High-conductance State Research Articles

Comparison of the dynamics of neural interactions between current-based and conductance-based integrate-and-fire recurrent networks

Models of networks of Leaky Integrate-and-Fire (LIF) neurons are a widely used tool for theoretical investigations of brain function. These models have been used both with current- and conductance-based synapses. However, the differences in the dynamics expressed by these two approaches have been so far mainly studied at the single neuron level. To investigate how these synaptic models affect network activity, we compared the single neuron and neural population dynamics of conductance-based networks (COBNs) and current-based networks (CUBNs) of LIF neurons. These networks were endowed with sparse excitatory and inhibitory recurrent connections, and were tested in conditions including both low- and high-conductance states. We developed a novel procedure to obtain comparable networks by properly tuning the synaptic parameters not shared by the models. The so defined comparable networks displayed an excellent and robust match of first order statistics (average single neuron firing rates and average frequency spectrum of network activity). However, these comparable networks showed profound differences in the second order statistics of neural population interactions and in the modulation of these properties by external inputs. The correlation between inhibitory and excitatory synaptic currents and the cross-neuron correlation between synaptic inputs, membrane potentials and spike trains were stronger and more stimulus-modulated in the COBN. Because of these properties, the spike train correlation carried more information about the strength of the input in the COBN, although the firing rates were equally informative in both network models. Moreover, the network activity of COBN showed stronger synchronization in the gamma band, and spectral information about the input higher and spread over a broader range of frequencies. These results suggest that the second order statistics of network dynamics depend strongly on the choice of synaptic model.

Investigation on properties of ultrafast switching in a bulk gallium arsenide avalanche semiconductor switch

Properties of ultrafast switching in a bulk gallium arsenide (GaAs) avalanche semiconductor switch based on semi-insulating wafer, triggered by an optical pulse, were analyzed using physics-based numerical simulations. It has been demonstrated that when a voltage with amplitude of 5.2 kV is applied, after an exciting optical pulse with energy of 1 μJ arrival, the structure with thickness of 650 μm reaches a high conductivity state within 110 ps. Carriers are created due to photons absorption, and electrons and holes drift to anode and cathode terminals, respectively. Static ionizing domains appear both at anode and cathode terminals, and create impact-generated carriers which contribute to the formation of electron-hole plasma along entire channel. When the electric field in plasma region increases above the critical value (∼4 kV/cm) at which the electrons drift velocity peaks, a domain comes into being. An increase in carrier concentration due to avalanche multiplication in the domains reduces the domain width and results in the formation of an additional domain as soon as the field outside the domains increases above ∼4 kV/cm. The formation and evolution of multiple powerfully avalanching domains observed in the simulations are the physical reasons of ultrafast switching. The switch exhibits delayed breakdown with the characteristics affected by biased electric field, current density, and optical pulse energy. The dependence of threshold energy of the exciting optical pulse on the biased electric field is discussed.

Negative Differential Electrolyte Resistance in a Solid-State Nanopore Resulting from Electroosmotic Flow Bistability

A solid-state nanopore separating two aqueous solutions containing different concentrations of KCl is demonstrated to exhibit negative differential resistance (NDR) when a constant pressure is applied across the nanopore. NDR refers to a decrease in electrical current when the voltage applied across the nanopore is increased. NDR results from the interdependence of solution flow (electroosmotic and pressure-engendered) with the distributions of K+ and Cl- within the nanopore. A switch from a high-conductivity state to a low-conductivity state occurs over a very narrow voltage window (<2 mV) that depends on the nanopore geometry, electrolyte concentration, and nanopore surface charge density. Finite element simulations based on a simultaneous solution of the Navier-Stokes, Poisson, and Nernst-Planck equations demonstrate that NDR results from a positive feedback mechanism between the ion distributions and electroosmotic flow, yielding a true bistability in fluid flow and electrical current at a critical applied voltage, i.e., the NDR "switching potential". Solution pH and Ca2+ were separately employed as chemical stimuli to investigate the dependence of the NDR on the surface charge density. The NDR switching potential is remarkably sensitive to the surface charge density, and thus to pH and the presence of Ca2+, suggesting possible applications in chemical sensing.

