arXiv subjects

C. A. Kuntscher

Publications and source records attributed to C. A. Kuntscher.

At least 55 records · Page 3Linked to original sources

arXiv2010

Metal-insulator transition in NiS$_{2-x}$Se$_x$

The origin of the gap in NiS2 as well as the pressure- and doping-induced metal-insulator transition in the NiS2-xSex solid solutions are investigated both theoretically using the first-principles band structures combined with the dynamical mean-field approximation for the electronic correlations and experimentally by means of infrared and x-ray absorption spectroscopies. The bonding-antibonding splitting in the S-S (Se-Se) dimer is identified as the main parameter controlling the size of the charge gap. The implications for the metal-insulator transition driven by pressure and Se doping are discussed.

cond-mat.str-el↗

arXiv2009

Infrared spectroscopic studies on unoriented single-walled carbon nanotube films under hydrostatic pressure

The electronic properties of as-prepared and purified unoriented single-walled carbon nanotube films were studied by transmission measurements over a broad frequency range (far-infrared up to visible) as a function of temperature (15 K - 295 K) and external pressure (up to 8 GPa). Both the as-prepared and the purified SWCNT films exhibit nearly temperature-independent properties. With increasing pressure the low-energy absorbance decreases suggesting an increasing carrier localization due to pressure-induced deformations. The energy of the optical transitions in the SWCNTs decreases with increasing pressure, which can be attributed to pressure-induced hybridization and symmetry-breaking effects. We find an anomaly in the pressure-induced shift of the optical transitions at $\sim$2 GPa due to a structural phase transition.

cond-mat.mes-hall↗

arXiv2009

Deconfinement transition and dimensional crossover in the Bechgaard-Fabre salts: pressure- and temperature-dependent optical investigations

The infrared response of the organic conductor (TMTSF)$_2$PF$_6$ and the Mott insulator (TMTTF)$_2$PF$_6$ are investigated as a function of temperature and pressure and for the polarization parallel and perpendicular to the molecular stacks. By applying external pressure on (TMTTF)$_2$PF$_6$, the Mott gap rapidly diminishes until the deconfinement transition occurs when the gap energy is approximately twice the interchain transfer integral. In its deconfined state (TMTTF)$_2$PF$_6$ exhibits a crossover from a quasi-one-dimensional to a higher-dimensional metal upon reducing the temperature. For (TMTSF)$_2$PF$_6$ this dimensional crossover is observed either with increase in external pressure or with decrease in temperature. We quantitatively determine the dimensional crossover line in the pressure-temperature diagram based on the degree of coherence in the optical response perpendicular to the molecular stacks.

cond-mat.str-el↗

arXiv2009

Pressure-induced structural phase transition in the Bechgaard-Fabre salts

The crystal structures of the quasi-one-dimensional organic salts (TMTTF)$_2$PF$_6$ and (TMTSF)$_2$PF$_6$ were studied by pressure-dependent x-ray diffraction up to 10 GPa at room temperature. The unit-cell parameters exhibit a clear anomaly due to a structural phase transition at 8.5 and 5.5 GPa for (TMTTF)$_2$PF$_6$ and (TMTSF)$_2$PF$_6$, respectively.

cond-mat.str-el↗

arXiv2009

Effect of high pressure on multiferroic BiFeO3

We report experimental evidence for pressure instabilities in the model multiferroic BiFeO3 and namely reveal two structural phase transitions around 3 GPa and 10 GPa by using diffraction and far-infrared spectroscopy at a synchrotron source. The intermediate phase from 3 to 9 GPa crystallizes in a monoclinic space group, with octahedra tilts and small cation displacements. When the pressure is further increased the cation displacements (and thus the polar character) of BiFeO3 is suppressed above 10 GPa. The above 10 GPa observed non-polar orthorhombic Pnma structure is in agreement with recent theoretical ab-initio prediction, while the intermediate monoclinic phase was not predicted theoretically.

