Sivakumar Vishnuvardhan Mambakkam

The discovery of topological insulators (TIs) and their unique electronic properties has motivated research into a variety of applications, including quantum computing. It has been proposed that TI surface states will be energetically discretized in a quantum dot nanoparticle. These discretized states could then be used as basis states for a qubit that is more resistant to decoherence. In this work, prototypical TI Bi2Se3 nanoparticles are grown on GaAs (001) using the droplet epitaxy technique, and we demonstrate the control of nanoparticle height, area, and density by changing the duration of bismuth deposition and substrate temperature. Within the growth window studied, nanoparticles ranged from 5 to 15 nm in height with an 8–18 nm equivalent circular radius, and the density could be relatively well controlled by changing the substrate temperature and bismuth deposition time.

Topological insulator

Bismuth

Surface States

10.1116/6.0001157

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Observation of nonlinear planar Hall effect in magnetic insulator/topological insulator heterostructures

arXiv (Cornell University) (2022)

Yang Wang Sivakumar Vishnuvardhan Mambakkam Yue-Xin Huang Yong Wang Ji Yi

Interfacing topological insulators (TIs) with magnetic insulators (MIs) has been widely used to study the interaction between topological surface states and magnetism. Previous transport studies typically interpret the suppression of weak antilocalization or appearance of the anomalous Hall effect as signatures of magnetic proximity effect (MPE) imposed to TIs. Here, we report the observation of nonlinear planar Hall effect (NPHE) in Bi2Se3 films grown on MI thulium and yttrium iron garnet (TmIG and YIG) substrates, which is an order of magnitude larger than that in Bi2Se3 grown on nonmagnetic gadolinium gallium garnet (GGG) substrate. The nonlinear Hall resistance in TmIG/Bi2Se3 depends linearly on the external magnetic field, while that in YIG/Bi2Se3 exhibits an extra hysteresis loop around zero field. The magnitude of the NPHE is found to scale inversely with carrier density. We speculate the observed NPHE is related to the MPE-induced exchange gap opening and out-of-plane spin textures in the TI surface states, which may be used as an alternative transport signature of the MPE in MI/TI heterostructures.

Topological insulator

Yttrium Iron Garnet

Magnetism

10.48550/arxiv.2203.06293

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Spectroscopy of van der Waals nanomaterials: Opportunities and Challenges

arXiv (Cornell University) (2023)

Sivakumar Vishnuvardhan Mambakkam Stephanie Law

The study of van der Waals (vdW) materials has seen increased interest in recent years, due to the wide range of uses for these materials because of their unique mechanical, electronic, and optical properties. This area has recently expanded further into studying the behavior of vdW nanomaterials, as decreasing dimensions open up opportunities to interact with these materials in new ways. However, measuring the band structures of nanomaterials, which is key to understanding how confinement affects material properties and interactions, comes with several challenges. In this review, we survey a range of techniques for synthesizing and characterizing vdW nanomaterials, in order to outline the key material and characterization challenges. This includes controlling the Fermi level in vdW nanoparticles, preparing these particles for either ensemble or individual particle measurement, as well as protecting the pristine surface from oxidation.

Nanomaterials

Characterization

10.48550/arxiv.2310.10811

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Alternative Applications of SPL: Testing Ideas Through Experiments and Mathematical Modeling

The minerals, metals & materials series (2017)

Dawei Yu Sivakumar Vishnuvardhan Mambakkam Donghui Li Kinnor Chattopadhyay Lei Gao

Ferrous metallurgy

10.1007/978-3-319-51541-0_72

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Citations (14)

Growth of topological insulator nanoparticles by droplet epitaxy

Sivakumar Vishnuvardhan Mambakkam Stephanie Law

Topological insulators (TIs) are a class of materials which exhibit unique electronic states delocalized on the outer surface of the material. Due to the strong spin-orbit coupling in TIs, these states are spin-polarized and robust against scattering. The robust nature of these states could present a potential solution to the problem of decoherence in qubits. This has motivated research into creating quantum information storage/transport devices using TIs. In order to create such a device, we first must understand how to isolate these states energetically, and this can be done using quantum confinement. Our first step involves the creation of quantum confined TIs. Here, we report on the growth of TI quantum dots grown by droplet epitaxy. We chose the TI Bi2Se3, a commonly studied 3D topological insulator, and growths were performed on GaAs in a molecular beam epitaxy chamber. We varied the growth conditions of bismuth deposition time and substrate temperature to determine the degree to which TI nanoparticle dimensions could be controlled. Within the growth window, we observed particles in the range of 5-15 nm tall and 500-3000 nm2 in area (equivalent circular radius ~12-30 nm). We observed various trends in particle height, area, density and polydispersity as a result of varying growth conditions and adjusting the growth procedure (ex. adding a long anneal step between bismuth and selenium deposition steps). Overall, these results will be helpful in aiding further development of topological insulator devices.

