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Zi-Jie Yan

Publications and source records attributed to Zi-Jie Yan.

At least 19 recordsLinked to original sources
arXiv2026

Signatures of nodal superconductivity in stoichiometric FeTe

Superconductivity in stoichiometric FeTe opens access to the FeTe endpoint of the Fe(Se,Te) phase diagram, yet the nature of its superconducting pairing state remains unresolved. In this work, we combine scanning superconducting quantum interference device (SQUID) microscopy, electrical transport, scanning tunneling microscopy and spectroscopy (STM/S), and mean-field calculations to investigate the local superfluid response and pairing state of FeTe thin films with tunable stoichiometry. Even in stoichiometric FeTe, we observe micrometer-scale spatial variations in both superfluid stiffness and superconducting transition temperature $T_c$, while the London penetration depth remains non-saturating down to 0.02$T_c$ and follows a power-law temperature dependence with an exponent of approximately 1-1.5. Together with a V-shaped low-energy density of states and two-gap modeling, these results indicate a superconducting state with gap nodes or deep minima, consistent with either a $d$-wave or nodal $s$-wave superconducting state. Our findings establish stoichiometric FeTe as a distinct superconducting regime that departs from the trend toward more isotropic gaps at intermediate Se/Te compositions, providing a new benchmark for modern microscopic theories of iron-chalcogenide superconductivity. Our work also reveals a crossover from weak to rapid suppression of $T_c$ as superfluid stiffness decreases, connecting FeTe to the broader phenomenology observed in unconventional superconductors.

cond-mat.supr-con↗
arXiv2026

Interface-Confined Superconductivity with Thickness-Independent Superfluid Stiffness in (Pb,Sn)Te/FeTe Bilayers

Interface-induced superconductivity in FeTe-based heterostructures provides a promising route toward topological superconductivity, yet the roles of the neighboring layers topology, symmetry, and electronic structure remain unresolved. In this work, we employ molecular beam epitaxy to grow Pb1-xSnxTe/FeTe bilayers and use angle-resolved photoemission spectroscopy to track the evolution of the Pb1-xSnxTe layer from a trivial insulator to a topological crystalline insulator hosting multiple Dirac surface states. Electrical transport measurements reveal robust superconductivity throughout the entire composition range, with a nearly constant superconducting transition temperature of ~12 K despite substantial changes in the electronic structure and topology of Pb1-xSnxTe. Double-coil mutual-inductance measurements further reveal comparable superfluid stiffness across the topological phase transition and nearly thickness-independent superfluid stiffness despite large variations in the constituent-layer thicknesses, demonstrating that superconductivity is confined near the interface. These results establish that superconductivity in FeTe-based heterostructures is largely insensitive to the topology, crystal symmetry, and detailed electronic structure of the neighboring layer, supporting a primary origin in modifications to the FeTe layer. The coexistence of interface-confined superconductivity and tunable multiple Dirac surface states in Pb1-xSnxTe/FeTe bilayers provides a versatile platform for exploring topological superconductivity and interactions among multiple Majorana zero modes.

cond-mat.supr-con↗
arXiv2026

Imaging Surface Magnetization in Altermagnetic MnTe Films

Altermagnets with pronounced spin-splitting band structure, unconventional magnetic and crystal symmetries, and exotic magneto-transport properties have received immense interest in cutting-edge spintronics, materials science, and condensed matter physics research. Microscopic imaging of spontaneous magnetic domains and phases in altermagnets constitutes an important step for investigating their underlying material properties, mechanisms, and spin behaviors. Taking advantage of scanning-probe quantum microscopy, here we report nanoscale quantum sensing of a prototypical altermagnet candidate $α$-MnTe. We visualize evanescent magnetization and the associated magnetic domains in epitaxial MnTe films, which allows external magnetic fields to control the intrinsic altermagnetic order and configurations. By evaluating a series of MnTe films with different thicknesses down to the atomic scale, we further present evidence for the interfacial origin of the observed weak magnetization and show its correlation with the anomalous Hall effect in MnTe film. Our results advance the current understanding of emergent altermagnetism, providing insights into future material design of altermagnet-integrated spintronic devices.

