www Group
 
 
 
 

Highlights


Unusual Dirac Fermions on the Surface of a Noncentrosymmetric α-BiPd Superconductor, S. Thirupathaiah, Soumi Ghosh, Rajveer Jha, E. D. L. Rienks, Kapildeb Dolui, V. V. Ravi Kishore, B. Büchner, Tanmoy Das, V. P. S. Awana, D. D. Sarma, and J. Fink,
Phys. Rev. Lett. 117, 177001, 2016.

Combining multiple emergent correlated properties such as superconductivity and magnetism within the topological matrix can have exceptional consequences in garnering new and exotic physics. Here, we study the topological surface states from a noncentrosymmetric α-BiPd superconductor by employing angle-resolved photoemission spectroscopy and first-principles calculations. We observe that the Dirac surface states of this system have several interesting and unusual properties, compared to other topological surface states. The surface state is strongly anisotropic and the in-plane Fermi velocity varies rigorously on rotating the crystal about the y axis. Moreover, it acquires an unusual band gap as a function of ky, possibly due to hybridization with bulk bands, detected upon varying the excitation energy. The coexistence of all the functional properties in addition to the unusual surface state characteristics make this an interesting material.

Is CH3NH3PbI3 Polar?, Sharada G, Pratibha Mahale, Bhushan P. Kore, Somdutta Mukherjee, Mysore S. Pavan, Chandan De, Somnath Ghara, A. Sundaresan, Anshu Pandey, Tayur N. Guru Row, and D. D. Sarma, J. Phys. Chem. Lett., 7 (13), 2412, 2016.

In view of the continued controversy concerning the polar/nonpolar nature of the hybrid perovskite system, CH3NH3PbI3, we report the first investigation of a time-resolved pump-probe measurement of the second harmonic generation efficiency as well as using its more traditional form as a sensitive probe of the absence/presence of the center of inversion in the system both in its excited and ground states, respectively. Our results clearly show that SHG efficiency, if nonzero, is below the limit of detection, strongly indicative of a nonpolar or centrosymmetric structure. Our results on the same samples, based on temperature dependent single crystal X-ray diffraction and P-E loop measurements, are entirely consistent with the above conclusion of a centrosymmetric structure for this compound in all three phases, namely the high temperature cubic phase, the intermediate temperature tetragonal phase and the low temperature orthorhombic phase. It is important to note that all our experimental probes are volume averaging and performed on bulk materials, suggesting that basic material properties of CH3NH3PbI3 are consistent with a centrosymmetric, nonpolar structure.

 

High photon energy spectroscopy of NiO: Experiment and theory, S. K. Panda, Banabir Pal, Suman Mandal, Mihaela Gorgoi, Shyamashis Das, Indranil Sarkar, Wolfgang Drube, Weiwei Sun, I. Di Marco, Andreas Lindblad, P. Thunström, A. Delin, Olof Karis, Y. O. Kvashnin, M. van Schilfgaarde, O. Eriksson, and D. D. Sarma, Phys. Rev. B 93, 235138, 2016.

We have revisited the valence band electronic structure of NiO by means of hard x-ray photoemission spectroscopy (HAXPES) together with theoretical calculations using both the GW method and the local density approximation + dynamical mean-field theory (LDA+DMFT) approaches. The effective impurity problem in DMFT is solved through the exact diagonalization (ED) method. We show that the LDA+DMFT method in conjunction with the standard fully localized limit (FLL) and around mean field (AMF) double-counting alone cannot explain all the observed structures in the HAXPES spectra. GW corrections are required for the O bands and Ni-s and p derived states to properly position their binding energies. Our results establish that a combination of the GW and DMFT methods is necessary for correctly describing the electronic structure of NiO in a proper ab initio framework. We also demonstrate that the inclusion of photoionization cross section is crucial to interpret the HAXPES spectra of NiO. We argue that our conclusions are general and that the here suggested approach is appropriate for any complex transition metal oxide.

