Van Hove Singularity Research Articles

Prediction of dual quantum spin Hall insulator in NbIrTe4 monolayer

Abstract Dual quantum spin Hall insulator (QSHI) is a newly discovered topological state in the 2D material TaIrTe4, exhibiting both a traditional Z 2 band gap at the charge neutrality point and a Van Hove singularity (VHS) induced correlated Z 2 band gap with weak doping. Inspired by the recent progress in theoretical understanding and experimental measurements, we predicted a promising dual QSHI in the counterpart material of the NbIrTe4 monolayer by first-principles calculations. In addition to the well-known band inversion at the charge neutrality point, two new band inversions were found after a CDW phase transition when the chemical potential is near the VHS, one direct and one indirect Z 2 band gap. The VHS-induced non-trivial band gap is around 10 meV, much larger than that from TaIrTe4. Furthermore, since the new generated band gap is mainly dominated by the 4d orbitals of Nb, electronic correlation effects should be relatively stronger in NbIrTe4 as compared to TaIrTe4. Therefore, the dual QSHI state in the NbIrTe4 monolayer is expected to be a good platform for investigating the interplay between topology and correlation effects.

Van Hove singularity, flat bands, Dirac states, and superconductivity in van der Waals–bonded and covalently bonded bilayer borophene with a coloring triangular lattice

Van Hove singularity, flat bands, Dirac states, and superconductivity in van der Waals–bonded and covalently bonded bilayer borophene with a coloring triangular lattice

Electric Field-Tunable Superconductivity with Competing Orders in Twisted Bilayer Graphene near the Magic Angle.

Superconductivity (SC) in twisted bilayer graphene (tBLG) has been explored by varying carrier concentrations, twist angles, and screening strength, with the aim of uncovering its origin and possible connections to strong electronic correlations in narrow bands and various resulting broken symmetries. However, the link between the tBLG band structure and the onset of SC and other orders largely remains unclear. In this study, we address this crucial gap by examining in situ band structure tuning of a near magic-angle (θ ≈ 0.95°) tBLG device with a displacement field (D) and reveal competition between SC and other broken symmetries. At zero D, the device exhibits superconducting signatures without the resistance peak at half-filling, a characteristic signature with a strong electronic correlation. As D increases, the SC is suppressed, accompanied by the appearance of a resistance peak at half-filling. Hall density measurements reveal that at zero D, SC arises around the van Hove singularity (vHs) from an isospin or spin-valley unpolarized band. At higher D, the suppression of SC coincides with broken isospin symmetry near half-filling with lifted degeneracy (gd ∼ 2). Additionally, as the SC phase becomes weaker with D, vHs shifts to higher fillings, highlighting the modification of the underlying band structure with the applied electric field. These findings, with recent theoretical study on SC in tBLG, highlight the competition, rather than being connected concomitantly, between SC and other orders promoted by broken symmetries.

About the hump in the low-temperature isochoric heat capacity of Ne cryocrystals

The low-temperature dependence of the heat capacity of Ne cryocrystals was analyzed. A hump that appears as a local maximum in the heat capacity curve is shown as C/T3 vs T at Tmax. It was observed that with increasing molar volume, the Tmax shifts to lower temperatures. A universal empirical relation Δ∗ was proposed to describe the excess (non-Debye) heat capacity at low temperatures. It was shown that the value of the universal function Δ∗ depends on both the magnitude of the heat capacity anomaly and the temperature at which the hump occurs, but remains universal and independent of molar volume for noble gas cryocrystals. This study provides new information about the potential origins of the hump in the heat capacity as a manifestation of the first van Hove singularity. The study’s results may be useful in addressing the issue of the calorimetric boson peak in amorphous and disordered solids.

Arbitrary order transfer matrix exceptional points and Van Hove singularities

Arbitrary order transfer matrix exceptional points and Van Hove singularities

Artificial Fine-Tuned van Hove Singularity in Twisted Bilayer and Double-Twist Trilayer Graphene with Enhanced Absorption for Photodetection and Photoemission in the Near-Infrared II Range.

