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1. Zero-point entropies of spin-jam and spin-glass states in a frustrated magnet

Author

Piyakulworawat, Chairote, Thennakoon, Asiri, Yang, Junjie, Yoshizawa, Hideki, Ueta, Daichi, Sato, Taku J, Sheng, Kuan, Chen, Wei-Tin, Pai, Woei-Wu, Matan, Kittiwit, and Lee, Seung-Hun

Subjects
Condensed Matter - Strongly Correlated Electrons, Condensed Matter - Disordered Systems and Neural Networks
Abstract

Thermodynamics studies of a prototypical quasi-two-dimensional frustrated magnet Ba$_2$Sn$_2$ZnCr$_{7p}$Ga$_{10-7p}$O$_{22}$ where the magnetic Cr$^{3+}$ ions are arranged in a triangular network of bipyramids show that the magnetic zero-point entropy for $p=0.98$ is 55(1)\% of the entropy expected when the Cr$^{3+}$ moments are fully disordered. Furthermore, when combined with a previous neutron scattering study and the perimeter scaling entropy of a spin jam, the analysis reveals that with decreasing $p$, i.e., doping of the nonmagnetic Ga$^{3+}$ ions, the variation in the magnetic zero-point entropy can be well explained by the combined effects of the zero-point entropy of the spin jam state and that of weakly coupled orphan spins, shedding light on the coexistence of the two types of spin states in quantum magnetism., Comment: 8 pages, 4 figures, and Supplemental Material

Published
2023
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2. Quantum Trapping on MoS2 through Lifetimes of Resonant Electrons: Revealing Pauli Exclusion Principle

Author

Lu, Shin-Ming, Su, Wei-Bin, Jeng, Horng-Tay, Chan, Wen-Yuan, Chang, Ho-Hsiang, Pai, Woei Wu, Liu, Hsiang-Lin, and Chang, Chia-Seng

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics
Abstract

We demonstrate that the linewidth of the field emission resonance (FER) observed on the surface of MoS2 using scanning tunneling microscopy can vary by up to one order of magnitude with an increase in the electric field. This unusual phenomenon originates from quantum trapping, where the electron relaxed from the resonant electron in the FER is momentarily trapped in the potential well on the MoS2 surface. Because the relaxed electron and the resonant electron have the same spin and the action of the Pauli exclusion principle, the lifetimes of the resonant electrons can be substantially prolonged when the relaxed electrons engage in the resonance trapping. The linewidth of the FER is thus shrunk considerably to as narrow as 12 meV. Moreover, it was found that the valley intensities around the FER are zero, indicating that MoS2 has an energy gap above the vacuum level. By using the energies of FERs, the potential of the band bending in the MoS2 interior can be measured precisely., Comment: 23 pages (19 for main body, 4 for supplemental material), 5 figures (3 for main body, 2 for supplemental material)

Published
2019
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3. Emergence of charge density waves and a pseudogap in single-layer TiTe2

Author

Chen, P., Pai, Woei Wu, Chan, Y. -H., Takayama, A., Xu, C. -Z., Karn, A., Hasegawa, S., Chou, M. Y., Mo, S. -K., Fedorov, A. -V., and Chiang, T. -C.

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics
Abstract

Two-dimensional materials constitute a promising platform for developing nanoscale devices and systems. Their physical properties can be very different from those of the corresponding three-dimensional materials because of extreme quantum confinement and dimensional reduction. Here we report a study of TiTe$_2$ from the single-layer to the bulk limit. Using angle-resolved photoemission spectroscopy and scanning tunneling microscopy and spectroscopy, we observed the emergence of a (2 x 2) charge density wave order in single-layer TiTe$_2$ with a transition temperature of 92 $\pm$ 3 K. Also observed was a pseudogap of about 28 meV at the Fermi level at 4.2 K. Surprisingly, no charge density wave transitions were observed in 2- and multi-layer TiTe$_2$, despite the quasi-two-dimensional nature of the material in the bulk. The unique charge density wave phenomenon in the single layer raises intriguing questions that challenge the prevailing thinking about the mechanisms of charge density wave formation.

