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Magnetoresistance of One-Dimensional Subbands in Tunnel-Coupled Double Quantum Wires

Description: We study the low-temperature in-plane magnetoresistance of tunnel-coupled quasi-one-dimensional quantum wires. The wires are defined by two pairs of mutually aligned split gates on opposite sides of a < 1 micron thick AlGaAs/GaAs double quantum well heterostructure, allowing independent control of their widths. In the ballistic regime, when both wires are defined and the field is perpendicular to the current, a large resistance peak at ~6 Tesla is observed with a strong gate voltage dependence. The data is consistent with a counting model whereby the number of subbands crossing the Fermi level changes with field due to the formation of an anticrossing in each pair of 1D subbands.
Date: April 27, 1999
Creator: Blount, M.A.; Lyo, S.K.; Moon, J.S.; Reno, J.L.; Simmons, J.A. & Wendt, J.R.
Partner: UNT Libraries Government Documents Department

Electrically inactive poly-silicon grain boundaries

Description: Structures, energies, and electronic properties of symmetric [001] tilt grain boundaries in Si have been studied using Stillinger-Weber and Tersoff classical potentials, and semi-empirical (tight-binding) electronic structure methods. The calculated lowest energy (310) grain boundary structure and electronic properties are consistent with previous TEM measurement and calculations. For the controversial (710) grain boundaries, the tight-binding calculations do not show any electronic energy levels in the band gap. This indicates that with every atom fully fourfold coordinated, the (710) grain boundary should be electrically inactive. Some high-energy metastable grain boundaries were found to be electrically active by the presence of the levels introduced in the band gap. Also, the vacancy concentration at the (310) GB was found to be enhanced by many orders of magnitude relative to bulk. The dangling bond states of the vacancies should be electrically active.
Date: May 1, 1996
Creator: Chen, S.P.; Kress, J.D.; Voter, A.F. & Albers, R.C.
Partner: UNT Libraries Government Documents Department

Interband cascade light emitting devices based on type-II quantum wells

Description: The authors discuss physical processes in the newly developed type-II interband cascade light emitting devices, and review their recent progress in the demonstration of the first type-II interband cascade lasers and the observation of interband cascade electroluminescence up to room temperature in a broad mid-infrared wavelength region (extended to 9 {mu}m).
Date: June 1, 1997
Creator: Yang, Rui Q.; Lin, C.H. & Murry, S.J.
Partner: UNT Libraries Government Documents Department

Photoemission in strongly correlated crystalline f-electron systems: A need for a new approach

Description: The unusual properties of heavy fermion (or heavy electron) materials have sparked an avalanche of research over the last two decades in order to understand the basic phenomena responsible for these properties. Photoelectron spectroscopy (often referred to as PES in the following sections), the most direct measurement of the electronic structure of a material, should in principle be able to shed considerable light on this matter. In general the distinction between a localized and a band-like state is trivially observed in band dispersion. Much of the past work was performed on poly-crystalline samples, scraped in-situ to expose a clean surface for PES. There have since been considerable advances both in the quality of specimens as well as experimental resolution, which raise questions regarding these conclusions. Much of the past work on poly-crystalline samples has been reported in several review articles, most notably Allen et al., and it is not necessary here to review those efforts again, with the exception of subsequent work performed at high resolution. The primary focus of the present review will be on new measurements obtained on single crystals, cleaved or prepared in situ and measured at high resolution, which seem to suggest that agreement with the GS and NCA approximations is less than perfect, and that perhaps the starting models need to be modified, or that even an entirely new approach is called for. Of the promising new models the Periodic Anderson Model is most closely related to the SIM. Indeed, at high temperatures it reverts to the SIM. However, the charge polaron model of Liu (1997) as well as the two-electron band model of Sheng and Cooper (1995) cannot yet be ruled out. Inasmuch as the bulk of the single crystal work was performed by the Los Alamos group, this review will draw heavily on ...
Date: December 1, 1998
Creator: Arko, A.J.; Joyce, J.J. & Sarrao, J.
Partner: UNT Libraries Government Documents Department

Study of cyclotron resonance and magneto-photoluminescence of n-type modulation doped InGaAs quantum well layers and their characterizations

