arXiv subjects

Zhuo Bin Siu

Publications and source records attributed to Zhuo Bin Siu.

At least 19 recordsLinked to original sources

arXiv2026

Quantized Conductance through Surface States in High Quality Three-Dimensional Dirac Semimetal Cd$_3$As$_2$ Nanowire/Nanoribbon p-n Junctions

We report the observation of quantized conductance in high-mobility three-dimensional Dirac semimetal Cd$_3$As$_2$ nanowire and nanoribbon p-n junctions. By employing suspended device geometries with dual local gates, we form tunable p-n junctions and realize ballistic transport across sub-micron channel lengths. In a wide nanoribbon device with a channel width of $\sim 330$ nm, conductance plateaus appear at integer multiples of $2e^2/h$ in the n-n regime under high magnetic fields. Numerical simulations suggest that these features represent unresolved spin-split subbands due to the smaller subband spacing in wider channels, and support the interpretation that the observed quantization may originate from surface-state-dominated conduction. In contrast, narrower nanoribbons and nanowires exhibit conductance steps of $1e^2/h$, demonstrating spin-resolved subbands likely due to enhanced confinement effects. From spin-resolved subband spectroscopy, we extract an effective Landé $g$-factor of $\sim 43$ for the first subband in the bulk gap, establishing these nanostructures as a prospective platform for fault-tolerant quantum electronics.

cond-mat.mes-hall↗

arXiv2025

Radio-Frequency Detection of Fabry-Pérot Interference and Quantum Capacitance in Long-Channel Three-Dimensional Dirac Semimetal Cd3As2 Nanowires

We demonstrate phase-coherent transport in suspended long-channel Cd3As2 nanowire devices using both direct current (DC) transport and radio-frequency (RF) reflectometry measurements. By integrating Cd3As2 nanowires with on-chip superconducting LC resonators, we achieve sensitive detection of both resistance and quantum capacitance variations. In a long-channel device (L ~ 1.8 μm), clear Fabry-Pérot (FP) interference patterns are observed in both DC and RF measurements, provide strong evidence for ballistic electron transport. RF reflectometry reveals gate-dependent modulations of the resonance frequency, arising from quantum capacitance oscillations induced by changes in the density of states and FP interference. These oscillations exhibit a quasi-periodic structure that closely correlates with the FP patterns in DC transport measurements. In another device of a Cd3As2 nanowire Josephson junction (L ~ 730 nm, superconducting Al contacts), FP interference patterns are too weak to be resolved in DC conductance but are detectable using RF reflectometry. These results demonstrate the high quality of our Cd3As2 nanowires and the versatility of RF reflectometry, establishing their potential for applications in topological quantum devices, such as Andreev qubits or gatemon architectures.

cond-mat.mes-hall↗

arXiv2025

NHSE-Driven Coalescence of Topological Defect States in Non-Hermitian Systems

In this work, we describe a novel localization phenomena, the so-called topological defect accumulation, occurring in a non-Hermitian chain with an arbitrary number of defect sites. Specifically, it refers to the localization and coalescence of multiple defect eigenstates at a single defect site closest to the localization edge of the bulk non-Hermitian skin modes. This phenomenon is distinct from the conventional topological defect states in Hermitian systems, where the defect states are separately localized at their respective defect sites. The requirement for the onset of topological defect accumulation is the presence of non-reciprocal coupling which renders the chain non-Hermitian, as well as imaginary onsite potentials at the defect nodes. This allows the defect state accumulation and distribution to be modulated by both the defect site distribution and their corresponding onsite potentials paving the way for possible applications. Furthermore, this phenomenon is realizable in various synthetic and experimentally realizable platforms, such as topolectrical and photonic systems.

