Layer structure polytypism among niobium and tantalum selenides

Brown, B. E.; Beerntsen, D. J.

doi:10.1107/s0365110x65000063

Cited by 175 publications

(72 citation statements)

References 3 publications

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“…We used the experimentally measured lattice constants 20 , (a=3.44Å and c=12.55Å), which agrees well with other reports 21,22,23 and we relaxed the Se ions to their lowest energy positions along the c axis (z=0.1183c). This position is in good agreement with all experimental reports (z = 0.116 − 0.118c) 20,21,22 , except Ref. 23, which gives z = 0.125c.…”

Section: Electronic Structuresupporting

confidence: 81%

Fermi-surface nesting and the origin of the charge-density wave inNbSe2

2006

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We use highly accurate density functional calculations to study the band structure and Fermi surfaces of NbSe2. We calculate the real part of the non-interacting susceptibility, ℜχ0(q), which is the relevant quantity for a charge density wave (CDW) instability and the imaginary part,ℑχ0(q), which directly shows Fermi surface (FS) nesting. We show that there are very weak peaks in ℜχ0(q) near the CDW wave vector, but that no such peaks are visible in ℑχ0(q), definitively eliminating FS nesting as a factor in CDW formation. Because the peak in ℜχ0(q) is broad and shallow, it is unlikely to be the direct cause of the CDW instability. We briefly address the possibility that electronelectron interactions (local field effects) produce additional structure in the total (renormalized) susceptibility, and we discuss the role of electron-ion matrix elements.In 1964 V.L. Ginzburg proposed excitonic superconductivity in quasi-2D structures 1 composed of metal layers sandwiched between insulating layers. By that time, only two layered materials were known to be superconducting, 2,3 PdTe 2 and NbSe 2 . In the four decades since then, NbSe 2 and isostructural selenides have been intensively investigated (close to 1.5 thousand papers published to date). However, the main interest in this compound has shifted from the fact that it is a layered material (hundreds of layered superconductors are now known), to the existence of a nearly-commensurate charge density wave (CDW) 4 instability and it's possible interplay with the superconductivity that sets in at a lower temperature. We mainly forego discussion of the interesting issues surrounding the superconducting state, its origin, and its relationship to the CDW state, and instead concentrate on the mechanism behind the CDW transition itself.The first electronic structure calculations for NbSe 2 were presented by Mattheiss in 1973 5 . Using a non-selfconsistent potential he was able to produce a band structure with basic features in reasonable agreement with more recent self-consistent calculations 6,7,8 , but which showed only two bands crossing the Fermi energy (it is now known that there are three), and underestimated the energy depth of a saddle point at ∼ 1 2 ΓK. Fermi surfaces based on this band structure led to early suggestions that the CDW transition was driven by nesting 4 , an assumption that has carried through to the present time. The nearness of the saddle point to the Fermi energy (E F ) led Rice and Scott 9 to argue that CDW formation was driven, not by Fermi surface (FS) nesting in the conventional sense, but rather by saddle points lying within k B T CDW of E F and separated by the CDW wavevector, Q CDW = ( 1 3 , 0, 0). A significant amount of effort has been spent on resolving the 'controversy' between the nesting and saddle point theories for NbSe 2 and for related CDW compounds such as 2H-TaSe 2 , 1T-TaSe 2 , TaS 2 and others, but no specific feature that would give rise to an instability at Q CDW has been convincingly isolated. As early as 1978, Doran et al 10...

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Section: Electronic Structuresupporting

confidence: 81%

Fermi-surface nesting and the origin of the charge-density wave inNbSe2

2006

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“…The AC layer with the generalized composition [MX 2 ] is found in a very diverse range of materials such as metal sulfides MS 2 (M = Ti, Mo, Nb, Ta), [21,22] metal selenides MSe 2 (M = Mo, Ta), [23] divalent halides MX 2 (M = Ca, Mg, Fe, Co, Ni, Cd), [24,25] metal hydroxides M(OH) 2 (M = Mg, Ni, Co), [26,27] birnessite-type oxides MO 2 (M = Mn), [28,29] lithium oxides LiMO 2 [30,31] and basic salts M(OH) 2-x A x (M = Ni, Co; x = 0.66-0.25; A = Cl, NO 3 ). [32][33][34] We therefore call the AC layer a 'structural synthon', the term "structural" is used to describe the extended nature of the AC layer, as opposed to the sub.-molecular synthon originally proposed by Corey [35] and the supramolecular synthon of Desiraju [36,37] which are entities of finite dimension.…”

Section: Structural Synthon Approach To Predicting the Polytypesmentioning

confidence: 99%

Structural Synthon Approach to Predict the Possible Polytypes of Layered Double Hydroxides

Radha

Kamath

2012

Zeitschrift anorg allge chemie

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Abstract. The complete universe of possible polytypes of layered double hydroxides (LDH) is predicted on the basis of symmetry arguments. A single [MX 2 ] (X = OH) layer, also defined as a structural synthon, belongs to the layer group P32/m1. These layers can be stacked in such a way as to conserve the unique 3-axis of the layer in the resultant crystal. The different stacking sequences that facilitate symmetry conservation, yield the different possible polytypes of rhombohedral and hexagonal symmetries. More polytypes can be envisaged by including stacking sequences that systematically destroy the princi-

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“…In this work, we specifically examine 2H-TaSe 2 . The structure of this polytype has a two-layer repeat pattern (AcA BcB), within which the trigonal prismatic units are rotated 60 o with respect to each other [20][21][22].…”

Section: Introductionmentioning

confidence: 99%

Phonon and thermal properties of exfoliated TaSe2 thin films

Yan

Jiang

Pope

et al. 2013

Journal of Applied Physics

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We report on the phonon and thermal properties of thin films of tantalum diselenide (2H-TaSe 2 ) obtained via the "graphene-like" mechanical exfoliation of crystals grown by chemical vapor transport. The ratio of the intensities of the Raman peak from the Si substrate and the E 2g peak of TaSe 2 presents a convenient metric for quantifying film thickness. The temperature coefficients for two main Raman peaks, A 1g and E 2g , are -0.013 and -0.0097 cm -1 / o C, respectively. The Raman optothermal measurements indicate that the room temperature thermal conductivity in these films decreases from its bulk value of ~16 W/mK to ~9 W/mK in 45-nm thick films. The measurement of electrical resistivity of the field-effect devices with TaSe 2 channels indicates that heat conduction is dominated by acoustic phonons in these van der Waals films. The scaling of thermal conductivity with the film thickness suggests that the phonon scattering from the film boundaries is substantial despite the sharp interfaces of the mechanically cleaved samples. These results are important for understanding the thermal properties of thin films exfoliated from TaSe 2 and other metal dichalcogenides, as well as for evaluating self-heating effects in devices made from such materials.KEYWORDS: van der Waals materials, tantalum diselenide, Raman spectroscopy, thermal conductivity, metal dichalcogenide, thin film *Corresponding authors: salguero@uga.edu (TTS) and balandin@ee.ucr.edu (AAB) 2 | P a g e

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Layer structure polytypism among niobium and tantalum selenides

Cited by 175 publications

References 3 publications

Fermi-surface nesting and the origin of the charge-density wave inNbSe2

Fermi-surface nesting and the origin of the charge-density wave inNbSe2

Structural Synthon Approach to Predict the Possible Polytypes of Layered Double Hydroxides

Phonon and thermal properties of exfoliated TaSe2 thin films

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