Electronic Bandwidth Research Articles

Achieving Sub-100-ns Switching of Programmable Josephson Arrays

The accuracy of voltage waveforms, generated using programmable Josephson junction arrays, is limited by the time taken to switch between quantized levels. Relevant parameters are the time constants within the array chip, the characteristics of the cables, and the bandwidth of the bias electronics. A drive system for a 1-V SINIS array with 8192 junctions has been optimized, and a cable scheme has been devised which compensates for cable reflections. The measurements of the array's temporal response, made with this cable system, are compared to an RC model, both with 3- and 7-ns single time constants

Heavy electrons from Hund’s rule and short-range antiferromagnetism

We investigate the one-dimensional ferromagnetic Kondo lattice in the hole-rich region. Of interest to us is the intermediate situation where the ferromagnetic Kondo coupling (Hund's coupling) is comparable to the electron bandwidth. The forced alignment favors triplet states whereas singlet states enter in a quite low-density regime. The direct antiferromagnetic exchange between the core spins is assumed to still prevail over the double exchange. We discuss a solvable limit showing that short-range antiferromagnetism in the spin array will affect the coherent propagation of triplets, i.e., turns a light triplet into a heavy singlet, resulting in a heavy electron ground state.

Variable-range hopping conductivity and structure of density of localized states inLaMnO3+δ under pressure

The temperature dependence of the resistivity,ρ, ofceramic LaMnO3+δ with δ = 0.065 and 0.154 (or simply # 065 and # 154), is investigated under hydrostatic pressure,p, between thetemperatures T = 80 and 320 K in magnetic fields up to 10 T. The data forρ(T) are analysed within a Shklovskii–Efros-like variable-rangehopping conductivity model, governed by a soft Coulomb gap,Δ, and arigid gap, γ, in the density of localized states. The validity of this approach to the resistivity ofLaMnO3+δ under ambient pressure was established in recent investigations (Laiho et al 2005 J. Phys:Condens. Matter 17, 105). In the present work it is shown that upon increasingp up to 11 kbar (# 154) and 13 kbar (# 065) the magnetic Curie temperature,TC, is increasedby ∼12–13%.Correspondingly, Δ(p) is decreased by ∼7–8% and γ(p) by ∼8% in # 154 and 12% in # 065. Similarly, the electron localization radiusa(p) is increasedby ∼4% in # 154 and 8% in # 065, respectively. The influence of pressure onTC,Δ,γ and a is interpreted by increasing the electron bandwidth and decreasing thepolaron potential well, which stimulates the delocalization of electrons whenp is increased.

On The Theory of Isotope-effect in the d-wave Superconductors

The isotope-effect in a two-dimensional model of the d-wave phonon-exchange superconductor is studied. It is shown that the mean-field critical temperature, the Berezinskii–Kosterlitz–Thouless critical temperature and the superconducting gap strongly depend on the phonon frequency only in some range of the model parameters. These dependencies have different from the BCS theory forms. A qualitative comparison of the results with experimental data on some high-temperature superconductors is made. In particular, it is shown that the isotope-effect is significant in the case of strong electron–phonon coupling, when the phonon frequency is small compared to the free electron bandwidth.

Anchoring methane thiol on Cu(100) in different structural configurations: Electronic state dispersion

Self-assembled monolayers can be obtained by anchoring thiol molecules through their sulfur headgroup on metal substrates, obtaining long-range ordered configurations. Dimethyl-disulfide $[{(\mathrm{C}{\mathrm{H}}_{3}\mathrm{S})}_{2}]$ adsorbs in a symmetric fourfold chemisorption site as methane thiolate $(\mathrm{C}{\mathrm{H}}_{3}\mathrm{S})$ on the Cu(100) surface, and it builds up regular twofold reconstructed superstructures, $p(2\ifmmode\times\else\texttimes\fi{}2)$ at low dosing and $c(2\ifmmode\times\else\texttimes\fi{}6)∕c(2\ifmmode\times\else\texttimes\fi{}2)$ at higher coverage. Electronic state dispersion of the hybrid interface electronic states has been followed along the main symmetry directions of the surface Brillouin zone in the three different structural phases by means of high-resolution angular-resolved UV photoelectron spectroscopy. The electronic binding energy and bandwidth of the hybrid electronic states are mainly determined by the sulfur atom interaction with the metal surface. The bandwidths ranging from $400\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}780\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ for the twofold reconstructions depend on the different molecular packing. Comparison with previous experimental and theoretical results on different S-headgroup systems adsorbed on noble metal surfaces indicates a role of the radical tail group.