 

Origin and distribution of charge carriers in LaAlO3-SrTiO3 oxide heterostructures in the high carrier density limit, Sumanta Mukherjee, Banabir Pal, Debraj Choudhury, Indranil Sarkar, Wolfgang Drube, Mihaela Gorgoi, Olof Karis, H. Takagi, Jobu Matsuno, and D. D. Sarma, Phys. Rev. B 93, 245124, 2016.

Using hard x-ray photoelectron spectroscopy with variable photon energy (2-8 keV), we address the distribution of charge carriers in the prototypical LaAlO3 (LAO) and SrTiO3 (STO) oxide heterostructures with high carrier densities (1017 cm-2). Our results demonstrate the presence of two distinct charge distributions in this system: one tied to the interface with a ∼ 1-nm width and ∼ 2–5×1014 – cm-2 carrier concentration, while the other appears distributed nearly homogeneously through the bulk of STO corresponding to a much larger carrier contribution. Our results also establish bimodal oxygen vacancies, namely on top of LAO and throughout STO, quantitatively establishing these as the origin of the observed bimodal depth distribution of charge carriers in these highly doped sample.

 

Substrate Integrated Nickel-Iron Ultrabattery with Extraordinarily Enhanced Performances, Debasish Sarkar, Ashok Shukla, and D. D. Sarma, ACS Energy Lett., 1 (1), 82, 2016.

A substrate-integrated nickel-iron ultrabattery is realized using nickel oxide (NiO) nanoflakes and hematite (α-Fe2O3) nanorods as electroactive materials for its positive and negative electrodes, respectively. Direct growth of electroactive materials on a highly conductive stainless steel substrate enhances the mechanical stability of the system together with reducing its internal resistance. The proposed nanoarchitectural design of the electroactive materials provides a large number of interaction sites for the electrolytic ions with the electrode materials in conjunction with short ion-diffusion paths, which significantly improve the capacitive performance of individual electrodes and hence of the fabricated ultrabattery. As a consequence, the as-assembled ultrabattery exhibits a specific capacity value of ∼36 mAh/g (94 F/g and volumetric capacitance ∼0.77 F/cm3) at a current density of 0.5 A/g in a potential window between 0 and 1.4 V, with capacitance retention of ∼60% of its original value even when the load current density is increased 20 times.

 

Probing complex heterostructures using hard X-ray photoelectron spectroscopy (HAXPES), Banabir Pal, Sumanta Mukherjee, D.D. Sarma, Journal of Electron Spectroscopy and Related Phenomena 200, 332-339, 2015.

X-ray Photoelectron Spectroscopy (XPS) plays a central role in the investigation of electronic properties as well as compositional analysis of almost every conceivable material. However, a very short inelastic mean free path (IMFP) and the limited photon flux in standard laboratory conditions render this technique very much surface sensitive. Thus, the electronic structure buried below several layers of a heterogeneous sample is not accessible with usual photoemission techniques. An obvious way to overcome this limitation is to use a considerably higher energy photon source, as this increases the IMFP of the photo-ejected electron, thereby making the technique more depth and bulk sensitive. Due to this obvious advantage, Hard X-ray Photo Electron Spectroscopy (HAXPES) is rapidly becoming an extremely powerful tool for chemical, elemental, compositional and electronic characterization of bulk systems, more so with reference to systems characterized by the presence of buried interfaces and other types of chemical heterogeneity. The relevance of such an investigative tool becomes evident when we specifically note the ever-increasing importance of heterostructures and interfaces in the context of a wide range of device applications, spanning electronic, magnetic, optical and energy applications. The interest in this nondestructive, element specific HAXPES technique has grown rapidly in the past few years; we discuss critically its extensive use in the study of depth resolved electronic properties of nanocrystals, multilayer superlattices and buried interfaces, revealing their internal structures. We specifically present a comparative discussion, with examples, on two most commonly used methods to determine internal structures of heterostructured systems using XPS.