High-conductance State Research Articles

Specific features of the magnetic and electric properties and the magnetically inhomogeneous state of Nd0.5Sr0.5MnO3 single crystal

The hysteretic behavioral features of magnetization and resistance upon remagnetization in quasistatic (up to 9 T) and pulse (up to 14 T) magnetic fields have been investigated. The relaxation processes of magnetization and resistance after the effect of a 9 T magnetic field have also been studied. A mechanism of the remagnetization of antiferromagnetic insulating-ferromagnetic metallic (AFM/I-FM/M) phases and the existence of high-conductivity state of the sample after removal of the magnetizing field is proposed for low temperatures. This mechanism is due to slow relaxation of the nonequilibrium lattice corresponding to the FM phase (of larger volume) toward the equilibrium AFM lattice (of lesser volume).

Adjustment of conformation change and charge trapping in ion-doped polymers to achieve ternary memory performance

Ion-doped poly(4-vinylpyridine) derivatives (P4VPCz), where in the pendant chains the electron-withdrawing pyridine moiety and acceptor carbazole moiety are linked by a flexible alkyl spacer were designed and synthesized. Sandwiched ITO/P4VPCz/Al devices, made through simple spin-coating processes have shown that they could be tuned from a binary to ternary memory performance, by increasing the carbazole content. P4VPCz2 with a 20% mole ratio of the carbazole moiety shows a binary performance according to the charge trapping of the pyridine acceptor in the polymer, while P4VPCz5 which contains a 50% mole ratio of carbazole in the pendant chains, exhibits a ternary data storage ability, probably induced by double mechanisms (i.e. a conformational change and charge trapping). Interestingly, as the carbazole mole ratio reached 80%, the polymer P4VPCz8 has a high conductivity state, with no switching behavior, because of the large amount of doped ions improving the charge transfer mobility. Thus, we hope can offer a guideline to achieve a high-performance multilevel memory material, via combining different mechanisms as well as doping ions.

Atomistic Simulations Explain Mutational Effects on Ion Modulation and Kainate Receptor Activity

The most important excitatory neurotransmitter in the central nervous system is glutamate. Ionotropic glutamate receptors (iGluRs) thus play key roles in the brain; for example they are associated with learning and memory at the molecular level and implicated in various diseases. Binding of glutamate to the iGluR ligand-binding domain (LBD) triggers opening of the transmembrane ion channel, facilitating influx of cations. As opposed to other iGluRs, kainate-selective iGluRs (KARs) require binding of extracellular sodium and chloride ions to the LBD dimer interface in addition to agonist binding to obtain normal activity. We have examined the structural/dynamical effects of the presence/absence of these ions in conjunction with mutations at the dimer interface of GluK2 receptors. Electrophysiological studies of a double cysteine mutant (Y521C/L783C), cross-linking the LBD dimer, indicate that this mutant cannot be activated to wildtype(WT)-like high-conductance states but, as opposed to previous interpretations, does in fact reside mainly in a desensitized state. Atomistic molecular dynamics simulations of the WT and the cross-linked mutant reveal that whereas ions remain stably bound to the WT LBD dimer, they tend to leave early in the simulations of the cross-linked mutant. Interestingly, the mutant is also found to relax quickly to give a more open dimer interface than the WT, consistent with the current model of the desensitized state. Similar ion unbinding and interface opening is observed for a mutant (L783S) where the cation site is indirectly disrupted by disturbing a hydrophobic plug shielding the site. In the electrophysiological studies, this mutant appears non-functional. Overall, the results are consistent with the view that maximum conductance depends on a tightly packed dimer-interface regulated by the presence of extracellular ions.

Neuronal intrinsic properties shape naturally evoked sensory inputs in the dorsal horn of the spinal cord

