Single-particle Electronic Structure Research Articles

Vertex dominated superconductivity in intercalated FeSe

Bulk FeSe becomes superconducting below 9 K, but the critical temperature (Tc) is enhanced almost universally by a factor of ~4–5 when it is intercalated with alkali elements. How intercalation modifies the structure is known from in-situ X-ray and neutron scattering techniques, but why Tc changes so dramatically is not known. Here we show that there is one-to-one correspondence between the enhancement in magnetic instabilities at certain q vectors and superconducting pairing vertex, even while the nuclear spin relaxation rate 1/(T1T) may not reflect this enhancement. Intercalation modifies electronic screening both in the plane and also between layers. We disentangle quantitatively how superconducting pairing vertex gains from each such changes in electronic screening. Intercalated FeSe provides an archetypal example of superconductivity where information derived from the single-particle electronic structure appears to be insufficient to account for the origins of superconductivity, even when they are computed including correlation effects. We show that the five-fold enhancement in Tc on intercalation is not sensitive to the exact position of the dxy at Γ point, as long as it stays close to EF. Finally, we show that intercalation also significantly softens the collective charge excitations, suggesting the electron-phonon interaction could play some role in intercalated FeSe.

Spectroscopic Evidence for Electron Correlations in Epitaxial Bilayer Graphene with Interface-Reconstructed Superlattice Potentials

Superlattice potentials are theoretically predicted to modify the single-particle electronic structures. The resulting Coulomb-interaction-dominated low-energy physics would generate highly novel many-body phenomena. Here, by in situ tunneling spectroscopy, we show the signatures of superstructure-modulated correlated electron states in epitaxial bilayer graphene (BLG) on 6H-SiC(0001). As the carrier density is locally quasi-‘tuned’ by the superlattice potentials of a 6 × 6 interface reconstruction phase, the spectral-weight transfer occurs between the two broad peaks flanking the charge-neutral point. Such a detected non-rigid band shift beyond the single-particle band description implies the existence of correlation effects, probably attributed to the modified interlayer coupling in epitaxial BLG by the 6 × 6 reconstruction as in magic-angle BLG by the moiré potentials. Quantitative analysis suggests that the intrinsic interface reconstruction shows a high carrier tunability of ∼ 1/2 filling range, equivalent to the back gating by a voltage of ∼ 70 V in a typical gated BLG/SiO2/Si device. The finding in interface-modulated epitaxial BLG with reconstruction phase extends the BLG platform with electron correlations beyond the magic-angle situation, and may stimulate further investigations on correlated states in graphene systems and other van der Waals materials.

Electronic structures, charge transfer, and charge order in twisted transition metal dichalcogenide bilayers

Moir\'e superlattices of transition metal dichalcogenide (TMD) bilayers have been shown to host correlated electronic states, which arise from the interplay of long wavelength moir\'e potential and long-range Coulomb interaction. Here, we theoretically investigate structural relaxation and single-particle electronic structure of twisted TMD homobilayer. From the large-scale density functional theory calculation and continuum model with layer degrees of freedom, we find that the out-of-plane gating field creates a tunable charge transfer gap at the Dirac point between the first and second moir\'e valence bands. We further study the charge orders at the fractional band fillings. In the flat band limit, we find from Monte Carlo simulations a series of charge-ordered insulating states at various fillings $n=\frac{1}{4},\frac{1}{3},\frac{1}{2},\frac{2}{3},1$. We predict that gating field induces a phase transition between different electron crystals at fixed filling $n=\frac{1}{2}$ or $\frac{2}{3}$. At half-filling $n=1$, the ground state is a Mott insulator with electronically driven ferroelectricity. This work demonstrates that TMD homobilayer provides a powerful platform for the investigation of tunable charge transfer insulator and charge orders.

