Periodic Reporting for period 2 - HiggS2 (Higgs mode in Superconductors)
Reporting period: 2022-06-01 to 2023-11-30
Quantum materials host fascinating properties. Understanding their origins, the interaction between coexisting quantum phases and their tuning parameters is central for the future uses of such properties. Collective modes, that emerge when a phase transition to a quantum order sets, are witnesses of their underlying quantum order. Studying such collective modes is then crucial for a global understanding of the phenomena.
The HiggS² project particularly focus on the quest and the study of one type of collective mode of the superconducting state, the Higgs mode. A mode of Higgs type, of crucial importance in the standard model of elementary particle physics, is also a fundamental collective mode in quantum many-body systems. It is remarkable that such collective Higgs modes predicted by theory many years ago, and lying at the core of the electronic properties of several classes of materials (cold atoms, superconductors, magnetic materials ...) still remain elusive to experimental validation. Besides, many crucial questions remain open, like the mechanism of its observability, its mere existence in two-dimensional or unconventional superconductors, to name a few. Similarly to the Higgs boson, its detection remains a challenge for physicists. Our interest is then double: we aim at finding new example of Higgs mode in superconductors and at establishing a platform to use the Higgs mode as a probe of the superconducting state it-self. In a larger perspective, we expect cross-fertilization with High Energy Physics.
The project will use a potential mechanism of observability making the Higgs mode detectable by Raman spectroscopy, based on the interplay between an adjacent quantum order and superconductivity. The overall objective of the project is to contribute to the quest of new observations of Higgs mode in superconductors, to study them in a large variety of situations, by changing the type of adjacent quantum orders, by changing the type of superconducting orders or by changing the dimensionality. For this, we will use advanced Raman spectroscopy, notably under extreme conditions, beyond the state-of-the-art experiments. This project will establish definitive examples of the observation and mechanism of observability of the Higgs mode in superconductors, while developing its phenomenology to address major problems at the frontier of quantum materials research, with impact beyond mere superconductivity and even condensed matter.
The HiggS² project particularly focus on the quest and the study of one type of collective mode of the superconducting state, the Higgs mode. A mode of Higgs type, of crucial importance in the standard model of elementary particle physics, is also a fundamental collective mode in quantum many-body systems. It is remarkable that such collective Higgs modes predicted by theory many years ago, and lying at the core of the electronic properties of several classes of materials (cold atoms, superconductors, magnetic materials ...) still remain elusive to experimental validation. Besides, many crucial questions remain open, like the mechanism of its observability, its mere existence in two-dimensional or unconventional superconductors, to name a few. Similarly to the Higgs boson, its detection remains a challenge for physicists. Our interest is then double: we aim at finding new example of Higgs mode in superconductors and at establishing a platform to use the Higgs mode as a probe of the superconducting state it-self. In a larger perspective, we expect cross-fertilization with High Energy Physics.
The project will use a potential mechanism of observability making the Higgs mode detectable by Raman spectroscopy, based on the interplay between an adjacent quantum order and superconductivity. The overall objective of the project is to contribute to the quest of new observations of Higgs mode in superconductors, to study them in a large variety of situations, by changing the type of adjacent quantum orders, by changing the type of superconducting orders or by changing the dimensionality. For this, we will use advanced Raman spectroscopy, notably under extreme conditions, beyond the state-of-the-art experiments. This project will establish definitive examples of the observation and mechanism of observability of the Higgs mode in superconductors, while developing its phenomenology to address major problems at the frontier of quantum materials research, with impact beyond mere superconductivity and even condensed matter.
We have worked on three work packages (WP): WP-A, where we have focused on the study of various adjacent orders to the superconducting state; WP-B, where we have looked at collective modes in unconventional superconductor; and WP- C, where we have fabricated and characterized 2D heterostructures that are expected to host a Higgs mode.
The central enabling objective of all the WP of HiggS² project is the realization of a beyond state-of-the-art platform for Raman spectroscopic technique measurements under extreme conditions. So far, the main achievements of the HiggS² project are in fact related to technical advances for Raman spectroscopy under extreme conditions. We have developped and installed a new set-up able to measure Raman excitations down to 3 cm-1 at ambiant pressure and down to 5 cm-1 under pressure, both in condition of low temperature. In collaboration with the high field lab (LNCMI) in Grenoble, we developped an other unique set-up for optical measurements under pressure and under high magnetic field, down to low temperature. First results on the coupling between phonon and magnons using this set-up were obtained. We also offer to the scientific community a new software, fully accessible on-line, to calculate the selection rules of Raman spectroscopy, based on group theory.
