Showing papers by "Mildred S. Dresselhaus published in 1989"

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.

Transport properties and magnetic interactions in acceptor-type magnetic graphite intercalation compounds.

Abstract: Results on the resistivity of stage-1 and stage-2 ${\mathrm{CoCl}}_{2}$ graphite intercalation compounds (GIC's) as a function of temperature (T) and magnetic field (H) are reported. The anomalies observed in the resistivity measurements at the magnetic phase transitions are explained by an interaction based on \ensuremath{\pi}-d electron coupling. The contrasts in the T and H dependences of the resistivity between the stage-1 and stage-2 compounds for T${T}_{\mathrm{cl}}$ and H${H}_{c2}$ are attributed to the different correlation lengths in the c-axis antiferromagnetic ordering. The magnitude of the interplanar antiferromagnetic coupling constant (J') in stage-1 ${\mathrm{CoCl}}_{2}$ GIC's and the \ensuremath{\pi}-d exchange coupling constant (${J}_{\ensuremath{\pi}\mathit{\ensuremath{-}}d}$) are estimated from the transport measurements. Magnetic exchange mechanisms are proposed by considering the relative contributions of the superexchange, dipole-dipole, and Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. We conclude that the superexchange interaction is the dominant magnetic interplanar coupling mechanism in pristine ${\mathrm{CoCl}}_{2}$ and stage-1 ${\mathrm{CoCl}}_{2}$ GIC's, and is of comparable importance to the dipole-dipole interaction in stage-2 compounds. The dipole-dipole interaction is the dominant mechanism in higher stage GIC's (n\ensuremath{\ge}3). The RKKY interaction is always found to be negligibly small, due to the quasi-two-dimensional electronic properties of these acceptor GIC's.

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

Arthur von Hippel: The Man Behind the Von Hippel Award

Abstract: The Von Hippel Award, the most prestigious award of the Materials Research Society was first presented to its namesake, Arthur von Hippel, in 1976. On November 19, 1988, Prof. von Hippel celebrated his 90th birthday, an appropriate occasion to look back on a lifetime of achievement. Through the many testimonials that were heard at his 90th birthday celebration from family, friends, former students and colleagues, emerged a picture of materials science that reflects much that we treasure as the fabric of the Materials Research Society, namely the interdisciplinary approach to materials research. It has become an annual tradition of MRS to celebrate the achievement of the Von Hippel Award winner, and each year the celebrants begin the ceremony with an affirmation of the efficacy of the interdisciplinary approach. In the early years of the Von Hippel Award, our young students often heard greetings from Prof. von Hippel himself during the opening ceremony. But as the size of the meetings has grown, it has become increasingly difficult for “The Professor” to offer personal greetings. Many young people today yearn for contact with their intellectual heritage. Inspired by this need, the leadership of MRS recommended that a brief article be written for the MRS BULLETIN on reminiscences about Prof. Arthur von Hippel and his legacy of the interdisciplinary approach to materials research.

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.