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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., Comment: Accepted in Advanced Functional Materials

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., Comment: 18 pages, 5 figures, accepted by Advanced Materials

Here we present the discovery and characterization of the kagome metal TbTi$_3$Bi$_4$ in tandem with a new series of compounds, the Ln$_{2-x}$Ti$_{6+x}$Bi$_9$ (Ln: Tb-Lu) shurikagome metals. We previously reported on the growth of the LnTi$_3$Bi$_4$ (Ln: La-Gd$^{3+}$, Eu$^{2+}$, Yb$^{2+}$) family, a chemically diverse and exfoliable series of kagome metals with complex and highly anisotropic magnetism. However, unlike the La-Gd analogs, TbTi$_3$Bi$_4$ cannot be synthesized by our previous methodology due to phase competition with Ln$_{2-x}$Ti$_{6+x}$Bi$_9$ (x$\sim$1.7-1.2). Here we discuss the phase competition between the LnTi$_3$Bi$_4$ and Ln$_{2-x}$Ti$_{6+x}$Bi$_9$ families, helping to frame the difficulty in synthesizing LnTi$_3$Bi$_4$ compounds with small Ln species and providing a strategy to circumvent formation of Ln$_{2-x}$Ti$_{6+x}$Bi$_9$. Detailed characterization of the magnetic and electronic transport properties on single crystals of TbTi$_3$Bi$_4$ reveals a highly complex landscape of magnetic phases arising from an antiferromagnetic ground state. A series of metamagnetic transitions creates at least 5 unique magnetic phase pockets, including a 1/3 and 2/3 magnetization plateau. Further, the system exhibits an intimate connection between the magnetism and magnetotransport, exhibiting sharp switching from positive (+40%) to negative magnetoresistance (-50%). Like the LnTi$_3$Bi$_4$ kagome metals, the Ln$_{2-x}$Ti$_{6+x}$Bi$_9$ family exhibits quasi-2D networks of titanium and chains of rare-earth. We present the structures and some basic magnetic properties of the Ln$_{2-x}$Ti$_{6+x}$Bi$_9$ family alongside our characterization of the newly discovered TbTi$_3$Bi$_4$.

The current challenge to realizing continuously tunable magnetism lies in our inability to systematically change properties such as valence, spin, and orbital degrees of freedom as well as crystallographic geometry. Here, we demonstrate that ferromagnetism can be externally turned on with the application of low-energy helium implantation and subsequently erased and returned to the pristine state via annealing. This high level of continuous control is made possible by targeting magnetic metastability in the ultra-high conductivity, non-magnetic layered oxide PdCoO2 where local lattice distortions generated by helium implantation induce emergence of a net moment on the surrounding transition metal octahedral sites. These highly-localized moments communicate through the itinerant metal states which triggers the onset of percolated long-range ferromagnetism. The ability to continuously tune competing interactions enables tailoring precise magnetic and magnetotransport responses in an ultra-high conductivity film and will be critical to applications across spintronics., Comment: https://pubs.acs.org/doi/abs/10.1021/acs.nanolett.3c01065

The unique spin texture of quantum states in topological materials underpins many proposed spintronic applications. However, realizations of such great potential are stymied by perturbations, such as temperature and local fields imposed by impurities and defects, that can render a promising quantum state uncontrollable. Here, we report room-temperature observation of interaction between Rashba states and topological surface states, which manifests unique spin textures controllable by layer thickness of thin films. Specifically, we combine scanning tunneling microscopy/spectroscopy with the first-principles theoretical calculation to find the robust Rashba states coexisting with topological surface states along the surface steps with characteristic spin textures in momentum space. The Rashba edge states can be switched off by reducing the thickness of a topological insulator Bi2Se3 to bolster their interaction with the hybridized topological surface states. The study unveils a manipulating mechanism of the spin textures at room temperature, reinforcing the necessity of thin film technology in controlling quantum states.

