Flat Conduction Band Research Articles

Chemical and Electronic Structure of the i‐ZnO/InxSy:Na Front Contact Interface in Cu(In,Ga)(S,Se)2 Thin‐Film Solar Cells

AbstractThe chemical and electronic structure of the front contact i‐ZnO/InxSy:Na interface for Cu(In,Ga)(S,Se)2‐based thin‐film solar cells is investigated using a combination of x‐ray and electron spectroscopies. Upon i‐ZnO sputter deposition on the InxSy:Na buffer layer, we find an intermixed heterojunction and the formation of InOx and Na2SO4. The window layer is shown to consist of a mixture of Zn(OH)2 and ZnO, with decreasing relative Zn(OH)2 content for thicker window layers. Moreover, we observe diffusion of sodium to the surface of the window layer. We derive electronic surface band gaps of the i‐ZnO and InxSy:Na layers of 3.86 ± 0.18 eV and 2.60 ± 0.18 eV, respectively, and find a largely flat conduction band alignment at the i‐ZnO/InxSy:Na interface.

Study of structural, elastic, electronic, optical, magnetic and thermoelectric characteristics of Hexafluoromanganets A2MnF6 (A = Cs, Rb, K) cubic double perovskites

We analyse a detailed investigation of structure, electronic, optical, magnetic and thermoelectric properties of (Cs, Rb, K)2MnF6 double perovskites with cubic Fm3¯m space group. The calculation method was the augmented plane-wave functions plus local orbitals as implemented in the WIEN2k code and the GGA followed by the most accurate GGA-mBJ as exchange potentials. The precision of our K2MnF6, Rb2MnF6 and Cs2MnF6 lattice constant compared with their available experimental data is in the range 1.2 % to 3.6 %. The calculated band gap of K2MnF6, Rb2MnF6 and Cs2MnF6 materials advantages them for use in light-emitting diode technology. These materials exhibit ferromagnetic behavior. The negative formation energy, free Helmholtz energy and the dispersion of phonons confirm their thermal, thermodynamic and dynamic stability. The calculated band gap of K2MnF6, Rb2MnF6 and Cs2MnF6 materials advantages them for use in light-emitting diode technology. The p-type charge carriers, direct band gap, and flat conduction and valence bands make them as good thermoelectric materials. The major contribution to the magnetization comes from the unfilled Mn-3d orbital. The high static dielectric constant reduces the recombination rate of charge carriers and the presence of absorption peaks in the ultraviolet region are advantageous in the exploitation in the optoelectronic field. The flat valence and conduction bands, high p-type conductivity, good thermoelectric parameters, as well as non-toxicity make these compounds mainly attractive in the thermoelectric application.

Giant Correlated Gap and Possible Room-Temperature Correlated States in Twisted Bilayer MoS_{2}.

Moiré superlattices have emerged as an exciting condensed-matter quantum simulator for exploring the exotic physics of strong electronic correlations. Notable progress has been witnessed, but such correlated states are achievable usually at low temperatures. Here, we report evidence of possible room-temperature correlated electronic states and layer-hybridized SU(4) model simulator in AB-stacked MoS_{2} homobilayer moiré superlattices. Correlated insulating states at moiré band filling factors v=1, 2, 3 are unambiguously established in twisted bilayer MoS_{2}. Remarkably, the correlated electronic state at v=1 shows a giant correlated gap of ∼126 meV and may persist up to a record-high critical temperature over 285K. The realization of a possible room-temperature correlated state with a large correlated gap in twisted bilayer MoS_{2} can be understood as the cooperation effects of the stacking-specific atomic reconstruction and the resonantly enhanced interlayer hybridization, which largely amplify the moiré superlattice effects on electronic correlations. Furthermore, extreme large nonlinear Hall responses up to room temperature are uncovered near correlated electronic states, demonstrating the quantum geometry of moiré flat conduction band.

