184 research outputs found
Antiferromagnetic Domain Wall Engineering in Chromium Films
We have engineered an antiferromagnetic domain wall by utilizing a magnetic
frustration effect of a thin iron cap layer deposited on a chromium film.
Through lithography and wet etching we selectively remove areas of the Fe cap
layer to form a patterned ferromagnetic mask over the Cr film. Removing the Fe
locally removes magnetic frustration in user-defined regions of the Cr film. We
present x-ray microdiffraction microscopy results confirming the formation of a
90{\deg} spin-density wave propagation domain wall in Cr. This domain wall
nucleates at the boundary defined by our Fe mask.Comment: submitted to AP
Studies of superconductivity and structure for CaC6 to pressures above 15 GPa
The dependence of the superconducting transition temperature Tc of CaC6 has
been determined as a function of hydrostatic pressure in both helium-loaded gas
and diamond-anvil cells to 0.6 and 32 GPa, respectively. Following an initial
increase at the rate +0.39(1) K/GPa, Tc drops abruptly from 15 K to 4 K at 10
GPa. Synchrotron x-ray measurements to 15 GPa point to a structural transition
near 10 GPa from a rhombohedral to a higher symmetry phase
Muon spin rotation study of the magnetic penetration depth in the intercalated graphite superconductor CaC6
We report temperature- and magnetic field-dependent bulk muon spin rotation
measurements in a c-axis oriented superconductor CaC6 in the mixed state. Using
both a simple second moment analysis and the more precise analytical
Ginzburg-Landau model, we obtained a field independent in-plane magnetic
penetration depth {\lambda}ab (0) = 72(3) nm. The temperature dependencies of
the normalized muon spin relaxation rate and of the normalized superfluid
density result to be identical, and both are well represented by the clean
limit BCS model with 2\Delta/kB Tc = 3.6(1), suggesting that CaC6 is a fully
gapped BCS superconductor in the clean limit regime.Comment: Accepted for publication in PR
On the correct formula for the lifetime broadened superconducting density of states
We argue that the well known Dynes formula [Dynes R C {\it et al.} 1978 {\it
Phys. Rev. Lett.} {\bf 41} 1509] for the superconducting quasiparticle density
of states, which tries to incorporate the lifetime broadening in an approximate
way, cannot be justified microscopically for conventional superconductors.
Instead, we propose a new simple formula in which the energy gap has a finite
imaginary part and the quasiparticle energy is real. We prove that
in the quasiparticle approximation 2 gives the quasiparticle decay
rate at the gap edge for conventional superconductors. This conclusion does not
depend on the nature of interactions that cause the quasiparticle decay. The
new formula is tested on the case of a strong coupling superconductor
PbBi and an excellent agreement with theoretical predictions is
obtained. While both the Dynes formula and the one proposed in this work give
good fits and fit parameters for PbBi, only the latter formula
can be justified microscopically.Comment: 6 pages, 4 figure
Large Ca Isotope Effect in CaC6
We have measured the Ca isotope effect in the newly discovered superconductor
CaC6. The isotope effect coefficient is 0.50(7). If one assumes that this
material is a conventional electron-phonon coupled superconductor, this result
shows that the superconductivity is dominated by coupling of the electrons by
Ca phonon modes and that C phonons contribute very little. Thus, in contrast to
MgB2, where phonons in the B layers are responsible for the superconductivity,
in CaC6 the phonons are primarily modes of the intercalated Ca.Comment: 11 pages including 2 Figure
STM studies of CoxNbSe2 and MnxNbSe2
Cobalt and Manganese intercalated NbSe(2) single crystals have been synthesized and characterized by DC magnetization and scanning tunnelling microscopy (STM) at low temperatures. We observed a pronounced peak effect in magnetization for both Co and Mn intercalated samples that we further investigated by low temperature STM. A structural phase transition of the vortex lattice (VL) has been observed for applied magnetic fields corresponding to the peak in magnetization
Effect of magnetic impurities on the vortex lattice properties in NbSe2 single crystals
We report a pronounced peak effect in the magnetization of CoxNbSe2 single crystals with critical temperatures T-c ranging between 7.1 and 5.0 K, and MnxNbSe2 single crystals with critical temperatures down to 3.4 K. We correlate the peak effect in magnetization with the structure of the vortex lattice across the peak-effect region using scanning-tunneling microscopy. Magnetization measurements show that the amplitude of the peak effect in the case of CoxNbSe2 exhibits a nonmonotonic behavior as a function of the Co content, reaching a maximum for concentration of Co of about 0.4 at. % (corresponding to a T-c of 5.7 K) and after that gradually decreasing in amplitude with the increase in the Co content. The normalized value of the peak position H-p/H-c2 has weak dependence on Co concentration. In the case of MnxNbSe2 the features of the peak effect as a function of the Mn content are different and they can be understood in terms of strong pinning
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