Editorial

2 pages, 189 KiB

Open AccessEditorial

Advances in Topological Materials: Fundamentals, Challenges and Outlook

by Sławomir P. Łepkowski

Nanomaterials 2022, 12(19), 3522; https://0-doi-org.brum.beds.ac.uk/10.3390/nano12193522 - 08 Oct 2022

Viewed by 989

The discovery of topological insulators, characterized by an energy gap in bulk electronic band structures and metallic states on boundaries, has greatly inspired studies on the topological properties of the electronic band structures of crystalline materials [...] Full article

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

Research

Jump to: Editorial, Review

13 pages, 6397 KiB

Open AccessArticle

Polarization-Induced Phase Transitions in Ultra-Thin InGaN-Based Double Quantum Wells

by Sławomir P. Łepkowski and Abdur Rehman Anwar

Nanomaterials 2022, 12(14), 2418; https://0-doi-org.brum.beds.ac.uk/10.3390/nano12142418 - 14 Jul 2022

Cited by 2 | Viewed by 1396

Abstract

We investigate the phase transitions and the properties of the topological insulator in InGaN/GaN and InN/InGaN double quantum wells grown along the [0001] direction. We apply a realistic model based on the nonlinear theory of elasticity and piezoelectricity and the eight-band k·p method [...] Read more.

We investigate the phase transitions and the properties of the topological insulator in InGaN/GaN and InN/InGaN double quantum wells grown along the [0001] direction. We apply a realistic model based on the nonlinear theory of elasticity and piezoelectricity and the eight-band k·p method with relativistic and nonrelativistic linear-wave-vector terms. In this approach, the effective spin–orbit interaction in InN is negative, which represents the worst-case scenario for obtaining the topological insulator in InGaN-based structures. Despite this rigorous assumption, we demonstrate that the topological insulator can occur in InGaN/GaN and InN/InGaN double quantum wells when the widths of individual quantum wells are two and three monolayers (MLs), and three and three MLs. In these structures, when the interwell barrier is sufficiently thin, we can observe the topological phase transition from the normal insulator to the topological insulator via the Weyl semimetal, and the nontopological phase transition from the topological insulator to the nonlocal topological semimetal. We find that in InGaN/GaN double quantum wells, the bulk energy gap in the topological insulator phase is much smaller for the structures with both quantum well widths of 3 MLs than in the case when the quantum well widths are two and three MLs, whereas in InN/InGaN double quantum wells, the opposite is true. In InN/InGaN structures with both quantum wells being three MLs and a two ML interwell barrier, the bulk energy gap for the topological insulator can reach about

1.2 meV

. We also show that the topological insulator phase rapidly deteriorates with increasing width of the interwell barrier due to a decrease in the bulk energy gap and reduction in the window of In content between the normal insulator and the nonlocal topological semimetal. For InN/InGaN double quantum wells with the width of the interwell barrier above five or six MLs, the topological insulator phase does not appear. In these structures, we find two novel phase transitions, namely the nontopological phase transition from the normal insulator to the nonlocal normal semimetal and the topological phase transition from the nonlocal normal semimetal to the nonlocal topological semimetal via the buried Weyl semimetal. These results can guide future investigations towards achieving a topological insulator in InGaN-based nanostructures. Full article

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

► Show Figures

Figure 1

25 pages, 5230 KiB

Open AccessArticle

Quantum Phase Transition in the Spin Transport Properties of Ferromagnetic Metal-Insulator-Metal Hybrid Materials

by Musa A. M. Hussien and Aniekan Magnus Ukpong

Nanomaterials 2022, 12(11), 1836; https://0-doi-org.brum.beds.ac.uk/10.3390/nano12111836 - 27 May 2022

Cited by 3 | Viewed by 2348

Abstract

Perpendicular magnetic tunnel junctions provide a technologically important design platform for studying metal-insulator-metal heterostructure materials. Accurate characterization of the sensitivity of their electronic structure to proximity coupling effects based on first-principles calculations is key in the fundamental understanding of their emergent collective properties [...] Read more.