Optical properties of small polarons from dynamical mean-field theory

The optical properties of polarons are studied in the framework of the Holstein model by applying the dynamical mean-field theory. This approach allows to enlighten important quantitative and qualitative deviations from the limiting treatments of small polaron theory, that should be considered when interpreting experimental data. In the antiadiabatic regime, accounting on the same footing for a finite phonon frequency and a finite electron bandwidth allows to address the evolution of the optical absorption away from the well-understood molecular limit. It is shown that the width of the multiphonon peaks in the optical spectra depends on the temperature and on the frequency in a way that contradicts the commonly accepted results, most notably in the strong coupling case. In the adiabatic regime, on the other hand, the present method allows to identify a wide range of parameters of experimental interest, where the electron bandwidth is comparable or larger than the broadening of the Franck-Condon line, leading to a strong modification of both the position and the shape of the polaronic absorption. An analytical expression is derived in the limit of vanishing broadening, which improves over the existing formulas and whose validity extends to any finite-dimensional lattice. In the same adiabatic regime, at intermediate values of the interaction strength, the optical absorption exhibits a characteristic reentrant behavior, with the emergence of sharp features upon increasing the temperature -- polaron interband transitions -- which are peculiar of the polaron crossover, and for which analytical expressions are provided.

Excited-State Mixed Valence in Transition Metal Complexes

Mixed valence in the lowest-energy metal-to-ligand charge-transfer excited state of di-(4-acetylpyridine)tetraammineruthenium(II) complexes is defined and analyzed. The excited state has two interchangeably equivalent ligands with different oxidation states. The electronic absorption band energies, selection rules, and bandwidths are analyzed quantitatively in terms of the signs and orientations of the transition dipole moments, sign and magnitude of the coupling, and resonance Raman analysis of displaced normal modes.

Dual-signal heterodyne lock-in amplification with lasers

High-sensitivity heterodyne detection with lasers applied to radar and satellite communication is seriously hampered by the large electronic bandwidth due to Doppler shift and frequency instability. These drawbacks can be circumvented by dual-signal heterodyne detection. The system consists of mixing the local oscillator with two signal beams and the lock-in amplification at the beat frequency of the two intermediate frequencies. The signal modulation is demodulated by the lock-in amplification. A detailed analysis of the accompanying noise is given. The derived NEP is much smaller than that of a conventional heterodyne system. As examples, the system is applied to radar and space communication.

Collapse of Coherent Quasiparticle States inθ−(BEDT−TTF)2I3Observed by Optical Spectroscopy

Optical conductivity measurements on the organic metal theta-(BEDT-TTF)2I3 revealed that the system crosses over rapidly from a coherent quasiparticle state to an incoherent state with temperature. Despite the metallic temperature dependence of resistivity, a well-defined Drude peak at low temperatures turns into a far-infrared peak with temperature. The peak energy shifts to higher frequencies and, simultaneously, the spectral weight is transferred to high frequencies beyond the electron band width. These characteristics imply that theta-(BEDT-TTF)2I3, so far believed to be a typical metal, is in fact a strongly correlated electron system with "bad-metal" character.

One-dimensional electron lattice system with a long-range interelectron repulsion on a disordered host lattice

We study what happens to a generalized Wigner crystal, GWC (a regular structure formed by narrow-band electrons on a one-dimensional periodic host lattice), when the host lattice suffers a random distortion that does not break its long-range order. We show that an arbitrarily weak distortion of this kind gives rise to soliton-like GWC defects (discrete solitons, DS) in the ground state, and thereby converts the ordered GWC into a new disordered macroscopic state—lattice Wigner glass (LWG). The ground-state DS concentration is found to be proportional to λ4 (λ is the typical host-lattice strain). We show that the low-temperature LWG thermodynamics and kinetics are fully described in DS terms. A new phenomenon of a super-slow logarithmic relaxation in the LWG is revealed. Its time turns out to be tens orders of magnitude greater than the microscopic ones. Analytical dependences of LWG thermodynamic quantities on temperature and λ are obtained for an arbitrary relationship between the relevant Coulomb energies and the electron bandwidth.

