24 research outputs found
Type-III Dirac fermions in HfxZr1-xTe2 topological semimetal candidate (data)
No full textThis dataset contains the raw data files connected to the figures included in the paper "Type-III Dirac fermions in HfxZr1-xTe2 topological semimetal candidate" by S. Fragkos et al., Journal of Applied Physics 129, 075104 (2021); https://doi.org/10.1063/5.0038799
An Open Access version of the paper can be found here: https://zenodo.org/record/4562057#.YaDC4NBBxP
Author Correction: RNAs coordinate nuclear envelope assembly and DNA replication through ELYS recruitment to chromatin
No full textIn the original version of this Article, the affiliation details for Antoine Aze, Michalis Fragkos, Stéphane Bocquet, Julien Cau and Marcel Méchali incorrectly omitted ‘CNRS and the University of Montpellier’. This has now been corrected in both the PDF and HTML versions of the Article.</jats:p
Μελέτη τοπολογικών ιδιοτήτων των υλικών: τοπολογικοί μονωτές, ημιμέταλλα Weyl και Dirac
No full textTopological materials are materials whose properties remain invariant under continuous transformations. Topological insulators are insulating in bulk and conducting at surface. In another class of topological materials, the material is a semimetal where their valence and conduction bands touch at, or near the Fermi level. Depending on whether the bands are non or doubly-degenerate, a topological material is called topological Weyl or Dirac semimetal, respectively. Standard topological materials possess three-dimensional crystal structures that are often formed as bulk crystals. However, the research has been focused lately on two-dimensional van der Waals topological materials due to their unique quantum electronic properties and their reduced dimensionality. This doctoral dissertation aims on presenting theoretical and experimental results that contribute to the research on two-dimensional van der Waals topological materials. Firstly, we fabricated epitaxial (SnBi2Te4)n(Bi2Te3)m natural van der Waals superlattices. The existence of topological surface states is confirmed by using first-principles calculations in combination with in-situ and synchrotron angle-resolved photoemission spectroscopy. Their presence is also correlated with appearance of the weak antilocalization effect observed with magnetotransport measurements. In addition, we report on the crucial orthorhombic non-centrosymmetric Weyl semimetal Td-phase direct room-temperature observation in epitaxial three monolayers MoTe2 films generated layer by layer on InAs substrates via molecular beam epitaxy. The lattice constants of our epitaxial orthorhombic phase are found to be significantly greater than the experimental values from bulk Td-phase of MoTe2 that were previously published. In this study, we claim that the stabilization of the Td-phase at room temperature in epitaxial thin films is significantly influenced by the expanded lattice parameters, as well as the energy position of the Weyl points.Moreover, the family of group IV transition metal ditellurides is studied with theoretical calculations, where HfTe2 and ZrTe2 are classified as type-I and type-II topological Dirac semimetals, respectively. In addition, a new type-III phase is proposed, which is achieved through a topological phase transition, by alloying the two materials in combination with in-plane strain. This strain may be used to alternate between the two types of Dirac semimetals. We successfully fabricated the desired alloy by using molecular beam epitaxy, and performed angle-resolved photoemission spectroscopy, where the topological Dirac semimetal behavior is displayed.Finally, theoretical results of the WTe2, and to a lesser extend MoTe2, in a van der Waals heterostructure with the CrTe2 two-dimensional ferromagnet are presented. WTe2 and MoTe2 are topological Weyl semimetals in large thicknesses and two-dimensional topological insulators down to the single layer, holding great opportunities on spintronic applications. Our findings suggest that an interfacial Dzyaloshinskii-Moriya interaction strong enough to create Néel-type skyrmion lattice is produced whose dynamics are tested under external magnetic fields and temperature. This study also shows that the generation and annihilation of magnetic skyrmions in the CrTe2/WTe2 van der Waals heterostructure can be achieved and that the motion of the magnetic skyrmions can be controlled by ultra-low spin-polarized currents.