Nature of the quantum metal in a two-dimensional crystalline superconductor

Abstract

Two-dimensional (2D) materials are not expected to be metals at low temperature due to electron localization [1]. Consistent with this, pioneering studies on thin films reported only superconducting and insulating ground states, with a direct transition between the two as a function of disorder or magnetic field [2][3][4][5][6]. However, more recent works have revealed the presence of an intermediate quantum metallic state occupying a substantial region of the phase diagram [7-10] whose nature is intensely debated [11][12][13][14][15][16][17]. Here, we observe such a state in the disorder-free limit of a crystalline 2D superconductor, produced by mechanical co-lamination of NbSe 2 in inert atmosphere. Under a small perpendicular magnetic field, we induce a transition from superconductor to the quantum metal. We find a unique power law scaling with field in this phase, which is consistent with the Bose metal model where metallic behavior arises from strong phase fluctuations caused by the magnetic field [11][12][13][14].Global superconductivity emerges in a sample when conduction electrons form Cooper pairs and condense into a macroscopic, phase-coherent quantum state. In two dimensions, the phase coherence can be disrupted even at zero temperature by increasing disorder, either by degrading crystal quality or applying magnetic fields to create vortices [2]. Granular or amorphous superconducting thin films, for which disorder levels can be controlled during growth, have thus provided an established platform for the study of quantum phase transitions in 2D superconductors. Within the conventional theoretical framework, increasing disorder or magnetic field perpendicular to a strongly disordered film at T = 0 induces a direct transition to an insulating state as the normal state sheet resistance approaches the pair quantum resistance h/(2e) 2 = 6.4 kΩ [2,4]. As film quality has improved over time, however, an intervening metallic phase with resistance much lower than the normal state resistance has been observed in several systems with generally less disorder [7][8][9][10]. Its origin is not well understood, and the various theoretical treatments can be generally divided between purely bosonic-based models, in which Cooper pairing persists in the metallic phase but phase coherence is lost [11][12][13][14], and models that also incorporate other fermionic degrees of freedom [15][16][17].Recently, mechanical exfoliation has emerged as a technique to produce ultra-clean, crystalline 2D materials, with graphene being a well-known example [18]. Like amorphous films, the thickness of these samples can be easily controlled down to the level of individual atomic layers. In contrast to amorphous films, a 2D superconductor exfoliated from a lay- Figure 1. Environmentally controlled device fabrication. a) Schematic of heterostructure assembly process. Boron nitride (BN)/graphite (G) on a polymer stamp (PDMS) is used to electrically contact and encapsulate NbSe2 in inert atmosphere. The heterostructure is lithographically patte...

Quasiparticle energy bands and Fermi surfaces of monolayer NbSe2

“…It has been known for a long time 32,33 that threedimensional stacking structure of 2H-NbSe 2 is metallic at room temperature and undergoes a CDW transition at 33 K before becoming a superconductor 34,35 at 7.2 K although there has been the controversy regarding on the origin of CDW and the competition between CDW and superconducting (SC) states 32,33,[36][37][38][39][40][41][42][43][44][45] . After a few earlier attempts to investigate physical properties of its thin flakes 1,46,47 , a couple of recent works have reported successful isolations of its single layer form on top of various substrates and measure their CDW and SC phase transitions [27][28][29][30][31] . While all experiments [29][30][31]46,47 hitherto have shown that the superconducting transition temperature decreases but does not diminish completely when the thickness of 2H-NbSe 2 decreases to a single layer limit, the transition temperature (T CDW ) from metal to CDW phase differs from each other significantly 30,31 .…”

“…Moreover, the substrate screening also changes the nature of Coulomb interactions in graphene on top of either dielectric materials or metals and hence modifies the bands further 13,18 . Besides graphene, several recent experiments have revealed new interesting physical properties in a monoand a few-layer TMDs such as series of phase transitions and novel superconductivity, being different from those shown in their bulk forms [19][20][21][22][23][24][25][26][27][28][29][30][31] . Formation of charge density waves (CDWs) in three-dimensional metallic TMDs has attracted interests for the last couple of decades and origins of CDWs in some materials are still not settled yet 32,33 .…”

“…Formation of charge density waves (CDWs) in three-dimensional metallic TMDs has attracted interests for the last couple of decades and origins of CDWs in some materials are still not settled yet 32,33 . Therefore, the current efforts in investigating physical properties of thin flakes of TMDs may shed light on the origin of CDW phase in three dimensional TMDs and open a way to find the characteristic new collective phenomena in 2D crystals [24][25][26][27][28] . Among metallic TMDs, the stacked trigonal prismatic structure of niobium diselenide (2H-NbSe 2 ) is one of the most studied materials and an ideal system to study phase transitions as functions of temperature and dopings.…”

Nanoscale Superconducting Honeycomb Charge Order in IrTe2

“…The suppression of CDW order leads to the emerging superconductivity 1,2 , the CDW coexists with high temperature superconductivity in cupurates 3,4 , and the charge orderings are entangled with colossal magnetoresistance in manganese oxides 5,6 . In recent years, the charge ordering and CDW phenomena coupled with superconductivity are attracting renewed interest for the new functionality of two dimensional transition metal dichalogenides [7][8][9][10] . The recent development of microscopy techniques with the spectroscopic capability such as scanning tunneling microscopy/spectroscopy (STM/S) has made it possible to disclose atomic scale details of charge orderings in complex low dimensional systems in coexistence and competition with various electronic orders.…”

Dimensionality-driven orthorhombic MoTe2 at room temperature

“…1(b) shows an optical image of a representative device with the MoTe2 flake outlined by a dashed line. In order to avoid the effects of surface oxidation [26][27][28], MoTe2 was exfoliated onto a polymer stamp within a nitrogen-filled glovebox, transferred onto gold electrodes, and covered with hexagonal boron nitride (hBN) before being moved out to the ambient environment. Since hBN conforms to the features of the underlying surface, an atomic force microscope was used to determine the thickness of the buried MoTe2.…”