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A universal high energy anomaly in angle resolved photoemissionspectra of high temperature superconductors -- possible evidence ofspinon and holon branches

Description: A universal high energy anomaly in the single particlespectral function is reported in three different families of hightemperature superconductors by using angle-resolved photoemissionspectroscopy. As we follow the dispersing peak of the spectral functionfrom the Fermi energy to the valence band complex, we find dispersionanomalies marked by two distinctive high energy scales, E_1 approx 0.38eV and E_2 approx 0.8 eV. E_1 marks the energy above which the dispersionsplits into two branches. One is a continuation of the near parabolicdispersion, albeit with reduced spectral weight, and reaches the bottomof the band at the Gamma point at approx 0.5 eV. The other is given by apeak in the momentum space, nearly independent of energy between E_1 andE_2. Above E_2, a band-like dispersion re-emerges. We conjecture thatthese two energies mark the disintegration of the low energyquasiparticles into a spinon and holon branch in the high T_c cuprates.
Date: December 19, 2006
Creator: Graf, J.; Gweon, G.-H.; McElroy, K.; Zhou, S.Y.; Jozwiak, C.; Rotenberg, E. et al.
Partner: UNT Libraries Government Documents Department

Recombination and propagation of quasiparticles in cuprate superconductors

Description: Rapid developments in time-resolved optical spectroscopy have led to renewed interest in the nonequilibrium state of superconductors and other highly correlated electron materials. In these experiments, the nonequilibrium state is prepared by the absorption of short (less than 100 fs) laser pulses, typically in the near-infrared, that perturb the density and energy distribution of quasiparticles. The evolution of the nonequilibrium state is probed by time resolving the changes in the optical response functions of the medium that take place after photoexcitation. Ultimately, the goal of such experiments is to understand not only the nonequilibrium state, but to shed light on the still poorly understood equilibrium properties of these materials. We report nonequilibrium experiments that have revealed aspects of the cup rates that have been inaccessible by other techniques. Namely, the diffusion and recombination coefficients of quasiparticles have been measured in both YBa{sub 2}Cu{sub 3}O{sub 6.5} and Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+x} using time-resolved optical spectroscopy. Dependence of these measurements on doping, temperature and laser intensity is also obtained. To study the recombination of quasiparticles, we measure the change in reflectivity {Delta}R which is directly proportional to the nonequilibrium quasiparticle density created by the laser. From the intensity dependence, we estimate {beta}, the inelastic scattering coefficient and {gamma}{sub th} thermal equilibrium quasiparticle decay rate. We also present the dependence of recombination measurements on doping in Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+x}. Going from underdoped to overdoped regime, the sign of {Delta}R changes from positive to negative right at the optimal doping. This is accompanied by a change in dynamics. The decay of {Delta}R stops being intensity dependent exactly at the optimal doping. We provide possible interpretations of these two observations. To study the propagation of quasiparticles, we interfered two laser pulses to introduce a spatially periodic density of quasiparticles. Probing the evolution ...
Date: May 20, 2004
Creator: Gedik, Nuh
Partner: UNT Libraries Government Documents Department

Magneto-optical Signatures of a Cascade of Transitions in La 1.875Ba 0.125CuO 4

Description: Recent experiments on the original cuprate high temperature superconductor, La{sub 2-x}Ba{sub x}CuO{sub 4}, revealed a remarkable sequence of phase transitions. Here we investigate such crystals with polar Kerr effect which is sensitive to time-reversal-symmetry breaking. Concurrent birefringence measurements accurately locate the structural phase transitions from high-temperature tetragonal to low temperature orthorhombic, and then to lower temperature tetragonal, at which temperature a strong Kerr signal onsets. Hysteretic behavior of the Kerr signal suggests that time-reversal symmetry is already broken well above room temperature, an effect that was previously observed in high quality YBa{sub 2}Cu{sub 3}O{sub 6+x} crystals.
Date: September 14, 2012
Creator: Karapetyan, Hovnatan; Hucker, M.; Gu, G.D.; Tranquada, J.M.; Fejer, M.M.; Xia, Jing et al.
Partner: UNT Libraries Government Documents Department