Τα τοπολογικά υλικά είναι υλικά των οποίων οι ιδιότητες παραμένουν αμετάβλητες υπό συνεχείς μετασχηματισμούς. Οι τοπολογικοί μονωτές είναι μονωτές εσωτερικά και αγώγιμοι στην επιφάνεια. Σε μια άλλη κατηγορία τοπολογικών υλικών, το υλικό είναι ημιμέταλλο όπου οι ζώνες σθένους και αγωγιμότητάς τους τέμνονται πάνω ή κοντά στο επίπεδο Fermi. Ανάλογα με το αν οι ζώνες είναι μη ή διπλά εκφυλισμένες, ένα τοπολογικό υλικό ονομάζεται τοπολογικό ημιμέταλλο Weyl ή Dirac, αντίστοιχα. Τα τυπικά τοπολογικά υλικά διαθέτουν τρισδιάστατες κρυσταλλικές δομές που συχνά σχηματίζονται ως πολυστρωματικοί κρύσταλλοι. Ωστόσο, η έρευνα έχει επικεντρωθεί τελευταία σε δισδιάστατα τοπολογικά υλικά van der Waals λόγω των μοναδικών κβαντικών ηλεκτρονικών ιδιοτήτων τους και της μειωμένης τους διάστασης.Η παρούσα διδακτορική διατριβή στοχεύει στην παρουσίαση θεωρητικών και πειραματικών αποτελεσμάτων που συμβάλλουν στην έρευνα πάνω στα δισδιάστατα τοπολογικά υλικά. Πρώτον, κατασκευάσαμε επιταξιακά φυσικές υπερδομές van der Waals (SnBi2Te4)n(Bi2Te3)m. Η ύπαρξη τοπολογικών επιφανειακών καταστάσεων επιβεβαιώνεται με τη χρήση υπολογισμών πρώτων αρχών σε συνδυασμό με τη γωνιακός εξαρτώμενη φασματοσκοπία φωτοεκπομπής in-situ και σύγχροτρον. Η παρουσία τους συσχετίζεται επίσης με την εμφάνιση του φαινομένου ασθενούς αντιεντοπισμού που παρατηρείται με τις μετρήσεις μαγνητομεταφοράς.Επιπλέον, αναφέρουμε την άμεση παρατήρηση της κρίσιμης ορθορομβικής μη κεντροσυμμετρικής ημιμεταλλικής φάσης Weyl Td σε θερμοκρασία δωματίου, σε επιταξιακά υμένια MoTe2 τριών μονοστρωμάτων, που δημιουργούνται στρώμα προς στρώμα σε υποστρώματα InAs μέσω επιταξίας με μοριακές δέσμες. Οι πλεγματικές σταθερές της επιταξιακής ορθορομβικής μας φάσης βρέθηκαν να είναι σημαντικά μεγαλύτερες από τις πειραματικές τιμές από των πολυστρωματικών κρυστάλλων φάσης Td του MoTe2 που δημοσιεύθηκαν προηγουμένως. Σε αυτή τη μελέτη, υποστηρίζουμε ότι η σταθεροποίηση της φάσης Td σε θερμοκρασία δωματίου σε επιταξιακές λεπτές μεμβράνες επηρεάζεται σημαντικά από τις παραμέτρους του διογκωμένου πλέγματος, καθώς και η ενεργειακή θέση των σημείων Weyl.Επιπλέον, η οικογένεια των μεταβατικών μετάλλων διτελλουριδίων της ομάδας IV μελετάται με θεωρητικούς υπολογισμούς, όπου τα HfTe2 και ZrTe2 ταξινομούνται ως τοπολογικά ημιμέταλλα Dirac τύπου-Ι και τύπου-ΙΙ, αντίστοιχα. Επιπρόσθετα, προτείνεται μια νέα φάση τύπου-III, η οποία επιτυγχάνεται μέσω μιας τοπολογικής μετάβασης φάσης, με κράμα των δύο υλικών σε συνδυασμό με την εφαρμογή ενδοεπίπεδων τάσεων. Αυτή η τάση μπορεί να χρησιμοποιηθεί για την εναλλαγή μεταξύ των δύο τύπων ημιμετάλλων Dirac. Κατασκευάσαμε με επιτυχία το επιθυμητό κράμα χρησιμοποιώντας επιταξία με μοριακές δέσμες και πραγματοποιήσαμε γωνιακός εξαρτώμενη φασματοσκοπία φωτοεκπομπής, όπου παρατηρήθηκε η τοπολογική ημιμεταλλική συμπεριφορά Dirac.Τέλος, παρουσιάζονται θεωρητικά αποτελέσματα του WTe2, και σε μικρότερο βαθμό του MoTe2, σε μια ετεροδομή van der Waals με τον δισδιάστατο σιδηρομαγνήτη CrTe2. Το WTe2 και το MoTe2 είναι τοπολογικά ημιμέταλλα Weyl σε μεγάλα πάχη και δισδιάστατοι τοπολογικοί μονωτές σε πάχος ενός μονοστρώματος, και είναι πολλά υποσχόμενα για εφαρμογές στη σπιντρονική. Τα ευρήματά μας υποδηλώνουν ότι παράγεται μία διεπιφανειακή αλληλεπίδραση Dzyaloshinskii-Moriya αρκετά ισχυρή ώστε να δημιουργήσει πλέγμα σκυρμιονίων τύπου Néel του οποίου η δυναμική ελέγχεται υπό εξωτερικά μαγνητικά πεδία και τη θερμοκρασία. Αυτή η μελέτη δείχνει επίσης ότι η δημιουργία και η εξάλειψη των σκυρμιονίων στην ετεροδομή CrTe2/WTe2 μπορεί να επιτευχθεί και ότι η κίνηση των σκυρμιονίων μπορεί να επιτευχθεί με εξαιρετικά χαμηλά πολωμένα ρεύματα σπιν
Magnetic skyrmion manipulation in CrTe2/WTe2 2D van der Waals heterostructure