Published
2018
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4. Unique Gap Structure and Symmetry of the Charge Density Wave in Single-Layer VSe2

Author

Chen, P, Pai, Woei Wu, Chan, Y-H, Madhavan, V, Chou, MY, Mo, S-K, Fedorov, A-V, and Chiang, T-C

Subjects
Physical Sciences, Condensed Matter Physics, Mathematical Sciences, Engineering, General Physics, Mathematical sciences, Physical sciences
Abstract

Single layers of transition metal dichalcogenides (TMDCs) are excellent candidates for electronic applications beyond the graphene platform; many of them exhibit novel properties including charge density waves (CDWs) and magnetic ordering. CDWs in these single layers are generally a planar projection of the corresponding bulk CDWs because of the quasi-two-dimensional nature of TMDCs; a different CDW symmetry is unexpected. We report herein the successful creation of pristine single-layer VSe_{2}, which shows a (sqrt[7]×sqrt[3]) CDW in contrast to the (4×4) CDW for the layers in bulk VSe_{2}. Angle-resolved photoemission spectroscopy from the single layer shows a sizable (sqrt[7]×sqrt[3]) CDW gap of ∼100  meV at the zone boundary, a 220 K CDW transition temperature twice the bulk value, and no ferromagnetic exchange splitting as predicted by theory. This robust CDW with an exotic broken symmetry as the ground state is explained via a first-principles analysis. The results illustrate a unique CDW phenomenon in the two-dimensional limit.

Published
2018

5. Large quantum-spin-Hall gap in single-layer 1T' WSe2.

Author

Chen, P, Pai, Woei Wu, Chan, Y-H, Sun, W-L, Xu, C-Z, Lin, D-S, Chou, MY, Fedorov, A-V, and Chiang, T-C

Abstract

Two-dimensional (2D) topological insulators (TIs) are promising platforms for low-dissipation spintronic devices based on the quantum-spin-Hall (QSH) effect, but experimental realization of such systems with a large band gap suitable for room-temperature applications has proven difficult. Here, we report the successful growth on bilayer graphene of a quasi-freestanding WSe2 single layer with the 1T' structure that does not exist in the bulk form of WSe2. Using angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy/spectroscopy (STM/STS), we observe a gap of 129 meV in the 1T' layer and an in-gap edge state located near the layer boundary. The system's 2D TI characters are confirmed by first-principles calculations. The observed gap diminishes with doping by Rb adsorption, ultimately leading to an insulator-semimetal transition. The discovery of this large-gap 2D TI with a tunable band gap opens up opportunities for developing advanced nanoscale systems and quantum devices.

Published
2018

6. NbS$_{3}$: A unique quasi one-dimensional conductor with three charge density wave transitions

Author

Zybtsev, S. G., Pokrovskii, V. Ya., Nasretdinova, V. F., Zaitsev-Zotov, S. V., Pavlovskiy, V. V., Odobesco, A. B., Pai, Woei Wu, Chu, M. -W., Lin, Y. G., Zupanič, E., van Midden, H. J. P., Šturm, S., Tchernychova, E., Prodan, A., Bennett, J. C., Mukhamedshin, I. R., Chernysheva, O. V., Menushenkov, A. P., Loginov, V. B., Loginov, B. A., Titov, A. N., and Abdel-Hafiez, Mahmoud

Subjects
Condensed Matter - Strongly Correlated Electrons
Abstract

Through transport, compositional and structural studies, we review the features of the charge-density wave (CDW) conductor of NbS$_{3}$ (phase II). We highlight three central results: 1) In addition to the previously reported CDW transitions at $T_{P1}$ = 360\,K and $T_{P2}$ = 150\,K, another CDW transition occurs at a much higher temperature $T_{P0}$ = 620-650\,K; evidence for the non-linear conductivity of this CDW is presented. 2) We show that CDW associated with the $T_{P2}$ - transition arises from S vacancies acting as donors. Such a CDW transition has not been observed before. 3) We show exceptional coherence of the $T_{P1}$-CDW at room-temperature. Additionally, we report on the effects of uniaxial strain on the CDW transition temperatures and transport., Comment: 16 pages, 18 figures

Published
2016
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7. Emergence of charge density waves and a pseudogap in single-layer TiTe2.

