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We report evidence for a reentrant spin glass phase in electron-doped $\text{EuFe}_2\text{As}_2$ single crystals and first traces of the superconductivity re-entrance in optics. In the close-to-optimal doped $\text{Eu}(\text{Fe}_{0.91}\text{Ir}_{0.09})_2\text{As}_2$ and $\text{Eu}(\text{Fe}_{0.93}\text{Rh}_{0.07})_2\text{As}_2$ samples two magnetic transitions are observed below the superconducting critical temperature $T_c \approx 21$~K: the canted $A$-type antiferromagnetic order of the $\text{Eu}^{2+}$ ions sets in around $17\,\text{K}$; the spin glass behavior occurs another $2\,\text{K}$ lower in temperature. In addition, strong evidence for an additional transition is found far below the spin glass temperature. Our extensive optical and magnetic investigations provide new insight into the interplay of local magnetism and superconductivity in these systems and elucidate the effect of the spin-glass phase on the reentrant superconducting state.

We investigated transport, magnetotransport, and broadband optical properties of the half-Heusler compound YbPtBi. Hall measurements evidence two types of charge carriers: highly mobile electrons with a temperature-dependent concentration and low-mobile holes; their concentration stays almost constant within the investigated temperature range from 2.5 to 300 K. The optical spectra (10 meV - 2.7 eV) can be naturally decomposed into contributions from intra- and interband absorption processes, the former manifesting themselves as two Drude bands with very different scattering rates, corresponding to the charges with different mobilities. These results of the optical measurements allow us to separate the contributions from electrons and holes to the total conductivity and to implement a two-channel-conduction model for description of the magnetotransport data. In this approach, the electron and hole mobilities are found to be around 50000 and 10 cm$^{2}$/Vs at the lowest temperatures (2.5 K), respectively., Comment: 6 pages

Infrared optical investigations of $\alpha$-(BEDT-TTF)$_2$I$_3$ have been performed in the spectral range from 80 to 8000~cm$^{-1}$ down to temperatures as low as 10~K by applying hydrostatic pressure. In the metallic state, $T > 135$~K, we observe a 50\% increase in the Drude contribution as well as the mid-infrared band due to the growing intermolecular orbital overlap with pressure up to 11~kbar. In the ordered state, $T

We measured the optical reflectivity of [001]-oriented $n$-doped ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ in a broad frequency range (50--22 000 ${\mathrm{cm}}^{\ensuremath{-}1}$) for temperatures from 10 to 300 K. The optical conductivity, $\ensuremath{\sigma}(\ensuremath{\omega})={\ensuremath{\sigma}}_{1}(\ensuremath{\omega})+\mathrm{i}{\ensuremath{\sigma}}_{2}(\ensuremath{\omega})$, is isotropic within the (001) plane; its real part follows a power law, ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})\ensuremath{\propto}{\ensuremath{\omega}}^{1.65}$, in a large interval from 2000 to $8000{\mathrm{cm}}^{\ensuremath{-}1}$. This behavior is caused by interband transitions between two Dirac bands, which are effectively described by a sublinear dispersion relation, $E(k)\ensuremath{\propto}{|k|}^{0.6}$. The momentum-averaged Fermi velocity of the carriers in these bands is energy dependent and ranges from $1.2\ifmmode\times\else\texttimes\fi{}{10}^{5}$ to $3\ifmmode\times\else\texttimes\fi{}{10}^{5}$ m/s, depending on the distance from the Dirac points. We detect a gaplike feature in ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})$ and associate it with the Fermi level positioned around 100 meV above the Dirac points.

The electrodynamic response of Nb in the frequency range above and below the energy gap 2\ensuremath{\Delta} is studied in the normal and in the superconducting state. A coherent source interferometer is utilized in the spectral range between 5 and $30{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ to investigate the amplitude and the phase of the transmission through niobium films that allows the direct determination of both components of the complex conductivity. The optical conductivity up to $300{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ is evaluated using infrared reflection measurements. Below the 8.31-K superconducting transition temperature of the 150-\AA{}-thick film, the superconducting energy gap is observed to increase as the temperature decreases. The overall frequency dependence of the conductivity can be described using the BCS formalism and assuming finite scattering effects; however, at low temperatures we find deviations from the predicted behavior in the spectral range below the energy gap. Estimations of the gap at zero temperature $2\ensuremath{\Delta}(0)=24{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ or ${4.1k}_{B}{T}_{c}$ deviate from the weak-coupling BCS limit.

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.

We have measured the complex dynamical conductivity, $\ensuremath{\sigma}={\ensuremath{\sigma}}_{1}+i{\ensuremath{\sigma}}_{2}$, of superconducting $\text{Ba}{({\text{Fe}}_{0.9}{\text{Co}}_{0.1})}_{2}{\text{As}}_{2}$ $({T}_{c}=22\text{ }\text{K})$ at terahertz frequencies and temperatures 2--30 K. In the frequency dependence of ${\ensuremath{\sigma}}_{1}$ below ${T}_{c}$, we observe clear signatures of the superconducting energy gap opening. The temperature dependence of ${\ensuremath{\sigma}}_{1}$ demonstrates a pronounced coherence peak at frequencies below $15\text{ }{\text{cm}}^{\ensuremath{-}1}$ (1.8 meV). The temperature dependence of the penetration depth, calculated from ${\ensuremath{\sigma}}_{2}$, 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 ${\ensuremath{\Delta}}_{A}=3\text{ }\text{meV}$, while the larger gap is highly anisotropic with possible nodes and its rms amplitude is ${\ensuremath{\Delta}}_{0}=8\text{ }\text{meV}$. Overall, our results are consistent with a two-band superconductor with an ${s}_{\ifmmode\pm\else\textpm\fi{}}$ gap symmetry.

