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Proximity-Induced Superconductivity with Subgap Anomaly in Type II Weyl Semi-Metal WTe2

Due to the non-trivial topological band structure in type-II Weyl semimetal Tungsten ditelluride (WTe2), unconventional properties may emerge in its superconducting phase. While realizing intrinsic superconductivity has been challenging in the type-II Weyl semimetal WTe2, proximity effect may open an avenue for the realization of superconductivity. Here, we report the observation of proximity-induced superconductivity with a long coherence length along c axis in WTe2 thin flakes based on a WTe2/NbSe2 van der Waals heterostructure. Interestingly, we also observe anomalous oscillations of the differential resistance during the transition from superconducting to normal state. Theoretical calculations show excellent agreement with experimental results, revealing that such a sub-gap anomaly is the intrinsic property of WTe2 in superconducting state induced by the proximity effect. Our findings enrich the understanding of superconducting phase of type-II Weyl semimetals, and pave the way for their future applications in topological quantum computing.

Comparing to Type-I Weyl semimetals with a closed point-like Fermi surface, type-II Weyl semimetals possess unique features such as titled Weyl cone-like energy spectrum and paired Weyl nodes formed at the boundary of electron and hole pockets [27][28][29][30][31] , which lead to planar orientation-dependent chiral anomaly 32 . More interestingly, a type-II Weyl semimetal is expected to exhibit distinctive superconducting properties from type-I Weyl semimetals due to the tilted Weyl cone energy spectrum 33 . As a typical type-II Weyl semimetal 27 , Tungsten ditelluride (WTe2) possesses distorted structure of Td phase (space group Pnm21) which breaks the inversion symmetry and accommodates the existence of type-II Weyl points in film and topological edge state in monolayer [34][35][36][37][38][39] . With the unique topological bands, WTe2 is expected to offer a platform to study topological superconductivity. However, the intrinsic superconductivity in exfoliated WTe2 thin film has not yet been observed 40 .
In this work, we report proximity effect induced superconductivity in thin WTe2 flakes. The observed superconductivity is found to give rise to a long superconducting coherence length (above 30 nm) in WTe2 along its c axis. In addition, we observe anomalous oscillations in the differential resistance spectra as the bias current drives the system from superconducting to normal state. Moreover, our theoretical calculations show that the anomaly could be associated with the sub-gap features in the density of states (DOS) spectra in superconducting WTe2. Our work enriches the 4 understanding of superconducting phase in type-II Weyl semimetals and paves the way towards further exploration of topological superconducting states.
To realize proximity-induced superconductivity, a high-quality interface between thin WTe2 flake and superconductor is desirable. To that end, we chose layered niobium diselenide (NbSe2) as the superconductor (with Tc ~ 7.2 K and ∆ ~ 1.2 meV 41 ) and fabricated WTe2/NbSe2 van der Waals heterostructure via dry-transfer method (see Methods for details) 42 . To avoid device degradation, both NbSe2 and WTe2 samples were exfoliated and transferred onto pre-prepared electrodes in glovebox. The whole heterostructure was further covered by a hexagonal Boron Nitride (h-BN) flake to avoid undesirable oxidation (as shown in Figure 1b). In the fabrication process, we selected the thin WTe2 flakes with thickness of around 10 nm, to make sure that the flake was thick enough to exhibit band structures of type-II Weyl semimetal and thin enough to facilitate superconducting proximity effect. Besides, thin NbSe2 flake (around 20 nm) with smaller area than WTe2 sample was chosen to avoid NbSe2 contacting with electrodes.
We first examined the low temperature transport properties of thin WTe2 flakes by using a four-probe lock-in setup (with an AC current of 1 μA) (see Figure 1b). As shown in Figure 1c, we observe that the resistance of a typical device sharply drops to zero at a Tc (critical temperature) of 6.4 K at zero magnetic field, thus the superconducting phase in WTe2 is achieved, which has been verified in a different device setup (see the part I in Supporting Information). As the applied perpendicular magnetic field increases, the critical temperature Tc reduces until the superconductivity is completely suppressed at around 5 T. The magnetoresistance curves at four different temperatures (T=1.57, 3, 5 and 7 K) are shown in Figure 1d. As temperature arises, the critical magnetic field becomes smaller. When the temperature increases to 7 K, the superconductivity disappears. A feature of weak anti-localization effect in the nonlinear magnetoresistance curve was then observed, similar to what has been reported in the normal resistance state of WTe2 43 . Note that the proximity effect induced superconductivity in this work is related to the intrinsic band structure of WTe2 as a 5 type-II Weyl semimetal, different from high pressure-induced superconductivity in the 1T' phase WTe2 [44][45][46][47] and superconductivity in the epitaxial WTe2 thin film on a sapphire substrate 48 .
The achievement of superconductivity in the 10 nm-thick sample implies a long superconducting coherence length along c axis in WTe2. Based on the measured mobility along a axis ~2100 cm 2 /V −1 s −1 (see Fig.S6 in Supporting Information), the mobility along c axis is estimated to be around 210 cm 2 /V −1 s −1 since it is one order magnitude smaller than that along a axis 49 52,53 ) and is the temperature (6.4 K). is estimated to be above 30 nm, which results from the intrinsic band structure of superconducting WTe2 as a type-II Weyl semimetal, and will be discussed later.
