arXiv subjects

Yingdong Guan

Publications and source records attributed to Yingdong Guan.

15 recordsLinked to original sources

arXiv2026

High-resolution angle-resolved photoemission spectroscopy with tunable magnetic field

The control and perturbation of quantum phenomena with magnetic fields is an indispensable tool in materials research. Recently, in-situ field tuning was implemented into angle-resolved photoemission spectroscopy (magneto-ARPES), which provides direct momentum and energy-resolved information of the field-dependent electronic structure. However, aberrations of the electron trajectories were shown to be substantial, leading to significant spectral distortions and broadening even in small fields. Here we show that the electronic structure can be recovered from magneto-ARPES spectra with high accuracy while maintaining high momentum resolution even when strong trajectory aberrations are present. We studied Bi$_2$Se$_3$ in a dipole field of a coil using a laser-based ARPES system. The electronic structure was reconstructed in post-processing using detailed electron trajectory simulations. We identify two-dimensional (2D) momentum mapping, a micron beam spot size, and precise numerical field simulations as critical technical requirements for high-resolution magneto-ARPES. We show how circular dichroism can provide additional information about the coupling of the magnetic field to the spin. The experimental achievements and the scaling laws from our simulations provide a road map towards magneto-ARPES in larger fields.

cond-mat.str-el↗

arXiv2026

Defect Control via Cu Enrichment Enhances Multifunctional Properties in the Polar Semiconductor Cu1+xMn1-ySiTe3

Polar materials have recently attracted significant interest due to their rich multifunctional properties. The chalcogenide polar semiconductor Cu1-xMn1+ySiTe3 (Cu-deficient) is an emerging multiferroic system in which electric polarization is coupled to magnetization. However, its macroscopic ferroelectric polarization is strongly suppressed due to the presence of a high density of stacking faults. In this work, we demonstrate that these crystal defects, likely originating from non-stoichiometry, can be substantially reduced by increasing the Cu content. Cu-enriched samples, Cu1+xMn1-ySiTe3, crystallize in a noncentrosymmetric monoclinic structure (space group Pm) as the Cu-deficient counterpart but show a nearly stacking-fault-free phase, which is attributed to the emergence of an interstitial site. Consequently, the Cu-enriched samples show a pronounced enhancement of the second-harmonic generation (SHG) response compared to Cu-deficient compositions. Magnetically, the Cu-enriched crystals retain long-range antiferromagnetic order with a Neel temperature of TN ~ 33 K without a glassy state but manifest a distinct spin-flop transition along the polar b-axis that is absent in the Cu-deficient compositions. Furthermore, the electronic ground state evolves from insulating to doped semiconducting behavior upon Cu enrichment. Together, these results establish this material system as a unique and versatile platform for elucidating the interplay among composition, crystal defects, and multifunctional properties, offering a route to design magnetic polar systems with tunable quantum functionalities.

cond-mat.mtrl-sci↗

arXiv2026

Evidence of Spin-Valley Coupling in Dirac Material BaMnBi2 Probed by Quantum Hall Effect and Nonlinear Hall Effect

Valleytronics is a rapidly advancing field that explores the use of the valley degree of freedom in electronic systems to encode and process information. It relies on electronic states with spin valley locking, first predicted and observed in monolayer transition metal dichalcogenides such as MoS2. However, very few bulk materials have been reported to host spin valley locked electronic states. In this work, we present experimental evidence for a predicted, unique spin valley locked electronic state generated by Bi zigzag chains in the layered compound BaMnBi2. We observe remarkable quantum transport properties in this material, including a stacked quantum Hall effect (QHE) and a nonlinear Hall effect (NLHE). From the analysis of the QHE, we identify a spin valley degeneracy of four, while the NLHE provides supporting evidence for the anticipated valley contrasted Berry curvature, a typical signature of a spin valley locked state. This spin valley locked state contrasts with that observed in the sister compound BaMnSb2, where the degeneracy is two. This difference arises from significant variations in their orthorhombic crystal structures and spin orbit coupling. These findings establish a new platform for exploring coupled spin valley physics in bulk materials and highlight its potential for valleytronic device applications.

cond-mat.mes-hall↗

arXiv2026

Successful growth of low carrier density $α$-In$_2$Se$_3$ single crystals using Se-flux in a modified Bridgman furnace

