KEYWORDS: NbSe 2 , transition-metal dichalcogenides, macro-size monolayer film, ultralow temperature and high magnetic field electrical transport, Ising superconductivity, quantum phase transition 2 ABSTRACT Two-dimensional (2D) transition metal dichalcogenides (TMDs) have a range of unique physics properties and could be used in the development of electronics, photonics, spintronics and quantum computing devices. The mechanical exfoliation technique of micro-size TMD flakes has attracted particular interest due to its simplicity and cost effectiveness. However, for most applications, large area and high quality films are preferred. Furthermore, when the thickness of crystalline films is down to the 2D limit (monolayer), exotic properties can be expected due to the quantum confinement and symmetry breaking. In this paper, we have successfully prepared macro-size atomically flat monolayer NbSe 2 films on bilayer graphene terminated surface of 6H-SiC(0001) substrates by molecular beam epitaxy (MBE) method. The films exhibit an onset superconducting critical transition temperature (T c onset ) above 6 K, 2 times higher than that of mechanical exfoliated NbSe 2 flakes. Simultaneously, the transport measurements at high magnetic fields reveal that the parallel characteristic field B c// is at least 4.5 times higher than the paramagnetic limiting field, consistent with Zeeman-protected Ising superconductivity mechanism. Besides, by ultralow temperature electrical transport measurements, the monolayer NbSe 2 film shows the signature of quantum Griffiths singularity when approaching the zero-temperature quantum critical point. TEXTQuasi-2D superconductors such as ultrathin films with thickness down to monolayer [1][2][3][4][5][6][7] 10, 11,13 , the coexistence of charge density wave (CDW) and the superconducting phase was observed down to the monolayer limit but the T c of monolayer NbSe 2 got significantly suppressed (less than 3.1 K) compared with its bulk value (7.2 K).Superconductor-insulator (metal) transition (SIT/SMT), a paradigm of quantum phase transition, is an important topic in condensed matter physics. In the 2D limit regime, the orbital effect is restricted in parallel magnetic field. Calculations show that in anisotropic superconductors, the FFLO state might lead to an enhancement of the upper critical field between 1.5 and 2.5 times of the Pauli paramagnetic limit field 32,35 . In perpendicular magnetic field cases, the characteristic field B c (0) of NbSe 2 film is estimated to be 2.87 T, smaller than the Pauli paramagnetic limit field (8.21 T). Besides, the 8 Maki parameter α⊥ = 0.44 is smaller than 1.8. In parallel magnetic field cases, the characteristic field B c// (0) ~ 37.22 T is at least 4.5 times of Pauli paramagnetic limit field, which exceeds the theoretical predictions of 1.5 ~ 2.5 times 32,35 . Therefore, the chance of the existence of FFLO state in monolayer NbSe 2 is little.The sample 3 (with T c onset ~ 6 K, Figure S5 For SIT, the sample critical resistance on phase transition is th...

Some theories predict that the filling factor 5/2 fractional quantum Hall state can exhibit non-Abelian statistics, which makes it a candidate for fault-tolerant topological quantum computation. Although the non-Abelian Pfaffian state and its particle-hole conjugate, the anti-Pfaffian state, are the most plausible wave functions for the 5/2 state, there are a number of alternatives with either Abelian or non-Abelian statistics. Recent experiments suggest that the tunneling exponents are more consistent with an Abelian state rather than a non-Abelian state. Here, we present edge-current-tunneling experiments in geometrically confined quantum point contacts, which indicate that Abelian and non-Abelian states compete at filling factor 5/2. Our results are consistent with a transition from an Abelian state to a non-Abelian state in a single quantum point contact when the confinement is tuned. Our observation suggests that there is an intrinsic non-Abelian 5/2 ground state but that the appropriate confinement is necessary to maintain it. This observation is important not only for understanding the physics of the 5/2 state but also for the design of future topological quantum computation devices.fractional quantum Hall effect | 5/2 fractional quantum Hall state | edge-current tunneling | quantum point contact | non-Abelian statistics A mong the roughly 100 known fractional quantum Hall (FQH) states, the filling factor ν = 5/2 state is special. It is an evendenominator state with elementary excitations that may have nonAbelian statistics (1-10). If the ground state is non-Abelian, it would be insensitive to environmental decoherence (11) and would therefore be useful for fault-tolerant topological quantum computation. Because of this potential application, much theoretical effort has focused on the 5/2 state, and a variety of wave functions have been proposed (1-3, 5-7, 12-15). There have also been several experiments on the 5/2 state, with more of them supporting non-Abelian than Abelian statistics (16,17). All of the proposed wave functions for the 5/2 state have an effective charge e* of the quasiparticles that is a quarter of the elementary charge e, and this effective charge has been confirmed experimentally (18-21). However, the wave function of the 5/2 state is still under discussion.The proposed wave functions of the 5/2 state can be distinguished by the strength of the interaction between quasiparticles, described by a coupling constant g. This coupling constant can be obtained from experiments using weak-tunneling theory (22). The FQH effect emerges in a highly interacting 2D electron gas (2DEG) at high magnetic field and ultralow temperature. The FQH states have gapless conducting edge currents at the boundaries of the 2DEG, in which fractionally charged quasiparticles carry the current. If the counter-propagating edge currents of a FQH state are brought close enough together in a quantum point contact (QPC), back-scattering is induced. In the weak-tunneling regime, the rate of quasiparticle tunneling depend...

Superconductor-insulator/metal transition (SIT/SMT) represents a prototype of quantum phase transition, where quantum fluctuation plays a dominant role and dramatically changes the physical properties of low-dimensional superconducting systems. Recent observation of quantum Griffiths singularity (QGS) offers an essential perspective to understand the subtleties of quantum phase transition in two-dimensional superconductors. Here we study the magnetic field induced SMT in ultrathin crystalline Pb films down to ultralow temperatures. The divergent critical exponent is observed when approaching zero temperature quantum critical point, indicating QGS. Distinctively, the anomalous phase boundary of SMT that the onset critical field decreases with decreasing temperatures in low temperature regime distinguishes our observation from previous reports of QGS in various two-dimensional superconductors. We demonstrate that the anomalous phase boundary originates from the superconducting fluctuations in ultrathin Pb films with pronounced spin-orbit interaction. Our findings reveal a novel aspect of QGS of SMT in two-dimensional superconductors with anomalous phase boundary.