/Tag: TiSe2

Negative electronic compressibility and nanoscale inhomogeneity in ionic-liquid gated two-dimensional superconductors. (arXiv:1706.01274v2 [cond-mat.mes-hall] UPDATED)

2018-10-12T04:30:18+00:00October 12th, 2018|Categories: Publications|Tags: , |

When the electron density of highly crystalline thin films is tuned by chemical doping or ionic liq- uid gating, interesting effects appear including unconventional superconductivity, sizeable spin-orbit coupling, competition with charge-density waves, and a debated low-temperature metallic state that seems to avoid the superconducting or insulating fate of standard two-dimensional electron systems. Some experiments also find a marked tendency to a negative electronic compressibility. We suggest that this indicates an inclination for electronic phase separation resulting in a nanoscopic inhomo- geneity. Although the mild modulation of the inhomogeneous landscape is compatible with a high electron mobility in the metallic state, this intrinsically inhomogeneous character is highlighted by the peculiar behaviour of the metal-to-superconductor transition. Modelling the system with super- conducting puddles embedded in a metallic matrix, we fit the peculiar resistance vs. temperature curves of systems like TiSe2, MoS2, and ZrNCl. In this framework also the low-temperature debated metallic state finds a natural explanation in terms of the pristine metallic background embedding non-percolating superconducting clusters. An intrinsically inhomogeneous character naturally raises the question of the formation mechanism(s). We propose a mechanism based on the interplay be- tween electrons and the charges of the gating ionic liquid.

Published : "arXiv Mesoscale and Nanoscale Physics".

Intrinsic Insulating Ground State in Transition Metal Dichalcogenide TiSe2. (arXiv:1809.09467v1 [cond-mat.str-el])

2018-09-26T02:29:19+00:00September 26th, 2018|Categories: Publications|Tags: |

The transition metal dichalcogenide TiSe2 has received signifcant research attention over the past four decades, in large part due to the uniqueness of its charge-ordered state. Different work has presented ways to suppress the charge density wave transition, vary low temperature resistivity by orders of magnitude, and stabilize magnetic or superconducting states. Here we give the results of a new synthesis technique whereby samples were grown in a an argon gas environment at pressures as high as 180 bar. Above 100 K, the properties of these samples are unchanged from previous reports, but a hysteretic resistance region that begins around 80 K accompanied by insulating low temperature behavior are distinct from anything previously observed. This new feature is linked to a decrease in carrier concentration and may allow access to a nonmetallic ground state in a material long speculated to be an excitonic insulator.

Published in: "arXiv Material Science".

Aspects of symmetry and topology in the charge density wave phase of 1T-TiSe2. (arXiv:1807.11107v1 [cond-mat.mtrl-sci])

2018-07-31T02:29:23+00:00July 31st, 2018|Categories: Publications|Tags: |

The charge density wave (CDW) in 1T-TiSe2 accompanied by the periodic lattice distortion has a nontrivial symmetry configuration. The symmetry is important as an indication of the mechanism and a cue for experimental probes. We examine the symmetry of the system and clear up the connection of electronic structures between the normal and the CDW states. Especially, we unravel the consequential irreducible representations (IRs) of electronic states and, more ahead, those of gap functions among bands when the CDW occurs. Normally symmetry-related topology will be achieved directly, so we assert that the theory is valuable and practical for the search of topological CDW insulators.

Published in: "arXiv Material Science".

Accessing new 2D semiconductors with optical band gap: synthesis of iron-intercalated titanium diselenide thin films via LPCVD

2018-06-20T12:32:50+00:00June 20th, 2018|Categories: Publications|Tags: |

RSC Adv., 2018, 8,22552-22558DOI: 10.1039/C8RA03174F, Paper Open Access &nbsp This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.Clara Sanchez-Perez, Caroline E. Knapp, Ross H. Colman, Carlos Sotelo-Vazquez, Raija Oilunkaniemi, Risto S. Laitinen, Claire J. CarmaltFe-doped TiSe2 thin-films were synthesized via

Published in: "RSC Advances".