cond-mat.mtrl-sci↗
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↗
arXiv2026

Stoichiometric FeTe is a Superconductor

Iron-based superconductors are a fascinating family of materials in which multiple electronic bands and strong antiferromagnetic (AFM) correlations are key ingredients for competing ground states, including antiferromagnetism, electronic nematicity, and unconventional superconductivity. FeTe, unlike its superconducting isostructural counterpart FeSe, has long been regarded as an AFM metal sans superconductivity. In this work, we employ molecular beam epitaxy to grow FeTe films and perform post-growth annealing under a Te flux. By performing spin-polarized scanning tunneling microscopy and spectroscopy, we demonstrate that the AFM order in as-grown FeTe films is induced by interstitial Fe atoms that disrupt the ideal 1:1 stoichiometry. Remarkably, the removal of these interstitial Fe atoms through Te annealing yields stoichiometric FeTe films that show no AFM order and instead exhibit robust superconductivity with a critical temperature of ~13.5K. This superconducting state is further confirmed by the observation of Cooper pair tunneling, zero electrical resistance, and the Meissner effect. Therefore, our results demonstrate that stoichiometric FeTe is inherently a superconductor, overturning a long-held view that it is an AFM metal. This work clarifies the origin of superconductivity in FeTe-based heterostructures and demonstrates the importance of stoichiometry control in understanding the competition between AFM and superconductivity in iron-based superconductors.

cond-mat.supr-con↗
arXiv2026

Moire Engineering of Cooper-Pair Density Modulation States

Cooper-pair density modulation (CPDM) states are superconducting phases in which the order parameter varies periodically in real space without breaking translational symmetry. Recently, moire superlattices in layered materials have emerged as powerful platforms for engineering charge density with tunable lattice symmetry, offering a new route to creating and controlling CPDM states. In this work, we demonstrate moire-induced CPDM states in a bilayer heterostructure formed by epitaxially stacking one quintuple layer (1 QL) of topological insulator Sb2Te3 on a six-unit-cell (6 UC) antiferromagnetic FeTe layer. Scanning tunneling microscopy and spectroscopy (STM/S) measurements reveal a moiré superlattice formed between the hexagonal Te lattice of Sb2Te3 and the square Te lattice of FeTe, which spatially modulates the two superconducting gaps of the 1 QL Sb2Te3/6 UC FeTe bilayer. Our Josephson STM/S measurements provide direct real-space imaging of the CPDM states with a wavelength corresponding to the periodicity of the moire superlattice. By substituting Sb2Te3 with Bi2Te3, we achieve control over both the periodicity and magnitude of the CPDM states. Our work demonstrates an epitaxial strategy for synthesizing moire superlattices from materials with different crystal symmetries and reveals a new mechanism for engineering CPDM states in designer bilayer heterostructures.

cond-mat.supr-con↗
arXiv2026

Surface-State-Driven Anomalous Hall Effect in Altermagnetic MnTe Films

Altermagnets have recently emerged as a new class of magnetic materials that combine compensated magnetic order with spin-split electronic band structures. In this work, we employ molecular beam epitaxy to grow MnTe thin films with controlled thickness on InP(111)A substrates. By performing angle-resolved photoemission spectroscopy measurements, we observe a large spin splitting of ~230 meV for bulk bands well below the Fermi level and identify surface states that cross the Fermi level. Electrical transport measurements reveal that a robust anomalous Hall (AH) effect persists down to 2 K and an AH sign reversal occurs near 175 K. By systematically tuning film thickness, growth conditions, and interfacial structure, we demonstrate that the AH response in MnTe films originates from the Berry curvature of surface states rather than from bulk bands. Our first-principles calculations reveal that this surface-state-driven AH effect is imprinted by the bulk altermagnetic order and remains unchanged for terminations with opposite Mn magnetic orientations. Our results establish a unique surface transport probe of bulk altermagnetism, demonstrate interface engineering as an effective route to generate and control the AH effect in altermagnets, and provide a unified understanding of the AH response in altermagentic MnTe films.