Crystal structure, mechanical, electronic, optical and thermoelectric characteristics of Cs2MCl6 (M = Se, Sn, Te and Ti) cubic double perovskites

The crystal structure, mechanical, electronic, optical and thermoelectric characteristics of Cs2MCl6 (M = Se, Sn, Te and Ti) cubic double perovskites are studied within GGA, GGA-mBJ and EV-GGA functionals. The M − Cl bond lengths are shorter and especially in Cs2TiCl6 double perovskite, which reflects the strong interaction between M and Cl atoms and this is correlated with its better chemical stability. The negativity of formation energy and Helmholtz free energy and no imaginary phonon modes throughout the Brillouin zone confirm the thermal, thermodynamic and dynamical stability of these double perovskites. Semiconductors Cs2MCl6 (M = Se, Sn, Te and Ti) double perovskites with flat conduction and valence bands, and an indirect band gap are p-type carriers. A high Seebeck coefficient, adequate ZT values ​​and non-toxicity make these compounds attractive for thermoelectric applications at high temperature and spintronic technology. The empty first conduction band corresponds to their band gap, and the transition occurs from Cl-p to (Se-p, Sn-p, Te-p and Ti-d). The high static dielectric constant and the intense peak of the real part in the ultraviolet energy range favor less the recombination rate of charge carriers and their use in optoelectronic devices. The indirect band gap, high absorption in ultraviolet energy, high static refractive index make these cubic double perovskites as ideal materials for solar cell applications.

Polarization-matching and carrier confinement in III-nitride deep-ultraviolet light-emitting diodes

The polarization-induced quantum confined Stark effect has been recognized as a significant factor contributing to the Internal Quantum Efficiency (IQE) droop in light-emitting diodes (LEDs). This study focuses on the design of LEDs by investigating the InAlN/AlGaN interface. By incorporating InAlN quantum wells, a polarization-matched (PM) multi-quantum well (MQW) LED architecture was developed. While the flat conduction and valence bands on PM MQWs indicate an improved recombination rate, it is crucial to examine the impact on IQE, considering carrier confinement and injection efficiency influenced by the band offsets. This paper presents a numerical analysis comparing two LEDs emitting at 245 and 275 nm, respectively. The results demonstrate that the PM LED operating at 275 nm exhibits enhanced performance, benefiting from high probability density overlap. Conversely, the PM LED emitting at 245 nm demonstrates poor confinement, resulting in an overall low performance, regardless of polarization matching.

First principles study of stability and thermoelectric response of GeH monolayer

Thermoelectric characteristics of two-dimensional GeH structure have been studied using first-principles density functional technique and Boltzmann transport theory. Positive phonon modes and negative formation energy both attest to the stability of structure. A high value of elastic constant suggests strong chemical bonding and hence high mechanical strength. A direct band gap semiconducting character of GeH monolayer with 1.64 eV band gap is revealed by examination of electronic band structure. Flat conduction band in one direction and degenerate valence band near Fermi level improves density of states. As a consequence, both Seebeck coefficient and electrical conductivity shows excellent response and results in high power factor. At room temperature, the ZeT value of 5.51 (5.45) is obtained for p-(n) type doping, which shows its high practical capability of thermoelectric conversion. These findings suggest that GeH monolayer is an exciting possibility for 2D thermoelectric applications.

Excitonic Condensate in Flat Valence and Conduction Bands of Opposite Chirality.

Excitonic Bose-Einstein condensation (EBEC) has drawn increasing attention recently with the emergence of 2D materials. A general criterion for EBEC, as expected in an excitonic insulator (EI) state, is to have negative exciton formation energies in a semiconductor. Here, using exact diagonalization of a multiexciton Hamiltonian modeled in a diatomic kagome lattice, we demonstrate that the negative exciton formation energies are only a prerequisite but insufficient condition for realizing an EI. By a comparative study between the cases of both conduction and valence flat bands (FBs) versus that of a parabolic conduction band, we further show that the presence and increased FB contribution to exciton formation provide an attractive avenue to stabilize the excitonic condensate, as confirmed by calculations and analyses of multiexciton energies, wave functions, and reduced density matrices. Our results warrant a similar many-exciton analysis for other known and/or new candidates of EIs and demonstrate the FBs of opposite parity as a unique platform for studying exciton physics, paving the way to material realization of spinor BEC and spin superfluidity.

