Many transition-metal oxides show very large ("colossal") magnitudes of the dielectric constant and thus have immense potential for applications in modern microelectronics and for the development of new capacitance-based energystorage devices. In the present work, we thoroughly discuss the mechanisms that can lead to colossal values of the dielectric constant, especially emphasising effects generated by external and internal interfaces, including electronic phase separation. In addition, we provide a detailed overview and discussion of the dielectric properties of CaCu3Ti4O12 and related systems, which is today's most investigated material with colossal dielectric constant. Also a variety of further transition-metal oxides with large dielectric constants are treated in detail, among them the system La2−xSrxNiO4 where electronic phase separation may play a role in the generation of a colossal dielectric constant. a
Multiferroics, showing simultaneous ordering of electrical and magnetic degrees of freedom, are remarkable materials as seen from both the academic and technological points of view 1,2 . A prominent mechanism of multiferroicity is the spin-driven ferroelectricity, often found in frustrated antiferromagnets with helical spin order 1,3,4,5 . There, similar to conventional ferroelectrics, the electrical dipoles arise from an off-centre displacement of ions. However, recently a different mechanism, namely purely electronic ferroelectricity, where charge order breaks inversion symmetry, has attracted considerable interest 6 . Here we provide evidence for this exotic type of ferroelectricity, accompanied by antiferromagnetic spin order, in a two-dimensional organic charge-transfer salt, thus representing a new class of multiferroics. Quite unexpectedly for electronic ferroelectrics, dipolar and spin order arise nearly simultaneously. This can be ascribed to the loss of spin frustration induced by the ferroelectric ordering. Hence, here the spin order is driven by the ferroelectricity, in marked contrast to the spin-driven ferroelectricity in helical magnets.In the present work, we have investigated single crystalline -(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl (-Cl), where BEDT-TTF stands for bis(ethylenedithio)-tetrathiafulvalene (often abbreviated as ET). Two crystals with different geometries and contact materials were investigated (see methods section). In these compounds, dimers of ET molecules form an anisotropic triangular lattice with a halffilled dimer band, where the strong on-dimer Coulomb interaction U drives the system to a Mott insulating state 7,8 . In addition, the importance of intra-dimer degrees of freedom and inter-site interactions V have been pointed out 9,10,11 . -Cl consists of alternating conducting ET layers and insulating anion sheets (see Supplementary Information (SI), Fig. S1). Within the ET layers, adjacent molecules form dimers on which a single electron hole is located. Below T N 27 K, intralayer antiferromagnetic and interlayer ferromagnetic ordering of hole spins occur, followed by weak ferromagnetic canting below 23 K (refs. 12,13). -Cl becomes superconducting below 12.8 K, when applying pressures of 300 bar 14 . Figure 1 shows the temperature dependence of the conductivity ' of crystal 1, measured at 2.1 Hz, providing a good estimate of the dc conductivity dc , see SI. Aside of the wellknown overall semiconducting characteristics of dc (T) 14,15 , we find a jump-like decrease by two decades at around 27 K, about the same temperature where long-range antiferromagnetic ordering is reported 7,8 . A corresponding jump was also found in sample 2 (see SI Figure 2 shows the dielectric constant '(T) of sample 1 for various frequencies. Pronounced peaks reaching absolute values up to several hundreds are revealed. While the peak positions are nearly frequency independent, their amplitudes become strongly suppressed with increasing frequency. The overall behaviour is typical for order-di...
In the present work, we provide broadband dielectric spectra showing strong electrode polarization effects for various materials, belonging to very different material classes. This includes both ionic and electronic conductors as, e.g., salt solutions, ionic liquids, human blood, and colossal-dielectricconstant materials. These data are intended to provide a broad data base enabling a critical test of the validity of phenomenological and microscopic models for electrode polarization. In the present work, the results are analyzed using a simple phenomenological equivalent-circuit description, involving a distributed parallel RC circuit element for the modeling of the weakly conducting regions close to the electrodes. Excellent fits of the experimental data are achieved in this way, demonstrating the universal applicability of this approach. In the investigated ionically conducting materials, we find the universal appearance of a second dispersion region due to electrode polarization, which is only revealed if measuring down to sufficiently low frequencies. This indicates the presence of a second charge-transport process in ionic conductors with blocking electrodes.
Domain walls in ferroelectric semiconductors show promise as multifunctional two-dimensional elements for next-generation nanotechnology. Electric fields, for example, can control the direct-current resistance and reversibly switch between insulating and conductive domain-wall states, enabling elementary electronic devices such as gates and transistors. To facilitate electrical signal processing and transformation at the domain-wall level, however, an expansion into the realm of alternating-current technology is required. Here, we demonstrate diode-like alternating-to-direct current conversion based on neutral ferroelectric domain walls in ErMnO. By combining scanning probe and dielectric spectroscopy, we show that the rectification occurs at the tip-wall contact for frequencies at which the walls are effectively pinned. Using density functional theory, we attribute the responsible transport behaviour at the neutral walls to an accumulation of oxygen defects. The practical frequency regime and magnitude of the direct current output are controlled by the bulk conductivity, establishing electrode-wall junctions as versatile atomic-scale diodes.
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...
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