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Chen L.-B.,Qingdao Technological University | Yang W.,Beijing Computational Science Research Center
Laser Physics Letters | Year: 2014

We propose a two-qubit optically controlled phase gate in quantum dot molecules via adiabatic passage and hole tunnelling. Our proposal combines the merits of the current generation of vertically stacked self-assembled InAs quantum dots and adiabatic passage. The simulation shows an implementation of the gate with a fidelity exceeding 0.98. © 2014 Astro Ltd.


Miao M.-S.,Beijing Computational Science Research Center | Miao M.-S.,University of California at Santa Barbara | Hoffmann R.,Cornell University
Accounts of Chemical Research | Year: 2014

ConspectusElectrides, in which electrons occupy interstitial regions in the crystal and behave as anions, appear as new phases for many elements (and compounds) under high pressure. We propose a unified theory of high pressure electrides (HPEs) by treating electrons in the interstitial sites as filling the quantized orbitals of the interstitial space enclosed by the surrounding atom cores, generating what we call an interstitial quasi-atom, ISQ.With increasing pressure, the energies of the valence orbitals of atoms increase more significantly than the ISQ levels, due to repulsion, exclusion by the atom cores, effectively giving the valence electrons less room in which to move. At a high enough pressure, which depends on the element and its orbitals, the frontier atomic electron may become higher in energy than the ISQ, resulting in electron transfer to the interstitial space and the formation of an HPE.By using a He lattice model to compress (with minimal orbital interaction at moderate pressures between the surrounding He and the contained atoms or molecules) atoms and an interstitial space, we are able to semiquantitatively explain and predict the propensity of various elements to form HPEs. The slopes in energy of various orbitals with pressure (s > p > d) are essential for identifying trends across the entire Periodic Table. We predict that the elements forming HPEs under 500 GPa will be Li, Na (both already known to do so), Al, and, near the high end of this pressure range, Mg, Si, Tl, In, and Pb. Ferromagnetic electrides for the heavier alkali metals, suggested by Pickard and Needs, potentially compete with transformation to d-group metals. © 2014 American Chemical Society.


Ke S.-H.,Tongji University | Ke S.-H.,Beijing Computational Science Research Center
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

An efficient all-electron G0W0 method and a quasiparticle self-consistent GW (QSGW) method for molecules are proposed in the molecular-orbital space with the full random-phase approximation. The convergence with the basis set is examined. As an application, the ionization energy and electron affinity of a series of conjugated molecules (up to 32 atoms) are calculated and compared to the experiment. The QSGW result improves the G0W0 result and both of them are in significantly better agreement with experimental data than those from Hartree-Fock (HF) and hybrid density-functional calculations, especially for electron affinity. The nearly correct energy gap and suppressed self-interaction error by the HF exchange make our method a good candidate for investigating electronic and transport properties of molecular systems. © 2011 American Physical Society.


Li Y.-C.,Beijing Computational Science Research Center | Lin H.-Q.,Beijing Computational Science Research Center
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011

Pairwise quantum discord (QD) and classical correlation (CC) are studied in the XY spin chain with three-spin interaction. We analyze their capability in detecting quantum phase transitions (QPTs) at both zero and finite temperatures and find that the pairwise QD of two neighboring spins is more reliable than that of any other distances in identifying QPTs. Both the QD and CC detect quantum critical points associated with first- and higher-order QPTs caused by field and three-spin interactions at finite temperatures. In addition, we find a different finite-size scaling behavior for QD from previous reports for the transverse field Ising case and show some interesting phenomena of QD and entanglement of formation for finite temperatures. © 2011 American Physical Society.


Chen Y.,Beijing Computational Science Research Center
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011

We investigate the high-order harmonic generation of atoms and molecules exposed in strong and short-wavelength (shorter than 800 nm) laser fields. Our simulations show that the electronic rescattering trajectory depends strongly on the property of the Coulomb potential. Using numerical schemes, we identify the important role of excited states in the emission times of harmonics from molecules. We propose a model, which considers the initial position when electrons tunnel out from the potential, to explain the electronic response in intense and relatively high-frequency laser fields. © 2011 American Physical Society.


Miao M.-S.,University of California at Santa Barbara | Miao M.-S.,Beijing Computational Science Research Center
Nature Chemistry | Year: 2013

The periodicity of the elements and the non-reactivity of the inner-shell electrons are two related principles of chemistry, rooted in the atomic shell structure. Within compounds, Group I elements, for example, invariably assume the +1 oxidation state, and their chemical properties differ completely from those of the p-block elements. These general rules govern our understanding of chemical structures and reactions. Here, first-principles calculations show that, under pressure, caesium atoms can share their 5p electrons to become formally oxidized beyond the +1 state. In the presence of fluorine and under pressure, the formation of CsF n (n > 1) compounds containing neutral or ionic molecules is predicted. Their geometry and bonding resemble that of isoelectronic XeF n molecules, showing a caesium atom that behaves chemically like a p-block element under these conditions. The calculated stability of the CsF n compounds shows that the inner-shell electrons can become the main components of chemical bonds. © 2013 Macmillan Publishers Limited. All rights reserved.


