Research Interests

Optical Spectroscopy of Semiconductors and Superconductors, with an emphasis on High-Pressure Studies

My research has focused on studying the electronic and vibrational properties of materials, using hydrostatic pressure as a tuning mechanism.Frequently the fundamental nature of new materials cannot be fully understood unless one tunes a basic parameter.  When the spacing between atoms is changed by 1 to 5%, the structure of the material remains the same, while there are drastic changes in the internal spring constants that govern the vibrational properties, and the overlap of electronic orbitals which govern their electronic properties.  Hydrostatic pressure produces this change in the range of 0-200 kbar.

In my laboratory we study semiconductors, their mullilayerheterostructures, high temperature superconductors and more recently, polymers. The semiconductors and polymers are of interest due to their excellent optical properties, which are used in laser diodes, light emitting diodes, and as other electronic devices.  The new opto-electronic polymers are cheap and easy to produce, and are expected to be the building blocks for a new wave of electronic devices.  The high temperature superconductors have exciting applications in various industries.

We use several optical techniques to study these materials, notably laser induced photoluminescence, and Raman scattering.  All of these materials have rich spectra arising from several physical mechanisms.  For example, in the superconductors one can observe Raman scattering due to dopant related effects and learn about the structure of the material; or study the temperature behavior to observe changes near the superconducting transition temperature. One can study effects on the vibrational spectra due to planarization of a polymer chain.  In semiconductor heterostructures one can study direct and indirect transitions, impurity effects, crossovers of bands and energy levels, and changing internal strain, to name a few. 

Physical Sciences for K-12 students. I have a strong interest in teaching, both at the University and K-12 levels.  In 1992 I initiated Exploring Physics, an after school program at Columbia's elementary and middle schools. Exploring Physics  targets female students and aims to get them interested in Physics at an early stage.  More recently we have expanded to provide new programs to grades 5-12, with an emphasis on the physical sciences.  These programs are in collaboration with other faculty at the University and the Columbia Public Schools, and are funded by the National Science Foundation.

Polarized photomodulated reflectivity and photoluminescence studies of ordered InGaP2 under pressure, R.J. Thomas, H.R. Chandrasekhar, M. Chandrasekhar, E.D. Jones, and R.P. Schneider, J. Phys. Chem. Solids 56 357 (1995).

Optical, electronic, magnetic and superconducting properties of quasiperiodic quantum dot arrays synthesized by a novel electrochemical technique, S. Bandyopadhyay, A.E. Miller, and M. Chandrasekhar, Invited paper at the SPIE International Symposium on Optoelectronic, Microphotonic and Laser Technologies, San Jose, CA, Feb 1995, Proc. SPEE 2397, 12 (1995).

Polarized Optical studies of Ordered InGaP2 under pressure, R.J. Thomas, H.R. Chandrasekhar, M. Chandrasekhar, E.D. Jones, and R.P. Schneider, Superlattices and Microstructures 18, 1, 1995.

Temperature dependence of strain in ZnSe/GaAs, R.J. Thomas, B.A. Rockwell, H.R. Chandrasekhar, M. Chandrasekhar, A.K. Ramdas, M. Kobayashi and R.L. Gunshor, J. Appl. Phys. 78, 6569 (1995).

Evidence for chain site substitution of Ni in YBa2(Cu'lxNix)4)8, D.J. Payne, H.R. Chandrasekhar, M. Chandrasekhar, B. Jayaram and J. Ulanday, Solid State Commun. 98, 971 (1996).

Quasiperiodic quantum dot arrays produced by electrochemical synthesis, S. Bandyopadhyay, A.E. Miller, DF Yue, G. Banerjee, R.E. Ricker, S. Jones, J.A. Eastman, E. Baugher, and M. Chandrasekhar, Nanotechnology, 7, 360 (1996).

Structure and Raman studies of HgBa2CuO4+x, S.H. Liou, R. Li, Q. Cai, M. Chandrasekhar, H. Luo, M. Chen, W.B. Yelon, A.L. Vasiliev, and E.P. Kvam, J. of Superconductivity, 11, 59 (1998).

Hydrostatic Pressure study of Raman phonons in YBa2(Cu0.96Ni0.04)4O8, D.J. Payne, S. Guha, M. Chandrasekhar, H.R. Chandrasekhar, B. Jayaram, and J. Ulanday, Review of High Pressure Science and Technology,7, 535, (1998).

Structural properties of poly (paraphenylenes) under high pressure, S.Guha, W. Graupner, R. Resel, M. Chandrasekhar, H.R. Chandrasekhar, and G. Leising, Proceedings of the Materials Research Society, 1997.

Electronic properties of poly (paraphenylenes) under high pressure, W. Graupner, S.Guha, S. Yang, M. Chandrasekhar, H.R. Chandrasekhar, G. Leising, U. Scherf, and K. Mifilen, Proceedings of the Materials Research Society Fall Meeting, 1997.

Temperature dependence of the intervalley deformation potential of GaAs/AlAs superlattices under hydrostatic pressure, S. Guha, Q. Cai, M. Chandrasekhar, H.R. Chandrasekhar, H. Kim, A.D. Alvarenga, R. Vogelgesang, A.K. Ramdas, and M.R. Melloch, Proceedings of the Materials Research Society Fall Meeting, 1997.

Photoluminescence of short period superlattices: a hydrostatic pressure and temperature study, S. Guha, Q. Cai, M. Chandrasekhar, H.R. Chandrasekhar, H. Kim, A.D. Alvarenga, R. Vogelgesang, A.K. Ramdas, and M.R. Melloch, submitted to Phys Rev.

Raman phonons under hydrostatic pressure in YBa2(CulxNix)408,, D.J. Payne, S. Guha, Q. Cai, M. Chandrasekhar, H.R. Chandrasekhar, B. Jayaram, and J. Ulanday, submitted to Phys. Rev.

Neutron transmutation doping as an experimental probe for AsSe in ZnSe, E.D. Wheeler, Jack L. Boone, J.W. Farmer and H.R. Chandrasekhar, Phys. Rev. B53, 15617 (19961)

Neutron transmutation doping as an experimental probe for CuZn in ZnSe, E.D. Wheeler, Jack L. Boone, J.W. Farmer and H.R. Chandrasekhar, J. Appl. Phys. 81, 524 (1997).

Incorporation Of CuZn centers in ZnSe far from equilibrium, E.D. Wheeler, Jack L. Boone, J.W. Farmer and H.R. Chandrasekhar, J. Phys. Chem. Solids, 58, 79 (1997).

Temperature dependence of Interband transitions in short period GaAs/AlAs superlattices, S. Guha, Q. Cai, M. Chandrasekhar, H.R. Chandrasekhar, H.Kim, A.D. Alvarenga, R.Vogelgesang, A.K. Ramdas and M.R. Melloch, Physics of Low Dimensional Structures, I.,. 57(1997).

Emission Spectra of Colonic Tissue and Endogenous Fluorophores, B. Banerjee, Brent Miedema and H.R. Chandrasekhar, submitted to American Journal of Medical Science.

High Pressure studies of Semiconductor Heterostructures, in Advances in High Pressure Research in Condensed Matter, H.R. Chandrasekhar, edited by S.K. Sikka, NISCOM, New Delhi, India, 1997, pp 166172.

BANDGAP TUNING IN SEMICONDUCTOR HETEROSTRUCTURES, H.R. Chandrasekhar and M. Chandrasekhar, a book chapter in the Encyclopedia of Electrical and Electronic Engineering, ed; John G. Webster, Wiley & sons,(in print)