Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session E05: Topological Superconductivity: TMD, Quantum WellsFocus
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Sponsoring Units: DMP Chair: Vidya Madhavan, Univ of Illinois - Urbana Room: BCEC 108 |
Tuesday, March 5, 2019 8:00AM - 8:36AM |
E05.00001: Quantum spin Hall state in monolayer 1T’-TMDCs Invited Speaker: Shujie Tang Quantum spin Hall (QSH) insulator, also known as 2D topological insulator (TI), hosts quantized helical edge state, which is a superior candidate to construct artificial 1D superconductor utilizing the superconductivity proximity effect. Among the rapidly developing 2D TI family, group VI transition metal dichalcogenides (TMDCs) in 1T’ structural phases are unique in their van der Waals layered structure, which can be easily integrated into vertical heterostructures. Bottom-up synthesis of monolayer 1T’-TMDCs (WTe2, WSe2 and MoTe2) on graphitized SiC substrate were achieved using molecular beam epitaxy. Together with theoretical calculation and ARPES measurement, the characteristic signatures of QSH states such as band inversion and bulk gap opening have been confirmed in 1T’-WTe2 and WSe2 [1, 2]. Scanning tunneling spectroscopy (STS) measurements further provide evidence for the robust edge states reside in the bulk gap. It is also found that, although conduction and valence band degeneracy is lifted in 1T’-MoTe2, the spin-orbital coupling is not strong enough to further separate them, leaving 1T’-MoTe2 a semimetal [3]. |
Tuesday, March 5, 2019 8:36AM - 8:48AM |
E05.00002: Point-contact spectroscopic study of Weyl semimetals MoTe2 and WTe2 Yurii Naidyuk, Dima L. Bashlakov, Oksana E. Kvitnitskaya, Saicharan Aswartham, Igor V. Morozov, Ivan Chernyavskii, Grigory Shipunov, Günter Fuchs, Stefan-Lüdwig Drechsler, Dr.Ruben Hühne, Kornelius Nielsch, Bernd Büchner, Dmitri Efremov The Andreev-reflection (AR) double-minimum structure was observed in dV/dI curves of “hard” MoTe2/Ag point contacts (PCs) with enhanced. Tc up to 5 K. The extracted superconducting (SC) gap Δ has a BCS-like dependence with 2Δ/kBTc =3.7+/-0.4 averaged over 9 PCs. Remarkably, the observation of a single zero-bias minimum in dV/dI of “soft” PCs (i.e. using silver paint instead of Ag wire) may indicate a topological “gapless-like” SC state of the MoTe2 surface, since “soft” contacts probe mainly the interface and avoid pressure effect. The Yanson point-contact (PC) electron-phonon interaction (EPI) spectra of MoTe2 demonstrate only a broad maximum around 18 meV. Contrary, dV/dI curves of WTe2 PCs show tiny SC features if at all, but display an EPI structure with maxima at 8 and 16 meV. Thus, Yanson PC spectra demonstrate the presence of solely phononic excitations for both MoTe2 and WTe2, while AR spectra show a “gapless-like” SC state only in MoTe2 (at least above 2.5K). Interestingly. For both compounds a broad maximum in dV/dI at large voltages (>100mV) was observed indicating some phase transition. The latter might be induced by the high current density in the PC and/or local heating, thus enabling the manipulation of the quantum electronic states at the interface in the PC core. |
Tuesday, March 5, 2019 8:48AM - 9:00AM |
E05.00003: Nontrivial superconductivity signatures in topological MoTe2−xSx crystals Yanan Li, Qiangqiang Gu, Chen Chen, Nitin Samarth, Tong Zhang, Ji Feng, Jian Wang Topological Weyl semimetals (TWSs) and topological Dirac semimetals have broadened the classification of topological phases and provide new platforms for studying topological superconductivity. Here, we report a systematic study of superconductivity in sulfur-doped Td-phase MoTe2 with enhanced Tc compared with type-II TWS MoTe2. We found that Td-phase S-doped MoTe2 (MoTe2−xSx, x ∼ 0.2) is a two-band s-wave bulk superconductor (superconducting gap∼0.13 meV and 0.26 meV), where the superconducting behavior can be explained by the s+− pairing model. We also used scanning tunneling spectroscopy to detect a relatively large surface superconducting gap (∼1.7 meV), suggestive of topologically nontrivial superconductivity based on the pairing of Fermi arc surface states. Indeed, a comparison of the quasi-particle interference patterns with band-structure calculations indicates Fermi arcs in MoTe2−xSx. Our results demonstrates that the Td-phase MoTe2−xSx might be a promising topological superconductor candidate and also a unique material for studying s+− superconductivity. |
