Evidence For Topological Surface States Research Articles

Anisotropic topological surface states in thin-film monoclinic Ag2Se

Silver chalcogenide systems have recently attracted significant attention due to their promising topological insulating properties. Here we conducted systematic low-temperature local scanning tunneling microscopy/spectroscopy and first-principles studies on the surface states of monoclinic Ag2Se thin films grown using molecular beam epitaxy. Through the use of quasi-particle interference patterns, we have observed evidence for topological surface states on the selenium-terminated surfaces with different types of defect densities. The results of ab initio calculations confirm the existence of nontrivial topological surface states in the monoclinic Ag2Se structure, for which such properties have not been previously reported. The energy dispersion determined using voltage-dependent standing wave patterns suggests that these topological states have an anisotropic Dirac cone structure. This discovery may lead to new applications for monoclinic Ag2Se in the rapidly growing fields of nanoelectronics and spintronics.

Evidence of topological surface states in dypresium monopnictides compounds

Recently, the rock salt structures of rare earth monopnictides have attracted a lot of attention as reference materials to show Dirac-like surface states and high carrier mobility which are very important for fundamental study and technological applications. Using density functional theory and maximally localized Wannier functions, the electronic and topological properties of dypresium monopnictides DyΥ (Υ = P,As,Sb) are explored and discussed. The existing band inversion along the Γ−X direction of the Brillouin zone as well as the calculation of the Z2 topological invariant corresponding to the three compounds reveal their non-trivial topological character. Our calculations also show the occurrence of a highly anisotropic Dirac cone to the surface spectrum of these compounds at bulk X-points projected onto the surface. The present results can predict the emergence of several promising properties in dypresium monopnictides, which constitute a potential class of realistic materials exhibiting topological surface states.

1D topological systems for next-generation electronics

1D topological systems for next-generation electronics

2D weak anti-localization in thin films of the topological semimetal Pd_{3}Bi_{2}S_{2}

Pd_{3}Bi_{2}S_{2} (PBS) is a recently proposed topological semimetal candidate. However, evidence for topological surface states have not yet been revealed in transport measurements due to the large mobility of bulk carriers. We report the growth and magneto-transport studies of PBS thin films where the mobility of the bulk carriers is reduced by two orders of magnitude, revealing for the first time, contributions from the 2-dimensional (2D) topological surface states in the observation of the 2D weak anti-localization (WAL) effect in magnetic field and angle dependent conductivity measurements. The magnetotransport data is analysed within the 2D Hikami-Larkin-Nagaoka (HLN) theory. The analysis suggests that multiple conduction channels contribute to the transport. It is also found that the temperature dependence of the dephasing length can’t be explained only by electron-electron scattering and that electron-phonon scattering also contributes to the phase relaxation mechanism in PBS films.

Experimental Evidence of Topological Surface States in Mg3Bi2 Films Grown by Molecular Beam Epitaxy**Supported by the Science Challenge Project under Grant No TZ2016004, the Opening Foundation of State Key Laboratory of High Performance Computing under Grant No 201601-02, the Foundation of President of CAEP under Grant No 201501040, the Natural Science Foundation of Hunan Province under Grant No 2016JJ1021, the

Nodal line semimetal (NLS) is a new quantum state hosting one-dimensional closed loops formed by the crossing of two bands. The so-called type-II NLS means that these two crossing bands have the same sign in their slopes along the radial direction of the loop, which requires that the crossing bands are either right-tilted or left-tilted at the same time. According to the theoretical prediction, Mg3Bi2 is an ideal candidate for studying the type-II NLS by tuning its spin-orbit coupling (SOC). High-quality Mg3Bi2 films are grown by molecular beam epitaxy (MBE). By in-situ angle resolved photoemission spectroscopy (ARPES), a pair of surface resonance bands around the point are clearly seen. This shows that Mg3Bi2 films grown by MBE are Mg(1)-terminated by comparing the ARPES spectra with the first principles calculations results. Moreover, the temperature dependent weak anti-localization effect in Mg3Bi2 films is observed under magneto-transport measurements, which shows clear two-dimensional (2D) e–e scattering characteristics by fitting with the Hikami–Larkin–Nagaoka model. Therefore, by combining with ARPES, magneto-transport measurements and the first principles calculations, this work proves that Mg3Bi2 is a semimetal with topological surface states. This paves the way for Mg3Bi2 to be used as an ideal material platform to study the exotic features of type-II nodal line semimetals and the topological phase transition by tuning its SOC.

