316 research outputs found
Observation of insulator-metal transition in EuNiO under high pressure
The charge transfer antiferromagnetic (T =220 K) insulator EuNiO
undergoes, at ambient pressure, a temperature-induced metal insulator MI
transition at T=463 K. We have investigated the effect of pressure (up
to p~20 GPa) on the electronic, magnetic and structural properties of
EuNiO using electrical resistance measurements, {151}^Eu nuclear
resonance scattering of synchrotron radiation and x-ray diffraction,
respectively. With increasing pressure we find at p =5.8 GPa a transition
from the insulating state to a metallic state, while the orthorhombic structure
remains unchanged up to 20 GPa. The results are explained in terms of a gradual
increase of the electronic bandwidth with increasing pressure, which results in
a closing of the charge transfer gap. It is further shown that the
pressure-induced metallic state exhibits magnetic order with a lowervalue of
T (T ~120 K at 9.4 GPa) which disappears between 9.4 and 14.4 GPa.Comment: 10 pages, 3 figure
Structure and superconductivity in the binary ReMo alloys
The binary ReMo alloys, known to cover the full range of solid
solutions, were successfully synthesized and their crystal structures and
physical properties investigated via powder x-ray diffraction, electrical
resistivity, magnetic susceptibility, and heat capacity. By varying the Re/Mo
ratio we explore the full ReMo binary phase diagram, in all its
four different solid phases: hcp-Mg (), -Mn
(), -CrFe (), and bcc-W (),
of which the second is non-centrosymmetric with the rest being centrosymmetric.
All ReMo alloys are superconductors, whose critical temperatures
exhibit a peculiar phase diagram, characterized by three different
superconducting regions. In most alloys the is almost an order of
magnitude higher than in pure Re and Mo. Low-temperature electronic
specific-heat data evidence a fully-gapped superconducting state, whose
enhanced gap magnitude and specific-heat discontinuity suggest a moderately
strong electron-phonon coupling across the series. Considering that several
-Mn-type Re alloys ( = transition metal) show time-reversal
symmetry breaking (TRSB) in the superconducting state, while TRS is preserved
in the isostructural MgIrB or NbOs, the
ReMo alloys represent another suitable system for studying the
interplay of space-inversion, gauge, and time-reversal symmetries in future
experiments expected to probe TRSB in the Re family.Comment: 8 pages, 7 figures, accepted for publication on Physical Review
Material
Spin-state transition in LaCoO3: direct neutron spectroscopic evidence of excited magnetic states
A gradual spin-state transition occurs in LaCoO3 around T~80-120 K, whose
detailed nature remains controversial. We studied this transition by means of
inelastic neutron scattering (INS), and found that with increasing temperature
an excitation at ~0.6 meV appears, whose intensity increases with temperature,
following the bulk magnetization. Within a model including crystal field
interaction and spin-orbit coupling we interpret this excitation as originating
from a transition between thermally excited states located about 120 K above
the ground state. We further discuss the nature of the magnetic excited state
in terms of intermediate-spin (IS, S=1) vs. high-spin (HS, S=2) states. Since
the g-factor obtained from the field dependence of the INS is g~3, the second
interpretation looks more plausible.Comment: 10 pages, 4 figure
Nodeless superconductivity in the noncentrosymmetric MoRhN superconductor: a SR study
The noncentrosymmetric superconductor MoRhN, with K,
adopts a -Mn-type structure (space group 432), similar to that of
MoAlC. Its bulk superconductivity was characterized by magnetization
and heat-capacity measurements, while its microscopic electronic properties
were investigated by means of muon-spin rotation and relaxation (SR). The
low-temperature superfluid density, measured via transverse-field (TF)-SR,
evidences a fully-gapped superconducting state with , very close to 1.76 - the BCS gap value for
the weak coupling case, and a magnetic penetration depth nm.
The absence of spontaneous magnetic fields below the onset of
superconductivity, as determined by zero-field (ZF)-SR measurements, hints
at a preserved time-reversal symmetry in the superconducting state. Both TF-and
ZF-SR results evidence a spin-singlet pairing in MoRhN.Comment: 5 figures and 5 pages. Accepted for publication as a Rapid
Communication in Phys. Rev.
