24 research outputs found
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
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.
Signatures of van Hove singularities in the anisotropic in-plane optical conductivity of the topological semimetal NbSiTe
We present a temperature-dependent infrared spectroscopy study on the layered
topological semimetal NbSiTe combined with density-functional theory
(DFT) calculations of the electronic band structure and optical conductivity.
Our results reveal an anisotropic behavior of the in-plane (-plane) optical
conductivity, with three pronounced excitations located at around 0.15, 0.28,
and 0.41~eV for the polarization of the incident radiation along the axis.
These excitations are well reproduced in the theoretical spectra. Based on the
\textit{ab initio} results, the excitations around 0.15 eV and 0.28 eV are
interpreted as fingerprints of van Hove singularities in the electronic band
structure and compared to the findings for other topological semimetals.Comment: 9 pages, 7 figures, accepted for publication in Phys. Rev.
Infrared spectroscopy study of the nodal-line semimetal candidate ZrSiTe under pressure: Hints for pressure-induced phase transitions
We studied the effect of external pressure on the optical response of the
nodal-line semimetal candidate ZrSiTe by reflectivity measurements. At
pressures of a few GPa, the reflectivity, optical conductivity, and loss
function are strongly affected in the whole measured frequency range (200-16500
cm), indicating drastic changes in the electronic band structure. The
pressure-induced shift of the electronic bands affects both the intraband and
interband transitions. We find anomalies in the pressure dependence of several
optical parameters at the pressures 4.1 GPa and
6.5 GPa, suggesting the occurrence of two phase transitions of
either structural or electronic type.Comment: 5 pages, 2 figures, accepted for publication in Phys. Rev.
High-pressure versus isoelectronic doping effect on the honeycomb iridate NaIrO
We study the effect of isoelectronic doping and external pressure in tuning
the ground state of the honeycomb iridate NaIrO by combining optical
spectroscopy with synchrotron x-ray diffraction measurements on single
crystals. The obtained optical conductivity of NaIrO is discussed in
terms of a Mott insulating picture versus the formation of quasimolecular
orbitals and in terms of Kitaev-interactions. With increasing Li content ,
(NaLi)IrO moves deeper into the Mott insulating regime and
there are indications that up to a doping level of 24\% the compound comes
closer to the Kitaev-limit. The optical conductivity spectrum of single
crystalline -LiIrO does not follow the trends observed for the
series up to . There are strong indications that -LiIrO
is less close to the Kitaev-limit compared to NaIrO and closer to the
quasimolecular orbital picture. Except for the pressure-induced hardening of
the phonon modes, the optical properties of NaIrO seem to be robust
against external pressure. Possible explanations of the unexpected evolution of
the optical conductivity with isolectronic doping and the drastic change
between and are given by comparing the pressure-induced changes
of lattice parameters and the optical conductivity with the corresponding
changes induced by doping.Comment: 12 pages, 6 figures, accepted for publication in Phys. Rev.
Two pressure-induced structural phase transitions in TiOCl
We studied the crystal structure of TiOCl up to pressures of =25~GPa at
room temperature by x-ray powder diffraction measurements. Two pressure-induced
structural phase transitions are observed: At 15~GPa emerges
an 22 superstructure with -axis unique monoclinic
symmetry (space group P2/). At 22~GPa all lattice
parameters of the monoclinic phase show a pronounced anomaly. A fraction of the
sample persists in the ambient orthorhombic phase (space group ) over the
whole pressure range.Comment: 5 pages, 5 figures; accepted for publication in Phys. Rev.
Chemical pressure effect on the optical conductivity of the nodal-line semimetals ZrSi (=S, Se, Te) and ZrGe (=S, Te)
ZrSiS is a nodal-line semimetal, whose electronic band structure contains a
diamond-shaped line of Dirac nodes. We carried out a comparative study on the
optical conductivity of ZrSiS and related compounds ZrSiSe, ZrSiTe, ZrGeS, and
ZrGeTe by reflectivity measurements over a broad frequency range combined with
density functional theory calculations. The optical conductivity exhibits a
distinct U shape, ending at a sharp peak at around 10000~cm for all
studied compounds, except for ZrSiTe. The U shape of the optical conductivity
is due to transitions between the linearly dispersing bands crossing each other
along the nodal line. The sharp high-energy peak is related to transitions
between almost parallel bands, and its energy position depends on the
interlayer bonding correlated with the / ratio, which can be tuned by
either chemical or external pressure. For ZrSiTe, another pair of crossing
bands appears in the vicinity of the Fermi level, corrugating the nodal-line
electronic structure and leading to the observed difference in optical
conductivity. The findings suggest that the Dirac physics in Zr compounds
with =Si, Ge and =S, Se, Te is closely connected to the interlayer
bonding.Comment: 9 pages, 6 figure
Polaron physics and crossover transition in magnetite probed by pressure-dependent infrared spectroscopy
The optical properties of magnetite at room temperature were studied by
infrared reflectivity measurements as a function of pressure up to 8 GPa. The
optical conductivity spectrum consists of a Drude term, two sharp phonon modes,
a far-infrared band at around 600 cm, and a pronounced mid-infrared
absorption band. With increasing pressure both absorption bands shift to lower
frequencies and the phonon modes harden in a linear fashion. Based on the shape
of the MIR band, the temperature dependence of the dc transport data, and the
occurrence of the far-infrared band in the optical conductivity spectrum the
polaronic coupling strength in magnetite at room temperature should be
classified as intermediate. For the lower-energy phonon mode an abrupt increase
of the linear pressure coefficient occurs at around 6 GPa, which could be
attributed to minor alterations of the charge distribution among the different
Fe sites.Comment: 7 pages, 7 figure
Pressure-induced excitations in the out-of-plane optical response of the nodal-line semimetal ZrSiS
The anisotropic optical response of the layered, nodal-line semimetal ZrSiS
at ambient and high pressure is investigated by frequency-dependent
reflectivity measurements for the polarization along and perpendicular to the
layers. The highly anisotropic optical conductivity is in very good agreement
with results from density functional theory calculations and confirms the
anisotropic character of ZrSiS. Whereas the in-plane optical conductivity shows
only modest pressure-induced changes, we found strong effects on the
out-of-plane optical conductivity spectrum of ZrSiS, with the appearance of two
prominent excitations. These pronounced pressure-induced effects can neither be
attributed to a structural phase transition according to our single-crystal
x-ray diffraction measurements, nor can they be explained by electronic
correlation and electron-hole pairing effects, as revealed by theoretical
calculations. Our findings are discussed in the context of the recently
proposed excitonic insulator phase in ZrSiS.Comment: 6 pages, 3 figures, accepted for publication in Phys. Rev. Let
Pressure-Induced Excitations in the Out-of-Plane Optical Response of the Nodal-Line Semimetal ZrSiS
The anisotropic optical response of the layered, nodal-line semimetal ZrSiS at ambient and high pressure is investigated by frequency-dependent reflectivity measurements for the polarization along and perpendicular to the layers. The highly anisotropic optical conductivity is in very good agreement with results from density-functional theory calculations and confirms the anisotropic character of ZrSiS. Whereas the in-plane optical conductivity shows only modest pressure-induced changes, we found strong effects on the out-of-plane optical conductivity spectrum of ZrSiS, with the appearance of two prominent excitations. These pronounced pressure-induced effects can neither be attributed to a structural phase transition according to our single-crystal x-ray diffraction measurements, nor can they be explained by electronic correlation and electron-hole pairing effects, as revealed by theoretical calculations. Our findings are discussed in the context of the recently proposed excitonic insulator phase in ZrSiS