arXiv subjects

E. van Heumen

Publications and source records attributed to E. van Heumen.

32 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

The cleavage surface of the BaFe_(2-x)Co_(x)As_(2) and Fe_(y)Se_(1-x)Te_(x) superconductors: from diversity to simplicity

We elucidate the termination surface of cleaved single crystals of the BaFe_(2-x)Co_(x)As_(2) and Fe_(y)Se_(1-x)Te_(x) families of the high temperature iron based superconductors. By combining scanning tunneling microscopic data with low energy electron diffraction we prove that the termination layer of the Ba122 systems is a remnant of the Ba layer, which exhibits a complex diversity of ordered and disordered structures. The observed surface topographies and their accompanying superstructure reflections in electron diffraction depend on the cleavage temperature. In stark contrast, Fe_(y)Se_(1-x)Te_(x) possesses only a single termination structure - that of the tetragonally ordered Se_(1-x)Te_(x) layer.

cond-mat.supr-con↗

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

Optical probes of electron correlations in solids

Classically the interaction between light and matter is given by the Maxwell relations. These are briefly reviewed and will be used as a basis to discuss several techniques that are used in optical spectroscopy. We then discuss the quantum mechanical description of the optical conductivity based on the Kubo formalism. This is used as a basis to understand how strong correlation effects can be observed using optical techniques. We will discuss the use of sum rules in the interpretation of optical experiments. Finally, we describe the effect of including interactions between electronic and collective degrees of freedom on optical spectra.

cond-mat.str-el↗

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

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↗

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↗

arXiv2006

Doping Dependence of the Redistribution of Optical Spectral Weight in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$

We present the ab-plane optical conductivity of four single crystals of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ (Bi2212) with different carrier doping levels from the strongly underdoped to the strongly overdoped range with $T_c$=66, 88, 77, and 67 K respectively. We focus on the redistribution of the low frequency optical spectral weight (SW) in the superconducting and normal states. The temperature dependence of the low-frequency spectral weight in the normal state is significantly stronger in the overdoped regime. In agreement with other studies, the superconducting order is marked by an increase of the low frequency SW for low doping, while the SW decreases for the highly overdoped sample. The effect crosses through zero at a doping concentration $δ$=0.19 which is slightly to the right of the maximum of the superconducting dome. This sign change is not reproduced by the BCS model calculations, assuming the electron-momentum dispersion known from published ARPES data. Recent Cluster Dynamical Mean Field Theory (CDMFT) calculations based on the Hubbard and t-J models, agree in several relevant respects with the experimental data.

cond-mat.str-el↗

arXiv2006

In-plane optical spectral weight transfer in optimally doped Bi$_{2}$Sr$_{2}$Ca$_{2}$Cu$_{3}$O$_{10}$

We examine the redistribution of the in-plane optical spectral weight in the normal and superconducting state in tri-layer \bbb (Bi2223) near optimal doping ($T_c$ = 110 K) on a single crystal via infrared reflectivity and spectroscopic ellipsometry. We report the temperature dependence of the low-frequency integrated spectral weight $W(Ω_c)$ for different values of the cutoff energy $Ω_c$. Two different model-independent analyses consistently show that for $Ω_c$ = 1 eV, which is below the charge transfer gap, $W(Ω_c)$ increases below $T_c$, implying the lowering of the kinetic energy of the holes. This is opposite to the BCS scenario, but it follows the same trend observed in the bi-layer compound \bb (Bi2212). The size of this effect is larger in Bi2223 than in Bi2212, approximately scaling with the critical temperature. In the normal state, the temperature dependence of $W(Ω_c)$ is close to $T^2$ up to 300 K.

cond-mat.supr-con↗