Showing papers by "Hans P. Jenssen published in 1989"

Neutron scattering study of soft optical phonons in La2-xSrxCuO4-y

Abstract: The soft optical phonons associated with the tetragonal-to-orthorhombic structural phase transition in La/sub 2-//sub x/Sr/sub x/CuO/sub 4-//sub y/ have been investigated using inelastic-neutron-scattering techniques. Single crystals with Sr concentration of x = 0, 0.08, and 0.14 were studied. Both branches of the doubly degenerate X-point phonon which go soft at the tetragonal-to-orthorhombic transition have been observed in the orthorhombic phase. In all three samples one branch hardens, while the other remains relatively soft with behavior indicative of a further incipient structural phase transition. We identify the hard and soft branches of the orthorhombic phase as zone center and zone-boundary phonons, respectively. The zone-center mode has A/sub g/ symmetry and is, consequently, Raman active. The relation of these measurements to other structural properties and to the superconductivity in La/sub 2-//sub x/Sr/sub x/CuO/sub 4-//sub y/ is discussed.

Conductivity and Hall coefficient in La2CuO4+y near the insulator-metal transition.

Abstract: Measurements are reported of the conductivity, its anisotropy, and the in-plane Hall coefficient for a single crystal of La/sub 2/CuO/sub 4+/ital y// with /ital y/ varied from 0%--1.5%. The conductivity is thermally activated with a single activation energy at high temperatures and with a continuously decreasing activation energy below /similar to/50 K. The anisotropy is as large as 500 at high temperatures, falls sharply to 10--20 at /similar to/50 K, and then is only weakly /ital T/ dependent at lower temperatures. The Hall coefficient is simply activated above /similar to/50 K (with an activation energy similar to that of the in-plane conductivity), and has a peak at lower temperatures. All these features are evidence for a change in the transport mechanism near 50 K. The low-temperature conductivity is consistent with thermally assisted tunneling between localized states at the Fermi energy. The high-temperature process arises from thermal activation of carriers into a band of highly anisotropic states. The low mobility associated with these states makes it impossible, however, to determine whether they arise from impurities or are states near a band or mobility edge.

Frequency dependence of the conductivity and dielectric constant of La2CuO4+y near the insulator-metal transition.

Abstract: Measurements are reported of the conductivity and dielectric constant at frequencies up to 10 MHz for a single crystal of La{sub 2}CuO{sub 4+{ital y}} with {ital y}{similar to}0.001 to 0.01. The reported behavior is typical of conventional doped semiconductors, with dielectric constant (at {ital y}=0) and effective mass that are not exceptionally large. The dielectric constant grows as the insulator-to-metal transition is approached, but only for the electric field in the CuO{sub 2} layers, indicating that the transition is two dimensional. The crossover to weak localization occurs close to the antiferromagnetic phase boundary.

Thermal properties of single-crystal La2CuO4- Delta.

Abstract: We report measurements of the thermal conductivity, Raman spectrum, magnetic susceptibility, and specific heat of a single crystal of ${\mathrm{La}}_{2}\mathrm{Cu}{\mathrm{O}}_{4\ensuremath{-}\ensuremath{\Delta}}$ from room temperature down to 10 K. We observe a sharp kink in the heat conductivity near 250 K in the [001] direction and a broader anomaly in the range 130-200 K in the [110] direction. These features have been correlated, in terms of a strongly temperature-dependent phonon-magnon scattering mechanism, with changes in the magnetic structure as identified through susceptibility studies. These results differ substantially from experiments on sintered samples, which show a thermal conductivity more reminiscent of a tunneling system. It is suggested that the amorphouslike behavior of the sintered materials is an artifact of their rather poor crystalline nature, which masks the intrinsic effects associated with the single crystal as reported here.

Raman study of pure and doped (Li,Sr) single crystals of La2CuO4-y.