Indium selenide (In$_2$Se$_3$) has garnered significant attention for its intriguing properties and applications in batteries, solar cells, photodetectors and ferroelectric devices. However, the controlled synthesis of single phase $α$-In$_2$Se$_3$ remains challenging owing to its complex phase diagram, presence of multiple polymorphs and the high volatility of selenium that induces non-stoichiometry and unintentional carrier doping. For ferroelectric α-In2Se3, minimizing the carrier density is essential because leakage current can obscure polarization switching. Here, we report the growth of $α$-In$_2$Se$_3$ single crystals using a unique approach, the Se-flux assisted modified vertical Bridgman technique combined with liquid encapsulation under high pressure. This approach creates a high-pressure, Se-rich environment that effectively minimizes Se-vaporization. Structural and compositional analysis using X-ray diffraction, transmission electron microscopy and energy-dispersive X-ray spectroscopy confirm the formation of pure $α$-In$_2$Se$_3$ single crystals with 3R stacking. Furthermore, the crystals exhibit remarkably low carrier density of 1.5-3.2 $\times$ 10$^{16}$ cm$^{-3}$ at 300K$-$the lowest reported to date, reflecting a significant suppression of Se-vacancies relative to the conventional Bridgman or melt-grown crystals. Through transport and ARPES measurements on different batches of crystals, we also demonstrate that the amount of Se-flux plays a crucial role in controlling Se-vacancies. Our results thus establish this modified Bridgman method as an effective strategy for synthesizing large $α$-In$_2$Se$_3$ single crystals with reduced intrinsic defects. This technique can be broadly applied to grow other volatile chalcogenides with reduced defects and controlled stoichiometry.

cond-mat.mtrl-sci↗

arXiv2025

Building 3D superconductor-based Josephson junctions using a via transfer approach

The coupling of superconductivity to unconventional materials may lead to novel quantum states and potential applications. Controlling the quality of the superconductor-normal metal interface is of crucial importance to the understanding and engineering of the superconducting proximity effect. In many cases, conventional lithography-based deposition methods introduce undesirable effects. Using the concept of via contact and dry transfer, we have constructed smooth, van der Waals-like contact between 3D superconducting NbN/Pd and graphene with low contact resistance of approximately 130 $Ω\cdot μm$. Gate-tunable supercurrent, Fraunhofer pattern, and Andreev reflections are observed, the properties of which can be understood using an induced superconducting gap $Δ$' in this planar contact geometry. We discuss potential mechanisms impacting the magnitude of $Δ$' and suggest ways of further increasing the proximity coupling. This gentle, lithography-free contacting method can be applied to air- and damage-sensitive surfaces to engineer novel superconducting heterostructures.

cond-mat.mes-hall↗

arXiv2025

Double-Crucible Vertical Bridgman Technique for Stoichiometry-Controlled Chalcogenide Crystal Growth

Precise stoichiometry control in single-crystal growth is essential for both technological applications and fundamental research. However, conventional growth methods often face challenges such as non-stoichiometry, compositional gradients, and phase impurities, particularly in non-congruent melting systems. Even in congruent melting systems like Bi2Se3, deviations from the ideal stoichiometric composition can lead to significant property degradation, such as excessive bulk conductivity, which limits its topological applications. In this study, we introduce the double-crucible vertical Bridgman (DCVB) method, a novel approach that enhances stoichiometry control through the combined use of continuous source material feeding, traveling-solvent growth, and liquid encapsulation, which suppresses volatile element loss under high pressure. Using Bi2Se3 as a model system, we demonstrate that crystals grown via DCVB exhibit enhanced stoichiometric control, significantly reducing defect density and achieving much lower carrier concentrations compared to those produced by conventional Bridgman techniques. Moreover, the continuous feeding of source material enables the growth of large crystals. This approach presents a promising strategy for synthesizing high-quality, large-scale crystals, particularly for metal chalcogenides and pnictides that exhibit challenging non-congruent melting behaviors.

cond-mat.mtrl-sci↗

arXiv2025

Discovery of a Highly Anisotropic Type-II Ferromagnetic Weyl State Exhibiting a 3D Quantum Hall Effect

Topological semimetals, particularly Weyl semimetals (WSMs), are crucial platforms for exploring emergent quantum phenomena due to their unique electronic structures and potential to transition into various topological phases. In this study, we report the discovery of a ferromagnetic (FM) type-II WSM in Mn(Bi1-xSbx)4Te7, which exhibits a remarkable three-dimensional (3D) quantum Hall effect (QHE). By precisely tuning the chemical potential through Sb doping, we obtained samples with the Fermi level near the charge neutrality point for x = ~ 0.27. This was confirmed by spectroscopy measurements (ARPES and STS), and these samples showed strong quantum oscillations along with a key transport signature of a Weyl state - chiral anomaly, and Fermi surface reconstruction driven by FM ordering. Our theoretical analysis indicates that this Weyl state evolves from a parent nodal ring state, where higher-order k-terms split the nodal line into type-II Weyl nodes. The Weyl state exhibits significant anisotropy, characterized by a pronounced reduction in Fermi velocity along the kz-axis, likely accounting for the observed 3D QHE. These results not only highlight the exceptional tunability of the Mn(Bi1-xSbx)4Te7 system, where precise control of the chemical potential and magnetic properties opens access to novel quantum phases, but also advance the understanding of FM WSMs.

