arXiv subjects

An-Hsi Chen

Publications and source records attributed to An-Hsi Chen.

18 recordsLinked to original sources

arXiv2026

Tunable anharmonicity in Sn-InAs nanowire transmons beyond the short junction limit

The anharmonicity of a transmon qubit, defined as the difference in energy level spacing, is a key design parameter. In transmons built from hybrid superconductor-semiconductor Josephson elements, the anharmonicity is tunable with gate voltages that control both the Josephson energy and the weak link transparency. In Sn-InAs nanowire transmons, we use two-tone microwave spectroscopy to extract anharmonicity ranging in absolute value from the transmon charging energy $E_c$ to values smaller than $E_c/10$. This behavior contrasts with the predictions of the multi-channel short-junction model, which sets a lower limit on anharmonicity at $E_c/4$. Coherent operation of the qubit is still possible at the point of the lowest anharmonicity. These findings demonstrate the potential of quantum circuits that benefit from widely electrically tunable anharmonicity.

cond-mat.mes-hall↗

arXiv2026

Intrinsic Even-Odd Thickness-Driven Anomalous Hall in Epitaxial MnBi2Te4 Thin Films

We demonstrate precise control of magnetism in MnBi2Te4 thin films through careful synthesis by molecular beam epitaxy, achieving minimal defects and accurate layer thickness control. By optimizing Mn-Bi-Te ratios and growth temperatures, we minimize detrimental self-doping effects and accurately target integer-layer films. X-ray diffraction and reflectivity provide quantitative measures of film quality and thickness. When these macroscale probes of structure and thickness are integrated with magnetotransport measurements, a striking even-odd layer dependence of the anomalous Hall effect is revealed. Odd-layer films exhibit a large hysteresis up to the N\'eel temperature (~25K), consistent with non-compensated antiferromagnetism, while even-layer films show minimal response, as expected for an antiferromagnet. The sign of the anomalous Hall effect exhibits a sign reversal for intrinsic magnetism versus magnetism associated with defects. This work identifies critical factors for inducing pure, non-compensated ferromagnetism and reveals the character of the intrinsic anomalous Hall effect in MnBi2Te4, which together is a step towards realizing the zero-field quantum anomalous Hall effect in topological materials.

cond-mat.mtrl-sci↗

arXiv2026

Strain patterning of flexomagnetism

Flexomagnetism, the coupling of magnetic ordering to strain gradients, provides access to novel symmetry-broken magnetic phases that cannot be accessed via uniform strain. However, flexomagnetism is hard to understand because it is extremely difficult to control a spatially varying strain. Here, we develop a top-down strategy to pattern transverse strain gradients using helium ion implantation through a lithographically defined mask. Using epitaxial films of the antiferromagnetic nodal line semimetal GdAuGe, we demonstrate that transverse strain gradients $\partial \varepsilon_{zz}/\partial x$ induce near-room-temperature ferromagnetic response, compared to the retained para or antiferromagnetism for homogeneously strained GdAuGe. We spatially correlate the magnetic response with the regions of largest strain gradient, via magnetic force microscopy and nanobeam x-ray diffraction, respectively, to confirm the flexomagnetic response. Our approach opens new avenues for the precise control of magnetic phases in thin films of quantum materials via a patterned strain gradient.

cond-mat.mtrl-sci↗

arXiv2025

Weak localization and universal conductance fluctuations in large area twisted bilayer graphene

We study diffusive magnetotransport in highly p-doped large area twisted bilayer graphene in 1{\deg}, 7{\deg}, 9{\deg} and 20{\deg} samples. We report weak localization in twisted bilayer graphene for the first time. All samples exhibit weak localization, from which we extract the phase coherence length and intervalley scattering lengths, and from that determine that dephasing is caused by electron-electron scattering and intervalley scattering is caused by point defects. We observe signatures of universal conductance fluctuations in the 9{\deg} sample, which has high mobility and is near the van Hove singularity. Further improvements in sample quality and applications to large area moire materials will open new avenues to observe quantum interference effects.

