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"Signatures of two-dimensional superconductivity emerging within a three-dimensional host superconductor" — Carolina Parra: Francis C. Niestemski, Alex W. Contryman, Paula Giraldo-Gallo, Theodore H. Geballe, Ian R. Fisher, and Hari C. Manoharan; Proceedings of the Nationa Academy of Sciences, 04/12/21.
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Francis C. Niestemski, Alex W. Contryman, Paula Giraldo-Gallo, Theodore H. Geballe, Ian R. Fisher, and Hari C. Manoharan
Abstract
Spatial disorder has been shown to drive two-dimensional (2D) superconductors to an insulating phase through a superconductor–insulator transition (SIT). Numerical calculations predict that with increasing disorder, emergent electronic granularity is expected in these materials—a phenomenon where superconducting (SC) domains on the scale of the material’s coherence length are embedded in an insulating matrix and coherently coupled by Josephson tunneling. Here, we present spatially resolved scanning tunneling spectroscopy (STS) measurements of the three-dimensional (3D) superconductor BaPb1−xBixO3 (BPBO), which surprisingly demonstrate three key signatures of emergent electronic granularity, having only been previously conjectured and observed in 2D thin-film systems. These signatures include the observation of emergent SC domains on the scale of the coherence length, finite energy gap over all space, and strong enhancement of spatial anticorrelation between pairing amplitude and gap magnitude as the SIT is approached. These observations are suggestive of 2D SC behavior embedded within a conventional 3D s-wave host, an intriguing but still unexplained interdimensional phenomenon, which has been hinted at by previous experiments in which critical scaling exponents in the vicinity of a putative 3D quantum phase transition are consistent only with dimensionality d = 2.
"Quantum engineered Kondo lattices" — Jeremy Figgins: Laila S. Mattos, Warren Mar, Yi-Ting Chen, Hari C. Manoharan & Dirk K. Morr ; Nature Communications, 12/06/19.
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Laila S. Mattos, Warren Mar, Yi-Ting Chen, Hari C. Manoharan & Dirk K. Morr
Abstract
Atomic manipulation techniques have provided a bottom-up approach to investigating the unconventional properties and complex phases of strongly correlated electron materials. By engineering artificial systems containing tens to thousands of atoms with tailored electronic or magnetic properties, it has become possible to explore how quantum many-body effects emerge as the size of a system is increased from the nanoscale to the mesoscale. Here we investigate both theoretically and experimentally the quantum engineering of nanoscale Kondo lattices – Kondo droplets – exemplifying nanoscopic replicas of heavy-fermion materials. We demonstrate that by changing a droplet’s real-space geometry, we can not only create coherently coupled Kondo droplets whose properties asymptotically approach those of a quantum-coherent Kondo lattice, but also markedly increase or decrease the droplet’s Kondo temperature. Furthermore we report on the discovery of a new quantum phenomenon – the Kondo echo – a signature of droplets containing Kondo holes functioning as direct probes of spatially extended, quantum-coherent Kondo cloud correlations.
"Phase Separation of Dirac Electrons in Topological Insulators at the Spatial Limit" — Carolina Parra: Thiago Henrique Rodrigues da Cunha, Alex W. Contryman, Desheng Kong, Francisco Montero-Silva† Pedro Henrique Rezende Gonçalves, Diogo Duarte Dos Reis, Paula Giraldo-Gallo, Rodrigo Segura , Fernanda Olivares, Francis Niestemski, Yi Cui, Rogerio Magalhaes-Paniago, and Hari C. Manoharan; American Chemical Society, 12/27/16.
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Thiago Henrique Rodrigues da Cunha, Alex W. Contryman, Desheng Kong, Francisco Montero-Silva† Pedro Henrique Rezende Gonçalves, Diogo Duarte Dos Reis, Paula Giraldo-Gallo, Rodrigo Segura , Fernanda Olivares, Francis Niestemski, Yi Cui, Rogerio Magalhaes-Paniago, and Hari C. Manoharan
Abstract
In this work we present unique signatures manifested by the local electronic properties of the topological surface state in Bi2Te3 nanostructures as the spatial limit is approached. We concentrate on the pure nanoscale limit (nanoplatelets) with spatial electronic resolution down to 1 nm. The highlights include strong dependencies on nanoplatelet size: (1) observation of a phase separation of Dirac electrons whose length scale decreases as the spatial limit is approached, and (2) the evolution from heavily n-type to lightly n-type surface doping as nanoplatelet thickness increases. Our results show a new approach to tune the Dirac point together with reduction of electronic disorder in topological insulator (TI) nanostructured systems. We expect our work will provide a new route for application of these nanostructured Dirac systems in electronic devices.

