139 research outputs found
Orbitally induced hierarchy of exchange interactions in zigzag antiferromagnetic state of honeycomb silver delafossite Ag3Co2SbO6
We report the revised crystal structure, static and dynamic magnetic
properties of quasi-two dimensional honeycomb-lattice silver delafossite
Ag3Co2SbO6. The magnetic susceptibility and specific heat data are consistent
with the onset of antiferromagnetic long range order at low temperatures with
N\'eel temperature TN ~ 21.2 K. In addition, the magnetization curves revealed
a field-induced (spin-flop type) transition below TN in moderate magnetic
fields. The GGA+U calculations show the importance of the orbital degrees of
freedom, which maintain a hierarchy of exchange interaction in the system. The
strongest antiferromagnetic exchange coupling was found in the shortest Co-Co
pairs and is due to direct and superexchange interactions between the
half-filled xz+yz orbitals pointing directly to each other. The other four out
of six nearest neighbor exchanges within the cobalt hexagon are suppressed,
since for these bonds active half-filled orbitals turned out to be parallel and
do not overlap. The electron spin resonance (ESR) spectra reveal a Gaussian
shape line attributed to Co2+ ion in octahedral coordination with average
effective g-factor g=2.3+/-0.1 at room temperature and shows strong divergence
of ESR parameters below 120 K, which imply an extended region of short-range
correlations. Based on the results of magnetic and thermodynamic studies in
applied fields, we propose the magnetic phase diagram for the new
honeycomb-lattice delafossite
The superconducting gaps in LiFeAs: Joint study of specific heat and ARPES
We present specific heat, c_P, and ARPES data on single crystals of the
stoichiometric superconductor LiFeAs. A pronounced anomaly is found in c_P at
the superconducting transition. The electronic contribution can be described by
two s-type energy gaps with magnitudes of approximately Delta1 = 1.2 meV and
Delta2 = 2.6 meV and a normal-state gamma coefficient of 10 mJ/mol K^2. All
these values are in remarkable agreement with ARPES results.Comment: 4 pages, 3 figure
High-pressure behavior of superconducting boron-doped diamond
This work investigates the high-pressure structure of freestanding
superconducting ( = 4.3\,K) boron doped diamond (BDD) and how it affects
the electronic and vibrational properties using Raman spectroscopy and x-ray
diffraction in the 0-30\,GPa range. High-pressure Raman scattering experiments
revealed an abrupt change in the linear pressure coefficients and the grain
boundary components undergo an irreversible phase change at 14\,GPa. We show
that the blue shift in the pressure-dependent vibrational modes correlates with
the negative pressure coefficient of in BDD. The analysis of x-ray
diffraction data determines the equation of state of the BDD film, revealing a
high bulk modulus of =51028\,GPa. The comparative analysis of
high-pressure data clarified that the sp carbons in the grain boundaries
transform into hexagonal diamond.Comment: 7 pages, 4 figure
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