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Inducing Strong Superconductivity in WTe2 by Proximity Effect Ce Huang, Awadhesh Narayan, Enze Zhang, Yanwen Liu, Xiao Yan, Jiaxiang Wang, Cheng Zhang, Weiyi Wang, Tong Zhou, Changjiang Yi, Shanshan Liu, Jiwei Ling, Huiqin Zhang, Ran Liu, Raman Sankar, Fang-Cheng Chou, Yihua Wang, Youguo Shi, Kam Tuen Law, Stefano Sanvito, Peng Zhou, Zheng Han, and Faxian Xiu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b03102 • Publication Date (Web): 14 Jun 2018 Downloaded from http://pubs.acs.org on June 14, 2018
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Inducing Strong Superconductivity in WTe2 by Proximity Effect Ce Huang1,2, Awadhesh Narayan3, Enze Zhang1,2, Yanwen Liu1,2, Xiao Yan5, Jiaxiang Wang1,2, Cheng Zhang1,2, Weiyi Wang1,2, Tong Zhou6, Changjiang Yi7, Shanshan Liu1,2, Jiwei Ling1,2, Huiqin Zhang1,2, Ran Liu1,2, Raman Sankar8,9, Fangcheng Chou9,10,11, Yihua Wang1,2, Youguo Shi7,12, Kam Tuen Law6, Stefano Sanvito4, Peng Zhou5*, Zheng Han13*, Faxian Xiu1,2,14* 1
State Key Laboratory of Surface Physics and Department of Physics, Fudan University, Shanghai 200433, China
2
Institute for Nanoelectronic Devices and Quantum Computing, Fudan University, Shanghai 200433, China
3
Materials Theory, ETH Zurich, Wolfgang-Pauli-Strasse 27, CH 8093 Zurich, Switzerland 4
School of Physics, AMBER and CRANN Institute, Trinity College, Dublin 2, Ireland State Key Laboratory of ASIC and System, Department of Microelectronics, Fudan University, Shanghai 200433, China 6 Department of Physics, The Hong Kong University of Science and Technology, Clear 5
Water Bay, Hong Kong, China 7 Institute of Physics and Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100190, China 8 Institute of Physics, Academia Sinica, Taipei 11529, Taiwan 9
Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan
10
National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan. Taiwan Consortium of Emergent Crystalline Materials, Ministry of Science and Technology, Taipei 10622, Taiwan. 12 School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100190, China 13 Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P. R. China. 14 Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, 11
China *
Correspondence and requests for materials should be addressed to F. X. (E-mail:
[email protected]), Z. H. (Email:
[email protected]), P. Z. (Email:
[email protected]). 1 / 27
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ABSTRACT The search for proximity-induced superconductivity in topological materials has generated widespread interest in the condensed matter physics community. The superconducting states inheriting nontrivial topology at interfaces are expected to exhibit exotic phenomena such as topological superconductivity and Majorana zero modes, which hold promise for applications in quantum computation. However, a practical realization of such hybrid structures based on topological semimetals and superconductors has hitherto been limited. Here, we report the strong proximity-induced superconductivity in type-II Weyl semimetal WTe2, in a van der Waals hybrid structure obtained by mechanically transferring NbSe2 onto various thicknesses of WTe2. When the WTe2 thickness ( ) reaches 21 nm, the superconducting transition occurs around the critical temperature ( ) of NbSe2 with a gap amplitude ( ) of 0.38 meV and an
unexpected ultra-long proximity length ( ) up to . With the thicker 42nm WTe2 layer, however, the proximity effect yields ~. , .
