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Searching for the dispersionless flat band (FB) in quantum materials, especially in topological systems, becomes an interesting topic. The kagome lattice is an ideal platform for such exploration because the FB can be naturally induced by the underlying destructive interference. Nevertheless, the magnetic kagome system that hosts the FB close to the Fermi level (E F) is exceptionally rare. Here, we study the electronic structure of a kagome magnet LuMn6Sn6 by combining angle-resolved photoemission spectroscopy and density functional theory calculations. The observed Fermi-surface topology and overall band dispersions are similar to previous studies of the X Mn6Sn6 (X = Dy, Tb, Gd, Y) family of compounds. We clearly observe two kagome-derived FBs extending through the entire Brillouin zone, and one of them is located just below E F. The photon-energy-dependent measurements reveal that these FBs are nearly dispersionless along the kz direction as well, supporting the quasi-two-dimensional character of such FBs. Our results complement the X Mn6Sn6 family and demonstrate the robustness of the FB features across this family. [ABSTRACT FROM AUTHOR]

Dispersionless flat bands (FBs) in momentum space, given rise to electron destructive interference in frustrated lattices, offer opportunities to enhance electronic correlations and host exotic many‐body phenomena, such as Wigner crystal, fractional quantum hall state, and superconductivity. Despite successes in theory, great challenges remain in experimentally realizing FBs in frustrated lattices due to thermodynamically structural instability. Here, the observation of electronic FB in a potassium distorted colouring triangle (DCT) lattice is reported, which is supported on a blue phosphorene‐gold network. It is verified that the interaction between potassium and the underlayer dominates and stabilizes the frustrated structures. Two‐dimensional electron gas is modulated by the DCT lattice, and in turn results in a FB dispersion due to destructive quantum interferences. The FB exhibits suppressed bandwidth with high density of states, which is directly observed by scanning tunneling microscopy and confirmed by the first‐principles calculation. This work demonstrates that DCT lattice is a promising platform to study FB physics and explore exotic phenomena of correlation and topological matters. [ABSTRACT FROM AUTHOR]

The heavy fermion (HF) state of d-electron systems is of great current interest since it exhibits various exotic phases and phenomena that are reminiscent of the Kondo effect in f -electron HF systems. Here, we present a combined infrared spectroscopy and first-principles band structure calculation study of the 3d-electron HF compound YFe2Ge2. The infrared response exhibits several charge-dynamical hallmarks of HF and a corresponding scaling behavior that resemble those of the f -electron HF systems. In particular, the low-temperature spectra reveal a dramatic narrowing of the Drude response along with the appearance of a hybridization gap (Δ ~50 meV) and a strongly enhanced quasiparticle effective mass. Moreover, the temperature dependence of the infrared response indicates a crossover around T* ~100K from a coherent state at low temperature to a quasi-incoherent one at high temperature. Despite of these striking similarities, our band structure calculations suggest that the mechanism underlying the HF behavior in YFe2Ge2 is distinct from the Kondo scenario of the f -electron HF compounds and even from that of the d-electron iron-arsenide superconductor KFe2As2. For the latter, the HF state is driven by orbital-selective correlations due to a strong Hund's coupling. Instead, for YFe2Ge2 the HF behavior originates from the band flatness near the Fermi level induced by the combined effects of kinetic frustration from a destructive interference between the direct Fe-Fe and indirect Fe-Ge-Fe hoppings, band hybridization involving Fe 3d and Y 4d electrons, and electron correlations. This highlights that rather different mechanisms can be at the heart of the HF state in d-electron systems. [ABSTRACT FROM AUTHOR]

Flat-band periodic materials are characterized by a linear spectrum containing at least one band where the propagation constant remains nearly constant irrespective of the Bloch momentum across the Brillouin zone. These materials provide a unique platform for investigating phenomena related to light localization. Meantime, the interaction between flat-band physics and nonlinearity in continuous systems remains largely unexplored, particularly in continuous systems where the band flatness deviates slightly from zero, in contrast to simplified discrete systems with exactly flat bands. Here, we use a continuous superhoneycomb lattice featuring a flat band in its spectrum to theoretically and numerically introduce a range of stable flat-band solitons. These solutions encompass fundamental, dipole, multi-peak, and even vortex solitons. Numerical analysis demonstrates that these solitons are stable in a broad range of powers. They do not bifurcate from the flat band and can be analyzed using Wannier function expansion leading to their designation as Wannier solitons. These solitons showcase novel possibilities for light localization and transmission within nonlinear flat-band systems. [ABSTRACT FROM AUTHOR]

