We theoretically investigated the chiral phonons of honeycomb-type bilayer Wigner crystals recently discovered in van der Waals structures of layered transition metal dichalcogenides. These chiral phonons can emerge u...We theoretically investigated the chiral phonons of honeycomb-type bilayer Wigner crystals recently discovered in van der Waals structures of layered transition metal dichalcogenides. These chiral phonons can emerge under the inversion symmetry breaking introduced by an effective mass imbalance between the two layers or a moiré potential in one layer, as well as under the time-reversal symmetry breaking realized by applying a magnetic field. Considering the wide tunability of layered materials, the frequencies and chirality of phonons can both be tuned by varying the system parameters. These findings suggest that bilayer honeycomb-type Wigner crystals can serve as an exciting new platform for studying chiral phonons.展开更多
The physics of flat band is novel and rich but difficult to access.In this regard,recently twisting of bilayer van der Waals(vd W)-bounded two-dimensional(2 D)materials has attracted much attention,because the reducti...The physics of flat band is novel and rich but difficult to access.In this regard,recently twisting of bilayer van der Waals(vd W)-bounded two-dimensional(2 D)materials has attracted much attention,because the reduction of Brillouin zone will eventually lead to a diminishing kinetic energy.Alternatively,one may start with a 2 D kagome lattice,which already possesses flat bands at the Fermi level,but unfortunately these bands connect quadratically to other(dispersive)bands,leading to undesirable effects.Here,we propose,by first-principles calculation and tight-binding modeling,that the same bilayer twisting approach can be used to isolate the kagome flat bands.As the starting kinetic energy is already vanishingly small,the interlayer vd W potential is always sufficiently large irrespective of the twisting angle.As such the electronic states in the(connected)flat bands become unstable against a spontaneous Wigner crystallization,which is expected to have interesting interplays with other flat-band phenomena such as novel superconductivity and anomalous quantum Hall effect.展开更多
We investigate the spin-orbit coupling effect in a two-dimensional （2D） Wigner crystal. It is shown that sufficiently strong spin-orbit coupling and an appropriate sign of g-factor could transform the Wigner crystal...We investigate the spin-orbit coupling effect in a two-dimensional （2D） Wigner crystal. It is shown that sufficiently strong spin-orbit coupling and an appropriate sign of g-factor could transform the Wigner crystal to a topological phonon system. We demonstrate the existence of chiral phonon edge modes in finite size samples, as well as the robustness of the modes in the topological phase. We explore the possibility of realizing the topological phonon system in 2D Wigner crystals confined in semiconductor quantum wells/heterostructure. It is found that the spin-orbit coupling is too weak for driving a topological phase transition in these systems. It is argued that one may look for topological phonon systems in correlated Wigner crystals with emergent effective spin-orbit coupling.展开更多
Dirac materials,starting with graphene,have drawn tremendous research interest in the past decade.Instead of focusing on the p_(z)orbital as in graphene,we move a step further and study orbital-active Dirac materials,...Dirac materials,starting with graphene,have drawn tremendous research interest in the past decade.Instead of focusing on the p_(z)orbital as in graphene,we move a step further and study orbital-active Dirac materials,where the orbital degrees of freedom transform as a two-dimensional irreducible representation of the lattice point group.Examples of orbital-active Dirac materials occur in a broad class of systems,including transition-metal-oxide heterostructures,transition-metal dichalcogenide monolayers,germanene,stanene,and optical lattices.Different systems are unified based on symmetry principles.The band structure of orbital-active Dirac materials features Dirac cones at K(K′)and quadratic band touching points at Γ,regardless of the origin of the orbital degrees of freedom.In the strong anisotropy limit,i.e.,when theπ-bonding can be neglected,flat bands appear due to the destructive interference.These features make orbital-active Dirac materials an even wider playground for searching for exotic states of matter,such as the Dirac semi-metal,ferromagnetism,Wigner crystallization,quantum spin Hall state,and quantum anomalous Hall state.展开更多
基金supported by Tencent’s Program of Aspiring Explorers in Sciencesupport by the National Natural Science Foundation of China (Grant No. 12274477)the Department of Science and Technology of Guangdong Province in China (Grant No. 2019QN01X061)。
文摘We theoretically investigated the chiral phonons of honeycomb-type bilayer Wigner crystals recently discovered in van der Waals structures of layered transition metal dichalcogenides. These chiral phonons can emerge under the inversion symmetry breaking introduced by an effective mass imbalance between the two layers or a moiré potential in one layer, as well as under the time-reversal symmetry breaking realized by applying a magnetic field. Considering the wide tunability of layered materials, the frequencies and chirality of phonons can both be tuned by varying the system parameters. These findings suggest that bilayer honeycomb-type Wigner crystals can serve as an exciting new platform for studying chiral phonons.
