arXiv subjects

Deyi Zhuo

Publications and source records attributed to Deyi Zhuo.

15 recordsLinked to original sources

arXiv2026

Designer Quantum States in Magnetic Topological Insulator Multilayers

Magnetic topological insulators (TIs) provide a highly tunable platform for engineering quantum states that emerge from the interplay between topology and magnetism. In this review article, we summarize experimental progress over the past decade in designing magnetic TI multilayers by molecular beam epitaxy (MBE). By treating magnetically doped and undoped TI layers as topological Legos, we discuss how layer thickness, magnetic doping, heterostructure architecture, and stacking sequence can be used to control magnetic exchange gaps, interlayer coupling, and the Chern number C with atomic-layer precision. We first briefly review the realization of the C = 1 quantum anomalous Hall (QAH) effect in uniformly Cr-doped (Bi,Sb)2Te3 films in 2013 and uniformly V-doped (Bi,Sb)2Te3 films in 2015. We then discuss how Cr-doped and undoped (Bi,Sb)2Te3 layers can be combined to realize the C = 1 QAH effect in magnetically modulation-doped trilayers, including its extension into the three-dimensional (3D) regime. Next, we review the development of high-C QAH states, engineered plateau phase transitions, mesoscopic QAH devices, and electrical switching of chiral edge-current chirality. Finally, we discuss the realizations of axion insulator and C = 1/2 parity anomaly states in asymmetric magnetic TI trilayers. These advances establish magnetic TI multilayers as a versatile materials platform for creating new designer quantum states, including synthetic Weyl semimetal and QAH metal phases, and for probing the topological magnetoelectric effect in thick axion insulators and 3D QAH insulators.

cond-mat.mes-hall↗

arXiv2026

Designing Magnetic Topological Insulator Trilayers for Highly-Efficient Spin-Orbit Torque Switching

Spin-orbit torque (SOT) enables efficient electrical control of magnetization, offering a pathway towards low-power spintronic devices. Magnetic topological insulators (TIs), with spin-momentum-locked surface states and intrinsic ferromagnetism, provide a unique platform for realizing SOT switching of edge current chirality in quantum anomalous Hall (QAH) insulators. In this work, we employ molecular beam epitaxy to synthesize a series of magnetic TI trilayers with controlled layer thicknesses on heat-treated SrTiO3(111) substrates. Electrical transport measurements reveal that SOT-driven magnetization reversal and the associated switching of edge current chirality are governed by the SrTiO3(111) substrate-induced charging effect, which generates an asymmetric chemical-potential alignment between the top and bottom magnetic TI layers. Furthermore, we demonstrate that the switching polarity and efficiency can be tuned through heterostructure design, gate voltage, and in-plane magnetic field, consistent with SOT symmetry. These findings identify chemical potential asymmetry as the origin of the large SOT switching ratio in magnetic TI trilayers and establish a route for electrical control of edge current chirality in QAH insulators. This work advances the understanding of SOT switching mechanism in magnetic topological materials and paves the way for next-generation, energy-efficient QAH-based logic and memory devices.

cond-mat.mes-hall↗

arXiv2025

Boundary-Bulk Interplay in Nonlinear Topological Transport

Nonlinear transport has emerged as a powerful approach to probe the quantum geometry of electronic wavefunctions, such as Berry curvature and quantum metric, in topological materials. While nonlinear responses governed by bulk quantum geometry and band topology are well understood, the role of boundary modes (e.g., edge, surface, and hinge states) in nonlinear transport of topological materials remains largely unexplored. In this work, we demonstrate boundary-bulk interplay in nonlinear transport, including second-harmonic Hall and nonreciprocal longitudinal responses, in molecular beam epitaxy-grown magnetic topological insulator heterostructures. We find that the nonlinear transport is maximized when the sample is tuned slightly away from the well-quantized states, including the quantum anomalous Hall and axion insulator states. The sign and amplitude of the nonlinear transport depend on electrode configuration, magnetic order, and carrier type, establishing boundary mode transport as the dominant contributor. These findings, supported by symmetry analysis and nonlinear Landauer-Büttiker formalism, demonstrate that nonlinear transport in topological materials is governed by the interplay between boundary and bulk states. We further derive a universal relation between different lead voltages from electrode geometry symmetry, which allows us to distinguish nonlinear boundary transport from bulk contributions. Our work highlights the critical role of electrodes in nonlinear transport, which is absent in nonlinear optics, and establishes boundary modes as a key origin of the giant nonlinear response in nearly bulk-insulating topological materials. This insight opens new opportunities for engineering nonlinear transport through boundary-bulk interplay in future device applications of topological materials.