Electrical bistability, negative differential resistance and carrier transport in flexible organic memory device based on polymer bilayer structure

Bistable nonvolatile memory devices containing two different layers of polymers, viz. MEH-PPV (poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenyl vinylene]) and PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)) has been fabricated by a simple spin-coating technique on flexible polyimide (PI) substrates with a structure Al/MEH-PPV/PEDOT:PSS/Ag-Pd/PI. The current–voltage measurements of the as-fabricated devices showed a nonvolatile electrical bistability with electric field induced charge transfer through the polymer layers and negative differential resistance (NDR) which is attributed to the charge trapping in the MEH-PPV layer. The current ON/OFF ratio between the high-conducting state (ON state) and low-conducting state (OFF state) is found to be of the order of 103 at room temperature which is comparable to organic field effect transistor based memory devices. We propose that such an improvement of rectification ratio (ON/OFF ratio) is caused due to the inclusion of PEDOT:PSS, which serves as a conducting current path for carrier transport; however, NDR is an effect of the trapped charges in the MEH-PPV electron confinement layer. The device shows excellent stability over 104s without any significant degradation under continuous readout testing in both the ON and OFF states. The carrier transport mechanism of the fabricated organic bistable device has been explained on the basis of different conduction mechanisms such as thermionic emission, space-charge-limited conduction, and Fowler–Nordheim tunneling. A band diagram is proposed to explain the charge transport phenomena. These bilayer structures are free from the drawbacks of the single organic layer based memory devices where the phase separation between the nanoparticles and polymers leads to the degradation of device stability and lifetime.

Core level photoemission studies on conducting polypyrrole polymer nanotubes showing switching transitions

Conducting polypyrrole (PPY) nanotubes is a classical model system for strongly correlated disordered materials showing intriguing switching transitions from low to high conductivity states at low temperature. This switching behaviour can be tuned by incorporating gold nanoclusters to form composite nanotubes (AuPPY). Here, we present core level electronic structure studies on PPY and AuPPY nanotubes with different diameters using hard X-ray photoemission spectroscopy at room temperature. The spectroscopic data provide information on the role of diameter as well as metal cluster incorporation for a modification of the electronic structure of this important class of nanotubes. Furthermore, electrical transport measurements were performed at low temperature to extract the change in localization length and doping level in these nanotubes. Based on these results, we explain the observed electrical transport behavior of these nanotubes as the interplay of disorder and carrier concentration.

High-Performance Memory Device Using Graphene Oxide Flakes Sandwiched Polymethylmethacrylate Layers

Organic bistable devices (OBDs) using graphene oxide (GO) flakes sandwiched polymethylmethacrylate (PMMA) films were fabricated. These devices exhibited two accessible conducting states, that is, a low-conductivity (OFF) state and a high-conductivity (ON) state. The devices can be switched to ON state under a negative electrical sweep; it can also be reset to the initial OFF state by a reverse (positive) electrical sweep. Detailed I-V measurements have shown that in ITO/PMMA/GO/PMMA/Al sandwiches the resistive switching originates from the formation and rupture of conducting filaments. The ON/OFF ratio of the OBDs was approximately 5 x 10(3), reproducibility of more than 10(5) cycles, and retention time of 10(4) s. These properties show that the device is promising for high-density, low-cost memory application.