Description: Two-dimensional natures of energy-band and the effective mass of conduction subband in narrow InGaAs/InAlAs quantum well layers have been clarified via magneto-photoluminescence, cyclotron resonance, Shubnikov-de Haas oscillations and quantum Hall effect, interband optical transmittance, and photoluminescence. Heavy effective masses of 0.07m{sub 0} were determined in 5- and 10-nm-wide quantum wells, which were 70% larger than the bulk bandedge mass, 0.041m{sub 0}. Sheet carrier concentration in the quantum wells was as high as 1 x 10{sup 12} cm{sup {minus}2}.
Date: June 1, 1997
Creator: Kotera, N.; Tanaka, K. & Jones, E.D.
Partner: UNT Libraries Government Documents Department

Spectral ellipsometry of GaSb: Experiment and modelling

Description: The optical constants {epsilon}(E)[{equals}{epsilon}{sub 1}(E) + i{epsilon}{sub 2}(E)] of single crystal GaSb at 300K have been measured using spectral ellipsometry in the range of 0.3--5.3 eV. The {epsilon}(E) spectra displayed distinct structures associated with critical points (CPs) at E{sub 0}(direct gap), spin-orbit split E{sub 0} + {Delta}{sub 0} component, spin-orbit split (E{sub 1}), E{sub 1} + {Delta}{sub 1} and (E{sub 0}{prime}), E{sub 0}{prime} + {Delta}{sub 0}{prime} doublets, as well as E{sub 2}. The experimental data over the entire measured spectral range (after oxide removal) has been fit using the Holden model dielectric function [Phys.Rev.B 56, 4037 (1997)] based on the electronic energy-band structure near these CPs plus excitonic and band-to-band Coulomb enhancement effects at E{sub 0}, E{sub 0} + {Delta}{sub 0}and the E{sub 1}, E{sub 1} + {Delta}{sub 1} doublet. In addition to evaluating the energies of these various band-to-band CPs, information about the binding energy (R{sub 1}) of the two-dimensional exciton related to the E{sub 1}, E{sub 1} + {Delta}{sub 1} CPS was obtained. The value of R{sub 1} was in good agreement with effective mass/{rvec k} {center_dot} {rvec p} theory. The ability to evaluate R{sub 1} has important ramifications for recent first-principles band structure calculations which include exciton effects at E{sub 0}, E{sub 1}, and E{sub 2}.
Date: May 1, 1999
Creator: Charache, G.W.; Mu {tilde n}oz, M.; Wei, K.; Pollak, F.H. & Freeouf, J.L.
Partner: UNT Libraries Government Documents Department

Exciton and biexciton signatures in the femtosecond transient absorption of phenylene-based polymers and oligomers

Description: The femtosecond transient absorption of phenylene-based polymers at low intensities is dominated by singlet excitons. In films, inter-chain excitons are created at high intensities via two-exciton states. In solution, the authors observe signatures of stable biexcitons.
Date: March 1, 1998
Creator: McBranch, D.; Klimov, V. & Kraabel, B.
Partner: UNT Libraries Government Documents Department

A combined experimental and theoretical approach to atomic scale characterization

Description: Recently, the scanning transmission electron microscope has become capable of forming electron probes of atomic dimensions. Through the technique of Z-contrast imaging, it is now possible to form atomic resolution images with high compositional sensitivity from which atomic column positions can be directly determined. An incoherent image of this nature also allows atomic resolution chemical analysis to be performed, by locating the probe over particular columns or planes seen in the image while electron energy loss spectra are collected. Such data represents either an ideal starting point for first principles theoretical calculations or a test of theoretical predictions. The authors present several examples where theory and experiment together give a very complete and often surprising atomic scale view of complex materials.
Date: February 1, 1998
Creator: Pennycook, S.J.; Chisholm, M.F.; Yan, Y.; Duscher, G. & Pantelides, S.T.
Partner: UNT Libraries Government Documents Department

Implications of Hund`s second rule for the insulating behavior of FeO and CoO

Description: A formal expression for Hund`s second rule correlations is applied to energy band calculations for FeO, CoO, and NiO. In all 3 cases, insulating gaps are found, in contrast to standard band calculations, where an insulating gap is only found for NiO. The calculated orbital moments, though, are too large, due to the correction being based on the atomic limit. 2 tabs, 12 refs.
Date: November 1, 1990
Creator: Norman, M. R.
Partner: UNT Libraries Government Documents Department

