12 research outputs found
Competing interactions in correlated heterostructures
In the last decade, the field of heterostructures involving transition-metal oxides
as building blocks has grown to become one of the most active areas in the
field of correlated materials and, more in general, in condensed matter. The interest
in these systems is motivated by the possibility to artificially design and
manipulate electronic phases inaccessible in the bulk constituents. The prototypical
and most studied heterojuncion is formed by the two band insulator
LaAlO3 (LAO) and SrTiO3 (STO) where an insulator-metal transition occurs at
the interface as a function of the thickness of the LAO layer. When the latter exceeds
a universal threshold, a few-layer thick two-dimensional electron gas establishes
on the STO side. A similar phenomenology is realized at the interface
between STO and the Mott insulator LaTiO3. In both cases the 2DEG turns into
a superconductor at 300mK. The phenomenology of these systems, which are only an example of the many opportunities offered by heterostructures formed
by transition-metal oxides and correlated materials, reveals immediately that a
number of physical effects conspire to determine their fascinating properties.
Electron-electron correlations are certainly expected to play a role because of
the narrow bands arising from the d electrons of transition-metal oxides. Moreover,
there are strong evidences of an important role of electron-phonon coupling
already in bulk STO, and the phonon-driven interaction is the most likely
candidate for the two-dimensional superconductivity found at the interfaces.
Moreover, an important role of spin-orbit coupling is expected at interfaces, and
it will be particularly important for 5d systems like iridates. All these competing
interactions should be treated on the same footing without assuming a clear
hierarchy in order to disentangle their effects in the rich phenomenology. This
means that any theoretical treatment should be able to handle competing interactions.
This can be realized using the Dynamical Mean-Field Theory, a powerful
approach which freezes spatial fluctuations in order to fully account for
the local quantum dynamics arising from the different relevant interactions. In
order to treat layered systems, Dynamical Mean-Field Theory must be extended
in order to allow for different physics on different layers. In this thesis we contribute
to the theoretical understanding of heterostructures of transition-metal
oxides and correlated materials touching all the above-mentioned points. We
now briefly introduce the structure of the thesis and the content of the different
chapters. The first Chapter is devoted to an introduction about transition-metal
heterostructures with some emphasis on the LTO/STO and LAO/STO systems.
In the second Chapter we introduce the several theoretical models we use in
the rest of the thesis, namely single-band and multi-band Hubbard modeling of strong correlations, electron-phonon interaction and spin-orbit coupling.
Chapter 3 briefly introduces DMFT and its extensions to treat all the interactions
discussed in the second Chapter.
The fourth chapter contains a novel extensions of DMFT to layered systems
which minimizes finite-size effects and approximation, as well as an application
of the method to the attractive Hubbard model, which allows us to study the
proximity effects as a function of the various model parameters.
In Chapter 5 we discuss the interplay between strong correlations and electronphonon
interaction, identifying the conditions under which an s-wave superconductor
can be realized in the presence of strong correlations. An application
to a model version of the LTO/STO system is presented.
Finally, in Chapter 6 we study the interplay between strong correlations,
Hund\u2019s coupling and spin-orbit interaction in a three-fold degenerate model
for d electrons. A study of the magnetic phase of the iridate compound Sr2IrO4
is finally presented.
All these result contribute to improve our understanding of the complex
interplay underlying the physics of transition-metal oxides and will represent
the basis to build a more complete modelization of there systems
Time-resolved photoemission and RIXS study of a site-selective Mott insulator
Inspired by the physics of rare earth nickelates, we study the photoemission
(PES) and resonant inelastic X-ray scattering (RIXS) spectra of a correlated
electron system with two types of insulating sublattices. Sublattice A is
characterized by a hybridization gap and a low-spin state, while sublattice B
features a Mott gap and a local magnetic moment. We show how the coupling of
these two qualitatively different insulating states affects the dynamics of
photo-induced charge carriers and how the nonequilibrium states manifest
themselves in the PES and RIXS signals. In particular, we find that charge
carriers created on the B sublattice migrate to the A sublattice, where they
contribute to the creation of in-gap states in the PES signal, and to
characteristic peaks in the nonequilibrium RIXS spectrum. While the
contributions from the two sublattices cannot be easily distinguished in the
local photoemission spectrum, the weights of the RIXS signals in the
two-dimensional - space provide
information on the local state evolution on both sublattices
Fermion quartets on the square lattice
We study a microscopic model for four spinless fermions on the square lattice
which exhibits a quartet bound state in the strong coupling regime. The
four-particle quantum states are analyzed using symmetry arguments and by
introducing a zoo of relevant lattice animals. These considerations, as well as
variational and exact diagonalization calculations demonstrate the existence of
a narrow quartet band at small hopping and a first order transition to
delocalized fermions at a critical hopping parameter, in qualitative contrast
to, e. g., the BCS-BEC crossover in the attractive Hubbard model. In the case
of pure attraction, an intermediate phase is found, in which a more extended
and presumably more mobile hybrid quartet dominates the ground state. We
comment on the relevance of the spin degree of freedom and on the reasons why
electron quartetting is rarely observed in real materials
Nature of the photo-induced metallic state in monoclinic VO
The metal-insulator transition of VO, which in equilibrium is associated
with a structural phase transition, has been intensively studied for decades.
