Van Hove Singularity Research Articles

Enhanced optical conductivity and many-body effects in strongly-driven photo-excited semi-metallic graphite

The excitation of quasi-particles near the extrema of the electronic band structure is a gateway to electronic phase transitions in condensed matter. In a many-body system, quasi-particle dynamics are strongly influenced by the electronic single-particle structure and have been extensively studied in the weak optical excitation regime. Yet, under strong optical excitation, where light fields coherently drive carriers, the dynamics of many-body interactions that can lead to new quantum phases remain largely unresolved. Here, we induce such a highly non-equilibrium many-body state through strong optical excitation of charge carriers near the van Hove singularity in graphite. We investigate the system’s evolution into a strongly-driven photo-excited state with attosecond soft X-ray core-level spectroscopy. We find an enhancement of the optical conductivity of nearly ten times the quantum conductivity and pinpoint it to carrier excitations in flat bands. This interaction regime is robust against carrier-carrier interaction with coherent optical phonons acting as an attractive force reminiscent of superconductivity. The strongly-driven non-equilibrium state is markedly different from the single-particle structure and macroscopic conductivity and is a consequence of the non-adiabatic many-body state.

Polarization dependence of intensities Raman peaks in twisted graphene bilayer

The polarization dependence of the Raman 2D peak intensity and the polarization independence of the G Raman peak intensity have already been established in single-layer and AB-stacked bilayer graphene. Nevertheless, this study has not been conducted for twisted bilayers. In this study, we investigate the polarization properties of Raman peaks in three distinct twisted bilayer graphene samples, each exhibiting unique characteristics: G peak resonance, the presence of R′ peaks, and D-like peaks, respectively. Our analysis reveals anisotropic behavior in the G peak only under resonance enhancement conditions and consistent polarization-dependent behavior in the 2D peak across all twisted bilayers. Furthermore, we observe a pronounced symmetric and asymmetric polarization dependence in the intensity of R′ and D-like peaks, respectively. These observations have been attributed to the strong coupling between the two layers resulting from the resonance between the van Hove singularities, which affects the behavior of the G peak, and to a double resonance effect that enhances the coupling between the nodal structure and the electron–phonon anisotropy in the intravalley and intervalley regions for R′ and D-like peaks, respectively. These findings contribute to the advancement of our understanding of twisted bilayer systems and offer opportunities for exploring more profound studies.

Prediction of Van Hove singularity systems in ternary borides

A computational search for stable structures among both α and β phases of ternary ATB4 borides (A = Mg, Ca, Sr, Ba, Al, Ga, and Zn, T is 3d or 4d transition elements) has been performed. We found that α-ATB4 compounds with A = Mg, Ca, Al, and T = V, Cr, Mn, Fe, Ni, and Co form a family of structurally stable or almost stable materials. These systems are metallic in non-magnetic states and characterized by the formation of the localized molecular-like state of 3d transition metal atom dimers, which leads to the appearance of numerous Van Hove singularities in the electronic spectrum. The closeness of such singularities to the Fermi level can be easily tuned by electron doping. For the atoms in the middle of the 3d row (Cr, Mn, and Fe), these singularities led to magnetic instabilities and magnetic ground states with a weakly metallic or semiconducting nature. Such states appear as non-trivial coexistence of the different spin ladders formed by magnetic dimers of 3d elements. These magnetic states can be characterized as an analog of the spin glass state. Experimental attempts to produce MgFeB4 and associated challenges are discussed, and promising directions for further synthetic studies are formulated.

Janus breathing-Kagome lattice Nb3TeX7 (X = Cl, Br and I) with low thermal conductivity and high thermoelectric figure of merit

Kagome lattices have attracted much attention because of their potential in superconductivity and topological applications. In this work, we proposed three two-dimensional Janus breathing-Kagome lattices, Nb3TeCl7, Nb3TeBr7 and Nb3TeI7, and further investigated their stability, electronic and thermoelectric (TE) transport performance. We illustrated that the three materials all possess high stability, and low cleavage energy of 0.21–0.22 J/m2. Besides, they exhibit common features of Kagome lattices, such as fat bands and van Hove singularities near band-edges, which lead to high Seebeck coefficient (0.37–0.57 mV/K) and TE power factor of (2.79 − 3.71 mW/K2m). Further, owing to their asymmetric Janus structures, the strong coupling between acoustic and optical phonons, as well as low frequency for lying-phonons, the three lattices also possess low lattice thermal conductivity of 1.06–2.76 W/mK. Ultimately, they deliver promising TE figure of merit of {0.28, 0.39, 0.43} at 300 K, and increase further to {1.09, 1.39, 1.40} at 700 K, demonstrating breathing-Kagome lattice Nb3TeX7 are suitable candidates in TE application.

