29 research outputs found
Raman Scattering Spectra of Boron Imidazolate Frameworks Containing Paramagnetic Ions
We present a Raman scattering spectroscopic study of boron imidazolate
metal-organic frameworks (BIFs) with a number of paramagnetic ions in a wide
frequency range from 25 to 1700 cm, which covers local vibrations of the
linkers and well as collective lattice vibrations. We show that the spectral
region above 800 cm belongs to the local vibrations of the linkers,
which have the same frequencies for the studied BIFs without any dependence on
the structure of the BIFs, and are easily interpreted based on the spectra of
imidazolate linkers. In contrast, collective lattice vibrations, observed below
100 cm, show a distinction between cage and two-dimensional BIFs
structures, with a weak dependence on the metal node. We identify the range of
vibrations around 200 cm, which are distinct for each MOF, depending on
a metal node. Our work demonstrates the energy hierarchy in the vibrational
response of BIFs
Dynamical Bonding Driving Mixed Valency in a Metal Boride
Samarium hexaboride is an anomaly, having many exotic and seemingly mutually
incompatible properties. It was proposed to be a mixed-valent semiconductor,
and later - a topological Kondo insulator, and yet has a Fermi surface despite
being an insulator. We propose a new and unified understanding of SmB
centered on the hitherto unrecognized dynamical bonding effect: the coexistence
of two Sm-B bonding modes within SmB, corresponding to different oxidation
states of the Sm. The mixed valency arises in SmB from thermal population
of these distinct minima enabled by motion of B. Our model simultaneously
explains the thermal valence fluctuations, appearance of magnetic Fermi
surface, excess entropy at low temperatures, pressure-induced phase
transitions, and related features in Raman spectra and their unexpected
dependence on temperature and boron isotope
Raman study of magnetic excitations and magneto-elastic coupling in alpha-SrCr2O4
Using Raman spectroscopy, we investigate the lattice phonons, magnetic
excitations, and magneto-elastic coupling in the distorted triangular-lattice
Heisenberg antiferromagnet alpha-SrCr2O4, which develops helical magnetic order
below 43 K. Temperature dependent phonon spectra are compared to predictions
from density functional theory calculations which allows us to assign the
observed modes and identify weak effects arising from coupled lattice and
magnetic degrees of freedom. Raman scattering associated with two-magnon
excitations is observed at 20 meV and 40 meV. These energies are in general
agreement with our ab-initio calculations of exchange interactions and earlier
theoretical predictions of the two-magnon Raman response of triangular-lattice
antiferromagnets. The temperature dependence of the two-magnon excitations
indicates that spin correlations persist well above the N\'eel temperature