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Correlating Structural, Electronic, and Magnetic Properties of Epitaxial VSe2 Thin Films. (arXiv:1912.12798v1 [cond-mat.mtrl-sci])

2020-01-01T02:29:56+00:00January 1st, 2020|Categories: Publications|Tags: |

The electronic and magnetic properties of transition metal dichalcogenides are known to be extremely sensitive to their structure. In this paper we study the effect of structure on the electronic and magnetic properties of mono- and bilayer $VSe_2$ films grown using molecular beam epitaxy. $VSe_2$ has recently attracted much attention due to reports of emergent ferromagnetism in the 2D limit. To understand this important compound, high quality 1T and distorted 1T films were grown at temperatures of 200 $^text{o}$C and 450 $^text{o}$C respectively and studied using 4K Scanning Tunneling Microscopy/Spectroscopy. The measured density of states and the charge density wave (CDW) patterns were compared to band structure and phonon dispersion calculations. Films in the 1T phase reveal different CDW patterns in the first layer compared to the second. Interestingly, we find the second layer of the 1T-film shows a CDW pattern with 4a $times$ 4a periodicity which is the 2D version of the bulk CDW observed in this compound. Our phonon dispersion calculations confirm the presence of a soft phonon at the correct wavevector that leads to this CDW. In contrast, the first layer of distorted 1T phase films shows a strong stripe feature with varying periodicities, while the second layer displays no observable CDW pattern. Finally, we find that the monolayer 1T $VSe_2$ film is weakly ferromagnetic, with ~ $3.5 {mu}_B$ per unit similar to previous reports.

Published in: "arXiv Material Science".

Two Phonon Interactions and Charge Density Wave in Single Crystalline VSe2 Probed by Raman Spectroscopy. (arXiv:1912.05802v1 [cond-mat.mtrl-sci])

2019-12-13T02:29:19+00:00December 13th, 2019|Categories: Publications|Tags: |

Charge density wave (CDW) is a unique phenomenon mostly realized in two-dimensional (2D) metallic layered transition metal dichalcogenides (TMDCs). Here, we report on Raman spectroscopy of single crystal 1T-VSe2 and observed signature of commensurate CDW (C-CDW) and incommensurate (I-CDW) transition, in the temperature range of 50-120 K. The room temperature Raman spectra showed a sharp A1g mode ~ 206 cm-1 along with two new weak and broad Raman modes associated to Eg mode ~ 257 cm-1 and two-phonon mode (2ph) ~ 332 cm-1. The onset of I-CDW and C-CDW is estimated from resistance measurements supported by magnetic measurements. Remarkably, at the onset of I-CDW ~ 115 K, a significant enhancement in the intensity of weak Eg mode is observed along with emergence of a doubly degenerate Eg(2) mode ~ 144 cm-1. Below 70 K, a weak A1g mode ~ 170 cm-1 emerged signifying the onset of C-CDW. Anomalous phonon softening of Eg mode ~ 257 cm-1 and 2ph process mode below I-CDW has been also observed, which is signifying the involvement of electron-phonon coupling.

Published in: "arXiv Material Science".

Chemically Exfoliated VSe2 Monolayers with Room‐Temperature Ferromagnetism

2019-10-18T10:33:40+00:00October 18th, 2019|Categories: Publications|Tags: |

Large‐size 1T‐VSe2 monolayers are successfully produced at high yield by electrochemical exfoliation of bulk crystal. To guard against air‐induced degradation, thiol molecules are introduced to passivate the VSe2 flakes, allowing the observation of robust room‐temperature ferromagnetism in monolayer VSe2. Abstract Among van der Waals layered ferromagnets, monolayer vanadium diselenide (VSe2) stands out due to its robust ferromagnetism. However, the exfoliation of monolayer VSe2 is challenging, not least because the monolayer flake is extremely unstable in air. Using an electrochemical exfoliation approach with organic cations as the intercalants, monolayer 1T‐VSe2 flakes are successfully obtained from the bulk crystal at high yield. Thiol molecules are further introduced onto the VSe2 surface to passivate the exfoliated flakes, which improves the air stability of the flakes for subsequent characterizations. Room‐temperature ferromagnetism is confirmed on the exfoliated 2D VSe2 flakes using a superconducting quantum interference device (SQUID), X‐ray magnetic circular dichroism (XMCD), and magnetic force microscopy (MFM), where the monolayer flake displays the strongest ferromagnetic properties. Se vacancies, which can be ubiquitous in such materials, also contribute to the ferromagnetism of VSe2, although density functional theory (DFT) calculations show that such effect can be minimized by physisorbed oxygen molecules or covalently bound thiol molecules.