cond-mat.mes-hall↗

arXiv2025

Evidence for Half-Quantized Chiral Edge Current in a C = 1/2 Parity Anomaly State

A single massive Dirac surface band is predicted to exhibit a half-quantized Hall conductance, a hallmark of the C = 1/2 parity anomaly state in quantum field theory. Experimental signatures of the C = 1/2 parity anomaly state have been observed in semi-magnetic topological insulator (TI) bilayers, yet whether it supports a half-quantized chiral edge current remains elusive. Here, we observe a robust half-quantized Hall conductance plateau in a molecular beam epitaxy (MBE)-grown asymmetric magnetic TI trilayer under specific in-plane magnetic field regimes, corresponding to the C = 1/2 parity anomaly state. Within this state, both nonlocal and nonreciprocal transport signals are greatly enhanced, which we identify as direct evidence for a half-quantized chiral edge current localized at the boundary of the top gapped surface. Our numerical simulations demonstrate that this half-quantized chiral edge channel is the essential carrier of the observed half-quantized Hall conductance plateau, analogous to the quantized chiral edge channel in the C = 1 quantum anomalous Hall state. Our results provide experimental evidence for the half-quantized chiral edge transport in a C = 1/2 parity anomaly state. This work establishes asymmetric magnetic TI trilayers as a platform for probing single Dirac fermion physics and paves the way to explore a series of exciting phenomena in the C = 1/2 parity anomaly state, including the topological magnetoelectric effect and quantized magneto-optical response.

cond-mat.mes-hall↗

arXiv2025

Meissner Effect and Nonreciprocal Charge Transport in Non-Topological 1T-CrTe2/FeTe Heterostructures

Interface-induced superconductivity has recently been achieved by stacking a magnetic topological insulator layer on an antiferromagnetic FeTe layer. However, the mechanism driving this emergent superconductivity remains unclear. Here, we employ molecular beam epitaxy to grow a 1T-CrTe2 layer, a two-dimensional ferromagnet with a Curie temperature up to room temperature, on a FeTe layer. These 1T-CrTe2/FeTe heterostructures show superconductivity with a critical temperature of ~12 K. Through magnetic force microscopy measurements, we observe the Meissner effect on the surface of the 1T-CrTe2 layer. Our electrical transport measurements reveal that the 1T-CrTe2/FeTe heterostructures exhibit nonreciprocal charge transport behavior, characterized by a large magneto-chiral anisotropy coefficient. The enhanced nonreciprocal charge transport in 1T-CrTe2/FeTe heterostructures provides a promising platform for exploring the magnetically controllable superconducting diode effect.

cond-mat.supr-con↗

arXiv2025

Gate-Tunable Ambipolar Josephson Current in a Topological Insulator

Dirac surface states in a topological insulator (TI) with proximity-induced superconductivity offer a promising platform for realizing topological superconductivity and Majorana physics. However, in TIs, the Josephson effect is usually observed in regimes where transport is dominated by either substantial bulk conduction channels or unipolar surface states. In this work, we demonstrate gate-tunable ambipolar Josephson current in lateral Josephson junction (JJ) devices based on bulk-insulating (Bi,Sb)2Te3 thin films grown by molecular beam epitaxy (MBE). For thinner films, the supercurrent exhibits pronounced gate-tunable ambipolar behavior and is significantly suppressed as the chemical potential approaches the Dirac point, yet persists across it. In contrast, thicker films exhibit a much weaker ambipolar response. Moreover, we find that the supercurrent becomes significantly less resilient to external magnetic fields when the chemical potential is tuned near the Dirac point in both thickness regimes. Our numerical simulations demonstrate the ambipolar behavior of these TI JJ devices and attribute the asymmetric supercurrent observed in thicker TI films to the coexistence of Dirac surface states and bulk conduction channels. The demonstration of gate-tunable ambipolar Josephson transport in MBE-grown TI films paves the way for realizing Dirac-surface-state-mediated topological superconductivity and establishes a foundation for future exploration of electrically tunable Majorana modes.

