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Charge density wave transition in single-layer titanium diselenide.

Chen P, Chan YH, Fang XY, Zhang Y, Chou MY, Mo SK, Hussain Z, Fedorov AV, Chiang TC - Nat Commun (2015)

Bottom Line: Angle-resolved photoemission spectroscopy measurements reveal a small absolute bandgap at room temperature, which grows wider with decreasing temperature T below TC in conjunction with the emergence of (2 × 2) ordering.The results are rationalized in terms of first-principles calculations, symmetry breaking and phonon entropy effects.The observed Bardeen-Cooper-Schrieffer (BCS) behaviour of the gap implies a mean-field CDW order in the single layer and an anisotropic CDW order in the bulk.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, University of Illinois at Urbana-Champaign, 1110 West Green Street, Urbana, Illinois 61801-3080, USA.

ABSTRACT
A single molecular layer of titanium diselenide (TiSe2) is a promising material for advanced electronics beyond graphene-a strong focus of current research. Such molecular layers are at the quantum limit of device miniaturization and can show enhanced electronic effects not realizable in thick films. We show that single-layer TiSe2 exhibits a charge density wave (CDW) transition at critical temperature TC=232±5 K, which is higher than the bulk TC=200±5 K. Angle-resolved photoemission spectroscopy measurements reveal a small absolute bandgap at room temperature, which grows wider with decreasing temperature T below TC in conjunction with the emergence of (2 × 2) ordering. The results are rationalized in terms of first-principles calculations, symmetry breaking and phonon entropy effects. The observed Bardeen-Cooper-Schrieffer (BCS) behaviour of the gap implies a mean-field CDW order in the single layer and an anisotropic CDW order in the bulk.

No MeSH data available.


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Film structure and electronic bands.(a) Atomic structure of a single layer of TiSe2. In bulk TiSe2 the layer spacing is c as indicated. (b) Brillouin zones of the (1 × 1) and (2 × 2) structure outlined in black and red, respectively. (c) A reflection high-energy electron diffraction pattern after film growth. (d) Core-level scans taken with 100 eV photons. (e) ARPES maps taken from a single layer of TiSe2 along the  direction for the (1 × 1) normal phase at room temperature and the (2 × 2) CDW phase at 10 K. All data were taken with 58 eV photons. (f) Calculated DFT band structure of the (1 × 1) and (2 × 2) phases with the HSE hybrid functional.
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f1: Film structure and electronic bands.(a) Atomic structure of a single layer of TiSe2. In bulk TiSe2 the layer spacing is c as indicated. (b) Brillouin zones of the (1 × 1) and (2 × 2) structure outlined in black and red, respectively. (c) A reflection high-energy electron diffraction pattern after film growth. (d) Core-level scans taken with 100 eV photons. (e) ARPES maps taken from a single layer of TiSe2 along the direction for the (1 × 1) normal phase at room temperature and the (2 × 2) CDW phase at 10 K. All data were taken with 58 eV photons. (f) Calculated DFT band structure of the (1 × 1) and (2 × 2) phases with the HSE hybrid functional.

Mentions: Our single layers of TiSe2 were grown in situ on a bilayer-graphene-terminated 6H-SiC (0001) (refs 14, 15). The interfacial interaction is expected to be of the van der Waals type, resulting in a nearly decoupled TiSe2 overlayer. The crystal structure (Fig. 1a) consists of a hexagonal planar net of Ti atoms sandwiched between two Se atomic layers. The (1 × 1) and (2 × 2) Brillouin zones are also hexagonal (Fig. 1b). Reflection high-energy electron diffraction (Fig. 1c) reveals a well-ordered layer with the same in-plane lattice constant as that of bulk TiSe2 within our experimental accuracy. Scans of the core levels (Fig. 1d) show a much stronger Se 3d signal than the Ti 3d signal partly because the top atomic layer is made of Se atoms.