Visualizing near-coexistence of massless Dirac electrons and ultra-massive saddle point electrons

Strong singularities in the electronic density of states amplify correlation effects and play a key role in determining the ordering instabilities in various materials. Recently high order van Hove singularities (VHSs) with diverging power-law scaling have been classified in single-band electron models. We show that the 110 surface of Bismuth exhibits high order VHS with an usually high density of states divergence \sim (E)^{-0.7}∼(E)−0.7. Detailed mapping of the surface band structure using scanning tunneling microscopy and spectroscopy combined with first-principles calculations show that this singularity occurs in close proximity to Dirac bands located at the center of the surface Brillouin zone. The enhanced power-law divergence is shown to originate from the anisotropic flattening of the Dirac band just above the Dirac node. Such near-coexistence of massless Dirac electrons and ultra-massive saddle points enables to study the interplay of high order VHS and Dirac fermions.

Electron-Correlation-Induced Charge Density Wave in FeGe

As the first magnetic kagome material to exhibit the charge density wave (CDW) order, FeGe has attracted much attention in recent research. Similar to AV3Sb5 (A = K, Cs, Rb), FeGe exhibits the CDW pattern with an in-plane 2×2 structure and the existence of van Hove singularities near the Fermi level. However, sharply different from AV3Sb5 which has phonon instability at M point, all the theoretically calculated phonon frequencies in FeGe remain positive. Based on first-principles calculations, we surprisingly find that the maximum of nesting function is at K point instead of M point. Two Fermi pockets with Fe-dxz and Fe-d x 2–y 2 /dxy orbital characters have large contribution to the Fermi nesting, which evolve significantly with kz , indicating the highly three-dimensional (3D) feature of FeGe in contrast to AV3Sb5. Considering the effect of local Coulomb interaction, we reveal that the instability at K point is significantly suppressed due to the sublattice interference mechanism. Meanwhile, the wave functions nested by vector M have many ingredients located at the same Fe site, thus the instability at M point is enhanced. This indicates that the electron correlation, rather than electron-phonon interaction, plays a key role in the CDW transition at M point.

Effect of van-Hove singularities in single-walled carbon nanotube leads on transport through T-shaped double quantum dot system

The T-shaped double quantum dot system with single-walled metallic armchair carbon nanotube leads has been studied using Green functions obtained by the equation of motion method. The effect of relative spacing between the energy levels of the dots, interdot tunneling matrix-element, interdot Coulomb interaction, and van-Hove singularities in density of states characteristics of quasi-one-dimensional carbon nanotube leads on the conductance of the double quantum dot system has been studied. The conductance and dot occupancies are calculated at finite temperatures. The density of states of the carbon nanotube leads is observed to play a significant role in determining the conductance profile. In particular, whenever the chemical potential of the isolated double quantum dot system is aligned with the position of a van-Hove singularity in the density of states of armchair carbon nanotube leads, the height of the corresponding conductance peak falls considerably. It is further observed that the suppression in the heights of the alternate peaks depends on the relative positions of the energy levels of the dots and their magnitude of separation.

Bilayer Kagome Borophene with Multiple van Hove Singularities.

The appearance of van Hove singularities near the Fermi level leads to prominent phenomena, including superconductivity, charge density wave, and ferromagnetism. Here a bilayer Kagome lattice with multiple van Hove singularities is designed and a novel borophene with such lattice (BK-borophene) is proposed by the first-principles calculations. BK-borophene, which is formed via three-center two-electron (3c-2e) σ-type bonds, is predicted to be energetically, dynamically, thermodynamically, and mechanically stable. The electronic structure hosts both conventional and high-order van Hove singularities in one band. The conventional van Hove singularity resulting from the horse saddle is 0.065eV lower than the Fermi level, while the high-order one resulting from the monkey saddle is 0.385eV below the Fermi level. Both the singularities lead to the divergence of electronic density of states. Besides, the high-order singularity is just intersected to a Dirac-like cone, where the Fermi velocity can reach 1.34 × 106m s-1. The interaction between the two Kagome lattices is critical for the appearance of high-order van Hove singularities. The novel bilayer Kagome borophene with rich and intriguing electronic structure offers an unprecedented platform for studying correlation phenomena in quantum material systems and beyond.

