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EXCHANGE INTERACTION BETWEEN CONDUCTION ELECTRONS AND MAGNETIC SHELL ELECTRONS IN RARE-EARTH METALS

Description: The interaction Hamiltonian for the Coulomb exchange effect between conduction electrons and magnetic-shell electrons in rare-earth metals was derived from first principles. The approximations under which the interaction can be represented by the product of electron and ion spin vectors are exhibited. (auth)
Date: January 15, 1961
Creator: Liu, S.H.
Partner: UNT Libraries Government Documents Department

The itinerant resonating-valence-bond model for superconductivity

Description: It has been proposed by Anderson that the pairing interaction in high temperature superconductors La/sub 2-x/ Sr/sub x/ Cuo/sub 4/ and Yba/sub 2/Cu/sub 3/O/sub 7-x/ is magnetic in origin, and the recent discovery of antiferromagnetic ordering in La/sub 2/CuO/sub 4/ has been regarded as strong evidence in support of this so-called resonating-valence-bond (RVB) model. Close examination of the ordered state of this material reveals that it is an itinerant antiferromagnet. Accordingly, the superconducting properties must also be studied using the itinerant model approach, rather than the local moment model discussed so far in literature. This paper reports an approximate solution of the itinerant RVB model of superconductivity. It is shown that superconductivity can take place in a narrow region of the parameter space, and that the fluctuating local exchange field causes the superconducting state to be gapless. 19 refs.
Date: August 1, 1987
Creator: Liu, S.H.
Partner: UNT Libraries Government Documents Department

(The latest developments of the physical aspects of electrochemistry)

Description: The author was one of 26 invited lecturers to discuss the latest developments of the physical aspects of electrochemistry. He interacted extensively with other lecturers and many participants from developing countries. He also visited with the Director of the Italian Synchrotron Radiation Source now under construction in Trieste, Italy.
Date: September 24, 1990
Creator: Liu, S.H.
Partner: UNT Libraries Government Documents Department

Simple model of the anisotropic penetration depth in high T{sub c} superconductors

Description: We present a simple model of some high {Tc} cuprates based upon superconducting (S) and normal (N) layers, which quantitatively fits the data of Bonn et al. for the low-temperature T dependence of the penetration depths {lambda}{sub a,b,c} in untwinned YBa{sub 2}Cu{sub 3}O{sub 7-{delta}}, assuming s-wave intralyer pairing. This SN model also leads to anisotropic surface states, which complicate analysis of photoemission and tunneling measurements.
Date: April 1, 1996
Creator: Klemm, R.A. & Liu, S.H.
Partner: UNT Libraries Government Documents Department

Energy gap structure and tunneling characteristics of layered superconductors

Description: The authors have analyzed the energy gaps and density-of-states (DOS) of layered superconductors with two inequivalent layers in a unit cell along the c-axis. In the physically interesting parameter range where the interlayer hopping strengths of the quasiparticles are comparable to the critical temperature, the peaks in the DOS curve do not correspond to the order parameters (OP`s) of each layer, but depend on the OP`s and the interlayer hopping strengths in a complex manner. In contrast to a BCS superconductor, the DOS of layered systems have logarithmic singularities. The simulated tunneling characteristics bear close resemblance to experimental results.
Date: June 1, 1993
Creator: Liu, S.H. & Klemm, R.A.
Partner: UNT Libraries Government Documents Department

Theory of bulk and interface constant phase elements in electrode- electrolyte systems

Description: This paper summarizes the progress gained in the last few years in our understanding of bulk and interface constant-phase-angle (CPA) behavior in electrode-electrolyte systems. It is now fairly well established that the interface constant-phase element originates from the fractal nature of the interface. The complex geometry gives rise to a fractal distribution of parallel current paths, and the competition between these paths results in the fractional power law behavior of the impedance across the interface. On the other hand, the early hope of relating the CPA exponent to the fractal dimension of the interface has been shown to be unattainable. Our understanding of the bulk CPA behavior, which is most prevalent in solid electrolytes, is only tentative. It is illustrated using a simple model that, under nonlinear dynamical laws that govern the flow of ions in the electrolyte, a current in the solid can generate a fractal distribution of vacancies which tend to impede the flow. The current is forced to negotiate a complex path through the solid, and the resulting fluctuation in path length and flow rate could be a source of the CPA behavior. 32 refs., 18 figs.
Date: January 1, 1991
Creator: Liu, S.H.
Partner: UNT Libraries Government Documents Department

Paramagnetic form factors from itinerant electron theory

Description: Elastic neutron scattering experiments performed over the past two decades have provided accurate information about the magnetic form factors of paramagnetic transition metals. These measurements have traditionally been analyzed in terms of an atomic-like theory. There are, however, some cases where this procedure does not work, and there remains the overall conceptual problem of using an atomistic theory for systems where the unpaired-spin electrons are itinerant. We have recently developed computer codes for efficiently evaluating the induced magnetic form factors of fcc and bcc itinerant electron paramagnets. Results for the orbital and spin contributions have been obtained for Cr, Nb, V, Mo, Pd and Rh based on local density bands. By using calculated spin enhancement parameters, we find reasonable agreement between theory and neutron form factor data. In addition, these zero parameter calculations yield predictions for the bulk susceptibility on an absolute scale which are in reasonable agreement with experiment in all treated cases except palladium.
Date: September 1, 1984
Creator: Cooke, J.F.; Liu, S.H. & Liu, A.J.
Partner: UNT Libraries Government Documents Department

