Tight-binding Research Articles

Graphene: Study in Electronic Structure, Superconductivity and Potential in Quantum Information Sciences

Graphene, a two-dimensional lattice of carbon atoms, exhibits remarkable electronic properties due to its Dirac-like electronic excitations. This paper reviews the elementary electronic properties of graphene, including the tight-binding model for single-layer graphene and the band structure formation. The tight-binding Hamiltonian provides a fundamental understanding of graphene’s linear energy dispersion near the Dirac points, leading to unique phenomena such as massless Dirac fermions and chiral tunneling. We explore the potential of superconductivity in graphene using BCS theory. We find that BCS theory cannot explain superconductivity in graphene and its forms. This is in contradiction with the discovery of unconventional superconductivity in twisted bilayer graphene (TBG). The study further explores the potential application of bilayer graphene quantum in quantum information sciences, highlighting the development of states with relaxation times exceeding 500 ms. These valley states, arising from the hexagonal lattice structure, can be used to encode information as robust valley qubits for fault-tolerant quantum computing

Topology in a one-dimensional plasmonic crystal: the optical approach

Abstract In this paper we study the topology of the bands of a plasmonic crystal composed of graphene and of a metallic grating. Firstly, we derive a Kronig–Penney type of equation for the plasmonic bands as function of the Bloch wavevector and discuss the propagation of the surface plasmon polaritons on the polaritonic crystal using a transfer-matrix approach considering a finite relaxation time. Second, we reformulate the problem as a tight-binding model that resembles the Su–Schrieffer–Heeger (SSH) Hamiltonian, one difference being that the hopping amplitudes are, in this case, energy dependent. In possession of the tight-binding equations it is a simple task to determine the topology (value of the winding number) of the bands. This allows to determine the existense or absence of topological end modes in the system. Similarly to the SSH model, we show that there is a tunable parameter that induces topological phase transitions from trivial to non-trivial. In our case, it is the distance d between the graphene sheet and the metallic grating. We note that d is a parameter that can be easily tuned experimentally simply by controlling the thickness of the spacer between the grating and the graphene sheet. It is then experimentally feasible to engineer devices with the required topological properties. Finally, we suggest a scattering experiment allowing the observation of the topological states.

Superconductivity in a background of topological spin texture

Abstract Motivated by a recent experiment on high-temperature superconductors [Nature 615, 405 (2023)], we perform a theoretical study to distinguish the nature of the topological spin texture in the superconducting phase of an underdoped cuperate. We propose a phenomenological tight-binding model of electrons with spin-singlet pairing hopping in the background of topological spin texture on the square lattice. Two types of topological spin texture relevant to the experiment are considered, i.e. Bloch Skyrmion and sinusoidal vortex, and the mean-field theory is employed. We discover an emergent $d_{xy}$-wave component in imaginary part of gap function for Skyrmion, but there is not for vortex. For Skyrmion, each coherent peak in the local density of states splits into two spatial uniform peaks, while for vortex, it splits into two spatial modulated peaks. Our study reveals the qualitatively different consequences of two types of topological spin texture, and can be possibly detected in the further STM expriment.

Non-Hermitian Skin Effect in Many-Body Thermophotonics.

The tight-binding model, foundational in depicting electronic behaviors in solid-state physics, has recently contributed to the understanding of non-Hermitian skin effects in optics, acoustics, and mechanics. However, tight-binding model is primarily built upon scalar nearest couplings, which in turn does not fit to describe the vectorial long-range interactions inherently in thermophotonics. Here, we report a strategy involving many-body radiative interactions in a two-dimensional thermophotonic lattice, and further reveal two types of orthogonal non-Hermitian skin modes in a reciprocal system. For in-plane modes, a pronounced geometry-dependent skin effect manifests at the edges, while for out-of-plane modes, skin effects induced by many-body interactions emerge at the corners instead. Our work provides a pioneering approach for understanding many-body-driven skin effect and unveils a mechanism for unexpected manipulation in thermophotonics.