Optical responses of twisted bilayer graphene at targeted wavelengths can be amplified by leveraging energy levels of van Hove singularities (VHS) via tuning periods of moiré superlattices. Therefore, precise control of twist angles as well as the moiré superlattices is necessary for fabricating integrated optoelectronic devices such as photodetectors and emitters. Although recent advances in twist angle control help the observation of correlated states in twisted magic-angle graphene structures, the impact of such precise control on enhanced optical absorption is still under investigation. Here, we employ a cut and stack method to construct tBLG with twist angles finely tuned between 4.6° and 6.6°, aligning the VHS near the telecom band with optimized optical absorption. The effective enhanced optical absorption and interlayer interaction uniformity are confirmed through Raman and reflection contrast spectroscopy. Additionally, we demonstrate increased photodetection responsivity and broadband upconversion photothermal emission, both enhanced by VHS, under resonant laser excitation. The study further explores a double-twist trilayer graphene configuration, resulting in better enhanced absorption due to the generation of additional VHS points. Our results underscore the potential of precisely engineered twisted structures in advancing the performance of optoelectronic devices.

Low-Frequency Resistance Noise in Near-Magic-Angle Twisted Bilayer Graphene.

The low-frequency resistance fluctuations, or noise, in electrical resistance not only set a performance benchmark in devices but also form a sensitive tool to probe nontrivial electronic phases and band structures in solids. Here, we report the measurement of such noise in the electrical resistance in twisted bilayer graphene (tBLG), where the layers are misoriented close to the magic angle (θ ∼ 1°). At high temperatures (T ≳ 60-70 K), the power spectral density (PSD) of the fluctuation inside the low-energy moiré bands is predominantly ∝1/f, where f is the frequency, being generally lowest close to the magic angle, and can be well-explained within the conventional McWhorter model of the '1/f noise' with trap-assisted density-mobility fluctuations. At low T (≲10 K), the measured noise exhibits a strong two-level random telegraphic signal (RTS), especially close to the moiré gap, which exhibits a ∝1/f2-like PSD that can be attributed to poorly screened resonances of the Fermi energy to specific bands of defects in the encapsulating boron nitride (hBN) layers. The low-T noise within the moiré band exhibits a series of minima at the integral as well as half-integral fillings, which align with the frequently observed van Hove singularities in the density-of-states driven by strong Coulomb interaction. Apart from providing a comprehensive account of the origin and the magnitude of noise in tBLG, our experiment also reveals noise to be significantly more sensitive to the underlying interaction effects in tBLG than the conventional time-averaged transport.

Theoretical Investigation of Two-Dimensional FeC4 Structures with Surface Van Hove Singularity for Electrochemical Nitric Oxide Reduction Reaction.

The electrochemical nitric oxide reduction reaction (eNORR) is an efficient method for converting aqueous NO into NH3. The pursuit of innovative electrocatalysts with enhanced activity, selectivity, durability, and cost-effectiveness for NORR remains a research focus. In this study, using particle swarm optimization (PSO) searches, density functional theory (DFT), and the constant-potential method (CPM), we predict two stable two-dimensional FeC4 monolayers, designated as α-FeC4 and β-FeC4, as promising electrocatalysts for the NORR. Our results demonstrate that both α-FeC4 and β-FeC4 monolayers possess intrinsic metallicity with surface Van Hove singularity (SVHS), showing remarkable NORR catalytic performance. Additionally, the substantial disparity in adsorption free energies between NO and H atom at 0 V ensures the high selectivity of these novel FeC4 monolayers toward NORR. These findings not only contribute to the expanding family of two-dimensional transition metal carbides but also provide a new idea for the design of highly efficient NORR electrocatalysts.

