Periodic Reporting for period 1 - PhononMoments (Phonon Magnetic Moments and Angular Momentum in Flexible Framework Materials)
Reporting period: 2021-07-01 to 2023-06-30
When heat is added to a crystal, its atoms vibrate, generating tiny quantum excitations known as “phonons”. Understanding the behavior and properties of these phonons is crucial for advancing materials science and technology. When a phonon causes atoms to move in circles (see attached image), the phonon has an associated handedness, or chirality. Essentially, this means that phonons wherein the atoms rotate counter-clockwise can have distinct properties from those wherein the atoms move clockwise. These “chiral phonons” have only been intensely studied recently, and much remains unknown about how they affect the bulk properties of crystals.
One of the most fascinating aspects of chiral phonons is that they can generate magnetic fields. If the atoms are electrically charged, their circular motions act like a tiny electric induction coil, producing magnetism. Since the charges of atoms are generally small in comparison to their mass, it was thought that this effect would be almost undetectably small. However, recent experiments have indicated much larger phonon-generated magnetic fields than initially thought, although the reason for this is still not perfectly understood.
Since chiral phonons are still a relatively unstudied phenomenon, our research into them is principally fundamental in nature. If the properties of chiral phonons, and how they affect, for example, the magnetism or heat transfer of a material, are better understood, this could lead to new applications which cannot currently be foreseen. Indeed, one of these applications was unexpectedly discovered during the project, as described below.
The first major goal of this research project was to use computational and theoretical techniques to study the magnetism of chiral phonons in a variety of materials, in order to determine if careful selection of the crystal structure could enhance the magnetic fields generated by phonons. A second goal was to study phonon magnetism in chiral materials (materials which have an inherent handedness). In these materials, phonons with non-zero momentum can naturally have a handedness and magnetism, whereas most previous studies had focused on phonons with near-zero momentum since these can be created using light. A third goal was to use the resulting theoretical predictions to detect chiral phonons experimentally.
One of the most fascinating aspects of chiral phonons is that they can generate magnetic fields. If the atoms are electrically charged, their circular motions act like a tiny electric induction coil, producing magnetism. Since the charges of atoms are generally small in comparison to their mass, it was thought that this effect would be almost undetectably small. However, recent experiments have indicated much larger phonon-generated magnetic fields than initially thought, although the reason for this is still not perfectly understood.
Since chiral phonons are still a relatively unstudied phenomenon, our research into them is principally fundamental in nature. If the properties of chiral phonons, and how they affect, for example, the magnetism or heat transfer of a material, are better understood, this could lead to new applications which cannot currently be foreseen. Indeed, one of these applications was unexpectedly discovered during the project, as described below.
The first major goal of this research project was to use computational and theoretical techniques to study the magnetism of chiral phonons in a variety of materials, in order to determine if careful selection of the crystal structure could enhance the magnetic fields generated by phonons. A second goal was to study phonon magnetism in chiral materials (materials which have an inherent handedness). In these materials, phonons with non-zero momentum can naturally have a handedness and magnetism, whereas most previous studies had focused on phonons with near-zero momentum since these can be created using light. A third goal was to use the resulting theoretical predictions to detect chiral phonons experimentally.
Using ab inito calculations of the vibrational properties of a variety of materials, we determined that the magnetic properties of phonons can be substantially amplified in flexible framework materials. These materials are stiff in some directions, giving them stability, but soft or floppy in others, giving them the ability to withstand large displacements. We found that as the flexibility of the materials we studied increased, so did the range of atomic displacements, ultimately allowing a greater number of phonon modes to exhibit phonon magnetism. Flexibility also allows a material to host more phonons (and therefore create a larger phonon-generated magnetic field) before it melts.