and a short of less than . Our theoretical calculations, based on the Bogoliubov-de Gennes equations in the clean limit, predict that the induced superconducting gap is a sizable fraction of the NbSe2 superconducting one when is less than 30 nm, and then decreases quickly as increases. This
agrees qualitatively well with the experiments. Such observations forms a basis in the search for superconducting phases in topological semimetals. KEYWORDS: (WTe2, Superconducting proximity effect, Andreev reflection, Bogoliubov-de Gennes equations, Topological semimetals) 2 / 27
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In recent years, the study of the materials with topological order or nontrivial band topology has been one of the most active fields in condensed matter physics.1-3 The topologically protected surface states represent two-dimensional Dirac particles on the surface of topological insulators (TIs) or open strings of surface states as Fermi arcs in the case of three-dimensional topological semimetals (TSMs). Different from TIs, TSMs possess isolated topologically protected points called Weyl nodes in the bulk spectrum.4-5 Their topological features have been confirmed through angle-resolved photoemission spectroscopy,6-9 scanning tunneling microscope10 and low-temperature transport.11-13 When these topological states become superconducting, exotic properties such as p-wave and nodal superconductivity,14 surface Majorana fermions15 and Weyl superconductivity16 have been predicted. In order to make these topological states superconducting, the key ingredient is a non-zero superconducting order parameter, which can be realized either by an intrinsic superconducting gap17 or through the superconducting proximity effect.18-19 For the former scenario, PtTe2 was demonstrated to have both superconductivity and type-II Dirac fermions, which turn the topological material into a new phase.20 High-pressure induced superconductivity was also found in Cd3As2 and WTe221-23 although the crystal lattices may be deformed, a feature that could jeopardize their topological properties. In contrast, the latter case of exploiting the proximity effect can be a more reliable approach to engineer topological superconducting systems by combining band-inverted and strongly spin-orbit coupled topological materials with robust superconductors. However, the experimental exploration and realization remain limited except for the study of point-contact24-26 and ion sputtering in which the superconducting proximity effect on NbAs is induced by Nb.27 Recently, the successful micromechanical exfoliation of two-dimensional (2D) layered materials28 makes it accessible to create 2D crystals by cleaving at the van der Waals (vdW) interface. The transfer of these 2D crystals onto other 2D materials enables the creation of high-quality hybrid structures without dangling bonds and lattice mismatch at the interface, which also prevents interface defects and inter-diffusion.29 Indeed, by transferring 2D superconducting materials such as NbSe2 and Bi2Sr2CaCu2O8+δ with this method one can, in practice, produce artificial superconductor-based
hybrid
structures
high-temperature superconductivity in Bi2Se3, 31-32
with 30
new
properties,
including
specular Andreev reflection (AR) in
33
graphene, vdW Josephson junctions, just to highlight the few. Type-II Weyl semimetal (WSM) WTe2 does not exhibit
intrinsic
34
superconductivity down to 0.3 K. Methods aimed at introducing superconductivity, such as applying high pressure,23, 35 may change the electronic band structure and destroy the topological Fermi arc states. Since WTe2 is a layered-material, artificial 3 / 27
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hybrid structure with superconducting layered materials can harvest high-quality interfacial superconductivity via proximity effect. Here, we present a transfer method to fabricate NbSe2/WTe2 hybrid structures to explore the proximity effect at the interface, which reveals a proximity-transfer of Andreev pairs (APs) into WTe2 accompanied by interface superconductivity. The transport properties show strong superconducting proximity effect in WTe2 with an ultra-long superconducting
proximity length approaching 7 μm. This proximity effect is found to strongly depend on the WTe2 thickness. Thin WTe2 films (< 30 nm) present a high transition temperature ( ), large superconducting gap (Δ ) and long proximity length (! ), while the thick ones exhibit a lower , smaller Δ and narrow superconducting