Growth of hexagonal polyaniline fibers with polyacrylamide pendants

A new concept of synthesizing hexagonal polyaniline (PANi) fibers with polyacrylamide (PAAm) pendants in PAAm oligomer (oligo-PAAm) colloid is proposed. The size and morphology of the composite fibers can be controlled by adjusting the synthesis conditions. A possible growth mechanism of the PANi microstructures based on typical PAAm–crystals interactions in a mild aqueous solution is proposed. The amide ligands decorated oligo-PAAm chains are able to coordinate with –C6H4–NH+C6H4NH+–C6H4–NH–C6H4–NH– sections, leading to a lower surface energy and growth along c-directions to form fibriform morphology. The fibers have uniform diameter ranging from 200 to 400 μm, and length varying from several millimeters to 6 cm. Fourier transform infrared measurement indicates that the fiber exhibits a main PANi chain with PAAm pendants, and ultraviolet–visible spectra reveal that the PANi is in high conductive emeraldine state. Acrylamide feed significantly affects the morphology, however, the conductivity and yield of the fiber depend on hydrochloric acid and potassium peroxydisulfate concentrations in the preparation process. The hexagonal composite fibers show an electrical conductivity of 2.2 S·cm−1 and yield of higher than 95%. Polymer-directed crystal growth may provide promising routes to rational synthesis of various ordered organic materials with complex form and structural specialization. POLYM. COMPOS., 35:253–262, 2014. © 2013 Society of Plastics Engineers

Incorporation of non-conjugated polymer chain in conjugated polymer matrix: A new single step strategy for free standing non-volatile polymer memory

A new single step strategy for polymer memory materials has been explored using free-standing polypyrrole (PPy) film in which non-conjugated polymer chains are incorporated as trap states during synthesis. The PPy film was synthesized by the acidic oxidation of 2,2′:5′,2′′-terpyrrole at the air/water interface. The free-standing PPy films show large hysteresis along with current peaks in opposite directions during current voltage (I–V) characteristics. Hysteric behavior has been utilized to show rewritable memory effect. Furthermore, once electrical state (high or low conduction state) is set, the state is stable for months in ambient condition unless the state is reset by applying a voltage of opposite polarity. Thus the PPy film can be used as read once memory. The memory effect of the film is due to the conformational changes of non-conjugated polymer chains in the PPy matrix. The changes in conformation were confirmed from UV–Vis and FTIR spectra. This new strategy leads free-standing film based all organic polymer memory devices at ultra low cost.

Switching of pulses with a power of 500 MW and subnanosecond rise front based on open discharge

The switching characteristics of open discharge in the plane geometry that occurs in a cell with opposed electron beams are studied. Switching time of 400 ps at voltage of 20 kV is obtained in helium. Current amplitude of 28 kA at current rise rate of 3.7 × 1013 A/s is achieved. The mechanism of the transition of the switcher to the high-conductivity state, which is based on photoemission induced by the resonance radiation of fast atoms, is considered.

Electrical bistability in monolayers of cobalt-doped ZnO nanocrystals: Effect of alignment of their magnetic domains

We grow a range of cobalt-doped ZnO nanocrystals, form a monolayer through electrostatic assembly approach, and characterize the nanostructures with a scanning tunneling microscope tip. The scanning tunneling spectroscopy results show electrical bistability, that is, presence of a low- and a high-conducting state, in such monolayers under application of a voltage pulse. The bistability depends on the content of cobalt in the nanocrystals. With the use of such ferromagnetic nanocrystals, we align the magnetization vector of the nanocrystals in a monolayer in order to study the effect of alignment of their magnetic domains on the electrical bistability. We observe that the transport gap of the nanocrystals decreases upon conductance switching. The gap decreases also due to alignment of the magnetic domains of the cobalt-doped nanocrystals. When the magnetically aligned nanocrystals undergo a conductance switching, the transport gap further decreases evidencing a correlation between the electrical bistability and the (change in) transport gap.

Mechanically controllable bi-stable states in a highly conductive single pyrazine molecular junction

We report the fabrication of a highly conductive single pyrazine molecular junction with Pt leads. Mechanically controllable break-junction measurements at low temperatures show two distinct high and low conductance states. These conductance values are two orders of magnitude larger than those of a conventional single molecular junction with anchoring groups because of direct binding of the π conjugated molecule to a metal electrode with large density of states at the Fermi energy. Inelastic electron tunneling spectroscopy combined with density functional theory calculations highlights the vibration modes of the system for the two regimes. Theory allows us to assign the high and low conductance states of the molecular junction to two configurations in which the pyrazine axis is tilted and parallel with respect to the junction axis, respectively. Finally, we show that the pyrazine junction can be reversibly switched between the two bi-stable conductance states by mechanically stretching and relaxing the junction.

