Electrical Tuning of Valley Magnetic Moment via Symmetry Control (1208.6069v1)

Abstract: Crystal symmetry governs the nature of electronic Bloch states. For example, in the presence of time reversal symmetry, the orbital magnetic moment and Berry curvature of the Bloch states must vanish unless inversion symmetry is broken. In certain 2D electron systems such as bilayer graphene, the intrinsic inversion symmetry can be broken simply by applying a perpendicular electric field. In principle, this offers the remarkable possibility of switching on/off and continuously tuning the magnetic moment and Berry curvature near the Dirac valleys by reversible electrical control. Here we demonstrate this principle for the first time using bilayer MoS2, which has the same symmetry as bilayer graphene but has a bandgap in the visible that allows direct optical probing of these Berry-phase related properties. We show that the optical circular dichroism, which reflects the orbital magnetic moment in the valleys, can be continuously tuned from -15% to 15% as a function of gate voltage in bilayer MoS2 field-effect transistors. In contrast, the dichroism is gate-independent in monolayer MoS2, which is structurally non-centrosymmetric. Our work demonstrates the ability to continuously vary orbital magnetic moments between positive and negative values via symmetry control. This represents a new approach to manipulating Berry-phase effects for applications in quantum electronics associated with 2D electronic materials.

• The paper shows that applying a perpendicular electric field continuously tunes the valley magnetic moment in bilayer MoS₂, enabling control of Berry-phase effects.
• It employs polarization-resolved micro-photoluminescence and DFT calculations to quantify orbital magnetic moments and optical dichroism variations.
• The findings have practical implications for quantum electronic and photonic devices, offering a new approach to manipulate valley polarization in 2D materials.

Electrical Tuning of Valley Magnetic Moment via Symmetry Control

The paper "Electrical Tuning of Valley Magnetic Moment via Symmetry Control" demonstrates an innovative experimental approach to manipulating Berry-phase effects in two-dimensional (2D) materials, particularly focusing on bilayer molybdenum disulfide (MoS₂) systems. Grounded in the intrinsic properties of the Bloch states and their dependency on crystal symmetry, the research explores the potential of using an external electric field to manipulate inversion symmetry and, consequently, control valley contrasting properties such as magnetic moments and Berry curvature.

Overview of Key Findings

In this paper, the application of a perpendicular electric field to bilayer MoS₂—a 2D material with an inherent bandgap in the visible spectrum—enables continuous tuning of the valley magnetic moment. By observing changes in optical circular dichroism as a function of gate voltage, the researchers have quantified the magnitude of orbital magnetic moments ranging from -15% to 15%. This contrasts with monolayer MoS₂, where the structurally non-centrosymmetric nature precludes similar electrical tuning of valley magnetic moments, resulting in a gate-independent optical dichroism.

The bilayer MoS₂ exhibits a symmetrically inversion-breaking capacity when subjected to an electric field, allowing distinct control over the valley contrasting optical properties. The degree of polarization, serving as a direct measure of the continuous tuning of the circular dichroism, reflects this ability to dynamically control reversal of valley magnetic moments. Notably, the paper underlines that this electrical manipulation of the Berry-phase-related features like the orbital magnetic moment is exclusively realized in bilayer configurations due to their inversion symmetric nature at equilibrium—a clear contrast to monolayer structures.

Methodological Insights

A crucial aspect of this research involved the fabrication of field-effect transistors (FETs) from mechanically exfoliated thin MoS₂ samples, enabling the control of perpendicular electric fields through back-gate voltages. The use of polarization-resolved micro-photoluminescence provided real-time insight into valley optical properties and served as the experimental basis for observing the magnetoelectric effect.

The paper leverages density functional theory (DFT) calculations to support the experimental observations, confirming that the valley-contrasting magnetoelectric effect can significantly alter the valley configurations by electric fields.

Practical and Theoretical Implications

The implications of these findings are substantial for the development of 2D material-based quantum electronic devices. The ability to dynamically control and tune Berry-phase effects such as the valley magnetic moment opens pathways for designing advanced photonic and electronic devices that exploit these quantum mechanical properties for enhanced functionality. These effects can potentially play pivotal roles in areas like topological quantum computing and spin-valleytronics, where gate-controllable valley polarization is highly desirable.

Moreover, the work provides theoretical grounding for future studies into the topological characteristics of 2D materials, particularly for the examination of inversion symmetry's role in dictating the electronic properties of Bloch states. It poses significant consequences for the construction of 2D heterostructures and van der Waals materials, suggesting new approaches for their structural and functional modulation.

Speculations for Future Directions

Looking forward, integrating these valley-contrast manipulation capabilities with other 2D materials might unveil new multi-functional devices. Further explorations could focus on enhancing control precision and scalability for practical device applications. Additionally, examining the interrelationship between other symmetry-breaking mechanisms and their effects on material properties will deepen understanding and potential applications of 2D quantum materials.

In sum, this paper provides a comprehensive assessment of electrical control in bilayer MoS₂, marking a step forward in the precise manipulation of quantum properties in 2D materials essential for future technological advances.