Chemical and Electronic Structure at the Interface between a Sputter‐Deposited Zn(O,S) Buffer and a Cu(In,Ga)(S,Se)2Solar Cell Absorber

The chemical and electronic structure of the interface between a sputter‐deposited Zn(O,S) buffer layer and an industrial Cu(In,Ga)(S,Se)2(CIGSSe) absorber for thin‐film solar cells is investigated with X‐ray and UV photoelectron spectroscopy, inverse photoemission spectroscopy, and X‐ray emission spectroscopy. We find a CIGSSe absorber surface band gap of 1.61 (±0.14) eV, which is significantly increased as compared to the minimal value derived with bulk‐sensitive methods (≈1.1 eV). We find no indication for diffusion of absorber elements into the buffer layer. Surface‐ and bulk‐sensitive measurements of the buffer layer suggest the presence of S‐Zn and S‐O bonds in the Zn(O,S) layer. We find that the naturally existing downward band bending toward the CIGSSe absorber surface is increased by the formation of the interface, likely enhancing carrier separation under illumination. We also derive a flat conduction band alignment, in line with the reported high conversion efficiencies of corresponding large‐area solar cells.

Theory of triangulene two-dimensional crystals

Abstract Equilateral triangle-shaped graphene nanoislands with a lateral dimension of n benzene rings are known as [n] triangulenes. Individual [n] triangulenes are open-shell molecules, with single-particle electronic spectra that host n − 1 half-filled zero modes and a many-body ground state with spin S = ( n − 1 ) / 2 . The on-surface synthesis of triangulenes has been demonstrated for n = 3 , 4 , 5 , 7 and the observation of a Haldane symmetry-protected topological phase has been reported in chains of [3]triangulenes. Here, we provide a unified theory for the electronic properties of a family of two-dimensional honeycomb lattices whose unit cell contains a pair of triangulenes with dimensions n a , n b . Combining density functional theory and tight-binding calculations, we find a wealth of half-filled narrow bands, including a graphene-like spectrum (for n a = n b = 2 ), spin-1 Dirac electrons (for n a = 2 , n b = 3 ), p x , y -orbital physics (for n a = n b = 3 ), as well as a gapped system with flat valence and conduction bands (for n a = n b = 4 ). All these results are rationalized with a class of effective Hamiltonians acting on the subspace of the zero-energy states that generalize the graphene honeycomb model to the case of fermions with an internal pseudospin degree of freedom with C 3 symmetry.

Floquet engineering the band structure of materials with optimal control theory

We demonstrate that the electronic structure of a material can be deformed into Floquet pseudobands with arbitrarily tailored shapes. We achieve this goal with a combination of quantum optimal control theory and Floquet engineering. The power and versatility of this framework is demonstrated here by utilizing the independent-electron tight-binding description of the $\ensuremath{\pi}$ electronic system of graphene. We show several prototype examples focusing on the region around the $K$ (Dirac) point of the Brillouin zone: creation of a gap with opposing flat valence and conduction bands, creation of a gap with opposing concave symmetric valence and conduction bands (which would correspond to a material with an effective negative electron-hole mass), and closure of the gap when departing from a modified graphene model with a nonzero field-free gap. We employ time-periodic drives with several frequency components and polarizations, in contrast to the usual monochromatic fields, and use control theory to find the amplitudes of each component that optimize the shape of the bands as desired. In addition, we use quantum control methods to find realistic switch-on pulses that bring the material into the predefined stationary Floquet band structure, i.e., into a state in which the desired Floquet modes of the target bands are fully occupied, so that they should remain stroboscopically stationary, with long lifetimes, when the weak periodic drives are started. Finally, we note that although we have focused on solid state materials, the technique that we propose could be equally used for the Floquet engineering of ultracold atoms in optical lattices and for other nonequilibrium dynamical and correlated systems.

Electronic Structure of Zinc-5,10,15,20-tetraethynylporphyrin: Evolution from the Molecule to a One-Dimensional Chain, a Two-Dimensional Covalent Organic Framework, and a Nanotube

Zinc-5,10,15,20-tetraethynylporphyrin (Zn-TEP) has been used as a building block to prepare one-dimensional (1D) chains, two-dimensional (2D) covalent organic frameworks, as well as nanotubes that represent molecular analogues of carbon nanotubes. It is of interest then to evaluate the electronic-structure evolution of Zn-TEP from the zero-dimensional (0D) molecule to the 1D chain, the 2D COF, and the nanotubes. Here, based on density functional theory calculations, we discuss the effects of dimensionality on the electronic structure of Zn-TEP by describing the fundamental relationship between the frontier molecular orbitals of Zn-TEP and the electronic bands in periodic lattices. Wavevector-independent flat bands appear in all the periodic systems due to the absence of wave function contribution on the meso carbons in the Zn-TEP frontier molecular orbitals. A zone-folding approach coming from a tight-binding model effectively captures the connection between the 2D COF and the nanotube when applying periodic boundary conditions along the circumferential direction around the nanotubes. Importantly, the Zn-TEP nanotube has a totally flat lowest conduction band, which provides an organic material platform to explore a variety of many-body phenomena.