Universal versus Materials-Dependent Two-Gap Behaviour of the High-Tc Cuprate Superconductors: Angle-Resolved Photoemission Study of La2-xSrxCuO4

Description: We have investigated the doping and temperature dependences of the pseudogap and superconducting gap in the single-layer cuprate La{sub 2-x}Sr{sub x}CuO{sub 4} by angle-resolved photoemission spectroscopy. The results clearly exhibit two distinct energy and temperature scales, namely, the gap around ({pi}, 0) of magnitude {Delta}* and the gap around the node characterized by the d-wave order parameter {Delta}{sub 0}. In comparison with Bi2212 having higher T{sub c}'s, {Delta}{sub 0} is smaller, while {Delta}* and T* are similar. This result suggests that {Delta}* and T* are approximately material-independent properties of a single CuO{sub 2} plane, in contrast to the material-dependent {Delta}0, representing the pairing strength.
Date: November 30, 2008
Creator: Yoshida, T.; Hashimoto, Makoto; Ideta, S.; Fujimori, Atsushi; Tanaka, K.; Mannella, Norman et al.
Partner: UNT Libraries Government Documents Department


Description: The 2010 Gordon Conference on Correlated Electron Systems will present cutting-edge research on emergent properties arising from strong electronic correlations. The Conference will feature a wide range of topics, such as the role of topology in condensed matter systems, quantum Hall interferometry and non-Abelian statistics, quantum criticality, metal-insulator transition, quantum effects in conductivity, Dirac quasiparticles, and superconductivity in cuprates and pnictides. In addition, we are reserving two sessions for new developments in this field that may arise in the coming year. The Conference will bring together a collection of investigators who are at the forefront of their field, and will provide opportunities for junior scientists and graduate students to present their work in poster format and exchange ideas with leaders in the field. We intend to have talks by established leaders in the field and also by young researchers who have made seminal contributions to various aspects of correlated electron physics, The collegial atmosphere of this Conference, with programmed discussion sessions as well as opportunities for informal gatherings in the afternoons and evenings, provides an avenue for scientists from different disciplines to brainstorm and promotes cross-disciplinary collaborations in the various research areas represented.
Date: June 18, 2010
Creator: Basov, Dmitri
Partner: UNT Libraries Government Documents Department

2011 Superconductivity Gordon Research Conference (June 5-10, 2011, Waterville Valley Resort, Waterville Valley, New Hampshire)

Description: The 2011 Gordon Research Conference on Superconductivity will commemorate the 100th anniversary of the discovery of superconductivity by providing a forum for discussion of the latest experimental and theoretical advances in this field. The conference will bring together experts to address the current challenges in understanding correlated superconductors - from cuprates and pnictides to heavy fermion superconductors. The fundamental mechanisms of superconducting pairing, the underlying explanations for thermodynamic phase diagrams including potential importance of competing phases, the correspondence between these phenomena, and the transport and spectroscopic properties of these materials will be among the themes of the conference. We will also discuss the feasibility of using lessons learned from the study of known superconductors as a guide to the future discovery of novel and higher temperature superconductors. Speakers will be strongly encouraged to present new, unpublished work, which will ensure that discussions evoke and explore new research directions. The participation of young scientists at the graduate student or post-doctoral level will be encouraged by the offering of selected presentations, focused discussions with invited speakers, and poster sessions. In addition, the organizers have earmarked funds to facilitate attendance of members of groups underrepresented in science and engineering.
Date: June 10, 2011
Creator: Yazdani, Ali
Partner: UNT Libraries Government Documents Department