Our investigation into flexible framework materials also led to a significant finding: these materials can host chiral phonons with very low energies, which can be stable for relatively long times. This is attributed to the materials' structural flexibility, enabling circular atomic motions that extend into free space. Exploiting this discovery, we proposed a new approach for detecting dark matter. Dark matter is the unknown substance which makes up 85% of the mass of the Universe; it is thought to possibly be made up of almost undetectable particles which very rarely deposit energy into normal matter. Our detector concept would sense the magnetic moments of phonons created by dark matter–normal matter interactions using highly sensitive detectors of magnetic fields called superconducting quantum interference devices (SQUIDs). A preprint describing this proposal is available on the arXiv (arXiv:2301.07617).
Collaborating with experimentalists at the Paul Scherrer Institute, we realized another milestone by directly observing that chiral phonons possess angular momentum (or “spin”). This was accomplished by firing circularly polarized X-rays, which carry angular momentum, at a sample of quartz and measuring the change in energy of the X-rays after they deposit energy in the sample by creating phonons. We found that left- and right-handed light created different numbers of phonons, creating a pattern that was reversed when left-handed quartz was measured instead of right-handed quartz. This finding corresponded to our theoretical predictions, and showed that phonons in chiral materials are generally chiral. This experiment did not measure the phonon magnetism, but the “spin” of the phonon is thought to be able to interact with the spin of electrons in the material, thereby creating magnetic fields. This research was published in the journal Nature (vol. 618, p. 946–950 (2023)).
Our investigation into flexible framework materials also led to a significant finding: these materials can host chiral phonons with very low energies, which can be stable for relatively long times. This is attributed to the materials' structural flexibility, enabling circular atomic motions that extend into free space. Exploiting this discovery, we proposed a new approach for detecting dark matter. Dark matter is the unknown substance which makes up 85% of the mass of the Universe; it is thought to possibly be made up of almost undetectable particles which very rarely deposit energy into normal matter. Our detector concept would sense the magnetic moments of phonons created by dark matter–normal matter interactions using highly sensitive detectors of magnetic fields called superconducting quantum interference devices (SQUIDs). A preprint describing this proposal is available on the arXiv (arXiv:2301.07617).
Collaborating with experimentalists at the Paul Scherrer Institute, we realized another milestone by directly observing that chiral phonons possess angular momentum (or “spin”). This was accomplished by firing circularly polarized X-rays, which carry angular momentum, at a sample of quartz and measuring the change in energy of the X-rays after they deposit energy in the sample by creating phonons. We found that left- and right-handed light created different numbers of phonons, creating a pattern that was reversed when left-handed quartz was measured instead of right-handed quartz. This finding corresponded to our theoretical predictions, and showed that phonons in chiral materials are generally chiral. This experiment did not measure the phonon magnetism, but the “spin” of the phonon is thought to be able to interact with the spin of electrons in the material, thereby creating magnetic fields. This research was published in the journal Nature (vol. 618, p. 946–950 (2023)).
Our research has extended the state of the art in the field of phonon magnetism in several ways. Firstly, our results have greatly broadened the set of materials in which phonon magnetic effects have been studied, and suggests some new materials which could have significantly enhanced effects. One potential application of this result is to create faster electronic devices, where phonons can be quickly and easily generated using light and then used to switch the magnetic bits used in computers.
Secondly, our recognition of chiral phonons as potentially extremely sensitive detectors for dark matter (or other applications in particle physics) opens new prospects for understanding the fundamental nature of the universe. These detectors would have sensitivities many order of magnitude better than existing and proposed technology.
The experiment performed to detect chiral phonons using X-rays was the first of its kind, and therefore represents a new state of the art in the experimental investigation of phonons. This new technique could be used to validate theoretical predictions regarding phonon chirality, discover new physics, or even be applied to determine if an unknown material is chiral.
Secondly, our recognition of chiral phonons as potentially extremely sensitive detectors for dark matter (or other applications in particle physics) opens new prospects for understanding the fundamental nature of the universe. These detectors would have sensitivities many order of magnitude better than existing and proposed technology.
The experiment performed to detect chiral phonons using X-rays was the first of its kind, and therefore represents a new state of the art in the experimental investigation of phonons. This new technique could be used to validate theoretical predictions regarding phonon chirality, discover new physics, or even be applied to determine if an unknown material is chiral.