Showing 1–13 of 13 results for author: Sou, I K

In-situ Formation of Superconducting FeTe/layered-MnTe heterostructures

Authors: Zhihao He, Chen Ma, Jiannong Wang, Iam Keong Sou

Abstract: Manganese telluride (MnTe) has garnered strong interest recently for its antiferromagnetic semiconductor properties, which are promising for applications in spintronics, data storage, and quantum computing. In this study, we discovered that the deposition of FeTe at 300oC onto zinc-blende MnTe (ZB-MnTe) via molecular beam epitaxy (MBE) results in a phase transition from ZB-MnTe to a layered MnTe (… ▽ More Manganese telluride (MnTe) has garnered strong interest recently for its antiferromagnetic semiconductor properties, which are promising for applications in spintronics, data storage, and quantum computing. In this study, we discovered that the deposition of FeTe at 300oC onto zinc-blende MnTe (ZB-MnTe) via molecular beam epitaxy (MBE) results in a phase transition from ZB-MnTe to a layered MnTe (l-MnTe) phase with van der Waals (vdW) gaps, which is a previously unreported phase of MnTe. The l-MnTe phase was characterized using cross-sectional high-angle annular dark-field (HAADF) imaging, energy-dispersive X-ray spectroscopy (EDS) mapping, and X-ray photoelectron spectroscopy (XPS). The Fe/Te flux ratio during FeTe deposition was found to be critical to the phase transition, an increased Fe/Te flux ratio used for the FeTe growth leads to localized formation of layered Mn4Te3 (l-Mn4Te3), while a decreased Fe/Te flux ratio only generates a single monolayer of l-MnTe at the interface and the rest turns into a distorted ZB-MnTe (dZB-MnTe) phase. It was also found that FT-MT heterostructures grown at a lower substrate temperature of 250oC, as the Fe/Te flux ratio decreases, the ZB-MnTe layer was first transformed to dZB-MnTe and then to wurtzite MnTe (WZ-MnTe). The FeTe/l-MnTe heterostructure exhibits high-quality superconducting properties with a three-dimensional nature as demonstrated by its magneto-transport properties and there is evidence that l-MnTe seems to play a key role in inducing the observed superconductivity. Most importantly, this study reports the realization of layered structures of MnTe by an in-situ approach via chemical interactions, which might be further applied to generating unprecedented phases of materials under certain conditions. △ Less

Submitted 15 June, 2024; v1 submitted 13 April, 2024; originally announced April 2024.

Comments: 25 pages, 10 figures

Intra-Family Transformation of The Bi-Te Family via in-situ Chemical Interactions

Authors: Zhihao He, Tin Seng Manfred Ho, Rolf Lortz, Iam Keong Sou

Abstract: The Bi-Te binary system, characterized by the homologous series of the (Bi2)m(Bi2Te3)n, has always attracted research interest for its layered structures and potential in advanced materials applications. Despite Bi2Te3 has been extensively studied, exploration of other compounds has been constrained by synthesis challenges. This study reports the molecular beam epitaxy (MBE) growth of FeTe on Bi2T… ▽ More The Bi-Te binary system, characterized by the homologous series of the (Bi2)m(Bi2Te3)n, has always attracted research interest for its layered structures and potential in advanced materials applications. Despite Bi2Te3 has been extensively studied, exploration of other compounds has been constrained by synthesis challenges. This study reports the molecular beam epitaxy (MBE) growth of FeTe on Bi2Te3, demonstrating that varying growth conditions can turn the Bi2Te3 layer into different Bi-Te phases and form corresponding FeTe/Bi-Te heterostructures. Our combined analysis using reflection high-energy electron diffraction (RHEED), high-resolution X-ray diffraction (HRXRD), and high-resolution scanning transmission electron microscopy (HR-STEM), indicates that specific growth conditions used for the growth of the FeTe layer can facilitate the extraction of Te from Bi2Te3, leading to the formation of Bi4Te3 and Bi6Te3. Additionally, by lowering the FeTe growth temperature to 230 oC, Te extraction from the Bi2Te3 layer could be avoided, preserving the Bi2Te3 structure. Notably, all the three FeTe/Bi-Te structures exhibit superconductivity with the FeTe/Bi2Te3 heterostructure enjoying the highest superconductivity quality. These findings introduce a novel method for realizing Bi4Te3 and Bi6Te3 through Te extraction by growing FeTe on Bi2Te3, driven by the high reactivity between Fe and Te. This approach holds promise for synthesizing other members of the Bi-Te series, expanding the functional potential of these materials. △ Less

