Effect Of Thermal Excitations Research Articles

Strong-coupling expansion for the momentum distribution of the Bose-Hubbard model with benchmarking against exact numerical results

A strong-coupling expansion for the Green's functions, self-energies, and correlation functions of the Bose-Hubbard model is developed. We illustrate the general formalism, which includes all possible (normal-phase) inhomogeneous effects in the formalism, such as disorder or a trap potential, as well as effects of thermal excitations. The expansion is then employed to calculate the momentum distribution of the bosons in the Mott phase for an infinite homogeneous periodic system at zero temperature through third order in the hopping. By using scaling theory for the critical behavior at zero momentum and at the critical value of the hopping for the Mott insulator–to–superfluid transition along with a generalization of the random-phase-approximation-like form for the momentum distribution, we are able to extrapolate the series to infinite order and produce very accurate quantitative results for the momentum distribution in a simple functional form for one, two, and three dimensions. The accuracy is better in higher dimensions and is on the order of a few percent relative error everywhere except close to the critical value of the hopping divided by the on-site repulsion. In addition, we find simple phenomenological expressions for the Mott-phase lobes in two and three dimensions which are much more accurate than the truncated strong-coupling expansions and any other analytic approximation we are aware of. The strong-coupling expansions and scaling-theory results are benchmarked against numerically exact quantum Monte Carlo simulations in two and three dimensions and against density-matrix renormalization-group calculations in one dimension. These analytic expressions will be useful for quick comparison of experimental results to theory and in many cases can bypass the need for expensive numerical simulations.

Temperature dependence of critical velocity in a Bose gas in a moving optical lattice

We study the critical velocity of a Bose-condensed gas in a moving one-dimensional (1D) optical lattice potential at finite temperatures. Using a quasi-1D model of the Gross-Pitaeavskii equation and the Bogoliubov equations, we explicitly include effects of thermal excitations in the radial direction. Within the Popov approximation, we calculate the temperature dependence of the Bogoliubov excitations with varying lattice velocity. From the condition of the negative excitation energy, we obtain the critical velocity as a function of the lattice depth and the temperature. We find that the critical velocity decreases rapidly with increasing the temperature; this result is consistent with the experimental observations. Moreover, the critical velocity shows a rapid decrease with increasing lattice depth. This tendency is much more significant than in the previous works ignoring the effect of thermal excitations in the radial direction.

Damping of condensate oscillations of a trapped Bose gas in a one-dimensional optical lattice at finite temperatures

We study damping of the dipole oscillation in a Bose-condensed gas in a combined cigar-shaped harmonic trap and one-dimensional (1D) optical lattice potential at finite temperatures. In order to include the effect of thermal excitations in the radial direction, we derive a quasi-1D model of the Gross-Pitaevskii equation and the Bogoliubov equations. We use the Popov approximation to calculate the temperature dependence of the condensate fraction with varying lattice depth. We then calculate the Landau damping rate of the dipole oscillation as a function of the lattice depth and temperature. The damping rate increases with increasing lattice depth, which is consistent with experimental observations. The magnitude of the damping rate is in reasonable agreement with experimental data. We also find that the damping rate has a strong temperature dependence, showing a sharp increase with increasing temperature. Finally, we emphasize the importance of the radial thermal excitations in both equilibrium properties and the Landau damping.

Study of carrier behavior in La 0.9Ba 0.1MnO 3 − δ/SrTiO 3:Nb p-n heterojunction

The p-n junctions had been fabricated by depositing La 0.9Ba 0.1MnO 3 − δ (LBMO 3 − δ ) thin film on the Nb-doping SrTiO 3 (STON) substrates. Good rectifying properties of the LBMO 3 − δ /STON p-n junction have been confirmed in the temperature range of 70–300 K. It is found that the photovoltage ( V oc) peaks increase with increase of laser energy and temperature. The thermal excitation effects and the reduction of conduction-band mismatch with the rising of temperature are suggested to be responsible for the more carriers accumulating at edges of depletion layer, as well as the enhanced V oc peak.

