Band structures of strained kagome lattices

Materials with kagome lattices have attracted significant research attention due to their nontrivial features in energy bands.We theoretically investigate the evolution of electronic band structures of kagome lattices in response to uniaxial strain using both a tight-binding model and an antidot model based on a periodic muffin-tin potential.It is found that the Dirac points move with applied strain.Furthermore,the flat band of unstrained kagome lattices is found to develop into a highly anisotropic shape under a stretching strain along y direction,forming a partially flat band with a region dispersionless along ky direction while dispersive along kx direction.Our results shed light on the possibility of engineering the electronic band structures of kagome materials by mechanical strain.

Superconducting properties of the C15-type Laves phase ZrIr_(2) with an Ir-based kagome lattice

We report systematic studies on superconducting properties of the Laves phase superconductor ZrIr_(2).It crystallizes in a C15-type(cubic MgCu_(2)-type,space group Fd3m)structure in which the Ir atoms form a kagome lattice,with cell parameters a=b=c=7.3596(1)?.Resistivity and magnetic susceptibility measurements indicate that ZrIr_(2) is a type-Ⅱsuperconductor with a transition temperature of 4.0 K.The estimated lower and upper critical fields are 12.8 mT and 4.78 T,respectively.Heat capacity measurements confirm the bulk superconductivity in ZrIr_(2).ZrIr_(2) is found to possibly host strong-coupled s-wave superconductivity with the normalized specific heat change△C_(e)/γT_(c)~1.86 and the coupling strength△_(0)/k_BT_(c)~1.92.First-principles calculations suggest that ZrIr_(2) has three-dimensional Fermi surfaces with simple topologies,and the states at Fermi level mainly originate from the Ir-5d and Zr-4d orbitals.Similar to SrIr_(2) and ThIr_(2),spin–orbit coupling has dramatic influences on the band structure in ZrIr_(2).

Interaction and spin–orbit effects on a kagome lattice at 1/3 filling

We investigate the competing effects of spin-orbit coupling and electron--electron interaction on a kagome lattice at 1/3 filling. We apply the cellular dynamical mean-field theory and its real-space extension combined with the continuous time quantum Monte Carlo method, and obtain a phase diagram including the effects of the interaction and the spin-orbit coupling at T = 0. 1t, where T is the temperature and t is the hopping energy. We find that without the spin-orbit coupling, the system is in a semi-metal phase stable against the electron--electron interaction. The presence of the spin-orbit coupling can induce a topological non-trivial gap and drive the system to a topological insulator, and as the interaction increases, a larger spin--orbit coupling is required to reach the topological insulating phase.

Dynamic magnetic behaviors and magnetocaloric effect of the Kagome lattice:Monte Carlo simulations

Based on the Monte Carlo method,we examined the dynamic magnetic behaviors and magnetocaloric effect of a Kagome lattice subjected to the influence of time-dependent oscillating and time-independent magnetic fields.We used the Ising model to describe the Kagome lattice and study the dynamic order parameters,blocking temperature,internal energy,and phase diagrams.The results revealed that exchange coupling increases the stability of the system and the bias field induces order;however,the time-dependent oscillating magnetic field induces disorder.In addition,the magnetocaloric properties,changes in magnetic entropy,and relative cooling power of the Kagome lattice were investigated.

Electronic and topological properties of kagome lattice LaV3Si2

Topological kagome lattice at the frontier of fundamental physics plays a key role in non-trivial topological quantum state.Here,we predict and investigate kagome lattice rare-earth vanadium-based quantum material LaV3Si2using density functional theory calculations.Both phonon spectrum and crystal transformation show stability of this material,which may be grown by experimental method.Dirac fermions,flat bands,and van Hove points as some basic features are presented in band structure and surface states.Further,symmetry-based compatibility relations support enforced semi-metal for occupied electron numbers with strong Berry curvature.Our results suggest that rare-earth vanadium-based RV3Si2can be treated as a new family kagome lattice.

