arXiv subjects

Jongbae Hong

Publications and source records attributed to Jongbae Hong.

At least 19 recordsLinked to original sources

arXiv2026

Classifying coherent peaks in nanoelectronic devices by the presence or absence of spin exchange

Coherent peaks appearing in the differential conductance of quantum-dot single-electron transistors (QDSETs) and quantum point contact (QPC) devices are classified into two categories according to the scaling function onto which the temperature-scaled differential-conductance maxima collapse and the underlying spin dynamics. The zero-bias peaks (ZBPs) observed in QPCs and in the triplet state of the even-particle sector of QDSETs belong to the same category, whereas the ZBP in the odd-particle sector of a QDSET belongs to a different category together with all finite-bias coherent peaks observed in QPCs and in the even-particle sector of QDSETs. The spin dynamics of the former category involve spin exchange, a hallmark of Kondo dynamics, whereas those of the latter category involve only cotunneling of an up--down spin pair. Furthermore, for the former type of ZBP, the scaling temperature coincides with one-half of the full width at half maximum (FWHM), which corresponds to the Kondo temperature. In contrast, for the latter type, the scaling temperature does not coincide with the (1/2)FWHM-derived energy scale. To support these findings, the gate-voltage-dependent differential-conductance line shapes measured in the odd-particle sector of a QDSET are theoretically reproduced. The results demonstrate that the observed ZBP is a merging of two coherent side peaks generated solely by the cotunneling of up--down spin pairs.

cond-mat.mes-hall↗

arXiv2025

Classification of coherent peaks in two-terminal quantum devices into normal and anomalous Kondo peaks

Coherent peaks arising in the differential conductance of quantum dot (QD) and quantum point contact (QPC) devices are classified into two categories, normal and anomalous Kondo peaks, according to the underlying spin dynamics and the form of the scaling function to which the scaled temperature-dependent linear conductance collapses. The zero-bias peaks (ZBPs) observed in QPCs and in the triplet state of the even sector of quantum dot single-electron transistors (QDSETs) are identified as normal Kondo peaks, formed by spin dynamics involving spin exchange, a symbolic characteristic of the Kondo effect. For these ZBPs, the scaling temperature coincides with half the full width at half maximum (FWHM). In contrast, the ZBP observed in the odd sector of QDSETs and all finite-bias coherent peaks, including the coherent side peaks of QPCs and the split ZBP in the singlet state of the QDSET even sector, are identified as anomalous Kondo peaks, because they arise from spin dynamics without spin exchange, and their scaling temperature does not coincide with half the FWHM. To support these findings, we reproduce gate-voltage-dependent differential conductance line shapes measured in the odd sector of a QDSET, demonstrating that its ZBP originates from a combination of two coherent side peaks explicitly observed in QPCs.

cond-mat.mes-hall↗

arXiv2025

Gate-voltage-driven quantum phase transition at $0.7 (2e^2/h)$ in quantum point contacts

We investigate a quantum phase transition (QPT) in quantum point contacts by analyzing the gate-voltage-dependent quasiparticle energy at the Fermi level at zero temperature. This energy is computed using the local density of states at the site of the localized spin, which is extracted from the replicated gate-voltage-dependent differential conductance shaped by entangled-state tunneling. The QPT occurs between symmetric ($G \geq 0.7 G_0$) and asymmetric ($G < 0.7 G_0$) Kondo coupling states, where $G_0 = 2e^2/h$, and is driven by the migration of a localized spin in response to the side-gate voltage. The asymmetric state exhibits two distinct Kondo temperatures, while the symmetric state has only one. The existence of two Kondo temperatures in the $G < 0.7 G_0$ regime accounts for both the anomalous gate-voltage dependence of the zero-bias anomaly width and the inability to define a Kondo temperature in the $G < 0.7 G_0$ region.

cond-mat.mes-hall↗

arXiv2022

Determination of Plateau Widths and Energy Gaps in the Fractional Quantum Hall Effect by Multi-Particle Correlations

Determining plateau widths and energy gaps is the remaining task to fully understand the electron transport that gives the fractional quantum Hall effect at the lowest Landau level (LLL). We report that this determination is given by the degree of multi-particle correlations that governs the split distance of the fine structure of the LLL.We show that an electron flowing through an incompressible strip formed in a Hall bar behaves as a quasiparticle comprising the electron and its image, which replaces the confining potential of the incompressible strip. This quasiparticle is a composite boson of spin unity. Correlated quasiparticles having higher integral spins are induced by many-body interactions. The Zeeman effect for these integral spins of the quasiparticles yields fine splits in the LLL, which is responsible for the plateaus in Hall resistivity at fractional fillings. With such a scheme, we explicitly reproduce experimental Hall resistivity and energy gaps.