Carrier tuning of Stoner ferromagnetism in ThCr2Si2 -structure cobalt arsenides

CaCo$_{2-y}$As$_2$ is an unusual itinerant magnet with signatures of extreme magnetic frustration. The conditions for establishing magnetic order in such itinerant frustrated magnets, either by reducing frustration or increasing electronic correlations, is an open question. Here we use results from inelastic neutron scattering and magnetic susceptibility measurements and density functional theory calculations to show that hole doping in Ca(Co$_{1-x}$Fe$_{x}$)$_{2-y}$As$_{2}$ suppresses magnetic order by quenching the magnetic moment while maintaining the same level of magnetic frustration. The suppression is due to tuning the Fermi energy away from a peak in the electronic density of states originating from a flat conduction band. This results in the complete elimination of the magnetic moment by $x\approx0.25$, providing a clear example of a Stoner-type transition.

Structural, Electronic, and Mechanical Properties of 2D Oxidized Diamond (100) Nanofilms

AbstractIn this work, the structural, electronic, and mechanical properties of 2D atomically thick diamond (100) nanofilms with surface oxygen functionalization as a function of layer number (n) are investigated by first‐principles calculation. The phonon dispersion curves and ab‐initio molecular dynamics results prove the dynamical and thermal stabilities of the structures at n ≥ 6. The bandgaps of oxidized diamond nanofilms are independent of the layer number, attributed to the main contribution of surface atoms to conductance bands and valence bands near the Fermi energy. The Perdew–Burke–Ernzerhof (PBE) calculations with n from 6 to 22 show that the bandgaps of nanofilms with ether groups on both outmost sides (ether groups on one side and hydrogen functional groups on the other side) are in the region of 1.308–1.838 eV (1.803–1.884 eV). The bandgap values of nanofilms with methoxyacetone groups on both outmost sides (methoxyacetone groups on one side and hydrogen functional groups on the other side) localized in the region of 3.078–3.329 eV (3.135–3.302 eV), and the flat valence bands and conduction bands near the Fermi energy make the easy transition between direct to indirect bandgaps. The calculated elastic constants and acoustic velocities of nanofilms have a significantly parity‐dependent oscillatory phenomenon and increase with increasing film thickness. This work provides new ideas for fabricating 2D diamond‐based nanodevices (i.e., optoelectrical semiconductor devices, micro‐ and nano‐electro‐mechanical systems) with high performances applied in practical fields.

Competing correlated states and abundant orbital magnetism in twisted monolayer-bilayer graphene

Flat band moiré superlattices have recently emerged as unique platforms for investigating the interplay between strong electronic correlations, nontrivial band topology, and multiple isospin ‘flavor’ symmetries. Twisted monolayer-bilayer graphene (tMBG) is an especially rich system owing to its low crystal symmetry and the tunability of its bandwidth and topology with an external electric field. Here, we find that orbital magnetism is abundant within the correlated phase diagram of tMBG, giving rise to the anomalous Hall effect in correlated metallic states nearby most odd integer fillings of the flat conduction band, as well as correlated Chern insulator states stabilized in an external magnetic field. The behavior of the states at zero field appears to be inconsistent with simple spin and valley polarization for the specific range of twist angles we investigate, and instead may plausibly result from an intervalley coherent (IVC) state with an order parameter that breaks time reversal symmetry. The application of a magnetic field further tunes the competition between correlated states, in some cases driving first-order topological phase transitions. Our results underscore the rich interplay between closely competing correlated ground states in tMBG, with possible implications for probing exotic IVC ordering.

