Discrete vortex bound states with a van Hove singularity in the vicinity of the Fermi level

A theoretical study on discrete vortex bound states is carried out near a vortex core in the presence of a van Hove singularity(VHS) near the Fermi level by solving Bogoliubov–de Gennes(Bd G) equations. When the VHS lies exactly at the Fermi level and also at the middle of the band, a zero-energy state and other higher-energy states whose energy ratios follow integer numbers emerge. These discrete vortex bound state peaks undergo a splitting behavior when the VHS or Fermi level moves away from the middle of the band. Such splitting behavior will eventually lead to a new arrangement of quantized vortex core states whose energy ratios follow half-odd-integer numbers.

Observation of Van Hove Singularities and Temperature Dependence of Electrical Characteristics in Suspended Carbon Nanotube Schottky Barrier Transistors

A Van Hove singularity(VHS) is a singularity in the phonon or electronic density of states of a crystalline solid. When the Fermi energy is close to the VHS, instabilities will occur, which can give rise to new phases of matter with desirable properties. However, the position of the VHS in the band structure cannot be changed in most materials. In this work, we demonstrate that the carrier densities required to approach the VHS are reached by gating in a suspended carbon nanotube Schottky barrier transistor. Critical saddle points were observed in regions of both positive and negative gate voltage, and the conductance flattened out when the gate voltage exceeded the critical value. These novel physical phenomena were evident when the temperature is below 100 K. Further, the temperature dependence of the electrical characteristics was also investigated in this type of Schottky barrier transistor.

Effects of van Hove Singularities on Transport of Quantum Dot Systems in Kondo Regime

In the present paper, we study the effect of van Hove singularities of conduction electron on the transport of a single quantum dot system in the Kondo regime. By using both the equation-of-motion and the noncrossing approximation techniques, we show that the corrections caused by these singularities are actually minor. It can be explained by observing that the singularities in the equations, which determine the electronic DOS on the dot, are integrable. Furthermore, we find that, although each line width function is divergent at van Hove singular points, the total divergence is canceled out in the final formula to calculate the current through the system. Therefore, as far as the qualitative properties of the system is concerned, these singularities can be ignored and the wide-band approximation can be safely used in calculation.

Spontaneous isospin polarization and quantum Hall ferromagnetism in a rhombohedral trilayer graphene superlattice

Moiré superlattices in van der Waals heterostructures have recently attracted enormous interests, due to the highly controllable electronic correlation that gives rise to superconductivity, ferromagnetism, and nontrivial topological properties. To gain a deep understanding of such exotic properties, it is essential to clarify the broken symmetry between spin and valley flavors which universally exists in these ground states. Here in a rhombohedral trilayer graphene crystallographically aligned with a hexagonal boron nitride, we report various kinds of symmetry-breaking transition tuned by displacement fields(D) and magnetic fields:(ⅰ) While it is well known that a finite D can enhance correlation to result in correlated insulators at fractional fillings of a flat band, we find the correlation gap emerges before the flavor is fully filled at a positive D, but the sequence is reversed at a negative D.(ⅱ) Around zero D, electronic correlation can be invoked by narrow Landau levels, leading to quantum Hall ferromagnetism that lifts all the degeneracies including not only spin and valley but also orbital degrees of freedom. Our result unveils the complication of transitions between symmetry-breaking phases, shedding light on the mechanisms of various exotic phenomena in strongly correlated systems.

Correlated states in alternating twisted bilayer–monolayer–monolayer graphene heterostructure

Highly controlled electronic correlation in twisted graphene heterostructures has gained enormous research interests recently,encouraging exploration in a wide range of moirésuperlattices beyond the celebrated twisted bilayer graphene.Here we characterize correlated states in an alternating twisted Bernal bilayer–monolayer–monolayer graphene of~1.74°,and find that both van Hove singularities and multiple correlated states are asymmetrically tuned by displacement fields.In particular,when one electron per moiréunit cell is occupied in the electron-side flat band,or the hole-side flat band(i.e.,three holes per moiréunit cell),the correlated peaks are found to counterintuitively grow with heating and maximize around 20 K–a signature of Pomeranchuk effect.Our multilayer heterostructure opens more opportunities to engineer complicated systems for investigating correlated phenomena.

