Research Projects
Professor L. Ramdas RamMohan
Worcester Polytechnic Institute, Worcester, MA 01609, USA
The recent projects with Professor RamMohan at Worcester Polytechnic Institute involve fundamental theoretical issues in optoelectronics and the modeling and simulation of physical systems. The initial 14 years of his career in Relativistic Field Theory and ManyBody Theory has provided the backdrop to the methods he continues to employ in the analysis of physical problems. Over the past 25 years he has worked in Condensed Matter Physics with emphasis on quantum semiconductor heterostructures and their linear and nonlinear optical properties. The present ongoing research projects he is involved in are listed below. All the projects involve theory and also numerical modeling. Besides a strong mathematical physics background, knowledge of Cprogramming for scientific applications would be useful for graduate work on these projects.
Graduate Research Assistantships are available for suitable candidates. Interested applicants should email the Physics Department at WPI and contact either Professor G. Iannacchione (gsiannac@wpi.edu), the head of the physics department, or Professor RamMohan (lrram@wpi.edu).
Undergraduate students interested in pursuing Major Qualifying Projects on the following topics are encouraged to contact Professor RamMohan directly. Sophomore/Junior level students in Physics, ECE and Applied Mathematics would find their backgrounds more suitable for such MQP topics than others.

"Investigation of Physical Mechanisms in Multiband Tunneling in Layered Semiconductor Structures." This project was supported by the NSF.(Principal Investigator: L. R. RamMohan, WPI)
Aim of the project: The aim of this project is to model layered semiconductor structures that can be grown using molecularbeamepitaxy. If the individual layers are compound semiconductors such as GaAs/AlGaAs or CdTe/ZnSe we model the heterostructure using the energy band structure of the individual layers. The mathematical issue is the development of solutions for energy eigenvalues, wavefunctions, and tunneling currents through these structures, by solving the multicomponent Schrödinger equations for the carriers in the energy bands in solids. The ability to model heterostructures with accuracy has led to the paradigm of wavefunction engineering, an approach through which he has successfully designed midIR (26 µm). These lasers are in the GaSb/InAs/AlSb system which has narrdow effective energy band gaps because they are TypeII heterostructures due to their energy band alignments. In the present project, the tunneling current and the IV characteristics are being modeled in a multiband framework. This is a theoretically challenging and computationally demanding project.
Outcomes: A proper appreciation of the mechanisms playing a role in multiband tunneling will help us design better quantum heterostructure devices. Most devices work under external bias with carriers moving from one region of the device to another.

"Spintronics and carrier induced ferromagnetism in Antimonide Quantum Heterostructures: Simulation and Device Modeling." This project was funded by DARPA. (P.I.: L. R. RamMohan, WPI)
Aim of the project: The aim of this project is to understand the carrierinduced ferromagnetism in semiconducting materials that are doped with magnetic ions. When Manganese (Mn) ions are inserted into the crystal matrix of IIIV compound semiconductors, the Mn go in substitutionally replacing the Group III cations. They also enter the crystal as acceptors, releasing holes in the valence band. The carriers interact with the Mn ions via the magnetic exchange interaction and align their spins, or magnetic dipole moments. We can then generate ferromagnetic behavior in ordinary semiconductors and control their ferromagnetic behavior by applying external bias to control the carrier densities in the heterostructures.
Outcomes: Spin oriented carrier injection would allow us to design spinLEDs (lightemitting diodes) and when the spinpolarized lifetime of the carrier is large it could be used for quantum computation.

"Sensors: A New Class of Devices Based on Interfacial Effects in MetalSemiconductor hybrid Structures." Funded by NSF. (P.I.: S. Solin, Washington University at St. Louis; CoPI: L. R. RamMohan, WPI)
Aim of the project: Work at NEC Research Institute by S. Solin led to the discovery of the phenomenon of Extraordinary Magnetoresistance (EMR) in semiconductors with metallic inclusions in them. The resistance changes in the presence of a magnetic field and typical structures have responses of 100% to 700,000%. This level of sensitivity to magnetic fields and the display of the property down to nanoscopic sizes imply that one could use them to make magnetic sensors. What is remarkable about this is that the semiconductormetal structures are free of magnetic metals that are typically used to make readheads for computer harddrives. Also, the property is dependent on the geometry of the distribution of the metal in the semiconductor. The theory and modeling is done at WPI, and Professor Solin who is now at Washington University at St. Louis will do the experiments.
Outcomes: The design and modeling of these readheads together with their experimental performance suggests that storage capacity of harddrives can be extended from the present 15Gbit/sq.in. upto 1Terabit/sq.in. We have now proposed the development of a new class of sensors for strain, electric field, and optical detectors that exploit the geometry dependent properties. The design and simulation of new devices is done using novel applications of the finite element method.

