How the interacting electronic states and phases of layered transition-metal dichalcogenides evolve when thinned to the single-layer limit is a key open question in the study of two-dimensional ...materials. Here, we use angle-resolved photoemission to investigate the electronic structure of monolayer VSe2 grown on bilayer graphene/SiC. While the global electronic structure is similar to that of bulk VSe2, we show that, for the monolayer, pronounced energy gaps develop over the entire Fermi surface with decreasing temperature below T c = 140 ± 5 K, concomitant with the emergence of charge-order superstructures evident in low-energy electron diffraction. These observations point to a charge-density wave instability in the monolayer that is strongly enhanced over that of the bulk. Moreover, our measurements of both the electronic structure and of X-ray magnetic circular dichroism reveal no signatures of a ferromagnetic ordering, in contrast to the results of a recent experimental study as well as expectations from density functional theory. Our study thus points to a delicate balance that can be realized between competing interacting states and phases in monolayer transition-metal dichalcogenides.
Signatures of the Kondo effect in VSe2 Barua, Sourabh; Hatnean, M. Ciomaga; Lees, M. R. ...
Scientific reports,
09/2017, Letnik:
7, Številka:
1
Journal Article
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VSe2 is a transition metal dichaclogenide which has a charge- density wave transition that has been well studied. We report on a low-temperature upturn in the resistivity and, at temperatures below ...this resistivity minimum, an unusual magnetoresistance which is negative at low fields and positive at higher fields, in single crystals of VSe2. The negative magnetoresistance has a parabolic dependence on the magnetic field and shows little angular dependence. The magnetoresistance at temperatures above the resistivity minimum is always positive. We interpret these results as signatures of the Kondo effect in VSe2. An upturn in the susceptibility indicates the presence of interlayer V ions which can provide the localized magnetic moments required for scattering the conduction electrons in the Kondo effect. The low-temperature behaviour of the heat capacity, including a high value of γ, along with a deviation from a Curie-Weiss law observed in the low-temperature magnetic susceptibility, are consistent with the presence of magnetic interactions between the paramagnetic interlayer V ions and a Kondo screening of these V moments.
In this report, we scrutinize the thickness dependent resistivity data from the recent literature on electrical transport measurements in topological insulators. A linear increase in resistivity with ...increase in thickness is expected in the case of these materials since they have an insulating bulk and a conducting surface. However, such a trend is not seen in the resistivity versus thickness data for all the cases examined, except for some samples, where it holds for a range of thickness.
This review article covers the growth and characterization of two-dimensional (2D) crystals of transition metal chalcogenides, h-BN, graphene, etc. The chemical vapor transport method for bulk single ...crystal growth is discussed in detail. Top-down methods like mechanical and liquid exfoliation and bottom-up methods like chemical vapor deposition and molecular beam epitaxy for mono/few-layer growth are described. The optimal characterization techniques such as optical, atomic force, scanning electron, and Raman spectroscopy for identification of mono/few-layer(s) of the 2D crystals are discussed. In addition, a survey was done for the application of 2D crystals for both creation and deterministic transfer of single-photon sources and photovoltaic systems. Finally, the application of plasmonic nanoantenna was proposed for enhanced solar-to-electrical energy conversion and faster/brighter quantum communication devices.
Quasi-2-dimensional (2D) halide perovskites have recently attracted attention due to their higher operational stability as alternatives to 3-dimensional (3D) perovskites having exceptional ...optoelectronic and charge transport properties. To reduce the lead content, here following the double perovskite approach lead is substituted with silver and bismuth simultaneously, and three quasi-2D perovskites, with general formula (C7H10N)2Pb(1-2x)AgxBixBr4, (0 ≤ x ≤ 0.5) were prepared. The optical studies show that the partially lead substituted sample has the lowest optical band gap, aptly supported by the theoretical calculations. The powder X-ray diffraction technique along with field-emission scanning electron microscopy suggests enhancement in crystallinity along with the decrease in grain boundaries with the substitution of lead. The improvement in crystallinity with concomitant reduction in grain boundaries has led to the decrease in point defects as identified from the positron annihilation lifetime spectroscopy and coincidence Doppler broadening analysis. The tuned band gap, improved crystal quality along with lower defects jointly contributed to the enhancement in electrical properties of the perovskites with varying lead percentages. Finally, the photoresponse of all the materials was studied after fabricating metal (Al)-semiconductor (MS) junction thin film photodetector devices.
