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Unfolding the complexity of quasi-particle physics in disordered materials
Authors:
Sai Mu,
Raina Olsen,
B. Dutta,
L. Lindsay,
G. D. Samolyuk,
T. Berlijn,
E. D. Specht,
K. Jin,
H. Bei,
T. Hickel,
B. C. Larson,
G. M. Stocks
Abstract:
The concept of quasi-particles forms the theoretical basis of our microscopic understanding of emergent phenomena associated with quantum mechanical many-body interactions. However, quasi-particle theory in disordered materials has proven difficult, resulting in the predominance of mean-field solutions. Here we report first-principles phonon calculations and inelastic x-ray and neutron scattering…
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The concept of quasi-particles forms the theoretical basis of our microscopic understanding of emergent phenomena associated with quantum mechanical many-body interactions. However, quasi-particle theory in disordered materials has proven difficult, resulting in the predominance of mean-field solutions. Here we report first-principles phonon calculations and inelastic x-ray and neutron scattering measurements on equiatomic alloys (NiCo, NiFe, AgPd, and NiFeCo) with force constant dominant disorder - confronting a key 50-year-old assumption in the Hamiltonian of all mean-field quasi-particle solutions for off-diagonal disorder. Our results have revealed the presence of a large, and heretofore unrecognized, impact of local chemical environments on the distribution of the species-pair-resolved force constant disorder that can dominate phonon scattering. This discovery not only identifies a critical analysis issue that has broad implications for other elementary excitations such as magnons and skyrmions in magnetic alloys, but also provides an important tool for the design of materials with ultra-low thermal conductivity.
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Submitted 12 June, 2019;
originally announced June 2019.
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Nuclear spin correlations and collective excitations in supercritical H$_2$
Authors:
Raina J. Olsen,
Jon W. Taylor,
Cristian I. Contescu,
James R. Morris
Abstract:
Nuclear spins are known to experience spontaneous long-range correlations only below 2.5 mili-Kelvin in superfluid $^3$He. Here we present the first evidence of nuclear spin coupling in molecular hydrogen (H$_2$) at 74-92 Kelvin using neutron scattering, showing a fundamental change in nature from the incoherent scattering universally expected from hydrogen, which reflects single particle properti…
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Nuclear spins are known to experience spontaneous long-range correlations only below 2.5 mili-Kelvin in superfluid $^3$He. Here we present the first evidence of nuclear spin coupling in molecular hydrogen (H$_2$) at 74-92 Kelvin using neutron scattering, showing a fundamental change in nature from the incoherent scattering universally expected from hydrogen, which reflects single particle properties of uncorrelated nuclear spins, to coherent, with a peak materializing on the elastic line, indicating H$_2$-H$_2$ nuclear spin correlations. In this novel phase, the dynamic response of the system also changes nature, and collective excitations with an effective mass of nine H$_2$ are observed with inelastic scattering at momentum transfers up to 37 Å$^{-1}$, corresponding to length scales smaller than the H-H bond, where previous experiments have always found single atom excitations. This novel behavior has only been observed from H$_2$ within the subnanometer sized graphitic pores of a carbon material, marking the first demonstration that a confined materials environment can be used to control nuclear spin correlations.
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Submitted 21 January, 2016;
originally announced February 2016.
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High temperature Bose-Einstein condensation into an excited state at equilibrium
Authors:
Raina J. Olsen
Abstract:
We describe Bose-Einstein condensation of strongly interacting particles into a quantum state which is an excited single-particle state, but becomes the ground state as density increases because it minimizes the interaction energy compared to other states. Mean field calculations for a graphene potential just wide enough for two closely interacting layers of molecular hydrogen show condensation at…
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We describe Bose-Einstein condensation of strongly interacting particles into a quantum state which is an excited single-particle state, but becomes the ground state as density increases because it minimizes the interaction energy compared to other states. Mean field calculations for a graphene potential just wide enough for two closely interacting layers of molecular hydrogen show condensation at temperatures up to 60 K. In the condensed state, molecules hop between layers, increasing the first peak in the pair-correlation function just past the hard core repulsion diameter.
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Submitted 4 November, 2013; v1 submitted 29 April, 2013;
originally announced April 2013.