Charge density wave transition in single-layer titanium diselenide.

Chen P, Chan YH, Fang XY, Zhang Y, Chou MY, Mo SK, Hussain Z, Fedorov AV, Chiang TC - Nat Commun (2015)

Film structure and electronic bands.(a) Atomic structure of a single layer of TiSe2. In bulk TiSe2 the layer spacing is c as indicated. (b) Brillouin zones of the (1 × 1) and (2 × 2) structure outlined in black and red, respectively. (c) A reflection high-energy electron diffraction pattern after film growth. (d) Core-level scans taken with 100 eV photons. (e) ARPES maps taken from a single layer of TiSe2 along the  direction for the (1 × 1) normal phase at room temperature and the (2 × 2) CDW phase at 10 K. All data were taken with 58 eV photons. (f) Calculated DFT band structure of the (1 × 1) and (2 × 2) phases with the HSE hybrid functional.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4660365&req=5

f1: Film structure and electronic bands.(a) Atomic structure of a single layer of TiSe2. In bulk TiSe2 the layer spacing is c as indicated. (b) Brillouin zones of the (1 × 1) and (2 × 2) structure outlined in black and red, respectively. (c) A reflection high-energy electron diffraction pattern after film growth. (d) Core-level scans taken with 100 eV photons. (e) ARPES maps taken from a single layer of TiSe2 along the direction for the (1 × 1) normal phase at room temperature and the (2 × 2) CDW phase at 10 K. All data were taken with 58 eV photons. (f) Calculated DFT band structure of the (1 × 1) and (2 × 2) phases with the HSE hybrid functional.
Mentions: Our single layers of TiSe2 were grown in situ on a bilayer-graphene-terminated 6H-SiC (0001) (refs 14, 15). The interfacial interaction is expected to be of the van der Waals type, resulting in a nearly decoupled TiSe2 overlayer. The crystal structure (Fig. 1a) consists of a hexagonal planar net of Ti atoms sandwiched between two Se atomic layers. The (1 × 1) and (2 × 2) Brillouin zones are also hexagonal (Fig. 1b). Reflection high-energy electron diffraction (Fig. 1c) reveals a well-ordered layer with the same in-plane lattice constant as that of bulk TiSe2 within our experimental accuracy. Scans of the core levels (Fig. 1d) show a much stronger Se 3d signal than the Ti 3d signal partly because the top atomic layer is made of Se atoms.

Bottom Line: Angle-resolved photoemission spectroscopy measurements reveal a small absolute bandgap at room temperature, which grows wider with decreasing temperature T below TC in conjunction with the emergence of (2 × 2) ordering.The results are rationalized in terms of first-principles calculations, symmetry breaking and phonon entropy effects.The observed Bardeen-Cooper-Schrieffer (BCS) behaviour of the gap implies a mean-field CDW order in the single layer and an anisotropic CDW order in the bulk.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, University of Illinois at Urbana-Champaign, 1110 West Green Street, Urbana, Illinois 61801-3080, USA.

ABSTRACT
A single molecular layer of titanium diselenide (TiSe2) is a promising material for advanced electronics beyond graphene-a strong focus of current research. Such molecular layers are at the quantum limit of device miniaturization and can show enhanced electronic effects not realizable in thick films. We show that single-layer TiSe2 exhibits a charge density wave (CDW) transition at critical temperature TC=232±5 K, which is higher than the bulk TC=200±5 K. Angle-resolved photoemission spectroscopy measurements reveal a small absolute bandgap at room temperature, which grows wider with decreasing temperature T below TC in conjunction with the emergence of (2 × 2) ordering. The results are rationalized in terms of first-principles calculations, symmetry breaking and phonon entropy effects. The observed Bardeen-Cooper-Schrieffer (BCS) behaviour of the gap implies a mean-field CDW order in the single layer and an anisotropic CDW order in the bulk.

No MeSH data available.


Related in: MedlinePlus