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Advances in Topological Materials: Fundamentals, Challenges and Outlook, Volume II

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A special issue of Nanomaterials (ISSN 2079-4991). This special issue belongs to the section "Nanoelectronics, Nanosensors and Devices".

Deadline for manuscript submissions: closed (20 January 2024) | Viewed by 4652

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Prof. Dr. Sławomir P. Łepkowski

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Institute of High Pressure Physics, "Unipress", Polish Academy of Sciences, ul. Sokołowska 29/37, 01-142 Warszawa, Poland
Interests: topological insulators; topological phase transition; theory of semiconductor nanostructures; k·p method; ab-initio calculations
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Dear Colleagues,

The discovery of the time-reversal topological insulators in two and three dimensions has greatly inspired the study of topological properties of the electronic band structure of crystalline materials. Topological insulators are characterized by an energy gap in the bulk electronic band structure and metallic states on the boundaries. Closing of the band gap by the surface or edge states is caused by nontrivial topology of the bulk states, originating from an inversion in the order in the valence and conduction bands at time reversal invariant wave vectors in the Brillouin zone. The search for other materials with nontrivial topological properties has led to the invention of the topological crystalline insulators in which the topological nature of the electronic structure arises from crystal symmetries. Subsequently, the Dirac and Weyl semimetals with topologically protected, linearly dispersing bands in the bulk band structure have joined the family of topological materials. Recently, the higher-order topological insulators in which the gapless states appear on the boundary with dimensions two or more lower than that of the bulk have been discovered. Higher-order topological phases have been observed both in solid-state materials and in photonic and acoustic metamaterials.

This is the second volume of the Special Issue on “Advances in Topological Materials: Fundamentals, Challenges, and Outlook”. In this Special Issue, we focus on topological nanomaterials, nanostructures, and nano-metamaterials. Research on the topological effects at the nanoscale not only leads to the observation of new phenomena, such as Majorana fermions in topological nanowires but is also primarily important for the application of topological materials and metamaterials in modern electronic, acoustic, and optical devices. This Special Issue aims to highlight the latest state-of-the-art studies on the topological effects in nanomaterials, nanostructures, and nano-metamaterials.

Prof. Dr. Sławomir P. Łepkowski
Guest Editor

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Keywords

topological insulators
Dirac semimetals
Weyl semimetals
topological phase transition
nanocrystals
nanowires
Majorana fermions
quantum dots
metamaterials
topological photonics
topological acoustics
topological devices

Published Papers (4 papers)

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Research

17 pages, 804 KiB

Open AccessArticle

Majorana Excitons in a Kitaev Chain of Semiconductor Quantum Dots in a Nanowire

by Mahan Mohseni, Hassan Allami, Daniel Miravet, David J. Gayowsky, Marek Korkusinski and Pawel Hawrylak

Nanomaterials 2023, 13(16), 2293; https://doi.org/10.3390/nano13162293 - 09 Aug 2023

Cited by 3 | Viewed by 735

Abstract

We present here a theory of Majorana excitons, photo-excited conduction electron-valence band hole pairs, interacting with Majorana Fermions in a Kitaev chain of semiconductor quantum dots embedded in a nanowire. Using analytical tools and exact diagonalization methods, we identify the presence of Majorana [...] Read more.

(This article belongs to the Special Issue Advances in Topological Materials: Fundamentals, Challenges and Outlook, Volume II)

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15 pages, 3250 KiB

Open AccessFeature PaperArticle

Quantum Spin Hall Effect in Two-Monolayer-Thick InN/InGaN Coupled Multiple Quantum Wells

by Sławomir P. Łepkowski

Nanomaterials 2023, 13(15), 2212; https://doi.org/10.3390/nano13152212 - 30 Jul 2023

