Abhishek Banerjee

The use of hydrothermal methods in the synthesis of novel open-framework materials

The preparation of inorganic compounds, exhibiting open-framework structures, by hydrothermal methods has been presented. To illustrate the efficacy of this approach, few select examples encompassing a wide variety and diversity in the structures have been provided. In all the cases, good quality single crystals were obtained, which were used for the elucidation of the structure. In the first example, simple inorganic network compounds based on phosphite and arsenate are described. In the second example, inorganic-organic hybrid compounds involving phosphite/arsenate along with oxalate units are presented. In the third example, new coordination polymers with interesting structures are given. The examples presented are representative of the type and variety of compounds one can prepare by careful choice of the reaction conditions.

Accessing Rashba states in electrostatically gated topological insulator devices

We study the low temperature electrical transport in gated BiSbTe1.25Se1.75/hexagonal-Boron Nitride van der Waals heterostructure devices. Our experiments indicate the presence of Rashba spin-split states confined to the sample surface. While such states have been observed previously in photo-emission spectroscopy and STM experiments, it has not been possible to unambiguously detect them by electrical means and their transport properties remain mostly unknown. We show that these states support high mobility conduction with Hall effect mobilities ∼2000 to 3000 cm2/V-s that are paradoxically much larger than the mobilities of the topological surface states ∼300 cm2/V-s at T = 2 K. The spin-split nature of these states is confirmed by magneto-resistance measurements that reveal multi-channel weak anti-localization. Our work shows that Rashba spin split states can be electrically accessed in Topological insulators paving the way for future spintronic applications.

Emergence of a weak topological insulator from the BixSey family

We report the emergence of a weak topological insulator (WTI), BiSe, of the Bi-chalcogenide family with an indirect bandgap of 42 meV. Its structural unit consists of a bismuth bilayer (Bi-2), a known quantum spin hall insulator sandwiched between two units of Bi2Se3 which are three dimensional strong topological insulators. Our density functional theory calculations confirm the WTI phase and angle resolved photo-emission spectroscopy measurements carried out on cleaved single crystal flakes show Rashba states that closely agree with our theoretical predictions. Finally, we present a comparison between electronic and magneto-transport properties measured on single crystal flakes and thin films of BiSe. Published by AIP Publishing.

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.

Growth of high quality single crystals of Bi2Se3 topological insulator via solid state reaction method

Recently discovered, Topological Insulators (TIs) have garnered enormous amount of attention owing to its unique surface properties which has potential applications in the field of spintronics and other modern technologies. For all this, it should require a very good quality samples. There are a number of techniques suggested by people for the growth of good quality TIs. Here, we are reporting the growth of high quality single crystals of Bi2Se3 (a TI) by slow cooling solid-state reaction method. X-ray diffraction measurements performed on a cleaved flake of single crystal Bi2Se3 showed up with proper orientations of the crystal planes. High energy X-ray diffraction has been performed to confirm the stoichiometry of the compound and also recorded Laue patterns prove the single crystalline nature of Bi2Se3. Moreover, angle resolved photo-emission spectroscopy (ARPES) carried out on a flat crystal flake shows distinct Dirac dispersion of surface bands at the gamma point clarifying it as a 3D topological insulator.

Enhanced coherence and decoupled surface states in topological insulators through structural disorder

To harness the true potential of topological insulators as quantum materials for information processing, it is imperative to maximise topological surface state conduction while simultaneously improving their quantum coherence. However, these goals have turned out to be contradictory. Surface dominated transport in topological insulators has been achieved primarily through compensation doping of bulk carriers which introduces tremendous electronic disorder and drastically deteriorates electronic coherence. In this work, we use structural disorder instead of electronic disorder to manipulate the electrical properties of thin films of topological insulator Bi2Se3. We achieve decoupled surface state transport in our samples and observe significantly suppressed carrier dephasing rates in the coupled surface state regime. As the film thickness is decreased, the dephasing rate evolves from a linear to a super-linear temperature dependence. While the former is consistent with Nyquist electron-electron interactions, the latter leads to significantly enhanced coherence at low temperatures and is indicative of energy exchange due to frictional drag between the two surface states. Our work opens up the way to harness topological surface states, without being afflicted by the deleterious effects of compensation doping.To harness the true potential of topological insulators as quantum materials for information processing, it is imperative to maximise topological surface state conduction while simultaneously improving their quantum coherence. However, these goals have turned out to be contradictory. Surface dominated transport in topological insulators has been achieved primarily through compensation doping of bulk carriers which introduces tremendous electronic disorder and drastically deteriorates electronic coherence. In this work, we use structural disorder instead of electronic disorder to manipulate the electrical properties of thin films of topological insulator Bi2Se3. We achieve decoupled surface state transport in our samples and observe significantly suppressed carrier dephasing rates in the coupled surface state regime. As the film thickness is decreased, the dephasing rate evolves from a linear to a super-linear temperature dependence. While the former is consistent with Nyquist electron-electron interaction...

On Auslander’s formula and cohereditary torsion pairs

For a small abelian category C, Auslander’s formula allows us to express C as a quotient of the category mod −C of coherent functors on C. We consider an abelian category with the added structure of a cohereditary torsion pair τ = (𝒯 ,ℱ). We prove versions of Auslander’s formula for the torsion-free class ℱ of C, for the derived torsion-free class Db(ℱ) of the triangulated category Db(C) as well as the induced torsion-free class in the ind-category Ind −C of C. Further, for a given regular cardinal α, we also consider the category modα −C of α-presentable objects in the functor category Fun(Cop,Ab). Then, under certain conditions, we show that the torsion-free class ℱ can be recovered as a subquotient of modα −C.