Experimental evidence for topological surface states wrapping around a bulk SnTe crystal

We demonstrate that the metallic topological surface states wrap on all sides the 3D topological crystalline insulator SnTe. This is achieved by studying oscillatory quantum magneto-transport and magnetization at tilted magnetic fields which enables us to observe simultaneous contributions from neighbouring sample sides. Taking into account pinning of the Fermi energy by the SnTe reservoir we successfully describe theoretically the de Haas-van Alphen oscillations of magnetization. The determined \pi-Berry phase of surface states confirms their Dirac fermion character. We independently observe oscillatory contributions of magneto-transport and magnetization originating from the bulk SnTe reservoir of high hole density. It is concluded that the bulk and surface Landau states exist in parallel. Our main result that the bulk reservoir is surrounded on all sides by the topological surface states has an universal character.

Bulk and in-gap states in SmB6 revealed by high-field magnetotransport

We report on temperature-dependent magnetotransport experiments of ${\mathrm{SmB}}_{6}$ single crystals in high magnetic fields up to 33 T. Above the low-temperature plateau region in the zero-field resistivity, we find two distinct gapped regimes. The first is characterized by a temperature-independent gap ${\mathrm{\ensuremath{\Delta}}}_{2}$ that closes in the presence of a high magnetic field while the second regime, above $\ensuremath{\sim}8\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, features an energy gap ${\mathrm{\ensuremath{\Delta}}}_{1}$ that is temperature dependent. In the entire temperature range, we observe an overall negative magnetoresistance that does not depend on the orientation of the crystalline plane with respect to the applied magnetic field with a maximum value at $T\ensuremath{\sim}6.2\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The regime of in-gap states is characterized by a low mobility, an anomalously high carrier concentration, and an enhanced nonlocal resistance that exceeds the expected classical value for a two-dimensional metallic conductor. Overall, our data can be interpreted in terms of a transition from bulk to in-gap dominated transport, but evidence of topological surface states is not observed.

Multiple Dirac cones at the surface of the topological metal LaBi

The rare-earth monopnictide LaBi exhibits exotic magneto-transport properties, including an extremely large and anisotropic magnetoresistance. Experimental evidence for topological surface states is still missing although band inversions have been postulated to induce a topological phase in LaBi. In this work, we have revealed the existence of surface states of LaBi through the observation of three Dirac cones: two coexist at the corners and one appears at the centre of the Brillouin zone, by employing angle-resolved photoemission spectroscopy in conjunction with ab initio calculations. The odd number of surface Dirac cones is a direct consequence of the odd number of band inversions in the bulk band structure, thereby proving that LaBi is a topological, compensated semimetal, which is equivalent to a time-reversal invariant topological insulator. Our findings provide insight into the topological surface states of LaBi’s semi-metallicity and related magneto-transport properties.

Scanning tunneling microscopy study of the possible topological surface states in BiTeCl

Recently, the non-centrosymmetric bismuth tellurohalides such as BiTeCl are being studied as possible candidates for topological insulators. While some photoemission studies showed that BiTeCl is an inversion asymmetric topological insulator, others showed that it is a normal semiconductor with Rashba splitting. Meanwhile, first-principle calculations have failed to confirm the existence of topological surface states in BiTeCl so far. Therefore, the topological nature of BiTeCl requires further investigation. Here we report a low-temperature scanning tunneling microscopy study on the surface states of BiTeCl single crystals. On the tellurium (Te) -terminated surfaces with relatively low defect density, evidence for topological surface states is observed in the quasi-particle interference patterns, both in the anisotropy of the scattering vectors and the fast decay of the interference near the step edges. Meanwhile, on the samples with much higher defect densities, we observed surface states that behave differently. Our results may help to resolve the current controversy on the topological nature of BiTeCl.

Surface-state-dominated transport in crystals of the topological crystalline insulator In-dopedPb1−xSnxTe

Three-dimensional topological insulators and topological crystalline insulators represent new quantum states of matter, which are predicted to have insulating bulk states and spin-momentum-locked gapless surface states. Experimentally, it has proven difficult to achieve the high bulk resistivity that would allow surface states to dominate the transport properties over a substantial temperature range. Here we report a series of indium-doped Pb$_{1-x}$Sn$_x$Te compounds that manifest huge bulk resistivities together with strong evidence of topological surface states, based on thickness-dependent transport studies and magnetoresistance measurements. For these bulk-insulating materials, the surface states determine the resistivity for temperatures approaching 30 K.

Evidence for topological surface states in metallic single crystals of Bi2Te3

Bi2Te3 is a member of a new class of materials known as topological insulators which are supposed to be insulating in the interior and conducting on the surface. However, experimental verification of the conductive qualities of the surface states has been hindered by parallel bulk conductions. We report low temperature magnetotransport measurements on single crystal samples of Bi2Te3. We observe metallic character in our samples and large and linear magnetoresistance from 1.5 K to 290 K with prominent Shubnikov–de Haas (SdH) oscillations whose traces persist up to 20 K. Even though our samples are metallic, we are able to obtain a Berry phase close to the value of π, which is expected for Dirac fermions of the topological surface states. This indicates that we have obtained evidence for the topological surface states in metallic single crystals of Bi2Te3. Other physical measurements obtained from the analysis of the SdH oscillations are also in close agreement with those reported for the topological surface states. The linear magnetoresistance observed in our sample, which is considered as a signature of the Dirac fermions of the surface states, lends further credence to the existence of topological surface states.