region around the interface. Our theoretical analysis explains the thickness-dependent proximity-induced superconducting gap and returns a qualitative agreement with the experimental observations. The demonstrated strong proximity effect in fabricated artificial hybrid structure will further help in the research of superconductivity in topological semimetals. RESULTS AND DISCUSSION High-quality bulk NbSe2 and WTe2 crystals were grown by chemical vapor transport (CVT) methods. NbSe2/WTe2 hybrid structures were fabricated using polyvinylalcohol (PVA)36 (see Methods for details). NbSe2 flakes were transferred onto WTe2 to form the hybrid heterostructures (Fig. 1a, device #01). The thickness of NbSe2 and WTe2 was measured by atomic force microscope (AFM) and verified through the optical contrast method. WTe2, shown in blue, is found to be around 7 nm thick (~10 layers). Standard e-beam lithography and magnetron sputtering were used
to fabricate the electric contacts with Ti(5nm)/Au(100nm) or Ag(120nm). The details of the transport measurement setup and the difference between the two types of metal electrodes are elaborated in Supporting Section 1 and 2 (see Fig. S1 and S2), respectively. We characterized the electronic transport properties across the junction and
measured the temperature-dependent resistance (R-T) curves at ≥ 1.9 K as shown in Fig. 1b-d. The thickness of the NbSe2 is larger than 50 nm. A current annealing method37 was employed to improve the interface of the NbSe2/WTe2 heterostructure prior to the low-temperature transport measurements. This, in general, results in a significant decrease of the contact resistance. The zero-field resistance of NbSe2
across electrode 1 and 2, denoted as ()* , drops suddenly at ) 7.2 K and the magnetic field-temperature phase diagram (H-T diagram) agrees well with the / Ginzburg-Landau (GL) model25 ,- . *
01
*2345 6-)7
9
61 − ), as shown in Fig. 1b and 9:
inset, where Φ- and 60) represent the flux quantum and the GL coherence 4 / 27
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/ length at 0 K , respectively. Here . * is the upper critical field along the perpendicular direction, and 60) is extracted to be 7 nm. We define the onset of
the resistance drop as the critical temperature (see Supporting Section 3). The resistance (JK in Fig. 1c consists of three components, including the resistance of
NbSe2, that of WTe2 and the interface contact resistance. At zero magnetic field, (JK first drops at ) 7.2 K which is due to the superconducting transition of NbSe2.
Interestingly, when cooling down to * 6.2 K, the resistance experiences another pronounced drop down to 5.4 K. The onset transition temperature for each step gradually decreases with the applied magnetic field and merges together at about 0.6 T. It is noteworthy that in WTe2 the normal state resistance increases as the magnetic field B rises, as a result of the positive magnetoresistance (MR) by perfect n-p compensation.38 Then, it is indispensable to carry out the R-T measurements in different WTe2 regions. The residual resistivity of WTe2 thin films shown in Fig. 1d is larger than its bulk counterpart presumably owing to the slight oxidization in air as reported previously for thin flakes22, 39 although we tried to minimize the exposure time of the device. Very strikingly, while being distant on the metallic WTe2 side, the
signatures of the superconductivity proximity effect induced resistance drop in (KM , (KN and (MN were resolved. These are also magnetic field dependent (see Supporting Fig. S1a for more details). 4-terminal measurement method can exclude the effect of current redistribution which is inevitable in 2-terminal method. Even though theoretically the possible current redistribution may change the current direction in hybrid structure, it should make strong effect on the superconducting temperature (7.2 K) of NbSe2. However, the transition in WTe2 are observed at 6.2 K which is different. The current flows from the top right corner electrode to the bottom left one as shown in Fig. 1a and the interface region is parallel to all electrodes, guaranteeing that the current flows following the path or direction from interface to electrode 4 and then arrives at electrode 5 area. We can exclude the possibility of current redistribution effect in this device. Besides, electrodes 5 and 7 are far away from interface, eliminating any current redistribution effect around the interface region. Additionally, we also measured the device with contacts traversing the entire width of the WTe2 flake in Fig. S4 (device #05) which shows the same long proximity effect. Moreover, we performed a series of contrast experiments to prove the proximity effect and will describe it in the later discussion section and the Supporting Section 10. The critical temperature at zero field for (KM , (KN and (MN is 6.2 K, which is the same as the
