Revisiting Coulomb diamond signatures in quantum Hall interferometers
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2022-03-31 |
| Journal | Physical review. B./Physical review. B |
| Authors | Nicolas Moreau, SĂ©bastien Faniel, Frederico Martins, L. Desplanque, X. Wallart |
| Institutions | Centre National de la Recherche Scientifique, Ăcole Centrale de Lille |
| Citations | 2 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research revisits the interpretation of Coulomb diamond signatures observed in nanoscale quantum Hall interferometers (QHIs) using scanning gate microscopy (SGM).
- Core Achievement: Demonstrated that apparent Coulomb diamond patterns, typically associated with the Coulomb Dominated (CD) regime, can occur purely within the coherent Aharonov-Bohm (AB) regime.
- Mechanism: A simple Fabry-PĂ©rot (FP) model accurately reproduces the transition from checkerboard (AB signature) to diamond patterns by smoothly varying the reflection and transmission amplitudes (r, t) at the QHI tunneling paths.
- Device Structure: The QHI forms spontaneously due to an impurity-induced quantum dot located within a Quantum Point Contact (QPC) constriction in an InGaAs/InAlAs 2DEG heterostructure.
- Size and Coherence: The QHI achieved a small radius of approximately 125 nm (Area < 0.05 ”m2), while maintaining pure AB coherence, contrary to expectations that such small devices should be dominated by charging effects.
- Validation: Temperature dependence measurements of the oscillation amplitude confirmed the exponential decay characteristic of the AB regime, decisively ruling out the T-1 dependence expected for the CD regime.
- Impact: This finding necessitates revisiting previous data where diamond patterns were automatically ascribed to Coulomb blockade, and opens the path toward engineering robust, nanoscale QHIs for anyonic braiding and fundamental quantum physics tests.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Heterostructure Material | In0.7Ga0.3As/In0.52Al0.48As QW | N/A | Grown by Molecular Beam Epitaxy (MBE) on InP. |
| Quantum Well Depth | 25 | nm | Distance below the surface. |
| 2DEG Density (Ns) | 5.7 x 1015 | m-2 | Bare electronic density. |
| 2DEG Mobility (”) | 5.3 x 104 | cm2/Vs | N/A |
| QPC Constriction Width | ~350 | nm | Defined by wet etching. |
| Base Temperature (T) | ~100 | mK | Dilution refrigerator operating temperature. |
| Magnetic Field Range (B) | 5.2 to 7.7 | T | Range studied for QHI configurations. |
| SGM Tip Distance (dtip) | ~60 | nm | Distance above the 2DEG plane. |
| QHI Radius (A) | ~125 | nm | Estimated from AB oscillation fit (Area < 0.05 ”m2). |
| QHEC Velocity (v) | 0.5 x 105 | m/s | Used in Fabry-PĂ©rot model simulation. |
| Characteristic Energy Scale (Vsd) | ~0.2 | mV | Energy scale extracted from both checkerboard and diamond spectroscopies. |
| QPC Filling Factors (v, v*) | 4 (bulk), 3 (QPC) | N/A | Corresponds to the 2.2 kΩ plateau (Zeeman split LLs). |
| Reflection Probability (RL, RR) | 0.05 to 0.3 | N/A | Adjusted in model to transition from diamond (high R) to checkerboard (low R). |
Key Methodologies
Section titled âKey Methodologiesâ- Device Fabrication: Hall bar and QPC geometries were defined on the InGaAs/InAlAs quantum well using electron beam lithography and subsequent wet etching. Ge/Au ohmic contacts were used for electrical connection.
- Low-Temperature Transport: Measurements were conducted in a dilution refrigerator at a base temperature of approximately 100 mK, with a perpendicular magnetic field (B) applied to induce the Quantum Hall (QH) regime.
- Magnetoresistance (Rxx): Standard AC lock-in techniques were employed to measure the resistance Rxx = dV/dI, using a 2 nA excitation current.
- Scanning Gate Microscopy (SGM): An Atomic Force Microscope (AFM) with a metallic tip was used as a local gate. The tip, biased at voltage Vtip, was scanned ~60 nm above the 2DEG surface to locally modulate the electrostatic potential and QHI area, while Rxx was recorded.
- DC Bias Spectroscopy: Differential resistance (Rxx = dV/dI|Isd) maps were generated by adding a DC source-drain current (Isd) to the AC signal and sweeping Isd versus either the magnetic field (B) or the tip voltage (Vtip).
- Theoretical Modeling: Experimental spectroscopy maps were simulated using a basic Fabry-PĂ©rot (FP) interferometer model operating in the AB regime. The model calculates the conductance (G) through the dot based on the AB phase and tunneling probabilities (RL, RR).
- Visibility Decay Inclusion: To match experimental data, the model incorporated a visibility decay mechanism, modeled by a Gaussian function in Vsd, to account for the decrease in oscillation amplitude at higher source-drain bias.
Commercial Applications
Section titled âCommercial ApplicationsâThe successful demonstration of robust, coherent transport in nanoscale QHIs has direct implications for advanced quantum technologies:
- Topological Quantum Computing: The primary application is creating stable platforms for anyonic braiding. Achieving a pure AB regime in a QHI of this size (125 nm) is a critical step toward realizing fault-tolerant quantum computation based on non-Abelian statistics.
- Quantum Metrology and Standards: The highly controlled Aharonov-Bohm oscillations can be leveraged to develop ultra-precise quantum Hall-based current standards and magnetic flux sensors, offering enhanced sensitivity compared to classical devices.
- High-Coherence Quantum Circuitry: The methodology provides a blueprint for engineering complex, integrated quantum circuits where coherent transport must be maintained at the nanoscale, potentially leading to advanced quantum processors or simulators.
- Advanced Semiconductor Characterization: The technique of using SGM combined with DC bias spectroscopy offers a powerful tool for characterizing potential landscapes and identifying impurity-induced quantum dots in novel semiconductor heterostructures, crucial for quality control in advanced fabrication processes.
- Quantum Hall Effect Devices: The ability to tune the QHI transmission probability (r, t) via local gating (Vtip) allows for the dynamic control of electron beam splitting, enabling the design of reconfigurable quantum switches and modulators.
View Original Abstract
Coulomb diamonds are the archetypal signatures of Coulomb blockade, a\nwell-known charging effect mainly observed in nanometer-sized âelectronic\nislandsâ tunnel-coupled with charge reservoirs. Here, we identify apparent\nCoulomb diamond features in the scanning gate spectroscopy of a quantum point\ncontact carved out of a semiconductor heterostructure, in the quantum Hall\nregime. Varying the scanning gate parameters and the magnetic field, the\ndiamonds are found to smoothly evolve to checkerboard patterns. To explain this\nsurprising behavior, we put forward a model which relies on the presence of a\nnanometer-sized Fabry-P\âerot quantum Hall interferometer at the center of the\nconstriction with tunable tunneling paths coupling the central part of the\ninterferometer to the quantum Hall channels running along the device edges.\nBoth types of signatures, diamonds and checkerboards, and the observed\ntransition, are reproduced by simply varying the interferometer size and the\ntransmission probabilities at the tunneling paths. The new proposed\ninterpretation of diamond phenomenology will likely lead to revisit previous\ndata, and opens the way towards engineering more complex interferometric\ndevices with nanoscale dimensions.\n