Viewed by 952

Abstract

In this study, we present a theoretical study of the quantum spin Hall effect in InN/InGaN coupled multiple quantum wells with the individual well widths equal to two atomic monolayers. We consider triple and quadruple quantum wells in which the In content in the interwell barriers is greater than or equal to the In content in the external barriers. To calculate the electronic subbands in these nanostructures, we use the eight-band k∙p Hamiltonian, assuming that the effective spin–orbit interaction in InN is negative, which represents the worst-case scenario for achieving a two-dimensional topological insulator. For triple quantum wells, we find that when the In contents of the external and interwell barriers are the same and the widths of the internal barriers are equal to two monolayers, a topological insulator with a bulk energy gap of 0.25 meV can appear. Increasing the In content in the interwell barriers leads to a significant increase in the bulk energy gap of the topological insulator, reaching about 0.8 meV. In these structures, the topological insulator can be achieved when the In content in the external barriers is about 0.64, causing relatively low strain in quantum wells and making the epitaxial growth of these structures within the range of current technology. Using the effective 2D Hamiltonian, we study the edge states in strip structures containing topological triple quantum wells. We demonstrate that the opening of the gap in the spectrum of the edge states caused by decreasing the width of the strip has an oscillatory character regardless of whether the pseudospin-mixing elements of the effective Hamiltonian are omitted or taken into account. The strength of the finite size effect in these structures is several times smaller than that in HgTe/HgCdTe and InAs/GaSb/AlSb topological insulators. Therefore, its influence on the quantum spin Hall effect is negligible in strips with a width larger than 150 nm, unless the temperature at which electron transport is measured is less than 1 mK. In the case of quadruple quantum wells, we find the topological insulator phase only when the In content in the interwell barriers is larger than in the external barriers. We show that in these structures, a topological insulator with a bulk energy gap of 0.038 meV can be achieved when the In content in the external barriers is about 0.75. Since this value of the bulk energy gap is very small, quadruple quantum wells are less useful for realizing a measurable quantum spin Hall system, but they are still attractive for achieving a topological phase transition and a nonlocal topological semimetal phase. Full article

(This article belongs to the Special Issue Advances in Topological Materials: Fundamentals, Challenges and Outlook, Volume II)

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13 pages, 2070 KiB

Open AccessArticle

Evolution of Mn_1−xGe_xBi₂Te₄ Electronic Structure under Variation of Ge Content

by Tatiana P. Estyunina, Alexander M. Shikin, Dmitry A. Estyunin, Alexander V. Eryzhenkov, Ilya I. Klimovskikh, Kirill A. Bokai, Vladimir A. Golyashov, Konstantin A. Kokh, Oleg E. Tereshchenko, Shiv Kumar, Kenya Shimada and Artem V. Tarasov

Nanomaterials 2023, 13(14), 2151; https://doi.org/10.3390/nano13142151 - 24 Jul 2023

Cited by 1 | Viewed by 1148

Abstract

One of the approaches to manipulate MnBi

_{2}

_{4}

properties is the magnetic dilution, which inevitably affects the interplay of magnetism and band topology in the system. In this work, we carried out angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory [...] Read more.

One of the approaches to manipulate MnBi

_{2}

_{4}

_{1 - x}

_{x}

_{2}

_{4}

that occur under parameter x variation. We consider two ways of Mn/Ge substitution: (i) bulk doping of the whole system; (ii) surface doping of the first septuple layer. For the case (i), the experimental results reveal a decrease in the value of the bulk band gap, which should be reversed by an increase when the Ge concentration reaches a certain value. Ab-initio calculations show that at Ge concentrations above 50%, there is an absence of the bulk band inversion of the Te

p_{z}

and Bi

p_{z}

contributions at the

Γ

-point with significant spatial redistribution of the states at the band gap edges into the bulk, suggesting topological phase transition in the system. For case (ii) of the vertical heterostructure Mn

_{1 - x}

_{x}

_{2}

_{4}

/MnBi

_{2}

_{4}

, it was shown that an increase of Ge concentration in the first septuple layer leads to effective modulation of the Dirac gap in the absence of significant topological surface states of spatial redistribution. The results obtained indicate that surface doping compares favorably compared to bulk doping as a method for the Dirac gap value modulation. Full article

(This article belongs to the Special Issue Advances in Topological Materials: Fundamentals, Challenges and Outlook, Volume II)

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12 pages, 6326 KiB

Open AccessArticle

Enhancement of the Surface Morphology of (Bi_0.4Sb_0.6)₂Te₃ Thin Films by In Situ Thermal Annealing

by Liesbeth Mulder, Hanne van de Glind, Alexander Brinkman and Omar Concepción

Nanomaterials 2023, 13(4), 763; https://doi.org/10.3390/nano13040763 - 17 Feb 2023

Cited by 1 | Viewed by 1196

Abstract

The study of the exotic properties of the surface states of topological insulators requires defect-free and smooth surfaces. This work aims to study the enhancement of the surface morphology of optimally doped, high-crystalline (Bi_0.4Sb_0.6)₂Te₃ films deposited by molecular beam epitaxy on Al₂O₃ (001) substrates. Atomic force microscopy shows that by employing an in situ thermal post anneal, the surface roughness is reduced significantly, and transmission electron microscopy reveals that structural defects are diminished substantially. Thence, these films provide a great platform for the research on the thickness-dependent properties of topological insulators. Full article

(This article belongs to the Special Issue Advances in Topological Materials: Fundamentals, Challenges and Outlook, Volume II)

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