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Superconductivity in doped insulators

Description: It is shown that many synthetic metals, including high temperature superconductors are ``bad metals``, with such a poor conductivity that the usual meanfield theory of superconductivity breaks down because of anomalously large classical and quantum fluctuations of the phase of the superconducting order parameter. It is argued that the supression of a first order phase transition (phase separation) by the long-range Coulomb interaction leads to high temperature superconductivity accompanied by static or dynamical charge inhomogeneIty. Evidence in support of this picture for high temperature superconductors is described.
Date: December 31, 1995
Creator: Emery, V.J. & Kivelson, S.A.
Partner: UNT Libraries Government Documents Department


Description: The magnetic excitation spectrum of the ideal S=1/2 1-dimensional quantum antiferromagnet (AF), contains no single-mode contributions, and, in fact, can be described as a 2-particle continuum of spinons. Chain-MF theory predicts that a weakly coupled chains system becomes ordered at low temperatures. In the ordered state a sharp magnon excitation is present. The continuum persists, but is separated from the magnon branch by an appreciable energy gap. On the other hand, if weakly coupled chains are described through the 1/S expansion of the conventional Holstein-Primakov spin wave theory, one finds a longitudinal 2-magnon continuum and a transverse 3-magnon continuum that start immediately above the spin wave energy.
Date: March 16, 2000
Partner: UNT Libraries Government Documents Department

Magnetic excitations in a random exchange system Fe{sub x}Mn{sub 1-x}TiO{sub 3}

Description: In order to examine the influence of exchange frustration on spin wave excitations, inelastic neutron scattering experiments have been performed on a random exchange system Fe{sub x}Mn{sub l-x}TiO{sub 3} with x = 0.00, 0.10, 0.25 and 0.33. For all three mixed systems, the magnetic excitation spectra consist of two components: a well-defined spin wave component and a broad damped component. The latter can be well fitted to a spectral weight function for a damped harmonic oscillator.
Date: August 24, 1994
Creator: Kawano, H.; Yoshizawa, H.; Nicklow, R.M. & Ito, A.
Partner: UNT Libraries Government Documents Department

Reduced size of ordered moments of a quasi 1d antiferromagnet Sr{sub 2}CuO{sub 3}

Description: The quasi one-dimensional antiferromagnet Sr{sub 2}CuO{sub 3} exhibits Neel order of Cu{sup 2+} moments (S = 1/2) at T{sub N} = 5.41(1) K, as demonstrated by (1) almost resolution-limited magnetic Bragg reflections in neutron scattering measurements, and (2) spontaneous muon spin precession in zero-field muon spin relaxation ({mu}SR) measurements. The temperature dependence of the order parameters are consistent between the two experimental techniques. From the neutron data, the authors obtained an upper-limit for the ordered moment size of {approximately} 0.06 {mu}{sub B}. This indicates a significant moment reduction from quantum fluctuations.
Date: September 1, 1996
Creator: Kojima, K.; Larkin, M. & Luke, G.M.
Partner: UNT Libraries Government Documents Department

Theoretical studies of magnetic systems. Final report, August 1, 1994--November 30, 1997

Description: During the grant period the authors have studied five areas of research: (1) low dimensional ferrimagnets; (2) lattice effects in the mixed valence problem; (3) spin compensation in the one dimensional Kondo lattice; (4) the interaction of quasi particles in short coherence length superconductors; and (5) novel effects in angle resolved photoemission spectra from nearly antiferromagnetic materials. Progress in each area is summarized.
Date: December 31, 1997
Creator: Gor`kov, L.P.; Novotny, M.A. & Schrieffer, J.R.
Partner: UNT Libraries Government Documents Department

Random-field critical scattering at high magnetic concentration in the Ising antiferromagnet Fe{sub 0.93}Zn{sub 0.07}F{sub 2}