No full textMagnetic skyrmions in two-dimensional van der Waals materials provide an ideal platform to push skyrmion technology to the ultimate atomically thin limit. In this work, we theoretically demonstrate the Dzyaloshinskii–Moriya interaction and the formation of a Néel-type skyrmion lattice at the CrTe2/WTe2 bilayer van der Waals heterostructure. Our calculations suggest a field-controlled Néel-type skyrmion lattice – ferromagnet transition cycle. In addition, a spin-torque induced by spin-polarized current injection was simulated in order to study the motion of a skyrmion on a racetrack, where an increase of the skyrmion Hall angle is observed at high temperatures. Consequently, this study suggests that generation and annihilation of skyrmions can be achieved with temperature or field control and also manipulate the velocity and the direction of the Néel-type skyrmions, through ultra-low current densities and temperature, thus, shedding light to the general picture of magnetic skyrmion control and design on two-dimensional van der Waals heterostructures.The following article has been accepted by Applied Physics Letters. After it is published, it will be found at https://doi.org/10.1063/5.0089999
This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. and may be found at https://doi.org/10.1063/5.0089999.
For Creative Commons licensed material, please use: "Copyright 2022 Author(s). This article is distributed under a Creative Commons Attribution (CC BY) License.
Ultrathin epitaxial Bi film growth on 2D HfTe2 template (data)
No full textThis dataset contains the raw data files connected to the figures included in the paper "Ultrathin epitaxial Bi film growth on 2D HfTe2 template" by E. Xenogiannopoulou et al., 2022 Nanotechnology 33 015701; https://doi.org/10.1088/1361-6528/ac2d08
An Open Access version of the paper can be found here: https://zenodo.org/record/5720132#.YaDKWNBBxP
Ultrafast Nonequilibrium Enhancement of Electron-Phonon Interaction in 2 H − MoTe 2
No full textInternational audienceUnderstanding nonequilibrium electron-phonon interactions at the microscopic level and on ultrafast timescales is a central goal of modern condensed matter physics. Combining time- and angle-resolved extreme ultraviolet photoemission spectroscopy with constrained density functional perturbation theory, we demonstrate that photoexcited carrier density can serve as a tuning knob to enhance electron-phonon interactions in nonequilibrium conditions. Specifically, nonequilibrium band structure mapping and valley-resolved ultrafast population dynamics in semiconducting transition-metal dichalcogenide 2 H − MoTe 2 reveal band-gap renormalizations and reduced population lifetimes as photoexcited carrier densities increase. Through theoretical analysis of photoinduced electron and phonon energy and linewidth renormalizations, we attribute these transient features to nonequilibrium modifications of electron-phonon coupling matrix elements. This Letter advances our understanding of microscopic coupling mechanisms enabling control over relaxation pathways in driven solids
Massless Dirac Fermions in ZrTe 2 Semimetal Grown on InAs(111) by van der Waals Epitaxy
No full textInternational audienc
Direct Observation at Room Temperature of the Orthorhombic Weyl Semimetal Phase in Thin Epitaxial MoTe 2
No full textInternational audienceThe direct observation at room temperature (RT) of the noncentrosym-metric orthorhombic topological Weyl semimetal phase in epitaxial thin films of MoTe 2 grown on InAs(111)/Si(111) substrates by molecular beam epitaxy (MBE) is reported. The orthorhombic phase is typically found at lower temperatures but its observation at RT in this work is attributed to the enlarged lattice parameters, influenced by the substrate, which stabilize an interlayer antibonding state compatible with the orthorhombic stacking. First-principles calculations predict eight type II Weyl nodes which are located below (but near) the Fermi energy making them accessible to charge transport and creating the prospect for practical applications exploiting the nontrivial topological properties. The orthorhombic phase coexists with an unconventional triclinic layer stacking which is different than the monoclinic or orthorhombic structures but it is centrosymmetric and topologically trivial
Type-III Dirac fermions in HfxZr1-xTe2topological semimetal candidate