Van Hove Singularities as a Result of Quantum Confinement:The Origin of Intriguing Physical Properties in Pb Thin Films

In situ angle-resolved photoemission spectroscopy(ARPES)and scanning tunneling spectroscopy(STS)have been used to study the electronic structure of Pb thin films grown on a Si(111)substrates.The experiments reveal that the electronic structure near the Fermi energy is dominated by a set of m-shaped subbands because of strong quantum confinement in the films,and the tops of the m-shaped subbands form an intriguing ring-like Van Hove singularity.Combined with theoretical calculations,we show that it is the Van Hove singularity that leads to an extremely high density of states near the Fermi energy and the recently reported strong oscillations(with a period of two monolayers)in various properties of Pb films.

Density wave and topological superconductivity in the magic-angle-twisted bilayer-graphene

The model dependence in the study of the magic-angle twisted bilayer-graphene(MA-TBG)is an important issue in the research area.It has been argued previously that the two-band tight-binding(TB)model(per spin and valley)cannot serve as a start point for succeeding studies as it cannot correctly describe the topological aspect of the continuumtheory model near the Dirac nodes in the mini Brillouin zone(MBZ).For this purpose,we adopt the faithful TB model[Phys.Rev.B 99195455(2019)]with five bands(per spin and valley)as our start point,which is further equipped with extended Hubbard interactions.Then after systematic random-phase-approximation(RPA)based calculations,we study the electron instabilities of this model,including the density wave(DW)and superconductivity(SC),near the van Hove singularity(VHS).Our results are as follows.In the case neglecting the tiny inter-valley exchange interaction,the exact SU(2)K×SU(2)K symmetry leads to the degeneracy between the inter-valley charge DW(CDW)and the spin DW(SDW)(which would be mixed then),and that between the singlet d+id-wave and triplet p+ip-wave topological SCs.When a realistic tiny inter-valley exchange interaction is turned on with nonzero coefficient(J_H=0),the SDW or CDW is favored respectively at the critical point,determined by JH→0-or JH→0+.In the mean time,the degeneracy between the singlet d+id-wave and triplet p+ip-wave topological SCs is also lifted up by the tiny JH.These results are highly similar to the results of our previous study[arXiv:2003.09513]adopting the two-band TB model,with the reason lying in that both models share the same symmetry and Fermi-surface(FS)nesting character near the VHS.Such a similarity suggests that the low-energy physics of the doped MA-TBG is mainly determined by the symmetry and the shape of the FS of the doped system,and is insensitive to other details of the band structure,including the topological aspects near the Dirac nodes in the MBZ.

Tunable correlation in twisted monolayer–trilayer graphene

Flat-band physics of moirésuperlattices,originally discovered in the celebrated twisted bilayer graphene,have recently been intensively explored in multilayer graphene systems that can be further controlled by electric field.In this work,we experimentally find the evidence of correlated insulators at half filling of the electron moiréband of twisted monolayer–trilayer graphene with a twist angle around 1.2°.Van Hove singularity(VHS),manifested as enhanced resistance and zero Hall voltage,is observed to be distinct in conduction and valence flat bands.It also depends on the direction and magnitude of the displacement fields,consistent with the asymmetric crystal structure.While the resistance ridges at VHS can be enhanced by magnetic fields,when they cross commensurate fillings of the moirésuperlattice in the conduction band,the enhancement is so strong that signatures of correlated insulator appear,which may further develop into an energy gap depending on the correlation strength.At last,Fermi velocity derived from temperature coefficients of resistivity is compared between conduction and valence bands with different displacement fields.It is found that electronic correlation has a negative dependence on the Fermi velocity,which in turn could be used to quantify the correlation strength.

