Strongly Correlated Materials

Carme Sousa: Optical properties

1. Photo-magnetic switching materials

The main objective of this research line is to understand the microscopic behaviour of transition metal containing photo-magnetic switching materials. Some molecular species containing transition metal ions, mainly iron and cobalt, may exhibit a transition between states of different magnetic moment. This spin crossover interconversion can be triggered by a change in temperature, by preassure or by irradiation with light. The later phenomenon, called Light-Induced Excited Spin State Trapping (LIESST), is the most appealing for technological applications, like optical data storage and processing devices.

The aim of our research is to determine the key parameters of spin crossover materials containing Fe(II), Fe(III) and Co(II) transition metal ions, study the optical properties of such systems, and  elucidate the mechanism of the LIESST phenomenon. To achieve this goal we apply a multiconfigurational wave function approach (CASSCF/CASPT2) which includes the three esential ingredients for this purpose: (i) a proper description of the spin of the various electronic states involved in the process, (ii) a direct access to excited states and (iii) the inclusion of spin-orbit effects, which are important to explain the intersystem crossings. 

         [Fe^II(bpy)₃]²⁺ complex                                      Spin crossover parameters for the [Fe^II(tz)₆]²⁺ complex                           Proposed LIESST mechanism for [Fe^II(bpy)₃]²⁺     

2. Spectroscopy of impurities and defects 

It is nowadays well-known that the presence of defects and impurities controls many properties of solid materials. The existence of impurities, mainly transition metal ions, in several materials, like oxides and halides, change the optical, magnetic and electric properties of the host system. These changes open the possibility to use these materials in several technological applications like electronic devices or solid state lasers.

In addition, crystalline lattices contain defects. Among all possible defects, oxygen vacancies are the most common point defects in metal oxides. Both, defects and impurities are often difficult to identify from the experimental point of view because of the low concentration and the coexistence of several kind of defects and/or impurities.

The goal of our research is to characterize the structural, chemical, electric and optical properties of the defective centers by means of ab initio calculations. This implies to study the ground and low-lying excited states of these systems.

Among the properties and materials we are interested in are:

• Structural, electronic and optical properties of transition metal ions, like Ag⁺, Ag²⁺, Ni⁺ or Cu²⁺, in alkaline earth oxides and halides.
• Optical and geometric properties of oxygen vacancies (F centers) and divacancies (M centers) in bulk and surface MgO, CaO and α-Al₂O₃.
• Optical properties of supported metal clusters on different sites of the MgO(001) surface, including regular sites at the flat (001) terraces, monoatomic steps and oxygen vacancies.

Schematic representation of the energy levels involved in the electronic transitions of bulk F and M centers in MgO

Ibério de P. R. Moreira: Magnetic properties

The LaMnO₃ oxide is the parent material for a series of manganites which show the colossal magnetoresistance property upon doped with cations such as Ca, Ba or Sr. This property is related to a metal-insulator transition, in which magnetic degrees of freedom are involved. The study of colossal magnetoresistance in these compounds has prompted the understanding of the electronic and magnetic structure of the parent oxide LaMnO₃, which in the ground state is an antiferromagnetic insulator. In this compound an orbital ordering shows up due to the break in the degeneration of the Mn³⁺ d orbitals in the t_2g³e_g¹ configuration. This particular orbital ordering is the responsible for the A-type antiferromagnetic structure present in the undoped material, as shown by different scientific groups.

Modelling high T_c cuprate superconductivity

	local model
J(n e-) or t((n-1) e-)	t and J effective parameters

Accurate estimates of electronic structure parameters provide new insight in High-Tc superconductivity in cuprates.

Magnetic coupling models

		Mapping procedures provide rigurous schemes to extract magnetic coupling parameters form ab initio calculations and its interpretation by means of effective Hamiltonian theory offers a detailed insigth from the electronic structure of the system.