Krishnakumar S. R. Menon

Electronic structure investigation of MoS2 and MoSe2 using angle-resolved photoemission spectroscopy and ab initio band structure studies

Angle-resolved photoemission spectroscopy (ARPES) and ab initio band structure calculations have been used to study the detailed valence band structure of molybdenite, MoS(2) and MoSe(2). The experimental band structure obtained from ARPES has been found to be in good agreement with the theoretical calculations performed using the linear augmented plane wave (LAPW) method. In going from MoS(2) to MoSe(2), the dispersion of the valence bands decreases along both k(parallel) and k(perpendicular), revealing the increased two-dimensional character which is attributed to the increasing interlayer distance or c/a ratio in these compounds. The width of the valence band and the band gap are also found to decrease, whereas the valence band maxima shift towards the higher binding energy from MoS(2) to MoSe(2).

Finite size versus surface effects on magnetic properties of antiferromagnetic particles

The observation of finite magnetic moment in antiferromagnetic materials is quite unusual and has been immensely investigated in nanoparticle systems. Here, the structural and magnetic properties of NiO particles are explored by x-ray diffraction, extended x-ray absorption fine structure, and magnetization measurements. Using similar-sized particles with different surface defect structure, we show that the observed magnetic enhancement, which is present even beyond finite-size limit, is due to the surface effects. However, the well known spin glass freezing is found to occur only in nano-regime.

Direct view at colossal permittivity in donor-acceptor (Nb, In) co-doped rutile TiO2

Topical observations of colossal permittivity (CP) with low dielectric loss in donor-acceptor cations co-doped rutile TiO2 have opened up several possibilities in microelectronics and energy-storage devices. Yet, the precise origin of the CP behavior, knowledge of which is essential to empower the device integration suitably, is highly disputed in the literature. From spectromicroscopic approach besides dielectric measurements, we explore that microscopic electronic inhomogeneities along with the nano-scale phase boundaries and the low temperature polaronic relaxation are mostly responsible for such a dielectric behavior, rather than electron-pinned defect-dipoles/grain-boundary effects as usually proposed. Donor-acceptor co-doping results in a controlled carrier-hopping inevitably influencing the dielectric loss while invariably upholding the CP value.

Inhomogeneous band bending on MoS2(0001) arising from surface steps and dislocations

We study the observed inhomogeneous band bending effects on cleaved MoS(2)(0001) single-crystal surfaces. Both Mo 3d and S 2p core levels were found to shift to lower binding energy in regions of the MoS(2) crystal with high step densities, as suggested by spot splitting of the LEED (low energy electron diffraction) pattern. Surface electronic band structure measurements also reveal a rigid shift of the valence bands in these regions, resulting from local Fermi level pinning effects. A surface electric field gradient on the MoS(2) crystals caused by the charged dislocations from the regions of high step densities generated by the cleaving process is found to explain most of the experimental observations.

Quantum well states in Ag thin films on MoS2(0001) surfaces

In-plane dispersions of quantum well (QW) states originating from the electron confinement of Ag sp electrons within the MoS(2) band gap region are investigated by means of angle-resolved photoemission spectroscopy (ARPES). A number of QW resonances have been observed in the ARPES spectra in a binding energy range lying outside the MoS(2) energy gap which is required for full confinement of the Ag sp electrons. In spite of having the expected free electron-like behavior, these QW states show a significant increase of in-plane effective mass with increasing binding energy due to the hybridization of Ag sp electrons with the MoS(2) valence bands. The binding energy dependence of the bottom of the QW states (k(//) = 0) as a function of the Ag film thickness has been analyzed. The well-established phase accumulation model has been applied for calculating the phase shifts of electrons at the boundaries. Our observations show that the total phase shift behaves differently for energies above and below the MoS(2) valence band maxima, due to the hybridizations being different in nature. The structure plot calculated considering the different quantum number dependent total phase shifts provides a good description of the experimental observations.

Microscopic investigation of surface and interfacial magnetic domain structure of Fe–NiO(1 0 0) system

We report our experimental observation of microscopic modification, reconstruction and evolution of the antiferromagnetic domain structure of the NiO(1?0?0) surface in view of the exchange bias effect. Some domain patterns (called non-equilibrium domains) as observed on the as-cleaved surface do not follow the well-known bulk symmetry traces on the (1?0?0) surface. But, bulk-terminated domains (called equilibrium domains) are found to be renucleated up on cooling the sample from above the N?el temperature, unless domains are strongly pinned by crystalline defects. We also observe certain domain evolution after annealing the crystal at various temperatures above the N?el temperature. Influence of growth condition on the ferromagnetic domain structure of the Fe film deposited on NiO(1?0?0) has been followed in situ. While for thin film case (7?ML), no influence of growth has been observed, for thicker film (21?ML) the Fe domain structure is found to be determined by both growth-induced and exchange anisotropy. Thus, our observations depict a thickness-dependent interplay between growth-induced and exchange anisotropy in ferromagnetic film grown on the antiferromagnetic substrate.