second transition of (JK . By looking at the distance- and field-dependent R-T curves in Fig. 1d and Supporting Fig. S3c, it is concluded that the WTe2 in-between the electrodes 4 and 5 becomes superconducting due to the interface proximity effect. At the same time, the resistance between the furthermost electrodes 7 and 6 retains a typical value for the normal state. Such experiments clearly demonstrate an ultra-long 5 / 27
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proximity length of over 6 μm away from the interface, which is beyond the expected value, as it will be discussed in the following sections. Several devices show
a similar proximity length reaching 6~7 μm (see Supporting Section 5 and 6). When a superconductor is placed in contact with a normal metal, APs will flow
through the normal metal with a proximity length that in the clean limit (4 mV), the difference of the differential conductance between the 6.5 K-spectrum and the
lower-temperature ones is the resistance drop of superconducting NbSe2 since it always remains superconducting in our applied bias range (see Supporting Fig. S18d). At zero bias, when decreasing the temperature from 6.5 to 2 K, initially the resistance quickly drops, then slowly increases, which is consistent with the R-T curve in Fig. 3b. A low-bias fit to the spectrum using the standard BTK theory42 further confirms the observation of AR (see Supporting Fig. S14). For a finite transparency, the ratio of the zero-bias to the high-bias resistances,
qr
qs qr
qs
, is given by43 *t)uv 7 w
≈ 6)u*v7 )7,
where Z is the barrier parameter. From the measurements x 0.62 can be extracted (x has a range of 0.67 − 0.69 by a systematic fitting, see Supporting Section 7.1 for details). A similar behavior has been observed in the field-dependent differential conductance spectra at 1.9 K, as shown in Fig. 4b. It is clear that at 2.5 T the DCPs degenerate into a BICP and then they are completely suppressed at 4.5 T, in good agreement with the critical field of the heterostructure shown in Fig. 3d. In order to provide a quantitative analysis of dI/dV, the half width of the temperature- and field-dependent DCPs are plotted in Fig. 4c. The half width of zero bias dip,
corresponding to the superconducting gap,44 is ~1.1 meV , comparable to the superconducting gap of NbSe2 flakes.45 Also, the temperature dependence of Δ- fits well to the BCS theory (red dashed line in Fig. 4c). Now we consider the observed ZBCP in the lower temperature regime (0.05 K-1.8 K). Figure 4d-e show the dI/dV spectra normalized to the 7 K data and stacked 9 / 27
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for convenience at different temperatures and magnetic fields. The ZBCP has been reported in various systems such as hybrid structures and single superconductors.1, 46 As an indirect tool to examine in-gap states, the ZBCP serves as an important signature for Majorana fermion and p-wave superconductivity.17, 46 Such a ZBCP is commonly observed for ab-plane tunneling in d-wave superconductors, where it is attributed to the zero-energy Andreev bound states.47 Here, the ZBCP is pronounced and becomes stronger and sharper at low temperatures; its emergence agrees well with the drop of the R-T curve below 1.2 K in Fig. 2b. Figure 4e illustrates the magnetic field dependence of the ZBCP, showing a similar trend with the temperature dependence; and the critical field is the same as that in Fig. 3e. This strongly suggests that the ZBCP is associated with the proximity-induced WTe2 superconducting states. The half width of ZBCP is plotted in Fig. 4f where its strong dependence on the magnetic field excludes the possibility of Majorana zero mode originating from topological superconductivity,1, 46 in which the magnetic field should not significantly affect the ZBCP. Nevertheless, the ZBCP arises due to the proximity-induced superconductivity in the WTe2 flakes. More excitingly, when the temperature decreases to 0.3 K, the ZBCP starts to split into two symmetric peaks. Meanwhile, the width of the splitting becomes wider and the dip turns to be sharper as the temperature decreases (Fig. 4d). The double peak structure gradually evolves into a single peak at 600 Oe (Fig. 4e), indicating that the splitting is associated with the proximity-induced superconducting states. Then,
two symmetric peaks at a larger bias around ±0.36mV emerge when < 1 K. Since WTe2 is anisotropic and has a Fermi surface with both two electron and hole pockets, two-gap or anisotropic s-wave superconductivity can induce such behavior.48 However, the large bias peak positions do not change much with temperature and are at the same location even upon 1500 Oe magnetic field, which contradicts with two-gap or anisotropic s-wave superconductor. At first glance, the behavior seems to be a re-entrance effect,49 which occurs in proximity-induced superconducting region at low temperature. It is expected for a clean NS (normal metal-superconductor) contact, which corresponds to the superconducting WTe2 and normal WTe2 in our