Alloying indium into the topological insulator Sn1-xInxBi2Te4 induces bulk superconductivity with critical temperatures Tc up to 1.85 K and upper critical fields up to about 14 kOe. This is confirmed by electrical resistivity, heat capacity, and magnetic susceptibility measurements. The heat capacity shows a discontinuity at Tc and temperature dependence below Tc consistent with weak coupling BCS theory, and suggests a superconducting gap near 0.25 meV. The superconductivity is type-II and the topological surface states have been verified by photoemission. A simple picture suggests analogies with the isostructural magnetic topological insulator MnBi2Te4, in which a natural heterostructure hosts complementary properties on different sublattices, and motivates new interest in this large family of compounds. The existence of both topological surface states and superconductivity in Sn1-xInxBi2Te4 identifies these materials as promising candidates for the study of topological superconductivity.

The interface between two-dimensional topological Dirac states and an s-wave superconductor is expected to support Majorana bound states that can be used for quantum computing applications. Realizing these novel states of matter and their applications requires control over superconductivity and spin-orbit coupling to achieve spin-momentum locked topological surface states which are simultaneously superconducting. While signatures of Majorana bound states have been observed in the magnetic vortex cores of bulk FeTe0.55Se0.45, inhomogeneity and disorder from doping makes these signatures unclear and inconsistent between vortices. Here we report superconductivity in monolayer FeTe1-ySey (Fe(Te,Se)) grown on Bi2Te3 by molecular beam epitaxy. Spin and angle resolved photoemission spectroscopy directly resolve the interfacial spin and electronic structure of Fe(Te,Se)/Bi2Te3 heterostructures. We find that for y = 0.25 the Fe(Te,Se) electronic structure overlaps with the topological Bi2Te3 interfacial states which disrupts the desired spin-momentum locking. In contrast, for y = 0.1 a smaller Fe(Te,Se) Fermi surface allows for clear spin-momentum locking observed in the topological states. Hence, we demonstrate the Fe(Te,Se)/Bi2Te3 system is a highly tunable platform for realizing Majorana bound states where reduced doping can improve characteristics important for Majorana interrogation and potential applications.

Understanding the effects of interfacial modification to the functional properties of magnetic topological insulator thin films is crucial for developing novel technological applications from spintronics to quantum computing. Here, we report that a large electronic and magnetic response is induced in the intrinsic magnetic topological insulator MnBi2Te4 by controlling the propagation of surface oxidation. We show that the formation of the surface oxide layer is confined to the top 1-2 unit cells but drives large changes in the overall magnetic response. Specifically, we observe a dramatic reversal of the sign of the anomalous Hall effect driven by finite thickness magnetism, which indicates that the film splits into distinct magnetic layers each with a unique electronic signature. These data reveal a delicate dependence of the overall magnetic and electronic response of MnBi2Te4 on the stoichiometry of the top layers. Our study suggests that perturbations resulting from surface oxidation may play a non-trivial role in the stabilization of the quantum anomalous Hall effect in this system and that understanding targeted modifications to the surface may open new routes for engineering novel topological and magnetic responses in this fascinating material., Comment: Accepted in Advanced Functional Materials

Topological materials hosting kagome lattices have drawn considerable attention due to the interplay between topology, magnetism, and electronic correlations. The (Fe$_{1-x}$Co$_x$)Sn system not only hosts a kagome lattice but has a tunable symmetry breaking magnetic moment with temperature and doping. In this study, angle resolved photoemission spectroscopy and first principles calculations are used to investigate the interplay between the topological electronic structure and varying magnetic moment from the planar to axial antiferromagnetic phases. A theoretically predicted gap at the Dirac point is revealed in the low temperature axial phase but no gap opening is observed across a temperature dependent magnetic phase transition. However, topological surface bands are observed to shift in energy as the surface magnetic moment is reduced or becomes disordered over time during experimental measurements. The shifting surface bands may preclude the determination of a temperature dependent bulk gap but highlights the intricate connections between magnetism and topology with a surface/bulk dichotomy that can affect material properties and their interrogation., Comment: 11 pages, 4 figures