Author

Chen, P, Pai, Woei Wu, Chan, Y-H, Takayama, A, Xu, C-Z, Karn, A, Hasegawa, S, Chou, MY, Mo, S-K, Fedorov, A-V, and Chiang, T-C

Abstract

Two-dimensional materials constitute a promising platform for developing nanoscale devices and systems. Their physical properties can be very different from those of the corresponding three-dimensional materials because of extreme quantum confinement and dimensional reduction. Here we report a study of TiTe2 from the single-layer to the bulk limit. Using angle-resolved photoemission spectroscopy and scanning tunneling microscopy and spectroscopy, we observed the emergence of a (2 × 2) charge density wave order in single-layer TiTe2 with a transition temperature of 92 ± 3 K. Also observed was a pseudogap of about 28 meV at the Fermi level at 4.2 K. Surprisingly, no charge density wave transitions were observed in two-layer and multi-layer TiTe2, despite the quasi-two-dimensional nature of the material in the bulk. The unique charge density wave phenomenon in the single layer raises intriguing questions that challenge the prevailing thinking about the mechanisms of charge density wave formation.Due to reduced dimensionality, the properties of 2D materials are often different from their 3D counterparts. Here, the authors identify the emergence of a unique charge density wave (CDW) order in monolayer TiTe2 that challenges the current understanding of CDW formation.

Published
2017

9. Sodium Trimer Ordering on NaxCoO2 Surface

Author

Pai, Woei Wu, Huang, S. H., Meng, Ying S., Chao, Y. C., Lin, C. H., Liu, H. L., and Chou, F. C.

Subjects
Condensed Matter - Materials Science, Condensed Matter - Strongly Correlated Electrons
Abstract

Sodium ion ordering on in situ cleaved NaxCoO2 (x=0.84) surface has been studied by ultra high vacuum scanning tunneling microscopy (UHV-STM) at room temperature. Three main phases, with p(3x3), (root 7 x root 7), and (2 root 3 x 2 root 3) hexagonal unit cells and surface Na concentration of 1/3, 3/7, 1/2, respectively, were identified. One surprising finding is that Na trimers act as the basic building blocks that order in long range. The stability of Na trimers is attributed to the increased Na coordination with oxygen as indicated by ab initio calculations, and possibly at finite temperature by configuration entropy.

Published
2008
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10. Sodium vacancy ordering and the co-existence of localized spins and itinerant charges in NaxCoO2

Author

Chou, F. C., Chu, M. -W., Shu, G. J., Huang, F. T., Pai, Woei Wu, Sheu, H. S., Imai, T., Ning, F. L., and Lee, Patrick A.

Subjects
Condensed Matter - Strongly Correlated Electrons
Abstract

The sodium cobaltate family (NaxCoO2) is unique among transition metal oxides because the Co sits on a triangular lattice and its valence can be tuned over a wide range by varying the Na concentration x. Up to now detailed modeling of the rich phenomenology (which ranges from unconventional superconductivity to enhanced thermopower) has been hampered by the difficulty of controlling pure phases. We discovered that certain Na concentrations are specially stable and are associated with superlattice ordering of the Na clusters. This leads naturally to a picture of co-existence of localized spins and itinerant charge carriers. For x = 0.84 we found a remarkably small Fermi energy of 87 K. Our picture brings coherence to a variety of measurements ranging from NMR to optical to thermal transport. Our results also allow us to take the first step towards modeling the mysterious ``Curie-Weiss'' metal state at x = 0.71. We suggest the local moments may form a quantum spin liquid state and we propose experimental test of our hypothesis., Comment: 16 pages, 5 figures

Published
2007
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12. Dislocation Emission around Nanoindentations on a (001) fcc Metal Surface Studied by STM and Atomistic Simulations

Author

de la Fuente, O. Rodriguez, Zimmerman, J. A., Gonzalez, M. A., de la Figuera, J., Hamilton, J. C., Pai, Woei Wu, and Rojo, J. M.

