Dipolar excitonic insulator in a moiré lattice

Two-dimensional (2D) moiré materials provide a highly-controllable solid-state platform for studies of correlated quantum phenomena. To date, experimental studies have focused on the correlated electronic states; the correlated bosonic states in moiré materials have remained practically unexplored. Here, we report a correlated dipolar excitonic insulator--a charge insulating state driven by exciton formation--in a Coulomb-coupled WSe2 monolayer and WSe2/WS2 moiré bilayer at total hole doping density equal to the moiré density. The system is a Mott insulator when all the holes reside in the moiré layer. Under an out-of-plane electric field, the holes can be continuously transferred to the WSe2 monolayer, but remain strongly bound to the empty moiré sites. This is effectively an interlayer exciton fluid in the moiré lattice under a particle-hole transformation. We identify the phase space and determine the charge gap energy of the excitonic insulating state by optical spectroscopy and capacitance measurements, respectively. We further observe the emergence of local magnetic moments in the WSe2 monolayer induced by the strong interlayer Coulomb correlation. Our result paves the path for realizing correlated bosonic quantum phenomena described by the Bose-Hubbard model in a solid-state system.

A semi exact solution for a metallic phase in a Holstein-Hubbard chain at half filling with Gaussian anharmonic phonons

The nature of phase transition from an antiferromagnetic SDW polaronic Mott insulator to the paramagnetic bipolaronic CDW Peierls insulator is studied for the half-filled Holstein-Hubbard model in one dimension in the presence of Gaussian phonon anharmonicity. A number of unitary transformations performed in succession on the Hamiltonian followed by a general many-phonon averaging leads to an effective electronic Hamiltonian which is then treated exactly by using the Bethe-Ansatz technique of Lieb and Wu to determine the energy of the ground state of the system. Next using the Mott–Hubbard metallicity condition, local spin-moment calculation, and the concept of quantum entanglement entropy and double occupancy, it is shown that in a plane spanned by the electron–phonon coupling coefficient and onsite Coulomb correlation energy, there exists a window in which the SDW and CDW phases are separated by an intermediate phase that is metallic.

Fe(phen)2(NCS)2 on Al(100): Influence of AlN layer on spin crossover barrier

We study how a nitride layer affects spin crossover (SCO) in a single Fe(phen)2(NCS)2 (Fephen) molecule adsorbed on the Al(100) surface through ab initio calculations. The Coulomb correlation of the...