225 research outputs found
Tunneling spectroscopy for probing orbital anisotropy in iron pnictides
Using realistic multi-orbital tight-binding Hamiltonians and the T-matrix
formalism, we explore the effects of a non-magnetic impurity on the local
density of states in Fe-based compounds. We show that scanning tunneling
spectroscopy (STS) has very specific anisotropic signatures that track the
evolution of orbital splitting (OS) and antiferromagnetic gaps. Both
anisotropies exhibit two patterns that split in energy with decreasing
temperature, but for OS these two patterns map onto each other under 90 degree
rotation. STS experiments that observe these signatures should expose the
underlying magnetic and orbital order as a function of temperature across
various phase transitions.Comment: 12 pages, 9 figures, replacement with minor changes suggested by
referee
Efficient extraction of a collimated ultra-cold neutron beam using diffusive channels
We present a first experimental demonstration of a new method to extract a
well-collimated beam of ultra-cold neutrons (UCN) from a storage vessel.
Neutrons with too large divergence are not removed from the beam by an
absorbing collimation, but a diffuse or semidiffuse channel with high Fermi
potential reflects them back into the vessel. This avoids unnecessary losses
and keeps the storage time high, which may be beneficial when the vessel is
part of a UCN source with long buildup time of a high UCN density
An Improved Neutron Electric Dipole Moment Experiment
A new measurement of the neutron EDM, using Ramsey's method of separated
oscillatory fields, is in preparation at the new high intensity source of
ultra-cold neutrons (UCN) at the Paul Scherrer Institute, Villigen, Switzerland
(PSI). The existence of a non-zero nEDM would violate both parity and time
reversal symmetry and, given the CPT theorem, might lead to a discovery of new
CP violating mechanisms. Already the current upper limit for the nEDM
(|d_n|<2.9E-26 e.cm) constrains some extensions of the Standard Model.
The new experiment aims at a two orders of magnitude reduction of the
experimental uncertainty, to be achieved mainly by (1) the higher UCN flux
provided by the new PSI source, (2) better magnetic field control with improved
magnetometry and (3) a double chamber configuration with opposite electric
field directions.
The first stage of the experiment will use an upgrade of the RAL/Sussex/ILL
group's apparatus (which has produced the current best result) moved from
Institut Laue-Langevin to PSI. The final accuracy will be achieved in a further
step with a new spectrometer, presently in the design phase.Comment: Flavor Physics & CP Violation Conference, Taipei, 200
An Improved Search for the Neutron Electric Dipole Moment
A permanent electric dipole moment of fundamental spin-1/2 particles violates
both parity (P) and time re- versal (T) symmetry, and hence, also charge-parity
(CP) symmetry since there is no sign of CPT-violation. The search for a neutron
electric dipole moment (nEDM) probes CP violation within and beyond the Stan-
dard Model. The experiment, set up at the Paul Scherrer Institute (PSI), an
improved, upgraded version of the apparatus which provided the current best
experimental limit, dn < 2.9E-26 ecm (90% C.L.), by the RAL/Sussex/ILL
collaboration: Baker et al., Phys. Rev. Lett. 97, 131801 (2006). In the next
two years we aim to improve the sensitivity of the apparatus to sigma(dn) =
2.6E-27 ecm corresponding to an upper limit of dn < 5E-27 ecm (95% C.L.), in
case for a null result. In parallel the collaboration works on the design of a
new apparatus to further increase the sensitivity to sigma(dn) = 2.6E-28 ecm.Comment: APS Division for particles and fields, Conference Proceedings, Two
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Effective one-dimensionality of AC hopping conduction in the extreme disorder limit
It is argued that in the limit of extreme disorder AC hopping is dominated by
"percolation paths". Modelling a percolation path as a one-dimensional path
with a sharp jump rate cut-off leads to an expression for the universal AC
conductivity, that fits computer simulations in two and three dimensions better
than the effective medium approximation.Comment: 6 postscript figure
Cubic boron nitride: a new prospective material for ultracold neutron application
For the first time, the neutron optical wall-potential of natural cubic boron
nitride (cBN) was measured at the ultracold neutron (UCN) source of the
research reactor TRIGA Mainz using the time-of-flight method (TOF). The samples
investigated had a wall-potential of (305 +/- 15) neV. This value is in good
agreement with the result extracted from neutron reflectometry data and
theoretical expectations. Because of its high critical velocity for UCN and its
good dielectric characteristics, cubic boron nitride coatings (isotopically
enriched) will be useful for a number of applications in UCN experiments
Gravitational depolarization of ultracold neutrons: comparison with data
We compare the expected effects of so-called gravitationally enhanced depolarization of ultracold neutrons to measurements carried out in a spin-precession chamber exposed to a variety of vertical magnetic-field gradients. In particular, we have investigated the dependence upon these field gradients of spin-depolarization rates and also of shifts in the measured neutron Larmor precession frequency. We find excellent qualitative agreement, with gravitationally enhanced depolarization accounting for several previously unexplained features in the data
Direct characterization of photo-induced lattice dynamics in BaFe₂As₂
Ultrafast light pulses can modify electronic properties of quantum materials by perturbing the underlying, intertwined degrees of freedom. In particular, iron-based superconductors exhibit a strong coupling among electronic nematic fluctuations, spins and the lattice, serving as a playground for ultrafast manipulation. Here we use time-resolved X-ray scattering to measure the lattice dynamics of photoexcited BaFeAs. On optical excitation, no signature of an ultrafast change of the crystal symmetry is observed, but the lattice oscillates rapidly in time due to the coherent excitation of an A mode that modulates the Fe–As–Fe bond angle. We directly quantify the coherent lattice dynamics and show that even a small photoinduced lattice distortion can induce notable changes in the electronic and magnetic properties. Our analysis implies that transient structural modification can be an effective tool for manipulating the electronic properties of multi-orbital systems, where electronic instabilities are sensitive to the orbital character of bands
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