Flat bands – single-particle energy bands – in tight-binding lattices, aka networks, have attracted attention due to the presence of macroscopic degeneracies and their sensitivity to perturbations. They support compact localized eigenstates protected by destructive interference. This makes them natural candidates for emerging exotic phases and unconventional orders. In this review we consider the recently proposed systematic ways to construct flat band networks based on symmetries or fine-tuning. We then discuss how the construction methods can be further extended, adapted or exploited in presence of perturbations, both single-particle and many-body. This strategy has lead to the discovery of non-perturbative metal-insulator transitions, fractal phases, nonlinear and quantum caging and many-body nonergodic quantum models. We discuss what implications these results may have for the design of fine-tuned nanophotonic systems including photonic crystals, nanocavities, and metasurfaces. [ABSTRACT FROM AUTHOR]

We study the broadening of initially localized wave packets in a quasi one-dimensional diamond ladder with interacting, spinless fermions. The lattice possesses a flat band causing localization. We place special focus on the transition away from the flat band many-body localized case by adding very weak dispersion. By doing so, we allow propagation of the wave packet on significantly different timescales which causes anomalous diffusion. Due to the temporal separation of dynamic processes, an interaction-induced, prethermal equilibrium becomes apparent. A physical picture of light and heavy modes for this prethermal behavior can be obtained within Born–Oppenheimer approximation via basis transformation of the original Hamiltonian. This reveals a detachment between light, symmetric and heavy, anti-symmetric particle species. We show that the prethermal state is characterized by heavy particles binding together mediated by the light particles. [ABSTRACT FROM AUTHOR]

A unique feature of non-Hermitian systems is the extreme sensitivity of the eigenspectrum to boundary conditions with the emergence of the non-Hermitian skin effect (NHSE). A NHSE originates from the point-gap topology of complex eigenspectrum, where an extensive number of eigen-states are anomalously localized at the boundary driven by nonreciprocal dissipation. Two different approaches to create localization are disorder and flat-band spectrum, and their interplay can lead to the anomalous inverse Anderson localization, where the Bernoulli anti-symmetric disorder induces mobility in a full-flat band system in the presence of Aharonov–Bohm (AB) Cage. In this work, we study the localization–delocalization transitions due to the interplay of the point-gap topology, flat band and correlated disorder in the one-dimensional rhombic lattice, where both its Hermitian and non-Hermitian structures show AB cage in the presence of magnetic flux. Although it remains the coexistence of localization and delocalization for the Hermitian rhombic lattice in the presence of the random anti-symmetric disorder, it surprisingly becomes complete delocalization, accompanied by the emergence of NHSE. To further study the effects from the Bernoulli anti-symmetric disorder, we found the similar NHSE due to the interplay of the point-gap topology, correlated disorder and flat bands. Our anomalous localization–delocalization property can be experimentally tested in the classical physical platform, such as electrical circuit. [ABSTRACT FROM AUTHOR]

Benefited from the lower dimensionality compared to their 3D counterpart, 2D flat‐band systems provide cleaner lattice models, easier experimental verification, and higher tunability, which make the 2D van der Waals (vdW) system an ideal playground for exploring flat‐band physics as well as their potential applications. Given the vast amount of research in the field of flat bands, a simple and efficient approach to search for realistic vdW materials with flat bands is still missing. Here, a two‐tier framework to filter and diagnose high‐quality flat‐band vdW materials by combining high‐throughput first‐principles calculations and the proposed 2D flat‐band score criterion is presented. Based on systematic geometrical analysis, 861 potential monolayer vdW materials are initially obtained amounting to 187,093 structures as stored in the Inorganic Crystal Structure Database. By applying the 2D flat‐band score criterion, 229 flat‐band candidates are efficiently identified, among which a sub‐catalog of 74 materials with flat bands right next to the Fermi level is further provided to facilitate experimental verification. All these efforts to screen experimentally available flat‐band candidates will certainly motivate continuing exploration toward the realization of this class of special materials and their applications in material science. [ABSTRACT FROM AUTHOR]

Twist-induced lattice misalignment of bilayer graphene leads to moiré patterns that generate electronic flat bands with strongly correlated electronic states, but it still requires a sophisticated process to precisely control the twist angle. Here, we propose a different way to generate hexagonal moirés in bilayer graphene by uniaxially stretching the two layers along two distinct armchair directions, respectively. The heterostrain-induced moiré gives rise to flat bands near the Fermi level due to the deformation-induced equivalent misaligned angle between two graphene layers, featuring an electronic equivalence of twisted bilayer graphene. We demonstrate the flat bands at a heterostrain of 2.1%, equivalent to the first magic angle of 1.05°. Yet, a slight shift of Dirac point from K point due to the uniaxial strain splits the flat bands into two van Hove singularity peaks that are separated by 18 meV and located above and below the Fermi level, respectively. Our results suggest a potential way to control the electronic strong correlation in bilayer graphene of natural stacking. [ABSTRACT FROM AUTHOR]

