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Condens. Matter, Volume 2, Issue 4 (December 2017)

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Research

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Open AccessArticle A Field-Theoretical Approach to the P vs. NP Problem via the Phase Sign of Quantum Monte Carlo
Condens. Matter 2017, 2(4), 33; doi:10.3390/condmat2040033
Received: 18 August 2017 / Revised: 17 September 2017 / Accepted: 18 October 2017 / Published: 19 October 2017
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Abstract
I present here a new method that allows the introduction of a discrete auxiliary symmetry in a theory in such a way that the eigenvalue spectrum of the fermion functional determinant is made up of complex conjugated pairs. The method implies a particular
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I present here a new method that allows the introduction of a discrete auxiliary symmetry in a theory in such a way that the eigenvalue spectrum of the fermion functional determinant is made up of complex conjugated pairs. The method implies a particular way of introducing and integrating over auxiliary fields related to a set of artificial shift symmetries. Gauge fixing the artificial continuous shift symmetries in the direct and dual sectors leads to the appearance of direct and dual Becchi–Rouet–Stora–Tyutin (BRST)-type global symmetries and of a symplectic structure over the field space. Such a method may allow the extension of the applicability of quantum Monte Carlo methods to some problems plagued by the fermionic sign problem. Full article
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Open AccessArticle Enhanced Manifold of States Achieved in Heterostructures of Iron Selenide and Boron-Doped Graphene
Condens. Matter 2017, 2(4), 34; doi:10.3390/condmat2040034
Received: 5 September 2017 / Revised: 17 October 2017 / Accepted: 25 October 2017 / Published: 29 October 2017
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Abstract
Enhanced superconductivity is sought by employing heterostructures composed of boron-doped graphene and iron selenide. Build-up of a composite manifold of near-degenerate noninteracting states formed by coupling top-of-valence-band states of FeSe to bottom-of-conduction-band states of boron-doped graphene is demonstrated. Intra- and intersubsystem excitons are
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Enhanced superconductivity is sought by employing heterostructures composed of boron-doped graphene and iron selenide. Build-up of a composite manifold of near-degenerate noninteracting states formed by coupling top-of-valence-band states of FeSe to bottom-of-conduction-band states of boron-doped graphene is demonstrated. Intra- and intersubsystem excitons are explored by means of density functional theory in order to articulate a normal state from which superconductivity may emerge. The results are discussed in the context of electron correlation in general and multi-band superconductivity in particular. Full article
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Open AccessArticle Interior Melting of the C3B16 and C2B14 Clusters Between 1000 K and 2000 K
Condens. Matter 2017, 2(4), 35; doi:10.3390/condmat2040035
Received: 24 October 2017 / Revised: 20 November 2017 / Accepted: 23 November 2017 / Published: 27 November 2017
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Abstract
For bulk three-dimensional materials, it is common for the surface to melt at a slightly lower temperature than the bulk. This is known as surface melting, and is typically due to the fact that there are fewer bonds to surface atoms. However, for
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For bulk three-dimensional materials, it is common for the surface to melt at a slightly lower temperature than the bulk. This is known as surface melting, and is typically due to the fact that there are fewer bonds to surface atoms. However, for small clusters, this picture can change. In recent years, there have been investigations of the B19 and B19 clusters, which show striking diffusive behavior as they are heated to 1000 K. We wondered what the effect of substituting a few carbon atoms would be on the properties of these small clusters. To this end, we carried out extensive structural searches and molecular dynamics simulations to study the properties of C3B16 and C2B14 at elevated temperatures. The ground state structures and lowest energy isomers for these clusters were determined and calculated. The lowest energy structures are two-dimensional with vacancies inside. The C atoms are located in the outer ring in the ground state. At 1400 K, the outer rim containing the carbon atoms has fixed bonding, while the interior atoms are able to diffuse freely. Therefore, both of these clusters display interior melting at 1400 K. This interior melting is explained by the larger bond strength of the rim atoms. Molecular dynamics simulations at 3000 K showed complete melting and we observed a wide variety of configurations in both clusters. Full article
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Open AccessArticle Role of H Distribution on Coherent Quantum Transport of Electrons in Hydrogenated Graphene
Condens. Matter 2017, 2(4), 37; doi:10.3390/condmat2040037
Received: 7 October 2017 / Revised: 28 November 2017 / Accepted: 29 November 2017 / Published: 4 December 2017
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Abstract
Using quantum mechanical methods, in the framework of non-equilibrium Green’s function (NEGF) theory, we discuss the effects of the real space distribution of hydrogen adatoms on the electronic properties of graphene. Advanced methods for the stochastic process simulation at the atomic resolution are
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Using quantum mechanical methods, in the framework of non-equilibrium Green’s function (NEGF) theory, we discuss the effects of the real space distribution of hydrogen adatoms on the electronic properties of graphene. Advanced methods for the stochastic process simulation at the atomic resolution are applied to generate system configurations in agreement with the experimental realization of these systems as a function of the process parameters (e.g., temperature and hydrogen flux). We show how these Monte Carlo (MC) methods can achieve accurate predictions of the functionalization kinetics in multiple time and length scales. The ingredients of the overall numerical methodology are highlighted: the ab initio study of the stability of key configurations, on lattice matching of the energetic configuration relation, accelerated algorithms, sequential coupling with the NEGF based on calibrated Hamiltonians and statistical analysis of the transport characteristics. We demonstrate the benefit to this coupled MC-NEGF method in the study of quantum effects in manipulated nanosystems. Full article
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Open AccessFeature PaperArticle Nanoscale Phase Separation and Lattice Complexity in VO2: The Metal–Insulator Transition Investigated by XANES via Auger Electron Yield at the Vanadium L23-Edge and Resonant Photoemission
Condens. Matter 2017, 2(4), 38; doi:10.3390/condmat2040038
Received: 28 September 2017 / Revised: 27 November 2017 / Accepted: 7 December 2017 / Published: 10 December 2017
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Abstract
Among transition metal oxides, VO2 is a particularly interesting and challenging correlated electron material where an insulator to metal transition (MIT) occurs near room temperature. Here we investigate a 16 nm thick strained vanadium dioxide film, trying to clarify the dynamic behavior
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Among transition metal oxides, VO2 is a particularly interesting and challenging correlated electron material where an insulator to metal transition (MIT) occurs near room temperature. Here we investigate a 16 nm thick strained vanadium dioxide film, trying to clarify the dynamic behavior of the insulator/metal transition. We measured (resonant) photoemission below and above the MIT transition temperature, focusing on heating and cooling effects at the vanadium L23-edge using X-ray Absorption Near-Edge Structure (XANES). The vanadium L23-edges probe the transitions from the 2p core level to final unoccupied states with 3d orbital symmetry above the Fermi level. The dynamics of the 3d unoccupied states both at the L3- and at the L2-edge are in agreement with the hysteretic behavior of this thin film. In the first stage of the cooling, the 3d unoccupied states do not change while the transition in the insulating phase appears below 60 °C. Finally, Resonant Photoemission Spectra (ResPES) point out a shift of the Fermi level of ~0.75 eV, which can be correlated to the dynamics of the 3d// orbitals, the electron–electron correlation, and the stability of the metallic state. Full article
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Review