cond-mat.mes-hall↗

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

Transmon qubit using Sn as a junction superconductor

Superconductor qubits typically use aluminum-aluminum oxide tunnel junctions to provide the non-linear inductance. Junctions with semiconductor barriers make it possible to vary the superconductor material and explore beyond aluminum. We use InAs semiconductor nanowires coated with thin superconducting shells of beta-Sn to realize transmon qubits. By tuning the Josephson energy with a gate voltage, we adjust the qubit frequency over a range of 3 GHz. The longest energy relaxation time, T1 = 27 microseconds, is obtained at the lowest qubit frequencies, while the longest echo dephasing time, T2 = 1.8 microseconds, is achieved at higher frequencies. We assess the possible factors limiting coherence times in these devices and discuss steps to enhance performance through improvements in materials fabrication and circuit design.

cond-mat.mes-hall↗

arXiv2025

Quantum confinement effect in Sb thin films

Antimony (Sb), an element with strong spin-orbit coupling, is predicted to undergo a topological phase transition from a topological semimetal to a topological insulator as its dimensionality approaches the two-dimensional limit, driven by the quantum confinement effect. In this study, we investigate this transition in Sb thin films grown by molecular beam epitaxy, employing electrical transport measurements and angle-resolved photoemission spectroscopy (ARPES). Electrical transport measurements revealed signatures of a modified electronic band structure, including a Hall response with multiple carrier types, a decreasing carrier concentration, and a transition in the curvature of the longitudinal resistance from quadratic to linear with decreasing film thickness. Temperature-dependent magnetoresistance further showed weak antilocalization below 16 K, indicating strong spin-orbit coupling and suggesting the presence of non-trivial topological states. Analysis of the WAL characteristics revealed a single coherent conducting channel and a thickness-dependent change in the phase decoherence mechanism. Complementary ARPES measurements confirmed that reducing the film thickness lifts the conduction band at the M-point, consistent with the emergence of a band gap. These findings support theoretical predictions of a thickness-dependent band structure evolution driven by the quantum confinement effect, providing a foundation for further exploration of topological phase transitions in Sb as well as Bi1-xSbx. The realization of an elemental topological material with simplified stoichiometry and semiconductor compatibility presents a promising avenue for next-generation hybrid systems and applications in spintronics and quantum technologies.

cond-mat.mes-hall↗

arXiv2025

Programmable phase selection between altermagnetic and non-centrosymmetric polymorphs of MnTe on InP via molecular beam epitaxy

Phase selecting nearly degenerate crystalline polymorphs during epitaxial growth can be challenging yet is critical to targeting physical properties for specific applications. Here, we establish how phase selectivity of altermagnetic and non-centrosymmetric polymorphs of MnTe with high structural quality and phase purity can be programmed by subtle changes to the surface of lattice-matched InP substrates in molecular beam epitaxial (MBE) growth. Bulk altermagnetic MnTe is thermodynamically stable in the hexagonal NiAs-structure and is synthesized here on the (111)A surface (In-terminated) of InP, while the non-centrosymmetric, cubic ZnS-structure with wide band gap (> 3eV) is stabilized on the (111)B surface (P-terminated). Here we use electron microscopy, photoemission spectroscopy, and reflection high-energy electron diffraction, which together indicate that the phase selection is triggered at the interface and proceeds along the growing surface. First principles calculations suggest that interfacial termination and strain have a significant effect on the interfacial energy; stabilizing the NiAs polymorph on the In-terminated surface and the ZnS structure on the P-terminated surface. Selectively grown, high-quality films of MnTe polymorphs are key platforms that will enable our understanding of the novel properties of these materials, thereby facilitating their use in new applications ranging from spintronics to microelectronic devices.

cond-mat.mtrl-sci↗

arXiv2025

Reversible Modification of Rashba States in Topological Insulators at Room Temperature by Edge Functionalization

Quantum materials with novel spin textures from strong spin-orbit coupling (SOC) are essential components for a wide array of proposed spintronic devices. Topological insulators have necessary strong SOC that imposes a unique spin texture on topological states and Rashba states that arise on the boundary, but there is no established methodology to control the spin texture reversibly. Here, we demonstrate that functionalizing Bi2Se3 films by altering the step-edge termination directly changes the strength of SOC and thereby modifies the Rashba strength of 1D edge states. Scanning tunneling microscopy/spectroscopy shows that these Rashba edge states arise and subsequently vanish through the Se functionalization and reduction process of the step edges. The observations are corroborated by density functional theory calculations, which show that a subtle chemical change of edge termination fundamentally alters the underlying electronic structure. Importantly, we experimentally demonstrated fully reversible and repeatable switching of Rashba edge states across multiple cycles at room temperature. The results imply Se functionalization as a practical method to control SOC and spin texture of quantum states in topological insulators.

