The hysteretic and reversible polarity-dependent resistive switch driven by electric pulses is studied in both Ag/Pr0.7Ca0.3MnO3/YBa2Cu3O7 sandwiches and single-layer Pr0.7Ca0.3MnO3 strips. The data demonstrate that the switch takes place at the Ag-Pr0.7Ca0.3MnO3 interface. A model, which describes the data well, is proposed. We further suggest that electrochemical diffusion is the cause for the switch.Pr 0.7 Ca 0.3 MnO 3 (PCMO) has attracted extensive interest recently. Below 150 K, its free energies corresponding to the paramagnetic, the charge-ordered, and the ferromagnetic states differ only slightly. Therefore, a slight external disturbance, e.g. magnetic field, light, isotope mass, pressure, or electric field, may lead to a large resistivity (ρ) change, but only at low temperatures.1 Therefore, it is interesting to note the report of Liu et al.2 that the two-lead resistance, R, of a PCMO layer sandwiched between an Ag top-electrode and a YBa 2 Cu 3 O 7 (YBCO) or a Pt bottom-electrode can be drastically and repeatably alternated at room temperature by applying electric pulses with different polarities.3 This R-switch may thus offer potential device applications, e.g. nonvolatile memory. Similar R-changes in single-layer PCMO films with the four-lead configuration were also reported. The R-switch has therefore been attributed to bulk properties of PCMO, in terms of the alignment of the presumed ferromagnetic clusters by the electric field.2 The interpretation, if confirmed, presents a major challenge to the physics of manganites and, possibly, to the basic law of parity conservation. The reported R-change of ∆R ≥ 3000 Ω across a 600 nm thick PCMO film represents a ρ-increase of ∆ρ ≈ 105 Ω cm, and suggests a novel state with a ρ(297 K) far greater than the ρ(297 K) << 10 1 Ω cm ever reported in PCMO. According to the commonly accepted polaron model, ρ(297 K) of manganites is controlled by the polaron mobility and should be ultimately restricted by the hopping barrier (10 −1 eV ≈ k B T at 297 K) associated with the Jahn-Teller distortion, which is only a few eV. 4 The experimental ρ(297 K) is only 10 −2 to 10 0 Ω cm in (La y Pr 1−y ) 1−x Ca x MnO 3 for 0.2 ≤ x ≤ 0.5 and 0 ≤ y ≤ 0.7, 5 and ≤ 10 4 Ω cm even in extreme cases, such as Nd 0.7 Ba 0.3 MnO 3 and LaMnO 3 .6 A ρ(297 K) of 10 5 Ω cm or higher would suggest a new insulating state never observed before and challenge the polaron model commonly accepted. In a more general sense, this polarity-dependent ρ in a uniform material reported, if proven, represents a violation of the law of parity conservation in the electromagnetic field. It may occur without parity violation only if the sample is asymmetric due to either an inhomogeneity in the thickness direction or poling by electric pulses ("training"); neither bears any obvious relation to the alignment model proposed.2 The present study is motivated by our attempt to elucidate the mechanism responsible for, and the nature of, the R-switch. Our data demonstrate that the switch occurs at the Ag-PCMO interface, ...