Hofstadter Butterflies and Metal/Insulator Transitions for Moiré Heterostructures

AbstractWe consider a tight-binding model recently introduced by Timmel and Mele (Phys Rev Lett 125:166803, 2020) for strained moiré heterostructures. We consider two honeycomb lattices to which layer antisymmetric shear strain is applied to periodically modulate the tunneling between the lattices in one distinguished direction. This effectively reduces the model to one spatial dimension and makes it amenable to the theory of matrix-valued quasi-periodic operators. We then study the charge transport and spectral properties of this system, explaining the appearance of a Hofstadter-type butterfly and the occurrence of metal/insulator transitions that have recently been experimentally verified for non-interacting moiré systems (Wang et al. in Nature 577:42–46, 2020). For sufficiently incommensurable moiré lengths, described by a diophantine condition, as well as strong coupling between the lattices, which can be tuned by applying physical pressure, this leads to the occurrence of localization phenomena.

Topological material in the III–V family: Heteroepitaxial InBi on InAs

InBi(001) is formed epitaxially on InAs(111)-A by depositing Bi onto an In-rich surface. Angle-resolved photoemission measurements reveal topological electronic surface states, close to the M¯ high symmetry point. This demonstrates a heteroepitaxial system entirely in the III–V family with topological electronic properties. InBi shows coexistence of Bi and In surface terminations, in contradiction with other III–V materials. For the Bi termination, the study gives a consistent physical picture of the topological surface electronic structure of InBi(001) terminated by a Bi bilayer rather than a surface formed by splitting to a Bi monolayer termination. Theoretical calculations based on relativistic density functional theory and the one-step model of photoemission clarify the relationship between the InBi(001) surface termination and the topological surface states, supporting a predominant role of the Bi bilayer termination. Furthermore, a tight-binding model based on this Bi bilayer termination with only Bi–Bi hopping terms, and no Bi–In interaction, gives a deeper insight into the spin texture. Published by the American Physical Society 2024

Electron transport and quantum phase transitions in cumulene-connected C80H20 fulleryne: DFT and tight-binding studies

Molecular bridges are opening up exciting new applications in diverse fields, improving the efficiency of conductive inks, enhancing the performance of devices such as organic light-emitting diodes and low-cost solar cells, and advancing the development of highly sensitive sensors, chemical reactions, drug delivery systems, and more. In this paper, we study the electron transport properties of a C80H20 fulleryne (dodecahedryne) connected to two cumulene electrodes. Using density functional theory (DFT), we determine the optimal molecular bridge structure. Based on the IR vibration spectra, different stable phases of the molecular bridge are obtained. The corresponding tight-binding (TB) parameters of the cage are obtained by assigning appropriate values of the length and type of bonds for the fulleryne cage through matching the HOMO-LUMO gap between the DFT calculations and the TB parameters. The electron transport for the desired structures is investigated using the obtained tight-binding parameters and the non-equilibrium Green's function (NEGF) method. Finally, it is concluded that among the five possible configurations for the cumulene-dodecahedryne -cumulene molecular bridge, only one specific configuration—where the electrodes are one edge apart—exhibits metallic behavior, while other positions act as insulators. In addition, the system exhibits quantum phase transitions from metal to semiconductor and from insulator to metal in the presence of critical electric fields. The ability to control quantum phase transitions in these molecular systems can be leveraged to develop qubits for quantum computing. The unique properties can be utilized to design advanced molecular electronic devices.

Coherent states of graphene layer with and without a [formula omitted]-symmetric chemical potential

In this paper we construct different classes of coherent and bicoherent states for the graphene tight-binding model in presence of a magnetic field, and for a deformed version where we include a PT-symmetric chemical potential V. In particular, the problems caused by the absence of a suitable ground state for the system is taken into account in the construction of these states, for V=0 and for V≠0. We introduce ladder operators which work well in our context, and we show, in particular, that there exists a choice of these operators which produce a factorization of the Hamiltonian. The role of broken and unbroken PT-symmetry is discussed, in connection with the strength of V.

Structural Insights into Helix–Loop–Helix Peptides for “Ligand-Targeting” Intracellular Drug Delivery via VEGF Receptor-Mediated Endocytosis

As a new alternative to antibody-drug conjugates, we developed “ligand-targeting” peptide-drug conjugates (PDCs), in which conformationally constrained helix-loop-helix (HLH) peptide M49 targeting human vascular endothelial growth factor-A (VEGF) was used as a drug carrier. The biochemical study showed that HLH peptide M49 made a complex with VEGF in the extracellular environment, and then the M49/VEGF complex interacts with the receptor on the cell surface to induce cellular internalization via the endocytic pathway. Here, we present an X-ray crystal structure of the M49/VEGF complex at 1.5 Å resolution using a protein crystal grown in microgravity. The structure illustrated the “ligand-targeting” cellular uptake mechanism for intracellular drug delivery and the molecular basis on the peptide-VEGF binding mode with tight binding and high target specificity. In addition, mutational studies and thermodynamic analysis provided information on the driving forces of the complex formation. This work would contribute to the design of mid-size molecular-targeting peptides as well as HLH peptides, advancing the research in drug discovery and chemical biology.