cond-mat.str-el↗

arXiv2022

Correlated composite approach to fractional quantum Hall effect via edge current

The fractional quantum Hall effect (FQHE) is extensively studied, but the explanation for Hall plateau widths and excitation energy gaps remains elusive. We study the effective theory of FQHE built upon experimental inputs of Hall current distribution, edge dynamics, and many-body correlations. We argue that correlated composites of integer spin, comprising electrons and their images, localized at the edge of the incompressible strip are the basic transport entity. We show in the lowest Landau level that Zeeman interactions of these composites produce all odd denominator plateaus and effective fractional charges. Utilizing field-dependent chemical potential and effective g-factor, we fully explain the observed Hall resistivity curve and excitation energy gaps of the half-filling family. The plateau heights are systematically generated by multi-particle correlations, whereas the plateau widths and excitation energy gaps are determined by the correlation strengths. We explicitly show that the Drude-like behavior at half-filling follows from equal strength of multi-particle correlations.

cond-mat.mes-hall↗

arXiv2022

Fractional quantum Hall effect driven by multi-particle correlations in edge current

The fractional quantum Hall effect has been considered as a puzzling quantum many-body phenomenon that has yet to be fully explained. The plateau width and excitation energy gap are particularly problematic. We report here that those two are determined by degrees of multi-particle correlations among the skipping electrons forming the edge current flowing in incompressible strips (ISs). Consideration of the total angular momentum of correlated skipping electrons and their images, which are introduced to eliminate the confining potential within the IS, yields additional Zeeman energies that hierarchically split the Landau levels (LLs) by correlation order. This level splitting produces all the odd-denominator plateaus and explains the occurrence of fractional charges, while the split distances representing the correlation strengths determine both plateau widths and excitation energy gaps. With such a scheme, we explicitly reproduce an experimental Hall resistivity curve for the lowest LL and reveal the characteristics of the half-filling state.

cond-mat.mes-hall↗

arXiv2022

Energy Gaps and Plateau Characteristics in the Fractional Quantum Hall Effect Derive from Multi-particle Correlations

The energy gaps appearing in the fractional quantum Hall effect (FQHE) remain an essential aspect of the investigation. Moreover, the plateau widths in the Hall resistance have been considered simply an effect of disorder as in the integral quantum Hall effect. The existing theories could neither explain the Hall resistance curve owing to plateau widths nor calculate the energy gaps. This study reveals that both the energy gaps and plateau widths contain fundamental many-body aspects of the FQHE. They are found to be connected via the strengths of multi-particle correlations, which do not affect the plateau heights. They are automatically quantized just by the presence of multi-particle correlations. This work focuses on correlated skipping electrons moving through the edge of an incompressible strip formed within a Hall bar. Consequently, a single-particle Hamiltonian was constructed incorporating the Zeeman energies of multiply-correlated skipping electrons. The resulting energy spectrum exhibits hierarchical splits of the Landau levels according to correlation order. The lowest Landau level is examined. Based on such level splitting, a previously measured Hall resistance curve and energy gaps are quantitatively explained by determining the parameters that describe the degrees of multi-particle correlations. The chemical potential and effective $g$-factors are additionally predicted for the Hall resistance. Furthermore, the fractional electron charge $e/(2n+1)$ for an electron participating in $n$-particle correlation was obtained by identifying the Fermi distribution function of $n$ correlated basic transport entities moving through the edge of the incompressible strip. Finally, the ideal-like Hall resistance was obtained at half-filling using the strengths of multi-particle correlations given in a regular pattern.

cond-mat.mes-hall↗

arXiv2019

Analyzing scanning tunneling spectroscopy for Fe-based superconductor Ba$_{1-x}$K$_x$Fe$_2$As$_2$ and extracting $s$-wave density of states

