[Paper Review] Observation of a topological 3D Dirac semimetal phase in high-mobility Cd3As2

Madhab Neupane, Su‐Yang Xu+11 more|arXiv (Cornell University)|Sep 30, 2013

Topological Materials and Phenomena7 citations

TL;DR

This paper experimentally identifies a three-dimensional topological Dirac semimetal phase in stoichiometric Cd3As2 using high-resolution angle-resolved photoemission spectroscopy (ARPES), revealing a bulk Dirac cone with exceptionally high Fermi velocity (~1.5 × 10⁶ m/s) and electron mobility up to 42,850 cm² V⁻¹ s⁻¹. The phase is protected by C₄ crystalline symmetry and strong spin-orbit coupling, establishing Cd3As2 as a high-mobility platform for studying 3D relativistic Dirac fermion physics.

ABSTRACT

Experimental identification of three-dimensional (3D) Dirac semimetals in solid state systems is critical for realizing exotic topological phenomena and quantum transport such as the Weyl phases, high temperature linear quantum magnetoresistance and topological magnetic phases. Using high resolution angle-resolved photoemission spectroscopy, we performed systematic electronic structure studies on well-known compound Cd3As2. For the first time, we observe a highly linear bulk Dirac cone located at the Brillouin zone center projected onto the (001) surface which is consistent with a 3D Dirac semimetal phase in Cd3As2. Remarkably, an unusually high Dirac Fermion velocity up to 10.2 extrmÅ{\cdot}$eV (1.5 imes 10^{6} ms^-1) is seen in samples where the mobility far exceeds 40,000 cm^2/V.s suggesting that Cd3As2 can be a promising candidate as a hypercone analog of graphene in many device-applications which can also incorporate topological quantum phenomena in a large gap setting. Our experimental identification of this novel topological 3D Dirac semimetal phase, distinct from a 3D topological insulator phase discovered previously, paves the way for exploring higher dimensional relativistic physics in bulk transport and for realizing novel Fermionic matter such as a Fermi arc nodal metal.

Motivation & Objective

• To experimentally confirm the existence of a 3D topological Dirac semimetal (BDS) phase in a stoichiometric, non-metastable material with strong spin-orbit coupling.
• To identify and characterize the bulk Dirac cone in Cd3As2 using high-resolution ARPES, focusing on its dispersion and velocity.
• To demonstrate that high electron mobility and massless Dirac fermion behavior coexist in a single material, enabling exploration of exotic quantum transport phenomena.
• To contrast the crystalline-symmetry-protected BDS phase in Cd3As2 with previously studied 2D Dirac systems and Bi-based topological insulators.
• To establish Cd3As2 as a robust platform for realizing higher-dimensional topological phases, such as Weyl semimetals and topological insulators, via symmetry breaking or doping.

Proposed method

• High-resolution angle-resolved photoemission spectroscopy (ARPES) was performed at synchrotron light sources (SRC, ALS, HiSOR) to map the electronic band structure of cleaved (001) surfaces of Cd3As2.
• Spin-resolved ARPES measurements were conducted at the ESPRESSO endstation using VLEED spin detectors to probe spin texture and confirm topological protection.
• In situ sample cleaving at temperatures between 10–80 K and ultra-high vacuum (<1×10⁻¹⁰ torr) ensured surface stability and minimal degradation during measurements.
• First-principles density functional theory (DFT) calculations with spin-orbit coupling were performed using the VASP package and projector-augmented wave method on the experimental crystal structure.
• A 4×4×2 Monkhorst-Pack k-mesh was used for Brillouin zone sampling, and the generalized gradient approximation (GGA) was employed for exchange-correlation effects.
• Carrier density and mobility were measured via transport and ARPES to correlate electronic structure with macroscopic transport properties.

Experimental results

Research questions

• RQ1 Can a gapless three-dimensional topological Dirac semimetal phase be experimentally realized in a stoichiometric, high-mobility material like Cd3As2?
• RQ2 What is the electronic dispersion and Fermi velocity of the bulk Dirac cone in Cd3As2, and how does it compare to graphene and other 3D Dirac systems?
• RQ3 Is the Dirac semimetal phase in Cd3As2 protected by crystalline symmetry and spin-orbit coupling, and how does this differ from surface states in topological insulators?
• RQ4 Can the Dirac node in Cd3As2 be gapped or split into Weyl nodes via symmetry breaking or doping, and what topological phases would emerge?
• RQ5 How does the high mobility and linear dispersion in Cd3As2 enable novel quantum Hall-like phenomena compared to 2D Dirac systems?

Key findings

• A highly linear bulk band crossing forming a three-dimensional Dirac cone was observed at the Brillouin zone center in Cd3As2, confirming the presence of a topological Dirac semimetal phase.
• The in-plane Fermi velocity of the Dirac cone reaches up to 1.5 × 10⁶ m/s, indicating a highly relativistic, massless Dirac fermion behavior.
• Electron mobility in the samples reaches up to 42,850 cm² V⁻¹ s⁻¹ at 130 K, confirming high carrier mobility in a stoichiometric system.
• The Dirac semimetal phase is protected by C₄ crystalline symmetry and strong spin-orbit coupling, distinguishing it from 2D Dirac systems and weak spin-orbit 3D Dirac materials.
• The observed Dirac cone is robust and stable over 20-hour measurement periods, indicating high surface quality and minimal degradation.
• Theoretical calculations confirm the bulk Dirac cone and its topological protection, with excellent agreement between measured and calculated band dispersions.