cond-mat.mtrl-sci↗

arXiv2009

Magnetic order induced crystal symmetry lowering in ACr$_2$O$_4$ ferrimagnetic spinels

We demonstrate that the onset of complex spin orders in ACr$_2$O$_4$ spinels with magnetic A$=$Co, Fe and Cu ions lowers the lattice symmetry. This is clearly indicated by the emergence of anisotropic lattice dynamics -- as evidenced by the pronounced phonon splittings -- even when experiments probing static distortions fail. We show that the crystal symmetry in the magnetic phase is reduced from tetragonal to orthorhombic for FeCr$_2$O$_4$ and CuCr$_2$O$_4$ with Jahn-Teller active A-site ions. The conical spin structure in FeCr$_2$O$_4$ is also manifested in the phonon frequencies. In contrast, the multiferroic CoCr$_2$O$_4$ with no orbital degrees of freedom remains nearly cubic in its ground state.

cond-mat.str-el↗

arXiv2008

Mott-Hubbard gap closure and structural phase transition in the oxyhalides TiOBr and TiOCl under pressure

Pressure-dependent transmittance and reflectance spectra of TiOBr and TiOCl single crystals at room temperature suggest the closure of the Mott-Hubbard gap, i.e., the gap is filled with additional electronic states extending down to the far-infrared range. According to pressure-dependent x-ray powder diffraction data the gap closure coincides with a structural phase transition. The transition in TiOBr occurs at slightly lower pressure ($p$=14 GPa) compared to TiOCl ($p$=16 GPa) under hydrostatic conditions, which is discussed in terms of the chemical pressure effect. The results of pressure-dependent transmittance measurements on TiOBr at low temperatures reveal similar effects at 23 K, where the compound is in the spin-Peierls phase at ambient pressure.

cond-mat.str-el↗

arXiv2007

A Raman study of the Charge-Density-Wave State in A$_{0.3}$MoO$_3$ (A = K,Rb)

We report a comparative Raman spectroscopic study of the quasi-one-dimensional charge-density-wave systems \ab (A = K, Rb). The temperature and polarization dependent experiments reveal charge-coupled vibrational Raman features. The strongly temperature-dependent collective amplitudon mode in both materials differ by about 3 cm, thus revealing the role of alkali atom. We discus the observed vibrational features in terms of charge-density-wave ground state accompanied by change in the crystal symmetry. A frequency-kink in some modes seen in \bb between T = 80 K and 100 K supports the first-order lock-in transition, unlike \rb. The unusually sharp Raman lines(limited by the instrumental response) at very low temperatures and their temperature evolution suggests that the decay of the low energy phonons is strongly influenced by the presence of the temperature dependent charge density wave gap.

cond-mat.str-el↗

arXiv2007

Pressure-induced phase transitions in the multiferroic perovskite BiFeO3 studied by far-infrared micro-spectroscopy

We present the results of pressure-dependent far-infrared reflectivity measurements on the multiferroic perovskite BiFeO3 at room temperature. The observed behavior of the infrared-active phonon modes as a funtion of pressure clearly reveals two structural phase transitions around 3.0 and 7.5 GPa, supporting the results of recent Raman and x-ray diffraction studies under pressure. Based on the pressure-dependent frequency shifts of the infrared-active phonon modes we discuss the possible character of the phase transitions.

cond-mat.other↗

arXiv2007

Pressure-induced metallization and structural phase transition of the Mott-Hubbard insulator TiOBr

We investigated the pressure-dependent optical response of the low-dimensional Mott-Hubbard insulator TiOBr by transmittance and reflectance measurements in the infrared and visible frequency range. A suppression of the transmittance above a critical pressure and a concomitant increase of the reflectance are observed, suggesting a pressure-induced metallization of TiOBr. The metallic phase of TiOBr at high pressure is confirmed by the presence of additional excitations extending down to the far-infrared range. The pressure-induced metallization coincides with a structural phase transition, according to the results of x-ray powder diffraction experiments under pressure.