Role of cell type and synaptic connections during functionally relevant network states in vitro

Cortical neurons are embedded in networks, whose activity has distinct features depending on the brain state and on the involved cell types. Two distinct types of functionally relevant activities are asynchronous firing containing spike correlations and temporally-structured, oscillatory activity. In the neocortex, network synchronization is achieved by the tight control of principal pyramidal, excitatory neuron (PN) firing by fast-spiking (FS), parvalbumin-positive inhibitory neurons. However, how different neuron types process and convey information in different cortical states is not completely understood. We simultaneously recorded from pairs of neurons in acute mouse cortical slices. Using dynamic clamp, we combined computer modeling with experimental electrophysiology and we studied how cortical neurons compute and integrate diverse sequences of synaptic inputs (both excitatory and inhibitory) into specific spike-train outputs. In particular, we used three types of inputs: i) asynchronous, correlated spike trains in the form of post-synaptic currents, as originating from a network of pre-synaptic randomly connected neurons; ii) sinusoidal currents of various frequencies (ranging from the delta to the high-gamma bands); iii) realistic excitatory and inhibitory oscillating currents in the gamma frequency band. For all these stimuli, we also tested the effect of an in vivo-like synaptic high-conductance state on neuronal response [1]. For asynchronous input containing correlations, we measured output spike correlations between pairs of two PNs, two FS cells, or one PN and one FS cell at different degrees of correlation. Our results show that inhibition desynchronizes the output of PNs, preventing supra-linear correlation [2]. Moreover, this occurred also when neurons were injected with additional in vivo-like synaptic conductances. FS cells showed similar sensitivity to inhibition, but with a much stronger ability to correlate their output. When sinusoidal input was injected, both PNs and FS cells followed the stimuli, with FS cells capable of firing reliably in response to high-frequency (>100 Hz) oscillatory input. Interestingly, in the presence of in vivo-like synaptic activity, PNs significantly increased their ability of following intracellular injections of sinusoidal input across a broader range of frequencies. Finally, we injected realistic gamma input, by simulating and injecting runs of EPSCs and IPSCs occurring in temporally structured manner [3]. In addition, neurons were connected by artificial, albeit realistic, unitary excitatory, inhibitory and self-inhibiting synapses (autapses), thus allowing the investigation of local network effects in eliciting and maintaining oscillations. Our preliminary results show that, in pairs of FS cells, the presence of bidirectional inhibitory synapses strongly decreased the strength of output oscillations, with neurons alternatively silencing each other. Interestingly, additional autaptic GABAergic conductances balanced the network, allowing reemergence of oscillations. Overall, our results deepen our understanding of the features and the processing capabilities of excitatory and inhibitory networks of neurons in the presence of functionally relevant brain activities.

Quantification of Steady-State Ion Transport through Single Conical Nanopores and a Nonuniform Distribution of Surface Charges

Electrostatic interactions of mobile charges in solution with the fixed surface charges are known to strongly affect stochastic sensing and electrochemical energy conversion processes at nanodevices or devices with nanostructured interfaces. The key parameter to describe this interaction, surface charge density (SCD), is not directly accessible at nanometer scale and often extrapolated from ensemble values. In this report, the steady-state current-voltage (i-V) curves measured using single conical glass nanopores in different electrolyte solutions are fitted by solving Poisson and Nernst-Planck equations through finite element approach. Both high and low conductivity state currents of the rectified i-V curve are quantitatively fitted in simulation at less than 5% error. The overestimation of low conductivity state current using existing models is overcome by the introduction of an exponential SCD distribution inside the conical nanopore. A maximum SCD value at the pore orifice is determined from the fitting of the high conductivity state current, while the distribution length of the exponential SCD gradient is determined by fitting the low conductivity state current. Quantitative fitting of the rectified i-V responses and the efficacy of the proposed model are further validated by the comparison of electrolytes with different types of cations (K(+) and Li(+)). The gradient distribution of surface charges is proposed to be dependent on the local electric field distribution inside the conical nanopore.

