Terahertz and optical response of novel quantum materials

Quantum materials are a class of systems that host low energy emergent properties due to the strong correlations between the lattice, charge, spin and orbital degrees of freedom. In recent years, this classification has encompassed the world of strongly correlated materials, like high-temperature superconductors and Mott-Hubbard insulators, and the topological states of matter, like Dirac/Weyl semimetals and topological insulators. This doctoral thesis discusses the study of the optical characterization of novel quantum materials, with a special focus on the terahertz (THz) spectral range, where low energy emergent features arise as a consequence of strong electronic correlations, symmetry breaking and/or topological transitions. The low energy photons associated to THz radiation grant an easy access to the quasiparticles occupying the low energy modes of quantum materials. The recent progress in the generation and detection of THz radiation have enabled the production of ultrashort pulses at the picosecond scale and their exploitation in time-domain spectroscopy measurements, like pump-probe and nonlinear spectroscopy, from which it is possible to study the out-of-equilibrium dynamics of electrons and dipole-coupled low energy states, along with the appearance of nonlinear responses at the presence of high external electric fields. This thesis shows that quantum materials can host a plethora of THz-related responses which can be tuned in terms of their thickness, temperature and external drivings, like the perturbation associated to an ultrashort optical pulse. It addresses the optical and THz study of novel magnetic quantum materials like Co2MnGa, a magnetic nodal line semimetal, MnBi2Te4, the first intrinsic magnetic topological insulator discovered, and CrI3, a layered ferromagnetic insulator with research interests ranging from spintronics to topological Majorana modes. In particular, the interplay between the magnetic, electronic and phononic states is addressed through linear and nonlinear spectroscopy, along with the direct sampling of the topological features by optical means. As a final purpose, this thesis also reports the novel findings for the direct optical sampling of the superconductive gap in the Sr-doped nickelate NdNiO2, a strain-induced superconductor. The results show how the electrodynamic properties of this material are quantitatively different from the ones found in cuprate superconductors, sharing a similar infinite layer structural phase

Spatially Resolved Spectral Imaging by A THz-FEL

Using the unique characteristics of the free-electron-laser (FEL), we successfully performed high-sensitivity spectral imaging of different materials in the terahertz (THz) and far-infrared (FIR) domain. THz imaging at various wavelengths was achieved using in situ spectroscopy by means of this wavelength tunable and monochromatic source. In particular, owing to its large intensity and directionality, we could collect high-sensitivity transmission imaging of extremely low-transparency materials and three-dimensional objects in the 3–6 THz range. By accurately identifying the intrinsic absorption wavelength of organic and inorganic materials, we succeeded in the mapping of spatial distribution of individual components. This simple imaging technique using a focusing optics and a raster scan modality has made it possible to set up and carry out fast spectral imaging experiments on different materials in this radiation facility

Anomalous Hall effect and magnetoresistance in micro-ribbons of the magnetic Weyl semimetal candidate PrRhC2

PrRhC2 belongs to the rare-earth carbides family whose properties are of special interest among topological semimetals due to the simultaneous breaking of both inversion and time-reversal symmetry. The concomitant absence of both symmetries grants the possibility to tune the Weyl nodes chirality and to enhance topological effects like the chiral anomaly. In this work, we report on the synthesis and compare the magnetotransport measurements of a poly- and single crystalline PrRhC2 sample. Using a remarkable and sophisticated technique, the PrRhC2 single crystal is prepared via focused ion beam cutting from the polycrystalline material. Our magnetometric and specific heat analyses reveal a non-collinear antiferromagnetic state below 20K, as well as short-range magnetic correlations and/or magnetic fluctuations well above the onset of the magnetic transition. The transport measurements on the PrRhC2 single crystal display an electrical resistivity peak at 3K and an anomalous Hall effect below 6K indicative of a net magnetization component in the ordered state. Furthermore, we study the angular variation of magnetoresistivities as a function of the angle between the in-plane magnetic field and the injected electrical current. We find that both the transverse and the longitudinal resistivities exhibit fourfold angular dependencies due to higher-order terms in the resistivity tensor, consistent with the orthorhombic crystal symmetry of PrRhC2. Our experimental results may be interpreted as features of topological Weyl semimetallic behavior in the magnetotransport properties

Optical Properties of Superconducting Nd0.8Sr0.2NiO2 Nickelate

The intensive search for alternative non-cuprate high-transition-temperature ($T_c$) superconductors has taken a positive turn recently with the discovery of superconductivity in infinite layer nickelates. This discovery is expected to be the basis for disentangling the puzzle behind the physics of high $T_c$ in oxides. In the unsolved quest for the physical conditions necessary for inducing superconductivity, we report an optical study of a Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ film measured using optical spectroscopy, at temperatures above and below the critical temperature $T_c\sim 13$ K. The normal-state electrodynamics of Nd$_{0.8}$Sr$_{0.2}$NiO$_2$, is described by the Drude model characterized by a scattering time just above $T_c$ ($\tau \sim 1.7\times 10^{-14}$ s) and a plasma frequency $\omega_p = 8500$ cm$^{-1}$ in combination with an absorption band in the Mid-Infrared (MIR) around $\omega_0 \sim 4000$ cm$^{-1}$. The MIR absorption indicates the presence of strong electronic correlation effect in the NiO$_2$ plane similarly to cuprates. Below $T_c$, a superconducting energy gap ($2\Delta$) of $\sim 3.2$ meV is extracted from the Terahertz reflectivity using the the Mattis-Bardeen model. From the Ferrel-Glover-Thinkam Rule applied to the real part of the optical conductivity, we also estimate a London penetration depth of about 490 nm, in agreement with a type-II superconductivity in Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ Nickelate

