On-site Energy Difference Research Articles

Engineering Interlayer Hybridization in Energy Space via Dipolar Overlayers

The interlayer hybridization (IH) of van der Waals (vdW) materials is thought to be mostly associated with the unignorable interlayer overlaps of wavefunctions (t) in real space. Here, we develop a more fundamental understanding of IH by introducing a new physical quantity, the IH admixture ratio α. Consequently, an exotic strategy of IH engineering in energy space can be proposed, i.e., instead of changing t as commonly used, α can be effectively tuned in energy space by changing the on-site energy difference (2Δ) between neighboring-layer states. In practice, this is feasible via reshaping the electrostatic potential of the surface by deposing a dipolar overlayer, e.g., crystalline ice. Our first-principles calculations unveil that IH engineering via adjusting 2Δ can greatly tune interlayer optical transitions in transition-metal dichalcogenide bilayers, switch different types of Dirac surface states in Bi2Se3 thin films, and control magnetic phase transition of charge density waves in 1H/1T-TaS2 bilayers, opening new opportunities to govern the fundamental optoelectronic, topological, and magnetic properties of vdW systems beyond the traditional interlayer distance or twisting engineering.

Rectified Bloch oscillations in dynamically modulated waveguide arrays.

We study the dynamics of excitations in dynamically modulated waveguide arrays with an external spatial linear potential. Longitudinally periodic modulation may cause a significant change in the width of the quasi-energy band and leads to the dynamical band suppression with a linear dispersion relation. This substantially affects the Bloch oscillation dynamics. Novel dynamical phenomena with no analogue in ordinary discrete waveguides, named rectified Bloch oscillations, are highlighted. Due to the interplay between directional coupling between adjacent waveguides and diffraction suppression by the introduced onsite energy difference, at odd times of half Bloch oscillations period, the new submodes are continuously excited along two opposite rectification directions and experience same oscillation evolution, and eventually lead to the formation of a diamondlike intensity network. Both the amplitude and direction of the rectified Bloch oscillations strongly depend on the coupling strength. When coupling strength passes the critical value at which dynamical band suppression with a linear dispersion relation occurs, the direction of Bloch oscillations is inverted.

Atomistic Hartree theory of twisted double bilayer graphene near the magic angle

Twisted double bilayer graphene (tDBLG) is a moiré material that has recently generated significant interest because of the observation of correlated phases near the magic angle. We carry out atomistic Hartree theory calculations to study the role of electron–electron interactions in the normal state of tDBLG. In contrast to twisted bilayer graphene, we find that such interactions do not result in significant doping-dependent deformations of the electronic band structure of tDBLG. However, interactions play an important role for the electronic structure in the presence of a perpendicular electric field as they screen the external field. Finally, we analyze the contribution of the Hartree potential to the crystal field, i.e. the on-site energy difference between the inner and outer layers. We find that the on-site energy obtained from Hartree theory has the same sign, but a smaller magnitude compared to previous studies in which the on-site energy was determined by fitting tight-binding results to ab initio density-functional theory (DFT) band structures. To understand this quantitative difference, we analyze the ab initio Kohn–Sham potential obtained from DFT and find that a subtle interplay of electron–electron and electron–ion interactions determines the magnitude of the on-site potential.

Designing nickelate superconductors with d8 configuration exploiting mixed-anion strategy

Inspired by a recently proposed superconducting mechanism for a new cuprate superconductor Ba$_2$CuO$_{3+\delta}$, we theoretically design an unconventional nickelate superconductor with $d^8$ electron configuration. Our strategy is to enlarge the on-site energy difference between $3d_{x^2-y^2}$ and other $3d$ orbitals by adopting halogens or hydrogen as out-of-plane anions, so that the $3d$ bands other than $d_{x^-y^2}$ lie just below the Fermi level for the $d^8$ configuration, acting as incipient bands that enhance superconductivity. We also discuss a possible relevance of the present proposal to the recently discovered superconductor (Nd,Sr)NiO$_2$.

Electrical conductivity of statically perturbed topological crystalline insulators

We theoretically investigate the electrical conductivity (EC) of topological crystalline insulators SnTe (001) and related alloys in the presence of external static local perturbations employing a continuum model and the Kubo-Greenwood formalism. Particularly, we have found the conditions for which the Dirac and saddle points can be altered in the presence of the perpendicular electric field, the on-site energy difference and the exchange field. Then, the EC of these alterations is addressed for potentials smaller and larger than the intervalley scattering parameter. Numerical results show that the band gap opens by the exchange field, while the electric field and on-site energy difference shift the Dirac/saddle points and the Fermi level, respectively. These phenomena result in the increasing and decreasing of the EC with electric field/on-site energy difference and exchange field, respectively. Finally, we find that the EC becomes non-zero at zero temperature when the Fermi level is shifted. These findings will bring applications to topological electronics.

