89 research outputs found
Quantum fluctuations in high field magnetization of 2D square lattice J1-J2 antiferromagnets
The J1-J2 square lattice Heisenberg model with spin S=1/2 has three phases
with long-range magnetic order and two unconventionally ordered phases
depending on the ratio of exchange constants. It describes a number of recently
found layered vanadium oxide compounds. A simple means of investigating the
ground state is the study of the magnetization curve and high-field
susceptibility. We discuss these quantities by using the spin-wave theory and
the exact diagonalization in the whole J1-J2 plane. We compare both results and
find good overall agreement in the sectors of the phase diagram with magnetic
order. Close to the disordered regions the magnetization curve shows strong
deviations from the classical linear behaviour caused by large quantum
fluctuations and spin-wave approximation breaks down. On the FM side (J1<0)
where one approaches the quantum gapless spin nematic ground state this region
is surprisingly large. We find that inclusion of second order spin-wave
corrections does not lead to fundamental improvement. Quantum corrections to
the tilting angle of the ordered moments are also calculated. They may have
both signs, contrary to the always negative first order quantum corrections to
the magnetization. Finally we investigate the effect of the interlayer coupling
and find that the quasi-2D picture remains valid up to |J_\perp/J1| ~ 0.3.Comment: 13 pages, 6figure
MuSR studies of RE(O,F)FeAs (RE = La, Nd, Ce) and LaOFeP systems: possible incommensurate/stripe magnetism and superfluid density
Muon spin relaxation (MuSR) measurements in iron oxy-pnictide systems have
revealed: (1) commensurate long-range order in undoped LaOFeAs; (2) Bessel
function line shape in La(O0.97F0.03)FeAs which indicates possible
incommensurate or stripe magnetism; (3) anomalous weak magnetism existing in
superconducting LaOFeP, Ce(O0.84F0.16)FeAs, and Nd(O0.88F0.12)FeAs but absent
in superconducting La(O0.92F0.08)FeAs; and (4) scaling of superfluid density
and Tc in the Ce, La, and Nd-FeAs superconductors following a nearly linear
relationship found in cuprates.Comment: 4 pages, 5 figures (color
Muon Spin Rotation Measurement of the Magnetic Field Penetration Depth in Ba(Fe0.93 Co0.07)2 As2 : Evidence for Multiple Superconducting Gaps
We have performed transverse field muon spin rotation measurements of single
crystals of Ba(FeCoAs with the applied magnetic field
along the direction. Fourier transforms of the measured spectra
reveal an anisotropic lineshape characteristic of an Abrikosov vortex lattice.
We have fit the SRSR spectra to a microscopic model in terms of the
penetration depth and the Ginzburg-Landau parameter .
We find that as a function of temperature, the penetration depth varies more
rapidly than in standard weak coupled BCS theory. For this reason we first fit
the temperature dependence to a power law where the power varies from 1.6 to
2.2 as the field changes from 200G to 1000G. Due to the surprisingly strong
field dependence of the power and the superfluid density we proceeded to fit
the temperature dependence to a two gap model, where the size of the two gaps
is field independent. From this model, we obtained gaps of
and , corresponding to roughly 6 meV
and 3 meV respectively
Superfluid Density and Field-Induced Magnetism in Ba(Fe1-xCox)2As2 and Sr(Fe1-xCox)2As2 Measured with Muon Spin Relaxation
We report muon spin rotation (SR) measurements of single crystal
Ba(FeCo)As and Sr(FeCo)As. From
measurements of the magnetic field penetration depth we find that for
optimally- and over-doped samples, varies monotonically
with the superconducting transition temperature T. Within the
superconducting state we observe a positive shift in the muon precession
signal, likely indicating that the applied field induces an internal magnetic
field. The size of the induced field decreases with increasing doping but is
present for all Co concentrations studied.Comment: 7 pages, accepted for publication in Phys. Rev.
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