Josephson current signatures of Majorana flat bands on the surface of time-reversal-invariant Weyl and Dirac semimetals

Chen, Anffany; Pikulin, Dmitry I.; Franz, Marcel

doi:10.1103/physrevb.95.174505

Cited by 24 publications

(23 citation statements)

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“…The induced pairing in these topological superconducting surface states can give rise to topologically protected gapless Andreev bound states, i.e., Majorana zero modes. Second, to our knowledge, Cd 3 As 2 is the only topological semimetal system where the evidence of Majorana flat bands [31] have been observed [28]. This provides high confidence that localized Majorana zero modes can also be realized in Cd 3 As 2 .…”

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confidence: 59%

“…Below, we try to explain the data by considering two alternative mechanisms for ZBCP formation: (1) the existence of MZMs at the interface between a superconductor and a quantum spin Hall insulator stack/Dirac semimetal [31,[36][37][38] and (2) a supercurrent mechanism that may give rise to the observed large ZBCP [25]. Our detailed theoretical modeling will show that the latter origin is the one most likely to apply.…”

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confidence: 99%

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Zero-bias conductance peak in Dirac semimetal-superconductor devices

Haenel

Rodriguez

et al. 2020

Phys. Rev. Research

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Majorana zero modes (MZMs), fundamental building blocks for realizing topological quantum computers, can appear at the interface between a superconductor and a topological material. One of the experimental signatures that has been widely pursued to confirm the existence of MZMs is the observation of a large, quantized zero-bias conductance peak (ZBCP) in the differential conductance measurements. In this Letter, we report observation of such a large ZBCP in junction structures of normal metal (titanium/gold Ti/Au)-Dirac semimetal (cadmiumarsenide Cd 3 As 2)-conventional superconductor (aluminum Al), with a value close to four times that of the normal state conductance. Our detailed analyses suggest that this large ZBCP is most likely not caused by MZMs. We attribute the ZBCP, instead, to the existence of a supercurrent between two far-separated superconducting Al electrodes, which shows up as a zero-bias peak because of the circuitry and thermal fluctuations of the supercurrent phase, a mechanism conceived by Ivanchenko and Zil'berman more than 50 years ago [Ivanchenko and Zilberman, JETP 28, 1272 (1969)]. Our results thus call for extreme caution when assigning the origin of a large ZBCP to MZMs in a multiterminal semiconductor or topological insulator/semimetal setup. We thus provide criteria for identifying when the ZBCP is definitely not caused by an MZM. Furthermore, we present several remarkable experimental results of a supercurrent effect occurring over unusually long distances and clean perfect Andreev reflection features.

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confidence: 59%

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confidence: 99%

Zero-bias conductance peak in Dirac semimetal-superconductor devices

Haenel

Rodriguez

et al. 2020

Phys. Rev. Research

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“…Due to their anomalous electromagnetic responses 7–14 , as well as the interesting interplay between bulk and surface Fermi arcs 15,16 , topological semimetals have been studied intensively in recent years 5 . In parallel to the rapid exploration of Dirac properties in topological semimetals, the study of their superconducting states has become an important topic aiming for possible unconventional superconductivity 17–20 . It was predicted that Fermi-arc states in a Dirac semimetal such as Cd 3 As 2 could be used to realize Majorana fermions 20,21 .…”

Section: Introductionmentioning

confidence: 99%

“…In parallel to the rapid exploration of Dirac properties in topological semimetals, the study of their superconducting states has become an important topic aiming for possible unconventional superconductivity 17–20 . It was predicted that Fermi-arc states in a Dirac semimetal such as Cd 3 As 2 could be used to realize Majorana fermions 20,21 . This is due to the fact that the Fermi-arc states in Cd 3 As 2 stem from helical edge states of quantum spin Hall insulators embedded in the time-reversal invariant semimetal 22 .…”

