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Fast, reversible, and low-power manipulation of the spin texture is crucial for next generation spintronic devices like non-volatile bipolar memories, switchable spin current injectors or spin field effect transistors. Ferroelectric Rashba semiconductors (FERSC) are the ideal class of materials for the realization of such devices. Their ferroelectric character enables an electronic control of the Rashba-type spin texture by means of the reversible and switchable polarization. Yet, only very few materials have been established to belong to this class of multifunctional materials. Here, Pb1-xGexTe is unraveled as a novel FERSC system down to nanoscale. The ferroelectric phase transition and concomitant lattice distortion is demonstrated by temperature dependent X-ray diffraction, and its effect on electronic properties are measured by angle-resolved photoemission spectroscopy. In few nanometer-thick epitaxial heterostructures, a large Rashba spin-splitting is exhibiting a wide tuning range as a function of temperature and Ge content. Our work defines Pb1- xGexTe as a high-potential FERSC system for spintronic applications., Comment: 18 pages, 6 figures

$\alpha$-Sn is an elemental topological material, whose topological phases can be tuned by strain and magnetic field. Such tunability offers a substantial potential for topological electronics. However, InSb substrates, commonly used to stabilize $\alpha$-Sn allotrope, suffer from parallel conduction, restricting transport investigations and potential applications. Here, the successful MBE growth of high-quality $\alpha$-Sn layers on insulating, hybrid CdTe/GaAs(001) substrates, with bulk electron mobility approaching 20000 cm$^2$V$^{-1}$s$^{-1}$ is reported. The electronic properties of the samples are systematically investigated by independent complementary techniques, enabling thorough characterization of the 3D Dirac (DSM) and Weyl (WSM) semimetal phases induced by the strains and magnetic field, respectively. Magneto-optical experiments, corroborated with band structure modeling, provide an exhaustive description of the bulk states in the DSM phase. The modeled electronic structure is directly observed in angle-resolved photoemission spectroscopy, which reveals linearly dispersing bands near the Fermi level. The first detailed study of negative longitudinal magnetoresistance relates this effect to the chiral anomaly and, consequently, to the presence of WSM. Observation of the $\pi$ Berry phase in Shubnikov-de Haas oscillations agrees with the topologically non-trivial nature of the investigated samples. Our findings establish $\alpha$-Sn as an attractive topological material for exploring relativistic physics and future applications., Comment: Accepted version; Main text: 37 pages, 7 figures; Supplementary Materials: 29 pages, 13 figures

The ferroelectric semiconductor $\alpha$-SnTe has been regarded as a topological crystalline insulator and the dispersion of its surface states has been intensively measured with angle-resolved photoemission spectroscopy (ARPES) over the last decade. However, much less attention has been given to the impact of the ferroelectric transition on its electronic structure, and in particular on its bulk states. Here, we investigate the low-energy electronic structure of $\alpha$-SnTe with ARPES and follow the evolution of the bulk-state Rashba splitting as a function of temperature, across its ferroelectric critical temperature of about $T_c\sim 110$ K. Unexpectedly, we observe a persistent band splitting up to room temperature, which is consistent with an order-disorder contribution to the phase transition that requires the presence of fluctuating local dipoles above $T_c$. We conclude that no topological surface state can occur at the (111) surface of SnTe, at odds with recent literature., Comment: 26 pages, 8 figures

In topological semimetals, nodes appear at symmetry points in the Brillouin zone as a result of band inversion, and yield quasi-relativistic massless fermions at low energies. Cd3As2 is a three-dimensional topological semimetal that hosts two Dirac cones responsible for a variety of quantum phenomena. In this work, we demonstrate the strain tuning of the Dirac nodes of Cd3As2 through a combination of magnetooptical infrared spectroscopy and high-resolution X-ray diffraction studies performed on epitaxial films. In these thin films, we observe a giant enhancement of the node separation in momentum space by close to a factor of 4. A combination of experimental measurements and theoretical modelling allows relate the origin of this enhancement to a strengthening of the topological band inversion driven by lattice strain. Our results demonstrate how strain can be used as a knob to tune the topological properties of semimetals and to potentially enhance their performance and response for various applications.

Topological interface states in multivalley systems are studied to unravel their valley sensitivity. For this purpose, multivalley IV-VI topological crystalline insulator (TCI) heterostructures are explored using magneto-optical Landau level spectroscopy up to 34 teslas. We characterize the topological interface states emerging from the distinct L-valleys in Pb1-xSnxSe multi quantum wells grown along the [111] direction. It is shown that the shape of the 2D Fermi surfaces of topological interface states residing at the TCI/trivial insulator interfaces are strongly affected by the valley anisotropy of topologically trivial Pb1-yEuySe barriers. This phenomenon is shown to be due to the deep penetration of the topological interface states into the barriers. For the valleys tilted with respect to the confinement direction, a significant interaction between topological states and the conventional massive quantum well states is observed, evidenced by the resulting large anti-crossings between Landau levels. These are theoretically well-described by a k.p model that takes into account tilt and anisotropy of the valleys in two dimensions. Therefore, our work provides a precise characterization of the topological interface state valley splitting, as well as an accurate determination of the anisotropy of their Dirac cone dispersion., Comment: 19 pages, 6 figures

InAs/GaSb hosts a broken gap band alignment that has been shown to generate helical topological edge states. Upon the introduction of Mn into the structure, it has been predicted to host a quantized anomalous Hall effect. Here, we show that dilute Mn doping on InAs in InAs/GaSb, allows a tuning of the Fermi level, the introduction of paramagnetism, but also has a non-trivial impact on the band alignment of the system. The measurement of Shubnikov-de-Haas oscillations, cyclotron resonance, and a non-linear Hall effect in Mn-doped samples indicate the coexistence of a high mobility two-dimensional electron gas and a hole gas. Conversely, in undoped InAs/GaSb, pure-n-type transport is observed. We hypothesize that Mn acceptor levels can pin the Fermi energy near the valence band edge of InAs, far from the interface, which introduces a strong band bending to preserve the band offset at the InAs/GaSb interface. The realization of the QAHE in this structure will thus require a careful control of the band alignment to preserve topological insulating character.