arXiv subjects

R. P. S. M. Lobo

Publications and source records attributed to R. P. S. M. Lobo.

At least 19 recordsLinked to original sources

arXiv2024

Revealing the KH2PO4 soft-mode coupling mechanism with infrared spectroscopy under pressure

We measured the far-infrared reflectivity of a KH2PO4 single crystal up to pressures of 2 GPa in the ferroelectric and paraelectric phases. We find that the nu4 vibrational mode of the PO4 tetrahedron is strongly affected by the applied pressure. At ambient pressure this phonon is destabilized by the presence of the H ions and hence shows a highly damped character, beyond the phonon propagation threshold. Applying a pressure close to 0.6 GPa makes this phonon clearly underdamped. Its behavior closely follows the soft-mode behavior observed in Raman spectroscopy. Our results solve a long standing open problem, demonstrating that the nu4 mode is the excitation mediating the coupling of the hydrogen network to the lattice modes that create the ferroelectic polarization in KH2PO4.

cond-mat.mtrl-sci↗

arXiv2021

Pressure-induced structural phase transition and suppression of Jahn-Teller distortion in the quadruple perovskite structure

By means of in situ synchrotron X-ray diffraction and Raman spectroscopy under hydrostatic pressure, we investigate the structural stability of the quadruple perovskite LaMn7O12. At 34 GPa, the data unveil a first-order structural phase transition from monoclinic I2/m to cubic Im-3 symmetry characterized by a pronounced contraction of the unit cell and by a significant modifications in the Raman phonon modes. The phase transition is also marked by the suppression of Jahn-Teller distortion which is present in the ambient monoclinic phase. In addition, above 20 GPa pressure, a sudden and simultaneous broadening is observed in several Raman modes which suggests the onset of a sizable electron-phonon interaction and incipient charge mobility. Considering that LaMn7O12 is paramagnetic insulator at ambient, and Jahn-Teller distortion is frozen in the high-pressure Im-3 phase, we argue that this phase could be a potential candidate to host a purely electronic insulator-metal transition with no participation of the lattice

cond-mat.mtrl-sci↗

arXiv2020

Metavalent bonding in crystalline solids: how does it collapse?

The chemical bond is one of the most powerful, yet controversial concepts in chemistry, explaining property trends in solids. Recently, a novel type of chemical bonding has been identified in several higher chalcogenides, characterized by a unique property portfolio, unconventional bond breaking and sharing of about one electron between adjacent atoms. Metavalent bonding is a fundamental type of bonding besides covalent, ionic and metallic bonding, raising the pertinent question, if there is a well-defined transition between metavalent and covalent bonding. For three different pseudo-binary lines, namely GeTe1-xSex, Sb2Te3(1-x)Se3x and Bi2-2xSb2xSe3, a sudden drop in several properties, including the optical dielectric constant, the Born effective charge, the electrical conductivity as well as the bond breaking is observed once a critical Se or Sb concentration is reached. This finding provides a blueprint to explore the impact of metavalent bonding on attractive properties utilized in phase change materials and thermoelectrics.

cond-mat.mtrl-sci↗

arXiv2018

Scaling of the Fano effect of the in-plane Fe-As phonon and the superconducting critical temperature in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$

By means of infrared spectroscopy we determine the temperature-doping phase diagram of the Fano effect for the in-plane Fe-As stretching mode in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$. The Fano parameter $1/q^2$, which is a measure of the phonon coupling to the electronic particle-hole continuum, shows a remarkable sensitivity to the magnetic/structural orderings at low temperatures. More strikingly, at elevated temperatures in the paramagnetic/tetragonal state we find a linear correlation between $1/q^2$ and the superconducting critical temperature $T_c$. Based on theoretical calculations and symmetry considerations, we identify the relevant interband transitions that are coupled to the Fe-As mode. In particular, we show that a sizable $xy$ orbital component at the Fermi level is fundamental for the Fano effect and possibly also for the superconducting pairing.

cond-mat.supr-con↗

arXiv2018

Linear behavior of the optical conductivity and incoherent charge transport in BaCoS2

Optical conductivity measurements on a BaCoS2 single crystal show an unusual linear behavior over a broad spectral range. In the paramagnetic phase above 300 K, the spectrum shows no gap, which contradicts the previously proposed scenario of a charge-transfer Mott insulator. Ab initio dynamical mean field theory calculations including a retarded Hubbard interaction explain the data in terms of an incipient opening of a Co(3d)-S(3p) charge-transfer gap concomitant to incoherent charge transport driven by electronic correlations. These results point to a non-Fermi liquid scenario with Hund's metal properties in the paramagnetic state, which arises from an incipient Mott phase destabilized by low-energy charge fluctuations across the vanishing 3d-3p charge-transfer gap.