Ferromagnetic nanorings

Ferromagnetic metal rings of nanometre range widths and thicknesses exhibitfundamentally new spin states, switching behaviour and spin dynamics, whichcan be precisely controlled via geometry, material composition and applied field.Following the discovery of the ‘onion state’, which mediates the switching toand between vortex states, a range of fascinating phenomena has been found inthese structures. In this overview of our work on ring elements, we first show howthe geometric parameters of ring elements determine the exact equilibrium spinconfiguration of the domain walls of rings in the onion state, and we show howsuch behaviour can be understood as the result of the competition between theexchange and magnetostatic energy terms. Electron transport provides an extremelysensitive probe of the presence, spatial location and motion of domain walls, whichdetermine the magnetic state in individual rings, while magneto-optical measurementswith high spatial resolution can be used to probe the switching behaviour of ringstructures with very high sensitivity. We illustrate how the ring geometry hasbeen used for the study of a wide variety of magnetic phenomena, including thedisplacement of domain walls by electric currents, magnetoresistance, the strength of thepinning potential introduced by nanometre size constrictions, the effect of thermalexcitations on the equilibrium state and the stochastic nature of switching events.

Effective method to compute vibrationally resolved optical spectra of large molecules at finite temperature in the gas phase and in solution

The authors present a new method for the computation of vibrationally resolved optical spectra of large molecules, including the Duschinsky rotation of the normal modes and the effect of thermal excitation. The method automatically selects the relevant vibronic contributions to the spectrum, independently of their frequency, and it is able to provide fully converged spectra with moderate computational times, both in vacuo and in solution. By describing the electronic states in the frame of the density functional theory and its time-dependent extension, they computed the room temperature absorption spectra of coumarin C153 and trans-stilbene in cyclohexane and the phosphorescence spectrum of porphyrazine in gas phase, showing that the method is fast and efficient. The comparison with experiment for trans-stilbene and coumarin C153 is very satisfactory, confirming the progress made toward a reliable method for the computation and interpretation for the optical spectra of large molecules.

IR photon enhanced dissociative electron attachment to SF 6: Dependence on photon, vibrational, and electron energy

The influence of IR photon excitation of the ν 3 vibrational mode in SF 6 on dissociative electron attachment ( SF 5 - formation ) is investigated at high electron energy resolution (down to 1 meV) over the energy range E = 0–0.5 eV with the laser photoelectron attachment method. The molecules are contained in a collimated seeded supersonic beam (nozzle temperatures T 0 = 300–600 K, corresponding to vibrational temperatures T V ≈ T 0 − 100 K) and transversely excited by the 10PX lines (X = 10–40) in the 10.6 μm band of a continuous CO 2 laser at intensities up to about 400 W cm −2. The IR excitation and the attachment regions are separated by 5 cm. The IR photon induced enhancement of the SF 5 - yield is found to be optimal on the 10P28 line (936.8 cm −1) at all nozzle temperatures (with the maximum reached for T 0 ≈ 390 K) and monotonically decreasing with rising electron energy from 0 eV over a range of about 0.3 eV. For a fixed spatial profile of the exciting IR beam, the enhancement at E ≈ 0 eV follows a near-square-root dependence on laser power. With reference to previous work on the excitation of supersonic SF 6 beams by CO 2 laser light, the fraction of laser-excited SF 6( ν 3 ⩾ 1) molecules is estimated, and the absolute cross sections σ L( E) for SF 5 - formation involving the IR-excited molecules are determined; they exceed the cross sections σ 0( E) for thermal molecules in a way which strongly depends on electron energy and initial vibrational energy. In contrast, the cross section for SF 6 - formation is found to be almost independent of laser-excitation and temperature. The experimental findings are discussed with regard to the multiphoton character of the IR excitation, and comparisons are made with the effects of thermal excitation. The mechanisms for SF 6 - and SF 5 - formation and the responsible potential energy surfaces are discussed in the light of the available experimental data.

