Strain-induced Topological Phase Transitions Research Articles

Strain-Induced Topological Phase Transitions Covering the Indicator in Orthorhombic Li2AuBi.

The symmetry indicator is widely used to classify topological materials hosting inversion symmetry. We find orthorhombic Li2AuBi in space group Cmcm is a topological insulator with under no strain via first-principles calculations. Due to small band gaps in the kz = 0 plane, the band inversions can be selectively induced by moderate external strains to realize phases covering all values of = 1, 2, 3, and 0. Detailed phase diagrams are plotted under various moderate strains. The (001) surface states and their associated Fermi surfaces and spin textures are calculated. The topological surface states have different connectivities and different spin textures for the four different phases. The tunability of topological surface states via moderate strain suggests Li2AuBi as an attractive topological material for device applications.

Strain-induced topological phase transition in graphene nanoribbons

Strain-induced topological phase transition in graphene nanoribbons

Strain-induced topological phase transition in ferromagnetic Janus monolayer MnSbBiS2Te2.

We investigate a strain-induced topological phase transition in the ferromagnetic Janus monolayer MnSbBiS2Te2 using first-principles calculations. The electronic, magnetic, and topological properties are studied under biaxial strain within the range of -8 to +8%. The ground state of monolayer MnSbBiS2Te2 is metallic with an out-of-plane magnetic easy axis. A band gap is opened when a compressive strain between -4% and -7% is applied. We observe a topological phase transition at a biaxial strain of -5%, where the material becomes a Chern insulator exhibiting a quantum anomalous hall (QAH) effect. We find that biaxial strain and spin-orbit coupling (SOC) are responsible for the topological phase transition in MnSbBiS2Te2. In addition, we find that biaxial strain can alter the direction of the magnetic easy axis of MnSbBiS2Te2. The Curie temperature is calculated using the Heisenberg model and is found to be 24 K. This study could pave the way to the design of topological materials with potential applications in spintronics, quantum computing, and dissipationless electronics.

Thickness-dependent topological phase transition and Rashba-like preformed topological surface states of α-Sn(001) thin films on InSb(001)

Topological materials, possessing spin-momentum locked topological surface states (TSS), have attracted much interest due to their potential applications in spintronics. \ensuremath{\alpha}-phase Sn (\ensuremath{\alpha}-Sn), being one of them, displays enriched topological phases via band-gap engineering through a strain or confinement effect. In this work, we investigated the band evolution of in-plane compressively strained \ensuremath{\alpha}-Sn(001) thin films on InSb(001) in a wide range of thickness from 3 bilayers (BL) to 370 BL by combining angle-resolved photoemission spectra and first-principles calculations. Gapped surface states evolved to a linearly dispersive TSS at a critical thickness of 6 BL, indicating that the system undergoes a phase transition from topologically trivial to nontrivial. For films thicker than 30 BL, additional Rashba-like surface states (RSS) were identified. These RSS served as preformed TSS in another strain-induced topological phase transition. In thick films, 370-BL \ensuremath{\alpha}-Sn(001), so as to preclude the confinement effect in thin films, our results were consistent with a Dirac semimetal phase with Dirac nodes located along $\mathrm{\ensuremath{\Gamma}}--Z$. This thickness-dependent band-structure study deepens our understanding of topological phase transitions and the evolution of Dirac states. Furthermore, the coexistence of TSS and RSS in a Dirac semimetal \ensuremath{\alpha}-Sn might significantly enhance the potential for spintronic applications.

