arXiv subjects

A. B. Kuzmenko

Publications and source records attributed to A. B. Kuzmenko.

At least 37 records · Page 2Linked to original sources

arXiv2009

Optical properties of BaFe$_{2-x}$Co$_x$As$_2$

We present detailed temperature dependent optical data on BaFe$_{2-x}$Co$_{x}$As$_{2}$ (BCFA), with x = 0.14, between 4 meV and 6.5 eV. We analyze our spectra to determine the main optical parameters and show that in this material the interband conductivity already starts around 10 meV. We determine the superfluid density to be 2.2 10^{7}$ cm^{-2}, which places optimally doped BFCA close to the Uemura line. Our experimental data shows clear signs of a superconducting gap with 2$Δ_{1}$ = 6.2 $\pm$ 0.8 meV. In addition we show that the optical spectra are consistent with the presence of an additional band of strongly scattered carriers with a larger gap, 2$Δ_{2}$ = 14 $\pm$ 2 meV.

cond-mat.supr-con↗

arXiv2009

Gate tunable charged-phonon effect in bilayer graphene

We observe a giant increase of the infrared intensity and a softening of the in-plane antisymmetric phonon mode Eu (~0.2 eV) in bilayer graphene as a function of the gate-induced doping. The phonon peak has a pronounced Fano-like asymmetry. We suggest that the intensity growth and the softening originate from the coupling of the phonon mode to the narrow electronic transition between parallel bands of the same character, while the asymmetry is due to the interaction with the continuum of transitions between the lowest hole and electron bands. The growth of the peak is a manifestation of the "charged-phonon" effect observed previously in organic chain conductors and doped fullerenes, which can now be tuned with the gate voltage.

cond-mat.str-el↗

arXiv2009

Determination of the gate-tunable bandgap and tight-binding parameters in bilayer graphene using infrared spectroscopy

We present a compelling evidence for the opening of a bandgap in exfoliated bottom-gated bilayer graphene by fitting the gate-voltage modulated infrared reflectivity spectra in a large range of doping levels with a tight-binding model and the Kubo formula. A close quantitative agreement between the experimental and calculated spectra is achieved, allowing us to determine self-consistently the full set of Slonczewski-Weiss-McClure tight-binding parameters together with the gate-voltage dependent bandgap. The doping dependence of the bandgap shows a good agreement with the existing calculations that take the effects of self-screening into account. We also identify certain mismatches between the tight-binding model and the data, which can be related to electron-electron and electron-phonon interactions.

cond-mat.str-el↗

arXiv2009

Evidence for strongly coupled charge-density-wave ordering in three-dimensional RE5Ir4Si10 compounds by optical measurements

We report optical spectra of Lu$_5$Ir$_4$Si$_{10}$ and Er$_5$Ir$_4$Si$_{10}$, exhibiting the phenomenon of coexisting superconductivity or antiferromagnetism and charge density wave (CDW) order. We measure the maximum value of the charge density wave gap present on part of the Fermi surface of Lu5Ir4Si10, corresponding to a ratio 2Δ/k_B T_CDW \approx 10, well above the value in the limit of weak electron-phonon coupling. Strong electron-phonon coupling was confirmed by analyzing the optical conductivity with the Holstein model describing the electron-phonon interactions, indicating the coupling to phonons centered at 30 meV, with a coupling constant λ\approx 2.6. Finally we provide evidence that approximately 16 % of the Fermi surface of Lu5Ir4Si10 becomes gapped in the CDW state.

cond-mat.str-el↗

arXiv2009

Optics clues to pairing glues in high T$_{c}$ cuprates

We analyze optical spectra of high temperature superconductors using a minimal model of electrons coupled to bosons. We consider the marginal Fermi liquid theory and the spin fluctuation theory, as well as a histogram representation of the bosonic spectral density. We find that the two theories can both be used to describe the experimental data provided that we allow for an additional scattering channel with an energy of 55 meV.

cond-mat.supr-con↗

arXiv2009

Doping dependent optical properties of Bi2201

An experimental study of the in-plane optical conductivity of (Pb$_{x}$,Bi$_{2-x}$)(La$_{y}$Sr$_{2-y}$)CuO$_{6+δ}$ (Bi2201) is presented for a broad doping and temperature range. The in-plane conductivity is analyzed within a strong coupling formalism. We address the interrelationship between the optical conductivity ($σ(ω)$), the single particle self energy, and the electron-boson spectral function. We find that the frequency and temperature dependence can be well described within this formalism. We present a universal description of optical, ARPES and tunneling spectra. The full frequency and temperature dependence of the optical spectra and single particle self-energy is shown to result from an electron-boson spectral function, which shows a strong doping dependence and weak temperature dependence.