cond-mat.str-el↗

arXiv2018

Optical study of Dirac fermions and related phonon anomalies in antiferromagnetic compound CaFeAsF

We performed optical studies on CaFeAsF single crystals, a parent compound of the 1111-type iron-based superconductors that undergoes a structural phase transition from tetragonal to orthorhombic at $T_s$ = 121 K and a magnetic one to a spin density wave (SDW) state at $T_N$ = 110 K. In the low temperature optical conductivity spectrum, after the subtraction of a narrow Drude peak, we observe a pronounced singularity around 300 cm$^{-1}$ that separates two regions of quasilinear conductivity. We outline that these characteristic absorption features are signatures of Dirac fermions, similar to what was previously reported for the BaFe$_2$As$_2$ system [1]. In support of this interpretation, we show that for the latter system this singular feature disappears rapidly upon electron and hole doping, as expected if it arises from a van Hove singularity in-between two Dirac cones. Finally, we show that one of the infrared-active phonon modes (the Fe-As mode at 250 cm$^{-1}$) develops a strongly asymmetric line shape in the SDW state and note that this behaviour can be explained in terms of a strong coupling with the Dirac fermions.

cond-mat.supr-con↗

arXiv2017

Infrared probe of the gap evolution across the phase diagram of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$

We measured the optical conductivity of superconducting single crystals of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ with $x$ ranging from 0.40 (optimal doping, $T_c = 39$ K) down to 0.20 (underdoped, $T_c = 16$ K), where a magnetic order coexists with superconductivity. In the normal state, the low-frequency optical conductivity can be described by an incoherent broad Drude component and a coherent narrow Drude component: the broad one is doping-independent, while the narrow one shows strong scattering in the heavily underdoped compound. In the superconducting state, the formation of the condensate leads to a low-frequency suppression of the optical conductivity spectral weight. In the heavily underdoped region, the superfluid density is significantly suppressed, and the weight of unpaired carriers rapidly increases. We attribute these results to changes in the superconducting gap across the phase diagram, which could show a nodal-to-nodeless transition due to the strong interplay between magnetism and superconductivity in underdoped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$.

cond-mat.supr-con↗

arXiv2016

Optical observation of spin-density-wave fluctuations in Ba122 iron-based superconductors

In iron-based superconductors, a spin-density-wave (SDW) magnetic order is suppressed with doping and unconventional superconductivity appears in close proximity to the SDW instability. The optical response of the SDW order shows clear gap features: substantial suppression in the low-frequency optical conductivity, alongside a spectral weight transfer from low to high frequencies. Here, we study the detailed temperature dependence of the optical response in three different series of the Ba122 system [Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$, Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ and BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$]. Intriguingly, we found that the suppression of the low-frequency optical conductivity and spectral weight transfer appear at a temperature $T^{\ast}$ much higher than the SDW transition temperature $T_{SDW}$. Since this behavior has the same optical feature and energy scale as the SDW order, we attribute it to SDW fluctuations. Furthermore, $T^{\ast}$ is suppressed with doping, closely following the doping dependence of the nematic fluctuations detected by other techniques. These results suggest that the magnetic and nematic orders have an intimate relationship, in favor of the magnetic-fluctuation-driven nematicity scenario in iron-based superconductors.

cond-mat.supr-con↗

arXiv2015

Optical conductivity of URu$_2$Si$_2$ in the Kondo Liquid and Hidden-Order Phases

We measured the polarized optical conductivity of URu$_2$Si$_2$ from room temperature down to 5 K, covering the Kondo state, the coherent Kondo liquid regime, and the hidden-order phase. The normal state is characterized by an anisotropic behavior between the ab plane and c axis responses. The ab plane optical conductivity is strongly influenced by the formation of the coherent Kondo liquid: a sharp Drude peak develops and a hybridization gap at 12 meV leads to a spectral weight transfer to mid-infrared energies. The c axis conductivity has a different behavior: the Drude peak already exists at 300 K and no particular anomaly or gap signature appears in the coherent Kondo liquid regime. When entering the hidden-order state, both polarizations see a dramatic decrease in the Drude spectral weight and scattering rate, compatible with a loss of about 50 % of the carriers at the Fermi level. At the same time a density-wave like gap appears along both polarizations at about 6.5 meV at 5 K. This gap closes respecting a mean field thermal evolution in the ab plane. Along the c axis it remains roughly constant and it "fills up" rather than closing.