Visualization of the strain-induced topological phase transition in a quasi-one-dimensional superconductor TaSe3

Control of the phase transition from topological to normal insulators can allow for an on/off switching of spin current. While topological phase transitions have been realized by elemental substitution in semiconducting alloys, such an approach requires preparation of materials with various compositions. Thus it is quite far from a feasible device application, which demands a reversible operation. Here we use angle-resolved photoemission spectroscopy and spin- and angle-resolved photoemission spectroscopy to visualize the strain-driven band-structure evolution of the quasi-one-dimensional superconductor TaSe3. We demonstrate that it undergoes reversible strain-induced topological phase transitions from a strong topological insulator phase with spin-polarized, quasi-one-dimensional topological surface states, to topologically trivial semimetal and band insulating phases. The quasi-one-dimensional superconductor TaSe3 provides a suitable platform for engineering the topological spintronics, for example as an on/off switch for a spin current that is robust against impurity scattering.

Strain-induced topological phase transition with inversion of the in-plane electric polarization in tiny-gap semiconductor SiGe monolayer

Our density functional theory calculations show that tiny-gap semiconductor SiGe monolayer is a quantum valley Hall insulator with a spontaneous electric polarization and, under a small biaxial strain, undergoes a topological phase transition between the states with opposite valley Chern numbers. The topological phase transition entails abrupt inversion of the in-plane electric polarization corresponding to inversion of the sublattice pseudospin polarization, while the out-of-plane electric polarization shows a linear response to the biaxial strain as well as to the perpendicular electric field regardless of the phase transition. Thus, the quantum valley Hall state entails in-plane ferroelectricity corresponding to a sublattice pseudospin ferromagnetism.

Strain effects on novel quantum materials

Novel quantum materials have been one of the most important topics in condensed matter physics due to their unique physical properties and potential applications. Many effective methods, especially for strain, have been developed to tune the properties of solid state materials. This paper systematically reviews several novel quantum materials discovered in recent years with a focus on their strain effects. Firstly, we introduce strain-induced topological phase transitions in topological materials. Secondly, we discuss the effects on electron transport in monolayer MoS2 and superconductivity in phosphorene. Finally, we analyze the tunable magnetic moment or spontaneous electrical polarization in two-dimensional ferromagnetic or ferroelectric materials.

Designing Flexible Quantum Spin Hall Insulators through 2D Ordered Hybrid Transition-Metal Carbides

Quantum spin Hall (QSH) insulators have attracted much attention due to their potential applications ranging from electronic devices to quantum computing. In general, a large band gap is regarded as a critical descriptor in the design of QSH insulators; however, it faces challenges when additional factors such as strain and surface oxidation are involved in practical applications. In this work, taking M″2M′C2O2 (M′ = Ti, Zr, Hf; M″ = Mo, W) as a representative, results reveal that 2D ordered transition-metal carbides (MXenes) are promising candidates for flexible spintronic devices, which is ascribed to the mechanical flexibility and robust QSH states under strain. Although a large bulk band gap is shown in M″2HfC2O2, a strain-induced topological phase transition may limit its flexible application. On the contrary, M″2TiC2O2 has a smaller gap, and its topological nontrivial state survives under strain. When n changes from 0 to 4 in M″2TinCn+1O2, a topologically nontrivial–trivial phase transition is obser...

Artificial gravity field, astrophysical analogues, and topological phase transitions in strained topological semimetals

Effective gravity and gauge fields are emergent properties intrinsic for low-energy quasiparticles in topological semimetals. Here, taking two Dirac semimetals as examples, we demonstrate that applied lattice strain can generate warped spacetime, with fascinating analogues in astrophysics. Particularly, we study the possibility of simulating black-hole/white-hole event horizons and gravitational lensing effect. Furthermore, we discover strain-induced topological phase transitions, both in the bulk materials and in their thin films. Especially in thin films, the transition between the quantum spin Hall and the trivial insulating phases can be achieved by a small strain, naturally leading to the proposition of a novel piezo-topological transistor device. Possible experimental realizations and analogue of Hawking radiation effect are discussed. Our result bridges multiple disciplines, revealing topological semimetals as a unique table-top platform for exploring interesting phenomena in astrophysics and general relativity; it also suggests realistic materials and methods to achieve controlled topological phase transitions with great potential for device applications.