cond-mat.supr-con↗

arXiv2009

Optical determination of the relation between the electron-boson coupling function and the critical temperature in high T$_c$ cuprates

We take advantage of the connection between the free carrier optical conductivity and the glue function in the normal state, to reconstruct from the infrared optical conductivity the glue-spectrum of ten different high-Tc cuprates revealing a robust peak in the 50-60 meV range and a broad con- tinuum at higher energies for all measured charge carrier concentrations and temperatures up to 290 K. We observe that the strong coupling formalism accounts fully for the known strong temperature dependence of the optical spectra of the high Tc cuprates, except for strongly underdoped samples. We observe a correlation between the doping trend of the experimental glue spectra and the critical temperature. The data obtained on the overdoped side of the phase diagram conclusively excludes the electron-phonon coupling as the main source of superconducting pairing.

cond-mat.supr-con↗

arXiv2008

Infrared spectroscopy of electronic bands in bilayer graphene

We present infrared spectra (0.1-1 eV) of electrostatically gated bilayer graphene as a function of doping and compare it with tight binding calculations. All major spectral features corresponding to the expected interband transitions are identified in the spectra: a strong peak due to transitions between parallel split-off bands and two onset-like features due to transitions between valence and conduction bands. A strong gate voltage dependence of these structures and a significant electron-hole asymmetry is observed that we use to extract several band parameters. Surprisingly, the structures related to the gate-induced bandgap are much less pronounced in the experiment than predicted by the tight binding model.

cond-mat.str-el↗

arXiv2008

Observation of a robust peak in the glue function of the high-Tc cuprates in the 50-60 meV range

cond-mat.supr-con↗

arXiv2008

Far-infrared probe of superconductivity in SmO$_{1-x}$F$_{x}$FeAs}

We report far-infrared reflectance measurements on polycrystalline superconducting samples of SmO$_{1-x}$F$_{x}$FeAs ($x$ = 0.12, 0.15 and 0.2). We clearly observe superconductivity induced changes of reflectivity in a broad range of energies, which resembles earlier optical measurements on high $T_{c}$ cuprates. The superconducting-to-normal reflectivity ratio $R_{s}/R_{n}$ grows for the photon energies below 18 meV and shows a complicated structure due to the presence of a strong infrared-active phonon at about 10 meV.

cond-mat.supr-con↗

arXiv2007

Optical Integral in the Cuprates and the Question of Sum Rule Violation

Much attention has been given to a possible violation of the optical sum rule in the cuprates, and the connection this might have to kinetic energy lowering. The optical integral is composed of a cut-off independent term (whose temperature dependence is a measure of the sum rule violation), plus a cut-off dependent term that accounts for the extension of the Drude peak beyond the upper bound of the integral. We find that the temperature dependence of the optical integral in the normal state of the cuprates can be accounted for solely by the latter term, implying that the dominant contribution to the observed sum rule `violation' in the normal state is due to the finite cut-off. This cut-off dependent term is well modeled by a theory of electrons interacting with a broad spectrum of bosons.

cond-mat.supr-con↗

arXiv2007

Electron-phonon interaction and charge carrier mass enhancement in SrTiO3

We report a comprehensive THz, infrared and optical study of Nb doped SrTiO3 as well as DC conductivity and Hall effect measurements. Our THz spectra at 7K show the presence of a very narrow (<2 meV) Drude peak, the spectral weight of which shows approximately a factor of three enhancement of the band mass for all carrier concentrations. The missing spectral weight is regained in a broad 'mid-infrared' band which originates from electron-phonon coupling. We find no evidence of a particularly large electron-phonon coupling that would result in small polaron formation. Analysis of the results yields an electron-phonon coupling parameter of an intermediate strength, $α\approx 4$.

cond-mat.str-el↗

arXiv2007

Universal dynamical conductance in graphite

We find experimentally that the optical sheet conductance of graphite per graphene layer is very close to $(π/2)e^2/h$, which is the theoretically expected value of dynamical conductance of isolated monolayer graphene. Our calculations within the Slonczewski-McClure-Weiss model explain well why the interplane hopping leaves the conductance of graphene sheets in graphite almost unchanged for photon energies between 0.1 and 0.6 eV, even though it significantly affects the band structure on the same energy scale. The f-sum rule analysis shows that the large increase of the Drude spectral weight as a function of temperature is at the expense of the removed low-energy optical spectral weight of transitions between hole and electron bands.

