arXiv subjects

Xiong Yao

Publications and source records attributed to Xiong Yao.

At least 19 recordsLinked to original sources

arXiv2026

Superconducting PdTe Thin Film Via Topotactic Transformation, Toward Topological Superconductors

Topological superconductors (TSCs) hosting Majorana zero modes (MZMs) offer a pathway to fault-tolerant quantum computation. PdTe is a promising TSC candidate due to its topological surface states and a reasonable superconducting critical temperature of ~4.5 K. However, it has been challenging to grow PdTe thin films with bulk-like superconducting properties. Here, we show that high-quality, superconducting PdTe thin films can be grown using molecular beam epitaxy (MBE). The films exhibit a sharp superconducting transition (T_onset = 4.43 K with transition width of 0.06 K), comparable to that of bulk crystals. This was made possible via a topotactic transformation from a PdTe_2 buffer layer to a PdTe phase by growing Pd on top under Te-deficient conditions. Structural and transport analyses confirm the NiAs-type structure of PdTe, as well as its two-dimensional superconducting behavior and excellent air stability. These findings suggest that the MBE-grown PdTe films and their heterostructures are a promising platform for topological superconductivity and Majorana physics.

cond-mat.supr-con↗

arXiv2026

Interface-Enhanced Superconductivity in Ultrathin TiN Proximitized by Topological Insulators

High-quality topological insulator-superconductor (TI-SC) heterostructure with an atomically sharp and well-controlled interface is crucial for realizing topological superconductivity and topological quantum qubit. In particular, many studies of TI-SC heterostructures have focused on inducing superconducting gap in the TI layer via proximity effect, while the active manipulation of superconductivity in the SC layer remains largely unexplored. In this work, we fabricated TI/TiN heterostructures using highly air-stable, ultrathin TiN films as the SC layer, and observed an interface-enhanced superconductivity that contrasts with the conventional proximity effect in superconductor-normal metal interface. Band structure measurements reveal a consistent shift of Dirac point with Tc enhancement. Interfacial charge transfer provides a plausible explanation for this shift based on the systematic analysis and is therefore a likely contributor to the observed Tc enhancement. First principles calculations elucidate the charge transfer pathways, highlighting the critical role of the interfacial BiTe (BiSe)bilayer. Our results not only provide a tunable TI-SC hybrid system with robust superconductivity at ultrathin thickness, but also offer a potential route for manipulating superconductivity in TI-SC heterostructures via interface engineering.

cond-mat.mtrl-sci↗

arXiv2026

Long-range optomechanical interactions in SiN membrane arrays

Optomechanical systems using a membrane-in-the-middle configuration can exhibit a long-range type of interaction similar to how atoms show collective motion in an optical potential. Photons bounce back and forth inside a high-finesse Fabry-Pérot cavity and mediate the interaction between multiple membranes over a significant distance compared to the wavelength. Recently, it has been demonstrated that off-resonant coupling between light and the inter-membrane cavity can lead to coherent mechanical noise cancellation. On-resonance coupling of light with both the Fabry-Pérot and inter-membrane cavities, predicted to enhance the single photon optomechanical coupling, have to date not been experimentally demonstrated, however. In our experiment, a double-membrane system inside a Fabry-Pérot cavity resonantly enhances the cavity field, resulting in a stronger optomechanical coupling strength from the increased radiation pressure. The resonance condition is first identified by analyzing the slope of the dispersion relation. Then, the optomechanical coupling is determined at various chip positions over one wavelength range. The optimum coupling conditions are obtained and enhancement is demonstrated for double membrane arrays with three different reflectivites, reaching nearly four-fold enhancement for the collective motion of $R=65\%$ double membranes. The cavity losses at the optimum coupling are also characterized and the potential of reaching the single-photon strong coupling regime is discussed.

physics.optics↗

arXiv2025

Tuning Carrier Type and Density in Highly Conductive and Infrared-Transparent (Bi1-xSbx)2Te3 films

