44 research outputs found
Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3
The charge response in the barium vanadium sulfide (BaVS3) single crystals is
characterized by dc resistivity and low frequency dielectric spectroscopy. A
broad relaxation mode in MHz range with huge dielectric constant ~= 10^6
emerges at the metal-to-insulator phase transition TMI ~= 67 K, weakens with
lowering temperature and eventually levels off below the magnetic transition
Tchi ~= 30 K. The mean relaxation time is thermally activated in a manner
similar to the dc resistivity. These features are interpreted as signatures of
the collective charge excitations characteristic for the orbital ordering that
gradually develops below TMI and stabilizes at long-range scale below Tchi.Comment: 6 pages, 3 figures, submitted to PR
Pressure-dependent optical investigations of -(BEDT-TTF)I: tuning charge order and narrow gap towards a Dirac semimetal
Infrared optical investigations of -(BEDT-TTF)I have been
performed in the spectral range from 80 to 8000~cm down to temperatures
as low as 10~K by applying hydrostatic pressure. In the metallic state, ~K, we observe a 50\% increase in the Drude contribution as well as the
mid-infrared band due to the growing intermolecular orbital overlap with
pressure up to 11~kbar. In the ordered state, , we extract how
the electronic charge per molecule varies with temperature and pressure:
Transport and optical studies demonstrate that charge order and metal-insulator
transition coincide and consistently yield a linear decrease of the transition
temperature by ~K/kbar. The charge disproportionation
diminishes by /kbar and the optical gap between
the bands decreases with pressure by -47~cm/kbar. In our high-pressure
and low-temperature experiments, we do observe contributions from the massive
charge carriers as well as from massless Dirac electrons to the low-frequency
optical conductivity, however, without being able to disentangle them
unambiguously.Comment: 13 pages, 17 figures, submitted to Phys. Rev.
Two-step transition in a magnetoelectric ferrimagnet Cu2OSeO3
We report a detailed single crystal investigation of a magnetoelectric
ferrimagnet Cu2OSeO3 using dc magnetization and ac susceptibility along the
three principal directions [100], [110] and [111]. We have observed that in
small magnetic fields two magnetic transitions occur, one at Tc = 57 K and the
second one at TN = 58 K. At Tc the non-linear susceptibility reveals the
emergence of the ferromagnetic component and below Tc the magnetization
measurements show the splitting between field-cooled and zero-field-cooled
regimes. Above 1000 Oe the magnetization saturates and the system is in a
single domain state. The temperature dependence of the saturation below Tc can
be well described by m(T) = m(0)[1 - (T/Tc)^2]^{\beta}, with m(0) = 0.56
(mu)B/Cu, corresponding to the 3-up-1-down configuration. The dielectric
constant measured on a thin single crystal shows a systematic deviation below
the transition, indicating an intrinsic magnetoelectric effect.Comment: accepted for publication in PR
Anisotropic Charge Modulation in Ladder Planes of Sr_14-xCa_xCu_24O_41
The charge response of the ladders in Sr_14-xCa_xCu_24O_41 is characterized
by dc resistivity, low frequency dielectric and optical spectroscopy in all
three crystallographic directions. The collective charge-density wave screened
mode is observed in the direction of the rungs for x=0, 3 and 6, in addition to
the mode along the legs. For x=8 and 9, the charge-density-wave response along
the rungs fully vanishes, while the one along the legs persists. The transport
perpendicular to the planes is always dominated by hopping.Comment: 4 pages, 3 figures, submitted to PRB R
Anisotropic charge dynamics in the quantum spin-liquid candidate -(BEDT-TTF)Cu(CN)
We have in detail characterized the anisotropic charge response of the dimer
Mott insulator -(BEDT-TTF)\-Cu(CN) by dc conductivity, Hall
effect and dielectric spectroscopy. At room temperature the Hall coefficient is
positive and close to the value expected from stoichiometry; the temperature
behavior follows the dc resistivity . Within the planes the dc
conductivity is well described by variable-range hopping in two dimensions;
this model, however, fails for the out-of-plane direction. An unusually broad
in-plane dielectric relaxation is detected below about 60 K; it slows down much
faster than the dc conductivity following an Arrhenius law. At around 17 K we
can identify a pronounced dielectric anomaly concomitantly with anomalous
features in the mean relaxation time and spectral broadening. The out-of-plane
relaxation, on the other hand, shows a much weaker dielectric anomaly; it
closely follows the temperature behavior of the respective dc resistivity. At
lower temperatures, the dielectric constant becomes smaller both within and
perpendicular to the planes; also the relaxation levels off. The observed
behavior bears features of relaxor-like ferroelectricity. Because
heterogeneities impede its long-range development, only a weak tunneling-like
dynamics persists at low temperatures. We suggest that the random potential and
domain structure gradually emerge due to the coupling to the anion network.Comment: 14 pages, 13 figure