Session Y15: 2D Materials: Superconductivity and Correlations III

Superconductivity in monolayer FeSe and quantum Griffiths singularity in 2D superconductors

By direct transport and magnetic measurements, we provide first direct evidence for high temperature superconductivity in monolayer FeSe films on insulating SrTiO3 (STO) substrates with the onset Tc and critical current density much higher than those for bulk FeSe [1]. Besides, the thickness dependent superconductivity in ultrathin FeSe films on STO has been investigated [2,3]. By both in situ scanning tunneling microscopy/spectroscopy and ex situ transport and magnetization measurements, we find that the two-atomic-layer Ga film with graphene-like structure on wide band-gap semiconductor GaN is superconducting with Tc up to 5.4 K [4]. Furthermore, in three-atomic-layer Ga films, we observe for the first time the quantum Griffiths singularity of superconductor-metal transition in two dimensional (2D) superconductors [5]. References: 1. Chin. Phys. Lett. 31, 017401 (2014) (Editors' choice in Science 343, 230 (2014)) 2. Scientific Reports 4, 6040 (2014) 3. arXiv:1507.08431 (accepted by 2D Materials) 4. Physical Review Letters 114, 107003 (2015) (Editors' Suggestion) 5. Science 350, 542 (2015) (accompanied with a perspective paper: Science 350, 509)   

High-Tc superconductivity at 40 K emerged in ultrathin FeSe electric-double-layer transistors

A few unit-cell (UC) FeSe films on SrTiO3 substrates have recently attracted much attentions owing to emergence of high temperature superconductivity (high-Tc) about 65 K compared to the bulk value of 8 K. Modulation of electronic structure, charge transfer from SrTiO3, and electron-phonon coupling between the film and substrate are proposed as possible origins for high-Tc. Although the in-situ scanning tunneling and photoemission spectroscopies have been intensively studied [1], systematic thickness, carrier density and substrate material dependences of electrical measurements have been limited so far. Here we report on high-Tc in FeSe films on SrTiO3 and MgO in electric-double-layer transistor (EDLT) [2]. Both the film thickness and electric field can be tuned by electrochemical etching and electrostatic doping in EDLT. The systematic thickness dependences reveal that the onset Tc of 40 K appears from around 10 nm to 1 UC under the electric field while the initial 18-nm-thick FeSe shows no high-Tc. Our results point out the importance of electron accumulation or electronic band modulation for high-Tc in FeSe rather than electron-phonon coupling. [1] Q. Y. Wang et al., Chin. Phys. Lett. 29, 037402 (2012). [2] J. Shiogai et al., Nature Physics (2015).   

Ferromagnetism and d+id superconductivity in 1/2 doped correlated systems on triangular lattice

We investigate the quantum phase diagrams of t-J model on triangular lattice at 1/2 doping with various lattice sizes by using a combination of density matrix renormalization group (DMRG), variational Monte Carlo and quantum field theories. To sharply distinguish different phases, we calculated the symmetry quantum numbers of the ground state wave functions, and the results are further confirmed by looking into correlation functions. Our results show there is a first order phase transition from ferromagnetism to d+id superconductor, with the transition taking place at $J/t=0.4\pm0.2$.   

Topological superconductivity in magnetic adatom chains on a superconductor- multiple topological phases and disorder

Recent experimental efforts to realize topological superconductors and Majorana bound states in magnetic chains on top of a superconductor has stimulated lots of associated theory work. In this talk I will present recent results on topological superconductivity in dilute ferromagnetic chains on a superconducting surface with a Rashba spin-orbit coupling. We develop a theoretical framework that allows us to study the properties of magnetic chains at arbitrary subgap energies. Our analysis reveals that the system can support at least five distinct topological phases within realistic parameter regime. Our findings also show that the isolated bound-state energies do not need to be fine-tuned close to the gap centre and that the topological phases may be surprisingly robust towards various sources of disorder.   

Iron-Based Superconductors as topological matter

We show the existence of non-trivial topological properties in Iron-based superconductors. Several examples are provided, including (1) the single layer FeSe grown on SrTiO3 substrate, in which an topological insulator phase exists due to the band inversion at M point; (2) CaFeAs2, a staggered intercalation compound that integrates both quantum spin hall and superconductivity in which the nontrivial topology stems from the chain-like As layers away from FeAs layers; (3) the Fe(Te,Se) thin films in which the nontrivial Z2 topological invariance originates from the parity exchange at$\Gamma $ point that is controlled by the Te(Se) height; (4 nontrivial topology that is driven by the nematic order in FeSe. These results lay ground for integrating high Tc superconductivity with topological properties to realize new emergent phenomena, such as majorana particles, in iron-based high temperature superconductors Reference: (1) NingNing Hao and Jiangping Hu, Topological phases in the Single Layer FeSe"; Phys. Rev. X 4, 031053 (2014). (2) X Wu, C Le, Y Liang, S Qin, H Fan and J. P. Hu Effect of As-chain layers in CaFeAs 2" Phys. Rev. B 89 205102 (2014) (3) X. Wu, S. Qin, Y. Liang, C. Le, H. Fan, and J. Hu, CaFeAs\textbf{2}: a Staggered Intercalation of Quantum Spin Hall and High Temperature Superconductivity," Physics. Rev. B (Rapid Communication), 91, 081111 (2015) (4) X Wu, Y Liang and JP Hu, unpublished.   

