Yinong Zhou’s research while affiliated with University of Utah and other places

Flat bands (FBs), presenting a strongly interacting quantum system, have drawn increasing interest recently. However, experimental growth and synthesis of FB materials have been challenging and have remained elusive for the ideal form of monolayer materials where the FB arises from destructive quantum interference as predicted in 2D lattice models. Here, we report surface growth of a self-assembled monolayer of 2D hydrogen-bond (H-bond) organic frameworks (HOFs) of 1,3,5-tris(4-hydroxyphenyl)benzene (THPB) on Au(111) substrate and the observation of FB. High-resolution scanning tunneling microscopy or spectroscopy shows mesoscale, highly ordered, and uniform THPB HOF domains, while angle-resolved photoemission spectroscopy highlights a FB over the whole Brillouin zone. Density-functional-theory calculations and analyses reveal that the observed topological FB arises from a hidden electronic breathing-kagome lattice without atomically breathing bonds. Our findings demonstrate that self-assembly of HOFs provides a viable approach for synthesis of 2D organic topological materials, paving the way to explore many-body quantum states of topological FBs.

Quantum anomalous Hall effect (QAHE) and quantum spin Hall effect (QSHE) are two interesting physical manifestations of 2D materials that have an intrinsic nontrivial band topology. In principle, they are ground-state equilibrium properties characterized by Fermi level lying in a topological gap, below which all the occupied bands are summed to a non-zero topological invariant. Here, we propose theoretical concepts and models of “excited” QAHE (EQAHE) and EQSHE generated by dissociation of an excitonic insulator (EI) state with complete population inversion (CPI), a unique many-body ground state enabled by two yin-yang flat bands (FBs) of opposite chirality hosted in a diatomic Kagome lattice. The two FBs have a trivial gap in between, i.e., the system is a trivial insulator in the single-particle ground-state, but nontrivial gaps above and below, so that upon photoexcitation the quasi-Fermi levels of both electrons and holes will lie in a nontrivial gap achieved by the CPI-EI state, as demonstrated by exact diagonalization calculations. Then dissociation of singlet and triplet EI state will lead to EQAHE and EQSHE, respectively. Realization of yin-yang FBs in real materials are also discussed.

A fractional quantum Hall effect (FQHE) has been predicted in a topological flat band (FB) by a single-particle band structure combined with phenomenological theory or solution of a many-body lattice Hamiltonian with fuzzy parameters. A long-standing roadblock toward the realization of a FB-FQHE is lacking the many-body solution of specific materials under realistic conditions. We demonstrate a combined study of single-particle Floquet band theory with exact diagonalization (ED) of a many-body Hamiltonian. We show that a time-periodic circularly polarized laser inverts the sign of second-nearest-neighbor hopping in a kagome lattice and enhances spin-orbit coupling in one spin channel to produce a Floquet FB with a high flatness ratio of bandwidth over band gap, as exemplified in monolayer Pt 3 C 36 S 12 H 12. The ED of the resultant Floquet-kagome lattice Hamiltonian gives a one-third-filling ground state with a laser-dependent excitation gap of a FQH state, up to an estimated temperature above 70 K. Our findings pave the way for exploring the alluding high-temperature FB-FQHE.

Flat bands (FBs), presenting a strongly interacting quantum system have drawn increasing interest recently, as partly reflected by its discovery in twisted bilayer graphene in association with superconductivity. However, experimental realization of topological FBs has been hampered by lacking FB materials, and especially remained elusive in the ideal platform of monolayer materials where they arise from destructive quantum interference as predicted in 2D lattice models. Here, we report experimental synthesis of mesoscale ordered self-assembled monolayer of 2D hydrogen-bond (H-bond) organic frameworks (HOFs) of 1,3,5-tris(4-hydroxyphenyl)benzene (THPB) on Au(111) surface and the observation of related topological FB. Its formation, atomic and electronic structures have been characterized with a suite of experimental and theoretical techniques in excellent agreement. High-resolution scanning tunneling microscopy/spectroscopy (STM/STS) shows mesoscale, highly-ordered and uniform THPB-HOF domains, while angle-resolved photoemission spectroscopy (ARPES) highlights a FB over the whole Brillouin zone (BZ). Density-functional-theory (DFT) calculations and analyses reveal that the observed topological FB arises from a hidden electronic breathing-Kagome lattice without atomically breathing bonds. Our findings demonstrate that self-assembly of HOFs provides a viable approach for synthesis of 2D organic topological materials, paving the way to explore many-body quantum states of topological FBs.

