Program

Rebecca Alexander, Ph. D. | Senior Associate Dean for Research and Community Engagement 

Fred Salsbury, Ph. D. | Graduate Director of the Department of Physics

Behzad Raofi, Ph.D. — Space Systems Engineer and Navigato, NASA JPL 

Behzad Raofi was born in Tehran, Iran. Watching Neil Armstrong’s first moon landing in 1969 instilled the love of space exploration in him since a young age. He graduated as valedictorian of Dr. Hashtroody High School and subsequently came to the United States to pursue a career in aerospace engineering (ASE). He completed his Bachelor of Science degree at the University of Texas at Austin with highest honors, followed by Masters Degree in ASE.

In his early career he supported projects such as The Space Shuttle Rendezvous Techniques Development at NASA Johnson Space Center and the Gamma Ray Observatory at NASA Goddard Space Flight Center. Since joining NASA/Jet Propulsion Laboratory (JPL) after obtaining his PhD in 1998, Behzad has worked on many deep space missions as a navigator, including Mars Exploration Rovers Spirit and Opportunity, team lead for the trajectory control for Mars Phoenix Lander Mission, and early navigation support for Mars Science Laboratory Mission (Curiosity). Starting in 2009, as a Mission Interface Manager in the NASA/JPL Deep Space Network directorate, Behzad managed the provision of Deep Space Telecommunication Support to over 17 missions, including European (ESA), Japanese (JAXA), and Indian (ISRO) missions. In 2017 he joined the joint NASA-ISRO mission NISAR as the lead V&V engineer and Chief Validation Engineer (NISAR is an Earth Orbiting Mission expected to launch in June 2025 to help improve our understanding of the Earth System Processes). As a Senior Flight Mechanics Systems Engineer, Dr. Raofi is currently supporting NASA’s Artemis Program, Lunar Ascent Segment, at Marshall Space Flight Center (MSFC).

Arezoo Nameny, Ph.D. Candidate — Guthold's Lab 

Background. Fibrin polymerizes into a fibrous mesh that provides mechanical stability to blood clots. Ultimately, this mesh needs to be dissolved, in a process called fibrinolysis, to allow normal blood flow. Mesh structure and lysis depend on fibrinogen and thrombin concentrations; however, quantitative relationships of these dependencies are lacking. Aim. This study aims to investigate the effects of fibrinogen and thrombin concentrations on the structural properties of fibrin clots, including fiber density, pore size, fractal dimension, and fiber diameter. Additionally, we seek to derive mathematical relationships that describe how these structural properties influence clot lysis time, providing insights into the relative contributions of fibrinogen and thrombin to clot formation and their implications for thrombosis, hemostasis, and lysis.

Methods. We used confocal and electron microscopy to quantitatively analyze the relationship between structural properties of plasma clots – fiber density, pore size, fractal dimension, fiber diameter – and fibrinogen and thrombin concentrations; we used a lysis assay to determine lysis time for all clot conditions.

Result. We found that higher concentrations of fibrinogen and thrombin led to clots with increased fiber density, reduced pore size, and greater fractal dimension (branching). Fiber diameter was observed to increase with increasing fibrinogen and decreasing thrombin. From these observations, power-law equations were derived to describe the relationship between clot structural properties, lysis time, and the concentrations of fibrinogen and thrombin. 

Conclusion. The equations revealed that fibrinogen concentration has a more pronounced effect on clot structure than thrombin concentration. These findings hold significant implications for understanding clot formation and their relevance in the context of thrombosis, hemostasis, and lysis.

Motahhare Mirhosseini, Ph.D. Candidate — NANOTEQ 

Two-dimensional hexagonal Bismuth Telluride (Bi2Te3) nanoplates can exhibit a chiral edge current due to the presence of a metallic edge state within the bulk band structure, reflecting their topological nature. The precise control over the morphology and composition of crystalline Bi2Te3 nanoplates using solution-based synthesis is well-established. Introducing a pore at the center of these nanoplates results in a Corbino geometry, providing a platform to observe edge currents at both the inner and outer edges and explore correlations between them. In this study, we present a solvothermal synthesis method to create and control pore sizes at the center of a few quintuple-layered Bi2Te3 nanoplates. We characterize the nanopore distribution, size, and morphology using transmission electron microscopy and atomic force microscopy. We show that the removal of tellurium (Te) nanorods, which act as nucleation sites for nanoplate growth, leaves behind nanopores. Furthermore, as the thickness of the Te rods increases, the nanopore size also increases. Finally, we employ magnetic force microscopy to map the magnetic forces at the inner and outer edges, providing insights into the topological properties in a Corbino geometry.