Surface state and normal layer effects

Description: In addition to the conducting CuO{sub 2} (S) layers, most high-T{sub c} superconductors also contain other conducting (N) layers, which are only superconducting due to the proximity effect. The combination of S and N layers can give rise to complicated electronic densities of states, leading to quasilinear penetration depth and NMR relaxation rate behavior at low temperatures. Surface states can also complicate the analysis of tunneling and, photoemission measurements. Moreover, geometrical considerations and in homogeneously trapped flux axe possible explanations of the paramagnetic Meissner effect and of corner and ring SQUID experiments. Hence, all of the above experiments could be consistent with isotropic s-wave superconductivity within the S layers.
Date: August 1, 1995
Creator: Klemm, R.A.; Ledvij, M. & Liu, S.H.
Partner: UNT Libraries Government Documents Department

Spin and charge dynamics of chromium alloys

Description: Both the spin- and charge-density waves of Cr alloys are produced by the Coulomb attraction between electrons and holes on nearly nested Fermi surfaces. Driven by quasi-particle transitions, transverse spin- wave and longitudinal phason modes are associated with rotational and translational symmetries of pure Cr and its dilute alloys. At low frequencies, both spin and charge phasons have a nearly linear dispersion with a mode velocity which approaches the spin-wave velocity as T approaches T{sub N} or as the mismatch between the Fermi surfaces increases.
Date: July 1, 1996
Creator: Fishman, R.S.; Viswanath, V.S. & Liu, S.H.
Partner: UNT Libraries Government Documents Department

Comment on {open_quotes}superfluid anisotropy in YBCO: Evidence for pair tunneling superconductivity{close_quotes}

Description: Recently, Xiang and Wheatley extended our proximity coupling SN model of the (S) CuO{sub 2} plane and (N) CuO chain layers in YBCO compounds, using the single particle bands {epsilon}{sup 0}{sub 1k} = -t(cos k{sub a} + cos k{sub b}) - {mu}{sub 1} and {epsilon}{sup 0}{sub 2k} = -t{sub 2y}cos k{sub b} - {mu}{sub 2}, respectively. Although we showed that it was possible to fit the penetration depth {lambda}{sub ab}(T) in twinned YBCO with either s- or d-wave pairing and both bands free-particle-like, they claimed that: (1) It is impossible to fit the quasi-linear {lambda}{sub a}(T) in untwinned YBCO without order parameter line nodes, such as for d{sub x2-y2} pairing. (2) Even d-wave SN fits to the {lambda}{sub a,b,c} data are inferior to those obtained from a model of d-wave interlayer pair tunneling (DIPT). We remark that their claim is false, and ignores other data favoring proximity coupling.
Date: January 17, 1996
Creator: Klemm, R.A.; Ledvij, M. & Liu, S.H.
Partner: UNT Libraries Government Documents Department

Energy gap structure of layered superconductors

Description: We report the energy gap structure and density-of-states (DOS) of a model layered superconductor with one superconducting layer and one normal layer in a unit cell along the c-axis. In the physically interesting parameter range where the interlayer hopping strengths of the quasiparticles are comparable to the critical temperature, the peaks in the DOS curve do not correspond to the order parameter (OP) of the superconducting layer, but depend on the OP and the band dispersion in the c-direction in a complex manner. In contrast to a BCS superconductor, the DOS of layered systems have logarithmic singularities. Our simulated tunneling characteristics bear close resemblance to experimental results.
Date: November 1, 1993
Creator: Liu, S. H. & Klemm, R. A.
Partner: UNT Libraries Government Documents Department

Threshold photoelectron spectrum of the Argon 3s satellites

Description: Lately a variety of techniques have studied the electron correlation satellites with binding energies between the Argon 3s binding energy (29.24 eV) and the 2p/sup /minus/2/ ionization potential (43.38 eV). One of these techniques, Threshold Photoelectron Spectroscopy, with /approximately/90 meV FWHM resolution, revealed at least 25 individual electronic states. All of these could contribute to any other satellite spectrum, and this helped explain some discrepancies between previous measurements. This technique has been applied to the same region with higher resolution (<60 meV at the Ar 3s/sup /minus/1/peak). In this higher resolution spectrum at least 29 individual electronic states are present. In some cases the multiplet splitting is observed. 12 refs., 2 figs.
Date: July 1, 1989
Creator: Medhurst, L.J.; Von Wittenau, A.S.; van Zee, R.D.; Zhang, J.S.; Liu, S.H.; Shirley, D.A. et al.
Partner: UNT Libraries Government Documents Department