Thermal excitation effects of photoluminescence of annealed GaInNAs∕GaAs quantum-well laser structures grown by plasma-assisted molecular-beam epitaxy

Ga In N As ∕ Ga As quantum well laser structures have been grown by plasma-assisted molecular beam epitaxy. Rapid thermal annealing was applied to suppress the nitrogen-related localized states in the material. These nitrogen-related localized states significantly quench the photoluminescence due to its low radiative recombination efficiency, compared to band-to-band transitions. Further, the thermal excitation processes of carriers from localized states to extended states result in the high temperature-sensitivity of light emission, which may lead to a low characteristic temperature if such structures are used in a laser diode. Our experiments have shown that annealing at 760°C for 120s is insufficient to totally eliminate the nitrogen-related localized states, which may require a higher temperature anneal process.

Strain interactions and defect formation in stacked InGaAs quantum dot and dot-in-well structures

The growth of high-quality stacked quantum dot (QD) structures represents one of the key challenges for future device applications. Electronic coupling between QDs requires closely separated electronic levels and thin barrier layers, requiring near identical composition and shape, despite strong strain interactions. This paper presents a detailed characterization study of stacked InGaAs QD and InAs/InGaAs dot-in-well (DWELL) structures using cross-sectional transmission electron microscopy. For In .5Ga .5As/GaAs QD structures we have observed optimized stacking using a barrier thickness ∼12 nm. We also report studies of stacking in DWELL laser structures. Despite reports of very low threshold currents in such lasers, designed for 1.3 μm emission, performance is limited by gain saturation and thermal excitation effects. We have explored solutions to these problems by stacking multiple DWELL layers of three, five and 10 repeats. Initial attempts at stacked multilayer structures, particularly samples with a large number of repeats, produced variable results, with a number of the final devices characterized by poor emission and electrical characteristics. Analysis by transmission electron microscopy has identified the presence of large defective regions arising from the complex interaction of dots on several planes and propagating threading dislocations into the cladding layers. The origin of this defect is identified as the coalescence of QDs at very high density and the resulting dislocation propagating to higher dot planes. An effective modified method to reduce the defect density by growing the barrier layer at higher temperature will be discussed. Finally, we report the growth of a stacked 10-layer structure using relatively thin barriers, grown using this technique.

Effect of thermal excitation on intermolecular charge transfer efficiency in conducting polyaniline

A conductometric gas-sensing platform with temperature control has been adopted to probe temperature-dependent phenomena in conducting polymers. This letter describes our recent observation of thermal effects on the intermolecular charge transfer efficiency of conducting polyaniline at both ends of the charge transfer—either as a donor or as an acceptor, upon exposure to gaseous analytes. When polyaniline is serving as a donor, the thermal excitation of electrons in polyaniline facilitates the charge transfer. However, if polyaniline assumes the role of an acceptor, the charge transfer becomes less efficient at higher temperatures. Furthermore, a simplified band structure model is employed to elucidate such discriminative behavior.

An equation of state for molybdenum and tungsten

An equation of state (EOS) recently formulated by Shanker and Kushwah has been used for calculating the volumes of molybdenum (Mo) and tungsten (W) at simultaneously elevated pressures and temperatures. The results are obtained up to a pressure of 300 GPa and up to a temperature of nearly 7000 K. The values of thermal pressure have been calculated using the formulation given by Anderson and Isaak which takes into account the effect of thermal excitation of electrons. The Shanker–Kushwah EOS has been found most suitable for the materials for which the pressure derivative of isothermal bulk modulus at P=0, i.e. K 0′ is less than four. The solids Mo and W satisfy this condition. The values of volumes and isothermal bulk modulus calculated at different temperatures and pressures are found to present close agreement with the experimental data based on shock wave measurements reinforcing the validity of the equation of state used. The results based on the Shanker–Kushwah EOS are found to compare well with the corresponding values derived from the Vinet EOS.

Onset of magnetic anisotropy in epitaxial Fe films on GaAs(001)

It was shown previously that ultrathin Fe films epitaxially grown on GaAs(001) exhibit a strong in-plane uniaxial magnetic anisotropy which turns out to be a pure interface contribution with an anisotropy constant KUS, expressed as an energy per unit area, which is constant in a wide thickness range. However, for films thinner than ∼10 monolayers (ML), KUS decreases with decreasing thickness when measured at 300 K. In order to eliminate effects of thermal excitations, Fe(001) films grown on GaAs(001) by molecular beam epitaxy were investigated by superconducting quantum interference device magnetometry at low temperature. The extrapolated room temperature values and the ground state data both indicate that KUS vanishes at t=2.5 ML. This is the thickness at which the onset of ferromagnetism takes place, i.e., where the Curie temperature TC becomes nonzero. This might be interpreted as a verification of the Mermin–Wagner theorem in the sense that long-range ferromagnetic order is stabilized by the magnetic anisotropy. It is discussed whether the onset of ferromagnetism is indeed triggered by the appearance of magnetic anisotropy or if there is a common origin of both phenomena. Finally, it is found that the uniaxial anisotropy does not vanish at TC, but persists up to temperatures of ∼1.5 TC. This means that KUS does not scale with a certain power of the spontaneous magnetization. The disparity between the persistence of the anisotropy above TC and its disappearance below the critical thickness of 2.5 ML is discussed.

The Dawn of Bronchoscopy

The Dawn of Bronchoscopy

Carrier transport characteristics of small-size field effect transistors

The quantum wave transport of carriers driven by an external electric field (drain bias) is studied numerically in a typical GaAs/AlGaAs heterojunction field effect transistor as a function of the gate bias. We have shown that when the size of the transistor is reduced so that quantum effects become important, the operational characteristics of the transistor are dominated by the conductance quantization effect at infinitely small drain bias and zero temperature. The conductance quantization is diminished when the temperature is increased due to the thermal excitation effect. Normal I–V characteristics of the transistor performance are obtained for small-size field effect transistor where quantum wave transmission is the principal carrier transport mechanism.

Transport properties of CeTe 2

Two anomalous peaks of electrical resistivity around 65 and 5.6 K have already been reported for CeTe 2. To make the origin of the anomalies clear, the magnetoresistivity, electrical resistivity and Hall coefficient in CeTe 2 are measured. From these measurements, the carrier concentration and the mobilities of the electron and hole are obtained. The former peak around 65 K in the resistivity seems to be due to the competition between a thermal excitation effect in the low carrier system of p-f mixing. The latter peak around 5.6 K seems to be due to a pair Wigner crystal, charge-density wave, magnetic scattering, and so on. In addition, the carrier concentration of CeTe 2 as an extremely low carrier system is estimated to be about 10 20−10 21 m −3.

Nonlinear Meissner effect in unconventional superconductors.

We examine the long-wavelength current response in anisotropic superconductors and show how the field-dependence of the Meissner penetration length can be used to detect the structure of the order parameter. Nodes in the excitation gap lead to a nonlinear current-velocity constitutive equation at low temperatures which is distinct for each symmetry class of the order parameter. The effective Meissner penetration length is linear in $H$ and exhibits a characteristic anisotropy for fields in the $ab$-plane that is determined by the positions of the nodes in momentum space. The nonlinear current-velocity relation also leads to an intrinsic magnetic torque for in-plane fields that are not parallel to a nodal or antinodal direction. The torque scales as $H^3$ for $T\rightarrow 0$ and has a characteristic angular dependence. We analyze the effects of thermal excitations, impurity scattering and geometry on the current response of a $d_{x^2-y^2}$ superconductor, and discuss our results in light of recent measurements of the low-temperature penetration length and in-plane magnetization of single-crystals of $YBa_2Cu_3O_{7-\delta}$ and $LuBa_2Cu_3O_{7-\delta}$.

