Session S15: 2D Materials: Superconductivity and Correlations II

2D superconductivity by ionic gating

2D superconductivity is attracting a renewed interest due to the discoveries of new highly crystalline 2D superconductors in the past decade. Superconductivity at the oxide interfaces triggered by LaAlO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ has become one of the promising routes for creation of new 2D superconductors. Also, the MBE grown metallic monolayers including FeSe are also offering a new platform of 2D superconductors. In the last two years, there appear a variety of monolayer/bilayer superconductors fabricated by CVD or mechanical exfoliation. Among these, electric field induced superconductivity by electric double layer transistor (EDLT) is a unique platform of 2D superconductivity, because of its ability of high density charge accumulation, and also because of the versatility in terms of materials, stemming from oxides to organics and layered chalcogenides. In this presentation, the following issues of electric filed induced superconductivity will be addressed; (1) Tunable carrier density, (2) Weak pinning, (3) Absence of inversion symmetry. (1) Since the sheet carrier density is quasi-continuously tunable from 0 to the order of 10$^{\mathrm{14}}$ cm$^{\mathrm{-2}}$, one is able to establish an electronic phase diagram of superconductivity, which will be compared with that of bulk superconductors. (2) The thickness of superconductivity can be estimated as 2 -- 10 nm, dependent on materials, and is much smaller than the in-plane coherence length. Such a thin but low resistance at normal state results in extremely weak pinning beyond the dirty Boson model in the amorphous metallic films. (3) Due to the electric filed, the inversion symmetry is inherently broken in EDLT. This feature appears in the enhancement of Pauli limit of the upper critical field for the in-plane magnetic fields. In transition metal dichalcogenide with a substantial spin-orbit interactions, we were able to confirm the stabilization of Cooper pair due to its spin-valley locking.   

Electrochemical manipulation of two-dimensional materials

The electronic properties of a variety of two-dimensional (2D) materials can be modulated by electrochemical modifications on mesoscopic scale. Here we demonstrate a proof-of-concept ionic field-effect transistor (iFET), which is based on reversible modifications of the electronic properties of a wide range of layered materials (e.g. 1T-TaS$_2$ and 2H-TaS$_2$) through gate-controlled electrochemical reactions with mobile ions in the electrolyte. By fine-tuning the electrochemical reactions, we can switch between Mott phase, superconducting phase, metallic phase and insulating phase in a single 1T-TaS$_2$ iFET device. Such technique opens up new possibilities in searching for the novel state of matter in 2D materials.   

Griffiths singularity of quantum phase transition in ion-gated ZrNCl

Recent technological advances of thin films fabrication, especially mechanical exfoliation, led to discoveries of less-disordered highly-crystalline two-dimensional (2D) superconductors; atomically thin NbSe2 and ion-gated 2D materials, which show intrinsic properties of 2D superconductors with minimal disorder; for example, metallic ground state [1,2], and unconventional 2D Ising superconductivity due to pure spin-valley locking effect [3-5]. In this talk, we focus on magnetotransport properties of an ionic-liquid gated ZrNCl, which exhibited Griffiths singularity-like behavior in superconductor-metal-insulator transition induced by magnetic fields at low carrier concentrations. The overall behavior is quite similar to the recent results of superconducting Ga thin films, in which quantum Griffiths singularity was observed in vortex-glass state [6]. We will discuss the relationship between Griffiths singularity and quantum tunneling or flux flow of vortices phase (vortex liquid) in our system. [1] Y. Saito et al. Science 350, 409 (2015). [2] A. W. Tsen et al. arXiv 1507.08639 [3] Y. Saito et al. Nature Phys. doi: 10.1038/nphys3580. (arXiv:1506.04146). [4] X. Xi et al. arXiv:1507.08731. [5] J. M. Lu et al. arXiv:1506.07620. [6] Y. Xing et al. Science 350, 542 (2015).   