We investigate the polarity-dependent field-induced resistive switching phenomenon driven by electric pulses in perovskite oxides. Our data show that the switching is a common occurrence restricted to an interfacial layer between a deposited metal electrode and the oxide. We determine through impedance spectroscopy that the interfacial layer is no thicker than 10 nm and that the switch is accompanied by a small capacitance increase associated with charge accumulation. Based on interfacial I − V characterization and measurement of the temperature dependence of the resistance, we propose that a field-created crystalline defect mechanism, which is controllable for devices, drives the switch.Recent observation of room-temperature resistive switching driven by electric fields in various perovskite oxides has garnered attention due to the potential for nonvolatile memory applications. This fieldinduced resistive switch has been reported in several compounds. 1,2,3,4,5,6 The switching observed shares several features: a moderate switch speed; an altered resistance inconsistent with the well-accepted bulk resistivity; and a sensitivity to surface treatments. Various models have been put forth, but inconsistencies remain. For instance, bulk charge ordering in Pr 0.7 Ca 0.3 MnO 3 (PCMO) has been suggested, but this creates an apparent conflict with both the spatial symmetry and the reported bulk properties.1 Although interface models have been proposed involving either lattice defects or carrier concentration, 2,5,7 the exact nature of this interface remains vague. We therefore investigate the interface properties associated with the resistive switch and have observed: a) the switching is a common phenomenon to the metal-oxide interfacial layer; b) the interfacial transport properties are rather different from that of the bulk; c) the interfacial resistance is dominated by the carrier trapping with no indication of Schottky barriers; and d) the capacitance of the interfacial layer, on the order of 1000 nF/cm 2 , changes with switching, which is indicative of a change in the space-charge. These observations can be self-consistently understood using a carrier-trapping model and suggest that the field-induced switch is not restricted to a small class of materials. Therefore, the potential for controlling this phenomenon for device applications is considerable.Ceramic samples synthesized by standard solid-state reactions were determined to be single-phase based on X-ray powder diffraction patterns taken on a Rigaku DMAX-IIIB diffractometer. PCMO thin films were ac sputtered on LaAlO 3 substrates at 760• C in an Ar:O 2 = 2 : 3 mixed atmosphere at 140 mTorr. The films were found to be highly epitaxial. Pt leads were attached to the ceramic samples using Ted Pella Leitsilber 200 Ag paint. The thin film samples received sputtered Ag electrodes to which Pt leads were also attached using Ag paint.Ceramic LaCoO 3 , La 0.7 Ca 0.3 MnO 3 , Pr 0.7 Ca 0.3 MnO 3 , SrFeO 2.7 , RuSr 2 GdCu 2 O 3 , and YBa 2 Cu 3 O 7 were tested. We adopted ...

We have measured the dc and ac electrical and magnetic properties in various magnetic fields of the recently reported superconducting ferromagnet RuSr 2 GdCu 2 O 8 . Our reversible magnetization measurements demonstrate the absence of a bulk Meissner state in the compound below the superconducting transition temperature. Several scenarios that might account for the absence of a bulk Meissner state, including the possible presence of a spongelike non-uniform superconducting or a crypto-superconducting structure in the chemically uniform Ru-1212, have been proposed and discussed.

Electric-field-induced resistive switching in metal-oxide interfaces has attracted extensive recent interest. While many agree that lattice defects play a key role, details of the physical processes are far from clear. There is debate, for example, regarding whether the electromigration of pre-existing point defects or the field-created larger lattice defects dominates the switch. We investigate several Ag-Pr 0.7 Ca 0.3 MnO 3 samples exhibiting either submicrosecond fast switching or slow quasistatic dc switching. It is found that the carrier trapping potentials are very different for the pre-existing point defects associated with doping ͑and/or electromigration͒ and for the defects responsible for the submicrosecond fast switching. Creation/removal of the defects with more severe lattice distortions and spatial spreading ͑trapping potential Ն0.35 eV͒, therefore, should be the dominating mechanism during submicrosecond switching. On the other hand, the shallow defects ͑trapping potential Ӷ0.2 eV͒ associated with doping/annealing are most likely responsible for the resistance hysteresis ͑slow switch͒ during quasistatic voltage sweep.