Lateral topological crystalline insulator heterostructure

The emergence of lateral heterostructures fabricated by two-dimensional building blocks brings many exciting realms in material science and device physics. Enriching available nanomaterials for creating such heterostructures and enabling the underlying new physics is highly coveted for the integration of next-generation devices. Here, we report a breakthrough in lateral heterostructure based on the monolayer square transition-metal dichalcogenides MX2 (M = W, X = S/Se) modules. Our results reveal that the MX2 lateral heterostructure (1S-MX2 LHS) can possess excellent thermal and dynamical stability. Remarkably, the highly desired two-dimensional topological crystalline insulator phase is confirmed by the calculated mirror Chern number . A nontrivial band gap of 65 meV is obtained with SOC, indicating the potential for room-temperature observation and applications. The topologically protected edge states emerge at the edges of two different nanoribbons between the bulk band gap, which is consistent with the mirror Chern number. In addition, a strain-induced topological phase transition in 1S-MX2 LHS is also revealed, endowing the potential utilities in electronics and spintronics. Our predictions not only introduce new member and vitality into the studies of lateral heterostructures, but also highlight the promise of lateral heterostructure as appealing topological crystalline insulator platforms with excellent stability for future devices.

Surface-state spin textures in strained bulk HgTe: Strain-induced topological phase transitions

The opening of a band gap due to compressive uniaxial strain renders bulk HgTe a strong three-dimensional topological insulators with protected gapless surface states at any surface. By employing a six-band k.p model, we determine the spin textures of the topological surface states of strained HgTe using their close relations with the mirror Chern numbers of the system and the orbital composition of the surface states. We show that at surfaces with C2v point group symmetry an increase in the strain magnitude triggers a topological phase transition where the winding number of the surface state spin texture is flipped while the four topological invariants characterizing the bulk band structure of the material are unchanged.

Strain-induced topological phase transition at zigzag edges of monolayer transition-metal dichalcogenides

The effect of strain in zigzag ribbons of monolayer transition-metal dichalcogenides with induced superconductivity is studied using a minimal 3-band tight-binding model. The unstrained system shows a topological phase with Majorana zero modes localized at the boundaries of the one-dimensional (1D) zigzag edges. By direct inspection of the spectrum and wave functions we examine the evolution of the topological phase as an in-plane, uniaxial deformation is imposed. It is found that strain shifts the energy of 1D edge states, thus causing a topological phase transition which eliminates the Majorana modes. For realistic parameter values, we show that the effect of strain can be changed from completely destructive -- in which case a small built in strain is enough to destroy the topological phase -- to a situation where strain becomes an effective tuning parameter which can be used to manipulate Majorana zero modes. These two regimes are accessible by increasing the value of the applied Zeeman field within realistic values. We also study how strain effects are affected by the chemical potential, showing in particular how unwanted effects can be minimized. Finally, as a cross-check of the obtained results, we reveal the connection between 1D Majorana zero modes in the zigzag edge and the multi-band Berry phase, which serves as a topological invariant of this system.

Strain-induced topological phase transition in phosphorene and in phosphorene nanoribbons

Using the tight-binding (TB) approximation with inclusion of the spin-orbit interaction, we predict a topological phase transition in the electronic band structure of phosphorene in the presence of axial strains. We derive a low-energy TB Hamiltonian that includes the spin-orbit interaction for bulk phosphorene. Applying a compressive biaxial in-plane strain and perpendicular tensile strain in ranges where the structure is still stable leads to a topological phase transition. We also examine the influence of strain on zigzag phosphorene nanoribbons (zPNRs) and the formation of the corresponding protected edge states when the system is in the topological phase. For zPNRs up to a width of 100 nm the energy gap is at least three orders of magnitude larger than the thermal energy at room temperature.

Stacked bilayer phosphorene: strain-induced quantum spin Hall state and optical measurement.