Submitted 7 June, 2024; v1 submitted 16 December, 2023; originally announced December 2023.

Comments: 25 pages, 10 figures

Interfacial superconductivity and zero bias peak in quasi-one-dimensional Bi2Te3/Fe1+yTe heterostructure nanostructures

Authors: Man Kit Cheng, Cheuk Yin Ng, Sui Lun Ho, Omargeldi Atanov, Wai Ting Tai, Jing Liang, Rolf Lortz, Iam Keong Sou

Abstract: Bi2Te3/Fe1+yTe heterostructures are known to exhibit interfacial superconductivity between two non-superconducting materials: Fe1+yTe as the parent compound of Fe-based superconducting materials and the topological insulator Bi2Te3. Here, we present a top-down approach starting from two-dimensional (2D) heterostructures to fabricate one-dimensional (1D) Bi2Te3/Fe1+yTe nanowires or narrow nanoribbo… ▽ More Bi2Te3/Fe1+yTe heterostructures are known to exhibit interfacial superconductivity between two non-superconducting materials: Fe1+yTe as the parent compound of Fe-based superconducting materials and the topological insulator Bi2Te3. Here, we present a top-down approach starting from two-dimensional (2D) heterostructures to fabricate one-dimensional (1D) Bi2Te3/Fe1+yTe nanowires or narrow nanoribbons. We demonstrate that the Bi2Te3/Fe1+yTe heterostructure remains intact in nanostructures of widths on the order of 100 nm and the interfacial superconductivity is preserved, as evidenced by electrical transport and Andreev reflection point contact spectroscopy experiments measured at the end of the nanowire. The differential conductance shows a similar superconducting twin-gap structure as in two-dimensional heterostructures, but with enhanced fluctuation effects due to the lower dimensionality. A zero-bias conductance peak indicates the presence of an Andreev bound state and given the involvement of the topological Bi2Te3 surface state, we discuss a possible topological nature of superconductivity with strong interplay with an emerging ferromagnetism due to the interstitial excess iron in the Fe1+yTe layer, developing in parallel with superconductivity at low temperatures. △ Less

Submitted 1 December, 2022; v1 submitted 8 August, 2022; originally announced August 2022.

Highly active hydrogen evolution facilitated by topological surface states on a Pd/SnTe metal/topological crystalline insulator heterostructure

Authors: Qing Qu, Bin Liu, Wing Sum Lau, Ding Pan, Iam Keong Sou

Abstract: Recently, topological quantum materials have emerged as a promising electrocatalyst for hydrogen evolution reaction (HER). However, most of their performance largely lags behind noble metals such as benchmark platinum (Pt). In this work, a Pd(20nm)/SnTe(70nm) heterostructure, fabricated by molecular beam epitaxy and electron beam evaporation, is found to display much higher electrocatalytic activi… ▽ More Recently, topological quantum materials have emerged as a promising electrocatalyst for hydrogen evolution reaction (HER). However, most of their performance largely lags behind noble metals such as benchmark platinum (Pt). In this work, a Pd(20nm)/SnTe(70nm) heterostructure, fabricated by molecular beam epitaxy and electron beam evaporation, is found to display much higher electrocatalytic activity than that of a pure Pd(20nm) thin film and even higher than that of a commercial Pt foil. This heterostructure adopts an extracted turnover frequency value more than two times higher than that of the Pd(20nm) thin film at a potential of 0.2 V, indicating a much higher intrinsic activity per Pd site. Density functional theory calculations show that the conventional d-band theory, which works well for many transition metal heterostructures, cannot explain the enhancement of electrocatalytic performance. Instead, we found that the topological surface states (TSSs) of the SnTe (001) underlayer play a key role; electrons transfer from both the Pd surface and the adsorbed H atoms to the TSSs of SnTe (001), resulting in weaker Pd-H binding strength and more favorable hydrogen adsorption free energies. Our work demonstrates for the first time that a metal/topological quantum material heterostructure could be a prominent catalyst to enjoy HER activity outperforming that of a commercial Pt foil and offers a promising direction to optimize the performance of electrocatalysts based on topological quantum materials. △ Less

