Surface plasmon-phonon-magnon polariton in a topological insulator-antiferromagnetic bilayer structure. (arXiv:2205.07367v1 [cond-mat.mtrl-sci])

2022-05-17T02:29:45+00:00May 17th, 2022|Categories: Publications|Tags: , , |

We present a robust technique for computationally studying surface polariton modes in hybrid materials. We use a semi-classical model that allows us to understand the physics behind the interactions between collective excitations of the hybrid system and develop a scattering and transfer matrix method that imposes the proper boundary conditions to solve Maxwell equations and derive a general equation describing the surface polariton in a heterostructure consisting of N constituent materials. We apply this method to a test structure composed of a topological insulator (TI) and an antiferromagnetic material (AFM) to study the resulting surface Dirac plasmon-phonon-magnon polariton (DPPMP). We find that interactions between the excitations of the two constituents result in the formation of hybridized modes and the emergence of avoided-crossing points in the dispersion relations for the DPPMP. For the specific case of a Bi2Se3 TI material, the polariton branch with low frequency below 2 THz redshifts upon increasing the thickness of TI thin film, which leads to an upper bound on the thickness of the TI layer that will allow an observable signature of strong coupling and the emergence of hybridized states. We also find that the strength of the coupling between the TI and the AFM, which is parameterized by the amplitude of the avoided-crossing splitting between the two polariton branches at the magnon resonance frequency, depends on the magnitude of the magnetic dipole and the line width of the magnon in the AFM material as well as on the Fermi energy of Dirac plasmon in

Published in: "arXiv Material Science".

Multifunctional Two-dimensional van der Waals Janus Magnet Cr-based Dichalcogenide Halides. (arXiv:2205.04053v1 [cond-mat.mtrl-sci])

2022-05-10T02:29:35+00:00May 10th, 2022|Categories: Publications|Tags: , , |

Two-dimensional van der Waals Janus materials and their heterostructures offer fertile platforms for designing fascinating functionalities. Here, by means of systematic first-principles studies on van der Waals Janus monolayer Cr-based dichalcogenide halides CrYX (Y=S, Se, Te; X=Cl, Br, I), we find that CrSX (X=Cl, Br, I) are the very desirable high TC ferromagnetic semiconductors with an out-of-plane magnetization. Excitingly, by the benefit of the large magnetic moments on ligand S2- anions, the sought-after large-gap quantum anomalous Hall effect and sizable valley splitting can be achieved through the magnetic proximity effect in van der Waals heterostructures CrSBr/Bi2Se3/CrSBr and MoTe2/CrSBr, respectively. Additionally, we show that large Dzyaloshinskii-Moriya interactions give rise to skyrmion states in CrTeX (X=Cl, Br, I) under external magnetic fields. Our work reveals that two-dimensional Janus magnet Cr-based dichalcogenide halides have appealing multifunctionalities in the applications of topological electronic and valleytronic devices.

Published in: "arXiv Material Science".

Crossover of Ising- to Rashba-Type Superconductivity in Epitaxial Bi2Se3/Monolayer NbSe2 Heterostructures. (arXiv:2112.14623v3 [cond-mat.supr-con] UPDATED)

2022-04-20T04:30:33+00:00April 20th, 2022|Categories: Publications|Tags: , , |

A topological insulator (TI) interfaced with an s-wave superconductor has been predicted to host an unusual form of superconductivity known as topological superconductivity (TSC). Molecular beam epitaxy (MBE) has been the primary approach in the scalable synthesis of the TI/superconductor heterostructures. Although the growth of epitaxial TI films on s-wave superconductors has been achieved, it remains an outstanding challenge for synthesizing atomically thin TI/superconductor heterostructures, which are critical for engineering the TSC phase. Here, we used MBE to grow Bi2Se3 films with the controlled thickness on monolayer NbSe2 and performed in-situ angle-resolved photoemission spectroscopy and ex-situ magneto-transport measurements on these Bi2Se3/monolayer NbSe2 heterostructures. We found that the emergence of Rashba-type bulk quantum well bands and spin-nondegenerate surface states coincides with a marked suppression of the in-plane upper critical magnetic field of the superconductivity in Bi2Se3/monolayer NbSe2 heterostructures. This is the signature of a crossover from Ising- to Rashba-type superconducting pairings, induced by altering Bi2Se3 film thickness. Our work opens a new route for exploring a robust TSC phase in TI/Ising superconductor heterostructures.

