Periodic Reporting for period 1 - CavityMag (Cavity quantum electrodynamics control of magnetic phases in twisted van der Waals heterostructures)
Período documentado: 2023-05-01 hasta 2025-05-31
The interaction between light and matter is the foundation for how we percieve the world. Information about our surroundings reach our eyes in the form of photons, the fundamental unit of light, where it is transformed into electrical impulses that can be interpreted by our brain. However, to collect information is not the only utility of light, as it can also be emitted to affect objects around us. Specifically, by tailored laser pulses inducing electric currents and mechanical vibrations, it is possible to control material properties with great precision, and to make materials magnetic or electrically conducting.
One problem with laser light is that the material is easily heated, and that the induced properties only survive for a short time after the pulse has passed. This makes laser light best suited to control electrically insulating materials, which are not as easily heated as conducting materials. It is also best adapted to “classical” states, which are states that are well described by classical statistical physics, and where quantum mechanical coherence (i.e. that particles move in a coordinated as opposed to independent fashion) plays a minor role. Since laser driving is not well adapted to manipulating quantum mechanical states, it is of great importance to find new and alternative ways to optically control such states. This could enable the development of more fault-tolerant components for quantum computing, which are predicted to be able to solve certain problems (in particular complex scientific problems involving quantum mechanics) more efficiently than ordinary computers.
In this project, I have developed methods where optical cavities are used to stabilize quantum mechanical states in materials embedded in the cavity. An optical cavity is like a small container for light, and consists in the simplest case of two parallel (high quality) mirrors. Because of quantum mechanical effects, the space between the mirrors is never empty, but filled with constant fluctuations of the electromagnetic field. By carefully designing the cavity, e.g. by varying its geometry and the constituent materials, it is possible to control the fluctuations of the field. As a material can sense the changes in the electromagnetic fluctuations, and adapt its properties in response to it, cavities can be used to control the embedded material. The key difference with laser based methods is that the cavity does not need to contain “real” photons, but will remain in its quantum mechanical ground state. The change in the material is thereby a purely quantum mechanical effect, in the sense that classical physics would describe the cavity as empty, and its effect on the material as none.
One problem with laser light is that the material is easily heated, and that the induced properties only survive for a short time after the pulse has passed. This makes laser light best suited to control electrically insulating materials, which are not as easily heated as conducting materials. It is also best adapted to “classical” states, which are states that are well described by classical statistical physics, and where quantum mechanical coherence (i.e. that particles move in a coordinated as opposed to independent fashion) plays a minor role. Since laser driving is not well adapted to manipulating quantum mechanical states, it is of great importance to find new and alternative ways to optically control such states. This could enable the development of more fault-tolerant components for quantum computing, which are predicted to be able to solve certain problems (in particular complex scientific problems involving quantum mechanics) more efficiently than ordinary computers.
In this project, I have developed methods where optical cavities are used to stabilize quantum mechanical states in materials embedded in the cavity. An optical cavity is like a small container for light, and consists in the simplest case of two parallel (high quality) mirrors. Because of quantum mechanical effects, the space between the mirrors is never empty, but filled with constant fluctuations of the electromagnetic field. By carefully designing the cavity, e.g. by varying its geometry and the constituent materials, it is possible to control the fluctuations of the field. As a material can sense the changes in the electromagnetic fluctuations, and adapt its properties in response to it, cavities can be used to control the embedded material. The key difference with laser based methods is that the cavity does not need to contain “real” photons, but will remain in its quantum mechanical ground state. The change in the material is thereby a purely quantum mechanical effect, in the sense that classical physics would describe the cavity as empty, and its effect on the material as none.
To describe the interaction between light and matter, inside a cavity, I have developed new advanced computational methods that account both for the material’s atomic structure, and for the specific form and nature of the cavity. In particular, I have developed methods to derive effective models for the low energy properties of magnetic materials, containing the magnetic and vibrational excitations of a solid, and their coupling to light. These model have been fully parameterized from first principles calculations, thereby enhancing their predictive power.
Using these methods, we have studied how cavities can be used to modify the properties of the antiferromagnetic materials RuCl3 and FePS3, both belonging to a class of layer quasi two-dimensional materials held together by weak interlayer forces, and known as van der Waals materials. For both these materials, we have found that the interaction with cavity fluctuations can turn the material ferromagnetic under suitable conditions. Importantly, these changes in the magnetic state correspond to a reconfiguration of material ground state, such that the effect is thermodynamically stable.
In developing these methods, we have also been able to address systems with strong interaction between the magnetic and vibrational excitations (called magnons and phonons), and systems driven by short laser pulses. In particular, we found that the coherent coupling between magnons and phonons can lend magnetic properties to the phonons, such that these vibrations respond to magnetic fields as if they carrier a magnetic moment. Such a coupling has been measured and found to agree well with our calculations in the materials FePS3, FePSe3 and FeBr2.
An even more exotic situation appears in the material NiI2, which is a so-called multiferroic with an intertwined magnetic order and electric polarization. Here the magnons aqcuire an electric dipole moment, such that their magnetic properties can be controlled with electric fields. By comparing detailed calculations with state of the art experiments we found this magnetoelectric coupling survives to ultrashort time scales, allowing fast manipulation of magnetic states via electric fields.
Using these methods, we have studied how cavities can be used to modify the properties of the antiferromagnetic materials RuCl3 and FePS3, both belonging to a class of layer quasi two-dimensional materials held together by weak interlayer forces, and known as van der Waals materials. For both these materials, we have found that the interaction with cavity fluctuations can turn the material ferromagnetic under suitable conditions. Importantly, these changes in the magnetic state correspond to a reconfiguration of material ground state, such that the effect is thermodynamically stable.
In developing these methods, we have also been able to address systems with strong interaction between the magnetic and vibrational excitations (called magnons and phonons), and systems driven by short laser pulses. In particular, we found that the coherent coupling between magnons and phonons can lend magnetic properties to the phonons, such that these vibrations respond to magnetic fields as if they carrier a magnetic moment. Such a coupling has been measured and found to agree well with our calculations in the materials FePS3, FePSe3 and FeBr2.
An even more exotic situation appears in the material NiI2, which is a so-called multiferroic with an intertwined magnetic order and electric polarization. Here the magnons aqcuire an electric dipole moment, such that their magnetic properties can be controlled with electric fields. By comparing detailed calculations with state of the art experiments we found this magnetoelectric coupling survives to ultrashort time scales, allowing fast manipulation of magnetic states via electric fields.
With the methods developed, we have established a general framework to describe the interaction of light and matter inside the cavity, and between magnetic and vibrational properties of matter. Our results have given new insights into the interaction between light and matter, and will be of great use to better understand how cavities can be used to control the state of embedded materials, and to stabilize exotic quantum states. We are currently working on implementing these methods in the open source density functional theory code Octopus, developed and maintained by the host group, such that these development will be of use to the larger research community. In this way, our results will be of large relevance both for fundamental and applied research, and for the development of next-generation quantum information devices.