Evidence for topological surface states in epitaxial Bi2Se3 thin film grown by pulsed laser deposition through magneto-transport measurements

We report epitaxial growth via domain matching epitaxy of Bi2Se3 thin films on Al2O3 (0001) substrates with over 13% lattice misfit and critical thickness less than one monolayer. X-ray and electron diffraction patterns confirm that the layers are epitaxial with (0001) Bi2Se3 || (0001) Al2O3 and [21¯1¯0] Bi2Se3 || [21¯1¯0] Al2O3 (or) [21¯1¯0] Bi2Se3 || [112¯0] Al2O3 without the presence of an interfacial pseudomorphic layer. X-ray photoemission spectroscopy reveals that the films are Se-deficient, in agreement with electrical transport data showing n-type carriers and metallic behavior. Magneto-resistance (MR) measurements show a cusp feature corresponding to weak antilocalization and linear-MR shows a non-saturating trend up to 9T. These results suggest topological surface states in PLD-grown Bi2Se3 films.

Spin–orbit coupling and weak antilocalization in the thermoelectric material -K2Bi8Se13

We have studied the effect of spin–orbital coupling (SOC) on the electronic transport properties of the thermoelectric material -K2Bi8Se13 via magnetoresistance measurements. We found that the strong SOC in this material results in the weak antilocalization (WAL) effect, which can be described well by a three-dimensional weak localization model. The phase coherence length extracted from theoretical fitting exhibits a power-law temperature dependence, with an exponent around 2.1, indicating that the electron dephasing is governed by electron–transverse phonon interactions. As in topological insulators, the WAL effect in -K2Bi8Se13 can be quenched by magnetic impurities (Mn) but is robust against non-magnetic impurities (Te). Although our magnetotransport studies provide no evidence for topological surface states, our analyses suggest that SOC plays an important role in determining the thermoelectric properties of -K2Bi8Se13.

Experimental evidences of topological surface states of β-Ag2Te

We present evidence of topological surface states in β-Ag2Te through first-principles calculations, periodic quantum interference effect and ambipolar electric field effect in single crystalline nanoribbon. Our first-principles calculations show that β-Ag2Te is a topological insulator with a gapless Dirac cone with strong anisotropy. To experimentally probe the topological surface state, we synthesized high quality β-Ag2Te nanoribbons and performed electron transport measurements. The coexistence of pronounced Aharonov-Bohm oscillations and weak Altshuler-Aronov-Spivak oscillations clearly demonstrates coherent electron transport around the perimeter of β-Ag2Te nanoribbon and therefore the existence of topological surface states, which is further supported by the ambipolar electric field effect for devices fabricated by β-Ag2Te nanoribbons. The experimental evidences of topological surface states and the theoretically predicted anisotropic Dirac cone of β-Ag2Te suggest that the material may be a promising candidate of topological insulator for fundamental study and future spintronic devices.

Aharonov–Bohm interference in topological insulator nanoribbons

The existence of topological conducting surfaces on insulators has been demonstrated by angular photoemission spectroscopy, but the number of transport experiments on these systems have so far been scarce. Transport evidence of topological surface states is now shown in Bi2Se3 nanoribbons through the observation of Aharonov–Bohm oscillations. Topological insulators represent unusual phases of quantum matter with an insulating bulk gap and gapless edges or surface states. The two-dimensional topological insulator phase was predicted in HgTe quantum wells1 and confirmed by transport measurements2. Recently, Bi2Se3 and related materials have been proposed as three-dimensional topological insulators with a single Dirac cone on the surface3,4, protected by time-reversal symmetry5,6,7. The topological surface states have been observed by angle-resolved photoemission spectroscopy experiments4,8. However, few transport measurements9 in this context have been reported, presumably owing to the predominance of bulk carriers from crystal defects or thermal excitations10. Here we show unambiguous transport evidence of topological surface states through periodic quantum interference effects in layered single-crystalline Bi2Se3 nanoribbons, which have larger surface-to-volume ratios than bulk materials and can therefore manifest surface effects. Pronounced Aharonov–Bohm oscillations11 in the magnetoresistance clearly demonstrate the coherent propagation of two-dimensional electrons around the perimeter of the nanoribbon surface, as expected from the topological nature of the surface states. The dominance of the primary h/e oscillation, where h is Planck’s constant and e is the electron charge, and its temperature dependence demonstrate the robustness of these states. Our results suggest that topological insulator nanoribbons afford promising materials for future spintronic devices at room temperature12.