Superconductivity in kagome metal ThRu_(3)Si_(2)

We report the physical properties of ThRu_(3)Si_(2)featured with distorted Ru kagome lattice.The combined experiments of resistivity,magnetization and specific heat reveal bulk superconductivity with T_(c)=3.8 K.The specific heat jump and calculated electron–phonon coupling indicate a moderate coupled BCS superconductor.In comparison with LaRu_(3)Si_(2),the calculated electronic structure in ThRu_(3)Si_(2)shows an electron-doping effect with electron filling lifted from 100 meV below flat bands to 300 meV above it.This explains the lower superconducting transition temperature and weaker electron correlations observed in ThRu_(3)Si_(2).Our work suggests the Tc and electronic correlations in the kagome superconductor could have an intimate connection with the flat bands.

Crystal growth, magnetic and electrical transport properties of the kagome magnet RCr_(6)Ge_(6)(R= Gd–Tm)

Kagome magnets have attracted considerable research attention due to the interplay between topology,magnetism and electronic correlations.In this study we report single-crystal synthesis of a series of the kagome magnets.RCr_(6)Ge_(6)(R=Gd-Tm) that possess defect-free Cr kagome lattices and systematically study their magnetic and electrical transport properties.The transition from a canted ferrimagnetic to a paramagnetic state in GdCr_(6)Ge_(6),TbCr_(6)Ge_(6),DyCr_(6)Ge_(6),HoCr_(6)Ge_(6),ErCr_(6)Ge_(6) and TmCr_(6)Ge_(6) occurs at 11.3 K,10.8 K,4.3 K,2.5 K,3.3 K and below 2 K,respectively,due to R-R interactions within the compounds.Magnetization measurements reveal highly anisotropic magnetism with canted magnetic moments in these compounds.In electrical transport,both negative and positive magnetoresistances at different magnetic fields and temperatures have been observed due to the competition between different scattering mechanisms.This work enriches our understanding of the Cr-based kagome magnets and paves the way to search for possible topological responses in this family.

Layered kagome compound Na_(2)Ni_(3)S_(4)with topological flat band

We report structural and electronic properties of Na_(2)Ni_(3)S_(4),a quasi-two-dimensional compound composed of alternating layers of[Ni_(3)S_(4)]^(2-)and Na^(+).The compound features a remarkable Ni-based kagome lattice with a square planar configuration of four surrounding S atoms for each Ni atom.Magnetization and electrical measurements reveal a weak paramagnetic insulator with a gap of about 0.5 eV.Our band structure calculation highlights a set of topological flat bands of the kagome lattice derived from the rotated dxz-orbital with C_(3)+T symmetry in the presence of crystal-field splitting.

Formation of honeycomb-Kagome hexagonal superlattice pattern with dark discharges in dielectric barrier discharge

A honeycomb-Kagome hexagonal superlattice pattern with dark discharges is observed in a dielectric barrier discharge system for the first time.The spatiotemporal structure of the honeycomb-Kagome hexagonal superlattice pattern with dark discharges is investigated by an intensified charge-coupled device and the photomultipliers show that it is an interleaving of three different sub-lattices,which are bright-spot,invisible honeycomb lattice,and Kagome lattice with invisible frameworks and dim-spots,respectively.The invisible honeycomb lattices and Kagome lattices are actually composed of dark discharges.By using the optical emission spectra method,it is found that the plasma parameters of the three different sub-lattices are different.The influence of the dark discharges on pattern formation is discussed.The results may have significance for the investigation of the dark discharges and will accelerate the development of self-organized pattern dynamics.