Phase diagram and isotope effects of the quasi-one-dimensional electron gas coupled to phonons

Using a multistep renormalization group method, we study the low-temperature phases of the interacting one-dimensional (1D) electron gas coupled to phonons. We obtain analytic expressions for the weak-coupling quantum phase boundaries of the 1D extended Holstein-Hubbard model and the 1D extended Peierls-Hubbard model for general band-filling and phonon frequency. Away from half-filling, the phase diagrams are characterized by a delicate competition between spin density wave, charge density wave, and superconducting orders. We study the dependence of the ground state on the electron-phonon (el-ph) and electron-electron (el-el) coupling strengths, the screening length, electron bandwidth, phonon frequency, doping, and type of phonon. Unlike the case in Fermi liquids, in 1D the el-ph coupling is strongly renormalized, often to stronger values. Even when the bare phonon-induced attraction is weak compared to the bare el-el repulsion, a small amount of retardation can cause the renormalized el-ph interaction to dominate the problem. We find cases in which a repulsive el-el interaction enhances the superconducting susceptibility in the presence of a retarded el-ph interaction. The spin gap and superconducting susceptibility are found to be strongly dependent on the deviation from half-filling (doping). In some cases, the superconducting susceptibility varies nonmonotonically with doping and exhibits a maximum at a particular doping. For a quasi-1D array of weakly coupled, fluctuating 1D chains, the superconducting transition temperature T_c also exhibits a maximum as a function of doping. The effect of changing the ion mass (isotope effect) on T_c is found to be largest near half-filling and to decrease rapidly with doping.

Phase noise-tolerant synchronous QPSK/BPSK baseband-type intradyne receiver concept with feedforward carrier recovery

Quadrature phase-shift keying (QPSK) is attractive to increase transmission lengths and capacity, especially when it is combined with polarization division multiplex. Baseband processing at the symbol rate allows to keep the required electronic bandwidth low. So far, external cavity lasers seemed to be indispensable for such transmission systems due to linewidth requirements. We propose a feedforward carrier recovery scheme based on regenerative intradyne frequency dividers, i.e., the well-known regenerative frequency divider is extended to process baseband in-phase and quadrature (I and Q) signals. An IF linewidth tolerance of up to 0.001 times the QPSK symbol rate is predicted, 2 decades more than for an optical phase locked loop with a realistic loop delay. This means that commercially available DFB lasers shall suffice for synchronous optical QPSK/BPSK transmission.

Quantum filtering of electron emission from ultrasharp tips

This paper derives an exact fully three-dimensional wave-function solution and presents representative calculations for the problem of an electron confined in an atomically sharp three-dimensional tip. The presented solution has important implications in technologies including self-collimated electron sources, ultrafine electron-beam lithography, electron holography, and cold cathode radio-frequency generation. The tip is modeled in the prolate-spheroidal coordinate system because the principal characteristics of an ultrasharp tip, including cone angle and tip radius, are naturally represented. This quantum-mechanical analysis corroborates several experimental observations on atomically sharp tips, including unexpectedly narrow energy bandwidth for the emitted electrons and pronounced collimation, or self-focusing, of the emitted electron beam. The three-dimensional analysis developed in this paper indicates that these experimental phenomena are attributable to a ``quantum filtering'' effect that arises in the geometric constriction characteristic of ultrasharp vacuum field emitters. The results of this paper suggest that the free-electron supply function, routinely invoked in classical field-emission analysis, may be an inaccurate approximation for the electronic states of ultrasharp tips having quantum-filtering characteristics. Surface plots of the electron wave-function density for various total electron energies and tip geometries demonstrate the tip selectivity with respect to electron states.

The Measurement of Radiative Lifetimes Using Laser-Induced Fluorescence: Experimental Review and Astrophysical Application