Description: The high magnetic concentration ising antiferromagnet Fe{sub 0.93}Zn{sub 0.07}F{sub 2} does not exhibit the severe critical scattering hysteresis at low temperatures observed in all lower concentration samples studied. The system therefore provides equilibrium neutron scattering line shapes suitable for determining random-field Ising model critical behavior.
Date: March 24, 1997
Creator: Slanic, Z.; Belanger, D. P. & Fernandez-Baca, J. A.
Partner: UNT Libraries Government Documents Department

Phase diagram of the random-field Ising system Fe{sub 0.60}Zn{sub 0.40}F{sub 2} at intense fields

Description: The critical and irreversibility phase boundaries of the d = 3 diluted uniaxial antiferromagnet Fe{sub 0.60}Zn{sub 0.40}F{sub 2} have been determined under strong external magnetic fields by means of magnetization measurements. The data reveal that the random-field-induced glassy phase, previously observed in the upper part of the (H,T) phase diagram for highly diluted samples (x {approx_equal} 0.3), is extended to higher values of x.
Date: October 1, 1997
Creator: Montenegro, F.C.; Lima, K.A.; Torikachvili, M.S. & Lacerda, A.H.
Partner: UNT Libraries Government Documents Department

Charge inhomogeneity and high temperature superconductivity

Description: When the condensation of a gas of fermions into a (self-bound) liquid state is frustrated by the long-range Coulomb interaction, the consequence is a large local fluctuation of the charge density, together with pairing on a high energy scale. The competition between these two effects at long length scales determines the nature of the ordered state at low temperatures. Evidence for the central role of this competition in determining the physical properties of the high temperature superconductors is provided by the delicate interplay of superconductivity, charge and spin ordering, and structural phase transformations in the La{sub 2}CuO{sub 4} family of materials. There the gas-liquid transition corresponds to the phase separation of holes doped into an antiferromagnetic insulator. Because of the low superfluid density and poor conductivity, the critical temperature for the superconducting transition in underdoped and optimally doped materials is governed by the onset of phase coherence and not by the pairing energy scale.
Date: May 1, 1996
Creator: Emery, V.J. & Kivelson, S.A.
Partner: UNT Libraries Government Documents Department

Crystallographic and magnetic structure of UCu{sub 1.5}Sn{sub 2}

Description: We report on the crystallographic and magnetic structures of the antiferromagnet UCu{sub 1.5}Sn{sub 2}, as determined by x-ray and neutron powder diffraction. It forms in the tetragonal CaBe{sub 2}Ge{sub 2} structure type, with space group P/4nmm, and we find no site disorder between two different Sn 2c sites, in contrast with a previous report. UCu{sub 1.5}Sn{sub 2} orders antiferromagnetically with a Neel temperature of about 110 K. This is unusually high amongst uranium intermetallics. The uranium moments align along the c-axis in a collinear arrangement but alternating along the c-axis. The low-temperature uranium moment is 1.95{mu}{sub B}.
Date: September 1995
Creator: Purwanto, A.; Robinson, R. A.; Nakotte, H.; Swainson, I. & Torikachvili, M.
Partner: UNT Libraries Government Documents Department

Heavy fermions in high magnetic fields

Description: This is the final report of a three-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). The National High Magnetic Field Laboratory (NHMFL) has established major new facilities at LANL. This project sought to explore some exciting new problems in condensed matter physics that could be studied using these facilities. We studied the behavior of heavy-fermion compounds in high-magnetic fields. The unusual properties of these materials are governed by small energy scales arising from strong many-body correlations, demonstrating that the fields that can be achieved in the NHMFL can be used to probe these correlations.
Date: May 1, 1996
Creator: Trugman, S.; Bedell, K.; Bonca, J.; Gulacsi, M. & Yu, C.
Partner: UNT Libraries Government Documents Department

