124 research outputs found
Spin-Waves in the Mid-Infrared Spectrum of Antiferromagnetic YBaCuO
The mid-infrared spin-wave spectrum of antiferromagnetic
YBaCuO\ was determined by infrared transmission and reflection
measurements (\bbox{k} \!\! \parallel c) at ~K.\@ Excitation of
single magnons of the optical branch was observed at
~meV.\@ Two further peaks at ~meV
() and ~meV
() both belong to the two-magnon spectrum. Linear
spin wave theory is in good agreement with the measured two-magnon spectrum,
and allows to determine the exchange constant to be about ~meV,
whereas the intrabilayer coupling is approximately .Comment: 3 figures in uuencoded for
Unifying Magnons and Triplons in Stripe-Ordered Cuprate Superconductors
Based on a two-dimensional model of coupled two-leg spin ladders, we derive a
unified picture of recent neutron scattering data of stripe-ordered
La_(15/8)Ba_(1/8)CuO_4, namely of the low-energy magnons around the
superstructure satellites and of the triplon excitations at higher energies.
The resonance peak at the antiferromagnetic wave vector Q_AF in the
stripe-ordered phase corresponds to a saddle point in the dispersion of the
magnetic excitations. Quantitative agreement with the neutron data is obtained
for J= 130-160 meV and J_cyc/J = 0.2-0.25.Comment: 4 pages, 4 figures included updated version taking new data into
account; factor in spectral weight corrected; Figs. 2 and 4 change
Self-normalizing phase measurement in multimode terahertz spectroscopy based on photomixing of three lasers
Photomixing of two near-infrared lasers is well established for
continuous-wave terahertz spectroscopy. Photomixing of three lasers allows us
to measure at three terahertz frequencies simultaneously. Similar to Fourier
spectroscopy, the spectral information is contained in an nterferogram, which
is equivalent to the waveform in time-domain spectroscopy. We use one fixed
terahertz frequency \nu_ref to monitor temporal drifts of the setup, i.e., of
the optical path-length difference. The other two frequencies are scanned for
broadband high-resolution spectroscopy. The frequency dependence of the phase
is obtained with high accuracy by normalizing it to the data obtained at
\nu_ref, which eliminates drifts of the optical path-length difference. We
achieve an accuracy of about 1-2 microns or 10^{-8} of the optical path length.
This method is particularly suitable for applications in nonideal environmental
conditions outside of an air-conditioned laboratory.Comment: 5 pages, 5 figure
Mott-Hubbard exciton in the optical conductivity of YTiO3 and SmTiO3
In the Mott-Hubbard insulators YTiO3 and SmTiO3 we study optical excitations
from the lower to the upper Hubbard band, d^1d^1 -> d^0d^2. The multi-peak
structure observed in the optical conductivity reflects the multiplet structure
of the upper Hubbard band in a multi-orbital system. Absorption bands at 2.55
and 4.15 eV in the ferromagnet YTiO3 correspond to final states with a triplet
d^2 configuration, whereas a peak at 3.7 eV in the antiferromagnet SmTiO3 is
attributed to a singlet d^2 final state. A strongly temperature-dependent peak
at 1.95 eV in YTiO3 and 1.8 eV in SmTiO3 is interpreted in terms of a Hubbard
exciton, i.e., a charge-neutral (quasi-)bound state of a hole in the lower
Hubbard band and a double occupancy in the upper one. The binding to such a
Hubbard exciton may arise both due to Coulomb attraction between
nearest-neighbor sites and due to a lowering of the kinetic energy in a system
with magnetic and/or orbital correlations. Furthermore, we observe anomalies of
the spectral weight in the vicinity of the magnetic ordering transitions, both
in YTiO3 and SmTiO3. In the G-type antiferromagnet SmTiO3, the sign of the
change of the spectral weight at T_N depends on the polarization. This
demonstrates that the temperature dependence of the spectral weight is not
dominated by the spin-spin correlations, but rather reflects small changes of
the orbital occupation.Comment: Strongly extended version; new data of SmTiO3 included; detailed
discussion of temperature dependence include
Optical absorption spectra in SrCu_2O_3 two-leg spin ladder
We calculate the phonon-assisted optical-absorption spectra in SrCu_2O_3
two-leg spin-ladder systems. The results for two models proposed for SrCu_2O_3
are compared. In the model including the effects of a cyclic four-spin
interaction, the shoulder structure appears at 978 cm^{-1} and the peak appears
at 1975 cm^{-1} in the spectrum for polarization of the electric field parallel
to the legs. In the other model which describes a pure two-leg ladder, the peak
appears around the lower edge of the spectrum at 1344 cm^{-1}. The feature can
be effective in determining the proper model for SrCu_2O_3.Comment: 5 pages, 5 figures, to appear in PRB vol. 67 (2003
Optically probing symmetry breaking in the chiral magnet Cu2OSeO3
We report on the linear optical properties of the chiral magnet Cu2OSeO3,
specifically associated with the absence of inversion symmetry, the chiral
crystallographic structure, and magnetic order. Through spectroscopic
ellipsometry, we observe local crystal-field excitations below the
charge-transfer gap. These crystal-field excitations are optically allowed due
to the lack of inversion symmetry at the Cu sites. Optical polarization
rotation measurements were used to study the structural chirality and magnetic
order. The temperature dependence of the natural optical rotation, originating
in the chiral crystal structure, provides evidence for a finite
magneto-electric effect in the helimagnetic phase. We find a large
magneto-optical susceptibility on the order of V(540nm)~10^4 rad/(T*m) in the
helimagnetic phase and a maximum Faraday rotation of ~165deg/mm in the
ferrimagnetic phase. The large value of V can be explained by considering spin
cluster formation and the relative ease of domain reorientation in this
metamagnetic material. The magneto-optical activity allows us to map the
magnetic phase diagram, including the skyrmion lattice phase. In addition to
this, we probe and discuss the nature of the various magnetic phase transitions
in Cu2OSeO3.Comment: 9 pages, 10 figure
Revealing puddles of electrons and holes in compensated topological insulators
Three-dimensional topological insulators harbour metallic surface states with
exotic properties. In transport or optics, these properties are typically
masked by defect-induced bulk carriers. Compensation of donors and acceptors
reduces the carrier density, but the bulk resistivity remains disappointingly
small. We show that measurements of the optical conductivity in BiSbTeSe
pinpoint the presence of electron-hole puddles in the bulk at low temperatures,
which is essential for understanding DC bulk transport. The puddles arise from
large fluctuations of the Coulomb potential of donors and acceptors, even in
the case of full compensation. Surprisingly, the number of carriers appearing
within puddles drops rapidly with increasing temperature and almost vanishes
around 40 K. Monte Carlo simulations show that a highly non-linear screening
effect arising from thermally activated carriers destroys the puddles at a
temperature scale set by the Coulomb interaction between neighbouring dopants,
explaining the experimental observation semi-quantitatively. This mechanism
remains valid if donors and acceptors do not compensate perfectly.Comment: 11 pages with 7 figures plus supplemental material (3 pages
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