Session S15: Superconductivity in One and Two Dimensions I

Exploring Unconventional Superconductivity in 2M-WS2 thin layers

Superconductivity, characterized by zero electrical resistance, has been a central theme of condensed matter physics since its discovery. While conventional superconductivity can be understood through the BCS theory with electron-phonon coupling, deviations from this introduce the enigmatic field of unconventional superconductivity, offering potential breakthroughs in quantum technologies. Among transition metal dichalcogenides, 2M-WS2 is particularly notable, achieving the highest transition temperature (8.9 K) in this class. Both experimental results and theoretical predictions affirm the topological superconducting properties of bulk 2M-WS2. In this work, we explore deeply into the unconventional superconducting of thin 2M-WS2 flakes, emphasizing the important role of magnetic fields. Remarkably, when the flake thickness decreases below a certain threshold, the extracted in-plane upper critical field at zero temperature surpasses the Pauli paramagnetic limit. Our results underscore the existence of unconventional superconductivity in 2M-WS2 thin layers, illuminating the intricate connection between layer thickness and 2D superconducting characteristics of 2M-WS2.   

Probing the Superconducting Order Parameter Symmetry in Monolayer WTe2

Monolayer WTe2 exhibits a complex phase diagram, which includes the presence of superconductivity at low carrier densities adjacent to a quantum spin hall insulating phase. This provides a unique setting to explore the interplay between superconducting and topological insulating phases, where unconventional superconductivity is expected to emerge, potentially hosting p-wave or spin triplet pairing. Nevertheless, the symmetry of the superconducting order parameter remains unexplored. Here, we measure the complex conductivity of the complete phase diagram of WTe2 using microwave reflectometry measurements, allowing us to extract the temperature-dependent superfluid density while concurrently measuring the DC transport characteristics of the device. Our findings mark the initial steps toward understanding the pairing symmetry in WTe2 and provide insights into the quantum phase transition inherent to this material.   

Two-Fold Anisotropic Superconductivity in Bilayer Td-MoTe2

Non-centrosymmetric 2D superconductors offer an opportunity to explore superconducting behaviors with strong spin-orbit coupling. Among the non-centrosymmetric families, T_d-MoTe₂ is a representative material because of its rich phases. Notably, T_d-MoTe₂ is the first 2D materials that demonstrated a coupling between ferroelectricity and superconductivity, and this ferroelctric switching can be simply controlled by electrical gating. Here, we will present on the superconducting behavior in bilayer T_d-MoTe₂ under an applied magentic field along different directions in-plane, and under different displacement fields and doping densities. We find that bilayer T_d-MoTe₂ has a two-fold symmetric superconducting behavior as a function of in-plane magnetic field angle that maximizes along the a-axis, parallel to the mirror plane. Importantly, large violations of Pauli limiting are observed, and DFT calculation suggests the anisotropic superconductivity in bilayer MoTe₂ is likely driven by Ising-like spin-orbit coupling. In addition, the two-fold anisotropy is preserved in the entire superconducting region, even with the interaction of strong Rashba spin-orbit coupling, and we find that the two-fold symmetric superconductivity remains after the ferroelectric switching. Our findings generally agree with previously observed results in multilayer and monolayer T_d-MoTe₂ and the expected spin-orbit enhanced upper critical fields as found in DFT calculations.   

Dry-Patterning Chemically Sensitive Quantum Materials Using a Computer Numerical Control Router Machine

Accurately and consistently patterning quantum materials for electrical transport measurements is crucial for the field of quantum device manufacturing. Nevertheless, the most used technique, photolithography, poses a risk of degrading or even damaging chemically sensitive quantum materials during the fabrication process. In this work, we developed a dry-patterning approach for device fabrication, achieving lateral etching resolution as fine as 5 µm. The method harnesses the capabilities of a desktop computer numerical control (CNC) router machine to etch patterns into thin films, leaving behind the desired devices on the substrate. Using this technique, we have successfully produced Hall bars with conductive channel widths of less than 60 µm on films of superconducting single-layer FeSe/SrTiO₃ capped with approximately 20 layers of FeTe. Transport measurements have demonstrated a superconducting zero-resistance temperature of 9.6 K across several device geometries, including a sample-wide Van der Pauw (vdP). However, the highest onset temperature (T_onset) of 23.2 K is exhibited in the vdP configuration and gradually decreases as the Hall bar dimensions decrease, reaching 12.2 K for the smallest device produced. Our approach offers a time-efficient, cost-effective, and chemical-free strategy to fabricate devices for understanding heterogeneous quantum phenomena in quantum materials.   