Spin-valley locking of the bulk transition-metal dichalocogenide superconductor NbSe$_{2}$

2H-NbSe$_{2}$ is a metallic transition metal dichalcogenide, which hosts instabilities to a charge density wave phase, and a superconducting phase at low temperatures [1]. To date, it has been assumed that these phases are largely unaffected by the spin degree of freedom. In contrast, from spin- and angle-resolved photoemission measurements, supported by first principles calculations, we reveal that the normal state Fermi surface hosts a complex spin texture. We uncover a rich spin-valley locking of the form also observed in the semiconducting materials of the same family [2], consistent with the recent observation of Ising pairing in the superconducting state of monolayer NbSe$_{2}$ [3]. We find that in the normal state of the bulk compound there is persistent spin polarisation which becomes intrinsically linked to the electronic dimensionality, showing a significant dependence on the out-of-plane momentum. This prompts a reinterpretation of the complex phases that emerge in this, and related materials. [1] Wilson JA et al, Phys. Rev. Lett. 32, 882 (1974). [2] Xiao D et al, Phys. Rev. Lett. 108, 196802 (2012); Xu X et al, Nature Phys. 10, 343–350 (2014); Riley JM et al, Nature Phys. 10, 835 (2014). [3] Xi X et al, arXiv:1507.08731.   

Superconductivity in few-layer NbS$_{2}$ and TaS$_{2}$ prepared by mechanical exfoliation

2D materials with novel physical properties are useful for scientific inquiries and technological applications. The superconducting 2D materials provide an opportunity to explore the superconductivity in the 2D limit. In this work, the superconductivity in few layer 2H-NbS$_{2}$ and 2H-TaS$_{2}$ are studied. Single crystals are obtained by vapor transport method and flakes are obtained by mechanical exfoliation. In NbS$_{2}$ flakes, the superconducting transition temperature (T$_{c}$) monotonically decreases with decreasing thickness. On the other hand, T$_{c}$ of TaS$_{2}$ flakes appears to monotonically increase as the flake gets thinner and the signature of CDW transition in R vs. T curves eventually disappear. The electric double layer transistors (EDLTs) of NbS$_{2}$ and TaS$_{2}$ flakes are also being fabricated to tune superconductivity in these 2D crystals. The results on these experiments will be presented.   

Characterizing the electronic ground states of single-layer NbSe2 via STM/STS

Layered transition metal dichalcogenides (TMDs) are ideal systems for exploring collective electronic phases such as charge density wave (CDW) order and superconductivity. In bulk NbSe2 the CDW sets in at TCDW $=$ 33K and superconductivity sets in at Tc $=$ 7.2K. Below Tc these electronic states coexist but their microscopic formation mechanisms remain controversial. Here we present an electronic characterization study of a single 2D layer of NbSe2 by means of low temperature scanning tunneling microscopy/spectroscopy (STM/STS), angle-resolved photoemission spectroscopy (ARPES), and electrical transport measurements. We demonstrate that the CDW order remains intact in 2D and exhibits a robust 3 x 3 superlattice. Superconductivity also still occurs but its onset is depressed to 1.6K. Our STS measurements at 5K reveal a CDW gap of $\Delta \quad =$ 4 meV at the Fermi energy, which is accessible via STS due to the removal of bands crossing the Fermi surface in the 2D limit. Our observations are consistent with the predicted simplified (compared to bulk) electronic structure of single-layer NbSe2, thus providing new insight into CDW formation and superconductivity in this model strongly-correlated system.   

Studies of the epitaxial monolayer NbSe$_2$ by ultra-low-temperature scanning tunnelling microscope

Monolayer NbSe$_2$ has been successfully synthesized by molecular beam epitaxy on the graphitized SiC(0001) surface. Wide substrate temperature window from 200$^{\circ}$C to 650$^{\circ}$C for the epitaxial growth has been observed. The polycrystalline nature of the epitaxial sheet, which is caused by the weak Van der Waals interaction with substrate, has been evidenced by reflection high-energy electron diffraction and locally by scanning tunnelling microscope. Under the high temperature growth condition, grain size could reach as large as hundreds of nanometers. The shape of grain boundary is strongly depended on the misaligned angle between adjacent grains. Mainly, three type grain boundaries have been identified at the atomic scale by the local scanning probe. The BCS-like superconducting gap and the spatial fluctuation of order parameter have been revealed by ultra-low temperature scanning tunnelling microscope in the sub-Kelvin range.   

