Quantum matter is one of the most pertinent and active research areas in condensed matter physics today.
The QuantumMatter@PT network currently encompasses portuguese scientists interested in a wide variety of physical problems, ranging from strongly interacting electron physics to the simulation of quantum transport in real-life nano-devices. You can find below some of the main research themes, accompanied by a brief description. |
Strongly Correlated Quantum Materials
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Systems made of a macroscopically large number of degrees of freedom that strongly interact with each other often behave very differently from a non-interacting collection of their constitutive parts. This phenomenon - called "emergence" - is responsible for electronic phases of matter, such as magnets or superconductors, where electron-electron or electron-phonon interactions are strong. Nevertheless, collective excitations can be even more exotic, such as in quantum spin liquids, characterized by long-range quantum entanglement, fractionalized excitations, and absence of magnetic order. Research in this area aims at explaining or predicting properties of quantum materials in terms of their emergent collective degrees of freedom. Topological Quantum Phases of Matter
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Robust material properties under perturbations is a characteristic feature of topological quantum phases of matter. This is an appealing property from both a fundamental and applied point-of-view. Understanding this robustness, which evades standard Landau theory, requires topological concepts. A paradigmatic seminal example is the quantum Hall insulator in 2D where the transverse conductivity is a topological invariant. Nowadays topological quantum phases have been extended to 3D insulators, metals and semimetals, and even superconductors.
Research in this area focuses on two main lines: search for topological phases in novel quantum materials; understanding topological quantum phases, either already present or induced, when the system departs from the ideal clean, non-interacting limit. Picture: The Fermi surface of the Weyl Nodal-Loop is a continuous line in the plane kz = 0. The ground-state wavefunction has a width Γ(W, L) around the loop, for fixed linear system size and disorder strength. [Taken from M. Gonçalves et al, Phys. Rev. Lett. 124, 136405 (2020)] High-Performance Computing for
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The use of computational methods is essential to study physical phenomena in real-life quantum materials. Computers make possible the simulation of physical observables in more realistic models, where analytical tools cannot be employed. This opens up a whole avenue of research where the chance of comparing theoretical results with experimental measurements is greatly enhanced. The resources needed to fully simulate a quantum system in a classical computer grow exponentially with its size. This obstructs any dreams of getting complete answers. Nevertheless, models and approximations designed to capture the most relevant features can be developed in the interplay of theoretical and computational approaches. Computational Physics is, in its own right, a branch of theoretical physics pivotal for the current development of condensed matter physics, as a whole. Finally, the use of computational methods in Physics has been a driving force that paved the way for new methodologies which are now applied in the most varied contexts, from chemistry/pharmacology to the social sciences. |
Two-Dimensional Quantum Materials
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The isolation of graphene in 2004 opened the field of research on two-dimensional materials. The family of two-dimensional materials is vast including metals, semimetals, semiconductors, insulators, superconductors, and magnetic materials. The two-dimensional nature of these materials make their properties easily tunable - either by applying gate voltages, strain or chemical modification - making them a rich playground for both fundamental and applied research. Furthermore, two-dimensional materials can be stacked on top of each other, forming the so called van der Waals structures. These structures can combine the properties of different materials or give origin to completely new ones. The prime example of this is twisted bilayer graphene: where two layers of graphene placed on top of each other, can be made superconducting by controlling the rotation angle between the two. Non-Equilibrium Quantum Transport
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Electron transport in materials and solid state devices is a cornerstone of almost all modern technology. At high temperatures and for large enough samples, these electrodynamic phenomena are well explained by a semiclassical picture. Thereby, conduction electrons behave as clouds of classical particles that are accelerated by external electromagnetic fields, diffuse due to density inhomogeneities and collide with crystal defects. Moreover, if the working time scales are large enough, all these effects happen while the system remains permanently in an equilibrium state. As faster, cooler and more miniaturized devices are developed, the wave-like nature of conductions electrons gets unveiled and transport becomes a non-equilibrium and purely quantum phenomenon. This justifies a growing interest in a quantum transport theory where the competition between electron matter-wave interference and thermal decoherence effects is considered within an out-of-equilibrium microscopic theory capable of describing current dynamics at the shortest time scales. Light-Matter Interaction
and Optoelectronics MORE INFO
We are all familiar with the process by which radiation of frequency ω interacts with matter and induces a transition between energy levels i→ f, provided Bohr 's famous rule is met,
ℏω = Ef - Ei. This process is seen as an absorption of a single radiation photon and is, by far, the dominant one for low intensity radiation. But when the radiation intensity increases, processes with two, three or more photons become increasingly important. This is the realm of non-linear optics. Many technologically important processes, like intensity dependent index of refraction, optical switching, third harmonic generation, are non-linear optical effects. This mature field received renewed attention when it was found that the new 2D materials, like graphene, transition metal dichalcogenides (TMDs) and other layered materials, have much stronger non-linear response than previously known materials. Calculations of non-linear response are notoriously complicated, fraught with pitfalls, and we have been looking in detail on how to streamline these calculations. The case shown in the figure refers to doped graphene, where Pauli's exclusion principle should prevent response for frequencies ℏω < 2μ . It has proven possible to separate one, two and three photon response (the three plots shown above on the left) where the thresholds for ℏω = 2μ/3 (3 photons), ℏω = μ (2 photons) and ℏω = 2μ (1 photon) are visible. The full third order response is the sum of the three. Picture: Transitions to empty states in graphene require an energy equals to twice the Fermi energy, 2μ . These can be achieved by three, two and one-photon processes. [Taken from D. Passos et. al., unpublished]. |