cond-mat.mes-hall↗

arXiv2025

Interlayer Exchange Coupling-Induced Critical-Metal-to-Insulator Phase Transition in Quantum Anomalous Hall Insulators

Interlayer exchange coupling (IEC) between two magnetic layers sandwiched by a nonmagnetic spacer layer plays a critical role in shaping the magnetic properties of such heterostructures. The quantum anomalous Hall (QAH) effect has been realized in a structure composed of two magnetically doped topological insulator (TI) layers separated by an undoped TI layer. In this work, we employ molecular beam epitaxy to synthesize a series of magnetic TI sandwiches with varying thicknesses of the middle TI spacer layer. The well-quantized QAH effect is observed in all these samples, and the IEC modulates its critical behavior between the top and bottom magnetic TI layers. Near the plateau phase transition (PPT), thinner QAH samples exhibit a two-dimensional critical metal behavior with nearly temperature-independent longitudinal resistance. In contrast, thicker QAH samples behave as a three-dimensional insulator with reduced longitudinal resistance at higher temperatures. We employ a magnetic TI Hamiltonian with random magnetic domains to understand the IEC-induced critical-metal-to-insulator transition observed near QAH PPT.

cond-mat.mes-hall↗

arXiv2025

Interlayer Coupling-Induced Quantum Phase Transition in Quantum Anomalous Hall Multilayers

A quantum phase transition arises from competition between different ground states and is typically accessed by varying a single physical parameter near absolute zero temperature. The quantum anomalous Hall (QAH) effect with high Chern number C has recently been achieved in magnetic topological insulator (TI) multilayers. In this work, we employ molecular beam epitaxy to synthesize a series of magnetic TI penta-layers by varying the thickness of the middle magnetic TI layer, designated as m quintuple layers. Electrical transport measurements demonstrate a quantum phase transition between C = 1 and C = 2 QAH states. For m 1 and m 2, the sample exhibits the well-quantized C = 1 and C = 2 QAH states, respectively. For 1 m 2, we observe a monotonic decrease in Hall resistance from h/e2 to h/2e2 with increasing m, accompanied by a peak in the longitudinal resistance. The quantum phase transition between C = 1 and C = 2 QAH states is attributed to the weakening of the interlayer coupling between the top and the bottom C = 1 QAH layers. Our findings provide a scalable strategy for engineering QAH devices with a tunable Chern number. This approach enables precise control and enhanced functionality in chiral edge current-based electronic devices.

cond-mat.mes-hall↗

arXiv2024

Engineering Plateau Phase Transition in Quantum Anomalous Hall Multilayers

The plateau phase transition in quantum anomalous Hall (QAH) insulators corresponds to a quantum state wherein a single magnetic domain gives way to multiple magnetic domains and then re-converges back to a single magnetic domain. The layer structure of the sample provides an external knob for adjusting the Chern number C of the QAH insulators. Here, we employ molecular beam epitaxy (MBE) to grow magnetic topological insulator (TI) multilayers with an asymmetric layer structure and realize the magnetic field-driven plateau phase transition between two QAH states with odd Chern number change ΔC. In multilayer structures with C=+-1 and C=+-2 QAH states, we find two characteristic power-law behaviors between temperature and the scaling variables on the magnetic field at transition points. The critical exponents extracted for the plateau phase transitions with ΔC=1 and ΔC=3 in QAH insulators are found to be nearly identical, specifically, k1~0.390+-0.021 and k2~0.388+-0.015, respectively. We construct a four-layer Chalker-Coddington network model to understand the consistent critical exponents for the plateau phase transitions with ΔC=1 and ΔC=3. This work will motivate further investigations into the critical behaviors of plateau phase transitions with different ΔC in QAH insulators and provide new opportunities for the development of QAH chiral edge current-based electronic and spintronic devices.