Band structure parameters for quantum wells: Magnetoluminescence determinations

Description: We report on low-temperature magnetoluminescence determinations of bandstructure parameters for an 8.5nm-wide n-type In{sub 0.20}Ga{sub 0.80}As/GaAs strained single-quantum well structure. We find that the conduction-band is almost parabolic, with its mass varying from 0.067m{sub 0} at zone center to 0.069m{sub 0} at a Fermi energy E{sub f} {approx} 50 meV, and that the valence-bands are highly non-parabolic with the valence band masses m, varying from about 0.1 m{sub 0} at zone center to about 0.3m{sub 0} for large k-vectors. A comparison with a k x p calculation for the valence-band mass and dispersion curve show good agreement. An accurate and unambiguous determination for the band-gap energy E{sub gap} is made by extrapolating the magnetic field dependent Landau level transition energies to zero field.
Date: April 1, 1997
Creator: Jones, E.D.
Partner: UNT Libraries Government Documents Department

Electronic states in Cd{sub 1{minus}x}Zn{sub x}Te/CdTe strained layer coupled double quantum wells and their photoluminescence

Description: Experimental and theoretical investigation of electronic states in a strained-layer CdTe/CdZnTe coupled double quantum well structure are presented. The optical properties of this lattice-mismatched heterostructure were characterized with photoluminescence (PL), PL excitation and polarization spectroscopies. Influence of electrical field on exciton states in the strained-layer CdTe/CdZnTe coupled double quantum well structure is experimentally studied. The confined electronic states were calculated in the framework of the envelope function approach, taking into account the strain effect induced by the lattice-mismatch. Experimental results are compared with the calculated transition energies.
Date: December 31, 1994
Creator: Li, T.; Lozykowski, H.J. & Reno, J.
Partner: UNT Libraries Government Documents Department

Strongly correlated f-electron systems: A PES study

Description: The term heavy fermions refers to materials (thus far only compounds with elements having an unfilled 4f or 5f shells) whose large specific heat {gamma}-values suggest that the conduction electrons at low temperatures have a very heavy effective mass. Magnetic susceptibility measurements, {chi}, generally yield a Curie-Weiss behavior at high temperatures with a well developed moment, which would be consistent with localized behavior of the f-electrons. Thus, the f-electrons appear to behave as non-interacting single impurities at elevated temperature. Below a characteristic Kondo temperature, T{sub K}, the susceptibility levels off or even decreases. This is interpreted as a compensation of the f-moment by the ligand conduction electrons that are believed to align anti-parallel to form a singlet state and has led to the widespread use of the Anderson Impurity Hamiltonian and the Single Impurity Model (SIM). Weak hybridization with these conduction electrons yields a narrow, highly temperature dependent, DOS at the Fermi energy, often referred to as the Kondo resonance (KR). At still lower temperatures it is generally agreed that in stoichiometric compounds a lattice of these singlet states finally results in extremely narrow bands at the Fermi energy, whose bandwidth is of the order k{sub B}T{sub K}. Clearly coherent bands cannot form above T{sub K} owing to the narrow width. A model for periodic Kondo systems will inevitably have to include the lattice. Preliminary PAM calculations indicate that this inclusion yields results differing qualitatively, rather than just quantitatively, from the SIM predictions. The photoemission data on single crystal heavy fermions are consistent with the following PAM predictions: (1) the temperature dependence of the KR is much slower than expected from the SIM; indeed, it is primarily7 due to broadening and Fermi function truncation; (2) the spectral weight of the KR relative to the localized 4f feature (not discussed here) ...
Date: December 31, 1998
Creator: Arko, A.J.; Joyce, J.J.; Sarrao, J.; Thompson, J.D.; Morales, L.; Fisk, Z. et al.
Partner: UNT Libraries Government Documents Department

Multi-band Bloch equations and gain spectra of highly excited II-VI semiconductor quantum wells