Topological insulator

Bismuth

Surface States

10.1117/12.2596418

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Large Bilinear Magnetoresistance from Rashba Spin-Splitting on the Surface of a Topological Insulator

arXiv (Cornell University) (2022)

Yang Wang Binbin Liu Yue-Xin Huang Sivakumar Vishnuvardhan Mambakkam Yong Wang

In addition to the topologically protected linear dispersion, a band-bending-confined two-dimensional electron gas with tunable Rashba spin-splitting (RSS) was found to coexist with the topological surface states on the surface of topological insulators (TIs). Here, we report the observation of large bilinear magnetoresistance (BMR) in Bi2Se3 films decorated with transition metal atoms. The magnitude of the BMR sensitively depends on the type and amount of atoms deposited, with a maximum achieved value close to those of strong Rashba semiconductors. Our first-principles calculations reproduce the quantum well states and reveal sizable RSS in all Bi2Se3 heterostructures with broken inversion symmetry. Our results show that charge-spin interconversion through RSS states in TIs can be fine-tuned through surface atom deposition and easily detected via BMR for potential spintronic applications.

Topological insulator

Surface States

Rashba effect

Point reflection

10.48550/arxiv.2209.07666

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Observation of nonlinear planar Hall effect in magnetic-insulator–topological-insulator heterostructures

Physical review. B./Physical review. B (2022)

Y. Wang Sivakumar Vishnuvardhan Mambakkam Yue-Xin Huang Yong Wang Yi Ji

Interfacing topological insulators (TIs) with magnetic insulators (MIs) has been widely used to study the interaction between topological surface states and magnetism. Previous transport studies typically interpret the suppression of weak antilocalization or appearance of the anomalous Hall effect as signatures of magnetic proximity effect (MPE) imposed to TIs. Here, we report the observation of nonlinear planar Hall effect (NPHE) in Bi2Se3 films grown on MI thulium and yttrium iron garnet (TmIG and YIG) substrates, which is an order of magnitude larger than that in Bi2Se3 grown on nonmagnetic gadolinium gallium garnet (GGG) substrate. The nonlinear Hall resistance in TmIG/Bi2Se3 depends linearly on the external magnetic field, while that in YIG/Bi2Se3 exhibits an extra hysteresis loop around zero field. The magnitude of the NPHE is found to scale inversely with carrier density. We speculate the observed NPHE is related to the MPE-induced exchange gap opening and out-of-plane spin textures in the TI surface states, which may be used as an alternative transport signature of the MPE in MI/TI heterostructures.

Topological insulator

Yttrium Iron Garnet

Magnetism

10.1103/physrevb.106.155408

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Citations (14)

In-Plane Plasmon Coupling in Topological Insulator Bi$_2$Se$_3$ Thin Films.

arXiv (Cornell University) (2021)

Saadia Nasir Zhengtianye Wang Sivakumar Vishnuvardhan Mambakkam Stephanie Law

The surface states of the 3D topological insulator (TI), Bi$_2$Se$_3$, are known to host two-dimensional Dirac plasmon polaritons (DPPs) in the terahertz spectral range. In TI thin films, the DPPs excited on the top and bottom surfaces couple, leading to an acoustic and an optical plasmon mode. Vertical coupling in these materials is therefore reasonable well-understood, but in-plane coupling among localized TI DPPs has yet to be investigated. In this paper, we demonstrate in-plane DPP coupling in TI stripe arrays and show that they exhibit dipole-dipole type coupling. The coupling becomes negligible with the lattice constant is greater than approximately 2.8 times the stripe width, which is comparable to results for in-plane coupling of localized plasmons excited on metallic nanoparticles or graphene plasmon polaritons. This understanding could be leveraged for the creation of TI-based metasurfaces.

Topological insulator

Localized surface plasmon

Nanophotonics

Lattice (music)

Source

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In-plane plasmon coupling in topological insulator Bi2Se3 thin films

Applied Physics Letters (2021)

Saadia Nasir Zhengtianye Wang Sivakumar Vishnuvardhan Mambakkam Stephanie Law

The surface states of the 3D topological insulator (TI), Bi$_2$Se$_3$, are known to host two-dimensional Dirac plasmon polaritons (DPPs) in the terahertz spectral range. In TI thin films, the DPPs excited on the top and bottom surfaces couple, leading to an acoustic and an optical plasmon mode. Vertical coupling in these materials is therefore reasonable well-understood, but in-plane coupling among localized TI DPPs has yet to be investigated. In this paper, we demonstrate in-plane DPP coupling in TI stripe arrays and show that they exhibit dipole-dipole type coupling. The coupling becomes negligible with the lattice constant is greater than approximately 2.8 times the stripe width, which is comparable to results for in-plane coupling of localized plasmons excited on metallic nanoparticles or graphene plasmon polaritons. This understanding could be leveraged for the creation of TI-based metasurfaces.

Topological insulator

Localized surface plasmon

Nanophotonics

10.1063/5.0071895

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Citations (6)

Spent Potlining (SPL): A myriad of opportunities

Aluminium international today (2015)

Dawei Yu Sivakumar Vishnuvardhan Mambakkam Andre Hernandez Rivera Donghui Li Kinnor Chattopadhyay

Source

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Citations (6)