Mott metal-insulator transition Research Papers (original) (raw)

The Falicov-Kimball model of spinless quantum electrons hopping on a 1-dimensional lattice and of immobile classical ions occupying some lattice sites, with only intrasite coupling between those particles, have been studied at zero temperature by means of well-controlled numerical procedures. For selected values of the unique coupling parameter U the restricted phase diagrams (based on all the periodic configurations of localized particles (ions) with period not greater than 16 lattice constants, typically) have been constructed in the grand-canonical ensemble. Then these diagrams have been translated into the canonical ensemble. Compared to the diagrams obtained in other studies our ones contain more details, in particular they give better insight into the way the mixtures of periodic phases are formed. Our study has revealed several families of new characteristic phases like the generalized most homogeneous and the generalized crenel phases, a first example of a structural phase transition and a tendency to build up an additional symmetry - the hole-particle symmetry with respect to the ions (electrons) only, as U decreases.

- by and +2
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- Mathematical Physics, Quantum Physics, Mott metal-insulator transition, Dimensional

We report the effect of average A-site cation radius on the structural, magnetic and electrical properties of electron doped manganite La 0.7 Ce 0.3 MnO 3 thin films. A site cation radius hr A i is varied systematically by partially replacing La C3 ions by smaller Y C3 ions in the parent compound. The carrier doping, i.e. the fraction of tetravalent Ce atoms at the A-site was kept at 30%. A series of La 0.7Kx Y x Ce 0.3 MnO 3 (xZ0, 0.05, 0.1, 0.15 and 0.25) thin films were prepared under identical conditions by using pulsed laser deposition technique. Metal-insulator transition temperature (T p ) and the ferromagnetic Curie temperature (T c ) are found to be decreasing significantly with increasing yttrium concentration, i.e. decreasing hr A i. The resistivity (r) at T p increases by one order of magnitude when the yttrium concentration is increased from xZ0 to 0.25, while magnetization decreases with decreasing hr A i. Magnetoresistance as measured under field of 1 T is significant near T c . Structural analysis reveals the films are having single phase and c-axis lattice parameter decreasing linearly as hr A i decreases. There is no marked difference in surface morphology of samples with different yttrium concentration. From EDX the Y:Ce:Mn ratio were consistent within 5% of the nominal composition. q

The ground state phase diagram of the 1D Hubbard chain with pair-hopping interaction is studied. The analysis of the model is performed using the continuum-limit field theory approach and exact diagonalization studies. At half-filling the phase diagram is shown to consist of two superconducting states with Cooper pair center-of-mass momentum Q = 0 (BCS-η0 phase) and Q = π (ηπ-phase) and four insulating phases corresponding to the Mott antiferromagnet, the Peierls dimerized phase, the charge-density-wave (CDW) insulator as well as an unconventional insulating phase characterized by the coexistence of a CDW and a bond-located staggered magnetization. Away from half-filling the phase diagram consists of the superconducting BCS-η0 and ηπ phases and the metallic Luttingerliquid phase. The BCS-η0 phase exhibits smooth crossover from a weak-coupling BCS type to a strong coupling local-pair regime. The ηπ phase shows properties of the doublon (zero size Cooper pair) superconductor with Cooper pair center-of-mass momentum Q = π. The transition into the ηπ-paired state corresponds to an abrupt change in the groundstate structure. After the transition the conduction band is completely destroyed and a new ηπ-pair band corresponding to the strongly correlated doublon motion is created.

Ultra-thin tungsten films were prepared using hotwire assisted atomic layer deposition. The film thickness ranged from 2.5 to 10 nm, as determined by spectroscopic ellipsometry and verified by scanning electron microscopy. The films were implemented in conventional Van der Pauw and circular transmission line method (CTLM) test structures, to explore the effect of film thickness on the sheet and contact resistance, temperature coefficient of resistance (TCR), and external electric field applied. All films exhibited linear current-voltage characteristics. The sheet resistance was shown to considerably vary across the wafer, due to the film thickness non-uniformity. The TCR values changed from positive to negative with decreasing the film thickness. A field-induced modulation of the sheet resistance up to ∼4.6•10 −4 V −1 was obtained for a 2.5 nm thick film, larger than that generally observed for metals.

- by R. Wolters
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- Engineering, Atomic Layer Deposition, Thin Films, Mott metal-insulator transition

A simulation based comparative study of the polarization hysteresis of the ferroelectric capacitor using various ferroelectric models is presented. A 2-dimensional finite element device-level model was implemented using SILVACO's ATLAS device simulator to generate the polarization hysteresis characteristics for Au/Poly(vinylidene fluoride-trifluoroethylene)/Au metal-insulator-metal (MIM) device. Landau free energy expression for electric field in terms of polarization is also implemented in the MATLAB to produce the polarization hysteresis curve of polycrystalline ferroelectrics. The main drawback of these models was their inability to predict polarization saturation at the same electric field limits for which the experimental device was saturating. A new model for ferroelectric hysteresis based on curve fitting algorithm is derived that forces the polarization to be saturated at desired electric field. The MATLAB simulation of this model and the experimental hysteresis is compared which shows an excellent level of agreement.

- by bikash shrestha
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- Finite Element, Hysteresis, Mott metal-insulator transition, Electric Fields

Mixed HfTaOx dielectric has been deposited by radio frequency (RF) magnetron co-sputtering of HfO2 and Ta2O5 targets on Si substrates and with Pt bottom electrode for metal-oxide-semiconductor (MOS) and metal-insulator-metal (MIM) structures, respectively. HfTaOx layers are characterized by X-ray photoelectron spectroscopy (XPS) to examine the chemical composition. Surface morphology and crystallinity of the deposited film were examined, using by atomic force microscopy (AFM) and grazing incidence X-ray diffraction (GIXRD), respectively. For both the structures electrical properties have been studied in detail. MOS capacitor capacitance-voltage (C-V) characteristics have been utilized to determine the interface trap density and trap distribution in the silicon band gap. A small capacitance non-linearity and low dissipation factor were found in the Pt/HfTaOx/Pt MIM structures.

- by Chandan Sarkar
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- Engineering, Materials Engineering, Materials Science, Technology

The as-deposited and annealed radio frequency reactive magnetron sputtered tantalum oxide (Ta 2 O 5 ) films were characterized by studying the chemical binding configuration, structural and electrical properties. X-ray photoelectron spectroscopy and X-ray diffraction analysis of the films elucidate that the film annealed at 673 K was stoichiometric with orthorhombic b-phase Ta 2 O 5 . The dielectric constant values of the tantalum oxide capacitors with the sandwich structure of Al/Ta 2 O 5 /Si were in the range from 14 to 26 depending on the post-deposition annealing temperature. The leakage current density was o 20 nA cm À 2 at the gate bias voltage of 0.04 MV/cm for the annealed films. The electrical conduction mechanism observed in the films was Poole-Frenkel.

- by Chandan Sarkar
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- Engineering, Materials Engineering, Materials Science, Technology
- by Simon Redfern
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- Mott metal-insulator transition, Physical sciences, CHEMICAL SCIENCES

The current-voltage (I-V) characteristics of metal/insulator/metal (MIM) diodes illuminated at optical frequencies are modeled using a semiclassical approach that accounts for the photon energy of the radiation. Instead of classical small-signal rectification, in which a continuous span of the dc I-V curve is sampled during rectification, at optical frequencies, the radiation samples the dc I-V curve at discrete voltage steps separated by the photon energy (divided by the electronic charge). As a result, the diode resistance and responsivity differ from their classical values. At optical frequencies, a diode with even a moderate forward-to-reverse current asymmetry exhibits high quantum efficiency. An analysis is carried out to determine the requirements imposed by the operating frequency on the circuit parameters of antenna-coupled diode rectifiers, which are also called rectennas. Diodes with low resistance and capacitance are required for the RC time constant of the rectenna to be smaller than the reciprocal of the operating frequency and to couple energy efficiently from the antenna. Existing MIM diodes do not meet the requirements to operate efficiently at visible-to-near-infrared wavelengths.

- by Garret Moddel
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- Solar Cell, Near Infrared, Mott metal-insulator transition, Resistance

We present an extended x-ray absorption fine structure study at the Fe K edge of the local structural changes in Fe 3 O 4 across the Verwey transition at T V ϳ 120 K. The local structure of the FeO 6 octahedra is distorted below T V and remains unaltered during the transition, the local distortions being already present in the pseudocubic phase above T V . The phonon modes responsible for the lattice distortion are those associated with displacements of the octahedral Fe-Fe chains. We propose that the metal-insulator transition is then caused by a change in the regime of the local distortions from a static regime at low temperatures to a dynamical one at high temperatures, the lattice dynamics above T V being the origin for the electrical conductivity.

The requirements and development of high-k dielectric films for application in storage cells of future generation flash and Dynamic Random Access Memory (DRAM) devices are reviewed. Dielectrics with kvalue in the 9-30 range are studied as insulators between charge storage layers and control gates in flash devices. For this application, large band gaps (>6 eV) and band offsets are required, as well as low trap densities. Materials studied include aluminates and scandates. For DRAM metal-insulator-metal (MIM) capacitors, aggressive scaling of the equivalent oxide thickness (with targets down to 0.3 nm) drives the research towards dielectrics with k-values >50. Due to the high aspect ratio of MIMCap structures, highly conformal deposition techniques are needed, triggering a substantial effort to develop Atomic Layer Deposition (ALD) processes for the deposition of metal gates and high-k dielectrics. Materials studied include Sr-and Ba-based perovskites, with SrTiO 3 as one of the most promising candidates, as well as tantalates, titanates and niobates.

We review studies of the electromagnetic response of various classes of correlated electron materials including transition metal oxides, organic and molecular conductors, intermetallic compounds with d-and f -electrons as well as magnetic semiconductors. Optical inquiry into correlations in all these diverse systems is enabled by experimental access to the fundamental characteristics of an ensemble of electrons including their self-energy and kinetic energy. Steady-state spectroscopy carried out over a broad range of frequencies from microwaves to UV light and fast optics timeresolved techniques provide complimentary prospectives on correlations. Because the theoretical understanding of strong correlations is still evolving, the review is focused on the analysis of the universal trends that are emerging out of a large body of experimental data augmented where possible with insights from numerical studies. arXiv:1106.2309v1 [cond-mat.str-el]

- by Dimitri Basov
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- Modern physics, Mott metal-insulator transition, Thin Film, Physical sciences
- by Douglas Natelson
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- Engineering, Superconductivity, Quantum Theory, Advanced Materials

A term first coined by Mott back in 1968 a “pseudogap” is the depletion of the electronic density of states at the Fermi level, and pseudogaps have been observed in many systems. However, since the discovery of the high-temperature superconductors (HTSC) in 1986, the central role attributed to the pseudogap in these systems has meant that by many researchers now associate the term pseudogap exclusively with the HTSC phenomenon. Recently, the problem has got a lot of new attention with the rediscovery of two distinct energy scales (“two-gap scenario”) and charge density waves patterns in the cuprates. Despite many excellent reviews on the pseudogap phenomenon in HTSC, published from its very discovery up to now, the mechanism of the pseudogap and its relation to superconductivity are still open questions. The present review represents a contribution dealing with the pseudogap, focusing on results from angle resolved photoemission spectroscopy (ARPES) and ends up with the conclusion that the pseudogap in cuprates is a complex phenomenon which includes at least three different “intertwined” orders: spin and charge density waves and preformed pairs, which appears in different parts of the phase diagram. The density waves in cuprates are competing to superconductivity for the electronic states but, on the other hand, should drive the electronic structure to vicinity of Lifshitz transition, that could be a key similarity between the superconducting cuprates and iron-based superconductors. One may also note that since the pseudogap in cuprates has multiple origins there is no need to recoin the term suggested by Mott.

We present a 90-dB spurious-free dynamic range sigma-delta modulator (ΣΔM) for asymmetric digital subscriber line applications (both ADSL and ADSL+), with up to a 4.4-MS/s digital output rate. It uses a cascade (MASH) multibit architecture and has been implemented in a 2.5-V supply, 0.25-μm CMOS process with metal-insulator-metal capacitors. The prototypes feature 78-dB dynamic range (DR) in the 30-kHz to 2.2-MHz band (ADSL+) and 85-dB DR in the 30-kHz to 1.1-MHz band (ADSL). Integral and differential nonlinearity are within ±0.85 and ±0.80 LSB14 b, respectively. The ΣΔ modulator and its auxiliary blocks (clock phase and reference voltage generators, and I/O buffers) dissipate 65.8 mW. Only 55 mW are dissipated in the ΣΔ modulator.

Vanadium dioxide single-crystal precipitates with controlled particle sizes were produced in an amorphous, fused SiO 2 host by the stoichiometric coimplantation of vanadium and oxygen ions and subsequent thermal processing. The effects of the vanadium dioxide nanocrystal size, nanocrystal morphology, and particle/host interactions on the VO 2 semiconductor-to-metal phase transition were characterized. VO 2 nanoparticles embedded in amorphous SiO 2 exhibit a sharp phase transition with a hysteresis that is up to 50°C in width-one of the largest values ever reported for this transition. The relative decrease in the optical transmission in the near-infrared region in going from the semiconducting to the metallic phase of VO 2 ranges from 20% to 35%. Both the hysteresis width and the transition temperature are correlated with the size of the precipitates. Doping the embedded VO 2 particles with ions such as titanium alters the characteristics of the phase transition, pointing the way to control the hysteresis behavior over a wide range of values and providing insight into the operative physical mechanisms.

- by Rene Lopez
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- Engineering, Mechanical Engineering, Condensed Matter Physics, Applied Physics

We study the origin of the temperature-induced Mott transition in Ca2RuO4. As a method we use the local-density approximation+dynamical mean-field theory. We show the following. (i) The Mott transition is driven by the change in structure from long to short c-axis layered perovskite (L-Pbca to S-Pbca); it occurs together with orbital order, which follows, rather than produces, the structural transition. (ii) In the metallic L-Pbca phase the orbital polarization is ~0. (iii) In the insulating S-Pbca phase the lower energy orbital, ~xy, is full. (iv) The spin-flip and pair-hopping Coulomb terms reduce the effective masses in the metallic phase. Our results indicate that a similar scenario applies to Ca_{2-x}Sr_xRuO_4 (x<0.2). In the metallic x< 0.5 structures electrons are progressively transferred to the xz/yz bands with increasing x, however we find no orbital-selective Mott transition down to ~300 K.

- by eva pavarini
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- Mott metal-insulator transition, Physical sciences, Mott Transition, Effective mass

The architecture, materials choice and process technology for stacked-capacitors in embedded-DRAM applications are a crucial concern for each new technology node. An overview of the evolution of capacitor technology is presented from the early days of planar PIS (poly/insulator/silicon) capacitors to the MIM (metal/insulator/metal) capacitors used for todays 65 nm technology node. In comparing Ta2O5, HfO2 and Al2O3 as high-k dielectric for use in 65 nm eDRAM technology, Al2O3 is found to give a good compromise between capacitor performance and manufacturability. The use of atomic layer deposition (ALD) is identified to be an enabling technology for both high-k dielectrics and capacitor electrodes.

Recent developments in nanostructure self-assembly from gold and silver particles are reviewed. A brief historical background of the field is given, followed by a selection of topics which are of particular current interest. An overview of the preparation of thiol-stabilised gold and silver nanoparticles and their spontaneous self-organisation into well-ordered superlattices is presented. Distance-dependent metal insulator transitions in ensembles of nanoparticles are discussed, along with a previously unpublished measurement of optical properties of dithiol-linked thin films of gold nanoparticles. Recent approaches to more complex nano-architectures are reviewed, including the use of various templates and of DNA base pair recognition. Some aspects of nanoscopic surface chemistry of gold particles including the evolution of molecular recognition sites are reviewed. Current and potential future applications are discussed.

- by KHALED HAMID
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- Engineering, Carbon Nanotubes, Self Assembly, Carbon Nanotube

A tri-band dual-mode chip filter, which is fabricated in a commercial pHEMT GaAs technology and suitable for millimeter-wave applications, is presented in this paper. This filter is constructed by using stacked ring resonators with individual perturbations and feeding capacitors; thus, fractional bandwidths and center frequencies of three pass bands can be flexibly controlled. A model of the tri-band filter that considers coupling effects between stacked ring resonators is first proposed, and then a synthesis method that can determine values of all elements of a typical dual-mode ring filter based on specifications of required characteristics is also described. According to the proposed design method, filter designers can quickly determine the layout pattern of the tri-band dual-mode filter which employs proper metal-insulator-metal (MIM) capacitors to obtain the desired filter performance. To verify the proposed design concept, an experimental prototype, locating three pass bands at 60 GHz, 77 GHz and 100 GHz, respectively, was fabricated on GaAs substrate. The size of the filter is about 0.46×0.91 mm2 and the measured insertion losses in the three passbands are less than 2.4 dB, 2.7 dB and 3.5 dB. The associated return losses are greater 18 dB, 17 dB and 8 dB over the frequency bands of interest.

- by Yi-Chyun Chiang
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- Millimeter Wave Antennas, Design method, Mott metal-insulator transition, Mmic

We review the first decade of extensive optical studies of ferromagnetic, III-Mn-V diluted magnetic semiconductors. Mn introduces holes and local moments to the III-V host, which can result in carrier mediated ferromagnetism in these disordered semiconductors. Spectroscopic experiments provide direct access to the strength and nature of the exchange between holes and local moments; the degree of itineracy of the carriers; and the evolution of the states at the Fermi energy with doping. Taken together, diversity of optical methods reveal that Mn is an unconventional dopant, in that the metal to insulator transition is governed by the strength of the hybridization between Mn and its p-nictogen neighbor. The interplay between the optical, electronic and magnetic properties of III-Mn-V magnetic semiconductors is of fundamental interest and may enable future spin-optoelectronic devices. arXiv:0810.3669v1 [cond-mat.mtrl-sci]

- by Dimitri Basov
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- Mechanical Engineering, Condensed Matter Physics, Hybridization, Doping

The effect of pressure on the conduction of the NbS3 quasi-one-dimensional conductor is studied. A pressure-induced insulator-metal transition is observed. The transition is accompanied by an increase in conductivity by six orders of magnitude at room temperature. Under pressures of 3–4 GPa, an additional phase transition appears in the temperature dependences of resistance. This transition manifests itself in an increase in the local conduction activation energy. The quantity dln(R)/d(1/T) reaches its maximum under pressures of 4–5 GPa, and the temperature position of the maximum of dln(R)/d(1/T) depends on the pressure as T* ≈ 7.5P + 202 K.

For use in millimeter-wave integrated circuits, theoretical and experimental design data for various useful capacitive transmission lines in coplanar waveguide on gallium arsenide are presented. Multifinger and metal-insulator-metal (MIM) lines enabling low characteristic impedance in the order of 10 as well as broad-band capacitive coupled transmission lines are investigated. Simple design rules and accurate models validated with measurements up to 120 GHz demonstrate that such planar transmission lines can be used in monolithic microwave integrated circuits (MMIC's) to extend the impedance range and the design flexibility with respect to the conventional CPW.

We investigate the highly incoherent regime of hole-doped two-dimensional Mott-Hubbard insulators at moderately small doping ␦ and temperatures տ0.1J, where J is the exchange coupling. Within an extended dynamical mean-field theory of the t-J model and a generalized noncrossing approximation we calculate the single-particle spectral function, the dynamical susceptibility, and thermodynamic and transport quantities. Short-ranged antiferromagnetic fluctuations lead to strongly incoherent single-particle dynamics, large entropy, and large electrical resistivity. At low doping a pseudogap is found to open up in both the single-particle and spin excitation spectra, leading to a decrease in entropy and resistivity. The Hall coefficient changes sign to positive values upon lowering the doping level and increases inversely proportional to ␦.

We reconsider the strong-coupling expansion for the Hubbard model recently introduced by Sarker and Pairault et al. By introducing slave particles that act as projection operators onto the empty, singly occupied and doubly occupied atomic states, the perturbation theory around the atomic limit distinguishes between processes that do conserve or do not conserve the total number of doubly occupied sites. This allows for a systematic t/U expansion that does not break down at low temperature (t being the intersite hopping amplitude and U the local Coulomb repulsion). The fermionic field becomes a two-component field, which reflects the presence of the two Hubbard bands. The single-particle propagator is naturally expressed as a function of a 2 × 2 matrix self-energy. Furthermore, by introducing a time-and space-fluctuating spin-quantization axis in the functional integral, we can expand around a "non-degenerate" ground-state where each singly occupied site has a well defined spin direction (which may fluctuate in time). This formalism is used to derive the effective action of charge carriers in the lower Hubbard band to first order in t/U. We recover the action of the t-J model in the spin-hole coherent-state path integral. We also compare our results with those previously obtained by studying fluctuations around the large-U Hartree-Fock saddle point.

- by H. Eusebe
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- Mott metal-insulator transition

We examine the percolation threshold for composites made by dispersing anisometric, single-crystal graphite flakes in either an epoxy resin or a polyurethane polymer matrix. Analysis is based on empirical and excludedvolume approaches and... more

We solve by Dynamical Mean Field Theory a toy-model which has a phase diagram strikingly similar to that of high Tc superconductors: a bell-shaped superconducting region adjacent the Mott insulator and a normal phase that evolves from a conventional Fermi liquid to a pseudogapped semi-metal as the Mott transition is approached. Guided by the physics of the impurity model that is self-consistently solved within Dynamical Mean Field Theory, we introduce an analytical ansatz to model the dynamical behavior across the various phases which fits very accurately the numerical data. The ansatz is based on the assumption that the wave-function renormalization, that is very severe especially in the pseudogap phase close to the Mott transition, is perfectly canceled by the vertex corrections in the Cooper pairing channel. A remarkable outcome is that a superconducting state can develop even from a pseudogapped normal state, in which there are no low-energy quasiparticles. The overall physical scenario that emerges, although unraveled in a specific model and in an infinite-coordination Bethe lattice, can be interpreted in terms of so general arguments to suggest that it can be realized in other correlated systems.

The present paper focuses on a quantitative analysis of the metallic and semiconducting behaviour of electrical resistivity in perovskite manganites La1−xNaxMnO3 (x = 0.1, 0.17). An effective interionic interaction potential (EIoIP) with the long-range Coulomb, van der Waals (vdW) interaction and short-range repulsive interaction up to second neighbour ions within the Hafemeister and Flygare approach was formulated to estimate the Debye and Einstein temperature and was found to be consistent with the available experimental data. For both doping concentration x = 0.1 and 0.17 the electron-phonon, electron-electron and electron-magnon interactions are effective to describe the resistivity behaviour for temperatures less than the metal-insulator transition (TP ). For temperatures, T > TP , the semiconducting nature is discussed with Mott's variable range hopping (VRH) model and small polaron conduction (SPC) model. The fitted density of states as revealed from VRH differs drastically from the experimental value and therefore means the VRH model is not a viable option for describing the resistivity behaviour in high temperature region, T > TP . The SPC model consistently retraces the higher temperature resistivity behaviour (T > θD/2). The metallic and semiconducting resistivity behaviour of sodium substituted manganites are analysed, to our knowledge, for the first time highlighting the importance of electron-phonon, electron-electron, electron-magnon interactions and small polaron conduction.

The trivalent alkali fullerides solids of generic composition A 3 C 60 , where C 60 is the fullerene molecule and A = K, Rb, and Cs, are a well established family of molecular superconductors. The superconductive electron pairing is of regular s-wave symmetry and is accounted for by conventional coupling of electrons to phonons, in particular by well understood Jahn Teller intramolecular C 60 vibrations. A source of renewed interest in these systems are alarming indications of strong electron-electron repulsion phenomena, which emerged especially in compounds where the C 60 -C 60 distance is expanded, by either a large cation size or by other chemical or physical means. Several examples are now known where this kind of expansion, while leading to a high superconducting temperature at first, gradually or suddenly causes a decline of superconductivity and its eventual disappearance in favor of a Mott insulating state. This kind of insulating state is the hallmark of strong electron correlations in cuprate and organic superconductors, and its appearance suggests that fullerides might also be members at large of that family.

We demonstrate a new flexible metal-insulator-metal capacitor using 9.5-nm-thick ZrO 2 film on a plastic polyimide substrate based on a simple and low-cost sol-gel precursor spin-coating process. The surface morphology of the ZrO 2 film was investigated using scan electron microscope and atomic force microscope. The as-deposited ZrO 2 film under suitable treatment of oxygen (O 2 ) plasma and then subsequent annealing at 250°C exhibits superior low leakage current density of 9.0 Â 10 À9 A/cm 2 at applied voltage of 5 V and maximum capacitance density of 13.3 fF/lm 2 at 1 MHz. The as-deposited sol-gel film was completely oxidized when we employed O 2 plasma at relatively low temperature and power (30 W), hence enhancing the electrical performance of the capacitor. The shift (Zr 3d from 184.1 eV to 184.64 eV) in X-ray photoelectron spectroscopy of the binding energy of the electrons towards higher binding energy; clearly indicates that the O 2 plasma reaction was most effective process for the complete oxidation of the sol-gel precursor at relatively low processing temperature.

Traditional routes to Charge-Density-Wave in transition metal dichalcogenides, relying on Fermi surface nesting or Jahn-Teller instabilities have recently been brought into question. While this calls for exploration of alternative views, paucity of theoretical guidance sustains lively controversy on the origin of, and interplay between CDW and superconductive orders in transition metal dichalcogenides. Here, we explore a preformed excitonic liquid route, heavily supplemented by modern correlated electronic structure calculations, to an excitonic-CDW order in 1T-TiSe2. We show that orbital-selective dynamical localisation arising from preformed excitonic liquid correlations is somewhat reminiscent to states proposed for d-and f -band quantum criticality at the border of magnetism. Excellent quantitative explication of a wide range of spectral and transport responses in both normal and CDW phases provides strong support for our scenario, and suggests that soft excitonic liquid fluctuations mediate superconductivity in a broad class of transition metal dichalcogenides on the border of CDW. This brings the transition metal dichalcogenides closer to the bad actors (where the metallic state is clearly not a Fermi liquid) in d-and f -band systems, where anomalously soft fluctuations of electronic origin are believed to mediate unconventional superconductivity on the border of magnetism.

- by Mukul Laad
- •
- Band Structure, Mott metal-insulator transition, Physical sciences, CHEMICAL SCIENCES

The X-ray diffraction, electrical resistivity (ρ), piezoresistance, thermoelectric power (TEP, S) and magnetisation experiments have been performed on polycrystalline La 1 − x Ce x MnO 3 (x = 0-0.6) samples over a temperature range (20-320 K). All the compounds except x = 0.1 display a metal-insulator (MI) transition at T MI . From the analysis of the resistivity data we find that the double peaks observed in the ρ(T) data of ceramic samples are originating from the grain boundary effects. Moreover, the charge conduction at higher temperature occurs by means of a thermally activated polaron hopping mechanism. The application of pressure suppresses the resistivity and enhances the resistive transition temperature (T MI ) and hence the Curie temperature (T C ). The thermopower (TEP, S) of all the samples is positive and above T C , both the ρ(T) and S(T) curves show that charge conduction at high temperatures takes place according to Emin and Holstein's theory of adiabatic polaron hopping. At low temperatures, thermal variation of the magnetization (M) can be explained considering the conventional spin wave theory containing T 3/2 and T 2 terms. In the paramagnetic region, the M(T) data fits well with the Curie-Weiss law.

The optical/infrared properties of films of vanadium dioxide (VO2) and vanadium sesquioxide (V2O3) have been investigated via ellipsometry and near-normal incidence reflectance measurements from far infrared to ultraviolet frequencies. Significant changes occur in the optical conductivity of both VO2 and V2O3 across the metal-insulator transitions at least up to (and possibly beyond) 6 eV. We argue that such changes in optical conductivity and electronic spectral weight over a broad frequency range is evidence of the important role of electronic correlations to the metal-insulator transitions in both of these vanadium oxides. We observe a sharp optical transition with possible final state (exciton) effects in the insulating phase of VO2. This sharp optical transition occurs between narrow a1g bands that arise from the quasi-one-dimensional chains of vanadium dimers. Electronic correlations in the metallic phases of both VO2 and V2O3 lead to reduction of the kinetic energy of the charge carriers compared to band theory values, with paramagnetic metallic V2O3 showing evidence of stronger correlations compared to rutile metallic VO2.

The general low-energy theory of electrons interacting via repulsive short-range interactions on graphene's honeycomb lattice at half filling is presented. The exact symmetry of the Lagrangian with local quartic terms for the Dirac four-component field dictated by the lattice is identified as D2 × Uc(1)×time reversal, where D2 is the dihedral group, and Uc(1) is a subgroup of the SUc(2) "chiral" group of the non-interacting Lagrangian, that represents translations in Dirac language. The Lagrangian describing spinless particles respecting this symmetry is parameterized by six independent coupling constants. We show how first imposing the rotational, then Lorentz, and finally chiral symmetry to the quartic terms -in conjunction with the Fierz transformations -eventually reduces the set of couplings to just two, in the "maximally symmetric" local interacting theory. We identify the two critical points in such a Lorentz and chirally symmetric theory as describing metalinsulator transitions into the states with either time-reversal or chiral symmetry being broken. The latter is proposed to govern the continuous transition in both the Thirring and Nambu-Jona-Lasinio models in 2+1 dimensions and with a single Dirac field. In the site-localized, "atomic", limit of the interacting Hamiltonian, under the assumption of emergent Lorentz invariance, the low-energy theory describes the continuous transitions into the insulator with either a finite Haldane's (circulating currents) or Semenoff's (staggered density) masses, both in the universality class of the Gross-Neveu model. The simple picture of the metal-insulator transition on a honeycomb lattice emerges at which the residue of the quasiparticle pole at the metallic, and the mass-gap in the insulating phase both vanish continuously as the critical point is approached. In contrast to these two critical quantities, we argue that the Fermi velocity is non-critical as a consequence of the dynamical exponent being fixed to unity by the emergent Lorentz invariance near criticality. Possible effects of the long-range Coulomb interaction, and the critical behavior of the specific heat and conductivity are discussed.

A strategy has been proposed recently to design plasmonic nanostructures capable of efficient harvesting of light over a broadband spectrum. Applying a singular conformal transformation to a metal-insulator-metal infinite structure, the optical response of two kissing nanowires can be deduced analytically. This nanostructure is shown to exhibit a large and continuous absorption cross-section relative to its physical size over the whole visible spectrum. Considerable field enhancement and confinement at the nano-scale are also expected at the touching point. Actually, instead of transporting the energy out to infinity, like in a metal slab geometry, the surface plasmon modes here propagate towards the singularity of the structure where their velocity vanishes and energy accumulates. The field enhancement is then a balance between this energy accumulation and dissipation losses. The asymptotic case of a nanowire placed on top of a metal plate is shown to be of great interest for nanofocusing. Finally, numerical simulations are performed to investigate the effect of radiative losses when the structure dimension becomes comparable to the wavelength.

- by Dangyuan Lei and +1
- •
- Nanowires, Numerical Simulation, Mott metal-insulator transition, Physical sciences

Optimized geometries, HOMO-LUMO gaps, vertical ionization potentials and electron affinities are obtained using HF, and B3LYP methods with 6-311G, B3LYP calculations are performed with LANL2DZ effective core potential. Electron correlation is included by doing MP2 calculation. The harmonic frequencies of all the compounds are obtained using B3LYP with 6-311G** and/or LANL2DZ basis sets. The force field and vibrational spectra are analyzed and 74 symmetry unique non-redundant local force constants are evaluated. Probable assignments are proposed for all the fundamentals based on the potential energy distribution. q

- by Ramachandran Gnanasekaran
- •
- Density-functional theory, Crystal structure, Geometry, X Rays

We report the evolution of superconducting properties with disorder, in 3-dimensional homogeneously disordered epitaxial NbN thin films. The effective disorder in NbN is controlled from moderately clean limit down to Anderson metal–insulator transition by changing the deposition conditions. We propose a phase diagram for NbN in temperature-disorder plane. With increasing disorder, we observe that as kF l→1 the superconducting transition temperature (Tc) and normal state conductivity in the limit T →0 (σ0) go to zero. The phase diagram shows that in homogeneously disordered 3-D NbN films, the metal–insulator transition and the superconductor–insulator transition occur at a single quantum critical point, kF l ∼ 1.

- by Mintu Mondal
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- Condensed Matter Physics, Mott metal-insulator transition, Thin Film, Dimensional

A new model is presented of current transport in Metal Insulator Metal (MIM) structures by quantum mechanical tunnelling. In addition to direct tunnelling through an insulating layer, tunnelling via defects present in the insulating layer plays an important role. Examples of the influence of the material and thickness of the insulating layer, energy distribution of traps, and metal work functions are also provided.

- by Juraj Racko and +2
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- Quantum Mechanics, Mott metal-insulator transition, Work Function, Energy Distribution

We discuss an experiment for determining the heat capacities of various solids based on a calorimetric approach where the solid vaporizes a measurable mass of liquid nitrogen. We demonstrate our technique for copper and aluminum, and compare our data with Einstein's model of independent harmonic oscillators and the more accurate Debye model. We also illustrate an interesting material property, the Verwey transition in magnetite.

- by waqas mahmood
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- Physics Education, Student Experience, Mathematical Sciences, Copper

Silver nanoparticles embedded in a dielectric matrix are investigated for their potential as broadbandabsorbing optical sensor materials. This contribution focuses on the electrical properties of silver nanoparticles on glass substrates at various morphological stages. The electrical current through thin films, consisting of silver nanoparticles, was characterized as a function of film thickness. Three distinct conductivity zones were observed. Two relatively flat zones (''dielectric'' for very thin films and ''metallic'' for films thicker than 300-400 Å ) are separated by a sharp transition zone where percolation dominates. The dielectric zone is characterized by isolated particle islands with the electrical conduction dominated by a thermally activated tunneling process. The transition zone is dominated by interconnected silver nanoclusters-a small increase of the film thickness results in a large increase of the electrical conductivity. The metallic conductivity zone dominates for thicknesses above 300-400 Å .

- by Haoyan Wei
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- Materials Engineering, Condensed Matter Physics, Microstructure, Nanoparticles

The LDA+ DMFT ͑local density approximation combined with dynamical mean-field theory͒ computation scheme has been used to study spectral and magnetic properties of FeSi and Fe 1−x Co x Si. Having compared different models, we conclude that a correlated band insulator scenario in contrast to Kondo insulator model agrees well with FeSi band structure and experimental data. Coulomb correlation effects lead to band narrowing of the states near the Fermi level with mass renormalization parameter m ‫ء‬ Ϸ 2 in agreement with the results of angle-resolved photoemission spectroscopy. Temperature dependence of spectral functions and magnetic susceptibility calculated in DMFT reproduces transition from nonmagnetic semiconductor to metal with local magnetic moments observed experimentally. Cobalt doping leads to ferromagnetism that has itinerant nature and can be successfully described by the LDA+ DMFT method.

Single crystal neutron diffraction studies were carried out for (TMTSF),BF4 at 20 K to accurately determine the methyl group H atom positions of the TMTSF molecule. At 20 K, this salt has the (20 X 26 X 2c) superstructure, with respect to the (a X b X c) structure observed above its anion ordering temperature -40 K. The structural features of the anion ordering were described in terms of short He-F contacts between BF4-anions and TMTSF molecules. Also, the anion ordering in (TMTSF),BF, is apparently independent of cooling rate, since the completely ordered superstructures obtained upon fast ( -6 deg min-') and slow (-0.2 deg m i d ) cooling of the crystal to 20 K do not vary significantly. The nature of the metal-insulator transition in (TMTSF),BF,, which is a consequence of the anion ordering, was studied by performing tight binding band calculations for the 125 and 20 K crystal structures of (TMTSF),BF4. This salt is a narrow band gap (-0.1 eV) semiconductor at 20 K, which is a result of the Peierls distortion involving the half-filled valence band of (TMTSF),BF, above -40 K.

- by Mark Beno
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- Neutron Diffraction, Crystal structure, Band Structure, Mott metal-insulator transition

The oxygen content of La 0.35 Ca 0.65 MnO y was decreased by annealing the samples at high temperatures in vacuum with graphite powder nearby. Insulator-metal (I-M) transition occurred upon dramatic decrease of oxygen content. Correspondingly the samples showed a large magnetoresistance (MR) and MR ¼ CðM=M s Þ 2 at 4.5 K up to relatively large M=M s : The results were explained in terms of the variation of Mn 3+ /Mn 4+ ratio and oxygen vacancy ordering induced percolation of the ferromagnetic regions. Spin-dependent hopping model can account for the variation of MR with M=M s and the variation of C with the decrease of oxygen content. r

- by satish shinde
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- Engineering, Mechanical Engineering, Condensed Matter Physics, Applied Physics

We show that the pressure-temperature phase diagram of the Mott insulator Ca2RuO4 features a metal-insulator transition at 0.5GPa: at 300K from paramagnetic insulator to paramagnetic quasi-two-dimensional metal; at T ≤ 12K from antiferromagnetic insulator to ferromagnetic, highly anisotropic, three-dimensional metal. We compare the metallic state to that of the structurally related p-wave superconductor Sr2RuO4, and discuss the importance of structural distortions, which are expected to couple strongly to pressure. PACS numbers: 71.30+h, 75.30Kz, 74.70Pq, and 74.62Fj

- by Yoshiteru Maeno and +2
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- Mott metal-insulator transition, Superconductors, Three Dimensional, Pressure Effect

We review some of the recent advances in the simulation of plasmonic devices, drawing examples from our own work in metal-insulator-metal (MIM) plasmonic waveguide components and networks. We introduce the mode-matching technique for modeling of MIM waveguide devices. We derive the complete set of orthogonal modes that the MIM waveguide supports and use it to apply the mode-matching technique to the analysis of plasmonic waveguide networks. We also introduce several different equivalent models for plasmonic waveguide components, such as the characteristic impedance model for deep subwavelength MIM waveguides, the scattering matrix description of MIM waveguide junctions, and equivalent circuit models. The model abstraction provided by these equivalent models is important for the analysis and synthesis of device functions, as illustrated with the design of a waveguide mode converter.