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Far-infrared measurements, between 3 and 450 cm ', of absorption spectra of highly oriented films of Li-DNA and Na-DNA in the temperature range 5 -300 K are reported. Five lowfrequency infrared-active vibrational modes are observed. The lowest infrared-active mode at 45 cm ' for Li-DNA and 41 cm ' for Na-DNA, as observed for the first time, is found to soften upon sample hydration. Studies of the hydration-induced absorption at the low-frequency end of the spectrum also show a pronounced absorption band at about 10 cm ' that is attributed to relaxation processes. Based on a simple lattice-dynamics model, the description of low-lying vibrational modes of DNA is presented. Eigenvectors of the lowest infrared-active and Raman-active modes are calculated. In addition, the model semiquantitatively describes the influence of hydration on mode frequencies.

We have performed ellipsometric measurements of the far-infrared c-axis dielectric response of underdoped YBa 2 Cu 3 O 7−δ single crystals. Here we report a detailed analysis of the temperature-dependent renormalization of the oxygen bending phonon mode at 320 cm −1 and the formation of the additional absorption peak around 400-500 cm −1 . For a strongly underdoped YBa 2 Cu 3 O 6.5 crystal with T c =52 K we find that, in agreement with previous reports based on conventional reflection measurements, the gradual onset of both features occurs well above T c at T*∼150 K. Contrary to some of these reports, however, our data establish that the phonon anomaly and the 1 formation of the additional peak exhibit very pronounced and steep changes right at T c . For a less underdoped YBa 2 Cu 3 O 6.75 crystal with T c =80 K, the onset temperature of the phonon anomaly almost coincides with T c . Also in contrast to some previous reports, we find for both crystals that a sizeable fraction of the spectral weight of the additional absorption peak cannot be accounted for by the spectral-weight loss of the phonon modes but instead arises from a redistribution of the electronic continuum. Our ellipsometric data are consistent with a model where the bilayer cuprate compounds are treated as a superlattice of intra-and inter-bilayer Josephson-junctions.

In this paper, we address a number of outstanding issues concerning the nature and the role of magnetic inhomogenities in the iron chalcogenide system FeTe 1-x Se x and their correlation with superconductivity in this system. We report morphology of superconducting single crystals of FeTe 0.65 Se 0.35 studied with transmission electron microscopy, high angle annular dark field scanning transmission electron microscopy and their magnetic and superconducting properties characterized with magnetization, specific heat and magnetic resonance spectroscopy. Our data demonstrate a presence of nanometre scale hexagonal regions coexisting with tetragonal host lattice, a chemical disorder demonstrating non homogeneous distribution of host atoms in the crystal lattice, as well as hundreds-of-nanometres-long iron-deficient bands. From magnetic data and ferromagnetic resonance temperature dependence, we attribute magnetic phases in Fe-Te-Se to Fe 3 O 4 inclusions and to hexagonal symmetry nanometre scale regions with structure of Fe 7 Se 8 type. Our results suggest that nonhomogeneous distribution of host atoms might be an intrinsic feature of superconducting Fe-Te-Se chalcogenides and we find a surprising correlation indicating that faster grown crystal of inferior crystallographic properties is a better superconductor.

We analyze the infrared reflectivity of Lal.85Sr0.~sCuO4 single crystals with E parallel to the c-axis. The plasma edge at around 6 meV (50 cm-I), which occurs only for T < T¢, is due to Cooper-pair tunneling. This low value of the plasma edge is shown to be consistent with the c-axis plasma frequency obtained from LDA band structure calculations (> 0.1 eV) if we take into account that the single-particle charge transport along the c-axis is strongly incoherent in the normal state, and remains so in the superconducting state. From a comparison of the optical conductivity with model calculations based on sand d-wave weak-coupling theory in the dirty limit, we find no evidence for a reduction of the c-axis quasi-particle scattering rate below T~. The c-axis scattering rate, the normal-state c-axis plasma frequency, and Tc obey hT > hvp >> 3.5kBTc, which is exactly opposite to the clean limit.