Published in: "Advanced Materials".

Morphology Control of Epitaxial Monolayer Transition Metal Dichalcogenides. (arXiv:1910.03307v1 [cond-mat.mtrl-sci])

2019-10-09T02:29:26+00:00October 9th, 2019|Categories: Publications|Tags: , , |

To advance fundamental understanding, and ultimate application, of transition-metal dichalcogenide (TMD) monolayers, it is essential to develop capabilities for the synthesis of high-quality single-layer samples. Molecular beam epitaxy (MBE), a leading technique for the fabrication of the highest-quality epitaxial films of conventional semiconductors has, however, typically yielded only small grain sizes and sub-optimal morphologies when applied to the van der Waals growth of monolayer TMDs. Here, we present a systematic study on the influence of adatom mobility, growth rate, and metal:chalcogen flux on the growth of NbSe2, VSe2 and TiSe2 using MBE. Through this, we identify the key drivers and influence of the adatom kinetics that control the epitaxial growth of TMDs, realising four distinct morphologies of the as-grown compounds. We use this to determine optimised growth conditions for the fabrication of high-quality monolayers, ultimately realising the largest grain sizes of monolayer TMDs that have been achieved to date via MBE growth.

Published in: "arXiv Material Science".

Chemically Exfoliated VSe2 Monolayers with Room‐Temperature Ferromagnetism

2019-10-02T02:39:10+00:00October 2nd, 2019|Categories: Publications|Tags: |

Large‐size 1T‐VSe2 monolayers are successfully produced at high yield by electrochemical exfoliation of bulk crystal. To guard against air‐induced degradation, thiol molecules are introduced to passivate the VSe2 flakes, allowing the observation of robust room‐temperature ferromagnetism in monolayer VSe2. Abstract Among van der Waals layered ferromagnets, monolayer vanadium diselenide (VSe2) stands out due to its robust ferromagnetism. However, the exfoliation of monolayer VSe2 is challenging, not least because the monolayer flake is extremely unstable in air. Using an electrochemical exfoliation approach with organic cations as the intercalants, monolayer 1T‐VSe2 flakes are successfully obtained from the bulk crystal at high yield. Thiol molecules are further introduced onto the VSe2 surface to passivate the exfoliated flakes, which improves the air stability of the flakes for subsequent characterizations. Room‐temperature ferromagnetism is confirmed on the exfoliated 2D VSe2 flakes using a superconducting quantum interference device (SQUID), X‐ray magnetic circular dichroism (XMCD), and magnetic force microscopy (MFM), where the monolayer flake displays the strongest ferromagnetic properties. Se vacancies, which can be ubiquitous in such materials, also contribute to the ferromagnetism of VSe2, although density functional theory (DFT) calculations show that such effect can be minimized by physisorbed oxygen molecules or covalently bound thiol molecules.

Published in: "Advanced Materials".

Structural phase transitions in VSe2: energetics, electronic structure and magnetism. (arXiv:1909.11134v1 [cond-mat.str-el])

2019-09-26T05:29:28+00:00September 26th, 2019|Categories: Publications|Tags: , |

First principles calculations of magnetic and electronic properties of VSe2 describing the transition between two structural phases(H,T) were performed. Results of the calculations evidence rather low energy barrier ( 0.60 eV for monolayer) for transition betweenthe phases. The energy required for the deviation of Se atom or whole layer of selenium atoms on a small angle up to 10 degrees from initial positions is also rather low, 0.32 and 0.19 eV/Se, respectively. The changes in band structure of VSe2 caused by these motions of Se atoms should be taken into account for analysis of the experimental data. Simulations of the strain effects suggest that the experimentally observed T phase of VSe2 monolayer is the ground state due a substrate-induced strain. Calculations of the difference in total energies of ferromagnetic and antiferromagnetic configurations evidence that the ferromagnetic configuration is the ground state of the system for all stable and intermediate atomic structures. Calculated phonon dispersions suggest visible influence of magnetic configurations on vibrational properties.

