We carry out a computational study on the geometric and electronic properties of multi-layers of silicene in different stacking configurations using a state-of-art abinitio density functional theory based calculations. In this work we investigate the evolution of these properties with increasing number of layers (n) ranging from 1 to 10. Though, mono-layer of silicene possesses properties similar to those of graphene, our results show that the geometric and electronic properties of multilayers of silicene are strikingly different from those of multi-layers of graphene. We observe that there exist strong inter-layer covalent bondings between the layers in multi-layers of silicene as opposed to weak van der Waal's bonding which exists between the graphene layers. The inter-layer bonding strongly influences the geometric and electronic structures of these multi-layers. Like bilayers of graphene, silicene with two different stacking configurations AA and AB exhibits linear and parabolic dispersions around the Fermi level, respectively. However, unlike graphene, for bi-layers of silicene, these dispersion curves are shifted in band diagram; this is due to the strong inter-layer bonding present in the latter. For n > 3, we study the geometric and electronic properties of multi-layers with four different stacking configurations namely, AAAA, AABB, ABAB and ABC. Our results on cohesive energy show that all the multi-layers considered are energetically stable. Furthermore, we find that the three stacking configurations (AAAA, AABB and ABC) containing tetrahedral coordination have much higher cohesive energy than that of Bernal (ABAB) stacking configuration. This is in contrast to the case of multi-layers of graphene where ABAB is reported to be the lowest energy configuration. We also observe that bands near the Fermi level in lower energy stacking configurations AAAA, AABB and ABC correspond to the surface atoms and these surface states are responsible for the semi-metallic character of these multi-layers.
Despite a tremendous interest on molybdenum disulfide as a thinnest direct band gap semiconductor, single step synthesis of a large area purely monolayer MoS2 film has not yet been reported. Here, we report a CVD route to synthesize a continuous film of strictly monolayer MoS2 covering an area as large as a few cm2 on a variety of different substrates without using any seeding material or any elaborate pretreatment of the substrate. This is achieved by allowing the growth to take place in the naturally formed gap between a piece of SiO2 coated Si wafer and the substrate, when the latter is placed on top of the former inside a CVD reactor. We propose a qualitative model to explain why the MoS2 films are always strictly monolayer in this method. The photoluminescence study of these monolayers shows the characteristic excitonic and trionic features associated with monolayer MoS2. In addition, a broad defect related luminescence band appears at ∼1.7 eV. As temperature decreases, the intensity of this broad feature increases, while the band edge luminescence reduces.
Abstract:We present the results of Synchrotron XRD measurements on powdered single crystal samples of BaFe2-xRuxAs2, as a function of Ru content, and as a function of temperature, across the spin density wave transition in BaFe1.9Ru0.1As2. The Rietveld refinements reveal that with Ru substitution, while the a-axis increases, the c-axis decreases. In addition the variation of positional co-ordinates of As (zAs), the Fe-As bond length and the As-Fe-As bond angles have also been determined. In the sample with x=0.1, temperature dependent XRD measurements, indicate that the orthorhombicity shows the characteristic increase with decrease in temperature, below the magnetic transition. It is seen that the c-axis, the As-FeAs bond angles, Fe-As bond length and positional co-ordinate of the As show definite anomalies close to the structural transition. The observed anomalies in structural parameters are analysed in conjunction with geometric optimization of the structure using ab-initio electronic structure calculations.
The electronic structure of CeAg 2 Ge 2 single crystal has been investigated by using valence-band photoemission at different photon energies ranging from 110 to 150 eV. Resonant photoemission has been observed near the 4d threshold of Ce at 121 eV. The constant initial-state spectra shows two photoemission features having 4f character near the Fermi level at −0.4 and −1.7 eV which exhibits Fano-type sharp resonance character. The experimental spectra have been interpreted with the help of calculations based on full-potential linearized augmented plane-wave method using density-functional theory. Excellent agreement has been obtained between the theory and the experiment. The origin of the feature near to Fermi level is related to the Ce 4f states and the feature at −1.7 eV is related to the strong hybridization between the Ce 4f and 5d, Ag 4d and Ge 4p states.Resonant photoemission spectroscopy ͑RPES͒ has emerged as a very powerful tool to understand the electronic states of rare earths. From last few decades, Ce-based intermetallic compounds have attracted much attention for their various ground-state properties, such as the magnetism, nonmagnetic heavy Fermion, and quantum criticality, etc. 1-6 The diverse ground states are due to the competition between the Ruderman-Kittel-Kasuya-Yosida interaction and the Kondo effect. 7 CeAg 2 Ge 2 belongs to a wide class of cerium compounds that crystallize in the ThCr 2 Si 2 structure having a body-centered tetragonal lattice. 6 At room temperature it exhibits a paramagnetic phase while at low temperature an antiferromagnetic phase with T N = 7 K is reported. 5,6 More recently, detailed studies were performed on a single crystal, which has shown that CeAg 2 Ge 2 orders antiferromagnetically at 4.6 K. 8 The neutron-diffraction experiment shows a sine modulated structure with a magnetic moment equal to 1.85 B at T = 1.5 K. 6 In the Ce based intermetallic alloys it is believed that the ground-state properties depend on the strength of hybridization of the f electrons with the delocalized band states and this motivated us to understand the electronic structure of CeAg 2 Ge 2 . In the present work, we investigate the occupied electronic states of CeAg 2 Ge 2 at room temperature and try to provide a clear understanding of the hybridization between the local Ce 4f electrons and the itinerant conduction electrons by the systematic Ce 4d-4f RPES study.CeAg 2 Ge 2 single crystal was grown by the self-flux method. 8 For the PE measurements the sample has been mechanically polished to mirror finish using quarter micron diamond paste. The RPES measurements on this sample were carried out at the angle-integrated PE beamline on the Indus-1 synchrotron radiation source. 9 The valence-band ͑VB͒ photoemission spectra were recorded using a photon energy of 110-130 eV in very small steps of 1 eV. The energy analyzer from Omicron ͑EA125͒ is used to measure the spectra at room temperature. The spectra were normalized by the photon flux estimated from the photocurrent from the post mirror of th...
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