Stable topological insulators achieved using high energy electron beams

Topological insulators are transformative quantum solids with immune-to-disorder metallic surface states having Dirac band structure. Ubiquitous charged bulk defects, however, pull the Fermi energy into the bulk bands, denying access to surface charge transport. Here we demonstrate that irradiation with swift ($\sim 2.5$ MeV energy) electron beams allows to compensate these defects, bring the Fermi level back into the bulk gap, and reach the charge neutrality point (CNP). Controlling the beam fluence we tune bulk conductivity from \textit{p}- (hole-like) to \textit{n}-type (electron-like), crossing the Dirac point and back, while preserving the Dirac energy dispersion. The CNP conductance has a two-dimensional (2D) character on the order of ten conductance quanta $G_0 =e^2/h$, and reveals, both in Bi$_2$Te$_3$ and Bi$_2$Se$_3$, the presence of only two quantum channels corresponding to two topological surfaces. The intrinsic quantum transport of the topological states is accessible disregarding the bulk size.Comment: Main manuscript - 12 pages, 4 figures; Supplementary file - 15 pages, 11 figures, 1 Table, 4 Note

Direct observation of electronic band gap and hot carrier dynamics in GeAs semiconductor

Germanium arsenide (GeAs) is a layered semiconductor with remarkably anisotropic physical, thermoelectric and optical properties, and a promising candidate for multifunctional devices based on in-plane polarization dependent response. Understanding the underlying mechanism of such devices requires the knowledge of GeAs electronic band structure and of the hot carrier dynamics in its conduction band, whose details are still unclear. In this work, we investigated the properties of occupied and photoexcited states of GeAs in energy-momentum space, by combining scanning tunneling spectroscopy (STS), angle-resolved photoemission spectroscopy (ARPES) and time-resolved ARPES. We found that, GeAs is an indirect gap semiconductor having an electronic gap of 0.8 eV, for which the conduction band minimum (CBM) is located at the Gamma point while the valence band maximum (VBM) is out of Gamma. A Stark broadening of the valence band is observed immediately after photoexcitation, which can be attributed to the effects of the electrical field at the surface induced by inhomogeneous screening. Moreover, the hot electrons relaxation time of 1.56 ps down to the CBM which is dominated from both inter-valley and intra-valley coupling. Besides their relevance for our understanding of GeAs, these findings present general interest for the design on high performance thermoelectric and optoelectronic devices based on 2D semiconductors

Electronic dispersion, correlations and stacking in the photoexcited state of 1T-TaS2_2

Here we perform angle and time-resolved photoelectron spectroscopy on the commensurate Charge Density Wave (CDW) phase of 1T-TaS$_2$. Data with different probe pulse polarization are employed to map the dispersion of electronic states below and above the chemical potential. Upon photoexcitation, the fluctuations of CDW order erase the band dispersion near to the chemical potential and halve the charge gap size. This transient phase sets within half a period of the coherent lattice motion and is favored by strong electronic correlations. The experimental results are compared to Density-Functional Theory (DFT) calculations with a self-consistent evaluation of the Coulomb repulsion. Our simulations indicate that the screening of Coulomb repulsion depends on the stacking order of the TaS$_2$ layers. The entanglement of such degrees of freedom suggest that both the structural order and electronic repulsion are locally modified by the photoinduced CDW fluctuations

Ultrafast dynamics with time-resolved ARPES: photoexcited electrons in monochalcogenide semiconductors

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Dynamics of electronic states in the Intermediate phase of 1T-TaS2_2

This article reports a comparative study of bulk and surface properties in the transition metal dichalcogenide 1T-TaS$_2$. When heating the sample, the surface displays an intermediate insulating phase that persists for $\sim 10$ K on top of a metallic bulk. The weaker screening of Coulomb repulsion and stiffer Charge Density Wave (CDW) explain such resilience of a correlated insulator in the topmost layers. Both time resolved ARPES and transient reflectivity are employed to investigate the dynamics of electrons and CDW collective motion. It follows that the amplitude mode is always stiffer at the surface and displays variable coupling to the Mott-Peierls band, stronger in the low temperature phase and weaker in the intermediate one

Electronic structure of the α-(BEDT-TTF)2I3 surface by photoelectron spectroscopy

We report anomalies observed in photoelectron spectroscopy measurements performed on α-(BEDT-TTF)2I3 crystals. In particular, above its metal-insulator transition temperature (T ' 135 K), we observe the lack of a sharp Fermi edge in contradiction with the metallic transport properties exhib- ited by this quasi-bidimensional organic material. We interpret these unusual results as a signature of a one-dimensional electronic behavior confirmed by DFT band structure calculations. Using photoelectron spectroscopy we probe a Luttinger liquid with a large correlation parameter (α ą 1) that we interpret to be caused by the chain-like electronic structure of α-(BEDT-TTF)2I3 surface doped by iodine defects. These new surface effects are inaccessible by bulk sensitive measurements of electronic transport techniques