Intrinsic electrophysiological properties arising from specific combinations of voltage-gated channels are fundamental for the performance of small neural networks in invertebrates, but their role in large-scale vertebrate circuits remains controversial. Although spinal neurons have complex intrinsic properties, some tasks produce high-conductance states that override intrinsic conductances, minimizing their contribution to network function. Because the detection and coding of somato-sensory information at early stages probably involves a relatively small number of neurons, we speculated that intrinsic electrophysiological properties are likely involved in the processing of sensory inputs by dorsal horn neurons (DHN). To test this idea, we took advantage of an integrated spinal cord–hindlimbs preparation from turtles allowing the combination of patch-clamp recordings of DHN embedded in an intact network, with accurate control of the extracellular milieu. We found that plateau potentials and low threshold spikes (LTS) -mediated by L- and T-type Ca2+channels, respectively- generated complex dynamics by interacting with naturally evoked synaptic potentials. Inhibitory receptive fields could be changed in sign by activation of the LTS. On the other hand, the plateau potential transformed sensory signals in the time domain by generating persistent activity triggered on and off by brief sensory inputs and windup of the response to repetitive sensory stimulation. Our findings suggest that intrinsic properties dynamically shape sensory inputs and thus represent a major building block for sensory processing by DHN. Intrinsic conductances in DHN appear to provide a mechanism for plastic phenomena such as dynamic receptive fields and sensitization to pain.

Dependencies of Donor–Acceptor Memory on Molecular Levels

This study reports the variation in the switch-on voltages of resistive switching devices using an electron-donating conjugated polymer, poly(3-hexylthiophene), P3HT, and hexaazatriphenylene-derivative acceptors. Memory devices fabricated using the spin-coated blends demonstrated electrical bistability with a conductivity modulation of ∼2 orders in magnitude and with negligible data loss after a stress test of 500 consecutive readouts conducted at 0.2 V. The switch-on voltage of the resistive switching device can be altered by using molecular acceptors with different LUMO levels. The charge transfer between the donor and acceptor was shown by quenching the fluorescence emission from P3HT after adding the acceptors. UV–vis absorption and Raman spectroscopic characterization revealed electrostatic doping of P3HT after switching the device from its OFF to ON state. The charge injection mechanisms of the two different conductive states were also analyzed and were found to be due to thermionic emission and Poole–Frenkel tunneling when the device is in the low and high conductive states, respectively.

TARP‐associated AMPA receptors display an increased maximum channel conductance and multiple kinetically distinct open states

Fast excitatory synaptic transmission in the CNS is mediated mainly by AMPA-type glutamate receptors (AMPARs), whose biophysical properties are dramatically modulated by the presence of transmembrane AMPAR regulatory proteins (TARPs). To help construct a kinetic model that will realistically describe native AMPAR/TARP function, we have examined the single-channel properties of homomeric GluA1 AMPARs in combination with the TARPs, γ-2, γ-4 and γ-5. In a saturating concentration of agonist, each of these AMPAR/TARP combinations gave rise to single-channel currents with multiple conductance levels that appeared intrinsic to the receptor-channel complex, and showed long-lived subconductance states. The open time and burst length distributions of the receptor complexes displayed multiple dwell-time components. In the case of γ-2- and γ-4-associated receptors, these distributions included a long-lived component lasting tens of milliseconds that was absent from both GluA1 alone and γ-5-associated receptors. The open time distributions for each conductance level required two dwell-time components, indicating that at each conductance level the channel occupies a minimum of two kinetically distinct open states. We have explored how these data place novel constraints on possible kinetic models of TARP-associated AMPARs that may be used to define AMPAR-mediated synaptic transmission.

Preparation of polyaniline and self-doped polyaniline–clay nanocomposites in supercritical CO2: Synthesis and conductivity study

Supercritical CO2 (ScCO2) was used as processing medium to prepare polyaniline (PANI) and self-doped polyaniline (SPANI)–clay nanocomposites. Interfacial copolymerization of aniline (ANI) and m-aminobenzene sulfonic acid (SAN) in the presence of Cloisite 30B nano-clay powder was performed in ScCO2/water to produce PANI and SPANI–clay nanocomposites. Different ratio of aniline to SAN was used and the temperature profile during copolymerization was measured to monitor the copolymerization process. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis, Fourier transform infrared spectroscopy (FTIR) and UV–vis spectroscopy were used to characterize the morphology and the structure of the resultant nanocomposites. SEM and TEM results reveal that nano-clays are fully dispersed in the ScCO2 synthesized nanocomposites, while in the atmospheric synthesize ones the nano-clays have tightly bonded agglomerated and are still stack to each others. FTIR and UV–vis results revealed that the products are in high conductive emeraldine salt state. The influences of clay loading on the conductivity of resulting nanocomposites were studied. It was found that the conductivity of nanocomposites is slightly lower than pristine polymers and decreases by nano-clay loading. Conductivity of compressed pellets of pristine PANI and SPANI were measured during the storage for different periods of times to evaluate aging properties. It was found that resulted SPANI has stable conductivity compare to the PANI nanocomposite.