cond-mat.str-el↗

arXiv2007

Impact of a finite cut-off for the optical sum rule in the superconducting state

A single band optical sum rule derived by Kubo can reveal a novel kind of superconducting state. It relies, however, on a knowledge of the single band contribution from zero to infinite frequency. A number of experiments over the past five years have used this sum rule; their data has been interpreted in support of 'kinetic energy-driven superconductivity'. However, because of the presence of unwanted interband optical spectral weight, they necessarily have to truncate their sum at a finite frequency. This work examines theoretical models where the impact of this truncation can be examined first in the normal state, and then in the superconducting state. The latter case is particularly important as previous considerations attributed the observed anomalous temperature dependence as an artifact of a non-infinite cutoff frequency. We find that this is in fact not the case, and that the sign of the corrections from the use of a non-infinite cutoff is such that the observed temperature dependence is even more anomalous when proper account is taken of the cutoff. On the other hand, in these same models, we find that the strong observed temperature dependence in the normal state can be attributed to the effect of a non-infinite cutoff frequency.

cond-mat.supr-con↗

arXiv2007

Model-Independent Sum Rule Analysis Based on Limited-Range Spectral Data

Partial sum rules are widely used in physics to separate low- and high-energy degrees of freedom of complex dynamical systems. Their application, though, is challenged in practice by the always finite spectrometer bandwidth and is often performed using risky model-dependent extrapolations. We show that, given spectra of the real and imaginary parts of any causal frequency-dependent response function (for example, optical conductivity, magnetic susceptibility, acoustical impedance etc.) in a limited range, the sum-rule integral from zero to a certain cutoff frequency inside this range can be safely derived using only the Kramers-Kronig dispersion relations without any extra model assumptions. This implies that experimental techniques providing both active and reactive response components independently, such as spectroscopic ellipsometry in optics, allow an extrapolation-independent determination of spectral weight 'hidden' below the lowest accessible frequency.

cond-mat.other↗

arXiv2007

Multiband and impurity effects in infrared and optical spectra of MgB2

A short review of the optical and far-infrared measurements on MgB$_{2}$ is given. Multiband and multigap effects are analyzed by comparing optical properties with other experiments and {\em ab initio} calculations. The covered topics are: the plasma frequency, electron-phonon interaction, impurity scattering, the effects of C and Al substitution, interband transitions and the far-infrared signatures of the superconducting gaps.

cond-mat.supr-con↗

arXiv2007

Optical properties and electronic structure of ZrB$_{12}$

We report optical (6 meV - 4 eV) properties of a boride superconductor ZrB$_{12}$ ($T_c$ = 6 K) in the normal state from 20 to 300 K measured on high-quality single crystals by a combination of reflectivity and ellipsometry. The Drude plasma frequency and interband optical conductivity calculated by self-consistent full-potential LMTO method agree well with experimental data. The Eliashberg function $α_{tr}^2F(ω)$ extracted from optical spectra features two peaks at about 25 and 80 meV, in agreement with specific heat data. The total coupling constant is $λ_{tr}=1.0\pm0.35$. The low energy peak presumably corresponds to the displacement mode of Zr inside $B_{24}$ cages, while the second one involves largely boron atoms. In addition to the usual narrowing of the Drude peak with cooling down, we observe an unexpected removal of about 10 % of the Drude spectral weight which is partially transferred to the region of the lowest-energy interband transition ($\sim$ 1 eV). This effect may be caused by the delocalization of the metal ion from the center of the $B_{24}$ cluster.

cond-mat.supr-con↗

arXiv2006

Optical and thermodynamic properties of the high-temperature superconductor HgBa_2CuO_4+delta

In- and out-of-plane optical spectra and specific heat measurements for the single layer cuprate superconductor Hg-1201 at optimal doping (Tc = 97 K) are presented. Both the in-plane and out-of-plane superfluid density agree well with a recently proposed scaling relation rho_{s}=sigma_{dc}T_{c}. It is shown that there is a superconductivity induced increase of the in-plane low frequency spectral weight which follows the trend found in underdoped and optimally doped Bi-2212 and optimally doped Bi-2223. We observe an increase of optical spectral weight which corresponds to a change in kinetic energy of approximately 0.5 meV/Cu which is more than enough to explain the condensation energy. The specific heat anomaly is 10 times smaller than in YBCO and 3 times smaller than in Bi-2212. The shape of the anomaly is similar to the one observed in YBCO showing that the superconducting transition is governed by thermal fluctuations.

cond-mat.supr-con↗