Subjects
Condensed Matter - Materials Science
Abstract

We present a combined study by Scanning Tunneling Microscopy and atomistic simulations of the emission of dissociated dislocation loops by nanoindentation on a (001) fcc surface. The latter consist of two stacking-fault ribbons bounded by Shockley partials and a stair-rod dislocation. These dissociated loops, which intersect the surface, are shown to originate from loops of interstitial character emitted along the <110> directions and are usually located at hundreds of angstroms away from the indentation point. Simulations reproduce the nucleation and glide of these dislocation loops., Comment: 10 pages, 4 figures

Published
2002
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13. Formation of Monolayer Charge Density Waves and Anomalous Edge Doping in Na Doped Bulk VSe2

Author

Chazarin, Ulysse, primary, Lezoualc'h, Mahé, additional, Chou, Jyh‐Ping, additional, Pai, Woei Wu, additional, Karn, Abhishek, additional, Sankar, Raman, additional, Chacon, Cyril C., additional, Girard, Yann, additional, Repain, Vincent, additional, Bellec, Amandine, additional, Rousset, Sylvie, additional, Smogunov, Alexander, additional, Dappe, Yannick J., additional, and Lagoute, Jérôme, additional

Published
2022
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16. Identification and Manipulation of Defects in Black Phosphorus

Author

Harsh, Rishav, primary, Mondal, Sourav, additional, Sharma, Devina, additional, Bouatou, Mehdi, additional, Chacon, Cyril, additional, Ilyn, Maxim, additional, Rogero, Celia, additional, Repain, Vincent, additional, Bellec, Amandine, additional, Girard, Yann, additional, Rousset, Sylvie, additional, Sankar, Raman, additional, Pai, Woei Wu, additional, Narasimhan, Shobhana, additional, and Lagoute, Jérôme, additional

Published
2022
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18. Supporting Information - Identification and Manipulation of Defects in Black Phosphorus

Author

Harsh, Rishav, Mondal, Sourav, Sharma, Devina, Bouatou, Mehdi, Chacon, Cyril, Ilyn, Max, Rogero, Celia, Repain, V., Bellec, Amandine, Girard, Yann, Rousset, S., Sankar, Raman, Pai, Woei Wu, Narasimhan, Shobhana, Lagoute, Jérôme, Harsh, Rishav, Mondal, Sourav, Sharma, Devina, Bouatou, Mehdi, Chacon, Cyril, Ilyn, Max, Rogero, Celia, Repain, V., Bellec, Amandine, Girard, Yann, Rousset, S., Sankar, Raman, Pai, Woei Wu, Narasimhan, Shobhana, and Lagoute, Jérôme

Published
2022

19. Identification and manipulation of defects in black phosphorus

Author

Indo-French Centre for the Promotion of Advanced Research, Agence Nationale de la Recherche (France), Sheikh Saqr Laboratory (India), Board of Research in Nuclear Sciences (India), Harsh, Rishav, Mondal, Sourav, Sharma, Devina, Bouatou, Mehdi, Chacon, Cyril, Ilyn, Max, Rogero, Celia, Repain, V., Bellec, Amandine, Girard, Yann, Rousset, S., Sankar, Raman, Pai, Woei Wu, Narasimhan, Shobhana, Lagoute, Jérôme, Indo-French Centre for the Promotion of Advanced Research, Agence Nationale de la Recherche (France), Sheikh Saqr Laboratory (India), Board of Research in Nuclear Sciences (India), Harsh, Rishav, Mondal, Sourav, Sharma, Devina, Bouatou, Mehdi, Chacon, Cyril, Ilyn, Max, Rogero, Celia, Repain, V., Bellec, Amandine, Girard, Yann, Rousset, S., Sankar, Raman, Pai, Woei Wu, Narasimhan, Shobhana, and Lagoute, Jérôme

Abstract

We identify and manipulate commonly occurring defects in black phosphorus, combining scanning tunneling microscopy experiments with density functional theory calculations. A ubiquitous defect, imaged at negative bias as a bright dumbbell extending over several nanometers, is shown to arise from a substitutional Sn impurity in the second sublayer. Another frequently observed defect type is identified as arising from an interstitial Sn atom; this defect can be switched to a more stable configuration consisting of a Sn substitutional defect + P adatom, by application of an electrical pulse via the STM tip. DFT calculations show that this pulse-induced structural transition switches the system from a non-magnetic configuration to a magnetic one. We introduce States Projected Onto Individual Layers (SPOIL) quantities which provide information about atom-wise and orbital-wise contributions to bias-dependent features observed in STM images.