Spin reorientation in easy-plane kagome ferromagnet Li_9Cr_3(P_2O_7)_3(PO_4)_2

We report the successful growth and characterization of Li_9Cr_3(P_2O_7)_3(PO_4)_2single crystal,and investigate its magnetic properties under external magnetic fields via magnetization and heat capacity measurements.Our study reveals that Li_9Cr_3(P_2O_7)_3(PO_4)_2 is an easy-plane kagome ferromagnet with S=3/2,as evidenced by the Curie–Weiss temperature of 6 K which implies a ferromagnetic exchange coupling in the material.Under zero magnetic field,Li_9Cr_3(P_2O_7)_3(PO_4)_2 undergoes a magnetic transition at TC=2.7 K from a paramagnetic state to a ferromagnetically ordered state with the magnetic moment lying in the kagome plane.By applying a c-axis directional magnetic field to rotate the spin alignment from the kagome plane to the c-axis,we observe a reduction in the magnetic transition temperature as the field is increased.We construct a magnetic phase diagram as a function of temperature and magnetic field applied parallel to the c-axis of Li_9Cr_3(P_2O_7)_3(PO_4)_2 and find that the phase boundary is linear over a certain temperature range.Regarding that theoretically,the field-induced phase transition of the spin reorientation in the easy-plane ferromagnet can be viewed as the ferromagnetic magnon Bose–Einstein condensation(BEC),the phase boundary scaling of field-induced(B c)magnetic transition in Li_9Cr_3(P_2O_7)_3(PO_4)_2 can be described as the quasi-2D magnon BEC,which has been observed in other ferromagnetic materials such as K_2CuF_4.

LnCu_(3)(OH)_(6)Cl_(3) (Ln=Gd, Tb, Dy): Heavy lanthanides on spin-1/2 kagome magnets

The spin-1/2 kagome antiferromagnets are key prototype materials for studying frustrated magnetism.Three isostructural kagome antiferromagnets LnCu_(3)(OH)_(6)Cl_(3)(Ln=Gd,Tb,Dy)have been successfully synthesized by the hydrothermal method.LnCu_(3)(OH)_(6)Cl_(3) adopts space group P3m1 and features the layered Cu-kagome lattice with lanthanide Ln3+cations sitting at the center of the hexagons.Although heavy lanthanides(Ln=Gd,Tb,Dy)in LnCu_(3)(OH)_(6)Cl_(3) provide a large effective magnetic moment and ferromagnetic-like spin correlations compared to light-lanthanides(Nd,Sm,Eu)analogues,Cu-kagome holds an antiferromagnetically ordered state at around 17 K like YCu_(3)(OH)_(6)Cl_(3).

Anisotropic magnetism and band evolution induced by ferromagnetic phase transition in titanium-based kagome ferromagnet SmTi_3Bi_4

Kagome magnets with diverse topological quantum responses are crucial for next-generation topological engineering.The anisotropic magnetism and band evolution induced by ferromagnetic phase transition(FMPT)is reported in a newly discovered titanium-based kagome ferromagnet Sm Ti3Bi4,which features a distorted Ti kagome lattice and Sm atomic zig-zag chains.Temperature-dependent resistivity,heat capacity,and magnetic susceptibility reveal a ferromagnetic ordering temperature Tc of23.2 K.A large magnetic anisotropy,observed by applying the magnetic field along three crystallographic axes,identifies the b axis as the easy axis.Angle-resolved photoemission spectroscopy with first-principles calculations unveils the characteristic kagome motif,including the Dirac point at the Fermi level and multiple van Hove singularities.Notably,a band splitting and gap closing attributed to FMPT is observed,originating from the exchange coupling between Sm 4 f local moments and itinerant electrons of the kagome Ti atoms,as well as the time-reversal symmetry breaking induced by the long-range ferromagnetic order.Considering the large in-plane magnetization and the evolution of electronic structure under the influence of ferromagnetic ordering,such materials promise to be a new platform for exploring the intricate electronic properties and magnetic phases based on the kagome lattice.

Imaging momentum-space Cooper pair formation and its competition with the charge density wave gap in a kagome superconductor

