Session J1: CM4: Condensed Matter

First principles calculations of the thermal properties of tin clathrate materials

Using a Local Density Approximation (LDA) approach, we have studied the energetics and the thermal properties (free energy, specific heat, lattice vibrational entropy) of the tin-based Type I semiconductor clathrates Sn$_{46}$, K$_{8}$Sn$_{46}$, K$_{8}$Sn$_{44}{\rm M}_{2}$ (${\rm M}$ is a Sn vacancy), Cs$_{8}$Ga$_{8}$Sn$_{38}$ and Cs$_{8}$Zn$_{4}$Sn$_{42}$. The clathrate lattices are open framework, cage-like structures. Our results predict that K$_{8}$Sn$_{46}$ is slightly less stable than K$_{8}$Sn$_{44}{\rm M}_{2}$, in agreement with other theories.\footnote{L. Mollnitz et. al., J. Chem. Phys. 117, 3 (2002)} We have optimized the geometry of each structure and have calculated the phonon density of states. The thermodynamic properties have then been calculated as a function of temperature. The localized vibrational (``rattler'') modes of the guests Cs and K have been calculated and their Einstein temperatures obtained. The Debye temperatures of each host clathrate have also been calculated. We use our results to help to explain the observed difference\footnote{G. S. Nolas et. al., Phys. Rev. B 53,165201 (2002)} in the lattice thermal conductivities of some of these materials.   

Experimental study of Ba$_{8}$Al$_{x}$Ge$_{46-x}$ clathrates by NMR and other techniques

Cage-structured group-IV clathrates exhibit interesting properties, for example very high thermoelectric efficiency. We have investigated Ba$_{8}$Al$_{x}$Ge$_{46-x}$, which is a ternary semiconductor for the case $x$ = 16. $^{27}$Al NMR studies for $x$ = 16 show metallic behavior, indicating doping by native defects into the metallic regime. For smaller $x$ the average relaxation rate increases, indicating an increased density of carriers. We also observe vacancies in low-$x$ samples via electron microprobe studies. For $x$ = 12 and 13, NMR exhibits a second line not seen for $x$ = 16, attributed to Al adjacent to vacancies. Magic angle spinning NMR and computer simulation of the first-order quadrupole line shape for these samples were used to analyze the observed NMR lines. In addition, we have used ab initio calculations of the electric field gradients to match the observed quadrupole broadening with the local structure. Evidence indicates that Al adjacent to vacancies are predominantly on 24$k$ sites. These sites have significantly different local electronic structure and smaller density of conduction electrons.   

NMR measurements of Al$_{20}$V$_{2}$La and Al$_{20}$V$_{2}$Eu

The Al$_{20}$V$_{2}$$R$ system, with $R$ = rare earth, features $R$ atoms in 16-atom Al cages. Our previous work on Al$_{20}$V$_{2}$Eu showed the development of underscreened Kondo behavior at low temperatures. From $^{27}$Al NMR measurements of nonmagnetic Al$_{20}$V$_{2}$La we identify the local electronic behavior of all three Al framework sites, and $T$$_{1}$ measurements indicate a significant difference between the behavior of two of the sites. At high temperatures, pseudogap behavior is observed, which is also supported by electronic calculations using the WIEN2k package. These calculations are also in excellent agreement with the measured specific heat. We conclude that the spin-fluctuation behavior in Al$_{20}$V$_{2}$Eu results from the rare-earth atom rather than from the Al-V framework. Transport measurements also could be fit to a Bloch-Gruneisen behavior, and by taking the difference, the magnetic contribution to the resistivity of Al$_{20}$V$_{2}$Eu could be identified, including a $T$-linear magnetoresistance which disappears at about 100 K, along with the apperance of Kondo behavior.   