The polarity-dependent resistive-switching across metal-Pr0.7Ca0.3MnO3 interfaces is investigated. The data suggest that shallow defects in the interface dominate the switching. Their density and fluctuation, therefore, will ultimately limit the device size. While the defects generated/annihilated by the pulses and the associated carrier depletion seem to play the major role at lower defect density, the defect correlations and their associated hopping ranges appear to dominate at higher defect density. Therefore, the switching characteristics, especially the size-scalability, may be altered through interface treatments.The renewed interest in various resistive switching phenomena is largely driven by recent market demands for nano-sized nonvolatile memory devices. 1 While the current boom of consumer electronics may largely be attributed to the successful miniaturization of both FLASH chips and mini hard drives, cheaper and smaller devices are called for.Various resistive hysteretic phenomena are consequently studied with the hope that the size limitations associated with the related physics/chemistry/technology might be less severe. 2 Our limited knowledge about the mechanisms so far, however, makes the evaluation difficult. This is especially true for the switching across metal-Pr 0.7 Ca 0.3 MnO 3 (PCMO) interfaces. 3,4,5,6 Several models, i.e. bulk phase-separation, 3 carrier-trapping in pre-existing metallic domains, 7,8 and field-induced lattice defects, 4 have been proposed. Each possesses its own distinguishable size-limitation, e.g. the statistics of the associated local mesostructures. Here, we report our mechanism investigation through both the trapped-carrier distribution and their hopping range. Our data suggest that the characteristics may largely be engineered through the mesostructure of the interfacial defects.Bulk PCMO, in great contrast with well known semiconductors, has a rather high nominal carrier concentration with its high resistivity mainly attributed to hopping barriers. 9 Local defects, therefore, appear as a natural cause of the resistive switching. Following this line of reasoning, a domain model has recently attracted much attention. 7,8 There, a tunneling from the electrode to some pre-existing interfacial metallic domains has been assumed to be the dominant process. Consequently, the carrier-occupation in the domains may change with the carrier-trapping during the write pulses, and cause the Rswitch between an on (low resistance) and an off (high resistance) state. This is realized through either the change of the tunneling probability 7 or a doping-induced metalinsulator transition. 8 Useful devices based on this mechanism, therefore, should typically be much larger than these interfacial domains. It is interesting to note that even if the "domains" can be reduced to individual lattice defects (or small clusters) as in the proposed defect modification model, 4 the fluctuation (inhomogeneity) of the defect density still sets a limit for the size scalability just like the dopant fl...

The existence of a negative static dielectric constant has drawn a great deal of theoretical controversy. Experimentally, one has never been observed. However, low-frequency negative capacitance has been widely reported in fields including physics, chemistry, biology, geology, and electronics. This wide variety of systems possesses an extremely diverse set of physical processes that, surprisingly, share similar characteristics. We present a general mechanism that unites the various instances of negative capacitance under a common framework. The mechanism demonstrates that the negative capacitance arises from dc/ac signal mixing across a nonlinear conductor. Verification of the model is performed in physically distinct samples: an electrorheological fluid, a fuel cell, and a solar cell. Furthermore, we argue that the negative capacitance, under appropriate conditions, can be associated with a negative-differential dielectric constant, possibly even in the static limit.

