Evidence for precursor superconducting pairing above T c in underdoped cuprates from an analysis of the in-plane infrared response

We performed calculations of the in-plane infrared response of underdoped cuprate superconductors to clarify the origin of a characteristic dip feature which occurs in the published experimental spectra of the real part of the in-plane conductivity below an onset temperature ${{T}^{{\rm ons}}}$ considerably higher than ${{T}_{{\rm c}}}$. We provide several arguments, based on a detailed comparison of our results with the published experimental data, confirming that the dip feature and the related features of the memory function $M(\omega )={{M}_{1}}(\omega )+{\rm i}{{M}_{2}}(\omega )$ (a peak in M1 and a kink in M2) are due to superconducting pairing correlations that develop below ${{T}^{{\rm ons}}}$. In particular, we show that (i) the dip feature, the peak and the kink of the low-temperature experimental data can be almost quantitatively reproduced by calculations based on a model of a d-wave superconductor. The formation of the dip feature in the experimental data below ${{T}^{{\rm ons}}}$ is shown to be analogous to the one occurring in the model spetra below ${{T}_{{\rm c}}}$. (ii) Calculations based on simple models, for which the dip in the temperature range from ${{T}_{{\rm c}}}$ to ${{T}^{{\rm ons}}}$ is unrelated to superconducting pairing, predict a shift of the onset of the dip at the high-energy side upon entering the superconducting state, that is not observed in the experimental data; (iii) the conductivity data in conjunction with the recent photoemission data (Reber et al 2012 Nat. Phys. 8 606, Reber et al 2013 Phys. Rev. B 87 060506) imply the persistence of the coherence factor characteristic of superconducting pairing correlations in a range of temperatures above ${{T}_{{\rm c}}}$

Signatures of the bonding-antibonding splitting in the cc-axis infrared response of moderately underdoped bilayer and trilayer cuprate superconductors

We report on results of our analysis of the c-axis infrared conductivity, σc(ω), of bilayer LnBa2Cu3O7−δ (Ln=La, Nd, Y) and trilayer Bi2Sr2Ca2Cu3O10+δ high-Tc superconductors. The analysis employs the multilayer model involving the conductivity of the bilayer or trilayer unit, σbl(ω), and that of the spacing layers separating the latter units, σint(ω). For the YBa2Cu3O7−δ sample with concentration of holes p=0.09, our fitting of the data strongly suggests that at low temperatures, the conductivity σbl(ω) possesses a pronounced and narrow Drude peak. For samples with p≥0.115 however, the fitting indicates that σbl(ω) is, at low temperatures, dominated by a mode at a finite energy in the range from 30 to 60 meV. The properties of this resonance are in accord with those of a collective mode that appears in the spectra of σbl(ω) calculated using a microscopic gauge-invariant theory of σc(ω) by J. Chaloupka and coworkers [Phys. Rev. B 79, 184513 (2009)]. The frequency and spectral weight of the latter mode are determined by the magnitude of the splitting between the bonding and the antibonding band of the bilayer or trilayer unit. Our results, in conjunction with the microscopic theory, thus demonstrate that in moderately underdoped bilayer and trilayer high-Tc cuprates the bilayer (or trilayer) splitting is already developed. The observed doping dependence is consistent with results from angular resolved photoemission spectroscopy

Infrared ellipsometry study of the confined electrons in a high-mobility <i>γ</i>-Al₂O₃/SrTiO₃ heterostructure

With infrared ellipsometry we studied the response of the confined electrons in γ-Al₂O₃/SrTiO₃ (GAO/STO) heterostructures in which they originate predominantly from oxygen vacancies. From the analysis of a so-called Berreman mode, that develops near the highest longitudinal optical phonon mode of SrTiO₃, we derive the sheet carrier density, N s , the mobility, μ, and the depth profile of the carrier concentration. Notably, we find that N s and the shape of the depth profile are similar as in LaAlO₃/SrTiO₃ (LAO/STO) heterostructures for which the itinerant carriers are believed to arise from a polar discontinuity. Despite an order of magnitude higher mobility in GAO/STO, as obtained from transport measurements, the derived mobility in the infrared range exhibits only a twofold increase. We interpret this finding in terms of the polaronic nature of the confined charge carriers in GAO/STO and LAO/STO which leads to a strong, frequency-dependent interaction with the STO phonons

