Abhishek Rai

(3 + 1)D superspace description of the incommensurate modulation in the premartensite phase of Ni2MnGa: a high resolution synchrotron x-ray powder diffraction study.

Le Bail and Rietveld analysis of high resolution synchrotron x-ray powder diffraction (SXRPD) data shows unambiguous signatures of the failure of the commensurate 3M modulation model. Using (3 + 1) dimensional superspace group formalism, we have not only confirmed the incommensurate modulation in the premartensite phase with a modulation wavevector of q = 0.337 61(5)c* but also determined the superspace group (Immm(00γ)s00), atomic positions and amplitude of modulations for the incommensurate premartensite phase of Ni2MnGa for the first time. Our results may have important implications in the understanding of the martensitic transition and hence the magnetic field induced strains.

High energy resolution bandpass photon detector for inverse photoemission spectroscopy

We report a bandpass ultraviolet photon detector for inverse photoemission spectroscopy with energy resolution of 82 ± 2 meV. The detector (Sr(0.7)Ca(0.3)F(2)/acetone) consists of Sr(0.7)Ca(0.3)F(2) entrance window with energy transmission cutoff of 9.85 eV and acetone as detection gas with 9.7 eV photoionization threshold. The response function of the detector, measured using synchrotron radiation, has a nearly Gaussian shape. The n = 1 image potential state of Cu(100) and the Fermi edge of silver have been measured to demonstrate the improvement in resolution compared to the CaF(2)/acetone detector. To show the advantage of improved resolution of the Sr(0.7)Ca(0.3)F(2)/acetone detector, the metal to semiconductor transition in Sn has been studied. The pseudogap in the semiconducting phase of Sn could be identified, which is not possible with the CaF(2)/acetone detector because of its worse resolution.

Intermediate stages of surface state formation and collapse of topological protection to transport in Bi2Se3

Surface states consisting of helical Dirac fermions have been extensively studied in three-dimensional topological insulators. Yet, experiments to date have only investigated fully formed topological surface states (TSS) and it is not known whether preformed or partially formed surface states can exist or what properties they could potentially host. Here, by decorating thin films of Bi2Se3 with nanosized islands of the same material, we show for the first time that not only can surface states exist in various intermediate stages of formation but they exhibit unique properties not accessible in fully formed TSS. These include tunability of the Dirac cone mass, vertical migration of the surface state wave-function and the appearance of mid-gap Rashba-like states as exemplified by our theoretical model for decorated TIs. Our experiments show that an interplay of Rashba and Dirac fermions on the surface leads to an intriguing multi-channel weak anti-localization effect concomitant with an unprecedented tuning of the topological protection to transport. Our work offers a new route to engineer topological surface states involving Dirac-Rashba coupling by nano-scale decoration of TI thin films, at the same time shedding light on the real-space mechanism of surface state formation in general.

Surface Study of Ni2MnGa(100)

The (100) surface of Ni2MnGa ferromagnetic shape memory alloy exhibits intrinsic surface property dissimilar to the bulk as well as influence of compositional variation at the surface. It is shown that by sputtering at room temperature and annealing at high temperature, it is possible to obtain a clean, ordered and stoichiometric surface. However, for even higher annealing temperatures, the surface becomes Mn rich. The (100) surface of Ni2MnGa is found to have Mn–Ga termination. A surface reconstruction to p4gm symmetry is observed in the austenite phase, while the expected bulk truncated symmetry at surface is p4mm. For the stoichiometric surface, the XPS valence band is compared with our calculations based on first principles density functional theory and good agreement is obtained. The ultraviolet photoelectron spectroscopy (UPS) valence band spectra depend sensitively on composition varying from Ni rich to Mn rich surfaces. A satellite feature observed in both Ni 2p core-level and valence band spectra is related to the narrow 3d valence band in Ni2MnGa.

Surface alloying in Sn/Au(111) at elevated temperature

On the basis of x-ray photoelectron spectroscopy, we show that when Sn is deposited on Au(111) single crystal surface at a substrate temperature TS=373 K, surface alloying occurs with the formation of AuSn phase. The evolution of the surface structure and the surface morphology has been studied by low energy electron diffraction and scanning tunneling microscopy, respectively as a function of Sn coverage and substrate temperatures.

Modulation on Ni2MnGa(001) surface

We report periodic modulation on (001) surface of Ni2MnGa ferromagnetic shape memory alloy. For the stoichiometric surface, analysis of the low energy electron diffraction (LEED) spot profiles shows that the modulation is incommensurate. The modulation appears at 200 K, concomitant with the first order structural transition to the martensitic phase.