225 research outputs found
Scaling of the microwave magneto-impedance in TlBaCaCuO thin films
We present measurements of the magnetic field-induced microwave complex
resistivity changes at 47 GHz in TlBaCaCuO (TBCCO)
thin films, in the ranges 58 K and 00.8 T. The large
imaginary part points to strong elastic response, but the
data are not easily reconciled with a rigid vortex model. We find that, over a
wide range of temperatures, all the pairs of curves and
can be collapsed on a pair of scaling curves
, , with the same
scaling field . We argue that is related to the loss of
vortex rigidity due to a vortex transformation.Comment: Two printed pages, Proceedings of M2S (Dresden, 2006), to appear in
Physica
Vortex state microwave resistivity in Tl-2212 thin films
We present measurements of the field induced changes in the 47 GHz complex
resistivity, , in TlBaCaCuO
(TBCCO) thin films with 105 K, prepared on CeO buffered
sapphire substrates. At low fields (10 mT) a very small irreversible
feature is present, suggesting a little role of intergranular phenomena. Above
that level exhibits a superlinear dependence with the
field, as opposed to the expected (at high frequencies) quasilinear behaviour.
We observe a crossover between predominantly imaginary to predominantly real
(dissipative) response with increasing temperature and/or field. In addition,
we find the clear scaling property , where the scaling field maps closely the melting
field measured in single crystals. We discuss our microwave results in terms of
loss of flux lines rigidity.Comment: 8 pages, 3 figures, proceedings of 9th HTSHFF, accepted for
publication on J. Supercon
Microwave vortex dynamics in Tl-2212 thin films
We present measurements of the effective surface impedance changes due to a
static magnetic field, \Delta Z(H,T)=\Delta R(H,T)+\rmi \Delta X(H,T), in a
Tl-2212 thin film with 103 K, grown on a CeO buffered sapphire
substrate. Measurements were performed through a dielectric resonator operating
at 47.7 GHz, for temperatures 60 K and magnetic fields T.
We observe exceptionally large field induced variations and pronounced
super-linear field dependencies in both and with
in almost the whole range explored. A careful
analysis of the data allows for an interpretation of these results as dominated
by vortex dynamics. In the intermediate-high field range we extract the main
vortex parameters by resorting to standard high frequency model and by taking
into proper account the creep contribution. The pinning constant shows a marked
decrease with the field which can be interpreted in terms of flux lines
softening associated to an incipient layer decoupling. Small vortex viscosity,
by an order of magnitude lower than in Y-123 are found. Some speculations about
these findings are provided.Comment: pdfTeX, 4 pages, 3 figures, VORTEX 2007 proceedings, to appear in
Physica
Spin-Wave and Electromagnon Dispersions in Multiferroic MnWO4 as Observed by Neutron Spectroscopy: Isotropic Heisenberg Exchange versus Anisotropic Dzyaloshinskii-Moriya Interaction
High resolution inelastic neutron scattering reveals that the elementary
magnetic excitations in multiferroic MnWO4 consist of low energy dispersive
electromagnons in addition to the well-known spin-wave excitations. The latter
can well be modeled by a Heisenberg Hamiltonian with magnetic exchange coupling
extending to the 12th nearest neighbor. They exhibit a spin-wave gap of 0.61(1)
meV. Two electromagnon branches appear at lower energies of 0.07(1) meV and
0.45(1) meV at the zone center. They reflect the dynamic magnetoelectric
coupling and persist in both, the collinear magnetic and paraelectric AF1
phase, and the spin spiral ferroelectric AF2 phase. These excitations are
associated with the Dzyaloshinskii-Moriya exchange interaction, which is
significant due to the rather large spin-orbit coupling.Comment: 8 pages, 6 figures, accepted for publication in Physical Review
Consequences of critical interchain couplings and anisotropy on a Haldane chain
Effects of interchain couplings and anisotropy on a Haldane chain have been
investigated by single crystal inelastic neutron scattering and density
functional theory (DFT) calculations on the model compound SrNiVO.