One of the standard methods for determining atomic transition probabilities is to combine branching fractions measured using Fourier-transform spectrometry with radiative lifetimes measurements using laser-induced fluorescence (LIF). This combination of techniques provides an efficient method for measuring large sets of accurate, absolute transition probabilities. The radiative lifetimes, which provide the overall scaling for the transition probabilities, can be measured routinely to ± 5% accuracy using time-resolved LIF. Although the time-resolved LIF technique we use does not achieve the accuracy of fast-beam LIF, the time-resolved technique does enable us to make measurements at a far greater rate (hundreds of level lifetimes per year). Care must be taken, however, to understand and control the systematic effects in time-resolved LIF measurements to maintain ± 5% accuracy over a wide dynamic range and hundreds of lifetime measurements.Over the last 25 years, we have measured lifetimes for 47 spectra using time-resolved LIF. Our atomic beam source can produce a slow beam of neutral and singly ionized atoms of nearly any element. Lifetimes from 2 ns to ~2 µs can be measured for energy levels ranging from 15,000 to ~60,000 cm-1. In this review we will describe our method of measuring radiative lifetimes with an emphasis on possible errors and techniques used for controlling them. The electronic bandwidth, linearity, and overall fidelity of the fast photomultiplier, cable connections, and transient waveform digitizer are concerns. Possible errors from atomic collisions, radiation trapping, Zeeman quantum beats, hyperfine quantum beats, atoms/ions escaping from the observation region before radiating, and from radiative cascade through lower levels must be understood and controlled.We will then present a recent example of the application of our transition probability data to abundance determinations in the sun and in metal-poor halo stars (Den Hartog E A et al 2003 Astrophys. J. Suppl. 148 543). Our results have significantly reduced the uncertainty of abundance determinations for many elements including the rare-earths: La, Nd, Eu, Tb, Dy, Ho, Tm, and Lu. Better laboratory data have made it possible to cleanly separate the enhanced abundance of r(apid)-process neutron capture elements in metal poor Galactic halo stars. The long-term scientific payoff from this attention to detail in laboratory experiments is a better understanding of the r-process, of the Galactic chemical evolution, of the age of the oldest stars in the Galaxy, and indeed of the origins of the non-primordial chemical elements.

Orbital physics in the perovskite Ti oxides

Titanate compounds have been recognized as key materials for understanding the coupling of magnetism and orbitals in strongly correlated electron systems. In the perovskite Ti oxide RTiO3 (where R represents the trivalent rare-earth ions), which is a typical Mott–Hubbard insulator, the Ti t2g orbitals and spins in the 3d1 state couple each other through the strong electron correlations, resulting in a rich variety of orbital–spin phases. One way of controlling the coupling is to change the tiltings of the TiO6 octahedra (namely the GdFeO3-type distortion) by varying the R ions, through which the relative ratio of the electron bandwidth to the Coulomb interaction is controlled. With this control, these Mott insulators exhibit an antiferromagnetic-to-ferromagnetic (AFM–FM) phase transition, which has turned out to be a consequence of rich orbital physics in these materials. The origin and nature of orbital–spin structures of these Mott insulators have been intensively studied both experimentally and theoretically. When the Mott insulators are doped with carriers, the titanates show touchstone properties of the filling controlled Mott transition. In this paper, we first review the state of the art on the studies for understanding physics contained in the properties of the perovskite titanates. On the properties of the insulators, we focus on the following three topics: (1) the origin and nature of the ferromagnetism as well as the orbital ordering in the compounds with relatively small R ions such as GdTiO3 and YTiO3, (2) the origin of the G-type antiferromagnetism and the orbital state in LaTiO3 and (3) the orbital–spin structures in other AFM(G) compounds with relatively large R ions (R = Ce, Pr, Nd and Sm). On the basis of these discussions, we discuss the whole phase diagram together with mechanisms of the magnetic phase transition. On the basis of the microscopic understanding of the orbital–spin states, we show that the Ti t2g degeneracy is inherently lifted in the titanates, which allows the single-band descriptions of the ground-state and the low-energy electronic structures as a good starting point. Our analyses indicate that these compounds offer good touchstone materials described by the single-band Hubbard model on the cubic lattice. From this insight, we also re-analyse the hole-doped titanates TiO3 (where A represents the divalent alkaline-earth ions). Experimentally revealed filling- and bandwidth-dependent properties and the critical behaviour of the metal–insulator transitions are discussed in the light of theories based on the single-band Hubbard models.

Pressure effects on charge, spin, and metal-insulator transitions in the narrow bandwidth manganitePr1−xCaxMnO3

Pressure effects on the charge and spin states and the relation between the ferromagnetic and metallic states were explored on the small bandwidth manganite ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ ($x=0.25$, 0.3, 0.35). Under pressure, the charge ordering state is suppressed and a ferromagnetic metallic state is induced in all three samples. The metal-insulator transition temperature $({T}_{\mathit{MI}})$ increases with pressure below a critical point $P*$, above which $({T}_{\mathit{MI}})$ decreases and the material becomes insulating as at the ambient pressure. The ${e}_{g}$ electron bandwidth and/or band filling mediate the pressure effects on the metal-insulator transition and the magnetic transition. In the small bandwidth and low doping concentration compound $(x=0.25)$, the ${T}_{\mathit{MI}}$ and the Curie temperature $({T}_{C})$ change with pressure in a reverse way and do not couple under pressure. In the $x=0.3$ compound, the relation of ${T}_{\mathit{MI}}$ and ${T}_{C}$ shows a critical behavior: They are coupled in the range of $\ensuremath{\sim}0.8--5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and decoupledoutside of this range. In the $x=0.35$ compound, ${T}_{\mathit{MI}}$ and ${T}_{C}$ are coupled in the measured pressure range where a ferromagnetic state is present.

