Interplay of orbital effects and nanoscale strain in topological crystalline insulators
TL;DR: Walkup et al. as mentioned in this paper used scanning-tunneling microscopy to study the effects of strain on the electronic structure of a heteroepitaxial thin film of a topological crystalline insulator, SnTe, and found that strain applied in one direction has the most pronounced influence on the band structure along the perpendicular direction.
Abstract: Orbital degrees of freedom can have pronounced effects on the fundamental properties of electrons in solids In addition to influencing bandwidths, gaps, correlation strength and dispersion, orbital effects have been implicated in generating novel electronic and structural phases Here we show how the orbital nature of bands can result in non-trivial effects of strain on band structure We use scanning–tunneling microscopy to study the effects of strain on the electronic structure of a heteroepitaxial thin film of a topological crystalline insulator, SnTe By studying the effects of uniaxial strain on the band structure we find a surprising effect where strain applied in one direction has the most pronounced influence on the band structure along the perpendicular direction Our theoretical calculations indicate that this effect arises from the orbital nature of the conduction and valence bands Our results imply that a microscopic model capturing strain effects must include a consideration of the orbital nature of bands The role of orbital degrees of freedom in determining the electronic structure remains obscured Here, Walkup et al report strain-induced band structure changes in a topological crystalline insulator SnTe, whose surprising behavior reflects the orbital nature of bands
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TL;DR: In this paper, it was shown that electronic nematicity can be locally decoupled from the underlying structural anisotropy in strain-engineered iron-selenide (FeSe) thin films.
Abstract: In a material prone to a nematic instability, anisotropic strain in principle provides a preferred symmetry-breaking direction for the electronic nematic state to follow. This is consistent with experimental observations, where electronic nematicity and structural anisotropy typically appear hand-in-hand. In this work, we discover that electronic nematicity can be locally decoupled from the underlying structural anisotropy in strain-engineered iron-selenide (FeSe) thin films. We use heteroepitaxial molecular beam epitaxy to grow FeSe with a nanoscale network of modulations that give rise to spatially varying strain. We map local anisotropic strain by analyzing scanning tunneling microscopy topographs, and visualize electronic nematic domains from concomitant spectroscopic maps. While the domains form so that the energy of nemato-elastic coupling is minimized, we observe distinct regions where electronic nematic ordering fails to flip direction, even though the underlying structural anisotropy is locally reversed. The findings point towards a nanometer-scale stiffness of the nematic order parameter. Here, in contrast to previous observations in Fe-based superconductors, scanning tunnelling microscopy of strain-patterned FeSe thin films reveals a local decoupling of electron nematicity and structural anisotropy, pointing towards a stiffness of the nematic order parameter at the nanoscale.
42 citations
TL;DR: In this article, various physical perturbations, subjects of lively discussions, have been uncovered to improve the thermoelectric power factor (PF) of SnTe (001) and related alloys.
Abstract: Topological crystalline insulators (TCIs) possess linearly dispersed metallic surface states, which are protected by crystal point group symmetries. The ability to fine-tune the effective mass of surface Dirac fermions by breaking their crystalline symmetry is highly desirable for thermoelectric applications. Given that the signatures of SnTe and its family originate from the (001) surface states, a natural question is: how does the thermoelectric performance of these states change due to the emergence of massive Dirac fermions? Herein, various physical perturbations, subjects of lively discussions, have been uncovered to improve the thermoelectric power factor (PF) of SnTe (001) and related alloys. Furthermore, orientation-dependent charge and heat currents are explored in detail. The surface-state Onsager transport calculations are performed using the Kubo–Greenwood approach. Highly dispersive and degenerate energy bands originating from the band gap opening are responsible for the enhancement of PF. While the x-direction has contributed mostly to the PF of the system, we report exceptional 74.65%, 121.67% and 110% enhancement of the PF compared with the pristine case at a temperature of 540 K when we perturb the crystalline mirror symmetry by strain, exchange field (stemming from proximity coupling to a ferromagnet, or the electric field, or Zeeman magnetic field) and Rashba spin–orbit coupling, respectively. The predicted PFs propose a new research direction to experimentalists to save time and to focus only on the thermal conductivity of SnTe (001) to achieve the highest thermoelectric efficiency.