Novel quantum materials have broad application prospects in spintronic devices and low‐power‐consumption devices. The kagome‐type lattice has special corner‐sharing triangle geometry, providing a rich platform to investigate the relationship of novel correlated topological states, geometry, and strong electron interactions. However, the orbital hybridization in the presence of flat band that originated from the electron localization due to the unique kagome lattice configuration is still unclear. Resonant‐angle‐resolved photoemission spectroscopy is utilized to study the valence band structures of a new vanadium‐based kagome material GdV6Sn6, combined with density functional theory calculation. Herein, a significant hybridization between V 3d and Sn 5p electrons, which paves the way to understand the formation of long‐range magnetic ordering of GdV6Sn6 and other kagome materials at low temperature, is unambiguously revealed in the results. [ABSTRACT FROM AUTHOR]

The flat electronic band has remarkable relevance in the strongly correlated phenomena mainly due to its reduced kinetic energy in comparison to the many-body potential energy. The formation of such bands in cubically structured nanowires is addressed in this article by means of a new independent channel method and a generalized convolution theorem developed for the Green's function including the first, second, and third neighbor interactions. A real-space renormalization method is further applied to address macroscopic-length aperiodic nanowires. We also determined the appearance condition of these flat bands, as well as their degeneracy and robustness in the face of perturbations, such as structural dislocations. Finally, the possible experimental detection of this flat band via the electronic specific heat is analyzed. [ABSTRACT FROM AUTHOR]

Copyright of Progress in Physics / Wulixue Jinzhan is the property of Progress in Physics Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Flat band systems are commonly associated with compact localized states (CLSs) that arise from the macroscopic degeneracy of eigenstates at the flat band energy. However, in the case of singular flat bands, conventional localized flat band states are incomplete, leading to the existence of noncontractible loop states (NLSs) with nontrivial real-space topology. In this study, we experimentally and analytically demonstrate the existence of NLSs in a 2D photonic borophene lattice without a CLS counterpart, owing to a band that is flat only along high-symmetry lines and dispersive along others. Our findings challenge the conventional notion that NLSs are necessarily linked to robust boundary modes due to a bulk-boundary correspondence. Protected by the band flatness that originates from band touching, NLSs play a significant role in investigating the fundamental physics of flat band systems. [ABSTRACT FROM AUTHOR]

We investigate a quantum-state transfer protocol through a quasi-1D spin-1/2 channel defined on a hexagonal layer featuring hopping disorder. In the absence of disorder, the quantum channel displays two flat bands hosting compact localized states. That is signalled by the appearance of singularities in the density of states. We show that an arbitrary qubit state can be transmitted with high fidelity between the two ends even in the presence of noise as long as they are not in resonance with energy levels surrounding the flat band. This higher sensitivity against disorder is due to the residual single-cell compact localized states accompanied by a degeneracy lifting. [ABSTRACT FROM AUTHOR]

We investigate theoretically the superfluidity of a one-dimensional boson system whose hopping energy is periodically modulated with a zero time average, which results in the suppression of first-order single-particle hopping processes. The dynamics of this Floquet-engineered flat-band system is entirely driven by correlations and described by exotic Hamiltonian and current operators. We employ exact diagonalization and compare our results with those of the conventional, undriven Bose–Hubbard system. We focus on the two main manifestations of superfluidity, the Hess-Fairbank effect and the metastability of supercurrents, with explicit inclusion of an impurity when relevant. Among the novel superfluid features, we highlight the presence of a cat-like ground state, with branches that have opposite crystal momentum but carry the same flux-dependent current, and the essential role of the interference between the collective components of the ground-state wave function. Calculation of the dynamic form factor reveals the presence of an acoustic mode that guarantees superfluidity in the thermodynamic limit. [ABSTRACT FROM AUTHOR]

We study the electronic structure of double-twisted few-layer graphite (DTFLG), which consists of three few-layer graphites (FLGs), i.e., an ABA-stacked graphene multilayer, stacked with two twist angles. We consider two categories of DTFLG, the alternately and chirally twisted cases, according to the rotation direction of the two twist angles. We show that, once the middle FLG of DTFLG is not thinner than the trilayer, both types of DTFLG can remarkably host two pairs of degenerate Moiré flat bands (MFBs) at EF, twice that of the magic angle twisted bilayer graphene (TBG). The doubled MFBs of DTFLG lead to a doubled density of states (DOS) at the Fermi level EF, which implies much stronger correlation effects than TBG. The degeneracy of MFBs can be lifted by a perpendicular electric field, and the isolated MFBs have a nonzero valley Chern number. We also reveal the peculiar wave function patterns of the MFBs in DTFLG. Our results establish a new family of Moiré systems that have a much higher DOS at EF and thus possibly much stronger correlation effects. [ABSTRACT FROM AUTHOR]