cond-mat.str-el↗

arXiv2015

Optical Conductivity Evidence of Clean-Limit Superconductivity in LiFeAs

We measured the optical conductivity of superconducting LiFeAs. In the superconducting state, the formation of the condensate leads to a spectral-weight loss and yields a penetration depth of 225 nm. No sharp signature of the superconducting gap is observed. This suggests that the system is likely in the clean limit. A Drude-Lorentz parametrization of the data in the normal state reveals a quasiparticle scattering rate supportive of spin fluctuations and proximity to a quantum critical point.

cond-mat.supr-con↗

arXiv2015

Anomalous phonon redshift in K-doped BaFe2As2 iron pnictides

The effect of K, Co and P dopings on the lattice dynamics in the BaFe$_2$As$_2$ system is studied by infrared spectroscopy. We focus on the phonon at $\sim$ 253 cm$^{-1}$, the highest energy in-plane infrared-active Fe-As mode in BaFe$_2$As$_2$. Our studies show that the Co and P dopings lead to a blue shift of this phonon in frequency, which can be simply interpreted by the change of lattice parameters induced by doping. In sharp contrast, an unusual red shift of the same mode was observed in the K-doped compound, at odds with the above explanation. This anomalous behavior in K-doped BaFe$_2$As$_2$ is more likely associated with the coupling between lattice vibrations and other channels, such as charge or spin. This coupling scenario is also supported by the asymmetric line shape and intensity growth of the phonon in the K-doped compound.

cond-mat.supr-con↗

arXiv2015

Optical conductivity of Ba0.6 K0.4 Fe2 As2 : The effect of in-plane and out-of-plane doping in the superconducting gap

We measured the in-plane optical conductivity of a nearly optimally doped (Ba,K)Fe2As2 single crystal with Tc = 39.1 K. Upon entering the superconducting state the optical conductivity below ~20 meV vanishes, strongly suggesting a fully gapped system. A BCS-like fit requires two different isotropic gaps to describe the optical response of this material. The temperature dependence of the gaps and the penetration depth suggest a strong interband coupling, but no impurity scattering induced pair breaking is present. This contrasts to the large residual conductivity observed in optimally doped Ba(Fe,Co)2As2 and strongly supports an s(+/-) gap symmetry for these compounds.

cond-mat.supr-con↗

arXiv2014

Eliashberg analysis of the optical conductivity in superconducting Pr$_2$CuO$_{x}$ ($x \simeq 4$)

Superconducting Pr$_2$CuO$_x$, $x\simeq 4$ films with $T^\prime$ structure and a $T_c$ of 27 K have been investigated by millimeter-wave transmission and broadband (infrared-to-ultraviolet) reflectivity measurements in the normal and superconducting state. The results obtained by both experimental methods show a consistent picture of the superconducting condensate formation below $T_c$. An Eliashberg analysis of the data proves $d$-wave superconductivity and unitary-limit impurity scattering of the charge carriers below $T_{c}$. The derived electron-exchange boson interaction spectral function $I^2χ(ω)$ shows only marginal changes at the superconducting transition with the mass enhancement factor $λ$, the first inverse moment of $I^2χ(ω)$, being equal to 4.16 at 30 K and to 4.25 at 4 K.

cond-mat.supr-con↗

arXiv2014

Lattice dynamics of the heavy fermion compound URu$_2$Si$_2$

We report a comprehensive investigation of the lattice dynamics of URu$_2$Si$_2$ as a function of temperature using Raman scattering, optical conductivity and inelastic neutron scattering measurements as well as theoretical {\it ab initio} calculations. The main effects on the optical phonon modes are related to Kondo physics. The B$_{1g}$ ($Γ_3$ symmetry) phonon mode slightly softens below $\sim$100~K, in connection with the previously reported softening of the elastic constant, $C_{11}-C_{12}$, of the same symmetry, both observations suggesting a B$_{1g}$ symmetry-breaking instability in the Kondo regime. Through optical conductivity, we detect clear signatures of strong electron-phonon coupling, with temperature dependent spectral weight and Fano line shape of some phonon modes. Surprisingly, the line shapes of two phonon modes, E$_u$(1) and A$_{2u}$(2), show opposite temperature dependencies. The A$_{2u}$(2) mode loses its Fano shape below 150 K, whereas the E$_u$(1) mode acquires it below 100~K, in the Kondo cross-over regime. This may point out to momentum-dependent Kondo physics. By inelastic neutron scattering measurements, we have drawn the full dispersion of the phonon modes between 300~K and 2~K. No remarkable temperature dependence has been obtained including through the hidden order transition. {\it Ab initio} calculations with the spin-orbit coupling are in good agreement with the data except for a few low energy branches with propagation in the (a,b) plane.

