Session F57: Superconductivity In Monolayers

Ultrafast Dynamics in Normal and CDW States of 2H-NbSe2

We have performed femtosecond pump-probe reflectivity study of the carrier and lattice dynamics using near-IR pump and white broadband probe at temperatures below and above the charge density wave transition in single crystal NbSe₂. The temperature dependence of the frequency of the coherent damped oscillations show a second order softening when approaching T_CDW. This mode can be described as a combination of two acoustic phonons, one of which has Kohn anomaly in a broad range of temperatures. We extracted basic characteristics of the soft mode and its anharmonic decay. Global analysis fits from femtosecond to nanosecond scale were applied to describe the relaxation dynamics from the broadband transient reflectivity data.   

Two-fold anisotropic superconducting properties of few-layer NbSe2

Transition metal dichalcogenides (TMDs) are a class of layered van der Waals materials that have recently attracted considerable interest due to a wide range of properties in the two-dimensional limit, including superconductivity, topological phases, and spin-valley-locked bands due to strong orbit coupling and inversion-symmetry breaking. Here, we present transport properties of few-layer niobium diselenide (NbSe₂), a TMD that exhibits signatures of an unusual Ising superconducting state as its thickness approaches the 2D limit. We perform magneto-transport experiments on high-quality few-layer NbSe₂ samples, which are fully encapsulated by hexagonal boron nitride in order to protect the NbSe₂ from the deleterious effects of exposure to oxygen and moisture. We find that, despite the three-fold symmetry of the lattice, the magneto-transport exhibits a two-fold in-plane anisotropy for in-plane magnetic fields up to 8 Tesla. This anisotropy is restricted to the temperature region between the T_c onset and T_c offset, and disappears in the normal state. We propose a phenomenological model to explain this unusual observation, discussing its implications to the elucidation of Ising superconductivity.   

Finite-momentum pairing in superconducting monolayer NbSe2

The nature of the so-called Ising superconducting state in non-centrosymmetric monolayer transition metal dichalcogenides remains a subject of intense debate. Here, we study the possibility of finite-momentum pairing within a low-energy interacting microscopic electronic model for monolayer 1H-NbSe₂. We find that, in the presence of both Rashba and Ising spin-orbit coupling (SOC), a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)-like state is expected to emerge for sufficiently strong in-plane fields. For special orientations of the magnetic field, this FFLO state displays topological Bogolyubov Fermi surfaces or nodes depending on the center-of-mass momentum of the Cooper pairs, protected by a crystalline mirror symmetry. We also show that out of the eight symmetry-allowed interactions involving the low-energy fermions in this model, four of them favor a pair-density wave (PDW) state involving momentum transfer K that may have a helical or chiral character. We investigate the stability and topology of these PDW states and their competition with the uniform superconducting phase in the presence of trigonal warping and mismatch between the K/K’ and the Γ pockets.   

Angle dependence of Hc2 with crossover between the orbital and paramagnetic limits in 2D NbSe2

Transition metal dichalcogenides (TMDs) are layered materials of abundant variety, providing emergent two-dimensional (2D) physical phenomena such as 2D superconductivity in few-layer NbSe₂. In the previous APS March Meeting, we reported layer-by-layer MBE growth of NbSe₂ thin films on insulating sapphire substrates [1], achieving relatively high T_c comparable to that of bulk in the thick-enough regime. With reducing thickness, superconducting properties turned out to exhibit 2D-like behavior, realizing Ising superconductivity with large-area samples. In this presentation, we will present the experimental data on the angle dependence of H_c2 in a bilayer NbSe₂ film measured with a 55 T pulsed magnet at ³He temperature. We observed cusp-like angle dependence of H_c2 around the parallel magnetic fields even far below T_c despite that H_c2 is purely dominated by the paramagnetic effect in this regime. In order to explain those results, we will propose a generalized Ginzburg-Landau model by taking into account the paramagnetic effect, which well describes the results with a microscopic physical picture. [1] H. Matsuoka et al, APS March Meeting 2018, K35.00009.   

Superfluid stiffness, optical spectral weight, and Tc bounds in multi-band superconductors

Recently we have used the optical sum rule for multi-band systems to bound the superfluid stiffness and derived upper bounds on the superconducting T_c in two dimensions. These bounds were shown [PRX 9, 031049 (2019)] to be particularly important for strongly interacting, narrow band systems, and found to give useful estimates for a variety of systems ranging from cold atoms to twisted bilayer graphene. We explore here various situations where one is forced to include bands far from the chemical potential in order to have localized Wannier functions, simple interactions, or to describe flat bands. Our goal is to compute the low-energy optical spectral weight and obtain tighter bounds on the superfluid stiffness, compared to the full spectral weight that includes all interband transitions. We also gain insight into how band topology impacts optical spectral weight. We will present results for model systems with flat bands, including Lieb and Kagome lattices, and estimate T_c bounds for monolayer FeSe on SrTiO₃ and for magic-angle twisted bilayer graphene.   

Proximity-induced superconducting gap in the quantum spin Hall edge state of monolayer WTe2

The quantum spin Hall (QSH) state was recently demonstrated in monolayers of the transition metal dichalcogenide 1T'-WTe₂ and is characterized by a band gap in the two-dimensional (2D) interior and helical one-dimensional (1D) edge states. Inducing superconductivity in the helical edge states would result in a 1D topological superconductor, a highly sought-after state of matter. In the present study, we use a novel dry-transfer flip technique to place atomically-thin layers of WTe₂ on a van der Waals superconductor, NbSe₂. Using scanning tunneling microscopy and spectroscopy (STM/STS), we demonstrate atomically clean surfaces and interfaces and the presence of a proximity-induced superconducting gap in the WTe₂ for thicknesses from a monolayer up to 7 crystalline layers. At the edge of the WTe₂ monolayer, we show that the superconducting gap coexists with the characteristic spectroscopic signature of the QSH edge state. Taken together, these observations provide conclusive evidence for proximity-induced superconductivity in the QSH edge state in WTe₂, a crucial step towards realizing 1D topological superconductivity and Majorana bound states in this van der Waals material platform.
  