We report that the twisted few layer graphite (tFL-graphite) is a new family of moiré heterostructures (MHSs), which has richer and highly tunable moiré flat 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 moiré band 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. [ABSTRACT FROM AUTHOR]

We investigate the origin of the ubiquitous existence of flat bands in the network superstructures of atomic chains, where one-dimensional (1D) atomic chains array periodically. While there can be many ways to connect those chains, we consider two representative ways of linking them, the dot-type and triangle-type links. Then, we construct a variety of superstructures, such as the square, rectangular, and honeycomb network superstructures with dot-type links and the honeycomb superstructure with triangle-type links. These links provide the wavefunctions with an opportunity to have destructive interference, which stabilizes the compact localized state (CLS). In the network superstructures, there exist multiple flat bands proportional to the number of atoms of each chain, and the corresponding eigenenergies can be found from the stability condition of the compact localized state. Finally, we demonstrate that the finite bandwidth of the nearly flat bands of the network superstructures arising from the next-nearest-neighbor hopping processes can be suppressed by increasing the length of the chains consisting of the superstructures. [ABSTRACT FROM AUTHOR]

Quantum anomalous Hall effect (QAHE) and quantum spin Hall effect (QSHE) are two interesting physical manifestations of 2D materials that have an intrinsic nontrivial band topology. In principle, they are ground-state equilibrium properties characterized by Fermi level lying in a topological gap, below which all the occupied bands are summed to a non-zero topological invariant. Here, we propose theoretical concepts and models of ‘excited’ QAHE (EQAHE) and EQSHE generated by dissociation of an excitonic insulator (EI) state with complete population inversion (CPI), a unique many-body ground state enabled by two yin-yang flat bands (FBs) of opposite chirality hosted in a diatomic Kagome lattice. The two FBs have a trivial gap in between, i.e. the system is a trivial insulator in the single-particle ground-state, but nontrivial gaps above and below, so that upon photoexcitation the quasi-Fermi levels of both electrons and holes will lie in a nontrivial gap achieved by the CPI-EI state, as demonstrated by exact diagonalization calculations. Then dissociation of singlet and triplet EI state will lead to EQAHE and EQSHE, respectively. Realizations of yin-yang FBs in real materials are also discussed. [ABSTRACT FROM AUTHOR]

Motivated by recent research works on the flat-band correlated electron systems, we construct a plaquette-Lieb lattice, which exhibits nearly flat bands close to the Fermi level around half-filling, when the inter-plaquette hopping integral is much smaller than the intra-plaquette one. A detailed comparison study with the well-known checkerboard lattice indicates that there exist significant differences for the pairing property between the two lattices, and the nearly flat bands on the plaquette-Lieb lattice strongly enhance the d -wave pairing correlation for repulsive on-site Hubbard interaction and s -wave pairing correlation for attractive on-site Hubbard interaction. Our study not only provides a new lattice with nearly flat bands, but also deepens the understanding of electronic structure on superconductivity. [ABSTRACT FROM AUTHOR]

This review considers the topological fermion condensation quantum phase transition (FCQPT) that explains the complex behavior of strongly correlated Fermi systems, such as frustrated insulators with quantum spin liquid and heavy fermion metals. The review contrasts theoretical consideration with recent experimental data collected on both heavy fermion metals (HF) and frustrated insulators. Such a method allows to understand experimental data. We also consider experimental data collected on quantum spin liquid in Lu 3 Cu 2 Sb 3 O 14 and quasi-one dimensional (1D) quantum spin liquid in both YbAlO 3 and Cu (C 4 H 4 N 2) (NO 3) 2 with the aim to establish a sound theoretical explanation for the observed scaling laws, Landau Fermi liquid (LFL) and non-Fermi-liquid (NFL) behavior exhibited by these frustrated insulators. The recent experimental data on the heavy-fermion metal α − YbAl 1 − x Fe x B 4 , with x = 0.014 , and on its sister compounds β − YbAlB 4 and YbCo 2 Ge 4 , carried out under the application of magnetic field as a control parameter are analyzed. We show that the thermodynamic and transport properties as well as the empirical scaling laws follow from the fermion condensation theory. We explain how both the similarity and the difference in the thermodynamic and transport properties of α − YbAl 1 − x Fe x B 4 and in its sister compounds β − YbAlB 4 and YbCo 2 Ge 4 emerge, as well as establish connection of these (HF) metals with insulators Lu 3 Cu 2 Sb 3 O 14 , Cu (C 4 H 4 N 2) (NO 3) 2 and YbAlO 3 . We demonstrate that the universal LFL and NFL behavior emerge because the HF compounds and the frustrated insulators are located near the topological FCQPT or are driven by the application of magnetic fields. [ABSTRACT FROM AUTHOR]