Abstract: Raman spectra are presented of pristine and doped (Li,Sr) single crystals of ${\mathrm{La}}_{2}\mathrm{Cu}{\mathrm{O}}_{4\ensuremath{-}y}$. Based on polarization measurements, we assign the 424-${\mathrm{cm}}^{\ensuremath{-}1}$ line to the symmetric ${A}_{1g}$ vibration of the out-of-plane oxygen atoms along the tetragonal axis. The line at 231 ${\mathrm{cm}}^{\ensuremath{-}1}$ is identified as the ${A}_{1g}$ vibration of the La atoms along the tetragonal axis and the line at 238 ${\mathrm{cm}}^{\ensuremath{-}1}$ as the ${E}_{g}$ vibration of the out-of-plane oxygen atoms parallel to the Cu-O layers. A symmetry-forbidden Raman line is observed at 680 ${\mathrm{cm}}^{\ensuremath{-}1}$ which, by comparison with infrared measurements, can be tentatively assigned to an in-plane oxygen vibration. The dependence of the Raman spectra upon Li and Sr doping is presented and discussed. Especially noteworthy are two strong and sharp Raman lines (at 757 and 773 ${\mathrm{cm}}^{\ensuremath{-}1}$ at room temperature) in the Sr-doped crystals, because of their higher frequencies, polarization sensitivities, and unusual temperature dependences.

Upconversion Processes in Er-Activated Solid State Laser Materials

Abstract: A quantitative model for Er-activated solid state laser materials has been developed to describe the population dynamics of the Er multiplets over a wide range of pump rates and Er concentrations. The model contains two adjustable parameters associated with the upconversion processes involved. The values of these upconversion rate parameters have been deduced by fitting the model to spectroscopic data.

Polarized reflectivity studies of single-crystal La 2 CuO 4

Abstract: Room-temperature polarized reflectivity measurements are reported for single-crystal La2CuO4 for the (E ⊥ ĉ) and (E || ĉ) polarizations, where ĉ denotes the tetragonal axis. A Kramers–Kronig analysis is performed to obtain the frequency-dependent dielectric functions. Frequencies for the seven zone-center IR-active phonons are obtained and the longitudinal-optic–transverse-optic splittings are used to evaluate the dynamic charge on the oxygen atoms. The reflectivity spectrum for the (E ⊥ ĉ) polarization is decidedly metallic and is fitted accordingly with an overdamped Drude model and a plasma frequency of 95 meV. In the high-frequency region of the spectrum, selection rules are observed for allowed and forbidden electronic transitions. The importance of this study to the interpretation of optical measurements on high-Tc, polycrystalline pellets of La2CuO4 and related materials is also discussed.

Frequency dependence of the conductivity and dielectric constant of La2CuO4+y near the insulator-metal transition

Abstract: Measurement are reported of the conductivity and the dielectric constant (ɛ 1 ) at frequencies (ω) up to 20 MHz and magnetic fields up to 15 T for a single crystal of La 2 CuO 4+y with y varied in the range 0.001 – 0.01. The frequency dependence of the conductivity at low temperatures follows a power law, which is typical of conventional doped semiconductors. ɛ 1 at high ω is independent of ω and grows as the insulator to metal transition (IMT) is approached, but only for the electric field in the CuO 2 layers, indicating that the IMT is two-dimensional. For the highest oxygen concentration, this excess ɛ 1 was reduced by a factor of 2 at a magnetic field of 15 T perpendicular to the CuO 2 layers.

Lattice instability in single-crystal La2−xSrxCuO4

Abstract: The structural phase transition from the tetragonal to the orthorhombic phase of doped and undoped samples of La2−xSrxCuO4 has been investigated by using inelastic neutron scattering techniques. The rotational nature of the soft mode leads to moderate electron-phonon coupling and the mode is unlikely to enhance significantly conventional phonon mediated superconductivity.

Magnetostriction of single-crystal La2CuO4.