A negative capacitance has been observed in a nano-colloid between 0.1 and 10 −5 Hz. The response is linear over a broad range of conditions. The low-ω dispersions of both the resistance and capacitance are consistent with the free-carrier plasma model, while the transient behavior demonstrates an unusual energy storage mechanism. A collective excitation, therefore, is suggested.The origin of the negative capacitance (NC) observed in many systems with mesostructures has been debated since 1988.1,2 Experimentally, this reactive response is probed by the phase shift between the applied ac voltage, V ac , and the resulting current, I ac , at a frequency ω. A negative phase-angle between I ac and V ac , therefore, represents a NC. Jonscher, 3 Ershov et al., 4 and others, however, repeatedly emphasize that the clue to these unusual observations lies in the equivalent negative tran-. Such a retarded current response, consequentially, is attributed to the inertia of the associated process. Penin 5 refers to such a process, which is characterized by the equation dI/dt+I/τ = bV with parameters τ and b, as a τ system. The equation, it should be pointed out, is actually that of a free-carrier plasma with the solution ofwhere R is the resistance, C is the capacitance, and 1/τ represents the plasma damping. It is therefore interesting to note that the proper mesostructures may theoretically lead to a negative dielectric response, 6 and any negative static ǫ ′ (which leads to NC at ω = 0) can be described as ghost-plasmons. 7A true plasma-like response, however, should also have a linear I − V characteristic as well as follow the causality correlation between R and C. The observed ∆I(0 + )/∆V , for example, should include a component of ∆I 1 = 1/R ω=∞ − 1/R ω=0 = −bτ from the Rdispersion in addition to the term of ∆I 2 = 2/π 1,2,3,4,5 In some cases (such as the impact ionization of gas in Ref.5), such a plasma interpretation is obviously an oversimplified approximation. Here we report the NC below 1 Hz in a giant electrorheological (ER) fluid.8 The I − V characteristic is linear across a broad V ac range, and all features, e.g. C(ω), ∆I, and the R(ω) contribution to it, are consistent with a plasma-like excitation with an extremely long τ .The ER fluid is a colloid of silicone oil and 20 nm urea-coated Ba 0.8 Rb 0.4 TiO(C 2 O 4 ) 2 nanoparticles. 8,9The nanoparticle to oil ratio is 10 g: 3 ml. The capacitor cells were constructed of two parallel gold or copper electrodes with dimensions of 6 mm × 13 mm and a gap distance of 0.1 mm. The ac measurements were performed via a serial connected ac voltage source, capacitor cell, and current meter. Only dc couplings were present. The phase angle was verified with resistors and capacitors of known values. Both the systematic error and the resolution of the phase angle are within 0.05• . Details of both the ER fluid preparation 9 and the measurements 8 have been reported before. The same hardware was used in the transient measurements with the pulse output and the voltage reading bein...

The significance of a negative dielectric constant has long been recognized. We report here the observation of a fieldinduced large negative dielectric constant of aggregates of oxide nano-particles at frequencies below ~ 1 Hz at room temperature. The accompanying induced charge detected opposes the electric field applied in the field-induced negative dielectric constant state. A possible collective effect in the nano-particle aggregates is proposed to account for the observations. Materials with a negative dielectric constant are expected to provide an attraction between similar charges and unusual scattering to electromagnetic waves with possible profound implications for high temperature superconductivity and communications.Keywords: negative dielectric constant, negative polarization, diaelectricity, nanoparticles, electrorheology A NEGATIVE STATIC DIELECTRIC CONSTANTAccording to the BCS theory, the superconducting transition temperature T c is equal to 1.14θ D exp [-1/N(0)V], where θ D is the Debye temperature (or the characteristic temperature of other quasi-particles), N(0) is the electron density of states at the Fermi energy, and V is the attractive interaction between the electrons (or other quasi-particles). It is evident that the greater the V is, the higher the T c is. Various propositions have been advanced over the years prior to the discovery of cuprate high temperature superconductivity to enhance V and thus T c . 1 The main interaction between two electric charges, q 1 and q 2 , separated by a distance r, is known to be V(r) = q 1 q 2 /ε'r, where ε' is the real part of the dielectric constant ε = ε' − iε". A negative static dielectric constant, ε'(k,ω) at a wave-vector k ≠ 0 and frequency ω = 0, has attracted great interest because of its implications for wave propagation, electrolyte behavior, colloidal properties, and bio-membrane functions in addition to possible high temperature superconductivity. For a negative ε', the force between two electrons will be attractive, leading to electron-pairing if other electron instabilities can be arrested.2 Electron-pairing has been demonstrated to be responsible for superconductivity in the conventional low temperature superconductors as well as in the newly discovered high temperature superconductors. A route via a negative ε' to higher T c may be worth exploring.The analogy between magnetism and electricity has long been established by Maxwell in the 19th century, in spite of their subtle difference. While magnetic materials display paramagnetism, ferromagnetism, antiferromagnetism, and diamagnetism, only paraelectricity, ferroelectricity, and antiferrolelectricity have been found in dielectric materials. The missing "diaelectricity" may therefore be found in a dielectric material that is characterized by a negative dielectric susceptibility ε'-1 and a polarization opposing the electric field.For the above reasons, we have decided to examine materials that may display a negative ε'.