Mechanisms of Surface Reaction in Fluorocarbon Dry Etching of Silicon Dioxide-An Effect of Thermal Excitation

Ion bombardment-induced thermal reaction between a fluorocarbon adlayer and a SiO2 surface in a reactive-ion-etching (RIE) environment which was simulated in an ultrahigh-vacuum thermal desorption mass spectroscopy (TDS) apparatus has been studied. The RIE-induced fluorocarbon chemisorbed layer, covering the SiO2 surface, was observed to be thermally stimulated to react chemically with the SiO2 in the TDS apparatus with an activation energy of approximately 1.9 eV. A terminal group, chemisorbed at the adlayer/SiO2 interface, could be an active participant in the thermal reaction. This observation indicates the possibility that chemical sputtering could occur in the actual RIE through a thermal excitation step, induced by ion bombardment. A significant difference in the RIE-induced mixing of fluorine atoms between SiO2 and Si also appeared in their TDS spectra.

Contributing bands to the optical absorption of reduced LiNbO3: thermal and optical excitation

The effect of thermal and optical excitation on the optical absorption band of reduced LiNbO3 has been investigated for crystals with different Li/Nb ratios. Fe- and Cu-doped samples were also studied. Although the main effect of the excitation is to enhance the infrared part of the spectra at the cost of the ultraviolet-visible region, a number of new features appear depending on stoichiometry and doping. In all cases, the effect of thermal excitation is reversible. The spectra for all undoped samples, before as well as after the various treatments were fitted with four gaussian bands peaking at 1.6, 2.3, 3.1 and 3.9 eV. Additional bands have to be used to fit the spectra for doped crystals (2.5 eV for Fe-doping and 2.8 eV and 3.3 eV for Cu-doping). The possible origin of the bands is discussed in relation to other available information, but a definite assignment cannot be offered yet.

Energy averaging and the flux-periodic phenomena in small normal-metal rings

The low-frequency ac conductivity, \ensuremath{\sigma}, is evaluated numerically with use of the Kubo formula with inelastic lifetimes for a small one-dimensional (1D) closed metallic ring with random potentials. The effects of thermal excitations at finite temperatures and of increasing inelastic scattering are considered. It is found that energy averaging over a range larger than the typical level spacing (in 1D) markedly reduces the fundamental (hc/e) and odd harmonic periodicities of \ensuremath{\sigma} as a function of the Aharonov-Bohm flux, \ensuremath{\varphi}, through the ring. Thus, energy averaging makes the lowest even harmonic (hc/2e) appear to be the fundamental period.

Competitive two-channel photodissociation of n-butylbenzene ions in the Fourier-transform ion cyclotron resonance mass spectrometer

The photochemistry of n-butylbenzene ions was studied in the ion cyclotron resonance ion trap using Fourier-transform and double-resonance techniques. The branching ratio for formation of the m/e 92 and 91 product ions was determined at several wavelengths. The effect of thermal excitation of the ions was analyzed, and values were derived for the branching ratio as a function of ion internal energy. The results are in agreement with dissociative charge-transfer ionization experiments, after similar correction for thermal excitation, but previous ion-beam photodissociation results are badly divergent as a result of large excess internal energy of the parent ions. RRKM calculations of the branching ratio suggest that the dissociation behavior is consistent with a quasiequilibrium-theory description, and that the activation energy for m/e 91 formation is 0.6 eV higher than that for m/e 92 formation. The actual values of these activation energies are less well determined, but the results are consistent with a value of 1.1 eV for m/e 92 formation, in agreement with existing thermochemical information, assuming a methylenecyclohexadiene product ion structure.