Electrotunable superlubricity of two-dimensional ZIF-8

Electric field control can actively, dynamically, and repeatably influence the interface friction behavior. The unique properties of two-dimensional (2D) ZIF-8 make it a promising lubricating material for electromechanical devices. The study on the electrotunable superlubricity of 2D ZIF-8 is carried out under longitudinal and transverse electric fields respectively, resulting in an order of magnitude variation in friction coefficient (μ: 0.0037∼0.0124). Through the experiments and simulation, the regulation mechanism of electric fields on the lubricating properties of 2D ZIF-8 is attributed to the coupling effect of adhesion regulation and out-of-plane deformation regulation: The weakening of anchoring effect reduces the adhesion between probe and 2D ZIF-8; the tight binding of interfacial charge under longitudinal electric field as well as the increase in surface stiffness caused by lattice tension under transverse electric field, both restrain the out-of-plane deformation during friction. The electrotunable superlubricity of 2D ZIF-8 helps achieve rapid and flexible adjustment of the friction interface in electro-mechanical systems under charged conditions, illuminating the future development prospects for intelligent control.

Tight-binding model of Pt-based jacutingaites as combination of the honeycomb and kagome lattices

We introduce a refined tight-binding (TB) model for Pt-based jacutingaite materialsPt2NX3, (N= Zn, Cd, Hg; X = S, Se, Te), offering a detailed representation of the low-energy physics of its monolayers. This model incorporates all elements with significant spin-orbit coupling contributions, which are essential for understanding the topological energy gaps in these materials. Through comparison with first-principles calculations, we meticulously fitted the TB parameters, ensuring an accurate depiction of the energy bands near the Fermi level. Our model reveals the intricate interplay between the Pt 3eandNmetal orbitals, forming distinct kagome and honeycomb lattice structures. Applying this model, we explore the edge states of Pt-based jacutingaite monolayer nanoribbons, highlighting the sensitivity of the topological edge states dispersion bands to the nanostructures geometric configurations. These insights not only deepen our understanding of jacutingaite materials but also assist in tailoring their electronic properties for future applications.

Antimicrobial Potency of Nor-Pyochelin Analogues and Their Cation Complexes against Multidrug-Resistant Pathogens.

The opportunistic pathogen Pseudomonas aeruginosa develops increasing resistance toward even the most potent antibiotics. Like other bacteria, the pathogen produces a number of virulence factors including metallophores, which constitute an important group. Pseudomonads produce the iron-chelating metallophore (siderophore) pyochelin, which, in addition to its iron-scavenging ability, is an effector for the transcriptional regulator PchR in its FeIII-bound form (ferripyochelin). In the present study, docking studies predicted a major ferripyochelin binding site in PchR, which prompted the exploration of nor-pyochelin analogues to produce tight binding to PchR, and thereby upregulation of the pyochelin metabolism. In addition, we investigated the effects of using the analogues to bind the antimicrobial cations GaIII and InIII. Selected analogues of nor-pyochelin were synthesized, and their GaIII- and InIII-based complexes were assessed for antimicrobial activity. The results indicate that the GaIII complexes inhibit the pathogens under iron-limited conditions, while the InIII-based systems are more effective in iron-rich media. Several of the GaIII complexes were shown to be highly effective against a multidrug-resistant P. aeruginosa clinical isolate, with minimum inhibitory concentrations (MICs) of ≤1 μg/mL. Similarly, two of the InIII-based systems were particularly effective against the isolate, with an MIC of 8 μg/mL. These results show high promise in comparison with other, traditionally potent antibiotics, as the compounds generally indicated low cytotoxicity toward mammalian cells. Preliminary mechanistic investigations using pseudomonal transposon mutants suggested that the inhibitory effects of the InIII-based systems could be due to acute iron deficiency as a result of InIII-bound bacterioferritin.

Effects of Crystallinity and Branched Chain on Thermal Degradation of Polyethylene: A SCC-DFTB Molecular Dynamics Study.