Infrared transparent conductors have long been sought due to their broad optoelectronic applications in the infrared wavelength range. However, the search for ideal materials has been limited by the inherent trade-off between electrical conductance and optical transmittance. Band engineering offers an effective approach to modulate carrier type and density, enabling concurrent tuning of both conductance and transmittance. In this work, we present a band engineering strategy that enables effective tuning of both infrared transmittance and electrical conductance in topological insulator (Bi1-xSbx)2Te3, bridging the gap and paving the way for applying topological insulators to infrared photoelectric devices. More importantly, with the combination of high carrier mobility and a large optical dielectric constant as suggested by previous report, Sb2Te3 achieves a high electrical conductance (~1000 S/cm) and outstanding infrared transmittance (92.3%) in the wavelength range of 8~13 um, demonstrating strong potential as an infrared transparent conductor. Our findings reveal that concurrent enhancement of both carrier mobility and optical dielectric constant is key to overcoming the conductance-transmittance trade-off. This work provides valuable insight for the exploration of high-performance infrared transparent conducting materials.

cond-mat.mtrl-sci↗

arXiv2025

Persistent Fluctuating Superconductivity and Planckian Dissipation in Fe(Te,Se)

Increasingly intricate phase diagrams in new classes of superconductors host fascinating interactions between superconductivity, diverse quantum phases, and quantum critical dynamics. The native superfluids, however, often exhibit much lower density and much greater inhomogeneity than conventional superfluids. This may render the superconductivity susceptible to fluctuations that are ordinarily assumed to be frozen out far below the superconducting transition temperature $T_c$, calling into question the degree to which the superconducting state is fully coherent. In this work, we leverage terahertz spectroscopy to demonstrate strongly fluctuating superconductivity in topological compositions of the multiband iron-based superconductor Fe(Te,Se). These fluctuations are found to persist undiminished far below $T_c$ and converge upon the limit of Planckian dissipation above $T_c$. These results indicate that extended quantum fluctuations dominate the electrodynamics of both the superconducting and Planckian-dissipative precursor states of Fe(Te,Se), and demonstrate that the assumption of phase coherence must be rigorously validated in emerging classes of unconventional superconductors.

cond-mat.supr-con↗

arXiv2025

Broad-band THz emission by Spin-to-Charge Conversion in Topological Material -- Ferromagnet Heterostructures

Terahertz spintronic devices combine ultrafast operation with low power consumption, making them strong candidates for next-generation memory technologies. In this study, we use time-domain terahertz emission spectroscopy to investigate spin-to-charge conversion (SCC) in bilayer heterostructures comprising topological insulators (TIs) or Weyl semimetals (WSMs) with ferromagnetic metals (FMs). SCC is studied in TI materials \ce{Bi2Se3}, Pb-doped \ce{Bi2Se3}, and (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$, and the WSM NbP. Our results reveal that the dependence of SCC on TI thickness varies with interface quality, indicating that thickness dependence alone is not a reliable criterion for distinguishing between inverse spin Hall effect and the inverse Rashba--Edelstein effect mechanisms. We find efficient SCC in TIs depends on both \textit{in-situ} growth to prevent surface oxidation and proper composition. In NbP$\vert$FM bilayers, we observe THz emission with efficiency and bandwidth comparable to that of TIs, highlighting the broader potential of topological materials. Finally, broadband spectral measurements demonstrate that both TIs and WSMs can generate THz pulses with frequencies extending up to 8\,THz. These findings underscore the promise of topological materials as efficient platforms for ultrafast, broadband spintronic applications.

cond-mat.mtrl-sci↗

arXiv2025

Single-layer magnet phase in intrinsic magnetic topological insulators, $[\mathrm{MnTe}][\mathrm{Bi}_{2}\mathrm{Te}_{3}]_{\mathrm{n}}$, far beyond the thermodynamic limit

The intrinsic magnetic topological insulator (IMTI) family $[\mathrm{MnTe}][\mathrm{Bi}_{2}\mathrm{Te}_{3}]_{\mathrm{n}}$ has demonstrated magneto-topological properties dependent on $n$, making it a promising platform for advanced electronics and spintronics. However, due to technical barriers in sample synthesis, their properties in the large $n$ limit remain unknown. To overcome this, we utilized the atomic layer-by-layer molecular beam epitaxy (ALL-MBE) technique and achieved IMTIs with $n$ as large as 15, far beyond the previously reported in bulk crystals or thin films. Then, we discover that the "single-layer magnet (SLM)" phase, primarily determined by intralayer ferromagnetic coupling, emerges for $n >$ $\sim 4$ and remains little affected up to $n = 15$. Nonetheless, still, non-zero, interlayer ferromagnetic coupling is necessary to stabilize the SLM phase, suggesting that the SLM phase eventually disappears in the $n\to\infty$ limit. This study uncovers the secrets of IMTIs beyond the thermodynamic limit and opens a door to diverse magneto-topological applications.

