Session L41: Focus Session: Search for New Superconductors - Heterostructures, Thin Films, Intercalated and High-Pressure Compounds

Could pnictide-type superconductivity occur in FeAs/GaAs superlattices?

A recurring feature in the iron pnictide superconductors is the existence of an FeAs layer. Here we use first-principles density functional calculations to examine whether the electronic structure characteristics believed to correlate with superconductivity in the pnictides are reproduced in superlattices containing alternating zincblende structure FeAs/GaAs layers. Such superlattices could offer important advantages in ease of fabrication using MBE techniques and integration with semiconductor devices.   

Hall effect in tungsten-bronze thin films

Hall effect in tungsten-oxide thin-films is investigated. Tungsten oxides (M$_{x}$WO$_{3-y})$ have long been studied for their interesting structural, electronic and electro-chromic properties. A pure tungsten trioxide (WO$_{3})$ is an insulator and doping of $M$ ($M$ = alkali or alkali-earth element) or oxygen deficiency makes the system conducting and sometimes superconducting. Indeed, even some very high $T_{c}$ anomalies have been reported. We have succeeded in growth of superconducting K$_{x}$WO$_{3-y}$ thin films and noticed that oxygen composition strongly affect the superconducting properties. Few reports for oxygen non-stoichiometry effects on the superconductivity in tungsten oxides are available, and the relationship between these normal state electronic properties and the superconducting properties of these materials has not yet been fully clarified. In this presentation, we will discuss relationship between superconductivity, oxygen non-stoichiometry and Hall coefficient.   

First-Principles Study of Superconductivity in Ultra- thin Pb Films

Recently, superconductivity in ultrathin layered Pb has been confirmed in samples with as few as two atomic layers [S. Qin, J. Kim, Q. Niu, and C.-K. Shih, Science 2009]. Interestingly, the prototypical strong-coupling superconductor exhibits different $T_c$'s for differing surface reconstructions in samples with only two monolayers. Additionally, $T_c$ is seen to oscillate as the number of atomic layers is increased. Using first principles techniques based on Wannier functions, we analyze the electronic structure, lattice dynamics and electron-phonon coupling for varying thicknesses and surface reconstructions of layered Pb. We discuss results as they relate to superconductivity in the bulk, for which accurate calculations of superconducting properties can be compared to experiment [W. L. McMillan and J.M. Rowell, PRL 1965]. This work was supported by National Science Foundation Grant No. DMR07-05941, the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Computational resources have been provided by the Lawrencium computational cluster resource provided by the IT Division at the Lawrence Berkeley National Laboratory (Supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231)   

Atomic-Layer Engineering of Oxide Superconductors

Using a unique molecular beam epitaxy system, we synthesize atomically smooth thin films, multilayers and superlattices that contain cuprates as well as other complex oxides, in order to enable novel experiments that probe the basic physics of HTS. We have also been trying to synthesize new artificial superconductors in form of superlattices made out of cuprates, nickelates, bismuthates, aluminates, etc. The samples are characterized in-situ by RHEED and TOF-ISARS, and ex-situ by XRD, AFM, transport measurements, high-resolution TEM, etc. In most cases we studied so far, one apparently encounters some competing instabilities (CDW or SDW formation, electron localization, etc.). The quest for hypothetic superconducting phases may lead one to search through a large portion of the compositional phase-space; to this end we have developed techniques for combinatorial synthesis and high-throughput characterization including parallel (multi-channel) measurements of resistivity, Hall Effect, and mutual inductance. Our most recent findings will be reported. *Work done in collaboration with G. Logvenov, A. Gozar, A. Bollinger and O. Pelleg (BNL), J. Clayhold (Miami Univ., OH) and S. Pennycook (ORNL).   

2D quantum oscillations in the normal state of a superconducting oxide heterostructure

We present a novel system showing 2D superconductivity and 2D Shubnikov-de Haas (SdH) oscillations, realized in a SrTiO$_{3}$ heterostructure using the delta-doping technique [1]. As the thickness of the dopant layer (1 at.{\%} Nb:SrTiO$_{3})$ inserted between insulating SrTiO$_{3}$ layers decreases, a 3D to 2D crossover in the superconducting state is observed from the anisotropy and temperature dependence of the upper critical field. In the normal state, the thinnest sample (5.5 nm) shows SdH oscillations that scale with the perpendicular magnetic field. These results demonstrate a new possibility to explore the interplay of quantum transport and superconductivity. \\[4pt] [1] Y. Kozuka \textit{et al.}, \textit{Nature}, in press.   