A Detailed Experimental and Theoretical Study into the Properties of C60 Dumbbell Junctions

A combined experimental and theoretical investigation is carried out into the electrical transport across a fullerene dumbbell one-molecule junction. The newly designed molecule comprises two C60 s connected to a fluorene backbone via cyclopropyl groups. It is wired between gold electrodes under ambient conditions by pressing the tip of a scanning tunnelling microscope (STM) onto one of the C60 groups. The STM allows us to identify a single molecule before the junction is formed through imaging, which means unambiguously that only one molecule is wired. Once lifted, the same molecule could be wired many times as it was strongly fixed to the tip, and a high conductance state close to 10(-2) G0 is found. The results also suggest that the relative conductance fluctuations are low as a result of the low mobility of the molecule. Theoretical analysis indicates that the molecule is connected directly to one electrode through the central fluorene, and that to bind it to the gold fully it has to be pushed through a layer of adsorbates naturally present in the experiment.

Flexible Nonvolatile Transistor Memory Devices Based on One‐Dimensional Electrospun P3HT:Au Hybrid Nanofibers

AbstractA novel flexible nonvolatile flash transistor memory devices on polyethylene naphthalate (PEN) substrate using 1D electrospun nanofiber of poly(3‐hexylthiophene) (P3HT):gold nanoparticles (Au NPs) hybrid as the channel is presented. The Au NPs are functionalized with self‐assembled monolayer (SAM) of para‐substituted amino (Au‐NH2), methyl (Au‐CH3) or trifluoromethyl (Au‐CF3) tail groups on the benzenethiol moiety. They are employed as localized charge traps across the nanofiber channel and program/erase the device towards low conductance (OFF)/high conductance (ON) states under the applied electrical field. With the low operation voltage of ±5 V, the hybrid nanofiber transistor memories exhibit a 3.5–10.6 V threshold voltage shifting and at least 104 s data retention, with a minimum effect on ≈100 programmed/erased stress endurances. The dipoles of the SAM probably modify the work function of the Au NPs associated with the P3HT nanofiber channel and manifest the degree of negative threshold voltage shifting in an order of Au‐NH2 &gt; Au‐CH3 &gt; Au‐CF3. The devices remain reliable and stable even under the bending conditions (radius: 5–30 mm) or 1000 repetitive bending cycles. The hybrid nanofiber can be used to obtain high‐performance digital nanoscale memories for flexible high density data storage devices.

Lateral Conduction Switching in Sputtered Ni-rich NiO Thin Films for Write-Once-Read-Many-Times Memory Applications

Magnetron sputtering has been used to deposit Ni-rich nickel oxide thin films. Based on the switching of lateral current conduction in the nickel oxide thin film between two in-plane electrodes, a planar write-once-read-many-times memory device has been demonstrated. The switching from a low-conductance state (i.e., the OFF state) to a high-conductance state (i.e., the ON state) is induced by a writing voltage, and it is irreversible due to the formation of tilted conductive filaments that are hard to be dissolved by the Joule heating effect. For 80 devices under test, the writing voltage is in a narrow range of 2.0−3.5 V and the ON/OFF resistance ratio is larger than 105 at the reading voltage of 0.3 V. An excellent reading endurance (106 readings) for both ON and OFF states is demonstrated. The device is promising in low-power applications as it can operate at ultra-low voltages (e.g., the reading voltage can be below 100 mV).