Chang K.,CAS Institute of Semiconductors | Lou W.-K.,CAS Institute of Semiconductors | Lou W.-K.,Beijing Computational Science Research Center
Physical Review Letters | Year: 2011

We investigate theoretically the electron states in HgTe quantum dots (QDs) with inverted band structures. In sharp contrast to conventional semiconductor quantum dots, the quantum states in the gap of the HgTe QD are fully spin-polarized and show ringlike density distributions near the boundary of the QD and spin-angular momentum locking. The persistent charge currents and magnetic moments, i.e., the Aharonov-Bohm effect, can be observed in such a QD structure. This feature offers us a practical way to detect these exotic ringlike edge states by using the SQUID technique. © 2011 American Physical Society.


Xiang Z.-L.,RIKEN | Xiang Z.-L.,Fudan University | Ashhab S.,RIKEN | Ashhab S.,University of Michigan | And 5 more authors.
Reviews of Modern Physics | Year: 2013

Hybrid quantum circuits combine two or more physical systems, with the goal of harnessing the advantages and strengths of the different systems in order to better explore new phenomena and potentially bring about novel quantum technologies. This article presents a brief overview of the progress achieved so far in the field of hybrid circuits involving atoms, spins, and solid-state devices (including superconducting and nanomechanical systems). How these circuits combine elements from atomic physics, quantum optics, condensed matter physics, and nanoscience is discussed, and different possible approaches for integrating various systems into a single circuit are presented. In particular, hybrid quantum circuits can be fabricated on a chip, facilitating their future scalability, which is crucial for building future quantum technologies, including quantum detectors, simulators, and computers. © 2013 American Physical Society.


News Article | April 18, 2016
Site: www.cemag.us

​Epitaxy, or growing crystalline film layers that are templated by a crystalline substrate, is a mainstay of manufacturing transistors and semiconductors. If the material in one deposited layer is the same as the material in the next layer, it can be energetically favorable for strong bonds to form between the highly ordered, perfectly matched layers. In contrast, trying to layer dissimilar materials is a great challenge if the crystal lattices don’t match up easily. Then, weak van der Waals forces create attraction but don’t form strong bonds between unlike layers. In a study led by the Department of Energy’s Oak Ridge National Laboratory, scientists synthesized a stack of atomically thin monolayers of two lattice-mismatched semiconductors. One, gallium selenide, is a “p-type” semiconductor, rich in charge carriers called “holes.” The other, molybdenum diselenide, is an “n-type” semiconductor, rich in electron charge carriers. Where the two semiconductor layers met, they formed an atomically sharp heterostructure called a p-n junction, which generated a photovoltaic response by separating electron-hole pairs that were generated by light. The achievement of creating this atomically thin solar cell, published in Science Advances, shows the promise of synthesizing mismatched layers to enable new families of functional two-dimensional (2D) materials. The idea of stacking different materials on top of each other isn’t new by itself. In fact, it is the basis for most electronic devices in use today. But such stacking usually only works when the individual materials have crystal lattices that are very similar, i.e., they have a good “lattice match.” This is where this research breaks new ground by growing high-quality layers of very different 2D materials, broadening the number of materials that can be combined and thus creating a wider range of potential atomically thin electronic devices. “Because the two layers had such a large lattice mismatch between them, it’s very unexpected that they would grow on each other in an orderly way,” says ORNL’s Xufan Li, lead author of the study. “But it worked.” The group was the first to show that monolayers of two different types of metal chalcogenides — binary compounds of sulfur, selenium or tellurium with a more electropositive element or radical — having such different lattice constants can be grown together to form a perfectly aligned stacking bilayer. “It’s a new, potential building block for energy-efficient optoelectronics,” Li says. Upon characterizing their new bilayer building block, the researchers found that the two mismatched layers had self-assembled into a repeating long-range atomic order that could be directly visualized by the Moiré patterns they showed in the electron microscope. “We were surprised that these patterns aligned perfectly,” Li says. Researchers in ORNL’s Functional Hybrid Nanomaterials group, led by David Geohegan, conducted the study with partners at Vanderbilt University, the University of Utah and Beijing Computational Science Research Center. “These new 2D mismatched layered heterostructures open the door to novel building blocks for optoelectronic applications,” says senior author Kai Xiao of ORNL. “They can allow us to study new physics properties which cannot be discovered with other 2D heterostructures with matched lattices. They offer potential for a wide range of physical phenomena ranging from interfacial magnetism, superconductivity and Hofstadter’s butterfly effect.” Li first grew a monolayer of molybdenum diselenide, and then grew a layer of gallium selenide on top. This technique, called “van der Waals epitaxy,” is named for the weak attractive forces that hold dissimilar layers together.  “With van der Waals epitaxy, despite big lattice mismatches, you can still grow another layer on the first,” Li says. Using scanning transmission electron microscopy, the team characterized the atomic structure of the materials and revealed the formation of Moiré patterns. The scientists plan to conduct future studies to explore how the material aligns during the growth process and how material composition influences properties beyond the photovoltaic response. The research advances efforts to incorporate 2D materials into devices. For many years, layering different compounds with similar lattice cell sizes has been widely studied. Different elements have been incorporated into the compounds to produce a wide range of physical properties related to superconductivity, magnetism and thermoelectrics. But layering 2D compounds having dissimilar lattice cell sizes is virtually unexplored territory. “We’ve opened the door to exploring all types of mismatched heterostructures,” Li says. The title of the paper is “Two-dimensional GaSe/MoSe misfit bilayer heterojunctions by van der Waals epitaxy.” Source: Oak Ridge National Laboratory


