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Electromagnon excitation in cupric oxide measured by Fabry-Pérot enhanced terahertz Mueller matrix ellipsometry
Here we present the use of Fabry-Pérot enhanced terahertz (THz) Mueller matrix ellipsometry to measure an electromagnon excitation in monoclinic cupric oxide (CuO). As a magnetically induced ferroelectric multiferroic, CuO exhibits coupling between electric and magnetic order. This gives rise to special quasiparticle excitations at THz frequencies called electromagnons. In order to measure the electromagnons in CuO, we exploit single-crystal CuO as a THz Fabry-Pérot cavity to resonantly enhance the excitation’s signature. This enhancement technique enables the complex index of refraction to be extracted. We observe a peak in the absorption coefficient near 0.705 THz and 215 K, which corresponds to the electromagnon excitation. This absorption peak is observed along only one major polarizability axis in the monoclinic a–c plane. We show the excitation can be represented using the Lorentz oscillator model, and discuss how these Lorentz parameters evolve with temperature. Our findings are in excellent agreement with previous characterizations by THz time-domain spectroscopy (THz-TDS), which demonstrates the validity of this enhancement technique
Electromagnon excitation in cupric oxide measured by Fabry-Pérot enhanced terahertz Mueller matrix ellipsometry
Here we present the use of Fabry-Pérot enhanced terahertz (THz) Mueller matrix ellipsometry to measure an electromagnon excitation in monoclinic cupric oxide (CuO). As a magnetically induced ferroelectric multiferroic, CuO exhibits coupling between electric and magnetic order. This gives rise to special quasiparticle excitations at THz frequencies called electromagnons. In order to measure the electromagnons in CuO, we exploit single-crystal CuO as a THz Fabry-Pérot cavity to resonantly enhance the excitation’s signature. This enhancement technique enables the complex index of refraction to be extracted. We observe a peak in the absorption coefficient near 0.705 THz and 215 K, which corresponds to the electromagnon excitation. This absorption peak is observed along only one major polarizability axis in the monoclinic a–c plane. We show the excitation can be represented using the Lorentz oscillator model, and discuss how these Lorentz parameters evolve with temperature. Our findings are in excellent agreement with previous characterizations by THz time-domain spectroscopy (THz-TDS), which demonstrates the validity of this enhancement technique
Electromagnon excitation in cupric oxide measured by Fabry-P\'erot enhanced terahertz Mueller matrix ellipsometry
Here we present the use of Fabry-P\'erot enhanced terahertz (THz) Mueller
matrix ellipsometry to measure an electromagnon excitation in monoclinic cupric
oxide (CuO). As a magnetically induced ferroelectric multiferroic, CuO exhibits
coupling between electric and magnetic order. This gives rise to special
quasiparticle excitations at THz frequencies called electromagnons. In order to
measure the electromagnons in CuO, we exploit single-crystal CuO as a THz
Fabry-P\'erot cavity to resonantly enhance the excitation's signature. This
enhancement technique enables the complex index of refraction to be extracted.
We observe a peak in the absorption coefficient near 0.705 THz and 215 K, which
corresponds to the electromagnon excitation. This absorption peak is observed
along only one major polarizability axis in the monoclinic a-c plane. We show
the excitation can be represented using the Lorentz oscillator model, and
discuss how these Lorentz parameters evolve with temperature. Our findings are
in excellent agreement with previous characterizations by THz time-domain
spectroscopy (THz-TDS), which demonstrates the validity of this enhancement
technique
Persistent coherence of quantum superpositions in an optimally doped cuprate revealed by 2D spectroscopy
Understanding of the precise mechanisms of high-temperature superconductivity
is elusive. In particular, in order to solve the puzzle of the pairing
mechanism, it is important to understand the detailed nature of the excitations
at energies around the superconducting gap. While measurements of the dynamics
of excited electronic populations have been able to give some insight, they
have largely neglected the intricate dynamics of quantum coherence. Here, we
apply multidimensional coherent spectroscopy for the first time to a
prototypical cuprate and report unprecedented coherent dynamics persisting for
~500 fs, originating directly from the quantum superposition of optically
excited states separated by 20 - 60 meV. These results reveal the correlation
between high and low energy excitations, and indicate that the interplay
between many-body states on different energy scales conserves phase coherence.
