Research

Research in the Zong Group aims to discover and understand microscopic processes that underlie novel phases of matter in a non-equilibrium system, probing open questions at the interface between strongly correlated materials and ultrafast science. Unlike photochemistry in molecular systems, light-induced transitions in correlated solids present additional complexities — alternatively viewed as opportunities — in achieving a surgical control of quantum matter. These complexities arise due to the presence of (i) multiple competing or coexisting orders, (ii) spatial heterogeneities, such as topological defects and phase separation in the nanoscopic or mesoscopic regime, and (iii) vastly different time- and energy scales ranging from electron-electron scattering (sub- to few fs) to structural rearrangement (up to µs).

To address these complexities, we design experiments to answer the following overarching questions:

How does spatial heterogeneity give rise to new light-induced order?

Creating nonequilibrium superconductivity or charge order seeded by transient topological defects
How to selectively manipulate competing or co-existing degrees of freedom to attain a specific phase or dynamics?

Nanomechanical engineering via ultrafast spin control in materials with strong spin-lattice coupling
What are design principles for integrating ultrafast dynamics into functional devices made out of quantum materials?

Bridging the timescale between electrical (≤ GHz) and optical (≥ THz) controls; tuning light-matter interaction via cavity QED

To address these complexities, we design experiments to answer the following overarching questions:

How does spatial heterogeneity give rise to new light-induced order?

Creating nonequilibrium superconductivity or charge order seeded by transient topological defects
How to selectively manipulate competing or co-existing degrees of freedom to attain a specific phase or dynamics?

Nanomechanical engineering via ultrafast spin control in materials with strong spin-lattice coupling
What are design principles for integrating ultrafast dynamics into functional devices made out of quantum materials?

Bridging the timescale between electrical (≤ GHz) and optical (≥ THz) controls; tuning light-matter interaction via cavity QED

Spatial heterogeneity in non-equilibrium transitions

Photoinduced phase transitions in solids are traditionally viewed as chemical reactions that proceed along a particular reaction coordinate in the phase space, where atoms move in a concerted manner towards their new positions. This picture is at odds with many equilibrium transitions where spatial heterogeneity is pronounced. Increasing evidence has shown that disorder plays a critical role in non-equilibrium transitions, where transient defects can even lead to new states of matter. We develop novel techniques to measure heterogeneities in either real or momentum space across a wide range of time scales, aiming to understand and control these spatial features as we explore hitherto unknown phases of matter.

Role of topological defects

Topological defects are singularities in an ordered medium. They constitute an essential feature of non-equilibrium transitions, such as in the celebrated Kibble-Zurek mechanism, and are known to host new states of matter upon their creation following intense photoexcitation. How do they form on the femtosecond timescale? What is their annihilation pathway? Can we control what type of defects are created by laser excitation? By studying defect dynamics, we aim to better understand non-equilibrium phase transitions and metastable states where topological defects play an important role in the property of quantum materials.

Pattern formation in correlated insulators

Along the time axis, the collapse of a Mott insulating gap represents the fastest known transition from an insulator to a metal. The fast timescale reflects the Coulomb nature of the energy gap, which is particularly sensitive to photoinduced carrier screening that occurs as fast as a few femtoseconds. Along the space axis, such insulator-metal transitions are often accompanied by a rich set of nanotextures, such as stripes observed in equilibrium, a result of the complex interaction between electrons and lattice ions. We are interested in a set of questions at the intersection between these time and space axes: What determines the characteristic speed that governs pattern formation? What sets the lengthscale of nanotextures and phase separation? How do we distinguish a continuous vs. discontinuous transition in this regime? Answers from our experiments help us construct more predictive theory about non-equilibrium phase transitions in the presence of spatial heterogeneity.

Role of topological defects

Pattern formation in correlated insulators

Ultrafast phase engineering using coupled degrees of freedom

A defining characteristic of quantum materials is the presence of proximal phases of matter involving co-existing or competing orders. We are interested in leveraging the interplay between coupled orders in a non-equilibrium setting to photo-engineer on-demand properties in correlated solids. Here, tailored pulses will be used in conjunction with tuning knobs in equilibrium, such as cavity quantum electrodynamics, static strain, and electric and magnetic fields. One particular focus is on 2D material platforms, whose electronic, lattice, and magnetic properties can be precisely controlled thanks to advances in thin film growth and device fabrication.

Selective enhancement of competing orders

Intense light pulses have emerged as a powerful tool to tune between neighboring phases of matter in quantum materials, where one phase is suppressed while another is enhanced. However, achieving the selectivity of which phase gets enhanced in a complex material is challenging. We are interested in integrating the ultrafast toolbox with special sample environment to target specific degrees of freedom in solids. Specifically, we utilize the effect of (i) strong mode coupling in a cavity and (ii) the extremely large electric field in ultrashort pulses to tune between proximal phases of matter, with a special focus on the lattice degree of freedom that underlies many phase transitions. This ultrafast cavity-based methodology combines the quantum nature of light and versatility of ultrashort pulses, offering a new avenue for precision control of microscopic degrees of freedom.

Nano-mechanical device via spin control

Recent discoveries of extraordinary spin-lattice and charge-lattice interactions in 2D crystals have opened new possibilities for controlling macroscopic structural properties. For example, spin configurations in 2D magnets sensitively depend on interlayer stacking and applied strain. In turn, nano-mechanical oscillators made of 2D magnets are found to exhibit unusual spin-dependent lattice responses. We are interested in new ways of nanomechanical control by using laser pulses to precisely manipulate spin in the ultrafast timescale. The goal is to design a specific light-triggered mechanical response in a nanomaterial, such as self-folding origami in a 2D sheet, where the ultrashort light pulse coupled with nanoscale size allows the mechanical motion to be extended to the GHz regime.

Overview

Research areas

Spatial heterogeneity in non-equilibrium transitions

Role of topological defects

Pattern formation in correlated insulators

Role of topological defects

Pattern formation in correlated insulators

Ultrafast phase engineering using coupled degrees of freedom

Selective enhancement of competing orders

Nano-mechanical device via spin control

Selective enhancement of competing orders

Nano-mechanical device via spin control