Published : "arXiv Mesoscale and Nanoscale Physics".

Weighing Dirac fermions by nonlinear Hall effect. (arXiv:2203.06293v1 [cond-mat.mes-hall])

2022-03-15T04:30:38+00:00March 15th, 2022|Categories: Publications|Tags: , |

Breaking the time-reversal symmetry on the surface of a topological insulator can open a gap for the linear dispersion and make the Dirac fermions massive. This can be achieved by either doping a topological insulator with magnetic elements or proximity-coupling it to magnetic insulators. While the exchange gap can be directly imaged in the former case, measuring it at the buried magnetic insulator/topological insulator interface remains to be challenging. Here, we report the observation of a large nonlinear Hall effect in iron garnet/Bi2Se3 heterostructures. Besides illuminating its magnetic origin, we also show that this nonlinear Hall effect can be utilized to measure the size of the exchange gap and the magnetic-proximity onset temperature. Our results demonstrate the nonlinear Hall effect as a spectroscopic tool to probe the modified band structure at magnetic insulator/topological insulator interfaces.

Published : "arXiv Mesoscale and Nanoscale Physics".

Observability of superconductivity in Sr-doped Bi2Se3 at the surface using scanning tunneling microscope. (arXiv:2202.07794v1 [cond-mat.supr-con])

2022-02-17T04:30:18+00:00February 17th, 2022|Categories: Publications|Tags: , |

The superconducting materials family of doped Bi2Se3 remains intensively studied in the field of condensed matter physics due to strong experimental evidence for topologically non-trivial superconductivity in the bulk. However, at the surface of these materials, even the observation of superconductivity itself is still controversial. We use scanning tunneling microscopy (STM) down to 0.4 K to show that on the surface of bulk superconducting SrxBi2Se3, no gap in the density of states is observed around the Fermi energy as long as clean metallic probe tips are used. Nevertheless, using scanning electron microscopy and energy-dispersive X-ray analysis, we find that micron-sized flakes of SrxBi2Se3 are easily transferred from the sample onto the STM probe tip and that such flakes consistently show a superconducting gap in the density of states. We argue that the superconductivity in SrxBi2Se3 crystals does not extend to the surface when the topological surface state (TSS) is intact, but in micro-flakes the TSS has been destroyed due to strain and allows the superconductivity to extend to the surface. To understand this phenomenon, we propose that the local electric field, always found in electron doped Bi2Se3 in the presence of the TSS due to an intrinsic upward band bending, works against superconductivity at the surface.

Published : "arXiv Mesoscale and Nanoscale Physics".

Observability of superconductivity in Sr-doped Bi2Se3 at the surface using scanning tunneling microscope. (arXiv:2202.07794v1 [cond-mat.supr-con])

2022-02-17T02:29:23+00:00February 17th, 2022|Categories: Publications|Tags: , |

The superconducting materials family of doped Bi2Se3 remains intensively studied in the field of condensed matter physics due to strong experimental evidence for topologically non-trivial superconductivity in the bulk. However, at the surface of these materials, even the observation of superconductivity itself is still controversial. We use scanning tunneling microscopy (STM) down to 0.4 K to show that on the surface of bulk superconducting SrxBi2Se3, no gap in the density of states is observed around the Fermi energy as long as clean metallic probe tips are used. Nevertheless, using scanning electron microscopy and energy-dispersive X-ray analysis, we find that micron-sized flakes of SrxBi2Se3 are easily transferred from the sample onto the STM probe tip and that such flakes consistently show a superconducting gap in the density of states. We argue that the superconductivity in SrxBi2Se3 crystals does not extend to the surface when the topological surface state (TSS) is intact, but in micro-flakes the TSS has been destroyed due to strain and allows the superconductivity to extend to the surface. To understand this phenomenon, we propose that the local electric field, always found in electron doped Bi2Se3 in the presence of the TSS due to an intrinsic upward band bending, works against superconductivity at the surface.

