Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session F09: Superconductivity in Graphene etc. |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Kenneth Burch, Boston College Room: BCEC 151A |
Tuesday, March 5, 2019 11:15AM - 11:27AM |
F09.00001: Nematic superconductivity stabilized by density wave fluctuations: Application to twisted bilayer graphene Vladyslav Kozii, Hiroki Isobe, Jorn Venderbos, Liang Fu Nematic superconductors possess unconventional superconducting order parameters that spontaneously break rotational symmetry of the underlying crystal. In this work we propose a mechanism for nematic superconductivity stabilized by strong density wave fluctuations in two dimensions. While the weak-coupling theory finds the fully gapped chiral state to be energetically stable, we show that strong density wave fluctuations result in an additional contribution to the free energy of a superconductor with multicomponent order parameters, which generally favors nematic superconductivity. Our theory shades light on the recent observation of rotational symmetry breaking in the superconducting state of twisted bilayer graphene. |
Tuesday, March 5, 2019 11:27AM - 11:39AM |
F09.00002: Antiferromagnetically ordered Mott insulator and d + id superconductivity in twisted bilayer graphene: A quantum Monte Carlo study Tongyun Huang, Lufeng Zhang, Tianxing Ma Using exact quantum Monte Carlo method, we examine the recent novel electronic states seen in magic-angle graphene superlattices. From the Hubbard model on a double-layer honeycomb lattice with a rotation angle θ=1.08°, we reveal that an antiferromagnetically ordered Mott insulator emerges beyond a critical U_{c} at half filling, and with a small doping, the pairing with d+id symmetry dominates over other pairings at low temperature. The effective d+id pairing interaction strongly increase as the on-site Coulomb interaction increases, indicating that the superconductivity is driven by electron electron correlation. Our non-biased numerical results demonstrate that the twisted bilayer graphene share the similar superconducting mechanism of high temperature superconductors, which is a new and ideal platform for further investigating the strongly correlated phenomena. |
Tuesday, March 5, 2019 11:39AM - 11:51AM |
F09.00003: Possible superconductivity through the magnetic exchange in transition-metal intercalated bilayer graphene Kevin Lucht, Aditi D Mahabir, Alexandria Alcantara, Alexander Balatsky, Jose L. Mendoza-Cortes, Jason Haraldsen This study examines the possibility of superconductivity in transition-metal intercalated bilayer graphene. Using density functional theory, we determine electronic and magnetic properties through the electronic structure and density of states for all ten 3d transition-metal elements in a honeycomb configuration between two layers of graphene. Through an analysis of the electron density, we assess the induction of the magnetic moment in each case, where we estimate the exchange coupling through a comparison of the ferromagnetic and antiferromagnetic configurations. Furthermore, we show that the electronic band structure of the transition-metal intercalated layers have similar characteristics to those graphene layers intercalated with alkali and alkaline-earth metals, where superconductivity has been observed. Using a similar analysis, we find that the carbon π bands are below the Fermi, which is a possible indicator of superconductivity. More interestingly, the π bands seem to be degenerate to the transition-metal d bands, which could indicate hybridization and may lead to unconventional superconductivity. |
Tuesday, March 5, 2019 11:51AM - 12:03PM |
F09.00004: Pairing symmetry and spontaneous vortex-antivortex lattice in superconducting twisted bilayer graphene: A Bogoliubov-de Gennes approach Shizeng Lin, Ying Su We study the superconducting pairing symmetry in twisted bilayer graphene by solving the Bogoliubov-de Gennes equation for all electrons in Moir\'{e} supercells. With increasing the pairing potential, the system evolves from the mixed non-topological $d+id$ and $p+ip$ phase to the $s+p+d$ phase via the first order phase transition. In the time-reversal symmetry breaking $d+id$ and $p+ip$ phase, vortex and antivortex lattice accompanying spontaneous supercurrent are induced by the twist. The superconducting order parameter is nonuniform in the Moir\'{e} unit cell. Nevertheless, the superconducting gap in the local density of states is identical in the unit cell. The twist induced vortices and non-topological nature of the mixed $d+id$ and $p+ip$ phase are not captured by the existing effective models. Our results suggest the importance of long-range pairing interaction for effective models. |
Tuesday, March 5, 2019 12:03PM - 12:15PM |