The superconducting ground state of kagome metals AV_(3)Sb_(5)(where A stands for K,Rb,or Cs)emerges from an exotic charge density wave(CDW)state that potentially breaks both rotational and time reversal symmetries.However,the specifics of the Cooper pairing mechanism,and the nature of the interplay between these two states remain elusive,largely due to the lack of momentum-space(k-space)superconducting energy gap structure.By implementing Bogoliubov quasiparticle interference(B QPI)imaging,we obtain k-space information on the multiband superconducting gap structureΔ_(SC)^(i)(k)in pristine CsV_(3)Sb_(5).We show that the estimated energy gap on the vanadium d_(xy/x^(2)-y^(2))orbital is anisotropic but nodeless,with a minimal value located near the M point.Interestingly,a comparison ofΔ_(SC)^(i)(k)with the CDW gapΔ_(CDW)^(i)(k)obtained by angle-re solved photoemission spectro scopy(ARPES)reveals direct k-space competition between the se two order parameters,i.e.,the opening of a large(small)CDW gap at a given momentum corresponds to a small(large)superconducting gap.When the long-range CDW order is suppressed by replacing vanadium with titanium,we find a nearly isotropic energy gap on both the V and Sb bands.This information will be critical for identifying the microscopic pairing mechanism and its interplay with intertwined electro nic orders in this kagome superconductor family.

Strong spin frustration and magnetism in kagoméantiferromagnets LnCu_(3)(OH)_(6)Br_(3)(Ln=Nd,Sm,and Eu)

To study the effects of lanthanide ions on the geometrically frustrated antiferromagnets and their magnetic properties,we grew high-quality single crystals of LnCu_(3)(OH)_(6)Br_(3)(Ln=Nd,Sm,and Eu)by hydrothermal method and studied their crystal structures and magnetic properties.The refinements of the crystal structure referred to the powder x-ray diffraction data show that LnCu_(3)(OH)_(6)Br_(3)adopt a Kapellasite-type layer structure,which is isostructural to their chlorine analogue.Magnetic susceptibilities demonstrate that LnCu_(3)(OH)_(6)Br_(3)have strong antiferromagnetic coupling and a pronounced magnetic frustration effect.Magnetization measurements indicate canted antiferromagnetic ordering of Cu^(2+)ions around 16 K within the kagoméplane and weak ferromagnetic coupling.Moreover,shoulder-like anomalies in specific heat around 16 K could be a signature of emergent of magnetic ordering.The low-temperature negative magnetization and specific heat of LnCu_(3)(OH)_(6)Br_(3)(Ln=Nd,Sm,and Eu)indicate that Ln^(3+)ions induce more exotic magnetic ground state properties.

Experimental observation of Fermi-level flat band in novel kagome metal CeNi_(5)

Kagome materials are a class of material with a lattice structure composed of corner-sharing triangles that produce various exotic electronic phenomena,such as Dirac fermions,van Hove singularities,and flat bands.However,most of the known kagome materials have a flat band detached from the Fermi energy,which limits the investigation of the emergent flat band physics.In this work,by combining soft x-ray angle-resolved photoemission spectroscopy(ARPES)and the first-principles calculations,the electronic structure is investigated of a novel kagome metal CeNi_(5) with a clear dispersion along the kz direction and a Fermi level flat band in theΓ–K–M–Γplane.Besides,resonant ARPES experimental results indicate that the valence state of Ce ions is close to 4^(+),which is consistent with the transport measurement result.Our results demonstrate the unique electronic properties of CeNi_(5) as a new kagome metal and provide an ideal platform for exploring the flat band physics and the interactions between different types of flat bands by tuning the valence state of Ce ions.

Selective flattening of magnon bands in kagome-lattice ferromagnets with Dzyaloshinskii-Moriya interaction

There is growing interest in revealing exotic properties of collective spin excitations in kagome-lattice ferromagnets such as magnon Hall effects,topological magnon insulators,and flat magnon bands.Using the well-established nearest-neighbor Heisenberg ferromagnet model with Dzyaloshinskii-Moriya interaction(DMI),in this study we uncover intriguing new aspects in the selectivity and topology of flat magnon bands.Among the three magnon bands(except for the top one,which is flat in the absence of DMI),we observe that each of the three bands can be selectively flattened at the critical DMI of D=±√3 J/3 and D=±√3 J.With a general DMI,the magnon bands become non-flat;however,there are nested lines that create a David star pattern for all three magnon bands whose flatness is robust during changing exchange coupling or DMIs.Contrary to prevailing belief,we show that each of the three flat bands is actually topologically trivial at critical DMIs.Furthermore,we show that while the middle band remains topologically trivial,for the other two bands,D=0 corresponds to the topological phase transition where their Chern numbers get interchanged;when D=±√3 J,the system undergoes a phase transition to the nonferromagnetic state.These central findings increase our understanding of spin excitations for future magnonics applications.

