Magnetically doped topological insulators (TIs) exhibit two distinct phases: the quantum anomalous Hall (QAH) phase when the Fermi level resides within the surface gap and a metallic phase outside the gap. The QAH phase hosts unidirectional transport channels known as chiral edge states, while the metallic phase exhibits nonreciprocal transport due to unbalanced bidirectional edge states. Using the chiral edge states in Cr-doped (Bi,Sb) 2 Te 3 sandwich structures, we realize non-Hermitian conductance matrices in a one-dimensional Corbino chain with well-defined chirality. By tuning the boundary conditions from periodic to open, we reveal the non-Hermitian skin effect, where eigenstates localize exponentially at one end of the chain. In the metallic phase, we further observe asymmetric, bidirectional coupling between the neighboring sites in the conductance matrix, a direct consequence of the system’s intrinsic nonreciprocity. These results establish magnetic TIs as a powerful platform for investigating emergent non-Hermitian phenomena in topological systems.

Altermagnets are a recently discovered class of materials with magnetic ordering that have a zero net magnetization and a momentum-dependent spin splitting in their band structure, arising from a collinear spin arrangement with alternating polarizations in the crystal lattice. The nickeline-structured manganese telluride (α-MnTe) is an attractive altermagnet candidate due to its predicted large spin splitting energy and a transition temperature near 300 K. In this work, we present a thorough investigation of the spin structure of α-MnTe thin films grown by molecular beam epitaxy with very high crystal quality and low residual magnetization. The epitaxial α-MnTe films have a full-width-at-half-maximum of 0.1° as measured by X-ray-diffraction rocking curves and a root-mean-square roughness below 1 nm. Neutron diffraction measurements confirm the antiferromagnetic order in the α-MnTe film and show a Néel temperature of 307 K. Polarized neutron reflectometry detects a vanishingly small net magnetization which may be confined to the MnTe/InP interface, which is evidence of the near-ideal stoichiometry in the sample. In vacuo angle resolved photoemission spectroscopy reveals that the bulk band spectrum of the MnTe films is consistent with the weak altermagnetic order as theoretically predicted and observed for the high symmetry nodal plane in the center of the Brillouin zone. This study establishes optimized growth conditions for the synthesis of stoichiometric α-MnTe thin films which exhibit exceptional structural and magnetic ordering, thereby providing a robust platform for evaluating the properties relevant to its consideration as an altermagnetic candidate.

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.

Two decades since its discovery, superconducting heavily boron-doped diamond (HBDD) still poses fundamental questions that need to be answered to unlock its full potential for quantum applications. We use electrical magnetotransport measurements of critically doped homoepitaxial single crystal HBDD films to reveal signatures of intrinsically granular superconductivity. By studying the dependence of electrical resistivity on temperature and magnetic field vector, we infer that this granularity arises from doping induced disorder. We observe an unexpected three-phase anisotropy in the magnetoresistance, accompanied by a spontaneous transverse voltage (Hall anomaly). Our findings indicate the emergence of an anisotropic order in an otherwise isotropic single crystal HBDD film, offering insights into the mechanism of superconductivity in this quantum material.

Multiterminal Josephson junctions offer a powerful playground for exploring exotic superconducting and topological phenomena beyond the reach of conventional two-terminal devices. In this work, we present the direct spectroscopic observation of Cooper quartet resonances, a signature of correlated tunneling of two Cooper pairs across the device, in a graphene three-terminal Josephson junction (3TJJ). Using tunneling spectroscopy, we visualize how Andreev bound states (ABS) evolve across a two-dimensional superconducting phase space, controlled by the two independent phase differences in the 3TJJ. These measurements reveal sharp local minima in the differential conductance spectra locked in a specific phase condition of superconducting phase variables. The resulting quantized trajectories around the compact torus of the superconducting phase variables reveal an underlying topological winding in the multipair transport. To interpret our results, we develop a theoretical model that connects the observed quartet resonances to the coherent hybridization of multiple ABS branches, a hallmark of the rich pairing process enabled by multiterminal geometries. Our results highlight the potential of multiterminal superconducting devices to host engineered superconducting states and pave the way for new approaches to topological band structure design based on phase-controlled, higher-order superconducting transport.