"Activating and optimizing MoS2 basal planes for hydrogen evolution through the formation of strained sulphur vacancies" — Hong Li: Charlie Tsai, Ai Leen Koh, Lili Cai, Alex W. Contryman, Alex H. Fragapane, Jiheng Zhao, Hyun Soon Han, Hari C. Manoharan, Frank Abild-Pedersen, Jens K. Nørskov & Xiaolin Zheng; Nature Materials, 11/09/15.
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Charlie Tsai, Ai Leen Koh, Lili Cai, Alex W. Contryman, Alex H. Fragapane, Jiheng Zhao, Hyun Soon Han, Hari C. Manoharan, Frank Abild-Pedersen, Jens K. Nørskov & Xiaolin Zheng
Abstract
As a promising non-precious catalyst for the hydrogen evolution reaction (HER; refs 1,2,3,4,5), molybdenum disulphide (MoS2) is known to contain active edge sites and an inert basal plane1, 6, 7, 8. Activating the MoS2 basal plane could further enhance its HER activity but is not often a strategy for doing so. Herein, we report the first activation and optimization of the basal plane of monolayer 2H-MoS2 for HER by introducing sulphur (S) vacancies and strain. Our theoretical and experimental results show that the S-vacancies are new catalytic sites in the basal plane, where gap states around the Fermi level allow hydrogen to bind directly to exposed Mo atoms. The hydrogen adsorption free energy (ΔGH) can be further manipulated by straining the surface with S-vacancies, which fine-tunes the catalytic activity. Proper combinations of S-vacancy and strain yield the optimal ΔGH = 0 eV, which allows us to achieve the highest intrinsic HER activity among molybdenum-sulphide-based catalysts.
"Stripe-like nanoscale structural phase separation in superconducting BaPb1−xBixO3" — P. Giraldo-Gallo: Y. Zhang, C. Parra, H.C. Manoharan, M.R. Beasley, T.H. Geballe, M.J. Kramer & I.R. Fisher; Nature Communications 6, 09/16/15.
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Y. Zhang, C. Parra, H.C. Manoharan, M.R. Beasley, T.H. Geballe, M.J. Kramer & I.R. Fisher
Abstract
The phase diagram of BaPb1−xBixO3 exhibits a superconducting dome in the proximity of a charge density wave phase. For the superconducting compositions, the material coexists as two structural polymorphs. Here we show, via high-resolution transmission electron microscopy, that the structural dimorphism is accommodated in the form of partially disordered nanoscale stripes. Identification of the morphology of the nanoscale structural phase separation enables determination of the associated length scales, which we compare with the Ginzburg–Landau coherence length. We find that the maximum Tc occurs when the superconducting coherence length matches the width of the partially disordered stripes, implying a connection between the structural phase separation and the shape of the superconducting dome.
"Unconventional molecule-resolved current rectification in diamondoid–fullerene hybrids" — Jason C. Randel: Francis C. Niestemski, Andrés R. Botello-Mendez, Warren Mar, Georges Ndabashimiye, Sorin Melinte, Jeremy E. P. Dahl, Robert M. K. Carlson, Ekaterina D. Butova, Andrey A. Fokin, Peter R. Schreiner, Jean-Christophe Charlier & Hari C. Manoharan; Nature Communications, 09/09/14.
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Francis C. Niestemski, Andrés R. Botello-Mendez, Warren Mar, Georges Ndabashimiye, Sorin Melinte, Jeremy E. P. Dahl, Robert M. K. Carlson, Ekaterina D. Butova, Andrey A. Fokin, Peter R. Schreiner, Jean-Christophe Charlier & Hari C. Manoharan
Abstract
The unimolecular rectifier is a fundamental building block of molecular electronics. Rectification in single molecules can arise from electron transfer between molecular orbitals displaying asymmetric spatial charge distributions, akin to p–n junction diodes in semiconductors. Here we report a novel all-hydrocarbon molecular rectifier consisting of a diamantane–C60 conjugate. By linking both sp3 (diamondoid) and sp2 () allotropes, this hybrid molecule opposingly pairs negative and positive electron affinities. The single-molecule conductances of self-assembled domains on (111), probed by low-temperature scanning tunnelling microscopy and spectroscopy, reveal a large rectifying response of the molecular constructs. This specific electronic behaviour is postulated to originate from the electrostatic repulsion of diamantane–C60 molecules due to positively charged terminal hydrogen atoms on the diamondoid interacting with the top electrode (scanning tip) at various bias voltages. Density functional theory computations scrutinize the electronic and vibrational spectroscopic fingerprints of this unique molecular structure and corroborate the unconventional rectification mechanism.
"Designer Dirac fermions and topological phases in molecular graphene" — Kenjiro K. Gomes: Warren Mar, Wonhee Ko, Francisco Guinea, Hari Manoharan; Nature, 03/14/12.