Published in: "arXiv Material Science".

Laser-patterned submicron Bi2Se3-WS2 pixels with tunable circular polarization at room temperature. (arXiv:2202.07495v1 [cond-mat.mtrl-sci])

2022-02-16T04:30:20+00:00February 16th, 2022|Categories: Publications|Tags: , , |

Characterizing and manipulating the circular polarization of light is central to numerous emerging technologies, including spintronics and quantum computing. Separately, monolayer tungsten disulfide (WS2) is a versatile material that has demonstrated promise in a variety of applications, including single photon emitters and valleytronics. Here, we demonstrate a method to tune the photoluminescence (PL) intensity (factor of x161), peak position (38.4meV range), circular polarization (39.4% range), and valley polarization of a Bi2Se3-WS2 2D heterostructure using a low-power laser (0.762uW) in ambient. Changes are spatially confined to the laser spot, enabling submicron (814nm) features, and are long-term stable (>334 days). PL and valley polarization changes can be controllably reversed through laser exposure in vacuum, allowing the material to be erased and reused. Atmospheric experiments and first-principles calculations indicate oxygen diffusion modulates the exciton radiative vs. non-radiative recombination pathways, where oxygen absorption leads to brightening, and desorption to darkening.

Published : "arXiv Mesoscale and Nanoscale Physics".

Equatorial magnetoplasma waves. (arXiv:2202.07164v1 [cond-mat.mes-hall])

2022-02-16T04:30:16+00:00February 16th, 2022|Categories: Publications|Tags: |

Due to its rotation, Earth traps a few equatorial ocean and atmospheric waves, including Kelvin, Yanai, Rossby, and Poincare modes. It has been recently demonstrated that the mathematical origin of equatorial waves is intricately related to the nontrivial topology of hydrodynamic equations describing oceans or the atmosphere. In the present work, we consider plasma oscillations supported by a two-dimensional electron gas confined at the surface of a sphere or a cylinder. We argue that in the presence of a uniform magnetic field, these systems host a set of equatorial magnetoplasma waves that are counterparts to the equatorial waves trapped by Earth. For a spherical geometry, the equatorial modes are well developed only if their penetration length is smaller than the radius of the sphere. For a cylindrical geometry, the spectrum of equatorial modes is weakly dependent on the cylinder radius and overcomes finite-size effects. We argue that this exceptional robustness can be explained by destructive interference effects. We discuss possible experimental setups, including grains and rods composed of topological insulators (e.g., Bi2Se3) or metal-coated dielectrics (e.g., Au2S).

Published : "arXiv Mesoscale and Nanoscale Physics".

Laser-patterned submicron Bi2Se3-WS2 pixels with tunable circular polarization at room temperature. (arXiv:2202.07495v1 [cond-mat.mtrl-sci])

2022-02-16T02:29:21+00:00February 16th, 2022|Categories: Publications|Tags: , , |

Characterizing and manipulating the circular polarization of light is central to numerous emerging technologies, including spintronics and quantum computing. Separately, monolayer tungsten disulfide (WS2) is a versatile material that has demonstrated promise in a variety of applications, including single photon emitters and valleytronics. Here, we demonstrate a method to tune the photoluminescence (PL) intensity (factor of x161), peak position (38.4meV range), circular polarization (39.4% range), and valley polarization of a Bi2Se3-WS2 2D heterostructure using a low-power laser (0.762uW) in ambient. Changes are spatially confined to the laser spot, enabling submicron (814nm) features, and are long-term stable (>334 days). PL and valley polarization changes can be controllably reversed through laser exposure in vacuum, allowing the material to be erased and reused. Atmospheric experiments and first-principles calculations indicate oxygen diffusion modulates the exciton radiative vs. non-radiative recombination pathways, where oxygen absorption leads to brightening, and desorption to darkening.

