Topological Crystalline Insulator SnTe Research Articles

Noncollinear twisted RKKY interaction on the optically driven SnTe(001) surface

The nontrivial spin texture on the (001) surface of topological crystalline insulator SnTe hosts exotic scientific importance and spintronic applications. Here, we study the effects of weak Floquet optical driving on the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between two magnetic impurities on a doped SnTe(001) surface. Due to peculiar spin-orbit hybridization, we find a noncollinear twisted RKKY interaction with XYZ-Heisenberg, symmetric in-plane, and asymmetric Dzyaloshinskii-Moriya (DM) terms. We see that contributions from the $z$ $(x)$ component of the XYZ-Heisenberg (DM) interaction are dominant for most parameters. The interactions, including DM terms that are responsible for interesting spin textures, require doping in most cases. We propose to modify the interactions in situ via optical control of band structure, and thereby doping. A notable aspect of this control protocol is breaking of electron-hole symmetry, which stems from the DM interaction.

Superior Performances of Self-Driven Near-Infrared Photodetectors Based on the SnTe:Si/Si Heterostructure Boosted by Bulk Photovoltaic Effect.

The upsurge of new materials that can be used for near-infrared (NIR) photodetectors operated without cooling is crucial. As a novel material with a small bandgap of ≈0.28eV, the topological crystalline insulator SnTe has attracted considerable attention. Herein, this work demonstrates self-driven NIR photodetectors based on SnTe/Si and SnTe:Si/Si heterostructures. The SnTe/Si heterostructure has a high detectivity D* of 3.3 × 1012 Jones. By Si doping, the SnTe:Si/Si heterostructure reduces the dark current density and increases the photocurrent by ≈1 order of magnitude simultaneously, which improves the detectivity D* by ≈2 orders of magnitude up to 1.59×1014 Jones. Further theoretical analysis indicates that the improved device performance may be ascribed to the bulk photovoltaic effect (BPVE), in which doped Si atoms break the inversion symmetry and thus enable the generation of additional photocurrents beyond the heterostructure. In addition, the external quantum efficiency (EQE) measured at room temperature at 850nm increases by a factor of 7.5 times, from 38.5% to 289%. A high responsivity of 1979mA W-1 without bias and fast rising time of 8 µs are also observed. The significantly improved photodetection achieved by the Si doping is of great interest and may provide a novel strategy for superior photodetectors.

Transport property of topological crystalline insulator SnTe (100) and ferrimagnetic insulator heterostructures

• High quality SnTe/RE3Fe5O12 (RE=Eu, Y) epitaxial heterostructures were prepared. • Domain disorder is induced by Fe atom diffusion in SnTe through magnetic interface. • Two different magnetoelectrical transport in two heterostructures are observed. • Linear magnetoresistance and nonlinear Hall appear in SnTe/Eu 3 Fe 5 O 12 . Topological crystalline insulator (TCI) SnTe is a potential material for quantum electronic devices because of its attractive inherent sensitivity of band topology and highly mobile characteristic of Dirac fermions. The proximity effect at the interface of SnTe film can affect the topological surface transport and may result in novel quantum magneto-electric effects. Here, we study the magnetoelectrical transport properties of SnTe thin films grown on ferrimagnetic insulators Eu 3 Fe 5 O 12 (110) (EuIG (110)) and Y 3 Fe 5 O 12 (111) (YIG (111)) single-crystal underlayers by molecular beam epitaxy. Linear magnetic resistance (LMR) is observed in SnTe/EuIG heterostructures in the low field range, which is different from the weak antilocalization (WAL) characteristic of SnTe/YIG heterostructures. Especially, the double carrier characteristic with the coexistence of holes and electrons in SnTe/EuIG heterostructure is quite different from the holes as main carriers in SnTe/YIG, although the SnTe layer remains the same crystal plane (100) in the two heterostructures. The LMR in SnTe/EuIG is attributed to the topological surface Dirac electrons and disordered domain distribution in the SnTe layer which is in sharp contrast to the WAL of SnTe/YIG with ordered domain distribution in the SnTe layer. The present studies of transport properties not only provide a fundamental understanding of the transport mechanism of TCI and magnetite insulator heterostructure but also display the promising application probability for tunable topological electronic devices.

