Weyl Semimetal in a Topological Insulator Multilayer (1105.5138v2)

Abstract: We propose a simple realization of the three-dimensional (3D) Weyl semimetal phase, utilizing a multilayer structure, composed of identical thin films of a magnetically-doped 3D topological insulator (TI), separated by ordinary-insulator spacer layers. We show that the phase diagram of this system contains a Weyl semimetal phase of the simplest possible kind, with only two Dirac nodes of opposite chirality, separated in momentum space, in its bandstructure. This particular type of Weyl semimetal has a finite anomalous Hall conductivity, chiral edge states, and occurs as an intermediate phase between an ordinary insulator and a 3D quantum anomalous Hall insulator with a quantized Hall conductivity, equal to $e^2/h$ per TI layer. We find that the Weyl semimetal has a nonzero DC conductivity at zero temperature and is thus an unusual metallic phase, characterized by a finite anomalous Hall conductivity and topologically-protected edge states.

• The paper proposes realizing a Weyl semimetal phase in a simple multilayer structure composed of magnetic topological insulators and insulator spacers.
• The Weyl semimetal phase emerges as an intermediate state between an ordinary insulator and a 3D quantum anomalous Hall insulator based on material properties.
• Key predictions include topologically protected surface chiral states and a nonquantized anomalous Hall conductivity that scales with Weyl node separation.

Weyl Semimetal in a Topological Insulator Multilayer

The paper authored by A.A. Burkov and Leon Balents explores an intriguing manifestation of the Weyl semimetal (WSM) phase in a multilayer structure composed of magnetically-doped topological insulators (TIs) and ordinary insulating spacer layers. This work contributes to the existing discourse on the realization of various topological phases of matter by presenting a theoretically simple yet experimentally feasible setup that stabilizes the WSM phase, characterized by distinct properties stemming from Weyl fermions.

Summary and Analysis

The authors propose a multilayer structure comprising thin films of 3D TIs, such as $\textrm{Bi}_2 \textrm{Se}_3$, interspersed with insulator layers that serves as spacers. The analysis reveals that the phase diagram of such a system harbors the WSM phase, characterized by the minimal presence of two Dirac nodes of opposite chirality, separated in momentum space. This separation protects the Weyl fermions topologically, ensuring their stability even in the presence of symmetry-breaking perturbations.

A key theoretical insight is the demonstration that Weyl nodes, conventionally viewed as isolated band crossing points, manifest here with topologically protected features such as nontrivial surface chiral states and a finite anomalous Hall conductivity. The simplistic Hamiltonian used in this analysis underscores the significance of adopting multilayer heterostructures for realizing WSMs without requiring complex or strongly-correlated materials.

Critical Findings

• Phase Diagram: The WSM phase emerges as an intermediate phase engineered between an ordinary insulator and a 3D quantum anomalous Hall (QAH) insulator. The critical transition parameters involve the manipulation of exchange splitting within the TI layers and interlayer tunneling amplitudes.
• Chirality and Band Structure: With magnetization-induced time-reversal symmetry breaking, the Weyl nodes are separated in momentum space. Their stability is ensured through their topological character, essentially serving as hedgehogs in momentum space due to the chirality of Weyl fermions.
• Anomalous Hall Conductivity: The paper predicts a nonquantized, distance-dependent anomalous Hall conductivity ($\sigma_{xy}$) in the WSM state. Notably, this conductivity scales proportionally with the separation between the Weyl nodes, a remarkable deviation from quantized conductivities observed in conventional Hall systems.

Implications and Future Directions

The proposed structure is envisioned to be a foundational platform for experimental realization due to the feasibility of thin-film growth techniques and magnetic doping that facilitates the necessary conditions for WSM emergence. The theoretical indications of nontrivial surface states and electrical conductivity properties position this multilayer system as an exciting candidate for applications in spintronics and novel electronic devices, where robustness against disorder and chiral anomaly-related effects might be exploited.

Future investigations could explore the effects of electron-electron interactions within these WSM phases, specifically examining how these interactions could modify transport properties or lead to emerging electronic orders. Moreover, understanding the transition dynamics and robustness under different symmetry-breaking scenarios could further inform the engineering of other topologically protected states.

In closing, this paper illuminates a clear trajectory for synthesizing WSM phases with minimal complexity, opening avenues for exploration in both fundamental research and applied physics. The simplicity and practical manufacturability of the proposed system underscore its relevance to ongoing advancements in topological materials research.