Research

2019 - 2024

Ex-researcher in Condensed Matter Physics at Sapienza University of Rome

In the condensed matter physics playground, the term quantum materials (QM) has found its place as a way to classify states of matter that cannot be described in terms of the low-level quantum mechanical theories, like the Fermi gas or the Landau liquid. On a microscopic level, these materials are intrinsically complex due to the strong correlations between the lattice, charge, spin, and orbital degrees of freedom. The resulting phases of matter can thus be linked to the concept of emergence, describing the appearance of low energy features as a consequence of the strong correlations, instead of a direct reductionist interpretation in terms of the single parts composing the material. From a historical point of view, the quantum materials label has encompassed the formerly known strongly correlated materials, like the high-temperature superconductors or the more general Slater and Mott-Hubbard insulators. However, since the discovery of the quantum Hall effect (QHE) in the 1980s, the new class of topological materials, whose electronic properties are related to the topological features of the band structure, has also found its place in the QM classification. Topological insulators, Dirac/Weyl semimetals, and low-dimensional materials like graphene all find their place in the QM class.

As highlighted by the Nobel laureate Philip W. Anderson in its seminal paper "More is Different", the concept of symmetry breaking is the core behind the variety of phenomena that appear in the condensed matter physics research branch. Different degrees of complexity can thus be created, ranging from perfectly ordered systems to chaotic ones, where no discernible order parameter can be identified as a consequence of the high complexity degree. The taste for these latter emergent properties coincided with the discovery of the topological states of matter, starting from the most trivial quantum Hall effect based on the quantization of Landau levels. Around the same time, a more clear picture was developed by Thouless, Kohmoto, Nightingale, and den Nijs (TKNN), highlighting the relationship of the electronic features to the nontrivial topological properties (Chern numbers or TKNN integers) of the Bloch states of an electron in a crystal. A few years later, Haldane gave an example of how the integer quantum Hall effect could arise in a simple model of a crystal with zero magnetic fields. The work of Haldane and Thouless was recognized by the 2016 Nobel prize in physics since it turned out to be crucial for many of the more recent developments of this area. The newfound quantum Hall state is fundamentally different from Landau's spontaneous symmetry breaking phases, like crystallization, magnetism, or superconductivity. In the case of topological phases, the transition does not break any symmetry and the phases are not associated with any local order parameter. In the past years, the most active research in the topological properties of condensed systems has revolved around topological insulators. However, intermediate phases between them and the normal insulators also show new topological features, characterized by the appearance of a semimetallic state, known as a Weyl/Dirac state. The quasiparticles describing this emergent phase are known as Dirac or Weyl electrons since they can be described by the homonym relativistic quantum equations. Due to the analogies with the high-energy physics theories, topological materials have become a tangible platform to study effects that have never been observed in the particle physics context, such as the chiral anomaly. Moreover, the absence of the strict Lorentz invariance in condensed matter physics permits the prediction of a broader range of new phenomena through the introduction of additional terms in the effective Hamiltonian of the systems.

The whole topological nature of matter can be described in terms of the geometric properties of the single electron's wavefunction, from which concepts like the Berry phase or the Berry connection arise. In other words, these properties arise from the way the wavefunctions get entangled inside the material. The simple thought about entanglement in a solid is mind-boggling since in a single chunk of metal we have more than 10^23 electrons. Even by taking in mind only the electrons at the Fermi surface, the ones that support the transport properties, the number is still enormous. These processes transcribe into remarkably robust properties of the topological phases, like the protection against back-scattering of the topological insulator surface states, supported by the time-reversal symmetry conservation, or the protection of the Weyl band structures against the formation of an energy gap. The combination of these topological states with further electronic correlations permits the production of the most intriguing phases of matter yet to be fully characterized, like topological superconductors or spin liquid phases, predicted to host fractionalized excitations in terms of spinons and Majorana quasiparticles. 

To achieve quantum control of this new physics by electrical and optical means, more studies are necessary, especially in the low frequency and nonlinear regime. Due to the low energy nature of emergent phenomena, the terahertz, or far-infrared, region of the electromagnetic spectrum (typically referred to as the frequencies between 100 GHz and 30 THz) is of critical importance in the spectroscopy of condensed matter systems. Many of the electronic properties of semiconductors and metals are greatly influenced by bound states whose energies are resonant with terahertz photons. For the case of quantum materials, this is true especially for the linear and nonlinear conductivity behavior of the emergent phases, which contains the information on the fundamental processes behind the quasiparticles transport. Despite its importance, terahertz spectroscopy developments have been hindered by the lack of suitable tools. However, recent technological innovations in photonics and nanotechnology are now enabling THz research to be applied in many more sectors, like communications and medical sciences. A milestone in this race has been the development of THz time-domain spectroscopy (THz-TDS), especially through the generation of THz waves by nonlinear optical effects, such as optical rectification in organic and inorganic nonlinear crystals. Gas is also one of the most promising and convenient nonlinear mediums for the generation of broadband pulsed terahertz radiation, with the highest generation efficiency in the case of high intensity, dual-frequency femtosecond laser pulses, focused to create an air plasma filament from which a broadband THz signal is emitted.

Publications

Attended Conferences

Invited speaker at "The 2nd 3D Graphene workshop: from fundamentals properties to applications", held at the University of Science and Technology of China, Hefei, Anhui

Co-chair of the THz Sapienza Workshop 2019: "Spectroscopy and Imaging with THz Radiation using Ultimate Radiation Sources" (10-11/12/2019), held at the University of Rome La Sapienza, Rome, Italy.  

Oral contributor at the SuperFOx2020 Conference on Superconductivity and Functional Oxides (10-12/02/2020), Santa Margherita Ligure, Italy  

Poster contributor at the 11th NGSCES 2021 virtual conference (07-10/06/2021)

Poster contributor at the LEES 2021 virtual conference (28/06-01/07/2021) (Best poster prize winner)

Invited speaker at the "Bilateral 3D Graphene workshop", held at Sapienza University, Rome, Italy (25/11/2021

Invited speaker at "Quantum Materials for Quantum Technologies workshop", held at Laboratori Nazionali di Frascati (LNF), Frascati, Rome, Italy (14-15/02/2022)

Invited speaker at the GEMCMP22 Conference (16-18/06/2022), Rome, Italy 

Invited speaker at the Superstripes 2022 Conference (20-24/06/2022), Laboratori Nazionali di Frascati (LNF), Frascati, Rome, Italy

Oral contributor at the LEES 2023 Conference (25-30/06/2023), Sankt Pölten, Austria

Invited speaker at the Superstripes 2023 Conference (26/06-01/07/2023), Ischia, Naples, Italy

Oral contributor at the FisMat2023 Conference (4-8/09/2023), Milan, Italy

Invited speaker at Physics & Topology Workshop (29-30/01/2023), Rome, Italy