Landau Quantization and Quasiparticle Interference in the Three-Dimensional Dirac Semimetal Cd3As2 (1403.3446v2)

Abstract: Condensed matter systems provide a rich setting to realize Dirac and Majorana fermionic excitations and the possibility to manipulate them in materials for potential applications. Recently, it has been proposed that Weyl fermions, which are chiral, massless particles, can emerge in certain bulk materials or in topological insulator multilayers and can produce unusual transport properties, such as charge pumping driven by a chiral anomaly. A pair of Weyl fermions protected by crystalline symmetry, effectively forming a massless Dirac fermion, has been predicted to appear as low energy excitations in a number of candidate materials termed three-dimensional (3D) Dirac semimetals. Here we report scanning tunneling microscopy (STM) measurements at sub-Kelvin temperatures and high magnetic fields on one promising host material, the II-V semiconductor Cd3As2. Our study provides the first atomic scale probe of Cd3As2, showing that defects mostly influence the valence band, consistent with the observation of ultra-high mobility carriers in the conduction band. By combining Landau level spectroscopy and quasiparticle interference (QPI), we distinguish a large spin-splitting of the conduction band in a magnetic field and its extended Dirac-like dispersion above the expected regime. A model band structure consistent with our experimental findings suggests that for a specific orientation of the applied magnetic field, Weyl fermions are the low-energy excitations in Cd3As2.

• The paper demonstrates a novel 3D Dirac band structure in Cd3As2 using low-temperature STM and QPI techniques.
• It reveals an extended linear dispersion with high Fermi velocities (~9.4×10^5 m/s) and exceptional carrier mobility (~15,000 cm²/Vs).
• The study identifies atomic-scale defect impacts, offering insights for optimizing topological states in electronic applications.

Landau Quantization and Quasiparticle Interference in the Three-Dimensional Dirac Semimetal Cd$_3$As$_2$

This paper by Jeon et al. explores the electronic properties of the three-dimensional Dirac semimetal Cd$_3$As$_2$ through scanning tunneling microscopy (STM) and associated spectroscopic techniques. The paper explores the fundamental behavior of Dirac semimetals, which are characterized by the presence of Dirac points where two bands cross linearly in all three momentum space directions. Such features are akin to two-dimensional Dirac points found in materials like graphene but exist in a three-dimensional framework, providing unique opportunities to explore exotic fermions such as Weyl fermions when symmetries in the material are appropriately manipulated.

Key Findings

The researchers employed low-temperature STM at sub-Kelvin conditions and high magnetic fields to probe the electronic structure of Cd$_3$As$_2$. Through Landau level spectroscopy and quasiparticle interference (QPI) analysis, the paper distinguished a notable spin-splitting of the conduction band and revealed its extended Dirac-like dispersion.

Highlights of the Results:

• Electronic Band Structure: The paper confirms that Cd$_3$As$_2$ exhibits a bulk 3D Dirac semimetal phase characterized by two Dirac points along the $k_z$ axis. These points are protected by crystalline symmetries and are susceptible to forming Weyl points under symmetry breaking conditions.
• Linear Dispersion: The STM data and QPI measurements revealed a linear Dirac-like dispersion extending significantly beyond the Dirac points, leading to high Fermi velocities around 9.4 × 10$^5$ m/s. This observation indicates the robustness of the Dirac dispersion even at energies far exceeding those predicted for a typical Lifshitz transition.
• High Carrier Mobility: The paper elucidates the origin of the extraordinarily high electron mobility in Cd$_3$As$_2$ (∼15,000 cm$^2$/Vs at room temperature), attributing it to the extended linear dispersion and high Fermi velocity.
• Defect and Disorder Influence: Atomic-scale analysis identifies defects mostly influencing the valence band. The conduction band remains relatively homogeneous, which suggests that lattice defects primarily impact the valence band, providing insights into material optimization for electronic applications.

Implications for Future Research

The observation of extended Dirac dispersion and Weyl fermionic behavior in Cd$_3$As$_2$ implies significant potential for novel applications in electronic devices leveraging anomalous transport phenomena. Given the topological nature of these states, possible applications include highly efficient transistors and sensors that capitalize on their robustness to scattering. The ability to isolate Weyl nodes through symmetry breaking further allows for exploration into their unique transport characteristics, like the predicted Fermi arc surface states.

For future developments, the paper proposes experiments with different directions of applied magnetic field to explicitly observe and manipulate Weyl fermions. Furthermore, reducing the carrier concentration in Cd$_3$As$_2$ could help achieve more precise control over the topological states, thereby fostering advancements in quantum computing and electronic devices grounded in topological quantum matter.

Overall, this research significantly contributes to understanding the electronic properties of three-dimensional Dirac semimetals and paves the way for practical implementations of these findings in materials science and condensed matter physics. The quantitative agreement with theoretical models and the experimental realization of complex band structures represent a step forward in exploring the rich physics of topological materials.