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Dirac matter

The fabrication and subsequent experiments on graphene created unexpected links between solid-state physics and quantum electrodynamics and triggered an intensive search for other condensed-matter systems with similarly structured electronic bands. At present, a number of such materials has been identified and they form a whole class of Dirac-type materials. These interesting systems host massless as well as massive particles described by relativistic-like Hamiltonians in one, two or three spatial dimensions. Several interesting systems from this class have recently been studied in our group using tools of optical magneto-spectroscopy in a broad, from THz up to visible, spectral range.

Massive Dirac electrons in bulk Bi2Se3

The intriguing Dirac-type surface states of topological insulators, protected by time-reversal symmetry, has been explored using a number of various experimental approches, including ARPES, STM/STS, transport or optical techniques. Much less attention has been paid to the bulk properties of topological insulators, even though the apperance of the surface states is governed by the general properties of their bulk Hamiltonians. Recently, we have probed bulk states in Bi2Se3 using Landau (...)

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Massless Kane electrons in gapless HgCdTe

The class of Dirac-type materials has been recently expanded to include systems hosting massless charge carriers within their 3D conical band structures. Using high-field magneto-transmission experiments, we have identified one of the very first such materials - HgCdTe, tuned to the point of the topological semiconductor-to-semimetal transition. The presence of a single 3D conical band located in the center of the Brilouin zone is shown by, e.g., specific absorption coefficient that (...)

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Kane versus Dirac massless electrons in cadmium arsenide

Instead of obeying Schrödinger equation, in some materials the low-energy excitations behave as massless Dirac particles. Three-dimensional (3D) Dirac semimetals represent one such class of materials, constitutuing the closest archetype of truly relativistic massless systems. Cadmium arsenide, Cd3As2, is the prime candidate for a 3D Dirac material that is stable at ambient conditions. Two stable Dirac cones are predicted to exist around the Gamma point of the Brillouin zone. However, a (...)

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Cyclotron emission from massless Kane electrons

When a magnetic field is applied to a solid, the continuous density of electronic states transforms into a set of discrete energy levels, known as Landau levels. Electrons excited in such a ladder may recombine, with emission of photons. This process can be viewed as an inverse of cyclotron resonance and it is referred to as ‘cyclotron emission’. The idea to construct the LL laser via stimulated cyclotron emission is as old as the experimental realization of the very first laser itself, and (...)

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