In particular, it is challenging to disentangle the role of Mott physics from
dimerization effects in the insulating phase. Femtosecond time-resolved
experiments showed that optical excitations can induce a transient metallic
state in the dimerized phase, which is distinct from the known equilibrium
phases. In this study, we combine non-equilibrium cluster dynamical mean-field
theory with realistic first principles modeling to clarify the nature of this
laser-induced metallic state. We show that the doublon-holon production by
laser pulses with polarization along the V-V dimers and the subsequent
inter-orbital reshuffling of the photo-carriers leads to a population of
orbital-mixed states and the filling of the gap. The photo-induced metal state
is qualitatively similar to a hot electronic state in the dimerized structure,
and does not involve a collapse of the Mott gap
Photo-induced charge dynamics in 1-TaS
Recent theoretical studies showed that the electronic structure of
1-TaS in the low-temperature commensurate charge density wave phase
exhibits a nontrivial interplay between band-insulating and Mott insulating
behavior. This has important implications for the interpretation of
photo-doping experiments. Here we use nonequilibrium dynamical mean-field
theory simulations of a realistic multi-layer structure to clarify the charge
carrier dynamics induced by a laser pulse. The solution is propagated up to the
picosecond timescale by employing a memory-truncation scheme. While long-lived
doublons and holons only exist in the surface state of a specific structure,
the disturbance of bonding states in the bilayers which make up the bulk of the
system explain the almost instantaneous appearance of in-gap states. Our
simulations consistently explain the coexistence of a doublon feature with a
prominent ``background" signal in previous time-resolved photoemission
experiments, and they suggest strategies for the selective population of the
ingap and doublon states by exploiting the sensitivity to the pump polarization
and pump frequency.Comment: 12 pages, 9 figure
Hund excitations and the efficiency of Mott solar cells
We study the dynamics of photoinduced charge carriers in semirealistic models of LaVO3 and YTiO3 polar heterostructures. It is shown that two types of impact ionization processes contribute to the carrier multiplication in these strongly correlated multiorbital systems: The first mechanism involves local spin state transitions, while the second mechanism involves the scattering of high-kinetic-energy carriers. Both processes act on the 10-fs timescale and play an important role in the harvesting of high-energy photons in solar cell applications. As a consequence, the optimal gap size for Mott solar cells is substantially smaller than for semiconductor devices
Screening from states and antiferromagnetic correlations in perovskites: A +EDMFT investigation
We perform a systematic {\it ab initio} study of the electronic structure of
Sr(V,Mo,Mn)O perovskites, using the parameter-free +EDMFT method. This
approach self-consistently calculates effective interaction parameters, taking
into account screening effects due to nonlocal charge fluctuations. Comparing
the results of a 3-band () description to those of a 5-band
(+) model, it is shown that the states have little effect on
the low-energy properties and the plasmonic features for the first two
compounds but play a more active role in SrMnO. In the case of SrMnO
paramagnetic +EDMFT yields a metallic low-temperature solution on the verge
of a Mott transition, while antiferromagnetic +EDMFT produces an insulating
solution with the correct gap size. We discuss the possible implications of
this result for the nature of the insulating state above the N\'eel
temperature, and the reliability of the +EDMFT scheme
Electronic structure of few-layer black phosphorus from -ARPES
Black phosphorus (BP) stands out among two-dimensional (2D) semiconductors
because of its high mobility and thickness dependent direct band gap. However,
the quasiparticle band structure of ultrathin BP has remained inaccessible to
experiment thus far. Here we use a recently developed laser-based micro-focus
angle resolved photoemission (-ARPES) system to establish the electronic
structure of 2-9 layer BP from experiment. Our measurements unveil ladders of
anisotropic, quantized subbands at energies that deviate from the scaling
observed in conventional semiconductor quantum wells. We quantify the
anisotropy of the effective masses and determine universal tight-binding
parameters which provide an accurate description of the electronic structure
for all thicknesses.Comment: Supporting Information available upon reques