arXiv subjects

Louis-Anne de Vaulchier

Publications and source records attributed to Louis-Anne de Vaulchier.

5 recordsLinked to original sources

arXiv2023

Energy gap of topological surface states in proximity to a magnetic insulator

Topological surface-states can acquire an energy gap when time-reversal symmetry is broken by interfacing with a magnetic insulator. This gap has yet to be measured. Such topological-magnetic insulator heterostructures can host a quantized anomalous Hall effect and can allow the control of the magnetic state of the insulator in a spintronic device. In this work, we observe the energy gap of topological surface-states in proximity to a magnetic insulator using magnetooptical Landau level spectroscopy. We measure Pb1-xSnxSe/EuSe heterostructures grown by molecular beam epitaxy exhibiting a record mobility and low Fermi energy. Through temperature dependent measurements and theoretical calculations, we show this gap is likely due to quantum confinement and conclude that the magnetic proximity effect is weak in this system. This weakness is disadvantageous for the realization of the quantum anomalous Hall effect, but favorable for spintronic devices which require the preservation of spin-momentum locking at the Fermi level.

cond-mat.mes-hall↗

arXiv2022

Enhanced Dirac node separation in strained Cd3As2 topological semimetal

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.

cond-mat.mes-hall↗

arXiv2022

Interaction between interface and massive states in multivalley topological heterostructures

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.

cond-mat.mes-hall↗

arXiv2017

Magnetooptical determination of a topological index

When a Dirac fermion system acquires an energy-gap, it is said to have either trivial (positive energy-gap) or non-trivial (negative energy-gap) topology, depending on the parity ordering of its conduction and valence bands. The non-trivial regime is identified by the presence of topological surface or edge-state dispersing in the energy gap of the bulk and is attributed a non-zero topological index. In this work, we show that such topological indices can be determined experimentally via an accurate measurement of the effective velocity of bulk massive Dirac fermions. We demonstrate this analytically starting from the Bernevig-Hughes-Zhang Hamiltonian (BHZ) to show how the topological index depends on this velocity. We then experimentally extract the topological index in Pb1-xSnxSe and Pb1-xSnxTe using infrared magnetooptical Landau level spectroscopy. This approach is argued to be universal to all material classes that can be described by a BHZ-like model and that host a topological phase transition.

cond-mat.mes-hall↗

arXiv2014

Disorder-perturbed Landau levels in high electron mobility epitaxial graphene

We show that the Landau levels in epitaxial graphene in presence of localized defects are significantly modified compared to those of an ideal system. We report on magneto-spectroscopy experiments performed on high quality samples. Besides typical interband magneto-optical transitions, we clearly observe additional transitions that involve perturbed states associated to short-range impurities such as vacancies. Their intensity is found to decrease with an annealing process and a partial self-healing over time is observed. Calculations of the perturbed Landau levels by using a delta-like potential show electronic states both between and at the same energies of the Laudau levels of ideal graphene. The calculated absorption spectra involving all perturbed and unperturbed states are in very good agreement with the experiments.

cond-mat.mes-hall↗