Chiral Floquet Engineering Induces Chirality-Dependent Momentum Shifts in Topological Fermions within CoSi Chiral Crystals

Keskeiset käsitteet

Circularly polarized light pumping can non-trivially manipulate the momentum of topological fermions in chiral crystals like CoSi, determined by a newly proposed Floquet chirality index, potentially enabling novel optoelectronic devices.

Tiivistelmä

Bibliographic Information:

Fan, B., Duan, W., Rubio, A., & Tang, P. (2024). Chiral Floquet Engineering on Topological Fermions in Chiral Crystals. arXiv preprint arXiv:2408.03115v2.

Research Objective:

This study investigates the impact of circularly polarized light (CPL) pumping on the electronic structure of topological fermions in chiral crystals, specifically focusing on the CoSi family. The authors aim to understand how the interplay between light chirality and crystal chirality affects the behavior of these fermions in a non-equilibrium state.

Methodology:

The researchers employ a two-pronged theoretical approach:

1. Floquet effective k · p model: This model, derived from perturbation theory, provides a simplified framework to analyze the light-dressed electronic structure under CPL pumping.
2. Floquet tight-binding Hamiltonian: This approach, based on ab initio calculations, offers a more robust and accurate description of the system, validating the findings of the k · p model.

Key Findings:

• CPL pumping induces momentum shifts in topological fermions located at the Γ and R points of the CoSi Brillouin zone.
• The direction of these shifts depends on the chirality of both the incident light and the topological fermion itself, captured by a newly introduced concept called the "Floquet chirality index (Ξk)."
• This index, defined as Ξk = γχk(η)A²/Ω, combines the chirality of the CPL (γ), the chirality of the topological fermion (χk(η)), the effective amplitude of the vector potential (A), and the frequency of the pumping laser (Ω).
• Despite the momentum shifts, the topological properties of the fermions, including their topological charges and the presence of Fermi arcs on the surface, remain preserved.

Main Conclusions:

• Chiral Floquet engineering offers a powerful tool to manipulate the electronic properties of topological materials in a non-equilibrium regime.
• The Floquet chirality index provides a framework to predict and control the direction of light-induced momentum shifts in topological fermions.
• This approach opens up possibilities for developing novel optoelectronic devices by leveraging the interplay between light and crystal chirality.

Significance:

This research significantly advances the field of Floquet engineering by introducing the concept of chirality as a new degree of freedom. The findings have profound implications for understanding and controlling the behavior of topological materials under light irradiation, potentially leading to advancements in ultrafast switching technologies and the development of innovative optoelectronic devices.

Limitations and Future Research:

• The study primarily focuses on the CoSi family of chiral crystals. Further investigation is needed to explore the applicability of these findings to other topological materials.
• Experimental validation of the predicted momentum shifts and the role of the Floquet chirality index is crucial. The authors propose techniques like time- and angle-resolved photoemission spectroscopy (TrARPES) and pump-probe Kerr or Faraday spectroscopy for this purpose.

Tilastot

The scattering time in CoSi single crystal is estimated to be around 131 fs.
A Mid-IR pumping laser with photon energy around 100 meV and an electric field intensity as large as 4.4 × 10^7 V/m is suggested for experimental validation.

Lainaukset

Tärkeimmät oivallukset

Chiral Floquet Engineering on Topological Fermions in Chiral Crystals

by Benshu Fan, ... klo arxiv.org 11-19-2024

https://arxiv.org/pdf/2408.03115.pdf

Syvällisempiä Kysymyksiä

How might the presence of defects or impurities in the chiral crystal affect the efficiency of chiral Floquet engineering and the observed momentum shifts?

Defects and impurities can significantly impact the efficiency of chiral Floquet engineering and the resulting momentum shifts of topological fermions in chiral crystals. Here's how:

Scattering and decoherence: Defects act as scattering centers, disrupting the coherent evolution of Floquet-Bloch states. This scattering introduces decoherence, effectively reducing the lifetime of these light-induced states and hindering the observation of clear momentum shifts. The efficiency of chiral Floquet engineering relies on the coherent interaction between light and the electronic system, and scattering directly diminishes this coherence.

Modification of electronic structure: Impurities and defects can locally alter the electronic band structure of the chiral crystal. This modification can lead to the emergence of localized states within the band gap or shifts in the energy and momentum of the original topological fermions. Consequently, the carefully engineered momentum shifts induced by CPL pumping might be obscured or deviate from the predicted values due to this altered electronic landscape.

Symmetry breaking: While chiral crystals inherently lack certain symmetries, defects can introduce further symmetry breaking. This localized symmetry breaking might lead to variations in the light-matter coupling strength and affect the Floquet chirality index Ξk in the vicinity of the defect. As a result, the momentum shifts of topological fermions could become spatially inhomogeneous, with variations depending on the proximity and nature of the defects.