Investigating the Cuprates as a platform for high-order Van Hove singularities and flat-band physics

Beyond the two-dimensional saddle-point Van Hove singularities (VHSs) with logarithmic divergences in the density of states, recent studies have identified higher-order VHSs with faster-than-logarithmic divergences that can amplify electron correlation effects. Here we show that the cuprate high-Tc superconductors harbor high-order VHSs in their electronic spectra and unveil a new correlation that the cuprates with high-order VHSs display higher Tc’s. Our analysis indicates that the normal and higher-order VHSs can provide a straightforward new marker for identifying propensity of a material toward the occurrence of correlated phases such as the excitonic insulators and supermetals. Our study suggests cuprates and related high-Tc superconductors as materials for exploring the interplay between high-order VHSs, superconducting transition temperatures, and electron correlation effects.

Prediction of two-dimensional materials: TeXO6 (X = transition metals):structure and electronic properties

Due to the extensive application of two-dimensional (2D) semiconductor materials in spintronics and tunable nano-mechanical devices, many researchers have invested in the research of 2D semiconductor materials, and they have obtained many excellent research results. Utilizing the first-principles calculations and Green’s function, we forecast the monolayer TeXO6 (X = Cr, Fe, Zn) as new 2D materials with various advantageous functional features. The calculation results for TeCrO6, TeFeO6, and TeZnO6 show good stability and promise prospective application potential if the materials are experimental synthesis. With applied strain, TeCrO6 possesses an indirect band gap of 1.096 eV, which is predicted to be strongly manipulabe in this work. We use the density functional theory to investigate the structure and electrical characteristics of the monolayer (X = transition metals). Detailed simulations of their energetics, atomic structures, and electronic structures under the impact of a biaxial strain have been performed. It is discovered that whereas TeCrO6 does not, TeXO6 (with X = Sc, Ti, V, Mn, Fe, Co, Ni, Cu, and Zn) exhibit spin splitting at the ground state. With the application of compressive strain, the electronic band gap narrows. These materials’ band topologies have been discovered to be more susceptible to biaxial stresses. If a strain of 8% is applied, TeFeO6 may change from semimetal to semiconductor. Under baxial strain, TeCrO6 exhibits interesting Van Hove singularities and Mexican-hat-like bands. Due to all these desirable characteristics, 2D TeXO6 is a prospective option for use in a number of technologies, particularly for spintronic and electrical devices.

Prediction of quantum spin Hall and Rashba effects in two-dimensional ilmenite oxides

Using first-principles calculations, we investigate the structural, electronic, and topological properties of two-dimensional (2D) pristine ilmenite oxides ABO3 (A = Au, Ag, or Cu; and B = Bi, Sb, or As) and their corresponding Janus structures. Phonon dispersions reveal the dynamic stability of these compounds. Interestingly, pristine CuBiO3 and AuBiO3, and Janus Cu0.5Ag0.5BiO3, Au0.5Cu0.5BiO3, Au0.5Ag0.5BiO3, and CuBi0.5As0.5O3 are topological insulators, while AuBi0.5As0.5O3, CuBi0.5Sb0.5O3, and AuBi0.5Sb0.5O3 are topological semimetals, as confirmed by their Z2 invariance and conducting edge states under the hybrid functional approach. Moreover, we found van Hove singularities in Au0.5Ag0.5BiO3 and Cu0.5Ag0.5BiO3 near the Fermi level, suggesting the coexistence of superconductivity and nontrivial topology. Finally, isotropic Rashba spin-splitting is studied in detail for Au0.5Ag0.5BiO3. Our findings demonstrate that 2D ilmenite oxides can be a new material playground for potential spintronic applications.