Spike phase locking in CA1 pyramidal neurons depends on background conductance and firing rate.

Oscillatory activity in neuronal networks correlates with different behavioral states throughout the nervous system, and the frequency-response characteristics of individual neurons are believed to be critical for network oscillations. Recent in vivo studies suggest that neurons experience periods of high membrane conductance, and that action potentials are often driven by membrane potential fluctuations in the living animal. To investigate the frequency-response characteristics of CA1 pyramidal neurons in the presence of high conductance and voltage fluctuations, we performed dynamic-clamp experiments in rat hippocampal brain slices. We drove neurons with noisy stimuli that included a sinusoidal component ranging, in different trials, from 0.1 to 500 Hz. In subsequent data analysis, we determined action potential phase-locking profiles with respect to background conductance, average firing rate, and frequency of the sinusoidal component. We found that background conductance and firing rate qualitatively change the phase-locking profiles of CA1 pyramidal neurons versus frequency. In particular, higher average spiking rates promoted bandpass profiles, and the high-conductance state promoted phase-locking at frequencies well above what would be predicted from changes in the membrane time constant. Mechanistically, spike rate adaptation and frequency resonance in the spike-generating mechanism are implicated in shaping the different phase-locking profiles. Our results demonstrate that CA1 pyramidal cells can actively change their synchronization properties in response to global changes in activity associated with different behavioral states.

Spatially Resolved Raman Spectroelectrochemistry of Solid-State Polythiophene/Viologen Memory Devices

A three terminal molecular memory device was monitored with in situ Raman spectroscopy during bias-induced switching between two metastable states having different conductivity. The device structure is similar to that of a polythiophene field effect transistor, but ethylviologen perchlorate was added to provide a redox counter-reaction to accompany polythiophene redox reactions. The conductivity of the polythiophene layer was reversibly switched between high and low conductance states with a "write/erase" (W/E) bias, while a separate readout circuit monitored the polymer conductance. Raman spectroscopy revealed reversible polythiophene oxidation to its polaron form accompanied by a one-electron viologen reduction. "Write", "read", and "erase" operations were repeatable, with only minor degradation of response after 200 W/E cycles. The devices exhibited switching immediately after fabrication and did not require an "electroforming" step required in many types of memory devices. Spatially resolved Raman spectroscopy revealed polaron formation throughout the polymer layer, even away from the electrodes in the channel and drain regions, indicating that thiophene oxidation "propagates" by growth of the conducting polaron form away from the source electrode. The results definitively demonstrate concurrent redox reactions of both polythiophene and viologen in solid-state devices and correlate such reactions with device conductivity. The mechanism deduced from spectroscopic and electronic monitoring should guide significant improvements in memory performance.

On Anomalies of Electrical Conductivity in Antistatic Plasticized Poly(vinyl chloride) Films

In plasticized poly(vinylchloride) (PVC) films, representing system of two chemically non-interacting insulators, PVC and plasticizer, at monotonous increase of concentration of one of the insulators (plasticizer) sharp transition insulator-semiconductor to a high conductive state is found out, which is accompanied by similar transition back to state with low conductivity at further increase of plasticizer concentration. The observed concentration range with a high (semiconductor) level of electric conductivity permits us to measure the resistance-thickness dependence for relatively wide films thickness set (20-200 μm), which is close to simple exponential. A phenomenological model that allows to interpret the observed phenomena as direct and inverse percolation transitions in two-phase medium with charge-transfer by nano-scale kinetic segments of macromolecules mainly along the borders between PVC-clusters and plasticizer is proposed.

Intrinsic and extrinsic resistive switching in a planar diode based on silver oxide nanoparticles

Resistive switching is investigated in thin-film planar diodes using silver oxide nanoparticles capped in a polymer. The conduction channel is directly exposed to the ambient atmosphere. Two types of switching are observed. In air, the hysteresis loop in the current–voltage characteristics is S-shaped. The high conductance state is volatile and unreliable. The switching is mediated by moisture and electrochemistry. In vacuum, the hysteresis loops are symmetric, N-shaped and exhibit a negative differential resistance region. The conductance states are non-volatile with good data retention, programming cycling endurance and large current modulation ratio. The switching is attributed to electroforming of silver oxide clusters.

