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 ...

Physical properties of semiconducting CdF2 crystals doped with In are determined from measurements of the radio-frequency response of a sample with Schottky barriers at frequencies 10 − 10 6 Hz. The dc conductivity, the activation energy of the amphoteric impurity, and the total concentration of the active In ions in CdF2 are found through an equivalent-circuit analysis of the frequency dependencies of the sample complex impedance at temperatures from 20 K 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 CdF2:In are compared with submillimeter (10 11− 10 12 Hz) measurements and with room-temperature infrared measurements of undoped CdF2. 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.

PACS 85.85.+j -Micro-and nano-electromechanical systems (MEMS/NEMS) and devices PACS 07.75.+h -Mass spectrometers PACS 03.67.-a -Quantum information Abstract -I establish the fundamental lower bound on the mass that can be measured with a nano-mechanical resonator in a given quantum state based on the fundamental quantum Cramér-Rao bound, and identify the quantum states which will allow the largest sensitivity for a given maximum energy. I show that with existing carbon nanotube resonators it should be possible in principle to measure a thousandth of the mass of an electron, and future improvements might allow to reach a regime where one can measure the relativistic change of mass due to absorption of a single photon, or the creation of a chemical bond.

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