F09.00005: Quantum Monte Carlo Study of Metal-Insulator Transitions on the Honeycomb Lattice Jingyao Wang, Tianxing Ma We investigate the band insulator-metal-Mott insulator transitions in the ionic Hubbard model on the 12*12 honeycomb lattice at half filling using determinant quantum Monte Carlo method. The phase diagram as a function of interaction U and staggered potential Δ indicates that the metallic state develops only if the interaction value varys in a moderate range. The Mott insulator state is antiferromagnetic. The conductivity decreases as the lattice size increases in metallic phase. A qualitative contrast to results obtained using the dynamical mean field theory is made at the end. |
Tuesday, March 5, 2019 12:15PM - 12:27PM |
F09.00006: Carbon-nanotube based highly flexible superconducting wire Jeong-gyun Kim, Haeyong Kang, Dongseok Suh In this work, we explored the feasibility of new fabrication of superconducting wire by using carbon nanotube (CNT) based yarn combined with superconducting materials. CNT yarn has been extensively studied due to its various outstanding mechanical properties. And it also could show multifunctional characteristics by combining with various guest materials. |
Tuesday, March 5, 2019 12:27PM - 12:39PM |
F09.00007: Theory of insulating phase and superconductivity in twisted bilayer graphene Xingyu Gu, Evan Laksono, Chuan Chen, Jia Ning Leaw, Nimisha Raghuvanshi, Shaffique Adam A correlated insulating phase and superconductivity have been recently observed in twisted bilayer graphene (tBG). We study theoretically these two phases in both the weak and strong coupling limit. In the weak coupling limit, using the random phase approximation, we find various possible combinations of density wave phases and pairing symmetries [1]. In the strong coupling limit, the insulating phase is anti-ferromagnetically (AFM) ordered, like in monolayer graphene [2]. However, we find there is no local magnetic moment in this AFM phase due to the special form of Wannier functions. Using the spin-fermion model, we find attractive electron-electron interaction mediated by the critical AFM fluctuations. This attractive interaction leads to chiral d+id superconductivity. Using a finite size tight-binding model, we show explicitly the existence of edge Majorana modes. |
Tuesday, March 5, 2019 12:39PM - 12:51PM |
F09.00008: Competing pairing propensities in twisted bilayer graphene Xianxin Wu, mario Fink, Michael Klett, Tim Wehling, Werner R Hanke, Ronny Thomale We employ a synopsis of random phase approximation (RPA) and functional renormalization group (fRG) to investigate twisted bilayer graphene based on a two-orbital effective model on the honeycomb lattice. Exploiting the ability of N-patch fRG to resolve the harmonic composition of a given superconducting instability channel, we develop a detailed profile of electronically mediated unconventional superconducting instabilities in twisted bilayer graphene, and highlight experimental signatures to distinguish between the resulting possible scenarios. |
Tuesday, March 5, 2019 12:51PM - 1:03PM |
F09.00009: Magnetic and superconducting correlation in monolayer and twisted-bilayer graphene Tianxing Ma, Tongyun Huang, Lufeng Zhang Using exact quantum Monte Carlo method, we identify the phase diagram of the half filled, the lightly doped and heavily doped graphene, which shows a rather rich physical properties. At half filling, the system is driven to a Mott insulator with antiferromagnetic long range order by increasing interaction, and a transition from a d+id pairing to a p+ip pairing is revealed, depends on the next-nearest hoping and the electronic fillings. We also examine the recent novel electronic states seen in magic-angle graphene superlattices. From the Hubbard model on a double-layer honeycomb lattice with a rotation angle θ=1.08, we reveal that an antiferromagnetically ordered Mott insulator emerges beyond a critical U c at half filling, and with a small doping, the pairing with d+id symmetry dominates over other pairings at low temperature. The effective d+id pairing interaction strongly increase as the on-site Coulomb interaction increases, indicating that the superconductivity is driven by electron-electron correlation. Our non-biased numerical results demonstrate that the twisted bilayer graphene is a new and idea platform for further investigating the strongly correlated phenomena. |
Tuesday, March 5, 2019 1:03PM - 1:15PM |