Can Cai, Ph.D. Candidate — Guthold's Lab 

Background

Observing fibrin fiber growth and maturation is critical for understanding the mechanisms of clot formation and stability. Traditional AFM approaches to study fibrin networks often involve diluting fibrinogen, which precludes or disrupts mature fibrin mesh formation. AFM offers high-resolution imaging of surface topography. SEM excels at visualizing structures above the surface, making it an ideal complement to AFM for studying 3D fibrin networks. Developing a method that preserves the native structure of fibrin fibers while enabling high-resolution imaging for both AFM and SEM is crucial for advancing our understanding of fibrin fiber formation.

Aims

We aimed to develop a sample preparation method that preserves fibrin fiber morphology for high-resolution imaging using AFM and SEM. The method should minimize fiber collapse and preserve native structure and enable determination of fibrin fiber growth across time points.

Methods

Fibrinogen/thrombin mixtures were directly deposited onto mica substrates. To preserve the fiber structure and stop the reaction, glutaraldehyde fixation was applied, followed by critical point drying (CPD) to minimize structural collapse. Samples were analyzed using AFM and SEM, allowing for cross-verification of fiber morphology. Time-series imaging (snapshots) was conducted to track fibrin fiber growth.

Results

We visualized fibrin fiber growth and maturation over time. Time-series imaging identified the gel point at 4–6 minutes. Post-gel-point analysis revealed lateral aggregation, where smaller units—fibrin multimers—were incorporated into pre-existing fibers. Fiber collapse was prevented, ensuring structural preservation for high-resolution imaging. Cross-verification between AFM and SEM confirmed the reliability of the method. Early fibers appeared as a disordered network of thin, short strands with numerous branch points; over time, these structures reorganized into cleaner, thicker, longer fibers with fewer branch points, prompting a revised model of fiber assembly.

Conclusions

The sample preparation workflow—direct mica deposition combined with optimized fixation and drying—preserves native fibrin morphology, enabling detailed AFM and SEM analysis. By characterizing the transition from highly branched, thin networks to thicker, less branched fibers, these observations underpin a new conceptual framework for fibrin fiber assembly and maturation.

Manikanta Makala, Ph.D. Candidate — Organic Electronics Lab 

Organic thin-film transistors (OTFTs) are key components in flexible and printed electronics, offering low-cost, large-area fabrication using solution-based processes. Although performance benchmarks have continually advanced, achieving long-term stability remains a critical challenge, particularly for n-channel OTFTs. Here, we outline a unique approach for achieving robust performance and operational stability in n-channel OTFTs, narrowing the gap with well-established p-channel devices. By combining the electronic advantages of donor–acceptor polymers with the structural benefits of ladder polymers, we synthesized a new class of copolymers derived from indacenodithiazole (IDTz) and diketopyrrolopyrrole (DPP) units, using which we demonstrated a simple yet effective approach to tune the charge transport behaviour from moderate ambipolar characteristics to efficient unipolar p- or n-type conduction by chemically modifying the electrode surface before polymer deposition. This selective optimization enhanced both the electron and hole injection, resulting in charge carrier mobilities exceeding 1 cm2/Vs. To minimize bias-induced device degradation, we incorporated a double-layer polymer dielectric[1] and subject the devices to 1000-minute stress test at consistently very high electric fields (VGS = VDS = 60V). This testing protocol, more aggressive than cycling tests, better represents real-world conditions requiring sustained, high-performance operation. Our devices showed excellent resilience, with negligible change in mobility and a small threshold voltage shift (ΔVth = 0.5 V), demonstrating their stability under prolonged stress.[2] A key finding of our study is the intricate relationship between the polymer structure, device performance and stability under bias stress. Although the polymers share a common backbone, those with the combination of linear and branched side chains exhibited the highest charge carrier mobility but also showed greater degradation (μt/ μ0 = 0.9, ΔVth = 1.2 V) compared to those with a high density of branched side chains, which maintained better stability (μt/ μ0 ≈ 1, ΔVth = 0.5 V). Furthermore, we investigate the role of thermal annealing on polymer morphology and its impact on the charge transport properties and density of electronic trap states. These trap states, which evolve over stress time, play a critical role in the performance decline of the devices. These findings contribute to a deeper understanding of the mechanisms governing charge transport and bias stress effects in OTFTs, providing valuable strategies for designing stable, high-performance organic electronic devices for next-generation optoelectronic applications.