Supercurrent in the quantum Hall regime: part I

The remarkable electronic quality of graphene/boron nitride heterostructures makes them an ideal medium to study induced superconductivity. Our Josephson junctions are made of encapsulated graphene demonstrate ballistic superconducting transport at the micron scale. In the hole-doped regime, a Fabry-Perot resonator is formed by PN junctions close to superconducting contacts, which causes quantum interference of the critical current. We study variations of the Fraunhofer pattern (I\textunderscore C vs. B) thought the gate voltage range. At higher magnetic fields, superconducting transport across the junctions becomes profoundly non-periodic. Despite demonstrating strong fluctuations as a function of density and magnetic field, we find that supercurrent persists in a wide range of parameters.   

Supercurrent in the quantum Hall regime, part II

A novel promising route for creating topological states and excitations is to combine superconductivity and the quantum Hall effect. Despite this potential, signatures of superconductivity in the quantum Hall regime remain scarce, and a superconducting current through a Landau-quantized two-dimensional electron gas has so far eluded experimental observation. High-mobility graphene/BN heterostructures exhibit the quantum Hall effect at relatively low field and are therefore particularly suitable to study the fate of the Josephson effect in that regime. Here, we report the observation of a superconducting current through graphene at fields as high as 2 Tesla. In that regime, the normal-state resistance is quantized but pockets of superconductivity still persist at small current bias. We will describe their bias and temperature dependence. Magnetic field interference patterns in the supercurrent inform on possible mechanisms mediating this supercurrent.   

Complex Stoichiometry reordering of PTCDA on Ag(111) upon K Intercalation

Alkali metal atoms are a simple yet efficient n-type dopant of organic semiconductors. However, the molecular crystal structures need be controlled and well understood in order to optimize the electronic properties (charge carrier density and mobility) of the target material. Here, we report that potassium intercalation into PTCDA monolayer domains on a Ag(111) substrate induces distinct stoichiometry-dependent structural reordering processes, resulting in highly ordered and large KxPTCDA domains. The emerging structures are analyzed by low temperature scanning tunneling microscopy (STM), scanning tunneling hydrogen microscopy (STHM), and low-energy electron diffraction (LEED) as a function of the stoichiometry and by density functional theory (DFT) calculations. Large stable monolayer domains are found for x=2,4. The epitaxy types for all intercalated stages are determined as point-on-line. The K atoms adsorb in the vicinity of the oxygen atoms of the PTCDA molecules, and their positions are determined with sub-Angstrom precision. This is a crucial prerequisite for the prospective assessment of the electronic properties of such composite films, as they depend on the mutual alignment between donor atoms and acceptor molecules.   

Uncovering fermionic zero-energy modes through a boundary-matrix approach

Given a non-interacting fermionic lattice system with arbitrary boundary conditions, we show how the problem of diagonalizing the single-particle Hamiltonian can be split into suitably defined bulk and boundary problems. Following this exact separation, a boundary matrix may be constructed, which contains complete information about the emergence and nature of zero-energy modes, even in the thermodynamic limit. Our approach is applicable to model Hamiltonians in arbitrary space dimensions of relevance to topological quantum matter. As a concrete illustration, we show how to correctly describe the zero-energy Majorana modes of a time-reversal-invariant two-band s-wave topological superconductor in a Josephson ring configuration, and also provide physical insight into the predicted unconventional Josephson effect.   

Interaction-driven strong topology on the boundary of a weak topological superconductor

We focus on a class of topological superconductors (TSCs) which exhibit a bulk energy gap and support Majorana flat bands (MFBs) on the surface. In contrast to previous proposals relying on strong TSCs with nodal bandstructure, here MFBs are solely protected by a weak topological invariant reflecting a global or local strong anisotropy. In the present case interactions play a dual role, on one hand driving the spontaneous symmetry breaking to an anisotropic superconducting phase and on the other, gapping out the arising MFBs yielding a strong topological phase on the boundary. The prototype system showing this kind of behavior is the nematic $p_z$-superconductor, which supports surface MFBs. While the interactions stabilize the $p_z$-SC phase in the bulk and induce the MFBs, suppressed bulk p-wave pairing terms occur on the surface, thus lifting the MFB-degeneracy. A similar situation can take place if the nematic features are only local, a scenario which is realizable in a heterostructure consisting of a conventional superconductor in proximity to a topological insulator surface with intrinsic magnetic order.