Submitted 1 September, 2022; v1 submitted 9 December, 2021; originally announced December 2021.

Comments: 40 pages, 17 figures. arXiv admin note: text overlap with arXiv:2102.08062

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

Authors: Qing Qu, Bin Liu, Hongtao Liu, Jing Liang, Jiannong Wang, Ding Pan, Iam Keong Sou

Abstract: 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 funct… ▽ More 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. △ Less

Submitted 28 July, 2021; v1 submitted 16 February, 2021; originally announced February 2021.

Comments: 78 pages, 35 figures, 4 tables

Studies on the origin of the interfacial superconductivity of Sb2Te3/Fe1+yTe heterostructures

Authors: Jing Liang, Yu Jun Zhang, Xiong Yao, Hui Li, Zi-Xiang Li, Jiannong Wang, Yuanzhen Chen, Iam Keong Sou

Abstract: The recent discovery of the interfacial superconductivity (SC) of the Bi2Te3/Fe1+yTe heterostructure has attracted extensive studies due to its potential as a novel platform for trapping and controlling Majorana fermions. Here we present studies of another topological insulator (TI)/Fe1+yTe heterostructure, Sb2Te3/Fe1+yTe, which also enjoys an interfacial two-dimensional SC. The results of transpo… ▽ More The recent discovery of the interfacial superconductivity (SC) of the Bi2Te3/Fe1+yTe heterostructure has attracted extensive studies due to its potential as a novel platform for trapping and controlling Majorana fermions. Here we present studies of another topological insulator (TI)/Fe1+yTe heterostructure, Sb2Te3/Fe1+yTe, which also enjoys an interfacial two-dimensional SC. The results of transport measurements support that the reduction of excess Fe concentration of the Fe1+yTe layer not only increases the fluctuation of its antiferromagnetic (AFM) order but also enhances the quality of the SC of this heterostructure system. On the other hand, the interfacial SC of this heterostructure was found to have a wider-ranging TI-layer thickness dependence than that of the Bi2Te3/Fe1+yTe heterostructure, which is believed to be attributed to the much higher bulk conductivity of Sb2Te3 that enhances indirect coupling between its top and bottom topological surface states (TSSs). Our results provide the evidence of the interplay among the AFM order, itinerant carries from the TSSs and the induced interfacial SC of the TI/Fe1+yTe heterostructure system. △ Less

Submitted 26 August, 2019; v1 submitted 15 August, 2019; originally announced August 2019.

Comments: 28 pages, 5 figures

Molecular Beam Epitaxy Grown $Cr_2Te_3$ Thin Films with Tunable Curie Temperatures

Authors: Hongxi Li, Linjing Wang, Tao Yu, Liang Zhou, Yang Qiu, Hongtao He, Fei Ye, Iam Keong Sou, Gan Wang