•Effect of Pb2+ substitution on structural, electronic and electrical properties were systematically investigated in quasi-2D perovskite.•Pb2+ substitution was carried out with a combination of a trivalent (Bi3+) and a monovalent (Ag+) cations following double perovskite approach.•Experimental and theoretical studies show that variation in Pb2+ content alters the band gap of perovskite; lowest for partially Pb2+ substituted sample.•Theoretical studies suggest that depending upon the orientation, semiconductor to metallic transition is possible in partially Pb2+ substituted perovskite.•Due to the smaller optical band gap and lower defect density, best device performances recorded for partially Pb2+ substituted sample.
Unraveling the origins of the electrocatalytic activity of composite nanomaterials is crucial but inherently challenging. Here, we present a comprehensive investigation of the influence of different ...orbitals’ interaction in the AuAgCu nanobowl model electrocatalyst during the hydrogen evolution reaction (HER). According to our theoretical study, AgAuCu exhibits a lower energy barrier than AgAu and AgCu bimetallic systems for the HER, suggesting that the trimetallic AgAuCu system interacts optimally with H*, resulting in the most efficient HER catalyst. As we delve deeper into the HER activity of AgAuCu, it was observed that the presence of Cu allows Au to adsorb the H* intermediate through the hybridization of s orbitals of hydrogen and s, d x 2 –y 2, and d z 2 orbitals of Au. Such orbital interaction was not present in the cases of AgAu and AgCu bimetallic systems, and as a result, these bimetallic systems exhibit lower HER activities.
We performed angle dependent magnetoresistance study of a metallic single crystal sample of Bi2Te3. We find that the magnetoresistance is highly asymmetric in positive and negative magnetic fields ...for small angles between the magnetic field and the direction perpendicular to the plane of the sample. The magnetoresistance becomes symmetric as the angle approaches 90°. The quantum Shubnikov de-Haas oscillations are symmetric and show signatures of topological surface states with Dirac dispersion in the form of non-zero Berry phase. However, the angular dependence of these oscillations suggests a complex three dimensional Fermi surface as the source of these oscillations, which does not exactly conform with the six ellipsoidal model of the Fermi surface of Bi2Te3. We attribute the asymmetry in the magnetoresistance to a mixing of the Hall voltage in the longitudinal resistance due to the comparable magnitude of the Hall and longitudinal resistance in our samples. This provides a clue to understanding the asymmetric magnetoresistance often seen in this and similar materials. Moreover, the asymmetric nature evolves with exposure to atmosphere and thermal cycling, which we believe is either due to exposure to atmosphere or thermal cycling, or both affecting the carrier concentration and hence the Hall signal in these samples. However, the quantum oscillations seem to be robust against these factors which suggests that the two have different origins.
•Angle dependent magnetoresistance (MR) and Shubnikov-de Haas oscillations are reported in Bi2Te3.•We report non-zero Berry phase which could be a signature of topological surface states.•Angle dependence shows three dimensional nature but differs from the expected variation.•We show asymmetric MR can arise from mixing of Hall and longitudinal resistance.•Immunity of oscillations from exposure and thermal cycling indicates they have a different origin.
Signatures of the Kondo effect in VSe 2 Barua, Sourabh; Hatnean, M Ciomaga; Lees, M R ...
Scientific reports,
09/2017, Letnik:
7, Številka:
1
Journal Article
Recenzirano
VSe
is a transition metal dichaclogenide which has a charge- density wave transition that has been well studied. We report on a low-temperature upturn in the resistivity and, at temperatures below ...this resistivity minimum, an unusual magnetoresistance which is negative at low fields and positive at higher fields, in single crystals of VSe
. The negative magnetoresistance has a parabolic dependence on the magnetic field and shows little angular dependence. The magnetoresistance at temperatures above the resistivity minimum is always positive. We interpret these results as signatures of the Kondo effect in VSe
. An upturn in the susceptibility indicates the presence of interlayer V ions which can provide the localized magnetic moments required for scattering the conduction electrons in the Kondo effect. The low-temperature behaviour of the heat capacity, including a high value of γ, along with a deviation from a Curie-Weiss law observed in the low-temperature magnetic susceptibility, are consistent with the presence of magnetic interactions between the paramagnetic interlayer V ions and a Kondo screening of these V moments.
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