arXiv subjects

Hyung Kook Choi

Publications and source records attributed to Hyung Kook Choi.

6 recordsLinked to original sources

arXiv2026

Entropy spectroscopy of a tunable two-site Hubbard molecule

Determining which microscopic states remain thermally active when tunneling and interactions compete is a central question in Hubbard physics. Here we develop an entropy measurement protocol for a double quantum dot, extending the charge-based approach established for a single dot, and measure the total entropy of a tunable two-site Hubbard molecule realized in a GaAs double dot. By shifting both dot levels together at fixed detuning, the entropy is correctly probed with only a single charge sensor. As interdot tunneling increases, the system evolves from two atomic-like dots through hybridized molecular states to a merged single dot. Across this evolution, the entropy decreases as tunnel-induced energy splitting exceeds the thermal energy and suppresses the occupation of higher-energy states. The measurements resolve the diminishing contribution of antibonding states and the changing thermal contributions of hybridized singlet and triplet states, in quantitative agreement with a two-site Hubbard model. By distinguishing states with the same charge configuration but different orbital and spin content, entropy reveals how tunneling and interactions determine the thermally active states of the minimal Hubbard system.

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arXiv2025

Method for rapid estimation of the energy-time covariance matrix of single electrons

The ability to emit and control single electrons in a dynamical manner enables their use in electron quantum optics and sensing. To characterize the electron states emitted with energy far above the Fermi energy, a dynamic barrier has been used. In this work, we extract the energy-time covariance matrix of single electrons by analyzing the energy variance obtained from the transconductance through the dynamic barrier. This method enables efficient and precise characterization of electron states, especially when a sinusoidal waveform is used. An effective phase-space area and the elliptical distribution are constructed from the covariance matrix and qualitatively compared with the distribution reconstructed from a tomographic method. The area constrained by the uncertainty relation serves as a measure of proximity to the quantum limit. Our results demonstrate an efficient method for characterizing electron states, paving the way for their application in quantum technologies.

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arXiv2024

Effective tuning methods for few-electron regime in gate-defined quantum dots

We present systematic methods for compensating gate crosstalk effects in gate-defined quantum dots (QDs), to allow the observation of Coulomb blockade peaks from the few-electron regime (N = 1) to N \approx 20. Gate crosstalk, where adjustments to one gate voltage unintentionally affect other gate-controlled parameters, makes it difficult to control tunneling rates and energy states of the QD separately. To overcome this crosstalk effect, we present two approaches: maintaining constant conductance of two quantum point contacts (QPCs) forming the QD by compensating the effect of the plunger gate voltage on the QPCs, and interpolating between gate voltage conditions optimized for QD observation at several electron numbers. These approaches minimize crosstalk effects by dynamically adjusting barrier gate voltages as a function of plunger gate voltage. Using these methods, we successfully observed Coulomb blockade peaks throughout the entire range from N = 1 to N \approx 20. Our methods provide a simple and effective solution for observing Coulomb blockade peaks over a wide range of electron numbers while maintaining control over the quantum states in the dot.

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arXiv2023

Observation of Electronic Modes in Open Cavity Resonator

The resemblance between electrons and optical waves has strongly driven the advancement of mesoscopic physics. However, electron waves have yet to be understood in open cavity structures which have provided contemporary optics with rich insight towards non-Hermitian systems and complex interactions between resonance mode. Here, we report the realization of an open cavity resonator in a two-dimensional electronic system. We studied the resonant electron modes within the cavity and resolved the signatures of longitudinal and transverse quantization, showing that the modes are robust despite the openness of the cavity being highly open to the background continuum. The transverse modes were investigated by applying a controlled deformation to the cavity, and their spatial distributions were further analyzed using magnetoconductance measurements and numerical simulation. These results lay the groundwork to exploring electronic wavefunctions in the context of modern optical systems, such as the dielectric microcavity.

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arXiv2020

Numerical Reconstruction of 2D Magnetic Focusing Experiments

Spatial aspects in quantum mechanics are often difficult to model in geometrically intricate settings that are typical of mesoscopic physics. In such cases, predicting the device behaviors is a vital but difficult challenge. Transverse magnetic focusing (TMF) is a prime example where a classically simple effect becomes difficult to approach in the quantum regime. Here, we have simulated a realistic TMF device and compared the results to those from experiments performed on GaAs/AlGaAs two-dimensional electron gas systems. Unlike previous studies, device features such as quantum point contacts and disorder were realized within the simulation. The simulated and experimental focusing spectra showed good agreement, and the analysis was extended to multichannel and energy-modulated scenarios. By revisiting the energy-modulated simulation with a quantum dot (QD) emitter, we confirmed that the unique geometry of a QD does not affect the focusing spectra, thereby validating the feasibility of such experiments in the study of monoenergetic excitations.

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arXiv2020

Ultrafast energy relaxation of quantum dot-generated 2D hot electrons

Through a series of transverse magnetic focusing experiments, we show that hot electrons in a two-dimensional electron gas system undergo an ultrafast relaxation when generated by a quantum dot (QD) instead of a quantum point contact (QPC). We find here that QPC hot electrons were well described by the non-interacting Fermi gas model for excitations up to 1.5 meV above the Fermi level of 7.44 meV, whereas QD hot electrons exhibited an energy loss quadratic to the excitation. The energy relaxation was a sizeable fraction of the tested excitations, up to about 55%. With the proposal that the hot electrons are relaxed by the QD immediately after emission, we present a toy model in which a capacitive coupling between the QD and its leads results in a finite, ultrafast energy relaxation.

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