Tuesday, March 5, 2019 9:00AM - 9:12AM |
E05.00004: Observation of Superconductivity in Few-layers WTe2 Induced by Normal Contacts* Artem Kononov, Gulibusitan Abulizi, Andreas Baumgartner, Kejian Qu, Jiaqiang Yan, David George Mandrus, Kenji Watanabe, Takashi Taniguchi, Christian Schonenberger WTe2 is a material possessing topological properties both as a bulk crystal and single layer. In the former case, it is a type-II Weyl semimetal and in the latter a quantum spin Hall state. Resent reports showing the possibility to induce superconductivity in WTe2 by proximity to a bulk superconductor or by electrostatic gating make it a promising platform for topological superconductivity and quantum computation. Here we report on a novel mechanism of inducing superconductivity within few-layers of WTe2 by normal metal contacts. In transport measurements we find that the induced superconductivity is highly anisotropic in magnetic field and survives up to 7 Tesla in in-plane field. Superconducting transport persists over µm distances at temperatures up to 1 Kelvin. Our results could provide insight to the superconducting state in WTe2 and its topological nature. |
Tuesday, March 5, 2019 9:12AM - 9:24AM |
E05.00005: Proximity-induced superconductivity with sub-gap anomaly in type-II Weyl semimetal WTe2 Qiao Li Tungsten ditelluride (WTe2), as one of material candidates in the family of type-II Weyl semimetals, exhibits many exotic properties, such as anisotropic chiral anomaly. Due to the intrinsic topological band structure and tilted Weyl cone in WTe2, it has been predicted that unconventional superconducting properties will emerge when WTe2 enters to superconducting phase. In this work, we report the observation of superconductivity in the type-II Weyl semimetal WTe2 based on WTe2/NbSe2 heterostructure induced by proximity effect. We find a long coherence length along c axis of WTe2. Furthermore, we observe anomalous oscillations of the differential resistance spectrum during the transition from superconducting to normal state. Theoretical calculations indicate that the staged change in the differential resistance spectrum can be associated with anomalous density of states in superconducting state and reveals that such a sub-gap anomaly is the intrinsic property of WTe2 in superconducting state induced by the proximity effect. Our findings can enrich the understanding of superconductivity mechanism in type-II Weyl semimetal and pave the way for their future applications in topological quantum computing. |
Tuesday, March 5, 2019 9:24AM - 9:36AM |
E05.00006: Quantum transport in MoTe2 and WTe2 topological monolayers Xirui Wang, Kenji Yasuda, Yafang Yang, Sanfeng Wu, Takehito Suzuki, Lin Zhou, Jing Kong, Joseph Checkelsky, Pablo Jarillo-Herrero There has been growing interest in the study of 2D topological quantum states. Recent experiments established monolayer WTe2 as a 2D topological insulator, which also becomes a superconductor via gating. This has triggered a lot of excitement in studying a wide range of topological properties of monolayer WTe2 and its cousins, such as MoTe2. We will report our investigations on the quantum transport behavior of monolayer WTe2 and MoTe2 in nano-fabricated devices, in the direction of searching for topological superconductivity. |
Tuesday, March 5, 2019 9:36AM - 9:48AM |
E05.00007: Procedural Determination of Novel Stoichiometric Topological Superconductors through Surface and Pressure Effects Abhishek Allamsetty Topological superconductors have attracted great interest as a groundbreaking platform to allow for fault-tolerant quantum computing and overcome the limitations of classical processors defined by Moore’s law. In this paper, we discover eight novel topological superconductors, ABX2 (A=Bi, In; B=Nb, Ta; X=Se, S) and propose a new systematic method to realize topological superconductors by combining of transition metal dichalcogenides with spin-orbit post-transitional elements. These eight compounds