In this work, we investigate the bound-state problem in a one-dimensional spin-1 Dirac Hamiltonian with a flat band. It is found that the flat band has significant effects on the bound states. For example, for Dirac delta potential gδ (x), there exists one bound state for both the positive and negative potential strength g. Furthermore, when the potential is weak, the bound-state energy is proportional to the potential strength g. For square well potential, the flat band results in the existence of infinite bound states for arbitrarily weak potential. In addition, when the bound-state energy is very near the flat band, the energy displays a hydrogen atom-like spectrum, i.e. the bound-state energies are inversely proportional to the square of the natural number n (e.g., E n ∝ 1/ n 2, n = 1, 2, 3, …). Most of the above nontrivial behaviors can be attributed to the infinitely large density of states of the flat band and its ensuing 1/ z singularity of the Green function. The combination of a short-ranged potential and flat band provides a new possibility to get an infinite number of bound states and a hydrogen atom-like energy spectrum. In addition, our findings provide some useful insights and further our understanding of the many-body physics of the flat band. [ABSTRACT FROM AUTHOR]

Copyright of Matériaux et Techniques is the property of EDP Sciences and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

We present an extension of the cell-construction method for the flat-band ferromagnetism. In a rather general setting, we construct Hubbard models with highly degenerate single-electron ground states and obtain a formal representation of these single-electron ground states. By our version of the cell-construction method, various types of flat-band Hubbard models, including the one on line graphs, can be designed and shown to have the unique ferromagnetic ground states when the electron number is equal to the degeneracy of the single-electron ground states. [ABSTRACT FROM AUTHOR]

The twistronics, which is arising from the moiré superlattice of the small angle between twisted bilayers of 2D materials like graphene, has attracted much attention in the field of 2D materials and condensed matter physics. The novel physical properties in such systems, like unconventional superconductivity, come from the dispersionless flat band that appears when the twist reaches some magic angles. By tuning the filling of the fourfold degeneracy flat bands, the desired effects are induced due to the strong correlation of the degenerated Bloch electrons. In this article, we review the twistronics in twisted bi- and multi-layer graphene (TBG and TMG), which is formed both by transfer assembly of exfoliated monolayer graphene and epitaxial growth of multilayer graphene on SiC substrates. Starting from a brief history, we then introduce the theory of flat band in TBG. In the following, we focus on the major achievements in this field: (a) van Hove singularities and charge order; (b) superconductivity and Mott insulator in TBG and (c) transport properties in TBG. In the end, we give the perspective of the rising materials system of twistronics, epitaxial multilayer graphene on the SiC. [ABSTRACT FROM AUTHOR]

We investigate the quantum tunneling properties through a square potential barrier in the α - T 3 model with an effective mass term. The additional mass term can be induced by the effective magnetic field or the site energy difference on different sublattices, in which the flat band is located at the center or edge of the band gap band, respectively. The linear dispersion in the α - T 3 model is modified in the presence of the mass term, leading to the destroying of the Klein tunneling for the normal incidence. It is demonstrated that the additional mass term in general suppresses the transmission. When the flat band is located at the band edge, the super-Klein tunneling and the resonant tunneling are considerably suppressed with the increase in the energy gap. For the dice lattice that α = 1 , the super-Klein tunneling is preserved when the flat band is located at the center of the band gap. [ABSTRACT FROM AUTHOR]

Featured Application: The proposed unidirectional backscattering-immune device may provide a potential way to realize robust optical delay lines and robust on-chip optical buffers. In conventional photonic crystal systems, extrinsic scattering resulting from random manufacturing defects or environmental changes is a major source of loss that causes performance degradation, and the backscattering loss is amplified as the group velocity slows down. In order to overcome the limitations in slow light systems, we propose a backscattering-immune slow light waveguide design. The waveguide is based on an interface between a square lattice of magneto-optical photonic crystal with precisely tailored rod radii of the first two rows and a titled 45 degrees square lattice of Alumina photonic crystal with an aligned band gap. High group indices of 77, 68, 64, and 60 with the normalized frequency bandwidths of 0.444%, 0.481%, 0.485%, and 0.491% are obtained, respectively. The corresponding normalized delay-bandwidth products remain around 0.32 for all cases, which are higher than previously reported works based on rod radius adjustment. The robustness for the edge modes against different types of interfacial defects is observed for the lack of backward propagation modes at the same frequencies as the unidirectional edge modes. Furthermore, the transmission direction can be controlled by the sign of the externally applied magnetic field normal to the plane. [ABSTRACT FROM AUTHOR]