Duality between atomic configurations and Bloch states in twistronic materials

The relative orientation (twist) of successive layers of stacked two-dimensional (2D) materials creates variations in the interlayer atomic registry. The variations often form a super lattice, called a moir\'e pattern, which can alter electronic properties. In this work we introduce a classification of the single-particle electronic structures that can occur in twisted stacks of 2D layers by characterizing them as "moir\'e molecules" or "moir\'e crystals". The molecules generate localized electronic states and moir\'e flat bands, while the crystals are sometimes unconventional and produce electronic banding in the configuration basis. The underpinning of this classification is the duality between interlayer configuration and monolayer Bloch momentum in moir\'e Hamiltonians. We apply this understanding to diagrams of local electron density in untwisted geometries to produce intuitive and quantitative predictions of twistronic properties. We provide a conceptual introduction to this framework through a one-dimensional model, and then apply it to 2D twisted bilayers of the semi-metal graphene, and of MoS$_2$, a representative material of the transition metal dichalcogenide (TMDC) family of semiconductors. This level of thorough understanding of twistronic phenomena is vital in the search for new material platforms for localized moir\'e electrons.

Many-Body Resonance in a Correlated Topological Kagome Antiferromagnet.

We use scanning tunneling microscopy to elucidate the atomically resolved electronic structure in the strongly correlated kagome Weyl antiferromagnet Mn_{3}Sn. In stark contrast to its broad single-particle electronic structure, we observe a pronounced resonance with a Fano line shape at the Fermi level resembling the many-body Kondo resonance. We find that this resonance does not arise from the step edges or atomic impurities but the intrinsic kagome lattice. Moreover, the resonance is robust against the perturbation of a vector magnetic field, but broadens substantially with increasing temperature, signaling strongly interacting physics. We show that this resonance can be understood as the result of geometrical frustration and strong correlation based on the kagome lattice Hubbard model. Our results point to the emergent many-body resonance behavior in a topological kagome magnet.

Exact continuum model for low-energy electronic states of twisted bilayer graphene

We introduce a complete physical model for the single-particle electronic structure of twisted bilayer graphene (tBLG), which incorporates the crucial role of lattice relaxation. Our model, based on $k \cdot p$ perturbation theory, combines the accuracy of DFT calculations through effective tight-binding Hamiltonians with the computational efficiency and complete control of the twist angle offered by continuum models. The inclusion of relaxation significantly changes the bandstructure at the first magic-angle twist corresponding to flat bands near the Fermi level (the "low-energy" states), and eliminates the appearance of a second magic-angle twist. We show that minimal models for the low-energy states of tBLG can be easily modified to capture the changes in electronic states as a function of twist angle.

Optical absorption in disordered monolayer molybdenum disulfide

We explore the combined impact of sulfur vacancies and electronic interactions on the optical properties of monolayer MoS$_2$. First, we present a generalized Anderson-Hubbard Hamiltonian that accounts for both randomly distributed sulfur vacancies and the presence of dielectric screening within the material. Second, we parameterize this energy-dependent Hamiltonian from first-principles calculations based on density functional theory and the Green function and screened Coulomb (GW) method. Third, we apply a first-principles-based many-body typical medium method to determine the single-particle electronic structure. Fourth, we solve the Bethe-Salpeter equation to obtain the charge susceptibility $\chi$ with its imaginary part being related to the absorbance $\mathcal{A}$. Our results show that an increased vacancy concentration leads to decreased absorption both in the band continuum and from exciton states within the band gap. We also observe increased absorption below the band gap threshold and present an expression, which describes Lifshitz tails, in excellent qualitative agreement with our numerical calculations. This latter increased absorption in the $1.0$--$2.5$\,eV makes defect engineering of potential interest for solar cell applications.

Effects of spontaneous and piezoelectric polarization fields on the electronic and optical properties in GaN/AlN quantum dots: multimillion-atomsp3d5s*tight-binding simulations

GaN quantum dots QDs grown on AlN substrates are strong candidates for UV and near-infrared applications. The wurtzite crystal symmetry in these dots induces internal fields arising from the following: i crystal atomicity;ii strained active region; iii piezoelectricity; and iv spontaneous polarization pyroelectricity. Accurate modeling of electronic and optical properties of these QDs must capture the interplay of these atomistic and long-range fields and the size quantization on an equal footing. In this work, single-particle electronic structure and interband optical transition rates of a GaN/AlN QD grown along the c-axis are studied using a coupled molecular mechanics-atomistic 20-band sp3d5s* tight-binding VFF-TB framework. To calculate piezoelectricity, a recently reported model that takes into account both the linear and the nonlinear dependence of polarization on the strain tensors has been employed. The simulated GaN/AlN dot is realistically sized containing ~3 million atoms and of hexagonal disk shape having height and base length of 4.1 and 17nm, respectively. It is found that, in contrast to the well-studied InN/GaN systems, the pyroelectric potential in GaN/AlN dot is larger than the piezoelectric counterpart, and the effects of piezoelectric and pyroelectric fields add up. The internal fields result in a large redshift in the electronic states near the Brillouin zone center known as quantum confined Stark effect, pronounced non-degeneracy in the excited states, strongly suppressed optical transition, and anisotropic emission spectra. Copyright © 2014 John Wiley & Sons, Ltd.