Quantum computation with two-dimensional graphene quantum dots

We study an array of graphene nano sheets that form a two-dimensional S = 1/2 Kagome spin lattice used for quantum computation. The edge states of the graphene nano sheets are used to form quantum dots to confine electrons and perform the computation. We propose two schemes of bang-bang control to combat decoherence and realize gate operations on this array of quantum dots. It is shown that both schemes contain a great amount of information for quantum computation. The corresponding gate operations are also proposed.

Designing topological and correlated 2D magnetic states via superatomic lattice constructions of zirconium dichloride

Magnetic materials could realize the intriguing quantum anomalous Hall effect and metal-to-insulator transition when combined with band topology or electronic correlation,which have broad prospects in quantum information,spintronics,and valleytronics.Here,we propose the approach of designing novel two-dimensional(2D)magnetic states via d-orbital-based superatomic lattices.Specifically,we chose triangular zirconium dichloride disks as superatoms to construct the honeycomb superatomic lattices.Using first-principles calculations,we identified a series of 2D magnetic states with varying sizes of superatoms.We found the non-uniform stoichiometries and geometric effect of superatomic lattice give rise to spin-polarized charges arranged in different magnetic configurations,containing ferromagnetic coloring triangles,antiferromagnetic honeycomb,and ferromagnetic kagome lattices.Attractively,these magnetic states are endowed with nontrivial band topology or strong correlation,forming an ideal Chern insulator or antiferromagnetic Dirac Mott insulator.Our work not only reveals the potential of d-orbital-based superatoms for generating unusual magnetic configurations,but also supplies a new avenue for material engineering at the nanoscale.

Quantum anomalous Hall effect in two-dimensional Cu-dicyanobenzene coloring-triangle lattice

Metrics details Abstract Magnetic two-dimensional(2D)topological insulators with spontaneous magnetization have been predicted to host quantum anomalous Hall effects(QAHEs).For organic topological insulators,the QAHE only exists in honeycomb or Kagome organometallic lattices based on theoretical calculations.Recently,coloring-triangle(CT)lattice has been found to be mathematically equivalent to a Kagome lattice,suggesting a potential 2D lattice to realize QAHE.Here,based on first-principles calculations,we predict an organometallic CT lattice,Cu-dicyanobenzene(DCB),to be a stable QAH insulator.It exhibits ferromagnetic(FM)properties as a result of the charge transfer from metal atoms to DCB molecules.Moreover,based on the Ising model,the Curie temperature of the FM ordering is calculated to be around 100 K.Both the Chern numbers and the chiral edge states of the semi-infinite Cu-DCB edge structure,which occur inside the spin-orbit coupling band gap,confirm its nontrivial topological properties.These make the Cu-DCB CT lattice an ideal candidate to enrich the family of QAH insulators.

Realization of photonic p-orbital higher-order topological insulators

The orbital degrees of freedom play a pivotal role in understanding fundamental phenomena in solid-state materials as well as exotic quantum states of matter including orbital superfluidity and topological semimetals.Despite tremendous efforts in engineering synthetic cold-atom,as well as electronic and photonic lattices to explore orbital physics,thus far high orbitals in an important class of materials,namely,higher-order topological insulators(HOTIs),have not been realized.Here,we demonstrate p-orbital corner states in a photonic HOTI,unveiling their underlying topological invariant,symmetry protection,and nonlinearity-induced dynamical rotation.In a Kagome-type HOTI,we find that the topological protection of p-orbital corner states demands an orbital-hopping symmetry in addition to generalized chiral symmetry.Due to orbital hybridization,nontrivial topology of the p-orbital HOTI is“hidden”if bulk polarization is used as the topological invariant,but well manifested by the generalized winding number.Our work opens a pathway for the exploration of intriguing orbital phenomena mediated by higher-band topology applicable to a broad spectrum of systems.