Significant effects on low energy excitation spectra are found where the
Haldane gap (; where is the intrachain exchange
interaction) is replaced by three energy minima at different antiferromagnetic
zone centers due to the complex interchain couplings. Further, the triplet
states are split into two branches by single-ion anisotropy. Quantitative
information on the intrachain and interchain interactions as well as on the
single-ion anisotropy are obtained from the analyses of the neutron scattering
spectra by the random phase approximation (RPA) method. The presence of
multiple competing interchain interactions is found from the analysis of the
experimental spectra and is also confirmed by the DFT calculations. The
interchain interactions are two orders of magnitude weaker than the
nearest-neighbour intrachain interaction = 8.7~meV. The DFT calculations
reveal that the dominant intrachain nearest-neighbor interaction occurs via
nontrivial extended superexchange pathways Ni--O--V--O--Ni involving the empty
orbital of V ions. The present single crystal study also allows us to
correctly position SrNiVO in the theoretical - phase
diagram [T. Sakai and M. Takahashi, Phys. Rev. B 42, 4537 (1990)] showing where
it lies within the spin-liquid phase.Comment: 12 pages, 12 figures, 3 tables PRB (accepted). in Phys. Rev. B (2015
Revisiting the ground state of CoAlO: comparison to the conventional antiferromagnet MnAlO
The A-site spinel material, CoAl2O4, is a physical realization of the
frustrated diamond-lattice antiferromagnet, a model in which is predicted to
contain unique incommensurate or `spin-spiral liquid' ground states. Our
previous single-crystal neutron scattering study instead classified it as a
`kinetically-inhibited' antiferromagnet, where the long ranged correlations of
a collinear Neel ground state are blocked by the freezing of domain wall motion
below a first-order phase transition at T* = 6.5 K. The current paper expands
on our original results in several important ways. New elastic and inelastic
neutron measurements are presented that show our initial conclusions are
affected by neither the sample measured nor the instrument resolution, while
measurements to temperatures as low as T = 250 mK limit the possible role being
played by low-lying thermal excitations. Polarized diffuse neutron measurements
confirm reports of short-range antiferromagnetic correlations and diffuse
streaks of scattering, but major diffuse features are explained as signatures
of overlapping critical correlations between neighboring Brillouin zones.
Finally, and critically, this paper presents detailed elastic and inelastic
measurements of magnetic correlations in a single-crystal of MnAl2O4, which
acts as an unfrustrated analogue to CoAl2O4. The unfrustrated material is shown
to have a classical continuous phase transition to Neel order at T_N = 39 K,
with collective spinwave excitations and Lorentzian-like critical correlations
which diverge at the transition. Direct comparison between the two compounds
indicates that CoAl2O4 is unique, not in the nature of high-temperature diffuse
correlations, but rather in the nature of the frozen state below T*. The higher
level of cation inversion in the MnAl2O4 sample indicates that this novel
behavior is primarily an effect of greater next-nearest-neighbor exchange.Comment: 13 pages, 8 figures, acccepted for publication in Physical Review
Role of commensurate and incommensurate low-energy excitations in the paramagnetic to hidden-order transition of URuSi
We report low-energy inelastic neutron scattering data of the paramagnetic
(PM) to hidden-order (HO) phase transition at in
URuSi. While confirming previous results for the HO and PM phases, our
data reveal a pronounced wavevector dependence of low-energy excitations across
the phase transition. To analyze the energy scans we employ a damped harmonic
oscillator model containing a fit parameter which is expected to
diverge at a second-order phase transition. Counter to expectations the
excitations at show an abrupt step-like suppression of
below , whereas excitations at , associated
with large-moment antiferromagnetism (LMAF) under pressure, show an enhancement
and a pronounced peak of at . Therefore, at the critical HO
temperature , LMAF fluctuations become nearly critical as well. This is
the behavior expected of a super-vector order parameter with nearly degenerate
components for the HO and LMAF leading to nearly isotropic fluctuations in the
combined order-parameter space.Comment: 6 pages; v3 accepted journal version; minor modifications compared to
v
Normal-State Spin Dynamics and Temperature-Dependent Spin Resonance Energy in an Optimally Doped Iron Arsenide Superconductor
The proximity of superconductivity and antiferromagnetism in the phase
diagram of iron arsenides, the apparently weak electron-phonon coupling and the
"resonance peak" in the superconducting spin excitation spectrum have fostered
the hypothesis of magnetically mediated Cooper pairing. However, since most
theories of superconductivity are based on a pairing boson of sufficient
spectral weight in the normal state, detailed knowledge of the spin excitation
spectrum above the superconducting transition temperature Tc is required to
assess the viability of this hypothesis. Using inelastic neutron scattering we
have studied the spin excitations in optimally doped BaFe1.85Co0.15As2 (Tc = 25
K) over a wide range of temperatures and energies. We present the results in
absolute units and find that the normal state spectrum carries a weight
comparable to underdoped cuprates. In contrast to cuprates, however, the
spectrum agrees well with predictions of the theory of nearly antiferromagnetic
metals, without complications arising from a pseudogap or competing
incommensurate spin-modulated phases. We also show that the temperature
evolution of the resonance energy follows the superconducting energy gap, as
expected from conventional Fermi-liquid approaches. Our observations point to a
surprisingly simple theoretical description of the spin dynamics in the iron
arsenides and provide a solid foundation for models of magnetically mediated
superconductivity.Comment: 8 pages, 4 figures, and an animatio
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