Published
2022

23. Modification of monolayer 1T-VSe2 by selective deposition of vanadium and tellurium.

Author

Karn, Abhishek, Chan, Yang Hao, Chazarin, Ulysse, Chen, Peng, and Pai, Woei Wu

Subjects
VANADIUM, CHARGE density waves, MONOMOLECULAR films, TRANSITION metals, TELLURIUM, SURFACE structure, MOLECULAR beams
Abstract

Hetero-structures of transition metal dichalcogenide (TMD) layers offer vast possibilities of new materials properties. The growth behaviors of such TMD hetero-structures can be complex. In this study, we report the structure modification of a VSe2 monolayer by the molecular beam deposition of Te, V, or both. With a typical growth temperature of 300 °C and a nominal flux rate, we found that Te deposition leaves the VSe2 intact. Vanadium deposition, in contrast, leads to small clusters ordered in stripes along well-defined directions. The charge density wave symmetry of monolayer VSe2 surprisingly changes in small regions enclosed by such vanadium stripes. With V and Te co-deposition, a dramatic change of the monolayer surface structure to a (2 × 1) ordered phase is observed. This study illustrates the unexpected complexities involved in preparing even a simple bilayer TMD hetero-structure, such as VTe2/VSe2. [ABSTRACT FROM AUTHOR]

Published
2022
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28. Pressure-induced reentrant transition in NbS3 phases: Combined Raman scattering and x-ray diffraction study

Author

Abdel-Hafiez, M., primary, Thiyagarajan, R., additional, Majumdar, A., additional, Ahuja, R., additional, Luo, W., additional, Vasiliev, A. N., additional, Maarouf, A. A., additional, Zybtsev, S. G., additional, Pokrovskii, V. Ya., additional, Zaitsev-Zotov, S. V., additional, Pavlovskiy, V. V., additional, Pai, Woei Wu, additional, Yang, W., additional, and Kulik, L. V., additional

Published
2019
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30. Pressure-induced reentrant transition in NbS3 phases : Combined Raman scattering and x-ray diffraction study

Author

Abdel-Hafiez, Mahmoud, Thiyagarajan, R., Majumdar, Arnab, Ahuja, Rajeev, Luo, Wei, Vasiliev, A. N., Maarouf, A. A., Zybtsev, S. G., Pokrovskii, V. Ya, V-Zaitsev-Zotov, S., Pavlovskiy, V. V., Pai, Woei Wu, Yang, W., Kulik, L. , V, Abdel-Hafiez, Mahmoud, Thiyagarajan, R., Majumdar, Arnab, Ahuja, Rajeev, Luo, Wei, Vasiliev, A. N., Maarouf, A. A., Zybtsev, S. G., Pokrovskii, V. Ya, V-Zaitsev-Zotov, S., Pavlovskiy, V. V., Pai, Woei Wu, Yang, W., and Kulik, L. , V

Abstract

We report the evolution of charge density wave states under pressure for two NbS3 phases: triclinic (phase I) and monoclinic (phase II) at room temperature. Raman and x-ray diffraction (XRD) techniques are applied. The x-ray studies on the monoclinic phase under pressure show a compression of the lattice at different rates below and above similar to 7 GPa but without a change in space group symmetry. The Raman spectra of the two phases evolve similarly with pressure; all peaks almost disappear in the similar to 6-8 GPa range, indicating a transition from an insulating to a metallic state, and peaks at new positions appear above 8 GPa. The results suggest suppression of the ambient charge-density waves and their subsequent recovery with new orderings above 8 GPa.

Published
2019
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32. Erratum: NbS3 : A unique quasi-one-dimensional conductor with three charge density wave transitions [Phys. Rev. B 95 , 035110 (2017)]

Author

Zybtsev, S. G., primary, Pokrovski, V. Ya., additional, Nasretdinova, V. F., additional, Zaitsev-Zotov, S. V., additional, Pavlovskiy, V. V., additional, Odobesco, A. B., additional, Pai, Woei Wu, additional, Chu, M.-W., additional, Lin, Y. G., additional, Zupanič, E., additional, van Midden, H. J. P., additional, Šturm, S., additional, Tchernychova, E., additional, Prodan, A., additional, Bennett, J. C., additional, Mukhamedshin, I. R., additional, Chernysheva, O. V., additional, Menushenkov, A. P., additional, Loginov, V. B., additional, Loginov, B. A., additional, Titov, A. N., additional, and Abdel-Hafiez, M., additional