Unveiling the Charge Density Wave Inhomogeneity and Pseudogap State in 1T-TiSe2. (arXiv:1611.10145v5 [cond-mat.str-el] UPDATED)

2018-04-02T19:58:59+00:00April 2nd, 2018|Categories: Publications|Tags: |

By using scanning tunneling microscopy (STM) / spectroscopy (STS), we systematically characterize the electronic structure of lightly doped 1T-TiSe2, and demonstrate the existence of the electronic inhomogeneity and the pseudogap state. It is found that the intercalation induced lattice distortion impacts the local band structure and reduce the size of the charge density wave (CDW) gap with the persisted 2×2 spatial modulation. On the other hand, the delocalized doping electrons promote the formation of pseudogap. Domination by either of the two effects results in the separation of two characteristic regions in real space, exhibiting rather different electronic structures. Further doping electrons to the surface confirms that the pseudogap may be the precursor for the superconducting gap. This study suggests that the competition of local lattice distortion and the delocalized doping effect contribute to the complicated relationship between charge density wave and superconductivity for intercalated 1T-TiSe2.

Published in: "arXiv Material Science".

Charge Transfer Effects in Naturally Occurring van der Waals Heterostructures (PbSe)1.16(TiSe2)m (m=1, 2). (arXiv:1803.03750v1 [cond-mat.str-el])

2018-03-13T20:00:31+00:00March 13th, 2018|Categories: Publications|Tags: , |

Van der Waals heterostructures (VDWHs) exhibit rich properties and thus has potential for applications, and charge transfer between different layers in a heterostructure often dominates its properties and device performance. It is thus critical to reveal and understand the charge transfer effects in VDWHs, for which electronic structure measurements have proven to be effective. Using angle-resolved photoemission spectroscopy, we studied the electronic structures of (PbSe)1.16(TiSe2)m(m=1, 2), which are naturally occurring VDWHs, and discovered several striking charge transfer effects. When the thickness of the TiSe2 layers is halved from m=2 to m=1, the amount of charge transferred increases unexpectedly by more than 250%. This is accompanied by a dramatic drop in the electron-phonon interaction strength far beyond the prediction by first-principles calculations and, consequently, superconductivity only exists in the m=2 compound with strong electron-phonon interaction, albeit with lower carrier density. Furthermore, we found that the amount of charge transferred in both compounds is nearly halved when warmed from below 10 K to room temperature, due to the different thermal expansion coefficients of the constituent layers of these misfit compounds. These unprecedentedly large charge transfer effects might widely exist in VDWHs composed of metal-semiconductor contacts; thus, our results provide important insights for further understanding and applications of VDWHs.

Published in: "arXiv Material Science".

Large-Area Atomic Layers of the Charge-Density-Wave Conductor TiSe2

2018-01-10T10:31:20+00:00January 10th, 2018|Categories: Publications|Tags: |

Abstract Layered transition metal (Ti, Ta, Nb, etc.) dichalcogenides are important prototypes for the study of the collective charge density wave (CDW). Reducing the system dimensionality is expected to lead to novel properties, as exemplified by the discovery of enhanced CDW order in ultrathin TiSe2. However, the syntheses of monolayer and large-area 2D CDW conductors can currently only be achieved by molecular beam epitaxy under ultrahigh vacuum. This study reports the growth of monolayer crystals and up to 5 × 105 µm2 large films of the typical 2D CDW conductor—TiSe2—by ambient-pressure chemical vapor deposition. Atomic resolution scanning transmission electron microscopy indicates the as-grown samples are highly crystalline 1T-phase TiSe2. Variable-temperature Raman spectroscopy shows a CDW phase transition temperature of 212.5 K in few layer TiSe2, indicative of high crystal quality. This work not only allows the exploration of many-body state of TiSe2 in 2D limit but also offers the possibility of utilizing large-area TiSe2 in ultrathin electronic devices. Large-area 2D charge-density-wave (CDW) conductor TiSe2 films are synthesized under ambient pressure by chemical vapor deposition. Low-temperature Raman measurement provides spectroscopic evidence that CDW order still emerges in monolayer TiSe2 and survives to a higher temperature than thick samples. This work offers the possibility of utilizing large-area TiSe2 in ultrathin electronic devices.

Published in: "Advanced Materials".

Critical role of the exchange interaction for the electronic structure and charge-density-wave formation in TiSe2. (arXiv:1704.05669v2 [cond-mat.mtrl-sci] UPDATED)

2017-10-19T19:58:56+00:00October 19th, 2017|Categories: Publications|Tags: |

We show that the inclusion of screened exchange via hybrid functionals provides a unified description of the electronic and vibrational properties of TiSe2. In contrast to local approximations in density functional theory, the explicit inclusion of exact, non-local exchange captures the effects of the electron-electron interaction needed to both separate the Ti-d states from the Se-p states and stabilize the charge-density-wave (CDW) (or low-T) phase through the formation of a p-d hybridized state. We further show that this leads to an enhanced electron-phonon coupling that can drive the transition even if a small gap opens in the high-T phase. Finally, we demonstrate that the hybrid functionals can generate a CDW phase where the electronic bands, the geometry, and the phonon frequencies are in agreement with experiments.