Evidence for weak electronic correlations in Fe-pnictides

Description: Using x-ray absorption and resonant inelastic x-ray scattering, charge dynamics at and near the Fe L edges is investigated in Fe pnictide materials, and contrasted to that measured in other Fe compounds. It is shown that the XAS and RIXS spectra for 122 and 1111 Fe pnictides are each qualitatively similar to Fe metal. Cluster diagonalization, multiplet, and density-functional calculations show that Coulomb correlations are much smaller than in the cuprates, highlighting the role of Fe metallicity and strong covalency in these materials. Best agreement with experiment is obtained using Hubbard parameters U {approx}< 2eV and J {approx} 0.8eV.
Date: April 29, 2010
Creator: Yang, W.L.
Partner: UNT Libraries Government Documents Department

Angle-resolved photoemission studies of lattice polaron formation in the cuprate Ca2CuO2Cl2

Description: To elucidate the nature of the single-particle excitations in the undoped parent cuprates, we have performed a detailed study of Ca{sub 2}CuO{sub 2}Cl{sub 2} using photoemission spectroscopy. The photoemission lineshapes of the lower Hubbard band are found to be well-described by a polaron model. By comparing the lineshape and temperature dependence of the lower Hubbard band with additional O 2p and Ca 3p states, we conclude that the dominant broadening mechanism arises from the interaction between the photohole and the lattice. The strength of this interaction was observed to be strongly anisotropic and may have important implications for the momentum dependence of the first doped hole states.
Date: May 3, 2010
Creator: Shen, K.M.
Partner: UNT Libraries Government Documents Department

Distinguishing Patterns of Charge Order: Stripes or Checkerboards

Description: In two dimensions, quenched disorder always rounds transitions involving the breaking of spatial symmetries so, in practice, it can often be difficult to infer what form the symmetry breaking would take in the 'ideal,' zero disorder limit. We discuss methods of data analysis which can be useful for making such inferences, and apply them to the problem of determining whether the preferred order in the cuprates is 'stripes' or 'checkerboards.' In many cases we show that the experiments clearly indicate stripe order, while in others (where the observed correlation length is short), the answer is presently uncertain.
Date: April 6, 2010
Creator: Robertson, J.A.
Partner: UNT Libraries Government Documents Department

Doping evolution of the electronic structure in the single-layer cuprates Bi2Sr2&#8722xLaxCuO6 delta: Comparison with other single-layer cuprates

Description: We have performed angle-resolved photoemission and core-level x-ray photoemission studies of the single-layer cuprate Bi{sub 2}Sr{sub 2-x}La{sub x}CuO{sub 6+{delta}} (Bi2201) and revealed the doping evolution of the electronic structure from the lightly-doped to optimally-doped regions. We have observed the formation of the dispersive quasi-particle band, evolution of the Fermi 'arc' into the Fermi surface and the shift of the chemical potential with hole doping as in other cuprates. The doping evolution in Bi2201 is similar to that in Ca{sub 2-x}Na{sub x}CuO{sub 2}Cl{sub 2} (Na-CCOC), where a rapid chemical potential shift toward the lower Hubbard band of the parent insulator has been observed, but is quite different from that in La{sub 2-x}Sr{sub x}CuO{sub 4} (LSCO), where the chemical potential does not shift, yet the dispersive band and the Fermi arc/surface are formed around the Fermi level already in the lightly-doped region. The (underlying) Fermi surface shape and band dispersions are quantitatively analyzed using tightbinding fit, and the deduced next-nearest-neighbor hopping integral t also confirm the similarity to Na-CCOC and the difference from LSCO.
Date: April 30, 2010
Creator: Hashimoto, M.
Partner: UNT Libraries Government Documents Department


