The selenide and telluride systems of niobium and tantalum

Revolinsky, E.; Brown, B. E.; Beerntsen, D. J.; Armitage, C.H.

doi:10.1016/0022-5088(65)90058-5

Cited by 56 publications

(12 citation statements)

References 14 publications

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“…Differently from the aforementioned TiSe 2 and MoSe 2 , two stable polymorphs of TaS 2 and TaSe 2 , the so-called "1T" and "2H" structures, exist in distinct temperature ranges. The 2H structure (P6 3 /mmc space group, NbS 2 structure type) is thermodynamically stable at low temperature (T < 840°C in the Ta-Se system) and the 1T structure (P-3m1 space group, CdI 2 structure type) at high temperature [14]. In this work, we show that, thanks to the improved CVT method based on chlorides, it is possible to grow selectively either polymorph of this compound, by simply tuning the temperature of the growth zone, either higher or lower than the phase transition temperature.…”

Section: Methodsmentioning

confidence: 99%

Improved chemical vapor transport growth of transition metal dichalcogenides

Ubaldini

Giannini

2014

Journal of Crystal Growth

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In the crystal growth of transition metal dichalcogenides by the Chemical Vapor Transport method (CVT), the choice of the transport agent plays a key role. We have investigated the effect of various chemical elements and compounds on the growth of TiSe 2 , MoSe 2 , TaS 2 and TaSe 2 and found that pure I 2 is the most suitable for growing TiSe 2 , whereas transition metal chlorides perform best with Mo-and Ta-chalcogenides. The use of TaCl 5 as a transport agent in the CVT process allows to selectively growth either polymorph of TaS 2 and TaSe 2 and the optimum growth conditions are reported. Moreover, by using TaCl 5 and tuning the temperature and the halogen starting ratio, it was possible to grow whiskers of the compounds

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Section: Methodsmentioning

confidence: 99%

Improved chemical vapor transport growth of transition metal dichalcogenides

Ubaldini

Giannini

2014

Journal of Crystal Growth

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“…2H structured niobium diselenide (NbSe 2 ) is one of the most studied materials for its prototypical superconductivity (SC) and two dimensional charge density wave (CDW) [1,2,3,4,5,6]. Based on experiments conducted on NbSe 2 , understanding on some of the most basic properties of SC and CDW was reached, such as the internal electronic structures of the vortex cores [7], the anisotropic s-wave [5,6,7] and multi-band SC [8,9,10,11], the anisotropic electron-phonon interaction [6,12], and the phonon softening at the CDW transition [13].…”

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Primary Role of the Barely Occupied States in the Charge Density Wave Formation ofNbSe2

et al. 2008

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NbSe2 is a prototypical charge-density-wave (CDW) material, whose mechanism remains mysterious so far. With angle resolved photoemission spectroscopy, we recovered the long-lost nesting condition over a large broken-honeycomb region in the Brillouin zone, which consists of six saddle band point regions with high density of states (DOS), and large regions away from Fermi surfaces with negligible DOS at the Fermi energy. We show that the major contributions to the CDW formation come from these barely occupied states rather than the saddle band points. Our findings not only resolve a long-standing puzzle, but also overthrow the conventional wisdom that CDW is dominated by regions with high DOS.

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“…NbSe2 is an anisotropic layered superconductor with a transition temperature of 7 K, a bulk superconducting energy gap Ao of 1.1 meV, and coherence lengths of 77 and 23 A parallel and perpendicular to the layers, respectively [12]. Single crystals were grown using an iodine-assisted transport reaction in a gradient furnace [13]. In order to minimize surface contaminants, the evaporation took place in a processing chamber (l.4xl0 -7 torr) coupled to a helium Dewar.…”

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confidence: 99%

Observation of bound quasiparticle states in thin Au islands by scanning tunneling microscopy

Tessmer

Lyding

1993

Phys. Rev. Lett.

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Cryogenic scanning tunneling microscopy has been used to measure the structure and the local electronic density of states of thin gold islands in metallic contact with a bulk superconducting NbSe2 crystal. Peaks are observed in the tunneling conductance below the energy gap corresponding to discrete bound quasiparticle states in the gold. The energies and spatial dependence of the bound states imply that the superconducting pair potential falls off abruptly at the Au-NbSe2 interface.PACS numbers: 74.50.+r, 61.16.ChAlthough the microscopic theory pertaining to spatial variations in superconductivity across normal-metal/superconductor (NS) interfaces has been well developed [1][2][3][4], direct electronic measurements on the microscopic scale near such an interface have been beyond the reach of experiment. Recently developed cryogenic scanning tunneling microscopes (STM's) have the spectroscopic capability to resolve the local electronic density of states N(E,T) by measuring the differential conductance dl/dV as a function of bias voltage V and tip position r. This allows the possibility of spatially characterizing NS systems with atomic resolution. At present, the only STM experiments dealing with regions of spatially varying superconducting parameters have studied magnetic vortices in the type II superconductor 2H-NbSe2 [5][6][7]. It would be advantageous to look instead at artificially prepared NS structures which offer a controlled geometry while decoupling screening currents and magnetic flux from the system.In this Letter we report spectroscopic STM measurements on a sample consisting of very thin normal-metal islands in good metallic contact with a large superconducting crystal. We find resonances in the tunneling conductance which can be identified as quasiparticle bound states spatially localized in the islands as a result of spatial variations in the superconducting pair potential A(r). Bound states have been detected previously in planar thin-film NS structures [8] and have been identified as giving rise to the enhanced conductance observed in the core of magnetic vortices [9][10][11]. From the energies and spatial variation of the bound states the spatial dependence of A(r) may be inferred. Our measurements are consistent with a pair potential falling off rapidly within a few angstroms across the NS interface.The sample consisted of a NbSe2 crystal (1 mmx2 mm x0.3 mm) onto which 7 A of gold were thermally evaporated as measured by a crystal monitor. Both materials were chosen for their inert surface properties. NbSe2 is an anisotropic layered superconductor with a transition temperature of 7 K, a bulk superconducting energy gap Ao of 1.1 meV, and coherence lengths of 77 and 23 A parallel and perpendicular to the layers, respectively [12]. Single crystals were grown using an iodine-assisted transport reaction in a gradient furnace [13]. In order to minimize surface contaminants, the evaporation took place in a processing chamber (l.4xl0 -7 torr) coupled to a helium Dewar. The sample was then transferred ...

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The selenide and telluride systems of niobium and tantalum

Cited by 56 publications

References 14 publications

Improved chemical vapor transport growth of transition metal dichalcogenides

Improved chemical vapor transport growth of transition metal dichalcogenides

Primary Role of the Barely Occupied States in the Charge Density Wave Formation ofNbSe2

Observation of bound quasiparticle states in thin Au islands by scanning tunneling microscopy

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