We use molecular beam epitaxy to develop a gate tunable p-n heterojunction that interfaces a canonical Dirac semimetal, Cd_3As_2, and a ferromagnetic semiconductor, In_1-xMn_xAs, with perpendicular magnetic anisotropy. Measurements of the anomalous Hall effect in top-gated Cd_3As_2/In_1-xMn_xAs devices show that the ferromagnetic Curie temperature (T_C) can be efficiently tuned using a modest gate voltage of ∼ 10 V, corresponding to a sensitivity to electric field (E) of ΔT_C/ΔE ∼ 10 K/MV/cm). The voltage tuning of T_C saturates near the charge neutrality point of Cd_3As_2 and vanishes at positive gate voltage in appropriately designed heterostructures. This non-monotonic behavior cannot be explained solely by hole-mediated ferromagnetism in the In_1-xMn_xAs alone, suggesting an interaction between the Dirac semimetal and the ferromagnetic semiconductor. Our results identify Cd_3As_2/In_1-xMn_xAs heterojunctions as a potentially attractive platform for studying emergent phenomena arising from the interplay between broken symmetry, topology, and magnetism in a topological semimetal.

The creation of topologically non-trivial matter across electronic, mechanical, cold-atom, and photonic platforms is advancing rapidly, yet understanding the breakdown of topological protection remains a major challenge. In this work, we use magnetic imaging combined with global electrical transport measurements to visualize the current-induced breakdown of the quantum anomalous Hall effect (QAHE) in a magnetically doped topological insulator. We find that dissipation emerges at localized hot spots near electrical contacts, where an abrupt change in Hall angle leads to significant distortions of the current density. Using the local magnetization as a proxy for electron temperature, we directly observe that the electrons are driven out of equilibrium with the lattice at the hot spots and throughout the device in the breakdown regime. By characterizing energy relaxation processes in our device, we show that the breakdown of quantization is governed entirely by electron heating, and that a vanishing thermal relaxation strength at millikelvin temperatures limits the robustness of the QAHE. Our findings provide a framework for diagnosing energy relaxation in topological materials and will guide realizing robust topological protection in magnetic topological insulators.

We investigate emergent superconductivity and nonreciprocal transport (magnetochiral anisotropy and superconducting diode effect) at the heterointerface of two nonsuperconducting van der Waals (vdW) materials, Dirac semimetal ZrTe2 and antiferromagnetic iron chalcogenide FeTe, grown using molecular beam epitaxy. We show from electrical transport measurements that two-dimensional (2D) superconductivity arises at the heterointerface below a critical temperature (Tc) of ∼10 K. In the superconducting transition region, nonreciprocal transport, characterized by magneto-chiral anisotropy, exhibits a magnitude comparable to that observed in topological insulators and is enhanced by a factor of 3 when the heterostructure is capped with a 2D vdW ferromagnet (CrTe2). Below Tc, the superconducting diode effect exhibits an efficiency of 29%. With strong spin-orbit coupling in ZrTe2, these epitaxial heterostructures provide an attractive epitaxial vdW platform for exploring unconventional superconductivity in Dirac semimetals and for developing nonreciprocal devices for superconducting electronics.

Indium selenide (In_2Se_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_2Se_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_2Se_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_2Se_3 single crystals with 3R stacking. Furthermore, the crystals exhibit remarkably low carrier density of 1.5-3.2 × 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_2Se_3 single crystals with reduced intrinsic defects. This technique can be broadly applied to grow other volatile chalcogenides with reduced defects and controlled stoichiometry.

In this work, we show that by coupling in situ spectroscopic ellipsometry with a molecular beam epitaxy growth chamber, the growth parameters of ternary compounds with potential for quantum and spintronic applications can be immediately ascertained during the entire growth cycle of a sample. Initially, several films of (BixIn1-x)(2)Se-3 with stoichiometries ranging from x = 0 to x = 1 were grown and characterized by X-ray reflectivity, X-ray photoelectron spectroscopy, and Rutherford backscattering. Using this information, ellipsometry spectra were fitted by representing the dielectric functions with Kramers-Kronig-consistent oscillators. Consequently, composition-dependent dielectric functions of (BixIn1-x)(2)Se-3 were parametrized to create a material file that determines the Bi content of an unknown (BixIn1-x)(2)Se-3 film. By using this material file, therefore, both the Bi content and thickness of a (BixIn1-x)(2)Se-3 film can be obtained immediately at any stage of the growth cycle. We tested the model for universality among MBE growth systems and found that the model was transferable between systems. Furthermore, the generalized model allowed us to monitor sticking and desorption coefficients for Bi2Se3 thin films in operando for the first time, with significant implications for quantum and spintronic applications by enabling more reproducible and controlled device fabrication, advancing the understanding of emergent physics, and helping address future societal bottlenecks in electronic performance and demand.