Published in: "arXiv Material Science".

Achieving high-quality van der Waals material on a semiconductor substrate. (arXiv:2202.02234v1 [cond-mat.mtrl-sci])

2022-02-07T02:30:18+00:00February 7th, 2022|Categories: Publications|Tags: |

Van der Waals (vdW) materials have emerged as a material class of significant interest for discovering new physics and for next-generation devices. To date, most research has been conducted on small exfoliated flakes or on thin films grown on inert substrates. However, in order to integrate these materials with existing semiconductor-based devices, they must be synthesized at a wafer scale on semiconductor substrates. Here, we explore the growth of Bi2Se3, a topological insulator and prototypical vdW material, on GaAs substrates. We explore surface treatments and find that an atomically-smooth GaAs surface is critical to achieving good-quality Bi2Se3 films despite the relatively weak film/substrate interaction. Calculations indicate that the interface is likely selenium-terminated and shows no evidence of covalent bonding. These results clearly demonstrate that the film/substrate interaction is important even in vdW epitaxy and provide a recipe for the growth of other vdW materials on semiconductor substrates.

Published in: "arXiv Material Science".

Crossover of Ising- to Rashba-Type Superconductivity in Epitaxial Bi2Se3/Monolayer NbSe2 Heterostructures. (arXiv:2112.14623v1 [cond-mat.supr-con])

2021-12-30T02:29:31+00:00December 30th, 2021|Categories: Publications|Tags: , , |

When two different materials are brought together, the resultant interface between them sometimes shows unexpected quantum phenomena. For example, the interface between a topological insulator (TI) and an s-wave superconductor has been predicted to host an unusual form of superconductivity known as topological superconductivity. Here, we synthesized Bi2Se3/monolayer NbSe2 heterostructures with different Bi2Se3 thicknesses using molecular beam epitaxy (MBE). We found that gapless Dirac surface states are formed for Bi2Se3 films as thin as 3 quintuple layers (QLs). Moreover, we observed Rashba-type bulk conduction bands for Bi2Se3 thickness greater than 2 QL. Our first-principles calculations show these phenomena are induced by the unique interface between Bi2Se3 films and monolayer NbSe2, where a BiSe bilayer with a cubic lattice structure exists. By performing magneto-transport measurements, we found that the emergence of Rashba-type bulk quantum well bands and spin-nondegenerate surface states coincides with a marked suppression of the in-plane upper critical magnetic field of the superconductivity in Bi2Se3/monolayer NbSe2 heterostructures. This indicates a crossover from Ising- to Rashba-type pairings. The synthesis of Bi2Se3/monolayer NbSe2 heterostructures and the demonstration of a crossover from Ising- to bulk Rashba-type superconductivity therein open a new route for exploring topological superconductivity in TI/superconductor heterostructures.

Published in: "arXiv Material Science".

Two-photon IR pumped UV-Vis transient absorption spectroscopy of Dirac fermions in the 2D and 3D topological insulator Bi2Se3. (arXiv:2112.10294v1 [cond-mat.mes-hall])