Description: Quasi-equilibrium excitation dependent optical probe spectra of II-VI semiconductor quantum wells at room temperature are investigated within the framework of multi-band semiconductor Bloch equations. The calculations include correlation effects beyond the Hartree-Fock level which describe dephasing, interband Coulomb correlations and band-gap renormalization in second Born approximation. In addition to the carrier-Coulomb interaction, the influence of carrier-phonon scattering and inhomogeneous broadening is considered. The explicit calculation of single particle properties like band structure and dipole matrix elements using k {center_dot} p theory makes it possible to investigate various II-VI material combinations. Numerical results are presented for CdZnSe/ZnSe and CdZnSe/MnZnSSe semiconductor quantum-well systems.
Date: April 21, 1997
Creator: Girndt, A.; Jahnke, F.; Knorr, A.; Koch, S.W. & Chow, W.W.
Partner: UNT Libraries Government Documents Department

First principles calculations of interlayer exchange coupling in bcc Fe/Cu/Fe structures

Description: The authors report on theoretical calculations of interlayer exchange coupling between two Fe layers separated by a modified Cu spacer. These calculations were motivated by experimental investigations of similar structures by the SFU group. The multilayer structures of interest have the general form: Fe/Cu(k)/Fe and Fe/Cu(m)/X(1)/Cu(n)/Fe where X indicates one AL (atomic layer) of foreign atoms X (Cr, Ag, or Fe) and k, m, n represent the number of atomic layers of Cu. The purpose of the experimental and theoretical work was to determine the effect of modifying the pure Cu spacer by replacing the central Cu atomic layer with the atomic layer of foreign atoms X. The first principles calculation were performed using the Layer Korringa-Kohn-Rostoker (LKKR) method. The theoretical thickness dependence of the exchange coupling between two semi-infinite Fe layers was calculated for pure Cu spacer thicknesses in the range of 0 < k < 16. The effect of the foreign atoms X on the exchange coupling was investigated using the structure with 9 AL Cu spacer as a reference sample. The calculated changes in the exchange coupling are in qualitative agreement with experiment.
Date: January 1, 1998
Creator: Kowalewski, M.; Heninrich, B.; Schulthess, T.C. & Butler, W.H.
Partner: UNT Libraries Government Documents Department

Electronic excitations and correlation effects in metals

Description: Theoretical descriptions of the spectrum of electronic excitations in real metals have not yet reached a fully predictive, first-principles stage. In this paper the authors begin by presenting brief highlights of recent progress made in the evaluation of dynamical electronic response in metals. A comparison between calculated and measured spectra--they use the loss spectra of Al and Cs as test cases--leads them to the conclusion that, even in weakly-correlated metals, correlation effects beyond mean-field theory play an important role. Furthermore, the effects of the underlying band structure turn out to be significant. Calculations which incorporate the effects of both dynamical correlations and band structure from first principles are not yet available. As a first step towards such goal, they outline a numerical algorithm for the self-consistent solution of the Dyson equation for the one-particle Green`s function. The self-energy is evaluated within the shielded-interaction approximation of Baym and Kadanoff. Their method, which is fully conserving, is a finite-temperature scheme which determines the Green`s function and the self-energy at the Matsubara frequencies on the imaginary axis. The analytical continuation to real frequencies is performed via Pade` approximant. They present results for the homogeneous electron gas which exemplify the importance of many-body self-consistency.
Date: July 1, 1997
Creator: Eguiluz, A.G. & Schoene, W.D.
Partner: UNT Libraries Government Documents Department

Optical constants of Ga{sub 1{minus}x}In{sub x}As{sub y}Sb{sub 1{minus}y} lattice-matched to GaSb(001): Experiment and modeling