Bulk-boundary correspondence is a foundational principle underlying the electronic band structure and physical behavior of topological quantum materials. Although it has been rigorously tested in topological systems where the physical properties involve charge currents, it remains unclear whether bulk-boundary correspondence should also hold for non-conserved spin currents. We study charge-to-spin conversion in a canonical topological insulator, Bi$_{1-x}$Sb$_x$, to address this fundamentally unresolved question. We use spin-torque ferromagnetic resonance measurements to accurately probe the charge-to-spin conversion efficiency in epitaxial Bi$_{1-x}$Sb$_x$~thin films of high structural quality spanning the entire range of composition, including both trivial and topological band structures, as verified using {\it in vacuo} angle-resolved photoemission spectroscopy. From these measurements, we deduce the effective spin Hall conductivity (SHC) and find excellent agreement with the values predicted by tight-binding calculations for the intrinsic SHC of the bulk bands. These results provide strong evidence that the strong spin-orbit entanglement of bulk states well below the Fermi energy connects directly to the SHC in epitaxial Bi$_{1-x}$Sb$_x$~films interfaced with a metallic ferromagnet. The excellent agreement between theory and experiment points to the generic value of analyses focused entirely on bulk properties, even for topological systems involving non-conserved spin currents.

We report charge-to-spin conversion in a hybrid heterostructure comprised of atomically thin bismuth (Bi) confined between a silicon carbide (SiC) substrate and epitaxial graphene (EG). We confirm the composition, the dimensionality, and a 96.5% intercalation coverage using x-ray photoelectron spectroscopy, scanning transmission microscopy, low-energy electron diffraction, and Raman spectroscopy. Electrical transport measurements show signs of weak antilocalization in the heterostructure. Spin-torque ferromagnetic resonance measurements in Permalloy/EG/2D-Bi heterostructures probe charge-to-spin conversion, revealing an in-plane polarization of the spin current, perpendicular to the charge current. The ratio of the in-plane to out-of-plane torque is 3.75 times higher than in hydrogenated graphene control samples.

Altermagnets are a newly identified family of collinear antiferromagnets with a momentum-dependent spin-split band structure of non-relativistic origin, derived from spin-group symmetry-protected crystal structures. Among candidate altermagnets, CrSb is attractive for potential applications because of a large spin-splitting near the Fermi level and a high Néel transition temperature of around 700 K. Molecular beam epitaxy is used to synthesize CrSb (0001) thin films with thicknesses ranging from 10 to 100 nm. Structural characterization, using reflection high energy electron diffraction, scanning transmission electron microscopy, and X-ray diffraction, demonstrates the growth of epitaxial films with good crystallinity. Polarized neutron reflectometry shows the absence of any net magnetization, consistent with antiferromagnetic order. In vacuo angle resolved photoemission spectroscopy (ARPES) measurements probe the band structure in a previously unexplored regime of film thickness, down to 10 nm. These ARPES measurements show a bulk-type, 3D momentum-dependent band splitting of up to 0.7 eV with g-wave symmetry, consistent with that seen in prior studies of bulk single crystals. The distinct altermagnetic band structure required for potential spin-transport applications survives down to the ∼10 nm thin film limit at room temperature.

Magnetic topological insulators (MTIs) host topologically protected edge states, but the role that these edge states play in electronic transport remains unclear. Using scanning superconducting quantum interference device (SQUID) microscopy, we performed local measurements of the current distribution in a quantum anomalous Hall (QAH) insulator at large bias currents, where the quantization of the conductivity tensor breaks down. We find that bulk currents in the channel interior coexist with edge currents at the sample boundary. While the position of the edge current changes with the reversal of the magnetic field, it does not depend on the current direction. To understand our observations, we introduce a model which includes contributions from both the sample magnetization and currents driven by chemical potential gradients. To parameterize our model, we use local measurements of the chemical potential induced changes in the sample magnetization. Our model reveals that the observed edge currents can be understood as changes in the magnetization generated by the electrochemical potential distribution in the sample under bias. Our work underscores the complexity of electronic transport in MTIs and highlights both the value and challenges of using magnetic imaging to disentangle various contributions to the electronic transport signatures.

We study nonequilibrium Andreev resonances in a voltage-biased graphene three-terminal Josephson junction (JJ). We observe periodic oscillations of resistance with maxima at multiples of the magnetic flux quantum (noninversion regime). As we increase the bias voltage, we further observe a transition point beyond which oscillations exhibit a $\pi$ phase shift (inversion regime) with maxima of resistance occuring at multiples of half-flux quantum. At this transition point, the frequency of the oscillations is doubled. We develop a model based on the coupling of the static Andreev bound states (ABSs) to the nonequilibrium Fermi surface of graphene to explain the observed noninversion to inversion crossovers. Our model associates these crossovers to microscopic phase-sensitive Andreev reflections which couple the normal and superfluid components of the current. Our findings show that multiterminal JJs can be used to engineer unconventional energy-phase relations such as those expected in the $\pi$-shifted ABSs without relying on quartet and Floquet physics. These nonequilibrium ABSs could potentially find applications in superconducting $\pi$ qubits.