No full textTopological semimetals host interesting new types of low-energy quasiparticles such as type-I and type-II Dirac and Weyl fermions. Type-III topological semimetals can emerge exactly at the border between type-I and II, characterized by a line-like Fermi surface and a flat energy dispersion near the topological band crossing. Here, we theoretically predict that 1T-HfTe2 and 1T-ZrTe2 transition metal dichalcogenides are type-I and type-II DSMs, respectively. By alloying the two materials, a new HfxZr1-xTe2 alloy with type-III Dirac cone emerges at x = 0.2, in combination with 1% in-plane compressive strain. By imaging the electronic energy bands with in situ angle-resolved photoemission spectroscopy of this random alloy with the desired composition, grown by molecular beam epitaxy on InAs(111) substrates, we provide experimental evidence that the tοp of type-III Dirac cone lies at - or very close to - the Fermi level. © 2021 Author(s)
Room Temperature Commensurate Charge Density Wave in Epitaxial Strained TiTe 2 Multilayer Films
No full textInternational audienceDespite a large number of studies [2,3] over the years since the first discovery [7] and a couple of comprehensive reviews [8,9] the actual mechanism for PLD/CDW formation is still under debate. The most recent experimental [10-13] and theoretical [14] works focus on the large area growth of the CDW phase [13] the thickness dependence , and the possible unconventional behavior in the ultimate 2D limit of a single layer TiSe 2. [10-12,14] On the other hand, the other Ti dichalcogenides namely TiS 2 and TiTe 2 did not show any clear evidence until very recently when a CDW state was reported only for 1 monolayer (ML)-thin TiTe 2 at temperatures lower than 92 K. [15] It is surprising that the CDW in TiTe 2 was found to be totally suppressed for films thicker than 1 ML, [15] unlike the case of other TMDs where 1 ML and bulk-like films both make the transition to a CDW at nearly the same temperature. The interest about TiTe 2 is continuously increasing in view of theoretical predictions [16] and more recent experimental evidence [17] about pressure induced topological phase transitions in TiTe 2. The possibility to also manipulate superconduc-tivity by external pressure as predicted [18] and more recently evidenced [19] in bulk TiTe 2 creates the prospect to explore the emergence of topological superconductivity in this material. In the latter work [19] it has been shown that under nonhydro-static pressure, a CDW-like state with estimated transition temperature above room temperature (RT) appears in bulk TiTe 2 at around 0.5-1.8 GPa. These results call for a re-examination of the possibility to obtain a CDW in multilayer TiTe 2 and indeed at RT with good potential for real world applications utilizing the properties of the CDW state. These applications include a voltage-controlled oscillator device operating at room temperature , [20] fast electronic resistance switching for nonvolatile memories, [21,22] and field-effect transistor devices potentially suitable for implementation of non-Boolean logic. [23] In this paper it is shown that multilayer films (50 ML ≈ 32 nm), as well as single layer TiTe 2 epitaxially grown on InAs(111)/ Si(111) substrates by molecular beam epitaxy exhibit, in ambient pressure conditions, a CDW distortion at room temperature which is sustained up to higher temperatures, at least 400 °C, as evidenced by reflection high energy electron diffrac-tion (RHEED) (Figure S1, Supporting Information). The results are explained in terms of anisotropic strain imposed by the substrate. The group IVB 2D transition metal dichalcogenides are considered to be stable in the high symmetry trigonal octahedral structure due to the lack of unpaired d-electrons on the metal site. It is found that multilayer epitaxial TiTe 2 is an exception adopting a commensurate 2 × 2 × 2 charge density wave (CDW) structure at room temperature with an ABA type of stacking as evidenced by direct lattice imaging and reciprocal space mapping. The CDW is stabilized by highly anisotropic strain imposed by the substrate with an out-off-plane compression which reduces the interlayer van der Waals gap increasing the coupling between TiTe 2 layers