Two new matrix-variate distributions with application in model-based clustering

Two matrix-variate distributions, both elliptical heavy-tailed generalization of the matrix-variate normal distribution, are introduced. They belong to the normal scale mixture family, and are respectively obtained by choosing a convenient shifted exponential or uniform as mixing distribution. Moreover, they have a closed-form for the probability density function that is characterized by only one additional parameter, with respect to the nested matrix-variate normal, governing the tail-weight. Both distributions are then used for model-based clustering via finite mixture models. The resulting mixtures, being able to handle data with atypical observations in a better way than the matrix-variate normal mixture, can avoid the disruption of the true underlying group structure. Different EM-based algorithms are implemented for parameter estimation and tested in terms of computational times and parameter recovery. Furthermore, these mixture models are fitted to simulated and real datasets, and their fitting and clustering performances are analyzed and compared to those obtained by other well-established competitors

Achromatic terahertz quarter-wave Fresnel rhomb retarder

Achromatic terahertz (THz) quarter-wave retarder is widely desired to manipulate the polarization states of broadband THz beams, which are essential for spectroscopic applications, such as circular dichroism spectroscopy and steering THz vortex beams. A retarder based on Fresnel reflection exhibits the potential for designing an achromatic THz quarter-wave retarder. However, special care should be taken to make a Fresnel retarder capable of manipulating the beam ellipticity by simply rotating its fast axis without affecting its propagation path. Hereby, we design a 4-bounce achromatic quarter-wave Fresnel rhomb retarder free of affecting beam propagation, which can easily change the input beam's ellipticity by simply rotating the retarder's fast axis

On the Use of the Matrix-Variate Tail-Inflated Normal Distribution for Parsimonious Mixture Modeling

Recent advances in the matrix-variate model-based clustering literature have shown the growing interest for this kind of data modelization. In this framework, finite mixture models constitute a powerful clustering technique, despite the fact that they tend to suffer from overparameterization problems because of the high number of parameters to be estimated. To cope with this issue, parsimonious matrix-variate normal mixtures have been recently proposed in the literature. However, for many real phenomena, the tails of the mixture components of such models are lighter than required, with a direct effect on the corresponding fitting results. Thus, in this paper we introduce a family of 196 parsimonious mixture models based on the matrix-variate tail-inflated normal distribution, an elliptical heavy-tailed generalization of the matrix-variate normal distribution. Parsimony is reached by applying the well-known eigen-decomposition of the component scale matrices, as well as by allowing the tailedness parameters of the mixture components to be tied across groups. An AECM algorithm for parameter estimation is presented. The proposed models are then fitted to simulated and real data. Comparisons with parsimonious matrix-variate normal mixtures are also provided

Terahertz Resonators Based on YBa₂Cu₃O₇ High-<i>T</i>_c Superconductor

Superconducting split-ring resonator arrays allow to overcome two main limitations affecting metallic metamaterial resonating in the terahertz (THz) range: ohmic losses and tunability of their optical response. In this work, we design and experimentally realize direct and complementary square arrays of superconducting YBa2Cu3O7 (YBCO) split-ring resonators working in the THz spectral range. The main purpose of this paper is to show how the metamaterial resonances can be tuned by temperature (T) when crossing the superconducting transition temperature Tc of YBCO. The tuning property can be quantified by describing the THz transmittance of the patterned YBCO films vs. T through a model of coupled resonators. This model allows us to estimate the THz resonances of split-ring arrays and their interaction, showing how the kinetic inductance Lk in the superconducting state is the main parameter affecting the metamaterial properties

Simultaneous elliptically and radially polarized THz from one-colour laser-induced plasma filament

THz-based technologies and research applications have seen a rapid increment in recent period together with the development of novel radiation sources based both on relativistic electrons and laser techniques. In this framework, laser-induced plasma filament plays an important role in generating intense and broadband THz radiation. Although many attentions have been paid to THz emission from two-color plasma filaments, one-color plasma emission has been scarcely investigated. In particular, the polarization state of one-color THz emission is still controversial due to the limitations of the existing THz detection techniques, which are incapable of simultaneously detecting elliptically and radially polarized THz radiation. In this manuscript, we develop a novel detection method and unambiguously demonstrate for the first time that one-color laser-induced plasma filament simultaneously emits elliptically and radially polarized THz radiation. These polarization states suggest that the generation mechanism results from electric quadrupole, showing a new route for producing more complex polarization states and THz vortex beams