Perpendicular magnetic tunnel junctions provide a technologically important design platform for studying metal-insulator-metal heterostructure materials. Accurate characterization of the sensitivity of their electronic structure to proximity coupling effects based on first-principles calculations is key in the fundamental understanding of their emergent collective properties at macroscopic scales. Here, we use an effective field theory that combines ab initio calculations of the electronic structure within density functional theory with the plane waves calculation of the spin polarised conductance to gain insights into the proximity effect induced magnetoelectric couplings that arise in the transport of spin angular momentum when a monolayer tunnel barrier material is integrated into the magnetic tunnel junction. We find that the spin density of states exhibits a discontinuous change from half-metallic to the metallic character in the presence of monolayer hexagonal boron nitride when the applied electric field reaches a critical amplitude, and this signals a first order transition in the transport phase. This unravels an electric-field induced quantum phase transition in the presence of a monolayer hexagonal boron nitride tunnel barrier quite unlike molybdenum disulphide. The role of the applied electric field in the observed phase transition is understood in terms of the induced spin-flip transition and the charge transfer at the constituent interfaces. The results of this study show that the choice of the tunnel barrier layer material plays a nontrivial role in determining the magnetoelectric couplings during spin tunnelling under external field bias. Full article

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

► Show Figures

Figure 1

13 pages, 16844 KiB

Open AccessEditor’s ChoiceArticle

Revisiting the van der Waals Epitaxy in the Case of (Bi_0.4Sb_0.6)₂Te₃ Thin Films on Dissimilar Substrates

by Liesbeth Mulder, Daan H. Wielens, Yorick A. Birkhölzer, Alexander Brinkman and Omar Concepción

Nanomaterials 2022, 12(11), 1790; https://0-doi-org.brum.beds.ac.uk/10.3390/nano12111790 - 24 May 2022

Cited by 2 | Viewed by 1764

Abstract

Ultrathin films of the ternary topological insulator (Bi

_{0.4}

Sb

_{0.6}

)

_{2}

Te

_{3}

are fabricated by molecular beam epitaxy. Although it is generally assumed that the ternary topological insulator tellurides grow by van der Waals epitaxy, our results show that the [...] Read more.

Ultrathin films of the ternary topological insulator (Bi

_{0.4}

Sb

_{0.6}

)

_{2}

Te

_{3}

are fabricated by molecular beam epitaxy. Although it is generally assumed that the ternary topological insulator tellurides grow by van der Waals epitaxy, our results show that the influence of the substrate is substantial and governs the formation of defects, mosaicity, and twin domains. For this comparative study, InP (111)A, Al

_{2}

O

_{3}

(001), and SrTiO

_{3}

(111) substrates were selected. While the films deposited on lattice-matched InP (111)A show van der Waals epitaxial relations, our results point to a quasi-van der Waals epitaxy for the films grown on substrates with a larger lattice mismatch. Full article

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

► Show Figures

Figure 1

9 pages, 2252 KiB

Open AccessArticle

Threefold Fermions, Weyl Points, and Superconductivity in the Mirror Symmetry Lacking Semiconductor TlCd₂Te₄

by Angus Huang, Chin-Hsuan Chen and Horng-Tay Jeng

Nanomaterials 2022, 12(4), 679; https://0-doi-org.brum.beds.ac.uk/10.3390/nano12040679 - 18 Feb 2022

Cited by 1 | Viewed by 1489

Abstract

The topological phase transition and exotic quasiparticles in materials have attracted much attention because of their potential in spintronics and mimic of elementary particles. Especially, great research interest has been paid to search for the Weyl fermions in solid-state physics. By using first-principles [...] Read more.

The topological phase transition and exotic quasiparticles in materials have attracted much attention because of their potential in spintronics and mimic of elementary particles. Especially, great research interest has been paid to search for the Weyl fermions in solid-state physics. By using first-principles calculations, we predict that the multinary semiconductor alloy

Tl {Cd}_{2} {Te}_{4}

exhibits threefold fermions and nodal-line fermions, which are protected by the

S_{4}

improper rotational symmetry. Moreover, owing to the lack of inversion and mirror symmetries, the threefold fermions split into Weyl fermions when the spin-orbit coupling is included. The chiral charge of Weyl points and the

Z_{2}

time-reversal topological invariant are investigated. The topological surface states, spin texture, and electron-phonon coupling analysis are presented. Our study demonstrates

Tl {Cd}_{2} {Te}_{4}

as a good platform to understand topological phase transitions as well as possible coexistance of topological Weyl semimetal and superconductivity in one single material. Full article