In WP-A, we have finalized the study of a superconducting compound whose adjacent order turns from incommensurate to commensurate when lowering the temperature or increasing pressure. We unveiled a new kind of collective mode of the adjacent order, a mixed of amplitude (Higgs-type) and phase mode. This was predicted in the 80's and revealed here for the first time. A second study we performed is related to the claimed "near ambient conditions superconductor" published in Nature in 2023. Motivated by the controversy, the ethical questions raised by this report as well as by the possibility to study a compound hosting an adjacent order (from the Raman results) and superconductivity at room temperature, we synthetized a similar compound as the authors with two cubic structures under pressure, one being the known cubic LuH2+x, whilst the other phase has been identified to be a novel type of cubic structure, which might be the result of the entrance in an adjacent order.
In WP-B, we have looked for collective mode in prototypical unconventional superconductor, the High-Tc cuprates. Using Raman spectroscopy under very high magnetic field, our attempts to reach the temperature-magnetic field region of interest to find a new Higgs mode failed. Nevertheless, striking results were obtained: we unveiled a Raman mode at low energy which presents unusual magnetic field and temperature dependencies. Our present understanding points to a mode due to the presence of the adjacent order, which will fold the momentum-space when established.
In WP-C, we have fabricated first few layers samples whose bulk system hosts demonstrated Higgs mode. We are currently investigating a new route to study Higgs modes in 2D in bulk compounds.
The central enabling objective of all the WP of HiggS² project is the realization of a beyond state-of-the-art platform for Raman spectroscopic technique measurements under extreme conditions. So far, the main achievements of the HiggS² project are in fact related to technical advances for Raman spectroscopy under extreme conditions. We have developped and installed a new set-up able to measure Raman excitations down to 3 cm-1 at ambiant pressure and down to 5 cm-1 under pressure, both in condition of low temperature. In collaboration with the high field lab (LNCMI) in Grenoble, we developped an other unique set-up for optical measurements under pressure and under high magnetic field, down to low temperature. First results on the coupling between phonon and magnons using this set-up were obtained. We also offer to the scientific community a new software, fully accessible on-line, to calculate the selection rules of Raman spectroscopy, based on group theory.
In WP-A, we have finalized the study of a superconducting compound whose adjacent order turns from incommensurate to commensurate when lowering the temperature or increasing pressure. We unveiled a new kind of collective mode of the adjacent order, a mixed of amplitude (Higgs-type) and phase mode. This was predicted in the 80's and revealed here for the first time. A second study we performed is related to the claimed "near ambient conditions superconductor" published in Nature in 2023. Motivated by the controversy, the ethical questions raised by this report as well as by the possibility to study a compound hosting an adjacent order (from the Raman results) and superconductivity at room temperature, we synthetized a similar compound as the authors with two cubic structures under pressure, one being the known cubic LuH2+x, whilst the other phase has been identified to be a novel type of cubic structure, which might be the result of the entrance in an adjacent order.
In WP-B, we have looked for collective mode in prototypical unconventional superconductor, the High-Tc cuprates. Using Raman spectroscopy under very high magnetic field, our attempts to reach the temperature-magnetic field region of interest to find a new Higgs mode failed. Nevertheless, striking results were obtained: we unveiled a Raman mode at low energy which presents unusual magnetic field and temperature dependencies. Our present understanding points to a mode due to the presence of the adjacent order, which will fold the momentum-space when established.
In WP-C, we have fabricated first few layers samples whose bulk system hosts demonstrated Higgs mode. We are currently investigating a new route to study Higgs modes in 2D in bulk compounds.
The technical work we have done so far will be crucial for the future studies in all WPs. We will continue to push the limits of the experiments, notably for measurements under stress and access to lower temperature. The HiggS² project progresses on technical advances for Raman spectroscopy under high pressure and high magnetic field are beyond the state-of-the-art. To our knowledge, this set-up is unique worldwide.
Till the end of the project, thanks to the access to lower temperature and by targeting the necessary conditions (T-P-H-stress), we will pursue our studies of Higgs modes in superconductors coexisting with various adjacent orders (WP-A) and various type of superconductivity (WP-B). In WP-B, the discovery of the low-energy mode in cuprates compounds opened a new interesting topic, which is clearly a step forward in the state of the art of the field of High-Tc cuprates. We will finalize this study to reach a full understanding of the nature of this mode. In WP-C, we will rely on our acquired know-how to fabricate the 2D heterostructures, while keep investigating the new route on bulk systems. We expect to be able to observe demanding Higgs mode in 2D world in at least one of the situations.
Till the end of the project, thanks to the access to lower temperature and by targeting the necessary conditions (T-P-H-stress), we will pursue our studies of Higgs modes in superconductors coexisting with various adjacent orders (WP-A) and various type of superconductivity (WP-B). In WP-B, the discovery of the low-energy mode in cuprates compounds opened a new interesting topic, which is clearly a step forward in the state of the art of the field of High-Tc cuprates. We will finalize this study to reach a full understanding of the nature of this mode. In WP-C, we will rely on our acquired know-how to fabricate the 2D heterostructures, while keep investigating the new route on bulk systems. We expect to be able to observe demanding Higgs mode in 2D world in at least one of the situations.