Ultrafast Anomalous Hall Current Generation and Detection in Bulk GaAs via Terahertz Emission: Influence of Landau Levels and Valence Band Contributions

Centrala begrepp

This research paper investigates the generation of anomalous Hall currents (AHCs) in bulk GaAs under the influence of Landau levels, revealing discrete AHC enhancements at Landau transition energies, even at room temperature, attributed to the energy dependence of geometric phases and valence band contributions.

Sammanfattning

• Bibliographic Information: Dresler, C., Priyadarshi, S., & Bieler, M. (n.d.). Anomalous Hall currents from optical excitation of Landau transitions in bulk GaAs.
• Research Objective: This study investigates the influence of Landau levels on anomalous Hall currents (AHCs) in bulk GaAs, utilizing terahertz (THz) radiation measurements to analyze the temporal and spectral dynamics of AHCs under varying magnetic fields and temperatures.
• Methodology: The researchers employed a pump-probe setup with ultrafast optical excitation to generate AHCs in (001) and (110) oriented GaAs samples. They applied varying magnetic fields (up to 7 T) and temperatures (down to 10 K) while measuring the emitted THz radiation, which directly correlates to the AHC dynamics. A theoretical model based on Boltzmann transport equations was developed to simulate and interpret the experimental results.
• Key Findings: The study reveals a discretization of AHCs corresponding to optical transitions between valence and conduction Landau bands. This discretization manifests as peaks in THz signal amplitude at specific magnetic field and photon energy combinations, directly aligning with Landau transition energies. Surprisingly, these Landau features are observable even at room temperature, attributed to the ultrafast, local probing nature of the THz detection technique. The research also highlights the distinct roles of valence and conduction bands in AHC generation. While the valence band contribution appears to follow the optical pulse envelope, the conduction band contribution involves cyclotronic motion, leading to observable differences in the THz spectra. Notably, the valence band contribution to AHC seems to decrease with increasing magnetic fields, potentially due to reduced valence band mixing.
• Main Conclusions: The study demonstrates the significant influence of Landau levels on AHCs in bulk GaAs, providing insights into the underlying physics of AHC generation. The observation of Landau features at room temperature using THz emission opens possibilities for exploring quantum phenomena in more accessible conditions. The distinct roles of valence and conduction bands, particularly the magnetic field dependence of valence band contributions, highlight the complex interplay of band structure and AHC dynamics.
• Significance: This research contributes significantly to the field of spintronics and the understanding of AHCs, particularly in the context of Landau quantization. The findings have implications for developing novel spintronic devices and exploring quantum transport phenomena in semiconductors.
• Limitations and Future Research: The theoretical model, while qualitatively explaining the observed phenomena, relies on certain assumptions and simplifications. Further refinement of the model, incorporating more detailed band structure calculations and scattering mechanisms, could provide a more accurate description of AHC dynamics. Additionally, investigating AHCs in other material systems and exploring the potential for manipulating AHCs through external stimuli could lead to new discoveries and applications.

Statistik

The GaAs samples used were 350 µm thick.
The cryogenic base temperature of the samples was assumed to be 10 K.
The pump pulses used had a pulse width of 140 fs and a repetition rate of 76 MHz.
The peak intensity of the pump pulses was 10 MW/cm2.
The suppression of THz signals from currents flowing in an unwanted direction was approximately 400:1.
At a magnetic field of 3 T, the cyclotron frequency of electrons is approximately 1.33 THz.

Citat

"In this paper, being motivated by the so far unstudied influence of Landau levels on AHCs, we extend the previous THz investigations of Refs. [16,18] to cryogenic temperatures down to 10 K and strong magnetic fields up to 7 T."
"Surprisingly, we find that the presence of Landau features in AHC measurements is not limited to cryogenic temperatures but appears in room temperature experiments, too."
"These observations endorse the sensitivity of THz-based studies of ultrafast photocurrents."
"Analyzing the shape of THz radiation from AHCs and comparing them to a model of AHCs based on the Boltzmann equations, we identify differences in time and frequency domain between the AHC contributions from the valance band (which follows the optical pulse envelope) and from the conduction band (which involves cyclotronic motion of electrons)."
"Moreover, our results strongly suggest that the valance band contribution to the AHC decreases for increasing magnetic fields."

Viktiga insikter från

Anomalous Hall currents from optical excitation of Landau transitions in bulk GaAs

by Christoph Dr... på arxiv.org 10-08-2024

https://arxiv.org/pdf/2410.05170.pdf

Djupare frågor

How might the observed Landau-quantized AHCs be leveraged in the development of novel spintronic devices, particularly those operating at room temperature?