"Wavefunction Engineering of Spintronic Devices in GaN/AlN and ZnO/MgO Quantum Structures doped with Transition Metal Ions." Funded by the AFOSR. (P.I.: L. R. RamMohan, WPI)
Aim of the project: The optoelectronic properties of layered heterostructures of the semiconductors with Wurtzite crystallographic structure are still being investigated. The material systems GaN/AlN and also ZnO/MgO have energy band gaps ranging from 3.5 eV to 6.25 eV, a range for laser operation from the blue region of the spectrum to the ultraviolet (UV). This project is for exploring theory and simulations for the carrierinduced ferromagnetic behavior of layered quantum semiconductor structures of IIIV and IIVI materials of Wurtzite structure doped with transition metal ions.
The two material systems behave differently. In GaN/AlN the Mn enter the lattice substituting the cation while also being an acceptor. On the other hand, Mn enters the lattice as a spindopant. The interaction of carriers with Mn spinsites in both systems has a large exchange constant that bodes well for carrier induced ferromagnetic behavior. In ZnO/MgO additional pdoping has been shown to induce ferromagnetic behavior. With 3~5% Mn concentrations the number of available carriers can be as large as ~1020 cm3 so that we have to account for (i) very large band bending by solving for SchrödingerPoisson selfconsistency, (ii) selfconsistency with the carrier induced ferromagnetism due to the exchange interaction, and (iii) internal spontaneous and piezoelectric polarization fields.
Outcomes: With the calculations developed in this work, designs for spin devices such as spinLEDs, tunable polarized lasers, spininjection structures will be simulated easily. At present there are no guidelines for the optimized design of such structures.

"A Systematic Study for the development of ZeroFlux Planes, Diffusion Paths, Diffusion Structures in Multicomponent, Multiphase Systems." Funded at Purdue University by the NSF. (Collaboration with Professor M. A. Dayananda).
Aim of the project: The phenomenon of interdiffusion in multicomponent, multiphase alloys is encountered in a wide variety of materials systems, ranging from high temperature alloys, coatings, claddings, nuclear fuels, nuclear wastes, thin films, composites, among others. The redistribution of the components, the development of the phase layers, and the evolution of the interdiffusion microstructure within the diffusion zone are governed not only by the relative diffusion behavior of the individual components in the different phases but also by the thermodynamic, kinetic and stereological characteristics of phase boundaries and interfaces within the diffusion structure. There had been no good way to extract the diffusion coefficients from the experimental concentration curves. The experimental work of Dayananda together with his approach of obtaining averaged diffusion coefficients has led to a breakthrough in the problem. With known diffusion coefficients, the aim is to predict the diffusion path and the concentration curves by theoretical and numerical methods to be developed in this collaboration.
Outcomes: The developments of the diffusion zone and diffusion structures in multiphase assemblies have both theoretical and practical implications. When our predictive approach is demonstrated to work, it will have a profound impact on the development of metallic alloys and their uses in industry.

"Development of a PhononMediated QuantumCascade Terahertz Laser." Funded at Univ. of Massachusetts at Lowell by DARPA. (Collaboration with Professors W. Goodhue and J. Waldman at UML.)
Aim of the project: The THz region of the electromagnetic spectrum is receiving a significant amount of research attention for numerous applications. THz radiation in this effort is defined as emission in the range of 16 THz (300  50 µm, or 4.13  24.8 meV, respectively). The modeling is being done at WPI while the experimental work is done at UMassLowell. The modeling uses finite element methods for the design of a quantum cascade structure in which the carrier depletion is done extremely efficiently using interfacephonons.
Outcomes: THz pulses have demonstrated the ability to nondestructively penetrate certain membranes, such as ceramic, paper, and cardboard, and provide image information behind these media. Proposed uses for this radiation have included airport and shipping security, and the medical and dental communities are already utilizing such systems for imaging through bones and other structures in a manner less harmful than Xray imaging. Furthermore, THz signals may prove extremely effective in tracking chemical and biological signatures for advanced threat detection. Water molecules and other commonly abundant gas species significantly attenuate THz signals. Nevertheless, THz radiation has been suggested for usage in exoatmospheric (above the stratosphere) radar applications. This radiation can potentially provide much higher resolution than traditional RF through much smaller apertures.

"A FiniteElement Approach to the Modeling of the Nematic Phase and the NematicIsotropic Phase Transition of Liquid Crystals." Proposal is being readied for submission to funding agencies. (PI: L. R. RamMohan; CoPI: Professor Germano Iannacchione, Physics Department, WPI.)
Aim of the project: A finiteelement modeling approach is proposed for simulating liquid crystal phase structures and transitions in a selfconsistent manner, so that parameters for the theory are determined by thermodynamic experiments on liquid crystals of finite volume with surface interaction energies accounted for. The project will begin by using as a model system, the nematic phase and the nematic to isotropic phase transition, in order to establish a full complement of tools necessary to describe phase structure and transitions at the meanfield level.
The benefits of this project include the advancement on several fundamental fronts concerning phase transitions and phase structure modeling across a spectrum of other condensed matter systems besides the modeling of liquid crystals that is used as the starting point in this proposal. In addition, several applications of liquid crystals would directly benefit, including electrooptical optimization for a variety of important photonic and "smartmaterial" applications.Outcomes: A unique and potentially dramatic new application is the use of liquid crystalline materials as sensors that will be active very near the nematic to isotropic phase transition. The idea would be to exploit the highly nonlinear electrooptical nematic responses in the transition region.

"Other diverse topics: Novel applications of the Finite Element Method to Nonlinear Systems; issues in Sparse Matrix Computation; Use of Variational Principles in Computations." Frequently, in research one has "targets of opportunity" that open up and are resolved with some modest effort. A number of such topics are available for exploration, development, and eventual publication as journal articles.