Excitability of jcBNST Neurons Is Reduced in Alcohol-Dependent Animals during Protracted Alcohol Withdrawal

Alcohol dependence and withdrawal has been shown to cause neuroadaptive changes at multiple levels of the nervous system. At the neuron level, adaptations of synaptic connections have been extensively studied in a number of brain areas and accumulating evidence also shows the importance of alcohol dependence-related changes in the intrinsic cellular properties of neurons. At the same time, it is still largely unknown how such neural adaptations impact the firing and integrative properties of neurons. To address these problems, here, we analyze physiological properties of neurons in the bed nucleus of stria terminalis (jcBNST) in animals with a history of alcohol dependence. As a comprehensive approach, first we measure passive and active membrane properties of neurons using conventional current clamp protocols and then analyze their firing responses under the action of simulated synaptic bombardment via dynamic clamp. We find that most physiological properties as measured by DC current injection are barely affected during protracted withdrawal. However, neuronal excitability as measured from firing responses under simulated synaptic inputs with the dynamic clamp is markedly reduced in all 3 types of jcBNST neurons. These results support the importance of studying the effects of alcohol and drugs of abuse on the firing properties of neurons with dynamic clamp protocols designed to bring the neurons into a high conductance state. Since the jcBNST integrates excitatory inputs from the basolateral amygdala (BLA) and cortical inputs from the infralimbic and the insular cortices and in turn is believed to contribute to the inhibitory input to the central nucleus of the amygdala (CeA) the reduced excitability of the jcBNST during protracted withdrawal in alcohol-dependent animals will likely affect ability of the jcBNST to shape the activity and output of the CeA.

Noninvasive Surface Coverage Determination of Chemically Modified Conical Nanopores that Rectify Ion Transport

Surface modification will change the surface charge density (SCD) at the signal-limiting region of nanochannel devices. By fitting the measured i-V curves in simulation via solving the Poisson and Nernst-Planck equations, the SCD and therefore the surface coverage can be noninvasively quantified. Amine terminated organosilanes are employed to chemically modify single conical nanopores. Determined by the protonation-deprotonation of the functional groups, the density and polarity of surface charges are adjusted by solution pH. The rectified current at high conductivity states is found to be proportional to the SCD near the nanopore orifice. This correlation allows the noninvasive determination of SCD and surface coverage of individual conical nanopores.

Organic bistable devices utilizing carbon nanotubes embedded in poly(methyl methacrylate)

The electrical and memory behavior of organic bistable memory devices in the form of metal-embedded insulator-metal (MIM) structure are described. The devices utilize layer-by-layer (LbL) deposited single walled carbon nanotubes (SWCNTs) as charge traps embedded between two polymethylmethacrylate (PMMA) insulating layers. The stack was sandwiched between two aluminium electrodes to form an Al/PMMA/SWCNTs/PMMA/Al structure. The current-voltage (I-V) characteristics of the devices exhibit electrical bistability and non-volatile memory characteristics in terms of switching between high conductive (ON) and low conductive (OFF) states. The different conductive states were programmed by application of a positive and negative voltage pulse for the ON and OFF states, respectively. A maximum ON/OFF ratio of 2 × 105 is achieved at low reading voltage of 1 V. Space-charge-limited-current (SCLC) conduction model was used to describe the carriers transport and the electrical bistability in the devices, which was attributed to the trapping and detrapping of electrons inside the SWCNTs.

Nanoscale observations of resistive switching high and low conductivity states on TiN/HfO2/Pt structures

Resistive Switching (RS) phenomenon in Metal–Insulator–Metal (MIM) structures with polycrystalline HfO2 layers as dielectric has been studied at the nanoscale using Conductive Atomic Force Microscope (CAFM). The CAFM measurements reveal that (i) the conductive filaments (CFs) created at very small areas are the origin of the RS phenomenon observed at device level and (ii) RS conductive filaments are primarily formed at the grain boundaries, which exhibit especially low breakdown voltage. CAFM images obtained on MIM structures at the Low and High Resistive states also show that, although the current in the Low Resistive State is mainly driven by a completely formed single CF, the cell area dependence of the conductivity in the High Resistive State could be explained by considering the presence of multiple partially formed CFs.