Published in: "arXiv Material Science".

Proximity-induced ferromagnetism and chemical reactivity in few layers VSe2 heterostructures. (arXiv:1909.01713v1 [cond-mat.mes-hall])

2019-09-05T04:30:36+00:00September 5th, 2019|Categories: Publications|Tags: , |

Among Transition-Metal Dichalcogenides, mono and few-layers thick VSe2 has gained much recent attention following claims of intrinsic room-temperature ferromagnetism in this system, which have nonetheless proved controversial. Here, we address the magnetic and chemical properties of Fe/VSe2 heterostructure by combining element sensitive absorption spectroscopy and photoemission spectroscopy. Our x-ray magnetic circular dichroism results confirm recent findings that both native mono/few-layer and bulk VSe2 do not show any signature of an intrinsic ferromagnetic ordering. Nonetheless, we find that ferromagnetism can be induced, even at room temperature, after coupling with a Fe thin film layer, with antiparallel alignment of the moment on the V with respect to Fe. We further consider the chemical reactivity at the Fe/VSe2 interface and its relation with interfacial magnetic coupling.

Published : "arXiv Mesoscale and Nanoscale Physics".

Chemically Exfoliated VSe2 Monolayers with Room‐Temperature Ferromagnetism

2019-08-19T08:46:33+00:00August 19th, 2019|Categories: Publications|Tags: |

Large size 1T‐VSe2 monolayers are successfully produced at high yield by electrochemical exfoliation of bulk crystal. To guard against air‐induced degradation, thiol molecules are introduced to passivate the VSe2 flakes, allowing the observation of robust room‐temperature ferromagnetism in monolayer VSe2. Abstract Among van der Waals layered ferromagnets, monolayer vanadium diselenide (VSe2) stands out due to its robust ferromagnetism. However, the exfoliation of monolayer VSe2 is challenging, not least because the monolayer flake is extremely unstable in air. Using an electrochemical exfoliation approach with organic cations as the intercalants, monolayer 1T‐VSe2 flakes are successfully obtained from the bulk crystal at high yield. Thiol molecules are further introduced onto the VSe2 surface to passivate the exfoliated flakes, which improves the air stability of the flakes for subsequent characterizations. Room‐temperature ferromagnetism is confirmed on the exfoliated 2D VSe2 flakes using a superconducting quantum interference device (SQUID), X‐ray magnetic circular dichroism (XMCD), and magnetic force microscopy (MFM), where the monolayer flake displays the strongest ferromagnetic properties. Se vacancies, which can be ubiquitous in such materials, also contribute to the ferromagnetism of VSe2, although density functional theory (DFT) calculations show that such effect can be minimized by physisorbed oxygen molecules or covalently bound thiol molecules.

Published in: "Advanced Materials".

Sub-room temperature ferromagnetism and its nature in VSe$_2$ monolayer. (arXiv:1907.04790v2 [cond-mat.mtrl-sci] UPDATED)

2019-07-24T02:29:34+00:00July 24th, 2019|Categories: Publications|Tags: |

Magnetic van der Waals materials provide an ideal platform for exploring two-dimensional magnetism of technological and scientific importance. Among them, 1T-VSe2 has been reported as one of the first room-temperature two-dimensional ferromagnets. However this conclusion remains elusive both theoretically and experimentally. By using charge self-consistent LDA+DMFT approach, suited for both itinerant and localized characters of electrons in the open-d subshells, we show that the monolayer is ferromagnetically ordered below 250K, implying the possible competition between charge density wave and ferromagnetic order. We predict that its ferromagnetism is vulnerable to interlayer coupling and extra charge dopings. The formation of local moment is attributed to the concerted effect of the quasiparticle pre-localization caused by reduced dimensionality and the electronic correlation. This study provides an important example where the reduced dimensionality is an essential factor to form local moments and subsequent magnetic orderings.

Published in: "arXiv Material Science".