physics.optics↗

arXiv2025

Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control

We investigate the size-dependent behavior of topological zero modes (TZMs) in finite non-Hermitian Su-Schrieffer-Heeger (SSH) chains implemented on a topolectrical circuit platform. By deriving exact analytical solutions for the eigenenergies and band gaps of TZMs, we reveal their sensitivity to system size and non-Hermitian parameters, such as asymmetric coupling and onsite gain or loss. Our results show that non-Hermiticity enables the recovery of exactly zero-energy TZMs at a critical system size, unlike Hermitian systems where finite-size effects cause energy splitting. These zero-energy modes produce pronounced impedance peaks in the circuit's admittance spectrum, providing a measurable signature of topological states. Additionally, a tunable grounding capacitor enables precise control of TZM energies at a fixed resonance frequency, enhancing practical tunability. Our findings offer insights into finite-size effects in non-Hermitian topological systems and guide the design of robust, reconfigurable topolectrical circuits for sensing and energy transfer applications.

cond-mat.mes-hall↗

arXiv2025

Protected chaos in a topological lattice

The erratic nature of chaotic behavior is thought to erode the stability of periodic behavior, including topological oscillations. However, we discover that in the presence of chaos, non-trivial topology not only endures but also provides robust protection to chaotic dynamics within a topological lattice hosting non-linear oscillators. Despite the difficulty in defining topological invariants in non-linear settings, non-trivial topological robustness still persists in the parametric state of chaotic boundary oscillations. We demonstrate this interplay between chaos and topology by incorporating chaotic Chua's circuits into a topological Su-Schrieffer-Heeger (SSH) circuit. By extrapolating from the linear limit to deep into the non-linear regime, we find that distinctive correlations in the bulk and edge scroll dynamics effectively capture the topological origin of the protected chaos. Our findings suggest that topologically protected chaos can be robustly achieved across a broad spectrum of periodically-driven systems, thereby offering new avenues for the design of resilient and adaptable non-linear networks.

cond-mat.mes-hall↗

arXiv2025

Reconfigurable Defect States in Non-Hermitian Topolectrical Chains with Gain and Loss

We investigate the interplay between the non-Hermitian skin effect (NHSE), parity-time (PT) symmetry, and topological defect states in a finite non-Hermitian Su-Schrieffer-Heeger (SSH) chain. In the conventional NHSE regime, non-reciprocal hopping leads to an asymmetric localization of all eigenstates at one edge of the system, including the bulk and topological edge states. However, the introduction of staggered gain and loss restores the symmetric localization of topological edge states while preserving the bulk NHSE. We further examine the response of defect states in this system, demonstrating that their spatial localization is dynamically controlled by the combined effects of NHSE and PT symmetry. Specifically, we identify three distinct regimes in which the defect states localize at the defect site, shift to the system's edges, or become completely delocalized. These findings extend beyond previous works that primarily explored the activation and suppression of defect states through gain-loss engineering. To validate our theoretical predictions, we propose an experimental realization using a topolectrical circuit, where non-Hermitian parameters are implemented via impedance converter-based non-reciprocal elements. Circuit simulations confirm the emergence and tunability of defect states through voltage and admittance measurements, providing a feasible platform for experimental studies of non-Hermitian defect engineering. Our results establish a route for designing reconfigurable non-Hermitian systems with controllable topological defect states, with potential applications in robust signal processing and sensing.

cond-mat.mes-hall↗

arXiv2024

Anomalous Non-Hermitian Skin Effects in Coupled Hermitian Chains with Cross-Coupling

In this work, we demonstrate the presence of an anomalous non-Hermitian skin effects which decay from both ends of a system consisting of two coupled Hermitian chains induced by non-reciprocal inter-chain cross-coupling. Another intriguing feature of the system is that its eigenenergy spectrum in thermodynamic limit deviates from the generalized Brillouin zone (GBZ), contrary to that of conventional non-Hermitian systems. The thermodynamic-limit energy spectrum is however restored to the GBZ by the presence of even an infinitesimal amount of gain and/or loss term to the system. In this case, the system exhibits a critical phenomena similar to that of coupled non-Hermitian chains, whereby the eigenspectrum starts to approach the GBZ beyond some critical size. Furthermore, the non Hermitian skin effect becomes less pronounced as the system size exceeds this critical size. We analytically explain these peculiar features which highlight the important role of gain and loss terms as well as inter-chain coupling in tuning non-Hermitian skin modes, thus suggesting a new avenue for the modulation of the skin mode characteristics of open systems.