cond-mat.mes-hall↗

arXiv2025

Nucleation and Antiphase Twin Control in Bi$_2$Se$_3$ via Step-Terminated Al$_2$O$_3$ Substrates

The epitaxial synthesis of high-quality 2D layered materials is an essential driver of both fundamental physics studies and technological applications. Bi$_2$Se$_3$, a prototypical 2D layered topological insulator, is sensitive to defects imparted during the growth, either thermodynamically or due to the film-substrate interaction. In this study, it is shown that step-terminated Al$_2$O$_3$ substrates with a high miscut angle (3{\deg}) can effectively suppress a particular hard-to-mitigate defect, the antiphase twin. Systematic investigations across a range of growth temperatures and substrate miscut angles confirm that atomic step edges act as preferential nucleation sites, stabilizing a single twin domain. First-principles calculations suggest that there is a significant energy barrier for twin boundary formation at step edges, supporting the experimental observations. Detailed structural characterization indicates that this twin-selectivity is lost through the mechanism of the 2D layers overgrowing the step edges, leading to higher twin density as the thickness increases. These findings highlight the complex energy landscape unique to 2D materials that is driven by the interplay between substrate properties, nucleation dynamics, and defect formation, and overcoming and controlling these are critical to improve material quality for quantum and electronic applications.

cond-mat.mtrl-sci↗

arXiv2025

Terahertz Landau level spectroscopy of Dirac fermions in millimeter-scale twisted bilayer graphene

Exotic electronic physics including correlated insulating states and fractional Chern insulators have been observed in twisted bilayer graphene in a magnetic field when the Fermi velocity vanishes, however a question remains as to the stability of these states which is controlled by the gap to the first excited state. Free-space terahertz magneto-optics can directly probe the gap to charge excitations which bounds the stability of electronic states, but this measurement has thus-far been inaccessible due to the micron size of twisted bilayer graphene samples, while the wavelength of terahertz light is up to a millimeter. Here we leverage advances in fabrication to create twisted bilayer graphene samples over 5 mm x 5 mm in size with a uniform twist angle and study the magnetic field dependence of the cyclotron resonance by a complex Faraday rotation experiment in p-doped large angle twisted bilayer graphene. These measurements directly probe charge excitations in inter-Landau level transitions and determine the Fermi velocity as a function of twist angle.

cond-mat.mes-hall↗

arXiv2025

Quantized Magneto-Terahertz Effects in the Antiferromagnetic Topological Insulator MnBi$_2$Te$_4$ Thin Films

MnBi$_2$Te$_4$ (MBT) is an ideal platform for studying the interplay between magnetism and topology. Many exotic topological phenomena, such as the quantum anomalous Hall effect and the axion insulator, have been observed in few-layer MBT. A key feature in MBT is the emergence of the surface exchange gap, which lies in the milli-electron-volt range (1 THz corresponds to 4.14 meV). This makes THz spectroscopy a powerful tool to probe the associated topological physics. In this study, we report the THz spectra of Faraday and Kerr rotations in MBT thin films grown by molecular beam epitaxy. By varying the external magnetic field, we observe three magnetic states: the antiferromagnetic state, the canted antiferromagnetic state, and the ferromagnetic state. Our terahertz results show a quantized Hall state under 6 T in both 6 SL and 7 SL samples without gate voltage. These findings provide new insights into the magneto-terahertz properties of MBT and its potential for topological spintronic applications.

cond-mat.mes-hall↗

arXiv2025

Sn-InAs nanowire shadow-defined Josephson junctions

Interest in hybrid electronic devices for quantum science is driving the research into superconductor-semiconductor materials combinations. Here we study InAs nanowires coated with shells of $\beta$-Sn. The wires grow via the vapor-liquid-solid mechanism out from (001) InAs substrates along two orientations, forming a criss-crossing landscape. This allows us to define nanowire-shadow junctions during the low temperature Sn shell deposition by carefully choosing the deposition angle. We find that the Sn shells are uniform in thickness and the grains have a preferential in-plane epitaxial relationship with InAs. The interface between Sn and InAs is abrupt and we do not observe interdiffusion. In our nanowire devices, Sn induces a superconducting gap of order 600 $\mu$eV, switching currents reaching values up to 500 nA, and critical magnetic fields along the nanowire of up to 1.3 T. These characteristics can be leveraged in the design of superconducting transmon qubits, parametric microwave amplifiers as well as for the investigation of triplet and topological superconductivity.

