Using far-infrared magnetospectroscopy in self-assembled InAs quantum dots, we have investigated the electronic transitions from the ground s levels to the excited p levels. The experiments consist of monitoring, by means of Zeeman tuning of the excited level, a resonant interaction between the discrete ( p, 0 LO phonon) state and the continuum of either (s, 1 LO phonon) or (s, 2 LO phonons). We show that the electrons and the LO phonons are always in a strong coupling regime and form an everlasting mixed electron-phonon mode. PACS numbers: 73.40.Kp, 73.20.Dx, 78.20.Ls Electrons in excited atomic states can relax towards lower lying levels by spontaneous emission of photons. Artificial atoms like semiconductor quantum dots display discrete levels. For electrons (or holes) placed in excited levels the spontaneous emission of photons is inefficient for the relaxation due to the characteristic energy splitting of the dot states (ϳ50 meV in a ϳ20 nm dot). The carriers bound to these artificial atoms are however in interaction with phonons which display a continuum of finite width, unlike photons. It has been shown that the intradot relaxation through acoustical phonons is totally inefficient, the energy mismatch between electron states being much too large [1,2]. In semiconductors, the most powerful energy relaxation channel is (by far) the irreversible emission of longitudinal optical (LO) phonons through the Fröhlich coupling. Despite its effectiveness, this electron-phonon coupling is weak, to the extent that the initial discrete level (e, 0 phonon) irreversibly decays into the continuum (g, 1 phonon) where e and g, respectively, denote an excited state and the ground electronic state. Such a weak coupling is very well described by the Fermi golden rule in bulk, quantum well (2D) or quantum wire (1D) structures. Because the optical phonons show very little dispersion, it has been argued that the LO phonon assisted relaxation in semiconductor quantum dots could be efficient only if the energy separation between the electronic states differs by one (or several) LO phonons. Here we present experimental evidence supported by theoretical modeling that the very idea of an electron emitting LO phonons and relaxing irreversibly to a less excited state (as in bulk, 2D, and 1D heterolayers) is wrong in a quantum dot. What happens in reality is that the electrons and the LO phonons are in a strong coupling regime and form everlasting mixed electron-phonon modes, as recently suggested by Inoshita and Sakaki in the case of one phonon [3]. Using far-infrared (FIR) magnetospectroscopy, we have investigated the g ! e transition in self-assembled doped InAs quantum dots. The experiments consist of monitoring, by means of Zeeman tuning of the dot excited level e, a resonant interaction between the discrete (e, 0 LO phonon) state and the continuum of either (g, 1 LO phonon) or (g, 2 LO phonons). We show that the (e, 0 LO phonon) state does not dissolve when entering into the continuum but forms a hybrid mode with (g, 1, or 2 LO p...

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Relativistic Dirac fermions are ubiquitous in condensed-matter physics. Their mass is proportional to the material energy gap and the ability to control and tune the mass has become an essential tool to engineer quantum phenomena that mimic high energy particles and provide novel device functionalities. In topological insulator thin films, new states of matter can be generated by hybridizing the massless Dirac states that occur at material surfaces. In this paper, we experimentally and theoretically introduce a platform where this hybridization can be continuously tuned: the Pb1-xSnxSe topological superlattice. In this system, topological Dirac states occur at the interfaces between a topological crystalline insulator Pb1-xSnxSe and a trivial insulator, realized in the form of topological quantum wells (TQW) epitaxially stacked on top of each other. Using magnetooptical transmission spectroscopy on high quality molecular beam epitaxy grown Pb1-xSnxSe superlattices, we show that the penetration depth of the TQW interface states and therefore their Dirac mass is continuously tunable with temperature. This presents a pathway to engineer the Dirac mass of topological systems and paves the way towards the realization of emergent quantum states of matter using Pb1-xSnxSe topological superlattices. I.

Dirac fermions in condensed matter physics hold great promise for novel fundamental physics, quantum devices and data storage applications. IV-VI semiconductors, in the inverted regime, have been recently shown to exhibit massless topological surface Dirac fermions protected by crystalline symmetry, as well as massive bulk Dirac fermions. Under a strong magnetic field (B), both surface and bulk states are quantized into Landau levels that disperse as B1/2, and are thus difficult to distinguish. In this work, magneto-optical absorption is used to probe the Landau levels of high mobility Bi-doped Pb0.54Sn0.46Te topological crystalline insulator (111)-oriented films. The high mobility achieved in these thin film structures allows us to probe and distinguish the Landau levels of both surface and bulk Dirac fermions and extract valuable quantitative information about their physical properties. This work paves the way for future magnetooptical and electronic transport experiments aimed at manipulating the band topology of such materials.