25 citations
TL;DR: It is reported that, besides a vast number of TCI applications, TCIs are versatile candidates for topological transistors with tunable ON and OFF states if appropriate tuning of the surface band gap can be performed experimentally.
Abstract: Topological crystalline insulators (TCIs) are particularly one of the most fascinating materials in current research. The gapless surface states protected by the crystal point group symmetries in TCIs entail the emergence of nontrivial physics and can be tailored by controlling the external perturbations. This paper is devoted to a detailed analysis of the perturbation effects on the quantum phase of SnTe(001) surface states. Generically, surface states are gradually perturbed so that the gapless phase dies out. In doing so, a numerical study of the perturbed · model is accomplished by the linear response theory and the Green's function technique. The model is experimentally accessible. The system displays a commensurate breaking of the mirror invariance imposed by external perturbations such as strain, magnetic proximity effect/electric field/Zeeman magnetic field, Rashba spin–orbit coupling, and dilute charged impurity. The interesting behaviors are explained by the variation of the gap with the above-mentioned perturbations (invoking the opening of the gap) at Dirac cones corresponding to the TCI phase. For suitably tuned parameters, SnTe(001) surface states realize gapped phases. The synergy of perturbations is responsible for breaking down the topologically non-trivial character of SnTe and related alloys. Further, the conditions under which the variations of the parameters maintain the topological properties are discussed. These findings and predictions report that, besides a vast number of TCI applications, TCIs are versatile candidates for topological transistors with tunable ON and OFF states if appropriate tuning of the surface band gap can be performed experimentally.
17 citations
TL;DR: In this paper, high-resolution transmission electron microscopy (TEEM) investigations showed that the nanowires grow on graphene in the van der Waals epitaxy mode induced when the catalyzing Au nanoparticles mix with Sn delivered from a SnTe flux, providing a liquid Au-Sn alloy.
Abstract: SnTe topological crystalline insulator nanowires have been grown by molecular beam epitaxy on graphene/SiC substrates. The nanowires have a cubic rock-salt structure, they grow along the [001] crystallographic direction and have four sidewalls consisting of {100} crystal planes known to host metallic surface states with a Dirac dispersion. Thorough high resolution transmission electron microscopy investigations show that the nanowires grow on graphene in the van der Waals epitaxy mode induced when the catalyzing Au nanoparticles mix with Sn delivered from a SnTe flux, providing a liquid Au-Sn alloy. The nanowires are totally free from structural defects, but their {001} sidewalls are prone to oxidation, which points out the necessity of depositing a protective capping layer in view of exploiting the magneto-electric transport phenomena involving charge carriers occupying topologically protected states.
15 citations
TL;DR: A recent review of strain-tunable quantum properties and functionalities can be found in this article , with a focus on low-dimensional quantum materials, including 2D van der Waals materials and heterostructures.
Abstract: Recent discoveries of exotic physical phenomena, such as unconventional superconductivity in magic-angle twisted bilayer graphene, dissipationless Dirac fermions in topological insulators, and quantum spin liquids, have triggered tremendous interest in quantum materials. The macroscopic revelation of quantum mechanical effects in quantum materials is associated with strong electron–electron correlations in the lattice, particularly where materials have reduced dimensionality. Owing to the strong correlations and confined geometry, altering atomic spacing and crystal symmetry via strain has emerged as an effective and versatile pathway for perturbing the subtle equilibrium of quantum states. This review highlights recent advances in strain-tunable quantum phenomena and functionalities, with particular focus on low-dimensional quantum materials. Experimental strategies for strain engineering are first discussed in terms of heterogeneity and elastic reconfigurability of strain distribution. The nontrivial quantum properties of several strain-quantum coupled platforms, including 2D van der Waals materials and heterostructures, topological insulators, superconducting oxides, and metal halide perovskites, are next outlined, with current challenges and future opportunities in quantum straintronics followed. Overall, strain engineering of quantum phenomena and functionalities is a rich field for fundamental research of many-body interactions and holds substantial promise for next-generation electronics capable of ultrafast, dissipationless, and secure information processing and communications.