News Article | April 18, 2016
Site: www.nanotech-now.com

Abstract: Epitaxy, or growing crystalline film layers that are templated by a crystalline substrate, is a mainstay of manufacturing transistors and semiconductors. If the material in one deposited layer is the same as the material in the next layer, it can be energetically favorable for strong bonds to form between the highly ordered, perfectly matched layers. In contrast, trying to layer dissimilar materials is a great challenge if the crystal lattices don't match up easily. Then, weak van der Waals forces create attraction but don't form strong bonds between unlike layers. In a study led by the Department of Energy's Oak Ridge National Laboratory, scientists synthesized a stack of atomically thin monolayers of two lattice-mismatched semiconductors. One, gallium selenide, is a "p-type" semiconductor, rich in charge carriers called "holes." The other, molybdenum diselenide, is an "n-type" semiconductor, rich in electron charge carriers. Where the two semiconductor layers met, they formed an atomically sharp heterostructure called a p-n junction, which generated a photovoltaic response by separating electron-hole pairs that were generated by light. The achievement of creating this atomically thin solar cell, published in Science Advances, shows the promise of synthesizing mismatched layers to enable new families of functional two-dimensional (2D) materials. The idea of stacking different materials on top of each other isn't new by itself. In fact, it is the basis for most electronic devices in use today. But such stacking usually only works when the individual materials have crystal lattices that are very similar, i.e., they have a good "lattice match." This is where this research breaks new ground by growing high-quality layers of very different 2D materials, broadening the number of materials that can be combined and thus creating a wider range of potential atomically thin electronic devices. "Because the two layers had such a large lattice mismatch between them, it's very unexpected that they would grow on each other in an orderly way," said ORNL's Xufan Li, lead author of the study. "But it worked." The group was the first to show that monolayers of two different types of metal chalcogenides--binary compounds of sulfur, selenium or tellurium with a more electropositive element or radical--having such different lattice constants can be grown together to form a perfectly aligned stacking bilayer. "It's a new, potential building block for energy-efficient optoelectronics," Li said. Upon characterizing their new bilayer building block, the researchers found that the two mismatched layers had self-assembled into a repeating long-range atomic order that could be directly visualized by the Moiré patterns they showed in the electron microscope. "We were surprised that these patterns aligned perfectly," Li said. Researchers in ORNL's Functional Hybrid Nanomaterials group, led by David Geohegan, conducted the study with partners at Vanderbilt University, the University of Utah and Beijing Computational Science Research Center. "These new 2D mismatched layered heterostructures open the door to novel building blocks for optoelectronic applications," said senior author Kai Xiao of ORNL. "They can allow us to study new physics properties which cannot be discovered with other 2D heterostructures with matched lattices. They offer potential for a wide range of physical phenomena ranging from interfacial magnetism, superconductivity and Hofstadter's butterfly effect." Li first grew a monolayer of molybdenum diselenide, and then grew a layer of gallium selenide on top. This technique, called "van der Waals epitaxy," is named for the weak attractive forces that hold dissimilar layers together. "With van der Waals epitaxy, despite big lattice mismatches, you can still grow another layer on the first," Li said. Using scanning transmission electron microscopy, the team characterized the atomic structure of the materials and revealed the formation of Moiré patterns. The scientists plan to conduct future studies to explore how the material aligns during the growth process and how material composition influences properties beyond the photovoltaic response. The research advances efforts to incorporate 2D materials into devices. For many years, layering different compounds with similar lattice cell sizes has been widely studied. Different elements have been incorporated into the compounds to produce a wide range of physical properties related to superconductivity, magnetism and thermoelectrics. But layering 2D compounds having dissimilar lattice cell sizes is virtually unexplored territory. "We've opened the door to exploring all types of mismatched heterostructures," Li said. The title of the paper is "Two-dimensional GaSe/MoSe2 misfit bilayer heterojunctions by van der Waals epitaxy." ### Research, including materials synthesis, was supported by the DOE Office of Science. Materials characterization was conducted in part at the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility at ORNL. ORNL Laboratory Directed Research and Development funds supported some of the device measurements in the study. About Oak Ridge National Laboratory UT-Battelle manages ORNL for DOE's Office of Science. The single largest supporter of basic research in the physical sciences in the United States, the Office of Science is working to address some of the most pressing challenges of our time. For more information, please click If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.

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