Lattice collective modes from a continuum model of magic-angle twisted bilayer graphene

We show that the insulating states of magic-angle twisted bilayer graphene support a series of collective modes corresponding to local particle-hole excitations on triangular lattice sites. Our theory is based on a continuum model of the magic angle flat bands. When the system is insulating at moir\'e band filling $\nu=-3$, our calculations show that the ground state supports seven low-energy modes that lie well below the charge gap throughout the moir\'e Brillouin zone, one of which couples strongly to THz photons. The low-energy collective modes are faithfully described by a model with a local $SU(8)$ degree of freedom in each moir\'e unit cell that we identify as the direct product of spin, valley, and an orbital pseudospin. Apart from spin and valley-wave modes, the collective mode spectrum includes a low-energy intra-flavor exciton mode associated with transitions between flat valence and conduction band orbitals.

Halide-modulated self-assembly of metal-free perovskite single crystals for bio-friendly X-ray detection

Halide-modulated self-assembly of metal-free perovskite single crystals for bio-friendly X-ray detection

Flat-Band-Enabled Triplet Excitonic Insulator in a Diatomic Kagome Lattice.

The excitonic insulator (EI) state is a strongly correlated many-body ground state, arising from an instability in the band structure toward exciton formation. We show that the flat valence and conduction bands of a semiconducting diatomic Kagome lattice, as exemplified in a superatomic graphene lattice, can possibly conspire to enable an interesting triplet EI state, based on density-functional theory calculations combined with many-body GW and Bethe-Salpeter equation. Our results indicate that massive carriers in flat bands with highly localized electron and hole wave functions significantly reduce the screening and enhance the exchange interaction, leading to an unusually large triplet exciton binding energy (∼1.1 eV) exceeding the GW band gap by ∼0.2 eV and a large singlet-triplet splitting of ∼0.4 eV. Our findings enrich once again the intriguing physics of flat bands and extend the scope of EI materials.

Exciton Band Topology in Spontaneous Quantum Anomalous Hall Insulators: Applications to Twisted Bilayer Graphene.

We uncover topological features of neutral particle-hole pair excitations of correlated quantum anomalous Hall (QAH) insulators whose approximately flat conduction and valence bands have equal and opposite nonzero Chern number. Using an exactly solvable model we show that the underlying band topology affects both the center-of-mass and relative motion of particle-hole bound states. This leads to the formation of topological exciton bands whose features are robust to nonuniformity of both the dispersion and the Berry curvature. We apply these ideas to recently reported broken-symmetry spontaneous QAH insulators in substrate aligned magic-angle twisted bilayer graphene.

Tunnel barrier engineering of spin-polarized mild band gap vertical ternary heterostructures.

The atomic and electronic structures and properties of advanced 2D ternary vertical spin-polarized semiconducting heterostructures based on mild band gap graphitic carbon nitride g-C3N4 and ferromagnetic single-layer CrI3 fragments, namely CrI3/g-C3N4/CrI3 and g-C3N4/CrI3/g-C3N4, were proposed and examined using the ab initio GGA PBE PBC technique. Both possible ferromagnetic (FM) and antiferromagnetic (AFM) spin ordering configurations of CrI3/g-C3N4/CrI3 were considered and found to be energetically degenerated, being significantly different in the density of states. Electronic structure calculations revealed that weak van der Waals interactions between the fragments are responsible for the main features of the atomic and electronic structures of both the types of heterostructures. The combination of flat valence and conduction bands and conductivity channels localized at spin-polarized semiconducting CrI3 fragments makes proposed heterostructures as magnetic tunnel junctions for spin- and photo-related applications such as spintronics, magnetoresistive random-access memory, photocatalysis, and as elements for highly efficient spin-polarized photovoltaic nanodevices.

Flat-band ferromagnetism in twisted bilayer graphene

We discuss twisted bilayer graphene (TBG) based on a theorem of flat band ferromagnetism put forward by Mielke and Tasaki. According to this theorem, ferromagnetism occurs if the single particle density matrix of the flat band states is irreducible and we argue that this result can be applied to the quasi-flat bands of TBG that emerge around the charge-neutrality point for twist angles around the magic angle $\theta\sim1.05^\circ$. We show that the density matrix is irreducible in this case, thus predicting a ferromagnetic ground state for neutral TBG ($n=0$). We then show that the theorem can also be applied only to the flat conduction or valence bands, if the substrate induces a single-particle gap at charge neutrality. Also in this case, the corresponding density matrix turns out to be irreducible, leading to ferromagnetism at half filling ($n=\pm2$).