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 $\omega_\text{in}$-$\omega_\text{out}$ 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 VO2_2

The metal-insulator transition of VO$_2$, 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 1TT-TaS2_2

Recent theoretical studies showed that the electronic structure of 1$T$-TaS$_2$ 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 ege_g states and antiferromagnetic correlations in d(1,2,3)d^{(1,2,3)} perovskites: A GWGW+EDMFT investigation

We perform a systematic {\it ab initio} study of the electronic structure of Sr(V,Mo,Mn)O$_3$ perovskites, using the parameter-free $GW$+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 ($t_{2g}$) description to those of a 5-band ($t_{2g}$+$e_g$) model, it is shown that the $e_g$ 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$_3$. In the case of SrMnO$_3$ paramagnetic $GW$+EDMFT yields a metallic low-temperature solution on the verge of a Mott transition, while antiferromagnetic $GW$+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 $GW$+EDMFT scheme

Electronic structure of few-layer black phosphorus from μ\mu-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 ($\mu$-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