Fate of density waves in the presence of a higher-order van Hove singularity

Fate of density waves in the presence of a higher-order van Hove singularity

Superconductivity in the bcc‐Type High‐Entropy Alloy TiHfNbTaMo

AbstractX‐ray powder diffraction, electrical resistivity, magnetization, and thermodynamic measurements are conducted to investigate the structure and superconducting properties of TiHfNbTaMo, a novel high‐entropy alloy possessing a valence electron count (VEC) of 4.8. The TiHfNbTaMo HEA is discovered to have a body‐centered cubic structure (space group ) with lattice parameters a = 3.445 (1) Å and a microscopically homogeneous distribution of the constituent elements. This material shows type‐II superconductivity with Tc = 3.42 K, lower critical field µ0Hc1(0) = 22.8 mT, and upper critical field µ0Hc2(0) = 3.95 T. Low‐temperature specific heat measurements show that the alloy is a conventional s‐wave type with a moderately coupled superconductor. First‐principles calculations show that the density of states (DOS) of the TiHfNbTaMo alloy is dominated by hybrid d orbitals of these five metal elements. Additionally, the TiHfNbTaMo HEA exhibits three van Hove singularities. Furthermore, the VEC and the composition of the elements (especially the Nb elemental content) affect the Tc of the bcc‐type HEA.

On the possibility of low-symmetry structures in Zr2CoSi

First principles calculation shows that the energy difference between the cubic L21 and Hg2CuTi inverse structure for Zr2CoSi is 1.454 eV/f.u., indicating than the L21 structure is more stable. The density of states and band structure of Zr2CoSi in the L21 structure do not support the half–metallic state. Moreover, van Hove singularity involving partially occupied Co 3d eg states occurs in minority states at the Fermi level and the Zr2CoSi in L21 structure satisfies the preconditions to exhibit a cubic-to-tetragonal instability according to a band-Jahn-Teller mechanism. The computed total energy landscapes of the system shows that there are two tetragonal and one orthorhombic distorted phases of Zr2CoSi that are energetically favored over the undistorted cubic L21 structure. However, only one tetragonal phase of Zr2CoSi with c/a > 1.0 fulfills necessary and sufficient elastic stability conditions for both GGA and LDA exchange–correlation potentials.

Different look at S-p bonding orbital in CdS, with Zn and Se series gap investigations

The electronic characteristics of Cadmium sulfide (CdS) series Zn x Cd16−x S8Se8 and Cd16Se16−x S x are investigated. According to the calculations, barely 12% Zn (X = 2) in the form Zn2Cd14Se16 is required to maximize the gap. To properly evaluate the excited states, the generalized gradient approximation (GGA) and modified Becke-Johnson (mBJ) potentials are utilized in the computations. The parent compound CdS exhibits three distinct forms of crystal structures, with the hexagonal structure being remarkably stable under the GGA and mBJ functionals. The partial density of states around the Fermi level demonstrates that the system’s electronic characteristics are determined by the interaction of S-p and Cd-s,p states. A substantial splitting of the valence band, shown by band structure analysis, suggests a greater polarization crystal field in the hexagonal CdS structure. The stereochemical activity of the S atom stimulated the CdS compound for extremely tiny displacements of 0.0014 Å and 0.043 Å for the Cd and S atoms in the [001] direction, respectively, as well as tilting of the Cd-S-Cd link. This compound exhibits lone pair properties, with three valance electrons of the p-orbital participating in covalent bonding and S-s electrons fully occupied. Furthermore, Van Hove singularities are discovered in the electron density of states due to the Cd-d atoms that support in the analysis of optical absorption spectra. Finally, the results suggest that CdS has potential for solar cells applications due to its direct wide band gap semiconductor features,

Direct Observation of Locally Modified Excitonic Effects within a Moiré Unit Cell in Twisted Bilayer Graphene.

Bilayer graphene, which forms moiré superlattices, possesses distinct electronic and optical properties owing to its hybridized energy band and the emergence of van Hove singularities depending on its twist angle. Extensive research has been conducted on the global characteristics of moiré superlattices induced by their long-range periodicity. However, the local properties, which differ owing to the variations in the three-dimensional atomic arrangement, within a moiré unit cell have been rarely explored. In this study, we demonstrate the highly localized excitation of carbon 1s electrons to unoccupied van Hove singularities in twisted bilayer graphene by electron energy loss spectroscopy using a monochromated transmission electron microscope. The core-level excitations associated with the van Hove singularities exhibit a systematic twist-angle dependence analogous to optical excitations. Furthermore, local variations in the core-level van Hove singularity peaks, which can originate from the core-exciton lifetimes and band modifications corresponding to the local stacking geometry within a moiré unit cell, are unambiguously corroborated.

