We have measured the complex dynamical conductivity, σ = σ1 + iσ2, of superconducting Ba(Fe0.9Co0.1)2As2 (Tc = 22 K) at terahertz frequencies and temperatures 2 -30 K. In the frequency dependence of σ1 below Tc, we observe clear signatures of the superconducting energy gap opening. The temperature dependence of σ1 demonstrates a pronounced coherence peak at frequencies below 15 cm −1 (1.8 meV). The temperature dependence of the penetration depth, calculated from σ2, shows power-law behavior at the lowest temperatures. Analysis of the conductivity data with a two-gap model, gives the smaller isotropic s-wave gap of ∆A = 3 meV, while the larger gap is highly anisotropic with possible nodes and its rms amplitude is ∆0 = 8 meV. Overall, our results are consistent with a two-band superconductor with an s± gap symmetry.

The B to A conformational transition of highly oriented DNA films due to a hydration change is observed with time-domain terahertz spectroscopy. Wet-spun films of calf thymus and salmon DNA are investigated for different film thicknesses and for different polarizations of the terahertz radiation relative to the DNA orientation. A clear polarization dependence is observed. Asynchronous optical sampling allows recording of terahertz absorption and background spectra in a few 10 s, permitting the tracking of the dehydration dynamics on a time scale of minutes. The observation of a phase transition is corroborated by Raman spectroscopy. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2743401͔The conformation of DNA is directly linked to important biological processes such as DNA replication and DNA repair. While DNA in aqueous solution is predominantly in the B form, it is also frequently seen to deviate from this conformation. Dehydration of B-type DNA to below 30% water content by weight converts it into A-type DNA. This conformational transition and its origins in terms of free energy difference between A-and B-type conformations have been intensively studied theoretically 1 and experimentally, e.g., by x-ray diffraction. 2-4Low-frequency vibrational excitations are expected to play an important role in conformational dynamics of DNA, as well as in the hydration shell, and might influence the dynamics of drug binding. 5 In particular, Raman and Brillouin scattering 6,7 as well as thermal neutron scattering 8 have been used to observe low-frequency excitation spectra. Far-infrared absorption spectroscopy in the terahertz frequency range provides another access to the low-frequency vibrational excitations of DNA. Besides Fourier-transform spectroscopy 9-11 and absorption spectroscopy using tunable terahertz light sources, 12,13 time-domain terahertz spectroscopy became available recently to study optical properties of biomolecules and DNA. [14][15][16][17] In this letter, we use a recently developed terahertz spectrometer based on high-speed asynchronous optical sampling ͑ASOPS͒ to measure the terahertz transmission characteristics of highly oriented wet-spun DNA films. We describe the dependences of the terahertz absorption in the range of 100 GHz-2.0 THz on the degree of hydration. Here, we take advantage of the fast sampling possibilities that are inherent to the ASOPS technique. The results from the terahertz experiments are corroborated by x-ray diffraction and Raman scattering experiments.Highly oriented films are produced from DNA sodium salt ͑Sigma-Aldrich Co.͒ using the wet-spinning method developed by Rupprecht. 18 A highly concentrated DNA solution is pumped through a spinneret and precipitated in 77% ethanol solution containing 0.3M NaCl at a temperature of 5°C such that thin DNA fibers are generated and spooled onto a rotating cylinder. Multilayered DNA films are taken off the cylinder and are dried at 5°C and are rehydrated at 75% relative humidity ͑r.h.͒ at room temperature for at least three d...

We report on an investigation of optical properties of multiferroic CoCr2O4 at terahertz frequencies in magnetic fields up to 30 T. Below the ferrimagnetic transition (94 K), the terahertz response of CoCr2O4 is dominated by a magnon mode, which shows a steep magnetic-field dependence. We ascribe this mode to an exchange resonance between two magnetic sublattices with different gfactors. In the framework of a simple two-sublattice model (the sublattices are formed by Co 2+ and Cr 3+ ions), we find the inter-sublattice coupling constant, λ = −(18 ± 1) K, and trace the magnetization for each sublattice as a function of field. We show that the Curie temperature of the Cr 3+ sublattice, Θ2 = (49 ± 2) K, coincides with the temperature range, where anomalies of the dielectric and magnetic properties of CoCr2O4 have been reported in literature.

We have measured the magnetization and specific heat of multiferroic CoCr2O4 in magnetic fields up to 14 T. The high-field magnetization measurements indicate a new phase transition at T * = 5 − 6 K. The phase between T * and the lock-in transition at 15 K is characterized by magnetic irreversibility. At higher magnetic fields, the irreversibility increases. Specific-heat measurements confirm the transition at T * , and also show irreversible behavior. We construct a field-temperature phase diagram of CoCr2O4. 1 At T S ≈ 26 K, a structural transition happens, below which a short-range-ordered spiral component develops in the spin system, resulting in a conical magnetic structure.2 The structural transition is also accompanied by the emergence of a spontaneous electric polarization, which direction can be reversed by applying a magnetic field.3 At T lock−in = 15 K, the period of the spin spiral becomes commensurate ("locks") to the lattice parameter.

Helium nanodroplets spectroscopy in combination with free electron laser. The combination is suitable for studying molecular clusters in 66–3600 cm−1 spectral range, covering the important low-frequency large amplitude intra and inter-molecular motions.