Q: What initially drew you to the APS?

Shanti: I’ve worked in high-pressure physics since my graduate student days, but early in my career, I wasn’t using synchrotron X-ray techniques. Structural studies under extreme pressure were far less advanced at the time, so my work focused on transport, magnetic, and spectroscopic measurements.

Structural information is essential for understanding materials under pressure, and as my career advanced, I recognized that such studies are also important to my research. Ambient-pressure structure provides only a single data point. Once pressure is applied, materials can undergo significant atomic rearrangements, and without tracking those changes, it’s difficult to interpret how their properties evolve.

Through conferences and interactions with beamline scientists, I learned what was becoming possible at APS high-pressure beamlines. The combination of technical capability and close engagement with the user community ultimately drew me in as a long-term APS user.

Q: Can you describe the research you’ve conducted at the APS? 

Shanti: My research at the APS focuses on how materials behave under extreme pressure, particularly in systems where the competition between electronic structure and atomic arrangement plays a decisive role in determining physical properties. One major area of my work involves materials composed of very light elements, where quantum nuclear effects are particularly strong. 

Lithium is exceptionally challenging to study with X-rays, making the APS's capabilities essential. Advances in beam quality and high-pressure diffraction techniques have allowed us to probe its structure under conditions previously inaccessible, providing direct insight into how pressure pathways influence phase transitions and material stability.

A second major thrust of my work involves correlated electron systems and functional materials. These materials can exhibit collective electronic states, such as charge density waves or superconductivity, that evolve or disappear under pressure. X-ray diffraction is critical for linking those electronic changes to underlying structural transformations.

Some of the key approaches and contributions from my work at the APS include:

• Comparative isotope studies in lithium using side-by-side samples within a single diamond anvil cell to eliminate experimental ambiguities
• High-pressure diffraction experiments that track structural evolution across multiple pressure–temperature pathways
• Use of advanced diamond anvil cell techniques, including pressure tuning at low temperature, to access metastable phases
• Integration of diffraction with complementary probes, such as spectroscopy and Raman measurements, to resolve complex phase transitions

Q: What role did the APS play in enabling or advancing your work? 

Shanti: In many cases, the APS didn’t just support our research; it made it possible. Over time, improvements in beam quality, detectors, background subtraction, and sample environments transformed what we could observe. Capabilities such as gas-loading systems and contrast-enhancing pressure media allowed us to distinguish the sample signal from its surroundings and extract meaningful structural information.

What began as a complementary tool became central to the research itself. The APS enabled us to connect structure, pressure pathways, and physical properties in a way that simply wasn’t feasible elsewhere.

Q: Has anything unexpected come out of your work with the APS, either in your results or in the process itself?

Shanti: One of the most unexpected outcomes was observing strong isotope effects and path dependence in lithium’s phase transitions. By following alternative pressure and temperature routes, we could reconcile discrepancies between theory and experiment that had persisted for over 70 years.

That work helped resolve a long-standing controversy in the field and was published in several high-impact journals. It highlighted how sensitive some materials are to experimental pathways, not just final conditions.

We saw similar surprises in our studies of water at high pressure, where combining multiple probes allowed us to clarify the nature of metastable phases that had been interpreted differently using single techniques.

Q: What impact has your research at the APS had on your work so far? What are you excited about exploring next?

Shanti: Structural information is fundamental to understanding material behavior under pressure. Atomic rearrangements drive changes in electronic and magnetic properties, and high-pressure experiments allow us to access states of matter that do not exist under ambient conditions.

With the upgraded APS, we can now probe materials with much finer spatial resolution. Smaller beam spots allow us to map structures micron by micron inside a pressure cell, effectively creating detailed internal images of materials under extreme conditions.

Looking ahead, I’m excited about how these capabilities will contribute to large structural databases. When combined with machine learning tools, these data can help accelerate materials discovery and design, opening new directions for high-pressure research.