cond-mat.mes-hall↗

arXiv2025

The Rise of Refractory Transition-Metal Nitride Films for Advanced Electronics and Plasmonics

The advancement of semiconductor materials has played a crucial role in the development of electronic and optical devices. However, scaling down semiconductor devices to the nanoscale has imposed limitations on device properties due to quantum effects. Hence, the search for successor materials has become a central focus in the fields of materials science and physics. Transition-metal nitrides (TMNs) are extraordinary materials known for their outstanding stability, biocompatibility, and ability to integrate with semiconductors. Over the past few decades, TMNs have been extensively employed in various fields. However, the synthesis of single-crystal TMNs has long been challenging, hindering the advancement of their high-performance electronics and plasmonics. Fortunately, progress in film deposition techniques has enabled the successful epitaxial growth of high-quality TMN films. In comparison to reported reviews, there is a scarcity of reviews on epitaxial TMN films from the perspective of materials physics and condensed matter physics, particularly at the atomic level. Therefore, this review aims to provide a brief summary of recent progress in epitaxial growth at atomic precision, emergent physical properties (superconductivity, magnetism, ferroelectricity, and plasmon), and advanced electronic and plasmonic devices associated with epitaxial TMN films.

cond-mat.mtrl-sci↗

arXiv2025

Universal Superconductivity in FeTe and All-Iron-Based Ferromagnetic Superconductor Heterostructures

Ferromagnetism (FM) and superconductivity (SC) are two of the most famous macroscopic quantum phenomena. However, nature normally does not allow SC and FM to coexist without significant degradation. Here, we introduce the first fully iron-based SC/FM heterostructures, composed of Fe(Te,Se) and Fe3GeTe2, and show that in this platform strong FM and high-temperature SC robustly coexist. We subsequently discover that chemical proximity effect from neighboring layers can universally drive the otherwise non-superconducting FeTe films into a SC state. This suggests that the ground state of FeTe is so close to the SC state that it could be driven in and out of the SC state with various other perturbations. Altogether, this shows that Fe-Te-based heterostructures provide a unique opportunity to manipulate magnetism, superconductivity and topological physics, paving the way toward new superconducting technologies.

cond-mat.supr-con↗

arXiv2024

High resistance of superconducting TiN thin films against environmental attacks

Superconductors, an essential class of functional materials, hold a vital position in both fundamental science and practical applications. However, most superconductors, including MgB$_2$, Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$, and FeSe, are highly sensitive to environmental attacks (such as water and moist air), hindering their wide applications. More importantly, the surface physical and chemical processes of most superconductors in various environments remain poorly understood. Here, we comprehensively investigate the high resistance of superconducting titanium nitride (TiN) epitaxial films against acid and alkali attacks. Unexpectedly, despite immersion in acid and alkaline solutions for over 7 days, the crystal structure and superconducting properties of TiN films remain stable, as demonstrated by high-resolution X-ray diffraction, electrical transport, atomic force microscopy, and scanning electron microscope. Furthermore, combining scanning transmission electron microscopy analysis with density functional theory calculations revealed the corrosion mechanisms: acid corrosions lead to the creation of numerous defects due to the substitution of Cl ions for N anions, whereas alkaline environments significantly reduce the film thickness through the stabilization of OH$^\ast$ adsorbates. Our results uncover the unexpected stability and durability of superconducting materials against environmental attacks, highlighting their potential for enhanced reliability and longevity in diverse applications.