A self-consistent theory of Fermi systems hosting flat bands is developed. Compared with an original model of fermion condensation, its key point consists in proper accounting for mixing between condensate and non-condensate degrees of freedom that leads to formation of a non-BCS gap Υ(p) in the single-particle spectrum. The results obtained explain: (1) the two-gap structure of spectra of single-particle excitations of electron systems of copper oxides, revealed in ARPES studies, (2) the role of violation of the topological stability of the Landau state in the arrangement of the T-x phase diagram of this family of high-Tc superconductors, (3) the topological nature of a metal–insulator transition, discovered in homogeneous two-dimensional low-density electron liquid of MOSFETs more than 20 years ago. [ABSTRACT FROM AUTHOR]

When doped with oxygen, the layered Y2O2Bi phase becomes a superconductor. This finding raises questions about the sites for doped oxygen, the mechanism of superconductivity, and practical guidelines for discovering new superconductors. We probed these questions in terms of first-principles calculations for undoped and O-doped Y2O2Bi. The preferred sites for doped O atoms are the centers of Bi4 squares in the Bi square net. Several Bi 6p x/ y bands of Y2O2Bi are raised in energy by oxygen doping because the 2p x/ y orbitals of the doped oxygen make antibonding possible with the 6p x/ y orbitals of surrounding Bi atoms. Consequently, the condition necessary for the 'flat/steep' band model for superconductivity is satisfied in O-doped Y2O2Bi. [ABSTRACT FROM AUTHOR]

Using one-dimensional tight-binding lattices and an analytical expression based on the Green's matrix, we show that anomalous minimum of the localization length near an isolated flat band, previously found for evanescent waves in a defect-free photonic crystal waveguide, is a generic feature and exists in the Anderson regime as well, i.e., in the presence of disorder. Our finding reveals a scaling behavior of the localization length in terms of the disorder strength, as well as a summation rule of the inverse localization length in terms of the density of states in different bands. Most interestingly, the latter indicates the possibility of having two localization minima inside a band gap, if this band gap is formed by two flat bands such as in a double-sided Lieb lattice. [ABSTRACT FROM AUTHOR]

We report on the transport properties of the super-honeycomb lattice, the band structure of which possesses a flat band and Dirac cones, according to the tight-binding approximation. The super-honeycomb model combines the honeycomb lattice and the Lieb lattice and displays the properties of both. It also represents a hybrid fermionic and bosonic system, which is rarely seen in nature. By choosing the phases of input beams properly, the flat-band mode of the super-honeycomb lattice will be excited and the input beams will exhibit strong localization during propagation. On the other hand, if the modes of Dirac cones of the super-honeycomb lattice are excited, one will observe conical diffraction. Furthermore, if the input beam is properly chosen to excite a sublattice of the super-honeycomb lattice and the modes of Dirac cones with different pseudospins, e.g., by the three-beam interference pattern, the pseudospin-mediated vortices will be observed. [ABSTRACT FROM AUTHOR]

The band structures of single and bilayer graphene with one-dimensional topological defects were calculated along the defect line, and appearance of the flat band near the Fermi level was observed. In addition, the influence of deformation (compression/expansion) on the flat band was studied. It was shown that compression across the grain boundary leads to disappearance of the flat band near the Fermi level, while the stretching along this direction does not significantly change the band structure. However, neither compression nor stretching along the grain boundary destroys the flat band. [ABSTRACT FROM AUTHOR]

In a semi-classical regime, we study a periodic magnetic Schrödinger operator in ℝ2. This is inspired by recent experiments on artificial magnetism with ultra cold atoms in optical lattices, and by the new interest for the operator on the hexagonal lattice describing the behavior of an electron in a graphene sheet. We first review some results for the square (Harper), triangular and hexagonal lattices. Then, we study the case when the periodicity is given by the kagome lattice considered by Hou. Following the techniques introduced by Helffer-Sjöstrand and Carlsson, we reduce this problem to the study of a discrete operator on ℓ2(ℤ2;ℂ3) and a pseudo-differential operator on L2(ℝ;ℂ3), which keep the symmetries of the kagome lattice. We estimate the coefficients of these operators in the case of a weak constant magnetic field. Plotting the spectrum for rational values of the magnetic flux divided by 2πh where h is the semi-classical parameter, we obtain a picture similar to Hofstadter's butterfly. We study the properties of this picture and prove the symmetries of the spectrum and the existence of flat bands, which do not occur in the case of the three previous models. [ABSTRACT FROM AUTHOR]