physics.optics↗

arXiv2024

Dynamic Manipulation of Non-Hermitian Skin Effect through Frequency in Topolectrical Circuits

One of the most fascinating phenomena in non-Hermitian systems is the extensive accumulation of the bulk eigenstates under open-boundary conditions which is known as the non-Hermitian skin effect (NSHE). Here, we propose a switchable NHSE in a topolectrical (TE) set-up which can be turned on or off simply by varying the driving frequency without any modification to the physical circuit. Specifically, we consider a coupled system consisting of two non-Hermitian Hatano-Nelson chains where each node of one chain is connected to neighboring node of the other chain via resistive couplings with opposite signs for the two coupling directions. Interestingly, the NHSE is switched on only if the driving frequency is greater than a certain critical frequency. Conversely, the NHSE in the coupled system is switched off when the frequency falls below the critical value, even though the individual uncoupled chains still exhibit the NHSE. This frequency-controlled NHSE may pave the way for many possible applications including non-Hermitian sensors where the driving frequency can manipulate the current and voltage localization.

cond-mat.mes-hall↗

arXiv2024

Exceptional Points and Braiding Topology in Non-Hermitian Systems with long-range coupling

We present a study of complex energy braiding in a 1D non-Hermitian system with $n$th order long range asymmetrical coupling. Our work highlights the emergence of novel topological phenomena in such systems beyond the conventional nearest-neighbor interaction. The modified SSH model displays $n$ distinct knots and links combinations in the complex energy-momentum space under periodic boundary conditions (PBC), which can be controlled by varying the coupling strengths. A topological invariant, namely the braiding index, is introduced to characterize the different complex energy braiding profiles, which depends on the zeros and poles of the characteristic polynomials. Furthermore, we demonstrate that the non-Hermitian skin effect can be localized at one or both ends, signifying conventional or bipolar localization, depending on the sign of the braiding index. Phase transitions between different braiding phases with the same (opposite) sign of the topological invariant occur at Type-1 (Type-2) exceptional points, with Type-1 (Type-2) phase transitions accompanied by single (multiple) exceptional points. We propose an experimental set-up to realize the various braiding schemes based on the RLC circuit framework, which provides an accessible avenue for implementation without recourse to high-dimensional momentum space required in most other platforms.

quant-ph↗

arXiv2024

Saturation Dynamics in Non-Hermitian Topological Sensing Systems

A class of non-Hermitian topological sensors (NTOSs) was recently proposed in which the NTOS comprises a non-Hermitian Su-Schrieffer-Heeger chain with a measurant-dependent coupling between the two ends of the chain. The smallest eigenenergy of the system, which serves as the readout signal, has an exponential dependence on the system size at small system sizes but saturates above a critical size. In this study, we further elucidate the dependence of the sensor sensitivity and saturation behavior on the system parameters. We explain how the behavior of the NTOS is characterized by a winding number, which indicates whether the smallest eigenenergy decreases to zero exponentially with the system size or grows exponentially up to a critical size. Interestingly, we further show that by imposing unidirectionality on the coupling between the two ends of a sensor, we can flip the size dependence of the smallest eigenenergy value from an exponentially increasing trend to an exponentially decreasing one. Our findings provide important insights into the saturation phenomenon and the impact of terminal couplings on the sensing characteristics of NTOSs.

quant-ph↗

arXiv2024

Engineering Higher-Order Dirac and Weyl Semimetallic phase in 3D Topolectrical Circuits

We propose a 3D topolectrical (TE) network that can be tuned to realize various higher-order topological gapless and chiral phases. We first study a higher-order Dirac semimetal phase that exhibits a hinge-like Fermi arc linking the Dirac points. This circuit can be extended to host highly tunable first- and second-order Weyl semimetal phases by introducing a non-reciprocal resistive coupling in the x-y plane that breaks time reversal symmetry. The first- and second-order Weyl points are connected by zero-admittance surface and hinge states, respectively. We also study the emergence of first- and second-order chiral modes induced by resistive couplings between similar nodes in the z-direction. These modes respectively occur in the midgap of the surface and hinge admittance bands in our circuit model without the need for any external magnetic field.

