Pranab Kumar Das
Email Address
slspkd@nus.edu.sg
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Publication Correlated plasmons in the topological insulator Bi2Se3 induced by long-range electron correlations(Nature Research, 2020) Whitcher, T.J.; Silly, M.G.; Yang, M.; Das, P.K.; Peyrot, D.; Chi, X.; Eddrief, M.; Moon, J.; Oh, S.; Castro-Neto, A.H.; Breese, Mark B. H.; Wee, A.T.S.; Silly, F.; Rusydi, A.; DEPT OF PHYSICS; CENTRE FOR ADVANCED 2D MATERIALS; SINGAPORE SYNCHROTRON LIGHT SOURCERecently, electron correlation has been shown to play an important role in unconventional plasmon generation in highly correlated electron systems. Electrons in topological insulators, on the other hand, are massless and insensitive to nonmagnetic scattering due to their protection by time-reversal symmetry, which makes these materials appealing platforms for hosting exotic plasmonic excitations. Here, using a combination of angle-dependent spectroscopic ellipsometry and angle-resolved photoemission spectroscopy as a function of temperature supported by first-principles calculations, we reveal a new pair of correlated plasmonic excitations at 1.04 and 1.52 eV and a significant Fermi level shift of 0.12 eV accompanied by spectral weight transfer in the topological insulator bismuth selenide (Bi2Se3). Interestingly, such a spectral weight transfer over a broad energy range causes a drastic change in the charge carrier density whereby the contribution of charge carriers in the bulk starts to rival those in the surface states and Bi2Se3 becomes more uniformly conducting. Our results show the importance of electronic correlations in determining the electronic structure and appearance of correlated plasmons in topological insulators and their potential applications in plasmonics. © 2020, The Author(s).Publication Giant Rashba effect at the topological surface of PrGe revealing antiferromagnetic spintronics(Nature Publishing Group, 2017) Banik, S; Das, P.K; Bendounan, A; Vobornik, I; Arya, A; Beaulieu, N; Fujii, J; Thamizhavel, A; Sastry, P.U; Sinha, A.K; Phase, D.M; Deb, S.K; SINGAPORE SYNCHROTRON LIGHT SOURCERashba spin-orbit splitting in the magnetic materials opens up a new perspective in the field of spintronics. Here, we report a giant Rashba spin-orbit splitting on the PrGe [010] surface in the paramagnetic phase with Rashba coefficient ?R = 5 eVÅ. We find that ?R can be tuned in this system as a function of temperature at different magnetic phases. Rashba type spin polarized surface states originates due to the strong hybridization between Pr 4f states with the conduction electrons. Significant changes observed in the spin polarized surface states across the magnetic transitions are due to the competition between Dzyaloshinsky-Moriya interaction and exchange interaction present in this system. Presence of Dzyaloshinsky-Moriya interaction on the topological surface give rise to Saddle point singularity which leads to electron-like and hole-like Rashba spin split bands in the Z?. ? - Z and X? - ? - X directions, respectively. Supporting evidences of Dzyaloshinsky-Moriya interaction have been obtained as anisotropic magnetoresistance with respect to field direction and first-order type hysteresis in the X-ray diffraction measurements. A giant negative magnetoresistance of 43% in the antiferromagnetic phase and tunable Rashba parameter with temperature makes this material a suitable candidate for application in the antiferromagnetic spintronic devices. © The Author(s) 2017.Publication Very efficient spin polarization analysis (VESPA): New exchange scattering-based setup for spin-resolved ARPES at APE-NFFA beamline at Elettra(International Union of Crystallography, 2017) Bigi, C; Das, P.K; Benedetti, D; Salvador, F; Krizmancic, D; Sergo, R; Martin, A; Panaccione, G; Rossi, G; Fujii, J; Vobornik, I; SINGAPORE SYNCHROTRON LIGHT SOURCEComplete photoemission experiments, enabling measurement of the full quantum set of the photoelectron final state, are in high demand for studying materials and nanostructures whose properties are determined by strong electron and spin correlations. Here the implementation of the new spin polarimeter VESPA (Very Efficient Spin Polarization Analysis) at the APE-NFFA beamline at Elettra is reported, which is based on the exchange coupling between the photoelectron spin and a ferromagnetic surface in a reflectometry setup. The system was designed to be integrated with a dedicated Scienta-Omicron DA30 electron energy analyzer allowing for two simultaneous reflectometry measurements, along perpendicular axes, that, after magnetization switching of the two targets, allow the three-dimensional vectorial reconstruction of the spin polarization to be performed while operating the DA30 in high-resolution mode. VESPA represents the very first installation for spin-resolved ARPES (SPARPES) at the Elettra synchrotron in Trieste, and is being heavily exploited by SPARPES users since autumn 2015.The VESPA new setup for spin-resolved angular-resolved photoemission spectroscopy is fully operational at the APE-NFFA beamline at the Elettra synchrotron, being a high-demand tool for the advanced investigation of the magnetic properties of materials, as well as the spin texture of complex non-magnetic low-dimensional systems (surfaces and nanostructures). Detailed descriptions of the detector and its characterization are reported. © 2017 Chiara Bigi et al.Publication Giant piezoelectricity in oxide thin films with nanopillar structure(AMER ASSOC ADVANCEMENT SCIENCE, 2020-07-17) Liu, Huajun; Wu, Haijun; Khuong, Phuong Ong; Yang, Tiannan; Yang, Ping; Das, Pranab Kumar; Chi, Xiao; Zhang, Yang; Diao, Caozheng; Wong, Wai Kong Alaric; Chew, Eh Piew; Chen, Yi Fan; Tan, Chee Kiang Ivan; Rusydi, Andrivo; Breese, Mark BH; Singh, David