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Warren Mar, Wonhee Ko, Francisco Guinea, Hari Manoharan
Abstract
The observation of massless Dirac fermions in monolayer graphene has generated a new area of science and technology seeking to harness charge carriers that behave relativistically within solid-state materials1. Both massless and massive Dirac fermions have been studied and proposed in a growing class of Dirac materials that includes bilayer graphene, surface states of topological insulators and iron-based high-temperature superconductors. Because the accessibility of this physics is predicated on the synthesis of new materials, the quest for Dirac quasi-particles has expanded to artificial systems such as lattices comprising ultracold atoms2, 3, 4. Here we report the emergence of Dirac fermions in a fully tunable condensed-matter system—molecular graphene—assembled by atomic manipulation of carbon monoxide molecules over a conventional two-dimensional electron system at a copper surface5. Using low-temperature scanning tunnelling microscopy and spectroscopy, we embed the symmetries underlying the two-dimensional Dirac equation into electron lattices, and then visualize and shape the resulting ground states. These experiments show the existence within the system of linearly dispersing, massless quasi-particles accompanied by a density of states characteristic of graphene. We then tune the quantum tunnelling between lattice sites locally to adjust the phase accrual of propagating electrons. Spatial texturing of lattice distortions produces atomically sharp p–n and p–n–p junction devices with two-dimensional control of Dirac fermion density and the power to endow Dirac particles with mass6, 7, 8. Moreover, we apply scalar and vector potentials locally and globally to engender topologically distinct ground states and, ultimately, embedded gauge fields9, 10, 11, 12, wherein Dirac electrons react to ‘pseudo’ electric and magnetic fields present in their reference frame but absent from the laboratory frame. We demonstrate that Landau levels created by these gauge fields can be taken to the relativistic magnetic quantum limit, which has so far been inaccessible in natural graphene. Molecular graphene provides a versatile means of synthesizing exotic topological electronic phases in condensed matter using tailored nanostructures.
"Structure of Cleaved (001) USb2 Single Crystal Surface" — Shao-Ping Chena: Marilyn Hawley, Phil B. Van Stockum, Hari C. Manoharan and Eric Bauer; MRS Proceedings, 04/15/11.
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Marilyn Hawley, Phil B. Van Stockum, Hari C. Manoharan and Eric Bauer
Abstract
We have achieved what we believe to be the first atomic resolution STM images for a uranium compound taken at room temperature. The a, b, and c lattice parameters in the images confirm that the USb2 crystals cleave on the (001) basal plane as expected. The a and b dimensions were equal, with the atoms arranged in a cubic pattern. Our calculations indicate a symmetric cut between Sb planes to be the most favorable cleavage plane and U atoms to be responsible for most of the DOS measured by STM. Some strange features observed in the STM will be discussed in conjunction with ab initio calculations.
"Laser-Synthesized Epitaxial Graphene" — Sangwon Lee: Michael F. Toney, Wonhee Ko, Jason C. Randel, Hee Joon Jung, Ko Munakata, Jesse Lu, Theodore H. Geballe, Malcolm R. Beasley, Robert Sinclair, Hari C. Manoharan, and Alberto Salleo; ACS Nano, 12/01/10.
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Michael F. Toney, Wonhee Ko, Jason C. Randel, Hee Joon Jung, Ko Munakata, Jesse Lu, Theodore H. Geballe, Malcolm R. Beasley, Robert Sinclair, Hari C. Manoharan, and Alberto Salleo
Abstract
Owing to its unique electronic properties, graphene has recently attracted wide attention in both the condensed matter physics and microelectronic device communities. Despite intense interest in this material, an industrially scalable graphene synthesis process remains elusive. Here, we demonstrate a high-throughput, low-temperature, spatially controlled and scalable epitaxial graphene (EG) synthesis technique based on laser-induced surface decomposition of the Si-rich face of a SiC single-crystal. We confirm the formation of EG on SiC as a result of excimer laser irradiation by using reflection high-energy electron diffraction (RHEED), Raman spectroscopy, synchrotron-based X-ray diffraction, transmission electron microscopy (TEM), and scanning tunneling microscopy (STM). Laser fluence controls the thickness of the graphene film down to a single monolayer. Laser-synthesized graphene does not display some of the structural characteristics observed in EG grown by conventional thermal decomposition on SiC (0001), such as Bernal stacking and surface reconstruction of the underlying SiC surface.
"Topological insulators: A romance with many dimensions" — Hari C. Manoharan: Nature Nanotechnology, 05/10/10.
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Abstract
Electric charges on the boundaries of certain insulators are programmed by topology to keep moving forward when they encounter an obstacle, rather than scattering backwards and increasing the resistance of the system. This is just one reason why topological insulators are one of the hottest topics in physics right now.