Although the vast majority of high-$T_c$ cuprate superconductors are hole-doped, a small family of electron-doped compounds exists. Under investigated until recently, there has been tremendous recent progress in their characterization. A consistent view is being reached on a number of formerly contentious issues, such as their order parameter symmetry, phase diagram, and normal state electronic structure. Many other aspects have been revealed exhibiting both their similarities and differences with the hole-doped compounds. This review summarizes the current experimental status of these materials, with a goal to providing a snapshot of our current understanding of electron-doped cuprates. When possible we put our results in the context of the hole-doped compounds. We attempt to synthesize this information into a consistent view on a number of topics important to both this material class as well as the overall cuprate phenomenology including the phase diagram, the superconducting order parameter symmetry, phase separation, pseudogap effects, the role of competing orders, the spin-density wave mean-field description of the normal state, and electron-phonon coupling.

Progress and perspectives on electron-doped cuprates

A quantitative theory of the isotropic electron‐nuclear spin interactions of carbon 13 in pi‐electron radicals is presented and applied to the hyperfine splittings observed in the electron spin resonance spectra of these substances. The splittings arise from sigma‐pi interactions which polarize both the 1s and 2s electrons. The 1s‐orbital spin polarization is shown to contribute a term of negative sign with a magnitude comparable to that from the 2s electrons. For an sp2 hybridized carbon atom that is bonded to three atoms, Xi (i=1, 2, 3), the hyperfine constant aC has the form aC=(SC+ ∑ i=13QCXiC)ρπ+ ∑ i=13QXiCCρiπ, where ρπ and ρiπ(i=1,2,3) are the pi‐electron spin densities on atoms C and Xi, respectively. The contribution of the 1s electrons is determined by SC and that of the 2s electrons by the Q's, where QBCA is the sigma‐pi parameter for the nucleus of atom A resulting from the interaction between the bond BC and the pi‐electron spin density on atom B. Calculations for a planar CHC2 fragment model...

THEORETICAL INTERPRETATION OF CARBON-13 HYPERFINE INTERACTIONS IN ELECTRON SPIN RESONANCE SPECTRA. Technical Note Report No. 9

A series of metal coordination polymers containing 1,1,2,2-ethenetetrathiolate (ett) linking bridge (poly[Ax(M-ett)]) are synthesized. The Seebeck coefficients of these conducting materials are high, and vary according to the center metals and counter cations. The TE device fabricated demonstrates the great potentials of these materials for TE applications.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/adma.201290076

Organic Thermoelectric Materials and Devices Based on p‐ and n‐Type Poly(metal 1,1,2,2‐ethenetetrathiolate)s

We review and compare electronic transport in different types of conducting polymer: conjugated organic polymers, the inorganic polymer polysulphur nitride, alkali-metal fulleride polymers, and carbon nanotubes. In each case, the transport properties show some unusual features compared to conventional metals. In conjugated organic conducting polymers, electronic transport shows a systematic pattern involving both metallic and non-metallic character. We discuss the physical conduction processes that can account for this behaviour. Key roles are played by the metal-semiconductor transition as the doping level is varied, and by the limited size of crystalline regions in the polymers, which gives rise to heterogeneous conduction. Transport data provide indirect evidence that the intrinsic conductivity of doped crystalline polyacetylene, in the absence of disordered regions, is higher than that of copper at room temperature; this high conductivity is consistent with the expected suppression of backscattering in highly anisotropic ('quasi-one-dimensional') metallic conduction. Bundles of single-wall carbon nanotubes have also been found to exhibit metallic behaviour. The temperature dependence of the conductivity of bulk samples is remarkably similar to the pattern characteristic of organic conducting polymers, typically showing a crossover from metallic to non-metallic behaviour as temperature decreases. Quantized one-dimensional conductance and other quantum effects are seen in individual nanotubes.

https://iopscience.iop.org/article/10.1088/0034-4885/64/1/201/pdf

Electronic transport properties of conducting polymers and carbon nanotubes

Doping is one of the most important methods to control charge carrier concentration in semiconductors. Ideally, the introduction of dopants should not perturb the ordered microstructure of the semiconducting host. In some systems, such as modulation-doped inorganic semiconductors or molecular charge transfer crystals, this can be achieved by spatially separating the dopants from the charge transport pathways. However, in conducting polymers, dopants tend to be randomly distributed within the conjugated polymer, and as a result the transport properties are strongly affected by the resulting structural and electronic disorder. Here, we show that in the highly ordered lamellar microstructure of a regioregular thiophene-based conjugated polymer, a small-molecule p-type dopant can be incorporated by solid state diffusion into the layers of solubilizing side chains without disrupting the conjugated layers. In contrast to more disordered systems, this allows us to observe coherent, free-electron-like charge transport properties, including a nearly ideal Hall effect in a wide temperature range, a positive magnetoconductance due to weak localization and the Pauli paramagnetic spin susceptibility.

/pdf/2d-coherent-charge-transport-in-highly-ordered-conducting-4vgt1jkrz5.pdf

2D coherent charge transport in highly ordered conducting polymers doped by solid state diffusion

The magnetic susceptibility and the electrical resistivity of dilute Au-V alloys, between 0.3 and 2.0 at.% of vanadium are measured in the temperature range from 1.4°K to 1000°K. The temperature variation of the magnetic susceptibility follows the Curie-Weiss type formula µ 2 /3 k ( T + A ), where A is 290±10°K. The electrical resistivity of the solute varies logarithmically at the high temperature and becomes parabolic, 1-( T / B ) 2 with B =280°K∼310°K, at the low temperature limit. These observations are independent of vanadium concentrations. The above experimental facts are discussed in comparison with the recently reported theories of the s - d interaction. The Kondo temperature is found to be about 300°K.

Anomalies Due to s - d Interaction in Dilute Au–V Alloys

Magnetic properties in polypyrrole doped by series of dopants

Frequency dependence of electron spin-lattice relaxation rate at 5–450 MHz in pristine trans-polyacetylene --- new evidence of one dimensional diffusive motion of electron spin (neutral soliton) ---

Si, Na, Ba, and Ba NMR experiments were carried out in the superconducting silicon clathrate compound NaxBaySi46 to study the electronic states above the superconducting transition temperature. We observed three distinct Si signals with different Knight shift ~2036, 862, and 720 ppm at 90 K with a small temperature dependence of ;10%!, which were ascribed to inequivalent Si sites in the Si46 structure. The results indicate that the conduction-electron spin and/or charge density is strongly site-dependent. At all the atomic sites, the Korringa relation in NMR relaxation was observed, indicative of metallic electronic structure with moderate electronic correlation. The Na and Ba atoms are partially ionized in NaxBaySi46 , which shows the noticeable difference from the system of alkali-metal-doped fullerides. The comparison with band calculations is discussed. @S0163-1829~96!06042-0#

/pdf/nmr-study-in-the-superconducting-silicon-clathrate-compound-2u93euxw07.pdf

Kiyoshi Kume

Papers

Anomalies Due to s - d Interaction in Dilute Au–V Alloys

Magnetic properties in polypyrrole doped by series of dopants

Frequency dependence of electron spin-lattice relaxation rate at 5–450 MHz in pristine trans-polyacetylene --- new evidence of one dimensional diffusive motion of electron spin (neutral soliton) ---

NMR study in the superconducting silicon clathrate compound NaxBaySi46

High resolution 13C NMR spectra in graphite chemical shift and diamagnetism