cond-mat.supr-con↗

arXiv2024

Mystery of superconductivity in FeTe films and the role of neighboring layers

Since the discovery of superconductivity in the Fe(Te,Se) system, it has been a general consensus that the end member of FeTe is not superconducting. Nonetheless, in recent years, there have been reports of superconducting FeTe films, but the origin of their superconductivity remains mysterious. Here, we provide the first comprehensive review of all the reported FeTe films regarding the relationship between their superconductivity and neighboring layers. Based on this review, we show that telluride neighboring layers are the key to superconducting FeTe films. Then, with additional new studies, we show that stoichiometric Te content, which can be readily achieved in FeTe films with the assistance of neighboring telluride layers, might be crucial to stabilizing the superconductivity in this system. This work provides insights into the underlying mechanism behind superconductivity in FeTe films and sheds light on the critical role of neighboring layers and stoichiometry control toward manipulating topological superconductivity in FeTe heterostructures.

cond-mat.supr-con↗

arXiv2024

Atomic-Layer-Controlled Magnetic Orders in MnBi2Te4-Bi2Te3 Topological Heterostructures

The natural van der Waals superlattice MnBi2Te4-(Bi2Te3)m provides an optimal platform to combine topology and magnetism in one system with minimal structural disorder. Here, we show that this system can harbor both ferromagnetic (FM) and antiferromagnetic (AFM) orders and that these magnetic orders can be controlled in two different ways by either varying the Mn-Mn distance while keeping the Bi2Te3/MnBi2Te4 ratio constant or vice versa. We achieve this by creating atomically engineered sandwich structures composed of Bi2Te3 and MnBi2Te4 layers. We show that the AFM order is exclusively determined by the Mn-Mn distance whereas the FM order depends only on the overall Bi2Te3/MnBi2Te4 ratio regardless of the distance between the MnBi2Te4 layers. Our results shed light on the origins of the AFM and FM orders and provide insights into how to manipulate magnetic orders not only for the MnBi2Te4-Bi2Te3 system but also for other magneto-topological materials.

cond-mat.mtrl-sci↗

arXiv2024

Resilient Growth of Highly Crystalline Topological Insulator-Superconductor Heterostructure Enabled by Ex-situ Nitride Film

Highly crystalline and easily feasible topological insulator-superconductor (TI-SC) heterostructures are crucial for the development of practical topological qubit devices. The optimal superconducting layer for TI-SC heterostructures should be highly resilient against external contaminations and structurally compatible with TIs. In this study, we provide a solution to this challenge by showcasing the growth of a highly crystalline TI-SC heterostructure using refractory TiN (111) as the superconducting layer. This approach can eliminate the need for in-situ cleaving or growth. More importantly, the TiN surface shows high resilience against contaminations during air exposure, as demonstrated by the successful recyclable growth of Bi2Se3. Our findings indicate that TI-SC heterostructures based on nitride films are compatible with device fabrication techniques, paving a path to the realization of practical topological qubit devices in the future.

cond-mat.mes-hall↗

arXiv2024

Observation of terahertz second harmonic generation from Dirac surface states in the topological insulator Bi$_2$Se$_3$

We report the observation of second harmonic generation with high conversion efficiency $\sim 0.005\%$ in the terahertz regime from thin films of the topological insulator Bi$_2$Se$_3$ that exhibit the linear photogalvanic effect, measured via time-domain terahertz spectroscopy and terahertz emission, respectively. As neither phenomena is observable from topologically trivial In-doped Bi$_2$Se$_3$, and since no enhancement is observed when subject to band bending, the efficient thickness-independent nonliear responses are attributable to the Dirac fermions of topological surface states of Bi$_2$Se$_3$. This observation of intrinsic terahertz second harmonic generation in an equilibrium system unlocks the full suite of both even and odd harmonic orders in the terahertz regime and opens new pathways to probing quantum geometry via intraband nonlinear processes. We hope our work will motivate the theoretical development of a full treatment of second harmonic generation for probing the quantum geometry in various inversion-breaking topological and twisted materials.

cond-mat.str-el↗

arXiv2024

Momentum-resolved electronic structures and strong electronic correlations in graphene-like nitride superconductors

Although transition-metal nitrides have been widely applied for several decades, experimental investigations of their high-resolution electronic band structures are rare due to the lack of high-quality single-crystalline samples. Here, we report on the first momentum-resolved electronic band structures of titanium nitride (TiN) films, a remarkable nitride superconductor. The measurements of crystal structures and electrical transport properties confirmed the high quality of these films. More importantly, with a combination of high-resolution angle-resolved photoelectron spectroscopy and the first-principles calculations, the extracted Coulomb interaction strength of TiN films can be as large as 8.5 eV, whereas resonant photoemission spectroscopy yields a value of 6.26 eV. These large values of Coulomb interaction strength indicate that superconducting TiN is a strongly correlated system. Our results uncover the unexpected electronic correlations in transition-metal nitrides, potentially providing a perspective not only to understand their emergent quantum states but also to develop their applications in quantum devices.