Published in: "arXiv Material Science".

S-Doped TiSe2 Nanoplates/Fe3O4 Nanoparticles Heterostructure

2017-09-18T18:29:53+00:00September 18th, 2017|Categories: Publications|Tags: , , |

2D Sulfur-doped TiSe2/Fe3O4 (named as S-TiSe2/Fe3O4) heterostructures are synthesized successfully based on a facile oil phase process. The Fe3O4 nanoparticles, with an average size of 8 nm, grow uniformly on the surface of S-doped TiSe2 (named as S-TiSe2) nanoplates (300 nm in diameter and 15 nm in thickness). These heterostructures combine the advantages of both S-TiSe2 with good electrical conductivity and Fe3O4 with high theoretical Li storage capacity. As demonstrated potential applications for energy storage, the S-TiSe2/Fe3O4 heterostructures possess high reversible capacities (707.4 mAh g−1 at 0.1 A g−1 during the 100th cycle), excellent cycling stability (432.3 mAh g−1 after 200 cycles at 5 A g−1), and good rate capability (e.g., 301.7 mAh g−1 at 20 A g−1) in lithium-ion batteries. As for sodium-ion batteries, the S-TiSe2/Fe3O4 heterostructures also maintain reversible capacities of 402.3 mAh g−1 at 0.1 A g−1 after 100 cycles, and a high rate capacity of 203.3 mAh g−1 at 4 A g−1. 2D sulfur-doped TiSe2/Fe3O4 heterostructures are synthesized successfully based on a facile oil phase process. These heterostructures combine the advantages of both S-doped TiSe2 with good electrical conductivity and Fe3O4 with high theoretical Li storage capacity. The heterostructured S-doped TiSe2 nanoplates/Fe3O4 nanoparticles exhibit high Li and Na storage capacities and rate capabilities.

Published in: "Small".

Interface-Driven Structural Distortions and Composition Segregation in 2D Heterostructures

2017-09-07T20:25:59+00:00September 7th, 2017|Categories: Publications|Tags: , |

The discovery of emergent phenomena in two-dimensional (2D) materials has sparked substantial research efforts in the materials community. A significant experimental challenge for this field is exerting atomistic control over the structure and composition of the constituent 2D layers and understanding how the interactions between layers drives both structure and properties. Segregation of Pb to the surface of three bilayer thick PbSe-SnSe alloy layers was discovered within [(PbxSn1-xSe)1+δ]n(TiSe2)1 heterostructures using electron microscopy. We demonstrate that this segregation is thermodynamically favored to occur when PbxSn1-xSe layers are interdigitated with TiSe2 monolayers. Density-functional theory (DFT) calculations indicate that the observed segregation depends on what is adjacent to the PbxSn1-xSe layers. The interplay between interface and volume free energies controls both the structure and composition of the constituent layers, which can be tuned using layer thickness.

Published in: "Angewandte Chemie International Edition".

Half-Metallic Behavior in 2D Transition Metal Dichalcogenides Nanosheets by Dual-Native-Defects Engineering

2017-09-01T06:29:56+00:00September 1st, 2017|Categories: Publications|Tags: |

Two-dimensional transition metal dichalcogenides (TMDs) have been regarded as one of the best nonartificial low-dimensional building blocks for developing spintronic nanodevices. However, the lack of spin polarization in the vicinity of the Fermi surface and local magnetic moment in pristine TMDs has greatly hampered the exploitation of magnetotransport properties. Herein, a half-metallic structure of TMDs is successfully developed by a simple chemical defect-engineering strategy. Dual native defects decorate titanium diselenides with the coexistence of metal-Ti-atom incorporation and Se-anion defects, resulting in a high-spin-polarized current and local magnetic moment of 2D Ti-based TMDs toward half-metallic room-temperature ferromagnetism character. Arising from spin-polarization transport, the as-obtained T-TiSe1.8 nanosheets exhibit a large negative magnetoresistance phenomenon with a value of −40% (5T, 10 K), representing one of the highest negative magnetoresistance effects among TMDs. It is anticipated that this dual regulation strategy will be a powerful tool for optimizing the intrinsic physical properties of TMD systems. A dual-native-defects (Ti atom self-doping and Se defects) engineering strategy is proposed to introduce a spin polarized current and local magnetic moment into 2D nonmagnetic TiSe2, bringing half-metallic behavior with larger negative magnetoresistance.

Published in: "Advanced Materials".