References:

[1] Iqbal, Hamna F., et al., Nature Communications, 2021, 12.1, p.2352.

[2] Makala, Manikanta., et al., Journal of Material Chemistry C, 2024, 12.42, pp.17089-17098.

Wells Graham, Ph.D. Candidate — Materials Simulation Group 

Metal-organic frameworks have garnered considerable interest due to their high porosity, large surface areas, and often easy synthesis, offering a vast structural diversity of interest to wide-ranging applications. Recently, the novel structure NU2100 showed exceptional uptake of CO2 and H2 compared to C2H4 and C2H6, a promising step towards integrated carbon capture and utilization. Here, we investigate NU2100 using ab initio techniques to understand the mechanism of adsorption observed experimentally. Our calculations uncover a surprising structural instability—associated with the strongly correlated Cu(I) sites within the structure—that drives a gate-opening effect. This structural instability has significant implications for the binding affinity and diffusion barriers of the guest molecules in NU2100. The thermodynamic, kinetic, and structural stability insights provided by our calculations lead to a complete mechanistic understanding of the adsorption behavior observed in NU2100 experimentally.

Nick Corak, Ph. D. — Environmental Dynamics Lab 

Banasree Sarkar Mou, Ph.D. Candidate — Winter Research Group 

Nonlinear spectroscopy opens up the possibility of exploring interesting physical phenomena in light-matter coupled materials. In particular, Second Harmonic Generation (SHG) intensity can identify magnetic order parameters and hidden order phases in studying 2D magnetic materials as suggested by recent studies [1]. We formulate quantitative analysis of the SHG intensity in terms of local many-body multiplet states including both polarization and current terms, which is more suitable for Mott insulators than previous DFT-based approaches. We then use our theory to predict the responses of a van der Waals material NiI2 and link them to the nature of the microscopic properties and magneto-optical coupling.

[1]. Zhao, Liuyan, et al. "Second harmonic generation spectroscopy of hidden phases." (2018): 207-226.

Evan Kumar, Ph.D. Candidate — Kandada Research Group 

Molecular polaritons, which are hybrid light-matter quasiparticles, have emerged as systems of intriguing research due to their ability to form non-equilibrium Bose-Einstein condensates at relatively low energy thresholds and at ambient temperatures. While many-body interactions are known as the key to driving such transitions, the nature and extent of these interactions, and as importantly, their timescales, are poorly understood. This is primarily due to the lack of unambiguous measurement of the buildup and decay dynamics of the condensate in these systems. Here, we investigate the time-resolved dynamics of a condensate of molecular polaritons in a prototypical chromophore system. We use Excitation Correlation Photoluminescence spectroscopy (ECPL): a nonlinear, lock-in based method to estimate ultrafast population evolution through measurement of time-integrated emission. Intriguingly, we observe the condensate populating via radiative scattering from the reservoir in hundreds of femtoseconds, a timespan longer than the polariton lifetime. The condensate then decays over several picoseconds, but shorter than the reservoir lifetime. Although phenomenological rate equations can reproduce the formation and decay dynamics of the condensate, they cannot account for the measured density dependent trends. Complementary transient reflection measurements and steady-state momentum space PL measurements indicate that contributions from higher-energy momentum states may play a role in the discrepancy, highlighting the need for a more precise theoretical model.