15 citations
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TL;DR: In this paper, the theoretical foundation for topological insulators and superconductors is reviewed and recent experiments are described in which the signatures of topologically insulators have been observed.
Abstract: Topological insulators are electronic materials that have a bulk band gap like an ordinary insulator but have protected conducting states on their edge or surface. These states are possible due to the combination of spin-orbit interactions and time-reversal symmetry. The two-dimensional (2D) topological insulator is a quantum spin Hall insulator, which is a close cousin of the integer quantum Hall state. A three-dimensional (3D) topological insulator supports novel spin-polarized 2D Dirac fermions on its surface. In this Colloquium the theoretical foundation for topological insulators and superconductors is reviewed and recent experiments are described in which the signatures of topological insulators have been observed. Transport experiments on $\mathrm{Hg}\mathrm{Te}∕\mathrm{Cd}\mathrm{Te}$ quantum wells are described that demonstrate the existence of the edge states predicted for the quantum spin Hall insulator. Experiments on ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$, ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$, and ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ are then discussed that establish these materials as 3D topological insulators and directly probe the topology of their surface states. Exotic states are described that can occur at the surface of a 3D topological insulator due to an induced energy gap. A magnetic gap leads to a novel quantum Hall state that gives rise to a topological magnetoelectric effect. A superconducting energy gap leads to a state that supports Majorana fermions and may provide a new venue for realizing proposals for topological quantum computation. Prospects for observing these exotic states are also discussed, as well as other potential device applications of topological insulators.
15,562 citations
TL;DR: Topological superconductors are new states of quantum matter which cannot be adiabatically connected to conventional insulators and semiconductors and are characterized by a full insulating gap in the bulk and gapless edge or surface states which are protected by time reversal symmetry.
Abstract: Topological insulators are new states of quantum matter which cannot be adiabatically connected to conventional insulators and semiconductors. They are characterized by a full insulating gap in the bulk and gapless edge or surface states which are protected by time-reversal symmetry. These topological materials have been theoretically predicted and experimentally observed in a variety of systems, including HgTe quantum wells, BiSb alloys, and Bi2Te3 and Bi2Se3 crystals. Theoretical models, materials properties, and experimental results on two-dimensional and three-dimensional topological insulators are reviewed, and both the topological band theory and the topological field theory are discussed. Topological superconductors have a full pairing gap in the bulk and gapless surface states consisting of Majorana fermions. The theory of topological superconductors is reviewed, in close analogy to the theory of topological insulators.
11,092 citations
TL;DR: In this paper, the authors studied three-dimensional generalizations of the quantum spin Hall (QSH) effect and introduced a tight binding model which realized the WTI and STI phases, and discussed its relevance to real materials including bismuth.
Abstract: We study three-dimensional generalizations of the quantum spin Hall (QSH) effect. Unlike two dimensions, where a single ${Z}_{2}$ topological invariant governs the effect, in three dimensions there are 4 invariants distinguishing 16 phases with two general classes: weak (WTI) and strong (STI) topological insulators. The WTI are like layered 2D QSH states, but are destroyed by disorder. The STI are robust and lead to novel ``topological metal'' surface states. We introduce a tight binding model which realizes the WTI and STI phases, and we discuss its relevance to real materials, including bismuth.
3,357 citations
TL;DR: In this paper, a method for measuring and mapping displacement fields and strain fields from high-resolution electron microscope (HREM) images is developed based upon centring a small aperture around a strong reflection in the Fourier transform of an HREM lattice image and performing an inverse Fourier transformation.
1,828 citations
TL;DR: A class of three-dimensional "topological crystalline insulators" which have metallic surface states with quadratic band degeneracy on high symmetry crystal surfaces is found.
Abstract: The recent discovery of topological insulators has revived interest in the band topology of insulators. In this Letter, we extend the topological classification of band structures to include certain crystal point group symmetry. We find a class of three-dimensional ``topological crystalline insulators'' which have metallic surface states with quadratic band degeneracy on high symmetry crystal surfaces. These topological crystalline insulators are the counterpart of topological insulators in materials without spin-orbit coupling. Their band structures are characterized by new topological invariants. We hope this work will enlarge the family of topological phases in band insulators and stimulate the search for them in real materials.
1,641 citations