In this work, the high entropy oxide A2CuO4 Ruddlesden-Popper (La0.2Pr0.2Nd0.2Sm0.2Eu0.2)2CuO4 is explored by charge doping with Ce+4 and Sr+2 at concentrations known to induce superconductivity in the simple parent compounds, Nd2CuO4 and La2CuO4. Electron doped (La0.185Pr0.185Nd0.185Sm0.185Eu0.185Ce0.075)2CuO4 and hole doped (La0.18Pr0.18Nd0.18Sm0.18Eu0.18Sr0.1)2CuO4 are synthesized and shown to be single crystal, epitaxially strained, and highly uniform. Transport measurements demonstrate that all as-grown films are insulating regardless of doping. Annealing studies show that resistivity can be tuned by modifying oxygen stoichiometry and inducing metallicity but without superconductivity. These results in turn are connected to extended x-ray absorption fine structure (EXAFS) results indicating that the lack of superconductivity in the high entropy cuprates likely originates from a large distortion within the Cu-O plane ({\sigma}2>0.015 {\AA}2) due to A-site cation size variance, which drives localization of charge carriers. These findings describe new opportunities for controlling charge- and orbital-mediated functional responses in Ruddlesden-Popper crystal structures, driven by balancing of cation size and charge variances that may be exploited for functionally important behaviors such as superconductivity, antiferromagnetism, and metal-insulator transitions, while opening less understood phase spaces hosting doped Mott insulators, strange metals, quantum criticality, pseudogaps, and ordered charge density waves.

The combination of two-dimensional Dirac surface states with s-wave superconductivity is expected to generate localized topological Majorana zero modes in vortex cores. Putative experimental signatures of these modes have been reported for heterostructures of proximitized topological insulators, iron-based superconductors or certain transition metal dichalcogenides. Despite these efforts, the Majorana nature of the observed excitation is still under debate. We propose to identify the presence of Majorana vortex modes using a nonlocal transport measurement protocol originally employed for one-dimensional settings. In the case of an isolated subgap state, the protocol provides a spatial map of the ratio of local charge- and probability-density which offers a clear distinction between Majorana and ordinary fermionic modes. We show that these distinctive features survive in the experimentally relevant case of hybridizing vortex core modes., Comment: 12 pages, 6 Figures

Ternary chalcogenides such as the parkerites and shandites are a broad class of materials exhibiting rich diversity of transport and magnetic behavior as well as an array of topological phases including Weyl and Dirac nodes. However, they remain largely unexplored as high-quality epitaxial thin films. Here, we report the self-regulated growth of thin films of the strong spin-orbit coupled superconductor Pd3Bi2Se2 on SrTiO3 by molecular beam epitaxy. Films are found to grow in a self-regulated fashion, where, in excess Se, the temperature and relative flux ratio of Pd to Bi controls the formation of Pd3Bi2Se2 due to the combined volatility of Bi, Se, and Bi-Se bonded phases. The resulting films are shown to be of high structural quality, the stoichiometry is independent of the Pd:Bi and Se flux ratio and exhibit a superconducting transition temperature of 800 mK and critical field of 17.7 +/- 0.5 mT, as probed by transport as well as magnetometry. Understanding and navigating the growth of the chemically and structurally diverse classes of ternary chalcogenides opens a vast space for discovering new phenomena as well as enabling new applications.

In the cuprates, one-dimensional chain compounds provide a unique opportunity to understand the microscopic physics due to the availability of reliable theories. However, progress has been limited by the inability to controllably dope these materials. Here, we report the synthesis and spectroscopic analysis of the one-dimensional cuprate Ba$_{2-x}$Sr$_x$CuO$_{3+\delta}$ over a wide range of hole doping. Our angle-resolved photoemission experiments reveal the doping evolution of the holon and spinon branches. We identify a prominent folding branch whose intensity fails to match predictions of the simple Hubbard model. An additional strong near-neighbor attraction, which may arise from coupling to phonons, quantitatively explains experiments for all accessible doping levels. Considering structural and quantum chemistry similarities among cuprates, this attraction will play a similarly crucial role in the high-$T_C$ superconducting counterparts

Research on two-dimensional materials has expanded over the past two decades to become a central theme in condensed matter research today. Significant advances have been made in the synthesis and subsequent reassembly of these materials using mechanical methods into a vast array of hybrid structures with novel properties and ever-increasing potential applications. The key hurdles in realizing this potential are the challenges in controlling the atomic structure of these layered hybrid materials and the difficulties in harnessing their unique functionality with existing semiconductor nanofabrication techniques. Here we report on high-quality van der Waals epitaxial growth and characterization of a layered topological insulator on freestanding monolayer graphene transferred to different mechanical supports. This templated synthesis approach enables direct interrogation of interfacial atomic structure of these as-grown hybrid structures and opens a route towards creating device structures with more traditional semiconductor nanofabrication techniques., Comment: 13 pages, 4 figures