Gap opening in topological-defect lattices in graphene

Ab initio calculations indicate that topological-defect networks in graphene display the full variety of single-particle electronic structures, including Dirac-fermion null-gap semiconductors, as well as metallic and semiconducting systems of very low formation energies with respect to a pristine graphene sheet. Corrugation induced by the topological defects further reduces the energy and tends to reduce the density of states at the Fermi level, to widen the gaps, or even to lead to gap opening in some cases where the parent planar geometry is metallic.

Asymmetry in negative differential resistance driven by electron–electron interactions in two-site molecular devices

We demonstrate that Coulomb interactions within the Hartree–Fock approximation can induce a strong asymmetry in the negative differential resistance of a two-site resonant molecular device. This sharp cutoff at high voltage is generic and independent of the detailed single-particle electronic structure of the system. The intersite coupling between distinct molecular subunits controls a transition from staircase to resonant current–voltage characteristics. These studies clarify the relationship between weak screening and strong nonlinearity.

Distortion of atoms around the neutral vacancy in a Si crystal

The single-particle electronic structure of a neutral lattice vacancy in a Si crystal is treated theoretically within the 16-atom supercell using the conventional tight-binding approximation. The physical mechanism leading to a spontaneous symmetric and tetragonal displacement of atoms surrounding a vacancy is discussed in detail. The elastic response of the lattice is calculated using the Keating valence force model applied to the cluster contained 441 atoms. The directions and amplitudes of atomic displacements are strongly symmetry-dependent. The amplitudes decrease with the distance R from the vacancy, approximately, as R-2, independently of the distortion symmetry. The influence of the vacancy defects on the lattice parameter is considered and estimated.

Charge ordering in rare-earth manganese oxides

Based on the interacting small polaron model, we have investigated charge ordering (CO) phenomena observed in half-doped manganites R 1/2A 1/2MnO 3. First, we have examined the charge gap formation in the single particle electronic structure in the CO phase. Second, we have studied the renormalization of the sound velocity around T CO, and compared the origin of the renormalization in the CO phase with that in the low doping metallic phase.

Interface hybridization and spectral distribution in strongly correlated itinerant magnetic systems.

We report the results of a theoretical study of the effect of the interface s-d hybridization on the photoemission spectral distribution in a one-monolayer Co on Cu(001) structure. The single-particle electronic structure and many-body interactions are treated on an equal footing in a numerical scheme that incorporates both band-structure and interaction effects. Using exact diagonalization method on small clusters with periodic boundary conditions, spin- and angle-resolved photoemission spectra are calculated. We study the effect of the interface hybridization on the spectral distribution by first calculating the spectra of an unsupported Co(001) monolayer in the absence of the substrate. Then we calculate the spectra with the substrate included. In the latter case, the s-d hybridization is first turned off to isolate the contribution from the crystal-field effect. Eventually, the one-monolayer Co on Cu(001) with full interface s-d hybridization, the realistic system, is studied. The calculated spectra are analyzed in a many-body picture and compared to available experimental data. Further experimental measurements are called for to test some theoretically predicted features.

ELECTRON SPECTROSCOPY OF STRONGLY CORRELATED ELECTRON MATERIALS—RECENT ISSUES AND RESULTS

An overview is given of some recent issues and results concerning the single-particle electronic structure of strongly correlated electron systems. Specific topics include (i) the α−γ transition in cerium metal, (ii) the Kondo resonance in Y 1−x U x Pd 3 and (iii) the insulator-to-metal transition in superconducting cuprates.

Effect of a nonuniform magnetic field on a two-dimensional electron gas in the ballistic regime.

