753 research outputs found
Cold N+NH Collisions in a Magnetic Trap
We present an experimental and theoretical study of atom-molecule collisions
in a mixture of cold, trapped atomic nitrogen and NH molecules at a temperature
of ~mK. We measure a small N+NH trap loss rate coefficient of
~cms.
Accurate quantum scattering calculations based on {\it ab initio} interaction
potentials are in agreement with experiment and indicate the magnetic dipole
interaction to be the dominant loss mechanism. Our theory further indicates the
ratio of N+NH elastic to inelastic collisions remains large () into the
mK regime
Kz selective scattering within quasiparticle interference measurements of FeSe
Quasiparticle interference (QPI) provides a wealth of information relating to the electronic structure of a material. However, it is often assumed that this information is constrained to two-dimensional electronic states. We show that this is not necessarily the case. For FeSe, a system dominated by surface defects, we show that it is actually all electronic states with negligible group velocity in the z axis that are contained within the experimental data. By using a three-dimensional tight-binding model of FeSe, fit to photoemission measurements, we directly reproduce the experimental QPI scattering dispersion, within a T-matrix formalism, by including both kz=0 and kz=Ï€ electronic states. This result unifies both tunnelling based and photoemission based experiments on FeSe and highlights the importance of kz within surface sensitive measurements of QPI.Publisher PDFPeer reviewe
FeSe and the missing electron pocket problem
LR acknowledges funding from the Royal Commission for the Exhibition 1851.The nature and origin of electronic nematicity remains a significant challenge in our understanding of the iron-based superconductors. This is particularly evident in the iron chalcogenide, FeSe, where it is currently unclear how the experimentally determined Fermi surface near the M point evolves from having two electron pockets in the tetragonal state, to exhibiting just a single electron pocket in the nematic state. This has posed a major theoretical challenge, which has become known as the missing electron pocket problem of FeSe, and is of central importance if we wish to uncover the secrets behind nematicity and superconductivity in the wider iron-based superconductors. Here, we review the recent experimental work uncovering this nematic Fermi surface of FeSe from both ARPES and STM measurements, as well as current theoretical attempts to explain this missing electron pocket of FeSe, with a particular focus on the emerging importance of incorporating the dxy orbital into theoretical descriptions of the nematic state. Furthermore, we will discuss the consequence this missing electron pocket has on the theoretical understanding of superconductivity in this system and present several remaining open questions and avenues for future research.Publisher PDFPeer reviewe
Cold heteromolecular dipolar collisions
We present the first experimental observation of cold collisions between two
different species of neutral polar molecules, each prepared in a single
internal quantum state. Combining for the first time the techniques of Stark
deceleration, magnetic trapping, and cryogenic buffer gas cooling allows the
enhancement of molecular interaction time by 10. This has enabled an
absolute measurement of the total trap loss cross sections between OH and
ND at a mean collision energy of 3.6 cm (5 K). Due to the dipolar
interaction, the total cross section increases upon application of an external
polarizing electric field. Cross sections computed from \emph{ab initio}
potential energy surfaces are in excellent agreement with the measured value at
zero external electric field. The theory presented here represents the first
such analysis of collisions between a radical and a closed-shell
polyatomic molecule.Comment: 7 pages, 5 figure
A general few-projection method for tomographic reconstruction of samples consisting of several distinct materials
We present a method for tomographic reconstruction of objects containing several distinct materials,
which is capable of accurately reconstructing a sample from vastly fewer angular projections than
required by conventional algorithms. The algorithm is more general than many previous discrete
tomography methods, as: (i) a priori knowledge of the exact number of materials is not required; (ii) the linear attenuation coefficient of each constituent material may assume a small range of a priori
unknown values. We present reconstructions from an experimental x-ray computed tomography scan of cortical bone acquired at the SPring-8 synchrotron
Revealing the single electron pocket of FeSe in a single orthorhombic domain
Authors acknowledge Diamond Light Source for time on beamline I05-ARPES under Proposal SI23890. L.C.R. acknowledges funding from the Royal Commission for the Exhibition of 1851.We measure the electronic structure of FeSe from within individual orthorhombic domains. Enabled by an angle-resolved photoemission spectroscopy beamline with a highly focused beam spot (nano-ARPES), we identify clear stripelike orthorhombic domains in FeSe with a length scale of approximately 1-5 μm. Our photoemission measurements of the Fermi surface and band structure within individual domains reveal a single electron pocket at the Brillouin zone corner. This result provides clear evidence for a one-electron-pocket electronic structure of FeSe, observed without the application of uniaxial strain, and calls for further theoretical insight into this unusual Fermi surface topology. Our results also showcase the potential of nano-ARPES for the study of correlated materials with local domain structures.Publisher PDFPeer reviewe
Mechanism of Collisional Spin Relaxation in Triplet-Sigma Molecules
We measure and theoretically determine the effect of molecular rotational
splitting on Zeeman relaxation rates in collisions of cold Triplet-Sigma
molecules with helium atoms in a magnetic field. All four stable isotopomers of
the imidogen (NH) molecule are magnetically trapped and studied in collisions
with 3He and 4He. The 4He data support the predicted inverse square dependence
of the collision induced Zeeman relaxation rate coefficient on the molecular
rotational constant B. The measured 3He rate coefficients are much larger than
4He and depend less strongly on B, and the theoretical analysis indicates they
are strongly affected by a shape resonance. The results demonstrate the
influence of molecular structure on collisional energy transfer at low
temperatures.Comment: 10 pages, 3 figures, revised introduction and focu
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