Pressure-induced topological and structural phase transitions in natural van der Waals heterostructures from the (SnTe)m(Bi2Te3)n homologous family: Raman spectroscopy, x-ray diffraction, and density functional theory

A new class of van der Waals heterostructures of ${(\mathrm{SnTe})}_{\mathrm{m}}{({\mathrm{Bi}}_{2}{\mathrm{Te}}_{3})}_{\mathrm{n}}$ (with $m=1,2,.\phantom{\rule{0.16em}{0ex}}.$ and $n=1,2,.\phantom{\rule{0.16em}{0ex}}.$), consisting of a topological crystalline insulator SnTe and a topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ are emerging with exciting properties and applications, such as in thermoelectrics. Our study examines the stability of these heterostructures ($m$ = 1 and $n$ = 1,2) under pressure using Raman scattering, synchrotron x-ray diffraction, and density functional theory. Raman studies as a function of pressure carried out at room temperature reveal a phase transition in the pressure regime of 3--5 GPa for both the compounds, which is shown to be associated with an electronic topological transition involving change in the ${\mathrm{Z}}_{2}$ topological invariant. In addition to the electronic changes, our Raman experiments indicate that rhombohedral ($R\overline{3}m$) ${\mathrm{SnBi}}_{2}{\mathrm{Te}}_{4}$ undergoes structural transition at $\ensuremath{\sim}6.0$ to a possible monoclinic phase and another transition at $\ensuremath{\sim}12.0$ GPa. Raman and x-ray diffraction experiments on trigonal ($P\overline{3}m1$) ${\mathrm{SnBi}}_{4}{\mathrm{Te}}_{7}$ show two structural transitions at $\ensuremath{\sim}9.5$ GPa to a monoclinic phase followed by one to cubic phase at $\ensuremath{\sim}14.1$ GPa. Our analysis of electronic structure reveals that the phase transition at 9.5 GPa in ${\mathrm{SnBi}}_{4}{\mathrm{Te}}_{7}$ is accompanied by an insulator to semimetal transition.

Direct Probe of the Ferromagnetic Proximity Effect at the Interface of SnTe/Fe Heterostructure by Polarized Neutron Reflectometry.

Introducing magnetic order into a topological insulator (TI) system has attracted much attention with an expectation of realizing exotic phenomena such as the quantum anomalous Hall effect (QAHE) and axion insulator states. The magnetic proximity effect (MPE) is one of the promising schemes to induce the magnetic order on the surface of a TI without introducing disorder accompanied by doping magnetic impurities in the TI. In this study, we investigate the MPE at the interface of a heterostructure consisting of the topological crystalline insulator (TCI) SnTe and Fe by employing polarized neutron reflectometry. The ferromagnetic order penetrates ∼2.2 nm deep into the SnTe layer from the interface with Fe, which persists up to room temperature. This is induced by the MPE on the surface of the TCI preserving the coherent topological states without introducing the disorder by doping magnetic impurities. This would open up a way for realizing next-generation spintronics and quantum computational devices.

Large Linear Magnetoresistance and Evidence of Degeneracy Lifting of Valence Bands in Rhombohedral Phase of Topological Crystalline Insulator SnTe

A comprehensive magnetotransport study on a p‐type single crystal of SnTe topological crystalline insulator is reported, across the cubic‐to‐rhombohedral (R3m) transition atTs≈64 K. SnTe exhibits a large unsaturated linear magnetoresistance (LMR) reaching a value of 42% at 5 K and 8 T for (100) plane. LMR is found to have a direct dependence on the mobility and a detailed analysis shows that it follows the classical Parish‐Littlewood model of conductivity fluctuations arising from macroscopic inhomogeneities of tellurium interstitial atoms. Profound SdH oscillations having aπBerry phase are observed in the rhombohedral (R3m) phase having significantly lower carrier density ≈5.32 × 1011 cm−2at 2 K, which provides direct evidence of protected topological surface states in theR3mphase. The Hall conductivity shows a transformation from one band to two‐band behavior across the structural transition, thus providing an experimental evidence for the degeneracy lifting of bulk valence bands below the cubic symmetry breaking point which is consistent with recent band structure calculations. The overall results indicate that magnetotransport studies can distinctly probe the surface and bulk sensitive properties of SnTe, and can also track the band‐splitting of degenerate bands across the cubic‐to‐rhombohedral (R3m) transition.

Persistence of symmetry-protected Dirac points at the surface of the topological crystalline insulator SnTe upon impurity doping.