DUV Double‐Resonant Raman Spectra and Interference Effect in Graphene: First‐Principles Calculations

ABSTRACTWe calculate double‐resonance Raman (DRR) spectra of monolayer graphene by first‐principles density functional calculation, for wide laser excitation energies from the near‐infrared (1.58 eV) to the deep‐ultraviolet (DUV, 5.41 eV) region. When laser excitation energy, , goes into the DUV region, Raman peak wavenumber for G band switches from red‐shift to blue‐shift and for 2D band switches from red‐shift to constant, in contrast to the continuous blue‐shift of G band. Raman intensity of the three bands generally decreases with increasing , except for around 4.08 eV where Raman intensity diverges due to van Hove singularity of electron density of states. The combined two‐phonon modes change with for both G and G bands (e.g., from 2LO to 2TO and back to 2LO for G and from LA + LO/TO to TA + LO/TO for G) but remain 2LO for 2D band. Further, the dominant DRR scattering process of G band changes from the electron‐hole ( or ) scattering processes to the scattering processes as goes into the DUV region, since the Dirac energy bands become asymmetric between and band that suppresses the process and the Raman intensity. Another factor to suppress the Raman intensity is the quantum interference effect between four scattering processes () which changes from constructive to destructive interference and finally to no interference with increasing . We calculate ‐dependent Raman tensor of the three bands and polarized Raman spectra, which further support the interference effect. The calculated results are directly compared and consistent with the experimental results.

Spin polarization detection via chirality-induced tunnelling currents in indium selenide.

Chirality, a basic property of symmetry breaking, is crucial for fields such as biology and physics. Recent advances in the study of chiral systems have stimulated interest in the discovery of symmetry-breaking states that enable exotic phenomena such as spontaneous gyrotropic order and superconductivity. Here we examine the interaction between light chirality and electron spins in indium selenide and study the effect of magnetic field on emerging tunnelling photocurrents at the Van Hove singularity. Although the effect is symmetric under linearly polarized light excitation, a non-symmetric signal emerges when the excitation is circularly polarized, making it possible to electrically detect light's chirality. Our study shows a negligible out-of-plane g-factor for few-layer indium selenide at the valence band edge, resulting in an unbalanced Zeeman splitting in hexagonal boron nitride spin bands. This finding allows us to measure the change in energy barrier height with exceptional resolution (~15 μeV). Furthermore, we confirm the long-standing theoretical prediction of spin-polarized hole accumulation in the flat valence band at increasing laser powers.

Emergent superconductivity driven by Van Hove singularity in a Janus Mo2PS monolayer.

Two-dimensional (2D) Janus structures with the breaking of out-of-plane mirror symmetry can induce many interesting physical phenomena, and have attracted widespread attention. Herein, we propose a Mo2PS monolayer with mirror asymmetry, identified by first-principles structural search calculations, which demonstrates high thermodynamic and dynamic stability. Our findings reveal that Mo 4d-orbitals dominate the metallicity, significantly enhancing the density of states near the Fermi level due to Van Hove singularities (VHSs), leading to the existence of phonon-mediated superconductivity. Notably, tensile strain elevates the critical temperature (Tc) nearly tenfold, driven by strong coupling between softened acoustic modes of Mo vibrations and a new saddle point VHS at point X. These results highlight that the Janus Mo2PS monolayer serves as a promising candidate for 2D straintronic applications with desirable physical properties.

Unveiling van Hove singularity modulation and fluctuated charge order in kagome superconductor CsV3Sb5 via time-resolved ARPES

