Thermodynamic explanation of the invar effect

Most materials expand when heated because atomic vibrations become stronger at higher temperatures, creating an inner pressure that causes the volume to expand. In 1895, however, a Swiss scientist called C.E. Guillaume combined iron and nickel to discover a material with near-zero thermal expansion, named invar. This discovery was awarded the 1920 physics Nobel prize and sparked thousands of scientific investigations. Since the anomalous invar effect is associated with magnetism, nearly all studies have focused on the electronic and spin structure of Fe-Ni. However, without including atomic vibrations (phonons), the proposed models have not been successful in describing the anomalous invar behavior. As a result, this material which has been widely used by the industry over the past 100 years, was only qualitatively understood by scientists.

Using synchrotron radiation, I proposed an experimental method able to isolate the effects of atomic vibrations and spins on the thermal expansion of invar, providing a holistic picture of this anomaly for the first time. These experiments show that the invar behavior stems from a competition between phonons and spins that precisely balance each other: atomic vibrations expand the material while the spin causes a contraction. The result is an unchanging volume characteristic of all invar materials. Additionally, with the support of calculations, we observe a coupling between spins and atomic vibration that is essential for this precise cancelation. Based on the experiments we propose a novel thermodynamic explanation of the invar effect, that can be read in full in the November 2023 edition of Nature Physics.

The role of phonons in the spin dynamics of molecular qubits & single-molecule magnets

Quantum bits:

Quantum information is fundamentally different from the binary system currently used for computation. Instead of a binary bit of information that is either 0 or 1, quantum bits (or qubits) can be 0 or 1, but also 0 and 1 at the same time, a mind-boggling quantum effect called superposition, that opens the possibility for much more powerful calculations. This idea has been around since the 80’s, so why are we seeing quantum computers in stores yet? The challenge is to find qubit-systems that reliably exist in these superposition states.

One idea is to use the spin of electrons as qubits because they intrinsically exist in a superposition of two states (spin-up and spin-down). What prevents them from being used in functional technology is the limited lifetime of their superposition states, known as the coherence time. In a material or molecule, electronic spins don’t live in isolation but are surrounded by other atoms, so they exchange energy with other spins nearby and lose energy to vibrational motion of atoms.

Using different spectroscopic techniques, I am currently investigating how such atomic vibrations affect the lifetime of molecular qubits.

Single-molecule magnets:

There is a related quantum phenomenon that could be harnessed as long-term quantum memory devices. If molecules have more than one unpaired electron at the central atom, these electrons interact with each other which can cause their spins to get trapped in a specific state (either 0 or 1). Such molecules are called single-molecule magnets that could be used for storing information, representing the ultimate miniaturization of storage devices. Here, atomic vibrations can affect the electronic states ‘un-trapping’ their spins and causing the information to get lost. I’m currently studying these vibrations to build strategies that minimize the transition between the 0 and 1 spin states.

Our precise understanding and control over quantum phenomena promises a future where quantum information, computing, sensing, and communication are part of our daily lives. We are still far away from such a reality, however, and scientists are currently looking for appropriate systems to carry out quantum operations.