are bulk superconductors with intrinsic topological surface states and can be realized without any tampering, a coveted quality for topological superconductors. Utilizing density functional theoretical calculations, we determined topological surface states and bulk superconductivity from the electronic band structures of our compounds. In addition, we demonstrated the robust nature of our materials under high-pressure conditions, as their non-trivial topology and superconductivity were maintained. Our eight novel compounds hold great promise towards the practical realization of fault-tolerant quantum computers through their ability to eliminate the longstanding problem of quantum decoherence within quantum computational systems. |
Tuesday, March 5, 2019 9:48AM - 10:00AM |
E05.00008: Enhancing Superconductivity in Indium Arsenide Quantum Well Heterostructures Andrew Saydjari, Michael Kosowsky, Andrew T Pierce, Joseph Yuan, Kaushini Wickramasinghe, Javad Shabani, Amir Yacoby Hybrid superconductor-semiconductor heterostructures containing indium arsenide quantum wells combine strong spin-orbit effects, s-wave superconductivity, and arbitrary lithographic confinement. This platform facilitates both study of induced superconductivity in the quantum well and topological states in quasi-one-dimensional systems. While epitaxial aluminum provides transparent contact to the quantum well, aluminum cannot sustain large magnetic fields (>1 T) required to explore many topological effects. Further, the small (150 µeV) superconducting gap of aluminum can complicate detection and inhibit manipulation of states required for applications to topological quantum computing. In this work, we present a method of coupling niobium-based superconductors to indium arsenide quantum wells to supplement the critical magnetic field and superconducting gap in this platform. Experimental progress on fabrication of superconductor-semiconductor-superconductor (SNS) junctions and quantum point contacts will be presented. |
Tuesday, March 5, 2019 10:00AM - 10:12AM |
E05.00009: Multi-terminal Josephson effect in epitaxial InAs/Al superconductor/semiconductor devices I: current-biased measurements Hanho Lee, Natalia Pankratova, Roman Kuzmin, Kaushini Wickramasinghe, Alex Levchenko, Maxim Vavilov, Javad Shabani, Vladimir Manucharyan Junctions of more than two topological superconductors are typically required for implementing braiding operations on Majorana fermions [1]. Here we report the first realization of 3- and 4-terminal Josephson junctions fabricated from a Majorana-compatible hybrid InAs/Al epitaxially-grown heterostructure [2]. We observed several novel phenomena: interaction of intersecting supercurrents, multi-terminal Fraunhofer effect in a magnetic field, and out-of-equilibrium multiple Andreev reflections (MAR) at bias voltages far exceeding the gap of Al. The data in a large number of channels regime can be modeled using random matrix circuit theory of superconducting transport. Progress towards realizing the phase-control of zero-energy quasiparticle states, predicted in such structures in the absence of Zeeman fields [3,4,5], will be discussed. |
Tuesday, March 5, 2019 10:12AM - 10:24AM |
E05.00010: Multi-terminal Josephson effect in epitaxial InAs/Al superconductor/semiconductor devices II: phase-biased measurements Natalia Pankratova, Hanho Lee, Roman Kuzmin, Kaushini Wickramasinghe, Alex Levchenko, Maxim Vavilov, Javad Shabani, Vladimir Manucharyan Junctions of more than two topological superconductors are typically required for implementing braiding operations on Majorana fermions [1]. Here we report the first realization of 3- and 4-terminal Josephson junctions fabricated from a Majorana-compatible hybrid InAs/Al epitaxially-grown heterostructure [2]. We observed several novel phenomena: interaction of intersecting supercurrents, multi-terminal Fraunhofer effect in a magnetic field, and out-of-equilibrium multiple Andreev reflections (MAR) at bias voltages far exceeding the gap of Al. The data in a large number of channels regime can be modeled using random matrix circuit theory of superconducting transport. Progress towards realizing the phase-control of zero-energy quasiparticle states, predicted in such structures in the absence of Zeeman fields [3,4,5], will be discussed. |