Abstract: Materials with perpendicular magnetic anisotropy (PMA) effect with high Curie temperature ($T_C$) is essential in applications. In this work, $Cr_2Te_3$ thin films showing PMA with $T_C$ ranging from 165 K to 295 K were successfully grown on $Al_2O_3$ by the molecular beam epitaxy (MBE) technique. The structural analysis, magneto-transport and magnetic characterizations were conducted to study the… ▽ More Materials with perpendicular magnetic anisotropy (PMA) effect with high Curie temperature ($T_C$) is essential in applications. In this work, $Cr_2Te_3$ thin films showing PMA with $T_C$ ranging from 165 K to 295 K were successfully grown on $Al_2O_3$ by the molecular beam epitaxy (MBE) technique. The structural analysis, magneto-transport and magnetic characterizations were conducted to study the physical origin of the improved $T_C$. In particular, ferromagnetic (FM) and antiferromagnetic (AFM) ordering competition were investigated. A phenomenological model based on the coupling degree between FM and AFM ordering was proposed to explain the observed $T_C$ enhancement. Our findings indicate that the $T_C$ of $Cr_2Te_3$ thin film can be tuned, which make it hold the potential for various magnetic applications. △ Less

Submitted 23 December, 2019; v1 submitted 26 May, 2019; originally announced May 2019.

Interfacial superconductivity induced by single-quintuple-layer Bi$_2$Te$_3$ on top of FeTe forming van-der-Waals heterostructure

Authors: Hailang Qin, Bin Guo, Linjing Wang, Meng Zhang, Bochao Xu, Kaige Shi, Tianluo Pan, Liang Zhou, Yang Qiu, Bin Xi, Iam Keong Sou, Dapeng Yu, Wei-Qiang Chen, Hongtao He, Fei Ye, Jia-Wei Mei, Gan Wang

Abstract: We report the first clear observation of interfacial superconductivity on top of FeTe(FT) covered by one quintuple-layer Bi$_2$Te$_3$(BT) forming van-der-Waals heterojunction. Both transport and scanning tunneling spectroscopy measurements confirm the occurrence of superconductivity at a transition temperature T$_c$ = 13~K, when a single-quintuple-layer BT is deposited on the non-superconducting F… ▽ More We report the first clear observation of interfacial superconductivity on top of FeTe(FT) covered by one quintuple-layer Bi$_2$Te$_3$(BT) forming van-der-Waals heterojunction. Both transport and scanning tunneling spectroscopy measurements confirm the occurrence of superconductivity at a transition temperature T$_c$ = 13~K, when a single-quintuple-layer BT is deposited on the non-superconducting FT surface. The superconductivity gap decays exponentially with the thickness of BT, suggesting it occurs at the BT-FT interface and the proximity length is above 5-6~nm. We also measure the work function's dependence on the thickness of BT, implying a charge transfer may occur at the BT/FT interface to introduce hole doping into the FT layer, which may serve as a possible candidate for the superconducting mechanism. Our BT/FT heterojunction provides a clean system to study the unconventional interfacial superconductivity. △ Less

Submitted 15 March, 2019; originally announced March 2019.

Reinforcement of interfacial superconductivity in a Bi2Te3/Fe1+yTe heterostructure under hydrostatic pressure

Authors: Junying Shen, Claire Heuckeroth, Yuhang Deng, Qinglin He, Hong Chao Liu, Jing Liang, Jiannong Wang, Iam Keong Sou, James S. Schilling, Rolf Lortz

Abstract: We investigate the hydrostatic pressure dependence of interfacial superconductivity occurring at the atomically sharp interface between two non-superconducting materials: the topological insulator (TI) Bi2Te3 and the parent compound Fe1+yTe of the chalcogenide iron based superconductors. Under pressure, a significant increase in the superconducting transition temperature Tc is observed. We trace t… ▽ More We investigate the hydrostatic pressure dependence of interfacial superconductivity occurring at the atomically sharp interface between two non-superconducting materials: the topological insulator (TI) Bi2Te3 and the parent compound Fe1+yTe of the chalcogenide iron based superconductors. Under pressure, a significant increase in the superconducting transition temperature Tc is observed. We trace the pressure dependence of a superconducting twin gap structure by Andreev reflection point contact spectroscopy (PCARS), which shows that a large superconducting gap associated with the interfacial superconductivity increases along with Tc. A second smaller gap, which is attributed to proximity-induced superconductivity in the TI layer, increases first, but then reaches a maximum and appears to be gradually suppressed at higher pressure. We interpret our data in the context of a pressure-induced doping effect of the interface, in which charge is transferred from the TI layer to the interface and the interfacial superconductivity is enhanced. This demonstrates the important role of the TI in the interfacial superconductivity mechanism. △ Less

Submitted 5 June, 2017; v1 submitted 13 March, 2017; originally announced March 2017.