Monolayer VTe2: Incommensurate Fermi surface nesting and suppression of charge density waves. (arXiv:1906.07855v1 [cond-mat.mtrl-sci])

2019-06-20T08:33:39+00:00June 20th, 2019|Categories: Publications|Tags: , |

We investigated the electronic structure of monolayer VTe2 grown on bilayer graphene by angle-resolved photoemission spectroscopy (ARPES). We found that monolayer VTe2 takes the octahedral 1T structure in contrast to the monoclinic one in the bulk, as evidenced by the good agreement in the Fermi-surface topology between ARPES results and first-principles band calculations for octahedral monolayer 1T-VTe2. We have revealed that monolayer 1T-VTe2 at low temperature is characterized by a metallic state whereas the nesting condition is better than that of isostructural monolayer VSe2 which undergoes a CDW transition to insulator at low temperature. The present result suggests an importance of Fermi-surface topology for characterizing the CDW properties of monolayer TMDs.

Published in: "arXiv Material Science".

Large tunneling magnetoresistance in VSe2/MoS2 magnetic tunnel junction. (arXiv:1904.07499v1 [cond-mat.mtrl-sci])

2019-04-17T02:29:36+00:00April 17th, 2019|Categories: Publications|Tags: , , |

Two-dimensional (2D) van der Waals (vdW) materials provide the possibility of realizing heterostructures with coveted properties. Here, we report a theoretical investigation of the vdW magnetic tunnel junction (MTJ) based on VSe2/MoS2 heterojunction, where the VSe2 monolayer acts as the ferromagnet with the room-temperature ferromagnetism. We propose the concept of spin-orbit torque (SOT) vdW MTJ with reliable reading and efficient writing operations. The non-equilibrium study reveals a large tunneling magnetoresistance (TMR) of 846 % at 300 Kelvin, identifying significantly its parallel and anti-parallel states. Thanks to the strong spin Hall conductivity of MoS2, SOT is promising for the magnetization switching of VSe2 free layer. Quantum-well states come into being and resonances appear in MTJ, suggesting that the voltage control can adjust transport properties effectively. The SOT vdW MTJ based on VSe2/MoS2 provides desirable performance and experimental feasibility, offering new opportunities for 2D spintronics.

Published in: "arXiv Material Science".

Giant dielectric difference in chiral asymmetric bilayers. (arXiv:1811.08262v1 [cond-mat.mes-hall])

2018-11-21T02:29:17+00:00November 21st, 2018|Categories: Publications|Tags: |

Twistronics rooted in the twist operation towards bilayer van der Waals crystals is of both theoretical and technological importance. The realize of the correlated electronic behaviors under this operation encourages enormous effort to the research on magic-angle systems which possess sensitive response to the external field. Here, a giant dielectric difference between 30 plus or minus degree twist case is observed in a typical magnetic system 2H-VSe2 bilayer. It is shown that due to the structural inversion asymmetry in its monolayer, the different stacking of the two cases corresponds to the two kind of valley polarized states: interlayer ferrovalley and interlayer antiferrovalley. Further investigations reveal that such different dielectric response between the two states stems from the different Fermi wave vectors coupled to the electric field. More interestingly, we even obtain the selective circularly polarized optical absorption by tuning the interlayer twist. These findings open an appealing route toward functional 2D materials design for electric and optical devices.

Published in: "arXiv Material Science".

Pseudogap, Fermi arc, and Peierls-insulating phase induced by 3D-2D crossover in monolayer VSe2. (arXiv:1810.02511v1 [cond-mat.mtrl-sci])

2018-10-08T02:29:16+00:00October 8th, 2018|Categories: Publications|Tags: , , |

One of important challenges in condensed-matter physics is to realize new quantum states of matter by manipulating the dimensionality of materials, as represented by the discovery of high-temperature superconductivity in atomic-layer pnictides and room-temperature quantum Hall effect in graphene. Transition-metal dichalcogenides (TMDs) provide a fertile platform for exploring novel quantum phenomena accompanied by the dimensionality change, since they exhibit a variety of electronic/magnetic states owing to quantum confinement. Here we report an anomalous metal-insulator transition induced by 3D-2D crossover in monolayer 1T-VSe2 grown on bilayer graphene. We observed a complete insulating state with a finite energy gap on the entire Fermi surface in monolayer 1T-VSe2 at low temperatures, in sharp contrast to metallic nature of bulk. More surprisingly, monolayer 1T-VSe2 exhibits a pseudogap with Fermi arc at temperatures above the charge-density-wave temperature, showing a close resemblance to high-temperature cuprates. This similarity suggests a common underlying physics between two apparently different systems, pointing to the importance of charge/spin fluctuations to create the novel electronic states, such as pseudogap and Fermi arc, in these materials.