Electric Field-Modulated Magnetic Phase Transition in van der Waals CrI3 Bilayers.

Two-dimensional van der Waals (vdW) magnetic materials are well-recognized milestones toward nanostructured spintronics. An interesting example is CrI3; its magnetic states can be modulated electrically, allowing spintronics applications that are highly compatible with electronics technologies. Here, we report the electric field alone induces the interlayer antiferromagnetic-to-ferromagnetic (AFM-to-FM) phase transition in CrI3 bilayers with critical field as low as 0.12 V/Å. The AFM-FM energy difference ΔE increases with electric field and is closely related to the field-induced on-site energy difference defined as the splitting between the electronic states of the two vdW layers. Our tight-binding model fits closely with ΔE as a function of electric field and gives a consistent estimation for orbital hopping, exchange splitting, and crystal field splitting. Furthermore, a CrI3-based spin field-effect device is suggested with the spin current switched on and off solely by the electric field. These findings not only reveal the physics underlying the transition but also provide guidelines for future discovery and design.

Topological Band Engineering of Lieb Lattice in Phthalocyanine-Based Metal-Organic Frameworks.

Topological properties of the Lieb lattice, i.e., the edge-centered square lattice, have been extensively studied and are, however, mostly based on theoretical models without identifying real material systems. Here, based on tight-binding and first-principles calculations, we demonstrate the Lieb-lattice features of the experimentally synthesized phthalocyanine-based metal-organic framework (MPc-MOF), which holds various intriguing topological phase transitions through band engineering. First, we show that the MPc-MOFs indeed have a peculiar Lieb band structure with 1/3 filling, which has been overlooked because of its unconventional band structure deviating from the ideal Lieb band. The intrinsic MPc-MOF presents a trivial insulating state, with its gap size determined by the on-site energy difference (ΔE) between the corner and edge-center sites. Through either chemical substitution or physical strain engineering, one can tune ΔE to close the gap and achieve a topological phase transition. Specifically, upon closing the gap, topological semimetallic/insulating states emerge from nonmagnetic MPc-MOFs, while magnetic semimetal/Chern insulator states arise from magnetic MPc-MOFs, respectively. Our discovery greatly enriches our understanding of the Lieb lattice and provides a guideline for experimental observation of the Lieb-lattice-based topological states.

A Lieb-like lattice in a covalent-organic framework and its Stoner ferromagnetism

Lieb lattice has been extensively studied to realize ferromagnetism due to its exotic flat band. However, its material realization has remained elusive; so far only artificial Lieb lattices have been made experimentally. Here, based on first-principles and tight-binding calculations, we discover that a recently synthesized two-dimensional sp2 carbon-conjugated covalent-organic framework (sp2c-COF) represents a material realization of a Lieb-like lattice. The observed ferromagnetism upon doping arises from a Dirac (valence) band in a non-ideal Lieb lattice with strong electronic inhomogeneity (EI) rather than the topological flat band in an ideal Lieb lattice. The EI, as characterized with a large on-site energy difference and a strong dimerization interaction between the corner and edge-center ligands, quenches the kinetic energy of the usual dispersive Dirac band, subjecting to an instability against spin polarization. We predict an even higher spin density for monolayer sp2c-COF to accommodate a higher doping concentration with reduced interlayer interaction.

A Dirac semimetal phase diagram of the binary compound CuI(R-3m)

Based on first-principles calculations, we study the properties and interactions of the existing binary compound CuI with the space group of R-3m. A pair of Dirac points with asymmetry high Fermi velocity is found to be distributed on the unique C3 symmetry axis in k space, which are formed by band inversion between I- |32,±32〉 and |32,±12〉 type states. CuI(R-3m) is located at the boundary of different topological phases. Anisotropic strain breaking C3 symmetry could switch CuI(R-3m) between topological Dirac semimetal state and strong topological insulator states. A Dirac semimetal phase diagram is obtained, and show that binary compound with the space group of R-3m is a good platform for getting different semi-metals states. We reveal that various topological phases can be achieved in the binary compounds with the space group of R-3m (NO.166) by regulating the on-site energy difference and the SOC strength. The compounds family of XY (X = Cu, Ag, Au; YI, Br, Cl, F) with the atomic structure of CuI(R-3m) are topological semimetals with one or two pairs of Dirac points around Fermi energy, except for AgCl and AgF.