Suppression of the Charge Density Wave State in Two-Dimensional 1T-TiSe2 by Atmospheric Oxidation

2017-07-20T20:25:57+00:00July 20th, 2017|Categories: Publications|Tags: |

Abstract Two-dimensional (2D) metallic transition-metal dichalcogenides (TMDCs), such as 1T-TiSe2, have recently emerged as unique platforms for exploring their exciting properties of superconductivity and the charge density wave (CDW). 2D 1T-TiSe2 undergoes rapid oxidation under ambient conditions, significantly affecting its CDW phase-transition behavior. We comprehensively investigate the oxidation process of 2D TiSe2 by tracking the evolution of the chemical composition and atomic structure with various microscopic and spectroscopic techniques and reveal its unique selenium-assisting oxidation mechanism. Our findings facilitate a better understanding of the chemistry of ultrathin TMDCs crystals, introduce an effective method to passivate their surfaces with capping layers, and thus open a way to further explore the functionality of these materials toward devices. The oxidation behavior and the effect of the oxidation on the charge density wave transition behavior of two-dimensional TiSe2 is revealed (see picture; Ti purple, Se yellow, O blue). This provides important insights into the oxidation mechanism of ultrathin layered materials, and paves the way for exploiting the unusual properties of these materials.

Published in: "Angewandte Chemie International Edition".

Electrical Stressing Induced Monolayer Vacancy Island Growth on TiSe2. (arXiv:1706.05096v1 [cond-mat.mes-hall])

2017-06-19T01:17:40+00:00June 19th, 2017|Categories: Publications|Tags: |

To ensure the practical application of atomically thin transition metal dichalcogenides, it is essential to characterize their structural stability under external stimuli such as electric fields and currents. Using vacancy monolayer islands on TiSe2 surfaces as a model system, for the first time we have observed a shape evolution and growth from triangular to hexagonal driven by scanning tunneling microscopy (STM) electrical stressing. The size of islands shows linear growth with a rate of (3.00 +- 0.05) x 10-3 nm/s, when the STM scanning parameters are held fixed at Vs = 1.0 V and I = 1.8 nA. We further quantified how the growth rate is related to the tunneling current magnitude. Our simulations of monolayer island evolution using phase-field modeling are in good agreement with our experimental observations, and point towards preferential edge atom dissociation under STM scanning driving the observed growth. The results could be potentially important for device applications of ultrathin transition metal dichalcogenides and related 2D materials subject to electrical stressing under device operating conditions.

Published : "arXiv Mesoscale and Nanoscale Physics".

Electronic and Magnetic Properties of 1T-TiSe2 Nanoribbons. (arXiv:1509.03467v1 [cond-mat.mes-hall])

2015-09-14T02:18:02+00:00September 14th, 2015|Categories: Publications|Tags: , |

Motivated by the recent synthesis of single layer TiSe2 , we used
state-of-the-art density functional theory calculations, to investigate the
structural and electronic properties of zigzag and armchair- edged nanoribbons
of this material. Our analysis reveals that, differing from ribbons of other
ultra-thin materials such as graphene, TiSe2 nanoribbons have some distinctive
properties. The electronic band gap of the nanoribbons decreases exponentially
with the width and vanishes for ribbons wider than 20 Angstroms. For
ultranarrow zigzag-edged nanoribbons we find odd-even oscillations in the band
gap width, although their band structures show similar features. Moreover, our
detailed magnetic-ground-state analysis reveals that zigzag and armchair edged
ribbons have nonmagnetic ground states. Passivating the dangling bonds with
hydrogen at the edges of the structures influences the band dispersion. Our
results shed light on the characteristic properties of T phase nanoribbons of
similar crystal structures.

Published : "arXiv Mesoscale and Nanoscale Physics".

Molecular beam epitaxy growth and scanning tunneling microscopy study of TiSe$_2$ ultrathin films. (arXiv:1412.8551v3 [cond-mat.mtrl-sci] UPDATED)

2016-10-15T12:03:53+00:00April 2nd, 2015|Categories: Publications|Tags: |

By Jun-Ping Peng, Jia-Qi Guan, Hui-Min Zhang, Can-Li Song, Lili Wang, Ke He, Qi-Kun Xue, Xu-Cun Ma Molecular beam epitaxy is used to grow TiSe2 ultrathin films on graphitized
SiC(0001) substrate. TiSe2films proceed via a nearly layer-by-layer growth mode
and exhibit two dominant types of defects, identified as Se vacancy and
interstitial, respectively. By means of scanning tunneling microscopy, we
demonstrate that the well-established charge density waves can survive in
single unit-cell …read more

Published in: arXiv Material Science

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