cond-mat.mes-hall↗

arXiv2023

Three-Dimensional Quantum Anomalous Hall Effect in Magnetic Topological Insulator Trilayers of Hundred-Nanometer Thickness

Magnetic topological states refer to a class of exotic phases in magnetic materials with their non-trivial topological property determined by magnetic spin configurations. An example of such states is the quantum anomalous Hall (QAH) state, which is a zero magnetic field manifestation of the quantum Hall effect. Current research in this direction focuses on QAH insulators with a thickness of less than 10nm. The thick QAH insulators in the three-dimensional(3D) regime are limited, largely due to inevitable bulk carriers being introduced in thick magnetic TI samples. Here, we employ molecular beam epitaxy (MBE) to synthesize magnetic TI trilayers with a thickness of up to ~106 nm. We find these samples exhibit well-quantized Hall resistance and vanishing longitudinal resistance at zero magnetic field. By varying magnetic dopants, gate voltages, temperature, and external magnetic fields, we examine the properties of these thick QAH insulators and demonstrate the robustness of the 3D QAH effect. The realization of the well-quantized 3D QAH effect indicates that the nonchiral side surface states of our thick magnetic TI trilayers are gapped and thus do not affect the QAH quantization. The 3D QAH insulators of hundred-nanometer thickness provide a promising platform for the exploration of fundamental physics, including axion physics and image magnetic monopole, and the advancement of electronic and spintronic devices to circumvent Moore's law.

cond-mat.mes-hall↗

arXiv2023

Axion Insulator State in Hundred-Nanometer-Thick Magnetic Topological Insulator Sandwich Heterostructures

An axion insulator is a three-dimensional (3D) topological insulator (TI), in which the bulk maintains the time-reversal symmetry or inversion symmetry but the surface states are gapped by surface magnetization. The axion insulator state has been observed in molecular beam epitaxy (MBE)-grown magnetically doped TI sandwiches and exfoliated intrinsic magnetic TI MnBi2Te4 flakes with an even number layer. All these samples have a thickness of ~10 nm, near the 2D-to-3D boundary. The coupling between the top and bottom surface states in thin samples may hinder the observation of quantized topological magnetoelectric response. Here, we employ MBE to synthesize magnetic TI sandwich heterostructures and find that the axion insulator state persists in a 3D sample with a thickness of ~106 nm. Our transport results show that the axion insulator state starts to emerge when the thickness of the middle undoped TI layer is greater than ~3 nm. The 3D hundred-nanometer-thick axion insulator provides a promising platform for the exploration of the topological magnetoelectric effect and other emergent magnetic topological states, such as the high-order TI phase.

cond-mat.mes-hall↗

arXiv2022

Environmental Doping-Induced Degradation of the Quantum Anomalous Hall Insulators

The quantum anomalous Hall (QAH) insulator is a topological quantum state with quantized Hall resistance and zero longitudinal resistance in the absence of an external magnetic field. The QAH insulator carries spin-polarized dissipation-free chiral edge current and thus provides a unique opportunity to develop energy-efficient transformative information technology. Despite promising advances on the QAH effect over the past decade, the QAH insulator has thus far eluded any practical applications. In addition to its low working temperature, the QAH state in magnetically doped topological insulator (TI) films/heterostructures usually deteriorates with time in ambient conditions. In this work, we prepare three QAH devices with similar initial properties and store them in different environments to investigate the evolution of their transport properties. The QAH device without a protection layer in air show clear degradation and becomes hole-doped with the charge neutral point shifting significantly to positive gate voltages. The QAH device kept in an argon glove box without a protection layer shows no measurable degradation after 560 hours and the device protected by a 3 nm AlOx protection layer in air shows minimal degradation with stable QAH properties. Our work shows a route to preserve the dissipation-free chiral edge state in QAH devices for potential applications in quantum information technology.