We investigate the effect of a non-magnetic donor impurity located at the surface of the SnTe topological crystalline insulator. In particular, the changes on the surface states due to a Sb impurity atom are analyzed by means of ab initio simulations of pristine and impurity-doped SnTe. Both semi-infinite and slab geometries are considered within the first-principles approach. Furthermore, minimal and Green's function continuum models are proposed with the same goal. We find that the Dirac cones are shifted down in energy upon doping; this shift strongly depends on the position of the impurity with respect to the surface. In addition, we observe that the width of the impurity band presents an even-odd behavior by varying the position of the impurity. This behavior is related to the position of the nodes of the wave function with respect to the surface, and hence it is a manifestation of confinement effects. We compare slab and semi-infinite geometries within the ab initio approach, demonstrating that the surface states remain gapless and their spin textures are unaltered in the doped semi-infinite system. In the slab geometry, a gap opens due to hybridization of the states localized at opposite surfaces. Finally, by means of a continuum model, we extrapolate our results to arbitrary positions of the impurity, clearly showing a non-monotonic behavior of the Dirac cone.

A multifaceted application of designed coulomb explosion occurring on oxidized topological crystalline insulator SnTe

Multifaceted application of designed coulomb explosion process occurred on the SnTe@oxide experimental model.

Anomalies in the temperature evolution of Dirac states in the topological crystalline insulator SnTe

Discovery of topologically protected surface states, believed to be immune to weak disorder and thermal effects, opened up a new avenue to reveal exotic fundamental science and advanced technology. While time-reversal symmetry plays the key role in most such materials, the bulk crystalline symmetries such as mirror symmetry preserve the topological properties of topological crystalline insulators (TCIs). It is apparent that any structural change may alter the topological properties of TCIs. To investigate this relatively unexplored landscape, we study the temperature evolution of the Dirac fermion states in an archetypical mirror-symmetry protected TCI, SnTe employing high-resolution angle-resolved photoemission spectroscopy and density functional theory studies. Experimental results reveal a perplexing scenario; the bulk bands observed at 22 K move nearer to the Fermi level at 60 K and again shift back to higher binding energies at 120 K. The slope of the surface Dirac bands at 22 K becomes smaller at 60 K and changes back to a larger value at 120 K. Our results from the first-principles calculations suggest that these anomalies can be attributed to the evolution of the hybridization physics with complex structural changes induced by temperature. In addition, we discover drastically reduced intensity of the Dirac states at the Fermi level at high temperatures may be due to complex evolution of anharmonicity, strain, etc. These results address robustness of the topologically protected surface states due to thermal effects and emphasize importance of covalency and anharmonicity in the topological properties of such emerging quantum materials.

Role of topological surface states and mirror symmetry in topological crystalline insulator SnTe as an efficient electrocatalyst.

The surface orientation dependence on the hydrogen evolution reaction (HER) performance of topological crystalline insulator (TCI) SnTe thin films is studied. Their intrinsic activities are determined by linear sweep voltammetry and cyclic voltammetry measurements. It is found that SnTe (001) and (111) surfaces exhibit intrinsic activities significantly larger than the (211) surface. Density functional theory calculations reveal that pure (001) and (111) surfaces are not good electrocatalysts, while those with Sn vacancies or partially oxidized surfaces, with the latter as evidenced by X-ray photoelectron spectroscopy, have high activity. The calculated overall performance of the (001) and (111) surfaces with robust topological surface states (TSSs) is better than that of the lowly symmetric (211) surface with fragile or without TSSs, which is further supported by their measured weak antilocalization strength. The high HER activity of SnTe (001) and (111) is attributed to the enhanced charge transfer between H atoms and TSSs. We also address the effect of possible surface facets and the contrast of the HER activity of the available active sites among the three samples. Our study demonstrates that the TSSs and mirror symmetry of TCIs expedite their HER activity.

Infrared optical spectrum of topological crystalline insulator SnTe (001) surface states

We investigate the effects of varying temperature and chemical potential on the optical absorption spectrum of (001) surface states of topological crystalline insulator SnTe using a four-band effective k ⋅ p Hamiltonian. The spectrum is characterized by a narrow peak at 52 meV and a shoulder feature at 160 meV. Both absorptions have maximal intensity at 0 K or when chemical potential is located at the charge neutrality point. Then, as temperature increases or as chemical potential diverges, they both decrease in intensity. The 52 meV peak originates from transitions between high density of states regions surrounding van Hove singularities and is the spectrum’s most prominent feature. Additionally, a third absorption from 110 meV to 150 meV, initially absent at 0 K or chemical potential at charge neutrality point, gradually builds in intensity as temperature increases or as chemical potential diverges. This absorption arises from transitions between low and high energy bands of opposite helicity. Importantly, we find that all distinct spectral features are diminished if the magnitude of chemical potential diverges to values above the van Hove singularity energies. If a given sample’s chemical potential is well-controlled, conventional infrared spectroscopy may be used to identify the spectral signatures of SnTe (001) surface states at room temperatures and without use of large magnetic fields.