cond-mat.str-el↗

arXiv2007

Metal-insulator transition in the low-dimensional organic conductor (TMTSF)2FSO3 probed by infrared microspectroscopy

We present measurements of the infrared response of the quasi-one-dimensional organic conductor (TMTSF)2$SO3 along (E||a) and perpendicular (E||b') to the stacking axis as a function of temperature. Above the metal-insulator transition related to the anion ordering the optical conductivity spectra show a Drude-like response. Below the transition an energy gap of about 1500 cm-1 (185 meV) opens, leading to the corresponding charge transfer band in the optical conductivity spectra. The analysis of the infrared-active vibrations gives evidence for the long-range crystal structure modulation below the transition temperature and for the short-range order fluctuations of the lattice modulation above the transition temperature. Also we report about a new infrared mode at around 710 cm-1 with a peculiar temperature behavior, which has so far not been observed in any other (TMTSF)2X salt showing a metal-insulator transition. A qualitative model based on the coupling between the TMTSF molecule vibration and the reorientation of electrical dipole moment of the FSO3 anion is proposed, in order to explain the anomalous behavior of the new mode.

cond-mat.str-el↗

arXiv2006

Electronic properties of correlated metals in the vicinity of a charge order transition: optical spectroscopy of $α$-(BEDT-TTF)$_2M$Hg(SCN)$_4$ ($M$ = NH$_4$, Rb, Tl)

The infrared spectra of the quasi-two-dimensional organic conductors $α$-(BEDT-TTF)$_2$$M$Hg(SCN)$_4$ ($M$ = NH$_4$, Rb, Tl) were measured in the range from 50 to 7000 \cm down to low temperatures in order to explore the influence of electronic correlations in quarter-filled metals. The interpretation of electronic spectra was confirmed by measurements of pressure dependant reflectance of $α$-(BEDT-TTF)$_2$KHg(SCN)$_4$ at T=300 K. The signatures of charge order fluctuations become more pronounced when going from the NH$_4$ salt to Rb and further to Tl compounds. On reducing the temperature, the metallic character of the optical response in the NH$_4$ and Rb salts increases, and the effective mass diminishes. For the Tl compound, clear signatures of charge order are found albeit the metallic properties still dominate. From the temperature dependence of the electronic scattering rate the crossover temperature is estimated below which the coherent charge-carriers response sets in. The observations are in excellent agreement with recent theoretical predictions for a quarter-filled metallic system close to charge order.

cond-mat.str-el↗

arXiv2006

Pressure-induced deconfinement of the charge transport in the quasi-one-dimensional Mott insulator (TMTTF)_2AsF_6

We studied the pressure dependence of the room-temperature infrared reflectivity of (TMTTF)_2AsF_6 along all three optical axes. This anisotropic organic compound consists of molecular stacks with orbital overlap along the a direction; due to electronic correlations the system is a quasi-one-dimensional Mott insulator with a charge gap Δ_ρ= 70 meV. The gap is gradually reduced with increasing external pressure, accompanied by the onset of a Drude contribution along the stacking direction. In the perpendicular b' direction a Drude-like optical response is observed for pressures above 2 GPa. This behavior is interpreted in terms of a deconfinement of the electrons in a one-dimensional Mott insulator, i.e. an insulator-to-metal transition which occurs when the interchain transfer integral t_b is approximately equal to half of the charge gap energy. We estimate the values of t_b and the Luttinger liquid parameter K_ρas a function of pressure.

cond-mat.str-el↗

arXiv2006

Effect of pressure on the polarized infrared optical response of quasi-one-dimensional LaTiO$_{3.41}$

The pressure-induced changes in the optical properties of the quasi-one-dimensional conductor LaTiO$_{3.41}$ were studied by polarization-dependent mid-infrared micro-spectroscopy at room temperature. For the polarization of the incident radiation parallel to the conducting direction, the optical conductivity spectrum shows a pronounced mid-infrared absorption band, exhibiting a shift to lower frequencies and an increase in oscillator strength with increasing pressure. On the basis of its pressure dependence, interpretations of the band in terms of electronic transitions and polaronic excitations are discussed. Discontinuous changes in the optical response near 15 GPa are in agreement with a recently reported pressure-induced structural phase transition and indicate the onset of a dimensional crossover in this highly anisotropic system.