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Screening in multilayer graphene
Authors:
R. van Gelderen,
R. Olsen,
C. Morais Smith
Abstract:
In this article we study the static polarization in ABC-stacked multilayer graphene. Since the density of states diverges for these systems if the number of layers exceeds three, screening effects are expected to be important. In the random phase approximation, screening can be included through the polarization. We derive an analytical integral expression for the polarization in both the full-band…
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In this article we study the static polarization in ABC-stacked multilayer graphene. Since the density of states diverges for these systems if the number of layers exceeds three, screening effects are expected to be important. In the random phase approximation, screening can be included through the polarization. We derive an analytical integral expression for the polarization in both the full-band model and an effective two-band model. Numerical evaluation of these integrals are very time consuming in the full-band model. Hence, for ABC-stacked trilayer graphene, we use the two-band model to calculate the low momentum part of the polarization. The results for the two-band model are universal, i.e. independent of doping. The high momentum part is linear and is determined by calculating two points, such that we can determine the slope. For ABC stacked trilayer graphene, the slope is given by three times the monolayer value. We compare our results to previous ones in the literature and discuss the similarities and discrepancies. Our results can be used to include screening in ABC-stacked multilayer systems in a way that all the characteristics of the polarization function are included. The numerical results for the polarization of trilayer graphene are used to sketch the screened potential.
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Submitted 22 September, 2013; v1 submitted 19 April, 2013;
originally announced April 2013.
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Ferromagnetism in ABC-trilayer graphene
Authors:
Richard Olsen,
Ralph van Gelderen,
C. Morais Smith
Abstract:
In this article we study the ferromagnetic behavior of ABC-stacked trilayer graphene. This is done using a nearest-neighbor tight-binding model, in the presence of long-range Coulomb interactions. For a given electron-electron interaction g and doping level n, we determine whether the total energy is minimized for a paramagnetic or ferromagnetic configuration of our variational parameters. The g v…
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In this article we study the ferromagnetic behavior of ABC-stacked trilayer graphene. This is done using a nearest-neighbor tight-binding model, in the presence of long-range Coulomb interactions. For a given electron-electron interaction g and doping level n, we determine whether the total energy is minimized for a paramagnetic or ferromagnetic configuration of our variational parameters. The g versus n phase diagram is first calculated for the unscreened case. We then include the effects of screening using a simplified expression for the fermion bubble diagram. We show that ferromagnetism in ABC-trilayer graphene is more robust than in monolayer, in bilayer, and in ABA-trilayer graphene. Although the screening reduces the ferromagnetic regime in ABC-trilayer graphene, the critical doping level remains one order of magnitude larger than in unscreened bilayer graphene.
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Submitted 19 December, 2012;
originally announced December 2012.
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Optimizing Smith-Waterman alignments
Authors:
Rolf Olsen,
Terence Hwa,
Michael Lassig
Abstract:
Mutual correlation between segments of DNA or protein sequences can be detected by Smith-Waterman local alignments. We present a statistical analysis of alignment of such sequences, based on a recent scaling theory. A new fidelity measure is introduced and shown to capture the significance of the local alignment, i.e., the extent to which the correlated subsequences are correctly identified. It…
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Mutual correlation between segments of DNA or protein sequences can be detected by Smith-Waterman local alignments. We present a statistical analysis of alignment of such sequences, based on a recent scaling theory. A new fidelity measure is introduced and shown to capture the significance of the local alignment, i.e., the extent to which the correlated subsequences are correctly identified. It is demonstrated how the fidelity may be optimized in the space of penalty parameters using only the alignment score data of a single sequence pair.
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Submitted 16 November, 1998;
originally announced November 1998.
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Direct subsurface absorption of hydrogen on Pd(111)
Authors:
Ole Martin Løvvik,
Roar Aspesæter Olsen
Abstract:
We summarize and discuss some of the available experimental and theoretical data important for understanding the role played by subsurface sites in dissociative chemisorption calculations for the H$_2$/Pd(111) system. Then we use a semi-empirical potential energy surface (PES) to model the interaction of a H$_2$ molecule impinging on a Pd(111) surface. The London-Eyring-Polanyi-Sato (LEPS) const…
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We summarize and discuss some of the available experimental and theoretical data important for understanding the role played by subsurface sites in dissociative chemisorption calculations for the H$_2$/Pd(111) system. Then we use a semi-empirical potential energy surface (PES) to model the interaction of a H$_2$ molecule impinging on a Pd(111) surface. The London-Eyring-Polanyi-Sato (LEPS) construction has been extended to make direct subsurface absorption possible. A 2-dimensional wave packet calculation is used to find qualitative trends in the direct subsurface absorption and to reveal the time scales involved. We suggest that a partial in-plane relaxation occurs for the slowest incoming particles, thus resulting in a higher direct subsurface absorption probability for low energies.
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Submitted 13 December, 1995;
originally announced December 1995.