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

► Show Figures

Figure 1

8 pages, 1533 KiB

Open AccessFeature PaperArticle

Thermoelectric Transport in a Three-Dimensional HgTe Topological Insulator

by Gennady M. Gusev, Ze D. Kvon, Alexander D. Levin and Nikolay N. Mikhailov

Nanomaterials 2021, 11(12), 3364; https://0-doi-org.brum.beds.ac.uk/10.3390/nano11123364 - 11 Dec 2021

Cited by 1 | Viewed by 2133

Abstract

The thermoelectric response of 80 nm-thick strained HgTe films of a three-dimensional topological insulator (3D TI) has been studied experimentally. An ambipolar thermopower is observed where the Fermi energy moves from conducting to the valence bulk band. The comparison between theory and experiment [...] Read more.

The thermoelectric response of 80 nm-thick strained HgTe films of a three-dimensional topological insulator (3D TI) has been studied experimentally. An ambipolar thermopower is observed where the Fermi energy moves from conducting to the valence bulk band. The comparison between theory and experiment shows that the thermopower is mostly due to the phonon drag contribution. In the region where the 2D Dirac electrons coexist with bulk hole states, the Seebeck coefficient is modified due to 2D electron–3D hole scattering. Full article

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

► Show Figures

Graphical abstract

11 pages, 1856 KiB

Open AccessArticle

Epitaxial Growth and Structural Characterizations of MnBi₂Te₄ Thin Films in Nanoscale

by Shu-Hsuan Su, Jen-Te Chang, Pei-Yu Chuang, Ming-Chieh Tsai, Yu-Wei Peng, Min Kai Lee, Cheng-Maw Cheng and Jung-Chung Andrew Huang

Nanomaterials 2021, 11(12), 3322; https://0-doi-org.brum.beds.ac.uk/10.3390/nano11123322 - 07 Dec 2021

Cited by 9 | Viewed by 3890

Abstract

The intrinsic magnetic topological insulator MnBi₂Te₄ has attracted much attention due to its special magnetic and topological properties. To date, most reports have focused on bulk or flake samples. For material integration and device applications, the epitaxial growth of MnBi [...] Read more.

The intrinsic magnetic topological insulator MnBi₂Te₄ has attracted much attention due to its special magnetic and topological properties. To date, most reports have focused on bulk or flake samples. For material integration and device applications, the epitaxial growth of MnBi₂Te₄ film in nanoscale is more important but challenging. Here, we report the growth of self-regulated MnBi₂Te₄ films by the molecular beam epitaxy. By tuning the substrate temperature to the optimal temperature for the growth surface, the stoichiometry of MnBi₂Te₄ becomes sensitive to the Mn/Bi flux ratio. Excessive and deficient Mn resulted in the formation of a MnTe and Bi₂Te₃ phase, respectively. The magnetic measurement of the 7 SL MnBi₂Te₄ film probed by the superconducting quantum interference device (SQUID) shows that the antiferromagnetic order occurring at the Néel temperature 22 K is accompanied by an anomalous magnetic hysteresis loop along the c-axis. The band structure measured by angle-resolved photoemission spectroscopy (ARPES) at 80 K reveals a Dirac-like surface state, which indicates that MnBi₂Te₄ has topological insulator properties in the paramagnetic phase. Our work demonstrates the key growth parameters for the design and optimization of the synthesis of nanoscale MnBi₂Te₄ films, which are of great significance for fundamental research and device applications involving antiferromagnetic topological insulators. Full article

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

► Show Figures

Figure 1

15 pages, 1163 KiB

Open AccessArticle

Thermo-Magneto-Electric Transport through a Torsion Dislocation in a Type I Weyl Semimetal

by Daniel Bonilla, Enrique Muñoz and Rodrigo Soto-Garrido

Nanomaterials 2021, 11(11), 2972; https://0-doi-org.brum.beds.ac.uk/10.3390/nano11112972 - 05 Nov 2021

Cited by 5 | Viewed by 1509

Abstract

Herein, we study electronic and thermoelectric transport in a type I Weyl semimetal nanojunction, with a torsional dislocation defect, in the presence of an external magnetic field parallel to the dislocation axis. The defect is modeled in a cylindrical geometry, as a combination [...] Read more.