Conductivity switching and memory effect in polymer brushes with carbazole pendant moieties

In this paper, poly(2-(N-carbazolyl)ethyl methacrylate) (PCEM) brushes have been prepared on the indium-tin oxide (ITO) surfaces via surface-initiated atom transfer radical polymerization (ATRP) using a silane coupling agent containing the initiator moiety. Films of PCEM brushes between bottom ITO electrode and Al top electrode are sandwiched to fabricate the ITO-g-PCEM/Al device. The device exhibits two conductivity states and can be switched from the initial low-conductivity (OFF) state to the high-conductivity (ON) state at the switching threshold voltages of −1.5V with the ON/OFF current ratio up to 106. The ON state of the device is nonvolatile and can withstand 106 pulse read cycles at −0.8V under ambient conditions. Upon reversing the bias, the ON state cannot be reset to the initial OFF states. The ITO-g-PCEM/Al device behaves as a write-once read-many-times (WORM) memory. Compared with that of the conventional ITO/PCEM/Al device fabricated by spin-coating, the switching voltage is lower in the ITO-g-PCEM/Al memory device.

Carrier Transport Mechanisms of Organic Bistable Devices Fabricated Utilizing Hybrid C60/Poly(methyl methacrylate) Nanocomposites

Organic bistable devices (OBDs) based on nanocomposites consisting of C60 embedded in the poly(methyl methacrylate) (PMMA) layer were fabricated by using a spin coating method. The current density–voltage (J–V) curves of the Al/C60 embedded in PMMA layer/indium–tin-oxide devices exhibited an electrical bistability with a low-conductivity state, a transition state, and a high-conductivity state. The J–V curves for OBDs containing hybrid C60 and PMMA except a low voltage range were reasonably fitted by using the space-charge-limited current conduction model. The trapped electron densities of the OBDs were attributed to the space charge relative to the internal electric field.

Carrier Transport Mechanisms of Organic Bistable Devices Fabricated Utilizing Hybrid C60/Poly(methyl methacrylate) Nanocomposites

Organic bistable devices (OBDs) based on nanocomposites consisting of C60 embedded in the poly(methyl methacrylate) (PMMA) layer were fabricated by using a spin coating method. The current density–voltage (J–V) curves of the Al/C60 embedded in PMMA layer/indium–tin-oxide devices exhibited an electrical bistability with a low-conductivity state, a transition state, and a high-conductivity state. The J–V curves for OBDs containing hybrid C60 and PMMA except a low voltage range were reasonably fitted by using the space-charge-limited current conduction model. The trapped electron densities of the OBDs were attributed to the space charge relative to the internal electric field.

The Behavior of the Ferromagnet/Wide-Band Polymer Heterostructure in Magnetic Field

The experimental results for huge magnetoresistance in polymer/ferromagnet system have been considered. The evidences have been established for the relation of the partially spin-polarized current in the structure and the magnetoresistance events. The model of charge and spin transport of the high-conductive state of wide-band polymer film/ferromagnet heterostructure have been proposed on the base of the experimental data totality.

Robustness, variability, phase dependence, and longevity of individual synaptic input effects on spike timing during fluctuating synaptic backgrounds: A modeling study of globus pallidus neuron phase response properties

A neuron’s phase response curve (PRC) shows how inputs arriving at different times during the spike cycle differentially affect the timing of subsequent spikes. Using a full morphological model of a globus pallidus (GP) neuron, we previously demonstrated that dendritic conductances shape the PRC in a spike frequency-dependent manner, suggesting different functional roles of perisomatic and distal dendritic synapses in the control of patterned network activity. In the present study we extend this analysis to examine the impact of physiologically realistic high conductance states on somatic and dendritic PRCs and the time course of spike train perturbations. First, we found that average somatic and dendritic PRCs preserved their shapes and spike frequency dependence when the model was driven by spatially-distributed, stochastic conductance inputs rather than tonic somatic current. However, responses to inputs during specific synaptic backgrounds often deviated substantially from the average PRC. Therefore, we analyzed the interactions of PRC stimuli with transient fluctuations in the synaptic background on a trial-by-trial basis. We found that the variability in responses to PRC stimuli and the incidence of stimulus-evoked added or skipped spikes were stimulus-phase-dependent and reflected the profile of the average PRC, suggesting commonality in the underlying mechanisms. Clear differences in the relation between the phase of input and variability of spike response between dendritic and somatic inputs indicate that these regions generally represent distinct dynamical subsystems of synaptic integration with respect to influencing the stability of spike time attractors generated by the overall synaptic conductance.