591 research outputs found
Heat capacity of the quantum magnet TiOCl
Measurements of the heat capacity C(T,H) of the one-dimensional quantum
magnet TiOCl are presented for temperatures 2K < T < 300K and magnetic fields
up to 5T. Distinct anomalies at 91K and 67K signal two subsequent phase
transitions. The lower of these transitions clearly is of first order and seems
to be related to the spin degrees of freedom. The transition at 92K probably
involves the lattice and/or orbital moments. A detailed analysis of the data
reveals that the entropy change through both transitions is surprisingly small
(~ 0.1R), pointing to the existence strong fluctuations well into the
non-ordered high-temperature phase. No significant magnetic field dependence
was detected.Comment: 4 pages, 2 figure
Group delay in THz spectroscopy with ultra-wideband log-spiral antennae
We report on the group delay observed in continuous-wave terahertz
spectroscopy based on photomixing with phase-sensitive homodyne detection. We
discuss the different contributions of the experimental setup to the phase
difference \Delta\phi(\nu) between transmitter arm and receiver arm. A simple
model based on three contributions yields a quantitative description of the
overall behavior of \Delta\phi(\nu). Firstly, the optical path-length
difference gives rise to a term linear in frequency. Secondly, the
ultra-wideband log-spiral antennae effectively radiate and receive in a
frequency-dependent active region, which in the most simple model is an annular
area with a circumference equal to the wavelength. The corresponding term
changes by roughly 6 pi between 100 GHz and 1 THz. The third contribution stems
from the photomixer impedance. In contrast, the derivative (d\Delta\phi / d\nu)
is dominated by the contribution of periodic modulations of \Delta\phi(\nu)
caused by standing waves, e.g., in the photomixers' Si lenses. Furthermore, we
discuss the Fourier-transformed spectra, which are equivalent to the waveform
in a time-domain experiment. In the time domain, the group delay introduced by
the log-spiral antennae gives rise to strongly chirped signals, in which low
frequencies are delayed. Correcting for the contributions of antennae and
photomixers yields sharp peaks or "pulses" and thus facilitates a
time-domain-like analysis of our continuous-wave data.Comment: 7 pages, 7 figure
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
Dielectric properties of charge ordered LuFe2O4 revisited: The apparent influence of contacts
We show results of broadband dielectric measurements on the charge ordered,
proposed to be mul- tiferroic material LuFe2O4. The temperature and frequency
dependence of the complex permittivity as investigated for temperatures above
and below the charge-oder transition near T_CO ~ 320 K and for frequencies up
to 1 GHz can be well described by a standard equivalent-circuit model
considering Maxwell-Wagner-type contacts and hopping induced AC-conductivity.
No pronounced contribution of intrinsic dipolar polarization could be found and
thus the ferroelectric character of the charge order in LuFe2O4 has to be
questioned.Comment: 4 pages, 3 figure
Anisotropy of the paramagnetic susceptibility in LaTiO: The electron-distribution picture in the ground state
The energy-level scheme and wave functions of the titanium ions in
LaTiO are calculated using crystal-field theory and spin-orbit coupling.
The theoretically derived temperature dependence and anisotropy of the magnetic
susceptibility agree well with experimental data obtained in an untwinned
single crystal. The refined fitting procedure reveals an almost isotropic
molecular field and a temperature dependence of the van Vleck susceptibility.
The charge distribution of the 3d--electron on the Ti positions and the
principle values of the quadrupole moments are derived and agree with NMR data
and recent measurements of orbital momentum and crystal-field splitting.
The low value of the ordered moment in the antiferromagnetic phase is
discussed.Comment: 6 pages, 2 figures, 3 table
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