Herein, we study electronic and thermoelectric transport in a type I Weyl semimetal nanojunction, with a torsional dislocation defect, in the presence of an external magnetic field parallel to the dislocation axis. The defect is modeled in a cylindrical geometry, as a combination of a gauge field accounting for torsional strain and a delta-potential barrier for the lattice mismatch effect. In the Landauer formalism, we find that due to the combination of strain and magnetic field, the electric current exhibits chiral valley-polarization, and the conductance displays the signature of Landau levels. We also compute the thermal transport coefficients, where a high thermopower and a large figure of merit are predicted for the junction. Full article

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

► Show Figures

Figure 1

20 pages, 2930 KiB

Open AccessArticle

Floquet Second-Order Topological Phases in Momentum Space

by Longwen Zhou

Nanomaterials 2021, 11(5), 1170; https://0-doi-org.brum.beds.ac.uk/10.3390/nano11051170 - 29 Apr 2021

Cited by 4 | Viewed by 2107

Abstract

Higher-order topological phases (HOTPs) are characterized by symmetry-protected bound states at the corners or hinges of the system. In this work, we reveal a momentum-space counterpart of HOTPs in time-periodic driven systems, which are demonstrated in a two-dimensional extension of the quantum double-kicked [...] Read more.

Higher-order topological phases (HOTPs) are characterized by symmetry-protected bound states at the corners or hinges of the system. In this work, we reveal a momentum-space counterpart of HOTPs in time-periodic driven systems, which are demonstrated in a two-dimensional extension of the quantum double-kicked rotor. The found Floquet HOTPs are protected by chiral symmetry and characterized by a pair of topological invariants, which could take arbitrarily large integer values with the increase of kicking strengths. These topological numbers are shown to be measurable from the chiral dynamics of wave packets. Under open boundary conditions, multiple quartets Floquet corner modes with zero and

π

quasienergies emerge in the system and coexist with delocalized bulk states at the same quasienergies, forming second-order Floquet topological bound states in the continuum. The number of these corner modes is further counted by the bulk topological invariants according to the relation of bulk-corner correspondence. Our findings thus extend the study of HOTPs to momentum-space lattices and further uncover the richness of HOTPs and corner-localized bound states in continuum in Floquet systems. Full article

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

► Show Figures

Figure 1

Review

Jump to: Editorial, Research

20 pages, 6240 KiB

Open AccessReview

Omnipresence of Weak Antilocalization (WAL) in Bi₂Se₃ Thin Films: A Review on Its Origin

by Rubén Gracia-Abad, Soraya Sangiao, Chiara Bigi, Sandeep Kumar Chaluvadi, Pasquale Orgiani and José María De Teresa

Nanomaterials 2021, 11(5), 1077; https://0-doi-org.brum.beds.ac.uk/10.3390/nano11051077 - 22 Apr 2021

Cited by 13 | Viewed by 3424

Abstract

Topological insulators are materials with time-reversal symmetric states of matter in which an insulating bulk is surrounded by protected Dirac-like edge or surface states. Among topological insulators,

{Bi}_{2} {Se}_{3}

has attracted special attention due to its simple surface band structure and [...] Read more.

Topological insulators are materials with time-reversal symmetric states of matter in which an insulating bulk is surrounded by protected Dirac-like edge or surface states. Among topological insulators,

{Bi}_{2} {Se}_{3}

has attracted special attention due to its simple surface band structure and its relatively large band gap that should enhance the contribution of its surface to transport, which is usually masked by the appearance of defects. In order to avoid this difficulty, several features characteristic of topological insulators in the quantum regime, such as the weak-antilocalization effect, can be explored through magnetotransport experiments carried out on thin films of this material. Here, we review the existing literature on the magnetotransport properties of

{Bi}_{2} {Se}_{3}

thin films, paying thorough attention to the weak-antilocalization effect, which is omnipresent no matter the film quality. We carefully follow the different situations found in reported experiments, from the most ideal situations, with a strong surface contribution, towards more realistic cases where the bulk contribution dominates. We have compared the transport data found in literature to shed light on the intrinsic properties of

{Bi}_{2} {Se}_{3}

, finding a clear relationship between the mobility and the phase coherence length of the films that could trigger further experiments on transport in topological systems. Full article

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

► Show Figures

Figure 1

Journal Menu

Journal Browser

Advances in Topological Materials: Fundamentals, Challenges and Outlook

Share This Special Issue

Special Issue Editor

Special Issue Information

Keywords

Published Papers (10 papers)

Editorial

Research

Review

Further Information

Guidelines

MDPI Initiatives

Follow MDPI