Published
2019
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33. Basic and charge density wave modulated structures of NbS3 -II

Author

Zupanič, E., primary, van Midden, H. J. P., additional, van Midden, M. A., additional, Šturm, S., additional, Tchernychova, E., additional, Pokrovskii, V. Ya., additional, Zybtsev, S. G., additional, Nasretdinova, V. F., additional, Zaitsev-Zotov, S. V., additional, Chen, W. T., additional, Pai, Woei Wu, additional, Bennett, J. C., additional, and Prodan, A., additional

Published
2018
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35. Single-layer dual germanene phases on Ag(111)

Author

Lin, Chung-Huang, primary, Huang, Angus, additional, Pai, Woei Wu, additional, Chen, Wei-Chuan, additional, Chen, Ting-Yu, additional, Chang, Tay-Rong, additional, Yukawa, Ryu, additional, Cheng, Cheng-Maw, additional, Mou, Chung-Yu, additional, Matsuda, Iwao, additional, Chiang, T.-C., additional, Jeng, H.-T., additional, and Tang, S.-J., additional

Published
2018
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36. Probing structural and electronic properties of h-BN by HRTEM and STM

Author

Van Dong Pham, Hakim Amara, Christian Ricolleau, Jérôme Lagoute, P. Lavenus, Damien Alloyeau, Guillaume Wang, Amandine Andrieux, Nelly Dorval, Pai Woei Wu, Frédéric Fossard, Annick Loiseau, Ouafi Mouhoub, Laboratoire d'étude des microstructures [Châtillon] (LEM - ONERA - CNRS), Centre National de la Recherche Scientifique (CNRS)-ONERA, Mathématique et Physique Quantique (MPQ), Centre National de la Recherche Scientifique (CNRS), Laboratoire Matériaux et Phénomènes Quantiques (MPQ (UMR_7162)), Université Paris Diderot - Paris 7 (UPD7)-Centre National de la Recherche Scientifique (CNRS), ONERA - The French Aerospace Lab [Palaiseau], ONERA-Université Paris Saclay (COmUE), ONERA - The French Aerospace Lab [Châtillon], National Taiwan University [Taiwan] (NTU), ONERA-Centre National de la Recherche Scientifique (CNRS), ONERA, Laboratoire d'étude des microstructures ( LEM - ONERA - CNRS ), ONERA-Centre National de la Recherche Scientifique ( CNRS ), Mathématique et Physique Quantique ( MPQ ), Centre National de la Recherche Scientifique ( CNRS ), Laboratoire Matériaux et Phénomènes Quantiques ( MPQ ), Université Paris Diderot - Paris 7 ( UPD7 ), Matériaux et Phénomènes Quantiques ( MPQ ), Université Paris Diderot - Paris 7 ( UPD7 ) -Centre National de la Recherche Scientifique ( CNRS ), ONERA - The French Aerospace Lab ( Palaiseau ), ONERA - The French Aerospace Lab ( Chatillon ), and National Taiwan University [Taiwan] ( NTU )

Subjects
Materials science, Microscope, aberration-corrected TEM, Stacking, Nanotechnology, 02 engineering and technology, 010402 general chemistry, 01 natural sciences, law.invention, Nanomaterials, electron beam irradiation, law, hexagonal boron nitride, [PHYS.COND]Physics [physics]/Condensed Matter [cond-mat], High-resolution transmission electron microscopy, [PHYS]Physics [physics], [ PHYS ] Physics [physics], Graphene, Doping, Heterojunction, 021001 nanoscience & nanotechnology, 0104 chemical sciences, Van Der Walls heterostructures, Scanning tunneling microscope, 0210 nano-technology, [ PHYS.COND ] Physics [physics]/Condensed Matter [cond-mat]
Abstract

