Our group have explored the ultrathin two-dimensional materials for less understood phenomena such as excitonic complexes and excitonic excited states, [Appl. Surf. Sci.(2019) 463, 52] many-body phenomena involving coupling of electron and phonon in charge density wave (CDW), [Phys Rev Res (2020) 2, 033118] local ferroelectric state in antiferroelectric semiconductors, [ Phys. Rev B (2020) 102 (20), 205308 and Adv. Func. Mater. (2020) 30, 2001387] and plasmon-phonon coupling leading to poor thermal conductivity in superionic thermoelectric material [ACS Appl. Ener. Mater. (2020) 3 (3), 2175]. We have emphasized and proposed energy band diagram with the unique observation of Rydberg states of A exciton and biexciton in PL spectra of monolayer MoS2, at 4 K. The understanding of such excitonic states and biexcitons is useful for future quantum information processing, optoelectronic, photonics and THz applications. [Appl. Surf. Sci.(2019) 463, 52] Plasmon-phonon coupling is another form of many-body interaction leading to poor thermal conductivity and thus improved thermoelectric performance of various thermoelectric materials. Such unique phenomena and fundamental understanding have important implications for the development of advanced technologies.
The correlation between electron-phonon coupling and topological quantum properties have got a very high attention by the scientific community across the globe. Majority of the materials studied for topological insulating behavior are also good thermoelectric materials, because of layered nature and nano band-gap. The champion Bi2Te3 is one of the example of such materials and in the similar series Bi2GeTe4, and Bi4GeTe7, also have topological insulating properties arising due to the spin-orbit interactions of Bi and Te, which influences the magneto-transport properties. We have addressed the dynamics of stretching of the Bi-Te bonds to understand the bridging phonons in the field of topological materials. We have united the two important aspects of a known strong topological insulator, Bi2GeTe4, with magneto transport and Raman spectroscopy studies. [PRB (2022) 105, 045134]
Associated with higher order of anharmonicity, the thermoelectric materials with large Grneisen parameter possesses ultralow thermal conductivity, which plays a decisive role in engineering of phonons. From the lower average phonon velocity, Gruneisen parameter and bond lengths, we suggested that the Bi2GeTe4, with ultralow thermal conductivity, has a great potential for thermoelectricity. [APL (2020) 117, 123901] In addition to this, we also have designed and prepared a complex Bi4GeTe7, which has the alternating quintuplet-septuplet atomic stacking separated by van der Waal gaps. The structural complexity, in natural heterostructures and mass-contrast by heavy elements, generates a sufficient amount of anharmonicity for poor phonon propagation resulting in poor thermal conductivity. [APL, (2021) 119, 223903] The unique concepts of crystalline anharmonicity for glassy thermal transport with appearance of soft phonon and impurity localized modes, [APL (2016), 109, 133904] electronic band convergence with doping and coupling of charge carriers with Magnetic Entropy [APL (2018), 113, 193904 and JMCC (2018) 6, 6489] for enhancement in thermoelectric performance of environment friendly crystalline SnTe. We showed that the thermoelectric power of SnTe can be increased not only by changing the charge carriers but also with modifying the effective thermal mass of charge carriers by band convergence and coupling of magnetic entropy. [JMCC (2018) 6, 6489] The new soft modes, observed in Raman spectra, have also been shown in other crystalline materials for the work done in collaborations. [Ener. Env. Sci. (2019) 12, 589; JACS (2019) 141, 51, 20293 and JACS (2019) 142 (36), 15595 (2020)]. We have demonstrated that the cubic phase of superionic Argyrodites (Ag8GeSn6) has an efficient thermoelectric property over the orthorhombic phase. [ACS Appl. Ener. Mater (2019) 2, 654]. Our findings signify the importance of the material and their relevance for energy research.
We employ first principles electronic calculations using density functional theory to understand the electronic and magnetic properties of different classes of materials including intermetallics, rare earth compounds, magnetic oxides, magnetoelectrics and so on. Bulk to thin films, nano clusters and heterostructures are dealt with to explore the potential use of these materials in various applications.
We use machine learning to understand magnetism in different materials.
