No abstract

In recent years, large effort has been put into the development and characterization of new colossal-ε' materials. For example, the recent discovery (1,2) of "colossal" values of the dielectric constant, ε', up to about 10 5 in CaCu 3 Ti 4 O 12 (CCTO) has aroused tremendous interest and a huge number of publications deals with its investigation and optimization. Aside of the extensively investigated CCTO, there are also some reports of other colossal-ε' materials (e.g., refs. (3,4,5,6,7)), mainly transition metal oxides. While there is no clear definition, the term "colossal" typically denotes values of ε' > 10 4 . Such materials are very appealing for the further miniaturization of capacitive components in electronic devices and also in giant capacitors that may replace batteries for energy storage.Of course, colossal dielectric constants are also found in ferroelectrics where close to the phase transition very large values are reached. However, ferroelectrics are characterized by a strong temperature dependence of ε' around their critical temperature, which restricts their applicability. In contrast, CCTO and other materials stand out due to their colossal-ε' values being nearly constant over a broad temperature range around room temperature. But in all these materials a strong frequency dependence is observed, revealing the signature of relaxational contributions, namely a steplike decrease of ε' above a certain, temperaturedependent frequency, accompanied by a peak in the dielectric loss. Intrinsic relaxations are commonly observed, e.g., in materials containing dipolar molecules, which reorient in accord with the ac field at low frequencies, but cannot follow at high frequencies. However, the extensive investigations of CCTO, have quite clearly revealed that the observed relaxation features are due to a nonintrinsic effect, termed Maxwell-Wagner (MW) relaxation (8,9,10). It arises from heterogeneity of the sample, which is composed of a bulk region with relatively high conductivity and one or several relatively insulating thin layers. The equivalent circuit describing such a sample leads to a relaxation-like frequency and temperature dependence (10). The insulating layers can arise, for example, from surface effects (e.g., depletion regions of Schottky diodes at the electrodes) or internal barriers (e.g., grain boundaries). However, this is rather irrelevant from an application point of view (e.g., external surface layers are used to enhance the capacitance in ferroelectrics-based multi-layer ceramic capacitors). Thus, although in CCTO the exact mechanism is not yet finally clarified, the interest in this material is still high. This is, amongst others, demonstrated by the fact that since its discovery in 2000, twelve socalled "highly-cited" papers on this topic have appeared (source: ISI Web of Science, Nov. 2008). Unfortunately, at room temperature the relaxation in CCTO leads to a decrease of ε' in the MHz region and around GHz only values of the order of 100 are observed (8,11,12). In contrast, electron...

ZrSiS exhibits a frequency-independent interband conductivity σ(ω)=const(ω)≡σ_{flat} in a broad range from 250 to 2500  cm^{-1} (30-300 meV). This makes ZrSiS similar to (quasi-)two-dimensional Dirac electron systems, such as graphite and graphene. We assign the flat optical conductivity to the transitions between quasi-two-dimensional Dirac bands near the Fermi level. In contrast to graphene, σ_{flat} is not universal but related to the length of the nodal line in the reciprocal space, k_{0}. Because of spin-orbit coupling, the discussed Dirac bands in ZrSiS possess a small gap Δ, for which we determine an upper bound max(Δ)=30  meV from our optical measurements. At low temperatures the momentum-relaxation rate collapses, and the characteristic length scale of momentum relaxation is of the order of microns below 50 K.

We measured the optical reflectivity of [001]-oriented n-doped Cd3As2 in a broad frequency range (50 -22 000 cm −1 ) for temperatures from 10 to 300 K. The optical conductivity, σ(ω) = σ1(ω) + iσ2(ω), is isotropic within the (001) plane; its real part follows a power law, σ1(ω) ∝ ω 1.65 , in a large interval from 2000 to 8000 cm −1 . This behavior is caused by interband transitions between two Dirac bands, which are effectively described by a sublinear dispersion relation, E(k) ∝ |k| 0.6 . The momentum-averaged Fermi velocity of the carriers in these bands is energy dependent and ranges from 1.2 × 10 5 to 3 × 10 5 m/s, depending on the distance from the Dirac points. We detect a gaplike feature in σ1(ω) and associate it with the Fermi level positioned around 100 meV above the Dirac points.

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.

Infrared optical investigations of α-(BEDT-TTF)2I3 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 < TCO, we extract how the electronic charge per molecule varies with temperature and pressure: Transport and optical studies demonstrate that charge order and metalinsulator transition coincide and consistently yield a linear decrease of the transition temperature TCO by 8 − 9 K/kbar. The charge disproportionation ∆ρ diminishes by 0.017 e/kbar and the optical gap ∆ between the bands decreases with pressure by -47 cm −1 /kbar. In our high-pressure and low-temperature experiments, we do observe contributions from the massive charge carriers as well as from massless Dirac electrons to the low-frequency optical conductivity, however, without being able to disentangle them unambiguously.

Accurate determination of the intrinsic electronic structure of thermoelectric materials is a prerequisite for utilizing an electronic band engineering strategy to improve their thermoelectric performance. Herein, with high‐resolution angle‐resolved photoemission spectroscopy (ARPES), the intrinsic electronic structure of the 3D half‐Heusler thermoelectric material ZrNiSn is revealed. An unexpectedly large intrinsic bandgap is directly observed by ARPES and is further confirmed by electrical and optical measurements and first‐principles calculations. Moreover, a large anisotropic conduction band with an anisotropic factor of 6 is identified by ARPES and attributed to be one of the most important reasons leading to the high thermoelectric performance of ZrNiSn. These successful findings rely on the grown high‐quality single crystals, which have fewer Ni interstitial defects and negligible in‐gap states on the electronic structure. This work demonstrates a realistic paradigm to investigate the electronic structure of 3D solid materials by using ARPES and provides new insights into the intrinsic electronic structure of the half‐Heusler system benefiting further optimization of thermoelectric performance.

Superconducting Pr2CuOx, x ≃ 4 (PCO) films with T ′ structure and a Tc of 27 K have been investigated by various optical methods in a wide frequency (7 -55000 cm −1 ) and temperature (2 to 300 K) range. The optical spectra do not reveal any indication of a normal-state gap formation. A Drude-like peak centered at zero frequency dominates the optical conductivity below 150 K. At higher temperatures, it shifts to finite frequencies. The detailed analysis of the low-frequency conductivity reveals that the Drude peak and a far-infrared (FIR) peak centered at about 300 cm −1 persist at all temperatures. The FIR-peak spectral weight is found to grow at the expense of the Drude spectral weight with increasing temperature. The temperature dependence of the penetration depth follows a behavior typical for d-wave superconductors. The absolute value of the penetration depth for zero temperature is 1.6 µm, indicating a rather low density of the superconducting condensate.