After the discovery of graphene and its consequences in the field of nanoscience and nanomaterials, there has been a growing interest in 2D materials and also their vertical stacking due to unique properties and potential applications.[1] For instance, it was shown the transport properties of exfoliated graphene supported by hexagonal boron nitride (h-BN) could approach the intrinsic graphene limits.[2] Nevertheless, studying the structural properties of 2D materials and 2D heterostructures is crucial to understand their physical and chemical properties. Our motivations have been to exploit state of the art aberration-corrected high resolution transmission electron microscopy (HRTEM) and scanning tunneling microscopy (STM) to study the structure and electronic properties of graphene (G), h-BN and G/h-BN heterostructures. HRTEM analyses were conducted with a JEOL ARM microscope equipped together with a cold FEG, an aberration corrector for the objective lens and a One view camera (Gatan). Notably, we used high-speed atomic-scale imaging to study with unprecedented dynamics (up to 25 fps) the nucleation and growth mechanisms of triangular holes in h-BN under beam irradiation (Figure 1). The direct observation of B and N atom sputtering and surface reconstruction processes allow understanding how the triangular shape and orientation of holes are maintained during the growth. Interestingly, by studying the effects of the electron dose and the number of BN layers, we demonstrate that these atomic-scale processes are simultaneously driven by kinetic and thermodynamic effects. Further works are in progress to study the stability of h-BN/G stacking under electron-beam irradiation. STM analyses were carried out with low temperature STM at 4 K, on 2D heterostructures that consist in a few layers of graphene doped with nitrogen on thick exfoliated flakes of BN deposited on SiO2. Remarkably, we show that STM allows identifying and characterizing ionization defects within the BN flakes below the graphene layers (Figure 2). This study opens new avenues to probe the electronic interactions between this two stacked materials. Keywords: hexagonal boron nitride; aberration-corrected TEM; electron beam irradiation; van der walls heterostructures

Published
2016
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40. Direct observation of methyl radicals islanding on copper surface and its effect on the kinetics of catalytic reactions

Author

Chan, Yuet Loy, Pai, Woei Wu, and Chuang, Tung J.

Subjects
Surface chemistry -- Research, Copper -- Research, Radicals (Chemistry) -- Research, Chemicals, plastics and rubber industries
Abstract

The formation of methyl radicals into densely packed two-dimensional islands on the Cu(111) surface is directly observed for the first time by scanning tunneling microscopy (STM). From this investigation on methyl-solid interactions, we infer that the islanding phenomenon may be quite general in catalytic systems.

Published
2004

41. Field Effect in the Linear and Nonlinear Conductivity of the Layered Quasi-One-Dimensional Semiconductor TiS3.

Author

Gorlova, I. G., Frolov, A. V., Orlov, A. P., Pokrovskii, V. Ya., and Pai, Woei Wu

Subjects
INDUCTIVE effect, SEMICONDUCTORS, FIELD-effect transistors, CURRENT-voltage characteristics, LOW temperatures, CRYSTAL whiskers
Abstract

Field-effect transistor structures based on whiskers of layered quasi-one-dimensional semiconductor TiS3 have been fabricated. The dependences of the conductivity σ on the gate voltage Vg, as well as the current-voltage characteristics of whiskers ("source-drain") at different Vg values, have been measured in the temperature range of 4.2-300 K. As the temperature decreases, the sensitivity of the conductivity to the gate voltage, α ≡ 1/σdσ/dVg, increases in the range from 300 to 80 K and decreases sharply below 80 K, where the nonlinear conductivity begins to depend on Vg. The results can be explained by the formation of an electronic crystal at low temperatures. [ABSTRACT FROM AUTHOR]

Published
2019
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43. NbS3: A unique quasi-one-dimensional conductor with three charge density wave transitions

Author

Zybtsev, S. G., primary, Pokrovskii, V. Ya., additional, Nasretdinova, V. F., additional, Zaitsev-Zotov, S. V., additional, Pavlovskiy, V. V., additional, Odobesco, A. B., additional, Pai, Woei Wu, additional, Chu, M.-W., additional, Lin, Y. G., additional, Zupanič, E., additional, van Midden, H. J. P., additional, Šturm, S., additional, Tchernychova, E., additional, Prodan, A., additional, Bennett, J. C., additional, Mukhamedshin, I. R., additional, Chernysheva, O. V., additional, Menushenkov, A. P., additional, Loginov, V. B., additional, Loginov, B. A., additional, Titov, A. N., additional, and Abdel-Hafiez, M., additional

Published
2017
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44. Spatially Extended Charge Density Wave Switching by Nanoscale Local Manipulation in a VTe2Monolayer

Author

Chazarin, Ulysse, Lezoualc’h, Mahé, Karn, Abhishek, Chou, Jyh-Ping, Pai, Woei Wu, Chacon, Cyril, Girard, Yann, Repain, Vincent, Bellec, Amandine, Rousset, Sylvie, González, César, Smogunov, Alexander, Lagoute, Jérôme, and Dappe, Yannick J.