Phase diagram of imperfect ferromagnetic/antiferromagnetic-bilayers

Description: The phase diagram for ferromagnetic/antiferromagnetic bilayers with imperfect interfaces is calculated, using a Ising spin-1/2 model which is solved numerically in the mean field approximation for finite temperatures. We identify 3 stable phases: domains in the ferromagnet, domains in the antiferromagnet, and domain walls near the interfaces with homogeneous order within the layers. Phase transitions between these phases occur as a function of temperature, relative film thicknesses, and step density.
Date: October 1, 1996
Creator: Berger, A. & Fullerton, E.E.
Partner: UNT Libraries Government Documents Department

Neutron study of fracton excitations in percolating antiferromagnets

Description: The authors report the results of an inelastic neutron scattering experiment on nearly-percolating Heisenberg antiferromagnets (RbMn{sub c}Mg{sub 1{minus}o}F{sub 3}), in which the Mn concentrations (C = 0.31, 0.34 and 0.39) are very close to the percolation threshold (c{sub p} = 0.312). A broad peak superimposed on Ising-cluster excitations was observed throughout the Brillouin zone. The intensity of a broad peak increased on approaching the percolation threshold. The origin of this broad peak is attributed to the excitation of fractons in a percolating network.
Date: June 27, 1997
Creator: Ikeda, H.; Takahashi, M.; Fernandez-Baca, J.A. & Nicklow, R.M.
Partner: UNT Libraries Government Documents Department

Magnetoresistance behavior of UNiGe

Description: The authors have measured the temperature dependences of the magnetoresistance of single-crystalline UNiGe for both parallel (i//B//c-axis) and perpendicular configurations (i//a-axis, B//c-axis) in magnetic fields up to 18 T. Both configurations yield similar magnetoresistance behavior, which emphasizes the strong magnetic contribution to the resistivity in all directions. Crossing magnetic-phase boundaries causes anomalies in the magnetoresistance, which allowed a completion of the previously proposed magnetic phase diagram.
Date: May 1, 1995
Creator: Nakotte, H.; Lacerda, A.; Purwanto, A.; Havela, L.; Sechovsky, V.; Prokes, K. et al.
Partner: UNT Libraries Government Documents Department

Chebyshev recursion methods: Kernel polynomials and maximum entropy

Description: The authors describe two Chebyshev recursion methods for calculations with very large sparse Hamiltonians, the kernel polynomial method (KPM) and the maximum entropy method (MEM). They are especially applicable to physical properties involving large numbers of eigenstates, which include densities of states, spectral functions, thermodynamics, total energies, as well as forces for molecular dynamics and Monte Carlo simulations. The authors apply Chebyshev methods to the electronic structure of Si, the thermodynamics of Heisenberg antiferromagnets, and a polaron problem.
Date: October 1, 1995
Creator: Silver, R.N.; Roeder, H.; Voter, A.F. & Kress, J.D.
Partner: UNT Libraries Government Documents Department

Magnetic properties of the S=1/2 quasi-one-dimensional antiferromagnet: Sr{sub 14-x}Y{sub x}Cu{sub 24}O{sub 41}

Description: Magnetic susceptibility and inelastic neutron scattering measurements have been performed on the S=1/2 quasi-one-dimensional system Sr{sub 14-x}Y{sub x}Cu{sub 24}O{sub 41}, which has both simple chains and two-leg ladder chains of copper ions. The experimental results show that the simple chain in Sr{sub 14}Cu{sub 24}O{sub 41} has a spin gap in the excitation spectrum, which originates from a dimerized state. We have also studied the effect of Y{sup 3+} substitution for Sr{sup 2+} site on the dimerized state. It was found that the yttrium substitution suppresses the gap energy drastically.
Date: July 1, 1996
Creator: Matsuda, M.; Katsumata, K.; Shapiro, S.M. & Shirane, G.
Partner: UNT Libraries Government Documents Department

Direct imaging of the first order spin flop transition in the layered manganite La{sub 1.4}Sr{sub 1.6}Mn{sub 2}O{sub 7}.