Description: One type of order that has been observed to compete with superconductivity in cuprates involves alternating charge and antiferromagnetic stripes. Recent neutron scattering studies indicate that the magnetic excitation spectrum of a stripe-ordered sample is very similar to that observed in superconducting samples. In fact, it now appears that there may be a universal magnetic spectrum for the cuprates. One likely implication of this universal spectrum is that stripes of a dynamic form are present in the superconducting samples. On cooling through the superconducting transition temperature, a gap opens in the magnetic spectrum, and the weight lost at low energy piles up above the gap; the transition temperature is correlated with the size of the spin gap. Depending on the magnitude of the spin gap with respect to the magnetic spectrum, the enhanced magnetic scattering at low temperature can be either commensurate or incommensurate. Connections between stripe correlations and superconductivity are discussed.
Date: August 22, 2005
Creator: TRANQUADA, J.M.
Partner: UNT Libraries Government Documents Department

Magic Doping Fractions in High-Temperature Superconductors

Description: We report hole-doping dependence of the in-plane resistivity {rho}{sub ab} in a cuprate superconductor La{sub 2-x}Sr{sub x}CuO{sub 4}, carefully examined using a series of high-quality single crystals. Our detailed measurements find a tendency towards charge ordering at particular rational hole doping fractions of 1/16, 3/32, 1/8, and 3/16. This observation appears to suggest a specific form of charge order and is most consistent with the recent theoretical prediction of the checkerboard-type ordering of the Cooper pairs at rational doping fractions x = (2m + 1)/2{sup n}, with integers m and n.
Date: January 15, 2010
Creator: Komiya, Seiki; Chen, Han-Dong; Zhang, Shou-Cheng & Ando, Yoichi
Partner: UNT Libraries Government Documents Department

A Twisted Ladder: Relating the Fe Superconductors to the High Tc Cuprates

Description: We construct a 2-leg ladder model of an Fe-pnictide superconductor and discuss its properties and relationship with the familiar 2-leg cuprate model. Our results suggest that the underlying pairing mechanism for the Fe-pnictide superconductors is similar to that for the cuprates.
Date: May 26, 2010
Creator: Berg, E.
Partner: UNT Libraries Government Documents Department

Parallel solution of the three-dimensional time-dependent Ginzburg- Landau equation

Description: The recent discovery of superconductivity in a class of copper-oxide compounds (the cuprate superconductors) at liquid nitrogen temperatures has generated a renewed interest in the magnetic properties of type-II superconductors. In our work, we are investigating these properties using the phenomenological time- dependent Ginzburg-Landau equation. This paper describes the parallelization of this equation.
Date: August 1, 1996
Creator: Galbreath, N.; Gropp, W.; Gunter, D.; Leaf, G. & Levine, D.
Partner: UNT Libraries Government Documents Department

Thermodynamic and nonstoichiometric behavior of the lead-doped and lead-free Bi-2212 systems

Description: EMF measurements of oxygen fugacities as a function of stoichiometry were made on pb-doped and Pb-free superconducting ceramics at 700-815 C using oxygen titration. Equations of oxygen partial pressure vs composition and temperature were derived from the EMF measurements. Thermodynamic assessments of the partial molar quantities {Delta}{bar H}(O{sub 2}) and {Delta}{bar S}(O{sub 2}) for Pb-doped Bi- 2212 and Pb-free Bi-2212 indicate that the solid-state decomposition of these Bi cuprates at low oxygen partial pressure can be represented by the diphasic CuO-Cu{sub 2}O system. Comparison of these results with Pb-doped Bi-2223 in powder and silver sheath form is presented.
Date: February 1, 1997
Creator: Tetenbaum, M.; Hash, M.; Tani, B.S.; Luo, J.S. & Maroni, V.A.
Partner: UNT Libraries Government Documents Department

