Phase diagram of the half-filled ionic Hubbard model

Abstract

We study the phase diagram of the ionic Hubbard model (IHM) at half-filling on a Bethe lattice of infinite connectivity using dynamical mean field theory (DMFT), with two impurity solvers, namely, iterated perturbation theory (IPT) and continuous time quantum Monte Carlo (CTQMC). The physics of the IHM is governed by the competition between the staggered ionic potential ∆ and the on-site Hubbard U . We find that for a finite ∆ and at zero temperature, long range antiferromagnetic (AFM) order sets in beyond a threshold U = UAF via a first order phase transition. For U smaller than UAF the system is a correlated band insulator. Both the methods show a clear evidence for a quantum transition to a half-metal phase just after the AFM order is turned on, followed by the formation of an AFM insulator on further increasing U . We show that the results obtained within both the methods have good qualitative and quantitative consistency in the intermediate to strong coupling regime at zero temperature as well as at finite temperature. On increasing the temperature, the AFM order is lost via a first order phase transition at a transition temperature TAF (U, ∆) (or, equivalently, on decreasing U below UAF (T, ∆)), within both the methods, for weak to intermediate values of U/t. In the strongly correlated regime, where the effective low energy Hamiltonian is the Heisenberg model, IPT is unable to capture the thermal (Neel) transition from the AFM phase to the paramagnetic phase, but the CTQMC does. At a finite temperature T , DMFT+CTQMC shows a second phase transition (not seen within DMFT+IPT) on increasing U beyond UAF . At UN > UAF , when the Neel temperature TN for the effective Heisenberg model becomes lower than T , the AFM order is lost via a second order transition. For2 ) where x = 2∆/U and thus TN increases with increase in ∆/U . In the 3-dimensional parameter space of (U/t, T /t and ∆/t), as T increases, the surface of first order transition at UAF (T, ∆) and that of the second order transition at UN (T, ∆) approach each other, shrinking the range over which the AFM order is stable. There is a line of tricritical points that separates the surfaces of first and second order phase transitions.

Unconventional superconductivity in a strongly correlated band-insulator without doping

“…For t = 0, the system shows a direct first order transition from an AF ordered phase to a correlated band insulator with a sliver of a half-metallic AF phase close to the AF transition point. This is consistent with a variational quantum Monte Carlo study of the half-filled IHM for t = 0 [32] as well as with most other earlier work [33,34]. When t is non-zero, due to the breaking of particle-hole symmetry as well as the frustration induced by the second neighbour spin-exchange coupling J , the system first attains ferrimagnetic order characterized by non-zero values of both the staggered (m s ) and the uniform (m f ) magnetizations, for a range of U/∆, beyond which it has pure AF order as shown in panel (a) of Fig.…”

Atomic reconstruction and flat bands in strain engineered transition metal dichalcogenide bilayer moiré systems

“…Our findings on the localization of the flat band wavefunctions at the conduction band edge in anti-parallelly stacked MoS 2 are consistent with recent spectroscopic measurements [1]. Furthermore, we show that these isolated flat bands in strained MSLs are ideal for studying the ionic Hubbard [43,44,45,46,47,48,49] and Hubbard models [50,51] on a honeycomb lattice and triangular lattice, respectively. We also fit the bands of the biaxially strained MSL to tight-binding models and a k.p continuum model.…”

Quantum Integrability of 1D Ionic Hubbard Model

“…The nature of transition between BI and MI and question of possible phases between BI and MI has been the subject of debates. This model in arbitrary dimension and at half‐filling has been suited by many authors employing various techniques …”