cond-mat.mtrl-sci↗

arXiv2024

Signatures of Majorana Bound States in the Diffraction Patterns of Extended Superconductor-Topological Insulator-Superconductor Josephson Junctions

In an extended superconductor-topological insulator-superconductor (S-TI-S) Josephson junction in a magnetic field, localized Majorana bound states (MBS) are predicted to exist at the cores of Josephson vortices where the local phase difference across the junction is an odd-multiple of $π$. These states contribute a supercurrent with a $4π$-periodic current-phase relation (CPR) that adds to the conventional $2π$-periodic sinusoidal CPR. In this work, we present a comprehensive experimental study of the critical current vs. applied magnetic field diffraction patterns of lateral Nb-Bi$_2$Se$_3$-Nb Josephson junctions. We compare our observations to a model of the Josephson dynamics in the S-TI-S junction system to explore what feature of MBS are, or are not, exhibited in these junctions. Consistent with the model, we find several distinct deviations from a Fraunhofer diffraction pattern that is expected for a uniform sin$(ϕ)$ CPR. In particular, we observe abrupt changes in the diffraction pattern at applied magnetic fields in which the current-carrying localized MBS are expected to enter the junction, and a lifting of the odd-numbered nodes consistent with a $4π$-periodic sin$(ϕ/2)$-component in the CPR. We also see that although the even-numbered nodes often remain fully-formed, we sometimes see deviations that are consistent with quasiparticle-induced fluctuations in the parity of the MBS pairs that encodes quantum information.

cond-mat.supr-con↗

arXiv2023

Broadband, High-Reflectivity Dielectric Mirrors at Wafer Scale: Combining Photonic Crystal and Metasurface Architectures for Advanced Lightsails

Highly ambitious initiatives aspire to propel a miniature spacecraft to a neighboring star within a human generation, leveraging the radiation pressure of lasers for propulsion. One of the main challenges to achieving this enormous feat is to build a meter-scale, ultra-low mass lightsail with broadband reflectivity. In this work, we present the design and fabrication of such a lightsail composed of two distinct dielectric layers and patterned with a photonic crystal structure covering a 4" wafer. We overcome the crucial challenge of achieving broad band reflection of >70% spanning over the full Doppler-shifted laser wavelength range during spacecraft acceleration, in combination with low total mass in the range of a few grams when scaled to meter size. Furthermore, we find new paths to reliably fabricate these sub-wavelength structures over macroscopic areas and then systematically characterize their optical performance, confirming their suitability for future lightsail applications. Our innovative device design and precise nanofabrication approaches represent a significant leap toward interstellar exploration.

physics.optics↗

arXiv2023

Active-feedback quantum control of an integrated low-frequency mechanical resonator

Preparing a massive mechanical resonator in a state with quantum limited motional energy provides a promising platform for studying fundamental physics with macroscopic systems and allows to realize a variety of applications, including precise sensing. While several demonstrations of such ground-state cooled systems have been achieved, in particular in sideband-resolved cavity optomechanics, for many systems overcoming the heating from the thermal bath remains a major challenge. In contrast, optomechanical systems in the sideband-unresolved limit are much easier to realize due to the relaxed requirements on their optical properties, and the possibility to use a feedback control schemes to reduce the motional energy. The achievable thermal occupation is ultimately limited by the correlation between the measurement precision and the back-action from the measurement. Here, we demonstrate measurement-based feedback cooling on a fully integrated optomechanical device fabricated using a pick-and-place method, operating in the deep sideband-unresolved limit. With the large optomechanical interaction and a low thermal decoherence rate, we achieve a minimal average phonon occupation of 0.76 when pre-cooled with liquid helium and 3.5 with liquid nitrogen. Significant sideband asymmetry for both bath temperatures verifies the quantum character of the mechanical motion. Our method and device are ideally suited for sensing applications directly operating at the quantum limit, greatly simplifying the operation of an optomechanical system in this regime.

quant-ph↗