Tuesday, March 5, 2019 10:24AM - 10:36AM |
E05.00011: Probing Spin-Orbit Coupling in InAs/Al SQUIDs William Andrew Mayer, SiChao Yu, Kaushini Wickramasinghe, Joseph Yuan, Narayan Mohanta, Alex Matos Abiague, Igor Zutic, Javad Shabani InAs/Al epitaxial heterostructures allow for transparent semiconductor/superconductor interfaces to study superconducting proximity effect, particularly in Josephson junctions. A superconducting quantum interference device (SQUID) constructed from such junctions opens the possibility of studying current-phase relations in these materials systems. One distinct property of this system is the gate-tunability where each junction can be depleted, effectively turning off the current in that junction. This allows for detailed analysis of the supercurrent properties not previously possible in a single device. The second important feature of this system is the presence of spin-orbit coupling in InAs, whose strength is gate-tunable. We study the SQUID signal as we tune the spin-orbit coupling in each arm using independent gate voltages. The comparison between experimental data and theoretical simulation suggests that spin-orbit coupling can be detected in SQUID signal. This direct probe of spin-orbit coupling in proximitized systems represents an important tool to study systems which potentially host Majorana fermions. |
Tuesday, March 5, 2019 10:36AM - 10:48AM |
E05.00012: Induced superconductivity in InSb 2DEGs Chung-Ting Ke, Christian M. Möhle, Folkert De Vries, Candice Thomas, Sara Metti, Charles R Guinn, Ray Kallaher, Geoffrey C. Gardner, Michael Manfra, Srijit Goswami Superconductor-semiconductor hybrid systems are promising candidates to create Majorana zero modes (MZMs). Using a semiconductor two-dimensional electron gas (2DEG) could allow one to create complex networks required for Majorana qubit schemes. In this regard, InSb 2DEGs stand out due to their large g-factor, strong spin-orbit coupling, and low disorder. We integrate such high-quality 2DEGs with NbTiN to create Josephson junctions (JJs), thus providing the first demonstration of induced superconductivity in InSb 2DEGs. Remarkably, the supercurrent persists over several microns and shows distinct signatures of ballistic transport. Applying an in-plane magnetic field produces a revival of the supercurrent due to a Zeeman induced 0-π transition, where the transition field scales linearly with the junction length. Moreover, we show that this transition can be tuned in-situ using gate voltages, which allows us to map out the free energy landscape of the JJ as a function of density and magnetic field. Our experiments are consistent with the expected behavior of 0-π transitions in ballistic JJs and are an important step toward creating robust MZMs in 2DEG-based π-junctions. |
Tuesday, March 5, 2019 10:48AM - 11:00AM |
E05.00013: Transport phenomena in epitaxial Al / InAs heterostructures with strong spin-orbit coupling in the presence of in-plane magnetic fields Michael Kosowsky, Andrew Saydjari, Andrew T Pierce, Hechen Ren, Kaushini Wickramasinghe, Joseph Yuan, Javad Shabani, Amir Yacoby Two-dimensional electron gas (2DEG) systems with strong spin-orbit coupling (SOC) and induced s-wave superconductivity are predicted to realize topological superconductivity when in the presence of in-plane magnetic fields. A major obstacle in studying these systems arises due to the tendency of magnetic fields to suppress superconductivity. By growing epitaxial aluminum on top of an InAs quantum well heterostructure, it is possible to create a high-transparency interface between the superconductor and the 2DEG. This interface results in induced superconductivity that is comparable with the true aluminum superconducting gap, thereby sustaining larger in-plane fields than achieved by ex situ aluminum deposition. Here, we discuss the devices that we have fabricated to explore this exciting system, as well as present initial millikelvin measurements studying Josephson phenomena in the presence of varying in-plane and perpendicular magnetic fields. |
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