Jawaharlal Nehru Centre for Advanced Scientific Research

Research Interests:

  • Theoretical and computational Condensed matter physics
  • Strongly-correlated electron systems; Quantum impurity and lattice models
  • Kondo Physics; Quantum phase transitions
  • Developing non-perturbative approaches for lattice based models
  • Heavy Fermion metals, Kondo insulators, transition metal oxides.
  • Quantum many body modeling for above materials and comparison to experiments.
  • Optoelectronic devices

 

 

Materials for which, a single-particle description of electronic properties fails, are classified as correlated electron systems. Interest in these derives from the wide range of phenomena they exhibit such as high temperature superconductivity in cuprates, heavy fermions in lanthanides/actinides, colossal magnetoresistance in manganites and  metal-insulator transitions in transition metal oxides. In recent years, there has been a resurgence of technological interest in these materials, which stems from the extraordinary sensitivity of transport and thermodynamic properties in these materials to external parameters such as temperature and pressure. Despite the decades of research in this area, enormous challenges remain for theoreticians and experimentalists. Our group employs diagrammatic perturbation theory based techniques of quantum many body theory to understand these materials in the framework of simple models; the focus being to address issues relating to transport and thermodynamics, especially of heavy fermion systems and  transition metal oxides.

 

       Electronic devices based on organic polymers are emerging as inexpensive and efficient alternatives to traditional inorganic semiconductor based devices. Modeling of such devices is challenging because of the inherent strong disorder in polymer thin films. We have developed discrete circuit level approaches as well as kinetic Monte Carlo based simulations for investigating charge transport and device modeling. This work is carried out in close collaboration with the molecular electronics lab in JNCASR.

 

Research Highlights:

 

  • Dynamical mean field theory investigations of strongly correlated models such as the periodic Anderson model and the Hubbard model in thenormal paramagnetic phase have been carried out using diagrammatic perturbation theory based methods. Using results from such studies, dynamics and transport properties of several heavy fermion systems and transition metal oxides have been quantitatively explained and understood. Several issues such as universality and scaling, large and negative magnetoresistance, anomalous features in resistivity and optics have been highlighted. Two PhD theses have resulted from this work.

 

 

  • We show that the unconventional charge-density wave in dichalcogenides can be understood as an instability of a strongly correlated excitonic liquid. Several properties such as photoemission and transport show excellent agreement with experiment. This work has been carried out in collaboration with the group of Prof. Arghya Taraphder at IIT Kharagpur. 

 

  • Found strong evidence for a quantum critical point within the superconducting dome in the phase diagram of the two-dimensional Hubbard model which is commonly used to model cuprate superconductors. This work was carried out in collaboration with Prof. Mark Jarrell at University of Cincinnati, USA (now at Louisiana State University).

 

 

  • Developed a new spreading impedance approach for modeling organic semiconductor devices: Application to organic position sensing devices has yielded excellent agreement and provided insight into the underlying microscopic length scales and time scales of these devices. The same approach was utilized in modeling a organic bulk heterojuction based solar cells with ITO/Graphene as electrodes. This work is being developed further and is being carried out in close collaboration with the experimental group of Prof. K.S.Narayan at JNCASR. One MS thesis has resulted from this work.

 

 

  • Developed a Kinetic Monte Carlo (KMC) technique for modeling average current and its fluctuations in a disordered organic bulk heterojuction matrix. The experimental results are the first measurements of noise spectrum in organic solar cells, and using general arguments combined with KMC simulations, we have been to explain all of the features in the noise spectrum. This includes a very unusual finite frequency peak, that we argue arises from dynamical space-charge effects. This work is carried out in close collaboration with the experimental group of Prof. K.S.Narayan at JNCASR. 

 

 

  • We have developed a theory for understanding the weak ferromagnetism and canting of the antiferromagnetic backbone in alloyed orthoferrites such as YFe$_{1-x}$Cr$_x$O$_3$. Experimental results from the group of Prof. Sundaresan at JNCASR have been consistently and quantitatively explained within this theory. We have argued that the interplay of various interactions that we have considered here is relevant for all B-site disordered perovskite compounds.

 

 

  •  We have carried out studies of electrical noise in carbon nanotubes. We have been able to identify a crossover from a diffusive to a quasi-ballistic regime through variation of length of the CNTs. Temperature dependence has also been studied. This work is in collaboration with the group of Prof. Timothy Fisher at Purdue.

 

 

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   Last modified date: 24-01-2017