cond-mat.mtrl-sci↗

arXiv2024

Crystallographic Disorder and Strong Magnetic Anisotropy in Dy3.1Pt2.0Sb3.8

We report the crystal growth and characterization of a rare-earth-containing material, Dy3.00(1)Pt2Sb4.48(2). This compound possesses a similar structure to the previously reported Y3Pt4Ge6, but lacks two layers of Pt atoms. Crystallographic disorder was found in Dy3.00(1)Pt2Sb4.48(2). Additionally, the Dy-Dy framework was found to have both square net and triangular lattices. Dy3.00(1)Pt2Sb4.48(2)8 was determined to be antiferromagnetically ordered around ~15 K while a competing antiferromagnetic sublattice also exists at lower temperature. Strong magnetic anisotropy was observed and several metamagnetic transitions were seen in the hysteresis loops. Furthermore, the Curie-Weiss fitting revealed unusually small effective moment of Dy, which is far below the expected value of Dy3+ (10.65 muB). This material might provide a new platform to study the relation between crystallographic disorder and magnetism.

cond-mat.mtrl-sci↗

arXiv2024

Structure and lattice excitations of the copper substituted lead oxyapatite Pb$_{9.06(7)}$Cu$_{0.94(6)}$(PO$_{3.92(4)}$)$_{6}$O$_{0.96(3)}$

The copper substituted lead oxyapatite, Pb$_{10-x}$Cu$_{x}$(PO$_{3.92(4)}$)$_{6}$O$_{0.96(3)}$ (x=0.94(6)) was studied using neutron and x-ray diffraction and neutron spectroscopy techniques. The crystal structure of the main phase of our sample, which has come to be colloquially known as LK-99, is verified to possess a hexagonal structure with space group $P 6_{3}/m$, alongside the presence of impurity phases Cu and Cu$_2$S. We determine the primary substitution location of the Cu as the Pb1 ($6h$) site, with a small substitution at the Pb2 ($4f$) site. Consequently, no clear Cu-doping-induced structural distortion was observed in the investigated temperature region between 10~K and 300~K. Specially, we did not observe a reduction of coordinate number at the Pb2 site or a clear tilting of PO$_4$ tetrahedron. Magnetic characterization reveals a diamagnetic signal in the specimen, accompanied by a very weak ferromagnetic component at 2 K. No long-range magnetic order down to 10 K was detected by the neutron diffraction. Inelastic neutron scattering measurements did not show magnetic excitations for energies up to 350 meV. There is no sign of a superconducting resonance in the excitation spectrum of this material. The measured phonon density of states compares well with density functional theory calculations performed for the main LK-99 phase and its impurity phases. Our study may shed some insight into the role of the favored substitution site of copper in the absence of structural distortion and superconductivity in LK-99.

cond-mat.mtrl-sci↗

arXiv2023

Coupling Between Magnetic and Transport Properties in Magnetic Layered Material Mn2-xZnxSb

We synthesized single crystals for Mn2-xZnxSb and studied their magnetic and electronic transport properties. This material system displays rich magnetic phase tunable with temperature and Zn composition. In addition, two groups of distinct magnetic and electronic properties, separated by a critical Zn composition of x = 0.6, are discovered. The Zn-less samples are metallic and characterized by a resistivity jump at the magnetic ordering temperature, while the Zn-rich samples lose metallicity and show a metal-to-insulator transition-like feature tunable by magnetic field. Our findings establish Mn2-xZnxSb as a promising material platform that offers opportunities to study how the coupling of spin, charge, and lattice degrees of freedom governs interesting transport properties in 2D magnets, which is currently a topic of broad interest.

cond-mat.mtrl-sci↗

arXiv2023

Layer-by-layer disentanglement of Bloch states via frequency-domain photoemission