To further investigate the proximity-induced superconducting properties in WTe2, we performed differential resistance measurements (dV/dI vs. V) at various temperatures, with typical results shown in Figure 2a. Interestingly, we observe an anomalous feature in the differential resistance spectra when the temperature is below Tc. In the differential resistance spectrum, besides two remarkable symmetric peaks appearing as expected at around 280 μV, several small peaks emerge at lower voltage biases. Similar differential resistance spectrum has been observed in other samples (see implies how WTe2 resistance changes as the bias current drives WTe2 from superconducting state to normal state. As temperature rises, these small peaks shrink in 6 both amplitude and bias gradually, and eventually vanish together with two main peaks after reaching Tc (6.4 K). The I-V characteristics at different temperatures can be obtained after integrating dV/dI-I curves, with results shown in Figure 2b. With increasing temperature, the superconductivity is completely suppressed at Tc~6.4 K, which agrees well with the differential resistance spectra. At 1.6 K, zero-resistance state occurs at a critical current ~125 μA ( is defined as the current at which the voltage reduces to zero), and superconductivity in WTe2 is suppressed at ~220 μA. Moreover, the measured (around 125 μA at ~2.5 Ω , with defined as the sample resistance above Tc) of the device is one order of magnitude smaller than the (1.8 mA at ~1 Ω) of NbSe2 and this has been observed in other measured devices (see Table.S1 and Fig.S5 in the Supporting Information). Furthermore, the nonlinear variation of I-V curve between superconducting and normal state is slowly-varying, which is quite different from the abrupt change in NbSe2.
To shed light on the observed anomaly during the transition from superconducting to normal state, we further investigated the magnetic field dependence of the differential resistance spectra and I-V characteristics at T=1.6 K (See Figure 2c and 2d).
Exceptionally, we find that the two main peaks become broad as B increases within and the dissipative component of the total driving current will be enhanced, leading to an enhancement of the differential resistance. We emphasize that the voltage is 8 measured under a bias current in our case, which is different from the usual case in which current is measured in response to a bias voltage.
To further prove that the unusual differential resistance spectra of superconducting WTe2 thin flake root in the unique DOS of WTe2, we carried out both theoretical and experimental studies on graphene/NbSe2 device with 10 nm thick graphene flake (the device structure is shown in the inset of Figure 4b). We first calculated the DOS of the graphene and NbSe2, with results shown in Figure 4a. The DOS of graphene resembles that of superconducting NbSe2 but differs from that of superconducting WTe2. We then measured the resistance of the graphene thin flake in a typical graphene/NbSe2 device at various temperatures. As shown in Figure 4b, with decreasing temperature, superconductivity can be also achieved in graphene at a transition temperature Tc of around 6.3 K, close to that of NbSe2. In contrast to the magnetoresistance results in WTe2, the normal resistance of graphene thin flake remains almost constant over the applied magnetic field range, which is consistent with the suppression of weak localization in graphene thin flake 54 . We further measured the differential resistance spectra at various temperatures and magnetic fields (see Figure 4c and Figure 4d). As expected, only two neat peaks are present in the scanning current range, without any sub-peaks emerging. When increasing temperature or magnetic field, the two peaks behave similarly to that of WTe2.
Compared with the properties of superconducting graphene, the DOS of WTe2 in the superconducting state is indeed unique and shows consistency with the experimental results. The trivial dV/dI spectra of graphene without any sub-peaks would result in one-step increase in resistance when driving superconducting to normal state. Such usual behavior indicates that Cooper pairs are broken into quasiparticles in an abrupt manner, different from multiple-step process occurring in the superconducting WTe2 thin flake. Combining all the evidence above, we believe that the nontrivial signature in the dV/dI spectra of WTe2 is unique and roots in its intrinsic band structure. Note that the sub-gap features in WTe2 may occur in another type-II Weyl semimetal MoTe2 55 , thus this should stimulate further exploration of the properties of type-II Weyl semimetals in superconducting phase.
In summary, we achieve superconductivity for the first time in type-II Weyl semimetal WTe2 by proximity effect based on a WTe2/NbSe2 heterostructure. The superconducting transition occurs at 6.4 K in WTe2, close to Tc of NbSe2. Most importantly, we discover anomalous oscillations of dV/dI spectra in superconducting WTe2. Furthermore, our theoretical calculations confirm that such peculiar feature in the dV/dI spectra in superconducting WTe2 results from its nontrivial DOS spectrum.
This work suggests that superconductivity in type-II Weyl semimetals exhibits distinctive feature from conventional superconductors and will stimulate further interest in exploration of topological superconductivity.
We grew the high-quality NbSe2 single crystals with a standard chemical vapor The devices were measured in an Oxford cryostat with a base temperature of about 1.6 K. The resistance signals were collected using a low-frequency Lock-in amplifier.
The differential resistance signals were collected by applying an AC excitation current of 1 nA added on top of a variable DC bias current.

Supporting information
The

Notes
The authors declare no competing financial interests.