Opportunities and Challenges in Twisted Bilayer Graphene:A Review

Two-dimensional(2D)materials exhibit enhanced physical,chemical,electronic,and optical properties when compared to those of bulk materials.Graphene demands significant attention due to its superior physical and electronic characteristics among different types of 2D materials.The bilayer graphene is fabricated by the stacking of the two monolayers of graphene.The twisted bilayer graphene(tBLG)superlattice is formed when these layers are twisted at a small angle.The presence of disorders and interlayer interactions in tBLG enhances several characteristics,including the optical and electrical properties.The studies on twisted bilayer graphene have been exciting and challenging thus far,especially after superconductivity was reported in tBLG at the magic angle.This article reviews the current progress in the fabrication techniques of twisted bilayer graphene and its twisting angle-dependent properties.

p+ip-wave pairing symmetry at type-II van Hove singularities

Based on the random phase approximation calculation in two-orbital honeycomb lattice model,we investigate the pairing symmetry of Ni-based transition-metal trichalcogenides by electron doping access to type-II van Hove singularities(vHs).We find that chiral even-parity d+id-wave(Eg)state is suppressed by odd-parity p+ip-wave(Eu)state when electron doping approaches the type-II vHs.The type-II vHs peak in density of states(DOS)enables to strengthen the ferromagnetic fluctuation,which is responsible for triplet pairing.The competition between antiferromagnetic and ferromagnetic fluctuation results in pairing phase transition from singlet to triplet pairing.The Ni-based transition-metal trichalcogenides provide a promising platform to unconventional superconductor emerging from electronic DOS.

具有高阶范霍夫奇点的自选轨道耦合体系中的鲁棒拓扑超导

Van Hove singularities in proximity to the Fermi level promote electronic interactions and generate diverse competing instabilities.It is also known that a nontrivial Berry phase derived from spin–orbit coupling can introduce an intriguing decoration into the interactions and thus alter correlated phenomena.However,it is unclear how and what type of new physics can emerge in a system featured by the interplay between van Hove singularities(VHSs)and the Berry phase.Here,based on a general Rashba model on the square lattice,we comprehensively explore such an interplay and its significant influence on the competing electronic instabilities by performing a parquet renormalization group analysis.Despite the existence of a variety of comparable fluctuations in the particle–particle and particle-hole channels associated with higher-order VHSs,we find that the chiral p±ip pairings emerge as two stable fixed trajectories within the generic interaction parameter space,namely the system becomes a robust topological superconductor.The chiral pairings stem from the hopping interaction induced by the nontrivial Berry phase.The possible experimental realization and implications are discussed.Our work sheds new light on the correlated states in quantum materials with strong spin–orbit coupling(SOC)and offers fresh insights into the exploration of topological superconductivity.

Angle-tunable intersubband photoabsorption and enhanced photobleaching in twisted bilayer graphene

Van der Waals heterostructures obtained by artificially stacking two-dimensional crystals represent the frontier of material engineering,demonstrating properties superior to those of the starting materials.Fine control of the interlayer twist angle has opened new possibilities for tailoring the optoelectronic properties of these heterostructures.Twisted bilayer graphene with a strong interlayer coupling is a prototype of twisted heterostructure inheriting the intriguing electronic properties of graphene.Understanding the effects of the twist angle on its out-of-equilibrium optical properties is crucial for devising optoelectronic applications.With this aim,we here combine excitation-resolved hot photoluminescence with femtosecond transient absorption microscopy.The hot charge carrier distribution induced by photo-excitation results in peaked absorption bleaching and photo-induced absorption bands,both with pronounced twist angle dependence.Theoretical simulations of the electronic band structure and of the joint density of states enable to assign these bands to the blocking of interband transitions at the van Hove singularities and to photo-activated intersubband transitions.The tens of picoseconds relaxation dynamics of the observed bands is attributed to the angle-dependence of electron and phonon heat capacities of twisted bilayer graphene.