Bulk-boundary correspondence, a foundational principle underlying the electronic band structure and physical behavior of topological quantum materials, has been rigorously tested in topological systems that involve conserved charge currents. However, it remains unclear whether bulk-boundary correspondence should hold for nonconserved spin currents. We address this unresolved question by using spin-torque ferromagnetic resonance to accurately probe the charge-to-spin conversion efficiency in epitaxial thin films of a canonical topological insulator, Bi1-xSbx. We find that the measured effective spin Hall conductivity (SHC) agrees well with tight-binding calculations for the intrinsic SHC of the bulk bands. These results indicate that the strong spin-orbit entanglement of bulk states well below the Fermi energy connects directly to the SHC of surface states in epitaxial Bi1-xSbx films interfaced with a metallic ferromagnet. The excellent agreement between theory and experiment affirms the generic value of analyses focused entirely on bulk properties, even for nonconserved topological spin currents.

Magnetic topological insulators (MTIs) host topologically protected edge states, but the role these edge states play in electronic transport remains unclear. Using scanning superconducting quantum interference device (SQUID) microscopy, we performed local measurements of the current distribution in a quantum anomalous Hall (QAH) insulator at large bias currents, where the quantization of the conductivity tensor breaks down. We find that bulk currents in the channel interior coexist with edge currents at the sample boundary. While the position of the edge current changes with the reversal of the magnetic field, it does not depend on the current direction. To understand our observations, we introduce a model that includes contributions from both the sample magnetization and currents driven by chemical potential gradients. To parametrize our model, we use local measurements of the chemical potential induced changes in the sample magnetization. Our model reveals that the observed edge currents can be understood as changes in the magnetization generated by the electrochemical potential distribution in the sample under bias. Our work underscores the complexity of electronic transport in MTIs and highlights both the value and challenges of using magnetic imaging to disentangle various contributions to the electronic transport signatures.

With rapid advances in qubit technologies, techniques for localizing, modulating, and measuring RF fields and their impact on qubit performance are of the utmost importance. Here, we demonstrate that flux-channeling from a permalloy nanowire can be used to achieve localized spatial modulation of an RF field and that the modulated field can be mapped with high resolution by using the Rabi oscillations of an NV center. Rabi maps reveal ∼100 mm wavelength microwaves concentrated in sub-300 nm regions with up to ∼16× power enhancement. This modulation is robust over a 20 dBm power range and has no adverse impact on NV T2 coherence time. Micromagnetic simulations confirm that the modulated field results from the nanowire's stray field through its constructive/destructive interference with the incident RF field. Our findings provide a new pathway for controlling qubits, amplifying RF signals, and mapping local fields in various on-chip RF technologies.

The interface between two different materials can show unexpected quantum phenomena. In this study, we used molecular beam epitaxy to synthesize heterostructures formed by stacking together two magnetic materials, a ferromagnetic topological insulator (TI) and an antiferromagnetic iron chalcogenide (FeTe). We observed emergent interface-induced superconductivity in these heterostructures and demonstrated the co-occurrence of superconductivity, ferromagnetism, and topological band structure in the magnetic TI layer-the three essential ingredients of chiral topological superconductivity (TSC). The unusual coexistence of ferromagnetism and superconductivity is accompanied by a high upper critical magnetic field that exceeds the Pauli paramagnetic limit for conventional superconductors at low temperatures. These 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.

The nature of the anomalous metal state has been a major puzzle in condensed matter physics for more than three decades. Here, we report systematic investigation and modulation of the anomalous metal states in high-temperature interface superconductor FeSe films on SrTiO3 substrate. Remarkably, under zero magnetic field, the anomalous metal state persists up to 20 K in pristine FeSe films, an exceptionally high temperature standing out from previous observations. In stark contrast, for the FeSe films with nano-hole arrays, the characteristic temperature of the anomalous metal state is considerably reduced. We demonstrate that the observed anomalous metal states originate from the quantum tunneling of vortices adjusted by the Ohmic dissipation. Our work offers a perspective for understanding the origin and modulation of the anomalous metal states in two-dimensional bosonic systems.