Description: The optical constants {epsilon}(E)[={epsilon}{sub 1}(E)+i{epsilon}{sub 2}(E)] of two epitaxial layers of GaInAsSb/GaSb have been measured at 300 K using spectral ellipsometry in the range of 0.35--5.3 eV. The {epsilon}(E) spectra displayed distinct structures associated with critical points (CPs) at E{sub 0} (direct gap), spin-orbit split E{sub 0}+{Delta}{sub 0} component, spin-orbit split (E{sub 1}, E{sub 1}+{Delta}{sub 1}) and (E{sub 0}{prime}, E{sub 0}{prime}+{Delta}{sub 0}{prime}) doublets, as well as E{sub 2}. The experimental data over the entire measured spectral range (after oxide removal) has been fit using the Holder model dielectric function [Phys.Rev.B 56, 4037 (1997)] based on the electronic energy-band structure near these CPs plus excitonic and band-to-band Coulomb enhancement effects at E{sub 0.}, E{sub 0}+{Delta}{sub 0} and the E{sub 1}, E{sub 1}+{Delta}{sub 1} doublet. In addition to evaluating the energies of these various band-to-band CPs, information about the binding energy (R{sub 1}) of the two-dimensional exciton related to the E{sub 1}, E{sub 1}+{Delta}{sub 1} CPs was obtained. The value of R{sub 1} was in good agreement with effective mass/k{sup {rightharpoonup}}{center_dot}p{sup {rightharpoonup}} theory. The ability to evaluate R{sub 1} has important ramifications for recent first-principles band structure calculations which include exciton effects at E{sub 0}, E{sub 1}, and E{sub 2} [M.Rohlfing and S.G.Louie, Phys.Rev.Lett. 81, 2312 (1998) and S. Albrecht et al., Phys.Rev.Lett. 80, 4510 (1998)]. The experimental absorption coefficients in the region of E{sub 0} were in good agreement with values obtained from a linear interpolation of the end point materials. The experimental results were compared to a recent evaluation and fitting (Holden model) of the optical constants of GaSb.
Date: June 1999
Creator: Charache, G. W.; Muñoz, M.; Wei, K.; Pollak, F. H.; Freeouf, J. L. & Wang, C. A.
Partner: UNT Libraries Government Documents Department

Photoluminescence-linewidth-derived exciton mass for InGaAsN alloys

Description: The authors report a measurement of the variation of the value of the linewidth of an excitonic transition in InGaAsN alloys (1 and 2% nitrogen) as a function of hydrostatic pressure using photoluminescence spectroscopy. The samples were grown by metal-organic chemical vapor deposition and the photoluminescence measurements were performed a 4K. The authors find that the value of the excitonic linewidth increases as a function of pressure until about 100 kbars after which it tends to saturate. This change in the excitonic linewidth is used to derive the pressure variation of the reduced mass of the exciton using a theoretical formalism which is based on the premise that the broadening of the excitonic transition is caused primarily by compositional fluctuations in a completely disordered alloy. The variation of the excitonic reduced mass thus derived is compared with that recently determined using a first-principles band structure calculation based on local density approximation.
Date: January 27, 2000
Creator: Jones, Eric D.; Allerman, Andrew A.; Kurtz, Steven R.; Modine, Normand A.; Bajaj, K. K.; Tozer, S. T. et al.
Partner: UNT Libraries Government Documents Department

Characteristic features of the exotic superconductors: A summary

Description: The authors summarize the results of a comprehensive examination of the characteristic features of the exotic superconductors, the superconductors so-labelled by Uemura and co-workers. In both the electronic and the crystal-chemistry properties, they find anomalous features which appear to be universal for these materials, as well as other features which are clearly not universal but common enough to be considered typical for these materials. Some implications of these anomalies are discussed.
Date: September 1, 1997
Creator: Brandow, B.
Partner: UNT Libraries Government Documents Department

First-principles exchange interactions between ferro and antiferromagnetic films: Co on NiMn, a case study

Description: Heisenberg exchange parameters at the interface of antiferromagnetic NiMn with ferromagnetic Co are calculated from first-principles. The authors use a layer version of the Korringa-Kohn-Rostocker multiple scattering approach and an expression, which is based on the expansion of the band energy, to calculate the exchange parameters from the underlying electronic structure. For bulk systems, the parameter sets yield Curie temperatures that are in good agreement with experimental values. In the interface region, the inter-layer interactions in NiMn change significantly compared to the bulk while the intra-layer interactions are almost unchanged.
Date: December 31, 1997
Creator: Schulthess, T.C. & Butler, W.H.
Partner: UNT Libraries Government Documents Department

Spectroscopic ellipsometry and band structure of Si{sub 1{minus}y}C{sub y} alloys grown pseudomorphically on Si (001)