Description: The spin-flop transition in the antiferromagnetic layered manganite La{sub 1.4}Sr{sub 1.6}Mn{sub 2}O{sub 7} was studied using magnetization measurements and a high-resolution magneto-optical imaging technique. We report the direct observation of the formation of ferromagnetic domains appearing at the first order spin-flop transition. The magnetization process proceeds through nucleation of polarized domains at crystal defect sites and not through the expansion of polarized domains due to domain wall motion. A small magnetic hysteresis is caused by the difference between the mechanisms of nucleation and annihilation of domains in the mixed state. These results establish a direct link between the magnetic structure on the atomic scale as seen in neutron scattering and the macroscopic properties of the sample as seen in magnetization and conductivity measurements.
Date: August 31, 1999
Creator: Berger, A.; Gray, K. E.; Miller, D. J.; Mitchell, J. F.; Vlasko-Vlasov, V. K. & Welp, U.
Partner: UNT Libraries Government Documents Department

Random fields and ordering in antiferromagnetic insulators

Description: Neutron scattering measurements of the effect on the long range order and phase transitions of random magnetic fields are reviewed. The results at low temperatures in Rb/sub 2/Co/sub x/Zn/sub 1-x/F/sub 4/ and Co/sub x/Zn/sub 1-x/F/sub 2/ show that the long range order is not established when samples are cooled in a magnetic field, but they form a state with unusually long magnetic correlations. In Mn/sub x/Zn/sub 1-x/F/sub 2/ a similar state is formed but only very close in temperature to T/sub N/.
Date: August 1, 1982
Creator: Cowley, R.A.
Partner: UNT Libraries Government Documents Department

Electronic phase separation and high temperature superconductors

Description: The authors review the extensive evidence from model calculations that neutral holes in an antiferromagnet separate into hole-rich and hole-poor phases. All known solvable limits of models of holes in a Heisenberg antiferromagnet exhibit this behavior. The authors show that when the phase separation is frustrated by the introduction of long-range Coulomb interactions, the typical consequence is either a modulated (charge density wave) state or a superconducting phase. The authors then review some of the strong experimental evidence supporting an electronically-driven phase separation of the holes in the cuprate superconductors and the related Ni oxides. Finally, the authors argue that frustrated phase separation in these materials can account for many of the anomalous normal state properties of the high temperature superconductors and provide the mechanism of superconductivity. In particular, it is shown that the T-linear resistivity of the normal state is a paraconductivity associated with a novel composite pairing, although the ordered superconducting state is more conventional.
Date: January 11, 1994
Creator: Kivelson, S. A. & Emery, V. J.
Partner: UNT Libraries Government Documents Department

Studies of the magnetic structure at the ferromagnet - antiferromagnet interface

Description: Antiferromagnetic layers are a scientifically challenging component in magneto-electronic devices such as magnetic sensors in hard disk heads, or magnetic RAM elements. In this paper we show that photo-electron emission microscopy (PEEM) is capable of determining the magnetic structure at the interface of ferromagnets and antiferromagnets with high spatial resolution (down to 20 nm). Dichroism effects at the L edges of the magnetic 3d transition metals, using circularly or linearly polarized soft x-rays from a synchrotron source, give rise to a magnetic image contrast. Images, acquired with the PEEM2 experiment at the Advanced Light Source, show magnetic contrast for antiferromagnetic LaFeO{sub 3}, microscopically resolving the magnetic domain structure in an antiferromagnetically ordered thin film for the first time. Magnetic coupling between LaFeO{sub 3} and an adjacent Co layer results in a complete correlation of their magnetic domain structures. From field dependent measurements a unidirectional anisotropy resulting in a local exchange bias of up to 30 Oe in single domains could be deduced. The elemental specificity and the quantitative magnetic sensitivity render PEEM a perfect tool to study magnetic coupling effects in multi-layered thin film samples.
Date: January 2, 2001
Creator: Scholl, A.; Nolting, F.; Stohr, J.; Luning, J.; Seo, J.W.; Locquet, J.-P. et al.
Partner: UNT Libraries Government Documents Department