Analytic modelling of quantum capacitance and carrier concentration for [formula omitted]-borophene FET based gas sensor

In this work, we investigate the physical and electronic properties of β12 - borophene FET-based gas sensor using a theoretical quantum capacitance model based on tight-binding approach. We study the impact of adsorbed NH3, NO, NO2 and CO gas molecule on its density of states, carrier concentration, quantum capacitance and I–V characteristics. We found a remarkable variation in the energy band structure and the density of states (DOS) of the β12 - borophene in the presence of the adsorbed gas molecule. The appearance of non-identical Van-Hove singularities in the DOS in the presence of adsorbed gas molecules strongly indicates the high sensitivity of β12 - borophene. We found a significant increase in the carrier concentration for NH3 gas while it decreases for all other gases. Moreover, a drastic change in quantum capacitance and current–voltage relation is also observed in adsorbed gases. The different properties of the given gas molecules are compared with the pristine borophene and found to exhibit distinct wrinkles in each case, thereby indicating the strong selectivity of our proposed gas sensor. Though β12 - borophene is found to be highly sensitive for all studied gases, the NO gas is found to be most sensitive compared to the others.

Superconductivity in twisted double bilayer graphene stabilized by WSe2.

Identifying the essential components of superconductivity in graphene-based systems remains a critical problem in two-dimensional materials research. This field is connected to the mysteries that underpin investigations of unconventional superconductivity in condensed-matter physics. Superconductivity has been observed in magic-angle twisted stacks of monolayer graphene but conspicuously not in twisted stacks of bilayer graphene, although both systems host topological flat bands and symmetry-broken states. Here we report the discovery of superconductivity in twisted double bilayer graphene (TDBG) in proximity to WSe2. Samples with twist angles 1.24° and 1.37° superconduct in small pockets of the gate-tuned phase diagram within the valence and conduction band, respectively. Superconductivity emerges from unpolarized phases near van Hove singularities and next to regions with broken isospin symmetry. Our results show the correlation between a high density of states and the emergence of superconductivity in TDBG while revealing a possible role for isospin fluctuations in the pairing.

Electronic structures and dielectric function of (5, 5) CNT-C2H4O system: A first-principles study on the detection capability of CNT for gas sensing applications.

Carbon nanotubes (CNTs) are known to have a wide range of applications in various fields of discipline. In this research, the ability of metallic armchair (5, 5) CNT to detect acetaldehyde (C2H4O) was investigated using first-principles density functional theory (DFT) as implemented in Quantum ESPRESSO with the Generalized Gradient Approximation (GGA). Accordingly, it was found that C2H4O preserved the metallic behavior of the CNT. However, some bands are found to have overlapped in both the valence and conduction regions of the electronic structures of the resulting system that are mainly caused by the p orbitals of the oxygen and carbon atoms of the compound. These are further confirmed by the projected density of states (pDOS). Moreover, optical transitions are observed in both the real and imaginary parts of the dielectric function caused by the interband transitions between the Van Hove singularities of the electronic structures. In all circumstances, this research has provided more insights into the potential gas sensing applications of metallic CNTs.

Electrical detection of the flat-band dispersion in van der Waals field-effect structures

Two-dimensional flat-band systems have recently attracted considerable interest due to the rich physics unveiled by emergent phenomena and correlated electronic states at van Hove singularities. However, the difficulties in electrically detecting the flat-band position in field-effect structures are slowing down the investigation of their properties. In this work, we use indium selenide (InSe) as a flat-band system due to a van Hove singularity at the valence-band edge in a few-layer form of the material without the requirement of a twist angle. We investigate tunnelling photocurrents in gated few-layer InSe structures and relate them to ambipolar transport and photoluminescence measurements. We observe an appearance of a sharp change in tunnelling mechanisms due to the presence of the van Hove singularity at the flat band. We further corroborate our findings by studying tunnelling currents as a reliable probe for the flat-band position up to room temperature. Our results create an alternative approach to studying flat-band systems in heterostructures of two-dimensional materials.