We extract the density of states (DOS) from the scanning tunneling spectroscopy data for Ba$_{1-x}$K$_x$Fe$_2$As$_2$ superconductor. The obtained sample DOS is composed of two ordinary $s$-wave types from the band at $Γ$ point and a linear-like DOS within the $s$-wave gap from the band at M point in the Brillouin zone, and is consistent with the corresponding data from angle-resolved photoemission spectroscopy. We clarify that the major peak of the tunneling conductance is not related to the DOS but is rather the effect of nonequilibrium coherent tunneling including all coherent spins in the tip and sample.

cond-mat.supr-con↗

arXiv2016

A universal explanation of tunneling conductance in exotic superconductors

A longstanding mystery in understanding cuprate superconductors is the inconsistency between the experimental data measured by scanning tunneling spectroscopy (STS) and angle-resolved photoemission spectroscopy (ARPES). In particular, the gap between prominent side peaks observed in STS is much bigger than the superconducting gap observed by ARPES measurements. Here, we reconcile the two experimental techniques by generalising a theory which was previously applied to zero-dimensional mesoscopic Kondo systems to strongly correlated two-dimensional (2D) exotic superconductors. We show that the side peaks observed in tunneling conductance measurements in all these materials have a universal origin: They are formed by coherence-mediated tunneling under bias and do not directly reflect the underlying density of states (DOS) of the sample. We obtain theoretical predictions of the tunneling conductance and the density of states of the sample simultaneously and show that for cuprate and pnictide superconductors, the extracted sample DOS is consistent with the superconducting gap measured by ARPES.

cond-mat.mes-hall↗

arXiv2014

Comparison between entangled and nonentangled two-reservoir Kondo systems

We clarify the difference between entangled and nonentangled two-reservoir mesoscopic Kondo systems and reveal the reason why theories using the Keldysh formalism, quantum Monte Carlo calculations, and the renormalization group approaches cannot explain the line shapes of tunneling conductance of mesoscopic Kondo systems measured by using a two-terminal setup but explain those of a three-terminal setup. We emphasize that the previous theories study a nonentangled system, while real two-reservoir mesoscopic Kondo systems are entangled systems in which two reservoirs are within the coherent region. We show that two coherent side peaks appearing in tunneling conductance signify the entanglement between two reservoirs. These side peaks are essential for explaining the experimental observations for tunneling conductance.

cond-mat.mes-hall↗

arXiv2014

Tunnelling of entangled Kondo singlet in two-reservoir nanocontact systems under bias

Tunnelling conductances observed for mesoscopic Kondo systems exhibit a zero-bias peak and two coherent side peaks. The former peak is usually understood as a Kondo effect and the latter side peak is recently clarified as the effect of inter-reservoir coherence. However, fitting the experimental $dI/dV$ line shapes, where $I$ and $V$ denote the current and bias voltage, respectively, has not been performed theoretically. Here, we fit the entire line shape range of the tunnelling conductance observed for a quantum dot, quantum point contact, and magnetized atom adsorbed on an insulating layer covering a metallic substrate by studying the tunnelling of entangled Kondo singlet (EKS) formed in a two-reservoir mesoscopic Kondo system. We also clarify the characteristic dynamics forming each coherent peak in terms of the processes comprising spin exchange, singlet hopping, and singlet partner changing. Tunnelling of entangled Kondo singlet can be applied to understanding the tunnelling conductance observed for a sample with strong electron correlation.

cond-mat.mes-hall↗

arXiv2013

Mesoscopic Transport of Entangled and Nonentangled Kondo Singlets under Bias

The tunneling conductances of a quantum point contact and a magnetized atom adsorbed on an insulating layer above a metallic substrate are obtained by considering the coherent transport of the entangled and nonentangled Kondo singlets, and these are compared with the experimental results. Spins of the entangled Kondo singlet flow unidirectionally in a sequential up-and-down manner. This transport does not follow linear response theory. The nonentangled Kondo singlet performs resonant tunneling through a coherent transport channel and yields coherent side peaks at a finite bias. The coherent transport channel is formed by two electron reservoirs within a coherent region.

cond-mat.mes-hall↗

arXiv2013

Reinterpretation of Scanning Tunneling Microscopy on an Adsorbed Magnetic Atom

The observation of the Kondo effect in mesoscopic systems under bias$^{1,2}$ has opened a new chapter in the physics of the Kondo phenomenon. Various types of $dI/dV$, where $I$ and $V$ denote current and source-drain (s-d) bias, respectively, line shapes have been measured by scanning tunneling microscopy (STM)$^{1,3-11}$. However, explanation by single Fano line shape$^{1,12-16}$ is not relevant and even misleading. Here, we provide consistent explanations for various asymmetric and symmetric line shapes in terms of a microscopic theory that shows the creation of two resonant tunneling levels (RTLs) when bias is applied$^{17}$. One side Kondo coupling between adatom and substrate does not create Kondo peak that appears only when the system has an overall Kondo coupling including both substrate and tip. The structure of an asymmetric line shape is mostly governed by the RTL peaks. Therefore, Kondo effect is negligible in most asymmetric line shapes.