The excitonic insulator (EI) state is a strongly correlated many-body ground state, arising from an instability in the band structure toward exciton formation. We show that the flat valence and conduction bands of a semiconducting diatomic Kagome lattice, as exemplified in a superatomic graphene lattice, can possibly conspire to enable an interesting triplet EI state, based on density-functional theory calculations combined with many-body GW and Bethe-Salpeter equation. Our results indicate that massive carriers in flat bands with highly localized electron and hole wave functions significantly reduce the screening and enhance the exchange interaction, leading to an unusually large triplet exciton binding energy (∼1.1 eV) exceeding the GW band gap by ∼0.2 eV and a large singlet-triplet splitting of ∼0.4 eV. Our findings enrich once again the intriguing physics of flat bands and extend the scope of EI materials.

Deposition of Bi on InSb(111)B reveals a striking Sierpiński-triangle (ST)-like structure in Bi thin films. Such a fractal geometric topology is further shown to turn off the intrinsic electronic topology in a thin film. Relaxation of a huge misfit strain of about 30% to 40% between Bi adlayer and substrate is revealed to drive the ST-like island formation. A Frenkel-Kontrova model is developed to illustrate the enhanced strain relief in the ST islands offsetting the additional step energy cost. Besides a sufficiently large tensile strain, forming ST-like structures also requires larger adlayer-substrate and intra-adlayer elastic stiffnesses, and weaker intra-adlayer interatomic interactions.

The excitonic insulator (EI) state is a strongly correlated many-body ground state, arising from an instability in the band structure towards exciton formation. We show that the flat valence and conduction bands of a semiconducting diatomic Kagome lattice, as exemplified in a superatomic graphene lattice, conspire to enable an interesting triplet EI state, based on density functional theory (DFT) calculations combined with many-body GW and Bethe-Salpeter Equation (BSE). As an intrinsic property of topological flat bands, massive carriers with highly localized electron and hole wavefunctions significantly reduce the screening and enhance the exchange interaction, leading to an unusually large triplet exciton binding energy (~1.1 eV) exceeding the GW band-gap by ~0.2 eV and a large singlet-triplet splitting of ~0.4 eV. Our findings enrich once again the intriguing physics of at bands and extend the scope of EI materials.

Using first-principles calculations, we investigate the electronic and topological properties of an antiferromagnetic (AFM) superatomic graphene lattice superimposed on a bipartite honeycomb lattice governed by Lieb's theorem of itinerant magnetism. It affords a concrete material realization of the AFM honeycomb model with a Dirac Mott insulating state, characterized by a gap opening at the Dirac point due to inversion symmetry breaking by long-range AFM order. The opposite Berry curvatures of the K and K′ valleys induces a circular dichroism (CD) Hall effect. Different from the valley Hall effect that activates only one valley, the CD Hall effect activates carriers from both K and K′ valleys, generating the opposite directions of transversal Hall currents for the left- and right-handed circularly polarized light, respectively.

π-Orbital bonding plays an important role not only in traditional molecular science and solid-state chemistry but also in modern quantum physics and materials, such as the relativistic Dirac states formed by bonding and antibonding π-bands in graphene. Here, we disclose an interesting manifestation of π-orbitals in forming the Yin–Yang Kagome bands, which host potentially a range of exotic quantum phenomena. Based on first-principles calculations and tight-binding orbital analyses, we show that the frontier π2- and π3-orbitals in anilato-based metal–organic frameworks form concurrently a conduction and valence set of Kagome bands, respectively, but with opposite signs of lattice hopping to constitute a pair of enantiomorphic Yin and Yang Kagome bands, as recently proposed in a diatomic Kagome lattice. The twisted configuration of neighboring benzene-derived organic ligands bridged by an octahedrally O-coordinated metal ion is found to play a critical role in creating the opposite sign of lattice hopping for the π2- versus π3-orbitals. Our finding affords a new material platform to study π-orbital originated quantum chemistry and physics.

Circular dichroism (CD) is generally observed in the optically active chiral molecules that originate from macroscopic electric and magnetic dipoles, which is usually quite small. In solid states, the so-called valley CD may arise microscopically from interband transitions between two chiral electronic valley bands of nonzero Berry curvatures at a given k point. However, generally, two sets of K and K′ valleys coexist in the Brillouin zone with opposite chiral selectivities, so that the net CD is zero for the whole material. Here, we demonstrate a giant CD originating from photoexcitation between two chiral Chern flat bands of opposite Chern numbers, namely, the enantiomorphic flat Chern bands. The dissymmetry factor g of such flat CD can reach the theoretical maximum value of 2 with the optimal spin-orbit coupling strength. Based on first-principles calculations, we identify that the Li intercalated bilayer π-conjugated nickel-bis(dithiolene) hosts a set of yin-yang kagome bands with an estimated large g=0.74 under magnetic field. Furthermore, based on the flat-CD mechanism, we propose two flatband devices of topological photodetectors and circularly polarized lasers.