Effects of electron-phonon coupling and magnetic field on electrical conductivity of monolayer graphene

Abstract Electronic properties of gapped graphene structure taking into account the effects of interaction between electrons and Einstein phonons have been addressed. Specially we study the temperature dependence of electrical conductivity and density of states of the structure. The effects of gap parameter, magnetic field and electron doping on electrical conductivity of the system have been studied. Gap parameter is defined as the on-site energy difference of atoms on two different sublattices of honeycomb structure. Green’s function method has been implemented to obtain electronic density of states of the system in the context of Holstein model Hamiltonian. One loop electronic self-energy of the model Hamiltonian has been obtained in order to find interacting electronic Green’s function. The electrical conductivity of graphene structure in the presence of electron-phonon coupling can be readily found using interacting Green’s function based on Kubo formula. We present the numerical results of energy dependence of density of states for different values of gap parameter and magnetic field in the presence of Holstein phonons. Finally the temperature dependence of electrical conductivity of gapped graphene structure due to electron phonon interaction and magnetic has been investigated. Our results show turning on magnetic field leads to appear double van Hov singularity for each value of electron phonon coupling.

Floquet Engineered Quadri-partite Lattice System as an Extension of the Topological Haldane Model

We theoretically study a two-dimensional quadri-partite system that is an extension of a topological Haldane model, in the sense that the topological properties change as a consequence of time-reversal and/or spatial-inversion symmetry breaking. The former symmetry is broken by the application of a circularly polarized laser field and the latter is broken by an on-site energy difference. The system differs from the Haldane model in that the Γ point solely governs its properties. Earlier tight binding studies of this system revealed that line degeneracies remain even after breaking of the time-reversal symmetry. These robust degeneracies preclude the use of a conventional Chern number. To overcome this difficulty, we here propose an energy bunch Chern number, which is defined for a bunch of connected energy bands. The validity of this number is demonstrated by its consistency with well-established previous results.

Polarons in organic ferromagnets

Abstract With an Anderson-like model including the electron-lattice interaction, the polaron formation in quasi-one-dimensional organic ferromagnets with spin radicals is investigated theoretically. In the presence of an additional electron, distortions of spin and charge densities are observed both in the main chain and radicals. A polaron-like lattice distortion confined with fractional charges and spins may be formed in the main chain, where the amplitude is tuned by the electron hopping between the main chain and radicals and the electron-electron interaction. The confined spins and charges are inconsistent in position, which is caused by the asymmetric modification of charges and spins from the low-energy electrons triggered by the polaron lattice distortion. A phase diagram of the polaron formation is given with different electron hopping and electron-electron strengths. The case of a nonzero on-site energy difference between the radicals and the main chain is also discussed. A background charge density wave is found in the main chain due to the charge transfer between the main chain and spin radicals, which hampers the formation of the polaron.

Optimal conditions for spatial adiabatic passage of a Bose-Einstein condensate

We investigate spatial adiabatic passage of a Bose-Einstein condensate in a triple well potential within the three-mode approximation. By rewriting the dynamics in the so-called time-dependent dark/dressed basis, we analytically derive the optimal conditions for the non-linear parameter and the on-site energies of each well to achieve a highly efficient condensate transport. We show that the non-linearity yields a high-efficiency plateau for the condensate transport as a function of the on-site energy difference between the outermost wells, favoring the robustness of the transport. We also analyze the case of different non-linearities in each well, which, for certain parameter values, leads to an increase of the width of this plateau.

Entanglement spectrum of fermionic bilayer honeycomb lattice: Hofstadter butterfly

We perform an analytical study of the energy and entanglement spectrum of non-interacting fermionic bilayer honeycomb lattices in the presence of trigonal warping in the energy spectrum, on-site energy difference and uniform magnetic field. Employing single particle correlation functions, we present an explicit form for a layer–layer entanglement Hamiltonian whose spectrum is the entanglement spectrum. We demonstrate that in the absence of trigonal warping, at zero on-site energy difference exact correspondence is established between the entanglement spectrum and energy spectrum of a monolayer which means that the entanglement spectrum perfectly reflects the edge state properties of the bilayer. We also show that trigonal warping breaks down such a perfect correspondence, however, in -K direction in the hexagonal Brillouin zone, their behaviors are remarkably the same for particular relevances of hopping parameters. In the presence of an on-site energy difference the symmetry of the entanglement spectrum is broken with opening an indirect entanglement gap. We also study the effects of a perpendicular magnetic field on both energy and the entanglement spectrum of the bilayer in the presence of trigonal warping and on-site energy difference. We demonstrate that the entanglement spectrum versus magnetic flux has a self similar fractal structure, known as the Hofstadter butterfly. Our results also show that the on-site energy difference causes a transition from the Hofstadter butterfly to a tree-like picture.

