Stephen Tsui

Field-driven hysteretic and reversible resistive switch at the Ag-Pr0.7Ca0.3MnO3 interface

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 migration is the cause for the switch.

Field-induced resistive switching in metal-oxide interfaces

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 10nm 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.

A possible crypto-superconducting structure in a superconducting ferromagnet

Abstract We have measured the dc and ac electrical and magnetic properties in various magnetic fields of the recently reported superconducting ferromagnet RuSr2GdCu2O8. 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 sponge-like nonuniform superconducting or a crypto-superconducting structure in the chemically uniform Ru-1212, have been proposed and discussed.

Mechanism and scalability in resistive switching of metal-Pr0.7Ca0.3MnO3 interface

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.

Plasmalike negative capacitance in nanocolloids

A negative capacitance has been observed in a nanocolloid between 0.1 and 10−5Hz. 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 a possible energy storage mechanism. A collective excitation, therefore, is suggested.

A negative dielectric constant in nano-particle materials under an electric field at very low frequencies

The significance of a negative dielectric constant has long been recognized. We report here the observation of a field-induced 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.

Crypto-superconductivity in RuSr2GdCu2O8

Abstract The susceptibility χ = χ ′ + iχ ″ of RuSr 2 GdCu 2 O 8 powders at 2 K has been deduced from ac and dc magnetizations with their particle size d from 800 to 10 μm and grain size 1–5 μm. It is observed that the loss part, χ″, scales with d over a field range of 0.0001 to 100 Oe, suggesting that the supercurrent is homogeneously distributed over particles as small as 10 μm. However, the inductance part, χ′, decreases with d systematically and drastically. An effective penetration depth as large as 30–50 μm is needed, which suggests a very small super-carrier concentration. Crypto-superconductivity is proposed to accomodate the data.

Suppression of multi-level bipolar resistive switching in Ag/Pr0.7Ca0.3MnO3 interfaces at low temperatures

Resistive switching phenomena induced by electric pulsing have been investigated for several years as a result of interest in memory technology development. We investigate the viability of bipolar resistive switching at cryogenic temperatures by cooling a Ag/Pr0.7Ca0.3MnO3 interface and subjecting it to varying applied voltage amplitudes. Upon cooling, the switching phenomenon is suppressed until it is no longer observable below 175 K. Varying the applied voltage reveals the existence of several discrete resistance states, and the cooling trends across different samples and pulsing amplitudes are similar. Our results indicate that an alteration in space charge density is the origin of the switch, and the mechanism for this low temperature suppression is the reduced mobility of local oxygen defects in this active interface layer.

Field-induced Giant Static Dielectric Constant in Nano-particle Aggregates at Room Temperature

The analogy between magnetism and electricity was established by Maxwell in the 19th century, despite the 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 be found if there exists a material that has a dc-polarization opposing the electric field or a negative dielectric susceptibility ε′ − 1, with ε′ being the real part of the relative dielectric constant. Both of these properties have been observed in nano-particle aggregates under a dc electric bias field at room temperature. A possible collective effect in the nano-particle aggregates is proposed to account for the observation. ‘Diaelectricity’ implies overscreening by polarization to the external charges. Materials with a negative static ε′ are expected to provide attraction to similar charges and unusual scattering to electromagnetic waves with possible profound implications for high temperature superconductivity and communications.

Generation of negative capacitance in a nanocolloid

Negative capacitance (NC) is a rather ubiquitous phenomenon that is found in many complex materials ranging from semiconductor devices to biological membranes. The underlying physical processes in this diverse collection differ considerably. However, we previously demonstrated that a relationship exists between NC and the conductivity of the material. Here, we examine and exploit this relationship in an effort to pinpoint the source of NC in a nanocolloid, composed of urea coated nanoparticles in silicone oil, which has previously been shown to exhibit the NC effect. This is accomplished by investigating the influence of several external parameters, such as temperature and moisture content, on the NC and conductance of the colloid as well as solid materials created from the nanoparticles used in the colloid. In addition to NC, the colloid demonstrates the electrorheological (ER) effect. It is shown that large scale particle motions, such as those that generate the ER effect, are not responsible for the NC...