cond-mat.mes-hall↗

arXiv2022

Creation of Chiral Interface Channels for Quantized Transport in Magnetic Topological Insulator Multilayer Heterostructures

One-dimensional (1D) topologically protected states are usually formed at the interface between two-dimensional (2D) materials with different topological invariants. Therefore, 1D chiral interface channels (CICs) can be created at the boundary of two quantum anomalous Hall (QAH) insulators with different Chern numbers. Such a QAH junction can function as a chiral edge current distributer at zero magnetic field, but its realization remains challenging. Here, by employing an in-situ mechanical mask, we use molecular beam epitaxy (MBE) to synthesize QAH insulator junctions, in which two QAH insulators with different Chern numbers are connected along a 1D junction. For the junction between C = 1 and C = -1 QAH insulators, we observe quantized transport and demonstrate the appearance of the two parallel propagating CICs along the magnetic domain wall at zero magnetic field. Moreover, since the Chern number of the QAH insulators in magnetic topological insulator (TI)/TI multilayers can be tuned by altering magnetic TI/TI bilayer periods, the junction between two QAH insulators with arbitrary Chern numbers can be achieved by growing different periods of magnetic TI/TI on the two sides of the sample. For the junction between C = 1 and C = 2 QAH insulators, our quantized transport shows that a single CIC appears at the interface. Our work lays down the foundation for the development of QAH insulator-based electronic and spintronic devices, topological chiral networks, and topological quantum computations.

cond-mat.mes-hall↗

arXiv2022

Confinement-Induced Chiral Edge Channel Interaction in Quantum Anomalous Hall Insulators

In quantum anomalous Hall (QAH) insulators, the interior is insulating but electrons can travel with zero resistance along one-dimensional conducting paths known as chiral edge channels (CECs). These CECs have been predicted to be confined to the one-dimensional (1D) edges and exponentially decay in the two-dimensional (2D) bulk. In this work, we present the results of a systematic study of QAH devices fashioned in a Hall bar geometry of different widths. At the charge neutral point, the QAH effect persists in a Hall bar device with a width of only ~72 nm, implying the intrinsic decaying length of CECs is less than ~36 nm. In the electron-doped regime, we find that the Hall resistance deviates quickly from the quantized value when the sample width is less than 1 um. Our theoretical calculations suggest that the deviation from the quantized Hall resistance in narrow QAH samples originates from the interaction between two opposite CECs mediated by disorder-induced bulk states in QAH insulators, consistent with our experimental observations.

cond-mat.mes-hall↗

arXiv2022

Electrical Switching of the Edge Current Chirality in Quantum Anomalous Hall Insulators

A quantum anomalous Hall (QAH) insulator is a topological state of matter, in which the interior is insulating but electrical current flows along the edges of the sample, in either clockwise (right-handed) or counter-clockwise (left-handed) direction dictated by the spontaneous magnetization orientation. Such chiral edge current (CEC) eliminates any backscattering, giving rise to quantized Hall resistance and zero longitudinal resistance. In this work, we fabricate mesoscopic QAH sandwich (i.e. magnetic topological insulator (TI)/TI/magnetic TI) Hall bar devices and succeed in switching the CEC chirality in QAH insulators through spin-orbit torque (SOT) by applying a current pulse and suitably controlled gate voltage. The well-quantized QAH states with opposite CEC chiralities are demonstrated through four- and three-terminal measurements before and after SOT switching. Our theoretical calculations show that the SOT that enables the magnetization switching can be generated by both bulk and surface carriers in QAH insulators, in good agreement with experimental observations. Current pulse-induced switching of the CEC chirality in QAH insulators will not only advance our knowledge in the interplay between magnetism and topological states but also expedite easy and instantaneous manipulation of the QAH state in proof-of-concept energy-efficient electronic and spintronic devices as well as quantum information applications.

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