cond-mat.mes-hall↗

arXiv2024

Second-order charge and spin transport in LaO/STO system in the presence of cubic Rashba spin orbit couplings

Certain non-centrosymmetric materials with broken time-reversal symmetry may exhibit non-reciprocal transport behavior under an applied electric field in which the charge and spin currents contain components that are second order in the electric field. In this study, we investigate the second-order spin accumulation and charge and spin responses in the LaAlO$_3$/SrTiO$_3$ (LaO/STO) system with magnetic dopants under the influence of linear and cubic Rashba spin-orbit coupling (RSOC) terms. We explain the physical origin of the second-order response and perform a symmetry analysis of the first and second-order responses for different dopant magnetization directions relative to the applied electric field. We then numerically solve the Boltzmann transport equation by extending the approach of Schliemann and Loss [Phys. Rev. B 68, 165311] to higher orders in the electric field. We show that the sign of the second-order responses can be switched by varying the magnetization direction of the magnetic dopants or relative strengths of the two cubic RSOC terms and explain these trends by considering the Fermi surfaces of the respective systems. These findings provide insights into the interplay of multiple SOC effects in a LaO/STO system and how the resulting first- and second-order charge and spin responses can be engineered by exploiting the symmetries of the system.

cond-mat.mes-hall↗

arXiv2023

Twisted topology and Bipolar Non-Hermitian Skin Effect induced by long-range asymmetric coupling

We investigate the twisted topology of the complex eigenspectrum of a one-dimensional non-Hermitian system under the influence of long-range unidirectional coupling. Unlike the complex energy spectrum of the conventional Hatano-Nelson chain, which takes the form of a single loop with a topological winding index of a definite sign, the introduction of long-range unidirectional hopping results in the creation of multiple twisted loops. These twisted loops exhibit opposite signs of the topological winding index, which correlate to alternating clockwise and anticlockwise energy windings. The simultaneous presence of both signs of the winding index translates into a bipolar non-Hermitian skin effect (NHSE), which challenges the conventional wisdom that the NHSE localization is dependent on the direction of the dominant nearest-neighbor interactions. In this bipolar NHSE, the exponents of the complex energy eigenvectors corresponding to clockwise and anti-clockwise windings, lie inside and outside of the complex unit circle, respectively. Interestingly, at the intersections of oppositely oriented energy loops where the sign of the topological winding index flips, the energy becomes real-valued, leading to a suppression of the NHSE. This marks the emergence of Bloch-like contact points, where both the bipolar NHSE and the traditional NHSE vanish. Based on the non-Hermitian model we provide analytical insights into the effects of long-range unidirectional coupling to the winding topology of its complex energy spectra and their broader implications for the field of condensed matter physics.

cond-mat.mes-hall↗

arXiv2023

Impedance responses and size-dependent resonances in topolectrical circuits via the method of images

Resonances in an electric circuit occur when capacitive and inductive components are present together. Such resonances appear in admittance measurements depending on the circuit's parameters and the driving AC frequency. In this study, we analyze the impedance characteristics of nontrivial topolectrical circuits such as one- and two-dimensional Su-Schrieffer-Heeger circuits and reveal that size-dependent anomalous impedance resonances inevitably arise in finite $LC$ circuits. Through the \textit{method of images}, we study how resonance modes in a multi-dimensional circuit array can be nontrivially modified by the reflection and interference of current from the structure and boundaries of the lattice. We derive analytic expressions for the impedance across two corner nodes of various lattice networks with homogeneous and heterogeneous circuit elements. We also derive the irregular dependency of the impedance resonance on the lattice size, and provide integral and dimensionally-reduced expressions for the impedance in three dimensions and above.