Katherine Koch, Ph.D. Candidate — Kandada Research Group 

Exciton-exciton interactions are fundamental to the light-emitting properties of semiconductors, influencing applications from lasers to quantum light sources. In this study, we investigate the spectroscopic signatures and binding energy of biexcitons in a metal halide two-dimensional Ruddlesden-Popper structure, which is known for hosting distinct excitonic resonances (X1 and X2) with unique lattice coupling. These multiple resonances enable the formation of both self and cross-coupled biexcitons (mixed biexcitons). Using three spectroscopic techniques—photoluminescence (PL) and two variations of two-dimensional electronic spectroscopy (2DES)—we map coherent one-quantum and two-quantum correlations to gain deeper insight into the biexciton characteristics. While PL spectroscopy is hindered by spectral broadening and reabsorption, 2DES provides a more accurate characterization, revealing multiple biexciton states and uncovering the mixed biexciton species arising from exciton cross-coupling. These findings highlight the importance of advanced spectroscopic approaches in accurately determining biexciton binding energies and offer new perspectives on many-body interactions in exciton-polarons within layered perovskites. 

Ramesh Dhakal, Ph.D. Candidate — Winter Research Group 

We have recently developed an ab initio method to compute spin-phonon couplings of magnetic insulators [1]. By implementing this approach, we model the thermal hall effect in α-RuCl3 as arising from spin-phonon couplings [2]. We show that intrinsic phonon thermal hall effect can essentially explain the observed low temperature thermal hall effect in this compound. Our findings suggest that spin-orbit interactions lead to momentum dependent spin-phonon coupling, which generates finite phonon Berry curvature giving rise to phonon thermal hall effect.

[1] R. Dhakal, S. Griffith, K. Choi, and S. M. Winter, Spin-phonon coupling in transition metal insulators: General computational approach and application to MnPSe3, arXiv:2407.00659.

[2] R. Dhakal, D. A. S. Kaib, K. Choi, S. Biswas, R. Valenti, and S. M. Winter, Theory of Intrinsic Phonon Thermal Hall Effect in α-RuCl3, arXiv:2407.00660v3.

Owen Ganter, Ph.D. Candidate — Winter Research Group 

We have recently developed an ab initio method to compute spin-phonon couplings of magnetic insulators [1]. By implementing this approach, we model the thermal hall effect in α-RuCl3 as arising from spin-phonon couplings [2]. We show that intrinsic phonon thermal hall effect can essentially explain the observed low temperature thermal hall effect in this compound. Our findings suggest that spin-orbit interactions lead to momentum dependent spin-phonon coupling, which generates finite phonon Berry curvature giving rise to phonon thermal hall effect.

[1] R. Dhakal, S. Griffith, K. Choi, and S. M. Winter, Spin-phonon coupling in transition metal insulators: General computational approach and application to MnPSe3, arXiv:2407.00659.

[2] R. Dhakal, D. A. S. Kaib, K. Choi, S. Biswas, R. Valenti, and S. M. Winter, Theory of Intrinsic Phonon Thermal Hall Effect in α-RuCl3, arXiv:2407.00660v3.

Vincent Siggia, Ph.D. Candidate — Carlson’ Lab 

The expansion of the Universe in f(R, T) gravity is studied. By focusing on functions of the form f(R, T) = f1(R) + f2(T), we assert that present-day acceleration can be achieved if the functional form of f2(T) either grows slowly or falls as a function of T. In particular, we demonstrate that when f2(T) ∝ Tε for ε ≤ 0, the Universe transitions to exponential growth at late times, just as it does in the standard cosmological model. A comparison of predictions of this model with type Ia supernovae shows that these models fit the data as well or even slightly better than the standard cosmological model without increasing the number of parameters.