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confidence: 99%

Proximity-induced surface superconductivity in Dirac semimetal Cd3As2

et al. 2019

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Cd 3 As 2 is a three-dimensional Dirac semimetal with separated Dirac points in momentum space. In spite of extensive transport and spectroscopic studies on its exotic properties, the evidence of superconductivity in its surface states remains elusive. Here, we report the observation of proximity-induced surface superconductivity in Nb/Cd 3 As 2 hybrid structures. Our four-terminal transport measurement identifies a pronounced proximity-induced pairing gap (gap size comparable to Nb) on the surfaces, which exhibits a flat conductance plateau in differential conductance spectra, consistent with our theoretical simulations. The surface supercurrent from Nb/Cd 3 As 2 /Nb junctions is also achieved with a Fraunhofer/SQUID-like pattern under out-of-plane/in-plane magnetic fields, respectively. The resultant mapping shows a predominant distribution on the top and bottom surfaces as the bulk carriers are depleted, which can be regarded as a higher dimensional analog of edge supercurrent in two-dimensional quantum spin Hall insulators. Our study provides the evidence of surface superconductivity in Dirac semimetals.

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“…By similar methods, it is feasible to also investigate Josephson junctions based on WSMs. Previous theoretical work [47][48][49][50][51] instead focused on either the inversion-symmetric case or the surface states where chirality is no longer a good quantum number [42,52].In this Letter, we focus on Josephson junctions to study the interplay of Zeeman fields, s-wave superconductivity, and chirality in inversion-asymmetric WSMs. We find that the Weyl nodes of opposite chirality display distinct spin textures.…”

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Chirality Josephson Current Due to a Novel Quantum Anomaly in Inversion-Asymmetric Weyl Semimetals

2018

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We study Josephson junctions based on inversion-asymmetric but time-reversal symmetric Weyl semimetals under the influence of Zeeman fields. We find that, due to distinct spin textures, the Weyl nodes of opposite chirality respond differently to an external magnetic field. Remarkably, a Zeeman field perpendicular to the junction direction results in a phase shift of opposite sign in the current-phase relations of opposite chirality. This leads to a finite chirality Josephson current (CJC) even in the absence of a phase difference across the junction. This feature could allow for applications in chiralitytronics. In the long junction and zero temperature limit, the CJC embodies a novel quantum anomaly of Goldstone bosons at π phase difference which is associated with a Z2 symmetry at low energies. It can be detected experimentally via an anomalous Fraunhofer pattern.Introduction.-Weyl semimetals (WSMs) have recently attracted intensive interest thanks to their realization in a number of materials [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] and to their novel physics associated with Weyl nodes [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33]. The Weyl nodes appear in pairs that carry opposite chirality [34] in the absence of time-reversal or inversion symmetry. Chirality is thus a defining emergent property of electrons in WSMs. The possibility to probe and manipulate chirality is one of the most intriguing aspects of WSMs. Recently, chirality-dependent physics has been discussed in various contexts [35][36][37].Josephson junctions provide a complementary method to probe the electronic properties of a particular material. They are the basic building blocks for superconducting electronics with applications ranging from electronic magnetometers to quantum computation [38][39][40][41]. Hence, it is of fundamental interest to study Josephson junctions based on WSMs. To date, most experimentally relevant WSMs preserve time-reversal symmetry but break inversion symmetry [5][6][7][8][9][10][11][12][13][14][15][16]. In these materials, s-wave superconductivity couples electrons of the same chirality [42,43]. Thus, chirality remains a well-defined property in those Josephson junctions. Hence, we could think about using chirality as a resource for electronics, just as spin in spintronics. We coin this idea chiralitytronics. Recently, Josephson junctions based on Dirac semimetals have been fabricated [44][45][46]. By similar methods, it is feasible to also investigate Josephson junctions based on WSMs. Previous theoretical work [47][48][49][50][51] instead focused on either the inversion-symmetric case or the surface states where chirality is no longer a good quantum number [42,52].In this Letter, we focus on Josephson junctions to study the interplay of Zeeman fields, s-wave superconductivity, and chirality in inversion-asymmetric WSMs. We find that the Weyl nodes of opposite chirality display distinct spin textures. Thus, they respond differently to an external magnetic field. As a result, a ...

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Josephson current signatures of Majorana flat bands on the surface of time-reversal-invariant Weyl and Dirac semimetals

Cited by 24 publications

References 60 publications

Zero-bias conductance peak in Dirac semimetal-superconductor devices

Zero-bias conductance peak in Dirac semimetal-superconductor devices

Proximity-induced surface superconductivity in Dirac semimetal Cd3As2

Chirality Josephson Current Due to a Novel Quantum Anomaly in Inversion-Asymmetric Weyl Semimetals

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