Moving Dirac nodes by chemical substitution

Dirac fermions play a central role in the study of topological phases, for they can generate a variety of exotic states, such as Weyl semimetals and topological insulators. The control and manipulation of Dirac fermions constitute a fundamental step toward the realization of novel concepts of electronic devices and quantum computation. By means of Angle-Resolved PhotoEmission Spectroscopy (ARPES) experiments and ab initio simulations, here, we show that Dirac states can be effectively tuned by doping a transition metal sulfide, BaNiS2, through Co/Ni substitution. The symmetry and chemical characteristics of this material, combined with the modification of the charge-transfer gap of BaCo1-xNixS2 across its phase diagram, lead to the formation of Dirac lines, whose position in k-space can be displaced along the Gamma - M symmetry direction and their form reshaped. Not only does the doping x tailor the location and shape of the Dirac bands, but it also controls the metal-insulator transition in the same compound, making BaCo1-xNixS2 a model system to functionalize Dirac materials by varying the strength of electron correlations

Stable topological insulators achieved using high energy electron beams

Topological insulators are potentially transformative quantum solids with metallic surface states which have Dirac band structure and are immune to disorder. Ubiquitous charged bulk defects, however, pull the Fermi energy into the bulk bands, denying access to surface charge transport. Here we demonstrate that irradiation with swift (B2.5MeV energy) electron beams allows to compensate these defects, bring the Fermi level back into the bulk gap and reach the charge neutrality point (CNP). Controlling the beam fluence, we tune bulk conductivity from p- (hole-like) to n-type (electron-like), crossing the Dirac point and back, while preserving the Dirac energy dispersion. The CNP conductance has a two-dimensional character on the order of ten conductance quanta and reveals, both in Bi2Te3 and Bi2Se3, the presence of only two quantum channels corresponding to two topological surfaces. The intrinsic quantum transport of the topological states is accessible disregarding the bulk size

Erratum: A microscopic view on the Mott transition in chromium-doped V 2 O 3

Nature Communications 1, Article number: 105 (2010); published: 02 November 2010; updated: 17 January 2012. In Figure 2 of this Article, panel labels c and d were inadvertently switched. A typographical error was also introduced in the last sentence of the legend, which should have read 'The scale bar in panel c represents 10 μm'

Dynamique des électrons et des phonons dans les systèmes fortement corrélés (transition de Mott dans V2O3 et supraconductivité dans les pnictures de fer)

Cette thèse concerne l'étude du rôle des électrons, des phonons et de leur dynamique dans les transitions de phase de deux systèmes fortement corrélés, le composé prototype de Mott V2O3 et le pnicture de fer supraconducteur Ba(Fe1-xCox)2As2. Ces systèmes ont été étudiés grâce à deux techniques complémentaires, la spectroscopie de photoélectrons et la réflectivité pompe-sonde. Les mesures de réflectivité transitoire dans (V1-xCrx)2O3 ont permis de mettre en évidence un durcissement photo-induit du réseau cristallin, dû a la modification ultra-rapide de la structure électronique du matériau. La mesure de phonons optiques et acoustiques cohérents a montré le lien entre excitation électronique, propriétés réticulaires et corrélations électroniques. Des mesures de photoémission résolue microscopiquement ont également permis, dans certaines parties du diagramme de phases, la visualisation de domaines isolants de Mott et métalliques. Concernant Ba(Fe1-xCox)2As2, la première mesure d'un phonon optique cohérent dans un pnicture de fer a permis d'interpréter la relaxation électronique transitoire de ce système comme gouvernée par un couplage électrons-phonons sélectif. La très faible valeur de cette constante de couplage, =0.12, met en évidence la nature non-conventionnelle de la supraconductivité dans ce système. La structure électronique de Ba(Fe1-xCox)2As2 a été mesurée par ARPES à basse énergie de photon, permettant la détermination des dispersions et des symétries des poches de trous. La modification de structure électronique induite par la transition supraconductrice a été mesurée, reflétant la complexité de la structure électronique des supraconducteurs multi-bandes.This thesis presents the study of electrons, phonons and their dynamics in the phase transitions of two strongly correlated materials, the prototype Mott compound (V1-xCrx)2O3 and the superconducting iron-pnictide Ba(Fe1-xCox)2As2. These systems were studied with two complementary techniques, photoelectron spectroscopy and pump-probe reflectivity. Transient reflectivity measurements in (V1-xCrx)2O3 show a photo-induced lattice stiffening, due to the ultrafast (at the femtosecond time-scale) electronic structure modification. Coherent optical and acoustic phonon measurements investigated the Iink between electronic excitation, lattice properties and electron correlations. Sub-micron spatially resolved photoemission allowed the measurement of coexisting Mott-insulating and metallic domains in some parts of the phase diagram. ln Ba(Fe1-xCox)2As2, the first measurement of a coherent optical phonon in an iron-pnictide compound made it possible to interpret the transient electronic relaxation as governed by a selective electron-phonon coupling. The coupling constant, =0.12, is too weak to explain the superconductivity by a phonon mediated mechanism. The electronic structure of Ba(Fe1-xCox)2As2 has been measured by angle-resolved photoemission at low photon energy, allowing an optimal wavevector and energy resolution. The band dispersion and their orbital origin close to the Brillouin zone center have been determined, and the changes in the electronic structure induced by superconductivity have been measured. These changes include a spectral weight transfer and selective gap opening, revealing the complex character of the electronic structure of this multi-band superconductor.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF