A surface-plasmon-polariton wavelength splitter based on a metal–insulator–metal waveguide

A surface-plasmon-polariton (SPP) wavelength splitter based on a metal–insulator–metal waveguide with multiple teeth is proposed. Using the transfer-matrix method, a plasmonic band gap is identified in the multiple-toothed structure, and the splitting wavelength of the SPP splitter can be easily adapted by adjusting the widths of the teeth and the gaps. The proposed wavelength splitter is further verified through finite-difference time-domain (FDTD) simulations, in which SPPs with incident wavelengths of 756 nm and 892 nm are successfully split and guided in opposite directions in the waveguide, with extinction ratios of 30 dB and 29 dB, respectively.     

Metal–insulator phase transition and topology in a three-component system

Due to the topology, insulators become non-trivial, particularly those with large Chern numbers which support multiple edge channels, catching our attention. In the framework of the tight binding approximation, we study a non-interacting Chern insulator model on the three-component dice lattice with real nearest-neighbor and complex next-nearest-neighbor hopping subjected to Λ-or V-type sublattice potentials. By analyzing the dispersions of corresponding energy bands, we find that the system undergoes a metal–insulator transition which can be modulated not only by the Fermi energy but also the tunable extra parameters. Furthermore, rich topological phases, including the ones with high Hall plateau, are uncovered by calculating the associated band’s Chern number. Besides, we also analyze the edge-state spectra and discuss the correspondence between Chern numbers and the edge states by the principle of bulk-edge correspondence. In general, our results suggest that there are large Chern number phases with C = ±3 and the work enriches the research about large Chern numbers in multiband systems.     

On the metal–insulator transition in vanadium dioxide

The conductivity σ of vanadium dioxide (VO₂) drops at a metal–insulator transition by four orders of magnitude due to the structural change between tetragonal and monoclinic crystals. In order to elucidate this conductivity drop, we introduce the semiclassical equation of motion to describe the dynamics of the conduction electron (wave packet), where the existence of a k-vector k is prerequisite for the conduction. We showed that the periodicity using the non-orthogonal bases does not legitimize the electron dynamics in solids. The theory suggests that the decrease in the dimensionality of the k-vectors due to the structural change is the cause of the conductivity drop.     

Strain-induced insulator–metal transition in ferroelectric BaTiO_3(001) surface:First-principles study

The electronic properties of TiO_2-terminated BaTiO_3(001) surface subjected to biaxial strain have been studied using first-principles calculations based on density functional theory. The Ti ions are always inward shifted either at compressive or tension strains, while the inward shift of the Ba ions occurs only for high compressive strain, implying an enhanced electric dipole moment in the case of high compressive strain. In particular, an insulator–metal transition is predicted at a compressive biaxial strain of 0.0475. These changes present a very interesting possibility for engineering the electronic properties of ferroelectric BaTiO_3(001) surface.     

Experimental observation of pseudogap in a modulation-doped Mott insulator: Sn/Si(111)-(√30×√30)R30°

Unusual quantum phenomena usually emerge upon doping Mott insulators. Using a molecular beam epitaxy system integrated with cryogenic scanning tunneling microscope, we investigate the electronic structure of a modulation-doped Mott insulator Sn/Si(111)-($\sqrt{3}\times \sqrt{3})R$30$^\circ$. In underdoped regions, we observe a universal pseudogap opening around the Fermi level, which changes little with the applied magnetic field and the occurrence of Sn vacancies. The pseudogap gets smeared out at elevated temperatures and alters in size with the spatial confinement of the Mott insulating phase. Our findings, along with the previously observed superconductivity at a higher doping level, are highly reminiscent of the electronic phase diagram in the doped copper oxide compounds.     

Composition rule for Al–transition metal binary quasicrystals

Compositions of Al–transition metal (TM) binary quasicrystals are revisited using the cluster-plus-glue-atom model. These compositions all conform to a unified cluster formula [icosahedron](glue)₁, where the icosahedron is taken from a crystalline approximant. In a given binary system, icosahedral and decagonal quasicrystals are expressed by the same icosahedron but glued, respectively, by one Al and one TM, and their e/a ratios fall close to 1.86 and 1.71–1.79, respectively. Their relative stabilities are discussed by referring to compositions satisfying the fully filled TM-3d states according to Häussler's resonance model.     

Flexible metal–insulator–metal (MIM) devices for plastic film AM-LCD

We developed new flexible metal–insulator–metal (MIM) devices subject to plastic film substrate. The structure of the MIM device is that a Ta₂O₅ insulator is covered with two flexible Al electrodes on both sides. The flexible structure of the MIM device was successfully fabricated applying our own etch-free process.     

IR thermometry: a new tool for contactless in situ investigations of metal–insulator transition

In the present work, we focus on an original study of metal–insulator transition in LaNiO₃ (LNO) compound by IR camera mounted on a pulsed laser deposition chamber. IR thermometry is based on the measurement of the radiated energy that is a function of the surface temperature and the emissivity (ε) of a material. Thus, at a fixed high temperature in an oxygen equilibrium state and by fixing arbitrary an emissivity value, we can follow the evolution of the “measured temperature” linked to the change of the real emissivity value during a reduction/reoxygenation treatment. The variation of emissivity is correlated to the change in optical constants, e.g. to the conductivity measured simultaneously by an in situ spectroscopic ellipsometry. The combination of both techniques offers a convenient way to observe in situ and contactless changes from metallic to insulator behavior and vice versa.     

Charge storage characteristics in Al/AlN/Si metal–insulator–semiconductor structure based on deep traps in AlN layer

Charge storage characteristics in an Al/AlN/p-Si metal–insulator–semiconductor (MIS) structure have been investigated by capacitance–voltage and long-term capacitance measurements. Good program/erase behavior is observed in the AlN/Si structure, which is attributed to the trapping and detrapping of charges in deep traps of the AlN layer. In the long-term retention mode, a clear memory window is found 2000 s after removing a program/erase voltage of ±3 V, indicating good charge retention capability of the MIS structure. Further investigation shows that for a program pulse width of 500 ms, the charge storage does not occur when the pulse amplitude is smaller than a threshold value of ∼1 V. The trapped charge density increases linearly with increase of the pulse amplitude (>1 V) and tends to saturate at 2.5 V. With increasing program pulse width, the trapped charged density increases a little more than logarithmically. PACS 73.40.Kp; 72.20.Jv; 71.55.Eq     

Investigation of electrical properties of HfO2 metal–insulator–metal (MIM) devices

This paper is devoted to the study of the electrical properties of Au/HfO₂/TiN metal–insulator–metal (MIM) capacitors in three distinctive modes: (1) alternative mode (C–f), (2) dynamic regime [thermally stimulated currents, TSCs I(T)] and (3) static mode [I(V)]. The electrical parameters are investigated for different temperatures. It is found that capacitance frequency C–f characteristic possesses a low-frequency dispersion that arises for high temperature (T > 300 °C). Accordingly, the loss factor exhibits a dielectric relaxation (with an activation energy E _a ~ 1.13 eV) which is intrinsically related to the diffusion of oxygen vacancies. The relaxation mechanisms of electrical defects in a dynamic regime (TSCs) analysis show that defect related to the TSC peak observed at 148.5 °C (E _a ~ 1 eV) is in agreement with impedance spectroscopy (C–f). On the other hand, when the MIM structures are analyzed in static mode, the I–V plots are governed by Schottky emission. The extrapolation of the curve at zero field gives a barrier height of 1.7 eV.     

Electric field induced metal–insulator transition in VO2 thin film based on FTO/VO2/FTO structure

A VO₂ thin film has been prepared using a DC magnetron sputtering method and annealing on an F-doped SnO₂ (FTO) conductive glass substrate. The FTO/VO₂/FTO structure was fabricated using photolithography and a chemical etching process. The temperature dependence of the I–V hysteresis loop for the FTO/VO₂/FTO structure has been analyzed. The threshold voltage decreases with increasing temperature, with a value of 9.2 V at 20 °C. The maximum transmission modulation value of the FTO/VO₂/FTO structure is 31.4% under various temperatures and voltages. Optical modulation can be realized in the structure by applying an electric field.     

Valence transition investigation in the O2–Yb–Si(111) system by means of the angle-resolved photoelectron spectroscopy method

Physics of the Solid State - Investigations of the Yb–Si(111) and O2–Yb–Si(111) structures are carried out by means of the angle-resolved photoelectron spectroscopy method, and...     

Electric-field induced ion-leveraged metal–insulator transition in conducting polymer-based field effect devices

The field effect devices prepared completely from conducting polymers, especially poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonic acid) (PEDOT/PSS), were studied. Normally in a conductive “on” state, the transistor-like device has a transition to a substantially less conductive “off” state at an applied positive gate voltage, typically ∼15–25 V. The current ratio I_off/I_on can exceed 10⁻⁴ at room temperature. We have found that the field effect is strongly temperature dependent and is substantially reduced upon decreasing the temperature by only a 10 °C. This loss of current reduction upon application of a gate voltage is not due to the temperature dependence of the electrical conductivity of polymers of which the devices are made. The temperature dependence of the dc conductivity of the PEDOT/PSS follows the variable range hopping law both before and after application of the gate voltage, though with an increased activation energy, T₀. We suggest that the conducting polymer is near the metal–insulator transition and that the field effect in the device is related to the electric field modulating this transition in the region underneath the gate electrode. The transition is controlled and leveraged by ion motion. The time dynamics of the current with the gate modulation strongly supports our conjecture. We demonstrate the generality of the phenomena by presenting similar results for devices fabricated from the conducting polypyrrole doped with Cl.     

Enhanced protein loading on a planar Si(111)–H surface with second generation NTA

A Si(111)–H surface was modified via a direct reaction between Si–H and 1-undecylenic acid (UA) under microwave irradiation to form molecular monolayers with terminal carboxyl groups. After esterifying carboxylic acid being esterified with N-hydroxysuccinimide (NHS), aminobutyl nitrilotriacetic acid (ANTA) was bound to the silicon surface through amidation (pH = 8.0) between its primary amino group and NHS-ester, producing nitrilotriacetic acid (NTA) anions. Then hexa-histidine tagged thioredoxin–urodilatin (his-tagged protein) and FITC-labeled hexa-histidine tagged thioredoxin–urodilatin (FITC-his-tagged protein) can be anchored after NTA was coordinated with Ni²⁺. Furthermore, the NTA-terminated chip was acidified with 0.1 M HCl and subsequently esterified with NHS and then amidated with ANTA again to produce a second generation NTA. Thus the surface density of nitrilotriacetic acid anions was improved and resultantly that of anchored proteins was also enhanced through the iterative reactions. Both multiple transmission-reflection infrared spectroscopy (MTR-IR) and fluorescence scanning measurements demonstrated a proximate 1.63 times of anchored proteins on the second generation NTA/Ni²⁺ as that on the first generation NTA/Ni²⁺ monolayer.     

Cs/GaAs(100) surface: Two-dimensional metal or Hubbard insulator?

The evolution of the characteristic electron energy loss spectra accompanying the deposition of cesium on GaAs(100) surfaces with various superstructural reconstructions is studied experimentally. It is shown that the appearance of loss peaks in the GaAs band gap for Cs coverages θ_Cs>0.5 monolayers is due to resonances in the longitudinal and transverse polarizability of two-dimensional metallic clusters of adatoms. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 8, 537–542 (25 October 1999)     

Types of surface modulation in a Ge–Si(111) heterosystem

Three types of elastic modulation in atomic layers of epitaxial Ge films on the surface of Si(111) observed in scanning tunnel microscopy are discussed. Two types of modulation are associated with misfit dislocations accumulating at the interface. Modulation of the third type arises on the surfaces of dislocation-free pseudomorphic films and could be due to the presence on a Si substrate surface of two-dimensional silicon islands with a height of three atomic bilayers and lateral dimensions of 15–20 nm. Measured bending values Δh for the third type of modulation (Δh ≤ 0.1 nm) agree with data calculated within the theory of elasticity     

Femtosecond mid-IR pump-probe spectroscopy of photoinduced insulator to metal transition in dimer Mott insulator κ-(BEDT-TTF)2X

Ultrafast dynamics of the photoinduced insulator (I) to metal (M) transition were investigated using femtosecond mid-infrared pump-probe spectroscopy in two-dimensional organic Mott insulators [bis (ethylenedithio)]-tetrathiafulvalene (BEDT-TTF) salts κ-(BEDT-TTF)₂X (κ-(ET)₂X, where X denotes anion). In κ-(d-ET)₂Cu[N(CN)₂]Br, a metallic state was photogenerated using a phonon-mediated mechanism: the effective bandwidth increases through the photoinduced molecular rearrangement. The mechanism differs fundamentally from the previously reported photoinduced filling control in one-dimensional Mott insulators.     

Thiophene on Si(111)2×1: Synchrotron radiation study of a desulfurization process

The adsorption and desulfurization of thiophene on cleaved silicon was studied at different temperatures. For substrate temperatures of 60–85 K, we found the co-existence of two different adsorption states at low exposures, which yield to a condensed thiophene multilayer at high exposures. For room-temperature substrates, we observed a desulfurization process. The process is probably followed by further fragmentation, and the fragmentation path depends on the substrate preparation process.     

Effect of annealing temperature on the electrical properties of Au/Ta2O5/n-GaN metal–insulator–semiconductor (MIS) structure

We report on the effect of an annealing temperature on the electrical properties of Au/Ta₂O₅/n-GaN metal–insulator–semiconductor (MIS) structure by current–voltage (I–V) and capacitance–voltage (C–V) measurements. The measured Schottky barrier height (Φ _bo) and ideality factor n values of the as-deposited Au/Ta₂O₅/n-GaN MIS structure are 0.93 eV (I–V) and 1.19. The barrier height (BH) increases to 1.03 eV and ideality factor decreases to 1.13 upon annealing at 500 ^°C for 1 min under nitrogen ambient. When the contact is annealed at 600 ^°C, the barrier height decreases and the ideality factor increases to 0.99 eV and 1.15. The barrier heights obtained from the C–V measurements are higher than those obtained from I–V measurements, and this indicates the existence of spatial inhomogeneity at the interface. Cheung’s functions are also used to calculate the barrier height (Φ _bo), ideality factor (n), and series resistance (R _s ) of the Au/Ta₂O₅/n-GaN MIS structure. Investigations reveal that the Schottky emission is the dominant mechanism and the Poole–Frenkel emission occurs only in the high voltage region. The energy distribution of interface states is determined from the forward bias I–V characteristics by taking into account the bias dependence of the effective barrier height. It is observed that the density value of interface states for the annealed samples with interfacial layer is lower than that of the density value of interface states of the as-deposited sample.