Interplay of Topological States on TI/TCI Interfaces.

Based on first-principles calculations, we study electronic structure of interfaces between a topological insulator (TI) SnBiTe and a topological crystalline insulator (TCI) SnTe. We consider two interface models characterized by the different atomic structure on the contact of the SnTe(111) and SnBiTe(0001) slabs: the model when two materials are connected without intermixing (abrupt type of interface) and the interface model predicted to be realized at epitaxial immersion growth on topological insulator substrates (smooth interface). We find that a strong potential gradient at the abrupt interface leads to the redistribution of the topological states deeper from the interface plane which prevents the annihilation of the Dirac states, predicted earlier. In contrast, a smooth interface is characterized by minor charge transfer, which promotes the strong interplay between TI and TCI Dirac cones leading to their complete annihilation.The topologically protected Dirac state of SnTe(111) survives irrespective of the interface structure.

Effective low-energy RKKY interaction in doped topological crystalline insulators

We study the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between two magnetic impurities resided on the (001) plane of topological crystalline insulator (TCI) SnTe associated with four anisotropic Dirac cones protected by the mirror symmetry. The lattice Green's functions governing the correlation of both the sublattices and spins are deduced by developing a two-band effective model. We explore the RKKY Heisenberg interaction of the system established by the interference between four Dirac cones. Our key finding is that, independent of the position of magnetic impurities on the same sublattices or different ones, multiple wave vectors with different orders describe the RKKY coupling physics in TCIs. Further, both the undoped and doped spin susceptibility give rise to the same decay rates like graphene. Magnetic impurities on the same and different sublattices propose the ferromagnetic and antiferromagnetic base phase of the undoped system, respectively. Here we demonstrate that RKKY exchange in TCIs, undergoes a ferromagnetic-to-antiferromagnetic transition and vice versa in response to switching on/off the electron and hole doping. Analytic expressions of doped TCIs are obtained to draw the magnetic phase diagram of the distance between two magnetic impurities and the doping concentration, clarifying the contributions of short and long distances as well as low and high concentrations to the RKKY coupling. Our results may tailor the magnetic features of TCIs for further research and application.

Electrical conductivity of statically perturbed topological crystalline insulators

We theoretically investigate the electrical conductivity (EC) of topological crystalline insulators SnTe (001) and related alloys in the presence of external static local perturbations employing a continuum model and the Kubo-Greenwood formalism. Particularly, we have found the conditions for which the Dirac and saddle points can be altered in the presence of the perpendicular electric field, the on-site energy difference and the exchange field. Then, the EC of these alterations is addressed for potentials smaller and larger than the intervalley scattering parameter. Numerical results show that the band gap opens by the exchange field, while the electric field and on-site energy difference shift the Dirac/saddle points and the Fermi level, respectively. These phenomena result in the increasing and decreasing of the EC with electric field/on-site energy difference and exchange field, respectively. Finally, we find that the EC becomes non-zero at zero temperature when the Fermi level is shifted. These findings will bring applications to topological electronics.

Electronic and topological properties of [formula omitted][formula omitted]Te

The electronic and topological properties of Sn1-xInxTe have been investigated based on first principles calculations. Sn1-xInxTe presents inverted bands at the L point, but the number of inverted bands depends on the In x composition. Nevertheless the computed mirror Chern number is always nM=-2 for all In concentrations studied here. Pairs of topologically protected states on the (001) surface of Sn1-xInxTe with ±i mirror eigenvalues have been identified. The character of these topological states is affected by In dopant as compared to the parent topological crystalline insulator SnTe. As the In content x increases, the Dirac crossing point moves further away from the L point, and the Fermi velocity of the topological states increases significantly. Also we verify that a hole to electron carrier-like transition takes place at x=0.25 due to depopulation of the In-5s orbital. As Sn1-xInxTe is a superconductor, there is a co-existence of topological states protected by mirror symmetry on the surface with superconductivity in the bulk.

Electrical and thermal properties of strain- and electric field-induced topological crystalline insulators

This paper elaborates theoretically the strain and electric field effects on the electronic density of states (DOS) and electronic heat capacity (EHC) of the topological crystalline insulator SnTe (001) and related alloys. We employ Green’s function calculations for DOS and the Boltzmann approach for EHC. Schottky anomalies are found in EHC based on the entropy analysis. Strain- and electric field-induced DOS indicates the band gap opening in the system. Relative trends with respect to the strain modulus (electric field strength) show an increasing (a fluctuating) trend for the Schottky temperature. The results may lead to wide-ranging applications in thermoelectrics and tunable topological electronics/spintronics.

