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Quantum materials exhibit remarkable emergent properties when they are excited by external sources. Functional applications of these properties rely heavily on their tunability in real time. However, these excited states decay rapidly once the excitation is removed, limiting their practical applications.

A collaborative team led by experimentalists from Harvard University and theorists from Emory University, in collaboration with the advanced X-ray facilities at the Paul Scherrer Institute (PSI), has now demonstrated an approach to stabilize these fleeting quantum states through a symmetry-protected metastable state. This exotic behavior is probed using bright X-ray flashes from the X-ray free electron laser SwissFEL at PSI. The findings in the journal Nature Materials.

Some materials exhibit fascinating quantum properties that can lead to transformative technologies, from lossless electronics to high-capacity batteries. However, when these materials are in their natural state, these properties remain hidden, and scientists need to gently ask for them to pop up.

One way they can do this is by using ultrashort pulses of light to alter the microscopic structure and electronic interactions in these materials so that these functional properties emerge in a predictive manner at an ultrashort timescale. But good things do not last forever—these light-induced states are transient, typically persisting only a few picoseconds, making them difficult to harness in practical applications. In rare cases, light-induced states become long-lived. Yet our understanding of these phenomena remains limited, and no general framework exists for designing excited states that last.

A team of scientists from Harvard University, Emory University, together with PSI colleagues overcame this challenge by manipulating the symmetry of electronic states in a copper oxide compound. Using the X-ray free electron laser SwissFEL at PSI, they demonstrated that tailored optical excitation can induce a 'metastable' non-equilibrium electronic state persisting for several nanoseconds—about a thousand times longer than they usually last for.

Steering electrons with light

The compound under study, Sr₁₄Cu₂₄O₄₁—a so-called cuprate ladder—is nearly one-dimensional. It is composed of two distinct structural units, the so-called ladders and chains, representing the shape in which copper and oxygen atoms organize. This one-dimensional structure offers a simplified platform to understand complex physical phenomena that also show up in higher-dimensional systems.

"This material is like our fruit fly. It is the idealized platform that we can use to study general quantum phenomena," comments experimental condensed matter physicist Matteo Mitrano from Harvard University, who led the study.

One way to achieve a long-lived ("metastable") non-equilibrium state is to trap it in an energy well from which it does not have enough energy to escape. However, this technique risks inducing structural phase transitions that change the material's molecular arrangement, and that is something Mitrano and his team wanted to avoid.

"We wanted to figure out whether there was another way to lock the material in a non-equilibrium state through purely electronic methods," explains Mitrano. For that reason, an alternative approach was proposed.

In this compound, the chain units hold a high density of electronic charge, while the ladders are relatively empty. At equilibrium, the symmetry mismatch of the electronic orbitals at low energies prevents any movement of charges between the two units.

"Such a symmetry mismatch is quite common in transitional-metal compounds, where the highest occupied orbital and lowest unoccupied orbital have opposite signs under a symmetry operation," comments theoretical physicist Yao Wang from Emory University, who established the underlying theory. "This mismatch is just more robust in cuprates, due to a special copper-oxide bonding state."

While such a symmetry mismatch prevents any charge transfer between the ladder and chain, a precisely engineered laser pulse breaks this symmetry, allowing charges to quantum tunnel from the chains to the ladders when the laser is on. "It's like switching on and off a valve," explains Mitrano.

Once the laser excitation is turned off, the tunnel connecting ladders and chains shuts down, cutting off the communication between these two units. This mechanism effectively traps the system in a new long-lived nonequilibrium state for some time that allows scientists to measure and use its properties.

Cutting-edge fast X-ray probes

The ultra-bright femtosecond X-ray pulses generated at the SwissFEL allowed the ultrafast electronic processes governing the formation and subsequent stabilization of the metastable state to be caught in action.

Using a technique known as time-resolved Resonant Inelastic X-ray scattering (tr-RIXS) at the SwissFEL Furka endstation, researchers can gain unique insight into magnetic, electric, and orbital excitations—and their evolution over time—revealing properties that often remain hidden to other probes.

"We can specifically target those atoms that determine the physical properties of the system," comments Elia Razzoli, group leader of the Furka endstation and responsible for the experimental setup.

This capability was key to dissecting the light-induced electronic motion that gave rise to the metastable state. "With this technique, we could observe how the electrons moved at their intrinsic ultrafast timescale and hence reveal electronic metastability," adds Hari Padma, postdoctoral scholar at Harvard and lead author of the paper.

The first of many more to come

tr-RIXS gives unique insight into energy and momentum dynamics of excited materials, opening new scientific opportunities for users of SwissFEL in studying quantum materials; indeed, these results come from the first experiment conducted by a user group at the new Furka endstation. It was the interest in the development of tr-RIXS at Furka that motivated the Harvard team to collaborate with scientists at PSI.

"It's a rare opportunity to get time on a machine where you can do these sorts of experiments," comments Mitrano.

To confirm this exotic symmetry-protected metastability mechanism, this collaborative team has carefully combined three different spectral techniques, including ultrafast optical reflectivity, ultrafast X-ray absorption, and tr-RIXS, with multiple theories and simulations to quantitatively confirm it. "In quantum materials, many simple assumptions can turn out impractical due to the complexity of the system. "We used both large-scale quantum many-body simulations and ab initio quantum chemistry methods to convince ourselves that the experimentally observed phenomena are exactly the symmetry-protected metastability," concludes Wang.

This work represents a major step forward in controlling quantum materials far from equilibrium, with broad implications for future technologies. By stabilizing light-induced non-equilibrium states, the study opens new possibilities for designing materials with tunable functionalities. This could enable ultrafast optoelectronic devices, including transducers that convert electrical signals to light and vice versa—key components for quantum communication and photonic computing. It also offers a pathway towards non-volatile information storage, where data is encoded in quantum states created and controlled by light.