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Mott Transition in Multi-Orbital Models for Iron Pnictides
The bad-metal behavior of the iron pnictides has motivated a theoretical
description in terms of a proximity to Mott localization. Since the parent
compounds of the iron pnictides contain an even number of 3d-electrons per Fe,
it is important to determine whether a Mott transition robustly exists and the
nature of the possible Mott insulating phases. We address these issues in a
minimal two-orbital model and a more realistic four-orbital model for the
parent iron pnictides using a slave-spin approach. In the two-orbital model
with two electrons per Fe, we identify a transition from metal to Mott
insulator. The critical coupling, , is greatly reduced by the Hund's
coupling. Depending on the ratio between the inter- and intra-orbital Coulomb
repulsions, the insulating state can be either a spin-Mott insulator or an
orbital-Mott insulator. In the four-orbital model with four electrons per Fe,
we find an orbitally selective metal-to-insulator transition in the case of
zero Hund's coupling; the transition to a Mott insulator in the and
orbitals takes place at the same critical coupling as the transition to a band
insulator in the and orbitals. In the presence of a finite
Hund's coupling, however, the localization transition is into a spin-Mott
state.Comment: 12 pages, 11 figures, to appear in Phys. Rev.
U(1) Slave-spin theory and its application to Mott transition in a multi-orbital model for iron pnictides
A U(1) slave-spin representation is introduced for multi-orbital Hubbard
models. As with the form of L. de'Medici et al. (Phys. Rev. B 72, 205124
(2005)), this approach represents a physical electron operator as the product
of a slave spin and an auxiliary fermion operator. For non-degenerate
multi-orbital models, our U(1) approach is advantageous in that it captures the
non-interacting limit at the mean-field level. For systems with either a single
orbital or degenerate multiple orbitals, the U(1) and slave-spin
approachs yield the same results in the slave-spin-condensed phase. In general,
the U(1) slave-spin approach contains a U(1) gauge redundancy, and properly
describes a Mott insulating phase. We apply the U(1) slave-spin approach to
study the metal-to-insulator transition in a five-orbital model for parent iron
pnictides. We demonstrate a Mott transition as a function of the interactions
in this model. The nature of the Mott insulating state is influenced by the
interplay between the Hund's rule coupling and crystal field splittings. In the
metallic phase, when the Hund's rule coupling is beyond a threshold, there is a
crossover from a weakly correlated metal to a strongly correlated one, through
which the quasiparticle speactral weight rapidly drops. The existence of such a
strongly correlated metallic phase supports the incipient Mott picture of the
parent iron pnictides. In the parameter regime for this phase and in the
vicinity of the Mott transition, we find that an orbital selective Mott state
has nearly as competitive a ground state energy.Comment: 7 pages, 2 figure
Orbital-selective Mott phase in multiorbital models for iron pnictides and chalcogenides
There is increasing recognition that the multiorbital nature of the 3d
electrons is important to the proper description of the electronic states in
the normal state of the iron-based superconductors. Earlier studies of the
pertinent multiorbital Hubbard models identified an orbital-selective Mott
phase, which anchors the orbital-selective behavior seen in the overall phase
diagram. An important characteristics of the models is that the orbitals are
kinetically coupled -- i.e. hybridized -- to each other, which makes the
orbital-selective Mott phase especially nontrivial. A U(1) slave-spin method
was used to analyze the model with nonzero orbital-level splittings. Here we
develop a Landau free-energy functional to shed further light on this issue. We
put the microscopic analysis from the U(1) slave-spin approach in this
perspective, and show that the intersite spin correlations are crucial to the
renormalization of the bare hybridization amplitude towards zero and the
concomitant realization of the orbital-selective Mott transition. Based on this
insight, we discuss additional ways to study the orbital-selective Mott physics
from a dynamical competition between the interorbital hybridization and
collective spin correlations. Our results demonstrate the robustness of the
orbital-selective Mott phase in the multiorbital models appropriate for the
iron-based superconductors.Comment: 10 pages, 2 figure
Orbital-selective Mott Phase in Multiorbital Models for Alkaline Iron Selenides K(1-x)Fe(2-y)Se2
We study a multiorbital model for the alkaline iron selenides
K(1-x)Fe(2-y)Se2 using a slave-spin method. With or without ordered vacancies,
we identify a metal-to-Mott-insulator transition at the commensurate filling of
six 3d electrons per iron ion. For Hund's couplings beyond a threshold value,
this occurs via an intermediate orbital-selective Mott phase, in which the 3d
xy orbital is Mott localized while the other 3d orbitals remain itinerant. This
phase is still stabilized over a range of carrier dopings. Our results lead to
an overall phase diagram for the alkaline iron selenides, which provides a
unified framework to understand the interplay between the strength of vacancy
order and carrier doping. In this phase diagram, the orbital-selective Mott
phase provides a natural link between the superconducting K(1-x)Fe(2-y)Se2 and
its Mott-insulating parent compound.Comment: 6 pages, 5 figures, including supplementary informatio
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