From Natural to Bioassisted and Biomimetic Artificial Water Channel Systems

Within biological systems, natural channels and pores transport metabolites across the cell membranes. Researchers have explored artificial ion-channel architectures as potential mimics of natural ionic conduction. All these synthetic systems have produced an impressive collection of alternative artificial ion-channels. Amazingly, researchers have made far less progress in the area of synthetic water channels. The development of synthetic biomimetic water channels and pores could contribute to a better understanding of the natural function of protein channels and could offer new strategies to generate highly selective, advanced water purification systems. Despite the imaginative work by synthetic chemists to produce sophisticated architectures that confine water clusters, most synthetic water channels have used natural proteins channels as the selectivity components, embedded in the diverse arrays of bioassisted artificial systems. These systems combine natural proteins that present high water conductance states under natural conditions with artificial lipidic or polymeric matrixes. Experimental results have demonstrated that natural biomolecules can be used as bioassisted building blocks for the construction of highly selective water transport through artificial membranes. A next step to further the potential of these systems was the design and construction of simpler compounds that maintain the high conduction activity obtained with natural compounds leading to fully synthetic artificial biomimetic systems. Such studies aim to use constitutional selective artificial superstructures for water/proton transport to select functions similar to the natural structures. Moving to simpler water channel systems offers a chance to better understand mechanistic and structural behaviors and to uncover novel interactive water-channels that might parallel those in biomolecular systems. This Account discusses the incipient development of the first artificial water channels systems. We include only systems that integrate synthetic elements in their water selective translocation unit. Therefore, we exclude peptide channels because their sequences derive from the proteins in natural channels. We review many of the natural systems involved in water and related proton transport processes. We describe how these systems can fit within our primary goal of maintaining natural function within bioassisted artificial systems. In the last part of the Account, we present several inspiring breakthroughs from the last decade in the field of biomimetic artificial water channels. Researchers have synthesized and tested hydrophobic, hydrophilic and hybrid nanotubular systems. All these examples demonstrate how the novel interactive water-channels can parallel biomolecular systems. At the same time these simpler artificial water channels offer a means of understanding the molecular-scale hydrodynamics of water for many biological scenarios.

In Situ Synthesis and Nonvolatile Rewritable‐Memory Effect of Polyaniline‐Functionalized Graphene Oxide

A new polyaniline (PANI)-functionalized graphene oxide (GO-PANI) was prepared by using an in situ oxidative graft polymerization of aniline on the surface of GO. Its highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), ionization potential (IP), and electron affinity (EA) values experimentally estimated by the onset of the redox potentials were -5.33, -3.57, 5.59, and 3.83 eV, respectively. A bistable electrical-switching effect was observed in electronic device with the GO-PANI film sandwiched between the indium tin oxide (ITO) and Al electrodes. This device exhibited two accessible conductivity states, that is, the low-conductivity (OFF) state and the high-conductivity (ON) state, and can be switched to the ON state under a negative electrical sweep, and can also be reset to the initial OFF state by a reverse (positive) electrical sweep. The ON state is nonvolatile and can withstand a constant voltage stress of -1 V for 3 h and 10(8) read cycles at -1 V under ambient conditions. The nonvolatile nature of the ON state and the ability to write, read, and erase the electrical states, fulfill the functionality of a rewritable memory. An ON/OFF current ratio of more than 10(4) at -1 V achieved in this memory device is high enough to promise a low misreading rate through the precise control of the ON and OFF states. The mechanism associated with the memory effects was elucidated from molecular simulation results.

Investigation of electrical characteristics of memory cells based on self-forming conducting nanostructures in a form of the TiN-SiO2-W open sandwich structure

The results of experimental investigations of electroforming and quasi-static current-voltage (I-V) characteristics of formed TiN-SiO2-W open sandwich structures in comparison to the Si-SiO2-W structures are presented. It is shown that similar memory and switching effects, which are associated with self-forming the conducting nanostructures on the surface of the open end of the dielectric film (silicon dioxide) about 20 nm thick, are observed in them. However, the features of the structures with the lower TiN electrode are the noticeably larger current, the lower threshold switching voltage from the low-conducting state into the high-conducting state, and a flatter I–V characteristic at voltages below the threshold one. These features can be explained by the decrease in the spreading resistance from the conducting structure, which is formed during electroforming, into the material of the lower electrode for titanium nitride compared to silicon (their resistivity values differ by a factor of 4) and the lower potential barrier at the TiN-SiO2 interface compared with the Si-SiO2 interface. The cell of energy-independent electrically reprogrammable memory with the TiN-SiO2-W structure possesses the better technical characteristics and manufacturability.

