7,947 research outputs found
High-Frequency Spin Waves in YBa2Cu3O6.15
Pulsed neutron spectroscopy is used to make absolute measurements of the
dynamic magnetic susceptibility of insulating YBa2Cu3O6.15. Acoustic and
optical modes, derived from in- and out-of-phase oscillation of spins in
adjacent CuO2 planes, dominate the spectra and are observed up to 250 meV. The
optical modes appear first at 74 meV. Linear-spin-wave theory gives an
excellent description of the data and yields intra- and inter-layer exchange
constants of J_parallel =125 meV and J_perp = 11 meV respectively and a
spin-wave intensity renormalization Z_chi = 0.4.Comment: postscript, 11 pages, 4 figures, Fig.2 fixe
A Comparison of the High-Frequency Magnetic Fluctuations in Insulating and Superconducting La2-xSrxCuO4
Inelastic neutron scattering performed at a spallation source is used to make
absolute measurements of the dynamic susceptibility of insulating La2CuO4 and
superconducting La2-xSrxCuO4 over the energy range 15<EN<350 meV. The effect of
Sr doping on the magnetic excitations is to cause a large broadening in
wavevector and a substantial change in the spectrum of the local spin
fluctuations. Comparison of the two compositions reveals a new energy scale of
22 meV in La1.86Sr0.14CuO4.Comment: RevTex, 7 Pages, 4 postscript figure
No Evidence for Orbital Loop Currents in Charge Ordered YBaCuO from Polarized Neutron Diffraction
It has been proposed that the pseudogap state of underdoped cuprate
superconductors may be due to a transition to a phase which has circulating
currents within each unit cell. Here, we use polarized neutron diffraction to
search for the corresponding orbital moments in two samples of underdoped
YBaCuO with doping levels and 0.123. In contrast to
some other reports using polarized neutrons, but in agreement with nuclear
magnetic resonance and muon spin rotation measurements, we find no evidence for
the appearance of magnetic order below 300 K. Thus, our experiment suggests
that such order is not an intrinsic property of high-quality cuprate
superconductor single crystals. Our results provide an upper bound for a
possible orbital loop moment which depends on the pattern of currents within
the unit cell. For example, for the CC- pattern proposed by Varma,
we find that the ordered moment per current loop is less than 0.013 for
.Comment: Comments in arXiv:1710.08173v1 fully addresse
Relativistically covariant state-dependent cloning of photons
The influence of the relativistic covariance requirement on the optimality of
the symmetric state-dependent 1 -> 2 cloning machine is studied. Namely, given
a photonic qubit whose basis is formed from the momentum-helicity eigenstates,
the change to the optimal cloning fidelity is calculated taking into account
the Lorentz covariance unitarily represented by Wigner's little group. To
pinpoint some of the interesting results, we found states for which the optimal
fidelity of the cloning process drops to 2/3 which corresponds to the fidelity
of the optimal classical cloner. Also, an implication for the security of the
BB84 protocol is analyzed.Comment: corrected, rewritten and accepted in PR
A neutron scattering study of the interplay between structure and magnetism in Ba(FeCo)As
Single crystal neutron diffraction is used to investigate the magnetic and
structural phase diagram of the electron doped superconductor
Ba(FeCo)As. Heat capacity and resistivity measurements have
demonstrated that Co doping this system splits the combined antiferromagnetic
and structural transition present in BaFeAs into two distinct
transitions. For =0.025, we find that the upper transition is between the
high-temperature tetragonal and low-temperature orthorhombic structures with
( K) and the antiferromagnetic transition occurs at
K. We find that doping rapidly suppresses the
antiferromagnetism, with antiferromagnetic order disappearing at . However, there is a region of co-existence of antiferromagnetism and
superconductivity. The effect of the antiferromagnetic transition can be seen
in the temperature dependence of the structural Bragg peaks from both neutron
scattering and x-ray diffraction. We infer from this that there is strong
coupling between the antiferromagnetism and the crystal lattice
Continuous variable private quantum channel
In this paper we introduce the concept of quantum private channel within the
continuous variables framework (CVPQC) and investigate its properties. In terms
of CVPQC we naturally define a "maximally" mixed state in phase space together
with its explicit construction and show that for increasing number of
encryption operations (which sets the length of a shared key between Alice and
Bob) the encrypted state is arbitrarily close to the maximally mixed state in
the sense of the Hilbert-Schmidt distance. We bring the exact solution for the
distance dependence and give also a rough estimate of the necessary number of
bits of the shared secret key (i.e. how much classical resources are needed for
an approximate encryption of a generally unknown continuous-variable state).
The definition of the CVPQC is analyzed from the Holevo bound point of view
which determines an upper bound of information about an incoming state an
eavesdropper is able to get from his optimal measurement.Comment: upper bound on information Eve can get was revised and substantially
lowered (chapter IV), part of chapter III rewritten, several typos correcte
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