28 research outputs found
Structural trends from a consistent set of single-crystal data of REFeAsO (RE = La, Ce, Pr, Nd, Sm, Gd, and Tb)
A new crystal growth technique for single-crystals of REFeAsO (RE = La, Ce,
Pr, Nd, Sm, Gd, and Tb) using NaI/KI as flux is presented. Crystals with a size
up to 300 m were isolated for single-crystal X-ray diffraction
measurements. Lattice parameters were determined by LeBail fits of X-ray powder
data against LaB6 standard. A consistent set of structural data is obtained and
interpreted in a hard-sphere model. Effective radii for the rare-earth metal
atoms for REFeAsO are deduced. The relation of the intra- and inter-plane
distances of the arsenic atoms is identified as limiter of the phase formation,
and its influence on Tc is discussed.Comment: 8 pages, 11 figures, 3 tables, fig. 6 changed to numerical plot,
minor changes to the text, accepted for publication in PR
Anisotropic electrical resistivity of LaFeAsO: evidence for electronic nematicity
Single crystals of LaFeAsO were successfully grown out of KI flux.
Temperature dependent electrical resistivity was measured with current flow
along the basal plane, \rho_perpend(T), as well as with current flow along the
crystallographic c-axis, \rho_parallel(T), the latter one utilizing electron
beam lithography and argon ion beam milling. The anisotropy ratio was found to
lie between \rho_parallel/\rho_perpend = 20 - 200. The measurement of
\rho_perpend(T) was performed with current flow along the tetragonal [1 0 0]
direction and along the [1 1 0] direction and revealed a clear in-plane
anisotropy already at T \leq 175 K. This is significantly above the
orthorhombic distortion at T_0 = 147 K and indicates the formation of an
electron nematic phase. Magnetic susceptibility and electrical resistivity give
evidence for a change of the magnetic structure of the iron atoms from
antiferromagnetic to ferromagnetic arrangement along the c-axis at T^\ast = 11
K.Comment: 10 pages, 6 figures, minor change
Magnetic Properties of an Effective Spin- Triangular-Lattice Compound LiYbS
Here, we report the synthesis and magnetic properties of a Yb-based
triangular-lattice compound LiYbS. At low temperatures, it features an
effective spin- state due to the combined effect of crystal
electric field and spin orbit coupling. Magnetic susceptibility measurements
and Li nuclear magnetic resonance experiments reveal the absence of
magnetic long range ordering down to 2~K, which suggests a possible quantum
spin liquid ground state. A dominant antiferromagnetic nearest neighbour
exchange interaction 5.3~K could be extracted form the
magnetic susceptibility. The NMR linewidth analysis yields the coupling
constant between the Li nuclei and Yb ions which was found to be purely
dipolar in nature.Comment: (accepted
Yb delafossites: unique exchange frustration of 4f spin 1/2 moments on a perfect triangular lattice
While the Heisenberg model for magnetic Mott insulators on planar lattice
structures is comparatively well understood in the case of transition metal
ions, the intrinsic spin-orbit entanglement of 4f magnetic ions on such
lattices shows fascinating new physics largely due to corresponding strong
anisotropies both in their single-ion and their exchange properties. We show
here that the Yb delafossites, containing perfect magnetic Yb triangular
lattice planes with pseudospin at low temperatures, are an ideal
platform to study these new phenomena. Competing frustrated interactions may
lead to an absence of magnetic order associated to a gapless spin liquid ground
state with a huge linear specific heat exceeding that of many heavy fermions,
whereas the application of a magnetic field induces anisotropic magnetic order
with successive transitions into different long ranged ordered structures. In
this comparative study, we discuss our experimental findings in terms of a
unified crystal-field and exchange model. We combine electron paramagnetic
resonance (EPR) experiments and results from neutron scattering with
measurements of the magnetic susceptibility, isothermal magnetization up to
full polarization, and specific heat to determine the relevant model
parameters. The impact of the crystal field is discussed as well as the
symmetry-compatible form of the exchange tensor, and we give explicit
expressions for the anisotropic g factor, the temperature dependence of the
susceptibility, the exchange-narrowed EPR linewidth and the saturation field.Comment: 20 pages, 7 figures, to appear in Physical Review
Magnetic behavior of EuCu2As2: Delicate balance between antiferromagnetic and ferromagnetic order
The Eu-based compound, EuCu2As2, crystallizing in the ThCr2Si2-type
tetragonal structure, has been synthesized and its magnetic behavior has been
investigated by magnetization (M), heat-capacity (C) and electrical resistivity
(rho) measurements as a function of temperature (T) and magnetic field (H) as
well as by 151Eu Moessbauer measurements. The results reveal that Eu is
divalent ordering antiferromagnetically below 15 K in the absence of magnetic
field, apparently with the formation of magnetic Brillouin-zone boundary gaps.
A fascinating observation is made in a narrow temperature range before
antiferromagnetism sets in: That is, there is a remarkable upturn just below 20
K in the plot of magnetic susceptibility versus T even at low fields, as though
the compound actually tends to order ferromagnetically. There are corresponding
anomalies in the magnetocaloric effect data as well. In addition, a small
application of magnetic field (around 1 kOe at 1.8 K) in the antiferromagnetic
state causes spin-reorientation effect. These results suggest that there is a
close balance between antiferromagnetism and ferromagnetism in this compoundComment: Phys. Rev. B, in pres
Electrical resistivity and tunneling anomalies in CeCuAs2
The compound CeCuAs2 is found to exhibit negative temperature (T) coefficient
of electrical resistivity (rho) under ambient pressure conditions in the entire
T-range of investigation (45 mK to 300 K), even in the presence of high
magnetic fields. Preliminary tunneling spectroscopic measurements indicate the
existence of a psuedo-gap at least at low temperatures, thereby implying that
this compound could be classified as a Kondo semi-conductor, though rho(T)
interestingly is not found to be of an activated type.Comment: To appear in the proceedings of SCES200