Influence on topological protection: Topological properties, while robust against perturbations, are not entirely immune to disorder. A high concentration of defects can induce transitions between different topological phases or even destroy the topological properties altogether. In such scenarios, the chiral Floquet engineering scheme might become ineffective as the underlying topological protection, crucial for the well-defined momentum shifts, is compromised.
In summary, the presence of defects and impurities in chiral crystals introduces scattering, modifies the electronic structure, and can potentially break symmetries and affect topological protection. These factors can significantly impact the efficiency of chiral Floquet engineering, leading to reduced coherence, altered momentum shifts, and even the loss of topological properties. Therefore, minimizing defects and maintaining high crystal quality are crucial for the successful implementation and observation of chiral Floquet engineering in these materials.

Could the manipulation of topological fermion momentum via CPL pumping be harnessed for information processing or storage in future quantum technologies?

The ability to manipulate topological fermion momentum using CPL pumping in chiral crystals presents intriguing possibilities for information processing and storage in future quantum technologies. Here are some potential avenues:

Chiral Qubits: The distinct momentum states of topological fermions under CPL pumping could serve as a basis for encoding quantum information. For instance, a qubit could be defined by the superposition of a topological fermion with momentum shifted in one direction (representing |0⟩) and the same fermion with momentum shifted in the opposite direction (representing |1⟩). The inherent robustness of topological states against disorder could potentially offer advantages in terms of qubit coherence and stability.

Optical Control of Quantum States: CPL pumping provides a means to optically control the momentum and, consequently, the energy of topological fermions. This optical control could be harnessed to manipulate the interaction between these fermions, enabling the realization of quantum logic gates. By carefully tuning the polarization and duration of the CPL pulses, one could envision implementing operations like entanglement generation or single-qubit rotations.

Valleytronic Devices: Chiral crystals often exhibit valley degrees of freedom, where electrons in different valleys possess distinct momenta. CPL pumping, with its inherent chirality, could selectively address and manipulate electrons in specific valleys. This selectivity opens up possibilities for valleytronic devices, where information is encoded and processed using the valley index as a degree of freedom.

Topological Quantum Memory: The robustness of topological states against perturbations makes them attractive candidates for quantum memory applications. By encoding information in the momentum states of topological fermions, one could potentially store quantum information with enhanced protection against decoherence. CPL pumping could then be used to write, read, and manipulate this stored information.

Chiral Optoelectronics: The interplay between CPL pumping and chiral crystals could lead to novel optoelectronic devices. For example, the momentum shift of topological fermions could be utilized to generate photocurrents with controllable direction and magnitude depending on the polarization of the incident light. This could lead to the development of highly sensitive chiral photodetectors or efficient spin-polarized current sources.
While these concepts are still in their early stages, the unique properties of topological fermions and the ability to manipulate them with light offer a promising platform for exploring novel quantum technologies. Further research is needed to fully understand and harness the potential of chiral Floquet engineering for information processing and storage.

If we consider the analogy of light acting as a "pump" and the chiral crystal as a "circuit," what other functionalities beyond momentum control might be achievable through this interplay?

The analogy of light as a "pump" and the chiral crystal as a "circuit" provides a useful framework for envisioning functionalities beyond momentum control in chiral Floquet engineering. Here are some possibilities:

Dynamic Band Structure Engineering: Beyond simply shifting existing bands, CPL pumping could be used to dynamically create and manipulate entirely new band structures within the chiral crystal. By tailoring the time-dependent electromagnetic field, one could induce the emergence of Floquet-Bloch bands with desired properties, such as tailored effective masses, band gaps, or even topological invariants. This dynamic band structure engineering could lead to on-demand control over the electronic and optical properties of the material.

Optical Switching of Topological Phases: CPL pumping could drive transitions between different topological phases in chiral crystals. By tuning the laser parameters, one could induce a transition from a trivial insulator to a topological insulator, or switch between different topological phases with distinct properties. This optical control over topological phases could lead to ultrafast switching devices for low-power electronics or novel platforms for exploring topological phase transitions.

Generation of Entangled States: The interaction between CPL photons and the chiral crystal could be harnessed to generate entangled states between light and matter. For instance, one could envision schemes where the polarization state of a reflected or transmitted photon becomes entangled with the momentum state of a topological fermion within the crystal. Such entangled states are crucial resources for quantum information processing and communication.

Control of Spin Textures: Chiral crystals often host non-trivial spin textures, such as spin spirals or skyrmions. CPL pumping, with its inherent spin angular momentum, could be used to manipulate these spin textures. This could involve controlling their size, position, or even inducing their motion within the crystal. Such capabilities could be relevant for spintronics applications, where information is encoded and processed using the spin of electrons.

Nonlinear Optical Responses: The strong light-matter coupling achievable in chiral Floquet engineering could lead to enhanced nonlinear optical responses. This could involve the generation of high-harmonic frequencies, efficient frequency mixing, or even the realization of novel nonlinear optical phenomena unique to the driven topological system. These nonlinear effects could be utilized for applications in ultrafast optics, spectroscopy, or optical sensing.
In essence, the interplay between light and chiral crystals in the framework of chiral Floquet engineering offers a rich playground for exploring and manipulating quantum phenomena. By moving beyond simple momentum control and embracing the full potential of this dynamic interplay, we can envision a future where light is not just a passive probe but an active tool for shaping and controlling the properties of quantum materials.