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

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

Controlling structural phases of Sn through lattice engineering

Topology and superconductivity, two distinct phenomena offer unique insight into quantum properties and their applications in quantum technologies, spintronics, and sustainable energy technologies if system can be found where they coexist. Tin (Sn) plays a pivotal role here as an element due to its two structural phases, $α$-Sn and $β$-Sn, exhibiting topological characteristics ($α$-Sn) and superconductivity ($β$-Sn). In this study we show how precise control of $α$ and $β$ phases of Sn thin films can be achieved by using molecular beam epitaxy grown buffer layers with systematic control over the lattice parameter. The resulting Sn films showed either $β$-Sn or $α$-Sn phases as the lattice constant of the buffer layer was varied from 6.10 A to 6.48 A, covering the range between GaSb (closely matched to InAs) and InSb. The crystal structures of the $α$- and $β$-Sn films were characterized by x-ray diffraction and confirmed by Raman spectroscopy and scanning transmission electron microscopy. The smooth and continuous surface morphology of the Sn films was validated using atomic force microscopy. The characteristics of $α$- and $β$-Sn phases were further verified using electrical transport measurements by observing resistance drop near 3.7 K for superconductivity of the $β$-Sn phase and Shubnikov-de Haas oscillations for the $α$-Sn phase. Density functional theory calculations showed that the stability of the Sn phases is highly dependent on lattice strain, with $α$-Sn being more stable under tensile strain and $β$-Sn becoming favorable under compressive strain, which is in good agreement with experimental observations. Hence, this study sheds light on controlling Sn phases through lattice engineering, enabling innovative applications in quantum technologies and beyond.

cond-mat.mtrl-sci↗

arXiv2024

Stoichiometry-induced ferromagnetism in altermagnetic candidate MnTe

The field of spintronics has seen a surge of interest in altermagnetism due to novel predictions and many possible applications. MnTe is a leading altermagnetic candidate that is of significant interest across spintronics due to its layered antiferromagnetic structure, high Neel temperature (TN ~ 310 K) and semiconducting properties. We present results on molecular beam epitaxy (MBE) grown MnTe/InP(111) films. Here, it is found that the electronic and magnetic properties are driven by the natural stoichiometry of MnTe. Electronic transport and in situ angle-resolved photoemission spectroscopy show the films are natively metallic with the Fermi level in the valence band and the band structure is in good agreement with first principles calculations for altermagnetic spin-splitting. Neutron diffraction confirms that the film is antiferromagnetic with planar anisotropy and polarized neutron reflectometry indicates weak ferromagnetism, which is linked to a slight Mn-richness that is intrinsic to the MBE grown samples. When combined with the anomalous Hall effect, this work shows that the electronic response is strongly affected by the ferromagnetic moment. Altogether, this highlights potential mechanisms for controlling altermagnetic ordering for diverse spintronic applications.

cond-mat.mtrl-sci↗

arXiv2024

Interfacially enhanced superconductivity in Fe(Te,Se)/Bi4Te3 heterostructures

Realizing topological superconductivity by integrating high-transition-temperature ($T_C$) superconductors with topological insulators can open new paths for quantum computing applications. Here, we report a new approach for increasing the superconducting transition temperature ($T_{C}^{onset}$) by interfacing the unconventional superconductor Fe(Te,Se) with the topological insulator Bi-Te system in the low-Se doping regime, near where superconductivity vanishes in the bulk. The critical finding is that the $T_{C}^{onset}$ of Fe(Te,Se) increases from nominally non-superconducting to as high as 12.5 K when $Bi_2Te_3$ is replaced with the topological phase $Bi_4Te_3$. Interfacing Fe(Te,Se) with $Bi_4Te_3$ is also found to be critical for stabilizing superconductivity in monolayer films where $T_{C}^{onset}$ can be as high as 6 K. Measurements of the electronic and crystalline structure of the $Bi_4Te_3$ layer reveal that a large electron transfer, epitaxial strain, and novel chemical reduction processes are critical factors for the enhancement of superconductivity. This novel route for enhancing $T_C$ in an important epitaxial system provides new insight on the nature of interfacial superconductivity and a platform to identify and utilize new electronic phases.

cond-mat.supr-con↗