cond-mat.mes-hall↗

arXiv2013

Theoretical Reproduction of the Lineshapes of Nonlinear Conductance Observed in a Quantum Point Contact

The nonlinear conductance observed in a quantum point contact is theoretically reproduced for the entire range of applied bias. The single-impurity Anderson model with two reservoirs at different chemical potentials is studied for a sequential change of the gate voltage. The imbalance in the left and right Kondo coupling strength is introduced by the displacement of the Kondo impurity by the electric field produced in the constriction of quantum point contact. We reveal the origin of the side peaks and study the behavior of the height and width of the zero-bias anomaly.

cond-mat.mes-hall↗

arXiv2010

Green's function technique for a two-electrode mesoscopic system under bias

We present a Green's function technique for studying the nonlinear conductance of a nanocontact system with two electrodes at different chemical potentials. The retarded Green's function for a single-impurity Anderson model with two reservoirs is obtained in terms of a $5\times 5$ matrix in which the effect of bias is contained. A complete set of basis vectors for the single-impurity Anderson model has been provided before formulating the Green's function. Finally, we present a self-consistent method to fix the undetermined quantities existing in the matrix elements for the retarded Green's function.

cond-mat.mes-hall↗

arXiv2009

Nonperturbative Green's function technique for nonequilibrium steady state

Nonperturbative dynamic theory has a particular advantage in studying the transport in a quantum impurity system in a steady state. Here, we develop a new approach for obtaining the retarded Green's function expressed in resolvent form. We use the Heisenberg picture to facilitate dynamic theory and propose a new systematic method of collecting the basis vectors spanning the Liouville space, which is the most crucial step in obtaining the resolvent Green's function. We obtain all the linearly independent basis vectors for studying the single-impurity Anderson models with one and two reservoirs. The latter is an appropriate model for studying the Kondo phenomenon in a steady state when a bias is applied. This is one of long standing subjects in theoretical condensed matter physics.

cond-mat.mes-hall↗

arXiv2009

Study of nonequilibrium Kondo phenomenon via nonperturbative dynamical theory

We develop a nonperturbative dynamical theory (NDT) to calculate the retarded Green's function under nonequilibrium conditions. The NDT is particularly useful for treating nonequilibrium transport problems in systems with strong correlation. We apply our NDT to the well-known single-impurity Anderson model at equilibrium to determine its feasibility. We then apply it to a nonequilibrium transport problem in a system with Kondo coupling. An Anderson model with two metallic reservoirs is studied to understand the phenomenon of Kondo-peak splitting in a single-electron transistor of mesoscopic size. We calculate the nonequilibrium retarded Green's function by using the NDT and analyze it in the atomic limit, where the novel coherent phenomenon manifested only under nonequilibrium conditions can be described in an analytical manner. We finally construct a self-consistent loop to calculate the retarded Green's function and present the results for spectral density and differential conductance obtained by the self-consistent method. Our results explain all the features of Kondo-peak splitting observed in experiments. One remarkable conclusion is that Kondo-peak splitting is not the splitting of a conventional Kondo peak, but the splitting of a novel coherent peak created under nonequilibrium steady-state conditions.

cond-mat.mes-hall↗

arXiv2007

Understanding Kondo Peak Splitting and the Mechanism of Cohernt Transport in a Single-Electron Transistor

The peculiar behavior of Kondo peak splitting under a magnetic field and bias can be explained by calculating the nonequilibrium retarded Green's function via the nonperturbative dynamical theory (NDT). In the NDT, the application of a lead-dot-lead system reveals that new resonant tunneling levels are activated near the Fermi level and the conventional Kondo peak at the Fermi level diminishes when a bias is applied. Magnetic field causes asymmetry in the spectral density and transforms the new resonant peak into a major peak whose behavior explains all the features of the nonequilibrium Kondo phenomenon. We also show the mechanism of coherent transport through the new resonant tunneling level.

cond-mat.str-el↗