case. The minimum of the zero field resistance corresponds to Z[ - ≈ ℏ/}~ , where
}~
1.33 × 10)) s is the diffusion time50 (see Supporting Section 9.2),
giving rise to the critical temperature - 0.57 K consistent with the experimental observations (~0.4 K in Fig. 3e). At low temperatures, the dip appears in the
differential conductance at a finite voltage
- where49
- ≈ Z[ ; 0.4 K is the temperature for the resistance minimum in Fig. 3e. From there, we can extract
- 0.05 mV that is almost half of the dip width. Nevertheless, the re-entrance effect is mostly observed in the case that the interface resistance is less than the 10 / 27
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resistance of the normal metal, a fact that is contradictory to our experiments where
the WTe2 resistance is much smaller than ( . Therefore, we attribute the
double peaks and central dip structure to be the hallmark of AR between
superconducting WTe2 and normal WTe2 with a finite Z value of 0.55. Symmetric peaks with dip at low bias can be simulated well by the modified BTK theory in Fig.S15 and the proximity-induced superconducting gap in WTe2 is 0.07 meV at 0.05 K. The temperature dependent Δ is plotted in Fig. 4f. However, the BCS fitting (red
dashed line in Fig. 4f) shows a deviation at higher temperature with a concave curvature, indicating either possible unconventional superconductivity25 or intrinsic disorder at the interface. For a quantitative modelling of the proximity effect, we used a modified BTK theory to calculate the c-axis NbSe2/WTe2 conductance spectra.48, 51 The differential
conductance below , divided by the normal state conductance g h
O
as
is expressed
u - 6_ −
) d 6
) 6_)d_ 6
) d
O
where 6_ ) is the Fermi-Dirac distribution at temperature . Here 6_) represents the BTK conductance at 0 K, 1 + }O | 6_)|* + 6}O − 1)| 6_ )* |* 6 _ ) }O |1 + 6}O − 1) 6_ )* |* where }O is the transparency of the barrier in the BTK approximation of current injection totally perpendicular to the NbSe2/WTe2 interface: 1 }O 1 + x* 6_ ) is a complex function
where ¡¢ 6_ )
£
√£ 7 ¥7
6_)
and ¡ 6_ )
¡¢ 6_ ) − 1 ¡ 6_) ¥
√£ 7 ¥7
, whose real parts are the BCS
quasiparticle and pair density of states, respectively. From more than 5 devices with 5-30 nm thick WTe2, the strong superconducting proximity effect occurs near 6O¦§¨7) ( 6O¦§¨7) − 6©9¨7 ) ≤ 1K ) with a long
proximity length ! , while the thick WTe2 (>30 nm) displays a shorter one with a lower 6©9¨7 ) . The physical parameters from the thin and thick WTe2 are
summarized in Table I. Generally, by increasing the thickness to the bulk limit the proximity length and the critical temperature are reduced significantly. Along the
c-axis of the NbSe2/WTe2 stack, the tunneling APs becomes unstable for thick WTe2 11 / 27
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and needs lower temperature for a coherent superconducting phase which indicates a low 6©9¨7 ) in thick WTe2. To completely rule out any theoretically possible current redistribution either due to the injection profile or due to inhomogeneity in the flake, we have performed a series of experiments by changing WTe2 to be a thick Cd3As2
and a ZrTe5 which are topological semimetals. Two Nb/ Cd3As2 and two Nb/ZrTe5 devices are displayed in Figs. S20-23, all of four devices show two transitions in
(« . However, no obvious transitions or kinks are observed in Cd3As2 and ZrTe5 using the similar electrode configuration as that of WTe2, which excludes the current redistribution effect or any other measurement inaccuracy issues. One NbSe2/WTe2 hybrid structure illustrated in Fig. S24 shows a bad interface without superconducting transition in R ®7 , indicating that the transition in WTe2 is related to the interface quality and we attribute to proximity effect.
Moreover, thickness dependence of ΔS /Δ- is displayed in Fig. 5. For a superconductor/normal metal structure in the clean limit, when the WTe2 is
sufficiently thinner than the critical thickness of ¯ , the tunneling of Cooper pairs (CPs) opens a gap in the normal layer which constitutes a sizable fraction of the
NbSe2 superconducting gap.52 On the contrary, the thickness of WTe2 is larger than
¯ , AR gives rise to a nonzero pairing amplitude but does not induce a superconducting gap. For an ideal junction (no Fermi surface mismatch and interfacial barrier),52 ¯ °