The enhanced superconductivity in monolayer FeSe on titanates opens a fascinating pathway towards the rational design of high-temperature superconductors. Utilizing the state-of-the-art oxide plus chalcogenide molecular beam epitaxy systems in situ connected to a synchrotron angle-resolved photoemission spectroscope, epitaxial LaTiO3 layers with varied atomic thicknesses are inserted between monolayer FeSe and SrTiO3, for systematic modulation of interfacial chemical potential.With the dramatic increase of electron accumulation at the LaTiO3-SrTiO3 surface, providing a substantial surge of work function mismatch across the FeSe-oxide interface, the charge transfer and the superconducting gap in the monolayer FeSe are found to remain markedly robust. This unexpected finding indicates the existence of an intrinsically anchored magic doping within the monolayer FeSe systems.

The interface between magnetic materials and topological insulators can drive the formation of exotic phases of matter and enable functionality through manipulation of the strong spin polarized transport. Here, we report that the spin-momentum-locked transport in the topological insulator Bi$_2$Se$_3$ is completely suppressed by scattering at a heterointerface with the kagome-lattice paramagnet, Co$_7$Se$_8$. Bi$_2$Se$_{3-}$Co$_7$Se$_{8-}$Bi$_2$Se$_3$ trilayer heterostructures were grown using molecular beam epitaxy. Magnetotransport measurements revealed a substantial suppression of the weak antilocalization effect for Co$_7$Se$_8$ at thicknesses as thin as a monolayer, indicating a strong dephasing mechanism. Bi$_{2-x}$Co$_x$Se$_3$ films, where Co is in a non-magnetic $3^+$ state, show weak antilocalization that survives to $x = 0.5$, which, in comparison with the heterostructures, suggests the unordered moments of the Co$^{2+}$ act as a far stronger dephasing element. This work highlights several important points regarding spin-polarized transport in topological insulator interfaces and how magnetic materials can be integrated with topological materials to realize both exotic phases as well as novel device functionality., Comment: Accepted APL Materials

Substrate engineering provides an opportunity to modulate the physical properties of quantum materials in thin film form. Here we report that TiSe$_2$ thin films grown on TiO$_2$ have unexpectedly large electron doping that suppresses the charge density wave (CDW) order. This is dramatically different from either bulk single crystal TiSe$_2$ or TiSe$_2$ thin films on graphene. The epitaxial TiSe$_2$ thin films can be prepared on TiO$_2$ via molecular beam epitaxy (MBE) in two ways: by conventional co-deposition using selenium and titanium sources, and by evaporating only selenium on reconstructed TiO$_2$ surfaces. Both growth methods yield atomically flat thin films with similar physical properties. The electron doping and subsequent suppression of CDW order can be explained by selenium vacancies in the TiSe$_2$ film, which naturally occur when TiO$_2$ substrates are used. This is due to the stronger interfacial bonding that changes the ideal growth conditions. Our finding provides a way to tune the chemical potential of chalcogenide thin films via substrate selection and engineering., Comment: 4 figures

A quantum spin Hall (QSH) insulator is a novel two-dimensional quantum state of matter that features quantized Hall conductance in the absence of magnetic field, resulting from topologically protected dissipationless edge states that bridge the energy gap opened by band inversion and strong spin-orbit coupling. By investigating electronic structure of epitaxially grown monolayer 1T'-WTe2 using angle-resolved photoemission (ARPES) and first principle calculations, we observe clear signatures of the topological band inversion and the band gap opening, which are the hallmarks of a QSH state. Scanning tunneling microscopy measurements further confirm the correct crystal structure and the existence of a bulk band gap, and provide evidence for a modified electronic structure near the edge that is consistent with the expectations for a QSH insulator. Our results establish monolayer 1T'-WTe2 as a new class of QSH insulator with large band gap in a robust two-dimensional materials family of transition metal dichalcogenides (TMDCs)., Comment: 19 pages, 4 figures; includes Supplemental Material (11 pages, 7 figures)