Infrared ellipsometry study of photogenerated charge carriers at the (001) and (110) surfaces of SrTiO3\mathrm{SrTi}{\mathrm{O}}_{3} crystals and at the interface of the corresponding LaAlO3/SrTiO3\mathrm{LaAl}{\mathrm{O}}_{3}/\mathrm{SrTi}{\mathrm{O}}_{3} heterostructures

With infrared (IR) ellipsometry and dc resistance measurements, we investigated the photodoping at the (001) and (110) surfaces of SrTiO3 (STO) single crystals and at the corresponding interfaces of LaAlO3/SrTiO3 (LAO/STO) heterostructures. In the bare STO crystals, we find that the photogenerated charge carriers, which accumulate near the (001) surface, have a similar depth profile and sheet carrier concentration as the confined electrons that were previously observed in LAO/STO (001) heterostructures. A large fraction of these photogenerated charge carriers persist at low temperature at the STO (001) surface even after the ultraviolet light has been switched off again. These persistent charge carriers seem to originate from oxygen vacancies that are trapped at the structural domain boundaries, which develop below the so-called antiferrodistortive transition at T∗=105K. This is most evident from a corresponding photodoping study of the dc transport in STO (110) crystals for which the concentration of these domain boundaries can be modified by applying a weak uniaxial stress. The oxygen vacancies and their trapping by defects are also the source of the electrons that are confined to the interface of LAO/STO (110) heterostructures, which likely do not have a polar discontinuity as in LAO/STO (001). In the former, the trapping and clustering of the oxygen vacancies also has a strong influence on the anisotropy of the charge carrier mobility. We show that this anisotropy can be readily varied and even inverted by various means, such as a gentle thermal treatment, UV irradiation, or even a weak uniaxial stress. Our experiments suggest that extended defects, which develop over long time periods (of weeks to months), can strongly influence the response of the confined charge carriers at the LAO/STO (110) interface

Macroscopic phase segregation in superconducting K_0.73Fe_1.67Se₂ as seen by muon spin rotation and infrared spectroscopy

Using muon spin rotation and infrared spectroscopy, we investigated the recently discovered superconductor K0.73Fe1.67Se₂ with Tc≈32 K. We show that the combined data can be consistently described in terms of a macroscopically phase-segregated state with a matrix of ∼88% volume fraction that is insulating and strongly magnetic and inclusions with an ∼12% volume fraction, which are metallic, superconducting, and nonmagnetic. The electronic properties of the latter, in terms of the normal state plasma frequency and the superconducting condensate density, appear to be similar as in other iron selenide or arsenide superconductors

Evidence of a Precursor Superconducting Phase at Temperatures as High as 180 K in RBa₂Cu₃O_7-δ (R=Y,Gd,Eu) Superconducting Crystals from Infrared Spectroscopy

We show that a multilayer analysis of the infrared c-axis response of RBa₂Cu₃O7-δ   (R=Y,Gd,Eu) provides important new information about the anomalous normal-state properties of underdoped cuprate high temperature superconductors. In addition to competing correlations which give rise to a pseudogap that depletes the low-energy electronic states below T*≫Tc, it enables us to identify the onset of a precursor superconducting state below Tons>Tc. We map out the doping phase diagram of Tons which reaches a maximum of 180 K at strong underdoping and present magnetic field dependent data which confirm our conclusions

Temperature dependence of the superconductivity-induced collective mode in the c-axis infrared spectra of bilayer cuprate superconductors

The low-temperature spectra of the c-axis infrared conductivity of bilayer high-Tc cuprate superconductors (HTCS) exhibit two superconductivity-induced modes [Li Yu et al., Phys. Rev. Lett. 100 (2008) 177004; and references therein]. Both can be understood in terms of a microscopic theory developed recently [J. Chaloupka, C. Bernhard, D. Munzar, Phys. Rev. B 79 (2009) 184513]. Here we summarize the elements of the theory and report on the temperature dependence (TD) of the low-energy mode and of the total optical spectral weight (SW). The calculated TD of the mode is consistent with experiment but the trends of the SW are not

Evidence for multiple superconducting gaps in optimally doped BaFe_1.87Co_0.13As₂ from infrared spectroscopy

We performed combined infrared reflection and ellipsometry measurements of the in-plane optical response of single crystals of the pnictide high-temperature superconductor BaFe1.87Co0.13As₂ with Tc=24.5 K. Our experimental data provide evidence for multiple superconducting energy gaps and can be well described in terms of three isotropic gaps with 2Δ/kBTc of 3.1, 4.7, and 9.2. The obtained low-temperature value of the in-plane magnetic penetration depth is 270 nm