arXiv subjects

Umang Soni

Publications and source records attributed to Umang Soni.

4 recordsLinked to original sources

arXiv2026

Tunnel-rate controlled local heat distribution in mesoscopic circuits

Solid-state quantum technologies, including qubits and quantum metrology circuits, demand milli-Kelvin operation to preserve fragile quantum states from classical noise. While the negligible electron-phonon coupling is the major impediment, reaching 50 mK electron temperature is further suffered by the high electrical resistance and sub-micron-scale dimensions of typical devices, limiting conventional heat dissipation. Though the phonons are effectively frozen, thermoelectric techniques could offer a viable path for heat management.This work explores thermally driven electrical transport in a gated quantum dot (QD) on a GaAs-AlGaAs two-dimensional electron gas (2DEG), to control heat flow between the source and drain reservoirs.By exploiting the QD's discrete energy spectrum and tuneable tunnel rates, a precise control over the polarity and magnitude of the resulting thermoelectric current is demonstrated. A temperature difference of 650 mK is maintained across the QD, a separation of 400 nm, by tuning the tunnel-rates. An experimental gate pulsing method is also introduced to directly measure the electron temperature differences across the QD, bypassing the need for any theoretical fits. The results presented here show that tuneable tunnel barriers can be used for local heat control, and could lead to advanced quantum refrigerators that work efficiently in mesoscopic circuits.

cond-mat.mes-hall↗

arXiv2026

Class-Specific Branch Attention for Mitigating Gradient Interference under Class Imbalance

Deep neural networks trained under severe class imbalance often exhibit degraded performance, typically attributed to statistical bias. In this work, we identify a complementary optimization-level pathology: inter-class gradient interference within shared representations, where gradients from majority classes suppress minority-class learning. To analyze this phenomenon, we introduce a diagnostic framework based on layer-wise gradient flow analysis and a Gradient Conflict Matrix, which quantifies interference using cosine similarity between class-specific gradients. Using this framework, we study multi-branch convolutional architectures and propose a lightweight modification, Class-Specific Branch Attention (CSBA), that enables branch-specific channel reweighting to reduce gradient coupling. This mechanism promotes implicit feature decoupling across branches while preserving architectural simplicity. Empirically, CSBA improves minority-class performance, increasing the F1 score for the Physical-Damage class from 0.261 to 0.522 under severe imbalance, while maintaining comparable overall accuracy. Validation on CIFAR-10-LT confirms that this behavior generalizes across imbalanced visual recognition settings, with Macro-F1 improving from 0.595 to 0.655. More broadly, our findings highlight the importance of considering optimization dynamics alongside statistical methods when designing architectures for imbalanced learning.

cs.AI↗

arXiv2025

Density-driven scattering and valley splitting in undoped Si/SiGe two-dimensional electron system

Undoped Si-SiGe two-dimensional electron gas (2DEG) provide an ideal platform for hosting quantum-dot spin-qubits owing enhanced spin dephasing times and compatibility with standard CMOS technology. The strained Si quantum well reduces the valley degeneracy into two closely spaced ones. The existence of a near-degenerate valley state act as a leakage channel and compromises gate fidelity. A robust and uniform valley splitting across the entire chip is crucial for achieving scalability in the architecture and reliability in operation. Imperfections such as broadened interfaces, alloy disorders and atomic steps significantly compromise the valley splitting. The associated scattering mechanisms play detrimental roles in the performance of the qubits. In this manuscript, exploiting low-temperature magnetotransport measurements, we investigate the scattering mechanisms and valley splitting in a high-mobility undoped Si-SiGe 2DEG. At lower carrier densities, transport is limited by remote impurity scattering, whereas at higher densities, background impurity scattering near the quantum well dominates. Both the transport and quantum lifetimes of the charge carriers increase with carrier concentration, due to the enhancement in the impurity screening. Magnetic-field-induced confinement effect also is found to improve the valley splitting. Current-biasing measurements reveals the role of carrier heating in the visibility of valley splitting and reveal a temperature limited valley splitting of approximately 100 micro-eV. These results provide critical insight into scattering-dominated regimes and valley splitting in undoped Si-SiGe, advancing its potential for silicon-based quantum devices.

cond-mat.mes-hall↗

arXiv2025

Refining Au/Sb alloyed ohmic contacts in undoped Si/SiGe strained quantum wells

Shallow undoped Si/SiGe quantum wells are the leading platforms for hosting quantum processors based on spin-qubits. The ohmic contacts to the electron gas in these systems are accomplished by ion-implantation technique since the conventional Au/Sb alloyed contacts present a rough surface consisting of sharp islands and pits. These sharp protrusions cause electrical discharge across the gate-dielectric between the ohmic contacts and the accumulation-gates causing device break-down. A clear understanding of the surface morphology, elemental, compositional and electrical characterization of the alloyed region would enable one to engineer a smoother post alloyed surface. In this work, we find that the rough surface morphology is a cumulative effect of the Au/Si eutectic reaction and the threading dislocations inherent in the heterostructure. The structural, elemental, and chemical-state analysis show that the inverted pyramidal pits are resulting from the enhanced Au/Si eutectic reaction at the threading dislocations stemming from the heterostructure interface, while, the sharp protrusions causing accumulation gate-leakage are gold-rich precipitations. The protrusions are removed using an aqua regia treatment prior to the deposition of the gate-oxide and gate electrode. Exploiting a Hall bar device, we analyse the mobility and carrier concentration of the undoped Si/SiGe consisting of Au/Sb alloyed contacts down to 1.5 K. The measured mobility ~10^5 cm^2/Vs and carrier concentration of ~10^11/cm^2are comparable to the reported values on similar high-mobility heterostructures confirming the efficacy of our modified Au/Sb alloy technique in accomplishing high-efficiency contacts to undoped Si/SiGe heterostructures.

cond-mat.mes-hall↗