Electronic structure of potassium graphite intercalation compounds KC8 and KC24

The recent discovery of high superconducting transitions in graphite intercalation compounds has sparked a new wave of interest in these materials. We report angle-resolved photoemission spectroscopy study of the first and second stage of graphite intercalated with potassium: KC8 and KC24. In both compounds, a complete charge transfer from the alkali metal to the graphene sheets was observed. Electron-phonon coupling on graphene-derived p* states with in-plane graphene-derived phonons is stronger in KC8, following a universal trend observed in other alkaline intercalated compounds where the coupling strengthens with doping of p* states. The observed coupling is therefore a good candidate for pairing interaction that causes superconductivity in graphite intercalation compounds.   

First Principles Study of Electronic and Vibrational Properties of BaHfN$_{2}$

The transition metal nitride BaHfN$_{2}$, which consists of weakly bonded neutral slabs of closed shell ions, has structural and chemical similarities to other layered nitrides which have impressive superconducting T$_{c}$'s when electron doped: A$_{x}$HfNCl, A$_{x}$ZrNCl, A$_{x}$TiNCl, with T$_{c}$= 26, 15.5 and 16 K, respectively for appropriate donor (A) concentrations x. These similarities suggest that BaHfN$_{2}$ may exhibit relatively high T$_{c}$ upon doping, with effects of structure and the role of specific transition metal ions yet to be understood. We carried out electronic structure calculations for stoichiometric BaHfN$_{2}$ and found a direct band gap of about 0.8 eV within Density Functional Theory, using the local density approximation. Doped electrons are expected to occupy the lowest conduction band, which has primarily Hf 5d$_{xy}$ character (similar to $\alpha $-TiNCl which has the lowest conduction band primarily composed of Ti 3d$_{xy})$. We also find that the two N sites, N1 in the Hf layer and N2 in the Ba layer, have very different Born effective charges (BEC). The deviations from the formal (-3) charge are opposite for the two sites. Comparison to the BEC's of the other compounds, and to the LO-TO splitting, will be discussed.   

Superconductivity in sol-gel prepared amorphous thin films of Na$_{x}$WO$_{3-y}$

Sol-gel prepared tungsten trioxide (WO$_{3-y})$ thin film (300 nm) electrochemically intercalated with sodium ions exhibits a pronounced diamagnetic and zero resistance onsets at 4.7 K. XRD spectrum of Na$_{x}$WO$_{3-y}$ film shows amorphous structure with very small peaks indicating to nucleation of hexagonal nanocrystals. Shift of $T_{c}$ at different applied currents and magnetic fields indicate on very high critical currents and critical magnetic fields. Despite of small resistivity onset starting at 6 K, the magnetic susceptibility measurements exhibit ZFC onset at 4.7 K, where the resistivity curve has second stronger temperature decline. This is a first observation of superconducting phase transition in amorphous tungsten bronzes.   

Pressure-induced Superconductivity in the Hydrogen-dense material YH$_3$

The results presented by us allow for an understanding of pressure-induced superconductivity of tri-hydrides with a particular focus on YH$_3$. We show that a structural phase transition from hexagonal to cubic structure occurs at 20 GPa, which is in good agreement with recent experiments. This structural phase transition is seen to be accompanied by an insulator-to-metal transition in our quasi-particle calculations. Furthermore, we present an analysis of the superconducting behavior in cubic YH$_3$. At the lowest possible pressure (17.7 GPa), cubic YH$_3$ is superconducting with a T$_c$ of 40 K and turns into the normal metallic phase at 25 GPa due to a change of s-d hybridization between hydrogen and yttrium. This hitherto unprecedented low pressure should make superconducting YH$_3$ a very attractive system to study experimentally among the hydrogen-rich superconductors. Finally, we also predict that the superconducting phase reemerges at 45 GPa. J. S. de Almeida, D. Y. Kim, C. Ortiz, M. Klintenberg, and R. Ahuja, Appl. Phys. Lett.{\bf 94} 251913 (2009). D. Y. Kim, R. H. Scheicher, R. Ahuja, Phys. Rev. Lett. {\bf 103}, 077002 (2009).   

