike the twisted bilayer graphene(TBG),the flat bands in twisted BPC are no longer restricted by"magic angles",i.e.,abnormal flat bands could be appeared as well at several specific large angles in addition to the small angles.The charge density of these flat bands possesses different local modes,indicating that they might be derived from different stacked modes and host different properties.The exotic physical properties presented in this work foreshow twisted BPC a promising material for the application of terahertz and infrared photodetectors and the exploration of strong correlation.

Wave function collapses and 1/n energy spectrum induced by a Coulomb potential in a one-dimensional flat band system

We investigate the bound state problem in a one-dimensional flat band system with a Coulomb potential.It is found that,in the presence of a Coulomb potential of type I(with three equal diagonal elements),similarly to that in the twodimensional case,the flat band could not survive.At the same time,the flat band states are transformed into localized states with a logarithmic singularity near the center position.In addition,the wave function near the origin would collapse for an arbitrarily weak Coulomb potential.Due to the wave function collapses,the eigen-energies for a shifted Coulomb potential depend sensitively on the cut-off parameter.For a Coulomb potential of type II,there exist infinite bound states that are generated from the flat band.Furthermore,when the bound state energy is very near the flat band,the energy is inversely proportional to the natural number,e.g.,E_(n)∝1/n,n=1,2,3,...It is expected that the 1/n energy spectrum could be observed experimentally in the near future.

Bilayer twisting as a mean to isolate connected flat bands in a kagome lattice through Wigner crystallization

The physics of flat band is novel and rich but difficult to access.In this regard,recently twisting of bilayer van der Waals(vd W)-bounded two-dimensional(2 D)materials has attracted much attention,because the reduction of Brillouin zone will eventually lead to a diminishing kinetic energy.Alternatively,one may start with a 2 D kagome lattice,which already possesses flat bands at the Fermi level,but unfortunately these bands connect quadratically to other(dispersive)bands,leading to undesirable effects.Here,we propose,by first-principles calculation and tight-binding modeling,that the same bilayer twisting approach can be used to isolate the kagome flat bands.As the starting kinetic energy is already vanishingly small,the interlayer vd W potential is always sufficiently large irrespective of the twisting angle.As such the electronic states in the(connected)flat bands become unstable against a spontaneous Wigner crystallization,which is expected to have interesting interplays with other flat-band phenomena such as novel superconductivity and anomalous quantum Hall effect.

The electronic structure of Nb＿3Al/Nb＿3Sn, a new test case for flat/steep band model of superconductivity

In this work, we choose Nb3Al/Nb3Sn as a new test case for flat/steep band model of superconductivity. Based on the density functional theory in the generalized gradient approximation, the electronic structure of Nb3Al/ Nb3Sn has been studied. The obtained results agree well with those of the earlier studies and show clearly fiat bands around the Fermi level. The steep bands as characterized in this work locate around the M point in the first Brillouin zone. The obtained results reveal that Nb3Al/Nb3Sn fits more to the ＂Flat/steep＂ band model than to the van-Hove singularity scenario. The fiat/steep band condition for superconductivity implies a different thermodynamic behavior of superconductors other than that predicted from the conventional BCS theory. This observation sets up an indicator for selecting a suitable superconductor when its large-scale industrial use is needed, for example, in superconducting maglev system or ITER project.

Itinerant Topological Magnons in SU (2) Symmetric Topological Hubbard Models with Nearly Flat Electronic Bands

We show that a suitable combination of flat-band ferromagnetism,geometry and nontrivial electronic band topology can give rise to itinerant topological magnons.An SU(2) symmetric topological Hubbard model with nearly flat electronic bands,on a Kagome lattice,is considered as the prototype.This model exhibits ferromagnetic order when the lowest electronic band is half-filled.Using the numerical exact diagonalization method with a projection onto this nearly flat band,we can obtain the magnonic spectra.In the flat-band limit,the spectra exhibit distinct dispersions with Dirac points,similar to those of free electrons with isotropic hoppings,or a local spin magnet with pure ferromagnetic Heisenberg exchanges on the same geometry.Significantly,the non-flatness of the electronic band may induce a topological gap at the Dirac points,leading to a magnonic band with a nonzero Chern number.More intriguingly,this magnonic Chern number changes its sign when the topological index of the electronic band is reversed,suggesting that the nontrivial topology of the magnonic band is related to its underlying electronic band.Our work suggests interesting directions for the further exploration of,and searches for,itinerant topological magnons.

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.

Superconductivity in kagome metal ThRu_(3)Si_(2)

We report the physical properties of ThRu_(3)Si_(2)featured with distorted Ru kagome lattice.The combined experiments of resistivity,magnetization and specific heat reveal bulk superconductivity with T_(c)=3.8 K.The specific heat jump and calculated electron–phonon coupling indicate a moderate coupled BCS superconductor.In comparison with LaRu_(3)Si_(2),the calculated electronic structure in ThRu_(3)Si_(2)shows an electron-doping effect with electron filling lifted from 100 meV below flat bands to 300 meV above it.This explains the lower superconducting transition temperature and weaker electron correlations observed in ThRu_(3)Si_(2).Our work suggests the Tc and electronic correlations in the kagome superconductor could have an intimate connection with the flat bands.

