Two-dimensional transition metal dichalcogenides with strong spin-orbit interactions and valley-dependentBerry curvature effects have attracted tremendous recent interests 1-7 . Although novel single-particle and excitonic phenomena related to spin-valley coupling have been extensively studied 1,3-6 , effects of spin-momentum locking on collective quantum phenomena remain unexplored. Here we report an observation of superconducting monolayer NbSe2 with an inplane upper critical field over six times of the Pauli paramagnetic limit by magnetotransport measurements. The effect can be understood in terms of the competing Zeeman effect and large intrinsic spin-orbit interactions in non-centrosymmetric NbSe2 monolayers, where the electronic spin is locked to the out-of-plane direction. Our results provide a strong evidence of unconventional Ising pairing protected by spin-momentum locking and open up a new avenue for studies of noncentrosymmetric superconductivity with unique spin and valley degrees of freedom in the exact two-dimensional limit.Monolayer transition metal dichalcogenide (TMD) of the hexagonal structure consists of a layer of transition metal atoms sandwiched between two layers of chalcogen atoms in the trigonal prismatic structure 8 (Fig. 1a). It possesses out-of-plane mirror symmetry and in-plane inversion asymmetry. The presence of the transition metal also gives rise to large spin-orbit interactions (SOIs). The mirror symmetry restricts the crystal field ( ⃗) to the plane. The SOIs split the spin states at finite momentum ⃗⃗ in the absence of inversion symmetry. They manifest as an effective magnetic field along the direction of ⃗⃗ × ⃗ , which is out-of-plane for the restricted two-dimensional (2D) motion of electrons in the plane. The electronic spin is thus oriented in the out-of-plane direction and in opposite directions for electrons of opposite momenta 1-3 (Fig. 1a). Such spinmomentum locking is destroyed in the bulk where inversion symmetry and spin degeneracy are restored 1,2,7 (Fig. 1b). Novel valley-and spin-dependent phenomena including optical orientation of the valley polarization 3,4 and the valley Hall effect 5 arisen from spin-momentum locking have been recently demonstrated in group-VI TMD

Exciton binding energy and excited states in monolayers of tungsten diselenide (WSe 2 ) are investigated using the combined linear absorption and two-photon photoluminescence excitation spectroscopy. The exciton binding energy is determined to be 0.37 eV, which is about an order of magnitude larger than that in III-V semiconductor quantum wells and renders the exciton excited states observable even at room temperature. The exciton excitation spectrum with both experimentally determined one-and two-photon active states is distinct from the simple two-dimensional (2D) hydrogenic model. This result reveals significantly reduced and nonlocal dielectric screening of Coulomb interactions in 2D semiconductors. The observed large exciton binding energy will also have a significant impact on next-generation photonics and optoelectronics applications based on 2D atomic crystals. DOI: 10.1103/PhysRevLett.113.026803 PACS numbers: 73.21.Fg, 71.35.Cc, 78.20.Ci, 78.55.Hx One of the most distinctive features of electrons in twodimensional (2D) semiconductors, such as single atomic layers of group VI transition metal dichalcogenides (TMDs) [1], is the significantly reduced dielectric screening of Coulomb interactions. An important consequence of strong Coulomb interactions is the formation of tightly bound excitons. Indeed, recent theoretical studies have predicted a large exciton binding energy between 0.5 and 1 eV in MoS 2 monolayers [2-10], a representative 2D direct gap semiconductor from the family of TMDs [11,12]. These values for the exciton binding energy are more than an order of magnitude larger than that in conventional III-V-based quasi-2D semiconductor quantum wells (QWs) [13,14]. Such tightly bound excitons are expected to not only dominate the optical response, but also to play a defining role in the optoelectronic processes, such as photoconduction and photocurrent generation in 2D semiconductors [1,15]. On the other hand, little is known about these tightly bound excitons from the experimental standpoint, except the energy of the lowest energy one-photon active exciton states [11] and an indirect evidence of large binding energies through recent studies on trions, quasiparticles of two electrons and a hole, or two holes and an electron [16][17][18]. Furthermore, a non-Rydberg series has been predicted for excitons in 2D semiconductors, arisen from the nonlocal character of screening of the Coulomb interactions [4,19]. While a Rydberg series for the exciton energy spectrum has been observed in bulk MoS 2 [20,21], similar experimental studies on monolayers of MoS 2 or other TMDs have not been reported [22].The challenge in experimental determination of the exciton binding energy in 2D TMDs by linear optical methods, commonly used for bulk semiconductors [23] or conventional semiconductor QWs [13], lies in the identification of the onset of band-to-band transitions in the optical absorption or emission spectrum. Such an onset of band-to-band transitions has not been observed in 2D TMDs presumably due to the sig...

