10 research outputs found
Rapid and Precise Determination of Zero-Field Splittings by Terahertz Time-Domain Electron Paramagnetic Resonance Spectroscopy
Zero-field splitting (ZFS) parameters are fundamentally tied to the
geometries of metal ion complexes. Despite their critical importance for
understanding the magnetism and spectroscopy of metal complexes, they are not
routinely available through general laboratory-based techniques, and are often
inferred from magnetism data. Here we demonstrate a simple tabletop
experimental approach that enables direct and reliable determination of ZFS
parameters in the terahertz (THz) regime. We report time-domain measurements of
electron paramagnetic resonance (EPR) signals associated with THz-frequency
ZFSs in molecular complexes containing high-spin transition-metal ions. We
measure the temporal profiles of the free-induction decays of spin resonances
in the complexes at zero and nonzero external magnetic fields, and we derive
the EPR spectra via numerical Fourier transformation of the time-domain
signals. In most cases, absolute values of the ZFS parameters are extracted
from the measured zero-field EPR frequencies, and the signs can be determined
by zero-field measurements at two different temperatures. Field-dependent EPR
measurements further allow refined determination of the ZFS parameters and
access to the g-factor. The results show good agreement with those obtained by
other methods. The simplicity of the method portends wide applicability in
chemistry, biology and material science.Comment: 36 pages, 30 figures, 1 tabl
Z topology and superconductivity from symmetry lowering of a 3D Dirac Metal AuPb
3D Dirac semi-metals (DSMs) are materials that have massless Dirac electrons
and exhibit exotic physical properties It has been suggested that structurally
distorting a DSM can create a Topological Insulator (TI), but this has not yet
been experimentally verified. Furthermore, quasiparticle excitations known as
Majorana Fermions have been theoretically proposed to exist in materials that
exhibit superconductivity and topological surface states. Here we show that the
cubic Laves phase AuPb has a bulk Dirac cone above 100 K that gaps out upon
cooling at a structural phase transition to create a topologically non trivial
phase that superconducts below 1.2 K. The nontrivial Z = -1 invariant in
the low temperature phase indicates that AuPb in its superconducting state
must have topological surface states. These characteristics make AuPb a
unique platform for studying the transition between bulk Dirac electrons and
topological surface states as well as studying the interaction of
superconductivity with topological surface states
Exciton-driven antiferromagnetic metal in a correlated van der Waals insulator
Collective excitations of bound electron-hole pairs -- known as excitons --
are ubiquitous in condensed matter, emerging in systems as diverse as band
semiconductors, molecular crystals, and proteins. Recently, their existence in
strongly correlated electron materials has attracted increasing interest due to
the excitons' unique coupling to spin and orbital degrees of freedom. The
non-equilibrium driving of such dressed quasiparticles offers a promising
platform for realizing unconventional many-body phenomena and phases beyond
thermodynamic equilibrium. Here, we achieve this in the van der Waals
correlated insulator NiPS by photoexciting its newly discovered
spin-orbit-entangled excitons that arise from Zhang-Rice states. By monitoring
the time evolution of the terahertz conductivity, we observe the coexistence of
itinerant carriers produced by exciton dissociation and the long-wavelength
antiferromagnetic magnon that coherently precesses in time. These results
demonstrate the emergence of a transient metallic state that preserves
long-range antiferromagnetism, a phase that cannot be reached by simply tuning
the temperature. More broadly, our findings open an avenue toward the
exciton-mediated optical manipulation of magnetism.Comment: 24 pages, 23 figure
Magnetic field-dependent low-energy magnon dynamics in α – RuCl 3
© 2019 American Physical Society. Revealing the spin excitations of complex quantum magnets is key to developing a minimal model that explains the underlying magnetic correlations in the ground state. We investigate the low-energy magnons in α-RuCl3 by combining time-domain terahertz spectroscopy under an external magnetic field and model Hamiltonian calculations. We observe two absorption peaks around 2.0 and 2.4 meV, which we attribute to zone-center spin waves. Using linear spin-wave theory with only nearest-neighbor terms of the exchange couplings, we calculate the antiferromagnetic resonance frequencies and reveal their dependence on an external field applied parallel to the nearest-neighbor Ru-Ru bonds. We find that the magnon behavior in an applied magnetic field can be understood only by including an off-diagonal Γ exchange term to the minimal Heisenberg-Kitaev model. Such an anisotropic exchange interaction that manifests itself as a result of strong spin-orbit coupling can naturally account for the observed mixing of the modes at higher fields strengths
Discovery of the soft electronic modes of the trimeron order in magnetite
Spectroscopic study of the low-energy excitations in magnetite Fe3O4 shows the signatures of its charge-ordered structure involved in the metal-insulator transition, whose building blocks are the three-site small polarons, termed trimerons.
The Verwey transition in magnetite (Fe3O4) is the first metal-insulator transition ever observed(1) and involves a concomitant structural rearrangement and charge-orbital ordering. Owing to the complex interplay of these intertwined degrees of freedom, a complete characterization of the low-temperature phase of magnetite and the mechanism driving the transition have long remained elusive. It was demonstrated in recent years that the fundamental building blocks of the charge-ordered structure are three-site small polarons called trimerons(2). However, electronic collective modes of this trimeron order have not been detected to date, and thus an understanding of the dynamics of the Verwey transition from an electronic point of view is still lacking. Here, we discover spectroscopic signatures of the low-energy electronic excitations of the trimeron network using terahertz light. By driving these modes coherently with an ultrashort laser pulse, we reveal their critical softening and hence demonstrate their direct involvement in the Verwey transition. These findings shed new light on the cooperative mechanism at the origin of magnetite's exotic ground state.Web of Scienc