Topological media are systems whose properties are protected by topology, and thus are robust to deformations of the system. In topological insulators and superconductors, the bulk-surface and bulk-vortex correspondence gives rise to the gapless Weyl, Dirac, or Majorana fermions on the surface of the system and inside vortex cores. In gapless topological media, the bulk-surface and bulk-vortex correspondence produce topologically protected gapless fermions without dispersion-the flat band. Fermion zero modes forming the flat band are localized on the surface of topological media with protected nodal lines and in the vortex core in systems with topologically protected Fermi points (Weyl points). Flat band has an extremely singular density of states, and this property may give rise in particular to surface superconductivity, which in principle could exist even at room temperature. [ABSTRACT FROM AUTHOR]

We propose a series of carbon nanostructures in the shape of tetrapod as a kind of three-dimensional junction for carbon nanotubes. The tetrapod junctions are such open networks that are made of sp2 carbon atoms only, have negative Gaussian curvature, and connect four nanotubes together. We define the structure of standard tetrapod junctions, the simplest one, that have 12 heptagons other than hexagons and have the Td symmetry.Our tight-binding energy-band calculations for the standard tetrapod junctions of smaller sizes show that their electronic property mainly depends on one particular topological factor: the junctions having a carbon atom in the center of each triangular face of tetrahedron exhibit metallic band structure while the junctions having a benzene ring in the center of the faces are semiconductors. We also find that tetrapod junctions connecting (6,0) nanotubes exhibit a flat band near the Fermi energy in a particular momentum region. The origin of the flat band states can be figured out from the wavefunction distribution. We also show the possibility to extend the standard tetrapod junctions to some non-standard ones that can connect nanotubes of different kinds and/or radii. [ABSTRACT FROM AUTHOR]

In this paper, an asymmetric photonic crystal (PC) waveguide is proposed for slow light transmission. A row of air holes is removed to form a line-defect waveguide, and the lateral symmetry of the waveguide is broken by shifting the holes in the PC cladding on one side along the waveguide axis. Two structural parameters are carefully adjusted: the amount of shift compared with the array of holes in the cladding on the other side, and the radius of the holes closest to the waveguide core in the shifted PC cladding. In the asymmetric waveguide, it is possible to obtain flat band modes with low group velocity (c/50) and low dispersion (on the order of 104 ps²/km) over a signal bandwidth of 40GHz. The delay-bandwidth product (DBP) of the proposed slow-light device is analyzed and compared with the DBP of the PC waveguides reported in literatures. We find that our structure yields a significant increase in DBP, and improves the effective bandwidth in which we can obtain slow modes with both low group velocity and vanishing dispersion. [ABSTRACT FROM AUTHOR]

The three dimensional (3D) electrostatics of carbon nanotube field-effect transistors (CNTFETs) is studied by solving the Poisson equation self consistently with equilibrium carrier statistics of CNTFETs. The 3D Poisson equation is solved using the method of moments. We examine how the 3D environment affects the electrostatics of a 30 nm intrinsic CNT under equilibrium conditions. We show that for a CNTFET with a planar gate, the scaling length (the distance by which the source and drain fields penetrate into the channel) is mostly determined by the gate oxide thickness. The contact geometry can also play an important role on the scaling length. A smaller contact results in shorter scaling length and better gate control. We finally show that the top gated geometry offers obvious advantage over the bottom gated geometry in terms of gate electrostatic control. [ABSTRACT FROM AUTHOR]

We theoretically evaluate the electronic band structures in carbon nanotubes with nanoscale periodic pores with a tight-binding approximation of π electrons, and demonstrate that band gaps of the carbon nanotubes with nanoscale periodic pores differ significantly from those of conventional carbon nanotubes. The band gaps of the carbon nanotubes with nanoscale periodic pores depend strongly on the size of pores and inter-pore distances. In some carbon nanotubes with nanoscale periodic pores, band gaps are constant as a function of their circumferences. In other ones, band gaps have the exact periodicity of three as a function of their circumferences. Those behaviors can be explained by taking properties of nanoscale periodic porous graphite into consideration. In some carbon nanotubes with relatively large nanoscale periodic pores, flat bands appear, which may cause singular properties about magnetism in one-dimensional porous carbon nanotubes. [ABSTRACT FROM AUTHOR]