As a widely used plastic, the aging and degradation of polyethylene (PE) are inevitable problems, whether the goal is to prolong the life of PE products or address the issue of white pollution. Molecular simulation is a vital scientific tool in elucidating the mechanisms and processes of chemical reactions. To obtain the distribution and evolution process of PE's thermal oxidation products, this work employs the self-consistent charge-density functional tight binding (SCC-DFTB) method to perform molecular simulations of the thermal oxidation of PE with different crystallinity and branched structures. We discovered that crystallinity does not affect the thermal oxidation mechanism of PE, but higher crystallinity makes PE more susceptible to cross-linking and carbon chain growth, reducing the degree of PE carbon chain breakage. The branched structure of PE results in differences in free volumes between the carbon chains, with larger pores leading to a concentrated distribution of O2 and chemical defects subsequently formed. The breakdown of PE is slowed down when chemical defects are localized in low-density regions of the carbon chain. The specifics and mechanism of PE's thermal oxidation are clearly revealed in this paper, which is essential for understanding the process in depth and for the development of anti-aging PE products.

Impact of valley degeneracy on the thermoelectric properties of zig-zag graphene nanoribbons with staggered sublattice potentials and transverse electric fields.

This study investigates the band inversion of flat bands in zig-zag graphene nanoribbons (ZGNRs) using a tight-binding model. The band inversion results from symmetry breaking in the transverse direction, achievable through deposition on specific substrates such as separated silicon carbide or hexagonal boron nitride sheets. Upon band inversion, ZGNRs exhibit electronic structures characterized by valley degeneracy and band gap properties, which can be modulated by transverse electric fields. To explore the impact of this level degeneracy on thermoelectric properties, we employ Green's function techniques to calculate thermoelectric quantities in ZGNR segments with staggered sublattice potentials and transverse electric fields. Two carrier transport scenarios are considered: the chemical potential is positioned above and below the highest occupied molecular orbital. We analyze thermionic-assisted transport (TAT) and direct ballistic transport (DBT). Level degeneracy enhances the electric power factors of ZGNRs by increasing electrical conductance, while the Seebeck coefficient remains robust in the TAT scenario. Conversely, in DBT, the enhancement of the power factor primarily stems from improvements in the Seebeck coefficient at elevated temperatures.

Anion regulation for surface passivation enables ultrahigh-stability perovskite nanocrystals.

All-inorganic perovskite CsPbBr3 nanocrystals (NCs) display high photoluminescence quantum yield and narrow emission, which show great potential application in optoelectronic devices. However, the poor environment stability of NCs will hinder their practical application. Herein, a series of ionic liquids with different anions (BF4-, Br-, and NO3-) were used as a sole capping ligand to synthesize NCs. Among the three samples, 1-hexadecyl-3-methylimidazolium tetrafluoroborate ([C16MIM]BF4) capped NCs have the highest stability in light, thermal, and water, possibly attributing to the in situ passivation of bromine vacancy via pseudohalogen BF4- and tight binding of ionic liquid ligands and lead atoms. In addition, green-emission [C16MIM]BF4 NCs were used to assemble a white light-emitting diode device, and it possessed a wide National Television System Committee color gamut of 124.5% and a stable emission peak at high driving currents of 380 mA. This work paves the way for resurfacing perovskite NCs with ultrahigh stability, thereby driving the perovskite NC display industry closer to real-world application.

Obtaining and Characterizing Thermostable α-Amylases Secreted by Bacillus subtilis, Originating from Bacillus amyloliquefaciens and Bacillus subtilis