cond-mat.mes-hall↗

arXiv2023

Anomalous fractal scaling in two-dimensional electric networks

Much of the qualitative nature of physical systems can be predicted from the way it scales with system size. Contrary to the continuum expectation, we observe a profound deviation from logarithmic scaling in the impedance of a two-dimensional $LC$ circuit network. We find this anomalous impedance contribution to sensitively depend on the number of nodes $N$ in a curious erratic manner, and experimentally demonstrate its robustness against perturbations from the contact and parasitic impedance of individual components. This impedance anomaly is traced back to a generalized resonance condition reminiscent of the Harper's equation for electronic lattice transport in a magnetic field, even though our circuit network does not involve magnetic translation symmetry. It exhibits an emergent fractal parametric structure of anomalous impedance peaks for different $N$ that cannot be reconciled with continuum theory and does not correspond to regular waveguide resonant behavior. This anomalous fractal scaling extends to the transport properties of generic systems described by a network Laplacian whenever a resonance frequency scale is simultaneously present.

cond-mat.mes-hall↗

arXiv2022

Valley Hall Effect and Kink States in Topolectrical Circuits

We investigate the emergence of topological valley Hall and kink states in a two-dimensional topolectrical (TE) model as a result of broken chiral and reflection symmetries. The TE system consists of two segments hosting distinct topological states with opposite signs of the valley Hall index, and separated by a heterojunction. In the practical circuit, the valley Hall index can be flipped between the two segments by modulating the onsite potential on the sublattice nodes of the respective segments. The presence of resistive coupling, which introduces non-Hermiticity in the system, subsequently leads to the emergence of gapped and gapless valley and kink states in the admittance spectra. These topological modes can be detected electrically by the impedance readouts of the system which can be correlated to its admittance spectra. Finally, we confirm the robustness of the valley Hall and kink states via realistic LTspice simulation taking into account the tolerance windows and parasitic effects inherent in circuit components. Our study demonstrates the applicability of TE circuit networks as a platform to realize and tune valley-dependent and kink topological phenomena.

cond-mat.mes-hall↗

arXiv2022

Semiclassical spin transport in LaO/STO system in the presence of multiple Rashba spin orbit couplings

The interaction between the linear and cubic spin-orbit coupling with magnetic moments and mobile spin-polarized carriers in the LaO/STO system provides new avenues for spin transport applications. We study the interplay between linear and cubic Rashba spin orbit coupling (RSOC) on in-plane magnetic moments in the LaO/STO system using the Boltzmann transport theory based on the relaxation time approximation (RTA) and the more refined Schliemann-Loss (SL) delta-potential scattering model. In general, both methods yield a linear (quadratic) relationship between the spin accumulation (spin current) when one of the three RSOC strengths is varied and the other two fixed. The simultaneous presence of multiple types of RSOC with distinct angular dependences is a key ingredient in breaking the k-space symmetry of the Fermi surface, thus ensuring a finite spin accumulation upon integration over the entire Fermi surface. While the oft-used RTA method is sufficiently accurate for spin accumulation calculations, the more refined SL model is required for spin current calculations because the RTA method neglects the anisotropy of the Fermi contour arising from the cubic RSOC terms. Based on the refined SL model and under optimal tuning of the RSOC parameters, the spin charge conversion values in LaO/STO is predicted to reach a remarkable efficiency of 30.

cond-mat.mes-hall↗

arXiv2022

System size dependent topological zero modes in coupled topolectrical chains

In this paper, we demonstrate the emergence and disappearance of topological zero modes (TZMs) in a coupled topolectrical (TE) circuit lattice. Specifically, we consider non-Hermitian TE chains in which TZMs do not occur in the individual uncoupled chains, but emerge when these chains are coupled by inter-chain capacitors. The coupled system hosts TZMs which show size-dependent behaviours and vanish beyond a certain critical size. In addition, the emergence or disappearance of the TZMs in the open boundary condition (OBC) spectra for a given size of the coupled system can be controlled by modulating the signs of its inverse decay length. Analytically, trivial and non-trivial phases of the coupled system can be distinguished by the differing ranks of their corresponding Laplacian matrix. The TE circuit framework enables the physical detection of the TZMs via electrical impedance measurements. Our work establishes the conditions for inducing TZMs and modulating their behavior in coupled TE chains.

cond-mat.mes-hall↗