Metal-Insulator Transition in thin Gadolinium Films

Two dimensional thin films of gadolinium were prepared in an ultra high vacuum chamber using electron-beam evaporation onto a cold substrate. The percolation limit was reached with the thinnest films, while thicker films show metallic behavior. Electronic measurements were conducted in-situ at low temperatures and nonlinear I-V curves were observed for the thinnest films. Progress in this ongoing study will be presented and applied to a better understanding of metal- insulator transitions in two dimensional disordered systems.   

Electronic Transport Properties of Mn$_{12}$-Acetate Film Measured with Self-assembled Tunneling Junction

We measure the differential conductance of Mn$_{12}$-Acetate 2 monolayer film and found it is about 10$^{-7}$-10$^{-8}$ S. We observed the Kondo resonance and transition from dip to peak as initial resistance decreases. We calculated the Kondo temperature of Mn$_{12}$-Acetate on the surface of Pt and it is 346$\pm $86 K. Sudden conductance change about 1.0$\times $10$^{-8}$ S was observed and as it is highly unlikely caused by mechanical instability, we speculate it is caused by the molecular configuration change between 2 states, which agree with Gregory's assumption.   

Anomalous Long-Range Proximity Effect Observed in Single-Crystal Superconducting Nanowires

An anomalous proximity effect has been observed in single-crystal Pb, Sn, and Zn nanowires, each in contact with a pair of macroscopic electrodes. With electrodes having a higher critical temperature Tc, superconductivity is induced at the Tc of the electrodes in Sn and Zn nanowires as long as 60 $\mu$m, which is more than 10 times the expected length defined by current theories. This effect is further confirmed by the field dependence of the resistive transitions and I-V characteristics. It is found to depend sensitively on the residual-resistance-ratio of the nanowires.   

RF Plasma Synthesis: A Novel Technique for Preparation of High Critical Field ``Dirty'' MgB$_{2}$

It has been shown, both theoretically as well as experimentally, that very high critical fields can be achieved in ``dirty'' MgB$_{2}$ superconductors. However, controlled homogeneous doping/alloying has thus far remained a challenge. A novel technique of rf plasma sputtering of ``dirty'' MgB$_{2}$ is hence being developed. This technique promises a potential for the synthesis of powders with homogeneous doping/alloying in the crystal lattice. Spatial temperature profile in the plasma plume is helpful, especially for the case of MgB$_{2}$ where the reacting species have very different vaporizing temperatures. Details of the design concept and preliminary results will be presented.   

Pinning Enhancement of Tb Doped TFA-MOD YBCO Film

In this study, minute rare earth Tb doped YBCO films were prepared by using Trifluoroacetic acid metalorganic deposition (TFA-MOD) method. The precursor solution was deposited on LAO single crystal substrate by using spinning coating method. The epitaxial YBCO films were obtained via a two-step heat treatment. The characterizations of films including microstructure, Tc, field dependent Jc will be reported. The relationship between dopant amount, Jc-H behavior, and microstructure evolution will be discussed.   

The interaction between superconductors and Mn$_{12}$-acetate single-molecule magnets

Possible applications of single-molecule magnets (SMMs), e.g. for ultra high density magnetic information storage device, quantum computation, and molecular electronics, have been suggested due to the unusual magnetic behavior. It is an important prerequisite for the applications to develop a reliable technique to organize the molecules on a surface and to detect the magnetic signals of the molecules. A solution evaporation technique combined with conventional lithography is a simple but reliable approach to generate Mn$_{12}$-acetate thin film patterns on the micro/nano-scale. The miniaturized SQUID is appropriate for sensing the magnetic flux from the film structure of the molecular magnets. A new interesting system, the so-called superconductor/SMM hybrid, results from the experimental configuration. Understanding this new type of hybrid system is important not only because of the expectation of new phenomena affecting on the functionality of superconducting devices, but also because the two coupled substances are fundamentally incompatible phases. In this presentation, the first experimental attempt to understand the interaction between an aluminum superconducting film and Mn$_{12}$-acetate SMMs will be discussed.