We propose an asymmetric photonic crystal (PC) waveguide, formed by moving the PC cladding on one side of the line defect along the waveguide core. By adjusting the structural parameters, we flatten the slow-mode dispersion curve, exclude the extreme points from the flat band, and avoid the unacceptably large dispersion induced by extreme points. We obtain an ultra-flat band with an inflection point corresponding to a slow-light mode with reduced distortion. We utilize the finite-difference time-domain method to verify the inflection-point ultraslow wave. [ABSTRACT FROM PUBLISHER]

In order to study the exceptional point (EP) properties in the photonic crystal waveguide, we construct the two-dimensional photonic crystal waveguide with the parity–time symmetry configuration. With the calculation of the eigen frequencies, the evolution of the waveguide mode bands discloses two EP-induced flat bands. The transmittance and reflectance at the two flat bands show a huge amplification property and the structure become a waveguide laser. The waveguide lasing is due to the common properties of the EP and the flat band. The results have been demonstrated by the transmission spectra, Poynting vector distribution and simulations of field propagation. [ABSTRACT FROM AUTHOR]

A Kene–Mele-type nearest-neighbor tight-binding model on a pyrochlore lattice is known to be a topological insulator in some parameter region. It is an important task to realize a topological insulator in a real compound, especially in an oxide that is stable in air. In this paper we systematically performed band structure calculations for six pyrochlore oxides A2B2O7 (A = Sn, Pb, Tl; B = Nb, Ta), which are properly described by this model, and found that heavily hole-doped Sn2Nb2O7 is a good candidate. Surprisingly, an effective spin–orbit coupling constant λ changes its sign depending on the composition of the material. Furthermore, we calculated the band structure of three virtual pyrochlore oxides, namely In2Nb2O7, In2Ta2O7 and Sn2Zr2O7. We found that Sn2Zr2O7 has a band gap at the k = 0 (Γ) point, similar to Sn2Nb2O7, though the band structure of Sn2Zr2O7 itself differs from the ideal nearest-neighbor tight-binding model. We propose that the co-doped system (In,Sn)2(Nb,Zr)2O7 may become a candidate of the three-dimensional strong topological insulator. [ABSTRACT FROM AUTHOR]

Kagome lattice is expected to be a perfect platform for hosting exotic emergent phenomena such as negative magnetism and unconventional superconductivity. However, the preparation of such a lattice on a scale of 100 nm or more, with a perfect crystalline order, especially in organic systems, is challenging. Herein, a facile method is presented to fabricate a large scale of epitaxial single‐layer organic Kagome lattice based on (BETS)2GaCl4 on Ag(111) (where BETS is bis(ethylenedithio)tetraselenafulvalene). The lattice accommodates guest molecules with size up to 1 nm, thus making it possible to host a variety of single molecular magnets. This possibility is demonstrated by the in situ selective encapsulation of GaCl4 radicals in the pores. Scanning tunneling microscopy and spectroscopy reveal that both the radical states and the trimer units exhibit sharp and spatially localized peaks at 1.08 and 1.42 eV, respectively. The persistence of these sharp features together with spatial localization, regardless of biasing conditions, suggests an evidence of a frustrated flat band with a possible tuning to incorporate topological order. [ABSTRACT FROM AUTHOR]

Using a first-principles electronic band calculation, we obtained a quasi flat-band near the Fermi level for the six pyrochlore oxides, A2B2O7. These quasi flat-bands are mostly characterized by the s-orbitals of the A-site. The band structures of these oxides are well described by the non-interacting Mielke model. Spin-polarized calculations showed that the ground state of these compounds was ferromagnetic after appropriate carrier doping, despite the absence of the magnetic element. [ABSTRACT FROM AUTHOR]

Quantum mechanics states that hopping integral between local orbitals makes the energy band dispersive. However, in some special cases, there are bands with no dispersion due to quantum interference. These bands are called as flat band. Many models having flat band have been proposed, and many interesting physical properties are predicted. However, no real compound having flat band has been found yet despite the 25 years of vigorous researches. We have found that some pyrochlore oxides have quasi-flat band just below the Fermi level by first principles calculation. Moreover, their valence bands are well described by a tight-binding model of pyrochlore lattice with isotropic nearest neighbor hopping integral. This model belongs to a class of Mielke model, whose ground state is known to be ferromagnetic with appropriate carrier doping and on-site repulsive Coulomb interaction. We have also performed a spin-polarized band calculation for the hole-doped system from first principles and found that the ground state is ferromagnetic for some doping region. Interestingly, these compounds do not include magnetic element, such as transition metal and rare-earth elements. [ABSTRACT FROM AUTHOR]