Layer-by-layer material engineering has enabled exotic quantum phenomena such as interfacial superconductivity and the quantum anomalous Hall effect. Meanwhile, deciphering electronic states layer-by-layer remains a fundamental scientific challenge. This is exemplified by the difficulty in understanding the layer origins of topological electronic states in magnetic topological insulators, which is key to understanding and controlling topological quantum phases. Here, we report a layer-encoded frequency-domain ARPES experiment on a magnetic topological insulator (MnBi2Te4)(Bi2Te3) to characterize the layer origins of electronic states. Infrared laser excitations launch coherent lattice vibrations with the layer index encoded by the vibration frequency; photoemission spectroscopy tracks the electron dynamics, where the layer information is decoded in the frequency domain. This layer-frequency correspondence reveals a surprising wavefunction relocation of the topological surface state from the top magnetic layer into the buried second layer, reconciling the controversy over the vanishing broken-symmetry energy gap in (MnBi2Te4)(Bi2Te3) and its related compounds. The layer-frequency correspondence can be harnessed to disentangle electronic states layer-by-layer in a broad class of van der Waals superlattices.

cond-mat.mes-hall↗

arXiv2022

High-throughput screening assisted discovery of a stable layered anti-ferromagnetic semiconductor: CdFeP2Se6

Recent advances in two-dimensional (2D) magnetism have heightened interest in layered magnetic materials due to their potential for spintronics. In particular, layered semiconducting antiferromagnets exhibit intriguing low-dimensional semiconducting behavior with both charge and spin as carrier controls. However, synthesis of these compounds is challenging and remains rare. Here, we conducted firstprinciples based high-throughput search to screen potentially stable mixed metal phosphorous trichalcogenides (MM'P2X6, where M and M' are transition metals and X is a chalcogenide) that have a wide range of tunable bandgaps and interesting magnetic properties. Among the potential candidates, we successfully synthesized a stable semiconducting layered magnetic material, CdFeP2Se6, that exhibits a short-range antiferromagnetic order at TN = 21 K with an indirect band gap of 2.23 eV. Our work suggests that highthroughput screening assisted synthesis be an effective method for layered magnetic materials discovery.

cond-mat.mtrl-sci↗

arXiv2021

Giant topological Hall effect in centrosymmetric tetragonal Mn2-xZnxSb

Topological magnetism typically appears in noncentrosymmetric compounds or compounds with geometric frustration. Here we report the effective tuning of magnetism in centrosymmetric tetragonal Mn2-xZnxSb by Zn substitution. The magnetism is found to be closely coupled to the transport properties, giving rise to to a very large topological hall effect is possibly associated with topological magnetism. Our finding suggests Mn2-xZnxSb is a candidate material for the centrosymmetric tetragonal topological magnetic system, offers opportunities for studying and tuning spin textures and developing near room temperature spin-based devices.

cond-mat.mtrl-sci↗

arXiv2019

Subtle metastability of the layered magnetic topological insulator MnBi2Te4 from weak interactions

The metastable layered compound MnBi2Te4 is the first experimentally realized intrinsic antiferromagnetic topological insulator, predicted to host the quantum anomalous Hall effect at high temperatures upon exfoliation to atomically thin layers. While its magnetic ordering and topological properties have generated intensive interest, the mechanism behind its metastability and the ideal crystal synthesis conditions have remained elusive. Here, using a combined first-principles-based approach that considers lattice, charge, and spin degrees of freedom, we investigate the metastability of MnBi2Te4 by calculating the Helmholtz free energy for the reaction Bi2Te3 + MnTe -> MnBi2Te4. We identify a narrow temperature range (767 K to 873 K) in which the compound is stable with respect to the competing binary phases and successfully synthesize high-quality MnBi2Te4 single crystals using the Bi-Te flux method within this range. We also predict the various contributions to the total specific heat, which is consistent with our experimental measurements. Our findings indicate that the degrees of freedom responsible for the van der Waals interaction, magnetic coupling, and nontrivial band topology in layered materials not only enable emergent phenomena but also determine thermodynamic stability. This conclusion lays the foundation for future computational material synthesis of novel layered systems.

cond-mat.mtrl-sci↗

arXiv2019

Ferromagnetic van der Waals compound MnSb$_{1.8}$Bi$_{0.2}$Te$_4$

The intersection of topology and magnetism represents a new playground to discover novel quantum phenomena and device concepts. In this work, we show that a van der Waals compound MnSb$_{1.8}$Bi$_{0.2}$Te$_4$ exhibits a ferromagnetic ground state with a Curie temperature of 26 K, in contrast to the antiferromagnetic order previously found for other members of the Mn(Sb, Bi)$_2$Te$_4$ family. We employ magneto-transport, bulk magnetization and neutron scattering studies to illustrate the magnetic and electrical properties of MnSb$_{1.8}$Bi$_{0.2}$Te$_4$ and report on the observation of an unusual anomalous Hall effect. Our results are an important step in the synthesis and understanding of ferromagnetic topological insulators.

cond-mat.mes-hall↗