Trion formation in monolayer transition metal dichalcogenides

We present three-body calculations for trions binding energy in monolayer transition metal dichalcogenides using the method of hyperspherical harmonics (HH). In numerical calculations for a proper treatment of Coulomb screening in two dimensions we assume that electrons and holes are interacted via Keldysh potential [1]. The convergences of binding energy calculations for the ground state of the trion as a function of the grand angular momentum are studied. For the trion binding energy in MoS$_{\mathrm{2}}$ we obtain 19.2 mev. This value is remarkably close to the experimental one of 18 meV. A comparison with results of other calculations are presented. We also study solutions of a hyperradial equation in a minimal approximation for the ground angular momentum to examine two regimes: a long range and a short range cases when the inter particle distance is much greater and much less than the screening length. For these cases, we find analytical expressions for the energy and wave function for trion states. [1] L. V. Keldysh, JETP Lett. \textbf{29}, 658 (1979   

Effects of the environment on the switching current in graphene-based Josephson Junctions.

The nature of the switching current and hysteresis (difference between switching and retrapping currents) in graphene-based Josephson junctions depends greatly on the interaction with the environment. Conventional devices result in underdamped Josephson junctions making the true critical current inaccessible. On the other hand, heavily isolating the Josephson junctions places them in the microscopic quantum tunneling regime even at high temperatures, also masking the critical current. We study the critical current, and the switching statistics in graphene Josephson junctions while varying the effects of the environment. Proper isolation of graphene Josephson junctions is necessary to measure the true critical current, especially so for the cases of small currents around the Dirac point. This is true for the case of conventional diffusive as well as the novel ballistic Josephson junctions.   

Sub-Kelvin lateral thermal transport in graphene with superconducting contacts

We studied thermal transport in graphene with superconducting contacts at low temperatures, $\sim$0.1 to 3 K, below the Bloch-Gr\"{u}neisen temperature. The lead (Pb) superconducting electrodes placed along the length of the graphene form a thermal barrier by preventing the outflow of hot electrons, thus allowing us to isolate and study other cooling pathways. We were able to observe a lateral thermal gradient by studying strips (5 x 50 $\mu$m) of CVD-grown graphene transferred onto a SiO$_{2}$ substrate. The characteristic length scale of the temperature profile is determined by the competition of the lateral heat flow within the electron system versus the local cooling of electrons by phonon emission. We anticipate extending this measurement in the near future to examine the outstanding question of electron-phonon cooling close to the Dirac point.   

Transport properties of high quality heterostructures from unstable 2D crystals prepared in inert atmosphere

Many layered materials can be cleaved down to individual atomic planes, similar to graphene, but only a small minority of them are stable under ambient conditions. The rest reacts and decomposes in air, which has severely hindered their investigation and possible uses. Here we introduce a remedial approach based on cleavage, transfer, alignment and encapsulation of airsensitive crystals, all inside a controlled inert atmosphere. To illustrate the technology, we choose two archetypal two-dimensional crystals unstable in air: black phosphorus and niobium diselenide. Our field-effect devices made from their monolayers are conductive and fully stable under ambient conditions, in contrast to the counterparts processed in air. NbSe$_{2}$ remains superconducting down to the monolayer thickness. Starting with a trilayer, phosphorene devices reach sufficiently high mobilities to exhibit Landau quantization. The approach offers a venue to significantly expand the range of experimentally accessible two-dimensional crystals and their heterostructures.   

Spin-orbit coupling induced by band hybridization in Graphene/WS2 heterostructures

Graphene are known to have a negligibly small intrinsic spin-orbit coupling (SOC), however, many novel physical phenomena such as the quantum spin Hall effect and the quantum anomalous Hall effect have been predicted if strong SOC exists in graphene. Despite that many theoretical studies have been carried out on the enhancement of the SOC strength in graphene, few experiments have been conducted to confirm the existence of and investigate the physical origin of the enhanced SOC in graphene. Here we demonstrated the introduction of SOC into graphene through the proximity effect by stacking WS2 onto graphene. We studied the magnetoconductance of graphene and found weak antilocalization emerges when graphene is covered by WS2. This is in a clear contrast with the weak localization behavior observed in bare graphene and thus provides an unambiguous evidence of the induced Rashba SOC. By focusing on a high carrier density region, we showed that it is possible to reliably extract the strength of Rashba SOC. Furthermore, via investigating the electric field dependence of the Rashba SOC with a dual-gate device, we found that the origin of this enhanced SOC is the band hybridization between graphene and WS2, in agreement with our theoretical calculations.