Pinaki Majumdar

Research Summary:

My work over the last one year concerns strong coupling itinerant electron magnetism, in the presence of disorder and/or competing interactions. The major experimental motivation comes from the manganites, but the results are generic to a wide class of materials. Our `self consistent renormalisation' based Monte Carlo (SCR-MC) allows a controlled approximation on the problems. Combined with the Kubo formulation of transport theory, it allows an essentially complete solution for a wide range of problems in statistical physics and many body theory.

1. DOUBLE EXCHANGE MODELS: SELF CONSISTENT RENORMALISATION
We proposed a scheme for constructing classical spin Hamiltonians from Hunds coupled spin-fermion models in the limit . The strong coupling between fermions and the core spins requires self-consistent calculation of the effective `exchange' in the model, either in the presence of inhomogeneities or with increasing temperature. The method can access system sizes much beyond the reach of `exact' simulations, and estimate transport and optical properties of the model without artificial broadening. It can be extended to include other variables, e.g, coupling to classical `phonons', and opens up the possibility of studying finite temperature transport and spectra in inhomogeneous strong coupling systems.

2. INHOMOGENEOUS FERROMAGNETISM AND UNCONVENTIONAL CHARGE DYNAMICS IN DISORDERED DOUBLE EXCHANGE MAGNETS
We solved the double exchange model in the presence of arbitrary substitutional disorder by using the SCR-MC. The magnetic properties are studied through classical Monte Carlo while the effective exchange, , are calculated by solving the disordered fermion problem, and renormalised self-consistently with increasing temperature. We obtained exact results on the conductivity, magnetoresistance, optical response and `real space' structure of the inhomogeneous ferromagnetic state, and compared our results with charge dynamics in disordered
LaSrMnO.

3. NANOSCALE PHASE COEXISTENCE AND PERCOLATIVE QUANTUM TRANSPORT
We studied the nanoscale phase coexistence of ferromagnetic metallic (FMM) and antiferromagnetic insulating (AFI) regions by including the effect of AF superexchange and weak disorder in the double exchange model. We used the SCR-MC, recovered `cluster coexistence', and clarified the cluster distribution for varying electron density, disorder, and temperature. We tracked the magnetic structure, obtained the density of states, with its `pseudogap' features, and, for the first time, provided a fully microscopic estimate of the resistivity in a phase coexistence regime.

4. ANTILOCALISATION TO STRONG LOCALISATION: THE INTERPLAY OF MAGNETIC SCATTERING AND STRUCTURAL DISORDER
We studied the effect of magnetic scattering on transport in a system with strong structural disorder, using exact finite size calculation of the low frequency optical conductivity. Weak magnetic scattering weakens localisation effects, while strong coupling enhances the localising effect of disorder. We map out the transport regimes and the phase diagram.

Publications:

1. Anderson-Mott Transition Driven by Spin Disorder: Spin Glass Transition and Magnetotransport in Amorphous GdSi, Pinaki Majumdar and Sanjeev Kumar, Phys. Rev. Lett. 90, 237202 (2003).

2. Novel Sound Phenomena in Superfluid Helium in Aerogel and Other Impure Superfluids, P. Brusov, P. Brusov, G. Lawes, C. Lee, A. Matsubara, O. Ishikawa and P. Majumdar, Phys. Lett. A 310, 311 (2003).

Preprints:

1. Double Exchange Models: Self Consistent Renormalisation, Sanjeev Kumar and Pinaki Majumdar, preprint, cond-mat 0305345.

2. Inhomogeneous Ferromagnetism and Unconventional Charge Dynamics in Disordered Double Exchange Magnets, Sanjeev Kumar and Pinaki Majumdar, preprint, cond-mat 0305352.

3. Nanoscale Phase Coexistence and Percolative Quantum Transport , Sanjeev Kumar and Pinaki Majumdar, preprint, cond-mat, 0305645.

4. Antilocalisation to Strong Localisation: the Interplay of Magnetic Scattering and Structural Disorder, Sanjeev Kumar and Pinaki Majumdar.

Invited Lectures/Seminars:

1. Two lectures at the ``Discussion Meeting on Strongly Correlated Systems'', Dec 5-8, 2002, at H.R.I

2. ``Layered Manganites'', lecture at the meeting on ``Colossal Magnetoresistance Manganites'', S. N. Bose Center, Calcutta, Jan 2003.

3. ``Self Consistent Renormalisation'' lecture at the IACS, Calcutta, Jan 2003.

4. ``Disordered Double Exchange Ferromagnets'', lecture at the DST Discussion Meeting at the Univ of Pune, March 2003.

Other Activities:

I  organised a ``Discussion Meeting on Strongly Correlated Systems'' between 5-8 Dec 2002. There were twenty participants, including twelve from other institutes.