Robust two-dimensional superconductivity and vortex system in Bi2Te3/FeTe heterostructures

Authors: Hong-Chao Liu, Hui Li, Qing Lin He, Iam Keong Sou, Swee K. Goh, Jiannong Wang

Abstract: The discovery of two-dimensional superconductivity in Bi2Te3/FeTe heterostructure provides a new platform for the search of Majorana fermions in condensed matter systems. Since Majorana fermions are expected to reside at the core of the vortices, a close examination of the vortex dynamics in superconducting interface is of paramount importance. Here, we report the robustness of the interfacial sup… ▽ More The discovery of two-dimensional superconductivity in Bi2Te3/FeTe heterostructure provides a new platform for the search of Majorana fermions in condensed matter systems. Since Majorana fermions are expected to reside at the core of the vortices, a close examination of the vortex dynamics in superconducting interface is of paramount importance. Here, we report the robustness of the interfacial superconductivity and 2D vortex dynamics in four as-grown and aged Bi2Te3/FeTe heterostructure with different Bi2Te3 epilayer thickness (3, 5, 7, 14 nm). After two years' air exposure, superconductivity remains robust even when the thickness of Bi2Te3 epilayer is down to 3 nm. Meanwhile, a new feature at ~13 K is induced in the aged samples, and the high field studies reveal its relevance to superconductivity. The resistance of all as-grown and aged heterostructures, just below the superconducting transition temperature follows the Arrhenius relation, indicating the thermally activated flux flow behavior at the interface of Bi2Te3 and FeTe. Moreover, the activation energy exhibits a logarithmic dependence on the magnetic field, providing a compelling evidence for the 2D vortex dynamics in this novel system. The weak disorder associated with aging-induced Te vacancies is possibly responsible for these observed phenomena. △ Less

Submitted 19 May, 2016; originally announced May 2016.

Comments: 21 pages, article+supplemental information, published in Scientific Reports

Journal ref: Sci. Rep. 6, 26168 (2016)

Tunable interaction-induced localization of surface electrons in antidot nanostructured Bi2Te3 thin films

Authors: Hong-Chao Liu, Hai-Zhou Lu, Hong-Tao He, Baikui Li, Shi-Guang Liu, Qing Lin He, Gan Wang, Iam Keong Sou, Shun-Qing Shen, Jiannong Wang

Abstract: Recently, a logarithmic decrease of conductivity has been observed in topological insulators at low temperatures, implying a tendency of localization of surface electrons. Here, we report quantum transport experiments on the topological insulator Bi2Te3 thin films with arrayed antidot nanostructures. With increasing density of the antidots, a systematic decrease is observed in the slope of the log… ▽ More Recently, a logarithmic decrease of conductivity has been observed in topological insulators at low temperatures, implying a tendency of localization of surface electrons. Here, we report quantum transport experiments on the topological insulator Bi2Te3 thin films with arrayed antidot nanostructures. With increasing density of the antidots, a systematic decrease is observed in the slope of the logarithmic temperature-dependent conductivity curves, indicating the electron-electron interaction can be tuned by the antidots. Meanwhile, the weak anti-localization effect revealed in magnetoconductivity exhibits an enhanced dominance of electron-electron interaction among decoherence mechanisms. The observation can be understood from an antidot-induced reduction of the effective dielectric constant, which controls the interactions between the surface electrons. Our results clarify the indispensable role of the electron-electron interaction in the localization of surface electrons and indicate the localization of surface electrons in an interacting topological insulator. △ Less

Submitted 8 October, 2014; originally announced October 2014.