cond-mat.mtrl-sci↗
arXiv2026

Interplay of Quantum Size Effect and Tensile Strain on Surface Morphology of Sn(100) Islands

The quantum size effect (QSE) and strain effect are two key factors influencing the surface morphology of thin films, which can increase film surface roughness through QSE-induced thickness oscillation and strain-induced island formation, respectively. Surface roughness usually manifests in the early stages of film growth and diminishes beyond a critical thickness. In this work, we employ molecular beam epitaxy (MBE) to grow Sn(100) islands with varying thickness N on bilayer graphene-terminated 6H-SiC(0001) substrates. Scanning tunneling microscopy and spectroscopy measurements reveal an inverse surface roughness effect that highlights the interplay of QSE and misfit strain in shaping the surface morphology of Sn(100) islands. For N =< 10, the islands exhibit flat surfaces, while for N >= 26, the island surfaces become corrugated and patterned. For the intermediate range, i.e., 12 =< N =<24, both flat and patterned surfaces coexist, with the percentage coverage of the patterned surface oscillating as a function of N. By performing density functional theory calculations, we demonstrate that the unusual surface pattern evolution in our MBE-grown Sn(100) islands is a result of the interplay between QSE-induced surface roughing and tensile strain-induced smoothening effect.

cond-mat.mtrl-sci↗
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

Orbital Hybridization-Induced Ising-Type Superconductivity in a Confined Gallium Layer

In low-dimensional superconductors, the interplay between quantum confinement and interfacial hybridization effects can reshape Cooper pair wavefunctions and induce novel forms of unconventional superconductivity. In this work, we employ a plasma-free, carbon buffer layer-assisted confinement epitaxy method to synthesize trilayer gallium (Ga) sandwiched between a graphene layer and a 6H-SiC(0001) substrate, forming an air-stable graphene/trilayer Ga/SiC heterostructure. In this confined light-element Ga layer, we demonstrate interfacial Ising-type superconductivity driven by atomic orbital hybridization between the Ga layer and the SiC substrate. Electrical transport measurements reveal that the in-plane upper critical magnetic field u0Hc2,|| reaches ~21.98T at T=400 mK, approximately 3.38 times the Pauli paramagnetic limit (~6.51T). Angle-resolved photoemission spectroscopy (ARPES) measurements combined with theoretical calculations confirm the presence of split Fermi surfaces with Ising-type spin textures at the K and K' valleys of the confined Ga layer strongly hybridized with SiC. Moreover, by incorporating finite relaxation time induced by impurity scattering into an Ising-type superconductivity model, we reproduce the entire temperature-dependent u0Hc2,|| phase diagram. This work establishes a new strategy to realize unconventional pairing wavefunctions by combining quantum confinement and interfacial hybridization effects in superconducting thin films. It also opens new avenues for designing scalable superconducting quantum electronic and spintronic devices through interfacial engineering.

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

Complex Magnetic Ordering in Candidate Topological Superconductors

The search for chiral topological superconductivity in magnetic topological insulator (TI)-FeTe heterostructures is a key frontier in condensed matter physics, with potential applications in topological quantum computing. The combination of ferromagnetism, superconductivity, and topologically nontrivial surface states brings together the key elements required for chiral Majorana physics. In this work, we examine the interplay between magnetism and superconductivity at the interfaces between FeTe and a series of TI overlayers. In superconducting MnBi$_2$Te$_4$/FeTe, any interfacial suppression of antiferromagnetism must affect at most a few nanometers. On the other hand, (Bi,Sb)$_2$Te$_3$/FeTe layers exhibit near-total suppression of antiferromagnetic ordering. Ferromagnetic Cr$_x$(Bi,Sb)$_{2-x}$Te$_3$ (CBST)/FeTe bilayers exhibit net magnetization in both CBST and FeTe layers, with evidence of interactions between superconductivity and ferromagnetism. These observations identify magnetic TI/FeTe interfaces as an exceptionally robust platform to realize chiral topological superconductivity.