We measure the penetration depth of optimally doped and underdoped ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4}$ in the millimeter frequency domain (4--7 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) and for temperatures $2\mathrm{K}l~Tl~300\mathrm{K}.$ The penetration depth as a function of temperature reveals significant changes on electron doping. It shows quadratic temperature dependence in underdoped samples, but increases almost exponentially at optimal doping. Significant changes in the gap anisotropy (or even in the gap symmetry) may account for this transition.

Experimental evidence is provided that colossal dielectric constants ${\ensuremath{\varepsilon}}^{\ensuremath{'}}g~1000,$ sometimes reported to exist in a broad temperature range, can often be explained by Maxwell-Wagner-type contributions of depletion layers at the interface between sample and contacts or at grain boundaries. We demonstrate this on a variety of different materials. We speculate that the largest intrinsic dielectric constant observed so far in nonferroelectric materials is of order ${10}^{2}.$

Physical properties of semiconducting ${\mathrm{CdF}}_{2}$ crystals doped with In are determined from measurements of the radio-frequency response of a sample with Schottky barriers at frequencies $10\ensuremath{-}{10}^{6} \mathrm{Hz}.$ The dielectric constant, the dc conductivity, the activation energy of the amphoteric impurity, and the total concentration of the active In ions in ${\mathrm{CdF}}_{2}$ are found through an equivalent-circuit analysis of the frequency dependencies of the sample complex impedance at temperatures from 20 to 300 K. Kinetic coefficients determining the thermally induced transitions between the deep and the shallow states of the In impurity and the barrier height between these states are obtained from the time-dependent radio-frequency response after illumination of the material. The results on the low-frequency conductivity in ${\mathrm{CdF}}_{2}:\mathrm{In}$ are compared with submillimeter ${(10}^{11}\ensuremath{-}{10}^{12} \mathrm{Hz})$ measurements and with room-temperature infrared measurements of undoped ${\mathrm{CdF}}_{2}.$ The low-frequency impedance measurements of semiconductor samples with Schottky barriers are shown to be a good tool for investigation of the physical properties of semiconductors.

We present optical conductivity data for Lu(Ni 1 - x Co x ) 2 B 2 C over a wide range of frequencies and temperatures for x = 0 and 0.09. Both materials show evidence of being good Drude metals with the infrared data in reasonable agreement with dc resistivity measurements at low frequencies. An absorption threshold is seen at approximately 700 cm - 1 . In the cobalt-doped material, we see a superconducting gap in the conductivity spectrum with an absorption onset at 24′2 cm - 1 = 3.9′0.4k B T, suggestive of weak to moderately strong coupling. The pure material is in the clean limit and no gap can be seen. We discuss the data in terms of the electron-phonon interaction and find that it can be fit below 600 cm - 1 with a plasma frequency of 3.3 eV and an electron-phoon coupling constant λ t r = 0.33 using an α 2 F(ω) spectrum fit to the resistivity.

Thorough Raman and infrared (IR) reflectivity investigations of nominally pure ${\mathrm{SrTiO}}_{3}$ ceramics in the 10--300 K range have revealed a clear presence of the polar phase whose manifestation steeply increases on cooling. The Raman strengths of the Raman-forbidden IR modes are proportional to ${\ensuremath{\omega}}_{\mathrm{TO}1}^{\ensuremath{-}\ensuremath{\alpha}}(\ensuremath{\alpha}\ensuremath{\approx}1.6)$ where ${\ensuremath{\omega}}_{\mathrm{TO}1}$ is the polar soft mode frequency. No pronounced permittivity dispersion is observed below the soft mode frequency so that, as in single crystals, the static permittivity is essentially determined by the soft mode contribution. A theory is suggested which assumes a frozen dipole moment connected with the grain boundaries which induces the polar phase in the grain bulk in correlation with the bulk soft-mode frequency. This stiffens slightly the effective soft mode response and reduces the low-temperature permittivity compared to that of single crystals. Moreover, the polar soft mode strongly couples to the ${E}_{g}$ component of the structural soft doublet showing that the polar axis is perpendicular to the tetragonal axis below the structural transition which is shifted to 132 K in our ceramics. Whereas the ${\mathrm{TiO}}_{6}$ octahedra tilt (primary order parameter) below the structural transition corresponds to that in single crystals, much smaller ${A}_{1g}\ensuremath{-}{E}_{g}$ splitting of the structural soft doublet shows that the tetragonal deformation (secondary order parameter) is nearly 10 times smaller, apparently due to the grain volume clamping in ceramics.

The dielectric response of semiconducting ${\mathrm{CdF}}_{2}$ crystals with bistable In centers in the range of ${10}^{1}\ensuremath{-}{10}^{6}\mathrm{Hz}$ reveals a quasi-Debye relaxation due to Schottky barriers at the Au/Ag contacts. These spectra can be modeled with a two- or tri-layer capacitor, the characteristics of which are determined by the conductivity and capacity of the crystal volume and the depletion layers at the contacts (the Maxwell-Wagner capacitance). Analyses of the temperature dependence of these parameters show that the volume conductivity is due to free-electron motion, whereas the depletion-layer conductivity probably is caused by electron jumps over the deep In centers. Illumination of the crystals in the photoionization absorption band of the deep centers has the same effect upon the dielectric response as an increase of the temperature since both factors increase the free-electron concentration.