J; Chen, Long-Qing; Pennycook, Stephen J; Yao, Kui; Mr Eh Piew Chew; PHYSICS; MECHANICAL ENGINEERING; MATERIALS SCIENCE AND ENGINEERING; SINGAPORE SYNCHROTRON LIGHT SOURCEHigh-performance piezoelectric materials are critical components for electromechanical sensors and actuators. For more than 60 years, the main strategy for obtaining large piezoelectric response has been to construct multiphase boundaries, where nanoscale domains with local structural and polar heterogeneity are formed, by tuning complex chemical compositions. We used a different strategy to emulate such local heterogeneity by forming nanopillar regions in perovskite oxide thin films. We obtained a giant effective piezoelectric coefficient d of ~1098 picometers per volt with a high Curie temperature of ~450°C. Our lead-free composition of sodium-deficient sodium niobate contains only three elements (Na, Nb, and O). The formation of local heterogeneity with nanopillars in the perovskite structure could be the basis for a general approach to designing and optimizing various functional materials. 33;fPublication Photoinduced metastable dd-exciton-driven metal-insulator transitions in quasi-one-dimensional transition metal oxides(Nature Research, 2020) Asmara, T.C.; Lichtenberg, F.; Biebl, F.; Zhu, T.; Das, P.K.; Naradipa, M.A.; Fauzi, A.D.; Diao, C.; Yang, P.; Lenzen, P.; Buchenau, S.; Grimm-Lebsanft, B.; Wan, D.; Trevisanutto, P.E.; Breese, Mark B. H.; Venkatesan, T.; Rübhausen, M.; Rusydi, A.; CENTRE FOR ADVANCED 2D MATERIALS; ELECTRICAL AND COMPUTER ENGINEERING; PHYSICS; SINGAPORE SYNCHROTRON LIGHT SOURCE; NUS NANOSCIENCE & NANOTECH INITIATIVEPhotoinduced phase transitions in matters have gained tremendous attention over the past few years. However, their ultrashort lifetime makes their study and possible control very challenging. Here, we report on highly anisotropic d-d excitonic excitations yielding photoinduced metal-insulator transitions (MITs) in quasi-one-dimensional metals Sr1-yNbOx using Mueller-Matrix spectroscopic ellipsometry, transient ultraviolet Raman spectroscopy, transient mid-infrared reflectivity and angular-resolved photoemission spectroscopy supported with density functional theory. Interestingly, the MITs are driven by photo-pumping of d-d excitons, causing the metallic a-axis to become insulating while the insulating b- and c-axis concomitantly become a correlated metal. We assign these effects to an interplay between the melting of charge and lattice orderings along the different anisotropic optical axes and Bose-Einstein-like condensation of the photoinduced excitons. The long lifetime in the order of several seconds of the metastable MITs gives greater flexibility to study and manipulate the transient excitonic state for potential applications in exciton-based optoelectronic devices. © 2020, The Author(s).Publication Layer-dependent quantum cooperation of electron and hole states in the anomalous semimetal WTe2(Nature Publishing Group, 2016) Das, P.K; Di Sante, D; Vobornik, I; Fujii, J; Okuda, T; Bruyer, E; Gyenis, A; Feldman, B.E; Tao, J; Ciancio, R; Rossi, G; Ali, M.N; Picozzi, S; Yadzani, A; Panaccione, G; Cava, R.J; SINGAPORE SYNCHROTRON LIGHT SOURCEThe behaviour of electrons and holes in a crystal lattice is a fundamental quantum phenomenon, accounting for a rich variety of material properties. Boosted by the remarkable electronic and physical properties of two-dimensional materials such as graphene and topological insulators, transition metal dichalcogenides have recently received renewed attention. In this context, the anomalous bulk properties of semimetallic WTe2 have attracted considerable interest. Here we report angle- and spin-resolved photoemission spectroscopy of WTe2 single crystals, through which we disentangle the role of W and Te atoms in the formation of the band structure and identify the interplay of charge, spin and orbital degrees of freedom. Supported by first-principles calculations and high-resolution surface topography, we reveal the existence of a layer-dependent behaviour. The balance of electron and hole states is found only when considering at least three Te-W-Te layers, showing that the behaviour of WTe2 is not strictly two dimensional.Publication Crystal growth and magnetic properties of equiatomic CeAl(IOP Publishing, 2014) Das, P.K; Thamizhavel, A; SINGAPORE SYNCHROTRON LIGHT SOURCESingle crystal of CeAl has been grown by flux method using Ce-Al self-flux. Several needle like single crystals were obtained and the length of the needle corresponds to the [001] crystallographic direction. Powder x-ray diffraction revealed that CeAl crystallizes in orthorhombic CrB-type structure with space group Cmcm (no. 63). The magnetic properties have been investigated by means of magnetic susceptibility, isothermal magnetization, electrical transport, and heat capacity measurements. CeAl is found to order antiferromagnetically with a Neel temperature TN =10 K. The magnetization data below the ordering temperature reveals two metamagentic transitions for fields less than 20 kOe. From the inverse magnetic susceptibility an effective moment of 2.66 ?B/Ce has been estimated, which indicates that Ce is in its trivalent state. Electrical resistivity data clearly shows a sharp drop at 10 K due to the reduction of spin disorder scattering of conduction electrons thus confirming the magnetic ordering. The estimated residual resistivity ratio (RRR) is 33, thus indicating a good quality of the single crystal. The bulk nature of the magnetic ordering is also confirmed by heat capacity data. From the Schottky anomaly of the heat capacity we have estimated the crystal field level splitting energies of the (2J + 1) degenerate ground state as 25 K and 175 K respectively for the fist and second excited states.