Self-organization of various “phase-separated” nanostructures in a single chemical vapor deposition

Chemical vapor deposition (CVD) is one of the most versatile techniques for the controlled synthesis of functional nanomaterials. When multiple precursors are induced, the CVD process often gives rise to the growth of doped or alloy compounds. In this work, we demonstrate the self-assembly of a variety of ‘phase-separated’ functional nanostructures from a single CVD in the presence of various precursors. In specific, with silicon substrate and powder of Mn and SnTe as precursors, we achieved self-organized nanostructures including Si/SiOx core-shell nanowire heterostructures both with and without embedded manganese silicide particles, Mn11Si19 nanowires, and SnTe nanoplates. The Si/SiOx core-shell nanowires embedded with manganese silicide particles were grown along the direction of the crystalline Si via an Au-catalyzed vapor-liquid-solid process, in which the Si and Mn vapors were supplied from the heated silicon substrates and Mn powder, respectively. In contrast, direct vapor-solid deposition led to particle-free -oriented Si/SiOx core-shell nanowires and -oriented Mn11Si19 nanowires, a promising thermoelectric material. No Sn or Te impurities were detected in these nanostructures down to the experimental limit. Topological crystalline insulator SnTe nanoplates with dominant {100} and {111} facets were found to be free of Mn (and Si) impurities, although nanoparticles and nanowires containing Mn were found in the vicinity of the nanoplates. While multiple-channel transport was observed in the SnTe nanoplates, it may not be related to the topological surface states due to surface oxidation. Finally, we carried out thermodynamic analysis and density functional theory calculations to understand the ‘phase-separation’ phenomenon and further discuss general approaches to grow phase-pure samples when the precursors contain residual impurities.

The strain and electric field modulation of magnetic properties in topological crystalline insulator thin films

In the present paper, the strain and the electric field effects on the Pauli spin paramagnetic susceptibility (PSPS) of topological crystalline insulator SnTe (0 0 1) thin films are studied, employing k→·p→ theory, the Green’s function approach, and the Boltzmann method. The intrinsic hybridization between the front and back surfaces induces a gap and shifts the crossover in PSPS. We demonstrate that the crossover intensity decreases with strain and electric field at a certain hybridization potential, while its position does not change (fluctuates) with the strain (electric field). Our results may provide useful information to design real magnetic devices.

Pristine and strained anisotropic group velocity and effective mass of surface Dirac fermions in the topological crystalline insulator SnTe

Abstract An analysis of the strain-induced group velocity(GV) and effective mass(EM) of massless topological Dirac fermions on the SnTe(001) surface, protected by crystalline symmetry, is presented. Using the traditional semi-classical k → ⋅ p → approach by associating fermions with wave-packets, we find that the expression for the energy dispersion surface as well as both GV and EM are anisotropic in the absence and presence of strain. The band structure calculations indicate that the massive Dirac fermions emerge for strained SnTe(001) surface. However, under strong strains, GVs and EMs of all branches of conduction and valence bands are the same quantitatively, but still anisotropic along different directions. On the other hand, depending on the special critical compressive and/or tensile strains, the direction of the fermionic waves is reversed. Further, the behavior of heavy and light masses are characterized qualitatively with strain modulus and direction. Our results are favorable for the thermoelectric properties of SnTe semiconductor since electronic features are coupled to the heat transports.

Thermodynamic properties of topological crystalline insulator SnTe (001) thin film in the presence of Zeeman magnetic field

Abstract Multiple surface Dirac cones in topological crystalline insulators motivate the researchers to tune the topological phase. In the present work, we theoretically study the longitudinal, transverse and in-plane Zeeman magnetic field effects on the electronic density of states (DOS), electronic heat capacity (EHC) and Pauli spin paramagnetic susceptibility (PSPS) [on behalf of the electrical, thermal and magnetical properties, respectively] of topological crystalline insulator SnTe (001) and related alloys thin films. In doing so, we implement the Dirac theory, Green’s function approach, and the Boltzmann method. First, we found that EHC and PSPS decrease with hybridization potential due to the hybridization-induced gap in DOS. Second, the Weyl cones appear [result from the van Hove singularities around the Fermi level] when the Zeeman field is greater than the hybridization potential. Third, the Zeeman splitting and shifting of bands are characterized by the van Hove singularities in the vicinity of the Fermi level and the degenerate states, the Schottky anomaly and the crossover in DOS, EHC and PSPS, respectively. Our findings show that the EHC (PSPS) increases (decreases) with Zeeman fields. The predicted results may be useful for designing nanoelectronic and spintronic TCI-based devices.