The single-particle electronic structure of a two-dimensional electron gas in a nonuniform magnetic field B consists of states that propagate perpendicularly to the field gradient \ensuremath{\nabla}B and have a remarkable time-reversal asymmetry. In one of the allowed directions the propagation has free-electron character and is confined to a narrow one-dimensional channel localized about the region where \ensuremath{\Vert}B\ensuremath{\Vert} is minimum. In the opposite direction, the Landau states propagate throughout the rest of the sample with a velocity proportional to \ensuremath{\Vert}\ensuremath{\nabla}B${\mathrm{\ensuremath{\Vert}}}^{1/2}$.

Exact Diagonalization Study of Real Materials

ABSTRACTWe report some recent developments in theoretical modeling of real 3d magnetic materials, particularly interface and ultrathin-film structures. We focus on many-body correlation effects and describe the method of modeling realistic strongly correlated electron systems using and going beyond die local-density-approximation single-particle electronic structures. Group-theoretical techniques and high-performance computing facilities are systematically used in this work. Fundamental spectroscopic properties of several prototypical 3d transition-metal systems have been obtained and new features induced by many-body correlation have been discovered. Results will be presented and discussed.

NOVEL MANY-BODY EFFECTS IN PHOTOEMISSION AND INVERSE PHOTOEMISSION SPECTRA OF ULTRATHIN TRANSITION-METAL FILMS

We present an overview of some recent developments in the theoretical modeling of transition-metal systems, particularly the ultrathin-film structures, focusing on the effects of electron-electron interactions. We describe the progress in the understanding of how to model realistic strongly correlated electron systems using and going beyond the local-density-approximation single-particle electronic structures. Results of exact many-body calculations of photoemission and inverse photoemission spectra of ultrathin nickel, iron and cobalt films are shown to illustrate the application of our approach. Interesting new features induced by many-body effects are found and discussed. Comparison with available experimental results is presented and further work, both experimental and theoretical, is suggested.

Electron spectroscopy of strongly correlated systems

An overview is given of current electron spectroscopy studies of the single-particle electronic structure of strongly correlated systems. Specific topics include (i) the Ce 4f spectrum of CeCu 2Si 2, (ii) the 5f spectrum of Y 1- x U x Pd 3, and (ii i) the doped metallic state of high-temperature superconductor materials, for which it is argued that in the Luttinger liquid scenario, the Fermi energy of the metal should lie in the parent insulator's gap, as observed experimentally.

The Electronic Structure of High Tc Copper-Oxide Superconductors from Photoemission Spectroscopy

This article will summarize the information gained from photoemission spectroscopy (PES) studies of the electronic structure of high temperature superconductor (HTSC) materials, with emphasis on distinguishing models for the normal state. Inverse photoemission spectroscopy (IPES) is a closely related measurement, and taken together PES and IPES yield the single-particle electronic structure, meaning the the spectrum to remove and add electrons, i.e., the electronic ionization and affinity spectrum. It is assumed that the sudden approximation holds, so that the photon event can be characterized by a photoemission cross section σ. The emission from a particular kind of electronic state i can often be identified because its cross section has a particular photon energy (hv) dependence σ (hv).We will emphasize a few recent results especially pertinent to the conceptual issues posed by these materials, and we will divide the discussion between angle-integrated spectra and angle-resolved spectra. For angle-integrated spectra, electrons are collected from the sample surface in a large solid angle, while for angle-resolved spectra, electrons are collected in a well-defined direction. Angle resolution can be used to select emitted electrons of a particular value of crystal momentum k.

Surface magnetism in an exactly soluble many-body periodic-cluster model of bcc iron

An exact solution of a two-dimensionally periodic two-site cluster, a {l brace}001{r brace} two-layer thin film with body-centered-cubic (bcc) crystal structure, is presented for iron. The purpose is to study the surface magnetism of bcc iron in a full many-body approach. The model consists of five {ital d} orbitals per site per spin, with interatomic hopping terms, a one-electron occupation energy for each orbital, and an on-site Coulomb interaction of the fullest generality allowed by atomic symmetry. A realistic local-density-approximation single-particle electronic structure is used. Crystal-field effects in the iron-film structure are discussed. The many-body energy-level spectrum and thermodynamic averages of energy and spin of the system are calculated. The physical picture for the enhancement of magnetization at the true iron surface is discussed.