In this work, we have investigated the precursor effects to superconductivity in BaPb0.75Bi0.25O3 using temperature dependent resistivity, x-ray diffraction technique and photoemission spectroscopy. The present compound exhibits superconductivity around 11 K (TC ). The synthesis procedure adopted is much simpler as compared to the procedure available in the literature. In the temperature range (10 K-25 K) i.e. above TC , our results show an increase in both the orthorhombic and tetragonal strain. The well screened features observed in Bi and Pb 4f7/2 core levels are indicative of the metallic nature of the sample. The compound exhibits finite intensity at the Fermi level at 300 K and this intensity decreases with decrease in temperature and develops into a pseudogap; the energy dependence of the spectral density of states suggests disordered metallic state. Furthermore, our band structure calculations reveal that the structural transition upon Pb doping results in the closing of the band gap at the Fermi level.
We investigate the effect of sample preparation conditions on the link between the structural and physical properties of polycrystalline spin-orbit Mott insulator, Sr2IrO4. The samples were prepared in two batches. With the first batch prepared as per the commonly adopted procedure in literature and the second batch prepared adopting the same procedure as the first batch but with an additional annealing in vacuum. Interestingly, our results show that without change in the value of the Curie temperature (TC), there occurs increase in the value of magnetization, resistivity, magneto-resistance (MR) and an increase in temperature range of stabilization of the canted antiferromagnetic structure. The temperature behaviour of the difference in the irreversible magnetization between the samples is in line with the difference in the Ir-O-Ir in-plane bond angle. At low temperatures, the conduction mechanism in the first batch of the sample is mainly governed by disorder while in the case of the other sample it is of Arrhenius type. The magneto-transport results have shown its strong link with the disorder and structural results. Although the nature and mechanism of the disorder needs to be investigated further, the present results throw light on the role of disorder and its connectivity between the structure and physical properties to understand its complex behaviors
We explore the structural and magnetic tunability of polycrystalline La0.2Sr0.8MnO3 sample by varying preparation conditions using solid state reaction method. Low temperature X-ray diffraction studies indicate that one of the samples shows a transition from cubic Pm-3m structure to dominant tetragonal I4/mcm structure below ?260K, while the other sample exhibits transformation from cubic structure to a dominant phase with fundamental reflections corresponding to P4/mmm tetragonal symmetry below ?260K, followed by an incomplete transition from this phase to tetragonal I4/mcm phase below ?225K. The Rietveld analysis at 260K indicates that the former sample exhibits co-existence of cubic Pm-3m, tetragonal I4/mcm and P4/mmm phases while the latter sample shows the co-existence of Pm-3m and tetragonal P4/mmm phases. Density functional theory(DFT+U) calculations suggest C-type antiferromagnetic and d3z2-r2 orbital ordering in I4/mcm structure and A-type antiferromagnetic with dx2-y2 orbital ordering in P4/mmm structure. The results show a possibility of tuning magnetic structure from C-type antiferromagnetic to A-type antiferromagnetic in an intermediate temperature range at fixed concentration, by varying the strain fields. At high temperatures(300-518K), the resistivity follows small polaron hopping model in both the samples and change in preparation conditions leads to a change over from adiabatic to non-adiabatic limit. Above 518K, both the samples exhibit metallic resistivity. Our results open a new avenue for tuning magnetic and structural phases in phase separated systems, and such tunability may be useful in antiferromagnetic spintronic, designing correlated electron-based electronic devices.
The electron correlations in the materials give rise to exotic low temperature quantum phenomena such as, Superconductivity, Topological surface states, Charge Density Wave, Metal to Insulator transition etc. We are interested in the study of the electrical and thermal transport properties of novel quantum materials to understand the underlying mechanism and behavior of these exciting phases of materials.
Some of the pyrochlore systems are known to exhibit spin ice magnetic states where the residual magnetic entropy of the system is equivalent to the entropy of the water ice. These systems are considered as the potential candidates for the magnetic monopole-like excitations. We are interested in the disordered pyrochlore system to understand the unusual spin freezing and the spin ice behavior of these materials.
We aim to investigate quantum coherent transport in novel two-dimensional materials such as graphene, topological insulators etc. Our aim is to evaluate the key quantities like Diffuson and the Cooperon by using many-body perturbation theory both in real-space and in momentum-space.
We wish to investigate the unconventional transport properties of topological material such as Weyl semimetals, Dirac semimetals, and other non trivial topological states of matter. We employ quasiclassical Boltzmann formalism and other many-body quantum techniques to solve this problem.