The high temperature superconductivity in single-unit-cell (1UC) FeSe on SrTiO3 (STO)(001) and the observation of replica bands by angle-resolved photoemission spectroscopy (ARPES) have led to the conjecture that the coupling between FeSe electron and the STO phonon is responsible for the enhancement of Tc over other FeSe-based superconductors1,2. However the recent observation of a similar superconducting gap in FeSe grown on the (110) surface of STO raises the question of whether a similar mechanism applies3,4. Here we report the ARPES study of the electronic structure of FeSe grown on STO(110). Similar to the results in FeSe/STO(001), clear replica bands are observed. We also present a comparative study of STO (001) and STO(110) bare surfaces, where photo doping generates metallic surface states. Similar replica bands separating from the main band by approximately the same energy are observed, indicating this coupling is a generic feature of the STO surfaces and interfaces. Our findings suggest that the large superconducting gaps observed in FeSe films grown on two different STO surface terminations are likely enhanced by a common coupling between FeSe electrons and STO phonons., Comment: 12 pages, 4 figures

In the high-temperature ($T_{c}$) cuprate superconductors, increasing evidence suggests that the pseudogap, existing below the pseudogap temperature $T$*, has a distinct broken electronic symmetry from that of superconductivity. Particularly, recent scattering experiments on the underdoped cuprates have suggested that a charge ordering competes with superconductivity. However, no direct link of this physics and the important low-energy excitations has been identified. Here we report an antagonistic singularity at $T_{c}$ in the spectral weight of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+{\delta}}$ as a compelling evidence for phase competition, which persists up to a high hole concentration $p$ ~ 0.22. Comparison with a theoretical calculation confirms that the singularity is a signature of competition between the order parameters for the pseudogap and superconductivity. The observation of the spectroscopic singularity at finite temperatures over a wide doping range provides new insights into the nature of the competitive interplay between the two intertwined phases and the complex phase diagram near the pseudogap critical point., Comment: 17 pages with 4 figures and supplementary information with 18 pages with 1 table and 4 figures

We characterize the occupied and unoccupied electronic structure of the topological insulator Bi$_2$Se$_3$ by one-photon and two-photon angle-resolved photoemission spectroscopy and slab band structure calculations. We reveal a second, unoccupied Dirac surface state with similar electronic structure and physical origin to the well-known topological surface state. This state is energetically located 1.5 eV above the conduction band, which permits it to be directly excited by the output of a Ti:Sapphire laser. This discovery demonstrates the feasibility of direct ultrafast optical coupling to a topologically protected, spin-textured surface state., Comment: Accepted to Physical Review Letters

The nature of metallicity and the level of electronic correlations in the antiferromagnetically ordered parent compounds are two important open issues for the iron-based superconductivity. We perform a temperature-dependent angle-resolved photoemission spectroscopy study of Fe1.02Te, the parent compound for iron chalcogenide superconductors. Deep in the antiferromagnetic state, the spectra exhibit a "peak-dip-hump" line shape associated with two clearly separate branches of dispersion, characteristics of polarons seen in manganites and lightly-doped cuprates. As temperature increases towards the Neel temperature (T_N), we observe a decreasing renormalization of the peak dispersion and a counterintuitive sharpening of the hump linewidth, suggestive of an intimate connection between the weakening electron-phonon (e-ph) coupling and antiferromagnetism. Our finding points to the highly-correlated nature of Fe1.02Te ground state featured by strong interactions among the charge, spin and lattice and a good metallicity plausibly contributed by the coherent polaron motion., Comment: 5 pages, 4 figures. Accepted by prl

Angle resolved photoemission spectroscopy (ARPES) studies were performed on two compounds (TlBiTe$_2$ and TlBiSe$_2$) from a recently proposed three dimensional topological insulator family in Thallium-based III-V-VI$_2$ ternary chalcogenides. For both materials, we show that the electronic band structures are in broad agreement with the $ab$ $initio$ calculations; by surveying over the entire surface Brillouin zone (BZ), we demonstrate that there is a single Dirac cone reside at the center of BZ, indicating its topological non-triviality. For TlBiSe$_2$, the observed Dirac point resides at the top of the bulk valance band, making it a large gap ($\geq$200$meV$) topological insulator; while for TlBiTe$_2$, we found there exist a negative indirect gap between the bulk conduction band at $M$ point and the bulk valance band near $\Gamma$, making it a semi-metal at proper doping. Interestingly, the unique band structures of TlBiTe$_2$ we observed further suggest TlBiTe$_2$ may be a candidate for topological superconductors., Comment: 5 pages, 4 figures