L-band增益平坦EDFA的研究

使用数值计算方法对L-band EDFA小信号增益特性和L-band EDFA中各种光束随EDF长度的变化进行了模拟,得到了L-band EDFA增益具有本征平坦特性的结论,并为L-band EDFA低泵浦转换效率提供了合理解释。在数值模拟的基础上,进行了增益平坦L-band EDFA的实验,获得L-band30nm波长范围内增益变化不超过0.5dB平坦增益的实验结果,并借助FBG有效地提高了泵浦转换效率。

超晶格结构在二维转角异质结中产生的新奇物态

二维量子材料由于其展现出的许多新奇特质而受到了普遍关注。当二维量子材料组成同质结或异质结时,晶格常数和层间旋转角度会使体系产生新的超晶格结构,从而导致一系列电子能带结构和物理性质的变化。文章首先围绕着超晶格结构所带来的晶格空间结构以及能带结构的变化展开讨论,主要介绍了由此产生的超晶格狄拉克点、莫尔激子和原子重构等新奇物性。除此之外,超晶格结构会使电子的费米速度在一组特定的旋转角度下消失,产生平带。这会导致该区域的物理性质由电子之间的相互作用能主导,从而产生显著地改变。这些由电子之间强关联现象所引起的新奇物性,成为了近些年凝聚态物理等研究领域的前沿课题之一。在这里,文章主要围绕着关联绝缘态、超导、电子晶体和轨道磁性等介绍了与电子之间强关联现象有关的内容,并在最后对超晶格结构未来的发展方向进行了展望。

增益光纤类型与L-band EDFA的性能关系

为分析增益光纤对L波段(L-band)掺铒光纤放大器(EDFA)增益特性的影响,基于G iles模型对不同增益光纤构成的EDFA进行了数值模拟和实验研究,模拟和实验表明,L-band EDFA增益值的大小与EDF掺杂浓度、泵浦功率及光纤长度有关,光纤长度越长,则达到平坦增益时需要的泵浦功率就越大,本征平坦增益值也越高。

Moiréflat bands of twisted few-layer graphite

We report that the twisted few layer graphite(tFL-graphite)is a new family of moiréheterostructures(MHSs),which has richer and highly tunable moiréflat band structures entirely distinct from all the known MHSs.A tFL-graphite is composed of two few-layer graphite(Bernal stacked multilayer graphene),which are stacked on each other with a small twisted angle.The moiréband structure of the tFL-graphite strongly depends on the layer number of its composed two van der Waals layers.Near the magic angle,a tFL-graphite always has two nearly flat bands coexisting with a few pairs of narrowed dispersive(parabolic or linear)bands at the Fermi level,thus,enhances the DOS at EF.This coexistence property may also enhance the possible superconductivity as been demonstrated in other multiband superconductivity systems.Therefore,we expect strong multiband correlation effects in tFL-graphite.Meanwhile,a proper perpendicular electric field can induce several isolated nearly flat bands with nonzero valley Chern number in some simple tFL-graphites,indicating that tFL-graphite is also a novel topological flat band system.

具有Kagome结构磁振子晶体中平带及带隙的研究

提出了一种具有Kagome结构的磁振子晶体模型,由铁(Fe)圆柱排列在氧化铕(EuO)基底中构成.利用平面波展开法,对磁振子晶体带结构做数值计算.研究发现,体积填充率一定时磁振子晶体有平带、狄拉克点和带隙出现,并且随着体积填充率的不断变化,平带的位置和数目以及带隙的位置和宽度均发生了变化.人工磁振子晶体中平带的发现完善了凝聚态物理学拓扑的研究内容,也可为局域性滤波器件的制作提供思路和理论基础.

转角双层石墨烯中的势杂质效应

近年来,平带在转角双层石墨烯(TBG)中的发现引起了越来越多的关注。在这项工作中,我们报告了我们对这个平带系统中杂质效应的研究,这对于真实材料来说是一个重要问题。通过采用Lanczos递归方法,我们求解了杂质附近的局域态密度(LDOS)。我们发现,对于大尺寸的杂质,一系列束缚态在杂质内部形成,LDOS中平带所对应的峰在杂质边界附近被压制,而在杂质内部由于杂质势的作用而发生平移。随着杂质尺寸变小,其对平带的影响变弱,这符合其大尺寸Wannier函数的预期。这个特性与通常具有小尺寸局域Wannier轨道的平带系统有所不同,表明TBG中的平带对小尺寸杂质更加稳定。

大尺寸非规则碲锌镉晶片双面抛光技术

碲锌镉(CdZnTe)晶体性能优越,是高性能碲镉汞(HgCdTe)外延薄膜的首选衬底材料。双面抛光是一种加工质量较高的碲锌镉晶片表面抛光方式,其具有效率高、平整度好、晶片应力堆积少的优点。但当碲锌镉晶片尺寸增大后,其加工难度也随之上升,易出现碎片多、加工速率慢、表面平整度差等问题。本文开展了大尺寸非规则碲锌镉晶片双面抛光技术研究,深入分析了面积大于50 cm^(2)的非规则碲锌镉晶片双面抛光工艺中,不同参数对抛光质量的影响,通过模拟并优化晶片运动轨迹,优化抛光液磨粒粒型、抛光压力、抛光液流量等抛光工艺参数,实现了具有较高抛光速率和较好表面质量的大尺寸非规则碲锌镉晶片双面抛光加工,对进一步深入研究双面抛光技术有着重要意义。