基金the National Natural Science Foundation of China(Grant No.11874314)supported by U.S.DOE under Grant No.DE-SC0002623。
文摘The physics of flat band is novel and rich but difficult to access.In this regard,recently twisting of bilayer van der Waals(vd W)-bounded two-dimensional(2 D)materials has attracted much attention,because the reduction of Brillouin zone will eventually lead to a diminishing kinetic energy.Alternatively,one may start with a 2 D kagome lattice,which already possesses flat bands at the Fermi level,but unfortunately these bands connect quadratically to other(dispersive)bands,leading to undesirable effects.Here,we propose,by first-principles calculation and tight-binding modeling,that the same bilayer twisting approach can be used to isolate the kagome flat bands.As the starting kinetic energy is already vanishingly small,the interlayer vd W potential is always sufficiently large irrespective of the twisting angle.As such the electronic states in the(connected)flat bands become unstable against a spontaneous Wigner crystallization,which is expected to have interesting interplays with other flat-band phenomena such as novel superconductivity and anomalous quantum Hall effect.
基金Supported by the National Basic Research Program of China under Grant No 2015CB921101the National Natural Science Foundation of China under Grant No 11325416
文摘We investigate the spin-orbit coupling effect in a two-dimensional （2D） Wigner crystal. It is shown that sufficiently strong spin-orbit coupling and an appropriate sign of g-factor could transform the Wigner crystal to a topological phonon system. We demonstrate the existence of chiral phonon edge modes in finite size samples, as well as the robustness of the modes in the topological phase. We explore the possibility of realizing the topological phonon system in 2D Wigner crystals confined in semiconductor quantum wells/heterostructure. It is found that the spin-orbit coupling is too weak for driving a topological phase transition in these systems. It is argued that one may look for topological phonon systems in correlated Wigner crystals with emergent effective spin-orbit coupling.
基金supported by the National Natural Science Foundation of China through Grant No.12174317,No.11729402 and No.12234016.
文摘Dirac materials,starting with graphene,have drawn tremendous research interest in the past decade.Instead of focusing on the p_(z)orbital as in graphene,we move a step further and study orbital-active Dirac materials,where the orbital degrees of freedom transform as a two-dimensional irreducible representation of the lattice point group.Examples of orbital-active Dirac materials occur in a broad class of systems,including transition-metal-oxide heterostructures,transition-metal dichalcogenide monolayers,germanene,stanene,and optical lattices.Different systems are unified based on symmetry principles.The band structure of orbital-active Dirac materials features Dirac cones at K(K′)and quadratic band touching points at Γ,regardless of the origin of the orbital degrees of freedom.In the strong anisotropy limit,i.e.,when theπ-bonding can be neglected,flat bands appear due to the destructive interference.These features make orbital-active Dirac materials an even wider playground for searching for exotic states of matter,such as the Dirac semi-metal,ferromagnetism,Wigner crystallization,quantum spin Hall state,and quantum anomalous Hall state.