F09.00010: Strain induced superconducting pair-density-wave states in graphene Chung-Yu Mou, Feng Xu, Chung-Hou Chung, Ting-Kuo Lee Graphene is known to be non-superconducting. However, surprising superconductivity is recently discovered in a flat-band in a twisted bi-layer graphene. Here we show that superconductivity can be more easily realized in topological flat-bands induced by strain in graphene through periodic ripples. Specifically, it is shown that by including correlation effects, the chiral d-wave superconductivity can be stabilized under strain even for slightly doped graphene. The chiral d-wave superconductivity generally coexists with charge density waves (CDW) and pair density waves (PDW) of the same period. Remarkably, a pure PDW state with doubled period that coexists with the CDW state is found to emerge at a finite temperature region under reasonable strain strength. The emergent PDW state is shown to be superconducting with non-vanishing superfluid density, and it realizes the long searched superconducting states with non-vanishing center of mass momentum for Cooper pairs. |
Tuesday, March 5, 2019 1:15PM - 1:27PM |
F09.00011: Effective model for Majorana modes in graphene Antonio Manesco, Durval Rodrigues Junior, Gabriel Weber It was recently proposed that the interface between a graphene nanoribbon in the canted antiferromagnetic quantum Hall state and an s-wave superconductor may present topological superconductivity, resulting in the appearance of Majorana zero modes [1]. However, a description of the low-energy physics in terms of system parameters was still missing. Starting from a mean-field continuum model for graphene in proximity to a superconductor, we derive the low-energy effective Hamiltonian describing the interface of this heterojunction from first principles. A comparison between tight-binding simulations and analytical calculations with effective masses suggests that normal reflections at the interface must be considered in order to fully describe the low-energy physics. |
Tuesday, March 5, 2019 1:27PM - 1:39PM |
F09.00012: Bulk supercurrent at high magnetic field in Graphene Josephson junctions Ruoyu Chen, Yulu Liu, Kenji Watanabe, Takashi Taniguchi, Chun Ning Lau Benefitted from its high mobility and gate-tunability, graphene could serve as an ideal platform hosting many innovative devices. Introducing superconductivity into graphene via proximity provides a path to complicated or large-scale graphene-based superconducting devices/circuits, and the combination of quantum Hall edge states and superconductivity may host Majorana Fermions which allows Fault-tolerant quantum computing. Here we report our efforts fabricating hBN-encapsulated graphene Josephson junctions in various geometries. While small supercurrent hosted by quantum Hall edge states has been reported in graphene before, we observed gate-tunable bulk supercurrent at moderate charge densities and magnetic fields up to 3 T. Latest data and comparison with theoretical models will be presented. |
Tuesday, March 5, 2019 1:39PM - 1:51PM |
F09.00013: Andreev reflection in Graphene–superconductor junctions in the quantum Hall regime Joseph Cuozzo, Stuart Thomas, Xiang Hu, Enrico Rossi We investigate the Andreev reflection at an interface between a graphene layer in the quantum Hall regime and a superconductor. In graphene due to the spin and valley degrees of freedom there is an approximate SU(4) symmetry. The breaking of this symmetry due to interactions and the Zeeman effect leads to a splitting of the Landau levels. In this talk I will discuss the effect on the Andreev reflection of the breaking of the Landau levels' spin and valley degeneracy. I will then show results for the Andreev reflection amplitude, and the Andreev reflection contribution to the interface conductance, for various degrees of transparency of the interface. |
Tuesday, March 5, 2019 1:51PM - 2:03PM |
F09.00014: Coherent transport and electron interference in cuprate superconductor/graphene junctions D Perconte, Christian Ulysse, D Bercioux, J Trastoy, Anke Sander, Sophie D'Ambrosio, P. R. Kidambi, S Hofmann, Bruno Dlubak, Pierre Seneor, F. Sebastian Bergeret, Javier Villegas Proximity-induced superconductivity is particularly interesting in graphene. Among other reasons, because that effect can be externally controlled by tuning the Fermi energy (and vector) via electrical gating. For example, using high-temperature superconducting YBa_{2}Cu_{3}O_{7}/graphene planar junctions, we recently demonstrated [1] gate-controlled superconducting electron interferences that allow modulating the Andreev reflection at the superconductor/graphene interface via Klein tunneling of electron/hole pairs. Here we will discuss new experiments in the same type of junctions, in which a different type of interferences –this time controlled by the bias voltage– are observed which are due to geometrical resonances and coherent propagation of electron/hole pairs across a graphene channel. This is substantiated by the relationship of the oscillations period and the graphene channel length (up to hundreds on nm), as well as by numerical simulations of the device conductance -which reproduce both the observed resonances and the background conductance. [1] D. Perconte et al. Nature Physics 14, 25 (2018) |
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