cond-mat.mtrl-sci↗

arXiv2006

Pressure-induced insulator-to-metal transition in low-dimensional TiOCl

We studied the transmittance and reflectance of the low-dimensional Mott-Hubbard insulator TiOCl in the infrared and visible frequency range as a function of pressure. The strong suppression of the transmittance and the abrupt increase of the near-infrared reflectance above 12 GPa suggest a pressure-induced insulator-to-metal transition. The pressure-dependent frequency shifts of the orbital excitations, as well as the pressure dependences of the charge gap and the spectral weight of the optical conductivity above the phase transition are presented.

cond-mat.str-el↗

arXiv2005

Doping Effects on the Charge-Density-Wave Dynamics in Blue Bronze

The temperature dependences of the dc resistivity and the nonlinear transport properties in pure, Rb-doped, and W-doped blue bronze K$_{0.3}$MoO$_{3}$ single crystals are presented. In comparison with the Rb doping, the W doping has larger effects on the electrical transport properties and the Peierls transition. In particular, the maximum in the temperature dependence of the threshold field for nonlinear transport, observed in pure and Rb-doped samples around 100 K, is absent in W-doped K$_{0.3}$MoO$_{3}$. These results are discussed with respect to the proposed incommensurate-commensurate transition of the charge-density-wave and its interaction with impurities and normal carriers.

cond-mat.mtrl-sci↗

arXiv2005

Electronic and vibrational properties of low-dimensional perovskites Sr$_{1-y}$La$_y$NbO$_{3.5-x}$

By angle-resolved photoemission spectroscopy and polarization-dependent infrared reflectivity measurements the electronic and vibrational properties of low-dimensional perovskites Sr$_{1-y}$La$_y$NbO$_{3.5-x}$ were studied along different crystal directions. The electronic behavior strongly depends on the oxygen and La content, including quasi-one-dimensional metallic and ferroelectric insulating behavior. An extremely small energy gap at the Fermi level is found for SrNbO$_{3.41}$ and SrNbO$_{3.45}$ along the conducting direction at low temperature, suggestive for a Peierls-type instability. Despite the similar nominal carrier density, for Sr$_{0.8}$La$_{0.2}$NbO$_{3.50}$ the quasi-one-dimensional metallic character is suppressed and no gap opening is observed, which can be explained by differences in the crystal structure. Electron-phonon interaction appears to play an important role in this series of compounds.

cond-mat.mtrl-sci↗

arXiv2004

Comparative magnetotransport and Tc measurements on kappa-(BEDT-TTF)2Cu(SCN)2 under pressure

We compare magnetotransport measurements under pressure on the organic superconductor $κ$-(BEDT-TTF)$_{2}$Cu(SCN)$_{2}$ with different pressure-media and discover that the results are pressure media dependent. This pressure-medium dependence is thought to originate from the difference in thermal contraction between the very soft and highly anisotropic sample and the isotropically contracting, but solid pressure medium, thus resulting in non-hydrostatic pressure on the sample. However, comparison of pressure measurements with different media reveals a pressure-medium independent correlation between the superconducting transition temperature, T$_{\rm c}$, and the size of the quasi 2-dimensional Fermi surface pocket and thus the quasi 2-dimensional carrier density in $κ$-(BEDT-TTF)$_{2}$Cu(SCN)$_{2}$. The observed pressure-induced increase in the quasi 2-dimensional carrier density can be interpreted as a transfer of carriers from quasi 1-dimensional Fermi surface sections, reminiscent of a mechanism in cuprate superconductors, where pressure is known to transfer carriers from the insulating charge reservoir layers into the conducting cuprate sheets.

cond-mat.supr-con↗