Room-temperature structural phase transition in the quasi-2D spin-1/2 Heisenberg antiferromagnet Cu(pz)(ClO)
Cu(pz)(ClO) (with pz denoting pyrazine CHN) is a
two-dimensional spin-1/2 square-lattice antiferromagnet with =
4.24 K. Due to a persisting focus on the low-temperature magnetic properties,
its room-temperature structural and physical properties caught no attention up
to now. Here we report a study of the structural features of
Cu(pz)(ClO) in the paramagnetic phase, up to 330 K. By employing
magnetization, specific heat, Cl nuclear magnetic resonance, and neutron
diffraction measurements, we provide evidence of a second-order phase
transition at = 294 K, not reported before. The absence of a
magnetic ordering across in the magnetization data, yet the
presence of a sizable anomaly in the specific heat, suggest a structural
order-to-disorder type transition. NMR and neutron-diffraction data corroborate
our conjecture, by revealing subtle angular distortions of the pyrazine rings
and of ClO counteranion tetrahedra, shown to adopt a configuration of
higher symmetry above the transition temperature.Comment: 10 pages, 12 figure
Probing the pairing symmetry in the over-doped Fe-based superconductor Ba_0.35Rb_0.65Fe_2As_2 as a function of hydrostatic pressure
We report muon spin rotation experiments on the magnetic penetration depth
lambda and the temperature dependence of lambda^{-2} in the over-doped Fe-based
high-temperature superconductor (Fe-HTS) Ba_{1-x}Rb_ xFe_2As_2 (x = 0.65)
studied at ambient and under hydrostatic pressures up to p = 2.3 GPa. We find
that in this system lambda^{-2}(T) is best described by d-wave scenario. This
is in contrast to the case of the optimally doped x = 0.35 system which is
known to be a nodeless s^{+-}-wave superconductor. This suggests that the
doping induces the change of the pairing symmetry from s^{+-} to d-wave in
Ba_{1-x}Rb_{x}Fe_{2}As_{2}. In addition, we find that the d-wave order
parameter is robust against pressure, suggesting that d is the common and
dominant pairing symmetry in over-doped Ba_{1-x}Rb_{x}Fe_{2}As_{2}. Application
of pressure of p = 2.3 GPa causes a decrease of lambda(0) by less than 5 %,
while at optimal doping x = 0.35 a significant decrease of lambda(0) was
reported. The superconducting transition temperature T_c as well as the gap to
T_c ratio 2Delta/k_BT_c show only a modest decrease with pressure. By combining
the present data with those previously obtained for optimally doped system x =
0.35 and for the end member x = 1 we conclude that the SC gap symmetry as well
as the pressure effects on the SC quantities strongly depend on the Rb doping
level. These results are discussed in the light of the putative Lifshitz
transition, i.e., a disappearance of the electron pockets in the Fermi surface
of Ba_{1-x}Rb_{x}Fe_{2}As_{2} upon hole doping.Comment: Accepted for publication in Physical Review
Metal-insulator transition in NdEuNiO compounds
Polycrystalline NdEuNiO () compounds
were synthesized in order to investigate the character of the metal-insulator
(MI) phase transition in this series. Samples were prepared through the sol-gel
route and subjected to heat treatments at 1000 C under oxygen
pressures as high as 80 bar. X-ray Diffraction (XRD) and Neutron Powder
Diffraction (NPD), electrical resistivity , and Magnetization
measurements were performed on these compounds. The results of NPD and XRD
indicated that the samples crystallize in an orthorhombic distorted perovskite
structure, space group . The analysis of the structural parameters
revealed a sudden and small expansion of 0.2% of the unit cell volume
when electronic localization occurs. This expansion was attributed to a small
increase of 0.003 \AA{} of the average Ni-O distance and a simultaneous
decrease of of the Ni-O-Ni superexchange angle. The
measurements revealed a MI transition occurring at temperatures
ranging from to 336 K for samples with and 0.50,
respectively. These measurements also show a large thermal hysteresis in
NdNiO during heating and cooling processes suggesting a first-order
character of the phase transition at . The width of this thermal
hysteresis was found to decrease appreciably for the sample
NdEuNiO. The results indicate that cation disorder
associated with increasing substitution of Nd by Eu is responsible for changing
the first order character of the transition in NdNiO.Comment: 19 pages, 9 figure
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