The parent phases of the Fe-arsenide superconductors harbor an antiferromagnetic ground state. Significantly, the Néel transition is either preceded or accompanied by a structural transition that breaks the four fold symmetry of the high-temperature lattice. Borrowing language from the field of soft condensed matter physics, this broken discrete rotational symmetry is widely referred to as an Ising nematic phase transition. Understanding the origin of this effect is a key component of a complete theoretical description of the occurrence of superconductivity in this family of compounds, motivating both theoretical and experimental investigation of the nematic transition and the associated in-plane anisotropy. Here we review recent experimental progress in determining the intrinsic in-plane electronic anisotropy as revealed by resistivity, reflectivity and ARPES measurements of detwinned single crystals of underdoped Fe arsenide superconductors in the "122" family of compounds.

We have measured the electrodynamic response in the metallic state of three highly anisotropic conductors, (TMTSF)2X, where X = PF6, AsF6, or ClO4, and TMTSF is the organic molecule tetramethyltetraselenofulvalene. In all three cases we find dramatic deviations from a simple Drude response. The optical conductivity has two features: a narrow mode at zero frequency, with a small spectral weight, and a mode centered around 200 cm −1 , with nearly all of the spectral weight expected for the relevant number of carriers and single particle bandmass. We argue that these features are characteristic of a nearly one-dimensional half-or quarter-filled band with Coulomb correlations, and evaluate the finite energy mode in terms of a one-dimensional Mott insulator. At high frequencies (hω > t ⊥ , the transfer integral perpendicular to the chains), the frequency dependence of the optical conductivity σ1(ω) is in agreement with calculations based on an interacting Tomonaga-Luttinger liquid, and is different from what is expected for an uncorrelated one-dimensional semiconductor. The zero frequency mode shows deviations from a simple Drude response, and can be adequately described with a frequency dependent mass and relaxation rate.

This review discusses the heavy-electron compounds and related materials from the perspective of their electrodynamic response. The investigation of the electrodynamic response by means of optical methods, extending over a very broad spectral range, should reveal, in principle, the complete excitation spectrum. This study incorporates several important sources of information on the intrinsic properties of the investigated materials. In particular, attention will be focused on the optical properties of prototype heavy-electron systems and on Kondo systems with low-temperature non-Fermi-liquid behavior or insulating characteristics. In the discussion, the electrodynamic response will be related to other relevant results arrived at by various experimental methods and to the theoretical state of the art. [S0034-6861(99)

Optical experiments were conducted on a series of organic linear chain conductors with different values of the interchain single-electron transfer integral tb, which quantifies the degree of anisotropy. Electron-electron interactions together with Umklapp scattering resulted in a correlation gap and an insulating state for small tb. An insulator-to-metal transition was observed when tb exceeded a critical value, on the order of the correlation gap Egap. The absence of a plasma edge on the insulator side of the transition for polarization perpendicular to the chains suggests that the electrons are confined to the chains. The optical features of the metallic state, when contrasted with the magnetic properties, are suggestive of spin-charge separation.

We present a comprehensive series of electrical transport ͑conductivity, magnetoresistance, and Hall effect͒, thermodynamic ͑specific heat, magnetic susceptibility, and magnetization͒, and optical ͑reflectivity͒ measurements in varying temperature ranges between 0.05 and 330 K on high-quality FeSi single crystals grown by vapor transport. The entire set of data can consistently be described with the usual relations for a ͑compensated n type͒ semiconductor if an unconventional band structure is assumed. Compared to the results of mean-field band-structure calculations, the height of the peaks in the total density of states around the energy gap is considerably enhanced, implying enhanced effective masses. Most likely correlation effects are the source of these features. At very low temperatures we encounter metallic behavior. A low concentration of correlated itinerant charge carriers coexists with interacting magnetic moments. ͓S0163-1829͑97͒05043-1͔

We investigate the optical conductivity with light polarized along the in-plane orthorhombic a and b axes of Ba(Fe1−xCox)2As2 for x = 0 and 2.5% under uniaxial pressure across their structural and magnetic transitions. The charge dynamics at low frequencies and temperatures on these detwinned, single-domain samples reveals an enhancement of both the scattering rate and Drude weight of the charge carriers along the antiferromagnetic a-axis with respect to the ferromagnetic b-axis. Our findings also allow us to estimate the dichroism, which extends to high frequencies and temperatures. These results reveal a nematic susceptibility as well as the electronic nature of the structural transition.

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