The atomic thickness of two-dimensional materials provides a unique opportunity to control their electrical and optical properties as well as to drive the electronic phase transitions by electrostatic doping. The discovery of two-dimensional magnetic materials has opened up the prospect of the electrical control of magnetism and the realization of new functional devices. A recent experiment based on the linear magneto-electric effect has demonstrated control of the magnetic order in bilayer CrI by electric fields. However, this approach is limited to non-centrosymmetric materials magnetically biased near the antiferromagnet-ferromagnet transition. Here, we demonstrate control of the magnetic properties of both monolayer and bilayer CrI by electrostatic doping using CrI-graphene vertical heterostructures. In monolayer CrI, doping significantly modifies the saturation magnetization, coercive force and Curie temperature, showing strengthened/weakened magnetic order with hole/electron doping. Remarkably, in bilayer CrI, the electron doping above ~2.5 × 10 cm induces a transition from an antiferromagnetic to a ferromagnetic ground state in the absence of a magnetic field. The result reveals a strongly doping-dependent interlayer exchange coupling, which enables robust switching of magnetization in bilayer CrI by small gate voltages.

Two-dimensional materials possess very different properties from their bulk counterparts. While changes in single-particle electronic properties have been investigated extensively, modifications in the many-body collective phenomena in the exact two-dimensional limit remain relatively unexplored. Here, we report a combined optical and electrical transport study on the many-body collective-order phase diagram of NbSe2 down to a thickness of one monolayer. Both the charge density wave and the superconducting phase have been observed down to the monolayer limit. The superconducting transition temperature decreases on lowering the layer thickness, but the newly observed charge-density-wave transition temperature increases from 33 K in the bulk to 145 K in the monolayer. Such highly unusual enhancement of charge density waves in atomically thin samples can be understood to be a result of significantly enhanced electron-phonon interactions in two-dimensional NbSe2 (ref. 4) and is supported by the large blueshift of the collective amplitude vibration observed in our experiment. Our results open up a new window for search and control of collective phases of two-dimensional matter, as well as expanding the functionalities of these materials for electronic applications.

These authors contributed equally: Tingxin Li, Shengwei Jiang.Stacking order can significantly influence the physical properties of two-dimensional (2D) van der Waals materials 1 . The recent isolation of atomically thin magnetic materials 2-22 opens the door for control and design of magnetism via stacking order. Here we apply hydrostatic pressure up to 2 GPa to modify the stacking order in a prototype van der Waals magnetic insulator CrI3. We observe an irreversible interlayer antiferromagnetic (AF) to ferromagnetic (FM) transition in atomically thin CrI3 by magnetic circular dichroism and electron tunneling measurements. The effect is accompanied by a monoclinic to a rhombohedral stacking order change characterized by polarized Raman spectroscopy. Before the structural change, the interlayer AF coupling energy can be tuned up by nearly 100% by pressure. Our experiment reveals interlayer FM coupling, which is the established ground state in bulk CrI3, but never observed in native exfoliated thin films. The observed correlation between the magnetic ground state and the stacking order is in good agreement with first principles calculations 23-27 and suggests a route towards nanoscale magnetic textures by moiré engineering 28 .Intrinsic magnetism in 2D van der Waals materials has received growing attention 2-22 . Of particular interest is the thickness-dependent magnetic ground state in atomically thin CrI3. In these exfoliated thin films, the magnetic moments are aligned (in the out-of-plane direction) in each layer, but anti-aligned in adjacent layers 3,12-22 . They are FM (or AF) depending on whether there is (or isn't) an uncompensated layer. The relatively weak interlayer coupling compared to the intralayer coupling allows effective ways to control the interlayer magnetism, which have led to interesting spintronics applications including voltage switching 12-14 , spin filtering 16-20 and spin transistors 21 . The origin of interlayer AF coupling is, however, not well understood since interlayer FM order is the ground state in the bulk crystals. Recent ab initio calculations 23-27 and experiments 22,29,30 have suggested that stacking order could provide an explanation but a direct correlation between stacking order and interlayer magnetism is lacking.In bulk CrI3, the Cr atoms in each layer form a honeycomb structure, and each Cr atom is surrounded by six I atoms in an octahedral coordination (Fig. 1a). The bulk crystals undergo a structural phase transition from a monoclinic phase (space group C2/m) at room temperature to a