Study of quantum magnetism is a research field of current interest. An important developments in this area is the discovery of quantum materials which exhibits novel ground states like quantum criticality, quantum spin liquid (QSL), spin ice, high temperature superconductivity, strange metal behaviour, nematic fluctuations, low dimensional magnetism, effective spin - 1/2 ground state etc. Compounds based on traditional magnetic ions such as Cu2+, Ni2+ and Cr5+ have provided a fertile ground for many novel quantum states. In recent years, the focus has slightly shifted to explore quantum magnetism in insulating materials, mainly in 4f rare-earth based magnetic systems. The aim of this problem is to investigate the nature of near neighbors' magnetic interactions and quantum magnetism in 3D spin systems
RScX alloys, where R = rare earths and X = Ga, Si, Ge and Sn have been subjected to rigorous investigation in the recent years in context of large magnetocaloric effect and magnetoresistance (MR) exhibited by them. This family of intermetallics crystallizes in hexagonal or tetragonal structures depending upon the constituents. The magnetic ordering temperature of these compounds varies from very low temperatures to temperatures well above room temperature. Members of this family show various physical properties such as Kondo effect, heavy fermionic behavior, spin-glass state, superconductivity, large positive and negative MR and multiple magnetic transitions. The aim of this problem is to investigate magnetic, magnetocaloric and electrical transport properties of a series of ternary intermetallics RScX (R = Rare earth and X = Ga, Si, Ge, Sn).
Weyl semimetals are exotic class of topological materials which provide a realization of chiral anomaly in condensed matter. The quasi-particle excitation of Weyl semimetals is Weyl fermion, which has a definite chirality. Heusler alloys have been subjected to extensive research because of the wide range of physical properties such as ferromagnetism, antiferromagnetism, spin polarization, half metallicity, topological phases, superconductivity etc., exhibited by them. It is feasible to search signatures of Weyl semimetals in either non centrosymmetric half Heusler alloys or the magnetic full Heusler alloys. The aim of this problem is to investigate potential Weyl semimetal candidates in the family of Heusler alloys.
Magnetic refrigeration technique has been suggested as a potential technique that has prominent advantages over the currently used gas compression-expansion method in the sense of its high-efficiency, environmentally friendly applications. Magnetocaloric effect (MCE) provides a unique way of realizing magnetic refrigeration. Additionally, investigation of magnetocaloric parameters of materials can help in better understanding about the complex magnetic phases present in the system, which may not be possible just by studying magnetization. The aim of this problem is to characterize and study novel magnetic materials. The investigations will be carried out on mixed metal oxides belonging to 3d/4d/5d electron systems and intermetallics belonging to 4f electron systems.
Our goal is to probe and understand the dynamics of underlying quasiparticle excitations and phonons in quantum materials such as quasi one dimensional magnets, spin frustrated magnetic system, putative Kitaev spin liquids etc.
In 2D nanosystems, we use inelastic light scattering probe and transport measurements to understand the role of phonons in layered magnetic as well as non-magnetic two-dimensional materials. Secondly, material engineering, through tuning their different physical, electronic and thermal properties as a function of controlled tuning parameters such as strain.
Cooled to nano-Kelvin temperatures, Bose-Einstein condensates acquire macroscopic quantum properties. Observable in experiments, quantum gases give direct access to quantum physics. Be it the components of novel quantum technologies, or as simulators of condensed matter systems, it is of immense value to know the role by quantum and thermal fluctuations to a fine detail. We explore the equilibrium and out-of-equilibrium properties of spinor Bose-Einstein condensates at finite temperatures.
One of the major concerns about organic solar cells is their low efficiency. Recent studies have pointed out that poor absorption and low carrier mobility of organic materials are responsible for the limited performance of organic photovoltaic cells. Molecular orbital mismatch that governs the open circuit voltage is also an important factor. Therefore, synthesis of novel organic materials (polymers and small molecules), developing new device architectures, spectroscopic investigation on materials and electrical characterization of devices are some of the aims of this project to address the efficiency issue.
Research on metal halide perovskites (both inorganic and hybrid organic-inorganic) has largely arisen out of rapid progress in their photovoltaic applications. However, these materials are potentially suitable for a diverse optoelectronic application such as light emitting diodes, photodetectors and lasers. These applications are enabled by the favorable material properties, which include long charge carrier diffusion lengths, high absorption coefficients with a sharp absorption edge and remarkably high photoluminescence (PL) quantum efficiency. Understanding the excited state photo-carrier dynamics and interactions are crucial for elucidating the working mechanisms of optoelectronic devices. Thus, our research group is extensively involved to develop the fundamental understanding of important properties of these materials like electronic structure, absorption, emission, carrier dynamics and transport, and other relevant photophysical processes that have propelled these materials to the forefront of modern optoelectronics research. Moreover, we develop new perovskite materials and study their application possibility in optoelectronic devices.