The observation of Landau-quantized anomalous Hall currents (AHCs) at room temperature in bulk GaAs holds exciting possibilities for spintronic devices. Spintronics aims to exploit the spin of electrons, in addition to their charge, for information processing and storage. Here's how this research could be leveraged:

Spin-polarized current sources: The AHC generation mechanism relies on the optical excitation of spin-polarized carriers. This suggests the possibility of developing efficient spin-polarized current sources operating at room temperature. By tuning the excitation laser's wavelength and the applied magnetic field, one could control the spin polarization of the generated current.
Spin transistors: One of the holy grails of spintronics is the development of a spin transistor, analogous to the ubiquitous charge-based transistor. The observed Landau quantization of AHCs could be exploited to create spin filters or spin valves, essential components of a spin transistor. By manipulating the magnetic field, one could potentially switch the AHC on or off, thereby controlling the flow of spin-polarized currents.
High-frequency spintronic devices: The ultrafast nature of the AHC generation and detection, on the order of picoseconds, opens up avenues for high-frequency spintronic devices. The THz frequencies involved could lead to faster data processing and communication speeds compared to conventional electronics.
Integration with existing semiconductor technology: The use of bulk GaAs, a well-studied and widely used semiconductor material, makes it potentially easier to integrate these findings with existing semiconductor technologies. This could accelerate the development and deployment of practical spintronic devices.
However, several challenges need to be addressed before realizing these applications. These include improving the efficiency of spin injection and transport, reducing the required magnetic field strengths, and developing scalable fabrication techniques.

Could other factors beyond valence band mixing, such as strain or interactions with phonons, contribute to the observed decrease in valence band AHC contributions at higher magnetic fields?

Yes, besides valence band mixing, other factors like strain and phonon interactions could contribute to the observed decrease in valence band AHC contributions at higher magnetic fields. Here's a breakdown:

Strain: Strain in the GaAs crystal lattice can modify the band structure, affecting band mixing and potentially reducing the Berry curvature and side-jump contributions from the valence band. As the magnetic field increases, the Landau levels become further apart in energy. Strain effects, which are generally more pronounced at smaller energy scales, might become less significant in determining the AHC contributions from these well-separated Landau levels.
Phonon interactions: Electron-phonon scattering plays a crucial role in carrier dynamics within semiconductors. At higher magnetic fields, the Landau level separation increases, potentially modifying the available phonon scattering pathways. This could lead to a suppression of specific scattering processes that contribute to the AHC, particularly those involving inter-Landau level transitions within the valence band.
Interplay of factors: It's important to note that these factors (strain, phonon interactions, and valence band mixing) are not mutually exclusive and can interplay in complex ways. For instance, strain can influence phonon modes and scattering rates, further impacting the AHC.
Further investigation, both experimental and theoretical, is needed to disentangle the individual contributions of these factors and gain a comprehensive understanding of their interplay in the observed magnetic field dependence of valence band AHC contributions.

Considering the interplay of light and electron spin in this research, how might these findings influence the development of quantum information processing technologies that rely on manipulating spin states?

The research highlights a strong interplay between light, electron spin, and Landau quantization, offering intriguing possibilities for quantum information processing technologies:

Optically controlled spin qubits: The ability to generate and manipulate spin-polarized currents using light suggests a pathway towards optically controlled spin qubits. By encoding quantum information in the spin state of electrons confined within Landau levels, one could potentially use ultrafast laser pulses to initialize, manipulate, and read out these qubits.
Long spin coherence times: The reduced dimensionality imposed by the magnetic field, leading to Landau quantization, can enhance spin coherence times. This is crucial for quantum information processing, as longer coherence times allow for more complex quantum operations to be performed before information is lost.
Scalability and integration: The use of a well-established semiconductor material like GaAs offers potential advantages in terms of scalability and integration with existing semiconductor fabrication techniques. This could be crucial for developing practical quantum information processing devices.
However, several challenges need to be overcome:

Controlling individual spins: While the research demonstrates control over ensembles of spins, manipulating individual spins within Landau levels remains a significant hurdle.
Enhancing spin coherence further: While Landau quantization can improve coherence, interactions with the environment still pose a limitation. Strategies to further suppress decoherence are needed.
Developing robust qubit architectures: Designing robust qubit architectures based on Landau-quantized systems that are resilient to noise and imperfections is essential.
Despite these challenges, the findings provide a promising platform for exploring novel quantum information processing schemes based on the interplay of light, spin, and Landau quantization in semiconductors.