Noise, detectors, and submillimeter–terahertz system performance in nonambient environments

Because many sources of radiation in the submillimeter–terahertz have relatively low power, the development of sensitive detectors has been important. Equally important is the selection of detector and source methodologies appropriate for specific scientific or technological applications. Discussed is the effect of Bose–Einstein statistics, blackbody mode structure, and detector optical and electronic bandwidths to develop closed-form expressions for the contributions from experimental backgrounds to overall system noise. The results obtained allay the oft-expressed concerns that many important experiments, especially those conducted at elevated temperatures or in harsh environments such as plasmas, are either difficult or not possible with high-resolution cw systems. To the contrary, in the large majority of cases this excess background noise is not observable. Experimental examples are provided.

The effect of a cation radii on structural, magnetic and electrical properties of doped manganites La 0.6− xPr xSr 0.4MnO 3

Structural, magnetic and transport properties of La 0.6− x Pr x Sr 0.4MnO 3 with x=0.0, 0.03, 0.06, 0.18, 0.3, 0.42, 0.54 and 0.6 are studied. The system exhibits a rhombohedrally distorted (R 3 ̄ c) perovskite structure for x⩽0.3. A rhombohedral–orthorhombic ( Pnma) structure transition is detected in the doping range from x=0.42 to 0.6. The structure refinement by Rietveld analysis of the X-ray powder diffraction data shows that the average distance Mn–O increases in the rhombohedral phases and decreases in the orthorhombic phases. Results show that the Curie temperature decreases from 374 to 310 K when 〈 r A 〉 varies from 1.254 to 1.231 Å. Electrical measurements show that all samples exhibit a metallic to semiconducting transition with increasing temperature. Meanwhile, the size of the resistivity ρ increases near T C. This phenomenon is interpreted as a gradual bending of the Mn–O–Mn bond angle, with decreasing 〈 r A 〉, which causes the narrowing of the electronic bandwidth and the effect of the A-site variance σ 2.

The observation of the insulator–metal transition inEuNiO3 under high pressure

The charge transfer antiferromagnetic(TN = 220 K)insulator EuNiO3 undergoes, at ambient pressure, a temperature-induced metal–insulator (MI) transition atTMI = 463 K. We have investigated the effect of pressure (up to ) on the electronic, magnetic and structural properties ofEuNiO3 using electricalresistance measurements, 151Eu nuclear resonance scattering of synchrotron radiation and x-raydiffraction, respectively. With increasing pressure we find atpc = 5.8 GPa a transition from the insulating state to a metallic state, while the orthorhombicstructure remains unchanged up to 20 GPa. The results are explained in terms ofa gradual increase of the electronic bandwidth with increasing pressure, whichresults in a closing of the charge transfer gap. It is further shown that thepressure-induced metallic state exhibits magnetic order with a lower value ofTN ( at 9.4 GPa) which disappears between 9.4 and 14.4 GPa.

Increase of Curie temperature in fixed ionic radius Ba $_\mathsf{1 + x}$ Sr $_\mathsf{1-3x}$ La $_\mathsf{2x}$ FeMoO $_\mathsf{6}$ double perovskites

We study the influence of electron doping in Ba1 + x Sr1-3x La2x FeMoO6 ( $x\le 0.3$ ) double perovskites, where the designed combination for the cationic site minimizes the influence of steric effects. We find an increase of the Curie temperature (T C ), which supports the claim of the important role played by the band filling in the value of T C . Other effects like changes in the bare electronic bandwidth (determined by structural parameters) and Fe/Mo antisite defects seem to play a less relevant role in the increase of T C in this series. Changes in structural parameters, magnetization and magnetotransport properties as a function of Ba/La doping are correlated with an increase of the density of states at the Fermi level and a moderate rise of antisites.