Published in: "arXiv Material Science".

Emergence of a Metal-Insulator Transition and High Temperature Charge Density Waves in VSe2 at the Monolayer Limit. (arXiv:1808.03034v1 [cond-mat.str-el])

2018-08-10T02:29:19+00:00August 10th, 2018|Categories: Publications|Tags: , , |

Emergent phenomena driven by electronic reconstructions in oxide heterostructures have been intensively discussed. However, the role of these phenomena in shaping the electronic properties in van der Waals heterointerfaces has hitherto not been established. By reducing the material thickness and forming a heterointerface, we find two types of charge-ordering transitions in monolayer VSe2 on graphene substrates. Angle-resolved photoemission spectroscopy (ARPES) uncovers that Fermi-surface nesting becomes perfect in ML VSe2. Renormalization group analysis confirms that imperfect nesting in three dimensions universally flows into perfect nesting in two dimensions. As a result, the charge density wave transition temperature is dramatically enhanced to a value of 350 K compared to the 105 K in bulk VSe2. More interestingly, ARPES and scanning tunneling microscopy measurements confirm an unexpected metal-insulator transition at 135 K, driven by lattice distortions. The heterointerface plays an important role in driving this novel metal-insulator transition in the family of monolayered transition metal dichalcogenides.

Published in: "arXiv Material Science".

Absence of ferromagnetism in VSe2 caused by its charge density wave phase. (arXiv:1804.07102v1 [cond-mat.mtrl-sci])

2018-04-20T19:59:09+00:00April 20th, 2018|Categories: Publications|Tags: |

In this study we present a detailed ab initio analysis of the magnetic properties of VSe2 . Ab initio calculations in the so-called 1T structure yield a ferromagnetic phase as most stable, with a magnetic moment of about 0.6 {mu} B /V. According to our calculations, and based on the Stoner criterion for itinerant ferromagnets, this ferromagnetic state is on the verge of instability. We have modeled ab initio the charge density wave state reported in the literature. This introduces a periodic lattice distortion leading to a supercell with periodicity 4a x 4a x 3c (4a x 4a for the monolayer) in which we have fully relaxed the atomic positions. We demonstrate that this structural rearrangement causes a strong reduction in the density of states at the Fermi level and the ground state of the system becomes non-magnetic. Experimental evidences that report a magnetic signal need to be understood on the light of this finding.

Published in: "arXiv Material Science".

Metallic few-layered VSe2 nanosheets: high two-dimensional conductivity for flexible in-plane solid-state supercapacitor

2018-03-07T01:18:02+00:00March 7th, 2018|Categories: Publications|Tags: , |

J. Mater. Chem. A, 2018, Accepted ManuscriptDOI: 10.1039/C8TA00089A, PaperChaolun Wang, Xing Wu, Yonghui Ma, Gang Mu, Yaoyi Li, Luo Chen, Hejun Xu, Yuanyuan Zhang, Jing Yang, Xiaodong Tang, Jian Zhang, Wenzhong Bao, Chun-Gang DuanMetallic two-dimensional (2D) 1T-VSe2 with high conductivity and large specific

Published in: "Journal of Materials Chemistry A".

Dynamic instabilities in strongly correlated ${text{VSe}}_{2}$ monolayers and bilayers

2017-12-27T18:30:43+00:00December 27th, 2017|Categories: Publications|Tags: , |

Author(s): Marco Esters, Richard G. Hennig, and David C. JohnsonWith the emergence of graphene and other two-dimensional (2D) materials, transition-metal dichalcogenides have been investigated intensely as potential 2D materials using experimental and theoretical methods. VSe2 is an especially interesting material since its bulk modification exhibits a charge-de…[Phys. Rev. B 96, 235147] Published Wed Dec 27, 2017

Published in: "Physical Review B".

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