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Open AccessReview Crystal Growth Techniques for Layered Superconductors
Condens. Matter 2017, 2(4), 32; doi:10.3390/condmat2040032
Received: 23 August 2017 / Revised: 12 September 2017 / Accepted: 16 October 2017 / Published: 16 October 2017
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Abstract
Layered superconductors are attractive because some of them show high critical temperatures. While their crystal structures are similar, these compounds are composed of many elements. Compounds with many elements tend to be incongruent melting compounds, thus, their single crystals cannot be grown via
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Layered superconductors are attractive because some of them show high critical temperatures. While their crystal structures are similar, these compounds are composed of many elements. Compounds with many elements tend to be incongruent melting compounds, thus, their single crystals cannot be grown via the melt-solidification process. Hence, these single crystals have to be grown below the decomposition temperature, and then the flux method, a very powerful tool for the growth of these single crystals with incongruent melting compounds, is used. This review shows the flux method for single-crystal growth technique by self-flux, chloride-based flux, and HPHT (high-pressure and high-temperature) flux method for many-layered superconductors: high-Tc cuprate, Fe-based and BiS2-based compounds. Full article
(This article belongs to the Special Issue Layered Superconductors)
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Open AccessReview d° Ferromagnetism of Magnesium Oxide
Condens. Matter 2017, 2(4), 36; doi:10.3390/condmat2040036
Received: 2 August 2017 / Revised: 11 October 2017 / Accepted: 22 November 2017 / Published: 29 November 2017
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Abstract
Magnetism without d-orbital electrons seems to be unrealistic; however, recent observations of magnetism in non-magnetic oxides, such as ZnO, HfO2, and MgO, have opened new avenues in the field of magnetism. Magnetism exhibited by these oxides is known as d° ferromagnetism,
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Magnetism without d-orbital electrons seems to be unrealistic; however, recent observations of magnetism in non-magnetic oxides, such as ZnO, HfO2, and MgO, have opened new avenues in the field of magnetism. Magnetism exhibited by these oxides is known as d° ferromagnetism, as these oxides either have completely filled or unfilled d-/f-orbitals. This magnetism is believed to occur due to polarization induced by p-orbitals. Magnetic polarization in these oxides arises due to vacancies, the excitation of trapped spin in the triplet state. The presence of vacancies at the surface and subsurface also affects the magnetic behavior of these oxides. In the present review, origins of magnetism in magnesium oxide are discussed to obtain understanding of d° ferromagnetism. Full article
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