Stacking nature and band gap opening of graphene: Perspective for optoelectronic applications

Abstract Using first principles density functional theory calculations, we have performed geometrical and electronic structure calculations of two-dimensional graphene(G) sheet on the hexagonal boron nitride (h-BN) with different stacking orders. We found that AB stacking appears as the ground state while AA-stacking is a local minima. Band gap opening in the hybrid G/h-BN is sensitive to the interlayer distance and stacking arrangement. Charge redistribution in the graphene sheet determined the band gap opening where the onsite energy difference between carbon lattice atoms of G-sheet takes place. Similar behavior can be observed for the proposed h-BN/G/h-BN tri-layer system. Stacking resolved calculations of the absorptive part of complex dielectric function and optical conductivity revealed the importance of the proposed hybrid systems in the optoelectronics.

Charge and Paramagnetic Spin Susceptibilities of Doped Gapped Graphene-Like Structures

We address spin polarization dependence of dynamical charge and spin susceptibilities of doped gapped graphene-like structures in the context of tight binding model Hamiltonian. The frequency behavior of both longitudinal and transverse spin susceptibilities has been calculated via calculating correlation function of spin density operators. Our results show the increase of electronic concentration corresponding to chemical potential leads to enhance the intensity of charge spectral function. We also show that longitudinal spin susceptibility reduces at fixed frequency with gap parameter associated with on-site energy difference between two types of sublattice atoms. Furthermore, the resonance peak in longitudinal spin susceptibility goes to higher frequencies with gap parameter. The effect of magnetization on the longitudinal spin susceptibility indicates two different behaviors depending on frequency region. Finally, the effects of magnetization and gap parameter on the frequency behavior of transverse spin susceptibility have been studied in details.

Transient quantum transport in double-dot Aharonov-Bohm interferometers

Real-time nonequilibrium quantum dynamics of electrons in double-dot Aharonov-Bohm (AB) interferometers is studied using an exact solution of the master equation. The building of the coherence between the two electronic paths shows up via the time-dependent amplitude of the AB oscillations in the transient transport current and can be enhanced by varying the applied bias on the leads, the on-site energy difference between the dots and the asymmetry of the coupling of the dots to the leads. The transient oscillations of the transport current do not obey phase rigidity. The circulating current has an antisymmetric AB oscillation in the flux. The nondegeneracy of the on-site energies and the finite bias cause the occupation in each dot to have an arbitrary flux dependence as the coupling asymmetry is varied.

Polarizability and screening in chiral multilayer graphene

We calculate the static polarizability of multilayer graphene and study the effect of stacking arrangement, carrier density, and onsite energy difference on graphene screening properties. At low densities, the energy spectrum of multilayer graphene is described by a set of chiral two-dimensional electron systems and the associated chiral nature determines the screening properties of multilayer graphene showing very different behavior depending on whether the chirality index is even or odd. As density increases, the energy spectrum follows that of the monolayer graphene and thus the polarizability approaches that of monolayer graphene. The qualitative dependence of graphene polarizability on chirality and layering indicates the possibility of distinct graphene quantum phases as a function of the chirality index.

Why the Band Gap of Graphene Is Tunable on Hexagonal Boron Nitride

The electronic properties of a graphene–boron nitride (G/BN) bilayer have been carefully investigated by first-principles calculations. We find that the energy gap of graphene is tunable from 0 to 0.55 eV and sensitive to the stacking order and interlayer distances of the G/BN bilayer. By electronic structure analysis and tight-binding simulations, we conclude that the charge redistribution within graphene and charge transfer between graphene and BN layers determine the energy gap of graphene, through modification of the on-site energy difference of carbon p orbitals at two sublattices. On the basis of the revealed mechanism, we also predict how to engineer the band gap of graphene.

Influence of backbone on the charge transport properties of G4-DNA molecules: amodel-based calculation

We put forward a model Hamiltonian to describe the influence of backbone energetics oncharge transport through guanine-quadruplex DNA (G4-DNA) molecules. Our analyticalresults show that an energy gap can be produced in the energy spectrum of G4-DNA byhybridization effects between the backbone and the base and by on-site energy difference ofthe backbone from the base. The environmental effects are investigated by introducingdifferent types of disorder into the backbone sites. Our numerical results suggest that thelocalization length of G4-DNA can be significantly enhanced by increasing the backbonedisorder degree when the environment-induced disorder is sufficiently large. There exists abackbone disorder-induced semiconducting–metallic transition in short G4-DNA molecules,where G4-DNA behaves as a semiconductor if the backbone disorder is weak andbehaves as a conductor if the backbone disorder degree surpasses a critical value.