Bilayer phosphorene attracted considerable interest, giving a potential application in nanoelectronics owing to its natural bandgap and high carrier mobility. However, very little is known regarding the possible usefulness in spintronics as a quantum spin Hall (QSH) state of material characterized by a bulk energy gap and gapless spin-filtered edge states. Here, we report a strain-induced topological phase transition from normal to QSH state in bilayer phosphorene, accompanied by band-inversion that changes number from 0 to 1, which is highly dependent on interlayer stacking. When the bottom layer is shifted by 1/2 unit-cell along zigzag/armchair direction with respect to the top layer, the maximum topological bandgap 92.5 meV is sufficiently large to realize QSH effect even at room-temperature. An optical measurement of QSH effect is therefore suggested in view of the wide optical absorption spectrum extending to far infra-red, making bilayer phosphorene a promising candidate for opto-spintronic devices.

Manipulating topological phase transition by strain

First-principles calculations show that strain-induced topological phase transition is a universal phenomenon in those narrow-gap semiconductors for which the valence band maximum (VBM) and conduction band minimum (CBM) have different parities. The transition originates from the opposite responses of the VBM and CBM, whose magnitudes depend critically on the direction of the applied strain. Our work suggests that strain can play a unique role in tuning the electronic properties of topological insulators for device applications, as well as in the achievement of new topological insulators.

Different topological insulating behavior inβ-GaS and GaS-II under uniaxial tension

The topological insulating behavior of \ensuremath{\beta}-GaS and GaS-II is investigated under uniaxial tensile strains, based on ab initio calculations with the Tran--Blaha modified Becke--Johnson potential. We found that the strain-induced topological phase transition only occurs in GaS-II, notwithstanding the same space group. The distinct topological insulating behavior of the two isomers are demonstrated to originate from the variation of the ${\ensuremath{\Gamma}}_{3}^{+}\ensuremath{-}{\ensuremath{\Gamma}}_{2}^{\ensuremath{-}}$ band gap, which is a direct consequence of the uniaxial-stretching-strengthened interlayer $s$-$s$ interactions. The present finding provides a new perspective in exploring the topological behavior in materials with similar compositions and space group.

Erratum: Atomic and Electronic Structure of Ultrathin Bi(111) Films Grown onBi2Te3(111)Substrates: Evidence for a Strain-Induced Topological Phase Transition [Phys. Rev. Lett.109, 227401 (2012)

Erratum: Atomic and Electronic Structure of Ultrathin Bi(111) Films Grown on<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>Bi</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>Te</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:mo stretchy="false">(</mml:mo><mml:mn>111</mml:mn><mml:mo stretchy="false">)</mml:mo></mml:math>Substrates: Evidence for a Strain-Induced Topological Phase Transition [Phys. Rev. Lett.<b>109</b>, 227401 (2012)

Anisotropic interactions and strain-induced topological phase transition in Sb2Se3and Bi2Se3

Based on first-principles calculations, we study the dependence of topological phase on anisotropic interactions in Bi${}_{2}$Se${}_{3}$-type materials. By applying different strains in order to vary interactions, we reveal that the topological phase is insensitive to lateral interaction but can be effectively tuned by longitudinal interaction. Longitudinal strain is inhomogeneous in the studied systems. The interquintuple interaction plays a dominant role in determining the topological phase. We explain the puzzling band-topology difference between Sb${}_{2}$Se${}_{3}$ and Bi${}_{2}$Se${}_{3}$ and propose an approach to tuning the topological phase by strain. It is found that Sb${}_{2}$Se${}_{3}$ can be converted into a topological insulator by applying compressive longitudinal strain while the converse strain can turn Bi${}_{2}$Se${}_{3}$ into a normal insulator. We have studied thin films of Sb${}_{2}$Se${}_{3}$ and Bi${}_{2}$Se${}_{3}$ and also observed a strain-induced topological phase transition.