2021-12-21T04:30:26+00:00December 21st, 2021|Categories: Publications|Tags: , |

It is often taken for granted that in pump-probe experiments on the topological insulator (TI) Bi2Se3 using IR pumping with a commercial Ti:Sapphire laser [~800 nm (1.55 eV photon energy)], carriers are excited in the one-photon absorption regime, even when pumped with absorbed fluences in the miliJ cm-2 range. Here, using UV-Vis transient absorption (TA) spectroscopy, we show that even at low-power IR pumping with absorbed fluences in the microJ cm-2 range, TA spectra of the 2D and 3D TI Bi2Se3 cover the entire visible and part of the UV region. This observation unambiguously indicates that the two-photon pumping regime accompanies the common one-photon pumping regime even at low laser powers applied. We attribute the high efficiency of two-photon pumping to the giant nonlinearity of Dirac fermions in Dirac surface states (SS). On the contrary, one-photon pumping is associated with the excitation of bound valence electrons in the bulk into the conduction band. We also identified two mechanisms of absorption bleaching, which manifest themselves in different spectral regions of probing and are associated with direct Pauli blocking in Dirac SS and an integrated filling of the phase-space in the bulk states. The effect of the film thickness and pumping power on the mass gain/loss dynamics of Dirac fermions upon relaxation and their vertical transport, as well as on the launching of coherent longitudinal optical (LO) phonon oscillations in the bulk states and Dirac SS is demonstrated. We have shown that the launching of coherent LO-phonon oscillations in the bulk is

Published : "arXiv Mesoscale and Nanoscale Physics".

Acidic TME‐Responsive Nano‐Bi2Se3@MnCaP as a NIR‐II‐Triggered Free Radical Generator for Hypoxia‐Irrelevant Phototherapy with High Specificity and Immunogenicity

2021-11-26T13:22:05+00:00November 26th, 2021|Categories: Publications|Tags: |

An acidity/near-infrared-II light dual-responsive theranostic platform is established for promoted cooperative CDT/PTT/TDT and immunogenicity elicitation with high tumor specificity. The multifunctional hybrid nanocomplex (Bi2Se3/AIPH@MnCaP) is developed by in situ mineralizing Mn-doped CaP onto the surface of AIPH-encapsulated mesoporous nano-Bi2Se3. Abstract Here, acidic tumor microenvironment (TME)-responsive nano-Bi2Se3@MnCaP, as a near-infrared-II (NIR-II) biowindow-triggered free radical generator for hypoxia‑irrelevant phototherapy, is elaborately developed by biomimetic mineralization of MnCaP onto 2, 2′-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (AIPH)-loaded mesoporous nano-Bi2Se3 to form Bi2Se3/AIPH@MnCaP (BAM). Surface mineral of MnCaP can be degraded under mild acidity, leading to the release of both Mn2+ and AIPH. The leached Mn2+ not only facilitates chemodynamic therapy (CDT) via hydroxyl radicals (•OH) from Mn2+-mediated Fenton-like reaction but also acts as contrast agent for magnetic resonance imaging. In another aspect, the splendid photothermal conversion capacity of BAM enables a rapid hyperthermia generation under NIR-II laser irradiation for photothermal therapy (PTT). Simultaneously, the local thermal shock can induce the disintegration of AIPH to generate alkyl radicals (•R) for thermodynamic therapy (TDT) and accelerate Fenton-like reaction rate to augment CDT efficacy. The strong synergistic effects from cooperative CDT/PTT/TDT are applied to 4T1 tumor suppression with minimal side effects. Importantly, the combination therapy can effectively trigger immunogenetic cell death and enhance antitumor immunity for systemic tumor eradication. Collectively, this proof-of-concept study demonstrates a more efficacious and safer strategy for oxygenation-independent phototherapy, which holds a good potential for clinical translation in cancer management.

Published in: "Small".

Coherent surface-to-bulk vibrational coupling in the 2D topologically trivial insulator Bi2Se3 monitored by ultrafast transient absorption spectroscopy. (arXiv:2111.05298v1 [cond-mat.mes-hall])

