251 research outputs found
The helium atom in a strong magnetic field
We investigate the electronic structure of the helium atom in a magnetic
field b etween B=0 and B=100a.u. The atom is treated as a nonrelativistic
system with two interactin g electrons and a fixed nucleus. Scaling laws are
provided connecting the fixed-nucleus Hamiltonia n to the one for the case of
finite nuclear mass. Respecting the symmetries of the electronic Ham iltonian
in the presence of a magnetic field, we represent this Hamiltonian as a matrix
with res pect to a two-particle basis composed of one-particle states of a
Gaussian basis set. The corresponding generalized eigenvalue problem is solved
numerically, providing in the present paper results for vanish ing magnetic
quantum number M=0 and even or odd z-parity, each for both singlet and triplet
spin symmetry. Total electronic energies of the ground state and the first few
excitations in each su bspace as well as their one-electron ionization energies
are presented as a function of the magnetic fie ld, and their behaviour is
discussed. Energy values for electromagnetic transitions within the M=0 sub
space are shown, and a complete table of wavelengths at all the detected
stationary points with respect to their field dependence is given, thereby
providing a basis for a comparison with observed ab sorption spectra of
magnetic white dwarfs.Comment: 21 pages, 4 Figures, acc.f.publ.in J.Phys.
Hydrogen atom moving across a strong magnetic field: analytical approximations
Analytical approximations are constructed for binding energies,
quantum-mechanical sizes and oscillator strengths of main radiative transitions
of hydrogen atoms arbitrarily moving in magnetic fields 10^{12}-10^{13} G.
Examples of using the obtained approximations for determination of maximum
transverse velocity of an atom and for evaluation of absorption spectra in
magnetic neutron star atmospheres are presented.Comment: 17 pages, 3 figures, 5 tables, LaTeX with IOP style files (included).
In v.2, Fig.1 and Table 5 have been corrected. In v.3, a misprint in the fit
for oscillator strengths, Eq.(21), has been correcte
The ground state of the Lithium atom in strong magnetic fields
The ground and some excited states of the Li atom in external uniform
magnetic fields are calculated by means of our 2D mesh Hartree-Fock method for
field strengths ranging from zero up to 2.35 10^8 T. With increasing field
strength the ground state undergoes two transitions involving three different
electronic configurations: for weak fields the ground state configuration
arises from the field-free 1s^22s configuration, for intermediate fields from
the 1s^22p_{-1} configuration and in high fields the 1s2p_{-1}3d_{-2}
electronic configuration is responsible for the properties of the atom. The
transition field strengths are determined. Calculations on the ground state of
the Li+ ion allow us to describe the field-dependent ionization energy of the
Li atom. Some general arguments on the ground states of multi-electron atoms in
strong magnetic fields are provided.Comment: 11 pages, 6 figures, submitted to Physical Review
The ground state of the carbon atom in strong magnetic fields
The ground and a few excited states of the carbon atom in external uniform
magnetic fields are calculated by means of our 2D mesh Hartree-Fock method for
field strengths ranging from zero up to 2.35 10^9 T. With increasing field
strength the ground state undergoes six transitions involving seven different
electronic configurations which belong to three groups with different spin
projections S_z=-1,-2,-3. For weak fields the ground state configuration arises
from the field-free 1s^2 2s^2 2p_0 2p_{-1}, S_z=-1 configuration. With
increasing field strength the ground state involves the four S_z=-2
configurations 1s^22s2p_0 2p_{-1}2p_{+1}, 1s^22s2p_0 2p_{-1}3d_{-2}, 1s^22p_0
2p_{-1}3d_{-2}4f_{-3} and 1s^22p_{-1}3d_{-2}4f_{-3}5g_{-4}, followed by the two
fully spin polarized S_z=-3 configurations 1s2p_02p_{-1}3d_{-2}4f_{-3}5g_{-4}
and 1s2p_{-1}3d_{-2}4f_{-3}5g_{-4}6h_{-5}. The last configuration forms the
ground state of the carbon atom in the high field regime \gamma>18.664. The
above series of ground state configurations is extracted from the results of
numerical calculations for more than twenty electronic configurations selected
due to some general energetical arguments.Comment: 6 figures,acc. Phys.Rev.
Charge transfer-induced Lifshitz transition and magnetic symmetry breaking in ultrathin CrSBr crystals
Ultrathin CrSBr flakes are exfoliated \emph{in situ} on Au(111) and Ag(111)
and their electronic structure is studied by angle-resolved photoemission
spectroscopy. The thin flakes' electronic properties are drastically different
from those of the bulk material and also substrate-dependent. For both
substrates, a strong charge transfer to the flakes is observed, partly
populating the conduction band and giving rise to a highly anisotropic Fermi
contour with an Ohmic contact to the substrate. The fundamental CrSBr band gap
is strongly renormalized compared to the bulk. The charge transfer to the CrSBr
flake is substantially larger for Ag(111) than for Au(111), but a rigid energy
shift of the chemical potential is insufficient to describe the observed band
structure modifications. In particular, the Fermi contour shows a Lifshitz
transition, the fundamental band gap undergoes a transition from direct on
Au(111) to indirect on Ag(111) and a doping-induced symmetry breaking between
the intra-layer Cr magnetic moments further modifies the band structure.
Electronic structure calculations can account for non-rigid Lifshitz-type band
structure changes in thin CrSBr as a function of doping and strain. In contrast
to undoped bulk band structure calculations that require self-consistent
theory, the doped thin film properties are well-approximated by density
functional theory if local Coulomb interactions are taken into account on the
mean-field level and the charge transfer is considered
Downfolding from Ab Initio to Interacting Model Hamiltonians: Comprehensive Analysis and Benchmarking
Model Hamiltonians are regularly derived from first-principles data to
describe correlated matter. However, the standard methods for this contain a
number of largely unexplored approximations. For a strongly correlated impurity
model system, here we carefully compare standard downfolding techniques with
the best-possible ground-truth estimates for charge-neutral excited state
energies and charge densities using state-of-the-art first-principles many-body
wave function approaches. To this end, we use the vanadocene molecule and
analyze all downfolding aspects, including the Hamiltonian form, target basis,
double counting correction, and Coulomb interaction screening models. We find
that the choice of target-space basis functions emerges as a key factor for the
quality of the downfolded results, while orbital-dependent double counting
correction diminishes the quality. Background screening to the Coulomb
interaction matrix elements primarily affects crystal-field excitations. Our
benchmark uncovers the relative importance of each downfolding step and offers
insights into the potential accuracy of minimal downfolded model Hamiltonians.Comment: 15 pages (+8 pages Supplemental Material), 8 figure
High-resolution ptychographic imaging at a seeded free-electron laser source using OAM beams
Electromagnetic waves possessing orbital angular momentum (OAM) are powerful
tools for applications in optical communications, new quantum technologies and
optical tweezers. Recently, they have attracted growing interest since they can
be harnessed to detect peculiar helical dichroic effects in chiral molecular
media and in magnetic nanostructures. In this work, we perform single-shot per
position ptychography on a nanostructured object at a seeded free-electron
laser, using extreme ultraviolet OAM beams of different topological charge
order generated with spiral zone plates. By controlling , we
demonstrate how the structural features of OAM beam profile determine an
improvement of about 30% in image resolution with respect to conventional
Gaussian beam illumination. This result extends the capabilities of coherent
diffraction imaging techniques, and paves the way for achieving time-resolved
high-resolution (below 100 nm) microscopy on large area samples.Comment: M. Pancaldi and F. Guzzi contributed equally to this wor
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