Stability of Tl-Ba-Ca-Cu-O Superconducting Thin Films

Description: We report the stability of TlBa{sub 2}CaCu{sub 2}O{sub 7} (Tl-1212) and Tl{sub 2}Ba{sub 2}CaCu{sub 2}O{sub 8} (T1-2212) thin films and by inference, the stability of TlBa{sub 2}Ca{sub 2}Cu{sub 3}O{sub 9} (Tl-1223) and Tl{sub 2}Ba{sub 2}Ca{sub 2}Cu{sub 3}O{sub 10} (Tl-2223) thin films, under a variety of conditions. In general, we observe that the stability behavior of the single Tl-O layer materials (Tl-1212 and Tl-1223)are similar and the double Tl-O layer materials (Tl-2212 and Tl-2223) are similar. All films are stable with repeated thermal cycling to cryogenic temperatures. Films are also stable in acetone and methanol. Moisture degrades film quality rapidly, especially in the form of vapor. Tl-1212 is more sensitive to vapor than Tl-2212. These materials are stable to high temperatures in either N{sub 2}, similar to vacuum for the cuprates, and O{sub 2} ambients. While total degradation of properties (superconducting and structural) occur at the same temperatures for all phases, 600 C in N{sub 2} and 700 C in O{sub 2}, the onset of degradation occurs at somewhat lower temperatures for Tl-1212 than for Tl-2212 films. In all cases, sample degradation is associated with Tl depletion from the films.
Date: August 23, 1999
Creator: Siegal, M.P.; Overmyer, D.L.; Venturini, E.L.; Padilla, R.R. & Provencio, P.N.
Partner: UNT Libraries Government Documents Department

Effects of c-axis Josephson coupling on dissipation, flux dynamics and the mechanism of high-T{sub c} superconductivity

Description: Measurements of the c-axis transport in highly anisotropic HTS materials strongly indicate that Josephson coupling is involved. This conclusion affects various properties of the HTS cuprates, including the irreversibility behavior for transport in the ab planes, the direct c-axis transport and potentially the mechanism of Cooper pairing.
Date: December 31, 1995
Creator: Gray, K.E. & Hettinger, J.D.
Partner: UNT Libraries Government Documents Department

Magnetic imaging of interlayer Josephson vortices.

Description: The authors have magnetically imaged interlayer Josephson vortices emerging parallel to the planes of single crystals of the organic superconductor {kappa}-(BEDT-TTF){sub 2} Cu(NCS){sub 2}, and the single layer cuprate high-T{sub c} superconductors Tl{sub 2}Ba{sub 2}CuO{sub 6+{delta}} (Tl-2201) and (Hg,Cu)Ba{sub 2}CuO{sub 6+{delta}} (Hg-1201), using a scanning Superconducting Quantum Interference Device (SQUID) microscope. These images provide a direct measurement of the interlayer penetration depth, which is approximately 63 {micro}m for {kappa}-(BEDT-TTF){sub 2} Cu(NCS){sub 2}, 18 {micro}m for Tl-2201 and 8 {micro}m for Hg-1201. The lengths for the cuprates are about a factor of 10 larger than originally predicted by the interlayer tunneling model for the mechanism of superconductivity in layered compounds, indicating that this mechanism alone cannot account for the high critical temperatures in these materials.
Date: January 15, 1999
Creator: Kirtley, J. A.
Partner: UNT Libraries Government Documents Department