Abstract: Longitudinal magnetostriction ($\frac{\ensuremath{\Delta}l}{l}$) experiments have been used to probe the magnetic phase diagram of single-crystal ${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$ at 42, 77, 145, and 191 K The geometry used is ($l\ensuremath{\parallel}\mathrm{H}\ensuremath{\parallel}\stackrel{^}{\mathrm{b}}$), where $\stackrel{\mathrm{^}}{\mathrm{b}}$ is a unit vector along the orthorhombic $b$ axis, perpendicular to the Cu${\mathrm{O}}_{2}$ planes The observed magnetic transition is associated with an induced alignment, along the field direction, of the small out-of-plane canting of the ${\mathrm{Cu}}^{2+}$ spins At all temperatures, the transition is marked by a small increase in $l$, with $\frac{\ensuremath{\Delta}l}{l}$ about 2 to 3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ A large hysteresis was observed at 42 K, with the transition field, ${H}_{t}$, in the up trace being 601 kOe, and in the down trace 458 kOe This hysteresis strongly decreases with increasing temperature At 191 K the jump in $l$ is still quite sharp indicating that the transition is still first order Our values of ${H}_{t}$ are in good agreement with those obtained on similar crystals using different techniques

Transient-thermoreflectance study of single-crystal lanthanum cuprate.

Abstract: Presentation d'analyses de thermoreflectance transitoire de l'ordre de la picoseconde du monocristal La 2 CuO 4 , entre 80 et 300 L. Etude de l'influence de la direction de polarisation de la lumiere

Weak localization effects in conductance measurements of La2−xSrxCuO4−y

Abstract: Conductivity data from many La 2−x Sr x CuO 4−y crystals is presented. All sufficiently conductive crystals (with conductances of order e 2 /h per square per CuO 2 layer) show a logarithmic temperature dependent correction to the conductance, of order e 2 /h, predicted by weak localization theory.

Magnetoresistance and the spin flop transition in pure single - Crystal La2CuO4+y

Abstract: Magnetoresistance (MR) measurements are reported for pure single crystals of La2CuO4+y at magnetic fields up to 23T With the magnetic field →H‖→c, parallel to the staggered moment, the MR shows evidence of a spin flop transition, which provides a measure of both the antisymmetric and the symmetric anisotropies of the nearest-neighbour exchange The MR, which arises from the interlayer hopping conductivity, is proportional to M†+, the order parameter of the antiferromagnetism (AF) with propagation vector →gt‖→c

Two Dimensional Quantum Spin Fluid — Progenitor of High Temperature Superconductivity —

Abstract: Neutron scattering probed novel two dimensional antiferromagnetic spin correlations in pure La2CuO4 which is the mother compound of high temperature superconductors of doped La2-xMxCuO4. Cu2+ moments are ordered instantaneously over very large areas in the CuO2 plane but there is no measurable time averaged staggered moment. Since the dynamical behavior observed is much akin to that in the quantum antigerromagnetic chain, we concluded that spin fluctuations observed at finite temperatures are quantum spin liquid state like that in quantum antiferromagnetic chain at T = 0.

NEUTRON SCATTERING STUDIES OF MAGNETIC EXCITATIONS IN La2−xSrxCuO4

Abstract: The results of recent neutron scattering studies on La 2−x Sr x CuO 4 are reviewed. For × = 0 the material is an antiferromagnetic insulator, and the spins in the CuO 2 sheets are well-described by the 2D S = 1/2 square lattice Heisenberg model. With increasing × the Neel state is destroyed and is replaced at low temperatures, at least for samples which are nonmetallic or have low superconducting T c 's, by a spin glass. The instantaneous correlations as well as those of the glass are incommensurate with the crystal lattice. The spin-spin correlation length in the doped CuO 2 sheets equals the average separation between the holes which ultimately carry the supercurrent.

Anisotropic Thermal Conductivity of Superconducting Lanthanum Cuprate

Abstract: The thermal conductivities of superconducting, Sr-doped lanthanum copper oxide single crystals have been measured from room temperature to below 100 mK parallel and perpendicular to the copper oxide planes. While the results indicate that the heat conduction is strongly anisotropic, the data have been analyzed in terms of a modified Bardeen-Rickhayzen-Tewordt theory of lattice thermal conductivity. It is shown that while electron scattering plays an important role in limiting the in-plane heat conductivity, this scattering channel is masked by other mechanisms for heat transport across the planes.