David Carchipulla-Morales, Ph.D. Candidate — Environmental Dynamics Lab 

Understanding land-atmosphere processes in tropical regions is important because these ecosystems store vast amounts of carbon and play a critical role in global water and energy cycles. Leaf Area Index (LAI) is a variable widely used in ecohydrology to model land-atmosphere processes, including evapotranspiration and gross primary productivity. Despite its crucial role in ecohydrology, high quality estimates of LAI have been elusive in tropical regions because of paucity of ground observations and persistent cloud cover that hamper remote sensing techniques. These challenges have yielded large variability in estimates of LAI and other remotely sensed vegetation indices in the tropics which result in underestimation and unrealistic seasonal trends. Furthermore, LAI algorithms for moderate spatial resolution satellite data (0.5 – 1 km) rely on lookup tables developed with little to no data from tropical ecosystems and cannot capture small scale heterogeneity in land cover. In this study, we present a method to produce a high spatial resolution LAI dataset for tropical ecosystems by using a random forest machine learning algorithm to downscale the Moderate Resolution Imaging Spectroradiometer (MODIS) MCD15A3H LAI product from 500 m to 30 m using Landsat satellite imagery and land use and land cover (LULC) datasets from Colombia, Costa Rica, Ecuador, and Peru. We produced a high-resolution LAI dataset for the dominant land cover types in the four study countries. Preliminary results in Costa Rica show that the machine learning algorithm can successfully produce LAI values at the spatial resolution of Landsat (i.e., 30 m pixel resolution). The spatial distribution of the high-resolution LAI estimates showed high values for forests, and low values for croplands and pasturelands. Thus, machine learning LAI estimates clearly distinguish between land cover classes not observed in MODIS LAI. Our LAI estimates also showed less temporal variability than MODIS which is expected for tropical forests. The results of this work will provide improved input data for Earth system models simulating carbon, energy, and water cycles in tropical ecosystems.

• Cumulative Effect of Orbital Resonances in Extreme-Mass-Ratio Inspirals

Edoardo Levati, Ph.D. Student — Cárdenas-Avendaño’s Lab 

• TMAO Osmoprotectant Interactions with E.coli 16S Ribosomal RNA using Molecular Dynamics 

Mitchell Turk, Ph.D. Student — Cho Group: Computational Biophysics

• Catalytic Formation of NO-Ferroheme

Thilini Karunarathna, Ph.D. Student — Kim-Shapiro’s Lab

• Investigating Tropical Forest Water Storage Dynamics across Climate and Topographic Gradients using a Water Mass Balance Approach

Conall O’Leary, Ph.D. Student — Environmental Dynamics Lab

• Protein Folding Transition States with Coarse-Grained Molecular Dynamics and Machine Learning

Gabriella Tamayo, Ph.D. Student — Cho Group: Computational Biophysics

• Approximate Killing Vectors on a General S2 Metric

Joseph Granlie, Ph.D. Student — Cook Group

• Polarization Signatures of Relativistic Hot Spots in the Kerr Spacetime

Pablo Ruales, Ph.D. Student — Cárdenas-Avendaño’s Lab

• Fluence dependence of ultrafast exciton dynamics in two dimensional perovskites

Bhargava Jogi, Ph.D. Student — Kandada Research Group

• Tuning the Adsorption Properties of Metal-Organic Frameworks through Coadsorbed Ammonia

Eric Chapman, Ph.D. Candidate — Materials Simulation Group

• Effects of Bad Cholesterol and Fibrin Stabilizing Factor on Blood Clot Crosslinking

Owais Kamran — Guthold's Lab

• Cumulative Effect of Orbital Resonances in Extreme-Mass-Ratio Inspirals

Edoardo Levati, Ph.D. Student — Cárdenas-Avendaño’s Lab 

• TMAO Osmoprotectant Interactions with E.coli 16S Ribosomal RNA using Molecular Dynamics 

Mitchell Turk, Ph.D. Student — Cho Group: Computational Biophysics

• Catalytic Formation of NO-Ferroheme

Thilini Karunarathna, Ph.D. Student — Kim-Shapiro’s Lab

• Investigating Tropical Forest Water Storage Dynamics across Climate and Topographic Gradients using a Water Mass Balance Approach

Conall O’Leary, Ph.D. Student — Environmental Dynamics Lab

• Protein Folding Transition States with Coarse-Grained Molecular Dynamics and Machine Learning

Gabriella Tamayo, Ph.D. Student  — Cho Group: Computational Biophysics

• Approximate Killing Vectors on a General S2 Metric

Joseph Granlie, Ph.D. Student  — Cook Group

• Polarization Signatures of Relativistic Hot Spots in the Kerr Spacetime

Pablo Ruales, Ph.D. Student  — Cárdenas-Avendaño’s Lab