The production of recombinant enzymes, primarily used for obtaining pure and functional target molecules, holds significant importance in modern biotechnology. This study aimed to obtain and characterize recombinant, extracellularly expressed α-amylases (Amy3500 and Amy1974), derived from B. amyloliquefaciens MDC1974 and B. subtilis MDC3500, respectively, using the pBE-S shuttle vector. Both α-amylase genes were molecularly cloned into the E. coli/B. subtilis pBE-S shuttle vector, both with (Amy1974sig and Amy3500sig) and without their signal peptides (Amy1974 and Amy3500), along with a signal peptide originating from the plasmid, and tested in flask fermentations. For recombinant Amy3500, the amylase variants resulted in similar levels of volumetric activity (700–750 U/mL). In contrast, the expression of Amy1974 nearly doubled compared to Amy1974sig with double signal peptides, reaching 2000 U/mL. SDS-PAGE estimated the molecular weight of Amy1974 α-amylase to be 54.6 kDa, which is consistent with the theoretical molecular mass calculations. However, the estimated molecular weight of Amy3500 α-amylase was significantly lower upon exiting the producer cells. Ca2⁺ ions exhibit a modest activating effect on the activities of Amy1974 and Amy3500 amylases, likely due to their tight binding to the protein scaffold. Both enzymes exhibited broad activity peaks between 45 and 70 °C, with a maximum at 65 °C. The Amy1974 and Amy3500 α-amylases demonstrated broad pH optima and pH-dependent thermostability, with optimum pH values at 6.5 and 5.8, and thermal stability peaks at pH 7.6 and 5.9, respectively. Both α-amylases displayed high relative activity against various starches, including corn amylopectin and potato amylose, while showing comparatively lower activity towards α-, β-, and γ-cyclodextrins. The Amy1974 amylase is effective in converting starch into dextrins of varying lengths, while Amy3500 primarily converts starch into glucose. These characteristics make them promising candidate enzymes for industrial applications.

Quantum Simulations of Radiation Damage in a Molecular Polyethylene Analog.

An atomic-level understanding of radiation-induced damage in simple polymers like polyethylene is essential for determining how these chemical changes can alter the physical and mechanical properties of important technological materials such as plastics. Ensembles of quantum simulations of radiation damage in a polyethylene analog are performed using the Density Functional Tight Binding method to help bind its radiolysis and subsequent degradation as a function of radiation dose. Chemical degradation products are categorized with a graph theory approach, and occurrence rates of unsaturated carbon bond formation, crosslinking, cycle formation, chain scission reactions, and out-gassing products are computed. Statistical correlations between product pairs show significant correlations between chain scission reactions, unsaturated carbon bond formation, and out-gassing products, though these correlations decrease with increasing atom recoil energy. The results present relatively simple chemical descriptors as possible indications of network rearrangements in the middle range of excitation energies. Ultimately, the work provides a computational framework for determining the coupling between nonequilibrium chemistry in polymers and potential changes to macro-scale properties that can aid in the interpretation of future radiation damage experiments on plasticmaterials.

Investigation of square-root higher-order topological insulator based on honeycomb-Kagome lattice of graphene plasmonic crystals

Abstract In this paper, a composite lattice of graphene plasmonic crystals consisting of breathing Kagome lattice and honeycomb lattice is constructed. The band structure of the proposed lattice is obtained by tight binding model, where the Hamiltonian demonstrate the square-root nature, and optical first principle calculation. The results from both methods agree well with each other. The square-root higher-order topological insulator in graphene plasmonic crystal is realized by constructing a sandwich structure, where two sets of corner states are observed. The proposed plasmonic structure might provide a new perspective for integrated plasmonic circuits and nano-topological lasers.

Atomically Precise Control of Topological State Hybridization in Conjugated Polymers.

Realization of topological quantum states in carbon nanostructures has recently emerged as a promising platform for hosting highly coherent and controllable quantum dot spin qubits. However, their adjustable manipulation remains elusive. Here, we report the atomically accurate control of the hybridization level of topologically protected quantum edge states emerging from topological interfaces in bottom-up-fabricated π-conjugated polymers. Our investigation employed a combination of low-temperature scanning tunneling microscopy and spectroscopy, along with high-resolution atomic force microscopy, to effectively modify the hybridization level of neighboring edge states by the selective dehydrogenation reaction of molecular units in a pentacene-based polymer and demonstrate their reversible character. Density functional theory, tight binding, and complete active space calculations for the Hubbard model were employed to support our findings, revealing that the extent of orbital overlap between the topological edge states can be finely tuned based on the geometry and electronic bandgap of the interconnecting region. These results demonstrate the utility of topological edge states as components for designing complex quantum arrangements for advanced electronic devices.

Edge-state-mediated RKKY coupling in graphene nanoflakes

We investigate the long-range behavior and size dependence of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in hexagonal and triangular graphene nanoflakes with zigzag and armchair edges. We employ the tight-binding model with exact diagonalization to calculate the RKKY interaction as a function of the distance between magnetic impurities, nanoflake size, and edge geometry. Our findings demonstrate a strong dependency of the RKKY interaction on edge geometry and flake size, with notable changes in the RKKY interaction strength. We further analyze the influence of structural defects on the interaction strength of exchange interactions.