Numerical Simulations of Quantum Many-body Systems

Description: The goals of our DOE work were to develop numerical tools in order to (1) determine the actual phase of particular many-electron models and (2) to understand the underlying mechanisms responsible for the observed phases. Over the years, DOE funds provided support for a number of graduate students and postdoctoral fellows who have gone on to continue and extend this effort. Looking back, they were more successful in determining the types of correlations that developed in particular models and less successful in establishing the underlying mechanisms. For example, they found clear evidence for antiferromagnetism, d{sub x{sup 3}-y{sup 2}}-pairing correlations, and stripes in various t-t{prime}-J and Hubbard models. Here, the stripes consisted of 1/2-filled domain walls of holes separated by {pi}-phase shifted antiferromagnetic regions. They found that a next-near-neighbor hopping t{prime} with t{prime}/t > 0 suppressed the stripes and favored the d{sub x{sup 3}-y{sup 2}}-pairing correlations. They studied a model of a CuO, 2-leg ladder and found that d{sub x{sup 3}-y{sup 2}} correlations formed when the system was doped with either electrons or holes. Another example that they studied was a two-dimensional spin 1/2 easy plane model with a near-neighbor exchange J and a four-site ring exchange K. In this J-K model, as K/J is increased, one moves from XY order to stripe order and to Ising antiferromagnetic order. They are still exploring the unusual transition between the Xy and striped phase. The key feature that we found was that strongly-correlated, many-electron systems are 'delicately balanced' between different possible phases. They also believe that their work provides strong support in favor of Anderson's suggestion that the Hubbard model contains the basic physics of the cuprates. That is, it exhibits antiferromagnetism, d{sub x{sup 3}-y{sup 2}}-pairing correlations, and stripes as the half-filled model is doped with holes. They were not as successful in ...
Date: April 20, 1998
Creator: Scalapino, Douglas J. Sugar, Robert L.
Partner: UNT Libraries Government Documents Department


Description: A flow trough type catalytic reactor system was adequately modified for NO related catalytic and adsorption measurements, including the on-line connection of a digital chemiluminescent NO-NO{sub x} analyzer to the reactor outlet system. Moreover, we have largely completed the installation of an FTIR coupled catalytic system containing a HTEC cell for high temperature DRIFT studies. Three different barium cuprate samples, Ba{sub 2}CuO{sub 3}, BaCuO{sub 2}, and Ba{sub 2}Cu{sub 3}O{sub 5} were synthesized and characterized by powder XRD for catalytic tests. Prior to catalytic studies over these cuprates, a new, liquid indium based supported molten metal catalyst (In-SMMC) was tested in the reduction of NO by various reductants. In the presence of excess O{sub 2} and H{sub 2}O, the In-SMMC proved to be more active for the selective catalytic reduction (SCR) of NO to N{sub 2} by ethanol than most other catalysts. Using C{sub 1}-C{sub 3} alcohols as reductants, self sustained periodic oscillations observed in the NO{sub x} concentrations of reactor effluents indicated the first time that radical intermediates can be involved in the SCR of NO by alcohols. Further, In-SMMC is the only effective and water tolerant SCR catalyst reported thus far which contains SiO{sub 2} support. Thus, this novel catalyst opens up a promising new alternative for developing an effective and durable catalyst for NO{sub x} abatement in stack emission.
Date: April 30, 1998
Partner: UNT Libraries Government Documents Department


Description: The year 2001 has been another highly productive year at the NSLS, with over 2500 users, including 720 first time users, conducting nearly 1200 experiments in fields ranging from the life, materials, chemical, and environmental sciences to applied science and technology. An impressive array of highlights from this scientific activity is included in this Activity Report. They include the first demonstration of a direct structural probe of the superconducting ground state in the cuprates by utilizing anomalous soft x-ray resonance effects to selectively enhance the scattering from doped holes. Another highly significant result was the determination of the structure of the potassium channel membrane protein. This is especially significant as it provides insight into how the channel functions and how it selects a particular kind of ion. In the nanoscience area, small angle x-ray scattering measurements played an essential role in determining that preferential sequestering of tailored metal nanocrystals into a self-assembled lamellar diblock copolymer can produce high quality metallodielectric photonic bandgap structures, demonstrating the potential of these nanocomposites for photonic crystal engineering. The infrared microscopy program continued to yield noteworthy results, including an important study that characterized the types and abundances of organic materials in contaminated and uncontaminated sediments from the New York/New Jersey Harbor. These results will be useful in devising improved methods for the destruction or removal of these environmental contaminants.
Date: May 1, 2002
Creator: CORWIN, M.A.
Partner: UNT Libraries Government Documents Department