Abstract

Monolayer transition metal dichalcogenide VTe2exhibits multiple charge density wave (CDW) phases, mainly (4 × 4) and (4 × 1). Here we report facile dynamic and tens-of-nanometer scale switching between these CDW phases with gentle bias pulses in scanning tunneling microscopy. Bias pulses purposely stimulate a reversible random CDW symmetry change between the isotropic (4 × 4) and anisotropic (4 × 1) CDWs, as well as CDW phase slips and rotation. The switching threshold of ∼1.0 V is independent of bias polarity, and the switching rate varies linearly with the tunneling current. Density functional theory calculations indicate that a coherent CDW phase switching incurs an energy barrier of ∼2.0–3.0 eV per (4 × 4) unit cell. While there is a challenge in understanding the observed large-area CDW random fluttering, we provide some possible explanations. The ability to manipulate electronic CDW phases sheds new light on tailoring CDW properties on demand.

Published
2024
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45. Tuning the Magnetic Anisotropy at a Molecule-Metal Interface

Author

Bairagi, K., Bellec, A., Repain, V., Chacon, C., Girard, Y., Garreau, Y., Lagoute, J., Rousset, S., Breitwieser, R., Hu, Yu-Cheng, Chao, Yen Cheng, Pai, Woei Wu, Li, D., Smogunov, A., Barreteau, Cyrille, Bairagi, K., Bellec, A., Repain, V., Chacon, C., Girard, Y., Garreau, Y., Lagoute, J., Rousset, S., Breitwieser, R., Hu, Yu-Cheng, Chao, Yen Cheng, Pai, Woei Wu, Li, D., Smogunov, A., and Barreteau, Cyrille

Abstract

We demonstrate that a C60 overlayer enhances the perpendicular magnetic anisotropy of a Co thin film, inducing an inverse spin reorientation transition from in plane to out of plane. The driving force is the C60/Co interfacial magnetic anisotropy that we have measured quantitatively in situ as a function of the C60 coverage. Comparison with state-of-the-art ab initio calculations show that this interfacial anisotropy mainly arises from the local hybridization between C60 pz and Co dz2 orbitals. By generalizing these arguments, we also demonstrate that the hybridization of C60 with a Fe(110) surface decreases the perpendicular magnetic anisotropy. These results open the way to tailor the interfacial magnetic anisotropy in organic-material-ferromagnet systems.

Published
2015

46. Tuning the Magnetic Anisotropy at a Molecule-Metal Interface

Author

Bairagi, K., primary, Bellec, A., additional, Repain, V., additional, Chacon, C., additional, Girard, Y., additional, Garreau, Y., additional, Lagoute, J., additional, Rousset, S., additional, Breitwieser, R., additional, Hu, Yu-Cheng, additional, Chao, Yen Cheng, additional, Pai, Woei Wu, additional, Li, D., additional, Smogunov, A., additional, and Barreteau, C., additional

Published
2015
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49. Geometric and electronic structure of a C60 monolayer on Ag(100)

Author

Zhang, Xieqiu, He, Wei, Zhao, Aidi, Li, Hongnian, Chen, Lan, Pai, Woei Wu, Hou, Jianguo, Loy, Michael Ming-tak, Yang, Jinlong, Xiao, Xudong, Zhang, Xieqiu, He, Wei, Zhao, Aidi, Li, Hongnian, Chen, Lan, Pai, Woei Wu, Hou, Jianguo, Loy, Michael Ming-tak, Yang, Jinlong, and Xiao, Xudong

Abstract

Using scanning tunneling microscopy and spectroscopy and first principles calculations, we have systematically studied the morphological and electronic structures of a C60 monolayer on Ag(100). Our results reveal that the bright-dim contrast has a definite geometric origin and there are two types of dim C60 molecules, one a monomer and the other a dimer. With distinctive electronic properties and distinctive charge transfers in the differently adsorbed C60 molecules, this C60 monolayer is highly inhomogeneous and aperiodic.

Published
2007

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