Franck-Condon blockade in suspended carbon nanotube quantum dots (0812.3826v1)

Abstract: Understanding the influence of vibrational motion of the atoms on electronic transitions in molecules constitutes a cornerstone of quantum physics, as epitomized by the Franck-Condon principle of spectroscopy. Recent advances in building molecular-electronics devices and nanoelectromechanical systems open a new arena for studying the interaction between mechanical and electronic degrees of freedom in transport at the single-molecule level. The tunneling of electrons through molecules or suspended quantum dots has been shown to excite vibrational modes, or vibrons. Beyond this effect, theory predicts that strong electron-vibron coupling dramatically suppresses the current flow at low biases, a collective behaviour known as Franck-Condon blockade. Here we show measurements on quantum dots formed in suspended single-wall carbon nanotubes revealing a remarkably large electron-vibron coupling and, due to the high quality and unprecedented tunability of our samples, admit a quantitative analysis of vibron-mediated electronic transport in the regime of strong electron-vibron coupling. This allows us to unambiguously demonstrate the Franck-Condon blockade in a suspended nanostructure. The large observed electron-vibron coupling could ultimately be a key ingredient for the detection of quantized mechanical motion. It also emphasizes the unique potential for nanoelectromechanical device applications based on suspended graphene sheets and carbon nanotubes.

• The paper demonstrates that strong electron-vibron coupling in CNT quantum dots suppresses current at low bias due to the Franck-Condon blockade.
• The paper employs precise electronic transport measurements to quantify vibron-assisted transport, with an observed electron-vibron coupling constant around 3.3 ± 0.9.
• The paper highlights the practical potential of leveraging FC blockade insights for developing advanced nanoelectromechanical and quantum devices using carbon nanomaterials.

Analysis of Franck-Condon Blockade in Suspended Carbon Nanotube Quantum Dots

This paper examines the Franck-Condon (FC) blockade as it manifests in suspended carbon nanotube (CNT) quantum dots. The intricate interplay between the vibronic excitations and electron transport in these nanostructures elucidates the complexity and potential of quantum devices.

Key Findings

The authors investigate how vibron-mediated electronic transport operates in a regime characterized by strong electron-vibron coupling. Utilizing carbon nanotubes as a quasi-one-dimensional model system, the study addresses several hallmarks of vibron-assisted transport. This involves a comprehensive exploration of the Coulomb blockade phenomena, marked by negative differential conductance (NDC) and quasi-periodic lines that signal vibron excitation.

The investigation confirms the presence of FC blockade using single-wall CNT quantum dots. The current is notably suppressed at low bias due to strong coupling between electrons and vibrons. This is quantified by the electron-vibron coupling constant ($g$), which, the researchers note, is approximately 3.3 ± 0.9, significantly higher than previous observations.

The study achieves a detailed description of the quantum transport mechanics, linking various observed peak magnitudes in conductance with the transitions described by the FC principle. This understanding aligns with theory and offers a quantitative match to experimental observations across a wide range of conditions, demonstrating the utility of the FC model in describing strong electron-vibron interactions in suspended quantum dots.

Methodology

The study utilizes a combination of electronic transport measurement techniques and structural characterization to comprehend the quantum dot's properties fully. This includes observing the four-fold degeneracies of electronic states indicative of the combined spin and valley degeneracies in clean CNTs. The methods showcase a thorough comprehension of differential conductance enhancements under varying thermal conditions and their connections to vibron excitation.

Implications

From a practical perspective, the findings emphasize the device potential of CNTs and graphene-based nanostructures in quantum technology. More broadly, the detection of vibronic effects within such scalable systems might propel the design of nanoelectromechanical systems where the electron-vibron coupling could be finely tuned for specific applications like sensors or energy conversion devices.

Theoretically, this research sustains the broader discourse in mesoscopic physics and material sciences regarding electronic-vibrational coupling and quantum coherence. It presents a core empirical grounding for further exploration into hybrid quantum systems, thus motivating future research on controlled vibron interactions.

Future Directions

The work invites several prospective research avenues, particularly in broadening the understanding of vibron transport phenomena. Future investigations might explore the influence of device geometry on FC blockade strength or look into ways to harness the observed strong electron-vibron coupling in practical applications. Exploring the interaction of different phonon modes with electronic degrees of freedom can further elucidate the dynamics in such nanostructures.

By advancing an empirical and theoretical understanding of FC blockade, this study sets a substantive chapter in the evolving landscape of quantum materials and nanoengineering, with implications extending into quantum computing and sensing technologies.

In conclusion, the study not only provides a solid framework for understanding FC interactions in CNT quantum dots but also strengthens the connection between theoretical predictions and empirical observations in quantum electronics. This foundational understanding is vital for progressing toward the practical realization of advanced quantum devices using carbon-based nanomaterials.