Experimental evidence is provided that colossal dielectric constants Ју1000, 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 .Materials exhibiting a colossal dielectric constant ͑CDC͒ ЈϾ10 3 have recently gained considerable attention. CDC behavior is of technical importance for applications using high-electronic materials, such as random access memories based on capacitive elements. Fundamental interest was initiated by the observation of CDC behavior in some high-T c parent compounds. 1,2 Indeed, CDC behavior may indicate a colossal polarizability, which was invoked in early polaronic and bipolaronic models as a possible mechanism for high-T c superconductivity. 3 During the last decade, similar observations of CDC behavior have been reported in an increasing number of materials, such as transition-metal oxides. 4 -6 Large dielectric constants are expected for ferroelectrics in a narrow temperature range close to T c or for systems with hopping charge carriers yielding a dielectric constant that diverges towards low frequencies. However, in various recent reports 1,2,4 -6 giant values of the dielectric constant were claimed to persist over broad temperature ranges and, when plotted as a function of frequency, revealing an almost constant low-frequency value and a steplike decrease of the dielectric constant towards higher frequencies. This steplike decrease, which is accompanied by a loss peak in the imaginary part of the permittivity, Љ, shifts exponentially to higher frequencies with increasing temperature, characteristic of Debye-like dipolar relaxation with a thermally activated relaxation rate. Several intrinsic physical interpretations have been given. Examples include almost incipient ferroelectricity in high-T c materials, 2 highly polarizable relaxation modes, 5 or a relaxorlike slowing down of dipolar fluctuations in nano-sized domains. 6 However, in Ref. 7 it was suspected that extrinsic effects may play a role in the CDC reported in Ref. 6.In the present paper we provide evidence that many of these observations are not intrinsic in origin and we speculate that most, if not all, of the CDC's reported so far are based on Maxwell-Wagner-type extrinsic effects. 8 We will promote the notion that the most natural explanation of apparent CDC's is contact effects and that in ceramic samples grain boundary effects may play a similar role and further ''enhance'' the dielectric constant. At these interfaces ͑metal-to-insulator contacts, intergrain boundaries͒ depletion layers are formed yielding Maxwell-Wagner-type relaxations when measured by standard dielectric techniques that use metallic electrodes and two-point contact configurations. Thus, while some of the reports may ...

We have performed direct measurements of the low-temperature dynamical conductivity and dielectric permittivity of single crystalline SmB 6 in the spectral range from 0.6 to 4.5 meV, i.e., below the hybridization gap. The obtained results together with the data of Hall-effect and infrared reflection measurements give evidence for a 19-meV energy gap in the density of states and an additional narrow donor-type band lying only 3 meV below the bottom of the upper conduction band. It is shown that at temperatures 5 KϽTϽ20 K the electrodynamic response and the dc conductivity of SmB 6 are determined by quasifree carriers thermally excited in the conduction band. We evaluate the microscopic parameters of these carriers: the spectral weight, the concentration, the effective mass, the scattering rate, and the mobility. Below 8 K the concentration of carriers in the conduction band freezes out exponentially and finally the electronic properties of SmB 6 are determined by the localized carriers in the narrow band with the typical signature of hopping conductivity.

The results of wide-range measurements of the low-frequency, rf, and microwave conductivity in the typical mixed-valent narrow-gap semiconductor samarium hexaboride are presented. The established steplike anomaly of conductivity ͑͒ around 10 GHz is discussed in the framework of the exciton-polaron approach and coherent-state formation in SmB 6 at helium temperatures. A combined analysis of the dc-and wide-range ac-transport characteristics and dielectric permittivity data at low temperatures is developed.

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