Local magnetic measurements of few-layer superconducting MoS2

Under ionic liquid gating, the transition metal dichalcogenide MoS₂ exhibits a superconducting dome with a maximum critical temperature of nearly 10 K in few-layer devices gated with an ionic liquid. Here, we report local magnetic measurements combined with electrical transport of this superconducting state as a function of temperature and over a limited range of carrier density. Using a scanning superconducting quantum interference device with an integrated field coil, we measure local diamagnetism at finite frequency. We report progress in extracting quantitative values of the Pearl length and superfluid density from these measurements.   

Superconducting properties of MBE-grown 3R-Ta1+xSe2 epitaxial thin films

2D materials research is one of the hot topics in condensed-matter science, but most studies are performed on nanometer-thick crystals fabricated by mechanical exfoliation. On the other hand, researches based on MBE-grown samples are very much limited in particular for transport studies presumably due to difficulties in making high quality thin films. We have recently established a versatile route to layer-by-layer epitaxial growth of a wide variety of 2D materials and their heterostructures on insulating sapphire substrates by MBE [1,2], opening a door for exploration of emergent transport phenomena even based on hardly-cleavable and/or thermodynamically-metastable compounds. In this presentation, we will report MBE growth of thermodynamically-metastable 3R-Ta_1+xSe₂ epitaxial thin films and their superconducting properties, which turned out to be very much different from those of thermodynamically-stable 2H-TaSe₂ in terms of T_c and superconducting anisotropy. [1] M. Nakano et al., Nano Lett. 17, 5595 (2017). [2] Y. Kashiwabara et al., Adv. Funct. Mater. 2019, 1900354 (2019).   

Type-II Ising superconductivity in two-dimensional materials with spin-orbit coupling

Centrosymmetric materials with spin-degenerate bands are generally considered to be trivial for spintronics and related physics. In two-dimensional (2D) materials with multiple degenerate orbitals, we find that the spin-orbit coupling can induce spin-orbital locking, generate out-of-plane Zeeman-like fields displaying opposite signs for opposing orbitals, and create novel electronic states insensitive to in-plane magnetic field, which thus enables a new type of Ising superconductivity applicable to centrosymmetric materials. Many candidate materials are identified by high-throughput first-principles calculations. Our work enriches the physics and materials of Ising superconductivity, opening new opportunities for future research of 2D materials.   

Insight into Two-Dimensional Borophene: Five-Center Bond and Phonon-Mediated Superconductivity

We report a previously unknown monolayer borophene allotrope and we call it super-B with a flat structure based on ab initio calculations. It has good thermal, dynamical, and mechanical stability compared with many other typical borophenes. We find that super-B has a fascinating chemical bond environment consisting of standard sp, sp² hybridizations, and delocalized five-center three-electron π bond, called π(5c-3e), relying on the natural bond orbital analysis. This exceptional distribution of electron orbitals plays a pivotal role in stabilizing the super-B chemically. Moreover, super-B has a critical temperature T_c of 20.8 K at ambient condition. We attribute this high T_c of super-B to the giant anharmonicity of two linear acoustic phonon branches and an unusually low optic phonon mode. These discoveries provide new insight into the chemical nature of low dimensional boron nanostructures and highlight the potential applications of designing flexible devices and high T_c superconductors.
[1] T. Kambe, et al. J. Am. Chem. Soc. 141, 12984-12988 (2019)
[2] Z. Gao, et al. Nano Lett 17, 772-777 (2017)
[3] Z. Gao, et al. Nanoscale 10, 12997-13003 (2018)
[4] G. Liu, et al. Phys. Rev. B 99, 195436 (2019)   

Atomic-layer Rashba superconductor protected by dynamic spin-momentum locking

The breaking of the space inversion symmetry at surfaces and interfaces leads to the Rashba-type spin-orbit coupling (SOC) in 2D materials, which can strongly affect superconductivity [1]. In this talk, I will focus on crystalline atomic-layer indium on silicon surfaces with a clear spin-splitting of the Fermi surface and in-plane spin polarizations in the momentum space [2,3]. Our electron transport measurements under ultrahigh vacuum environment reveal that its in-plane critical magnetic field exceeds the Pauli limit by a factor of 3-4 at zero temperature. Through quantitative analysis, we conclude that elastic scattering among spin-momentum-locked electronic states serves as an effective spin-orbit scattering, thereby strongly suppressing the paramagnetic pair-breaking effect [4]. The mechanism is referred to as dynamic spin-momentum locking, which is contrasted to static spin-valley locking in 2D superconductors with Zeeman-type SOC [5].

References
[1] T. Uchihashi, Supercond. Sci. Technol. 30, 013002 (2017) [2017 highlight].
[2] T. Uchihashi et al., Phys. Rev. Lett. 107, 207001 (2011) [Editors’ Suggestion/ Physics].
[3] U. Kobayashi et al., submitted.
[4] S. Yoshizawa et al., submitted.
[5] J. M. Lu et al., Science 350, 1353 (2015); Y. Saito et al. Nat. Phys. 12, 144 (2016).