cond-mat.supr-con↗
arXiv2024

Coexistence of Superconductivity and Antiferromagnetism in Topological Magnet MnBi2Te4 Films

The interface of two materials can harbor unexpected emergent phenomena. One example is interface-induced superconductivity. In this work, we employ molecular beam epitaxy to grow a series of heterostructures formed by stacking together two non-superconducting antiferromagnetic materials, an intrinsic antiferromagnetic topological insulator MnBi2Te4 and an antiferromagnetic iron chalcogenide FeTe. Our electrical transport measurements reveal interface-induced superconductivity in these heterostructures. By performing scanning tunneling microscopy and spectroscopy measurements, we observe a proximity-induced superconducting gap on the top surface of the MnBi2Te4 layer, confirming the interaction between superconductivity and antiferromagnetism in the MnBi2Te4 layer. Our findings will advance the fundamental inquiries into the topological superconducting phase in hybrid devices and provide a promising platform for the exploration of chiral Majorana physics in MnBi2Te4-based heterostructures.

cond-mat.supr-con↗
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

Interface-Induced Superconductivity in Magnetic Topological Insulator-Iron Chalcogenide Heterostructures

When two different electronic materials are brought together, the resultant interface often shows unexpected quantum phenomena, including interfacial superconductivity and Fu-Kane topological superconductivity (TSC). Here, we use molecular beam epitaxy (MBE) to synthesize heterostructures formed by stacking together two magnetic materials, a ferromagnetic topological insulator (TI) and an antiferromagnetic iron chalcogenide (FeTe). We discover emergent interface-induced superconductivity in these heterostructures and demonstrate the trifecta occurrence of superconductivity, ferromagnetism, and topological band structure in the magnetic TI layer, the three essential ingredients of chiral TSC. The unusual coexistence of ferromagnetism and superconductivity can be attributed to the high upper critical magnetic field that exceeds the Pauli paramagnetic limit for conventional superconductors at low temperatures. The magnetic TI/FeTe heterostructures with robust superconductivity and atomically sharp interfaces provide an ideal wafer-scale platform for the exploration of chiral TSC and Majorana physics, constituting an important step toward scalable topological quantum computation.

cond-mat.mes-hall↗
arXiv2023

Dirac-Fermion-Assisted Interfacial Superconductivity in Epitaxial Topological Insulator/Iron Chalcogenide Heterostructures

Over the last decade, the possibility of realizing topological superconductivity (TSC) has generated much excitement, mainly due to the potential use of its excitations (Majorana zero modes) in a fault-tolerant topological quantum computer 1,2. TSC can be created in electronic systems where the topological and superconducting orders coexist3, motivating the continued exploration of candidate material platforms to this end. Here, we use molecular beam epitaxy (MBE) to synthesize heterostructures that host emergent interfacial superconductivity when a non-superconducting antiferromagnet (FeTe) is interfaced with a topological insulator (TI) (Bi, Sb)2Te3 wherein the chemical potential can be tuned through varying the Bi/Sb ratio. By performing in-vacuo angle-resolved photoemission spectroscopy (ARPES) and ex-situ electrical transport measurements, we find that the superconducting transition temperature and the upper critical magnetic field are suppressed when the chemical potential approaches the Dirac point. This observation implies a direct correlation between the interfacial superconductivity and Dirac electrons of the TI layer. We provide evidence to show that the observed interfacial superconductivity and its chemical potential dependence is the result of the competition between the Ruderman-Kittel-Kasuya-Yosida-type ferromagnetic coupling mediated by Dirac surface states and antiferromagnetic exchange couplings that generate the bicollinear antiferromagnetic order in the FeTe layer. The Dirac-fermion-assisted interfacial superconductivity in (Bi,Sb)2Te3/FeTe heterostructures provides a new approach to probe TSC and Majorana physics in hybrid devices and potentially constitutes an alternative platform for topological quantum computation.

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↗