Electronic structure and superconductivity in rare and not so rare monatomic materials

We have calculated the electronic structure of Eu and P for dfferent crystal structures and volumes using the augmented-plane wave method with both the local-density and generalized-gradient approximations. Europium calculations used a frozen-core approximation with a semi-empirical shift of the logarithmic derivatives which moves the 5f states below the valence bands. This shift of the localized f-states yields the correct europium phase ordering with lattice parameters and bulk moduli in good agreement with experimental data. The calculated superconductivity properties under pressure for the bcc and hcp structures are also found to agree with and follow a T$_{c}$ trend similar to recent measurement by Debessai et al.\footnote{M. Debessai, T. Matsuoka, J. J. Hamlin, J. S. Schilling, and K. Smimizu, Phys. Rev. Lett. 102, 197002 (2009).} The phosphorus phase ordering has been found for high-symmetry allotropes, with lattice parameters and bulk moduli in good agreement with experimental data. The calculated superconductivity properties under pressure for the simple cubic structure is found to agree with measurements and we predict the bcc phase as a high pressure superconductor.   

Superconductivity in lithium under pressure from electron-phonon coupling

Lithium exhibits a very high superconducting transition temperature at pressures above 20 GPa. To explore the role of phonon induced pairing of the electrons, we apply first principle techniques based on the pseudopotential density functional approach and the Wannier interpolation method to calculate the electron-phonon coupling properties of lithium for the pressure range of 10-40 GPa. We are able to sample a very fine k-point grid with up to several million points in the Brillouin zone (BZ). The Fermi surface (FS) nesting features and phonon renormalizations were examined. Our results for the coupling strengths and phonon linewidths are consistent with previous studies where high resolution was not yet available. The pressure increase results in an increase in the electron-phonon coupling parameter throughout the BZ. We find that the pressure dependent coupling is also related to the shape of the FS. The calculated Tc is in agreement with experiment.   

First principles predictions of NaH$_{x}$ (x=2--6) at high pressures

In the last several years, the prediction of novel structures of materials in normal conditions and under pressure has become possible due in part to advances algorithm development. Moreover, the increasing availability of ever faster computers has made it possible to optimize the structures of the materials under investigation in reasonable time frames. In our talk, we will present the application of a genetic algorithm to predict the structures of several potentiallly superconducting hydrogen rich NaH$_{x}$ (x=2--6) systems. Our work follows up on the recent prediction of LiH$_{x}$, (x=2--8) which were found to be stable and metallic at 1/4th the pressure required to metallize hydrogen itself.[1] It may be that the addition of Na to H leads the formation of stable, metallic systems at pressures even lower than for LiH$_{x}$. We explore which structures are likely to form and be stable, under a wide range of pressures.\\[4pt] [1] E. Zurek, R. Hoffmann, N. W. Ashcroft, A. R. Oganov, A. O. Lyakhov, Proc. Natl. Acad. Sci. {\bf 106}, 17640-17643 (2009).   

Silicon Field-Effect Spin Lattices for Magnetism and Superconductivity Tests

Field-effect silicon spin lattices, two-dimensional structures patterned at the ten-nanometer scale, may work well for implementing strongly-coupled electronic systems. These metal-oxide-semiconductor devices have a subtly structured silicon-oxide interface that can produce substantial electronic structure effects. Comparing strong-coupling length and energy scales for semiconductors shows silicon a reasonable choice, with scales of 15.0 nm and 8.8 meV. The systems considered here for emulation with spin lattices include Lieb's ferromagnetic (\emph{PRL} 1989) and Mattis' high-$T_{c}$ superconductivity (\emph{Int.~J.~Mod.~Phys.~B} 2006) Hubbard models. Effective mass calculations for independent electrons in spin lattice devices with antidot geometries gave energy bands for fitting to bands from the Hubbard model with interactions turned off. The onsite tight-binding electron repulsion energy was estimated with the Coulomb charging energy of a spin lattice site. The calculations examined lattices with either electric field-dependent or -independent hopping parameters. For circular antidots on a square lattice, the optimal period-to-diameter ratio of $l/d\approx2.5$ maximizes Mattis' coupling parameter, giving 1.7 meV and 15 meV for diameters $d=10$ and $3$ nm. Technology issues will be discussed, including the results of ongoing fabrication efforts.