95 research outputs found
Suppression of the Charge-Density-Wave State in SrCaCuO by External Pressure
The influence of external pressure on the charge-density-wave (CDW) ground
state of the quasi-one-dimensional two-leg ladder compound
SrCaCuO has been studied by optical reflectivity
measurements as a function of temperature (10 - 300~K) and pressure (0.3 -
4.3~GPa) over the spectral range 580 - 6000 cm. With increasing pressure
the CDW transition temperature decreases with the linear pressure
coefficient -70~K/GPa, and above 3~GPa the CDW phase is
suppressed at all temperatures. This behavior is similar to that in compounds
SrCaCuO with increasing Ca content at ambient
pressure, with the simple scaling . The size of the
CDW gap decreases with increasing pressure, whereas the dimensionality of the
high-temperature insulating phase in SrCaCuO within
the ladder plane is hardly affected by external pressure.Comment: accepted for publication in Phys. Rev.
Pressure-induced changes in the optical properties of quasi-one-dimensional -NaVO
The pressure-induced changes in the optical properties of
-NaVO single crystals at room temperature were studied
by polarization-dependent Raman and far-infrared reflectivity measurements
under high pressure. From the changes in the Raman- and infrared-active phonon
modes in the pressure range 9 - 12 GPa a transfer of charge between the
different V sites can be inferred. The importance of electron-phonon coupling
in the low-pressure regime is discussed.Comment: 7 pages, 5 figure
Optical spectroscopy study on pressure-induced phase transitions in the three-dimensional Dirac semimetal CdAs
We report a room-temperature optical reflectivity study performed on
[112]-oriented CdAs single crystals over a broad energy range under
external pressure up to 10 GPa. The abrupt drop of the band dispersion
parameter (-parameter) and the interruption of the gradual redshift of the
bandgap at 4~GPa confirms the structural phase transition from a
tetragonal to a monoclinic phase in this material. The pressure-induced
increase of the overall optical conductivity at low energies and the continuous
redshift of the high-energy bands indicate that the system evolves towards a
topologically trivial metallic state, although a complete closing of the band
gap could not be observed in the studied pressure range. Furthermore, a
detailed investigation of the low-pressure regime suggests the possible
existence of an intermediate state between 2 and 4~GPa , that might be a
precursor of the structural phase transition or due to the lifted degeneracy of
the Dirac nodes. Several optical parameters show yet another anomaly at 8~GPa,
where low-temperature superconductivity was found in an earlier study.Comment: submitted to PR
Evolution of optical properties of chromium spinels CdCrO, HgCrS, and ZnCrSe under high pressure
We report pressure-dependent reflection and transmission measurements on
ZnCrSe, HgCrS, and CdCrO single crystals at room
temperature over a broad spectral range 200-24000 cm. The pressure
dependence of the phonon modes and the high-frequency electronic excitations
indicates that all three compounds undergo a pressure-induced structural phase
transition with the critical pressure 15 GPa, 12 GPa, and 10 GPa for
CdCrO, HgCrS, and ZnCrSe, respectively. The
eigenfrequencies of the electronic transitions are very close to the expected
values for chromium crystal-field transitions. In the case of the chalcogenides
pressure induces a red shift of the electronic excitation which indicates a
strong hybridization of the Cr d-bands with the chalcogenide bands.Comment: Accepted for publication in Phys. Rev.
Pressure-induced transition from the dynamic to static Jahn-Teller effect in (PhP)IC
High-pressure infrared transmission measurements on \PhC60 were performed up
to 9 GPa over a broad frequency range (200 - 20000 cm) to monitor the
vibrational and electronic/vibronic excitations under pressure. The four
fundamental T modes of \C60a\ are split into doublets already at the
lowest applied pressure and harden with increasing pressure. Several cation
modes and fullerene-related modes split into doublets at around 2 GPa, the most
prominent one being the G mode. The splitting of the vibrational modes
can be attributed to the transition from the dynamic to static Jahn-Teller
effect, caused by steric crowding at high pressure. Four absorption bands are
observed in the NIR-VIS frequency range. They are discussed in terms of
transitions between LUMO electronic states in \C60a, which are split because of
the Jahn-Teller distortion and can be coupled with vibrational modes. Various
distortions and the corresponding symmetry lowering are discussed. The observed
redshift of the absorption bands indicates that the splitting of the LUMO
electronic states is reduced upon pressure application.Comment: 10 pages, 17 figure
Optical signature of the pressure-induced dimerization in the honeycomb iridate -LiIrO
We studied the effect of external pressure on the electrodynamic properties
of -LiIrO single crystals in the frequency range of the phonon
modes and the Ir - transitions. The abrupt hardening of several phonon
modes under pressure supports the onset of the dimerized phase at the critical
pressure =3.8 GPa. With increasing pressure an overall decrease in
spectral weight of the Ir - transitions is found up to . Above
, the local (on-site) - excitations gain spectral weight with
increasing pressure, which hints at a pressure-induced increase in the
octahedral distortions. The non-local (intersite) Ir - transitions show a
monotonic blue-shift and decrease in spectral weight. The changes observed for
the non-local excitations are most prominent well above , namely for
pressures 12 GPa, and only small changes occur for pressures close to
. The profile of the optical conductivity at high pressures (20 GPa)
appears to be indicative for the dimerized state in iridates.Comment: 10 pages, 6 figures; accepted for publication in Phys. Rev.
Pressure dependence of the Verwey transition in magnetite: an infrared spectroscopic point of view
We investigated the electronic and vibrational properties of magnetite at
temperatures from 300 K down to 10 K and for pressures up to 10 GPa by
far-infrared reflectivity measurements. The Verwey transition is manifested by
a drastic decrease of the overall reflectance and the splitting of the phonon
modes as well as the activation of additional phonon modes. In the whole
studied pressure range the down-shift of the overall reflectance spectrum
saturates and the maximum number of phonon modes is reached at a critical
temperature, which sets a lower bound for the Verwey transition temperature
T. Based on these optical results a pressure-temperature phase
diagram for magnetite is proposed.Comment: 5 pages, 4 figures; accepted for publication in J. Appl. Phy
Stabilization of carbon nanotubes by filling with inner tubes: An optical spectroscopy study on double-walled carbon nanotubes under hydrostatic pressure
The stabilization of carbon nanotubes via the filling with inner tubes is
demonstrated by probing the optical transitions in double-walled carbon
nanotube bundles under hydrostatic pressure with optical spectroscopy.
Double-walled carbon nanotube films were prepared from fullerene peapods and
characterized by HRTEM and optical spectroscopy. In comparison to single-walled
carbon nanotubes, the pressure-induced redshifts of the optical transitions in
the outer tubes are significantly smaller below 10 GPa, demonstrating the
enhanced mechanical stability due to the inner tube already at low pressures.
Anomalies at the critical pressure P12 GPa signal the onset of the
pressure-induced deformation of the tubular cross-sections. The value of P
is in very good agreement with theoretical predictions of the pressure-induced
structural transitions in double-walled carbon nanotube bundles with similar
average diameters.Comment: 6 pages, 4 figures; to appear in Phys. Rev.
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