Description: The authors have measured the dielectric functions of three Si{sub 1{minus}y}C{sub y} alloys layers (y {le} 1.4%) grown pseudomorphically on Si (001) substrates using molecular beam epitaxy at low temperatures. From the numerical derivatives of the measured spectra, they determine the critical point energies E{sub 0}{prime} and E{sub 1} as a function of y (y {le} 1.4%) using a comparison with analytical line shapes and analyze these energies in terms of the expected shifts and splittings due to negative hydrostatic pressure, shear stress, and alloying. Their data agree well with the calculated shifts for E{sub 1}, but the E{sub 0}{prime} energies are lower than expected. They discuss their results in comparison with recent tight-binding molecular dynamics simulations by Demkov and Sankey predicting a total breakdown of the virtual-crystal approximation for such alloys.
Date: October 1, 1995
Creator: Zollner, S.; Herzinger, C.M.; Woollam, J.A.; Iyer, S.S.; Powell, A.P. & Eberl, K.
Partner: UNT Libraries Government Documents Department

Theoretical study of the structure and electronic properties of carbon and B{sub X}C{sub Y}N{sub Z} nanotubes

Description: Theoretical studies of the electronic and structural properties of carbon nanotubes and nanotubes composed of boron, carbon and nitrogen are presented. Structural stability, hybridization effects, static dielectric response, incorporation of metal atoms, and collapsed tube structures are calculated. Nanotubes of BN, BC{sub 3}, and BC{sub 2}N are predicted to form and have very different properties from those of the carbon systems. BN nanotubes are found to be constant band gap insulators with novel free-electron tubule states at the conduction band minimum. The possibility of chiral currents in doped BC{sub 2}N nanotubes is shown.
Date: June 1, 1995
Creator: Louie, S.G.
Partner: UNT Libraries Government Documents Department

Quasiparticle theory of electron excitations in solids

Description: A first-principles quasiparticle approach to electron excitation energies in solids is reviewed. The theory has been applied to explain and predict the spectroscopic properties of a variety of systems including bulk crystals, surfaces, interfaces, clusters, defects, and materials under pressure. Several illustrative applications are presented and some recent theoretical developments discussed.
Date: October 1, 1995
Creator: Louie, S. G.
Partner: UNT Libraries Government Documents Department

Resonant soft x-ray fluorescence studies of novel materials

Description: The authors are using resonant soft x-ray fluorescence at the Advanced Light Source to probe the electronic and geometric structure of novel materials. In the resonant process, a core electron is excited by a photon whose energy is near the core binding energy. In this energy regime the absorption and emission processes are coupled, and this coupling manifests itself in several ways. In boron nitride (BN), the resonant emission spectra reflect the influence of a ``spectator`` electron in an unoccupied excitonic state. The resonant emission can be used to distinguish between the various bulk phases of BN, and can also be used to probe the electronic structure of a monolayer of BN buried in a bulk environment, where it is inaccessible to electron spectroscopies. For highly-oriented pyrolytic graphite (HOPG) a coherent absorption-emission process takes place in the resonant regime, whereby crystalline momentum is conserved between the core excited electron and the valence hole which remains after emission.
Date: February 8, 1995
Creator: Carlisle, J.A.; Terminello, L.J.; Hudson, E.A.; Shirley, E.L.; Jia, J.J.; Callcott, T.A. et al.
Partner: UNT Libraries Government Documents Department

Electronic structure of C{sub 60} fullerites and nanotubes

Description: The authors present results on the electron excitation energies of solid C{sub 60} calculated using a first-principles quasiparticle theory. The effects of electron correlations and molecular orientational disorders are included. The single-particle properties of solid C{sub 60} are found to be well described in a quasiparticle band picture with orientation disorders. Electron-hole interactions are shown to be very strong in optical processes resulting in Frenkel excitons and sizable difference between the optical and photoemission gaps. The electronic and structural properties of carbon and other nanotubes are also investigated. Total energy results predict that the tubules are stable with respect to the formation of strips down to very small radii. Hybridization of the {sigma}{sup *} and {pi}{sup *} states is shown to be as important as band-folding effects in determining the metallicity of small radius tubules.
Date: May 1, 1994
Creator: Louie, S. G.
Partner: UNT Libraries Government Documents Department