Kagome superconductor CsV3Sb5, which exhibits intertwined unconventional charge-density wave (CDW) and superconductivity, has garnered significant attention recently. Despite extensive static studies, the nature of these exotic electronic orders remains elusive. In this study, we investigate the nonequilibrium electronic structure of CsV3Sb5 via time- and angle-resolved photoemission spectroscopy (ARPES). Our results reveal that upon laser excitation, the van Hove singularities immediately shift towards the Fermi level and subsequently oscillate in sync with a 1.3-THz coherent phonon mode. By analyzing the coherent intensity oscillations in the energy-momentum (-k) map, we find that this coherent phonon is strongly coupled with electronic bands from both Sb and V orbitals. While typically observable only in the CDW state, remarkably, we find that the 1.3-THz coherent phonon mode can be persistently excited at temperatures above TCDW, suggesting the potential existence of fluctuated CDW in CsV3Sb5. These findings enhance our understanding of the unconventional CDW in CsV3Sb5 and provide insights into the ultrafast control of kagome superconductivity. Published by the American Physical Society 2024

Unveiling van Hove Singularity-Boosted Photothermoelectric Response for Wearable Human-Radiation Detection.

Van Hove singularity (vHs), the singularity point of density of states (DOS) in crystalline solids, is a research hotspot in emerging phenomena such as light-matter interaction, superconducting, and quantum anomalous Hall effect. Although the significance of vHs in photothermoelectric (PTE) effect has been recognized, its integral role in electron excitation and thermoelectric effect is still unclear, particularly in the mid-infrared band that suffers from Pauli blockade in semimetals. Here, we unveil the Fermi-level-modulated PTE behavior in the vicinity of vHs in carbon nanotubes, employing ionic-liquid gating. The concurrent enhancement of optical absorption and thermoelectric effect effectively improves the overall photoresponse by tens of folds at the vHs point. Generally applicable to strongly correlated systems such as metallic 1D nanomaterials and 2D Moiré systems, a quantitative correlation between PTE photodetectivity and electronic DOS is derived in the vicinity of the vHs point. Finally, chemically doped PTE mid-infrared detectors with graded doping levels are demonstrated to exhibit human-radiation sensitivity, high flexibility, and high transparency, paving the way for wearable sensor networks in healthcare systems and the Internet of Things.

Topological Superconductivity in Heavily Doped Single-Layer Graphene.

The existence of superconductivity (SC) appears to be established in both twisted and nontwisted graphene multilayers. However, whether their building block, single-layer graphene (SLG), can also host SC remains an open question. Earlier theoretical works predicted that SLG could become a chiral d-wave superconductor driven by electronic interactions when doped to its van Hove singularity, but questions such as whether the d-wave SC survives the strong band renormalizations seen in experiments, its robustness against the source of doping, or if it will occur at any reasonable critical temperature (Tc) have remained difficult to answer, in part due to uncertainties in model parameters. Furthermore, doping of graphene beyond its van Hove singularity remained experimentally challenging and was not demonstrated until recently. In this study, we n dope SLG past the van Hove singularity by employing Tb intercalation and derive structural models from angle-resolved photoemission spectroscopy measurements. We adopt a reliable numerical framework based on a random-phase approximation technique to investigate the emergence of unconventional SC in the heavily doped monolayer. We predict that robust d + id topological SC could arise in SLG doped by Tb, with a Tc up to 600 mK. We also employ first-principles calculations to investigate the possibility of realizing d-wave SC with other dopants, such as Li or Cs. We find that dopants that change the lattice symmetry of SLG are detrimental to the d-wave state. The stability of the d-wave SC predicted here in Tb-doped SLG could provide a valuable insight for guiding future experimental efforts aimed at exploring topological superconductivity in monolayer graphene.

Pressure-Dependent Electronic Superlattice in the Kagome Superconductor CsV_{3}Sb_{5}.

We present a high-resolution single crystal x-ray diffraction study of kagome superconductor CsV_{3}Sb_{5}, exploring its response to variations in pressure and temperature. We discover that at low temperatures, the structural modulations of the electronic superlattice, commonly associated with charge-density-wave order, undergo a transformation around p∼0.7 GPa from the familiar 2×2 pattern to a long-range-ordered modulation at wave vector q=(0,3/8,1/2). Our observations align with inferred changes in the charge-density-wave pattern from prior transport and nuclear-magnetic-resonance studies, providing new insights into these transitions. Interestingly, the pressure-induced variations in the electronic superlattice correlate with two peaks in the superconducting transition temperature as pressure changes, hinting that fluctuations within the electronic superlattice could be key to stabilizing superconductivity. However, our findings contrast with the minimal pressure dependency anticipated by abinitio calculations of the electronic structure. They also challenge prevailing scenarios based on a Peierls-like nesting mechanism involving Van Hove singularities.

