30 research outputs found
Momentum distribution of the uniform electron gas: improved parametrization and exact limits of the cumulant expansion
The momentum distribution of the unpolarized uniform electron gas in its
Fermi-liquid regime, n(k,r_s), with the momenta k measured in units of the
Fermi wave number k_F and with the density parameter r_s, is constructed with
the help of the convex Kulik function G(x). It is assumed that , the on-top pair density g(0,r_s) and the kinetic energy t(r_s)
are known (respectively, from effective-potential calculations, from the
solution of the Overhauser model, and from Quantum Monte Carlo calculations via
the virial theorem). Information from the high- and the low-density limit,
corresponding to the random-phase approximation and to the Wigner crystal
limit, is used. The result is an accurate parametrization of n(k,r_s), which
fulfills most of the known exact constraints. It is in agreement with the
effective-potential calculation of Takada and Yasuhara [1991 {\it Phys. Rev.} B
{\bf 44} 7879], is compatible with Quantum Monte Carlo data, and is valid in
the density range . The corresponding cumulant expansions of
the pair density and of the static structure factor are discussed, and some
exact limits are derived
Screened exchange corrections to the random phase approximation from many-body perturbation theory
The random phase approximation (RPA) systematically overestimates the
magnitude of the correlation energy and generally underestimates cohesive
energies. This originates in part from the complete lack of exchange terms,
which would otherwise cancel Pauli exclusion principle violating (EPV)
contributions. The uncanceled EPV contributions also manifest themselves in
form of an unphysical negative pair density of spin-parallel electrons close to
electron-electron coalescence.
We follow considerations of many-body perturbation theory to propose an
exchange correction that corrects the largest set of EPV contributions while
having the lowest possible computational complexity. The proposed method
exchanges adjacent particle/hole pairs in the RPA diagrams, considerably
improving the pair density of spin-parallel electrons close to coalescence in
the uniform electron gas (UEG). The accuracy of the correlation energy is
comparable to other variants of Second Order Screened Exchange (SOSEX)
corrections although it is slightly more accurate for the spin-polarized UEG.
Its computational complexity scales as or
in orbital space or real space, respectively. Its memory requirement scales as
Editors’ Choice—4D Neutron and X-ray Tomography Studies of High Energy Density Primary Batteries: Part I. Dynamic Studies of LiSOCl2 during Discharge
The understanding of dynamic processes in Li-metal batteries is an important consideration to enable the full capacity of cells to be utilised. These processes, however, are generally not directly observable using X-ray techniques due to the low attenuation of Li; and are challenging to visualise using neutron imaging due to the low temporal resolution of the technique. In this work, complementary X-ray and neutron imaging are combined to track the dynamics of Li within a primary Li/SOCl2 cell. The temporal challenges posed by neutron imaging are overcome using the golden ratio imaging method which enables the identification of Li diffusion in operando. This combination of techniques has enabled an improved understanding of the processes which limit rate performance in Li/SOCl2 cells and may be applied beyond this chemistry to other Li-metal cells
Hellmann-Feynman theorem and fluctuation-correlation analysis of the Calogero-Sutherland model
Exploiting the results of the exact solution for the ground state of the
one-dimensional spinless quantum gas of Fermions and impenetrable Bosons with
the mu/x_{ij}^2 particle-particle interaction, the Hellmann-Feynman theorem
yields mutually compensating divergences of both the kinetic and the
interaction energy in the limiting case mu to -1/4. These divergences result
from the peculiar behavior of both the momentum distribution (for large
momenta) and the pair density (for small inter-particle separation). The
available analytical pair densities for mu=-1/4, 0, and 2 allow to analyze
particle-number fluctuations. They are suppressed by repulsive interaction
(mu>0), enhanced by attraction (mu<0), and may therefore measure the kind and
strength of correlation. Other recently proposed purely quantum-kinematical
measures of the correlation strength arise from the small-separation behavior
of the pair density or - for Fermions - from the non-idempotency of the
momentum distribution and its large-momenta behavior. They are compared with
each other and with reference-free, short-range correlation-measuring ratios of
the kinetic and potential energies.Comment: 30 pages, 9 figures, revised version, short version appeared as PRB
62, 15279-15282 (2000
Multi-Dimensional Characterization of Battery Materials
Demand for low carbon energy storage has highlighted the importance of imaging techniques for the characterization of electrode microstructures to determine key parameters associated with battery manufacture, operation, degradation, and failure both for next generation lithium and other novel battery systems. Here, recent progress and literature highlights from magnetic resonance, neutron, X-ray, focused ion beam, scanning and transmission electron microscopy are summarized. Two major trends are identified: First, the use of multi-modal microscopy in a correlative fashion, providing contrast modes spanning length- and time-scales, and second, the application of machine learning to guide data collection and analysis, recognizing the role of these tools in evaluating large data streams from increasingly sophisticated imaging experiments
Neutron imaging of lithium batteries
Advanced batteries are critical to achieving net zero and are proposed within decarbonization strategies for transport and grid-scale applications, alongside their ubiquitous application in consumer devices. Immense progress has been made in lithium battery technology in recent years, but significant challenges remain and new development strategies are required to improve performance, fully exploit power density capacities, utilize sustainable resources, and lower production costs. Suitable characterization techniques are crucial for understanding, inter alia, three-dimensional (3D) diffusion processes and formation of passivation layers or dendrites, which can lead to drastic capacity reduction and potentially to hazardous short circuiting. Studies of such phenomena typically utilize 2D or 3D imaging techniques, providing locally resolved information. 3D X-ray imaging is a widely used standard method, while time-lapse (4D) tomography is increasingly required for understanding the processes and transformations in an operational battery. Neutron imaging overcomes some of the limitations of X-ray tomography for battery studies. Notably, the high visibility of neutrons for light-Z elements, in particular hydrogen and lithium, enables the direct observation of lithium diffusion, electrolyte consumption, and gas formation in lithium batteries. Neutron imaging as a non-destructive analytical tool has been steadily growing in many disciplines as a result of improvements to neutron detectors and imaging facilities, providing increasingly higher spatial and temporal resolutions. Further, ongoing developments in diffraction imaging for mapping the structural and microstructural properties of battery components make the use of neutrons increasingly attractive. Here, we provide an overview of neutron imaging techniques, generally outlining advances and limitations for studies on batteries and reviewing imaging studies of lithium batteries. We conclude with an outlook on development methods in the field and discuss their potential and significance for future battery research