Bulk-Sensitive Angle-Resolved Photoemission Spectroscopy on TTF-TCNQ

TTF-TCNQ is a quasi-1D molecular conductor exhibiting metal-insulator transition at 54K accompanied by chargedensity-wave (CDW) formation. Its electronic properties in relation to the Peierls instability and CDW transitions are well investigated through the several decades. Past ARPES studies indicate the band dispersion consisting of TCNQ molecules (namely the TCNQ-band) shows the signature of spin–charge separation, together with the suppression of the quasiparticle excitation at the Fermi level. These observations have been discussed in terms of the 1D character of TTF-TCNQ, nevertheless, there are several mysteries remained. One is the band dispersions obtained by ARPES commonly indicating the band-width (or the band group velocity) larger by a factor of 2 with respect to band theory. To account for this discrepancy, the possibility of relaxed tilting of the topmost molecules and/or strong correlation effect have been raised until now. The second is the anomalous suppression of the spectral weight near the Fermi level (EF), which gives negligible amount of spectral intensity at EF even in the highly-conductive state above >60K. The electronic structure of the metallic quasi-1D state has been thus remained to be elucidated. To investigate the precise electronic structure of TTFTCNQ, here we utilized the laser-excited ARPES with h 1⁄4 6:994 eV. Owing to the relatively low photon-energy as compared with past ARPES studies (h 1⁄4 20{40 eV), the bulk-sensitive measurement becomes available. The effect of photoirradiation-induced damage peculiar to molecular compounds is also considered to be weaker compared to the measurements using higher-energy photons. The high cross section of s and p electrons for this photon-energy provides some advantage in studying the light-element based organic conductors. ARPES measurements were performed using a system constructed with VG-Scienta R4000 electron analyzer and an ultraviolet (h 1⁄4 6:994 eV) laser. He II (h 1⁄4 40:8 eV) light source was also used to check the correspondence with past reports. Single crystals of TTF-TCNQ grown by the diffusion method were cleaved in-situ to obtain fresh surfaces. The pressure was below 5 10 11 Torr throughout all the measurements. The energy resolution was E 1⁄4 4:0meV. The Fermi level EF of the sample was referred to that of a Au film evaporated on the sample substrate, with an accuracy of 0:1meV. Figure 1(a) shows the mapping of the ARPES intensity (h 1⁄4 6:994 eV) at the Fermi level, obtained by integrating the ARPES intensity in the energy window of 50meV. The shape of the Fermi surface thus observed makes a straight line at kF 1⁄4 0:24 0:01 A 1 1⁄4 ð0:29 0:01Þb , indicative of 1D electronic structure. The image of band dispersions along –Z direction obtained by h 1⁄4 6:994 and 40.8 eV are shown in Fig. 1(b,c). The peak positions of the ARPES intensity estimated from momentum distribution curve (MDC) and energy distribution curve (EDC) are plotted by circle and cross markers, respectively. The red and blue curves, on the other hand, represent the band dispersions consisting of TTF and TCNQ molecules, obtained by the first-principles calculation. By comparing with the band calculation, we can notice that the h 1⁄4 6:994 eV data is dominated by the signal from TTF bands, whereas the h 1⁄4 40:8 eV data shows both TTF and TCNQ bands that are well in accord with past reports. This h -dependence, which should be due to the photoionization cross section and/or matrix element effect, also explains the slight difference of the present ‘‘Fermi-surface’’ shape from that previously reported. The Fermi momentum obtained by ARPES, kF 1⁄4 0:24 0:01 A , is somewhat smaller compared with the band calculation, reflecting the overestimation of the charge transfer between TTF and TCNQ inevitably arising in the calculated data. Here, let us focus on the gradient of the TTF band dispersion. In the h 1⁄4 40:8 eV ARPES spectrum, the gradient of the TTF band dispersion is about 2 times larger as compared to the calculation. It shows that the observed High