2021-11-10T04:30:28+00:00November 10th, 2021|Categories: Publications|Tags: , |

Ultrafast carrier relaxation in the 2D topological insulator (TI) Bi2Se3 [gapped Dirac surface states (SS)] and how it inherits ultrafast relaxation in the 3D TI Bi2Se3 (gapless Dirac SS) remains a challenge for developing new optoelectronic devices based on these materials. Here ultrashort (100 fs) pumping pulses of ~340 nm wavelength (~3.65 eV photon energy) were applied to study ultrafast electron relaxation in the 2D TI Bi2Se3 films with a thickness of 2 and 5 quintuple layers (~2 and ~5 nm, respectively) using transient absorption (TA) spectroscopy in the ultraviolet-visible spectral region (1.65 – 3.85 eV). The negative and positive contributions of TA spectra were attributed to absorption bleaching that mostly occur in the bulk states and to the inverse bremsstrahlung type free carrier absorption in the surface states, respectively. Owing to this direct and selective access to the bulk and surface carrier dynamics, we were able to monitor coherent longitudinal optical (LO) phonon oscillations, which were successively launched in the bulk and surface states by the front of the relaxing electron population within the LO-phonon cascade emission. We have also recognized the coherent surface-to-bulk vibrational coupling that appears through the phase-dependent amplitude variations of coherent LO-phonon oscillations. This unique behavior manifests itself predominantly for the topologically trivial insulator phase of the 2D TI Bi2Se3 (2 nm thick film) in the photon energy range (~2.0 – 2.25 eV) where efficient energy exchange between the bulk and surface states occurs. We also found that the coherent surface-to-bulk vibrational coupling significantly weakens

Published : "arXiv Mesoscale and Nanoscale Physics".

Band Structure and Polarization Effects in Photothermoelectric Spectroscopy of a Bi2Se3 Device. (arXiv:2110.10799v1 [cond-mat.mtrl-sci])

2021-10-22T05:29:24+00:00October 22nd, 2021|Categories: Publications|Tags: , |

Bi2Se3 is a prototypical topological insulator which has a small band gap (~0.3 eV) and topologically protected conducting surface states. This material exhibits quite strong thermoelectric effects. Here we show in a mechanically exfoliated quasi-bulk nanoflake device that we can measure the energy dependent optical absorption through the photothermoelectric effect. Spectral signatures are seen for a number of optical transitions between the valence and conduction bands, as well as the 1.5 eV optical transition between the two topologically protected conducting surface states. We also observe a surprising linear polarization dependence in the response of the device that reflects the influence of the metal contacts.

Published in: "arXiv Material Science".

Visualizing Van der Waals Epitaxial Growth of Two-Dimensional Heterostructures. (arXiv:2110.04803v1 [cond-mat.mtrl-sci])

2021-10-12T02:30:09+00:00October 12th, 2021|Categories: Publications|Tags: , , , , |

Understanding the growth mechanisms of two-dimensional (2D) van der Waals (vdW) heterostructures is of great importance in exploring their functionalities and device applications. A custom-built system integrating physical vapor deposition and optical microscopy/Raman spectroscopy was employed to study the dynamic growth processes of 2D vdW heterostructures in situ. This allows us to identify a new growth mode with a distinctly different growth rate and morphology from those of the conventional linear growth mode. We propose a model that explains the difference in morphologies and quantifies the growth rates of the two modes by taking the role of surface diffusion into account. We have systematically investigated a range of material combinations including CdI2/WS2, CdI2/MoS2, CdI2/WSe2, PbI2/WS2, PbI2/MoS2, PbI2/WSe2 and Bi2Se3/WS2. These findings may be generalized to the synthesis of many other 2D heterostructures with controlled morphologies and physical properties, benefiting future device applications.

Published in: "arXiv Material Science".

Topological Magnetoelectric Response in Ferromagnetic Axion Insulators. (arXiv:2109.14160v1 [cond-mat.mes-hall])

2021-09-30T02:29:25+00:00September 30th, 2021|Categories: Publications|Tags: |

Topological magnetoelectric effect (TME) is a hallmark response of the topological field theory, which provides a paradigm shift in the study of emergent topological phenomena. However, its direct observation is yet to be realized due to the demanding magnetic configuration required to gap all the surface states. Here, we theoretically propose that the axion insulators with a simple ferromagnetic configuration, such as MnBi2Te4/(Bi2Se3)n family, provide an ideal playground to realize the linear TME. In a designed triangular prism geometry, all the surface states are magnetically gapped. Under a vertical electric field, the surface Hall currents give rise to a nearly half-quantized orbital moment, accompanied with a gapless chiral hinge mode circulating parallelly. Thus, the orbital magnetization from the two topological origins can be easily distinguished by reversing the electric field. Our work paves a new avenue towards the direct observation of TME in realistic axion-insulator materials.