Continuous tuning of spin-orbit coupled superconductivity in layered NbSe2

Ion intercalation has become a versatile and powerful tool to realize quantum phases. To fully explore the potential of lithium intercalation, we have successfully developed a lateral intercalation method via a solid state ion back-gate [1]. The substantially improved sample homogeneity is demonstrated in both low-temperature transport measurements and topographical imaging by in situ atomic force microscopy. We apply this technique to modulate superconductivity in NbSe₂ . In the few-layer case where Ising superconductivity prevails, lithium intercalation can further enhance the in-plane upper critical magnetic field by boosting the spin-splitting. In bulk-like NbSe₂, we observe a tentative Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state at intermediate lithium concentration and Ising pairing at higher doping. Our work helps provide a complete phase diagram of a spin-orbit coupled superconductor as a function of doping and layer separation.   

Ising superconductivity in bulk materials

Low-dimensional materials can display exceptional characteristics that diverge significantly from their larger-scale counterparts. For example, a single layer of NbSe₂ demonstrates Ising superconductivity, with its in-plane upper critical field surpassing the Pauli limit, a phenomenon absent in the bulk NbSe₂. However, it's worth noting that low-dimensional materials often suffer from instability and limited practicality in real-world applications. Through a series of experiments utilizing various techniques, we have discovered that bulk layered heterostructures composed of NbSe₂ mono- and bilayers sandwiched in-between spacer monolayers exhibit a two-dimensional band structure akin to a heavily doped NbSe₂ monolayer [1]. Notably, their in-plane upper critical fields exceed the Pauli limit by a factor of up to ten [2]. By combining first-principles calculations with experimental evidence, we have successfully derived precise band structure parameters, including the values for reduced interlayer coupling and significant spin-orbit splitting [3]. This quantitative analysis has provided us with insight into the fundamental underpinnings of Ising spin-orbit coupling in bulk superconductors.   

Investigation of the superconducting ground state of misfit layer transition metal dichalcogenide superconductor (PbS)1.13TaS2

Topological superconductors (TSC) such as P_x+iP_y chiral superconductors are predicted to be a hunting ground for Majorana zero modes. Although the experimental realization of such a system is scarce, a potential candidate is the transition metal dichalcogenide (TMDC) superconductor, 4Hb-TaS₂ . Misfit materials, close kin of TMDC, consist of TMDC layers sandwiched with rock salt monochalcogenides layers, namely (RQ)_1+x(TQ₂)_n (R = Pb, Bi, or Rare-Earth, Q = Chalcogene, T = Transition metal, and n = 1, 2) which exhibit an intergrowth structure. Due to the different symmetry of the RQ and TQ₂ layers, a misfit occurs along one crystallographic axis even if the other two directions are commensurate. The well-ordered natural heterostructure, significant charge reconstruction effects, along with strong spin-orbit coupling (soc) can lead to emergent properties distinct from those of individual layers, primarily making these families interesting candidates to explore. Here, we have studied the superconducting ground state of misfit layer material (PbS)_1.13TaS₂, which is an alternative stacking of PbS with TaS_2. This material shows superconductivity below 3.1 K, with a critical field exceeding the Pauli limit in the in-plane direction. The specific heat and upper critical field measurement suggest a two-gap superconductivity in this material. The ARPES data shows spin-polarized bands in the material with soc coupling strength 694 meVÅ. The weak interlayer coupling strength of 0.016 eV shows the highly two-dimensional nature of the electrons.   

Anisotropic Superconductivity in Atomically Thin (SnxIn1-x)Bi2Te4

Combining superconductivity with non-trivial band topology has been an emerging research area in recent years. It is especially appealing to identify material candidates in the van der Waals family of compounds where atomically thin films may allow the control of carrier density and disentanglement of bulk and topological states. In this talk, I will describe our progress in measuring and understanding the thickness-dependent electronic response of topological superconductor candidate (Sn_1-xIn_x)Bi₂Te₄. By decreasing the thickness from bulk to several layers we observe an increase in anisotropy of the superconducting state with high in-plane critical fields. Furthermore, I will discuss non-linear transport measurements above the critical temperature and their implications for the nature of superconductivity in this material.