Comments: 27 pages, article+supplemental materials, published in ACS Nano

Journal ref: ACS Nano 8, 9616-9621 (2014)

Large gap, a pseudogap and proximity effect in the Bi2Te3/Fe1+yTe interfacial superconductor

Authors: M. Q. He, Q. L. He, J. Y. Shen, H. C. Liu, Y. Zheng, C. H. Wong, Q. H. Chen, J. N. Wang, K. T. Law, I. K. Sou, A. P. Petrovic, R. Lortz

Abstract: We report directional point-contact spectroscopy data on the novel Bi2Te3/Fe1+yTe interfacial superconductor for a Bi2Te3 thickness of 9 quintuple layers, bonded by van der Waals epitaxy to a Fe1+yTe film at an atomically sharp interface. Our data show a very large superconducting twin-gap structure with an energy scale exceeding that of bulk FeSe or FeSe1-xTex by a factor of 4. While the larger g… ▽ More We report directional point-contact spectroscopy data on the novel Bi2Te3/Fe1+yTe interfacial superconductor for a Bi2Te3 thickness of 9 quintuple layers, bonded by van der Waals epitaxy to a Fe1+yTe film at an atomically sharp interface. Our data show a very large superconducting twin-gap structure with an energy scale exceeding that of bulk FeSe or FeSe1-xTex by a factor of 4. While the larger gap is isotropic and attributed to a thin FeTe layer in proximity of the interface, the smaller gap has a pronounced anisotropy and is associated with proximity-induced superconductivity in the topological insulator Bi2Te3. Zero resistance is lost above 8 K, but superconducting fluctuations are visible up to at least 12 K and the large gap is replaced by a pseudogap that persists up to 40 K. The spectra show a pronounced zero-bias conductance peak in the superconducting state, which may be a signature of an unconventional pairing mechanism. △ Less

Submitted 29 October, 2014; v1 submitted 22 July, 2014; originally announced July 2014.

Two-dimensional superconductivity at the interface of a Bi2Te3/FeTe heterostructure

Authors: Qing Lin He, Hongchao Liu, Mingquan He, Ying Hoi Lai, Hongtao He, Gan Wang, Kam Tuen Law, Rolf Lortz, Jiannong Wang, Iam Keong Sou

Abstract: Superconductivity at the interface of a heterostructure confined to nanometer-sized scale offers unique opportunities to study the exotic physics of two-dimensional superconductivity. The realization of superconductivity at the interface between a topological insulator and an iron-chalcogenide compound is highly attractive for exploring several recent theoretical predictions involving these two ne… ▽ More Superconductivity at the interface of a heterostructure confined to nanometer-sized scale offers unique opportunities to study the exotic physics of two-dimensional superconductivity. The realization of superconductivity at the interface between a topological insulator and an iron-chalcogenide compound is highly attractive for exploring several recent theoretical predictions involving these two new classes of materials. Here, we report transport measurements on a Bi2Te3/FeTe heterostructure fabricated via van der Waals epitaxy, which demonstrate superconductivity at the interface induced by the Bi2Te3 epilayer with thickness even down to one quintuple layer. The two-dimensional nature of the observed superconductivity with the highest transition temperature around 12 K was verified by the existence of a Berezinsky-Kosterlitz-Thouless transition and the diverging ratio of in-plane to out-plane upper critical field on approaching the superconducting transition temperature. With the combination of interface superconductivity and Dirac surface states of Bi2Te3, the heterostructure studied in this work provides a novel platform for realizing Majorana fermions. △ Less

Submitted 21 April, 2014; v1 submitted 24 September, 2013; originally announced September 2013.

Comments: 24 pages, 5 figures and supplementary information

Journal ref: Nature Communications 5, 4247 (2014)