10 research outputs found
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
Discovery of the soft electronic modes of the trimeron order in magnetite
The Verwey transition in magnetite (Fe3O4) is the first metal–insulator transition ever observed1 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 trimerons2. 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.United States. Department of Energy. Division of Materials Sciences and Engineering (Award DE-FG02-08ER46521)Gordon and Betty Moore Foundation EPiQS Initiative (Grant GBMF4541 (sample preparation and characterization))National Science Foundation (U.S.). Graduate Research Fellowship Program (Grant 1122374)Swiss National Science Foundation (Fellowships P2ELP2-172290 and P400P2-183842)National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (Award DMR-1720595
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
Observation of Selective Plasmon-Exciton Coupling in Nonradiative Energy Transfer: Donor-Selective versus Acceptor-Selective Plexcitons
We report selectively plasmon-mediated
nonradiative energy transfer between quantum dot (QD) emitters interacting
with each other via Förster-type resonance energy transfer
(FRET) under controlled plasmon coupling either to only the donor
QDs (i.e., donor-selective) or to only the acceptor QDs (i.e., acceptor-selective).
Using layer-by-layer assembled colloidal QD nanocrystal solids with
metal nanoparticles integrated at carefully designed spacing, we demonstrate
the ability to enable/disable the coupled plasmon-exciton (plexciton)
formation distinctly at the donor (exciton departing) site or at the
acceptor (exciton feeding) site of our choice, while not hindering
the donor exciton-acceptor exciton interaction but refraining from
simultaneous coupling to both sites of the donor and the acceptor
in the FRET process. In the case of donor-selective plexciton, we
observed a substantial shortening in the donor QD lifetime from 1.33
to 0.29 ns as a result of plasmon-coupling to the donors and the FRET-assisted
exciton transfer from the donors to the acceptors, both of which shorten
the donor lifetime. This consequently enhanced the acceptor emission
by a factor of 1.93. On the other hand, in the complementary case
of acceptor-selective plexciton we observed a 2.70-fold emission enhancement
in the acceptor QDs, larger than the acceptor emission enhancement
of the donor-selective plexciton, as a result of the combined effects
of the acceptor plasmon coupling and the FRET-assisted exciton feeding.
Here we present the comparative results of theoretical modeling of
the donor- and acceptor-selective plexcitons of nonradiative energy
transfer developed here for the first time, which are in excellent
agreement with the systematic experimental characterization. Such
an ability to modify and control energy transfer through mastering
plexcitons is of fundamental importance, opening up new applications
for quantum dot embedded plexciton devices along with the development
of new techniques in FRET-based fluorescence microscopy
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