Published in: "arXiv Material Science".

Orbital contributions in the element-resolved valence electronic structure of Bi2Se3. (arXiv:2108.01833v1 [cond-mat.mtrl-sci])

2021-08-05T02:29:19+00:00August 5th, 2021|Categories: Publications|Tags: |

In this work, we studied the bulk band structure of a topological insulator (TI) Bi2Se3 and determined the contributions of the Bi and Se orbital states to the valence bands using standing wave-excited hard X-ray photoemission spectroscopy (SW-HAXPES). This SW technique can provide the element-resolved information and extract individual Bi and Se contributions to the Bi2Se3 valence band. Comparisons with density functional theory (DFT) calculations (LDA and GW) reveal that the Bi 6s, Bi 6p, and Se 4p states are dominant in the Bi2Se3 HAXPES valence band. These findings represent a major step further in describing the Bi2Se3 band structure and improve the accuracy for future DFT predictions of emergent properties in this class of TIs.

Published in: "arXiv Material Science".

Optimization of the growth of the van der Waals materials Bi2Se3 and (Bi0.5In0.5)2Se3 by molecular beam epitaxy. (arXiv:2107.09771v1 [cond-mat.mtrl-sci])

2021-07-22T02:29:40+00:00July 22nd, 2021|Categories: Publications|Tags: , |

The naturally existing chalcogenide Bi2Se3 is topologically nontrivial due to the band inversion caused by strong spin-orbit coupling inside the bulk of the material. The surface states are spin polarized, protected by the time-inversion symmetry, and thus robust to the scattering caused by non-magnetic defects. A high purity topological insulator thin film can be easily grown via molecular beam epitaxy (MBE) on various substrates to enable novel electronics, optics, and spintronics applications. However, the unique surface state properties have historically been limited by the film quality, which is evaluated by crystallinity, surface morphology, and transport data. Here we propose and investigate different MBE growth strategies to improve the quality of Bi2Se3 thin films grown by MBE. In addition, growths of topological trivial insulator (Bi0.5In0.5)2Se3 (BIS) are also investigated. BIS is often used as a buffer layer or separation layer for topological insulator heterostructures. Based on the surface passivation status, we have classified the substrates into two categories, self-passivated or unpassivated, and determine the optimal growth mechanisms on the representative sapphire and GaAs, respectively. Growth temperature is a crucial control parameter for the van der Waals epitaxy for both types of substrates. For Bi2Se3 on GaAs, the surface passivation status determines the dominant growth mechanism.

Published in: "arXiv Material Science".

Twisted magnetic topological insulators. (arXiv:2104.13235v2 [cond-mat.str-el] UPDATED)

2021-07-08T02:29:22+00:00July 8th, 2021|Categories: Publications|Tags: |

Motivated by the discovery of the quantum anomalous Hall effect in Cr-doped ce{(Bi,Sb)2Te3} thin films, we study the generic states for magnetic topological insulators and explore the physical properties for both magnetism and itinerant electrons. First-principles calculations are exploited to investigate the magnetic interactions between magnetic Co atoms adsorbed on the ce{Bi2Se3} (111) surface. Due to the absence of inversion symmetry on the surface, there are Dzyaloshinskii-Moriya-like twisted spin interactions between the local moments of Co ions. These nonferromagnetic interactions twist the collinear spin configuration of the ferromagnet and generate various magnetic orders beyond a simple ferromagnet. Among them, the spin spiral state generates alternating counterpropagating modes across each period of spin states, and the skyrmion lattice even supports a chiral mode around the core of each skyrmion. The skyrmion lattice opens a gap at the surface Dirac point, resulting in the anomalous Hall effect. These results may inspire further experimental investigation of magnetic topological insulators.

Published in: "arXiv Material Science".

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