Surface spin orientation of NiO(100) and interfacial coupling of Fe/NiO(100) revisited with soft X-ray spectromicroscopy

Accurate retrieval of spin-axis orientation at the antiferromagnetic (AFM) surface requires involved consideration of the crystal field effect in X-ray magnetic linear dichroism (XMLD), which was neglected until recently. Here, we present a unique determination of surface spin-axes of the prototype antiferromagnet NiO(100) from detailed angular-dependent measurements using different polarizations of incident light by considering the recently developed angular dependence of the XMLD effect. The bulk twin domains terminating on the (100) surface have also been determined from the angular dependence of the experimental contrast at the oxygen K edge. The effect of Fe deposition on the AFM domain pattern was followed in situ and the interfacial exchange coupling of as-deposited Fe/NiO(100) has been explored using the recent formalism of XMLD. Unlike Co/NiO(100), we realize only the rough perpendicular coupling between Fe moments and compensated Ni spin-axes. The uncompensated spins (UCS) at the interface were also characterized and a mechanism of interfacial coupling is suggested.

Electronic structure of germanium selenide investigated using ultra-violet photo- electron spectroscopy

The valence band electronic structure of GeSe single crystals has been investigated using angle resolved photoemission spectroscopy (ARPES) and x-ray photoelectron spectroscopy. The experimentally observed bands from ARPES, match qualitatively with our LDA-based band structure calculations along the ??Z, ??Y and ??T symmetry directions. The valence band maximum occurs nearly midway along the ??Z direction, at a binding energy of ?0.5 eV, substantiating the indirect band gap of GeSe. Non-dispersive features associated with surface states and indirect transitions have been observed. The difference in hybridization of Se and Ge 4p orbitals leads to the variation of dispersion along the three symmetry directions. The predominance of the Se 4pz orbitals, evidenced from theoretical calculations, may be the cause for highly dispersive bands along the ??T direction. Detailed electronic structure analysis reveals the significance of the cation?anion 4p orbitals hybridization in the valence band dispersion of IV?VI semiconductors. This is the first comprehensive report of the electronic structure of a GeSe single crystal using ARPES in conjugation with theoretical band structure analysis.

Growth and structural evolution of Sn on Ag(001): Epitaxial monolayer to thick alloy film

The growth and structure of Sn on Ag(001), from submonolayer to thick film coverages at room temperature, are studied using low energy electron diffraction, x-ray photoemission spectroscopy and angle-resolved photoemission spectroscopy (ARPES) techniques. The authors observe different growth modes for submonolayer Sn coverages and for higher Sn coverages. Systematic surface structural evolution, consistent with the substitution of surface Ag atoms by Sn atoms, is observed for submonolayer Sn coverages while an ordered Ag-Sn bulk alloy film is formed for higher Sn coverages with an Ag overlayer. For monolayer coverage of Sn, a pseudomorphic growth of a Sn layer without alloying is determined. ARPES results also confirm the presence of an ordered Ag overlayer on the bulk Ag-Sn alloy film, suggesting the formation of an Ag/Ag3Sn/Ag(001) sandwich structure at the surface for higher Sn coverages. The present results illustrate the complex interplay of atomic mobilities, surface free-energies, and alloy formati...

Magnetic skin layer of NiO(100) probed by polarization-dependent spectromicroscopy

Using polarization-dependent x-ray photoemission electron microscopy, we have investigated the surface effects on antiferromagnetic (AFM) domain formation. Depth-resolved information obtained from our study indicates the presence of strain-induced surface AFM domains on some of the cleaved NiO(100) crystals, which are unusually thinner than bulk AFM domain wall widths (∼150 nm). Existence of such magnetic skin layer is substantiated by exchange-coupled ferromagnetic Fe domains in Fe/NiO(100), thereby evidencing the influence of this surface AFM domains on interfacial magnetic coupling. Our observations demonstrate a depth evolution of AFM structure in presence of induced surface strain, while the surface symmetry-breaking in absence of induced strain does not modify the bulk AFM domain structure. Realization of such thin surface AFM layer will provide better microscopic understanding of the exchange bias phenomena.