In revealing these dynamics we demonstrate that multidimensional coherent
spectroscopy can address electronic correlations and interrogate many-body
quantum systems in unprecedented ways
Avoided quasiparticle decay and enhanced excitation continuum in the spin-1/2 near-Heisenberg triangular antiferromagnet Ba3CoSb2O9
We explore the magnetic excitations of the spin-1/2 triangular
antiferromagnet Ba3CoSb2O9 in its 120 degree ordered phase using single-crystal
high-resolution inelastic neutron scattering. Sharp magnons with no decay are
observed throughout reciprocal space, with a strongly renormalized dispersion
and multiple soft modes compared to linear spin wave theory. We propose an
empirical parametrization that can quantitatively capture the complete
dispersions in the three-dimensional Brillouin zone and explicitly show that
the dispersion renormalizations have the direct consequence that one to two
magnon decays are avoided throughout reciprocal space, whereas such decays
would be allowed for the unrenormalized dispersions. At higher energies, we
observe a very strong continuum of excitations with highly-structured intensity
modulations extending up at least 4x the maximum one-magnon energy. The
one-magnon intensities decrease much faster upon increasing energy than
predicted by linear spin wave theory and the higher-energy continuum contains
much more intensity than can be accounted for by a two-magnon cross-section,
suggesting a significant transfer of spectral weight from the high-energy
magnons into the higher-energy continuum states. We attribute the strong
dispersion renormalizations and substantial transfer of spectral weight to
continuum states to the effect of quantum fluctuations and interactions beyond
the spin wave approximation, and make connections to theoretical approaches
that might capture such effects. Finally, through measurements in a strong
applied magnetic field, we find evidence for magnetic domains with opposite
senses for the spin rotation in the 120 degree ordered ground state, as
expected in the absence of Dzyaloshinskii-Moriya interactions, when the sense
of spin rotation is selected via spontaneous symmetry breaking.Comment: 20 pages, 13 figure
Coupling between spin and charge order driven by magnetic field in triangular Ising system LuFe2O4+δ
We present a study of the magnetic-field effect on spin correlations in the charge ordered triangular Ising system LuFe2O4+δ through single crystal neutron diffraction. In the absence of a magnetic field, the strong diffuse neutron scattering observed below the Neel temperature (TN = 240 K) indicates that LuFe2O4+δ shows short-range, two-dimensional (2D) correlations in the FeO5 triangular layers, characterized by the development of a magnetic scattering rod along the 1/3 1/3 L direction, persisting down to 5 K.We also found that on top of the 2D correlations, a long range ferromagnetic component associated with the propagation vector k1 = 0 sets in at around 240 K. On the other hand, an external magnetic field applied along the c-axis effectively favours a three-dimensional (3D) spin correlation between the FeO5 bilayers evidenced by the increase of the intensity of satellite reflections with propagation vector k2 = (1/3, 1/3, 3/2). This magnetic modulation is identical to the charge ordered superstructure, highlighting the field-promoted coupling between the spin and charge degrees of freedom. Formation of the 3D spin correlations suppresses both the rod-type diffuse scattering and the k1 component. Simple symmetry-based arguments provide a natural explanation of the observed phenomenon and put forward a possible charge redistribution in the applied magnetic field
Electric field control of the magnetic chiralities in ferroaxial multiferroic RbFe(MoO4)2
The coupling of magnetic chiralities to the ferroelectric polarisation in
multiferroic RbFe(MoO) is investigated by neutron spherical
polarimetry. Because of the axiality of the crystal structure below
= 190 K, helicity and triangular chirality are
symmetric-exchange coupled, explaining the onset of the ferroelectricity in
this proper-screw magnetic structure - a mechanism that can be generalised to
other systems with "ferroaxial" distortions in the crystal structure. With an
applied electric field we demonstrate control of the chiralities in both
structural domains simultaneously.Comment: 5 pages, 4 figure
A new topological insulator built from quasi one-dimensional atomic ribbons
A novel topological insulator with orthorhombic crystal structure is
demonstrated. It is characterized by quasi one-dimensional, conducting atomic
chains instead of the layered, two-dimensional sheets known from the
established Bi2(Se,Te)3 system. The Sb-doped Bi2Se3 nanowires are grown in a
TiO2-catalyzed process by chemical vapor deposition. The binary Bi2Se3 is
transformed from rhombohedral to orthorhombic by substituting Sb on ∼38% of
the Bi sites. Pure Sb2Se3 is a topologically trivial band insulator with an
orthorhombic crystal structure at ambient conditions, and it is known to
transform into a topological insulator at high pressure. Angle-resolved
photoemission spectroscopy shows a topological surface state, while Sb doping
also tunes the Fermi level to reside in the bandgap