cond-mat.str-el↗

arXiv2014

Optical study of superconducting Pr$_2$CuO$_{x}$ with $x\simeq 4$

Superconducting Pr$_2$CuO$_x$, $x\simeq 4$ (PCO) films with $T^\prime$ structure and a $T_c$ of 27 K have been investigated by various optical methods in a wide frequency (7 - 55000 cm$^{-1}$) and temperature (2 to 300 K) range. The optical spectra do not reveal any indication of a normal-state gap formation. A Drude-like peak centered at zero frequency dominates the optical conductivity below 150 K. At higher temperatures, it shifts to finite frequencies. The detailed analysis of the low-frequency conductivity reveals that the Drude peak and a far-infrared (FIR) peak centered at about 300 cm$^{-1}$ persist at all temperatures. The FIR-peak spectral weight is found to grow at the expense of the Drude spectral weight with increasing temperature. The temperature dependence of the penetration depth follows a behavior typical for $d$-wave superconductors. The absolute value of the penetration depth for zero temperature is 1.6 $μ$m, indicating a rather low density of the superconducting condensate.

cond-mat.supr-con↗

arXiv2013

Pseudogap in underdoped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ as seen via optical conductivity

We report the observation of a pseudogap in the \emph{ab}-plane optical conductivity of underdoped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ ($x = 0.2$ and 0.12) single crystals. Both samples show prominent gaps opened by a spin density wave (SDW) order and superconductivity at the transition temperatures $T_{\it SDW}$ and $T_c$, respectively. In addition, we observe an evident pseudogap below $T^{\ast} \sim$ 75 K, a temperature much lower than $T_{\it SDW}$ but much higher than $T_{c}$. A spectral weight analysis shows that the pseudogap is closely connected to the superconducting gap, indicating the possibility of its being a precursor of superconductivity. The doping dependence of the gaps is also supportive of such a scenario.

cond-mat.supr-con↗

arXiv2013

Hidden $T$-Linear Scattering Rate in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ Revealed by Optical Spectroscopy

The optical properties of Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ have been determined in the normal state for a number of temperatures over a wide frequency range. Two Drude terms, representing two groups of carriers with different scattering rates ($1/τ$), well describe the real part of the optical conductivity, $σ_{1}(ω)$. A "broad" Drude component results in an incoherent background with a $T$-independent $1/τ_b$, while a "narrow" Drude component reveals a $T$-linear $1/τ_n$ resulting in a resistivity $ρ_n \equiv 1/σ_{1n}(ω\rightarrow 0)$ also linear in temperature. An arctan($T$) low-frequency spectral weight is also a strong evidence for a $T$-linear 1/$τ$. Comparison to other materials with similar behavior suggests that the $T$-linear $1/τ_n$ and $ρ_n$ in Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ originate from scattering from spin fluctuations and hence that an antiferromagnetic quantum critical point is likely to exist in the superconducting dome.

cond-mat.supr-con↗

arXiv2012

Optical spectroscopy shows that the normal state of URu$_2$Si$_2$ is an anomalous Fermi liquid

Fermi showed that electrons, as a result of their quantum nature, form a gas of particles where the temperature and density follow the so called Fermi distribution. In a metal, as shown by Landau, that despite their strong Coulomb interaction with each other and the positive background ions, the electrons continue to act like free quantum mechanical particles but with enhanced masses. This state of matter, the Landau-Fermi liquid, is recognized experimentally by an electrical resistivity that is proportional to the square of the absolute temperature plus a term proportional to the square of the frequency of the applied field. Calculations show that, if electron-electron scattering dominates the resistivity in a Landau Fermi liquid, the ratio of the two terms, $b$ has the universal value of {\em b} = 4. We find that in the normal state of the heavy Fermion metal URu$_2$Si$_2$, instead of the Fermi liquid value of 4 the coefficient $b$ =1 $\pm$ 0.1. This unexpected result implies that the electrons in this material are experiencing a unique scattering process. This scattering is intrinsic and we suggest that, the uranium $f$ electrons do not hybridize to form a coherent Fermi liquid but instead act like a dense array of elastic impurities, interacting incoherently with the charge carriers. This behavior is not restricted to URu$_2$Si$_2$. Fermi liquid like states with $b \neq$ 4 have been observed in a number of disparate systems but the significance of this result has not been recognized.

cond-mat.str-el↗