Uniaxial strain tuning of charge modulation and singularity in a kagome superconductor

Tunable quantum materials hold great potential for applications. Of special interest are materials in which small lattice strain induces giant electronic responses. The kagome compounds AV3Sb5 (A = K, Rb, Cs) provide a testbed for electronic tunable states. In this study, through angle-resolved photoemission spectroscopy, we provide comprehensive spectroscopic measurements of the electronic responses induced by compressive and tensile strains on the charge-density-wave (CDW) and van Hove singularity (VHS) in CsV3Sb5. We observe a tripling of the CDW gap magnitudes with ~ 1% strain. Simultaneously, changes of both energy and mass of the VHS are observed. Combined, this reveals an anticorrelation between the unconventional CDW order parameter and the mass of the VHS, and highlight the role of the latter in the superconducting pairing. The substantial electronic responses uncover a rich strain tunability of the versatile kagome system in studying quantum interplays under lattice variations.

Manifestation of Layer-by-Layer Localization of van Hove Singularities in Tunneling between Bilayer Graphene Sheets

Tunneling between two sheets of bilayer graphene, the crystal lattices of which are rotated relative to each other by a small angle, has been studied. An anomalous behavior of the tunneling conductivity caused by van Hove singularities at the edges of the conduction and valence bands spatially localized in different sheets of bilayer graphene has been found.

Commensurate antiferromagnetic fluctuations with small electron doping

We investigate the presence of antiferromagnetic fluctuations in the longitudinal and transversal spin susceptibilities of a square lattice. The inclusion of both first and second neighbour hopping terms, along with exchange coupling, induces anti-ferromagnetic fluctuations over a finite range of fillings in both longitudinal and transversal static spin susceptibilities. In the absence of on-site Hubbard interaction, we observe incommensurate antiferromagnetic fluctuations between the two Van Hove fillings. Beyond the second Van Hove singularity at n=1.03, commensurate antiferromagnetic fluctuations dominate in both longitudinal and transversal spin susceptibilities. When incorporating a finite Hubbard interaction strength, U, we find that the commensurate anti-ferromagnetic fluctuations are preserved in both longitudinal and transversal dressed spin susceptibilities. However anti-ferromagnetic fluctuations vanish beyond a critical value of the Hubbard coupling strength, Uc. Despite the exchange field can be induced by a ferromagnetic substrate, we do not observe any ferromagnetic fluctuations in the system.

High-Order Van Hove Singularities and Their Connection to Flat Bands

High-Order Van Hove Singularities and Their Connection to Flat Bands

Three-dimensional higher-order saddle-point-induced flatbands in Co-based kagome metals

The saddle point (Van Hove singularity) exhibits a divergent density of states in two-dimensional systems, leading to fascinating phenomena such as strong correlations and unconventional superconductivity, yet it is seldom observed in three-dimensional (3D) systems. In this work we find two types of 3D higher-order saddle points (HOSPs) in emerging 3D kagome metals YbCo6Ge6 and MgCo6Ge6. Both HOSPs exhibit a singularity in their density of states, which is significantly enhanced compared to the ordinary saddle point. The HOSP near the Fermi energy generates a flatband extending a large area in the Brillouin zone, potentially amplifying the correlation effect and fostering electronic instabilities. Two types of HOSPs exhibit distinct robustness upon element substitution and lattice distortions in these kagome compounds. Our work paves the way for engineering exotic band structures, such as saddle points and flatbands, and exploring interesting phenomena in Co-based kagome materials. Published by the American Physical Society 2024